V o lu m e I I ALAN OSBOURNE SECOND EDITION EVERETT C. HUNT, W e b b I n s titu te E d ito r - in - C h ie f o f N a v a l A r c h ite c tu r e CONTRIBUTING EDITORS David Brown James Mercanti N e w S u lz e r D ie s e l L td . C a m a r C o r p o r a tio n , I n c . James A. Harbach William B. Morgan U .S . M e r c h a n t M a r in e A c a d e m y D a v id T a y lo r R e s e a r c h C e n te r R. D. Jacobs Alan L. Rowen C o n s u lta n t W e b b I n s titu te o f N a v a l A r c h ite c tu r e Aaron R. Kramer S ta te U n iv e r s ity o f N e w Y o r k M a r itim e C o lle g e Keith Wilson C o n s u lta n t Conrad C. Youngren S ta te U n iv e r s ity o f N e w Y o r k M a r itim e C o lle g e Copyright © 1943, 1991by Cornell Maritime Press, Inc. All rights reserved. No part of this book may be reproduced in any manner whatsoever without written permission except in the caHC of brief quotations embodiedin critical articles and reviews. For information, address Cornell Maritime Press, Inc., c,entreville, Maryland 21617. Library Modern Hunt. p. marine of Congress Cataloging-in-Publication engineer's manual.-2nd For the seamen ofthe U.s. Merchant Marine, who in times of national emergency have never been found wanting Data ed. / edited by Everett C. em. "Based on the original edition by Alan Osbourne." ISBN 0-87033-307-0 (v. 2) 1. Marine engineering. II. Osbourne, Alan. VM600.M65 623.8'7-dc20 I. Hunt, Everett C. 1990 89-71201 CIP Manufactured in the United States of America First edition, 1943. Second edition, 1991; second printing, 1994 C o n te n ts FOREWORD TO FIRST PREFACE EDITION xv : xvii CHAPTER 16 M a r in e D ie s e l E n g in e s Alan L. Rowen and R. D. Jacobs In trod uction 16-1 Operating Principles 16-3 Fuels for Diesel Engines 16-15 Turbocharging 16-22 Intake and Exhaust Systems 16-38 Fuel Injection and Combustion 16-42 Moments, Forces, and Vibration 16-50 Engine Performance: Matching Engines to Their Loads 16-70 Propulsion Engine Support Systems 16-81 Operating and Maintenance Procedures Vll 16-109 CONTENTS VllI CHAPTER CONTENTS 17 CHAPTER Engine Descriptions IX 20 Cryogenic Cargo Systems Keith Wilson, David Brown, and Alan L. Rowen James A. Harbach , Introduction 1 7 -1 Cryogenic Principles Sulzer RTA Two-Stroke Diesel Engines 1 7 -1 LNG Cargo Tanks SEMT-Pielstick PC2.5 Four-Stroke Diesel Engines CHAPTER 1 7 -4 1 r 20-1 20-5 LNG Cargo Systems 20-10 LNG Cargo Operations 20-21 Detailed Operating Procedures 20-22 18 Marine Refrigeration Systems CHAPTER James A. Harbach Refrigeration Principles 1 f l· 1 The Vapor-Compression Cycle and Refrigeration SystemH 1 8 ·7 H u ll 21 Machinery Everett C. Hunt Refrigeration System Components 1 1 4 ·1 6 Introduction 21-1 Operation and Maintenance 1 1 '1 ·4 3 General Requirements for Hull Machinery 21-1 Troubleshooting the Systems 1 1 4 ·1 5 7 Steering Gear CHAPTER 21-3 Windlasses, Capstans, and Gypsies 21-24 Winches 21-35 19 Heating, Ventilation, and A ir - C o n d it io n in g CHAPTER J ames A. Harbach Definitions and Principles 1 1 ·1 HVAC Systems 1 1 ·4 22 Marine Electrical Systems Conrad C. Youngren HVAC System Components 1 8 ·1 8 Shipboard Electrical Distribution Systems System Testing and Balancing 1 8 ·1 8 System Components 22-15 Electric Propulsion 22-43 Circuit Calculations 22-51 Glossary 22-61 22-1 x CONTENTS CONTENTS Xl Shipboard Approach to Vibration Analysis CHAPTER 23 Electrical Machinery Conrad C. Youngren Synchronous Alternators 23-1 Synchronous Motors 23-18 Three-Phase Induction Motors 23-25 Single-Phase Motors 23-57 DC Machinery 23-68 CHAPTER 25-11 Vibration Measuring Equipment 25-13 Programs for Preventive Maintenance 25-17 Interpreting Results of Vibration Measurements 25-21 Characteristics of Specific Machinery Defects 25-27 Marine Vibration Case Histories 25-29 CHAPTER 26 Inert Gas Systems and Crude Washing Machinery O il Everett C. Hunt And James Mercanti 24 Shipboard Central Operating Systems Aaron R. Kramer A History 24-1 Central Engine Room Operating System Components 24-3 Design Considerations 24-5 Central Operating System Types 24-8 Digital System Components 24-10 Digital Systems 24-17 Installation and Maintenance of Digital Systems 24-22 Use of Digital Central Operating, Monitoring, and Control Systems 24-30 Introduction and Background 26-1 Principles of Inert Gas Systems 26-3 Types of Inert Gas Systems 26-7 Design of Inert Gas Systems 26-8 Operation of the Inert Gas System 26-24 Maintenance and Testing 26-38 Instruction Manuals 26-40 Crude Oil Washing 26-41 Appendix: Extracts from IMO Regulations Concerning Crude Oil Washing 26-48 CHAPTER CHAPTER 25-8 Machinery Vibration Acceptable Limits 27 Coal Burning Technology 25 Everett C. Hunt Shipboard Vibration Analysis Everett C. Hunt Introduction 25-1 Design Engineer's Approach 25-7 Introduction 27-1 Coal Fuel 27-2 Combustion Engineering (C-E) Coal Fired Marine Boilers 27-18 XllI CONTENTS CONTENTS XlI Foster Wheeler (F-W) Coal Fired Boiler 27-31 Marine Coal Handling System 27 -37 Stoker System (Detroit Rotograte) 27-41 Combustion Control System 27 -54 CHAPTER 30 M a c h in e r y T e s ts a n d T r ia ls Everett C. Hunt CHAPTER 28 W a s te D is p o s a l S y ste m s Everett C. Hunt Introduction 30-1 Tests at Manufacturer's Plant 30-1 Dock Trials 30-2 Sea Trials 30-2 Introduction 28-1 Sea Trial Main Engine Testing 30-5 Sewage Treatment 28-2 Special Shipboard Instrumentation 30-7 Design of Sewage Treatment Systems 28-3 Shipboard Use of Trial Standardization Operation of a Sewage Treatment Plant 28-9 Discharge of Oily Water 28-10 Incineration of Oil Waste and Garbage 28-14 Design Features ofIncineration 28-15 Systems Feeding and Control of Incinerators 28-17 Operation of a Marine Incinerator 28-19 Glossary 28-19 CHAPTER P r o p e lle r s 29 a n d P r o p u ls io n William B. Morgan In trod uction 29-1 Ship Resistance and Propulsion 29-1 Propeller Design 29-8 Types of Propellers 29-26 Maintenance and Repair of Propellers 29-30 Glossary 29-33 INDEX 30-15 Data following Chapter 30 F o r e w o r d to F ir s t E d itio n HE first volume of this manual of Marine Engineering has received a gratifyingly wide acceptance among operating men. It is hoped that this second volume will also justify its place as a guide to the student and a companion to the older marine engineer. T JAMES L. BATES Director, Technical Division U. S. Maritime Commission W a s h in g to n , M arch D. C. 2, 1943 xv P r e fa c e HIS second edition of M o d e r n M a r in e E n g in e e r 's M a n u a l, Volume I I , published a half century after the first edition, will be useful to merchant marine engineer officers, superintendent and port engineers, ship repair specialists, and students. While this volume may be of some general interest to engineers engaged in ship design and shipbuilding, it is specifically directed to those involved in the operation and maintenance of shipboard machinery systems. The second edition is not a revision of the first edition. It is an entirely new manual prepared in the tradition of the first edition. In addition to the shipboard auxiliary machinery of the first edition, this edition places special and appropriate emphasis on diesel main propulsion, cargo systems, central operating systems, and vibration analysis as a monitoring and maintenance tool. A chapter on combustion of coal has been included in anticipation of a renewed interest in this fuel. While today's merchant ship retains most of the functional attributes of the machinery systems described in the first edition, the details are greatly different. Direct current electric power systems are rare except on some special vessels, such as cable vessels. High propulsion power ratings are common, providing higher speed for larger vessels. The modern slowspeed long-stroke diesel propulsion system has replaced the geared steam turbine as the most efficient and the most popular of available main propulsion systems. Unique cargo systems, such as LNG, container carriers, chemical carriers, very large crude oil carriers, and neo-bulk carriers, are in common use. Central operations, bridge control, unmanned machinery spaces, and special contract repair personnel are providing opportunities for reduction in the ship's force. The machinery associated with these changes is discussed in this edition. We have tried to incorporate metric measurements as well as the U. S. customary units. It is obviousthat most of the maritime world uses the S. 1.U. T XVll XVlll PREFACE system. Americans are long overdue in becoming comfortable with the S. I. U. system of measurements. The contributing editors of the second edition are all experienced in problems of ship operations and ship design. Most of them teach in accredited engineering schools with programs in marine engineering. A manual of this type would be impossible without the help and cooperation of the many industrial organizations that develop, design, and manufacture the wide array of shipboard machinery systems. These companies are fully acknowledged at the end of each chapter. ,'.t MODERN MARINE ENGINEER'S V o lu m e II MANUAL CHAPTER 16 Marine Diesel Engines ALAN L. ROWEN AND R. D. JACOBS INTRODUCTION Current Status N1990 diesel engines are by far the dominant choice for propulsion of merchant ships and naval auxiliary vessels. The radical increases in fuel oil prices which followedthe Middle East war of 1973 elevated the fuel component of ship operating cost to the point of dwarfing most of the other factors, including machinery maintenance. The higher efficiency of diesel engines relative to steam and gas turbine plants made them the obvious choice for new construction and many major conversions. In the years since, evolutionary developments in diesel engine design, which have not been matched in steam or gas turbine plants, have emphasized these differences. I Classification Diesel engines are probably best defined as reciprocating, c o m p r e s s io n ig n itio n engines, in which the fuel is ignited on injection by the hot, compressed charge of air in the cylinder. Beyond this they may be classified as follows: Speed. Traditionally, diesel engines are grouped into categories of lo w , and h ig h s p e e d , depending on crankshaft RPM and/or mean piston speed. Engine design appears to have overtaken the traditional definitions of the boundaries among these categories, however, especially when one attempts to distinguish between the medium and high speed groups, and a case can be made for additional categories. Low speed engines might best be defined as those whose crankshaft speeds are a suitable match for direct connection to a ship's propeller without reduction m e d iu m , 16-1 16-2 OPERATING MARINE DIESEL ENGINES PRINCIPLES 16-3 gearing, and so tend to have rated crankshaft speeds below 250 to 300 RPM. Most engineers would place the upper limit of the medium speed group, and the start of the high speed group, in the range of 900 to 1,200 RPM. With reference to the discussions which follow, low speed engines are usually two-stroke, in-line, crosshead engines with high stroke-to-bore ratios, while medium and high speed engines may be two- or four-stroke, in-line or V, and, with few exceptions, are trunk piston types with low stroke-to-bore ratios. bustion occurs alternately on both sides of the piston, producing power thrusts alternating toward and away from the crankshaft. All major engines of current design are single acting, although some double acting engines remain in service. Another type is the o p p o s e d p is to n e n g in e , in which combustion takes place between two pistons in each cylinder, each of which is single acting. Doxford opposed piston, low speed engines remained in production in Britain until 1981, while the Fairbanks Morse medium.speed engine remains in production in 1990. Thermodynamic cycle. Theoretical thermodynamic cycles for internal combustion engines include the O tto cycle, the d ie s e l cycle, and a combination of the two called the d u a l c o m b u s tio n , mixed, or Sabathe cycle. While these are theoretical cycles that are only approached in reality, it is the dual combustion cycle that most accurately represents the operation of most diesel engines of current design. Method of fuel injection. With the s o lid in je c tio n m e th o d , fuel is injected at very high pressure developed mechanically by an engine-driven fuel pump. Solid injection is the normal method offuel injection on engines of current design. A ir in je c tio n uses an engine-driven high pressure air compressor to inject the fuel, and is now generally obsolete. Combustion chamber design. In a d ir e c t or open chamber, the fuel is injected directly into the cylinder. Most engines of current design are of this type. In a p r e - c o m b u s tio n c h a m b e r design, a portion of the cylinder volume is partially isolated to receive the fuel injection. Some higher speed engines are so designed. Operating cycle. This can be tw o - s tr o k e , in which the entire sequence of events takes place in one revolution, or fo u r - s tr o k e , in which the sequence requires two revolutions. Cylinder grouping. Most engines of current design are vertical. There may be up to 12 cylinders in-line, or as many as 24 in a V configuration. Cylinder proportions. Cylinder proportions may be expressed as the Low speed engines may have very high ratios of 3:1 or more, but medium and high speed engines are usually constrained by air flow considerations to ratios close to one. s tr o k e - to - b o r e r a tio . Air supply. This can be provided in one of three ways: (1) T u r b o c h a r g e d , in which air is supplied to the engine at a pressure above atmospheric by a compressor driven by the exhaust gases. Most engines of current design are turbocharged. (2) Turbocharged and a fte r c o o le d , in which the air leaving the turbocharger, at high temperature as a result of compression, is cooled before entering the cylinders. Most engines of current design, especially the larger ones, are not only turbocharged but also aftercooled. ( 3 ) N a tu r a lly (or normally) a s p ir a te d , in which the engine draws its air directly from its surroundings at atmospheric pressure. Two-stroke cycle engines that are not turbocharged are incapable of drawing in air on their own, and so must be provided with some means of supplying air to the cylinders, such as under-piston scavenging or an engine-driven low pressure blower. Running gear can include a tr u n k p is to n , in which the cylinder wall must carry the side thrust of the connecting rod, or a c r o s s h e a d , in which the side thrust is transmitted directly to the engine structure by a crosshead and crosshead guide. Power pulses. Engines may be s in g le a c tin g , in which combustion produces one power thrust toward the crankshaft, or d o u b le a c tin g , in which com- II', I' -, Cooling. An engine may be w a te r c o o le d , in which case water is circulated through cooling passages around the combustion chamber, or a ir c o o le d , in which air is circulated over the external surfaces of the engine. Most marine engines are water cooled in a closed circuit by treated fresh water, which is then cooled in a closed heat exchanger by seawater, although for some applications, such as emergency generator engines, the heat exchanger may be an air-cooled radiator as in automotive applications. In any event, the lubricating oil serves as an intermediate coolant of the bearings and, in most cases, of the piston as well. OPERATING PRINCIPLES Thermodynamic Cycles Theoretical thermodynamic cycles for internal combustion engines include the Otto cycle, the diesel cycle, and a combination of the two, called the dual combustion, mixed or Sabathe cycle. While these are theoretical cycles that are only approached in reality, it is the dual combustion cycle that 16-4 MARINE DIESEL ENGINES OPERATING PRINCIPLES most accurately represents the operation of most diesel engines of current design. In the Otto cycle, a charge offuel and air is ignited by a spark and burns explosively, so rapidly that combustion is completed before the piston begins to move down, and therefore takes place at constant volume. Otto cycle engines usually operate on gasoline and are classified as spark-ignition engines. In the diesel and dual combustion cycles, ignition occurs when fuel is injected into the hot, compressed charge, and combustion continues after injection ceases until the fuel is consumed. Engines operating on these cycles are categorized as compression-ignition engines. The diesel cycle, in which the rate of combustion is so matched to the descent ofthe piston that pressure during the combustion period is constant, is difficult to achieve in practical engines. The dual combustion cycle assumes the initial combustion process to be explosive and the rest to occur at constant pressure, which more closely approximates conditions in diesel engines of current design. Most oil-burning diesel engines of current design are compressionignition types whose thermodynamic cycle is approximated by the dual combustion cycle. Otto and dual combustion cycles are related by the manner in which combustion takes place; if all other factors were equal, the theoretical thermal efficiency would be higher for an Otto cycle. Practical considerations prevent all these other factors from being equal, but the fact remains that the closer the dual combustion cycle can be made to approach an Otto cycle, with a large fraction of the fuel burning rapidly before the piston commences its downward stroke, the higher will be its theoretical thermal efficiency. A modification of the dual combustion cycle known as the Miller cycle or the modified Atkinson cycle has been used with diesel engines fitted with two-stage turbochargers. B a s ic T e r m in o lo g y Refer to Figure 16-1. The p is to n operates in the c y lin d e r b lo c k , which, in all but the smallest engines, is fitted with a replaceable cast iron c y lin d e r lin e r as well as a separate c y lin d e r h e a d . The liner and the head are usually water cooled, while the piston is usually oil cooled except in some of the large, low speed engines where it is water cooled. The reciprocating motion of the piston is converted to rotary motion of the crankshaft by the c o n n e c tin g r o d , which swivels about the w r is t (piston or gudgeon) p in at the top, and at its bottom end rotates about the c r a n k p in . The inside diameter of the cylinder is the b o r e . The uppermost position ofthe piston (and therefore of the crank) is to p d e a d c e n te r , or TDC, while the lowest is b o tto m d e a d c e n te r , or BDC. The distance travelled by the piston between TDC and BDC is the s tr o k e , which, when multiplied by the Figure 16-1. Two-stroke crosshead engine 16-5 16-6 MARINEDIESELENGINES cross-sectional area of the cylinder bore, yields the volume displaced by the piston during its stroke, called the d is p la c e m e n t. F o u r -S tr o k e Cycle E v e n t s Cycle events are most easily understood in relation to the four-stroke cycle, shown diagrammatically in Figures 16-2 and 16-3. The latter figure represents the pressure in the cylinder plotted against the piston position, which in turn is directly proportional to the cylinder volume displaced by the piston at that point in its travel. In four-stroke cycle engines the head contains passages connecting to air supply and exhaust manifolds, and also carries the air and exhaust valves as well as the fuel injector. Air and exhaust valves are opened into the cylinder mechanically by p u s h r o d s and rocker arms operated by the c a m s h a ft, and are closed by the combination of pressure within the cylinder and the force of the valve springs. The camshaft is gear- or chain-driven from the crankshaft at one-half of crankshaft R P M , in order to complete one cycle of events in two revolutions. (Each revolution causes two strokes of the piston: one up, one down.) Starting with the piston at top dead center at the start of the charging stroke, the events are as follows: OPERATINGPRINCIPLES 16-7 1. The charging stroke (in naturally aspirated engines, this is the intake or suction stroke). The air valve is open but the exhaust valve is closed. The piston has passed the top dead center position and is being moved down by the connecting rod as the crankshaft rotates. As the piston descends, air flows into the cylinder because the pressure in the cylinder is slightly less than that in the air manifold. Power to turn the crankshaft is provided by the other cylinders in a multiple-cylinder engine, or by energy stored in the flywheel. 2. The compression stroke. The air valve closes as the piston passes through bottom dead center, trapping the charge of air in the cylinder. The piston is driven up as the crankshaft rotates, compressing the charge to one-tenth to one-twentieth of its initial volume (the actual value, called the compression ratio, is at the lower end of this range in turbocharged engines). As the charge is compressed, its temperature rises until, toward the end of the stroke, it is well above the ignition temperature of the fuel. 3. Fuel injection. Fuel injection begins during the compression stroke, before the piston reaches top dead center. Ignition will occur as soon as the first droplets of fuel are heated to ignition temperature by the hot charge. The brief time between the beginning of injection and ignition is the ig n itio n d e la y period. (The fuel which accumulates during the ignition delay period accounts for the initial explosive combustion phase of the dual combustion cycle.) 4. The power stroke. After the piston passes through TDC, the pressure developed by the combustion of the fuel begins to force the piston down. As 16-8 MARINE DIESEL ENGINES the cylinder volume increases, however, the continued combustion maintains the pressure in the cylinder until injection and then combustion cease (points that are called, respectively, c u to ffa n d b u r n o u t) . M te r burnout, the piston continues to be forced down by the expanding gas. The power developed is related to the quantity of fuel burned in the cylinder.The quantity is in turn proportional to the length ofthe injection period,if the fuel is injected at a constant rate (as it is in most engines). If the beginning ofthe injection period is fixed (the more common case) then at light loads cutoff occurs early, followed by a long expansion period. 5. The exhaust stroke. The exhaust stroke actually begins just before the piston reaches bottom dead center, when the exhaust valve opens and the residual high pressure in the cylinder is relieved into the exhaust manifold as the gases b lo w d o w n . As the crankshaft pushes the connecting rod and piston up, most ofthe gas remaining in the cylinder is forced out. At top dead center only a fraction of the gas remains. In turbocharged engines this will be swept out as the air valve opens, just before the exhaust valve closes. This brief period when both valves are open is the o v e r la p period, and the process in which incoming air sweeps the cylinder clear of exhaust gas is called s c a v e n g in g . As the piston passes through top dead center, with the exhaust valve closingand the air valve opening, the cycle repeats. T w o -S tr o k e C y c le E v e n ts Engines operating on the two-stroke cycle may be lo o p - s c a v e n g e d or u n iflo w - s c a v e n g e d , as illustrated diagrammatically in Figures 16-4 and 16-5. Figure 16-5. Uniflow scavenging I n general, in two-stroke cycle engines, air is supplied to the cylinder through a row of ports arranged around the circumference of the cylinder liner just above the bottom dead center position of the piston crown, the piston and ports therefore serving the same function as the air valves of the four-stroke cycle engine. I n loop-scavenged engines, exhaust also takes place through a row of ports in the cylinder, these being arranged just above the air ports. Uniflow-scavenged engines (except opposed piston engines) exhaust through a valve (or two valves) in the cylinder head, which is operated by the camshaft. Since, in the two-stroke cycle, one cycle of events is completed in each revolution of the crankshaft, the camshaft speed is the same as that of the crankshaft. I n discussing events in the two-stroke cycle, it is important to bear in mind that air is always supplied to the cylinders under pressure, either the higher discharge pressure of a turbocharger or the lower pressure of a boost blower (or, in two-stroke engines without supercharging, of a scavenge air blower). With the piston at bottom dead center at the start of a cycle, events are as follows: 1. Scavenging and charging. As the piston passes through bottom dead center, the airports are open, as are the exhaust ports (or valves). Scavenging occurs 16-10 MARINE DIESEL ENGINES as the incoming air sweeps out the exhaust gases, a process which is likely to be more effective in a uniflow engine, especially in cylinders of high stroke-to-bore ratios. As the piston rises it closes off the air ports, then the exhaust ports in the loop-scavenged engine. In uniflow engines the exhaust valve is closed at this time. With the charge trapped in the cylinder, compression begins. 2. The compression stroke. As in the four-stroke cycle engine, as the piston rises, it compresses the charge to perhaps one-tenth to one-twentieth of its initial volume (the actual value, called the compression ratio, is at the lower end ofthe range in t].lrbocharged engines). As the charge is compressed, its temperature rises until, toward the end of the stroke, it is well above the ignition temperature of the fuel. 3. Fuel injection. Fuel injection begins during the compression stroke, before the piston reaches top dead center. Ignition will occur as soon as the first droplets of fuel are heated to ignition temperature by the hot charge. The brief time between the beginning of injection and ignition is the ig n itio n d e la y period. (The fuel which accumulates during the ignition delay period accounts for the initial explosive combustion phase in the dual combustion cycle.) 4. The power stroke. After the piston passes TDC the pressure developed by the combustion of the fuel begins to force the piston down. As the cylinder volume increases, however, the continued combustion will maintain the pressure in the cylinder until injection and then combustion cease (points which are called, respectively, cutoff and burnout). Subsequently, the piston continues to be forced down by the expanding gas. The power developed is related to the quantity of fuel burned in the cylinder. The quantity is in turn proportional to the length ofthe injection period, if the fuel is injected at a constant rate (as it is in most engines). If the beginning ofthe injection period is fixed (the more common case) then at light loads cutoff occurs early, followed by a long expansion period. 5. Exhaust. Exhaust begins in the loop-scavenged engine as soon as the descending piston exposes the exhaust ports, and the residual high pressure in the cylinder is relieved into the exhaust manifold as the gases b lo w d o w n . In the uniflow engine the exhaust valves are opened at about this time and the resulting action is similar. As the piston continues its descent, the air ports are exposed and incoming air begins to sweep the cylinder clear of exhaust gas. As the piston passes through bottom dead center, with air ports as well as exhaust ports (or valves) open, the cycle repeats. Deviations from the Norm Opposed piston engines. Opposed piston engines operate on the two-stroke cycle, generally as described above, but achieve uniflow scavenging OPERATING PRINCIPLES 16-11 without exhaust valves. See Figure 16-6. The cylinder liner has ports arranged at each end: one set is for air, the other for exhaust, and each set is controlled by one of the pistons. Loop-scavenged engines with exhaust valves. Some older loop-scavenged engines are fitted with rotating valves in the passage from the exhaust ports to the manifold so that, even when the exhaust ports remain exposed by the piston on the upstroke, compression can begin earlier. Glow plug and hot bulb engines. In these engines, generally considered obsolete, the compression ratio is insufficient to raise the temperature of the charge air above the ignition temperature of the fuel. The glow plug or hot bulb is an ignition source in the cylinder that will ignite the fuel as it is injected. G lo w p lu g s are heated electrically, while a h o t b u lb is simply an uncooled portion of the cylinder head that can be heated initially by a blowtorch to start the engine, after which it will be kept hot by the combustion process. Some small engines use glow plugs as a cold-starting aid, but run on a normal dual combustion cycle once warmed. 16-12 OPERATING PRINCIPLES MARINE DIESEL ENGINES Indicator Cards, IHP, BHP, Pressures Indicator cards. Figure 16-3 represents the pressure in an engine cylinder plotted against the piston position, which in turn is directly proportional to cylinder volume, and is therefore called a pressure-volume, or P-V, diagram. When the P-V diagram is obtained from the engine itself, using an engine indicator for low speed engines or electronic means for higher speed engines, it is called an in d ic a to r c a r d . IHP. In thermodynamic' terms, the work done during a cycle is the product of the pressure at any point in the cycle times the volume displaced by the piston at that point. It is therefore proportional to the area enclosed by the curve on the P-V diagram. The area enclosed can be determined by measurement with a planimeter, or by graphical or mathematical integration. Once multiplied by the appropriate constants, this area is the n e t w o r k (Wnet)done by the piston during the cycle; i.e., it is all the work delivered by the piston to the crankshaft during the power stroke, plus or including the work to overcome friction and to drive engine accessories, less the work obtained from the crankshaft to drive the piston on the other strokes. The mathematical expression is: where C is the constant of integration, P is cylinder pressure, and V is cylinder volume. When the net work is multiplied by the RPM, the result is the in d ic a te d p o w e r developed by the cylinder, expressed in horsepower as its IHP. The IHP of the engine is the sum of the IHP of the cylinders. Stated mathematically: IHP = L Wnet X RPM 16-13 during shop tests of an engine, when the engine is connected to drive a dynamometer. (One antiquated form of dynamometer is the prony brake, hence the terminology. The prony brake was named after the French engineer G. C. F. M. Riche, Baron de Prony.) The ratio ofBHP to IHP, expressed as a percentage, is the m e c h a n ic a l e ffic ie n c y of the engine. The mechanical efficiency can be accurately measured only when the BHP can be measured. However, once determined, values of mechanical efficiency provide a means of estimating BHP from measured values of IHP. Expressed mathematically: BHP = mechanical efficiency x IHP Maximum, boost, and mean effective pressures. The highest pressure reached in the combustion chamber during the cycle is the m a x im u m p r e s s u r e , also called the maximum firing pressure or the peak pressure. It can be readily measured in service with a special pressure gauge, and is therefore a useful diagnostic tool, especially for medium and high speed engines for which conventional indicator cards cannot easily be taken. The maximum pressure is usually reached shortly after injection begins, just beyond TDC. It is the maximum pressure developed when the engine is running at full load or rated output, which, with margin applied, the cylinder components must be designed to withstand. The b o o s t p r e s s u r e is the pressure in the charge air manifold of engines with turbochargers or blowers. The m e a n e ffe c tiv e p r e s s u r e (MEP) and the m e a n in d ic a te d p r e s s u r e CMIP)are the average pressures during the complete cycle. These values are calculated from measured data: When calculated from the indicated power the resulting value is the MIP, while a calculation from the BHP will yield the MEP. The two differ because of mechanical efficiency. The appropriate expressions are as follows: If all the cylinders were assumed to be performing equally, the expression would be: IHP = N x Wnet X RPM where N is the number of cylinders. It is important to stress that the IHP includes the power consumed by friction and by engine-driven accessories. However, since the IHP can be accurately measured by operators of low speed engines, it remains a primary tool for assessing the performance of these engines in service. BHP. The power that can actually be obtained from the engine is called the b r a k e p o w e r , or BHP when expressed in horsepower. It can be measured where C represents the appropriate unit conversion factors and Vdis is the displacement of the cylinderCs). Relationship of Power, MEP, MIP, and RPM. The expressions below relate power to MEP, MIP, and RPM: 16-15 FUELS FOR DIESEL ENGINES engines, as they have been matched to turbochargers ofincreasing efficiency, permitting the attainment of higher MEP. 5. Just as there is a limiting value ofMEP for a particular engine, so will there be a limiting value of torque. For this reason diesel engines are considered torque-limited machines; i.e., they are prevented, at reduced RPM, from reaching their rated BHP by a torque limit. where C = L= B= N= S p e c ific the appropriate unit conversion factor stroke bore RPM for two-stroke cylinders, or RPMl2 for four-stroke cylinders The amount of fuel consumed by an engine over a period of time, divided by the power output of the engine, is the specific fuel consumption (SFC). It will usually be measured on a test-bed at constant RPM and load, in accord with an established standard test code, for a fuel of given quality, and will be expressed as grams (or pounds) of fuel per metric (or British) brake horsepower (or kilowatt) per hour. Among the standards used in this country are those of ASME, SAE, ISO, and DEMA. Even for the same engine, the SFC will vary with ambient conditions. with load, with RPM, and with fuel quality. It is most important, in comparing values ofSFC, to ascertain that these factors are all the same, and to determine whether or not there are parasitic loads being imposed by such auxiliaries as enginedriven cooling or oil pumps. These relations can easily be remembered if one uses P for pressure 2 and notes that the cylinder cross-sectional area, A, is (TIl4)B . Then the expressions become: Power Among the important relations = conclusions F u e l C o n s u m p tio n PLAN which can be drawn from these are the following: 1. If all other things were equal, a two-stroke engine could deliver twice the power ofa similar size four-stroke engine. In actual fact, however, attainable levels of work per cycle or MEP in two-stroke engines are about half those of four-stroke engines, mainly because of the improved cooling possible between power strokes in the four-stroke cycle. The doubled number of power strokes per revolution of two-stroke engines therefore tends to compensate for their lower MEP. 2. For any given engine, power is proportional to the product ofMEP and RPM. The implication of this relation is that, should the RPM of the engine be reduced to a lower-than-rated value, the power output will have to be reduced in accord with the limiting values of MEP. For example, a main propulsion engine driving a fixed-pitch propeller will be forced to a lower RPM as the hull fouls over a period of time; an attempt to maintain engine output under these conditions results in high values of MEP, usually reflected in high exhaust temperatures. 3. To the extent that limiting values of MEP are indicative of limiting maximum pressure (therefore approaching maximum permissible component stress levels), it should be obvious from the relations that, if component strength were roughly equal for all engines, then a particular power output could be achieved by a cylinder of large dimensions at low RPM, or by a smaller cylinder at higher RPM. 4. Conversely, if component strength were increased to permit higher values ofMEP, then higher engine output could be achieved from an engine ofgiven size and RPM. This has been, in fact, the path of design evolution of most FUELS FOR D IE S E L E N G IN E S I n tr o d u c tio n Fuels are discussed in Chapter 8 of Volume I; the discussion below is limited to fuels for marine diesel engines. It should be noted that in the years since Volume I was published there have been substantial changes in sources of crude oil, in refining techniques, and in distribution and marketing procedures, changes that have had generally harmful effects on the characteristics of fuels used aboard motorships. Even distillate fuels are often at the limits of the specification. It remains true that a balance must be struck between the lower cost of the heavier fuel oils, and the inconvenience and greater cost of the fuel treatment combined with the increased engine maintenance associated with their use. At this point the great majority of low speed engines and a good number of medium speed engines are operated on heavy fuels, while an increasing number of high speed engines are proving capable of operation on at least lighter blends. T e r m in o lo g y : i H e a v y F u e ls versus L ig h t F u e ls Refining separates crude oil into a number of hydrocarbon products in a process based on their boiling points, with the lightest products having the lowest boiling points. At the light end of this spectrum are the distilled products, including the light distillate fuels known as gas oil or number 2. FUELS FOR DIESEL ENGINES 16-17 MARINE DIESEL ENGINES 16-16 settling, filtration, centrifuging, and atomization. Reasons for incorrect fuel temperature (and therefore higher viscosity) include inadequate steam supply, inadequate or fouled heating surfaces, damaged or missing insulation, and poorly calibrated or malfunctioning thermometers or viscosimeters. At the very high end of the viscosity spectrum problems may arise if the fuel must be heated to the point where it is subject to thermal cracking, or where thermal expansion of the injection pump components is sufficient to move their clearances outside intended limits. It is essential when burning heavy fuel in a diesel engine that the viscosity at the injection pumps and injectors be within design limits at all times. The volume offuel consumed by an engine will be small in relation to the volume available in the piping; therefore, in installations intended for operation on heavy fuels, the residence time between the heaters and the injectors can be sufficient, especially at low loads, for the fuel to cool. To prevent this cooling, a much larger flow rate is maintained, two or three times engine consumption at maximum continuous rating (MCR),with the unconsumed excess leaving the spill valves of the injection pumps and recirculating back to the booster pump suction (see "Fuel forwarding system" near the end of this chapter). These fuels are suitable for combustion in diesel engines without preheating (except in the coldest climates), so that fuel treatment can be limited to settling and filtration, although it is good practice to centrifuge even the distillate fuels. Present refining techniques are aimed at extracting the largest quantity of distilled products feasible from the crude. The resulting residual tends to be ofvery high viscosity, with most ofthe undesirable constituents of the crude, and it is frequently contaminated with the highly abrasive particles from catalytic eonverters called c a ta ly tic fin e s . This is the residual fuel used in most steamship boilers without any further treatment other than heating, settling, and rather coarse filtering. Residual fuel is rarely used alone as a fuel for diesel engines; far more frequently it is blended with a distilled product (the c u ttin g s to c k ) to produce a less viscous intermediate fuel, which, depending on the proportions used, can itselfbe described as light or heavy. Even the lighter blends will require preheating before pumping, settling, centrifuging, and combustion, so it is reasonable to refer to any intermediate fuel as heavy fuel. A blending chart can be used to determine the proportions necessary to produce a blend of selected viscosity, as shown, for example, in Table 16-l. It can be seen that relatively small fractions of distilled product can reduce the viscosity substantially, so that even the lighter blends will contain significant amounts of undesirable constituents. Specific gravity. The ability to separate water and solids from a fuel by settling and centrifuging is dependent primarily on their differences in weight from the fuel (and is also affected by the fuel viscosity). These differences increase as the fuel is heated. Conventional centrifuges can achieve adequate separation of water from suitably heated fuel with a specific gravity as high as 0.995 at ambient temperature. More sophisticated centrifuges with water-sensing controls can separate even heavier TABLE 16-1 Intermediate Fuels Produced by Blending C u ttin g S to c k ( d is tilla te ) Viscosity, cST at 50'C Percent cutting stock Percent residual Vanadium, ppm Sulfur, percent by weight B le n d s IF 8 0 IF 1 8 0 IF 2 8 0 40 100 80 27 180 14 280 10 o 73 86 90 290 3.1 345 3.5 360 3.7 nil 0.5 fuels. It should be borne in mind that an injection pump is a volume-measuring device: at constant engine output rack settings will vary depending on both the specific gravity and the heating value ofthe fuel. R e s id u a l 600 o 100 400 4.0 Heating value. The heating value (per unit mass or weight) ofresidual fuels is typically some 6 percent lower than that of distillates, a difference which carries over in proportion to the blended fuels. There is an inverse relation between the heating value and the specific gravity, as both properties are determined by the chemical composition ofthe fuel, i.e., the ratio of carbon to hydrogen, and the presence of other combustible elements, especially Fuel Properties and Constituents and Their Consequences Fuel properties are defined in Chapter 8 of Volume I. The following discussion is an amplification ofthat material as it applies to diesel engines of current design. Viscosity. Because fuel is usually sold according to its viscosity, viscosity is often considered an index of fuel quality. This can be misleading since full consideration must be given to undesirable constituents and properties. Viscosity offuel alone may present no problem as long as the fuel can be heated sufficiently at each point in the system to permit pumping, i\ sulfur. Engine builders' published data for specific fuel consumption, as well as most shop test data, are usually based on the use of distillate fuel of a standard heating value. The specific fuel consumption determined for an engine in service must therefore be corrected for the difference in heating value of the fuel actually used if comparison to such data is intended. 16-18 FUELS FOR DIESEL ENGINES MARINE DIESEL ENGINES Ignition quality. The ignition quality is an indication ofthe time necessary for the fuel to ignite after it has been injected into the cylinder of an engine: fuel oflow ignition quality will take longer to ignite, thus the ignition delay will be longer. The ignition quality of distillate fuels can be measured, and is usually presented as the c e ta n e n u m b e r . For heavy fuels the ignition quality is calculated and presented as an approximate c e ta n e in d e x . More recently, a Calculated Carbon Aromaticity Index (CCAI) has been introduced. The long ignition delay associated with fuels oflow ignition quality can result in a late and therefore more explosive start to the combustion period, with higher peak pressures, manifested as rough, noisy operation that, if sustained, can result in damage to cylinder heads, liners, pistons, and rings. The end of the combustion period can also be delayed, resulting in rough and incomplete combustion and, therefore, high fuel consumption and fouling ofthe combustion space. Because the ignition quality is related to time, slower turning engines are less affected by fuels of low ignition quality, and to some extent the injection timing can be advanced to compensate for the long ignition delay. Conversely, higher speed engines require fuels of higher ignition quality. Ignition delay is reduced at higher temperatures, and some manufacturers recommend that, for operation on low ignition quality fuel at low loads, the temperature of the jacket and piston coolants be maintained at high levels, and that the temperature of the charge air leaving the charge air cooler be increased. Carbon residue. The standard carbon residue tests are meant to provide an indication of carbon formation at high temperatures. Fuel with a high carbon residue index can be expected to leave more deposits after combustion, and fouling and wearing of cylinder liners, rings, ring grooves, exhaust valves, and turbocharger turbine nozzles. Effects on cylinder components can be reduced by the use of detergent cylinder oils. Turbocharger fouling is countered by frequent water washing. Carbon can also accumulate on the nozzle tips, interfering with the spray pattern, an effect best limited by frequent withdrawals of the injectors for cleaning. Solids and ash. Solid particles carried into the engine with the fuel can cause abrasive wear of fuel injection pumps, injectors, cylinder liners and rings, exhaust valve seats, and turbochargers. The larger solid particles will be removed in settling, filtration, and centrifugal purification. The solids that have proven particularly difficult to remove are the highly abrasive particles carried over into the residual from the silicaalumina-based catalyst used in catalytic cracking processes at many 16-19 refineries and called c a ta ly tic fin e s . The most effective procedure for reducing the presence of catalytic fines aboard ship includes the full-time use of multiple centrifuges arranged to process the fuel in series, with the first set up as a purifier (water and solids removal) and subsequent units as clarifiers (solids only). Fine filters, used alone, can provide adequate protection only for engines burning the cleanest of fuels. Fine filters are usually fitted to fuel systems that handle lower quality fuels only as a final backup in the event of purifier malfunction. When fitted as the sole means of protection, fine filters may clog at inconveniently frequent intervals. Sulfur. Sulfur is carried through to residual fuels from the crude, and consequently into blended fuels. In the combustion process the sulfur is reduced to sulfur dioxide, which can subsequently convert, in the presence of unused oxygen, to sulfur trioxide, which can then combine with water vapor to form gaseous sulfuric acid. At temperatures below about 150 C, condensation of the sulfuric acid begins. The presence of sulfur in the fuel therefore indicates a potential for c o ld e n d c o r r o s io n , i.e., acidic attack of surfaces exposed to the exhaust gas when they are at or below about 150 C. It can also cause contamination of the lubricating oil. Engine components that are most vulnerable to cold end corrosion can , include the lower ends of the cylinder liners and pistons, especially those of engines operated at low power for sustained periods. This problem can be countered by maintaining the temperature of the jacket and piston coolants at high levels, and by increasing the temperature of the charge air leaving the charge air cooler. In addition, the oil used for cylinder lubrication should have a high alkaline content (high to ta l b a s e n u m b e r , TBN) in order to neutralize the acid. A corollary problem exists in the use oflubricating oils of high TBN: if there is insufficient sulfur present in the fuel to neutralize the alkaline ingredients of the lubricating oil, the resulting deposits can cause scoring of the liner and wear on the rings. This problem can arise when an engine , that is normally operated on high sulfur fuel is later supplied with low sulfur fuel for an extended period, without a change of the oil lubricating the cylinders. In crosshead engines the purely vertical movement of the piston rod permits a packing gland to be fitted to separate the combustion space from \the crankcase, preventing combustion blowby and prohibiting excess cylinder oil from reaching the crankcase. Crosshead engines do not, thereI,fore,require a crankcase oil of high TBN, but use a high TBN cylinder oil :;:In a separate cylinder oil system. On the other hand, contamination of the a~nkcase oil can be a problem with trunk piston engines, which are ':usually supplied with crankcase oils having a high TBN. 0 0 i' 'i II, 16-20 MARINE DIESEL ENGINES Vanadium. Vanadium is carried through to the residual and blended fuels from the crude. During the combustion process, and especially in combination with sodium (see below), gaseous oxides will form, some of which will begin to change phase and form adhering deposits on combustion space surfaces whose temperatures exceed about 500°C. The surfaces most susceptible to such deposits are piston crowns, exhaust valves, and turbocharger turbine nozzles and blades. A more minor problem with vanadium deposits is corrosive attack and its ultimate effect on piston crowns and the bottom faces of exhaust valves. When the deposits occur on the seating surfaces of the exhaust valves, however, the results are more immediate, as the valve can overheat and burn, through the following mechanism offailure: 1. Most valves are cooled intermittently through contact with their seating surfaces when closed, and the deposits interfere with the good contact required. 2. Further, ifthe deposits prevent the valve from closing tightly, the hot gases will find passages between the valve and its seat during the combustion and expansion periods, eroding a groove in the valve ( w ir e - d r a w in g ) . Vanadium problems will be minimized in engines where the surfaces in question can be kept below about 500°C. Valve cooling is usually not a problem in the large low speed engines, but can be difficult in high speed engines and some of the medium speed engines. The two-stroke, loopscavenged engines are all but immune to vanadium attack. Manufacturers' limits specified for vanadium and sodium take these factors into account. Sodium. Most of the sodium in fuels is introduced through seawater contamination, and most will be removed with the water if settling and centrifuging procedures are adequate. The principal problem with sodium is in its combination with vanadium, described above. A rough rule of thumb limits sodium content to one-third of the vanadium content. It should be borne in mind that since sodium, unlike vanadium, can enter the fuel during transport to the ship or later, while stored aboard, analyses of samples from the fuel supplier or of samples taken during bunkering may give a false impression of the sodium content of the fuel reaching the engme. Flash point. The minimum permitted flash point, usually 60°C, is indicative ofthe maximum safe storage temperature for fuel oil. A problem can arise in a diesel plant burning heavy fuel ifthe fuel leaving the centrifuge, where it might be heated to 98°C, in turn raises the day tank temperature above the flash point. The problem can be solved if a cooler is fitted after the purifier in the line to the day tank. FUELS FOR DIESEL II ENGINES 16-21 In some cases, particularly when plants have been converted from distillate to heavy oil, the heated returns from the engine are returned to the day tank, heating it above the flash point. The correct arrangement includes a mixing tank of limited capacity, so that its high temperature represents less of a hazard. Crude oil, because it contains the light fractions, may have a flash point below the legal minimum, and may therefore be unacceptable for direct use as a fuel. Pour point. The pour point indicates the temperature to which fuel must be heated to permit pumping. The temperature of fuel can fall below the pour point not only in storage tanks and transfer lines, but also in the service system of an idle plant. Most plants burning heavy oil have fuel lines that are extensively steam-traced beneath the insulation. Incompatibility. Not all fuel constituents will mix compatibly with each other, so there is the possibility of constituents separating in tanks, often precipitating a heavy sludge, and leading to fluctuations in flow as the separated constituents reach key points in the system unevenly. It is the responsibility of the fuel supplier to ensure that fuels blended ashore do not contain incompatible constituents. Aboard ship, it is important to avoid mixing fuels from different deliveries, or blending fuels, without first undertaking a spot test for compatibility. Incompatibility in a fuel can reveal itself by increased sludge accumulations in tanks and at filters and centrifuges, by fluctuating pump discharge pressures, and by frequent viscosimeter excursions. Other than discharging the fuel ashore at the next opportunity, the only cure for the operator is to cope with the incompatibility as best he can until the fuel is consumed. Fuel Oil Analysis In order to treat the fuel properly, and because ofthe potential for damage to the engine, it is important that complete analyses of all fuel coming on board be available. A complete analysis includes all of the properties cited above. Many operators have found that analyses provided by fuel suppliers '\ are incomplete or otherwise unreliable, and have resorted to taking their own samples from the bunkering line during delivery and sending them ashore for independent analysis. The principal problem with such arrangements is in getting results of the analysis back to the ship before the newly bunkered fuel is needed. Care must be taken to ensure that a sample is truly representative. One recommended method is to drip-feed a reservoir from the bunkering line during the entire bunkering period, with the sample being extracted from the reservoir. 16-22 MARINE DIESEL ENGINES TURBOCHARGING 16-23 TURBOCHARGING Introduction Although some applications are best served by naturally aspirated or mechanically blown engines, the vast majority of main propulsion engines and generator drive engines are turbocharged and aftercooled. An engine and its turbocharger(s) are interdependent in their performance: a defective or mismatched turbocharger will preclude proper engine performance. Description and Classification Figure 16-7 shows a typical turbocharger. The important characteristic to note is that the rotor is freewheeling, driven only by the engine exhaust gases as they expand through the turbine. Turbochargers may be classified as follows: Number of stages. In general, turbochargers use a single stage compressor, driven by a single stage turbine. (Where engines have been fitted with two-stage turbocharging, two single stage units are fitted, with turbines and compressors in series.) Compressor type. Turbochargers almost always have centrifugal compres- sors. Turbine type. Turbines oflarge turbochargers are usually a x ia l flo w , as in Figure 16-7, while those of smaller units are usually Figure 16-8. r a d ia l flo w , as in Figure 16-7. Turbocharger with axial flow turbine and water-cooled casing of the shaft can have reduced diameters, outboard bearings will usually have lower friction losses. I n b o a r d b e a r in g s are located between the turbine disk and the compressor impeller as in Figure 16-8. Principles of Turbocharging Discharge pressure. Compressor discharge pressure is usually described by its ratio to intake pressure, called the p r e s s u r e r a tio . Currently, turbochargers are suitable for pressure ratios as high as 4.0. Turbine cooling. Traditionally, large turbocharger turbines are cooled by circulating engine jacket water through passages in the casing, as in Figure 16-7. Figure 16-9 shows an u n c o o le d turbocharger in which, while water may still be used to cool the turbine bearing, the turbine casing is not cooled,improving waste heat recovery from the exhaust gases downstream of the turbocharger. Bearing location. When the bearings are located at the extreme ends of the shaft, as in Figure 16-7, they are o u tb o a r d b e a r in g s . Because the ends and aftercooling. The principal reason for turbocharging is to increase the power output of an engine of given size and speed, by enabling the cylinders to be charged with air at high pressure, hence at higher density than atmospheric. Since the greater mass of air then present will permit a correspondingly greater mass offuel to be burned, the engine output will be higher. The effect on the cycle is an increase in the intensity or duration ofthe combustion period and an increase in the work per cycle and, therefore, the MEP. From the relations cited previously, it can be seen that an increase in MEP will result in an increase in power output. The power increase will be directly proportional to the increase in MEP if other factors, including cylinder dimensions and RPM, are unchanged. Reasons for turbocharging 16-24 TURBOCHARGING MARINE DIESEL ENGINES 16-25 Some ofthe aforementioned effect will be lost, however, if the air leaving the compressor is not cooled,because the temperature rise ofthe air during the compression process has the opposite effect, decreasing the density. Consequently, in most applications, charge air coolers are fitted after the compressor and are therefore often called aftercoolers. Turbocharging tends to reduce fuel consumption, in part because the friction losses ofthe turbocharged engine do not increase as rapidly as the power output, and in part because the improved charging results in better combustion conditions. r a t i o , a n d c o m p r e s s i o n r a t i o . The b o o s t r a tio compares the pressure in the air manifold to atmospheric pressure. The pressure ratio compares the pressure at the compressor discharge to intake pressure. The two ratios differ because of the pressure drops across the charge air cooler and intake air system. The c o m p r e s s io n r a tio , while actually a ratio of cylinder volume at BDC to cylinder volume at TDC, is also indicative of the pressure rise during the compression stroke. Turbocharged engines, which by definition have elevated boost ratios, tend to have relatively low compression ratios in order to avoid excessive maximum pressures. B o o s t r a tio , p r e s s u r e Figure 16-8. Turbocharger with radial flow turbine e f f i c i e n c y . A turbocharger is an energy recovery device: a more efficient turbine will recover more energy from the exhaust gas stream; low friction rotor shaft bearings will absorb less of the turbine output; and a more efficient compressor will better utilize the remaining energy to compress more air to a higher pressure. Expressed mathematically: T urbocharger l1tc where l1t x 11c x 11m overall turbocharger efficiency turbine efficiency l1c = compressor efficiency 11m = mechanical efficiency l1tc l1t Figure 16-9. Turbocharger with "uncooled" casing = = = Thus, small but simultaneous improvements in the efficiencies of components, through improved component design and manufacturing, have compounded effects on overall turbocharger efficiency. While these improvements tend to result in higher turbocharger cost, the environment of high fuel costs that has prevailed since the mid-Seventies makes the cost increase acceptable because of the resulting improvements in engine fuel consumption and power output. 16-26 MARINE DIESEL ENGINES Improvements in turbocharger efficiency lead to the attainability of higher boost pressures; if engine components are redesigned appropriately, the new generation engine that results can have higher MEP, higher power output, and lower SFC. Compressor characteristics and the surge limit. Centrifugal compressor characteristics are similar to those ofcentrifugal pumps. Most compressors used for turbocharging have essentially radial vanes, though slight backward curvature is increasingly used. In either event a plotted characteristic at constant RPM would appear similar to Figure 16-10. At constant speed the discharge pressure first rises as volumetric flow increases, then drops off rather sharply. The compressor efficiency curve also rises to a peak, although at any constant speed this peak is slightly to the right of the pressure peak. The power consumed by the compressor is related to the product of discharge pressure and flow rate. Thus, in the region to the right of the peak in the pressure curve, operation will be stable: in this region a momentary drop in volumetric flow rate, for example, perhaps brought on TURBO CHARGING 16-27 by a momentary reduction in engine speed, will be countered by a rise in pressure, with little or no effect on the turbine. In the region to the left of the pressure peak, a momentary drop in volumetric flow rate will be accompanied by a drop in discharge pressure and a reduction in compressor power consumption. Operation in the unstable area to the left of the pressure peak may result in c o m p r e s s o r s u r g e . As the pressure at the compressor discharge falls below that downstream, the flow can reverse. The result can simply be a pulsation ifthe situation is not severe or oflong duration, or the reversed flow can continue to the air intake and become audible, ranging in volume from a soft sneezing to a very loud backfiring sound. Obviously, operation in the surge region should be avoided; consequently, turbocharger designers establish a line, called the surge limit, through the pressure characteristic slightly to the right of the peak. Figure 16-10 represents compressor characteristics at only one speed. In order to completely define the characteristics of a particular compressor, similar data must be obtained at several constant speeds covering the range ofits operation, and plotted together on the same axes. The resulting diagram, of which Figure 16-11 is an example, is called a c o m p r e s s o r p e r fo r m a n c e m ap. Effects of wheel diameter and diffuser vane height. A map such as Figure 16-11 describes the performance of a particular compressor, comprising a wheel of given design and diameter, and a diffuser with vanes of given height. In practice, turbocharger manufacturers design a series or "family" of geometrically similar compressors with a range of compressor wheel diameters to cover a range offlow rates. When the compressor performance maps for the whole family are plotted together, the result will be similar to Figure 16-12. For each compressor wheel, a narrow range of performance variation is possible by exchanging the diffuser for one with a different vane height: higher vanes will shift compressor performance slightly to the right, while lower vanes will move the performance slightly to the left. In general, these adjustments of vane height away from the optimum are accompanied by a small penalty in compressor efficiency. Figure 16-10. Centrifugal compressor performance characteristics at constant RPM Turbocharger frame size and turbine characteristics. In general, turbine characteristics are more straightforward than compressor characteristics. Usually, for any given turbocharger series, selection of a compressor wheel diameter specifies the turbocharger; i.e., for each compressor wheel diameter there is a given compressor casing, turbine casing, and turbine disk. Adjustment of turbine performance is then obtained by selection of nozzle and blade characteristics. Figure 16-13 is an example of a selection curve for turbine characteristics, from which, given the exhaust gas flow TURBOCHARGING 16-29 and the expansion ratio across the turbine, the appropriate combination of nozzle plate and blade angle can be obtained. Then, from a curve such as Figure 16-14, the resulting turbine efficiency is estimated. T urbocharger M a tc h in g o f m a t c h i n g p r o c e d u r e . The procedure by which a turbocharger is selected to mate with an engine is called turbocharger matching. Usually this is done by the engine designer in the course of development of an engine design, or in upgrading an engine design to keep pace with advances in turbocharger or engine technology. On occasion, an existing engine will be rematched with a new or modified turbocharger, perhaps to suit new operating conditions. While the operating engineer will not normally be involved in turbocharger matching, a familiarity with the procedure will lead to a better understanding of the interdependent relationship between engines and their turbochargers, and ofthe effects, in service, of operation offthe design point. Figure 16-15 is provided to identify the terminology used in the simplified procedure outlined below. D e s c r ip tio n Figure 16-14. Turbine characteristics: efficiency versus blade speed/gas speed ratio 1. An estimate is made ofthe anticipated BHP that the engine will develop at a particular engine RPM. Normally the rated RPM will be selected, but under some conditions the turbocharger may best be matched to the engine at a different RPM. 2. Inlet conditions for the turbocharger, Po and To, are selected. Normally these will be standard atmospheric conditions, with Po corrected for a pressure drop across the intake filter. However, if operation in an abnormally hot or cold environment is expected, or if a long run of intake ducting is to be fitted, then conditions should be selected to suit. 3. An estimate might now be made of the amount of air, ma, that the engine will require at this condition. This can be obtained from a combination of basic principles and empirical data, including previous engine performance. Once ffia is determined, the volumetric air flow, Va, at standard conditions of pressure and temperature, can be calculated. 4. The engineer must now determine the air manifold pressure, Pl. As with the estimate of air flow, he will have a good idea ofthe approximate value to use as a first estimate. (Turbocharger matching is an iterative procedure in which the results of the first series of calculations become the assumptions for the next series; the calculations are repeated until the results equal the assumed values.) 5. Using the air cooler manufacturer's data for pressure drop versus air flow rate, the air cooler pressure drop can be added to PI to yield the compressor discharge pressure, P d, required. (The air cooler data will be appropriate for Figure 16-15. Schematic for turbocharger matching a clean air cooler only: obviously, if the air cooler is fouled, a higher P d would be needed to achieve the same value of PI). The compressor pressure ratio, PdIPo, can be calculated. 6. The compressor frame size can now be selected by entering the family of compressor performance maps (of which Figure 16-11 was an example) with the values ofPdlPo and Va just determined. These same data are now used to pinpoint the first estimate of operating point on the compressor performance map for the selected compressor, as in Figure 16-16. The operating point must have adequate margin from the surge limit; i.e., it must be 15 percent to 20 percent to the right of the surge limit at the value of PdiPo. Inadequate margin will invite turbocharger surge under service conditions, while excessive margin will place the compressor in a region oflow efficiency. If the surge margin is outside the recommended range, then maps for the same frame size but with higher or lower diffuser vanes, should be checked; if the margin against surge is still inadequate the next compressor frame size will be required. Once this is done, a preliminary selection of compressor, and therefore of turbocharger, has been made, based on the initial assumptions. What must be done next is to confirm that the power produced by the turbine will be sufficient to drive the compressor; i.e., that the initial assumptions were correct. TURBOCHARGING 16-33 the turbocharger RPM and the known characteristics of the particular bearings. The sum of bearing power absorption and compressor power is the amount of power that must be developed by the turbine: Wt, reg= Wbrg+ We where Wt,reg= r e q u ir e d turbine output Wbrg= power absorbed by bearings We = compressor power consumption At this point, a first estimate of the power r e q u ir e d from the turbine has been made. The next steps will determine, from the turbine characteristics for the turbocharger under consideration, whether this is available. 10. Estimates must be made of the gas conditions at the turbine inlet. For the simpler case of engines with constant pressure turbocharger systems it suffices to know the turbine inlet pressure and temperature, Pe and Te.(For engines with pulse charging systems the procedure is more complex, though similar in principle.) For any particular engine, Pe and Te can be estimated from basic principles and empirical data, including previous engine performance. 11. The exhaust pressure from the turbine, Pu, must also be estimated. Generally this can be done by adding an amount to the standard atmospheric pressure sufficient to allow for typical uptake losses. If, however, a waste heat recovery boiler or other device will be fitted in the uptake, or if the standard atmospheric pressure is not representative of anticipated operating conditions for the engine, more appropriate data should be used. Once these two pressure estimates are made, the expansion ratio, PeIPu, can be calculated. 12. The mass flow rate of the gas, mg, can be calculated by adding the mass flow rate of the fuel, mr, to the air mass flow, ma, previously estimated: mg = mr + ma The volumetric flow rate of the gases, Vg, can also be calculated. 13. In general, the selection of a compressor wheel diameter predetermines turbine characteristics, which may include wheel mean diameter and blade length. With the values of PelPu and Vg obtained in the previous steps, a turbine blade and nozzle angle selection curve, such as Figure 16-17, can be entered for the frame size under consideration, to select nozzle opening and blade angle. 14. The turbine efficiency can then be obtained from a curve such as Figure 16-18. However it will first be necessary to calculate the ratio ofblade speed to ideal gas speed from the following relations: 16-36 MARINE DIESEL ENGINES Turbocharger matching: conclusions. The most important conclusion to be reached in reviewing the turbocharger matching procedure is that a turbocharger is matched to an engine for a particular set of conditions. Operation at different conditions will be less than optimum and may, in extreme cases, be so unsatisfactory as to justify the retrofit of a new turbocharger, matched to the new conditions. Off-design-point operation may be permanent or temporary, intentional or inadvertent. A few examples follow. An over-pitchedpropeller,a heavilyfouledhull, single screwoperation ofa twin screw ship, or single engine operation of a pair of engines geared together are among those conditions that may require an engine to deliver higher than anticipated power at reduced RPM. Turbocharger surge wouldnot be surprising under such circumstances. Long or complex runs of intake ducting, elevated intake air temperatures, fouled intake air filters, or a dirty or damaged compressorare all likely to result in lower than expected air manifoldpressures. A fouled air coolercan force the compressorto operate at high RPM, close to the surge margin. Fouled turbine nozzles can sometimes force a turbocharger into surge; under other circumstances the result would be reflected in reduced air manifold pressures. Fouled turbine blades, a heavily fouled waste heat boiler, or a constricted uptake can prevent the turbine from reaching projected performance,and might first be reflected in low air manifoldpressures. The interdependent relation ofthe engine and turbocharger, and of the turbine and compressor, means that the system is prone to chain reactions. As an example, low air manifold pressure, which indicates lower air flow, can lead to dirty exhaust, resulting in turbine fouling, which can further aggravate the situation. Effect of improved turbocharger efficiency. With reference to step 16 of the matching procedure, if a turbocharger of improved efficiency became available, the balance between turbine output required and turbine output achieved would occur at a higher boost pressure or greater air flow rate. There are three possibilities: By changing exhaust timing of the engine or the configuration of its exhaust system, an engine's SFC can be improvedwithout altering its rating. The greater mass ofair trapped in the cylindercouldbe used forthe combustion of more fuel; i.e., engine output could be increased. Furthermore, the improved turbocharger efficiencywould be reflected in a reduced SFC for TURBOCHARGING 16-37 the engine. However,the engine would have to be capable of this greater output, as cyclepressures and therefore componentstress levelswouldrise. In most cases, in fact, as more efficientturbochargers have becomeavailable, engine components have had to be upgraded in order to permit the potential for higher ratings to be realized. The potential for excess turbine output can be realized in the provision of an exhaust gas turbine driving a mechanicalload. Three ofthe many possible configurations are (a) an exhaust gas turbine-driven generator; (b) an exhaust gas turbine geared to the engine output shaft, forming,in effect,a combinedcycleor turbo-compoundarrangement (the powercontributed by the turbine might be up to 5 percent);or (c)a combinationofthese, in which the exhaust gas turbine is connectedat the powertake-offgear ofan engine fitted with a shaft-driven generator. Boost Blowers As engine output is reduced, boost pressure falls. While four-stroke engines, by virtue of piston movement on charging and exhaust strokes, will continue to draw in their own charge air and expel most of the exhaust gases, two-stroke engines rely on elevated charge air pressure to scavenge and charge the cylinder. Below approximately half power, therefore, twostroke engines must be provided with an auxiliary means of pressurizing the air manifold. In most engines this takes the form of an electric motor-driven b o o s t b lo w e r , which is switched on automatically in response to air manifold pressure. In some smaller engines boost pressure is provided by mechanical drive ofthe turbocharger through an overrunning clutch and gear train from the crankshaft. Other methods, such as the use of the piston undersides as reciprocating pumps, or the provision of reciprocating pumps driven by links from the crossheads, may be considered obsolete, at least for the larger engines. Turbocharger Water Washing Systems In order to avoid the decline in performance that is caused by fouled turbines and compressors, many engines, including most intended for operation on heavier fuels, are fitted with water washing systems. Most commonly, these systems take the form of small tanks piped to the compressor inlet and the turbine inlet, fitted with water-filling and compressed air connections. In use, the engine load is reduced, and the charge of water, limited by the size of the tank, is injected over a period of about one minute. Solvents are usually not recommended: it is the impact of the water which does the cleaning. Frequency of use will depend on the rate of fouling, determined from experience. Water wash of the compressor will most likely be required infrequently. On the other hand it is not uncommon, in the case of engines run on the heaviest fuels, for the turbine to be washed daily. 16-38 MARINE DIESEL ENGINES Two-stage Turbocharging Two-stage turbocharging, in which two turbochargers are connected in series on both compressor and turbine ends (usually with charge air coolers at each compressor discharge), has been used to attain higher boost ratios and therefore higher MEP than even the most efficient turbochargers could make possible in a single stage. In most ofthe applications to date, it has been a matter of attaining very high power output on infrequent occasions from engines installed in. compartments where there were space and weight restrictions. Maximum pressures were limited by reducing the compression ratio. In part because of the rather recent availability of more efficient turbochargers, interest in two-stage turbocharging for merchant ships has diminished. INTAKE AND EXHAUST SYSTEMS Intake Systems The function of the intake system is to ensure a supply of clean air to the engine, within reasonable limits of temperature and pressure. The components of the system mounted on the engine may not be alone in achieving this: an engine installed in a clean, warm engine room may appear to have the most rudimentary intake system but in this case the engine room and its air supply system must be considered part of the engine air intake system as well. Not every installation will necessarily include all of the components described below. Direct versus external air intake. Typically, main propulsion engines and ship's service generator engines are installed in well-ventilated engine rooms, from which they draw their intake air. Care must be taken in laying out and operating the ventilating system to ensure that fresh air is supplied to the vicinity of the engine intakes. Location of the main engine intake in a poorly ventilated area ofthe engine room can result in air intake temperatures that are sufficiently in excess of conditions used in matching the turbochargers to bring on surge, air starvation, poor combustion, and high exhaust temperatures. Some engines are provided with external air intakes. As long as outside ambient temperatures are near the conditions for which the turbocharger was matched, and the location and configuration of the intakes are such as to avoid water ingestion, this is usually beneficial. However, very low intake air temperatures can cause the turbochargers to surge. Surge can also be caused by low intake pressures at the compressor brought about by excessive pressure drop in the intake system because of its length, or tight turns, or restricted air flow areas. INTAKE AND EXHAUST SYSTEMS 16-39 Intake filter and silencer. Engines with direct air intake have the filter mounted locally at the engine. Small, naturally aspirated engines will have intake filters of either the oil bath or the disposable dry media type mounted on the air intake manifold. In the most common configuration for large, turbocharged engines, washable dry media panels are mounted in an array surrounding the circumference of the compressor inlet. The design of these filters usually provides adequate silencing, but in some installations a plenum may be installed for further silencing. On engines fitted with external air intakes, a filter box may be mounted behind a set oflouvers that will provide a level of salt spray protection. The design of the filter box and the ducting to the engine must take silencing into account. Charge air cooler. Most marine charge air coolers are configured as a bank of finned water tubes over which the air flows, but sometimes compact heat exchangers of proprietary design are fitted. In either case, the air side will be prone to fouling and, because of the impact that this has on engine performance, maintaining cleanliness of the surface is of paramount importance, even though some compact cooler designs may be particularly difficult to clean. (Frequency of cleaning is best determined by observation of the air pressure drop across the cooler.) Proper cleaning of the oily residue that accumulates on charge air coolers requires the use of a solvent and time for the solvent to soak into the residue. Therefore, cleaning with the engine in operation is impractical. Ideally, charge air coolers would be arranged to allow cleaning with a minimum of dismantling, but this is not always the case and ad hoc arrangements are common. Often a blind flange is inserted to close off the lowest end of the cooler, allowing the entire external heat exchange surface to be immersed in solvent while the cooler remains in place. In other installations the cooler is broken out of its location and lowered into a solvent-filled tank, an arrangement which, after soaking, permits more thorough cleaning by use of a compressed-air hose. Most charge air coolers for large engines are cooled by seawater, but increasingly ships are being fitted with central seawater-to-freshwater coolers which then allow the charge air coolers to be circulated with fresh water. The advantage of the more complex central system is in its reducing the potential for fouling and corrosion ofthe charge air cooler water sides and water piping. Most smaller engines have charge air coolers that are freshwater cooled by the engine jacket water. The air entering the charge air cooler can exceed 200°C in the case of engines with high boost ratios and, in some more recent plants fitted with extensive waste heat recovery systems for turbogenerator drive, this source is used to preheat feedwater to the boiler (see Figures 16-53 and 16-40 MARINE DIESEL ENGINES 16-55). For this purpose the first rows of charge air cooler tubes are separated from the remainder of the bank in order to allow circulation by boiler feedwater. Charge air heating. The normal function of the charge air cooler is to reduce the temperature of the air leaving the turbocharger in order to increase its density. However, the incoming temperature of the air will vary with engine load, and consequently the cooling water flow to the charge air coolermust be regulated. This is usually done automatically in response to the temperature of the air leaving the cooler. At low engine loads the air leaving the charge air cooler may be too cool for optimal combustion conditions and may become saturated if it cools to below ambient temperature. In those engines where the cooler is normally circulated with jacket water, limited low load air heating is inherent in the design, but this can be increased using cooling water crossovers at low loads to circulate the charge air cooler with jacket water leaving the jackets. Air manifold. The air manifold serves to distribute the air uniformly to the cylinders. In turbocharged engines, it is typically located below both the turbochargers and the charge air coolers. It is important to note, therefore, that water leakage from the charge air cooler or turbocharger will accumulate in the intake manifold while the engine is stopped. If the engine is subsequently rotated without draining the manifold, the water will be drawn into the cylinders, where, because it is relatively incompressible, it can cause cracking of the piston crowns, skirts, liners, or heads, bending of the connecting rods, or damage to the bearings or crankshaft. Exhaust Systems The typical exhaust system of a turbocharged engine comprises ducting and manifolds, the turbocharger, often a waste heat boiler or other heat recovery device, and a silencer, and usually terminates with a spark arrestor at the top of the ship's stack. Exhaust systems of multiple engine installations are usually independent of each other. Pulse versus constant pressure turbocharging. A pressure probe located at the exhaust port of a cylinder will indicate a sharp pressure peak as the port first opens, called the b lo w d o w n p u ls e . A second, lesser, scavenging pulse OCcurswhen the air ports first open, and charge air sweeps through the cylinder to the exhaust port. When the exhaust piping is designed to maintain these pulses all the way to the turbine inlet nozzles so that their energy can be utilized in the turbine, the engine is said to be p u ls e tu r b o c h a r g e d . In its simplest form, pulse charging would require that separate exhaust ducts be led from each cylinder to separate groups of INTAKE AND EXHAUST SYSTEMS 16-41 turbine nozzles in order to avoid the interference of pulses from different cylinders, but in fact it is possible to combine exhaust branches from groups of two or more cylinders that are sufficiently far apart in their firing order (see Figure 16-19 for an example). If the selected cylinders are too close in firing order, or if the valve timing is incorrect, the exhaust pulse of one cylinder can interfere with the exhaust of another. Pulse charging uses small diameter piping from the cylinders to the turbine to prevent the pulses from dissipating en route. C o n s ta n t p r e s s u r e tu r b o c h a r g in g is characterized by a large diameter exhaust manifold running the length ofthe engine, into which the cylinders exhaust through short branches. Refer to Figure 16-19. The energy of the pulses can be partly recovered as the gas enters the manifold if these entrances are carefully designed as diffusers (sometimes called p u ls e c o n v e r te r s ) , which will elevate manifold pressure above what it would otherwise be. Generally, pulse charging permits energy to be recovered at lower engine output than constant pressure charging, and enables a somewhat more compact installation. These advantages must be weighed against the more efficient operation of the turbine in a constant pressure system, where the turbine benefits not only from the nearly constant inlet pressure but from full peripheral admission as well. Exhaust gas heat recovery. The energy in the gas leaving the turbocharger turbine, at temperatures ranging from a low of about 2 5 0 °C for some large two-stroke engines to a high of about 5 0 0 °C for some higher speed, fourstroke engines, is often recovered in waste heat boilers or other heat exchangers. The extent of waste heat recovery and the use to which the 16-42 MARINE DIESEL ENGINES recovered heat is put are matters which must be determined by examining the economic trade-offs involved; these, in turn, are affected by different operating patterns of the ship as well as fluctuations in costs, principally oHuel. A few ofthe many possibilities are described below. Almost all ships burning heavy fuel are fitted with waste heat boilers in the main engine uptakes sufficient to meet fuel oil heating requirements, plus domestic needs. It is usually not feasible to recover sufficient heat to meet more than a small portion of a tanker's cargo-heating requirements, for which an oil-fired boiler is necessary. In the case of ships with minimal electrical load (bulk carriers and tankers), sufficient heat can be recovered from main engines operated at as little as about 10,000 to 15,000 BHP to supply the ship's normal electrical and steam requirements from a waste heat boiler/steam turbogenerator plant (see Figures 16-53 and 16-55). These systems are common on high-powered ships. The BHP threshold will be lower in the case of four-stroke engines with their higher exhaust temperatures than for two-stroke engines, but in either case it can be reduced further by using charge air coolers for feedwater heating, using multiple pressure boilers and turbines, reducing the electrical load by using engine-driven auxiliaries, supplementing steam production with the oil-fired boiler, or supplementing electrical supply with shaft-driven generators. Supplementing the turbogenerator with diesel generators may be done only when the diesel generator can be kept sufficiently loaded for trouble-free operation, typically above about 35 percent load. Generally, waste heat boilers are fitted to the main engines, but under some circumstances-for example, passenger cruise ships with high electrical loads and relatively low utilization ofthe main engines-waste heat boilers are sometimes fitted to the auxiliary engines. In these cases much of the steam produced is used for fresh water production. Usually the waste heat recovery fluid is water but, in some special cases, other fluids, usually proprietary in composition, are more suitable. It should be noted that, in addition to the exhaust gases, the engine cooling water also contains recoverable heat. The use of the air cooler as a boiler feed heater is mentioned above. Most oceangoing motor ships utilize the jacket water as the heat source for the fresh water generators (i.e., the evaporators or distilling plant). FUEL INJECTION AND COMBUSTION Introduction The fuel injection system must accurately meter the fuel in response to required output, then inject it into the cylinder as a finely atomized spray FUEL INJECTION AND COMBUSTION 16-43 in order to enable complete combustion. Without exception, modern oilburning diesel engines achieve these goals with solid injection systems. Of the three types of solid injection systems, the most commonly applied is the jerk pump system. Common rail systems and distributor pump systems are confined in their application to the smaller, higher speed engines, although the large Doxford opposed piston engines, which remained in production until 1981, had common rail systems. Only the jerk pump system will be described. The fuel injection system is also the fuel metering system. Therefore, the first requirement of the system is: 1. The fuel injection system must accurately meter the fuel in response to required output. In addition, the following points are of absolute importance in obtaining good combustion: 2. The fuel must enter the cylinder at a precise moment during the compression stroke. 3. The fuel must enter as a finely atomized spray. This condition must obtain from the very beginning of the injection period through to the end. 4. The droplets must penetrate far enough into the combustion space to ensure that they are evenly distributed. 5. The fuel droplets must not penetrate so far that they impinge on the surrounding surfaces. 6. The fuel must be supplied to the cylinder at a predetermined rate (a constant rate is usually required). 7. At the end of the injection period the cutoff must be sharp and complete. Jerk Pump Injection System The jerk pump system comprises one in je c tio n p u m p and up to four in je c to r s for each cylinder. Fuel is delivered at nominal pressure to the injection pump-a reciprocating, positive displacement, plunger pumpwith the reciprocation provided by connecting the plunger directly to a cam follower (the term je r k p u m p derives from the short, sharp strokes which result). The principal types of injection pumps have a constant stroke, with metering provided by closing, then opening, spill ports during the stroke. Discharge of the injection pump is led directly to the injector, which comprises a spring-loaded fu e l v a lv e surmounting the fu e l n o z z le . The injector is enslaved to the pump, in that it is the discharge pressure of the fuel alone that forces the fuel valve open. In the jerk pump system the requirements for good combustion are obtained as follows: 16-44 FUEL INJECTION MARINE DIESEL ENGINES AND COMBUSTION 16-45 Timing the start and end of injection is dependent, first, on proper cam timing, and then on the correct internal calibration ofthe injection pump to ensure that spill port operation occurs correctly relative to plunger movement. Atomization and penetration are obtained by forcing the fuel through the holes ofthe fuel nozzle at very high pressure, typically on the order of300 to 1,000 or more atmospheres. Obtaining good atomization from the beginning ofthe injection period through to the end is dependent on the sharp rise and then fall of pump discharge pressure, as well as the rapid opening and closing of the injector. The spray pattern is a function ofthe configuration of the nozzle, which must be selected to avoid droplet impingement on the liner or the piston crown. Distribution of the fuel droplets will be assisted by air turbulence, which can be obtained by suitably shaping the piston crown and the cylinder head, and by orienting the air inlet ports to induce a swirling motion to the air. p u m p s . Figure 16-20 shows a typical example of a helix-controlled injection pump, which is the most common type (a valve-controlled pump is described in the next chapter). Note the helical recess in the periphery of the plunger. As the plunger rises, the spill port will close as the top of the plunger passes it. This traps the fuel above the plunger and initiates the effective portion ofthe stroke. The rise in fuel pressure as the plunger continues its stroke will be very sharp, since the fuel is almost incompressible. When the edge of the recess in the plunger exposes the spill port, the effective stroke terminates with a sharp pressure drop. Most injection pumps are fitted with a spring-loaded discharge check valve which will then close. Because of the helical shape of the recess, rotation of the plunger will alter the length of the effective stroke and therefore meter the amount of fuel injected: when the vertical edge of the recess is aligned with the spill port, no fuel is injected. Rotation of the plunger is achieved by lateral movement of the fu e l r a c k , which is in mesh with a pinion on the plunger shaft. The discharge check valve ensures that a residual pressure is maintained in the high pressure fuel line between injections. This residual pressure aids in ensuring a prompt beginning of each injection and also helps to avoid the cavitation that would be likely if line pressure dropped too low. The residual pressure will vary with engine speed and output, however, and many injection pumps are fitted with a relief valve that bypasses the check valve, enabling a constant residual pressure to be maintained over the whole load range, while also helping to prevent secondary injections. I n the injection pump of Figure 16-20 the top of the plunger closes the spill port at the same point regardless of its angular position, so that the injection always begins at the same time in the cycle regardless of engine output. It is increasingly common for the top of the injection pump plunger to be shaped to vary the beginning ofthe effective stroke, hence the timing I n je c tio n Figure 16-20. Fuel injection pump of the start of injection. A common pattern advances the injection furthest at settings corresponding to about 80 percent of engine output in order to maintain maximum cylinder pressure throughout the upper end of the engine output range. This favors specified fuel consumption by improving the thermodynamic cycle (injecting a larger fraction of the fuel before the piston reaches its TDC position; see "Thermodynamic cycle" at the beginning ofthis chapter) and also enables the high temperatures conducive to complete combustion to be maintained. Figure 16-21 shows a typical fuel injector. The shoulder on the plunger provides the lifting area on which the pressure ofthe fuel initially acts against the spring to start to open the injector. As soon as the plunger begins to rise, the additional lifting area at the bottom is exposed and the injector snaps open sharply. In je c to r s. 16-46 MARINE DIESEL ENGINES FUEL INJECTION AND COMBUSTION 16-47 Any irregularity in the interior of the high pressure passage can be sufficient to set up a reflectedpressure pulse which,on reaching the injectorafter it closes,can cause it to reopen,resulting in a s e c o n d a r y in je c tio n . High pressure fuel line leaks were one of the prevalent causes of fire in motorships, as the fuel lines were usually in the vicinity of hot exhaust surfaces. These lines are usually required to be fitted with shielding to reduce the fire risk. On some engines these problems are minimized or eliminated by uniting the injection pump and the injector in a single u n it in je c to r . Usually camshaft motion is then brought to the pump by push rods and rocker arms. Figure 16-21. Fuel injector The injector of Figure 16-21 is water cooled. The injector cooling circuit may be separate from the other engine cooling circuits to facilitate temperature control. In many engines sufficient cooling can be obtained by conduction to the cylinder head cooling circuit, simplifying injector manufacture and renewal. High pressure fuel line. The third component of the jerk pump system is the high pressure fuel line connecting the injection pump to the injector. In most engines the injection pumps are fitted on the side of the engine, convenient to the camshaft, thus necessitating piping of some length. Several possible problem areas must be addressed as a result: Even in the largest engines, the quantity of fuel injected per stroke is small relative to the volumeofthe high pressure line: the fuel dischargedby the pump displacesfuel already in the injector,whichopensin responseto the pressure pulse traveling the length of the line. The time needed for this pulse to travel the distance accounts for in je c tio n la g , the delay between spill port closureat the pump and the beginningofinjection. Jerk pump injection system problems. Problems in jerk pump systems on diesel engines of mature design are more likely related to component wear or to improper settings than to initial design. The cams, for example, must be correctly timed: it may not be sufficient to set the camshaft timing alone, since the cams are not always integral with the shaft. Because ofthe high loadings on the cam face, surface damage sufficient to affect the injection timing can occur. Injection pumps and injectors operate with close tolerances and are subject to wear from abrasive particles in the fuel. Poor quality fuels or weaknesses in the fuel treatment system will aggravate the situation. Items most subject to fuel abrasion are the injection pump plunger, barrel and valve seats, and, at the injector, the plunger, seat, and orifices. Cavitation erosion can be a problem in the high pressure parts of the system, affecting pump plunger, barrel, valve seating surfaces, high pressure line, and passages in the injector body. The erosion may have an obvious cause, such as cavitation induced at an irregularity or a change in the shape of a passage. Where there is no such obvious cause, the cavitation may be occurring in the wake of the pressure waves induced by the sharp closure of the injector, perhaps as the result of a failure by a discharge check valve to maintain an adequate residual pressure between injections. In engines operated on heavy fuels, the injector is likely to be the engine component most frequently removed for cleaning, testing, and parts renewal. On the test stand, the injector is checked for correct opening pressure, for leakage, and for spray pattern. Spring compression can be adjusted or a weak spring renewed; if the plunger seat or the orifices are worn, the nozzle must be replaced. Correct calibration of the injection pump is essential: spill port operation must be correct relative to plunger movement, the calibration between rack and plunger position must be correct, and the right number of shims must be inserted at adjustment points. 16-48 MARINE DIESEL ENGINES Combustion Fuel combustion in a diesel cylinder may be considered to occur in four phases (see Figure 16-22): 1. 2. 3. 4. Ignition delay period Rapid combustion period Steady burning period Mterburning period Combustion in a diesel cylinder does not take place at the tip of the injector, but rather at a distance away from it, as the individual fuel droplets will have to travel ( d iffu s e ) through the hot cylinder contents for sufficient time (the p r e p a r a tio n tim e ) to heat, begin to vaporize and mix with air, and finally ignite. FUEL INJECTION AND COMBUSTION 16-49 Phases of combustion 1. The ignition delay period. The ignition delay period is the interval between injector opening and the start ofignition. During this period the first droplets to enter the cylinder are heated by the surrounding charge of compressed air, begin to vaporize, and finally ignite. Until ignition occurs, there is no noticeable increase in the pressure in the cylinder above what it would be had no injection occurred. The ignition delay period is primarily a function of the ig n itio n q u a lity of the fuel, hence of its chemical composition. Fuels oflow ignition quality (i.e., of low c e ta n e n u m b e r ) will require more preparation time, and the delay period will therefore be longer. It is important to note that in a high speed engine the crankshaft rotates farther in a given period of time than in a low speed engine, which explains the generally lower tolerance of high speed engines for fuel oflow ignition quality. 2. The rapid combustion period. During this period, the fuel that has accumulated in the cylinder during the delay period before ignition burns rapidly. Because the fuel has already mixed with the charge air and begun the process of preparation for combustion, this is sometimes called the p r e m ix e d c o m b u s tio n p h a s e . The rapid combustion is accompanied by a sharp rise in cylinder pressure. If the pressure rises too sharply the combustion becomes audible, a phenomenon known as d ie s e l k n o c k . 3. The steady combustion period. Once combustion has been established in the cylinder, further fuel droplets entering the cylinder will burn as soon as they have penetrated, heated, vaporized, and mixed, so that the combustion rate lags behind the injection rate by the preparation time. Because the droplets burn as they diffuse into the cylinder, this is sometimes called the d iffu s io n c o m b u s tio n p h a s e . This period ends shortly after the injector closes ( c u to ff) , when the last of the fuel has burned. Cylinder pressure usually peaks just after TDC, near the middle of the steady combustion period, and then falls off smoothly after cutoff as the expansion stroke begins. 4. The afterburning period. If all the fuel has burned cleanly and completely by the end ofthe steady combustion period, the pressure trace will be smooth through the expansion stroke, and the afterburning period could be neglected. Typically, however, there will be some irregularities reflecting combustion of incompletely burned fuel or of intermediate combustion products, and some delayed c h e m ic a l e n d r e a c tio n s . It is during this period that soot and other pollutants are produced. Combustion problems. Difficulties in the combustion process are usually symptomatic of other problems, often related to the quality ofthe fuel and its preparation and injection, to air supply, or to maloperation; these are discussed under the appropriate headings. In engines of mature design such causes of combustion difficulties as component configuration are likely to have been eliminated. 16-50 MARINE DIESEL ENGINES MOMENTS, FORCES, AND VIBRATION Introduction The marine engineer is usually most concerned about the forces and moments generated by a diesel engine because oftheir potential for causing ( e x c itin g ) vibration of hull structure and such connected equipment as reduction gearing and propeller shafting. The engine is only one of severa] possible sources of vibr~ tion, however; propeller excitation is the most common cause of problems. The forces and moments ( d is tu r b a n c e s ) developed by an engine are entirely predictable in both magnitude (or a m p litu d e ) and frequency. Whether they will cause problems depends on the response of the ship's structure or connected equipment. This response will lie between two extremes: 1. If the frequency ofthe disturbance is even slightly different from the natural frequency of the structure or connected equipment, then, if they are sufficiently robust, the structure or connected equipment may absorb disturbances oflarge magnitude. 2. On the other hand, if the frequency of the disturbance (or an integral multiple of the frequency) is a sufficiently close match to the natural frequency of the structure or connected equipment, then even small disturbances can excite r e s o n a n t responses in the structure or connected equipment that are much larger in magnitude than the exciting disturbance. It is useful to consider two categories of diesel engine-induced disturbance: 1. External forces and moments that arise from the reciprocating motion of the pistons and running gear, and could cause an unrestrained engine to pitch, roll, or yaw. (With the engine installed in the ship, these disturbances can excite a response from the hull.) 2. Torsional vibration in the propulsion drive train that arises from the discrete power strokes of the engine and the resulting periodic application of torque, and generally affects only shaft-connected equipment, including reduction gearing and propeller shafting. Engine bearings and structure are designed to absorb internal forces and moments; thus, they are rarely transmitted to the hull. External Forces and Moments Overview. Because the source of external forces and moments generated by a diesel engine is in the reciprocating motion of the pistons and running gear, it can be noted that: MOMENTS, FORCES, AND VIBRATION 16-51 The magnitude ofthe individual forces and moments will be proportional to the masses involved and also to the square of the engine RPM. In multiple cylinder engines, it is possible to arrange the cylinders so that some ofthe external forces and moments generated by one cylinder are cancelled by other cylinders; in certain configurations the effect is complete. The lower the RPM of the engine, the lower will be the frequency of the disturbances it generates. Therefore, given the typically low natural frequencies of most hull structures, there is great likelihood that a response will be excited. Consequently: From the standpoint of exciting hull vibration, a worst case would be presented by a large bore, low speed engine with four, five, or six cylinders. A simple solution is available, however, in the form ofbalancers, which are frequently fitted to these engines. Smaller, high speed engines may generate disturbances which, while unlikely to excite hull vibration, may cause local vibration. Balancers may be used, and the engine may be installed on resilient, vibration-absorbing mountings. Piston motion and resulting forces. A simple analysis of the geometry of piston and crank motion (see Figure 16-23)will yield the following relationship between piston position and crank position: MOMENTS, FORCES, AND VIBRATION 16-57 the guide force is affected by the vertical reciprocating force,the moment will have higher order componentsoflow magnitude. I n addition, because the crankshaft is not absolutely rigid, the cranks deflect longitudinally under load. This will produce a pulsating axial force containing first and higher order components, all oflow magnitude. b y m u l t i p l e c y l i n d e r s . I t may be obvious from Figure 16-24 that if a second cylinder were arranged on the crankshaft, but with its crank 180 degrees out of phase with the first, as in Figure 16-27, then the first order vertical reciprocating force of the second cylinder would always balance that of the first cylinder, as shown in Figure 16-28. The second order vertical reciprocating forces, however, would always reinforce each other, creating a severe second order vertical reciprocating force imbalance. It is worth noting for this crank arrangement that, with the cylinders 180 degrees out of phase, equally spaced power strokes would occur if the cylinders operated on a two-stroke cycle: a two-cylinder, four-stroke cycle engine, with power strokes of each cylinder at 720-degree intervals, would require the cylinders to be 360 degrees out of phase, resulting in reinforced first and second order vertical reciprocating forces. While two-cylinder engines have practical application, it is confined to the very low output range. It is, therefore, worth moving on to examine more practical configurations. E x te r n a l fo r c e s a n d m o m e n ts g e n e r a te d C a s e s tu d y o f a fo u r -c y lin d e r , tw o -str o k e c y c le e n g in e Since each cylinder of a two-stroke engine will complete its cycle in 360 degrees, even application of torque to the shaft requires that a four-cylinder, two-stroke engine have the cylinders arranged 90 degrees apart, as in Figure 16-29. The firing order of the cylinders, in this case T im in g . Two conclusions can be drawn: A single cylinder engine would imposea vertical forceon its foundation that is made up ofthe engine weight plus the vertical reciprocatingforce,and that fluctuates in magnitude and direction at single and higher multiples of crankshaft RPM. Horizontally, although the fo r c e s are balanced, a to r q u e r e a c tio n couple (also called a guide forcemoment) is generated because the guide force and the horizontal component of bearing reaction are vertically displaced. This couplewouldtend to roll, capsize,or tip the single cylinderengine ofFigure 16-26counterclockwiseon the downstroke and clockwiseon the upstroke, more severely when F g is high, as in power and compressionstrokes. The torque reaction couple, therefore, has a major fluctuation at a frequency equal to crankshaft RPMin a single-cylinder,two-strokeengine, and equal to half crankshaft RPMin a single-cylinder,four-strokeengine,but because MOMENTS, FORCES, AND VIBRATION 16-59 R o ta tin g im b a la n c e . The longitudinal displacement of the eccentric masses of the crankpins and webs would produce a rotating couple, best eliminated by fitting counterweights to the crankshaft. V e r tic a l fo r c e s a n d th e ir e ffe c ts . With the cylinders 90 degrees out of phase, both first and second order vertical reciprocating forces are completely balanced, as shown in Figure 16-30. However, if the crankshaft is examined from the side (as in Figure 16-31, where it is positioned with cylinder 1 at TDC), it can be seen that, while the vertical reciprocating forces of cylinders 1 and 2 cancel each other, then because the forces act through the centerlines of the cylinders and are therefore longitudinally (or axially) displaced, a couple is generated tending to pitch the engine about a transverse axis. An analogous couple will be generated by cylinders 3 and 4. Thus, first, second, and higher order pitching couples will arise from the longitudinal displacements of first, second, ,and higher order vertical reciprocating forces. The first order pitching couple will be the largest in magnitude, as it arises from the largest force. Pairs of cylinders whose first order forces balance each other should be adjacent: to do otherwise would increase the pitching moment arm. It is this consideration that produces a firing order of 1-3-2-4 (although 1-4-2-3 would be equally suitable). The first order pitching couple can be countered by fitting additional counterweights to the crankshaft, i.e., in addition to the counterweights Figure 16-31. Axial displacement of vertical forces in a four-cylinder, two-stroke engine: cylinder 1 at TDC Figure 16-32. Principle of opposing counterweights to balance first order vertical couples fitted to balance the crankpins and webs. However, while the pitching couples are exclusively in the vertical plane, the counterweights would, as they rotate, generate a horizontal first order yawing couple. In fact, this is often an acceptable situation for main propulsion engines, as ships' hulls tend to be more rigid in the transverse direction. It is, therefore, the general practice of the engine designers to fit additional counterweights sufficient to cancel half of the first order pitching couple, thereby imposing a first order yawing couple of equal magnitude, i.e., half the magnitude of the original, vertical pitching couple. Alternatively, the first order pitching couple could be completely cancelled by two pairs of counterweights rotating in opposite directions at crankshaft RPM. Figure 16-32 illustrates an arrangement where one weight of such a pair is on the crankshaft, and the other is on a balance shaft geared to the crankshaft. Since the weights rotate in opposite directions (and in the same transverse plane), the horizontal components of the forces they generate will always cancel, leaving only a vertical force fluctuating at first order frequency. By fitting two pairs of such weight~ , longitudinally separated along the crankshaft, the first order pitching couple can be cancelled without generating a yawing couple. The same principle can be used to balance the second order pitching couples, ifthe pairs of opposing counterweights are driven at twice crankshaft RPM. This arrangement is called a Lanchester balancer, and a chain-driven example is shown in Figure 16-33. In principle, the higher order pitching couples could be countered by similar means; in fact they are usually not of sufficient magnitude to cause problems. A pulsating axial force will be produced by the deflection of the crank webs under load and will contain first and higher order components, all of low magnitude and therefore not normally a source of trouble. Occasionally, however, a higher order of this axial force may coincide with the natural frequency of the crankshaft itself. The usual solution in this case is to fit a damper, consisting of a dummy piston under engine oil pressure, at the free end of the crankshaft. T o r q u e r e a c tio n c o u p le s . Each of the cylinders will develop a torque reaction roll couple that will tend to rock the engine about a longitudinal axis because of the vertical displacement of the guide force from the horizontal bearing reaction. Because the frequency of the largest component of this disturbance is at crankshaft RPM for each cylinder of a multicylinder two-stroke engine, the engine will have a torque reaction roll couple whose largest component is at a frequency equal to the RPM multiplied by the number of cylinders. In addition to the torque reaction roll couple, the longitudinal displacement ofthe cylinders will cause the guide forces and the horizontal bearing reaction forces to generate equal but opposite yaw moments, one moment acting at the height of the wrist pins and the other moment acting at the height of the main bearings. The resulting torque reaction yawing couple MOMENTS, FORCES, AND VIBRATION 16-63 will tend to force the top of a tall engine into a racking movement (i.e., an X pattern when viewed from above) since the base of the engine is constrained. A four-cylinder, two-stroke engine may have a significant first order component ofthis racking couple, but for engines with more cylinders there are usually higher magnitude racking couples at higher order frequencies. Torque reaction couples of both types can be countered when necessary by bracing the top of the engine to an adjacent hull structure with transverse struts. S u m m a r y . Depending on the timing, the four-cylinder, two-stroke engine will have no external unbalanced forces of significance; unbalanced first, second, and higher order pitching couples, correctable by pairs of counter-rotating counterweights; an unbalanced first order yawing couple if additional counterweights have been fitted to reduce the first order vertical pitching couple; and unbalanced torque reaction couples, the most significant being a roll couple at four times engine RPM, countered when necessary by braces to hull structure. Case stu d y o f a fo u r -c y lin d e r , fo u r -str o k e e n g in e Since each cylinder of a four-stroke engine will complete its cycle in two revolutions or 720 degrees, even application of torque requires that a four-cylinder, four-stroke engine have the cylinders arranged 180 degrees apart, as in Figure 16-34. T im in g . MOMENTS, FORCES, AND VIBRATION 16-65 Ifthe crankshaft is viewed from the side, it can be seen that the couple generated by the longitudinal displacement of first order vertical reciprocating forces of cylinders 1 and 2 as they oppose each other is itself cancelled by an analogous couple from cylinders 3 and 4, so that no external first order pitching couple remains. With secondary reciprocating forces acting in phase for all cylinders, there will be no second order pitching couple either. As in the two-stroke engine, the pulsating axial force produced by the deflection ofthe crank webs may contain a higher order component whose frequency could coincide with the natural frequency of the crankshaft itself, necessitating the installation of an axial damper. T o r q u e r e a c tio n c o u p le s . Because the frequency of the largest component of the torque reaction couple is at half crankshaft RPM for each cylinder of a four-stroke engine, the four-cylinder in-line engine will have a torque reaction disturbance whose largest component is at a frequency equal to half of the RPM multiplied by the number of cylinders. S u m m a r y . Depending on the timing, the four-cylinder four-stroke engine will have a severe unbalanced second order external force, correctable by pairs of horizontally opposed counterweights rotating at twice crankshaft RPM; no unbalanced pitching couples of significance; an unbalanced first order yawing couple, if additional counterweights have been fitted to reduce the first order vertical pitching couple; and unbalanced torque reaction couples, the most significant being a roll couple at twice engine RPM. Summary of External Forces and Couples of In-line Engines It should be noted that pitching couples are independent of gas force and are influenced only by the weight and inertia of the components. If a cylinder must be cut out, balance will usually be less affected if piston and running gear remain in place. For in-line vertical engines with evenly spaced power strokes and no balancing gear or additional counterweights, the most serious external moments and forces generally encountered are described in the following paragraph. It should be noted that most of these disturbances can be corrected as described above. Higher order disturbances may be present in all planes including the longitudinal, but these only occasionally cause problems. Two-stroke in-line engines. Four-cylinder engines will usually have severe first and second order pitching couples and a severe fourth order roll couple. Five-cylinder engines will usually have a moderate first order pitching couple, but severe second order pitching and fifth order roll couples. Six-cylinder engines will usually have no first order pitching couple, but a severe second order pitching couple and a severe sixth order 16-66 MARINE DIESEL ENGINES roll couple. Engines with seven or more cylinders will usually have moderate or negligible first and second order pitching couples and moderate roll couples at an order equal to the number of cylinders. Eight- and twelvecylinder engines may have racking moments sufficient to require countermeasures, typically at third, fourth, and fifth orders. Four-stroke in-line engines. Four-cylinder engines will usually have a severe second order vertical force and a severe second order roll couple. Five-cylinder engines will usually have a moderate first order pitching couple, but a severe second order pitching couple and a severe 2.5 order roll couple. Engines with six or more cylinders will usually have moderate or negligible first and second order couples and moderate roll couples at an order equal to half the number of cylinders. MOMENTS, FORCES, AND VIBRATION 16-67 four-stroke engines, and at an order equal to the number of cranks for two-stroke engines. Because of the inherently more rigid structure ofthese engines compared with the tall crosshead engines, and also because of the higher frequencies resulting from the higher RPM and greater number of cylinders, these couples are not normally a source of concern. Described in the following paragraph are the most serious external moments and forces generally encountered in four-stroke V engines without balancing gear or additional counterweights beyond those necessary to reduce first order rotating couples, and with a typical bank angle of 45 to 50 degrees. Most first and second order disturbances can be corrected using combinations of opposite-rotating counterweights. Higher order components may be present in all planes but only occasionally cause problems. The uneven firing that results when the bank angle differs from the firing interval is not usually an important factor. Summary of External Forces and Couples of V Engines V engines are arranged with opposite cylinders in each bank acting on the same crank and therefore firing in succession. For even distribution of power strokes, the angle between the banks would have to be equal to the firing interval but, to simplify manufacturing, a constant VangIe (typically 45 or 50 degrees) is usually used regardless of the number of cylinders. Where an uneven distribution of power strokes results, the effect is mitigated by the large number of cylinders present. Several configurations will be perfectly balanced in regard to first and second order pitch and yaw, both forces and couples, regardless of the bank angle. Engines in this category include four-stroke V12, V16, and V24 engines. V8, V12, V16, and V20 engines with a 90-degree bank angle can, depending on firing order, have a rotating first order couple (i.e., equal yaw and pitch couples) and second order pitch and yaw balance. Since a rotating couple can be readily cancelled by a pair of opposing counterweights on the crankshaft, these engines can have perfect first and second order balance when so equipped. The 90-degree bank angle will also provide evenly distributed power strokes in a four-stroke V8, which accounts for the popularity ofthis configuration in automotive applications. With smaller bank angles the first order yaw couple diminishes while the first order pitch couple increases, but the opposing counterweights on the crankshaft can still be used to eliminate the yaw couple and reduce the pitch couple, or to convert the remaining pitch couple to a yaw couple. If a counter-rotating shaft, geared to run at crankshaft RPM, is fitted, the remaining first order imbalance can be completely eliminated. Torque reaction couples will be present in V engines, most significantly as a roll couple at an order equal to half the number of cranks for Four-stroke V engines. Eight-cylinder engines can be perfectly balanced in regard to first and second order pitching and yawing couples, but only ifthe bank angle is equal to the firing interval of90 degrees; smaller angles can yield first order disturbances sufficient to require correction. Ten cylinder engines will usually have moderate first and second order pitching and yawing couples and a moderate 2.5 order roll couple. Engines with 12, 16, or 24 cylinders will generally be perfectly balanced in regard to low order pitching and yawing couples, although they will usually have low to moderate roll couples at an order equal to half the number of cranks (i.e., at a quarter the number of cylinders). Engines with 14, 18, or 20 cylinders will generally have moderate or negligible first and second order pitching and yawing couples as well as low to moderate roll couples. Two-stroke V engines. There are far fewer designs oftwo-stroke V engines than four-stroke, but inasmuch as the Electro-Motive Division of General Motors has been building its engines exclusively to this configuration since the 1930s, there are vast numbers of them to be found in marine service. These engines have a firing order selected to eliminate secondary forces and couples, but with a 45-degree bank angle they would have a rather large primary pitching couple, even after being fitted with crankshaft counterweights, were it not for the use of the camshaft counterweights to provide first order moment compensation. This is possible because the camshafts run at engine RPM and are geared to run in opposite directions. The 45-degree bank angle and the selected firing order yield evenly distributed power strokes in the V8 and V16 engines, but not in the V12 and V20 engines, where the large number of cylinders provides adequate compensation. 16-68 MARINE DIESEL ENGINES Torsional Vibration Nature and effects. Torsional vibration arises from periodically varying torque superimposed upon the steady torque being transmitted to the load. The sources oftorque variation in propulsion systems are: The discrete power strokes ofthe engine, which generate torque pulsations once per crank throw per cycle, and at higher orders of this frequency, with a high magnitude at first order. In ships with direct-connected low speed diesels, this is usually the dominant source of torque variation. (A lower order torque variation' of significant magnitude can be imposed if one or more cylinders are unbalanced or not firing.) The discrete number of blades on the propeller, which imposes a torque variation at a frequency equal to the number of blades times the propeller RPM, with higher order components oflower magnitude. Gear tooth irregularities, with the frequency of the torque disturbance dependent on the particular irregularity. Given modern gear-manufacturing techniques, only damage is likely to cause the gearing to generate a torque variation of significant magnitude. In direct drive electrical generating sets, the source of torque variation is normally the engine, but under certain circumstances the generator can produce a torque variation as well. However the range of operating RPM of a generator set is normally predetermined and torsional vibration problems are usually avoided. When a propulsion system is operated in its normal range of RPM, torsional vibration becomes a problem if a source of excitation has a first or higher order frequency close to one of the natural frequencies for torsional vibration of the rotating system. Under these circumstances, the torque variation ofthe source will force the rotating system into resonance, causing it to oscillate torsionally at high angular amplitude, with corresponding high stresses that could result in shaft line component fatigue failure. In any event, the torsional oscillation can produce transverse components or can excite axial vibration of the shafting, either of which can in turn be transmitted to adjacent structures. In geared installations at low power levels, the oscillations can cause the gear tooth loaded faces to periodically separate and chatter. Determination of resonant speeds; barred speed ranges. Torsional vibration calculations are required by classification societies at an early stage in the design process, as soon as the engine has been selected and the configuration of the rotating system-including shafting, couplings, clutches, gearing, bearings, and propeller-is known. The natural frequencies for the rotating system are estimated for at least the three most likely MOMENTS, FORCES, AND VIBRATION 16-69 modes of torsional vibration. Traditionally, a method called a H o lz e r ta b u la tio n is used, but more sophisticated techniques are available. The resulting natural frequencies can each be divided by the orders of torque variation of each source in order to determine values of RPM at which resonances will occur. These are called c r itic a l s p e e d s . For example, for a direct-drive propulsion system driven by a five-cylinder, two-stroke engine, where the first mode natural frequency has been determined to be 40.1 radians/second or 383 RPM, there would be a major, or first order, critical speed at 76.6 RPM (383 divided by 5), and a second order critical speed at 38.3 RPM. If the stress levels arising at these speeds at any point in the shaft system-taking into account the energy absorbed in friction and by the entrained water surrounding the propeller ( d a m p in g ) - a r e in excess of permitted values, sustained operation near these speeds would be b a r r e d . Barred speed ranges may be an acceptable solution provided they are well below normal operating RPM. Avoiding torsional vibration. If the imposition of a barred speed range is not acceptable, usually because the barred range is too close to operating speed, the following solutions are available at the design stage: With direct connected engines, the intermediate shaft diameter can be increased. This lowers the operating stress on what is usually the weakest part of the rotating system, while raising the natural frequency of the system and moving the major critical speed above the operating RPM. This approach may be impractical for installations with long shaft lines. With direct connected engines and a long shaft line, a heavy flywheel (at the aft end of the crankshaft) can be fitted to reduce the natural frequency of the system, thereby shifting critical speeds to a lower RPM. A torsional vibration damper (or detuner) can be fitted at the forward end of the crankshaft to alter the natural frequency ofthe system and reduce the amplitude of the torque variation. The most frequently used dampers are of the spring-loaded or viscous fluid types. These dampers are occasionally used with direct connected engines and are commonly fitted to medium and high speed engines. Torsionally flexible couplings are almost always fitted to geared installations to isolate the gearing, and therefore the rest ofthe system aft ofthe gearing, from engine excitation. The most commonly fitted couplings are of the spring-loaded or rubber element types. These couplings are in addition to the quill shaft, which almost always connects the input flange of the reduction gear to the pinion, and is itself an effective torsionally flexible coupling. Flexible couplings and spring-loaded torsional dampers are both susceptible to low frequency excitation, such as that produced when one cylinder is cut out. Consequently, torsional vibration calculations are 16-70 MARINE DIESEL ENGINES ENGINE PERFORMANCE usually required for this mode of operation as well, and additional barred speed ranges may be imposed. E N G IN E PERFORM ANCE: T O T H E IR M A T C H IN G 16-71 these boundaries, and a familiarity with the load-matching procedure, will lead to a better understanding of what can be expected from a given installation. E N G IN E S E n g in e P e r fo r m a n c e LOADS I n tr o d u c tio n Diesel engines are typically furnished with a r a tin g or a maximum continuous rating (MCR), i.e., a stated value of power output at a corresponding RPM. This is an inadequate description ofthe performance characteristics ofthe engine, however, which will rarely be matched to its load, or operated continuously, at the rating. More complete information can take the form of an engine performance map, of which Figure 16-36 is a hypothetical example. The map defines an envelope within which operation is recommended by the engine designers. When an engine is initially selected for a particular installation, it must be correctly matched to its load so that conditions encountered in service do not force the engine to be operated outside of this envelope. Unfortunately, not all future service conditions can be foreseen, and operating engineers frequently find themselves struggling with engines that must be operated near or even beyond the boundaries of the envelope. An understanding of the implications of M aps Figure 16-36 defines the envelope in terms of power output, in this case B H P , versus RPM: some designers prefer to define the output in terms of MEP. The lines identified as 100 percent coincide with the MCR, and lines 1 through 6 with recommended limits for continuous operation under what the engine designer would define as normal conditions, with tolerable maintenance requirements. Normal conditions, including ambient pressures, temperatures, humidity, and fuel quality, must be accurately defined by the engine builder as they mayor may not comply with international standards. A point outside the limiting envelope at which an engine can be safely operated will, more than anything else, be a function of the particular engine design, reflecting design margins and the extent to which these have been proven by tests and by service experience, and the extent to which the design has evolved, been up-rated, or improved. Most designers have established their ratings and recommendations by probing the areas beyond the boundaries in prototype testing. Such testing, however, is usually confined to one or a few configurations of a particular model. For some models, therefore, margins may have been proven by, for example, several thousand hours of operation with an overload of20 or 30 percent, while for other models the margins may represent extrapolations of test data. E n g in e p e r fo r m a n c e lim its . The boundaries ofthe operating envelope are as follows: Rated engine RPM, Line 1 in Figure 16-36, is usually set by applying a design margin to the mechanical strength of those engine components which are subjected to the inertia forces. The turbocharger overspeed limit, Line 1b, is usually set by turbine blade root strength. Line 1a represents the setting of the overspeed governor or trip, usually 110 to 120 percent of rated RPM. Except for occasions when an engine driving a fixed pitch propeller is run at a modest overs peed on trials in order to maximize the load, operation beyond rated RPM is usually accidental. Lines 2 and 3 are, respectively, rated power (MCR) and rated MEP (or torque), and operation beyond these limits constitutes an overload. Overload operation results in increased stress and higher temperatures for combustion chamber surroundings (i.e., cylinder heads, liners, and piston crowns), a condition often referred to as high thermal load, as well as increased mechanical loadings on other engine components. Catastrophic failure in a properly maintained engine is considered unlikely because of the design 16-72 MARINE DIESEL ENGINES margins, and because inspections would reveal such effects as burning, cracking, or distortion in time for componentrenewal. In general, M E P and cylinder exhaust temperature are better parameters of thermal load than power output, with temperature being the preferred choiceofthe operator because of the ease with which it can be measured. Line4 represents marginal combustionair supply, and in turbocharged engines is set by applying a margin to the surge limit of the turbocharger, represented by Line 4a. Operation to the left ofLine 4 incurs increased likelihood of combustionchamber and turbine fouling, and smoke emission. Line 5 is the engine idle speed,typically 25 to 40 percent ofrated R P M . At lower engine speeds piston t;ing leakage during the compression stroke can so deprive the cylinders of air that combustionbecomesunreliable. Line 6 is a limit for the sustained use of heavy or sulfur-bearing fuels, below which operation on such fuels is not recommended. Combustion chamber temperatures at low loads are likely to be too low to effectivelyburn out some constituents of heavy fuels, and fouling of the combustion areas and the turbine can result. In addition, the lowtemperatures toward the bottom ofthe cylinder liners at such low loads can lead to sulfuric acid attack. Normal operation, fuel rate, and service recommendations. Generally, diesel engines are so matched to their loads that the normal operation in service is at some high fraction of rated output, normally under 90 percent ofMCR, and at, or somewhat below, rated RPM, i.e., in region A of Figure 16-36. This region usually coincides with the best range of specified fuel consumption. In addition, anticipated component lives, and therefore service recommendations for inspection, maintenance, renewal, and overhaul intervals, assume operation in this region. Nevertheless, most designers consider a certain amount of operation beyond Lines 2, 3, and 4 inevitable and take this into account in their service recommendations. Statements permitting time-limited operation in these regions, limited to perhaps one hour in ten or twelve, or a cumulative total of perhaps 500 to 2,000 hours per year, are typical. Variations in ratings. Apart from evolutionary effects, which usually result in an increase in rating as an engine design matures, identical or nearly identical engines are often offered with different ratings depending on the application. Some of the possibilities are listed below, where for purposes of this discussion the point corresponding to 100 percent RPM and 100 percent output in Figure 16-36 is referred to as the b a s ic rating. These variations from the basic rating may involve differences in the engine components or outfit. E c o n o m y r a tin g s . By derating an engine, i.e., by setting a new MCR lower than the basic rating, perhaps at a lower RPM as well, and by 16-73 ENGINE PERFORMANCE fine-tuning the turbocharger, fuel nozzles, and timing for this point, it is usually possible to obtain a lower fuel consumption, typically by 2 to 4 percent over the whole operating range. The basic rating may not then be achievable because ofthese changes. Since engine acquisition cost, weight, and volume will be essentially unchanged, the s p e c ific cost, weight, and volume (per horsepower or kilowatt) are all higher, but the long-term reduced fuel and maintenance often yield an economic gain. This is particularly true for direct-drive propulsion applications, where a reduced engine RPM be translated by the designer into a more efficient propeller. H ig h p e r fo r m a n c e r a tin g s . High performance operation is equivalent to sustained overload operation of the base engine, usually at the expense of increased maintenance requirements. The table below, for a particular line offour-stroke engines, is illustrative, and assumes no upgrading of engine components: P e r c e n t o f b a s ic M C R I n s p e c tio n / r e n e w a l in te r v a l a s p e r c e n t o f in te r v a l a t 8 0 to 9 0 p e r c e n t M C R 80 to 90 100 110 120 100 75 30 3 Fuel consumption will also suffer. The trade-off might be justified where low specific weight or volume is the overriding concern, for example, in naval combatants, or where engine use is limited, as in emergency generators. Upgrading of components-improving the materials of pistons and exhaust valves, for example-can improve the maintenance requirements but with the penalty of increased acquisition cost. In some cases the component changes are sufficient to justify a different model designation for the high performance engine. H e a v y fu e l r a tin g s . Low speed engines and the vast majority of medium speed engines of recent design, as well as some high speed engines, were intended from the beginning for heavy fuel operation. Other engines, however, may carry a lower rating for use with heavy fuels. The heavy fuel version ofthe engine may differ in components and outfit, with fuel nozzles, exhaust valves, and piston material being common changes. G e n e r a to r R P M r a tin g s . Engines driving alternating current generators are usually directly connected and must therefore run at a synchronous RPM. While the basic rated RPM may be a synchronous RPM in some cases, in other cases there will be a rated RPM for generator service, usually the closest synchronous RPM below the basic rated RPM. 16-74 MARINE DIESEL ENGINES Engine Matching for Ship Propulsion The case of engine matching most likely to be troublesome for the marine engineer, and the one which is least understood and most abused in service, is the selection of engines for ship propulsion. Hull power requirements. Figure 16-37 is a hypothetical but typical example of speed-power curves for a ship. The shape of a speed-power curve can be projected at the design stage from model tests or from a data base that includes standard series as well as trial run data from previously built ships of similar hull form. Line 1 in Figure 16-37 may represent the power required at loaded draft and trim, with the hull clean and smooth, and in good weather and calm seas (i.e., the expected performance on loaded trials). In service, the hull and propeller roughen through fouling by marine growth as well as general degradation (flaking paint, physical damage, corrosion, etc.); as this occurs, more power will be required for a given ship speed, so that the curve will shift to the left over time. Line 2 therefore represents loaded performance at a later time. An analogous curve for the ship in ballast would always lie lower and to the right of the corresponding curve for the loaded condition. Apart from weather and sea conditions, the rate at which performance deteriorates is a function of many factors, not all of which are predictable, but which include the quality of the antifouling paint and the time spent ENGINE PERFORMANCE 16-75 in warm water ports where fouling is most likely. For ships coated with conventional antifouling paints, the power required to maintain a constant ship speed might rise 30 to 50 percent or, if constant power is maintained, the ship speed may fall by some 5 to 15 percent over the interval before the next drydocking. If more sophisticated antifouling coatings with selfpolishing properties are used, the rise in power or drop in speed will be less. The typical interval between drydockings is 12 to 24 months, although the interval can be extended by partial hull cleaning and even repainting with the ship afloat. When the ship is drydocked the hull is cleaned and .repainted, and the propeller cleaned and perhaps polished. Not all of the general degradation is normally recoverable, however, so that even after cleaning and repainting the hull will normally be rougher than at the previous drydocking. As a consequence, the loaded performance curve after each drydocking will lie progressively to the left and above Line 1. After several drydocking cycles it may become economically attractive to attempt a more complete restoration to an "as-new" condition by blasting the hull to bare metal before repainting. Line 3 in Figure 16-37 represents a required speed to be achieved in service at loaded draft. If the engine were matched to the intersection of Line 3 and Line 1, where it would deliver MCR at the required service speed with the hull clean and smooth, it should be obvious that this speed could not be maintained as the hull and propeller roughen, even with good weather and in calm seas, except by overloading the engine. Consequently a propulsion engine must be matched so that only a portion of its rated power is required to meet the desired speed with the hull and propeller clean and smooth, leaving a power reserve, in addition to the engine margin, to offset the performance deterioration. Hull performance will also be affected by weather and sea conditions. Depending on the size of the total power reserve, as well as the trade route and the consequences of speed loss, an additional margin mayor may not be deemed necessary. The total margin, the difference between installed power or MCR and power required with hull and propeller clean and smooth, is the s e r v ic e m a r g in . It may be divided into constituent parts, as engine margin and s e a m a r g in . Propeller RPM. The efficiency of any propeller will peak at an optimal value of RPM, and propellers are usually designed so that this RPM coincides with the normal range of operation. That peak efficiency will be higher, however, for lower optimal values of RPM. A rough rule of thumb is that peak propeller efficiency rises 2 to 3 percent for every 10 RPM by which the optimal RPM can be reduced. It is largely this fact that has driven designers of low speed engines intended for direct connection to propellers to design for lower RPM. A limit on propeller RPM, and therefore on achievable efficiency, is set by the fact that the lower the RPM, the larger the propeller diameter. Propeller diameter is in turn limited by the neces- 16-76 MARINE DIESEL ENGINES sity for adequate propeller tip-to-hull clearance and by the need to keep the propeller immersed even in light ballast conditions, together with the desirability, at least for most merchant ships, of keeping the propeller above the baseline of the hull. (The achievable efficiency of a controllable pitch propeller is usually considered to be slightly lower than that of a fixed pitch propeller designed for the same application, because of the larger hub required.) The relation of the RPM of a fixed pitch propeller to the ship speed is shown in Figure 16-38 where Line 1 represents this relation at loaded draft and trim, with the hull clean and smooth, and in good weather and calm seas, i.e., the expected performance on loaded trials. In service, as the hull and propeller roughen, the RPM at any given ship speed will rise slightly; or, stated differently, the s lip would increase. The curve thus shifts upward with time, and Line 2 represents loaded performance at a later time. An analogous curve for a ballast condition would always lie lower than the curve for the loaded condition. Line 3 in Figure 16-38 represents a required ship speed to be achieved in service on loaded draft. If the engine were matched to the intersection of Line 3 and Line 1 , where it would run at 100 percent RPM at the required service speed with the hull clean and smooth, it should be obvious that the engine would exceed its rated RPM increasingly thereafter whenever the ship was at its service speed, as the hull and propeller roughen, even with good weather and in calm seas. Consequently a propulsion engine must be matched so that the ship speed achieved at 100 percent RPM, with hull and propeller clean and smooth, exceeds the required service speed. 16-80 MARINE DIESEL ENGINES might be needed with conventional antifouling protection. Nevertheless, this would be the course adopted if constant speed were required, for example, in the case of a container ship operated on a rigid schedule. On the other hand, if the power level were maintained at a constant value (Line 4 in the figure) the ship speed would decline. Cargo-carrying capability would be assessed on the basis of an average service speed, but service margins are typically lower than for constant speed applications. A representative range for motors hips with conventional antifouling protection may be taken as 10 to 20 percent. Where the trade permits, constant power operation is more economical; a high service margin means a higher installed power, hence higher acquisition cost and greater machinery weight and volume. In addition the fuel use will be greater in constant speed operation: a 50 percent rise in power to maintain constant speed might correspond to a 10 to 15 percent drop in speed at constant power. Case study: two engines geared to a single fixed pitch propeller. Figure 16-41 represents the combined performance envelope for two turbocharged engines with a single propeller matched to absorb 85 percent of the total PROPULSION ENGINE SUPPORT SYSTEMS 16-81 BHP at 100 percent RPM in the clean hull, loaded condition, in good weather, as indicated by the cube law, Line 1. Line 2 represents loaded hull performance at a later date. The maximum power output of one engine (50 percent ofthe total) intersects Line 1 at (.50/.85)1/3,or 84 percent RPM. The MEP would then be as follows: MEP = 1.0 BHP/.84 RPM = 1.19, or 119% The excessive MEP indicates a high overload and will be accompanied by excessive exhaust temperatures, forcing a reduction in output until Line 1 intersects the 100 percent MEP line for one engine. In this case the intersection is at about 76 percent RPM where the engine performance envelope is bounded by air supply rather than MEP. Here, it appears that sustained single engine use is precluded by the air limit, even with the hull clean and smooth, at least at loaded draft. If a ship is to be fitted with two engines geared to a single propeller, with the intention of single engine operation at reduced ship speed, there are the following possibilities: Fit a controllable pitch propeller, thereby enabling the engine to operate at or near its rated RPM, independent of ship speed. Use a lower match point; i.e, fit larger engines. In the example above, however, even a match point at 70 percent of combined MCR would not permit single engine operation at loaded draft with the hull clean. As an alternative to the lower match point, a "father-son" configuration is possible, with the larger engine meeting requirements for single engine operation. PROPULSION ENGINE SUPPORT SYSTEMS Introduction The arrangement of support systems will vary from ship to ship. These variations depend on many factors, among them the type of main and auxiliary engines, the trade, the arrangement of the ship, and the shipowner's preferences. The systems illustrated and discussed here are meant as typical examples only and may therefore fail to comply with systems recommended by a manufacturer, for example, or in place in a given plant. Starting Air/Compressed Air System A typical compressed air system is illustrated in Figure 16-42. The three segments of the system provide air for engine starting, for instrumentation and control, and for miscellaneous ship's services. Some engines, mostly smaller auxiliary engines, are started by cranking motors, which may be battery-, hydraulically, pneumatically, or PROPULSION ENGINE SUPPORT SYSTEMS 16-83 mechanically driven. Most larger auxiliary engines, however, as well as most propulsion engines, are started by the timed introduction of compressed air directly into those cylinders that were stopped in positions corresponding to the beginning of their power strokes in the selected direction of rotation. The compressed air drives those pistons down, in firing order sequence, thereby compressing air trapped in other cylinders. As one or more revolutions are completed, fuel is introduced in the normal manner into those cylinders whose pistons are completing a compression stroke, which then fire. The starting air is cut offand the engine accelerates to its idle speed, under control of the governer. Typically each cylinder of an in-line engine is fitted with a starting air valve, but commonly only the cylinders of one bank of a V engine are so fitted. The valves are usually opened by pilot air supplied via a camshaft-driven starting air distributor. In direct-reversing engines the distributor timing is shifted for reverse rotation (together with the timing of the inlet and exhaust valves and the fuel injection pumps), directing pilot air to those cylinders whose pistons have stopped just short of TDC on the upstrokes, so that the engine is rotated in the opposite direction for starting. Because the maneuverability of a ship is tied to the availability of starting air, the minimum number and size of starting air receivers must comply with regulatory body requirements. Typically, sufficient air must be stored to enable at least six consecutive starts of a nonreversing engine, or twelve of a direct-reversing engine, without recharging, in at least two receivers. Although the pressure may be reduced for admission to the cylinders, the pressure at which the air is stored will be twenty-five to thirty bars or more, with this higher pressure allowing smaller receivers. It is usually necessary to provide an independent means of starting the ship's service diesel generators. In the system shown, a small (and therefore rapidly recharged) receiver floats on the starting air main. In the absence of another source of compressed air, this emergency receiver can in turn be charged by an emergency air compressor, which may be driven by, for example, a hand-started diesel engine or a motor supplied from the emergency switchboard. Air for instrumentation and control should be dry and free from oil. It may be supplied from an independent, oil-free control air compressor with separate receiver; or, as shown, it may be bled from the starting air system and passed through a drier. In the latter case, a topping air compressor is advisable, matched to control air requirements, thereby relieving the large starting air compressors of this duty. In many geared diesel plants, the engines drive the pinions through air-actuated clutches, which are usually supplied with operating air from the starting air system through a reducing valve, and control and instrumentation air from the control air system. PROPULSION ENGINE SUPPORT SYSTEMS 16-85 Ship's service air supply may be taken from the starting air receivers as long as demand is limited. Where demand is high, a separate, low pressure compressor should be used in order to limit running hours on the large starting air compressors. The system of Figure 16-42 provides for either alternative. Compressed air systems on modern ships tend to be completely automated even in plants with attended machinery. Automatic drain traps are fitted at receivers and moisture separators, and compressors may be started and stopped automatically by pressure switches on the receivers, or they may start automatically, after which they run continuously until manually secured, being unloaded and loaded automatically as receiver pressure rises or falls. The pressure switches are set to start compressors successively, rather than simultaneously, as pressure falls in response to demand. F uel Systems A typical fuel system can be subdivided into separate filling and transfer systems for heavy fuel oil (Figure 16-43) and diesel oil, separate treatment systems (Figure 16-44), and a fuel oil forwarding system (Figure 16-45). Filling and transfer systems. The heavy fuel oil (HFO) filling and transfer system enables all HFO bunker tanks to be filled under pressure from pumps ashore or aboard a bunker barge. Good design practice calls for all the valves to be concentrated in one location to facilitate one-man operation. By using the valve at the foot of the filling line from deck to hold the pressure in the filling main below the static head of the overflow/vent pipes of the tanks, the possibility of an overflow to deck is reduced: overflow will be to the designated overflow tank instead, and this should be the last tank to be filled. The transfer pumps are normally used to transfer fuel from bunker tanks to the settling tanks, but can also serve between tanks or back up the filling line if it becomes necessary to discharge the contents of a tank ashore or to a barge. The suction from the distillate fuel oil (DO) transfer system enables the main engine to be run on DO for extended periods when necessary. Usually the suction main and the branches to the tanks will be steam traced and insulated. The transfer pumps are generally positive displacement rotary pumps, with coarse suction strainers for their own protection. The capacity ofthe transfer pumps is dictated by operational considerations: it may be reasonable to size each pump to fill the settling tank within an eight-hour workday. If the machinery arrangement permits, the duplicate transfer pump may be deleted, with standby provided by cross connections to one of the HFO booster pumps. The (DO) filling and transfer system is a PROPULSION ENGINE SUPPORT SYSTEMS 16-87 simplified version of the HFO system, with fewer tanks and no need for steam tracing or insulation. treatment systems. Fuel treatment systems include the settling tanks and purifiers, which enable most ofthe water and solids in the fuels to be removed. While clean distillate fuels are sometimes considered suitable for combustion in diesel engines without any treatment other than settling and filtration, given current refining practices it is advisable to centrifuge even the distillate fuel. In normal operation, fuel is transferred directly into the settling tanks from the bunker tanks, but passes to the day tanks only via the purifiers. Figure 16-44 shows a single HFO settler but two are preferable, each of 24-hour capacity, so that fuel can settle undisturbed for an extended period. To avoid drawing settled water and sediment into the purifier, the settling tank should be fitted with a sloping bottom, with the suction connection at the upper end, rising about 50 mm into the tank. HFO settling tank temperature will normally be 40°C to 50°C but should be kept well below the flash point. Most plants are fitted with centrifugal purifiers, with at least two units intended for full-time HFO operation either in series or in parallel, as described earlier. The rated capacity of each of the HFO purifiers should, at the very least, meet the main engine consumption at MCR with a 10 percent margin to allow for cleaning and other maintenance. The benefit of this apparent oversizing is more effective purification. (It should be noted that rated throughput of a given purifier when handling HFO may be only a fifth or less of its rating when handling DO.) Where existing piping precludes the flexibility of series or parallel operation of the HFO purifiers, a rearrangement of the piping should be undertaken. If existing purifiers are oflow capacity or are otherwise inadequate, installation of at least one new purifier should be considered. Modern purifiers tend to be self-cleaning, i.e., sludge-ejecting, and fully automated, with each HFO unit equipped with its own heater. Frequently, one of the HFO purifiers is arranged to stand by for a single DO purifier, as shown. Generally, purifier feed pumps are attached, positive displacement units, with throughput controlled by dumping back to the suction, in preference to throttling a suction valve. An alternative is to fit independent, motor-driven, variable displacement feed pumps. In most installations, oil leaves the purifiers under sufficient head to reach the day tanks, but where this is not the case discharge pumps must be fitted. HFO must be heated close to the boiling point of water to facilitate purification both by reducing the viscosity and by enhancing the difference in specific gravity between the fuel and the water; this hot oil input to the day tank can alone F uel PROPULSION ENGINE SUPPORT SYSTEMS 16-89 force the day tank temperature above the flash point. Consequently, some installations are fitted with a fuel cooler in the HFO purifier discharge line. Day tanks and settlers are likely to be fitted with level alarms, as well as local and remote level indicators. Settling tanks may be arranged so that a high level will stop the transfer pumps automatically. HFO tanks are fitted with steam heating coils, temperature indicators, and, often, alarms for low as well as high temperature. (DO will not normally require heating.) Remote operated valves at the tank outlets, that are capable of operation from outside the space containing the tank in the event of a fire, are a requirement of most regulatory bodies. Where the configuration permits, the day tank in each system may be arranged to overflow into the settlers, with the settlers in turn overflowing into the overflow tank in the double bottom. system. The fuel forwarding system of Figure 16-45 serves to supply HFO to engine injection pumps at the correct viscosity. Because the volume of fuel consumed will be small in relation to the volume available in the piping, the residence time between the heaters and the injectors would be sufficient, especially at low loads, for the fuel to cool. To prevent this cooling, a much larger flow rate is maintained, two or three times engine consumption at MCR, with the unconsumed excess leaving the spill valves ofthe injection pumps and recirculating back to the booster pump suction via the mixing tank. The amount drawn into the mixing tank from the day tank therefore corresponds to the amount consumed, while the oil temperature entering the heaters will reflect the amount of hot oil recirculated. The mixing tank is sometimes called the buffer tank since it enables the transition from hot HFO to cold DO (or vice versa) to occur gradually; it is also called a deaerating or degassing tank, since lighter fractions in the HFO can vent. In the typical configuration shown, the mixing tank is supplied by gravity from the day tank; its bottom, therefore, must be lower than the bottom of the day tank, while the top must be higher than the top of the day tank to enable degassing to occur. Nevertheless, to facilitate a reasonably rapid changeover from HFO to DO and vice versa, the volume of the mixing tank must be limited; consequently the mixing tank is most often configured as a pipe column. Because ofthe high temperatures to which the heaviest fuels must be heated and the increased amount of gas formation which would occur at atmospheric pressures, some engine manufacturers now recommend that the hot circuit of the forwarding system be designed as a closed and pressurized loop. This requires an additional set of low pressure booster pumps to feed the mixing tank from the day tank, but the mixing tank can then be more conveniently proportioned and located. Venting of gas that forms in the pressurized loop must be done through a relief valve. F u e l fo r w a r d in g 16-90 MARINE DIESEL ENGINES Booster pumps are fitted in duplicate and are usually of the positive displacement, rotary type. While the booster pumps will normally handle partly preheated oil, they must also be capable of handling DO on occasion, as well as cold HFO being recirculated while the system is brought up to operating conditions. At least two fuel heaters are fitted, each with sufficient capacity to heat the heaviest fuels likely to be encountered, and each conservatively rated in regard to fouling margins. The steam supply to the heaters is controlled by the viscosimeter, which must be sited in close proximity to the injection pumps, either just ahead of them or in the fuel return line as shown. When a ship's auxiliaty engines are fitted for operation on HFO, the forwarding system may be duplicated on an appropriate scale, and may incorporate an in-line blending unit, comprising metering pumps and mixing devices. To permit operation of the auxiliary engines on HFO when steam is unavailable, an electric fuel heater will be provided in parallel with the steam heaters. A final fuel filter is fitted after the heaters, where filtration is facilitated by the reduced viscosity. This filter serves principally as a backup to the purifiers. Modern installations are most likely to have a fine mesh, selfcleaning unit as shown, or a disposable-element type of even finer mesh. On older ships, fitted with duplex basket strainers, an upgrade may be advisable. All of the HFO piping in the forwarding system, sometimes including the filters and pumps, is likely to be steam traced and insulated. The extent of automation of the system will vary but will typically include automatic changeover for the pumps, low pressure alarms at pump discharge points, temperature alarms after the heaters, and a differential pressure alarm at the final filter. Lubrication Systems The complexity of motorship lubricating oil (LO) systems is the result of the number of different grades of oil required. A geared, medium speed diesel plant will usually require different grades of oil for the main and auxiliary engines, and other grades for gearing and miscellaneous uses. A LO system for a low speed diesel plant will be equally complex, involving at least two grades ofoil for the main engine (one for the circulating system and the other for the cylinders) and a third for the auxiliary engines. It is important to note that most trunk piston engines do not use a separate grade of oil for cylinder lubrication. Main engine LO circulating system. A typical circulating oil system appears in Figure 16-46. Oil draining from bearings and cooling passages to the bottom of the crankcase passes into a drain tank (or remote sump) built into the double bottom below the engine, from which it is drawn by 16-92 MARINE DIESEL ENGINES the LO circulating pump for redistribution via a cooler and a filter. Although the drain tank is shown as an integral part ofthe double bottom structure, in some cases independent tanks have been installed. In smaller engines the additional cost and complexity of a drain tank are often eliminated in favor of a wet sump formed by the crankcase bottom. The suction bell mouths clear the bottom by about 100 mm to avoid ingesting water or sediment, and may incorporate a foot valve to keep the suction line full when the pump is stopped. The drain tank or wet sump must be so designed that suction will be maintained at the most extreme conditions of list, trim, and sloshing .. The LO circulating pumps are most often positive displacement rotary pumps, and in larger plants are fitted in duplicate. Both pumps are motor driven in installations with low speed diesels, but higher speed engines are often fitted with an engine-driven pump, relying on the motor-driven pump for standby service. Each pump will have a coarse suction strainer for its own protection. Providing each pump with its own suction line is an advisable precaution. A full flow filter is provided in the pump discharge line before or after the cooler. It may be of the duplex, basket type, but better filtration will be provided by a disposable-element or self-cleaning simplex unit, with a standby filter in a bypass. Filtered oil is distributed to engine bearings, for governing and control service, to valve gear, and, on trunk piston engines as well as some crosshead engines, to pistons for cooling. Some of these services may require higher pressure oil, which can be obtained by fitting booster pumps in the line. In other cases some equipment is served by independent circulating systems that are scaled-down versions ofthe main system. Usually oil is also supplied to turbocharger bearings from the circulating oil pumps, but some turbochargers are provided with their own selfcontained LO system. In either case the turbochargers must be provided with an emergency supply ofLO, as they will continue to spin at high speed following a LO failure. The system of Figure 16-46 includes a gravity tank for this purpose. In crosshead engines, the crankcase, and therefore the circulating oil, is protected by the piston rod packing from contamination by combustion products blowing by the piston rings, whereas this kind of contamination in trunk piston engines is usually inevitable. Consequently, while a straight mineral oil with corrosion and oxidation inhibitors is usually recommended for the circulating oil of most crosshead engines regardless of the fuel in use, manufacturers oftrunk piston engines generally recommend a detergent oil with alkaline additives (measured as total base number or TBN) matched to the likely sulfur content of the fuel to be burned. While the circulating LO in a crosshead engine rarely requires replacement in the normal course of events, this is not usually the case PROPULSION ENGINE SUPPORT SYSTEMS 16-93 with trunk piston engines, where, in most cases, the LO must be renewed periodically. The life of trunk piston engine LO, already extended by the regular addition of fresh makeup oil to compensate for oil burned in cylinder lubrication, can be further extended by taking such extra measures as the fitting of addi tional extra -fine filtration loops and, if necessary, the occasional addition of chemical additive packages to the oil. There is the possibility of a crankcase overpressure condition or explosion in any diesel engine. A common pattern begins when an overheating bearing vaporizes oil, creating an explosive mixture which can then be ignited by further overheating of the bearing or, in trunk piston engines, by blowby. To give warning of overheating bearings, alarm systems oflarge engines include temperature probes at major bearings within the crankcase, while photoelectric oil mist detectors continuously sample the air drawn from likely accumulation points within the crankcase. Should an ignition occur, an initial rise in crankcase pressure will be limited by air available within the crankcase, but if this is followed by an influx of air, a much larger secondary explosion can occur. To minimize the possibility of the crankcase being breached by a primary pressure rise and of air entering to cause a secondary explosion, large engines are fitted with spring-loaded pressure relief valves (usually mounted on the crankcase doors) which open to relieve an overpressure and then snap shut. In addition, the crankcase vent pipe is restricted in size to limit the ingress of air. The extent of additional automation of the system will vary, but will typically include automatic changeover for the pumps, low pressure alarms, a low LO pressure trip, and a differential pressure alarm at the filter. LO temperature is usually controlled by a three-way, thermostatically controlled valve on the oil side ofthe cooler. Main engine cylinder oil system. Crosshead engines are fitted with an independent cylinder oil system for lubrication ofthe piston rings. A typical system is included in Figure 16-46. The cylinder oil is stored in one or, preferably, two tanks and is transferred daily to a small capacity measuring tank, from which it passes by gravity to the cylinder lubricators on the engine. The lubricators are precisely calibrated injectors, mechanically driven by the engine and timed to inject a metered quantity of the oil into the cylinder as the piston ring pack rises past the injection points. The oil is ultimately consumed. In crosshead engines in good condition the cylinder oil consumption may range from below 0.5 glhp-hr to below 1.0 glhp-hr. Because the quantities of oil injected per stroke are small, the measuring tank provides for consumption to be determined accurately as a drop in level over an elapsed time period. Cylinder oil is a high viscosity mineral oil, with a TBN matched to the anticipated sulfur content of the fuel. Two cylinder oil storage tanks provide flexibility in this regard by enabling cylinder oil of different TBN PROPULSION ENGINE SUPPORT SYSTEMS 16-95 to be carried. Cylinder oil storage tanks are filled from deck by gravity, a fact which may preclude filling the measuring tank from the storage tanks by gravity as well, necessitating a small hand- or motor-driven transfer pump. In trunk piston engines in good condition, cylinder lubrication consumes up to 1 g/hp-hr or more of circulating oil, which usually reaches the ring pack and cylinder liner walls by a controlled leakage from the wrist pin bearing. In some of the larger medium speed engines, circulating oil is injected for cylinder lubrication in the same manner as described above for cylinder oil in low speed engines. In these engines the oil is usually taken from the circulating system, but separate oil tanks and piping can be arranged to bring only clean, unused detergent oil of high TBN to the injectors. The arrangement will still fall short of what can be achieved in a crosshead engine, since an unburned portion of the injected oil, carrying entrained contaminants, will drain to the crankcase. storage, transfer, and purification system. While smaller high speed main engines may rely solely on filtration and occasional oil changes to maintain the quality of the circulating oil, most larger main engines are arranged for continuous bypass purification using centrifugal purifiers, as shown in Figure 16-47. Two purifiers are shown, although one is a standby unit, and it is often possible, by means of blanked cross connections or shared components, to rely on a reserve Fa purifier for standby. Normally, the purifier draws from a bell mouth in the suction well of the main engine drain tank and returns the oil to the forward end of the same tank, thereby avoiding short-circuiting. The purifier suction will generally be located aft of the circulating pump suction, and lower-about 50 mm above the tank bottom-to draw water before it reaches the circulating pump suctions. The main La storage tank, with a capacity equal to at least one charge for each engine it serves plus sufficient margin to meet miscellaneous needs, is filled from deck by gravity, while the settling tank is normally empty. Should an engine's circulating oil be massively contaminated, for example by water, it can be transferred to the settler using the transfer pump, and fresh oil brought down from the storage tank. Ifthe oil cannot then be redeemed by a combination of settling and purification, it can be discharged ashore or to a barge for reclamation. Modern purifiers tend to be self cleaning and fully automated. Generally purifier feed pumps are attached, positive displacement units, requiring the La purifiers to be located on the floor plates to minimize the suction lift required from the main engine drain tank. An alternative arrangement would place a motor-driven purifier feed pump on the floor plates, to provide more flexibility in purifier location. In most installations, oil leaves the purifiers under sufficient head to reach the settling tank, but where LO PROPULSION ENGINE SUPPORT SYSTEMS 16-97 this is not the case, a discharge pump must be fitted. Each purifier is normally sized to circulate the main engine drain tank three to five times per day. Purifier connections can be provided for batch purification of auxiliary engine LO but are unlikely to be used, since relatively small quantities are involved, since the alkaline-additive detergent oil will require renewal at intervals in any event, and since there is potential for contamination of the main engine oil. When auxiliary engines of substantial size are involved, however, a separate purifier may be justified. The fixed piping shown for filling and draining the auxiliary engine sumps may not be provided for smaller installations. An analogous problem exists in medium speed diesel plants, where the reduction gear contains a straight mineral oil that is subject to water contamination from condensation within the gear case, but which would otherwise last indefinitely. Again because ofthe danger of contamination, in this case of the gear oil by the alkaline main engine oil, even when purifier connections are fitted they are unlikely to be used. Some operators fit water-absorbing filters in the gear lubrication system, while others rely on occasional batch purification using a portable purifier or filter. F r e sh w a te r C o o lin g S y s te m s Figure 16-48 illustrates a typical freshwater cooling system for a plant with a crosshead engine with water-cooled pistons and three diesel generators. The system can be divided into separate subsystems for cooling main engine cylinder jackets, main engine pistons, main engine injectors, and the generator engines. (Charge air coolers of main engines are usually cooled by seawater; see below.) system. The jacket cooling system shown differs from that used on trunk piston engines only in the fact that the LO cooler and the charge air cooler, which are seawater cooled here, may be included in the same circuit. An elevated expansion tank maintains a static head on the suction side of the system and provides a convenient point for collecting vents, adding makeup feed, and adding chemicals to inhibit corrosion and formation of scale. Good venting is important: air carried with the coolant will enhance the potential for corrosion and can also accumulate at points to block coolant flow. The turbocharger supply and return lines are shown, since even turbochargers with uncooled casings usually require cooling water for the turbine-end bearing. In either event, water flow must be forced by an orifice in the bypass line, as shown. The jacket water circulating pumps are usually centrifugal pumps, and in larger plants are fitted in duplicate. Both pumps are motor driven in J a c k e t w a te r c o o lin g 16-98 MARINE DIESEL ENGINES installations with low speed diesels, but medium and high speed engines are often fitted with an engine-driven pump, relying on the motor-driven pump for standby service. Most seagoing ships recover heat from the main engine jacket water for fresh water generation. The fresh water generator is usually located ahead of the jacket cooler, and may be fitted with a supplemental steam or hot water heating coil for use when insufficient jacket water heat is available. The jacket water heater is used when the engine is idle. Maintaining the engine in a warm condition assists in minimizing corrosion. Automation is likely to include alarms for low pressure, high temperature, and low level in the expansion tank; automatic changeover of the pumps; and thermostatic control ofthe three-way valve at the cooler. Piston cooling water system. While all trunk piston engines, as well as some crosshead engines, use oil to cool the pistons, a number of crosshead engines use a cooling water system separate from the jacket water system. The water reaches and leaves the pistons through telescoping tubes enclosed within compartments inside the crankcase in order to avoid contamination of the LO should a gland fail. Because ofthe high temperature ofwater draining from the pistons and the resulting potential for flashing at the pump suction, some manufacturers recommend that deep well pumps immersed in the tank be used. Both pumps will be motor driven. Automation and other features will be similar to those described for the main engine jacket water system. Injector cooling system. Some engines are fitted with fuel injector cooling systems and others are not. In some cases injectors are cooled only through conduction to the heads; in other cases they are circulated with water as part of the jacket cooling system; and sometimes they are circulated with diesel oil in a closed loop. When a separate injector water circuit is fitted, it is a scaled-down version of the jacket cooling system, often without a cooler, and with a steam coil in the expansion tank to maintain temperature at low engine output when use of HFO is intended. Diesel generator cooling system. Auxiliary engines tend to have self-contained cooling circuits, with charge air cooler, cylinder jackets, and LO cooler circulated by a single cooling pump on each engine. The system of Figure 16-48 combines these circuits into a common system, with a central generator engine cooler and expansion tank, and with motor-driven pumps. This offers the advantage of circulating the idle generator engines with warm cooling water as protection against corrosion, and enables the standby engine to start and pick up load more rapidly. For reasons of reliability, however, many operators prefer that the auxiliary engines have 16-100 MARINE DIESEL ENGINES separate cooling systems. Automation and other features will be similar to those described for the main engine jacket water system. c o o l i n g systems. There is trend toward increased use of freshwater cooling for equipment previously cooled by seawater, with the logical extrapolation being a complete central freshwater cooling system in which fresh water is circulated from seawater-cooled central heat exchangers to main engine and auxiliary machinery coolers. The central system may be divided into separate high and low temperature subsystems. The benefit of a central freshwater cooling system is in reduced maintenance, since only the central coolers are exposed to seawater. C e n tr a l fr e sh w a te r S e a w a te r S y ste m s Included under this heading are not only the seawater cooling circuits but also such ship's service systems as ballast and fire main. Figure 16-49 is a typical example. In the example, seawater suction is taken from a crossover main connecting a low sea chest (suction box) on one side of the ship to a high sea chest on the other side. The low sea suction is used at sea where it is more likely to remain immersed as the ship rolls and pitches, while the high sea suction is used in port to reduce ingestion of sand and bottom debris. Both sea chests are vented by standpipes rising well above the highest waterline, and both are fitted with hose connections for blowing them out with compressed air or steam. Where operation in freezing water is frequent, the sea chests may be fitted with piped connections to introduce hot water from the overboard discharge or from separate steam heaters. The sea chests are covered with grids mounted flush with the hull; a reasonable precaution is to secure these with stainless steel bolts removable by divers. Further protection is provided by coarse basket strainers at each end of the crossover main. Seawater pumps in saltwater service tend to be high maintenance items and good design practice calls for pumps of similar capacity to be identical, simplifying spare parts requirements in service. In the illustrated example, three identical centrifugal pumps are provided, one for main engine cooling and two for ballast, with one ballast pump piped to stand by for engine cooling. This arrangement is suited to dry cargo ships and gas carriers, as most tankers with conventional cargo systems tend to locate the ballast pumps (which are most likely to be turbine driven) in the cargo pump room. In a tanker, however, the standby main engine cooling pump might serve to circulate the cargo and ballast pump condenser. The emergency bilge suction on the main engine cooling pump meets a regulatory body requirement. PROPULSION ENGINE SUPPORT SYSTEMS 16-101 In the illustrated example, two auxiliary cooling pumps are provided, drawing from the crossover main and discharging to meet all miscellaneous seawater cooling needs. The heated seawater leaving most of the auxiliaries is then combined with seawater leaving the main engine coolers; downstream is a three-way thermostatically controlled recirculating valve that can return some of the heated water to the crossover main and discharge the rest overboard. The recirculating system enables the seawater used for cooling to be maintained above 20°C, even in very cold ambient conditions. For the system to operate in port with the main engine secured, the thermostat should sense the seawater temperature in the auxiliary cooling system. It is usually good practice to limit the number of hull penetrations; to the extent practical, additional connections to the crossover main can provide sea suctions for fire, bilge, general service, after ballast, evaporator, and sanitary pumps. Where all these pumps cannot be clustered in the proximity of the crossover, however, an additional sea chest may be necessary. (In any event, one fire pump must have an independent suction.) Similar logic is applied to overboard discharge valves. Each overboard discharge is fitted with a hose connection for blowing out. Automation is likely to include, in addition to the recirculation valve, automatic starting of the standby pumps and low pressure alarms in both subsystems. S te a m S y ste m s Most diesel plants of any size are fitted with exhaust heat recovery systems to meet the heating requirements of the fuel and LO systems as well as hotel needs. Occasionally a thermal fluid is used but most often the working fluid is water. Where sufficient waste heat is available, a waste heat turbogenerator may be fitted. The design pressure of the steam system may be dependent on its use for fuel heating: the heaviest fuels require heating to about 170°C,in turn requiring saturated steam at the fuel heaters at a saturation pressure of about eight bars. Figure 16-50 illustrates a simple steam system frequently used on ships where steam requirements are limited, in which the oil-fired package boiler serves as the steam drum for a forced-circulation water tube waste heat boiler. This kind of system is easily automated: excess steam produced is dumped through a pressure-regulating valve to the seawater-circulated condenser, while a shortage of steam triggers a pressure switch to suppIementally fire the oil-fired boiler. The fact that the oil-fired boiler is filled with saturated steam and water at all times results in a rapid response. Sometimes gas bypasses are fitted to the waste heat boiler to enable control of steam production. 16-106 MARINE DIESEL ENGINES Some ships are fitted with waste heat boilers of the gas tube type in which steam production can be controlled by varying the water level. The oil-fired boiler would not necessarily be in the circuit, but it can be kept warm by a steam heating coil. Where the steam demand is in the range of about 2,000 kglhr or less (the usual case on dry cargo vessels), the oil-fired boiler is most often ofthe fire tube type. On tankers, where steam may be required for cargo heating, cargo and ballast pumping, and tank cleaning, the steam plant becomes more complicated. Figure 16-51 is a typical example, in which two oil-fired boilers supply steam at a pressure sufficient for cargo and ballast pump turbines, and also serve to supplement the output of the waste heat boiler. Because the waste heat boiler is operating at lower pressure it is provided with its own steam drum, and the oil-fired boilers are fitted with heating coils. The oil-fired boilers in this example are of the double circuit type: an oil-fired, closed, primary steam circuit generates steam in an attached secondary drum, precluding contamination of the primary circuit via a leaking Figure 16-54. Pinch point diagram for single pressure boiler 16-108 MARINE DIESEL ENGINES cargo-heating coil. The same results can be achieved using a contaminated evaporator (low pressure steam generator) to supply cargo-heating steam. Where sufficient waste heat is available for a turbogenerator to be fitted, the system can be as simple as in Figure 16-52, intended as a fly sheet to Figures 16-50 and 16-51. A problem with this system arises from the fact that a high condenser vacuum and its corresponding low condensate temperature will depress feedwater temperature and increase the heat required from the waste heat boiler, unless a feed heater is fitted. In Figure 16-53, a section of.the charge air cooler is arranged as a feed heater. Because of the danger of sulfuric acid attack on cold economizer tubes, the systems of Figures 16-53 and 16-55 incorporate injection points upstream of the economizer for saturated water, thereby enabling the feed temperature to be maintained even at low engine output. This is sometimes called e c o n o m iz e r r e c ir c u la tio n . The quantity of steam generated in a waste heat recovery cycle is governed by the available heat exchange surface in the waste heat boiler, and by the gas temperature, which must exceed the steam temperature at every point in the boiler, as illustrated in Figure 16-54. The point at which the temperatures come closest is the p in c h p o in t, for which a practical minimum may be about 15°C. The situation can be improved by lowering the design steam pressure, but an alternative means might then be OPERATING AND MAINTENANCE PROCEDURES 16-109 required for final fuel heating. Figure 16-55 illustrates a steam system for maximum heat recovery that incorporates a dual pressure waste heat boiler, enabling the steam temperatures to more closely match the gas temperature, as shown in Figure 16-56. There may be a trade-off involved when a more sophisticated system is adopted, as the increased complication may result in increased maintenance. A properly designed waste heat boiler may include the following features: Sootblowers. A vestibule design, enabling the V-bends to be outside the gas path (and thus not subject to external corrosion in addition to the internal corrosion and erosion), while improving access for isolating and plugging leaking tubes. Provision for gas side washdown when the main engine is secured, usually encompassing access hatches above and below the tube banks, a drain fitting at the bottom, and a means for blanking the gas inlet to protect the turbochargers. The ability to pass engine exhaust with the water side dry without damage (dry-steamed) in an emergency. Steam plants on modern motorships are likely to be fully automated for normal operations even ifthe machinery spaces are normally manned. All the normal procedures and precautions regarding the maintenance of water quality are necessary, including purity of makeup feed, regular sampling and testing, blowdown, and chemical treatment to preclude corrosion and scale formation. OPERATING AND MAINTENANCE PROCEDURES Introduction The procedures described here are general and meant to provide background information only. They are not intended to be comprehensive or all-inclusive. Recommended procedures for specific equipment should be sought out and followed. Normal Operations Starting. Prior to starting an engine that has been shut down for an extended period or has been overhauled, tools, eyebolts and other lifting gear, and rags and debris must be cleared from the crankcase, from the air and exhaust manifolds, from the air intakes, and from the generat'vicinity of the engine. Fluid levels and quality must be verified. If there is any doubt about the condition of the coolant, the LO, or the fuel, the system should be 16-110 MARINE DIESEL ENGINES drained and refilled. On main engines, the L a is circulated through the purifier and brought close to its normal temperature. Where motor-driven cooling pumps are fitted, one ofthese is started (in each circuit) and, on large engines, the cooling water is heated toward the operating temperature. High points and other air traps in the cooling system are vented. A seawater cooling pump is started and coolers are vented on their seawater sides. Where independent L a circulating pumps are fitted, one is started, and a flow to all bearings and" service points assured. On engines with prelubrication pumps, these are operated until all bearings receive oil. Crosshead engine cylinder lubricators are operated manually until all cylinders receive oil. Drains in air and exhaust manifolds are opened, as are indicator cocks on each cylinder. Where a compression release device is fitted, it should be used. If the engine is direct connected, permission to rotate the engine must be obtained from the bridge. M te r checking to see that all hands are clear, the engine is rotated through several revolutions with the turning gear or a barring device to ensure freedom from interference, to help establish oil films at bearings, and to ensure that cylinders and manifolds are free of water. The turning gear or barring device must then be disengaged. If access permits, turbochargers should be rotated by hand to ensure freedom of rotation. Fuel systems are primed and vented. Most engines that run on HFO are started on distillate fuel and shifted over to HFO only after starting; however, where the engine will1Jedirectly started on HFO, a booster pump is started with the recirculating line open. Steam is lined up to heaters, to steam-traced fuel lines, and, where appropriate, to the injector cooling circuit. The fuel is recirculated through the heaters (and injectors, on some engines) until hot fuel is present at the injectors, or as close to them as the system permits. On air-started engines the starting air receivers are charged and drained. An inspection is made to ensure that all gauge valves are open, that all sensors are in place and connected, and that standby pumps not in use are lined up and operable. Safety devices, including interlocks and alarms, are tested. Before starting propulsion engines that are directly connected to the propeller, permission must be obtained from the bridge. Further, if the propeller is fixed pitch the engine will have to be stopped immediately to avoid putting way on the ship and straining docking lines and bollards. On reversing engines, the camshaft and starting air distributor are set in the desired direction. The fuel rack is set to a starting position, usually about 25 percent of maximum. Starting air is lined up to the starting valve. OPERATING AND MAINTENANCE PROCEDURES 16-111 When the starting valve is opened, the engine will crank through one or two revolutions before firing. As the engine accelerates, the fuel rack is repositioned to the idle speed setting. If a decompression device is fitted and used, it is returned to its normal setting. When possible, the engine should be allowed to warm up at light load. When a higher load must be applied immediately, the lowest initial load should be applied and increases should be gradual. The engine manufacturer's suggestions should be followed. Immediately after starting, all systems are checked for proper operation. On engines fitted with attached pumps, but started using independent motor-driven pumps, the motor-driven pumps can be stopped once the engine stabilizes and "full away" is rung up on the telegraph. If the motor-driven pumps are fitted for automatic standby they must be left correctly lined up for this function. Maneuvering. Propulsion engine control may be effected by direct mechanical linkage or by pneumatic, hydraulic, or electrical servomotors, and may be applied from a local control stand at the engine or from a remote console in an engine control room or on the bridge. Once the engine has been prepared for starting, with support systems in operation, control can be turned over to a remote console. Automation interposed between the remote console and the engine may be such that the operator at the console need not understand, when he makes a change in speed or direction, the procedural details necessary to accomplish the change. Sufficient instrumentation is provided, however, to confirm his actions. Speed and load control of a diesel engine are achieved by positioning the fuel racks of the injection pumps, which are mechanically linked to operate simultaneously. In some installations, the racks may be positioned directly by the operator, but in most cases they are positioned by a governor, whose set point is adjusted by the operator. Engines that directly drive fixed pitch propellers are necessarily reversing engines. To comply with a stop order, the engine itself must be stopped by moving the fuel racks to the zero position. Then, when ordered to run ahead or astern, the camshaft and starting air distributor are positioned in the appropriate direction, the fuel racks are set to the starting position, and air is admitted to restart the engine. Ifthere is way on the ship when the fuel is cut offin response to a stop order, the engine will continue to be rotated by the propeller until starting air is applied in the reverse direction, or until the shaft brake, where fitted, is applied. In fixed pitch propeller installations where reversing engines are clutched to the propeller shaft (usually at gearing) the engines can be warmed up in advance of the maneuvering period. Normal maneuvering may then be as described above, with the clutches engaged throughout; or, alternatively, a stop may be achieved by declutching the engines and 16-112 MARINE DIESEL ENGINES allowing them to idle, while a shaft brake holds the propeller shaft stationary. To reverse propeller rotation, however, the engines must be stopped, the camshafts and starting air distributors shifted, the engines started again in the reverse direction, and the clutches engaged. Where nonreversing engines drive a fixed pitch propeller through reversing gearing, a shaft brake must be fitted. Reversing is achieved by reducing engine speed to idle, declutching the ahead gear train, applying the shaft brake to stop the shaft, engaging the clutch on the astern gear train, and then raising engine speed to the desired setting. In this arrangement the clutch movements and shaft brake application would normally be programmed to operate in sequence from a single lever. After each use of starting air, the compressors are started, usually automatically, to recharge the receivers. During intensive maneuvering periods the compressors may be running continuously, cycling on their unloaders. In most cases the receivers and moisture separators will be fitted with automatic drain traps, but if this is not the case they must be drained at frequent intervals. The minimum engine speed will typically be 25 to 40 percent of rated RPM and, unless a slip clutch is included in the drive train, or electric drive is used, this minimum RPM will determine minimum shaft speed. To maintain extremely low ship speeds in most ships with fixed pitch propellers requires that the engines be stopped and started repeatedly or, where clutches are fitted, that they be clutched in and out repeatedly. If there is a barred speed range, it is passed through quickly. Ships with controllable pitch propellers may be maneuvered entirely by pitch control, with the engine speed left constant throughout the maneuvering period, at about 75 to 90 percent ofrated RPM. During maneuvering periods, all support systems and engine instrumentation are monitored. Depending on the extent and condition of automation present, it may be necessary to make adjustments to keep temperatures and pressures within bounds. Boost blowers on two-stroke engines should be operating at low engine output. If an engine is idled or run at low loads for an extended period, the charge air cooling water is regulated (automatically or manually) to maintain the charge air temperature above the intake temperature to avoid condensation in the manifold. Where charge air coolers are cooledbyjacket water, it is frequently possible under these conditions to elevate the charge air temperature well above ambient, to help ensure prompt ignition, to aid combustion, and to help avoid sulfuric acid attack in the cylinders. For engines normally maneuvered on HFO it may be advisable, during a long idle period, to switch to distillate fuel, but this must be done gradually, usually through 'a mixing tank, since a sudden fuel temperature reduction can cause injection pump plungers to bind. OPERATING AND MAINTENANCE PROCEDURES 16-113 At the close of a departure maneuvering period, engines should be loaded in increments that extend over at least the first hour at sea. Running-in after an overhaul. Running-in is necessary to enable new piston rings to wear to conformity with the cylinder liner. (Modern bearing materials, on the other hand, usually need no running-in period.) An engine with one or more pistons fitted with new rings should be started and run on DO, with load applied gradually over a period of at least six hours. On engines with separate cylinder oil lubricators, a straight mineral cylinder oil should be fed to the re-ringed cylinders at an increased rate during the running-in period. Steady running. During steady running, all engine indicators and all support systems are monitored regularly, and adjustments made as required. Even in plants with extensive automation, engineers should regularly walk around the engines, listening for unusual noises, looking for leaks and sources of unusual vibration, and checking sight glasses and fluid levels. Trends are noted for their value in planning maintenance and in avoiding unexpected failure. Every day at sea, fuel is transferred to the settler, and, where two settlers are provided, purifier suction is changed over. Where HFO is used, tank heating and line tracing steam is supplied to the tank and lines that will be used in the following day's fuel transfer. Water and sediment are drained from settling tanks daily, and other tanks are checked for accumulations at least weekly. In crosshead engine installations, the cylinder oil measuring tank is refilled daily, and the levels before and after refilling noted in order to calculate the consumption. The cylinder oil injectors are then adjusted as required. Moisture is drained daily from charge air coolers and from the air manifold. On engines burning HFO, the turbocharger turbines are water washed, often every day, in accordance with manufacturer's instructions. The compressor will require cleaning less frequently. At least weekly, or once per passage for vessels on shorter voyages, indicator cards are taken on low speed engines, and maximum pressure readings on higher speed engines, in order to confirm that the load is balanced among the cylinders. Fuel oil consumption is calculated daily and lubricating oil drain tank (i.e., sump) levels checked at least as often. Self-cleaning purifiers will have to be opened for manual cleaning at intervals determined by experience. Until that experience is gained it may be reasonable to open HFO purifiers weekly, and LO and DO purifiers 16-114 OPERATING AND MAINTENANCE MARINE DIESEL ENGINES monthly. Older purifiers that do not eject accumulated sludge will require more frequent cleaning-at least daily for the HFO purifiers and perhaps weekly for the others. Stopping. Before stopping an engine normally run on HFO but started on DO, it will be necessary to change over to DO, although up to half an hour of operation at high output may be needed for the DO to fill the syste~ through to the injectors. Even for engines normally started on O ~ thIS may be advisable, as it will simplify any maintenance requmng the opening ofthe fuel system .. When steam for fuel heaters and for fuel line tracing is no longer needed, it is secured. On engines with independent cooling water and LO circulating pumps, these are kept in operation for about a half hour after the engine has been stopped .... Main propulsion engines cannot be secured until the bridge has given and confirmed the order "finished with engines." Once an engine is secured, fuel pumps are stopped, the starting air stop valve is shut, and the turning gear is engaged to prevent accidental rotation of the engine. Cylinder indicator cocks, drains at air coolers, and air and exhaust manifolds are opened and left open. LO purifiers on propulsion engines are kept in operation for at least 12 to 24 hours after the engine is stopped. Crankcase doors must not be opened until after the crankcase has cooled down because of the danger of a crankcase explosion. Emergency Operation It is often necessary to put an engine into temporary service with one or more cylinders cut out, or with a turbocharger secured. Det~ iled pro.cedures and precautions differ from one engine to another, and InstructIOn manuals should be consulted, but some general comments are in order: Depending on the nature ofthe problem, it is usually better from the stand~ oint of engine balance if the piston and running gear of the cut-out cyhnder remain in place. In this case, care must be taken to ensure adequate lubrication. If the piston must be hung up or removed, the starting air valve must be isolated. Load should be reduced to avoid overloading the remaining cylinders, and reduced further iflimits in operating parameters are approached. Where machinery is normally unattended, watches should be set. It must be ascertained ifbarred speed ranges exist for operation with a reduced number of cylinders, and operations in these ranges must be avoided. In any event, RPM and/or load must be changed if vibration intensifies. PROCEDURES 16-115 Care must be taken on stopping to see that the engine is in a position to be restarted; if not, a swing on air in the opposite direction will usually bring the engine to a starting position in the correct direction. The master must be advised of anticipated maneuvering constraints. Repairs should be undertaken as soon as possible. Maintenance Maintenance is necessary to keep machinery in sound condition, thereby reducing the likelihood of unexpected failure. P r e v e n tiv e m a in te n a n c e is performed according to a schedule recommended by the engine and auxiliary manufacturers as amended by the operator's experience and, in addition, as called for by an analysis of performance data ( c o n d itio n m o n ito r in g or tr e n d a n a ly s is ) . data analysis. Performance data analysis may be restricted to observing trends in pressures and temperatures read from thermometers and pressure gauges, and recorded in the logbook, but more sophisticated methods are being increasingly applied. These methods often enable recommended maintenance intervals to be exceeded with confidence, or can warn of early failure. As an example, some new engines are fitted with electronic pressure sensors in cylinders and high pressure fuel lines, together with position sensors on injectors and valve stems; these are keyed to crank angle sensors that enable the production of indicator cards, crank angle diagrams, fuel line pressure diagrams, and needle and valve lift diagrams accurately and instantly. It must be noted that for data analysis to be of any use, the data must be reasonably accurate. Periodically, sensors and instruments must be checked and calibrated, gauge lines blown clear, and circuits tested. Where readings are questionable and time permits, similar sensors or instruments may be interchanged between cylinders or locations to confirm a reading. One ofthe most important parameters providing an indication of engine condition is the exhaust gas temperature. Exhaust temperature will be indicated at the turbocharger for small engines, and, on larger engines, at individual cylinder exhaust branches as well. Rising exhaust temperatures overall are indicative of increasing load and should correlate with the fuel rack position. Where exhaust temperatures are rising overall at constant or declining rack setting, and without appreciable change in air temperatures, the cause might be air starvation because of fouled air coolers or fouled or damaged turbochargers. When individual cylinder exhaust temperatures are rising, the cause may be fouled or otherwise blocked ports, a maladjusted fuel rack, a defective injection pump or injector, a worn fuel cam, or a leaking or burned exhaust valve. When individual cylinder exhaust temperatures fall, a maladjusted fuel rack or a defective injection Performance 16-116 MARINE DIESEL ENGINES pump may be indicated, or a compression failure resulting from broken or stuck piston rings, a stuck or badly damaged exhaust valve, a stuck intake valve, or a failed gasket. At least weekly, indicator cards or maximum pressure readings should be taken. Indicator cards or their electronic equivalent can be used to detect problems with valves and combustion, and a fault-free card can provide sufficient information to balance the cylinder outputs. More discretion is required when relying on maximum pressure readings, which should be taken in conjunction with individual cylinder exhaust temperatures: a low maximum pressure accompanied by a high exhaust temperature usually reflects a leaking exhaust valve rather than an unbalanced cylinder. Cylinder compression readings can indicate that rings and valves are sealing tightly, independent of fuel rack position. The engine manual should be consulted for the recommended method and values: readings may be taken cold by barring orjacking the engine; or closely approximated using a maximum pressure indicator or electronic pressure sensors when the engine is idling without load; or they may be taken hot, with the engine near normal speed and the fuel cut off each cylinder in turn. With this confirmation that the cylinders are tight, maximum pressures or indicator cards taken shortly thereafter, with the engine loaded, can be used to balance the cylinders. Lubricating oil drain tank levels should be observed at least daily, and trends noted. A rising level may be because ofleakage ofLO from a storage tank, leakage of water from a heating coil or heat exchanger, or contamination by distillate fuel oil. A falling level may reflect a leak from the system or, in trunk piston engines, a rise in LO consumption, perhaps indicating, in turn, worn or stuck piston rings or worn valve guides. Lubricating oil samples provide useful information on engine condition, and may indicate a need for oil renewal: The presence of tin, copper, lead, or other bearing materials may be a warning of bearing wear. Traces of iron or chromium may indicate liner or ring wear; of aluminum, in engines with aluminum pistons, piston wear. Trace quantities ofwater treatment chemicals may indicate water leakage from cooling passages through cracks or seals. Traces of fuel constituents, a decrease in viscosity, or a drop in the flash point can be the result of fuel dilution, perhaps through leaking seals at fuel pumps. An abnormal drop in the TBN is usually the result of excessive blowby, indicating stuck or worn piston rings. A gradually rising pressure in the crankcase of trunk piston engines usually indicates blowby. OPERATING AND MAINTENANCE PROCEDURES 16-117 Fuel oil consumption should be calculated daily but, because of the many factors that can contribute to a rising trend in fuel consumption, it will not serve, on its own, to pinpoint problem areas adequately. , Smoke observations provide a coarse indication of problems that, in large, reasonably well instrumented engines, are more likely to have been detected earlier. Generally, dark smoke is composed of incompletely burned fuel, and so indicates overload, air starvation, defective or maladjusted injection equipment, or low fuel temperature. When there is a blue tinge to the smoke, the cause is usually excess lubricating oil in the combustion space, indicating worn or stuck piston rings, worn valve guides, or maladjusted cylinder lubricators. White smoke usually indicates low combustion temperature, caused by overcooling the cylinder or by an advanced or retarded injection, or it could be steam from a leaking jacket, cylinder head, turbocharger, or exhaust gas boiler. A smoke problem can be traced to a particular cylinder by successively cutting off fuel to each cylinder in turn. Falling pressures in pumped systems may be indicative of pump wear, leakage (including internal leakage through a bypass or relief valve), improperly set stop valves, or blockage at a strainer, heat exchanger, or elsewhere in the system. Falling pressures in lubricating oil systems could also be the result of excessive bearing wear, overheating of the oil, or fuel dilution. Discharge filters in LO and fuel systems, and charge air intake filters and coolers are normally fitted with pressure gauges or manometers indicating pressure drop. When the pressure drop rises to the prescribed limit, the unit must be cleaned. Vibration sensing and analysis have proven to be a very reliable moni toring technique for rotating machinery, indicating bearing deterioration or imbalances resulting from damage or deposits. The technique is widely applied to auxiliary machinery, but its usefulness on engine-driven or otherwise attached equipment is limited' by the extent to which the inherent vibration of the engine's many reciprocating components can be filtered out. Nevertheless, vibration-monitoring techniques have successfully detected bearing wear and piston skirt wear in high speed engines on an experimental basis, and wider application to large engines for these purposes is likely. As more sophisticated measuring techniques and instruments become available and are proven to be effective and reliable in service, the number of parameters monitored will increase and therefore the effectiveness of condition monitoring will grow. At present some engines are fitted with proximity detectors that locate piston rings and measure their wear, and others that measure valve and fuel plunger ,positions and accelerations. This information, taken together with data from temperature and pressure probes embedded at strategic points, can provide sufficient data to enable 16-118 MARINE DIESEL ENGINES a computer to track performance, detect ominous trends, diagnose likely causes, and suggest possible solutions. Maintenance schedules. Engine manufacturers provide schedules of routine maintenance tasks and their recommended frequencies. These schedules are usually predicated on the basis of a fuel that meets a certain specification and operation at 80 to 90 percent of rating. When fuels of poor quality are being used or the engine is operating under such adverse conditions as sustained overload or low load, more frequent maintenance may be necessary (see table in section on "Variations in ratings" earlier in this chapter) and would generally be indicated by an analysis of the performance data. The intervals cited below are for background information only and may be contradicted by manufacturers' requirements for specific engines, to which strict attention should be paid. In general, when run on fuel of poor quality, low speed engines will require less maintenance than higher speed engines. Major machinery maintenance tasks aboard ship are generally staggered to provide a tolerable amount of work during each port visit. Classification societies offer continuous machinery survey provisions to suit this practice. Operators of ships in seasonal trades usually attempt to restrict all planned maintenance to the lay-up period, when complete overhauls might be undertaken. Spares are kept in an overhauled, sub assembled condition, ready for use. For example, a fully outfitted cylinder head, immediately available for installation in place of one in service, will expedite both emergency repairs and staggered maintenance schemes. A used component, withdrawn from the engine, is either reconditioned to become the next spare or replaced. Each time bunkers are received, a sample should be obtained and analyzed, and the fuel thereby proven to be acceptable, before the fuel is used. Several times daily, temperatures, pressures, levels, and flows are monitored, and inspections made for noises, vibrations, and leaks. Daily, water is drained from the charge air cooler and the air manifold, and from points in the compressed air system not fitted with automatic traps; sludge is drained from the HFO settler; and other fuel tanks are checked for accumulated water or sediment. On crosshead engines, the cylinder oil measuring tank is refilled daily, the consumption calculated, and the lubricators inspected and adjusted as required. Weekly or more often, turbocharger turbines of engines run on HFO might be water washed. Compressors will require water washing less frequently. At intervals, typically of 500 to 1,000 hours, samples of LO are withdrawn from the main engine and auxiliary engines for analysis; oil must OPERATING AND MAINTENANCE PROCEDURES 16-119 be renewed if the analysis shows this is required (renewal will be more commonly necessary for trunk piston engines than for crosshead engines: see discussion, "Main engine LO circulating system," earlier in this chapter). At similar intervals, fresh water in each cooling circuit is sampled, tested, and treated or renewed as indicated. Every 1,000 hours or so, control levers and linkages are inspected, cleaned, and lubricated. At intervals of 1,000 to 2,000 hours on engines burning HFO, or as experience dictates, the fuel injectors must be removed, tested, cleaned, reset, and reconditioned or replaced as necessary. On medium and higher speed engines burning HFO and fitted with exhaust valves, exhaust valve performance may become suspect at intervals as short as 1,000 hours or less, but more usually by the time 5,000 or 6,000 hours have passed, depending on factors that include fuel quality, valve materials, valve cooling, and operating practices. (This problem is less often encountered in low speed engines, where valve cooling can generally be more effective.) On engines fitted with caged exhaust valves, the valves are withdrawn for inspection, cleaning, and lapping or grinding of the seats. Where the heads must be withdrawn to expose the valves, however, an effort should be made to confirm that the exhaust valve is indeed the problem: the air start and relief valves should be pulled and checked; the exhaust valve should be examined to see that it moves freely in its guide, and has the proper spring and sufficient tappet clearance to ensure closure; if a borescope is available it might be used to inspect the exhaust valve seats through the opening for the air start valve, the injector, or the relief valve. If the exhaust valve remains suspect, there is no alternative to pulling the head to inspect, clean, lap, grind, or replace the valve and/or the seats. At intervals of 1,000 to 3,000 hours air manifolds and, on crosshead engines, under-piston spaces are inspected and cleaned. On large twostroke engines the air ports usually permit a limited visual inspection of the cylinder liner, piston crown, rings, and skirt from the air manifold. At intervals of 1,000 to 3,000 hours, cam surfaces are inspected for pitting; timing gears or sprockets are inspected for tooth wear and cracks; timing chain tension is checked and adjusted; and tappet clearances are checked and adjusted. On engines with valves, the valve gear is inspected for free and proper operation; valves should be shown to move easily but not loosely in their guides; exhaust valve rotators must be checked for proper functioning; and valve and injector timing should be checked. Improper operation must be rectified and defective components replaced as necessary. At intervals of 3,000 to 6,000 hours, majbr nuts and bolts, including foundation bolts and tie-rods, are checked to ensure that they remain properly tightened; crankshaft deflection readings are taken and suspect 16-120 MARINE DIESEL ENGINES OPERATING AND MAINTENANCE main bearing clearances measured; telescopic tubes or swivel tubes s~ed for oil and water delivery to pistons and crossheads of cr~ sshead engInes are examined and glands are tightened or packed as re~ U Ired. At intervals of6,000 to 12,000 hours in engines runmng on HFO, or up to 25,000 hours in distillate fuel engines, cylinder h~ ads. are removed, inspected for cracks and leaks, and, if acceptable, rebUIlt wI~ h new parts. Pistons are pulled; inspected for scuffing,. cracking, burnmg, an? fu:l impingement; measured for circularity. and rmg groove w~ ar; and WrIstpm condition and clearances in trunk pIstons are det~ rmmed .. Accep~ able pistons will be returned to service, usually with new ru~ gs. Cyhnd~ r lmers are inspected for scuffing, fuel impingement, crackmg, and rIdges or shoulders' measured to determine wear; and dressed, honed, or replaced . 'd..The Cylinders are reassembled with new gaskets and seals. as reqUIre ', · At 6,000- to 12,000-hour intervals, usu.ally m coor.d matIon .~ I th th e cylinder overhaul described above, conne<:tmgrod bearmg C?ndltIOnand clearances are checked; in crosshead engInes, crosshead guIdes are also examined .. At intervals of 8,000 to 12,000 hours in engines runnmg on HFO, or up to 25 000 hours in engines burning distillate fuel, the turbochargers are disas~ embled, cleaned, inspected, and, if acceptable, balanced and reassembled with new bearings and seals .... At intervals of 12,000 to 25,000 hours the condItIOn.of the mam bearings, including the thrust bearing, is checked and theIr c~ earances measured. At similar intervals attached oil and water pumps are mspected and overhauled as required; injection pumps and the governor are overhauled and recalibrated. Special tools and maintenance aids. The premier maintenance aid in a diesel plant is an overhead crane. While a simple ov~ rhead beam fitted with a trolley for a chain hoist may sufficefor s~ all engInes, a pe~ m anen~ ly installed gantry crane, electrically or pneumatIcally power~ d: w~ th o ~bIle remote control, capable of longitudinal and transvers~ poslt~ onmg,.Is the norm in large plants. In plants with eight or more mam engIne cyhnders and short port stays, consideration might be given o ~ a second gantry carriage on reinforced rails, to enable two f~ferent y ~hnders to be work~ d simultaneously. By similar reasoning, auxIlIary en~ nes shoul? have the.lr own lifting gear, even when the arrangemen~ mIght permIt the mam engine gantry crane to plumb the auxiliary engInes.. Maintenance tasks are simplified by the use of s~ ecIaI tools and. access gear, usually supplied by the engine builder, sometI~ es as an optIOn~ ut often as standard equipment. Accessgear is usuall! mt~ n?ed for.use wIth the overhead crane and may include built-in ralls, hftmg pomt~ , and .acking pads within the crankcase to facilitate rem0.val of large Items. kquipment for a large engine might include the followmg: PROCEDURES 16-121 Lifting beams or brackets for heavy components, including heads, valve cages, pistons, liners, connecting rods, bearing caps, turbocharger rotor. Tensioning devices (hydraulic or electric) for all major studs and bolts, including cylinder head studs, tie rods, foundation bolts, bearing bolts. Strongbacks for removing components whose withdrawal might be difficult, such as valve cages, fuel injectors, air starting valves, bushings and guides, cylinder liners. Motor driven grinders for exhaust valves and seats. An injector testing unit. Mandrels for lapping fuel and starting air valve seats. Piston ring spreading and compression devices. Measuring devices, including trammels, bridge gauges, brackets for jacks, dial indicators used in measuring bearing clearances, extension rods for inside micrometer for cylinder liner, go/no-go gauges for valve guides and small bearings. Support devices, including such equipment as brackets to hold the crosshead in place while removing the connecting rod, brackets to support the connecting rod while removing bearing caps, hydraulic jacks and cradle for crankshaft to enable removal of main bearing bottom half. Blanks and caps, for openings and exposed threads, for example, to protect sensitive equipment from damage when adjacent parts are being serviced. Stud tensioning devices are common outfit. Smaller bolts and studs can be tensioned using a properly calibrated torque wrench, but larger bolts and studs are most often tensioned using a hydraulic jack to stretch the bolt or stud by an amount that coincides with the predetermined pressure developed by a hand pump. Figure 16-57 shows a typical unit. At the specified tension the nut is easily turned and may be backed offfor removal, or, when assembling, the nut is run up, the tension is released, and the bolt or stud remains prestressed. In an increasingly common application, a group of units is mounted in a ring to tension all cylinder head studs simultaneously, as in Figure 16-57. In some applications the tensioning device is an electric heating rod inserted in a central boring of the bolt or stud to elongate it. Related to the use oftensioning devices is the use of bolts and studs of calibrated length: when properly made up, the bolt or stud will have elongated by a predetermined amount, which can be measured with a micrometer (calibrated studs have an internal boring and are measured with a depth micrometer). Obviously, when a calibrated bolt retains its elongation with the nut slack, an overstress has occurred. Maintenance and repair procedures. Maintenance and repair procedures will differ from one engine to another, and instruction manuals should be consulted. Nevertheless, some general comments are in order: ACKNOWLEDGMENTS 16-123 running the engine, crankcase doors must not be opened until after the crankcase has cooled down, because of the danger of a crankcase explosion. Manufacturers' instruction manuals should be studied prior to undertaking a task. M te r Proper tools and materials must be on hand, including spares for any components that might, in the course of the job, be discovered to require replacement. Particularly in regard to such high speed precision equipment as turbochargers and purifiers, extreme care must be taken to avoid causing even minor damage that might lead to catastrophic consequences when the units are returned to service. Consumable components such as gaskets and lock washers should be renewed on reassembly. Consideration should be given to including piston rings in this category. Engines must be safely secured against rotation while work is underway. This is usually best done by engaging the jacking gear. On engines without jacking gear, brackets or blocks intended for this purpose must be installed. Individual components should be fastened in place. Before turning the engine on the jacking gear it must be ascertained that personnel and equipment are clear of moving parts. The temptation to use the gantry crane or a chain fall to jerk free an item which is difficult to withdraw must be resisted. Care must be taken to avoid contaminating LO or cooling water, by using only appropriate methods, such as draining it into clean tanks, and by shielding or blanking connections. Fire precautions must be observed. ACKNOW LEDGM ENTS The authors are indebted to Lawrence R. Wiersum (Computer Graphics of Northern Illinois) for the line drawings appearing in this chapter, and to the following manufacturers, who provided the remaining figures. Asea Brown Boveri Ltd. Colt Industries, Fairbanks Morse Engine Division M.A.N.-B&W Diesel A/S Sulzer Diesel, Ltd. GMT of America Corp. CHAPTER 17 E n g in e D e s c r ip tio n s KEITH WILSON, DAVID BROWN, AND ALAN L. ROWEN IN T R O D U C T IO N WO-STROKE, low speed, crosshead engines likely to be found in service today were manufactured to the designs ofBurmeister & Wain, Sulzer, Mitsubishi, MAN, Doxford, GMT, Gotaverken, and Stork. Today (1990) low speed, crosshead engine design is left to only the first three of these firms. A greater number of designs of medium speed, trunk piston engines are in manufacture, including those by SEMT-Pielstick, MANB&W, MaK (Krupp), Sulzer, GMT, EMD (General Motors), SWD, and many others. Different designs of high speed engines are in even greater number. No single design can be said to be typical of its type except in the most shallow sense, although low speed designs do tend to share more of the same characteristics than the medium speed and high speed engines, where differences extend even to the fact that both two-stroke and fourstroke cycles are represented. Space precludes descriptions of more than two examples: the Sulzer RTA program oftwo-stroke, low speed, crosshead engines, and the SEMT-Pielstick PC2.5 four-stroke, medium speed, trunk piston engines. T SULZER RTA T W O -S T R O K E I n tr o d u c tio n D IE S E L E N G IN E S a n d E v o lu tio n Figures 17-1, 17-2, and 17-3 show Sulzer RTA engines. Table 17-1 lists current characteristics of the RTA program, and Figure 17-4 illustrates the range of output and RPM covered, from 2,000 BHP for a 4RTA38 derated to 141 RPM, to 56,400 BHP for a 12RTA84M at its maximum rating. The RTA, introduced in 1981, represents the latest evolution in a 1'7 1 17-2 ENGINE DESCRIPTIONS series of two-stroke, low speed, crosshead engines, which began with the introduction ofthe SD in the 1940s. A comparison of the RTA84 with its immediate predecessor, the RLB90 (Figure 17-5), shows both basic similarities (attesting to the evolutionary development typical of diesel engines) and distinct differences: a much higher stroke-to-bore ratio and a uniflow scavenging arrangement. These differences result from market conditions prevailing toward the end of the 1970s when shipowners' fuel costs tended to be at least as important a consideration as acquisiti<:mcosts. Consequently, while rising power density had previously been the main parameter characterizing two-stroke engine development, improved specific fuel consumption (SFC) and lower RPM (to match the more efficient, slower turning propellers) became dominant development aims. The longer stroke made the lower RPM possible but necessitated the switch to uniflow scavenging; however, both of these features tended to improve SFC. These trends are illustrated by Figures 17-6 through 17-8, which trace Sulzer low speed engine evolution since 1960. The evolution ofthe competing Mitsubishi and B&W engines shows similar trends. There is a further point to be made in regard to evolutionary design of the RTA program. When initially introduced in 1981, it consisted of six cylinder sizes, designated by their bore in centimeters as RTA38, 48, 58, 17-8 ENGINE DESCRIPTIONS 68,76, and 84, all with stroke-to-bore ratios of about 2.9. The larger engines continued Sulzer practice in many details of construction and in having water-cooled pistons, while the smaller engines introduced several concepts not previously applied by Sulzer for low speed engines, including oil-cooling of the pistons. When additional engines were added to the program-the RTA52, 62, 72, and 84M, all with stroke-to-bore ratios of almost 3.5-sufficient satisfactory experience had been gained with oilcooled pistons and some of the other new concepts to incorporate these features in these new engine designs. F le x ib le R a tin g s A further reflection ofthe trade-offbetween acquisition cost and operating cost in the RTA program (as well as in competing B&W and Mitsubishi designs) is shown in the variety of different rating points available for each engine design. Referring to Table 17-1 and Figure 17-9, the R1 rating represents maximum continuous rating (MCR); the design is such that MEP can be maintained at the maximum value for selected RPM values down to 72 percent of maximum RPM, the R3 rating, to suit slower turning, more efficient propellers. Along the line from R1 to R3, therefore, the engine is derated in proportion to the RPM; the SFC will fall only slightly, but total fuel costs will be reduced because of the potential for improvements in propeller efficiency. Further reduction in fuel costs through reduced SFC can be had if the engine is derated to lower output, in the direction of the R2 and R4 ratings, as illustrated in Figure 17-10. Figure 17-9. RTA layout field E n g in e F u e l O il Q u a lity RTA engines are designed for operation on heavy fuels, with the following quality limits: Maximumviscosityat 50"C Maximumdensity at 1 5 " C MaximumConradsoncarbontest residue, by weight Maximumsulfur content,by weight Maximumash content,by weight Maximumvanadium content,by weight Maximumaluminum content,by weight Maximumwater content,by volume 700 cSt 0.991kg/l 22% 5% 0.2% 600 ppm 30 ppm 1% The density limit is dependent upon the capability of the fuel oil purifiers: if the latest generation of flow-controlled purifiers is used the limit is raised to 1.010 kg/l at 15"C. The aluminum content is used as an indication ofthe presence of catalytic fines. Figure 17-10. Reduction in fuel consumption achieved across the layout field S tr u c tu r e For the larger bore engines the bedplate is of welded steel construction; for the RTA38 and 48 cast iron is used. An integral thrust bearing is incorporated at the aft end. Cross members incorporate the main bearing saddles and tie bolt housings. Hold down bolts are accessible in a single line on either side of the bedplate, outside of the bedplate walls. The engines can be seated on either conventional cast iron chocks or on epoxy resin. Thrust transfer to the tank top can be through fitted hold down bolts or through welded thrust brackets (Figure 17-11), although if epoxy resin is used the welded brackets are mandatory. In the smaller bore engines the bedplate seating is arranged above the bottom of the bedplate to suit installation on smaller ships where the propeller centerline may be relatively closer to the baseline, while still allowing sufficient depth of double bottom. See Figure 17-12. The crankcase is assembled from A-frame columns-fabricated steel in the larger bore engines and steel-reinforced cast iron for the smaller engines-connected by steel plate and girder to form a box girder. The A-frames provide mountings for the crosshead guides. In way of the thrust SULZER RTA TWO-STROKE DIESEL ENGINES 17-11 blocks to the underside ofthe bearing saddles. The tie-rods are prestressed to ensure the rigidity ofthe structure and to avoid fretting and fatigue that would result from relative motion between the components. Cylinder blocks for the RTA38 and 48 are cast with an integral camshaft housing (see Figure 17-3). R u n n in g G ear Crankshafts are steel forgings, machined all over to avoid stress-raising surface irregularities. The larger engines use semibuilt crankshafts, with crankpin and webs forged from single ingots, shrink-fitted to separately forgedjournals, while the RTA38 crankshaft is a single forging. Depending on the number of cylinders, the crankshaft may be a single fabrication or may be divided by a flanged joint. Figure 17-13, a longitudinal section, shows the large diameters of journal and crankpin needed to handle the high torque, as well as the long webs resulting from the long stroke. Also evident is the close approach of the crosshead to the webs in the bottom dead center CEDC)position, a result of the short connecting rod used to limit engine height, and the close longitudinal spacing of the cylinders to limit engine length. Figure 17-12. Seating arrangements for the small bore RTA engines block the A-frames form a double walled enclosure for the camshaft drive gears. Depending on the number of cylinders, crankcase and bedplate may each be a single fabrication, or they may be divided to meet installation limitations. The cylinder blocks are individual cast iron units bolted together, and fh ••th",,, Q",{>".,''',.1 to t.h" h"flnlHt.f> hv t.ie holts extending- from the top of the Figure 17-13. Longitudinal section in way of running gear Main and bottom end bearings are thin-walled shells lined with white metal. Connecting rods are forged steel, with marine-type bottom ends. In engines with water-cooled pistons the crosshead bearing housing is divided, as illustrated in Figure 17-14, where the division permits the crosshead to clear the telescopic pipes carrying piston cooling water (see Figure 17-15). In engines with oil-cooled pistons the division is unneces- Figure 17-16. Cross section through crosshead of RTA engines with oil-cooledpistons sary and the piston rod is bolted into the top of the crosshead pin, permitting the full lower half of the pin to be supported in a full-width bearing, as shown in Figures 17-16, 17-17, and 17-18.The divided bearings are thin-walled steel shells with a tin-aluminum lining, a design evolved from succesful experience as illustrated in Figure 17-19, which also shows the increased bearing loadings and their extended duration on the later model engines. In the engines with a full-width lower half to the crosshead bearing, the specific loads are low enough to permit the use of white metal. All of the RTA engines incorporate hydrostatic lubrication for the crosshead bearings, using high pressure oil (12 to 16 bars). This has been a Sulzer practice since the early 1970s. B o r e C o o lin g The practice of bore cooling was originally patented by Sulzer in 1936, when it was applied to cooling the ribs between exhaust gas ports of the loop- scavenged engines. Since 1968 bore cooling has been applied progressively in the combustion chamber area, beginning with the liner ofthe RND engine, as illustrated by Figure 17-20; on the RTA the liner, the piston, and the head are all bore cooled. Bore cooling is a successful compromise between the conflicting requirements for, on the one hand, thick sections to withstand the gas pressure and, on the other, the thin sections needed for cooling. This success is illustrated by Figure 17-21, showing the reduced stress levels in cylinder heads of successive engine designs, even as gas pressures rose, and by Figure 17-22, which shows the modest temperatures of an RTA piston crown. A further advantage of bore cooling is in the simplification of castings and forgings and the improved quality control that results. SULZER RTA TWO-STROKE DIESEL ENGINES 17-17 The arrangement ofthe water-cooled pistons is shown in Figure 17-15. Stationary supply and return pipes are housed in a watertight enclosure in the crankcase, while telescopic tubes attached to the piston reciprocate over them. The watertight enclosure precludes the possibility oflubricating oil being contaminated by water leakage, a problem easily revealed, since the enclosure is open to the exterior at the bottom. Circulation of the water within the piston relies on the "cocktail shaker" effect. Figures 17-23, 17-24, 17-25, and 17-26 show the arrangement of the oil-cooled pistons. Oil is supplied to a connection on the crosshead through hinged pipes (see Figure 17-16) and then rises through a bored passage in the piston rod, which also carries the return pipe. Circulation of the oil to the ends of the bores is forced by jets at the piston rod top, shown in Figure 17-24. P is to n R o d P a c k in g G la n d Figures 17-15 and 17-27 show the packing gland and diaphragm separating the scavenge space from the crankcase. In way of the gland and extended out to one side of the engine, the diaphragm is doubled to form a neutral space, with separate packing rings at the scavenge space bottom and at the crankcase top. The neutral space permits the collection of dirty oil scraped from the rod. C y lin d e r H ead an d E xh au st V a lv e The bore-cooled, forged steel cylinder head used on the larger bore engines is illustrated in Figure 17-25. The major cooling passages are radial bores from the outer periphery, continued with nearly vertical bores into an Figure 17-22. Temperatures in an RTA58 water-cooled piston C y lin d e r L in e r a n d P is to n The cylinder liner is of alloy cast iron with a thick, bore-cooled collar at the top to provide the strength to withstand the gas pressures. Cylinder lubrication is by eight quills, each with its own spring-loaded accumulator, arranged around the liner about a fifth of the stroke below the TDC position ofthe top ring. The cylinder oil is metered by pumps, one for each cylinder, driven by hydraulic motors. The hydraulic motors are driven by bearing oil, with their speed regulated by an adjustable linkage from the fuel rack, giving adjustable but load-dependent oil feed rates. The discharge of each metering pump is separately adjustable, enabling a cylinder with new liner or rings to be fed at a greater rate. Piston crowns are forged alloy steel, carrying five cast iron rings free to float in their grooves. Figure 17-23. Underside of an oil-cooled piston crown for an RTA62 Figure 17-24. Top flange of an RTA62 piston rod showing jets for cooling oil Figure 17-25. Combustion chamber with exhaust valve assembly for larger bore RTA engines Figure 17-26. Combustion chamber with exhaust valve assembly for RTA38 and 48 annular space at the base of the exhaust valve pocket, as shown in detail in Figure 17-28. From the annular space blind bores in the exhaust valve cage carry the water close to the valve seat, while vertical bores lead up to the valve cage cooling jacket. The head used on the RTA38 and 48 is shown in Figure 17-26. It differs from the head of the larger bore engines at the junction with the liner, where it overlaps to provide added support to the liner, and to form an integral water guide. Bored pockets in the head carry the air starting valve, the fuel injectors, the relief valve, and the indicator cock. Depending on the bore, there will be two, three, or four injectors per cylinder. The exhaust valve and its cage and hydraulic actuator are shown in Figures 17-29 and 17-30. The concept is similar to that used on the latest B&W and Mitsubishi low speed engines: the valve is opened by hydraulic pressure from the earn-driven actuator and closed by compressed air (an "air spring"). The valve is rotated by vanes on the stem. The valve cage is cast iron with a hardened steel valve seat ring, while the valve itself is of nimonic alloy. The lower portion of the cage, the seat ring, and the valve itself have full rotational symmetry to avoid thermal or mechanical distor- 17-20 ENGINE DESCRIPTIONS tion. This symmetry, the rotation of the valve, and the intensive cooling of the seat ring ensure that seating surface temperatures are evenly distributed and sufficiently low to avoid vanadium deposition, as indicated in Figure 17-28. The exhaust valve stems are lubricated with cylinder oil metered by an additional pump on the cylinder oil pump shaft. C a m sh a ft The camshaft is gear-driven from the crankshaft, using three gears on the larger bore engines where the camshaft is at midheight on the engine, and four gears on the RTA38 and 48 (see Figure 17-31). For engines with up to eight or nine cylinders the drive is taken from the rim ofthe thrust collar; for large-bore engines with more than eight cylinders the drive is taken from the rim of the crankshaft coupling flange at midlength ofthe engine. To isolate the camshaft from the effect of torsional vibration, a Holset damper is fitted at the camshaft or, where engine-driven balancing gear is fitted, a Geislinger coupling is inserted in the drive train. The camshaft extends the full length of the engine and is assembled in segments, each of which carries cams for exhaust valve actuators and fuel injectors serving a pair of cylinders, and each of which is mounted in a separate earn box. The arrangement is shown in Figures 17-32 and -33. Figure 17-31. Camshaft drive gearing for the smaller bore engines Figure 17-33. Camshaft unit with fuel pumps and exhaust valve actuator 17 -22 ENGINE DESCRIPTIONS The exhaust valve timing is sufficiently symmetrical about BDC to remain unchanged when the engine is reversed, but this is not true for the injection timing. Consequently the fuel cams are rotated relative to the camshaft for reversing. To accomplish this the exhaust cams are arranged as the outer pair in each cam box, as shown in Figure 17-33. The fuel cams for each pair of cylinders are combined with a hydraulic servomotor which turns with the shaft (see Figure 17-34). Oil is supplied to the servomotor to rotate the fuel cams relative to the camshaft, through a stationary sleeve on the larger engines (Figure 17-34), or through drillings in the camshaft on the RTA38 and 48 (Figure 17-35). The oil supply is from the same high pressure lubrication circuit that serves the crosshead bearings. SULZER RTA TWO-STROKE DIESEL ENGINES 17 -23 The camshaft drives the governor, the pilot air distributor, the cylinder oil hydraulic pump, remote and local tachometers, and the rotation direction safeguard. F u e l I n je c tio n Unlike most diesel engines, which are fitted with injection pumps of the helix-controlled type, but continuing Sulzer's own long-standing practice, the larger bore RTA engines are fitted with valve-controlled pumps (see Figure 17-36). Injection begins when the suction valve is closed and continues until the spill valve is opened. I n a system first applied to the RLAengines and shown in Figure 17-37, the control shafts ofthe two valves are linked together, enabling the timing of the beginning of fuel injection, as well as its duration, to be varied according to the engine output. I n the Sulzer system, this variable injection timing (VIT) is done automatically by the governor, and enables the maximum combustion pressure to be held constant at the MCR value down to 85 percent output, and at higher values than would otherwise be obtained throughout the load range (see the upper curves of Figure 17-38), thereby achieving reduced SFC at all partial loads. Incorporated in the VIT linkage is a manually operated lever (the fuel quality setting lever in Figure 17-37) that will advance the fuel injection timing to compensate for the longer ignition delay period offuels with poor ignition quality. The effects are shown in the lower curves of Figure 17-38. The RTA38 and 48 are fitted with helix-controlled injection pumps. The recess in the plunger is shaped at the top as well as at the bottom to provide load-dependent VIT, while the spill helix is stepped to reduce injection pressure toward the end of the injection period and so avoid cavitation. For all engines, the tappet space below the pump plunger is sealed to avoid lubricating oil contamination from fuel leakage. Each pump is provided with a manually operated lever to lift the tappet clear of the camshaft, enabling individual cylinders to be isolated from service. Two to four fuel injectors are fitted to each cylinder, depending on the bore. The injectors are cooled by conduction through the cylinder head to cooling bores adjacent to the injector pocket: there is no separate injector cooling circuit. Figure 17-39 shows an injector equipped with a fuel circulation valve that is shown in detail in Figure 17-40. The circulation valve functions automatically to maintain flow under booster pump pressure through the high pressure piping and the injector, both when the engine is stopped and between injection strokes. This circulation ensures the availability of hot fuel right up to the nozzle at all times, facilitating pier-to-pier operation on heavy fuel. A ir a n d E x h a u s t S y ste m and T urbochargers The RTA engines are constant-pressure turbocharged, using either Brown Boveri or Mitsubishi turbochargers with uncooled casings. One or two Figure 17-38. Diagrams showing effects ofVIT and fuel quality control Figure 17-36.Valve-controlled injection pump used on the larger RTA engines turbochargers are fitted where possible but the 12-cylinder RTA84 and 84M engines require four. The uncooled casings result in a higher exhaust gas temperature after the turbine, which improves the potential for waste heat recovery. On the larger bore engines the turbochargers are mounted alongside the cylinders, above the exhaust manifold, as in Figure 17-2. For the RTA38 and 48, a single turbocharger is used in all cases and is mounted SULZER RTA TWO-STROKE Figure 17-39. RTA fuel injector Figure 17-40. Detail offuel circulation valve at one end ofthe engine or the other, as in Figure 17-3, to suit the confines of engine rooms on smaller ships. The air cooler is a finned tube unit, fitted one per turbocharger. The water side can be divided for circulation from up to three different sources to facilitate waste heat recovery. A water separator is fitted at the air manifold inlet from each air cooler. The air manifold is bolted to the cylinder blocks and, on the larger bore engines, provides structural support for the exhaust manifold. On these engines the manifold is divided longitudinally by a bulkhead carrying reed-type check valves (see Figure 17-2). A pair of motor-driven boost blowers, mounted one at each end ofthe air manifold, cuts in automatically as engine output is reduced below about 40 percent of MCR. The boost blowers draw from the upstream side of the bulkhead, discharging to the downstream side, and are therefore in series with the turbochargers. On the RTA38 and 48 the air cooler, water separator, boost blowers, and check valves are grouped into a compact module at the end of the engine, below the turbocharger, as shown in Figure 17-41. B a la n c in g DIESEL ENGINES 17-27 counterweights are fitted to the crankshaft to correct the first order pitching moment, thereby imposing a first order yawing moment. For engines with seven or more cylinders external couples will not normally excite hull vibration, but external, first order couples of four-cylinder engines, and second order couples offour-, five-, and six-cylinder engines can; consequently, these engines may be fitted with balancers: second order balancers will almost certainly be fitted to the largest bore engines with six or fewer cylinders and may be fitted to smaller bore engines; first order balancers may be required for four cylinder engines. Figure 17-42 shows the arrangement of combined first and second order balancing gear used on the larger bore four-cylinder engines, where it is built onto the engine and driven through Geislinger couplings and chains from the timing gears and camshaft; when balancing gear is required for the smaller bore engines, independent motor-driven balancers are installed. For installations where torque reaction moments excite hull or engine vibration, transverse stays, usually incorporating hydraulic dampers, are fitted at cylinder block height to connect the engine to adjacent ship's structure. Where torsional vibration calculations indicate the likelihood of dangerous resonance levels at critical speeds, shafting diameters or flywheel mass may be modified, a barred speed range may be imposed, or a damper may be fitted at the forward end ofthe crankshaft. A damper may also be a n d V ib r a tio n In common with competing B&Wand Mitsubishi engines, the RTA engines have no unbalanced free forces, and unbalanced external couples of first and second order are minimized. In the standard configuration, additional Figure 17-41. Modular arrangement ofturbocharger and air cooler used on the RTA38 and 48 Figure 17-42. Combined balancing gear to compensate for first and second order couples 17 -28 ENGINE DESCRIPTIONS fitted at the forward end ofthe crankshaft if axial vibration is likely to pose a problem. SULZER RTA TWO-STROKE DIESEL ENGINES t S u p p o r t S y ste m s Support systems recommended for the RTA engines differ in detail from the typical systems described in the previous chapter. The following features may be worthy of note: The remote-controlled starting air master valve incorporates a slow turning valve (item 17 in Figure 17-43) that will pass sufficient starting air to crank the engine over at a speed of five to ten RPM with the indicator cocks open. The lubricating oil system, shown schematically in Figure 17-44 for engines with water-cooled pistons, includes a high pressure circuit for crosshead bearing hydrostatic lubrication, for reversing servomotors, and for exhaust valve actuators. The following recommendations are made regarding the lubricating oil systems: 1. For engines with water-cooled pistons, the system oil should be an SAE 30 oil, rust and oxidation inhibiting; a light alkaline oil can be used. For engines with oil-cooled pistons where the oil is subject to higher temperatures, the system oil should be a mildly alkaline SAE 30 detergent oil. 2. For normal service the cylinder oil should be an SAE 50, with alkalinity matched to the sulfur content of the fuel, supplied at a feed rate ofOA to 0.6 g/BHP-hr. When running in new liners or rings a straight mineral oil should be used, at a feed rate of about 1.3 g/BHP-hr. The RTA engines use a pressurized fuel system (Figure 17-45) to prevent gas formation at the high temperatures to which the heaviest fuels must be heated for atomization. Fuel from the injection pump spill valves and from the injector circulation valves returns to the buffer tank via the venting unit. The seawater system, for engines with water-cooled pistons, is arranged so that seawater flows to the lubricating oil cooler first, then the charge air cooler; after this, it divides to flow through the jacket water cooler and the piston water cooler, which are arranged in parallel with each other. For engines with oil-cooled pistons the flow of seawater is first through the lubricating oil cooler, after which it divides to flow through the jacket water cooler and the charge air cooler, which are arranged in parallel with each other. The long-standing practices of waste heat recovery using jacket water for the evaporator and exhaust gas for steam generation may be supplemented at the charge air cooler. The water side of the cooler may be divided for circulation from up to three different sources for this purpose: in one ofmany possible schemes, the hottest section may be used to heat water for domestic needs and other low temperature heating services, while the second section can serve as a feed heater for the exhaust gas boiler; the third section is reserved for seawater or central freshwater cooling of the charge air. Figure 17-43. Starting air master valve Figure 17-44. Lubricating oil system for RTA engine with water-cooled pistons 17-29 SULZER RTA TWO-STROKE C o n tr o l a n d M o n ito r in g S y ste m s The standard control system supplied by Sulzer incorporates a remote pneumatic maneuvering unit, usually installed in the engine control room console, as well as in a local control stand at the engine. The system may be augmented with an electro-pneumatic bridge control system, with automatic programs for control of engine acceleration, deceleration, and crash maneuvers, or with special features to suit particular installations, such as those with controllable pitch propellers. The pneumatic maneuvering unit in the control room includes a reversing lever, a start button, a speed setting lever, and a control transfer button. In its ahead or astern position, the reversing lever sends an air signal to the reversing control valve, which controls the oil flow to the servomotors at the fuel cams and the starting air distributor; the servomotors set these into their ahead or astern positions. The reversing lever also has a stop position, in which an air signal is sent to release a pneumatic shutdown servomotor; this servomotor shifts the injection pump suction and spill valve shafts to the zero injection setting. The speed setting lever adjusts the set point of the governor through a pneumatic link; the governor then operates a mechanical linkage that adjusts the injection pump suction and spill valve settings. Pushing the start button sends an air signal to open the starting air master valve, permitting starting air to flow to the distributor and the appropriate starting air valves. A rotation direction safeguard, driven by the governor drive gear, will release the shutdown servomotor ifthe direction of engine rotation is not the same as the direction called for by the reversing lever and reversing control valve. DIESEL ENGINES 17-31 For a normal start, the reversing lever is set to the correct direction and the speed setting lever is set to a low speed; when the start button is pushed, the engine begins to turn as starting air is admitted, and the rotation direction safeguard activates the shut-down servomotor, permitting fuel flow to the injectors: the engine accelerates until the governor takes control. To reverse the engine, the reversing lever is changed over to the opposite direction; since the rotation direction safeguard then senses the conflict between engine direction and lever setting, it releases the shutdown servomotor and fuel injection ceases. The operator can use starting air to brake the engine against the rotation of the propeller by pushing the start button, and can subsequently start the engine turning on air in the new direction; when the rotation direction indicator senses this, it activates the shutdown servomotor to permit injection, and the engine will accelerate until the governor takes control. The local control stand can be activated using the control transfer switch. At this stand, the reversing lever, start'button, speed setting lever, and control transfer button are again present and have the same functions, but the pneumatic linkages are reduced or eliminated: the reverse lever is mechanically linked to the reverse control valve, the start button actuates the starting air solenoid mechanically, and the speed control lever can either operate via the governor or be latched to a direct mechanical linkage to the injection pump suction and spill valve shafts. In addition, the pneumatic control of the starting air master valve can be bypassed using an extended valve stem adjacent to the local control stand. When bridge control is fitted, the bridge console will have a single telegraph-style lever serving for starting, reversing, stopping, and speed control, and a control transfer switch. In addition to the shutdown servomotor, each pair of injection pumps is fitted with a spring-loaded safety cut-out that, unless restrained by air pressure, holds the suction valves open to prevent fuel injection. These are usually arranged to release if any ofthe following events occur: Overspeed Emergency stop button pressed Loss of air pressure in exhaust valve air springs Bearing lubricating oil failure Cylinder lubricating oil failure Cylinder cooling water failure Piston cooling water or oil failure Air is blocked from the starting air solenoid when the turning gear is engaged. Sensors on the charge air manifold act at high pressure to limit fuel injection, thereby preventing excess torque and smoke, and, at low 17-32 ENGINE DESCRIPTIONS pressure in consort with a sensor on the fuel linkage, to start the motordriven boost blowers. The slow turning facility, which uses starting air supplied through a bypass on the starting air master valve to turn the engine (Figure 17-43), is operated from the control room console by push buttons. Instrumentation and alarm points vary, depending on requirements of regulatory bodies and owner's preferences, but the standard engine design permits most parameters to be monitored locally or remotely, or to be fitted with alarms or other safeguards as appropriate. P ow er T a k e -O ff An RTA engine may be fitted with an integral power take-off from the crankshaft gear wheel of the camshaft drive train (Figure 17-46) to drive a generator, as shown in Figure 17-47. In this case the generator is driven at constant speed by a Sulzer-SLM drive unit. The constant speed drive unit, shown schematically in Figure 17-48, utilizes two epicyclic gear sets in tandem, and a superimposed hydraulic speed control system. The speed control comprises two hydraulic units that act as either pumps or motors, a constant displacement unit geared to an annulus of the first epicyclic, and a variable displacement unit on the planet carrier of the first set, which is also the annulus of the second set, together with a speed sensor and an electromechanical control unit. When the input speed is 100 percent of synchronous speed the annulus and planet carrier rotate at equal speed. If the main engine RPM is decreased so that input speed falls below synchronous, the displacement of the variable displacement unit is increased so that it acts as a pump, driving the other unit to maintain synchronous output speed; on an increase in input speed the variable displacement unit becomes a motor. For the RTA application, - ....- . - .... Figure 17-48. Schematic arrangement ofthe constant speed gear these units are self-contained and intended for mounting, together with coupling and generator, on a base plate supported by brackets on the main engine bedplate. E xh au st G a s T u r b in e The introduction in 1984 of turbochargers that were even more efficient than the BBC series-4 units, which the RTA program was originally designed to use, opened the possibilities of increasing engine ratings to match the increased air flow or, alternatively, of diverting some of the exhaust flow to a power turbine. It was the latter course that was adopted for the already high-rated RTA engines, under the name "efficiency booster 17 -34 ENGINE DESCRIPTIONS system." The standard configuration is shown schematically in Figure 17-49: the turbine is a single stage axial flow unit derived from turbocharger practice (Figure 17-50)that drives into the integral power take-off gear (Figure 17-51) via an epicyclic gear set and a fluid coupling. As can be seen in Figure 17-52, the reduction in SFC increases with the percent of gas diverted from the turbocharger, but so does the valve seat temperature. Diverted gas flow is limited to 10 percent for this reason. Figure 17-51. Integral PTO drive of the RTA engines For partial output operation, at about 50 percent or less, the gas flow to the turbine is closed so that all of the exhaust passes to the turbocharger. The step improvement in partial output fuel rates and airflow is evident in Figure 17-53. In Figure 17-54 the case is made that the additional power and improvement in efficiency resulting from the combination of a more efficient turbocharger with an exhaust gas turbine can render an engine so equipped competitive with a larger but derated engine: in this example a 6RTA62, derated to 75 percent output, can be replaced with a 5RTA62 matched at 90 percent output. M a in te n a n c e As with most other large ciiesel engines, the RTA engines are normally outfitted with an extensive array of hydraulic tools to facilitate maintenance tasks (see Figures 17-55 to 17-57). Such other specialized tools as valve seat grinders may be provided as well. Guideline maintenance intervals for an RTA with water-cooled pistons are as follows: Daily Check and top up governor oil level. Check drain outlet on air manifold. Wash turbocharger compressor. 500 hours Undertake spot test of system lubricating oil. Wash turbocharger turbine. Retension newly installed balancer drive chain (thereafter at 1,500-hour intervals). 1,500 hours Check hold-down bolts for the first time (thereafter, at 6,000- to 8,000-hour intervals). SULZER RTA TWO-STROKE DIESEL ENGINES 17 -39 Inspect fuel and exhaust cam surfaces. Check performance of starting air master valve. Check PTO gearbox externally for oil leakage and proper venting. Retension balancer drive chain. 3,000 hours Sample system lubricating oil for laboratory analysis. Test fuel injector spray pattern. Inspect piston visually through the air ports. Check condition of camshaft drive gear teeth. Change governor oil after flushing casing. Inspect, clean, or renew check valves in air manifold. Check condition of PTO gear teeth and determine bearing clearances. 6,000-8,000 hours Check hold-down bolt tension. Check main bearing clearance; adjust if necessary. Check tie-rod tension for the first time; adjust if necessary (subsequently at four-year intervals). Overhaul and regrind exhaust valve seats as needed. Measure crankshaft deflections. Check connecting rod bearing clearances. Check crosshead guide shoe clearances. Remove cylinder heads; withdraw and clean pistons; measure ring grooves; clean or renew piston rings; disassemble and inspect cooling water standpipes, telescoping tubes, and glands; measure liner wear; remove wear ridge from liner; check cylinder oil quills; inspect combustion side of head. Check backlash and clearance of intermediate camshaft drive gear. Check thrust bearing radial and axial clearances; check that bottom drain hole is clear. Dismantle, clean, and inspect valves of fuel injection pumps; check fuel pump timing and adjust if necessary; lubricate injection pump linkage. Clean air manifold. Open one piston annually to clean internal cooling spaces. Clean boost blower impeller and casing. Drain and wash out cylinder oil pumps. Check exhaust pipe bolts; clean sliding feet. 17-40 SEMT-PIELSTICK ENGINE DESCRIPTIONS Check or recalibrate pressure gauges and pyrometers. PC2.5 FOUR-STROKE PC2.5 S E M T -P IE L S T IC K F O U R -S T R O K E I n tr o d u c tio n 15,000-20,000 hours Check thrust bolt tension; adjust if necessary. Inspect connecting rod bearings and renew if necessary. Dismantle, clean, and inspect starting air distributor; check pilot valves for easy movement. Dismantle, clean, and inspect starting air master valve. of rotation direction Check performance safeguard. Dismantle and clean shutdown servomotor. Dismantle, clean, and inspect safety cut-out deVIces. Dismantle, clean, and inspect Geislinger coupling ofPTO gear. 40,000-60,000 hours As necessary or when required Remove sample of silicon fluid from viscous vibration damper on camshaft. Renew spring packs of Geislinger coupling ofPTO gear. Remove and inspect main bearings and thrust bearing. Remove and replace cylinder liners; renew rubber rings. Chamfer edges of air ports. Recut cylinder oil grooves. Clean or remachine fuel injector seat. Clean and overhaul fuel injectors. Check, top-up, or renew turning gear lubricating oil. Check and clean undersides of pistons. Check and clean water separators in air manifold. Clean turbocharger air filter. Clean air and water sides of charge air cooler. Measure stretch of balancer drive chain; renew if necessary. Clean or renew oil filter inserts. Renew starting air valves. D IE S E L E N G IN E S a n d E v o lu tio n The Pielstick-type PC medium speed diesel engine designed by the Societe d'Etudes de Machines Thermiques (SEMT) has been manufactured by licensees around the world since 1951. The PC2.5-400 series, rated at 650 BHP per cylinder at 514 to 520 RPM, is the successor to the previous PC2.3 of 12, 14, 16, and 18 cylinders by series, and is built in V configurations the Fairbanks Morse Division of Colt Industries, and in both V and in-line configurations by other licensees. The PC2.5 has a bore and stroke of 400 mm and 460 m m , respectively. Table 2 shows external dimensions and weights of the Fairbanks Morse engines. TABLE E x te r n a l F a ir b a n k s 35,000-40,000 hours 17 -41 DIESEL ENGINES 1 7 -2 D im e n s io n s a n d W e ig h ts o f M o r s e P C 2 .5 V M a r in e E n g in e s -_.---_ .. ~- N um ber of L e n g th W id th H e ig h t C y lin d e r s O v e r a ll O v e r a ll O v e r a ll W e ig h t, lb * A p p r o x im a te 12 !4 16 2 2 '7 W ' 1 1 '5 3 1 4 " 11'5" 144,200 25'0" 1 1 '5 3 1 4 " 11'5" 161,200 28'0" 12'13/4" 12'8114" 18 3 0 '5 1 1 4 " 12'13/4" 12'8114" 178,200 __ 2~0L_ *Dry weight for non-reversing engines without pumps A cross section through a PC2.5 engine appears as Figure 17-58. Comparison with a section of a PC2.3, with the higher-rated PC2.6, or with the much larger PC4.2 reveals the direct line of evolution of these engines. F ram e The engine frame is a welded fabrication of steel plate, castings, and forgings intended to provide great strength with minimum weight. The lower portion forms the crankcase, and is divided from the upper portion by a cast steel lower deck, clearly visible in Figure 17-58. The upper portion ofthe frame houses the bottom halves of the cylinders, which are inserted through the steel plate upper deck and secured by studs from the lower deck. Precision bored holes in the two decks, with a counterbore in the lower deck, ensure the 45-degree alignment of the cylinders. Upper and lower decks extend out to house the two camshafts. Figure 17-59. PC2.5 main bearings Figure 17-58. Section of a Pielstick PC2.5V A transverse cast steel arch supports each main bearing (Figure 17-59) and serves to divide the crankcase into separate compartments for each pair of cylinders. Steel plate sidewalls are welded to the arches and lower deck, and carry the baseplate mounting flanges. Large circular inspection holes are provided along both sides at crankshaft level of each compartment to facilitate inspection and service. The frame is extended at the drive end ofthe engine to house the timing gears and pump drives. ofthe arch and secured transversely by two lateral crossbolts. The bearing caps are each held in place by a single jack screw bearing down from the top of the arch. Thin wall precision bearings are used; the mild steel shell is lined with a copper-lead alloy and a lead-tin overlay; backs and sides of the shells are lead-tin plated. The bearings are positioned within the housings by tongues that fit into grooves in the saddle and cap. The lubrication arrangement can be traced in Figure 17-59. Oil from the external header passes down through the jack screw and cap to a circumferential groove extended over the upper 55 percent of the bearing, then through holes in the shell to a distribution groove in the lining. A radial bore in the crankshaft in way of the circumferential groove carries oil into an axial bore for distribution to connecting rod bearings, then through the connecting rods to the pistons. I n most applications an engine is fitted with an external bearing on the drive end to provide additional support for such external loads as a flywheel, a resilient coupling, shafting, or a generator rotor. The bearing is ofthe same materials as the internal main bearings, and is carried in a cast steel housing bolted to the frame. The sides of the shells are lined to serve as thrust faces operating against two collars machined onto the crankshaft, to maintain the longitudinal position of the crankshaft. C r a n k sh a ft M a in B e a r in g s A main bearing is shown in detail in Figure 17-59. Each saddle is inserted upward into its arch, and each is fastened by two vertical studs to the top The alloy steel crankshaft is forged in a single piece and machined all over. Coupling flanges are provided at both ends, with the flange at the free end available for barring gear, a torsional vibration damper, or power take-off. 17-44 ENGINE DESCRIPTIONS At the drive end the crankshaft carries a shoulder flange for mounting the timing gear and a journal with two collars for the external bearing. Passages are bored through the journals, webs, and crank pins to carry lubricating oil from the main bearings to the connecting rod bearings and the piston (see Figures 17-59 and 17-60). Dovetail slots for the balance weights are machined in the crank webs (visible in Figures 17-58 and 17-60). Each weight is secured against its slots by a single set screw in compreSSIOn. SEMT-PIELSTICK PC2.5 FOUR-STROKE DIESEL ENGINES 17-45 Torsional Vibration Damper Selection of the torsional vibration damper is made on the basis of a torsional vibration analysis of the engine and connected loads, including gearing, shafting propeller, and attached auxiliaries. Where a damper is required, either ofthe two types shown in Figure 17-61 might be fitted. In both types a heavy rim floats around a hub to which it is connected only by multiple spring packs; the rim is restrained axially by bolted side plates that overlap the hub. The mass of the rim and the stiffness of the springs are selected to provide the amount of damping needed. Camshafts, Reversing System, Timing Gears, and Auxiliary Drive Camshafts. Air and exhaust valves and fuel injection pumps are actuated by camshafts located outboard of each cylinder bank. The camshafts also drive the starting air distributor and a lubricating oil pump for the rocker arms. Individual cams for each service are keyed and shrunk onto the camshafts. The camshafts are underslung below the side extensions at the top of the frame (Figure 17-58) and are supported by sleeve bearings. At each cylinder, two sleeve bearings are fastened to the injection pump mounting, so that injection pump operating stresses are isolated from the engine frame. Bearings at the drive end aft of the timing gears include thrust faces for nonreversing engines; thrust faces for reversing engines are in the reversing lever bearings. The bearings are mounted to permit removal of a camshaft within its own length. Reversing System. For direct-reversing engines, each cam has two profiles, one for each direction, connected by a sloping surface to support the cam roller when the reversing lever shifts the camshaft axially. The camshaft drive gear is axially restrained on the shaft and moves with it when reversing, its teeth sliding on pinions whose face width is extended for this purpose. Figure 17-60. Connecting rod Timing, Timing Gears, and Auxiliary Drive. The camshafts are driven by two reduction gear trains, one for each camshaft, as shown in Figure 17-62. The large gear on the crankshaft drives two intermediate gears that turn at 1.6 times crankshaft speed. Each intermediate gear has an integral pinion that drives the larger camshaft gear at half crankshaft speed to achieve the cycle of events in Figure 17-63. The camshaft gears are resiliently mounted to isolate the camshaft from torsional vibration. The governor, the overspeed trip, and a tachometer are driven by one of the intermediate gears through a step-up pinion; attached pumps for lubricating oil,jacket water, and raw water, and the fuel oil booster pump may be driven through step-up pinions from either intermediate gear. 17-46 ENGINE DESCRIPTIONS SEMT-PIELSTICK PC2.5 FOUR-STROKE T im in g A u x lll.r y A. 8. - 1 6 8 to o th c ra n k 0 ", .n d Drlv•• 1 0 5 to o th w h e e ls -In te r m e d ia te 3 0 to o th p in io n s -In te r m e d ia te D. - 9 6 to o th w h e e ls E. - 3 6 to o th p in io n s F. _ 4 0 to o th p in io n 3 5 to o th p in io n H. - Speed I. L u b e - o ll p u m p - 17-47 gear C. - G. _ DIESEL ENGINES tr a in tr a in o n c a m s h a fts g o v e rn o r J. - Fuel K. - W a te r L - O v e rs p e e d b o o s te r pum p pum p tr ip Figure 17-62. Timing gear and auxiliary drives Cylinders Figure 17-61. PC2.5 torsional vibration dampers Figure 17-64 is a section through a complete cylinder assembly. Each cylinder consists of a centrifugally cast gray iron liner and a gray cast iron water jacket. The liner-to-jacket joint is made watertight at both ends by synthetic rubber O-rings that allow sufficient flexibility to accommodate relative movement due to thermal expansion and strain. The cylinder assembly fits into the frame to about half its length, and is clamped against the frame upper deck by the cylinder head, which is secured by eight studs Figure 17-63. Timing diagram threaded into the lower deck of the frame. The head bears on the liner, with a collar on the liner transmitting the clamping force to the jacket, which in turn has a shoulder bearing on the frame upper deck. A pilot shoulder on the liner fits a counterbore in the frame upper deck to maintain alignment. The liner extends below the jacket, through a bore in the lower deck of the frame. An a-ring seals the liner-to-Iower-deck joint against water leakage from the cylinder jacket, which would be revealed by a drain from the frame above the lower deck. Cooling water from the headers, exterior to the frame on each side of the engine, passes to the lower end of each jacket; it circulates around the liner to the top ofthe jacket, then through the bore-cooled collar of the liner, from which it passes into the cylinder head through an external duct. An air vent passage is provided between the jacket and the head at the high point, nearest the engine centerline. P is to n s Two types of pistons are used in the engine: a two-piece piston with a forged steel crown and an aluminum skirt, and a one-piece cast iron piston (see Figure 17-65). All pistons in an engine are ofthe same type. Figure 17-64. Section through cylinder Regardless of piston type, six piston rings are fitted, all located above the piston pin to ensure good lubrication ofthe skirt. The upper four rings are compression rings. The top ring is faced with chrome, the second with molybdenum, and the next two with copper. The lowest two rings are oil scraper rings, loaded by garter springs. The piston pin is hollow and mounts directly in the piston bosses with a slight interference fit. Drilled passages in the pin and the bosses carry oil from the connecting rod to cavities in the crown. Oil flow from these SEMT-PIELSTICK PC2.5 FOUR-STROKE DIESEL ENGINES 17-51 Oil for the piston pin bearing and for piston cooling and lubrication passes from the crankpin through holes in the bearing shells to a circumferential groove in the cap, then through drilled passages in the connecting rod, as shown in Figure 17-60. Cylinder Heads and Valves Each cylinder head assembly includes two air valves, two exhaust valves, an air start/check valve, a relief valve, and an indicator cock, as well as a fuel injector. Cylinder heads. Cylinder heads are hollow iron castings divided internally by intermediate decks, walls, bames, and passages (see Figure 17-66). Cooling water enters the lower chamber ofthe head via an external jumper from the cylinder jacket. In the lower chamber the water is forced to flow at high velocity over the hottest surfaces. It then passes to the upper chamber through two holes at opposite sides of the intermediate deck, cooling this area oflower heat emission at lower velocity. Most ofthe water then flows through the exhaust valve cages before rejoining the rest and flowing to the outlet header via an orifice. The orifice ensures that water enters all cavities in the head and valve cages. Figure 17-65. Pistons cavities is controlled to ensure that sufficient oil is retained to cool the crown by "cocktail shaker" action. Connecting Rods Each pair of cyli lders in a V shares a crankpin (see Figure 17-60) and, to accommodate two connecting rods side by side on a crankpin, cylinder transverse centerlines are staggered by a distance equal to the width of the connecting rod bottom end. Connecting rods are made of drop-forged steel with an I-section shank. The crank pin bearing is split diagonally and is serrated, with the bearing cap secured by four cap screws. The bearing itselfis ofthe thin-walled type, with steel shells lined with copper-lead, faced with a lead-tin flashing to reduce friction while running in. The backs of the shells are tin-plated to improve heat transfer. The piston pin bearing is a phosphor bronze bushing shrunk into the connecting rod. The diagonally split crankpin bearing on the PC2.5 is an evolutionary development from the horizontally split bearing used on the 2.3, permitting an enlarged bottom end that accommodates larger cap screws to absorb the increased stress, while still allowing the connecting rod to be withdrawn through the cylinder liner. Air and exhaust valves. Two air valves and two exhaust valves are fitted in each head. Each valve operates in an alloy iron bushing carried in a cast iron body or cage secured to the top of the head by three studs. The air valve seats are machined directly into the bottom of the head while each exhaust valve is carried in a cage that includes the valve seat (Figure 17-67). Each valve is closed by two helical springs of opposite pitch, acting on the stem via a collar and a conical, split lock. The air valves and bodies are identical, and the two exhaust valves in each cylinder head are also identical except for the cages, which are mirror images. Three types of exhaust valve assemblies are available for the PC2.5 engInes: 1. The standard assembly, with water-cooled valve guide and Stellite-faced valve seat, suitable for engines burning light distillate fuel. 2. An upgraded assembly, with the standard cage but with a valve with a nimonic valve head and a Rotocap, suitable with fuel containing up to about 150 ppm vanadium. 3. The further upgraded assembly shown in Figure 17-67, where the valve seat is intensively water cooled and the valve with nimonic head and Rotocap is used. This arrangement is used with fuel of higher vanadium content. The Rotocap device provides positive rotation of the valve to help keep seating surfaces free of deposits, to prevent hot spots, and to prevent Figure 17-67. Caged exhaust valve localized erosion. The nimonic valve head provides high strength and resistance to oxidation, corrosion, and creep at high temperatures. The water-cooled valve seat is designed to keep the temperature of the seating surfaces below that at which serious vanadium attack is likely. Air and exhaust valves are operated by cams through push rods and rocker arms. Each exhaust valve rocker arm is forked to operate both exhaust valves from a single cam and push rod (see Figure 17-68). Both air valves on each cylinder are operated simultaneously by one rocker arm, which is keyed to one end of a pivot shaft (visible in the figure), which carries a half arm at the other end for the second valve. Tappet adjustment is provided at the tips of the rocker arms for each valve. valves. Each cylinder head on marine propulsion engines is fitted with a relief valve that will open at cylinder pressures of2,100 to 2,200 psi. These valves can be removed for inspection and maintenance without disturbing the cylinder heads. R e lie f v a l v e s . Air start valves are normally fitted to only one bank of cylinders. These are pilot-activated valves, and when opened by air from A ir s ta r t/c h e c k SEMT-PIELSTICK Figure 17-68. PC2.5 cylinder head complete with rocker arms the starting air distributor they admit high pressure air from the main air header to the cylinders in correct sequence to rotate the engine. They also function as check valves, closing against pilot air pressure as soon as cylinder pressure exceeds starting air pressure. Air start/check valves can be removed for inspection and maintenance without disturbing the cylinder heads. Indicator cocks. Each cylinder head is fitted with an indicator cock for checking compression and firing pressures. These cocks can be removed for inspection and maintenance without disturbing the cylinder heads. Fuel Injection System A separate fuel injection system is fitted for each cylinder and consists of a earn-operated injection pump, a single injector centered in each cylinder head, and the interconnecting high pressure pipe. About twice the quantity offuel as is burned at full load is supplied at low pressure to the injection pump, which meters the fuel according to the load and returns the excess to the day tank or mixing tank. The injection pump discharges the metered fuel into the high pressure pipe, at pressures that reach about 14,000 psi at maximum RPM, at the proper time to initiate the injection process. When the pressure upstream of the injector builds up to the proper pressure for atomization (about 3,500 psi) the injector opens, allowing the fuel to spray into the cylinder. PC2.5 FOUR-STROKE DIESEL ENGINES 17 -55 Injection pumps. An injection pump is shown in Figure 17-69. The barrel has two ports on opposite sides for fuel inlet at low pressure from the booster pump. The plunger has two helical recesses on opposite sides to control the amount of fuel discharged at high pressure. When the earn forces the plunger up, the top of the plunger closes the inlet ports in the barrel and fuel is discharged at high pressure through the check valve at the top of the barrel, until the helical recesses rise into alignment with the inlet ports. The duration ofthe discharge stroke, and therefore the amount offuel discharged to the high pressure line, is established by the angular position of the plunger relative to the inlet ports in the barrel. This position is in turn established by rotating the plunger in the barrel by means of the pinion, engaged by the fuel rack, which is in turn positioned by the governor. The fuel rack is spring loaded to the zero position at each pump. Individual plungers and barrels are lapped together for a precision fit and operate without rings or seals. They are interchanged only as matched sets. Small amounts of fuel leaking past the plungers are collected via gravity drains. The opposing inlet ports and plunger recesses prevent lateral forces between the plunger and barrel that would cause excessive wear. The discharge check valve prevents the fuel in the high pressure line from returning to the pump during the down stroke. Timing of the start of the injection is adjustable at the tappet. Injectors. An injector is shown in Figure 17-70. Each injector consists of a body, a spring-loaded, fuel pressure-actuated needle valve, and a nozzle. Spring loading is factory set to permit opening at 3,500 psi. Two studs secure each injector to the head through a gasket at a shoulder in the cylinder head bore close to the bottom of the head. The injector body is sealed to the bore in the head at the top by O-rings to prevent contamination of rocker arm lubricating oil. Needles are precision fitted to nozzle center bores and are interchanged only in matched sets. Small amounts of fuel leaking past the needles are collected via gravity drains. The nozzle has ten orifices symmetrically oriented at a 70-degree angle to the vertical centerline. Orifice diameters of 0.60 mm and 0.65 mm are in use, with the smaller ones fitted where lower viscosity fuels will be burned. Injectors are cooled by water passages in the body and nozzle that connect to supply and return tubes above the top of the head. On heavy fuel engines the injector circuit is separate to enable the injector to be maintained at the correct temperature for injection, or to be heated in case the engine has been stopped without first changing over to distillate fuel. High pressure pipes. Each injection pump carries a forged steel delivery fitting that has bored fuel passages and connects, at a fitting on the side 17-56 ENGINE DESCRIPTIONS SEMT-PIELSTICK PC2.5 FOUR-STROKE DIESEL ENGINES 17-57 Figure 17-70. Fuel injector of the cylinder head, with a heavy steel tube run in a bore in the head to the injector. The annular space between the bore in the head and the heavy tube provides a passage for fuel leaking past the injector needle to the gravity drain system. T u r b o c h a r g in g Figure 17-69. Fuel injection pump Each bank of cylinders is fitted with a turbocharger, an aftercooler, a moisture separator, and an air manifold, all independent of the other bank. Exhaust gas may be led from selected groups of cylinders in each bank to separate groups of turbine nozzles in a pulse-charged configuration. AI- 17-58 ENGINE DESCRIPTIONS SEMT-PIELSTICK PC2.5 FOUR-STROKE DIESEL ENGINES ternatively, each cylinder may exhaust through a pulse converter to a common manifold for each bank, which is then led to an undivided nozzle block on the turbine. Turbochargers and aftercoolers may be mounted at either end of the engine but are usually at the drive end of marine propulsion engines. With reference to Figure 17-58, the air manifolds are outboard of each bank, while the exhaust lines are run in the insulated housing between the banks. Turbochargers may be fitted for water washing of both the turbine and the compressor. In the recommended system, a one-liter tank is installed for compressor washing and a 200-liter tank for turbine washing. The tanks are filled with pure, untreated fresh water injected by compressed air. The compressor is washed with the engine at full power, the full charge being injected over a period of four to ten seconds; washing must be followed by at least another hour of operation under load. The turbine wash is done with engine load adjusted until the turbine is running at 3,200 RPM, the water being injected over a period often to twenty minutes, with the turbine casing drain open. Mter a turbine wash the engine should be run under load for at least a further thirty minutes. C o n tr o l a n d M o n ito r in g Systems Propulsion engines are usually arranged for remote control from consoles in an engine control room and on the ship's bridge, with a local control fitted for maintenance and emergency use. Engine controls are part of a larger system that may include controls for clutches, shaft brakes, controllable propeller pitch, and attached auxiliaries, as appropriate. The remote control system mayor may not be supplied by the engine builder. The engine functions that are operated by the control system are starting, stopping, speed, and, on reversing engines, direction. Starting, stopping, and reversing controls are pneumatic. In some cases the pneumatic controls interface with solenoid-actuated pilot valves for electric control. Speed control may be pneumatic or electric. Starting system. An engine is started by admitting compressed, air to one bank of cylinders only, in the timed sequence of their firing order. The starting system of a nonreversing engine is shown schematically in Figure 17-71. Prior to starting, valves between the air tanks and the engine are opened, directing air under pressure to the main air control valve and the three-way solenoid start valve. Starting is initiated by energizing the three-way solenoid start valve, which directs air through the barring gear interlock to the pilot piston of the main air control valve, opening the valve to pressurize the main air header. Pilot air also passes to the air cylinder of the pneumatic/hydraulic actuator, which positions the fuel racks for starting. When the main air control valve is opened, a second pilot air circuit is pressurized, bringing air to the air start distributor. The air start Figure 17-71. Air start system 17 -59 17-60 ENGINE DESCRIPTIONS distributor is driven by the camshaft of the bank fitted with air start/check valves, and determine the sequence by which pilot air is sent to open air start/check valves; these admit air from the main air header to each cylinder in the bank during its power stroke, turning the engine over. Each air start/check valve functions as a pilot-actuated air admission valve until the cylinder begins to fire, then acts as a check valve to prevent the discharge of combustion gas into the main air header. The engine will accelerate until the governor takes control of the fuel rack setting. An engine can also be started manually by positioning the fuel racks using the lever visible in Figure 17-71, then using another lever to open the main air control valve. Manual operation bypasses the barring gear interlock. For reversing engines the starting system is similar, but incorporates additional interlocks operated by a multiported pilot air valve called a reversing gear selector, which is mechanically linked to the crosshead of the reversing gear. In addition there is a rotation detector which mechanically senses the direction ofrotation ofthe engine. One set of ports on the reversing gear selector is in series with the barring gear interlock, and ensures that air can reach the main air control valve pilot only if a camshaft shift is complete. The rotation detector is in series with the pneumatic! hydraulic actuator, and will prevent the fuel racks from being moved from the zero injection position unless the direction of rotation coincides with the direction for which the camshafts are set. Stopping. An engine is normally stopped by energizing the three-way solenoid stop valve (see Figure 17-71), which admits air to the air cylinder of the pneumatic/hydraulic actuator, forcing the fuel racks to the zero injection position. The reserve air tank and check valve ensure a supply of shutdown air independent of the starting air receivers. An engine can be manually stopped using the lever on the fuel rack operating linkage. Reversing system. The reversing levers for both camshafts are operated by a single hydraulic reversing cylinder mounted at the drive end of the engine. In the reversing cylinder the piston separates two closed oil supplies, one for each direction, each with its own small reservoir: control air pressure applied to either reservoir displaces oil into the cylinder, moving the piston from one end of the cylinder to the other, in turn shifting the reversing levers and the camshafts. Once its stroke is completed, the reversing piston, and therefore the levers and camshafts, is locked in position by an oil-actuated detent. Pneumatic interlocks prevent engine starting during camshaft shift and prevent camshaft shift while the engine . . IS runnmg. SEMT-PIELSTICK PC2.5 FOUR-STROKE DIESEL ENGINES 17-61 Speed control. In normal operation engine speed is controlled by adjusting the set point of the governor, which acts on the fuel racks through a hydraulic servo. A knob on the governor permits manual adjustment of the set point. The connection between the servo and the fuel rack operating linkage is made by a spring-loaded link that is rigid in normal operation but can be overpowered to move the racks to the zero injection position. , . Monitoring and automatic shutdown. Instrumentation and alarm points vary, depending on requirements of regulatory bodies and owner's preferences, but the standard engine design permits most parameters to be monitored locally or remotely, or to be fitted with alarms or other safeguards as appropriate. Each engine is fitted with an overspeed trip, a mechanical device comprising spring-loaded flyweights driven by the timing gear train (see Figure 17-62). On overs peed, the flyweights overpower their springs and trip a pawl, releasing a spring-loaded valve that directs lubricating oil to the oil cylinder of the pneumatic/hydraulic actuator. The actuator then forces the fuel racks to the zero injection position. An electric switch on the valve senses the trip, and energizes the three-way solenoid stop valve to simultaneously shift the air cylinder of the pneumatic/hydraulic actuator to the zero injection position, providing a redundant shutdown system. The overspeed trip must be manually reset after tripping. In addition to the overspeed trip, engines may be arranged to automatically shut down (depending on the requirements of the regulatory bodies involved) if any of the following events occur: Emergencystop button pressed Lowlubricatingoilpressure at engineinlet Lowrockerarm lubricatingoilpressure Lowjacket water pressure at engineinlet Highjacket water temperature at engine outlet Lowinjectorcoolingpump dischargepressure High injectorcoolingwater temperature at engineoutlet Lowreductiongear lubricatingoilpressure Usually an automatic shutdown will occur only after alarms have sounded and, in the case oflow pressure events, a standby pump has been started. Where appropriate the shutdown may occur only after a time delay. Also where appropriate, a manual override is provided to enable engine operation to continue, in situations that are critical to the ship, until the engine fails . 17 -62 ENGINE DESCRIPTIONS S u p p o r t S y ste m s Support systems recommended for the PC2.5 engines differ only in detail from the typical systems described in the previous chapter. The following features may be worthy of note: Some of the pumps in support systems may be engine driven from the timing gear train (Figure 17-62). Typically these would include the lubricating oil pump, the jacket water pump, and the seawater pump. In each case an electric motor-driven standby pump is provided, although in twin engine plants a single standby for each service may serve either engine. Engines run on distillate fuels only may have engine-driven fuel booster pumps, but heavy fuel engines are fitted with motor-driven booster pumps. Engines burning heavy fuels are fitted with separate closed-circuit injector cooling systems that allow the injector to be maintained at the correct temperature for injection, or to be heated in case the engine has been stopped without first having been changed over to distillate fuel. The cooling water from each injector is individually piped to an overhead expansion tank that is fitted with a steam coil, from which a motor-driven pump takes suction discharging to the injectors via a seawater-circulated cooler. On distillate fuel engines, cooling water reaches the injectors from the jacket water system via a separate seawater-circulated cooler. The seawater system is usually arranged with both of the air coolers and the injector water cooler in parallel, followed by the lubricating oil cooler and then the jacket water cooler in series. An independent lubricating oil system is provided for rocker arm lubrication to prevent contamination of the main system by water or fuel leakage in the cylinder head area. The system includes a tank, camshaft- and motor-driven pumps, filter, and pressure-regulating valve. Oil is fed to the rocker arm pivot bushings and then passes through drilled passages in the pivot pins and rocker arms to reach the push rods and tappets, returning to the tank by gravity. The recommended lubricating oil is an SAE 40 detergent, dispersant oil, with a principally paraffinic base. For heavy fuel engines the TBN should be in the 20 to 40 range, matched to neutralize the sulfur in the fuel oil. Waste heat can be recovered from the exhaust gases, from the jacket water, and from the charge air coolers. The engine crankcase is maintained at a very slight vacuum by a motor-driven fan that takes suction through an oil separator and discharges to a weather deck. F u e l O il S p e c ific a tio n PC2.5 engines may be operated on distillate supporting systems are correctly designed with the following quality limits: fuel or, when the engines and and outfitted, on heavy fuels SEMT-PIELSTICK Maximum Maximum Maximum Maximum Maximum Minimum Minimum Maximum Maximum PC2.5 FOUR-STROKE DIESEL ENGINES viscosity at 50'C Conradson carbon test residue, by weight sulfur content, by weight ash content, by weight bottom sediment and water, by volume cetane number gravity, degrees API sodium content, by weight vanadium content, by weight 17 -63 380 cSt 12.5% 3.5% 0.1% 2.0% 30 12.5 100 ppm 400 ppm Water and sediment must be reduced to 0.10 percent by purification before use. The specifications for sodium and vanadium are based on the presence of one or the other; if both are present lower limits may be applicable. M a in te n a n c e Guideline man-hour requirements for maintenance tasks, beginning with the engine in a fully assembled condition (except as otherwise noted), are as follows: M a n -h o u r r e q u ir e m e n ts . Dismantling and refitting a cylinder head 2 men, 3 hours Dismantling and refitting a connecting rod bearing (piston and connecting rod in place) 2 men, 1 hour Dismantling and refitting a main bearing (piston, connecting rod, and counterweights in place) 2 men, 3 hours Dismantling and refitting two exhaust valves (cylinder head in place) 2 men, 1 hour Dismantling and refitting a fuel injector (cylinder head in place) 1 man, 45 minutes Dismantling and refitting an injection pump 1 man, 1 hour Dismantling and refitting two air valves (cylinder head removed) • 2 men, 30 minutes Dismantling and refitting a piston and connecting rod (cylinder head already removed) 2 men, 45 minutes 17-64 ENGINE DESCRIPTIONS SEMT-PIELSTICK Changing cylinder liner (cylinder head and piston already removed) 1 man, 1 hour Cleaning and measuring cylinder liner (head, piston, and connecting rod already removed) 2 men, 1 hour S p e c i a l t o o l s . Tools normally furnished with the engines include hydraulic tighteners for main bearings and cylinder heads, extension gauges, component lifting attachments, alignment tools, valve refacing and grinding tools, pullers and extractors, fuel nozzle tester, maximum cylinder pressure indicator, special wrenches and pliers, special turbocharger tools, an engine barring lever, and conventional wrenches. Biweekly Analyze jacket and injector cooling water and treat as necessary. As required Clean or replace lubricating oil and fuel oil filters and strainers, and air filters when differential pressures reach limiting values. 1,250-1,500 hours Check freedom of air and exhaust valves by observation while barring engine. Check valve-to-rocker clearance. Change turbocharger lubricating oil. Inspect turbocharger cooling spaces for deposits and clean if necessary. Clean rocker arm lubricating oil tank. Take lubricating oil sample for spot test or analysis. Check exhaust valve Belleville washers. Examine gear train for tooth wear. Examine camshafts and camshaft bearings, cam surfaces, and push rods. 2,500-3,000 hours Check that main bearing jackscrews and crossbolts are tight. Take crank web deflections. Drain and refill governor oil. Clean air cooler water side if necessary. 17-65 5,000-6,000 hours Select two cylinders for disassembly: check crankpin bearing clearances with feeler gauge; pull both piston assemblies for inspection and determination of ring life; inspect crank pin bearing shells. Remove and inspect all exhaust valves; clean and reface if necessary; check wear of stems and guide bushings. Disassemble and clean main air control valve. Disassemble and clean starting air distributor. Remove air start valves; disassemble and clean. Clean turbocharger wheels and diffusers. Clean air and water sides of air coolers. Inspect relief valve on lubricating oil pump. 10,000-12,000 hours Inspect gear train; measure backlash. Check crankpin bearing clearances with feeler gauge. Pull piston assemblies (see Note 2 below);measure compression ring grooves; renew compression and scraper rings; renew piston pin seals and lock plates; check that piston pin tubes are tight; measure liner wear; deglaze liner; inspect crankpin bearing shells; inspect and measure piston pin bearing bushings; check air valve stem and bushing wear; reface air valves and seats ifnecessary; clean head seating surfaces; reassemble with new gaskets. Disassemble turbochargers for complete inspection; renew ball bearings and lubricating oil pumps. Disassemble engine-driven pumps for cleaning and complete inspection; renew ball bearings and seals if necessary. Inspect and clean overspeed trip. p r o g r a m . The following program is recommended for engines burning heavy fuel and operating at a high load factor. It is conservative and should be modified as experience is gained. Check engine lubricating oil level. DIESEL ENGINES Test alarms and automatic shutdown systems. Check movement of air starting valves and lubricate. Check exhaust valve Rotocaps. Remove fuel injectors and test for popping pressure and spray pattern; clean without disassembly. If no distress is noted, subsequent inspection may be at 5,000- to 6,000-hour intervals. M a in te n a n c e Daily PC2.5 FOUR-STROKE 17-66 20,000-24,000 hours ENGINE DESCRIPTIONS Remove cylinder heads, piston and connecting rod assemblies, liners, andjackets; separate liners from jackets for inspection and cleaning, then reassemble with new sealing rings; check crank pin bearing bolts by magnetic particle detection. Replace main bearing shells. Disassemble push rods and followers; check clearances at camshaft bearings and push rod guides. Inspect governor and governor drive. .Install replacement injection pumps and return used pumps to manufacturer for overhaul. CHAPTER 18 M a r in e R e f r ig e r a t io n S y ste m s JAMES A. HARBACH N o te s : 1. Log readings taken with accurate instruments are an important part ofthe maintenance program. Trends should be noted, for they serve as an early warning of problems developing. 2. Experience should be used to modify the program. While the program calls for piston overhauls at 10,000-12,000 hours, overhaul might not be required until 18,000-20,000 hours. The inspection of two pistons at 5,0006,000 hours should provide adequate indication of when an overhaul will be necessary. ACKNOW LEDGM ENTS The authors are indebted to Sulzer Diesel, Ltd., Colt Industries, and Fairbanks Morse Engine Division, for their assistance in preparing the text of this chapter, and for their provision of all of the figures. R E F R IG E R A T IO N P R IN C IP L E S R e fr ig e r a tio n R EFRIGERATION is defined as the branch of science and engineering that deals with the process of reducing and maintaining the temperature of a space or material below that of the surroundings. Since heat flows from a region of higher temperature to one oflower temperature, there is always a flow of heat from the surroundings into the refrigerated area. The amount of this flow of heat can be controlled by proper insulation. To maintain a constant temperature in the refrigerated space, the refrigeration system must remove heat from the space at the same rate as the heat is entering from the surroundings. H e a t T r a n sfe r Since refrigeration is essentially a heat transfer process, some understanding of heat transfer is necessary. Heat is a form of energy. The transfer of heat occurs in three ways: (1) by conduction, (2) by convection, and (3) by radiation. C o n d u c t i o n is the transfer of heat by direct contact between bodies. The heat energy is transferred from one molecule to those adjacent to it. The adjacent molecules can be part of the same body, or part of another body in direct contact with it. For example, a person holds one end of a steel bar in his hand and puts the other end in a flame. The heated end will transfer heat by conduction along the bar and eventually to the person's hand. The rate of heat transfer by conduction depends on the temperature difference between the high and low temperature parts and the thermal conductivity 18-1 18-2 MARINE REFRIGERATION SYSTEMS of the material. Materials with low values of thermal conductivity are used as heat insulators, and those with high values are used as heat conductors. is the transfer of heat by currents set up within a fluid medium. The currents can be created naturally, caused by the change in density of a heated fluid, or assisted mechanically by a fan, a pump, or other similar device. For example, a metal tank of water is heated at the center of its bottom by a flame. The heat of the flame is conducted through the tank bottom to the water inside. The adjacent water is heated, expands, and rises. It is replaced by cooler, denser water flowing in the sides. This water is then heated, rises, and continues the sequence. The heat from the flame is thus distributed throughout the tank by means ofthe natural currents. REFRIGERATION PRINCIPLES 18-3 vaporization of 970 Btu/lb (2256 kj/kg). Figure 18-1 shows the change of temperature and enthalpy (Btu/lb) for water from ice at O'F (-17.8'C) to superheated steam at 250'F (121'C). C o n v e c tio n is the transfer of heat from one body to another in the form of wave motion similar to light waves. There is no need for intervening matter. The best example of heat transfer by radiation is the heat the earth receives from the sun. The high level of molecular energy on the sun causes radiant energy waves to be transmitted to the cold of the surrounding space. An intervening body, such as the earth, will absorb some of the radiant energy. The amount of radiant energy absorbed depends on the nature of the material surface. For example, a rough, dark, dull surface absorbs almost all the radiant energy, while a highly polished surface, such as a mirror, reflects most of the radiant energy. R a d ia tio n S e n s ib le H eat Heat added or removed from a substance which changes its temperature but does not change its phase is called sensible heat. Sensible heat can be measured with a thermometer. Heat in the English system of units is measured in British Thermal Units (Btu). The unit of heat in the International Standard System (SI) is the joule. One Btu is equal to 1,055joules. A Btu is the amount of heat required to raise one pound of water one degree Fahrenheit. For different materials, a different amount of heat is required to raise one pound of the material one degree. This property is called its specific heat. For example, steel has a specific heat of 0.12. Ice has a specific heat of 0.5. Thus ice absorbs only half as much heat for a given temperature change as does water. L a te n t H e a t Heat added or removed from a substance which changes its phase, but not its temperature, is called latent heat. The heat involved in a change of phase from liquid to solid, or vice versa, is called the latent heat offusion. The heat involved in a change of phase from liquid to vapor, or vice versa, is called the latent heat of vaporization. Water at atmospheric pressure has a latent heat of fusion of 144 Btu/lb (335 kj/kg), and a latent heat of S a tu r a tio n T e m p e r a tu r e The temperature at which a substance changes from a liquid phase to the vapor phase, or vice versa, is called the saturation temperature. A liquid at the saturation temperature is called a saturated liquid, and vapor at the saturation temperature is called a saturated vapor. The saturation temperature of different fluids varies. For example, at atmospheric pressure, water vaporizes at 212'F (100'C) and ammonia vaporizes at-28'F (-33'C). For a particular fluid, the saturation temperature varies with pressure. For example, water vaporizes at 212'F (100'C) at 14.7 psia (101.3kPa) and 545'F (285'C) at 1000 psia (6895 kPa). The principle that boiling and condensing temperature of a fluid increases with increasing pressure, and vice versa, is important to an understanding of refrigeration systems. Vapors which have a temperature greater than the saturation temperature for their pressure are referred to as superheated. Liquids at a temperature below the saturation temperature for their pressure are referred to as subcooled. I c e R e fr ig e r a tio n Melting ice was not many years ago the only refrigerant available for use in domestic and small commercial refrigerators. Ice refrigeration has certain disadvantages: it cannot be used to maintain low temperatures; the 18-4 MARINE REFRIGERATION REFRIGERATION SYSTEMS ice must constantly be replenished; and the water resulting from the melting of the ice must be disposed of. However, even today, the capacity of a refrigeration system in tons is based on the cooling effect of melting ice. One ton of ice has a cooling effect of 288,000 Btu (144 Btu per pound times 2000 pounds = 288,000 Btu). One ton of refrigeration is the cooling effect of one ton of ice melting over 24 hours, i.e., 12,000 Btu/hour or 200 Btu/minute (3.52 kw). in the refrigerant) which simplifies the problem of oil return and improves the system efficiency due to its solvent action. The precautions regarding its use are to avoid liquid R-12 coming in contact with eyes or skin, to prevent freezing, and to avoid direct contact of R-12 with an open flame since it will decompose into toxic products. TABLE 18-1 Refrigerants A refrigerant is the working fluid which picks up the heat from the enclosed refrigerated space and transfers it to the surroundings. It should be understood that there is no such thing as an ideal refrigerant. Due to various applications and operating conditions, different refrigerants are suitable for different situations. A considerable number of fluids are available for use. However, only a few are employed as refrigerants in marine systems. Some which were popular a number of years ago (e.g., carbon dioxide and ammonia) are no longer used in marine practice because of the development of more suitable fluids. High on the list of desirable fluids are those that are nonflammable, nontoxic, and nonexplosive. In addition, the fluid should not react unfavorably with lubricating oil or any material commonly used in refrigeration systems, and should possess desirable thermodynamic properties that make it economical to use. Fluorocarbons The search by physical chemists in the late 1920s for desirable refrigerants led to the development of fluorocarbon refrigerants. Fluorocarbons are based on molecules of methane (CH4) or ethane (C2H6) in which one or more of the hydrogen atoms is replaced by fluorine or chlorine atoms. For example, replacing one of the hydrogen atoms with chlorine results in methyl chloride (CH3Cl), and replacing all four results in carbon tetrachloride (CCI4).The most common refrigerants in marine use are: refrigerant-12, dichlorodifluoromethane (CCI2F2); refrigerant-22, monochlorodifluoromethane (CHCIF2); and remgerant-ll, trichloromonofluoromethane (CCI3F). See Table 18-1 for a comparison of refrigerant properties. Refrigerant-12 (R-12) is the most widely used refrigerant in marine applications. It is completely safe (nontoxic, nonflammable, nonexplosive) and stable. It boils at -21.6°F (-29.8°C) at atmospheric pressure and is a liquid at ordinary temperatures under a pressure of70 to 75 psig (480 to 520 kPa). R-12 is oil miscible at all operating temperatures (oil dissolves 18-5 PRINCIPLES Comparative Refrigerant Characteristics* R e fr ig e r a n t B o ilin g P o in t @14. 7 p s ia R-12 (CCI2F2) R-22 (CHCIF2) R-ll (CCI3F) R-744 (CO2) R-717 (NH3) -21.6 -41.4 74.7 -109 -28.0 E v a p o r a to r C ondensor R e fr ig e r a n t P ressure P ressure F lo w D is p la c e - p s ig p s ig I b / m in m e n t, C F M 11.8 28.2 23.9t 317.5 19.6 *Based on 5°F evaporation tInches of Hg vacuum and 86°F condensation 93.3 158.2 3.5 1031.0 154.5 4.00 2.86 2.99 3.62 0.422 C om pressor 5.83 3.55 36.54 0.96 3.44 for a I-ton system Refrigerant-22 (R·22) has a boiling point of -41.4°F (-40.8°C) at atmospheric pressure. Developed originally as a low temperature refrigerant, it is used extensively in packaged air-conditioners. Modern marine reciprocating compressors are commonly designed to use either R-12 or R-22. For a given system, replacing R-12 with R-22 will increase the capacity approximately 60 percent. The operating pressures will be higher with R-22. R-22 is a safe refrigerant, but since it is not completely miscible with oil at low temperatures, precautions must be taken to insure oil return. Refrigerant-ll (R-1l) has a boiling point of74. 7°F (23. TC) at atmospheric pressure. Operating pressures are low with the suction below atmospheric and discharge typically 3 to 5 psig (20 to 35 kPa). Due to the large volume flow of refrigerant and the large compressor displacement required, R-ll is primarily used in higher tonnage centrifugal compressor air-conditioning systems. Like other fluorocarbon refrigerants, R-ll is nontoxic, nonflammable, and nonexplosive. Table of Saturation Properties Table 18-2 is a tabulation of saturation pressures and temperatures R-12 and R-22. for VAPOR-COMPRESSION THE CYCLE & REFRIGERATION V A P O R -C O M P R E S S IO N R E F R IG E R A T IO N CYCLE SYSTEMS 18-7 AND SYSTEM S T h e V a p o r -C o m p r e s s io n C y c le While other cycles such as the absorption are sometimes used for refrigeration systems, almost all marine refrigeration systems operate on the vapor-compression cycle. Any cycle consists of a repetitive series of thermodynamic processes. The operating fluid starts at a particular state or condition, passes through the series of processes, and returns to the initial condition. The vapor-compression cycle consists of the following processes: (1) expansion, (2) vaporization, (3) compression, and (4) condensation. A simple vapor-compression cycle is shown in Figure 18-2. Starting at the receiver, high temperature, high pressure liquid refrigerant flows from the receiver to the expansion valve. The pressure of the refrigerant is reduced by the expansion valve so that the evaporator temperature will be below the temperature of the refrigerated space. Some of the liquid refrigerant flashes to a vapor as the pressure is reduced. In the evaporator, the liquid vaporizes at a constant temperature and pressure as heat is picked up through the walls ofthe cooling coils. The compressor draws the vapor from the evaporator through the suction line into the compressor inlet. In the compressor, the refrigerant vapor pressure and temperature are increased and the high temperature, high pressure vapor is discharged into the hot gas line. The vapor then flows to the condenser where it comes in contact with the relatively cool condenser tubes. The refrigerant vapor gives up heat to the condenser cooling medium, condenses to a liquid, and drains from the condenser into the receiver, ready to be recirculated. H ig h S id e /L o w S id e A refrigerating system can be divided into two parts according to the pressure exerted by the refrigerant. The low pressure portion ofthe system (the "low side") consists of the expansion valve, the evaporator, and the suction line. This is the pressure at which the refrigerant is vaporized in the evaporator. The high pressure portion of the system (the "high side") consists of the compressor, the hot gas line, the condenser, the receiver, and the liquid line. This is the pressure at which the refrigerant is condensed in the condenser. The dividing points between the high side and the low side are the expansion valve and the compressor. E v a p o r a to r T e m p e r a tu r e In order for refrigeration to take place, the evaporator coil temperature must be below that ofthe refrigerated space. The evaporator temperature is controlled by varying the pressure in the evaporator since the vaporiza- VAFOR-COMPRESSION CYCLE & REFRIGERATION SYSTEMS 18-9 tion of the refrigerant occurs at the saturation temperature corresponding to the evaporator pressure. Raising the evaporator pressure raises the evaporator temperature, and lowering the pressure lowers the temperature. C o n d e n s in g T e m p e r a tu r e In order for the refrigerant gas to condense to a liquid in the condenser, its saturation temperature must be above that of the condenser cooling medium. Raising the condenser pressure raises the condensing temperature, and lowering the pressure lowers the temperature. To provide continuous refrigeration, the refrigerant vapor must be condensed at the same rate as the refrigerant liquid is vaporized in the evaporator. Obviously, any increase in the rate of vaporization will increase the required rate of heat transfer in the condenser. Heat transfer in the condenser is a function of (1) the condenser surface area, (2) the condenser heat transfer coefficient, and (3) the temperature difference between the condensing refrigerant vapor and condensing medium. Since the first two items are normally fixed, it follows that the condenser heat transfer varies with the temperature difference. The condensing temperature thus varies directly with the cooling medium temperature and the rate of refrigerant vaporization in the evaporator. D e v ia tio n s fr o m th e S im p le C y c le Actual refrigeration cycles deviate somewhat from the simple cycle discussed above. For example, pressure drops occur in the piping, across the condenser and evaporator, and across other components due to the flow of refrigerant through them. Sub cooling ofliquid refrigerant can occur in the receiver and liquid line. Superheating of the refrigerant gas can occur in the evaporator and suction line. Figure 18-3 compares an actual cycle with the simple cycle on a pressure-enthalpy diagram. In general, pressure drops are kept to a minimum consistent with economic pipe sizes. The effect of superheating of the refrigerant gas on cycle performance depends on where the superheating occurs. In general, superheating is kept to a minimum consistent with complete vaporization and avoiding liquid refrigerant flooding back to the compressor. Subcooling of the liquid refrigerant generally has a desirable effect on cycle performance. It is common on modern systems to install a liquid-suction heat exchanger (heat interchanger). The relatively coolrefrigerant gas coming from the evaporator is used to subcool the liquid refrigerant flowing to the expansion valve. This increases the efficiency and capacity ofthe system. M a r in e A p p lic a tio n s o f R e fr ig e r a tio n S y ste m s Mechanical refrigeration is used aboard ship for many purposes, including (1) refrigerated ship's stores, (2) air-conditioning, (3) refrigerated cargo, VAPOR-COMPRESSION CYCLE & REFRIGERATION SYSTEMS 18-11 the flow of refrigerant to the evaporator. Continued compressor operation reduces the suction pressure to the set point of the low pressure switch, stopping the compressor. Due to the stopping of refrigeration, the space temperature will slowly rise. When the temperature reaches the thermostat set point, the contacts close, energizing the solenoid valve, thus permitting liquid refrigerant to enter the evaporator. The refrigerant vaporizes, raising the suction pressure to the cut-in point of the low pressure switch, thus starting the compressor. The pump-down cycle reduces the possibility ofliquid flooding back to the compressor during start-up and substantially reduces the dilution of crankcase oil by the refrigerant. An understanding of the pump-down cycle control sequence is essential in troubleshooting refrigeration systems. R e fr ig e r a te d Figure 18-3. Comparison of simple cycle and actual cycle and (4) drinking water cooling. Most marine refrigeration systems use reciprocating compressors; however, systems using rotary and centrifugal compressors are becoming more common. The reciprocating and rotary types are positive displacement while the centrifugal type uses the centrifugal force created by a high speed impeller to provide the compression. Reciprocating compressors are especially flexible. They are used in high temperature (air-conditioning) as well as low temperature (cryogenic) applications, and in sizes from less than 1 ton to 250 tons or more. This flexibility when considered along with the reciprocating compressor's reliability and efficiency accounts for its widespread popularity. Rotary compressors are replacing reciprocating compressors in certain applications, while centrifugal compressors are used primarily in large tonnage air-conditioning or refrigerated cargo applications. A u to m a tic O p e r a tio n o f R e c ip r o c a tin g C om p ressor S y ste m s Most marine refrigeration systems operate on a method of control known as the "pump-down cycle." The refrigerated space temperature is monitored by a thermostat which acts to open and close the solenoid valve in the liquid line. The routine starting and stopping of the compressor is controlled by the low pressure cutout switch in the compressor suction line. As the space temperature is reduced to the set point of the thermostat, the thermostat contacts open, deenergizing the solenoid valve and stopping S h ip 's S to r e s Ship's stores refrigeration equipment is installed to preserve food required for consumption by the crew and passengers. Insulated walk-in type storage compartments are usually installed. A typical installation consists of a freeze room, dairy room, fruit and vegetable room, and two condensing units. Figure 18-4 is a diagram ofa typical system. The system is designed to maintain the freeze room at O°F(-18°C) and chill rooms at 33°F (0.5°C). Unit coolers or natural convection bare tube evaporators are commonly used. Each condensing unit consists of a compressor, condenser, receiver, 18-12 MARINE REFRIGERATION SYSTEMS VAPOR-COMPRESSION CYCLE & REFRIGERATION SYSTEMS 18-13 heat exchanger, controls, valves, and associated piping. One condensing unit can handle the entire plant load during holding operation while both units can be used for pull down operation. It is estimated that lowering the product temperatures after loading should not take more than two days under extreme design conditions. Provisions are made for the defrosting of freeze box evaporator coils to permit removal of accumulated ice to maintain efficiency. Defrosting is accomplished by electric heaters, or by use of hot refrigerant gas from the compressor. A ir -C o n d itio n in g Air-conditioning is the control of temperature and humidity of enclosed spaces to make the environment more comfortable for the people living and working there. While technically it includes winter heating, air-conditioning is normally taken to mean the cooling and dehumidifying of air in warm weather. A typical air-conditioning system is similar to that used for Figure 18-6. Condensing unit for 60-ton air-conditioning system, back view. Courtesy Carrier Corp. refrigerated ship's stores except that higher temperatures are involved and system tonnage is larger. The evaporators operate above 32'F (O'C), and therefore no defrosting provisions are required. Evaporators of smaller systems are usually of the direct expansion type while larger systems typically are of the chilled water type. The evaporator in a chilled water system is located in the refrigeration machinery room, and it cools water to 45'F to 50'F (TC to 10'C). The chilled water is then circulated to the remotely located cooling coils. Air-conditioning applications are different from refrigerated storage applications in that standby condensing capacity is normally not furnished. At peak loads, all condensing units are usually required. The plant is typically arranged to permit cross connection of condensing units thus allowing securing unneeded units. R e fr ig e r a te d Figure 18-5. Condensing unit for 60-ton air-conditioning system, front view. Courtesy Carrier Corp. C argo Refrigerated cargo spaces are installed to permit the shipment of perishable cargo. These systems vary in size from the small self-contained unit on a refrigerated container to a complex brine system on a refrigerated 18-14 MARINE REFRIGERATION cargo vessel. The systems are usually designed for maximum flexibility to permit the carriage of different cargoes at different temperatures. Defrosting provisions are required for all evaporators designed for below 32°F (O°C)operation. Where the installation is extensive and uses forced air cooling coils, hot seawater defrosting is common. Hot seawater is heated and sprayed over the coils to melt the frost and carry it away down the drains. The fans are shut down during defrosting to minimize the carryover of heat into the storage area. Some cargoes such as fruit and vegetables give off carbon dioxide (C02) during storage. To prev~ n t dangerous concentrations, ventilation systems are commonly provided to force fresh air into the refrigerated space and exhaust stale compartment air to the outside. Since the introduction of warm outside air imposes a significant load on the refrigeration system, the fan is sized for one air change per hour and is operated for 20 minutes per hour during normal operation (not pulldown) only. In large systems, it is frequently economical to employ brine as a secondary refrigerant in an indirect system. The primary refrigerant evaporator is employed to chill the secondary refrigerant, the brine, which is then circulated to the refrigerated spaces. Calcium chloride and sodium chloride are the most common types of brine used. Calcium chloride brine can achieve temperatures of6TF (-55°C) while sodium chloride brine can achieve temperatures of -6°F (-21°C). Tables 18-3 and 18-4 give the freezing points of calcium chloride and sodium chloride brine solutions. The density ofthe brine should be such that the freezing point is 10°F to 15°F (5°C to 8°C) below the required brine temperature. Note that the TABLE P r o p e r tie s S p e c ific G r a v ity (b y w e ig h t) ( 6 0 "F / 60 "F) o 5 10 15 20 25 29.87 30 32 34 1 8 -3 o f C a lc iu m C h lo r id e B r in e P ercent C aC l 1.000 1.044 1.087 1.133 1.182 1.233 1.290 1.295 1.317 1.340 lb C a C l2 / g a l C r y s ta lliz a tio n T e m p e r a t u r e ("F) 0.000 0.436 0.908 1.418 1.970 2.574 3.16 3.22 3.49 3.77 CYCLE & REFRIGERATION VAPOR-COMPRESSION SYSTEMS 32.0 27.7 22.3 13.5 -0.4 -21.0 -67.0 -50.8 -19.5 4.3 TABLE P r o p e r tie s P ercent N aC l (b y w e ig h t) o 5 10 15 20 23 25 1 8 -4 o f S o d iu m S p e c ific G r a v ity (59 "F / 39 "F) 1.000 1.035 1.072 1.111 1.150 1.175 1.191 18-15 SYSTEMS C h lo r id e B r in e lb N a C l / g a l C r y s ta lliz a tio n T e m p e r a tu r e 0.000 0.432 0.895 1.392 1.920 2.256 2.488 ("F) 32.0 27.0 20.4 12.0 1.8 -6.0 16.1 crystallization point of a brine solution decreases with increasing salt content, then starts to increase again. The concentration at which the crystallization temperature is at a minimum is called the eutectic point. At concentration below eutectic, ice crystals form on lowering temperature. At concentrations above eutectic, salt crystals form on lowering temperatures. The brine density should be checked at least once a month and additional salt added as required. Refrigerated containers are fitted with self-contained electric heating and cooling units. The typical unit is mounted flush with the front face of a standard-sized container. Cooling is provided by an R-12 refrigeration system with a semihermetic reciprocating compressor and an air-cooled condenser. Figure 18-7 is a typical diagram of the refrigeration system. Axial flow fans are installed to provide ventilation across the evaporator and condenser. Figure 18-8 shows the air flow through the unit. Heating and defrosting are accomplished with electric resistance heating elements located in the evaporator section. The heating and cooling cycles are controlled automatically by a thermostat, while the defrost cycle is initiated by a timer or a differential pressure switch monitoring the air pressure across the evaporator. The evaporator fan is secured automatically during the defrost cycle. Electric power to run the units can be supplied from the ship's electrical system, from deck-mounted packaged diesel generators, or from individual diesel generators mounted in the container front. Some units are built with a diesel as the primary drive and an electric motor backup. The units with diesel engines or diesel generators can be carried over land by trucks without generating capacity. Figure 18-8. Refrigerated container air flow. Courtesy Thermo King Corp. R e c ip r o c a tin g R E F R IG E R A T IO N R e fr ig e r a tio n SYSTEM COM PONENTS C om p ressor T yp es Three types of compressors are used in refrigeration systems: (1) reciprocating, (2) rotary, and (3) centrifugal. The reciprocating and rotary types are positive displacement machines. In the reciprocating compressor, the gas is compressed by a piston, while in a rotary compressor a roller, vane, or screw accomplishes it. The centrifugal compressor utilizes a high speed impeller to generate the compression force by centrifugal action. Each compressor type has certain advantages and disadvantages. The type selected depends on such factors as the size and type of the installation and on the refrigerant used. C om p ressors The reciprocating compressor is the most common type found in marine refrigeration systems. It is found in systems rated from less than one ton to several hundred tons and in high, medium, and low temperature applications. It is durable and efficient and can be manufactured economically. Marine reciprocating compressors are typically of the single-acting enclosed type. In single-acting compressors, vapor compression occurs on only one side of the piston, while in double-acting compressors, vapor compression occurs on both sides of the piston. Double-acting compressors are only found on older large industrial installations. Enclosed-type compressors drive the piston by a connecting rod driven by the crankshaft. The crankcase is airtight and exposed to the system refrigerant. Reciprocating compressors can be further classified as open, hermetic, or semihermetic. An open type unit has a separate motor and compressor. A hermetic unit is a sealed motor-compressor assembly and is common on smaller units. A semihermetic or "accessible hermetic" is a unit in which the shell of the assembly is bolted rather than welded. This allows field servicing. The number of cylinders varies from one to as many as twelve or more. In multicylinder units, the cylinders may be arranged in-line, radially, or in a V or W pattern. Figure 18-9 shows a modern open-type W pattern Figure 18-11. Compressor internal parts. Courtesy Carrier Corp. compressor. Figures 18-10 and 18-11 show the construction details of a typical compressor. Pistons are ofthe automotive or the double-trunk types. When automotive type pistons are used, the gas enters and exits through valves located in the cylinder head. In double-trunk piston units, the gas enters through ports in the cylinder wall and passes into the cylinder through suction valves located in the top of the piston. Figure 18-12 shows a compressor with double-trunk pistons. The valve types used in refrigeration compressors are the reed, the ring, and the poppet. All three types open and close automatically based on the pressure differential across the valve. Reed valves are flexible metal strips that fit over slots in the valve seat. Ring valves consist of one or more circular rings, valve springs, and a retainer. The poppet valve is similar in construction to an automobile valve that is spring loaded. Poppet valves are used only in low speed compressors while reed and ring valves can be used in low and high speed machines. Figure 18-13 shows the gas flow through a typical compressor. Figure 18-14 shows a typical suction and discharge valve assembly using ring valves. Most larger compressors are fitted with a spring loaded safety head. Under normal operating conditions, the head is held in place by heavy springs. In the event that a slug of liquid refrigerant should enter the cylinder, the safety head will rise and prevent damage to the compressor. REFRIGERATION SYSTEM COMPONENTS 18-23 Open type compressors require a crankshaft seal where the shaft exits the casing. The most popular type of crankshaft seal is the mechanical seal, Figures 18-15 and 18-16. It consists of a spring-loaded carbon or bronze rotating seal which runs against a stationary seal plate with a highly polished face. A rubber gasket seals the crankshaft and the rotating seal. The spring maintains the rotating seal in firm contact with the stationary seal plate. An oil film is required to lubricate the two smooth surfaces and form a vapor-tight seal. There are two basic types oflubrication systems found on refrigeration compressors: (1) splash and (2) forced feed. Splash lubrication is common on smaller compressors (10 HP and less) while larger compressors typically have some form of forced lubrication. A combination of splash and forced lubrication can be found on a single compressor. In splash lubrication, the crankcase is filled with oil to approximately the centerline of the crankshaft. As the crankshaft turns, oil is splashed onto the bearings, the cylinder walls, and other surfaces. Oil scoops are sometimes used to force oil through drilled passages to lubricate the connecting rod bearings. Oil cavities usually are located above the main bearings to collect oil which then feeds the bearings by gravity. In forced feed lubrication, a pump is used to deliver the oil to the various parts requiring lubrication. The pump is typically a small positive displace- Figure 18-17. Compressor lubrication system Figure 18-16. Compressor shaft seals. Courtesy Carrier Corp. ment unit driven by gears from the crankshaft. The oil is delivered under pressure through oil tubes and/or drilled passages in the crankshaft and connecting rods. Oil strainers are located at the pump suction, and oil filters are sometimes installed at the pump discharge. An oil pressure failure switch is installed to warn ofloss of proper lubrication. Figure 18-17 shows a typical compressor lubrication system. The oil selected for lubrication of refrigeration compressors must be chemically stable, have low pour and cloud points, and have the proper viscosity for the operating temperature range. In addition the oil used in hermetic units must have a high dielectric strength to avoid grounding or shorting of the motor windings. Consult the manufacturer's technical manual for the recommended oils for each compressor. A certain amount of refrigerant will always be dissolved in the lubricating oil. However, large amounts of refrigerant in the oil are undesirable. Excessive dilution can result in inadequate lubrication. In addition, during compressor start-up, the lowering ofthe crankcase pressure will cause oil foaming due to the vaporization ofthe refrigerant. In severe cases, this can disrupt lubrication and can cause carryover of the liquid refrigerant and oil into the cylinder. Since marine systems typically operate on the pumpdown cycle, the low crankcase pressure at shutdown limits refrigerant absorption by the oil. Crankcase heaters which come on automatically during the compressor offcycle can be used to keep the oil warm and reduce refrigerant absorption. A common cause of excessive oil dilution by liquid refrigerant is an improperly adjusted thermostatic expansion valve which allows overfeeding of the evaporator. The liquid refrigerant then carries over into the compressor crankcase. There are a number of methods of controlling the capacity ofreciprocating compressors. One method is to vary the speed of the compressor. Multispeed motors or steam turbines can be used to drive the unit. The most common method of varying the capacity of multi cylinder compressors is to vary the number of active cylinders by holding the suction valves open. The capacity control system unloads cylinders (i.e., cuts cylinders out of operation) in response to decreases in refrigeration load based on suction 18-26 MARINE REFRIGERATION SYSTEMS pressure. Under high loads (high suction pressures) none of the suction valves are held open, and all the cylinders are in operation. As the load decreases (and the suction pressure falls oft), the cylinders are cut out in sequence. Ifthe suction pressure continues to fall off, the compressor will stop on the low pressure switch. Figure 18-18 shows a typical capacity control system. Compressor lubricating oil is used to operate the valve lifting mechanism. Since oil pressure is required to load the cylinder, the compressor will start with all controlled cylinders unloaded, thus reducing the starting load on the compressor motor. 'R o ta r y C o m p r e s s o r s Rotary compressors are positive displacement machines. They have had limited usage in marine systems, but are becoming more common in industrial installations. The two basic types are vane and screw compressors. Vane compressors are commonly used in small domestic refrigerators, freezers, and room air conditioners. Vane compressors are similar in construction to a vane pump. Screw compressors first became popular for refrigeration service in Europe during the 19508 and 1960s. They are now popular in the United States for use in systems of200 to 500 tons capacity. Construction is similar to a double screw pump. While some early compressors used timing gears to avoid lubrication of the rotors, common practice today is to drive one rotor and then inject oil between the rotors for lubrication and sealing. An oil separator is located in the system REFRIGERATION SYSTEM COMPONENTS 18-27 after the compressor discharge. Many screw compressors are fitted with a sliding valve for capacity control. As the valve is opened, it delays the position at which compression begins along the rotor. C e n tr ifu g a l C o m p r e s s o r s Centrifugal compressors are commonly used on systems of 300 tons and larger. They are especially suitable for large chilled water air-conditioning systems. While not common on commercial vessels, centrifugal compressor systems are used on many larger naval vessels. Centrifugal compressors are similar in construction to centrifugal pumps. Figure 18-19 shows the construction of a typical centrifugal compressor. Due to the low density of the typical refrigerant gas, the pressure developed per stage is typically fairly low. Multistage compressors are common, even when relatively modest discharge pressures are required. Refrigerants such as R-ll which require a relatively large displacement per ton are especially suitable for these systems. Figure 18-20 is a diagram of a multistage centrifugal compressor system. Like most, it is closecoupled to minimize pressure drops, and employs a secondary refrigerant (chilled water or brine), It shows the use of flash gas removal during the expansion process which improves the cycle efficiency. Figure 18-21 shows a typical centrifugal compressor chiller unit. Centrifugal compressors are pressure lubricated either by a shaftdriven or external motor-driven oil pump. The system is arranged to avoid contact of the oil by the system refrigerant, thus simplifying lubrication. The system also includes oil coolers to maintain oil temperature during operation and oil filters to remove contaminants. Since many systems operate with low side pressures below atmospheric, a purge recovery unit is commonly installed to continuously remove noncondensable gases and to recover and return refrigerant mixed with the purged gases. The system consists of a small reciprocating compressor, a condenser, and a separating tank. The suction of the compressor is connected to the main system condenser. The gases flow to the purge condenser where the refrigerant condenses. In the separating tank, a float valve controls the return of the liquid refrigerant to the main system, and a relief valve vents the noncondensable gases to the atmosphere. Centrifugal compressor systems are controlled by sensing (or monitoring) the secondary refrigerant temperature. The system capacity is varied as required to maintain the chiller outlet temperature constant. The methods that can be employed to vary compressor capacity are: (1) pre rotation inlet vanes, (2) variable speed operation, (3) varying condenser pressure, and (4) bypassing discharge gas. The first two methods are the most popular, but the latter two are sometimes used in combination with one of the other methods. 18-30 MARINE REFRIGERATION SYSTEMS C on d en ser Most marine refrigeration condensers are of the water-cooled, multipass, shell-and-tube type as shown in Figure 18-22. Seawater is circulated through the tubes, and the hot gas from the compressor discharge is admitted to the shell and condenses on the outer surfaces of the tubes. The condenser is typically constructed of a steel shell, copper-nickel tubes and tube sheets, and bronze waterheads. Gas inlet, liquid outlet, purge, and water regulating valve control connections are provided. Small systems such as found in drinking water coolers are usually air-cooled, with finned tubes and a cooling fan; R e c e iv e r The receiver collects the liquid refrigerant draining from the condenser. It consists of a steel shell with steel dished heads welded on each end. Sight glasses or a liquid level indicator is installed to permit determination of the amount ofliquid refrigerant in the receiver. The receiver will typically have sufficient capacity to hold the entire system refrigerant charge and will retain a small liquid level during full load operation. High levels indicate overcharge and low levels indicate undercharge. REFRIGERATION SYSTEM COMPONENTS 18-31 E v a p o r a to r s Marine evaporators are usually either ofthe natural convection type or the forced convection type. Natural convection evaporators are bare or finned coils mounted on the walls or overhead of the refrigerated storage room. They are suitable for applications where low air velocities and minimum dehydration of the product are desired. Natural convection evaporators rely on the difference in density between cool and warm air to circulate air across the coils. Forced convection evaporators are referred to as "unit coolers," "fan coil units," or "cold diffusers." They are in common use in ship's stores refrigeration, cargo refrigeration, and air conditioning systems. The units consist of a cooling coil with finned tubes, a motor-driven fan, and drain pan, all enclosed in a sheet metal casing. Units designed for air-conditioning applications may also contain a heating coil so that the same unit may be used for winter heating as well as air-conditioning. Units designed for freeze room applications are usually fitted with an electric resistance defrost system. L iq u id -S u c tio n H eat E xchangers Liquid-suction heat exchangers (heat interchangers) are commonly installed in the suction line to prevent carryover of liquid refrigerant to the compressor. It also subcoolsthe liquid refrigerant flowing to the evaporators thus increasing the capacity and efficiency of the system. The heat exchanger is typically a shell-and-tube or shell-and-coil assembly with suction gas flowing through the tubes and the liquid flowing through the shell in a counterflow arrangement. T h e r m o s ta tic E x p a n s io n V a lv e While there are a number of devices available to control the flow of refrigerant to the evaporator, such as the capillary tube, the float valve, and the constant-pressure expansion valve, the thermostatic expansion valve is the device most commonly found in marine systems. The thermostatic expansion valve responds to the evaporator temperature and pressure and maintains a constant superheat at the outlet of the evaporator. As refrigerant is fed to the evaporator, the liquid boils off into a vapor. By the time the refrigerant gas reaches the end ofthe evaporator, it is superheated. Feeding more refrigerant to the evaporator will lower the superheat temperature, while feeding less refrigerant will raise the superheat temperature. Figure 18-23 shows a typical thermostatic expansion valve. A feeler bulb partially filled with a volatile liquid, typically the system refrigerant, is clamped to the outlet of the evaporator so that the bulb and the power fluid inside closely assume the suction gas temperature. The pressure of 18-32 MARINE REFRIGERATION SYSTEMS power fluid is applied to the top ofthe valve diaphragm, and the evaporator pressure pushes on the bottom. A spring applies a force and balances the valve assembly as shown in Figure 18-24. For the valve to open and feed refrigerant to the evaporator, the bulb pressure must be higher than the evaporator pressure. Since the bulb pressure responds to the evaporator outlet temperature, this can only occur when the suction gas is superheated. The spring is normally adjusted by the factory to obtain about 10°F (6°C) of superheat in service. Valves set for less than this will not operate properly and may flood back liquid refrigerant to the compressor. Valves set for less than 10°F (6°C) will reduce the effectiveness of the evaporator and will reduce the system capacity. Thermostatic expansion valves can be purchased as internally equalized or externally equalized. An internally equalized valve has a small port which admits the valve outlet pressure (i.e., the evaporator inlet pressure) to the bottom of the valve diaphragm. Some refrigeration systems have an appreciable pressure drop in the evaporator. Use of an internally equalized valve in such applications will result in unacceptably high superheat at the evaporator outlet. The solution is to use an externally equalized valve which has a fitting for connecting the evaporator outlet pressure rather than the inlet pressure to the bottom of the valve diaphragm. In general, an external equalizer should be used when the evaporator pressure drop is sufficient to change the saturation tempera- Figure 18-23. Thermostatic expansion valve. Courtesy Alco Controls Figure 18-24. Thermostatic expansion valve equilibrium ture 3°F (1.5°C)in air- conditioning applications, 2°F (1°C)in refrigeration systems, or reF CO.5°C) in low temperature (cryogenic) systems. There are some specialized types of thermostatic expansion valves available. Pressure-limiting or gas-charged valves can be installed in systems to limit the compressor load during pulldown conditions. Crosscharged valves which have a bulb fluid different than the system refrigerant are used in low temperature applications. Because ofthe wide variety of valves available, it is important to replace a thermostatic expansion valve with one of the same type and size. Check the manufacturer's manual. The superheat setting of most thermostatic expansion valves is adjustable. Remove the seal cap on the side of the valve and turn the adjusting stem with a screwdriver. Turning the stem to the right increases the spring pressure, reducing refrigerant flow, and increasing superheat. Turning the stem to the left decreases the spring tension, increasing refrigerant flow, and reducing superheat. Two turns of the stem will typically change the superheat about reF (O.5°C).One technique of measuring superheat is to use a contact thermometer to accurately measure the temperatures at the inlet and outlet of the evaporator. This method will yield good results only when the evaporator pressure drop is low.A second technique is to compare the evaporator outlet temperature with the saturation temperature corresponding to the compressor suction pressure. This will yield good results only when the pressure drop between the evaporator and the compressor is low. The best technique will involve taking pressure and temperature readings at the evaporator outlet. For example, the following data are taken for an R-12 system: 18-34 1. 2. 3. 4. MARINE REFRIGERATION SYSTEMS Evaporator outlet pressure (from gauge) = 37 psig (255 kPa) Evaporator outlet temperature (contact thermometer) = 50'F (lO'C) 37 psig = 40'F (4.4'C) (Table 18-2) Superheat = 50'F - 40'F = 10'F (5.6'C) If a check of superheat indicates a problem, valve adjustment is indicated. It should be noted, however, that improper installation of the feeler bulb is a common cause of expansion valve problems. Figure 18-25 shows some recommended bulb installations. It is most important that the bulb make good thermal contact with the suction line. Clean the line thoroughly and clamp the bulb securely. If the bulb is installed in a location outside the cooled space, it will probably be necessary to insulate the bulb as shown in Figure 18-26. H ig h a n d L o w P r e s s u r e S w itc h e s These devices are very similar in construction and operation, but perform very different functions in the refrigeration system. The high pressure switch is a safety device. It is actuated by the compressor discharge pressure and stops the compressor in the event of high pressure. The low pressure switch is actuated by the compressor suction pressure. It is the primary control for stopping and starting the compressor during normal operation. When the suction pressure has been pumped down to the Figure 18-26. Expansion valve bulb insulation desired level (the cut-out setting), the low pressure switch opens and stops the compressor. When the pressure rises to the desired level (the cut-in setting), the switch closes and the compressor starts. The high and low pressure switches can be supplied as separate units or as a single unit usually called a dual pressurestat. Each switch consists of a sensing element (either a diaphragm or a bellows) and a snap action switch with a set point (range) adjustment and a differential adjustment. Figure 18-27 shows a typical pressure control switch. Proceed as follows to set the switches. To set the high pressure switch: 1. Turn differential screw to minimum and range screw to maximum. 2. Start compressor and control discharge pressure by throttling condenser water flow. 3. Bring discharge pressure to cut-out point. Turn range screw until contacts open, stopping compressor. 4. Turn differential adjustment until contacts close, starting compressor when discharge pressure drops to cut-in point. To set the low pressure switch: 1. Turn differential screw to maximum and range screw to minimum. 2. Start compressor and control suction pressure by throttling the compressor suction stop valve. 3. Lower suction pressure to about 10 psi (69 kPa) below desired cut-in point. Turn range screw until contacts open, stopping compressor. Allow suction pressure to rise to desired cut-in point and close suction valve to hold it there. Turn range setting slowly until contacts close, starting compressor. Open suction valve. The cut-in point is now set. 4. Lower suction pressure to desired cut-out point and decrease differential setting until contacts open, stopping compressor. The cut-out point is now set. 18-36 MARINE REFRIGERATION T h e r m o s ta tic SYSTEMS S w itc h A thermostatic switch (or thermostat) is a temperature control switch used to open or close an electric contact based on sensing a temperature. The remote sensing bulb of the thermostat is filled with a volatile liquid charge. Changes in the sensed temperature cause changes in the pressure exerted by the remote bulb on a bellows which then operates the switch. On temperature rise, the pressure increases and the bellows closes the switch contact to complete the electric circuit. The circuit is interrupted on a reduction in temperature. A permanent magnet is typically part of the mechanism to impart a .snap action to the switch and prevent excessive arcing at the contacts. A typical thermostat will have a set point (range) adjustment and a differential adjustment. Changing the set point adjustment will affect the cut-in and cut-out points an equal amount. The differential adjustment affects the cut-out point only. To adjust a thermostatic switch proceed as follows: 1. Turn differential adjustment to maximum. 2. Turn range adjustment to minimum. 3. Bring the compartment down to the cut-in temperature and turn the range adjustment until the contacts close, then turn backwards until the contacts just open. This fixes the cut-in point. REFRIGERATION SYSTEM COMPONENTS 18-37 4. Lower the compartment to the cut-out temperature, then turn the differential adjustment until the contacts open. This fixes the cut-out point. S o le n o id V a lv e A solenoid is used where there is a requirement to start and stop remotely the flow of refrigerant. Solenoid valves can be operated by a thermostatic switch, a float switch, a high pressure switch, a low pressure switch, or some similar device. The most common use of solenoid valves in reciprocating compressor systems is to control the flow of remgerant to the evaporator using a thermostat. Figure 18-28 shows a cross section of a typical direct acting solenoid valve. When the coil is energized, the magnetic field draws a steel plunger towards the center of the coil, lifting the valve off its seat. When the coil is deenergized, the weight of the plunger, and in some designs a spring, causes it to fall and close the valve. The gravity-closing type must be installed with the plunger upright. I n larger capacities, a pilot-operated type is used to avoid large uneconomical coils. I n this type of valve, the solenoid operates a small pilot valve which then uses refrigerant pressure to open the main valve. A small pressure drop across the valve is required for operation. O il P r e s s u r e F a ilu r e S w itc h This device is a differential pressure switch designed to prevent operation of the compressor in the event of low oil pressure. It senses the pressure 18-38 MARINE REFRIGERATION SYSTEMS difference between the oil pump discharge and the crankcase (suction pressure). The switch stops the compressor motor in the event of low oil pressure. A time delay is incorporated into the control circuit to permit compressor start-up. A time delay is sometimes also incorporated in the cut-out mode. A manual reset is commonly installed. The switch is typically factory adjusted and if it does not operate properly, it must be replaced. Never operate the compressor if the oil pressure switch is inoperative. Correct the malfunction since improper lubrication will cause serious damage to the compressor. W a te r F a ilu r e S w itc h This switch is similar in construction to the low pressure switch. It stops the compressor in the event of loss of condenser cooling water supply. If the switch fails to function, the refrigerant pressure in the condenser increases to the point where the high pressure switch goes into operation. E v a p o r a to r P ressu re R e g u la to r Evaporator pressure regulators are commonly installed at the outlet ofthe higher temperature evaporators in a multibox system. Figure 18-29 shows a typical regulator. Its function is to prevent the evaporator pressure, and therefore its temperature, from falling below a predetermined minimum regardless of the compressor suction pressure. Note that the evaporator pressure regulator limits the minimum pressure, but does not maintain the evaporator pressure constant. The limiting of evaporator pressure and temperature allows the control of box humidity and prevents the dehydration of the stored product while permitting the compressor to satisfy the requirements of the coldest evaporator. S u c tio n P ressu re Figure 18-29. Evaporator pressure regulator. Courtesy Alco Controls and reduce the water flow. Valves on R-12 systems are typically adjusted for about 125 psig (682 kPa). Consult the manufacturer's manual for the proper set points. When the compressor stops, the condenser pressure gradually decreases to the saturated vapor pressure corresponding to the ambient temperature. This decrease in pressure is sufficient to close the regulating valve and stop the flow of cooling water. R e g u la to r The suction pressure regulator or "holdback valve" is sometimes installed to limit the suction pressure at the compressor inlet to a predetermined maximum. This prevents overload of the compressor driver during pulldown or other periods of high evaporator pressure. The valve is similar in construction to the evaporator pressure regulator but senses the downstream rather than the upstream pressure. W a te r -R e g u la tin g V a lv e s Most marine refrigeration systems use seawater-cooled condensers. Water-regulating valves are commonly installed to control the quantity of cooling water circulating through the condenser. Figure 18-30 is a typical water-regulating valve. The valve is actuated by the refrigerant pressure in the condenser. If the condenser pressure should increase, the valve will open, admitting more cooling water, and returning the pressure back to the set point. A decrease in condenser pressure will cause the valve to close Relief Valve Reliefvalves are installed in refrigeration systems to relieve unsafe system pressures. Marine systems will have at least one relief valve installed on the condenser with discharge overboard through a rupture disc. If there is an isolation valve between the condenser and the receiver, there will be a relief valve installed on the receiver with the outlet connected to the condenser. Small systems sometimes have a fusible plug installed in place of a reliefvalve. A fusible plug is simply a pipe plug which has been drilled and filled with a metal alloy designed to melt at a predetermined temperature. The design melting point depends on the pressure-temperature relationship of the system refrigerant. M a n u a l V a lv e s Manual valves are installed at various locations in the refrigeration system to facilitate system operation, to permit cutting units in and out, to isolate Figure 18-30. Water regulating valve. Courtesy Penn Valve components for maintenance, and for other purposes. Two basic types of valves are used, packed and packless. Most ofthe valves used in refrigeration systems today are of the packless type as shown in Figure 18-31. Packless valves employ a diaphragm to isolate the handwheel assembly from the refrigerant to minimize the possibility ofleakage. Packed valves are usually of the backs eating type as shown in Figure 18-32. When in the open position, the valve is backseated to minimize the possibility of leakage. The compressor service valves are usually of the packed type and commonly include a connection for attaching a service gauge. Backseating the valve permits attaching a gauge without loss of refrigerant. To measure the system pressure, the valve must be in an intermediate position between the front and back seats. L iq u id I n d ic a to r s (S ig h t G la s s e s ) Liquid indicators or sight glasses are commonly installed in the liquid line to indicate a proper refrigerant charge. Bubbles appearing in the liquid stream are an indication ofa shortage of refrigerant. Some indicators also include a moisture indicator. A portion of the indicator will change color based on the relative moisture content of the refrigerant. Figure 18-32. Packed valve. Courtesy Henry Valve Co. D e h y d r a to r A dehydrator or drier is usually installed in a bypass line downstream of the receiver. This permits replacement ofthe cartridge without shutdown ofthe system. A typical dehydrator is shown in Figure 18-33. The shell is Figure 18-34. Strainer. Courtesy Henry Valve Co. O P E R A T IO N Figure 18-33. Dehydrator. Courtesy Henry Valve Co. filled with a desiccant such as activated alumina or silica gel which adsorbs moisture and also acts as a filter. Even small amounts of moisture can cause problems such as frozen thermostatic expansion valves, so it is important to remove sufficient moisture to prevent the release of water in the low pressure portions ofthe system. Lower temperatures require lower moisture content. To obtain maximum service life, the dehydrator should only be in service during charging or when high moisture content is suspected. S tr a in e r Strainers are commonly installed in the liquid line before the solenoid valve and expansion valve to remove foreign particles such as scale, metal chips, and dirt. Since the refrigeration system is a closed circuit and most refrigerants are excellent solvents, any solid impurities will be circulated and cause clogging and sticking of the automatic valves. Fine mesh strainers such as are shown in Figure 18-34 will remove these solid impurities. I n addition to mesh strainers, filters with porous pad elements for the removal of very fine particles are available. The strainer element can be removed for cleaning without securing the system by isolating the strainer and then using the hand expansion valve. AND M A IN T E N A N C E S a fe ty P r e c a u tio n s The following precautions are general in nature. They apply to typical reciprocating compressor systems and to R-12, R-22, and other fluorocarbon refrigerants used in such systems. 1. Inspect compressor oil level and check oil pressure periodically. Typical oil level is from one-half to three-quarters up on the sight glass. The typical oil pressure is 45 to 55 psi above the suction pressure. 2. Do not start a compressor without ensuring that shutoff valves between the compressor and the condenser are open. 3. Do not jack or turn the compressor by hand when the power is on. 4. Monitor compressor operation carefully during initial start-up. Check for proper lubrication, liquid floodback, severe vibration, or unusual noise. 5. Do not attempt to add oil to compressor crankcase while compressor is in operation. 6. Do not bypass or jump any protective device because it is operating improperly. Find the trouble and make the necessary corrections. 7. Do not wipe down near moving parts. 8. Open circuit switch in case of electrical fire, and extinguish with C02 (never use water). 9. Be sure power is turned off before working on electrical equipment and circuits. Tag circuit breaker to prevent accidental energizing of circuit. 18-44 MARINE REFRIGERATION OPERATION AND MAINTENANCE SYSTEMS 10. Bleed piping section ofliquid refrigerant prior to opening for repairs. Close the inlet valve, wait until the piping warms indicating liquid refrigerant removal, then close the outlet valve. 11. Do not open any part of the system which is under a vacuum, or air and moisture will be drawn in. 12. Drain the cooling water system to prevent a freeze-up during system shutdown in freezing weather. 13. Expel all air from the system section by admitting a small amount of refrigerant into the section prior to final tightening after inspection or repair. 14. Do not use a torch on a line that has not been bled of refrigerant since R-12 and R-22 decompose into phosgene, a highly toxic gas, when exposed to high temperatures (above 1000"F [538"C]). The area should be well ventilated, and all isolation valves closed securely. 15. Always wear goggles when handling R-12 or R-22. If liquid refrigerant accidentally comes in contact with the eyes, obtain medical help immediately. Do not rub or irritate the eyes, and give the following first aid treatment immediately: (a) Introduce drops of sterile mineral oil into the eyes to irrigate; (b)Wash the eyes with a weak boric acid solution, or a sterile salt solution not to exceed 2 percent sodium chloride if irritation continues at all. 16. Treat as ifthe skin had been frostbitten ifliquid refrigerant comes in contact with the skin. 17. Do not work in a closed area where refrigerant may be leaking unless adequate ventilation is provided. Should a person be overcome due to lack of oxygen and high concentration of refrigerant, treat with artificial respiration. 18. Use care in handling and storing refrigerant cylinders. Don't subject cylinders to high temperatures. Take precautions to prevent mechanical damage. S ta r tin g To start up a system that has been secured, proceed as follows: 1. Check oil level in compressor crankcase. 2. Line up condenser seawater circulating system. Open valves in suction and discharge lines and close condenser water vents and drains. 3. Line up refrigerant system valves for normal system operation. Leave compressor suction stop valve closed. 4. Start seawater circulating through condenser. Vent air from condenser waterheads. 5. Start fans in refrigerated compartments (or pumps in brine or chilled water systems). 6. Check electrical power supply to compressor and solenoid valves. 7. Open compressor suction stop valve one full turn. 18-45 8. Start compressor in Auto mode. 9. Open suction valve slowly to prevent rapid pumping down of low pressure side to avoid oil foaming. If there is evidence of liquid floodback, throttle suction valve. If the compressor develops a knock, secure for five minutes, and restart with suction valve throttled. 10. Observe system operation carefully for five to ten minutes. O p e r a tio n 1. Follow carefully the "Preventive Maintenance Schedule" below, including a regular check of all pressures, temperatures, and crankcase oil level. 2. Adjust seawater flow to maintain proper discharge pressure. 3. Check for an open hand expansion valve and thermostatic valves for proper operation if frosting occurs. Frost on compressor cylinders and crankcase is caused by liquid refrigerant floodback. 4. Bypass the heat interchangers if the compressor discharge gas temperature exceeds 240"F. Otherwise, the unit should be kept in operation. 5. Crack valves and open gradually to avoid liquid floodback ifhand expansion valves must be used to permit inspection or repair. 6. Change over to standby compressor at least weekly during normal plant operation. S e c u r in g To secure a condensing unit, proceed as outlined below: 1. 2. 3. 4. 5. 6. Close main liquid line stop valve. Let compressor run until it cuts out on low pressure switch. Depress compressor motor controller Stop button. Close compressor suction and discharge stop valves. Secure condenser seawater circulating system. Close appropriate refrigerant valves. D e fr o s tin g Hot gas defrosting is performed by pumping hot discharge gas from the compressor directly into the cooling coil circuit to be defrosted. The hot gas is cooled and condensed, and the frost melts off the coils. The resulting liquid refrigerant is expanded and evaporated in a second coil through a hand expansion valve. During defrosting, the plant should be under normal operation. To defrost a coil by hot gas, proceed as follows: 1. Close cut-out valve on outlet side of thermal expansion valve for coil to be defrosted. 2. Allow sufficient time for all liquid refrigerant in coil being defrosted to be pumped out by the compressor. Close coil outlet valve. Open hot gas supply valve for coil to be defrosted. 18-46 MARINE REFRIGERATION 3. Stop compressor. Throttle condenser inlet valve, and open hot gas supply valve connecting compressor discharge to hot gas defrosting line. Throttling of condenser inlet valve should be regulated so as not to unduly prolong defrosting and also not to starve coils not involved in the defrosting. 4. Start compressor and slowly open hand expansion valve between coil being defrosted and coil used for expansion. Adjust hand expansion valve to prevent liquid refrigerant return to compressor. Close cut-out valve on outlet of thermal expansion for coil used for expansion. 5. Stop compressor when coil is defrosted. Close hot gas supply valve, and open condenser inlet valve fully. 6. Restart compressor: 7. Close hand expansion valve when all liquid refrigerant from defrosted coil has been removed and expanded. 8. Open coil outlet valve and both thermal expansion valve cut-out valves. R e fr ig e r a n t C harge The initial refrigerant charge for the system will be given in the system instruction manual. This quantity, however, is an estimate, and the actual quantity must be determined by trial and error. A refrigerant overcharge is indicated by high head pressure. A refrigerant undercharge is indicated by one or more of the following: low head pressure, low receiver level, vapor bubbles in the liquid line sight glass, compressor running continuously, short cycling, and hissing at control valves. Test for leaks if a loss of refrigerant occurs during normal operation. C h a r g in g R e fr ig e r a n t To add refrigerant to an operating system, proceed as follows: 1. 2. 3. 4. 5. 6. 7. 8. OPERATION SYSTEMS Close drier bypass and inlet valves. Check to see that solenoid valves are open. Weigh refrigerant drum. Connect refrigerant drum to charging valve with flexible charging line. Crack drum liquid valve before tightening line to blowout air. If drum is not fitted with separate liquid and vapor valves, tilt drum with valve end down. Close main liquid line valve and pump down system. Open charging valve and carefully open liquid valve on refrigerant drum. Liquid refrigerant will flow into the system. Start compressor. Continue charging until required amount of refrigerant has been charged. Check scale reading and observe liquid level in receiver. Close charging valve, close drier outlet valve, and open drier bypass valve. Open main liquid valve and observe liquid flow through sight glass. Bubbles indicate the need for further charging. If charge is complete, close drum valve, and disconnect charging line and drum. Store empty refrigerant drums for reuse. AND MAINTENANCE 18-47 R e m o v in g R e fr ig e r a n t With all refrigerant proceed as follows: system valves in their normal operating positions, 1. Shut off liquid supply to evaporators. Close drier bypass valve, open drier inlet valve, and check that drier outlet valve is closed. 2. Weigh empty refrigerant drum. Place on deck at an angle slightly above horizontal with drum valves at high end. 3. Connect drum to system drain valve with a flexible line and purge of air. 4. Open drum valve and drain valve with compressor operating automatically. Liquid refrigerant will flow into drum. Weigh drum while filling. Do not overfill. 5. Close drain valve and drum valve when required amount of refrigerant has been removed. Loosen drain line connections, allow any liquid refrigerant in drain line to evaporate, and remove line. Open liquid supply to evaporators. Store refrigerant drum. T e s tin g fo r R e fr ig e r a n t L eaks The most positive method for finding leaks in a refrigerant system is with an electronic or halide leak detector. Testing with oil or soapsuds will only detect larger leaks while the detectors will also locate the smaller ones. Do not attempt a leak test in a compartment where a leak is suspected until it has been well ventilated. Large concentrations of refrigerant in the air will affect the test. When using an electronic leak detector, first adjust the sensitivity according to the manufacturer's instructions. Move the detector probe tip around the suspected areas at about 1 or 2 inches per second. The probe continuously draws in air. If refrigerant is present in the sampled air, the detector will indicate it by a change in sound level, a flashing light, or a meter. The halide leak detector consists of a burner, needle valve, suction tube, and a chimney with a copper reaction plate. To operate, adjust the flame so that the top on the blue flame cone is level with or slightly above the reaction plate. Move the exploring tube slowly along the suspected area. I f refrigerant is present in the sample of air drawn in, it will react with the copper plate and the flame will change color. Small leaks give a greenish tint and large ones a vivid blue. If the system is losing refrigerant and a piping leak cannot be detected, the condenser should be checked for leaks. If the system has been secured for several hours, a small pocket of air may exist in the condenser head. To test, slowly open the head vent valves, one at a time, and insert the test tube of the leak detector. If a leak is indicated, test the tube sheets and 18-48 MARINE REFRIGERATION SYSTEMS tubes after draining water and removing the water heads. If the condenser cannot stand idle, flush and vent the water side of the condenser for at least fifteen minutes. Drain completely. Test for presence of refrigerant in the water heads through the drain or vent openings after closing up the water side for a brief period. E v a c u a tio n OPERATION AND MAINTENANCE 18-49 7. Disconnect refrigerant drum, and release system pressure to atmosphere. Reconnect vacuum line, gauge, and pump to charging valve. Repeat steps 4 and 5. Dehydrate system to 500 microns (69 Pa) instead of 1,500 microns (207 Pa). Close vacuum pump shutoff valve and stop pump. 8. Monitor vacuum gauge for 15 minutes to ensure system maintains vacuum. If system holds a vacuum, repeat step 6. The system is now ready for charging. a n d D e h y d r a tio n After initial installation or following extensive repairs, evacuation and dehydration of the system is required to prevent later troubles. Moisture in the system causes oil sludge and corrosion, and it is likely to freeze up the expansion valves of a low temperature system. Tests and field experience have shown that most troubles with seals and internal valves are caused by moisture in the system. Proper dehydration requires a vacuum pump capable of producing a vacuum of .02 inches Hg absolute (69 Pa) and an electronic vacuum gauge. Ambient temperature must be above 60'F (15.5'C) for proper dehydration. To evacuate and dehydrate a system proceed as follows: 1. Pressure-test the system to be sure it is free of leaks. Drain oil from the compressor crankcase. Replace with new oil after dehydration. 2. Release all refrigerant pressure from the system. Connect a tee to systemcharging valve. Connect vacuum pump to one side of tee and vacuum gauge to the other. Provide shutoff valves in branch lines. 3. Open compressor stop valves and all line stop valves in system. Close all valves and connections to atmosphere. Be sure to open hand expansion valves, coil return valves, and any other line valves which will connect the high and low sides of the system and allow the pump to draw a vacuum on the entire system. Ifnecessary, install a "jumper" line between the high and low sides of the system. 4. Open shutoff valve in vacuum pump suction line, start vacuum pump, and slowly open charging valve. 5. Open shutoff valve in gauge line occasionally and take a reading. Continue dehydrating until vacuum gauge indicates 1,500 microns (207 Pa). The operation will probably take 18 to 72 hours depending on system size and amount of moisture in the system. Failure to achieve a reading of 1,500 microns (207 Pa) may be due to one or more of the following problems: (a) Leak in system or connecting tubing; (b) Closed line valves; (c) Defective vacuum pump; (d) Defective vacuum gauge; and (e) Ambient temperature below 60'F (15.5'C). 6. Close refrigerant charging valve and vacuum gauge and pump valves, and stop the vacuum pump. Disconnect vacuum line from charging valve, and connect a drum of system refrigerant. Raise system pressure to 10 psig (69 Pa). T e s tin g fo r N o n c o n d e n s a b le Gases Air and noncondensable gases, if present in the system, are pumped through the system and discharged by the compressor into the condenser. These gases are trapped in the condenser and cause excessive condensing pressures. In order to check the condenser for the presence of air or noncondensable gases, it is essential that gauges and thermometers be accurate and that the system has sufficient charge so that the liquid refrigerant present in the receiver will seal the liquid line connection. To check for noncondensable gases, proceed as follows: 1. Close liquid line valve and allow system to pump down. 2. Shut off compressor and close suction line valves. 3. Determine the actual condensing temperature. A service gauge should be installed in the compressor discharge connection if a discharge pressure gauge is not already provided. An approximation of the actual condensing temperature will be reached when no further decrease is noted in the discharge pressure. The reduction in pressure can be accelerated on watercooled condensers by permitting circulation of cooling water until discharge pressure is reduced. The thermometer in the liquid line at the receiver provided on most vessels will then indicate the actual condensing temperature. Ifa thermometer is not installed in an air-cooled condenser application, a thermometer should be placed near the condenser to record the ambient temperature at that location. When the pressure and temperature of the air-cooled condenser have lowered to the ambient temperature, a reading of the thermometer will approximate the actual condensing temperature. 4. Record the condensing pressure. Refer to Table 18-2 or a similar table of properties for the system refrigerant, and look up the saturation temperature that corresponds to the condensing pressure. 5. Subtract the temperature recorded in Step 3 from the temperature recorded in Step 4. If the difference between these two temperatures is more than 5'F (2.8'C), it is necessary to purge. P u r g in g N o n c o n d e n s a b le Ifthe above test indicates as follows: the presence Gases of noncondensable gases, proceed 18-50 MARINE REFRIGERATION OPERATION AND MAINTENANCE SYSTEMS R e m o v in g 1. Stop the compressorfor 10 to 15 minutes. Leave all valves in their normal position. 2. Open purge valve on top of condenser, and slowlyrelease gases. 3. Since it is difficultto tell if excessiverefrigerant is being purged with the noncondensables,purge slowlyand check the condenser continually for the presence of noncondensablegases as explained above to minimize refrigerant loss whilepurging. C om p ressor 18-51 O il >roceed as follows to remove oil from a compressor crankcase: 1. Closeliquid line valve and pump down the system. 2. Loosencrankcase drain plug. Sincecrankcase is under light pressure, do not fully remove drain plug. Allowrequired amount of oil to be drained to seep slowlyaround the threads ofloosened plug. 3. Retighten drain plug. 4. Restart system. O il L e v e l A certain amount of compressor oil will always circulate through the system because oil is miscible in refrigerant. To allow for oil circulation, systems requiring a large refrigerant charge will need the addition of oil in excess ofthe normal compressor crankcase oil charge. When the system is first placed in operation, closely observe the oil level in the crankcase. Add oil whenever the oil level drops below normal (halfway up on the bull's-eye sight glass). Allow sufficient time for the system to balance after adding oil, since some of the circulating oil may return to the compressor. Then, again check oil level. Generally, the addition of one quart of oil for every fifty pounds of refrigerant charge will be an adequate allowance for oil circulation. M te r adding oil, if the oil level in the crankcase still falls below normal, oil is not returning to the compressor. The oil is probably being trapped in the cooling coils by an improperly adjusted thermal expansion valve. Mter the compressor has been stopped for several minutes, the oil level in the compressor crankcase should be about halfway up on the bull's-eye sight glass. During operation, the oil level will be slightly lower but will appear higher when oil is foaming. Check the oil level hourly. Add or remove oil to bring the level in the crankcase to the middle ofthe sight glass during steady operating conditions. A d d in g O il The method of adding oil given below, if properly followed, will prevent air and moisture from entering the system. Since refrigerant gas is heavier than air, and the crankcase is loaded with this gas, the position of the oil-charging hole is located to prevent the admission of air. Only clean oil from sealed containers should be used. Check the manufacturer's manual for the proper type of oil for the system. 1. Closethe liquidline valve and pump down the system. 2. Removeoil fillerplug slowly. 3. Add oil to centerofbull's-eye sight glass using a clean, well-driedfunnel or an oil pump and suitable connector. 4. Replaceoil fillerplug tightly. 5. Restart system. O il P r e s s u r e Correct oil pressure will insure adequate compressor lubrication and satisfactory operation of the compressor capacity control system. Compressors with a forced lubrication system are typically designed to operate with a normal oil pressure of 45 to 55 psi (310 to 379 kPa) above suction pressure. For example, if the compressor suction gauge reading is 40 psig (276 kPa), the oil pressure gauge reading should be 85 to 95 psig (586 to 655 kPa). During start-up, observe the oil pressure gauge to be sure that oil pressure develops during the first few minutes. Always check oil pressure when starting. The oil pressure should be normal after steady operating conditions have been reached, and oil has stopped foaming. Oil foaming may last fifteen minutes or longer. Do not allow compressor to run longer than one minute if oil pressure of at least 15 psig 003 kPa) over suction pressure does not develop. Reasons for low oil pressure include the following: 1. Insufficient oil in crankcase. 2. 3. 4. 5. 6. Oil pressure regulator not seating properly. Oil filter screen in bottom of crankcase cloggedwith dirt. Oil pump worn or defective or rotating in wrong direction. Faulty oil piping. Rapid pulldown of suction pressure on start-up causing excessiveoil foammg. C om p ressor O verhaul a n d R e p a ir Consult the manufacturer's manual for detailed disassembly and reassembly procedures. The following are general procedures to be followed in any compressor overhaul: 1. Be sure that faulty operation of the plant is not caused by trouble in some other part of the system before dismantling a compressor. 2. Dismantle only the part ofthe compressor necessary to correct the fault. 3. Sweep clean the deck in the vicinity ofthe compressor prior to any dismantling. Removefrom the area any spare parts, fittings, or tools not to be used. 18-52 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. MARINE REFRIGERATION SYSTEMS Obtain clean buckets or boxes in which to place disassembled components. Have on hand a supply of clean rags, as lint free as possible. Spread clean canvas or heavy paper on the deck to layout the larger parts (cylinder heads, crankshaft, etc.). Maintain cleanliness during the overhaul. Clean all parts with an approved solvent after disassembly. Use a stiff brush to remove dirt from grooves and crevices. Do not use carbon tetrachloride for cleaning polished steel parts because it removes the oil film, and the steel may rust. Coat all moving parts with compressor oil before reassembly. Dip dismantled parts to be left overnight in clean compressor oil, and wrap them in oil-soaked rags to prevent rusting. Use special tools furnished by the compressor manufacturer for the particular operation involved to dismantle or reassemble a compressor. Avoid damage to gaskets or gasket-seating surfaces when disassembling the compressor. These gaskets or new gaskets of identical thickness and material must be employed when reassembling. The use of discharge valve plate and compressor cylinder gaskets of proper thickness is particularly important since the thickness of these gaskets determines the clearance between the top of the pistons and discharge valve plate. Mark carefully when disassembling compressor parts, so that each part removed will be replaced in its original position when reassembling. Avoid filing, scraping, and grinding wherever possible when making compressor repairs or adjustments because of the danger of introducing emery or metal particles into the compressor. Place pistons upon reassembly on the proper rods facing the same direction as originally. Make certain that the oil dipper on lower connecting rod bearing of splash lubricated compressors is in correct position for dipping up oil when machine is in operation. Stagger the position of the ends of the piston rings so that all joints do not come on one side of the piston. Clean compressor crankcase and provide fresh charge of proper oil. A n a ly s is of Faulty C o m p r e s s o r V a lv e s Before opening a compressor for valve inspection or replacement, it should be definitely determined that the faulty operation of the system is due to the improper functioning of the valves and not to some other problem. Before assuming that compressor valves must be serviced, carefully check the system for all other possible causes of faulty operation. Faulty compressor valves may be indicated by either a gradual or a sudden decrease in the normal compressor capacity. Either the compressor will fail to pump or the suction pressure cannot be pumped down to the designed value. This will cause the compressor to run for abnormally long intervals or even continuously. Short shutdown periods may indicate leaky compressor valves provided the faulty operation is not due to some other fault in the system. 18-53 OPERATION AND MAINTENANCE T e s tin g C o m p r e s s o r The compressor discharge D is c h a r g e V a lv e s valve may be checked for leakage as follows: 1. Close liquid line valve and pump down system. 2. Stop compressor, and quickly close suction and discharge line valves. 3. If discharge pressure drops at a rate in excess of 3 psi per minute and crankcase (suction) pressure rises, there is evidence of discharge valve leakage. It may be necessary to pump down several times to remove refrigerant mixed with crankcase oil in order to obtain a true test. If the discharge valves are found to be defective in any way, it is advisable to replace the entire valve assembly with a spare. I f valve operation is faulty, chances are the discharge plate requires relapping. This process generally requires highly specialized machinery to produce a satisfactory surface and should not be attempted aboard ship except in an emergency. T e s tin g C o m p r e s s o r The compressor I' S u c tio n V a lv e s suction valves may be checked for leakage as follows: 1. Start the compressor under manual control. 2. Close the suction line stop valve gradually, exercising care to prevent violent oil foaming. 3. With the suction line stop valve finally closed, if a vacuum of approximately 20 inches Hg (34 kPa abs.) can be readily pumped, the suction valves may be considered satisfactory. Do not expect the vacuum to be maintained after the compressor stops due to the release of refrigerant from the oil. New valve assemblies may require a break-in period of several days before being checked. I f the test indicates a possible problem, the compressor should be pumped down, opened, and the valves inspected. Defective valves should be replaced with spare assemblies. Be sure all small pieces of a broken valve are accounted for. I f any pieces are not removed, the compressor may be damaged when put back in operation. Before installing a new suction valve assembly, the piston should be checked for damage. I f marred, the piston must be replaced as well as the suction valve. A lig n m e n t of C om p ressor C o u p lin g Couplings on direct drive units should be checked for proper alignment after repair or replacement. Both parallel and angular misalignment should be checked. Parallel misalignment in direct drive units is shown in Figure 18-35 and angular misalignment is shown in Figure 18-36. Angular misalignment can be checked with a feeler gauge or by clamping a dial 18-54 MARINE REFRIGERATION SYSTEMS OPERATION AND MAINTENANCE 18-55 Figure 18-35. Parallel misalignment in a direct drive unit Figure 18-37. Parallel misalignment in a belt drive unit Figure 18-38. Angular misalignment in a belt drive unit Figure 18-36. Angular misalignment in a direct drive unit indicator to the motor flange and adjusting it so the stem will contact the inside face of the compressor flange. Rotate the motor flange through 360 and record readings at 90 intervals. To check parallel misalignment, use a steel rule held against the coupling periphery, or a dial indicator clamped to the motor coupling with the stem adjusted to run on the compressor coupling periphery. Shim motor feet and shift motor position as necessary to correct the misalignments. Tighten all hold-down bolts and recheck. 0 0 B e lt D r iv e A d ju s tm e n t The belt drive must be aligned so that there is no angular or parallel misalignment. Both alignments can be checked with a straightedge or string. Parallel misalignment in belt drive units is shown in Figure 18-37, and angular misalignment is shown in Figure 18-38. Correct parallel misalignment by sliding the motor pulley on its shaft. Correct angular misalignment by loosening motor hold-down bolts and turning the motor frame. Check belt tension by depressing a single belt at the center of the span with one finger. A heavier belt with a 24-inch (61 cm) span should deflect 1;2 to 3/4 inch (1,14 to 2 cm). Lighter belts or longer spans should deflect proportionally more. Belts should always be replaced in sets, not singly. P r e v e n tiv e M a in te n a n c e S c h e d u le Mechanical equipment can best be kept in good repair and operating at top efficiency by strict adherence to a planned maintenance schedule. A systematic check of operation and condition of refrigeration systems should be maintained. A recommended preventive maintenance schedule is outlined below. This schedule is intended only as a guide. It is not intended to replace the recommendation of the equipment manufacturer, and may be altered to suit operating and maintenance conditions peculiar to the individual installation. 18-56 MARINE REFRIGERATION SYSTEMS H o u r ly 1. Enter all operating temperatures and pressures in a log. The log should contain columns for recording: (a) the time the check is made; (b) machinery room ambient temperature; (c) refrigerant suction pressure and temperature; (d) refrigerant discharge pressure and temperature; (e) oil pressure; (f) bull's-eye oil level (i.e., low, normal, high); (g) crankcase temperature (i.e., warm, cold, normal); (h) compressor noise (i.e., normal, knock, or other); (i) condenser seawater supply pressure; (j).condenser seawater entering and leaving temperatures; (k) liquid refrigerant temperature; (1) liquid refrigerant condition at sight glass (i.e., normal or vapor bubbles); and (m) temperature of refrigerated compartments. 2. Check motors for overheating. Get accustomed to the motor temperature by the way it feels to the hands. D a ily Review hourly operating log. Note any significant changes in plant performance and take corrective action, if necessary. W e e k ly 1. 2. 3. 4. Test refrigerant system for leaks. Check for noncondensable gases in condenser. Purge, if necessary. Check operation of all solenoid valves. Check glands on circulating pumps. TROUBLESHOOTING 3. Clean refrigerant liquid line strainers. 4. Check alignment of compressor and motor. 5. Check tightness of all bolts on equipment. A n n u a lly 1. Check calibration of all instrumentation. 2. Check onboard repair parts inventory and order parts as needed. T R O U B L E S H O O T IN G THE 1. Lubricate motor bearings, if necessary. 2. Blow dust out of motors and check lint screen. 3. Check contact points in motor controllers and control switches. Clean as required. 4. Check operation and settings of operating and safety control switches. Adjust as required. 5. Clean seawater strainers. 6. Check condenser zincs. Clean or replace as required. 7. If a brine is used, check brine density. H ig h C o m p a r tm e n t Controls not functioning Coils frosted Insufficient airflow through fan coil unit Infiltration of warm air Recent loading of warm product H ig h C o m p r e s s o r Q u a r te r ly P o s s ib le C a u s e 1. Clean c.ondenser water side. 2. Check all motors and starters. SYSTEM S The troubleshooting charts which follow are intended as a guide for locating malfunctions in a typical marine refrigeration or air-conditioning system with a reciprocating compressor. It is not intended to replace the detailed information provided by the manufacturer of a particular installation. When a system is not operating properly, first determine the symptoms, then consult the chart for the possible causes and the recommended actions to determine the actual reason for the problem. Once a problem is uncovered, it should be corrected using the procedures outlined by the manufacturer or by using the general procedures in this chapter. It is usually necessary to use a process of elimination in locating the trouble. Try the easier possibilities first, then move on to the more difficult ones. The order in which the symptoms and possible causes are presented does not necessarily indicate a suggested troubleshooting sequence. T e m p e r a tu r e P o s s ib le C a u s e M o n th ly 18-57 THE SYSTEMS Air in system A c tio n Check thermostat, solenoid valve, electrical circuits, and fuses. Check expansion valve and evaporator pressure adjustment and operation. Check and defrost, if necessary. Check fan operation. Check filters. Check for obstructions. Check for door ajar, poor door gaskets, or excessive traffic. Normal pulldown. Start second condensing unit, if required. D is c h a r g e P ressu re A c tio n Check for noncondensables. Purge, if present. 18-58 MARINE REFRIGERATION Insufficient cooling water flow Dirty condenser or corroded shell baffles Refrigerant overcharge (condenser tube partially covered) Compressor discharge stop valve partially closed TROUBLESHOOTING SYSTEMS Check water supply pressure. Check water regulating valve setting and operation. Check for obstruction or plugged strainer. Clean or replace as necessary. Check refrigerant level. Remove any excess. Check. Open fully. P o s s ib le C a u s e Liquid refrigerant flooding back Compressor suction stop valve partially closed Leaking compressor valves Worn piston rings P o s s ib le C a u s e Leaking compressor suction valves Compressor unloading at too high pressure Too much oil in circulation Clogged liquid line strainer Excessive suction temperature Leaking compressor valves Pump down, remove cylinder heads, and check. Pump down, disassemble, and check. P o s s ib le C a u s e Low voltage overload tripped Safety control switch open A c tio n Check expansion valve superheat setting and operation. Pump down, remove cylinder heads, and check. Check setting and operation of capacity control system. P o s s ib le C a u s e Restricted flow of refrigerant Compressor not unloading Low pressure cut-out not stopping compressor A c tio n Check refrigerant charge. Add as required. Check expansion valve superheat setting and operation. Check solenoid valve. Check strainers for clogging. Check setting and operation of capacity control system. Check setting and operation of low pressure cut-out. A c tio n Pump down and clean. Check expansion valve superheat setting and operation. Bypass heat in terchanger. Pump down, remove cylinder heads, and check. C o m p r e s s o r W ill N o t S ta r t Thermostat set too high Solenoid valve closed Lack of refrigerant L o w C o m p r e s s o r S u c tio n P r e s s u r e Low refrigerant charge A c tio n Check for open hand expansion valve. Check expansion valve superheat setting and operation. Check for proper oil level. H ig h C r a n k c a s e T e m p e r a tu r e No power to compressor H ig h C o m p r e s s o r S u c tio n P r e s s u r e Overfeeding refrigerant P o s s ib le C a u s e Liquid refrigerant flooding back A c tio n Check water regulating valve setting and operation. Check expansion valve superheat setting and operation. Check for open or leaking hand expansion valve. Check. Open fully. 18-59 C o ld (S w e a tin g o r F r o s tin g ) C o m p r e s s o r C r a n k c a s e P o s s ib le C a u s e L o w C o m p r e s s o r D is c h a r g e P r e s s u r e Excessive cooling water flow THE SYSTEMS A c tio n Check breaker, main switch, fuses, and wiring. Reset. Check and eliminate cause of low voltage. Check high pressure switch, water failure switch, and oil failure switch. Find and eliminate cause of switch operation. Check switch setting and operation. Reset. Turn on power to solenoid. Check valve for proper operation. Check for proper charge. If low, repair any leaks and recharge. C o m p r e s s o r R u n s C o n tin u o u s ly P o s s ib le C a u s e Low refrigerant charge Solenoid valve leaking Compressor valves leaking Worn piston rings and/or cylinder liner A c tio n Check for proper charge. Iflow, repair any leaks and recharge. Check solenoid for proper operation. Pump down, remove cylinder heads, and check. Pump down, disassemble, and then check 18-60 MARINE REFRIGERATION C om p ressor SYSTEMS TROUBLESHOOTING S h o r t-C y c le s o n H ig h P r e s s u r e P o s s ib le C a u s e P o s s ib le C a u s e Loose mounting bolts Compressor drive loose, worn, or impr..Qperlyaligned Check and reset. C o m p r e s s o r S h o r t-C y c le s o n L o w P r e s s u r e P o s s ib le C a l(s e S w itc h A c tio n See "Low Compressor Suction Pressure" above for possible causes and action. Reduced evaporator capacity Reliefvalve leaking slightly Internal leak in heat interchanger P o s s ib le C a u s e A c tio n Check superheat setting and operation of expansion valve. Pump down, disassemble, and check. O il N o t R e tu r n in g to C r a n k c a se A c tio n P o s s ib le C a u s e Expansion valve not flooding evaporator (oil trapped in coils) Oil return check valve stuck Discharge stop valve rattling Wear of compressor bearings, pistons rods, etc. L o u d H is s in g L o s s o f O il fr o m C o m p r e s s o r C r a n k c a s e Refrigerant flooding back to compressor Worn piston rings or cylinder liners Excess oil in circulation Liquid refrigerant flooding back Check for frosted coils. Check for proper operation of cooling coil fan. Check for obstructions to airflow. Test valve. Test for leaks. Adjust expansion valve. Pump down. Check valve and oil return passage for plugging and valve for proper operation. Insufficient refrigerant charge Obstruction in liquid line Insufficient oil in crankcase Faulty oil gauge Oil filter clogged Defective oil pump or improper rotation Defective oil relief valve Worn bearings Check oil level. Check gauge. Pump down and clean. Check direction of pump rotation. Disassemble pump and check for wear. Test valve. Pump down, disassemble, and check. N o is e in P ip in g T h e r m o E x p a n s io n A c tio n Check refrigerant charge. Check for plugged strainer. Check for partially closed valve. Check for other liquid line obstruction. V a lv e -H ig h P o s s ib le C a u s e Superheat adjustment too high Power a~ semblY failure Valve orIfice plugged Low inlet pressure A c tio n A c tio n Check bolts for proper tightness. Check flexible coupling and coupling flanges on direct drive units for looseness, wear, or misalignment. Check belts, pulleys, and flywheel on beltdrive units for looseness, wear, or misalignment. Check for high oil level. Check for refrigerant floodback. Check for open hand expansion valve. Check expansion valve superheat setting and operation. Backseat valve fully. Pump down, disassemble, and check compressor for wear. P o s s ib le C a u s e L o w O il P r e s s u r e P o s s ib le C a u s e 18-61 C o m p r e s s o r N o is y S w itc h A c tio n High pressure switch setting too low See "High Compressor Discharge Pressure" above for possible causes and action. THE SYSTEMS External equalizing line plugged T h e r m o E x p a n s io n P o s s ib le C a u s e Superheat adjustment too low Remote bulb not sensing evaporator outlet temperature correctly Superheat A c tio n Check superheat setting. Check operation of power assembly. Check for plugging. Wax or oil indicates improper oil type. Ice or dirt indicates need to cut in filter-drier. Check for proper head pressure to allow valve to pass rated flow. Check line for plugging. V a lv e -L o w Superheat A c tio n Check valve superheat setting. Check for proper clamping of remote bulb to suction line. Insulate if exposed to high ambient temperature. MARINE REFRIGERATION 18-62 Check for loss of charge by checking valve response to different bulb temperatures. Check line for plugging. Apply hot rags to valve to melt ice. If valve operation is restored, cut in filter-drier to remove moisture. Check for tight shutoff. Check for worn parts. Loss of charge from remote bulb/power assembly External equalizing line plugged Valve frozen open Valve pin and seat worn Valve improperly assembled Check that valve assembly is set up according to manufacturer's instructions. S o le n o id P o s s ib le V a lv e C o il B u r n o u t C ause A c tio n Electrical power Ambient conditions S o le n o id P o s s ib le V a lv e W ill N o t O p e n A c tio n C ause Check voltage and current at valve. If none or improper, check wiring, fuses, and breaker. Check thermostat setting and contacts. Check coil continuity and resistance. See "Solenoid Valve Coil Burnout" below for possible cause. Check for proper assembly. Check system differential against rated maximum (valve closed) and minimum (valve open). Check for corroded parts, foreign matter in valve, distorted enclosing tube or valve body, or oil trapped above piston. Electrical power Coil Burnout Valve improperly assembled Improper pressure differential Plunger movement restricted S o le n o id P o s s ib le V a lv e W ill N o t C lo s e C ause Plunger movement restricted Electrical power Manual valve operator in open position S o le n o id P o s s ib le A c tio n Check for corroded parts, foreign matter in valve, or distorted enclosing tube or valve body. Check for voltage at coil. Voltage indicates contact not opening or power coming from another source. Check wiring. Check position of manual operatorshould be backed out to allow valve to close. V a lv e C lo s e s b u t F lo w C o n tin u e s C ause Foreign material lodged under seat Seat chipped, broken, or deformed 18-63 ACKNOWLEDGMENTS SYSTEMS A c tio n Check all internal parts. Check seat. Valve remains closed with electrical power supplied Check for low or high voltage. Check for improper wiring resulting in voltage drop preventing valve opening. Check for high ambient temperature. Check for coil exposure to moisture or high humidity. Check for handling of fluids at temperature higher than valve rating. Check for reason valve fails to open. Check for corroded parts, foreign matter, distorted parts, or excessive differential pressure. REFERENCES Dossat, R. J. 1978. P r i n c i p l e s and Sons. o f R e fr ig e r a tio n , 2d ed. New York: John Wiley a n d P r o d u c t D i r e c t o r y . 4 vols. American Society of Heating, Refrigerating, and Air Conditioning Engineers. Atlanta, Ga. N a v a l S h i p s T e c h n i c a l M a n u a l . September 1967. Refrigerating systems, chap. 9590. In: Navships 0901-590-0002. Washington, D.C.: Naval Sea Systems Command. Stoecker, W. F., and Jones, J. W 1982. R e f r i g e r a t i o n a n d A i r C o n d i t i o n i n g . 2d ed. New York: McGraw-Hill. H andbook ACKNOW LEDGM ENTS The author is indebted to the following companies engaged in the manufacture of refrigeration systems and equipment who so kindly contributed photographs, drawings, and other material used in the preparation of this chapter: Carrier Corporation, Syracuse, New York The Trane Company, LaCrosse, Wisconsin Thermo King Corporation, Minneapolis, Minnesota Ako Controls Division, Emerson Electric Co., St. Louis, Missouri Henry Valve Company, Melrose Park, Illinois Detroit Switch, Inc., Pittsburgh, Pennsylvania CHAPTER H e a t in g , V e n t ila t io n , 19 a n d A ir - C o n d it io n in g JAMES A. HARBACH D E F IN IT IO N S AND P R IN C IP L E S Hv AC (heating, ventilation, and air-conditioning) functions on modern vessels are performed by complex systems which satisfy the requirements of controlling the temperature, humidity, and purity of the air in the various spaces on the vessel. In the past, the heating, ventilation, and air-conditioning functions were treated separately, while today integrated HVAC systems handle all requirements. In this chapter, the various systems and components in common use today will be described. Emphasis will be on operation and maintenance of existing systems rather than the design of new systems. Readers interested in system design should consult the list of references at the end of the chapter. A ir -C o n d itio n in g Air-conditioning is defined by ASHRAE (American Society of Heating, Refrigerating and Air Conditioning Engineers) as the process of treating air to control simultaneously its temperature, humidity, cleanliness, and distribution to meet the comfort requirements of the occupants of the conditioned space. Note that this definition includes not only cooling and dehumidification during warm weather, but the heating operation during cold weather, as well as the regulation of air velocity and removal offoreign particles. The system commonly referred to as the air-conditioning system is actually only part of the overall system and would more properly be called the cooling system. V e n tila tio n Ventilation may be defined as the process of supplying outside air to and exhausting air from an enclosed space. This is done to prevent the accumulation of dangerous or annoying gases, to reduce the humidity of spaces 19-1 19-2 HEATING, VENTILATION, DEFINITIONS AND AIR-CONDITIONING containing sources of moisture, or to reduce the temperature of spaces not mechanically cooledsuch as the engine room. Ventilation requirements for cargo spaces and living spaces are normally based on a given number of air changes per hour. In public areas such as dining saloons and messrooms, ventilation is normally based on a given CFM (cubic feet per minute) per occupant at maximum capacity. Engine room ventilation is normally sized to maintain the temperature at 10°F to 15°F (5°C to 8°C) above ambient. H e a tin g Heating may be defined as the process of adding heat to an enclosed space to maintain its temperature above ambient. The heat added makes up for losses due to heat transfer to the outside, and for losses due to ventilation or infiltration of outside air. " H e a t T r a n sfe r Refer to Chapter 18, "Marine Refrigeration Systems," for the definition of various terms related to heat transfer. These include convection, conduction, radiation, latent heat, sensible heat, and saturation temperature and pressure. P sy c h r o m e tr y a n d th e P s y c h r o m e tr ic C hart Psychrometry is the study of the properties of mixtures of air and water vapor. This subject is important to air-conditioning because the systems handle air-water vapor mixtures, not dry air. Some air-conditioning processes involve the removal of water from the air-water vapor mixture (dehumidification) while some involve the addition of water (humidification). A convenient way to represent the properties of air-water vapor mixtures is the psychrometric chart, Figure 19-1. On the chart, such properties as dry bulb temperature, wet bulb temperature, dew point, relative humidity, humidity ratio, specific volume, and enthalpy are presented in graphical form. D B (D r y B u lb T e m p e r a tu r e ) The temperature ofthe air as sensed by a standard thermometer is called the dry bulb temperature. W B (W e t B u lb T e m p e r a tu r e ) The temperature sensed by a thermometer whose bulb is wrapped in a water-soaked wick and moved rapidly in air is called the wet bulb temperature. A sling psychrometer is an inexpensive device containing a standard and a wet bulb thermometer mounted on a pivoting handle. Swinging the device rapidly produces the required air movement and allows determination of the wet and dry bulb temperatures. f AND PRINCIPLES 19-3 19-4 HEATING, VENTILATION, AND AIR-CONDITIONING HVAC SYSTEMS 19-5 DP (Dew Point Temperature) Dew point temperature is the temperature at which water vapor will begin to condense if the air were cooled at constant pressure. At the dew point, the air is saturated. RH (Relative Humidity) Relative humidity is the ratio of the actual water vapor pressure to the water vapor pressure that would exist ifthe air were saturated at the same dry bulb temperature. It is expressed in percent. W (Humidity Ratio) Humidity ratio is the ratio of the weight of water vapor in the air to the weight of dry air. It is expressed in pound/pound dry air or grains/pound of dry air (7,000 grains = 1 pound), or kilogram/kilogram of dry air. V (Specific Volume) Specific volume is the volume occupied by one pound of air. The units are cubic feet/pound or cubic meters/kilogram. h (Enthalpy) Enthalpy is the thermal energy content of the air expressed in BTU/pound or kilojoule/kilogram. Reading the Psychrometric Chart In order to locate any condition of air on the chart, two independent properties must be determined. The air condition can then be plotted on the chart, and all other properties can be read from the chart. Dry bulb temperatures are read along the horizontal scale at the bottom ofthe chart, humidity ratio is read along the right-hand vertical scale, and the wet bulb temperature, dew point temperature, and enthalpy are read along the diagonal scale at the upper left. Lines of constant relative humidity and specific volume are labeled in the body of the chart. Refer to Figure 19-2 for assistance in plotting and reading air conditions on the psychrometric chart. For example, sling psychrometer measurements are taken as 78° DB and 65° WB. Determine the relative humidity and dew point. (Answer: RH = 50 percent, DP = 58°F). HV AC SYSTEMS Heating and Cooling Loads One of the first steps in the design of an HVAC system is the calculation of the heating and cooling loads. While the detailed coverage of these Figure 19-2. Reading the psychrometric chart calculations is not within the scope of this book, a basic understanding of the various factors which affect the heating and cooling loads will be helpful to the operator of these systems. The references listed at the end of the chapter provide further details. The purpose of the HVAC system is to maintain a safe and comfortable internal environment despite variable outside conditions. The system must have sufficient capacity in terms of heating and cooling to meet the extremes of outside conditions which are anticipated. Some of the factors which can affect the cooling and heating loads are (1) design outside air conditions; (2) design inside air conditions; (3) materials and insulation in bulkheads, overheads, and decks; (4) infiltration and ventilation air; (5) lights, equipment, and occupants inside space (cooling only); and (6) solar gain (cooling only). The selection of design outside air conditions depends to some extent on the anticipated routes the ship will travel. Since it is difficult to predict the routes a ship will travel over its entire service life, it is common to base system design on 90° to 95° DB (32° to 35°C), 78° to 82° WB (2W to 28°C) cooling and 0°to 10°DB (-18° to-12°C)for heating. Inside design conditions are typically 6SO to 75° DB (20° to 24°C) for winter and 75° to 80° DB (24° to 2TC) for summer, with relative humidity varying from 20 to 75 percent. Figure 19-3 shows a recommended comfort zone. It is somewhat larger 19-7 HV AC SYSTEMS TABLE V e n tila tio n 1 9 -1 R e q u ir e m e n ts Space C F M per person Staterooms Dining rooms Toilet rooms Offices Kitchens 10-15 15-20 20-25 20-40 20-40 other machinery spaces where ventilation is used for heat removal, the ventilation rate is based on a given temperature rise in the space, typically 10°F to 20°F (6°C to 12°C), or an air change time of 1 to 2 minutes, whichever is greater. Mechanical exhaust should be 110 to 120 percent of the supply to compensate for expansion ofthe air and to create an indraft to prevent the escape of heat into conditioned spaces. Cargo space ventilation is normally based on two to three air changes per hour. Consideration is given to control of humidity in the holds to avoid condensation damage and to remove C02 and other gases given off by some cargoes. Figure 19-3. Design inside air conditions than the ASHRAE comfort zone, permitting lower winter and higher summer temperatures, in consideration of energy conservation criteria. The construction, and particularly the insulation used, of the bulkheads, decks, and overheads will affect heat transmission through them. Use ofadequate insulation can significantly reduce the heating and cooling loads. The introduction of outside air to the conditioned space by ventilation and infiltration will increase loads. While ventilation is required in many applications, excessive ventilation should be avoided. Infiltration can be minimized by proper weather stripping. In cooling load calculations, the additional load imposed by lighting, cooking facilities, various operating equipment, the occupants, and solar heat gain must also be included. V e n tila tio n H VA C S y ste m T ypes In order to maintain the desired temperature and humidity in the conditioned spaces, the HVAC system must add or remove thermal energy to or from the space. The heat transfer is accomplished typically by air or water. Some heating systems may use steam space heaters, and some cooling systems may use refrigerants directly. Air systems in common use are (1) terminal reheat, (2) dual duct, and (3) variable air volume. Water systems in common use are (1) one pipe, (2) two pipe, and (3) four pipe. These systems are designed to serve areas with more than one conditioned space or zone and where independent control of space air conditions is required. A single zone air system will be described first to illustrate the basic principles. C r ite r ia The introduction of outside air is required in some instances to prevent the accumulation of certain gases and contaminants to unhealthy levels. In densely occupied spaces, the buildup of carbon dioxide (C02) is a problem. Table 19-1 lists some typical criteria for ventilation based on CFM per occupant. In staterooms and other sparsely occupied spaces, infiltration is normally sufficient to maintain air quality, and total air supply rates are normally based on a number of air changes per hour. Use of filtered recirculated air in these instances can significantly reduce heating and cooling loads. In those applications such as the engine room and certain S in g le Z o n e S y s te m Figure 19-4 shows a single zone air system with provisions for heating, cooling, humidification, dehumidification, and the introduction of outside ventilation air. The supply fan delivers conditioned air to the space. An exhaust fan is commonly installed to avoid excessive positive air pressure in the space. The dampers allow the control of ventilation and recirculation air quantities. Air control can be manual or automatic based on outside air temperature. Automatic control can be used to increase ventilation air when the outside air temperature is somewhat lower than the conditioned HVAC SYSTEMS 19-9 cooling coil is maintained at a constant temperature, typically 55'F (13'C). The thermostat in each zone controls the reheat coil associated with that zone to maintain proper zone temperature. The reheat coil may be hot water, steam, or electric. The terminal reheat system provides good control oftemperature and humidity over a wide range ofloads, but is wasteful of energy since air is cooled, then heated. This can be reduced by raising the cool air temperature until the lightest loaded reheat coil turns off, or to use waste heat to operate the reheat coils. During cold weather, the reheat coils supply the heating requirements. D u a l D u c t S y ste m Figure 19-4. Single zone system space, thus reducing the cooling coil load. At cold or hot outside air temperatures, the ventilation air will automatically be set at the minimum. The cooling coil also performs the dehumidification function. If the coil temperature is below the air dew point, water vapor will condense and air humidity will be lowered. The humidifier admits steam directly into the air stream, increasing humidity with little increase in dry bulb temperature. The control system senses space temperature and humidity, then controls the cooling and heating coils and the humidifier. I n cold weather, heating and humidification will be required. I n hot weather, cooling and dehumidification are needed. The space humidity during cooling can be controlled in two ways: (1) varying the cooling coil temperature, or (2) maintaining a constant low cooling coil temperature and then using the heating coil to reheat the air. The second method wastes energy, and is normally only used on systems which require good humidity control or have high latent heat loads. M u ltip le Z o n e S y ste m s It is usually not economically feasible to provide a separate system for each zone. A zone is a room or group of rooms controlled by a single thermostat. The basic principles of temperature and humidity control outlined for the single zone system are expanded in the systems below to meet the requirements of multiple zone operation. T e r m in a l R e h e a t S y ste m Figure 19-5 is a diagram of a terminal reheat system. The fan delivers a fixed quantity of air, and the duct system distributes the air to the various conditioned spaces. During warm weather operation, the air leaving the Figure 19-6 is a diagram of a dual duct system. The constant quantity of air from the supply fan is split into two streams. Part of the air flows across the cooling coil and part across the heating coil. Each zone thermostat controls a mixing box which proportions the warm and cool air delivered to the zone to maintain the desired zone temperature. The dual duct system is responsive to load changes and can accommodate heating in some areas and cooling in others. Since two supply ducts are required, the system is commonly designed as a high velocity air system. However, this increases fan power requirements. As with the terminal reheat system, there are periods of simultaneous heating and cooling. During periods oflow (below 55'F, 13'C) outside air temperature, the cooling coil can be secured. During hot weather, the warm duct temperature can be lowered or perhaps the heating coil secured. Maintaining warm duct temperatures higher than that required to maintain comfort in all zones is a waste of energy. Some marine systems are arranged with the cooling coil and a reheater in series. While less efficient, this arrangement provides better dehumidification and reduces condensation problems. Figure 19-6. Dual duct system V a r ia b le A ir V o lu m e S y s te m s The terminal reheat and dual duct systems described above waste energy, especially at light cooling or heating loads. VAV (variable air volume) systems reduce this waste. While there are a number of different VAV systems, all utilize dampers in the supply duct to each zone to vary the air delivered to the zone. Figure 19-7 shows a simple VAV cooling system. The problem with this simple system is that, at low loads, the air supply is reduced to the point where poor air distribution and ventilation result. The solution to this problem is to combine the VAV technique with a terminal reheat or dual duct system. In a VAV reheat system, as zone load decreases, the damper will reduce air flow. Once a minimum flow (perhaps 25 to 30 percent of maximum) is reached, the reheat coil is energized, and air flow remains constant at the minimum. The VAV reheat system does incur some energy loss, but it is small compared to the constant volume terminal reheat system. It also overcomes the air distribution and ventilation problems ofthe simple VAV system. In the VAV dual duct system, the mixing box characteristics are modified to reduce the cool or warm air flows before mixing the other stream. During cold weather, the warm air duct flow is modulating with no addition of cool air. At moderate outside temperatures, warm and cool air flows are reduced, and air mixing ofthe reduced flows will occur. Like the VAV reheat system, there is still some energy waste, but it occurs at low flows and is small compared to the constant volume system. W a te r S y ste m s Water or hydronic systems accomplish heating and cooling through the distribution of hot and chilled water to the conditioned spaces. Water systems take up less space than air systems due to the higher density and specific heat of water. The required piping is much smaller than the Figure 19-7. Variable air volume system ductwork required for the same installation. While the initial cost of a water system may be less, the system usually lacks humidity control, ventilation is uncertain, and maintenance is higher. A simple one-pipe hydronic system is shown in Figure 19-8. Figure 19-9 shows a two-pipe reversed return system. In each system, hot water or chilled water is circulated to the conditioned spaces, which have fan coil units, convectors, or similar devices for exchange of heat to or from the air in the space. The two-pipe system has the advantage that the water temperature at each terminal unit is the same. The term reversed return as applied to Figure 19-9 refers to the fact that the water path to and from each terminal unit is about equal. This avoids flow imbalance which can occur in a direct return system. The disadvantage ofthe one- and two-pipe systems is that heating and cooling cannot occur simultaneously in different zones. The four-pipe system shown in Figure 19-10 overcomes that limitation. The terminal units are fitted with two coils, one for heating and one for cooling. The thermostat for each zone regulates the flow of hot and chilled water to the coils. Control is sequenced so that flow is not admitted to both at the same time. U n ita r y S y ste m s Factory assembled and packaged air-conditioning systems are available from the major air-conditioning manufacturers. The units are available as a single package containing condenser, compressor, evaporator, fan, controls, and filters, or as a split unit with the condenser and compressor located separately. The units are low in cost due to mass production, but do not have the flexibility of a custom-designed system. On ships, they are Figure 19-10. Four-pipe water system C a r g o H o ld D e h u m id ific a tio n Figure 19-9. Two-pipe water system sometimes found serving a single zone located remotely from the main conditioned spaces, such as the forward deckhouse on a tanker. Figure 19-11 shows a typical unitary system. S y ste m s Cargo holds on dry cargo vessels are generally ventilated with mechanical supply and exhaust, using one air change in 20 to 30 minutes. This is done to reduce hold temperatures and condensation of moisture on hold surfaces and cargo. Condensation is reduced by introducing dry air into the holds and exhausting an equal quantity of humid air. One or more automatic dehumidification systems may be provided for reducing the moisture level in the holds. Dehumidification systems utilize liquid or solid desiccants such as silica gel or lithium chloride. Figure 19-12 is a diagram of a liquid desiccant unit. Figure 19-13 shows a solid desiccant unit. A common feature ofthese systems is continuous regeneration of the water-laden desiccant by passing heated air through it. Moisture removed from the desiccant by the regeneration air is exhausted to the atmosphere. Since the moisture removal process involves the conversion of water vapor in the air to liquid in the desiccant, heat is released. A cooling coil is commonly installed to reduce the temperature of the dehumidified air. The dew point temperature in any cargo hold is normally maintained 10'F (5.6'C) below the surface temperature of the cargo or ship structure. The cargo hold dew point temperature is usually measured in the exhaust trunk and is thus an average of the different hold levels. The ventilation and dehumidification system is designed to operate in one of four modes: ventilation without dry air, ventilation with dry air, recirculation without dry air, and recirculation with dry air. The mode of operation can be Figure 19-13. Rotary Honeycombe dehumidifier. Courtesy Cargocaire Engineering Corp. selected from the wheelhouse by remote operation of pneumatically or electrically actuated control dampers. HV AC SYSTEM COMPONENTS Fans Fans are used to distribute air through the HVAC equipment and ducts to the air-conditioned spaces. Fans used in HVAC systems may be classified as either axial flow or centrifugal fans. In an axial flow fan, the air flow is in the same direction as the fan shaft. In centrifugal fans, the air discharge is radial relative to the fan shaft. Axial flow fans are termed propeller, tube-axial, or vane-axial depending on their construction. A propeller fan (Figure 19-14) is merely a propeller-type wheel mounted on a plate. A tube-axial fan consists of a vaned rotor mounted in a cylinder. The vane-axial fan (Figure 19-15) is similar, but fixed guide vanes improve air flow in the fan. Axial flow fans are limited in their ability to develop pressure, and thus cannot be used in large duct systems. Vane-axial fans are used in ducted systems with modest pressure drops where compact size is important. Centrifugal fans are the commonly used type in ducted air-conditioning systems. Centrifugal fans can be classified according to the shape of the impeller blades: forward curved, radial, or backward curved. In addition, high performance backward curved blade fans are called airfoil blade fans 19-16 HEATING, VENTILATION, AND AIR-CONDITIONING HV AC SYSTEM COMPONENTS 19-17 Figure 19-14. Propeller fan. Courtesy Buffalo Forge Co. Figure 19-16. Centrifugal fan. Courtesy Buffalo Forge Co. Figure 19-15. Vane-axial fan. Courtesy Buffalo Forge Co. due to the shape of the fan blades. Forward curved blade fans are characterized by low initial cost, lower efficiency, and an increasing power requirement with increasing air flow. Backward curved blade fans (especially airfoil types) are higher in initial cost, but have higher efficiency and thus lower operating costs, and have a nonoverloading head-capacity characteristic. Figure 19-16 shows a typical centrifugal fan. Fan performance characteristics. The fan is installed in the HVAC system in order to maintain a certain flow of air through the conditioned spaces. Because of fluid friction in the ducts and other components, the fan must develop pressure in order to move the air. The important fan performance characteristics are capacity (CFM), static pressure (inches of water), efficiency (percent), and power (BHP). Knowledge of fan performance is useful in fan selection, fan operation, and troubleshooting. Figures 19-17 and 19-18 are typical performance characteristics of a forward curved and backward curved blade centrifugal fan. Static pressure, efficiency, and brake horsepower are plotted as a function of flow rate. An inspection of the curves will reveal the following: 1. The static pressure curves for both types tend to peak in the middle range, and then tend to drop off with increasing flow. 2. The BHP for the forward curved blade fan increases with flow. The backward curved blade fan BHP increases gradually, peaks, then falls off. 3. Efficiency is highest in the middle ranges of flow. As previously mentioned, backward curved blade fans will usually have higher peak efficiency than forward curved blade fans. This, combined with the decreasing power requirements at high flow, rather than increasing as with forward curved types, explains the wide application of backward curved blade centrifugal fans in larger air-conditioning systems. This large increase in horsepower may overload the fan motor. The suitability of the motor, fan, and wiring should be checked. Fan capacity c o n t r o l . The capacity of a fan can be varied in several ways: (1) varying the fan speed, (2) using outlet dampers, and (3) using inlet vanes. The simplest means of changing fan speed is to change the belt pulley diameters. This is obviously unsuitable for a system that requires frequent capacity changes but is suitable for a permanent capacity change. Many types of variable speed drives are available for variable capacity applications. A recent development is solid-state variable frequency controls which permit the use of standard induction motors. The fan laws can be used to predict fan operation at different operating points. Variable speed operation is the most efficient means of controlling fan capacity, but can be expensive. Inlet vane control is preferable to outlet damper control from an energy efficiency point of view. F ilte r s Air filters are used in HVAC systems to protect human health and comfort, to maintain room cleanliness, and to protect system equipment. Filters used in marine HVAC systems are of three basic types: (1) roll, (2) panel, and (3) viscous. Roll-type air filters (Figure 19-19) with disposable dry media are popular for marine service. Their performance is better than the washable viscous type. Cleaning consists of rolling to a new clean section or replacing the roll when required. All filters on a particular ship are commonly selected with the same width. Heights of six feet can be accommodated. Panel-type air filters utilize dry media similar to that used in roll-type filters. They are common in small systems where use of the roll-type filters is not practical or economical. Viscous-type air filters utilize a washable element. The filter element is typically fabricated of bronze or copper mesh mounted in a hot-dipped galvanized frame. A common application of this type of filter is as a grease filter in galley ventilation systems and other locations where frequent replacement of disposable elements would be unacceptable. 19-20 HEATING, VENTILATION, AND AIR-CONDITIONING HV AC SYSTEM COMPONENTS 19-21 Steam coils are usually of the header type (a header at each end of straight tubes). This avoids return bends which may trap condensate, causing water hammer and possible freezing. Ifpossible, the tubes should be installed vertically. Each coil should be installed with a stop valve followed by a strainer and control valve on the steam supply and a dirt pocket or strainer, trap, and a cut-out valve on the condensate outlet. At least two feet of bare pipe for cooling should be provided between the coil and the trap. Condensate lines should be level or pitched in the direction of flow and typically are connected to an atmospheric or contaminated drain tank. Hot water coils are generally of the serpentine type. Header type coils have lower tube velocities. This reduces the pressure drop, but adversely affects the transfer of heat. Coils may be either bare or finned tube. Air chambers and vents must be installed to eliminate air and noncondensable gases. They reduce heat transfer in the coil and cause cavitation and noise in the piping system. The heating elements in electric resistance heating coils are usually of the tubular type and are Monel sheathed. On finned elements, the fins are Monel or aluminum alloy cast integral with the sheathing. Figure 19-19. Roll-type air filter. Courtesy The Trane Company. Air filters should be installed so that they are protected from the weather and are accessible for servicing by operating personnel. The length of time between filter servicing depends on many factors such as filter type, port conditions, and types of cargo handled. Filters should always be checked after dusty cargoes are handled. Pressure drop across the filter is the best means of determining the need for replacement or cleaning. A typical filter will have a pressure drop of 0.1 inches of water and should be serviced when the resistance reaches 0.5 inches of water. Some newer roll-type filters will automatically roll to a clean section when the pressure drop increases to the set point on a pressure switch. H e a tin g C o ils Heating coils in marine HVAC systems may be steam, hot water, or electric resistance. Steam heating is generally cost effective; however, it poses certain problems such as difficulty in controlling small heating capacities, condensate return, and water hammer. Hot water and electric resistance heating eliminate many of these problems. Medium temperature hot water (250°-350°F, 121°- 1 7 T C ) is replacing auxiliary steam in many applications. While not energy efficient, electric heating may be used wherever adequate generating capacity is available. Electric heating is particularly suited for electronics and other spaces where leakage from wet systems cannot be tolerated. Solid-state controls are available to provide excellent modulation of heat output. Figure 19-20. Hot water heating coil. Courtesy The Trane Company. 19-22 HEATING, VENTILATION, AND AIR-CONDITIONING C o o lin g C o ils Either chilled water or evaporating refrigerant may be used in cooling coils. DX (dry expansion) is the name given to coils using evaporating refrigerant. Cooling coils are typically made of copper tubing with aluminum or copper fins arranged in a serpentine arrangement. Figure 19-21 shows a DX coil while Figure 19-22 shows a chilled water cooling coil. An even number of rows, usually 6 to 10, are used so that piping connections can be made to the same end of the coil. Bare tube coils are not common in cooling coils due to the .small temperature difference. Fins increase the effective heat transfer area. A counterflow arrangement (Figure 19-23) is commonly used for chilled water coils. In this way, the coldest water is cooling the coldest air, resulting in increased heat transfer. R a d ia to r s a n d C o n v e c to r s Radiators and convectors are heat exchangers used to transfer the heat from the circulating hot water or condensing steam to the room air. Radiators are not common on U.S. flag vessels. However, hot water radiators of embossed steel plate construction are used on many foreign flag ships. Convectors have a finned pipe heating element enclosed in a sheet metal cabinet. The elements are made of either copper or hot-dip galvanized steel. Room air enters through an opening in the bottom and Figure 19-23. Counterflow cooling coil arrangement leaves through an outlet grille at the top. Figure 19-24 shows some typical convectors. Convectors should be installed along weather-exposed bulkheads, under windows, and near the deck. This puts the heat where the heat loss is the greatest, and cold downdrafts are prevented. Elevating convectors off the deck decreases their capacity, and stratification due to insufficient air circulation can occur. Figure 19-24. Convectors. Courtesy The Trane Company U n it H e a te r s A unit heater consists of a finned heating coil and a fan mounted in a compact assembly. See Figure 19-25. Forced circulation of air results in an increased rate of heat transfer. Unit heaters are most commonly used in steering gear rooms, shop areas, and machinery spaces of diesel ships (to prevent freezing when the main engine is secured). In large spaces where more than one is installed, unit heaters should be directed to circulate air in the space. F a n -C o il U n its A fan-coil unit consists of a finned tube coil, a small centrifugal fan, and a filter mounted in a cabinet (Figure 19-26). Floor-mounted units are similar in appearance to convectors. Fan-coil units can be used for both cooling and heating by supplying either chilled or hot water to the coil, or by providing both a cooling and a heating coil. Fan-coil units may also be mounted horizontally at ceiling levels. D u c ts a n d T r u n k s Conditioned air main ducts on ships are typically located in passageways with branches run to each space. The use offactory fabricated sheet metal ducting and fittings can reduce the cost of the installation. The ducting system is hung from existing structure. Small horizontal ductwork is Figure 19-26. Fan-coil unit commonly supported by sheet metal straps, while heavier ducts require angle iron cradles suspended from rods. Occasionally "built-in" trunks using ship's structure to form all or part of the air passage are employed in ventilation systems. The size oftrunks must be large enough to permit access to the interior for inspection and maintenance. Ducts in cargo spaces are usually made of steel plate to minimize the possibility of damage. Watertight ducts are constructed of the same material as the associated structure and must meet applicable test standards. Ducts carrying conditioned air are covered with thermal insulation such as glass fiber or mineral wool, and covered with a vapor barrier to eliminate 19-26 HEATING, VENTILATION, AND AIR-CONDITIONING condensation. Insulation is available in rigid board or blanket form. Ducts can be lined with acoustic insulation to absorb sound. The acoustic insulation can also serve as the thermal insulation. T e r m in a ls Conditioned air is distributed to the conditioned spaces by various types of supply terminals. Types in common use include (1) double-deflection registers, (2) fixed-pattern diffusers, and (3) perforated-panel distributors. Double-deflection registers consist of a frame with vertical and horizontal bars. The bars permit control of air direction. A volume control damper is also commonly fitted. Diffusers are available in round, square, or rectangular shapes. They consist of a series of concentric louvers to direct the air. A perforated-panel diffuser is merely a flat plate with a large number of holes in it. This type of diffuser is popular because it is inconspicuous when installed in hung ceilings. Exhaust terminals are similar in construction to supply terminals. Simple grilles are common. The supply and exhaust terminals should be located to maintain proper circulation in the space to prevent hot or cold air collections and avoid short circuiting. Specifications typically call for a maximum ambient temperature differential of 2°F (rC) and velocities of 35 to 50 feet per minute (11 to 15 meters per minute). HV AC C o n tr o l Systems The HVAC control system has the task of varying the amount of heating, cooling, dehumidification, and humidification to automatically maintain the conditioned space in a comfortable condition. In addition, the system must control the HVAC system in an efficient manner to minimize energy consumption. Figure 19-27 shows a simple closed loop control system. The temperature sensor senses the air temperature downstream ofthe coil. The control compares the sensed temperature with a set point and sends a signal to the control valve to vary the amount of hot water admitted to the coil. The change in air temperature caused by the change in valve position will be sensed and additional adjustments will automatically be made as necessary. While a control system may appear complex, it can be reduced to a series of simple system elements. The system elements are assembled from standard control devices: sensors, controllers, relays, and actuators. Pneumatic control devices are the most common in HVAC systems because of their simplicity and low cost. Pneumatic controls are powered by compressed air at 15 to 20 psig (103 to 138 kPa), although occasionally higher pressures are used to operate large valves and dampers. Self-actuated control valves are used in a few applications, primarily for controlling Figure 19-27. Simple closed loop control system steam coils. Electric controls are occasionally used for simple applications where compressed air and/or suitable self-actuated devices are available. Temperature sensors. The major types of temperature sensors include bimetallic, bellows, and bulb-and-capillary. The bimetallic sensor is most commonly used in room thermostats. It consists of two strips of dissimilar metals bonded together. Since the two metals expand at different rates, a temperature change causes the element to bend. The bimetallic element may be also shaped in a spiral to provide a rotary motion output. The bellows sensor is usually made of brass and filled with a vapor or gas. Temperature changes cause the bellows to expand and contract. A spring is used to adjust the set point. Bulb-and-capillary elements are commonly used to sense temperatures in remote locations. The element consists of a bulb, a capillary tube, and a spring-loaded diaphragm head. The element may be filled with a liquid, a gas, or a refrigerant depending on the application. As the bulb temperature changes, the pressure exerted by the liquid or gas changes. The bulb pressure is transmitted to the diaphragm by the capillary, where the pressure change is translated into movement. Pressure sensors. The major types of pressure sensors include diaphragms, bellows, and bourdon tubes. A diaphragm sensor is a flexible plate attached to a container. A spring balances the force caused by the pressure applied to the other side. 19-28 HEATING, VENTILATION, AND AIR-CONDITIONING A bellows sensor is a diaphragm connected to a container by a series of convolutions to permit an increased degree of movement. The bellows acts like a spring, or an external spring may be added to change the sensitivity. A bourdon tube sensor consists of a flattened tube bent in a circular shape. One end is connected to the pressure source, and the other end is free to move. An increase in pressure causes the tube to straighten out. Humidity sensors. Humidity sensors utilize hygroscopic materials which change their size in response to humidity changes. One element is made of two strips of different woods-it operates similarly to a bimetallic temperature sensor. Elements using such materials as hair, special cloth, or animal membrane which change length in response to humidity changes are common. Electric sensors which use compounds that change in resistance with a change in humidity are also available. Controllers. The purpose of a controller is to convert the output of the temperature, pressure, or humidity sensor into a pneumatic control signal, typically 3 to 15 psig (21 to 103 kPa). Controllers can be either direct acting or reverse acting. A controller is direct acting when an increase in the input parameter (temperature, pressure, humidity) results in an increase in the output air pressure. An increase in the input to a reverse-acting controller results in a decrease in the output air pressure. A bleed-type controller is shown in Figure 19-28. The flapper is moved by a sensor and varies the amount of air exiting the nozzle. If the flapper is moved towards the nozzle, the flow is decreased, and the output pressure rises. The linkage between the sensor and flapper determines whether the controller is direct or reverse acting. When a bleed-type controller must deliver large air flows, it is commonly combined with a relay. The bleedtype controller acts as a pilot to vary the output of the relay. Figure 19-29 is a simplified diagram of a nonbleed controller. It demands supply air only when the outlet pressure is increasing. A downward sensor motion will cause an inward movement of the diaphragm, raising the left end of the lever, and opening the supply val ve. The incoming air will increase the chamber pressure, pushing up on the diaphragm to offset the input from the sensor (negative feedback). When equilibrium is attained, the supply valve closes, and the new chamber pressure becomes the controller output pressure. A further downward sensor input will cause a rebalancing at some higher output pressure. An upward sensor movement will cause the bleed valve to open, and the controller will balance at a lower pressure. The type oflinkage between the sensor and the controller determines whether the controller is direct or reverse acting. Relays. Relays are used in a control circuit to perform a function that is beyond the ability or capacity of the controller. Most are variations of the Figure 19-29. Nonbleed controller nonbleed controller shown in Figure 19-29. Some ofthe functions that the various relays can perform include (1) amplification (or reduction) of input signal, (2) reversing of input signal, (3) difference (or sum) of input signals, (4) selection of higher (or lower) of two input signals, (5) average of two input signals, (6) conversion to on-off operation, (7) addition of reset (integral) action, and (8) sequencing oftwo or more actuators. Figure 19-30 is a diagram of a reversing relay. It may be used to change the output from a direct-acting controller to a reverse-acting output. Figure 19-31 is a HVAC SYSTEM COMPONENTS 19-31 made to a basic nonbleed controller so that it will perform various functions. The function of the actuator is to convert the pneumatic signal from the control system into a movement to position a fluid valve or an air damper. A pneumatic actuator is basically a piston or diaphragm on which the control air pressure acts on one side, and spring pressure acts on the other. See Figure 19-32. Increasing air pressure compresses the spring until the spring load and the load from the valve or damper balance. The spring range is typically set to provide full movement over the 3 to 15 psig (21 to 103 kPa) range, but other spring ranges are available. A c tu a to r s. P o s i t i o n e r s . Under certain conditions such as friction in the actuator or load, or wind load on a damper, accurate positioning of a simple actuator may not be possible. The solution is to use a positioner. A positioner (or positive positioning relay) will accurately position the actuator in response to the input pressure rather than a balance of forces within the actuator. A positioner is essentially a modification of a nonbleed controller with feedback for actuator position. Use of a positioner assures full supply air pressure is available to move the actuator even though the input control pressure may change only slightly. Dampers are used to control the flow of air in HVAC systems. Most dampers are ofthe multileaftype. They can be described as a parallelblade damper or an opposed-blade damper, and as normally open or normally closed. Figure 19-33 shows the operation of parallel-blade and opposed-blade dampers. Opposed-blade dampers are used in modulating applications while parallel-blade dampers are used in on-off applications. A normally open damper assumes an open position when no air pressure is applied to the actuator. A normally closed damper assumes a closed position when no air pressure is applied. The difference depends on the way the actuator is mounted, and on the way the linkage is connected to the damper. D am p ers. Figure 19-33. Damper types In marine applications, outside air dampers should be made of stainless steel. Other dampers may be stainless or hot-dip galvanized steel. Damper construction should be integrally airtight without the use of nonmetallic seals. C o n t r o l valves. Control valves are used to control the flow of steam and water to heating and cooling coils. The size of a coil is normally based on some maximum design load, and the control valve must pass the required flow to meet that maximum load condition. However, in most cases, the system is operating at part load, and the valve must satisfactorily control the fluid flow over the entire range of load conditions. Control valves are available with a number offlow-versus-lift characteristics to suit different applications. Figure 19-34 shows three common characteristics: quick opening, linear, and equal percentage. Standard globe valves typically have a quick -opening characteristic. Quick -opening val ves do not modulate at low loads satisfactorily, and are primarily used in on-off applications. Valves with linear or equal-percentage characteristics are especially suitable for controlling heating and cooling coils. Valves of this type are usually of the ported or cage-guided construction. See Figure 19-35. By changing the shape ofthe ports in the plug, the flow-versus-lift characteristic of the valve may be changed. Three-way valves are also available to control a heat exchanger. Threeway valves are available in two types: mixing and diverting. A mixing valve has two inlets and a single outlet. A diverting valve has one inlet and two Figure 19-35. Ported control valve outlets. Using either design in the wrong service will to cause valve chatter and other problems. Valves, like dampers, can be classified as normally open or normally closed. This characteristic depends on the way the actuator is connected to the valve body. A normally open valve will assume an open position with no air supply pressure to the actuator. A normally closed valve will assume a closed position with no air input. r e h e a t c o n t r o l system. In order to illustrate how the various control components may be connected into a complete system, a typical T e r m in a l 19-34 HEATING, VENTILATION, AND AIR-CONDITIONING control system for a terminal reheat air-conditioning installation will be described. Refer to Figure 19-36. The steam preheater coil and chilled water cooling coil are controlled by thermostats in the conditioned supply duct. The hot water reheater at the inlet to each conditioned space is controlled by the individual room thermostat. The preheater thermostat is set several degrees below the design chilled air (off-coil)temperature to prevent simultaneous operation of the preheater and cooling coil. The supply duct temperature thus remains relatively constant during both summer and winter operation, and the room temperature is maintained in all seasons by the reheat coil. The quantity of outside air is controlled by duct thermostats in the dutside air duct which positions the dampers in the exhaust, outside air, and recirculation ducts. By the use of normally closed dampers in the exhaust and outside air ducts and a normally open damper in the recirculation duct, the three dampers operate in sequence in response to a single control air input pressure. The duct thermostats are adjusted to reduce the quantity of outside air during hot or cold outside air conditions, and to increase the outside air during moderate conditions. This is done to reduce heating and cooling energy requirements. The control systems for other types of air conditioning systems will utilize many of the techniques described above, but, obviously, must be adapted to suit the particular application. By breaking the system down into a series of simple elements, even the most complex control system can be understood. An understanding ofthe operation of the control system is essential to any attempt to adjust or troubleshoot the HVAC system. SYSTEM TESTING AND BALANCING SYSTEM T E S T IN G AND 19-35 B A L A N C IN G The complexity of modern HVAC systems requires careful adjustment to achieve design performance. Failure to properly adjust and balance the system will result in uncomfortable conditions in the conditioned spaces and a waste of expensive energy. The balancing process requires an understanding of the HVAC principles described in this chapter and in the references at the end of the chapter. Organized procedures must be followed to insure a properly balanced system. I n s tr u m e n ta tio n The success of system testing and balancing depends on accurate, calibrated instrumentation. These instruments are used to measure such system parameters as temperature, pressure, flow, RPM, humidity, and electrical power. Instruments are available with various degrees of accuracy. In most cases, an accuracy of 1 to 5 percent is adequate. The calibration of each instrument should be checked before and after use. This can usually be done by comparison with an instrument of known accuracy. The instrument can then be adjusted, or a calibration curve prepared, to insure accurate readings. T e m p e r a tu r e Temperatures in HVAC systems are commonly measured by liquid-inglass, bimetallic, or vapor bulb thermometers. For thermometers installed permanently in pipes, a well is used to allow replacement without securing the system. An inexpensive pocket dial (bimetallic) thermometer is convenient for checking temperatures where permanent instrumentation is not installed. Contact thermometers are available to conveniently determine the approximate fluid temperature in a pipe by reading the outside temperature of the pipe. A recent development is portable digital instruments using thermocouple, thermistor, or RTD (resistance temperature detector) probes. Interchangeable probes are available for surface measurements, fluid measurements, and other applications. A permanent installation can utilize a number of probes, a selector switch, and a single readout, thus permitting the remote reading of many points from a central location. An RTD operates on the principle that the resistance of a coil of wire will change with temperature. The coil resistance is converted directly into a temperature by the readout circuitry. A thermocouple is ajoint oftwo wires of different materials. The joint generates a very small voltage, and this voltage varies with temperature. The readout converts the voltage directly into temperature. 19-38 HEATING, VENTILATION, a handheld mechanical or electronic tachometer can be used. The tip of the instrument is pressed into the countersink, and the RPM is read directly. The instruments are also supplied with a wheel which can replace the tip. By bringing the wheel in contact with rotating shaft or coupling of the rotating machine and multiplying by the ratio ofthe diameters, the RPM can be determined. A stroboscope can be used where contact with the rotating shaft is difficult. The instrument consists of a flashing light with an adjustable frequency. The light is directed at the rotating parts, and the frequency of the instrument adjusted until the equipment appears to stand still. This will occur when the frequency is equal to the speed ofrotation, or a multiple or fraction thereof. Some care must be taken to read the proper frequency. E le c tr ic a l P ow er The measurement of voltage, current, and electric power supplied to motors is commonly required. A standard portable multi meter can be used for checking voltages, currents, and resistances in HVAC circuits. A clamp-on ammeter is convenient for measuring the power delivered to motors. A U-shapedjaw is opened and clipped around a single wire of the circuit. The instrument operates by measuring the magnetic field created by the current flow. Clamp-on wattmeters which measure current, voltage, and power factor, and display kilowatts directly are also available. B a la n c in g a n A ir S y s te m Listed below are the recommended steps for balancing an air system: 1. Obtain a duct system drawing and list all design velocities and flow rates. If a system drawing is not available, prepare a one-line diagram. Show the location of all balancing dampers. If design velocities or flow rates are not available, Table 19-2, which lists typical design velocities in HVAC systems, can be used for estimating purposes. 2. Obtain design data for the system and all major components, i.e., fans, filters, coils, dampers, mixing boxes, etc. 3. Prepare data sheets. TABLE T y p ic a l V e lo c itie s L o c a tio n Main ducts, living spaces Main ducts, other spaces Branch ducts Coils Filters 19-2 in H V A C S y s te m s V e lo c ity 800-1500 FPM 1500-2500 FPM 800-1500 FPM 500-700 FPM 250-350 FPM 19-39 REFERENCES AND AIR-CONDITIONING 4. Decide on required instrumentation and on measurement locations. 5. Check the operation of all system components and controls. Check all damper positions. 6. Start fans. Measure fan speeds and adjust RPM to design values. 7. Measure fan capacities (CFM) using a Pitot traverse of the main duct or an anemometer reading across the coils. If flow is not within ±10 percent of design, check for possible problems such as closed dampers, equipment malfunction, etc. If system is operating properly, adjust the fan speed to obtain the proper CFM. 8. Measure flow in major supply and return ducts and adjust to design value (±10 percent) with dampers. 9. Measure and adjust the flow of each outlet terminal. Work either from the farthest outlet towards the fan or from the fan outwards. Use effective outlet areas when calculating flow rates. Repeat the cycle at least once, since later adjustments will affect earlier measured flows. 10. Record all data including flows, temperatures (including WB and DB temperatures at coil inlet and outlet), and pressures. Record all performance data for all operating equipment. B a la n c in g a W a te r S y ste m Listed below are the recommended steps for balancing a water system: 1. Obtain (or prepare) a piping system drawing and record design flows, temperatures, and pressures. 2. Obtain design data for the system and all major components, i.e., pumps, coils, chillers, hot water heaters, and terminal units. 3. Prepare data sheets. 4. Decide on required instrumentation and on measurement locations. 5. Check operation of all pumps, valves, and controls. Check the position of all valves. 6. Start the circulating pump. Check the flow with a flowmeter or by recording the suction and discharge pressures and referring to the design head-capacity curve. Adjust flow with discharge valve to about 110 percent of design. 7. Do not balance the branches if automatic control valves are installed to vary the flow through the system coils and terminal units. If automatic control valves are not fitted, the flows in the various branches must be measured and adjusted to ±10 percent of design. Measurement of the water temperatures in and out of the coils and terminal units can be used as an additional check. 8. Measure and record all operating data including performance of all major equipment. REFERENCES Dossat, R. J.1978. and Sons. P r in c ip le s o f R e fr ig e r a tio n . 2d ed. NewYork: John Wiley 19-40 HEATING, VENTILATION, AND AIR-CONDITIONING Haines, R. W. 1971. C o n t r o l S y s t e m s f o r H e a t i n g , V e n t i l a t i o n , a n d A i r C o n d i t i o n i n g . New York: Van Nostrand Reinhold Company. H a n d b o o k a n d P r o d u c t D i r e c t o r y . 4 vols. Atlanta, Ga.: American Society of Heating, Refrigerating, and Air Conditioning Engineers. Pita, E. G. 1981. A i r C o n d i t i o n i n g P r i n c i p l e s a n d S y s t e m s : A n E n e r g y A p p r o a c h . New York: John Wiley and Sons Stoecker, W. F., and Jones, J. W. 1982. R e f r i g e r a t i o n a n d A i r C o n d i t i o n i n g . New York: McGraw-Hill. CHAPTER C r y o g e n ic C a r g o S y s t e m s JAMES A. HARBACH AGKNOW LEDGM ENTS The author is indebted to the following companies engaged in the manufacture of HVAC systems and equipment who so kindly contributed photographs, drawings, and other material used in the preparation of this chapter: Carrier Corporation, Syracuse, New York The Trane Company, LaCrosse, Wisconsin Buffalo Forge Co., Rockville, Maryland Cargocaire Engineering Corp., Amesbury, Massachusetts 20 C R Y O G E N IC P R IN C IP L E S C RYOGENICS is the study of matter at very low temperatures. The primary interest of the marine industry in cryogenics is for the transportation of liquefied gases. If you liquefy gases you reduce their volume several hundred times, thus permitting economical transportation in specially designed tankers. While the transportation of liquefied gases at normal temperatures has been considered, the cost of the required high pressure tanks has prevented their use in large scale commercial operations. The liquefied gas tankers in service today utilize insulated tanks which carry the liquefied gases at atmospheric pressure. The properties of some liquefied gases at atmospheric pressure are shown in the table below: Properties of Liquefied Gases at Atmospheric Pressure F o r m u la Nitrogen Methane Ethylene Ethane Propane Butane N2 CH4 C2H4 C2H6 C3HS C4HIO B o ilin g -320'F -259'F -155'F -128'F -44'F +31'F P o in t (-196'C) (-162'C) (-104'C) (-89'C) (-42'C) (-0.5'C) L iq u id D e n s ity .81 .42 .57 .56 .59 .60 Two common liquefied gas cargoes are LPG (liquefied petroleum gas) and LNG (liquefied natural gas). LPG is primarily propane (boiling point -44'F, -42'C) while LNG is primarily methane (boiling point -259°F, -162°C. The actual makeup of LPG and LNG depends on the source of the gas field, and varies widely from field to field. This chapter will concentrate 20-1 20-2 CRYOGENIC CARGO SYSTEMS on LNG cargo systems, but most of the information will apply to other, higher temperature, and more easily handled cargoes. LNG tankers are usually built for dedicated service between specific ports such as Algeria (Arzew) to France (Le Havre) and the United Kingdom (Canvey), and Indonesia to Japan. The basic transportation cycle consists of liquefaction of the gas near the gas fields, marine transport of the liquefied gas in insulated tanks at atmospheric pressure, and vaporization of the liquefied cargo at the receiving terminal for distribution. Liquefaction facilities are normally not installed on the vessels. The gas produced by heat transfer through the tank insulation, termed boil-off,is used as fuel for the main engines. L iq u e fa c tio n Processes The first step in the transportation cycle of LNG is liquefaction. Natural gas from the supply field is delivered via high pressure pipeline to the loading terminal. Prior to liquefaction, impurities such as water, carbon dioxide, and heavier hydrocarbons which would solidify during the liquefaction process are removed. After liquefaction, the LNG is stored for transfer to the LNG tankers. There are a number of processes available for liquefying gases. These include the Linde-Hampson process, the cascade process, the autorefrigerated cascade (ARC) cycle, the expander cycle, the open Claude cycle, and the closed Brayton cycle. Two of the processes in common use in large natural gas liquefaction plants, the cascade process and the ARC cycle, will be described below. The cascade cycle. The cascade cycle consists of two or more vapor-com- pression refrigeration systems connected in series. Refer to Chapter 18 for a discussion of the vapor-compression cycle. The evaporator for the high temperature cycle serves as the condenser for the second cycle. The evaporatol' for the second cycle serves as the condenser for the third cycle, and so on. Figure 20-1 shows a simplified cascade system using propane in the high temperature cycle, ethylene in the second cycle, and liquefaction of methane in the third cycle. The use of several refrigeration cycles improves the overall system efficiency. Pressure differences between low and high sides are reduced and refrigerants are selected to suit the operating temperatures. The main disadvantage of this system is its complexity. Balancing and adjusting the system can be difficult, and maintenance costs are high. cascade (ARC) system. The ARC system is a simplification of the cascade cycle described above. A mixture of as many as six gases, including methane, ethane, propane, butane, pentane, and nitrogen, is used as the refrigerant. The composition of the refrigerant The auto-t'efrigerated Figure 20-1. Cascade liquefaction cycle mixture is chosen so that the partial condensation in anyone stage corresponds exactly to the refrigeration requirements of the next stage. Partial condensation of the refrigerant takes place at the high side pressure, while evaporation and the resultant cooling occurs at the low side pressure. Figure 20-2 is a diagram of a simplified ARC cycle. The refrigerant is condensed in a series of condensers. The liquid is sprayed into the column at several elevations, lowering the temperature and liquefying the natural gas flowing countercurrent to it. The ARC cycle eliminates many of the limitations of the cascade cycle. The simplification of the compression process results in lower capital costs. By installing compressors in parallel, system reliability and flexibility of operation are LNG CARGO TANKS 20-5 mal coefficient of expansion) at the operating temperature of -260'F (-162'C). The most important characteristic is no transition from ductile to brittle fracture modes at the lowest operating temperature. At temperatures below the transition temperature, i. e., the brittle fracture region, the material loses most of its ability to absorb energy, and catastrophic failure is more likely. Nickel is the most effective alloy in increasing the toughness of steel. Materials suitable for LNG service include stainless steel, certain aluminum alloys, 5 percent and 9 percent nickel steel, and Invar (36 percent nickel steel). Final selection of materials must consider the proposed application (tank structure, tank membrane, piping, etc.), ease of welding, amount of required aftertreatment, and, of course, cost. Stainless steel is typically used for LNG piping because it is readily available and has superior fire resistance compared to aluminum. Free-standing tanks are typically made of 5083 aluminum alloy. Membrane tank linings are typically stainless steel or Invar. LNG Figure 20-2. Auto-refrigerated cascade (ARC)liquefaction system increased. Failure of one compressor in the ARC cycle results in reduced capacity, while a compressor failure in the cascade system results in total shutdown. The main disadvantage of the ARC cycle is its higher power requirements compared to the cascade cycle. M a te r ia ls All materials that come in contact with LNG must possess appropriate mechanical and thermal properties (strength, ductility, fatigue life, ther- CARGO TANKS Many different designs have been proposed for the carriage of LNG cargo. They can be broken down into two categories: the freestanding or selfsupporting type, and the membrane or integrated type. The freestanding tank containment system incorporates tanks which are independent of the vessel's hull. The tanks are capable of supporting their own weight as well as withstanding the static and dynamic loads imposed by the liquid cargo. These loads are transferred to the ship structure through the tanks themselves and their key supporting structure. Membrane tanks, on the other hand, are made of thin metallic materials and are not self-supporting. The tanks are attached indirectly to the ship's structure through a load-bearing insulation. There is a definite tendency among shipyards and ship operators to select a proven tank design. Considering the extremely high costs of LNG vessels, the use of a proven, reliable design rather than anew, untried design is understandable. Unless a new design has significant advantages over the existing designs, it is unlikely to find customers. Safety is crucial when considering the advantages and disadvantages of the various tank designs. Classification societies have established regulations governing tank system design and construction. However, such considerations as survivability of collision or grounding are not covered. An important safety feature of most LNG tanks is the secondary barrier. It contains the LNG in the event of leakage of the primary containment, and prevents damage to the ship structure. Tanks designed as pressure vessels are not required to have a secondary barrier. Such features as ease ofinspection can also affect the overall safety and reliability of a design. 20-6 CRYOGENIC CARGO SYSTEMS LNG CARGO TANKS 3 Most LNG vessels of recent construction are ofthe 125,000 m size. This produces a vessel of about 935 ft (285 m) length overall with a loaded displacement of95,000 tons. Service speed is typically about 19 to 20 knots, requiring a propulsion plant of approximately 40,000 SHP. The cargo is carried in 5 or 6 tanks. The 4 most popular LNG cargo tank designs will be discussed in the following sections. C onch The current Conch tank design as shown in Figure 20-3 is a development of the pioneering work of Conch International Methane Company in the marine carriage of LNG. The tank is a prismatic, free-standing design constructed of 5083 aluminum alloy. A longitudinal liquid-tight bulkhead and a transverse swash bulkhead provide structural strength and reduce free surface and dynamic loading effects. The tank thickness is approximately 1;2 inch (13 mm) but increases to 111.2 inch (38 mm) at the chamfer panels. The tank is fitted to the hull shape and is located by keys and keyways at the bottom and top, allowing for the necessary expansion and 20-7 contraction. The keys are made of permali, a reinforced hardwood. The tank is supported by balsa-plywood insulation panels which are uniformly distributed under approximately 30 percent ofthe tank bottom. The spaces between the panels are filled with polyurethane foam and fiberglass, each 6 inches (15.25 cm) thick. The insulation system, which includes secondary barrier capabilities, consists of balsa-plywood frame panels along the corners of the cargo hold with a sprayed-in-place high density polyurethane covering the inner hull between the frames. The polyurethane foam is approximately 6 inches (15.25 cm) thick. Inboard ofthe foam is an attached layer of fiberglass of 6 inch (15.25 cm) thickness. The space between the tank wall and the insulation is maintained inert with nitrogen gas. T e c h n ig a z M em brane The Technigaz membrane containment system as shown in Figure 20-4 consists of wooden support beams, wooden insulation panels containing a 20-8 CRYOGENIC CARGO SYSTEMS LNG CARGO TANKS 20-9 wood secondary barrier, and a stainless steel primary barrier. The wooden support beams called grounds are made oflayers of Douglas fir and lauan plywood. They are mounted on the tank structure by steel studs and mastic. The space between the grounds is filled with glass wool to provide additional insulation and prevent formation of convection currents. Prefabricated laminated balsa and plywood insulation panels are glued to the grounds. Joints between insulation panels are filled with PVC, and plywood pieces are glued over the PVC joints to complete the secondary barrier. Balsa pads and plywood corner pieces complete the insulation. Stainless steel anchor points are bonded to wood insulation to provide a suitable surface for mounting the stainless steel primary barrier. The membrane is made of 1.2 mm (0.047 in.) low carbon stainless steel sheets. The sheets are corrugated to permit expansion and contraction. The intersection ofthe corrugations have a special geometry referred to as knots. Figure 20-5 taken during construction shows the details of the membrane. The individual membrane sheets are lapped and welded to complete the primary barrier. Figure 20-6 shows a completed tank. G az T ran sp ort M em brane The Gaz Transport containment system consists of two 0.5 mm (0.02 in.) thick Invar membranes and an insulation system made of a series of subdivided plywood boxes, factory-made on a production line and filled with perlite (see Figure 20-7). Invar was selected for the membranes because it has a very low coefficient of expansion, and thus eliminates the need for expansion joints or corrugations. The membranes are fabricated from strips of Invar approximately 0.4 m (13 .3 in.) wide. The edges are flanged, then welded to a tongue support attached to the insulation boxes. The system is unique in that the primary and secondary barriers and the primary and secondary insulation systems are identical. K v a e r n e r -M o ss S p h e r ic a l T ank The Kvaerner-Moss tank is a 5083 aluminum alloy, or 9 percent nickel steel, sphere supported by a cylindrical steel skirt attached to the equatorial ring of the tank. The skirt is welded to the ship structure at its base and bonded to the tank equatorial ring at its upper edge. No tank internal stiffeners or bulkheads are fitted (see Figure 20-8). The tanks and support skirts are insulated with a system of polyurethane foam panels and glass fiber. The panels and glass fiber are fixed to the tank by phenolic studs screwed into aluminum sockets welded to the tank shell. A rigid disk is threaded onto the outer end of the stud to secure each panel at six points. The panels and studs are oversprayed with a Hypalon rubber coating to minimize the permeation of moisture and other contaminants. Figure 20-9 illustrates the insulation system. Figure 20-5. Technigaz membrane during construction. Courtesy Newport News Shipbuilding and Drydock Co. A major advantage of this design is that it can be designed and analyzed according to pressure vessel standards. This permits the system to be built without a secondary barrier. However, a stainless steel drip pan, insulated from the h u ll structure, is installed under the tank as a precaution. The Figure 20-7. Gaz Transport containment system Figure 20-6. Completed Technigaz tank. Courtesy Newport News Shipbuilding and Drydock Co. ability to pressurize the tank provides an emergency means of discharging cargo. Some disadvantages given for this design are the relatively poor visibility from the bridge due to the protruding tanks, a relatively high center of gravity, and poor hull volume utilization. The design is claimed to be less susceptible to collision damage than membrane designs. LNG CARGO 20-11 shows a swiveljoint used in the arms. During loading, shore terminal pumps deliver the LNG to the ship's tanks. Vapor displaced by the loaded LNG is returned to the shore facility with the assistance of onboard compressors. Pressure in the tanks is maintained at a slight positive pressure at all times by control of the compressor capacity. Excess vapor produced by the loading operation is reliquefied or flared. Discharge operations are similar except shipboard cargo pumps and shore terminal compressors are used. During loading operations, ballast is discharged to maintain a constant draft. The ship is ballasted during discharging operations. Additional features of a typical shipboard cargo system include: SYSTEM S LNG and other cryogenic cargoes are loaded and discharged in a closed cycle as shown in Figure 20-10. Liquid and vapor lines with articulated loading arms are run between the ship and the shore terminal. Figure 1. Inerting of the space surrounding the cargo tank. 2. A means for controlled cool-down and warm-up ofthe tanks and piping. 3. Extensive instrumentation to monitor temperatures, pressures, tank levels, and other key parameters. Figure 20-10. Closed cycle cargo loading and discharge 4. Provisions for expansion and contraction of cargo piping. 5. Relief valves to prevent overpressure of the tanks and vapor main. Liquid piping which can be isolated is also fitted with relief valves which relieve to the vapor main. 6. Provisions for emergency discharge of the cargo. 7. Provisions for use of boil-off as fuel in the boilers. Figure 20-11. Loading arm swivel joint Figure 20-12. Liquid and vapor cargo systems L iq u id a n d V a p o r S y s te m s L N G C argo and Spray P um ps Figure 20-12 is a simplified diagram ofthe liquid and vapor systems on a typical LNG vessel. The systems are designed to perform the following functions: 1. Loading and discharging of LNG cargo. These portions of the systems include the liquid and vapor headers, port and starboard manifold stations, loading lines to each tank, discharge lines with submerged or deep well pumps for each tank, vapor lines to each tank, and the gas compressors and heaters. 2. Cool-down of the tanks. This is accomplished by the spray nozzles in each tank served by submerged or deep well spray pumps in each tank and a cool-down header. 3. Cool-down of the cargo lines. This is accomplished by the spray pumps and crosssovers between the liquid and cool-down headers. 4. Warm-up ofthe tanks. This is accomplished by using the compressors and heaters to deliver heated vapor to the tanks. 5. Vaporization of LNG or LN2 (liquid nitrogen). This is accomplished by the vaporizer which is connected between the cooldown and vapor headers. The pumps used for the transfer of LNG can be either the deep well or submerged type. The submerged type is the most common on LNG tankers. The deep well pump consists of an electric motor mounted on top of the tank. It operates a multistage centrifugal pump mounted on a long shaft extended deep into the tank. The drive shaft is supported by bearings inside the discharge pipe. Problems with binding ofthe drive shaft due to differential expansion and bearing contamination led to the development of suitable submerged pumps. Submerged electrically driven pumps can be used because the tanks are maintained at a slight positive pressure to exclude air. LNG has high electrical resistance, and the low temperature reduces the electrical resistance of the motor windings, thus increasing motor efficiency. The pump bearings are lubricated by the LNG. Submerged pumps are less expensive than the deep well type and are capable of pumping the tank down to less than 2 inches of LNG. No mechanical seal or packing gland is required since the pump is completely exposed to LNG. A typical single stage submerged pump is shown in Figure 20-13. A unique feature ofthis pump 20-16 CRYOGENIC CARGO SYSTEMS is the inlet inducer which is responsible for its low NPSH (net positive suction head). G as C om p ressors Two or three compressors are usually installed on a typical LNG tanker. The low capacity unit is commonly termed a boil-off compressor while the high capacity unit is termed a high duty or warm-up compressor. The compressor can be centrifugal or positive displacement driven by an electric motor or steam turbine. The compressor shafts are sealed with nitrogen to prevent gas leakage. The nitrogen seal pressure is typically adjusted to 20 psi (138 kPa) above the compressor casing pressure. Electric motor-driven compressors are normally installed with a bulkhead between the motor and compressor with a seal fitted where the drive shaft passes through the bulkhead. Compressor discharge pressures are typically in the range of 5 to 15 psig (35 to 105 kPa). G a s H e a te r s a n d V a p o r iz e r s Gas heaters are installed for heating boil-off gas for use in the boilers, and for heating gas during cargo tank warm-up. Gas enters the heaters from discharge of the gas compressors. The gas is heated either directly by steam in the heater or by a hot glycol-water mixture circulated through a LNG CARGO SYSTEMS 20-17 steam-to-water heat exchanger and the gas heater. Figure 20-14 is a diagram of a typical direct steam-heated gas heater. Steam is maintained at constant pressure in the heater shell, and gas is circulated through the tubes. Gas temperature is controlled by bypassing cold gas around the heater. A low temperature alarm and control on the condensate prevent freeze-up. Vaporizers are installed to perform the following functions: 1. Displace inert gas from the cargo tanks with vaporized LNG prior to cool-downand cargoloading. 2. Maintain cargo tank pressure during discharge when insufficientvapor is being returned from shore. 3. Supplementnatural boil-off. 4. Vaporizeliquid nitrogen. Vaporizers are similar in construction to gas heaters. Most are of the steam-heated shell-and-tube type. Separators are commonly installed at the vaporizer outlet to prevent liquid carryover into the vapor header. C o n tr o l V a lv e s Control valves used in cryogenic systems must be designed for reliable operation at the very low temperatures encountered. The valve must be 20-18 CRYOGENIC CARGO SYSTEMS LNG CARGO SYSTEMS 20-19 designed so that liquid cannot be trapped, or rupture may occur on warm-up. Most control valves in cryogenic service are pneumatically operated with a long warming extension. The warming extension allows the actuator to operate at close to ambient, rather than cryogenic, temperatures. Figure 20-15 shows a typical cryogenic control valve while Figure 20-16 shows a typical pneumatic controller used with the valve. N itr o g e n an d In ert Gas Systems The purpose of the nitrogen and inert gas systems on LNG tankers is to supply gases for the inerting of various spaces aboard the vessel. These systems include the hold spaces around the LNG storage tanks, cargo piping prior to loading or discharge, and the cargo tanks prior to gas freeing or preparation for cargo loading. The nitrogen system consists of insulated liquid nitrogen storage tanks, fill connections at the port and starboard loading stations, the vaporizer, and distribution piping to the various locations requiring nitrogen. Nitrogen gas is supplied automatically by a pressure control valve. The annular space is protected from possible overpressurization by relief valves. This is especially important on vessels with membrane tanks since overpressurization can cause membrane damage. Figure 20-16. Pneumatic controller. Courtesy Leslie Co. The inert gas system employs an inert gas generator similar to that used on many crude carriers for cargo tank inerting. Low sulphur (less than 1 percent) light marine diesel oil is burned with air at close to ideal (stoichiometric) conditions to produce an inert mixture of primarily nitrogen and carbon dioxide. Oxygen levels are typically 0.5 percent. The combustion gases are passed through a scrubbing tower to cool them and to lower the SOxcontent. The gases then pass through a refrigerant and/or desiccant dryer to remove water vapor and lower the dew point to about -50°F (-45°C). The system is fitted with extensive automatic controls to permit unattended continuous operation. The output from the inert gas plant is used primarily for inerting of the main cargo tanks. F u e l B o il-O ff Figure 20-15. LNG control valve. Courtesy Leslie Co. Systems Because of the normal heat leakage through the insulation of the cargo tanks, a certain amount of LNG cargo will vaporize. The gas produced is called boil-off and amounts to about 0.25 percent ofthe tank capacity per day for a typical tank design. This boil-off is used as fuel in the ship's boilers. The boiler fuel system is designed to use both boil-off and Bunker C fuel. Most systems are designed to maintain a minimum oil flow called pilot oil. At normal sea speed, the pilot oil plus boil-off is not sufficient to 20-20 CRYOGENIC CARGO SYSTEMS maintain boiler pressure, and additional Bunker C must be burned. In port, the boil-off plus pilot oil is more fuel than is required, and excess steam is dumped into the main condenser. A spray-type desuperheater is used to lower the steam temperature. Boil-off gas is removed from the cargo tanks by one of the compressors via the vapor header. Discharge pressure is 10 to 15 psig (69 to 103 kPa). The vapor is discharged to one of the gas heaters where the gas is heated to a controlled temperature of 70'F (21 'C). The compressor maintains the cargo tank pressure constant by monitoring the vapor header pressure and varying compressor capacity accordingly. The gas supply piping in the machinery space is douple-walled ventilated pipe. Gas is delivered through the inner pipe, and exhaust fans continuously vent the annulus between the inner and outer pipes. If the vent fans fail or gas is detected in the vent, the master shutoff valve will close automatically. Boilers designed for dual-fuel operation require some minor modifications. The main change is in the superheater temperature control. Since methane burns with less luminosity than oil, less energy is transferred by radiation in the furnace. This results in higher superheater outlet temperatures with gas firing than with oil. Although some boilers have been designed with double superheaters, most employ single superheaters with the control desuperheater, control valve, and piping sized to handle the 60' to 70'F (33' to 39'C) of additional temperature control required with gas firing, or with the superheater sized for gas firing and thus operating at less than design outlet temperature with oil firing. The registers must be modified to burn gas as well as oil. A series of gas nozzles are installed around the burner cone. A large number of nozzles are necessary to accommodate the large volume of gaseous fuel as compared to liquid fuel. The combustion control system for a dual-fuel system is obviously more complex than for a standard oil-fired boiler. I n s tr u m e n ta tio n Cargo operations are generally controlled from a central cargo control room. Remote instrumentation is available for measurement of many parameters including cargo and ballast tank levels, cargo density, and cargo tank pressures and temperatures. Control and monitoring of cargo and ballast valves are also provided. Some of the parameters monitored in the cargo control room on modern LNG carriers include: 1. Levelsofthe cargotank, both remote and local.Remoteinstruments include nitrogen bubbler and capacitance gauges. Local instruments include float gauges and viewingports with graduated scales.Reliabilityand redundancy are paramount in monitoring this key parameter because of the disastrous consequencesof an LNG spill. LNG CARGO OPERATIONS 20-21 2. Pressures ofliquid, vapor, and cool-downheaders; cargo tanks; LNG pump discharge; insulated and void spaces; inert gas system; compressor discharge; liquid nitrogen tank; compressed air system; and heating steam system. 3. Temperatures of cargo tanks and liquid cargo at various levels to monitor cool-down;secondary barrier for detecting primary barrier and insulation failure; nitrogen system; and vaporizer and gas heater outlets. 4. Monitoringof gas concentrations in insulation barriers, voids,and machinery spaces. Portable instruments are available for testing combustiblegas, oxygen,carbon dioxide,dew point, and halogen levels. LNG CARGO O P E R A T IO N S The procedures described in this section are general and do not apply to any particular class of vessel. They are not intended to be a substitute for the detailed procedures contained in the ship's cargo manual. Reference to Figure 20-12 will aid in understanding the procedures outlined. N o r m a l T r a n s p o r t C y c le An LNG vessel will travel with a small amount of LNG in the cargo tanks during the ballast leg. This maintains the cargo tanks at low temperatures and ready for loading without inerting operations. Tank temperatures are monitored and if a significant difference is noted between the top and bottom ofthe tank, the spray pump is started and residual LNG is sprayed into the top ofthe tank to equalize temperatures. Upon arrival at the loading terminal, the liquid and vapor loading arms are connected, the lines are inerted and cooled, and loading begins. Ship compressors are used for return of vapor to the shore terminal. After loading, the lines are drained, typically using a nitrogen gas purge, and the arms disconnected. During the loading operation, deballasting will maintain a constant draft. During the loaded voyage leg, tank pressure is maintained by the fuel boil-off system. Upon arrival at the discharge terminal, the liquid and vapor loading arms are connected, the lines are inerted and cooled, and discharge begins. The ship cargo pumps are started and vapor is returned by shore compressors. After loading, the lines are drained, and the arms disconnected. During the discharge operation, ballasting will maintain a constant draft. P r e p a r in g a C a r g o T a n k fo r L o a d in g Prior to loading LNG in a warm noninerted tank, the tank must be prepared. The preparation consists of three basic steps: (1) purging with inert gas, (2) purging with natural gas, and (3) cool-down with LNG. The initial purging is normally done using the inert gas generator, though 20-22 CRYOGENIC CARGO SYSTEMS liquid nitrogen from shore can be vaporized for the task. After inerting, the natural gas purging is accomplished by vaporizing LNG from shore and returning the vapor to shore. After purging with vaporized LNG, LNG is sprayed into the tank until the cool-down is complete. cally as the preselected level is reached. When all tanks are loaded, the shore isolation valves are closed. An emergency quick-closing valve system will stop filling automatically if the high tank levels are exceeded. D r a in in g P r e p a r in g a C a r g o T a n k fo r E n tr y Prior to entry for maintenance or inspection, the cargo tank must be warmed, inerted, and aerated. After stripping the tank of LNG, the tank is warmed by pulling vapor from the tank with the compressor, heating it with the gas heaters, and discharging the heated gas back to the tanks. The tanks are then inerted using the inert gas generator or vaporized liquid nitrogen. After inerting, the tank is purged with air. D E T A IL E D O P E R A T IN G PROCEDURES Listed below is more detailed information on the operating procedures (which have been outlined above) for the normal transport cycle, preparation for loading, and preparation for tank entry. S p r a y in g T a n k s d u r in g B a lla s t V o y a g e Some LNG is left in each tank for spraying inside the cargo tanks to keep them cold during the ballast voyage. Spray pumps are started to discharge LNG to the cool-down header to supply the spray nozzles in each tank. Vapor produced during the spraying operation is led through the vapor header to the boil-off compressor. The gas is then heated, and delivered to the engine room for use as fuel in the boilers. a n d C o o l-D o w n During the transit voyage, the cargo lines are isolated from the cargo tanks. Prior to loading or discharge, the lines are inerted with vaporized liquid nitrogen from the LN2 storage tank. After inerting, the spray pumps are used to deliver LNG to the warm liquid lines for cool-down. The pumps discharge to the cool-down header, and the crossover between the liquid and cool-down header and the fill valve on each tank is opened. Liquid for cooling flows through the lines back to the tank. Cooling will continue until the line temperatures are close to that of liquid methane. If not enough liquid is available in the ship's tanks, LNG will be supplied from shore. L o a d in g LNG Mter liquid line cool-down, the loading of LNG can begin. The ship and shore terminal isolation valves are opened, and liquid is delivered to the ship's tanks. The vapor generated during the fill operation and that displaced by the loaded liquid is returned to shore via the vapor header and the ship's compressors. Fill valves on each tank are closed automati- L N G L in e s Mter completion of a loading or discharging operation, the liquid in the lines will be returned to the tanks. The fill valves on each tank are closed, and the crossover between the liquid and cool-down headers is opened. A nitrogen purge is applied to the high point of the liquid fill lines, and the LNG is forced back through the spray nozzles to the tanks. The pressure at the spray nozzle indicates when liquid is no longer flowing. As an alternative, the nitrogen purge can be eliminated. Heat leakage will vaporize some of the LNG, pressurizing the line and forcing the remaining liquid back to the tank. D is c h a r g in g LNG After inerting and cool-down of the cargo lines, the cargo pumps are started. When pump operation is verified, the pump discharge valves are opened. LNG vapor is taken from shore during the discharging operation to maintain cargo tank pressure. The vapor valve to each tank can be used for throttling the gas supply to control tank pressure. If shore pressure is low, a gas compressor can be started to speed up discharging. If LNG vapor is not available from shore, vapor may be generated by vaporizing a small amount of LNG. At the end ofthe discharging operation, the cargo pumps are stopped and the discharge valves closed. I n e r tin g C a r g o L in e I n e r tin g 20-23 DETAILED OPERATING PROCEDURES C argo T anks This procedure applies to tanks that are being prepared for loading LNG. It does not apply to tanks that have been maintained cool and oxygen-free by LNG spraying operations. The inert gas plant is started and discharges to the tank via the vapor header and the gas heater. The gas is heated to about 150'F (66'C) to minimize the time and gas required. The tanks are vented to the atmosphere via the liquid header. The compressors may be used to aid in the venting process. Purging continues until the exhausting gas has an oxygen content of less than 3 percent. In lieu of using the inert gas generators, liquid nitrogen from shore may be vaporized and heated in the LNG vaporizer. Tank pressure must be carefully monitored during this process. P u r g in g C a r g o T a n k s w ith V a p o r iz e d LNG After inerting, the tanks are purged by vaporizing LNG from shore in the ship's LNG vaporizer. The vapors are delivered to the tanks via the vapor header. Purged vapors are exhausted through the liquid fill line and liquid 20-24 CRYOGENIC CARGO SYSTEMS header and returned to shore. Tank pressure is controlled by the LNG vaporizer inlet control valve. The gas being returned to shore is monitored for C02 content. When the C02 content reaches 1 percent, the purging operation is stopped. The ship's compressors may be used to aid in gas return. C a r g o T a n k C o o l-D o w n After inerting and purging with vaporized LNG, liquid LNG is routed to the spray header. The spray line valves on each tank are opened and LNG is discharged from the spray nozzles to cool the tanks. The spray rate will depend on the vapor-reGeiving capacity of the shore facility and the allowable cool-down rate ofthe tank, typically about 18'F (10'C) per hour. The spray rate is controlled by adjusting the spray control valves. Initially the tank pressure will decrease due to the cooling effect of the LNG. Spraying may have to be stopped for a period if the tank pressure goes too low. When the tank pressure increases, a compressor is started to assist in returning vapor to shore. C a r g o T a n k W a r m -U p The first step in preparing a tank for entry is warm-up. The compressors pull vapor from the tank via the vapor header. The vapor is delivered to the gas heaters, heated to about 140'F (60'C), and returned to the tank via the liquid header. Excess vapor is burned in the main boilers as usual. Warming continues until the tank temperatures are approximately ambient, thus insuring that freezing will not occur during the inerting operation. P u r g in g w ith I n e r t G a s After warm-up, the inert gas plant is started. Inert gas is delivered to the tank via the gas heater and liquid header. Gas is removed via the vapor header. Gas may be delivered to the boilers until the methane concentration drops below 50 percent, then it is vented to the atmosphere. Purging continues until the methane concentration drops to 5 percent. Liquid nitrogen from shore can be used instead of inert gas. C a r g o T a n k A e r a tin g After warm-up and inerting, the inert gas plants are used to deliver dry air to the tanks. Dry air is delivered via the vapor header, and the gas is exhausted to the atmosphere via the liquid header. Compressors may be used to aid in the venting process. Purging continues until the oxygen content in the exhausted gas reaches 18 percent. ACKNOWLEDGMENTS 20-25 REFERENCES Curt, R. P., and Delaney, T. D. 1972. M a r i n e T r a n s p o r t a t i o n o f L i q u e f i e d N a t u r a l G a s . Kings Point, N. Y.: National Maritime Research Center (NTIS PB249014ILK). Ffooks, R. 1979. N a t u r a l G a s B y S e a . London: Gentry Books. M a r i t i m e L N G M a n u a l . 1974. Kings Point, N. Y.: National Maritime Research Center (NTIS COM-75-10136/LK). Van Langen, J. R. 1973. L N G L i q u e f a c t i o n P l a n t s a n d A s s o c i a t e d S h o r e s i d e O p e r a t i o n s . Kings Point, N. Y.: National Maritime Research Center (NTIS COM-I0469ILK). ACKNOW LEDGM ENTS The author is indebted to Newport News Shipbuilding and Drydock Company, Leslie Company, the National Maritime Research Center, and Energy Transportation Corporation for their assistance in the preparation of this chapter. CHAPTER 21 H u ll M a c h in e r y EVERETT C. HUNT IN T R O D U C T IO N H ULLmachinery includes the equipments for steering, mooring, anchoring, and handling cargo aboard ship. The development of highly specialized cargo vessels, such as LASH, RO/RO, container, and others, has required the design of unique machinery, e.g., the elevators for LASH ships. The design and operation principles for most types of hull machinery are similar and related to the typical modern steering engines, windlasses, capstans, and winches which are discussed in this chapter. The historic development of hull machinery began with the manpowered mechanical advantage devices on sailing ships. The evolution to steam-powered vessels led to the common use of steam-powered engines for all hull machinery applications. Some of the steam engines are still in use. Electric machinery has been popular for several decades, but hydraulic motors now seem to be the most popular for new construction. The advent of the SCR (silicon controlled rectifier) for high power ratings has led to a renewed interest in DC electric drive machinery because ofthe desirable torque characteristics of such machinery. GENERAL R E Q U IR E M E N T S FOR HULL M A C H IN E R Y Hull machinery must have high reliability; i.e., the machinery must be ready to operate when needed and continue to operate without excessive out-of-service time for maintenance. The design and construction must be simple and rugged. Since the machinery location is frequently exposed to the sea environment, weatherproofing is an important consideration. Modern hull machinery is often designed with the mechanical power device ,)L 1 21-2 located belowdecks to improve the weather resistance. A simple design is easy to repair, which is an important aspect of hull machinery. To assure the longest possible trouble-free life, owners and ship designers should consider the following items when selecting hull machinery. C o r r o s io n P r o te c tio n The structural parts must be designed free of pockets which collect rain and seawater. Where appropriate, corrosion resistant materials should be selected. Fasteners should be galvanized or manufactured from corrosion resistant material. Access covers must have reliable, watertight seals. Finally, all metal parts must be covered with a modern coating system, including a white metal blast preparation, an appropriate primer, and a final epoxy coating. L u b r ic a tio n Deck machinery requires lubrication of the moving parts. The machinery designer can assure adequate lubrication by designing oil and grease fittings which are visible and accessible. The lubricating oil sumps should have clearly labeled and screened fill openings. Provision must be made to drain the sumps for oil replacement. Selection and use of a proper lubricant is another requirement for low maintenance. 21-3 STEERING GEAR HULL MACHINERY S T E E R IN G GEAR The steering engine provides the force necessary to position and hold the rudder against the fluid forces ofthe sea. In general, the forces acting on a rudder are proportional to the rudder angle, the velocity of the vessel squared, and the density of the water. The rudder forces are transferred to the rudder post as a torque which is transmitted to the tiller, a part of the steering engine. In modern practice, there are three types of steering engines available: (1) the electrohydraulic ram type, (2) the rotary vane type, and (3) the direct current high torque electric motor type. E le c tr o h y d r a u lic S te e r in g E n g in e A typical arrangement for a four-cylinder Rapson slide steering engine is shown in Figures 21-1 and 21-2. Two-cylinder arrangements are used on smaller vessels while the four-cylinder arrangement is typical for large vessels. The hydraulic fluid is delivered by a variable delivery pump. (The Hele-Shaw variable delivery pump will be described later.) The fluid under D e s c r ip tio n . M e c h a n ic a l A s s e m b lie s Bearings should be designed for easy shipboard replacement, and spares must be carried. Fittings must have locking or set screws and keys and caps to assure that the device is not assembled improperly, or that it comes adrift unexpectedly. The watertight integrity of all enclosures must be carefully designed and maintained. Adequate access for inspection and adjustment is essential. Unitized construction permits easy installation or subassembly change-out. E le c tr ic a l A s s e m b lie s Hull machinery electric motors should have conservative nameplate ratings. Electric circuits and switch gear must be designed and operated to protect the motor. Motors must be located belowdecks or have watertight enclosures for deck installation. Adequate cooling is needed for any design. Terminal wiring should always be clearly marked. H y d r a u lic A s s e m b lie s Cleanliness of the hydraulic fluid is necessary for acceptable life and operation of hydraulic operators. Fill screens, pump strainers, filters, and provision for cleaning and replacement are required in the design. The hydraulic pumps and motors must have conservative design ratings. Figure 21-1. Factory test view of typical four-ram Rapson slide steering engine STEERING GEAR 21-5 high pressure passes through a valve chest to either of the opposed cylinders. Each of the two pumps is continuously driven in one direction by an electric motor which runs at a light power load, except when the steering gear is actually driving the rudder. A floating lever control or a differential feedback connected to the pump stroke control causes the rudder to move in direct relationship to the steering wheel or other proportionate control and stops the pump delivery when the position of the rudder is at the required angle. Normally, only one pump or motor would be running; the second unit is a backup. The procedure for shifting from one pump to another will vary with the details of a particular design. The shipboard engineering officers must become familiar with the changeover procedure, so that it can be accomplished quickly under emergency circumstances. The tiller crosshead is made of forged or cast steel with a boss bored and keyed to fit the rudderstock. It is usually a single piece shrunk onto the rudderstock. In some installations, the tiller is made in two pieces which are bolted together on the rudderstock as shown in Figure 21-2. A cylindrical pin is formed on each side of the tiller boss to connect to the hydraulic cylinder crossheads. The four single-acting hydraulic rams act in pairs to provide the force on the tiller. Two cylinders are in line and carry rams bolted together at the juncture which forms upper and lower bearings for the trunnion arms of the swivel block. The swivel block has gunmetal bushing contact with the tiller pin. The ram translation movement is transmitted to the tiller through the swivel block, which moves longitudinally with the rams, turns in the bearings located at the rams juncture, and slides on the tiller arm to compensate the angular motion ofthe tiller. This arrangement is called a "Rapson slide" and is schematically illustrated in Figure 21-3. The Rapson slide has the desirable characteristic of increasing the torque available to move the rudder with increasing rudder angle. The torque available at large rudder angles is about 30 percent greater than at mid-rudder position. The hydraulic cylinders are manufactured from cast steel and the parallel sets are connected by beams which restrain cylinder movement due to side loading of the Rapson slide mechanisms. This side loading force, Fb, is illustrated in Figure 21-3. The hydraulic fluid is delivered by a variable delivery (i.e., stroke) pump through a valve chest to either of the opposed cylinders. Each pump is continuously driven in one direction by an electric motor which runs on essentially no load, except when the stroke of the pump is increased to provide fluid to move the rams in either direction. A welded steel tank, required by classification agencies, is installed to carry a reserve supply of oil for use in an emergency. A normally closed 21-6 HULL MACHINERY valve may be opened to permit the operating variable speed pump to deliver the reserve oil supply to the system. The hydraulic system pressure pipes are made of drawn steel with steel flanges. Valve chests are formed from forged steel blocks, machined externally, and bored out to form the required flow passages. Emergency rudder-locking valves are fitted in the supply line to each cylinder at the point of pipe connection. The system is fitted with pressure relief valves which relieve excessive pressure in the cylinders due to abnormal rudder forces, thus protecting the rudderstock and other parts from damage. The valve chest is arranged so that the rudder may be operated with two cylinders and reduced torque if a casualty puts one of the four cylinders out of service. system for electrohydraulic steering. Operation of the electrohydraulic steering system requires a constant speed variable stroke pump to supply hydraulic fluid to the rams. The Hele-Shaw pump is typical of the designs selected for this application. Figure 21-4 shows a sectional view of the Hele-Shaw pump construction. The following description of the pump operation refers to the parts identified in Figure 21-4. P u m p in g STEERING GEAR 21-7 Oil is drawn into and discharged from the pump through the flanges A and B mounted on the pipe connection cover D, in which is located a stationary central cylindrical valve E. The central valve is equipped with ports C which serve to transmit the fluid to and from the rotating cylinder body F. The cylinder body, mounted and free to turn on the cylindrical valve E, is supported by ball bearings P, and is rotated through the spindle G by the electric motor. In the periphery ofthe cylinder body, steel plungers H, usually 7 in number, are mounted. The end of each plunger carries a pair of slippers K, the outer faces of which are shaped to ride in the floating ring L. This floating ring is in turn mounted on two large ball bearings M situated on each side of the cylinder body, the outer races of these bearings being fitted in the guide blocks N. Guide rods 0 connect to the guide blocks and extend out through the case of the pump, and serve as the means to change the position ofthe floating ring with respect to the axis X-X of the pump. Therefore, by moving the guide rods in either direction, the stroke of the plungers H is altered, which changes the rate of discharge of the pump. Figure 21-5 shows a descriptive view of the Hele-Shaw pump with all parts in approximate relation to each other. Figure 21-6 also shows a cutaway section of the assembled pump. To illustrate the principle of the Hele-Shaw pump, a series of line drawings appear in Figure 21-7. It should be noted, for the purpose of simplicity, that only one plunger is shown in these diagrams, although 21-8 HULL MACHINERY there are usually 7 equally spaced about the periphery of the floating ring. Diagram A shows the floating ring in the neutral position; i.e., the axis of the floating ring coincides with the axis of the pump. In this position, as the cylinder body rotates, no pumping action occurs, since the plunger remains at a constant distance from the stationary central valve during the rotation. In diagram B, the floating ring has been moved to the right, causing the plunger, as it rotates toward the right side, to move toward the center or axis of the pump; and as it rotates to the right side, it is moving away from the axis of the pump. Accordingly, during the upper half of the rotation from X to Y, the piston is moving away from the central valve and produces the suction stroke, moving oil into the cylinder body. During the other half of the revolution, or from Y to X, as shown in diagram C, the plunger is being forced in toward the central valve, thereby discharging the oil. The pump suction occurs in the upper port ofthe central valve, and discharge occurs through the lower port of the central valve. Diagram D shows the action that would take place when the pump is stroked in the opposite direction, or to the left. Of course, when this occurs, the suction and discharge ports are reversed, and suction from the bottom port and discharge from the upper port of the valve are obtained. 21-10 HULL MACHINERY Moving the crosshead indicated in Figure 21-4 toward or away from the axis of the pump effects a change in the relationship of the floating ring with respect to the pump's axis X-X,causing a corresponding change in the discharge as well as the direction of flow of the fluid. The crosshead in Figure 21-4 connects to, and is actuated by, the differential follow-up control mechanism of the steering gear. Maintenance of electrohydraulic steering system. Maintenance between overhauls is normally limited to adjustment of packing glands on valves and the hydraulic rams. The classification agency will usually require periodic inspection ofthe variable stroke pumps. The bearings of these pumps and the motors are excellent subjects for vibration signature analysis as a means to detect change and potential for failure. Vibration signature analysis is described in Chapter 25. The following inspections of the steering gear should be made on a daily basis: 1. 2. 3. 4. 5. 6. 7. Check piping for leaks. Check that ram pressures are normal with gear in operation. Check all oil levels. Check pump temperature. Check motor temperature. Check hydraulic oil tank level. Check glands for leakage. On a weekly basis, the following routine should be observed for an operating hydraulic steering gear. 1. 2. 3. 4. 5. 6. 7. 8. Check communication system to bridge and engine central control station. Check air release valves. Check local control handwheel operation. Change over to alternative system of remote control and check operation. Check grease supply in fitting. Check filters. Change over power units. Log condition of steering gear. The following inspections should be made before leaving port: 1. 2. 3. 4. 5. Be sure machinery guards are in place. Switch C°:1 electric supply. Start one of the power units. Follow daily and weekly routines described above. Check sea gland. STEERING GEAR 21-11 Rotary Vane Steering Gear System The rotary vane type steering gear is a compact yet powerful steering gear available in a wide range of torque capability. Figure 21-8 illustrates the operating principle. The rotary vane steering gear has only two basic parts: the rotor and the stator. The stator is bolted to the ship's foundation through preloaded synthetic rubber shock absorbers. The hydraulic chamber which imparts the torque to the rotor is formed by vanes on the rotor and stator components. The fixed vanes are secured to the stator by high tensile steel dowel pins and cap screws. The moving vanes are secured to the rotor by cap screws and keys. The vanes are designed with adequate mechanical strength to permit them to act as rudder stops at the extremes of travel. Steel sealing strips backed by synthetic rubber are fitted in grooves along the sealing faces of the fixed and moving vanes to prevent leakage and 21-12 21-13 HULL MACHINERY STEERING GEAR provide high volumetric efficiency at the highest operating pressure. The single moving part, i.e., the rotor, is lubricated by the hydraulic fluid. Since the vanes impart only torque to the stator casing and rotor, wear is minimal. Furthermore, the rudderhead, bearing, and carrier are not subjected to any radial loading as a result of the steering gear action. A unit assembled for factory test is shown in Figure 21-9. The duplicate servo-controlled variable delivery pumps driven by electric motors shown in this figure provide the hydraulic power for the unit. torque motors operating a mechanical rotary actuator, which rotates the rudderstock through a balanced planetary gear. Either motor may operate the actuator. When the helm requires no rudder motion, the motor does not run, and the rudder is held in a locked position by brakes integral with the motor. Figure 21-11 is a basic electric diagram which illustrates the operating concept of the electromechanical steering gear. The operator selects either of the two redundant systems by operating a selector switch at the helm. When the starboard system is selected, power is supplied to the starboard position control and the SCR controller. The port steering system is isolated and inactive. When the steering wheel position indicates a call to reposition the rudder, the signal potentiometer develops a signal equivalent to the rudder position required by the helm. The follow-up potentiometer on the rudderstock transmits the feedback signal of actual rudder position. The two signals are compared in the SCR controller; if an error exists, that is, a difference between required and actual position, the comparator causes the SCR controller to energize the drive motor in the direction to eliminate the error. The SCR controller controls the starting, stopping, acceleration, and deceleration of the motor. While the motor is driving the rudder, the E le c tr o m e c h a n ic a l S te e r in g G e a r S y s te m Studies sponsored by the U. S. Maritime Administration have suggested the feasibility of using high torque electromechanical systems to replace the contemporary hydraulic steering systems. Small size of the machinery package, light weight, high efficiency, and high reliability are claimed for such systems. Figure 21-10 from a Maritime Administration report, published in 1978, shows historical data on the reliability of the components of a hydraulic steering gear system. These data compared to a mechanical system suggest that overall reliability could be improved by substituting suitable electromechanical components for the pumps, piping, and valves, and constantly operating motors ofthe hydraulic system. s t e e r i n g g e a r c o n c e p t . A proposed electromechanical steering gear consists of two independently powered direct current high E le c tr o m e c h a n ic a l Figure 21-12. Cutaway view of proposed electromechanical steering gear Figure 21-11. Electrical diagram for typical electromechanical steering gear SCR controller will limit maximum motor speed, torque, and power, and provide regenerative action when the motor experiences overhauling loads. The mechanical actuator illustrated in Figure 21-12 is proposed as a highly compact unitized design which includes four elements. The combining unit employs a differential gear arrangement to accept power from either electric motor while isolating the motor which is not in use. A torque relief unit limits the torque which can be applied to the actuator system by high externally applied sea and ship forces on the rudder. A balanced planetary rotary output actuator drives the rudderstock. The mechanical follow-up drive rod connected to the rudderstock provides input to the feedback potentiometer to stop the motor. The motor drives the actuator through bining unit. The idle drive gear holds one braked position. Ifboth motors are started, rudder at twice the normal rate, since the differential. I n te r n a tio n a l A greem ent the side the two o n S te e r in g spur gear differential comofthe differential gear in a differential would drive the inputs will be added by the G e a r R e q u ir e m e n ts S t e e r i n g g e a r f a i l u r e s . A series of steering gear failures occurred in the years 1960 to 1977, resulting in serious pollution by tankers and loss oflife and vessels for all types of ships. These calamities resulted in initiatives to establish international agreement on the requirements for steering systems. Prior to the international agreement, the reliability requirements for steering gear systems were minimal. 21-16 HULL MACHINERY The requirements of the International Convention for the Safety of Life at Sea, 1974 (SOLAS 74) is a detailed document for new and existing ships of different types and sizes. This document was revised and extended by the International Conference on Tanker Safety and Pollution Prevention, 1978. The following regulations for steering gears are quoted from SOLAS 74 as well as the amendments of 1981. SO L A S 74. Regulation 29 STEERING GEAR (a) Passenger Ships and Cargo Ships (i) Ships shall be provided with a main steering gear and an auxiliary steering gear to the satisfaction of the Administration. (ii) The main steering gear shall be of adequate strength and sufficient to steer the ship at maximum service speed. The main steering gear and rudder stock shall be so designed that they are not damaged at maximum astern speed. (iii) The auxiliary steering gear shall be of adequate strength and sufficient to steer the ship at navigable speed and capable of being brought speedily into action in an emergency. (iv) The exact position ofthe rudder, if power operated, shall be indicated at the principal steering station. (b) Passenger Ships only (i) The main steering gear shall be capable of putting the rudder over from 35 degrees on one side to 35 degrees on the other side with the ship running ahead at maximum service speed. The rudder shall be capable of being put over from 35 degrees on either side to 30 degrees on the other side in 28 seconds at maximum service speed. (ii) The auxiliary steering gear shall be operated by power in any case in which the Administration would require a rudder stock of over 228.6 millimetres (9 inches) diameter in way ofthe tiller. (iii) Where main steering gear power units and their connexions are fitted in duplicate to the satisfaction of the Administration, and each power unit enables the steering gear to meet the requirements of sub-paragraph (i) of this paragraph, no auxiliary steering gear need be required. (iv) Where the Administration would require a rudder stock with a diameter in way of the tiller exceeding 228.6 millimetres (9 inches) there shall be provided an alternative steering station located to the satisfaction of the Administration. The remote steering control systems from the principal and alternative steering stations shall be so arranged to the satisfaction of the Administration that failure of either system would not result in inability to steer the ship by means of the other system. (v) Means satisfactory to the Administration shall be provided to enable orders to be transmitted from the bridge to the alternative steering station. (c) Cargo Ships only (i) The auxiliary steering gear shall be operated by power in any case in which the Administration would require a rudder stock of over 355.6 21-17 STEERING GEAR millimetres (14 inches) diameter in way of the tiller. (ii) Where power-operated steering gear units and connexions are fitted in duplicate to the satisfaction of the Administration, and each unit complies with sub-paragraph (iii) of paragraph (a) of this Regulation, no auxiliary steering gear need be required, provided that the duplicate units and connexions operating together comply with sub-paragraph (ii) of paragraph (a) of this Regulation. Regulation 30 ELECTRIC AND ELECTROHYDRAULIC STEERING GEAR (a) Passenger Ships and Cargo Ships Indicators for running indication of the motors of electric and electrohydraulic steering gear shall be installed in a suitable location to the satisfaction of the Administration. (b) All Passenger Ships (irrespective of tonnage) and Cargo Ships of5,000 Tons Gross Tonnage and upwards (i) Electric and electrohydraulic steering gear shall be served by two circuits fed from the main switchboard. One of the circuits may pass through the emergency switchboard, if provided. Each circuit shall have adequate capacity for supplying all the motors which are normally connected to it and which operate simultaneously. If transfer arrangements are provided in the steering gear room to permit either circuit to supply any motor or combination of motors, the capacity of each circuit shall be adequate for the most severe load condition. The circuits shall be separated throughout their length as widely as is practicable. (ii) Short circuit protection only shall be provided for these circuits and motors. (c) Cargo Ships ofless than 5,000 Tons Gross Tonnage (i) Cargo ships in which electrical power is the sole source of power for both main and auxiliary steering gear shall comply with sub-paragraphs (i) and (ii) of paragraph (b) of this Regulation, except that ifthe auxiliary steering gear is powered by a motor primarily intended for other services, paragraph (b)(ii)may be waived, provided that the Administration is satisfied with the protection arrangements. (ii) Short circuit protection only shall be provided for motors and power circuits of electrically or electrohydraulically operated main steering gear. SO L A S 74 A s A m ended (1 9 8 1 ) Chapter II-I: Regulation 29 STEERING GEAR 1 Unless expressly provided otherwise, every ship shall be provided with a main steering gear and an auxiliary steering gear to the satisfaction of the Administration. The main steering gear and the auxiliary steering gear shall be so arranged that the failure of one of them will not render the other one inoperative. 21-18 HULL MACHINERY 2.1 All the steering gear components and the rudder stock shall be of sound and reliable construction to the satisfaction of the Administration. Special consideration shall be given to the suitability of any essential component which is not duplicated. Any such essential component shall, where appropriate, utilize anti-friction bearings such as ball bearings, roller bearings or sleeve bearings which shall be permanently lubricated or provided with lubrication fittings. 2.2 The design pressure for calculations to determine the scantlings of piping and other steering gear components subjected to internal hydraulic pressure shall be at least 1.25 times the maximum working pressure to be expected under the operational conditions specified in paragraph 3.2, taking into account any pressure which may exist in the low pressure side of the system. At the discretion ofthe Administration, fatigue criteria shall be applied for the design of piping and components, taking into account pulsating pressures due to dynamic loads. 2.3 Reliefvalves shall be fitted to any part of the hydraulic system which can be isolated and in which pressure can be generated from the power source or from external forces. The setting of the relief valves shall not exceed the design pressure. The valves shall be of adequate size and so arranged as to avoid an undue rise in pressure above the design pressure. 3 The main steering gear and rudder stock shall be: .1 of adequate strength and capable of steering the ship at maximum ahead service speed which shall be demonstrated; .2 capable of putting the rudder over from 35° on one side to 35° on the other side with the ship at its deepest seagoing draft and running ahead at maximum ahead service speed and, under the same conditions, from 35° on either side to 30° on the other side in not more than 28 seconds; .3 operated by power where necessary to meet the requirements of paragraph 3.2 and in any case when the Administration, excluding strengthening for navigation in ice, requires a rudder stock of over 120 mm diameter in way of the tiller; and .4 so designed that they will not be damaged at maximum astern speed; however, this design requirement need not be proved by trials at maximum astern speed and maximum rudder angle. 4 The auxiliary steering gear shall be: .1 of adequate strength and capable of steering the ship at navigable speed and of being brought speedily into action in an emergency; .2 capable of putting the rudder over £Tom15° on one side to 15° on the other side in not more than 60 seconds with the ship at its deepest seagoing draught and running ahead at one half of the maximum ahead service speed or 7 knots, whichever is the greater; and .3 operated by power where necessary to meet the requirements of paragraph 4.2 and in any case when the Administration, excluding strengthening for navigation in ice, requires a rudder stock over 230 mm diameter in way of the tiller. STEERING GEAR 21-19 5 Main and auxiliary steering gear power units shall be: .1 arranged to re-start automatically when power is restored after a power failure; and .2 capable of being brought into operation from a position on the navigating bridge. In the event of a power failure to anyone of the steering gear power units, an audible and visual alarm shall be given on the navigating bridge. 6.1 Where the main steering gear comprises two or more identical power units, an auxiliary steering gear need not be fitted, provided that: .1 in a passenger ship, the main steering gear is capable of operating the rudder as required by paragraph 3.2 while anyone of the power units is out of operation; .2 in a cargo ship, the main steering is capable of operating the rudder as required by paragraph 3.2 while operating with all power units; .3 the main steering gear is so arranged that after a single failure in its piping system or in one of the power units the defect can be isolated so that steering capability can be maintained or speedily regained. 6.2 The Administration may, until September 1986, accept the fitting of a steering gear which has a proven record of reliability but does not comply with the requirements of paragraph 6.1.3 for a hydraulic system. 6.3 Steering gears, other than of the hydraulic type, shall achieve standards equivalent to the requirements of this paragraph to the satisfaction of the Administration . 7 Steering gear control shall be provided: .1 for the main steering gear, both on the navigating bridge and in the steering gear compartment; .2 where the main steering gear is arranged in accordance with paragraph 6, by two independent control systems, both operable from the navigating bridge. This does not require duplication ofthe steering wheel or steering lever . Where the control system consists of an hydraulic telemotor, a second independent system need not be fitted, except in a tanker, chemical tanker or gas carrier of 10,000 tons gross tonnage and upwards; .3 for the auxiliary steering gear, in the steering gear compartment and, ifpower operated, it shall also be operable from the navigating bridge and shall be independent of the control system for the main steering gear. 8 Any main and auxiliary steering gear control system operable from the navigating bridge shall comply with the following: .1 if electric, it shall be served by its own separate circuit supplied from a steering gear power circuit from a point within the steering gear compartment, or directly from switchboard busbars supplying that steering gear power circuit at a point on the switchboard adjacent to the supply to the steering gear power circuit; .2 means shall be provided in the steering gear compartment for disconnecting any control system operable from the navigating bridge from the steering gear it serves; .3 the system shall be capable of being brought into operation £Toma 21-20 HULL MACHINERY position on the navigating bridge; .4 in the event of a failure of electrical power supply to the control system, an audible and visual alarm shall be given on the navigating bridge; and .5 short circuit protection only shall be provided for steering gear control supply circuits. 9 The electric power circuits and the steering gear control systems with their associated components, cables and pipes required by this Regulation and by Regulation 30 shall be separated as far as is practicable throughout their length. 10 A means ofcommunication shall be provided between the navigating bridge and the steering gear compartment. 11 The angular position of the rudder shall: .1 if the main steering gear is power operated, be indicated on the navigating bridge. The rudder angle indication shall be independent of the steering gear control system; .2 be recognizable in the steering gear compartment . 12 Hydraulic power operated steering gear shall be provided with the following: .1 arrangements to maintain the cleanliness of the hydraulic fluid taking into consideration the type and design of the hydraulic system; .2 a low level alarm for each hydraulic fluid reservoir to give the earliest practicable indication of hydraulic fluid leakage. Audible and visual alarms shall be given on the navigating bridge and in the machinery space where they can be readily observed; and .3 a fixed storage tank having sufficient capacity to recharge at least one power actuating system including the reservoir, where the main steering gear is required to be power operated. The storage tank shall be permanently connected by piping in such a manner that the hydraulic systems can be readily recharged from a position within the steering gear compartment and shall be provided with a contents gauge. 13 The steering gear compartment shall be: .1 readily accessible and, as far as practicable, separated from machinery spaces; and .2 provided with suitable arrangements to ensure working access to steering gear machinery and controls. These arrangements shall include handrails and gratings or other non-slip surfaces to ensure suitable working conditions in the event of hydraulic fluid leakage. 14 Where the rudder stock is required to be over 230 mm diameter in way of the tiller, excluding strengthening for navigation in ice, an alternative power supply, sufficient at least to supply the steering gear power unit which complies with the requirements of paragraph 4.2 and also its associated control system and the rudder angle indicator, shall be provided automatically, within 45 seconds, either from the emergency source of electrical power or from an independent source of power located in the steering gear compartment. This independent source of power shall be used only for this purpose. In every ship STEERING GEAR 21-21 of 10,000 tons gross tonnage and upwards, the alternative power supply shall have a capacity for at least 30 minutes of continuous operation and in any other ship for at least 10 minutes. 15 In every tanker, chemical tanker or gas carrier of 10,000 tons gross tonnage and upwards and in every other ship of70,000 tons gross tonnage and upwards, the main steering gear shall comprise two or more identical power units complying with the provisions of paragraph 6. 16 Every tanker, chemical tanker or gas carrier of 10,000 tons gross tonnage and upwards shall, subject to paragraph 17, comply with the following: .1 the main steering gear shall be so arranged that in the event of loss of steering capability due to a single failure in any part of one of the power actuating systems of the main steering gear, excluding the tiller, quadrant or components serving the same purpose, or seizure of the rudder actuators, steering capability shall be regained in not more than 45 seconds after the loss of one power actuating system. .2 The main steering gear shall comprise either: .2.1 two independent and separate power actuating systems, each capable of meeting the requirements of paragraph 3.2; or .2.2 at least two identical power actuating systems which, acting simultaneously in normal operation, shall be capable of meeting the requirements of paragraph 3.2. Where necessary to comply with this requirement, inter-connexion of hydraulic power actuating systems shall be provided . Loss ofhy-draulic fluid from one system shall be capable of being detected and the defective system automatically isolated so that the other actuating system or systems shall remain fully operational. .3 Steering gears other than ofthe hydraulic type shall achieve equivalent standards. 17 For tankers, chemical tankers or gas carriers of 10,000 tons gross tonnage and upwards, but ofless than 100,000 tonnes deadweight, solutions other than those set out in paragraph 16, which need not apply the single failure criterion to the rudder actuator or actuators, may be permitted provided that an equivalent safety standard is achieved and that: .1 following loss of steering capability due to a single failure of any part of the piping system or in one of the power units, steering capability shall be regained within 45 seconds; and .2 where the steering gear includes only a single rudder actuator, special consideration is given to stress analysis for the design including fatigue analysis and fracture mechanics analysis, as appropriate, to the material used, to the installation of sealing arrangements and the testing and inspection and to the provision of effective maintenance. In consideration of the foregoing, the Administration shall adopt regulations which include the provisions of the Guidelines for Acceptance of Non-Duplicated Rudder Actuators of Tankers of 10,000 tons Gross Tonnage and above but less than 100,000 tonnes Deadweight, adopted by the Organization. 18 For a tanker, chemical tanker or gas carrier of 10,000 tons gross tonnage and upwards, but less than 70,000 tonnes deadweight, the Administration may, 21-22 HULL MACHINERY until 1 September 1986, accept a steering gear system with a proven record of reliability which does not comply with the single failure criterion required for a hydraulic system in paragraph 16. 19 Every tanker, chemical tanker or gas carrier of 10,000 tons gross tonnage and upwards, constructed before 1 September 1984, shall comply, not later than 1 September 1986, with the following: .1 the requirements of paragraph 7.1,8.2,8.4,10,11,12.2,12.3 and 13.2; .2 two independent steering gear control systems shall be provided each of which can be operated from the navigating bridge. This does not require duplication of the steering wheel or steering lever; .3 ifthe steering gear control system in operation fails, the second system shall be capable of being brought into immediate operation from the navigating bridge; and .4 each steering gear control system, if electric, shall be served by its own separate circuit supplied from the steering gear power circuit or directly from switchboard busbars supplying that steering gear power circuit at a point on the switchboard adjacent to the supply to the steering gear power circuit. 20 In addition to the requirements of paragraph 19, in every tanker, chemical tanker or gas carrier of 40,000 tons gross tonnage and upwards, constructed before 1 September 1984, the steering gear shall, not later than 1 September 1988, be so arranged that, in the event of a single failure of the piping or of one of the power units, steering capability can be maintained or the rudder movement can be limited so that steering capability can be speedily regained. This shall be achieved by: .1 an independent means of restraining the rudder; or .2 fast acting valves which may be manually operated to isolate the actuator or actuators from the external hydraulic piping together with a means of directly refilling the actuators by a fixed independent power operated pump and piping system; or .3 an arrangement such that, where hydraulic power systems are interconnected, loss of hydraulic fluid from one system shall be detected and the defective system isolated either automatically or from the navigating bridge so that the other system remains fully operational. Regulation 30 ADDITIONAL REQUIREMENTS FOR ELECTRIC AND ELECTROHYDRAULIC STEERING GEAR 1 Means for indicating that the motors of electric and electrohydraulic steering gear are running shall be installed on the navigating bridge and at a suitable main machinery control position. 2 Each electric or electrohydraulic steering gear comprising one or more power units shall be served by at least two exclusive circuits fed directly from the main switchboard; however, one of the circuits may be supplied through the emergency switchboard. An auxiliary electric or electrohydraulic steering gear associated with a main electric or electrohydraulic steering gear may be connected 21-23 STEERING GEAR to one of the circuits supplying this main steering gear. The circuits supplying an electric or electrohydraulic steering gear shall have adequate rating for supplying all motors which can be simultaneously connected to them and may be required to operate simultaneously. 3 Short circuit protection and an overload alarm shall be provided for such circuits and motors. Protection against excess current, including starting current, if provided, shall be for not less than twice the full load current of the motor or circuit so protected, and shall be arranged to permit the passage of the appropriate starting currents. Where a three-phase supply is used an alarm shall be provided that will indicate failure of anyone of the supply phases. The alarms required in this subparagraph shall be both audible and visual and shall be situated in a conspicuous position in the main machinery space or control room from which the main machinery is normally controlled and as may be required by Regulation 51. 4 When in a ship of less than 1,600 tons gross tonnage an auxiliary steering gear which is required by Regulation 29.4.3 to be operated by power is not electrically powered or is powered by an electric motor primarily intended for other services, the main steering gear may be fed by one circuit from the main switchboard. Where such an electric motor primarily intended for other services is arranged to power such an auxiliary steering gear, the requirement of paragraph 2.2 may be waived by the Administration if satisfied with the protection arrangement together with the requirements of Regulation 29.5.1 and.2 and 29.7.3 applicable to auxiliary steering gear. Chapter V: Regulation 19-1 OPERATION OF STEERING GEAR In areas where navigation demands special caution, ships shall have more than one steering gear power unit in operation when such units are capable of simultaneous operation. Regulation 19-2 STEERING GEAR-TESTING AND DRILLS (a) Within 12 hours before departure, the ship's steering gear shall be checked and tested by the ship's crew. The test procedure shall include, where applicable, the operation of the following: (i) the main steering gear; (ii) the auxiliary steering gear; (iii) the remote steering gear control systems; (iv) the steering positions located on the navigating bridge; (v) the emergency power supply; (vi) the rudder angle indicators in relation to the actual position of the rudder; (vii) the remote steering gear control system power failure alarms; (viii) the steering gear power unit failure alarms; and 21-24 HULL MACHINERY (ix) automatic isolating arrangements and other automatic equipment. (b) The checks and tests shall include: (i) the full movement of the rudder according to the required capabilities of the steering gear; (ii) a visual inspection of the steering gear and its connecting linkage; and (iii) the operation of the means of communication between the navigating bridge and steering gear compartment. (c)(i) Simple operating instructions with a block diagram showing the changeover procedures for remote steering gear control systems and steering gear power units shall be permanently displayed on the navigating bridge and in the steering gear compartment. (ii) All ships officers concerned with the operation or maintenance of steering gear shall be familiar with the operation ofthe steering systems fitted on the ship and with the procedures for changing from one system to another. (d) In addition to the routine cheeks and tests prescribed in paragraphs (a) and (b), emergency steering drills shall take place at least once every three months in order to practise emergency steering procedures. These drills shall include direct control from within the steering gear compartment, the communications procedures with the navigating bridge and, where applicable, the operation of alternative power supplies. (e) The Administration may waive the requirement to carry out the checks and tests prescribed in paragraphs (a) and (b) for ships which regularly engage on voyages of short duration. Such ships shall carry out these checks and tests at least once every week. CDThe date upon which the checks and tests prescribed in paragraphs (a) and (b) are carried out and the date and details of emergency steering drills carried out under paragraph (d), shall be recorded in the log book as may be prescribed by the Administration. WINDLASSES, CAPSTANS, AND GYPSIES Considerable confusion has always existed as to the correct meaning of the words "windlass," "capstan," and "gypsy." While the word "gypsy" is of relatively recent usage, the words "capstan" and "windlass" have been used interchangeably in the past, and it has only been within the last century that the meaning of each term has been more definitely established, particularly in the case of the windlass. The gypsy, for instance, is still often referred to as a capstan, and vice versa. Present-day usage of the word "windlass" generally implies a chain sprocket or sprockets, known as wildcats, mounted on a vertical or horizontal shaft, hand- or power-operated, or both, and used primarily for handling an anchor by means of chain. The word "capstan" describes a vertical-barrelled, rotative device arranged for either hand operation or hand and power operation with pawls at its base to prevent it from reversing, and generally used for WINDLASSES, CAPSTANS, AND GYPSIES 21-25 warping or pulling objects in a horizontal direction. It is sometimes used for handling ground tackle. The "gypsy" is a vertical-barrelled device, direct-connected to a reduction gear and always power-operated. It is used largely for the rapid handling of two lines and for warping purposes. The name "gypsy" is not universally accepted, and many machines meeting this description are found designated as capstans, although no provision is made for hand operation. Windlass A ship's windlass is designed primarily for handling the anchor, but it is frequently used for handling lines and warps, as well. Its evolution stretches back several centuries, beginning when anchors, because ofthe increasing size of vessels, became too heavy to be hoisted hand over hand. From the vertical log, forerunner of the capstan, through the horizontal log, predecessor of the winch, the ship's windlass of today has emerged-a highly mechanized piece of equipment. Although it is used intermittently, and then only for brief periods, it must be of sturdy construction and capable of handling all loads required of it under the most severe conditions. Windlasses of many designs are used on shipboard. Hand-operated windlasses are found on very small vessels, but on the larger ships they are always power-operated, in view ofthe heavy anchors which must be raised. In addition to the wildcats, which handle the anchor chains, windlasses are usually equipped with warping heads (sometimes referred to as gypsy heads) for warping purposes and for the handling of cargo. Steam spur gear windlass. On older steam vessels, steam-driven spurgeared windlasses are sometimes employed. This type is illustrated in Figure 21-13. Assemblies and sectional views of the windlass are shown in Figures 21-14 through 21-17. The letter designations in all illustrations, including Figure 21-13, refer to the same parts. Figure 21-13. Steam-driven spur-geared windlass 21-26 HULL MACHINERY The windlass is a self-contained unit driven by a horizontal piston valve engine A mounted on the same bedplate. The unit is usually located on board ship so that the cylinders are set forward, thereby allowing the power unit to be placed well up in the bow. Operator control levers Band Care grouped conveniently together at the after end of the machine, as shown. The unit must, of course, be bolted securely to the ship's structure to avoid the possibility of its tearing loose during operation. The windlass is controlled through a piston-type reverse valve D, the engine driving the wildcats and warping heads through a double reduction spur gear. The gear is protected by shrouded gear guards, which also serve as a safety precaution. to those working close to the unit. Referring to Figures 21-14 through 21-18, the steam cylinders A drive the crankshaft E upon which the sleeve pinion F is freely mounted and which, in turn, carries the keyed gear G. In addition to the eccentrics H ofthe engine, the crankshaft E also carries the freely mounted pinion K and ajaw clutch L, the latter being provided to engage or disengage the engine from the windlass. When the clutch L has engaged the pinion K, the latter operates the gear M mounted and keyed on intermediate shaft N. Up to this point it is seen that the engine is driving only the warping heads 0 which are keyed to the intermediate shaft N. Shaft N, however, carries the sliding jaw clutch P and the freely mounted gear R. It, therefore, follows that when the clutch P engages gear R, the latter drives the gear G which in turn operates the main gear S through the sleeve pinion F. Gear S is keyed to the wildcat shaft U. In other words, when it is desired to operate the wildcats under power, gear R must be connected to the intermediate shaft N by the engagement of clutch P. When only the warping heads are to be operated by power, the clutch P must be in the disengaged position, and in this case the chain load must be held by applying the band brakes at the wildcat. The wildcats T are free to turn on the main shaft U but may be connected to it by a locking mechanism consisting of the locking head W, keyed to the shaft U, the locking ring X mounted on the locking head W, and two sliding block keys Y. As the locking ring X is turned (through the use of a bar inserted in holes AA) the annular lugs BB on its periphery draw the block keys Y into pockets in the side of the wildcat, thereby locking the wildcat to the locking head W. Turning the locking ring X in the opposite direction withdraws the block keys Y from the pocket and the wildcat is again free to turn on shaft U. Each wildcat is also equipped with a band brake of the screw compressor type. The brake consists of a forged steel band CC which encircles one flange of the wildcat, and connects to a screw shaft DD through a system of links and levers. Turning handwheel EE in one direction tightens the band CC around the flange of the wildcat and turning the wheel in the opposite direction releases the band. Holes FF are provided in the rim of the handwheel into WINDLASSES, CAPSTANS, AND GYPSIES 21-27 which a bar NN may be inserted to gain leverage in the braking operation. A chain stripper GG is provided at each wildcat to prevent the anchor chain from binding on the wildcat lugs as anchor is hove in. Figure 21-14. Plan view of steam windlass 21-28 WINDLASSES, HULL MACHINERY CAPSTANS, AND GYPSIES 21-29 r L ~ Figure 21-16. Transverse section view of windlass To operate winch heads: Figure 21-15. Section view of windlass wildcat shaft As mentioned previously, the engine is controlled by the piston-type reverse valve D, an internal view of which is shown in the drawing, Figure 21-17. The valve is operated by the hand lever C, engaging notches in the segment HH. When the lever latch engages the notch KK, the windlass operates in one direction and when the lever engages notch LL, rotation in the opposite direction results. When the center notch MM is engaged, no steam is passing to the engine. The fact that well-designed windlasses are generally of rugged construction enables them to give good service with little attention aside from regular lubrication. Adequate provision for proper lubrication of all working parts is usually found on all modern units. Operating instructions for the steam spur-geared windlass described are given below: The windlass is driven by the steam engine and is capable of heaving in both anchors and chains simultaneously, or each anchor and chain independently. The anchors can be paid out simultaneously or independently, either by the use ofthe mechanical brakes with the block keys disengaged from the pockets ofthe wildcat, or the use ofthe steam engine with the block keys engaged in the pockets ofthe wildcat. Before starting windlass: 1. Oil or grease all bearings as required. 2. Drain water from steam cylinders and valves. 3. Crack valve and turn engine over slowly until block keys are in line with slots in wildcats. 1. Make certain that the clutch P on the intermediate shaft is disengaged. 2. Open the throttle valve between the main steam supply and the steam engine reverse valve D. 3. Operate lever C of steam reverse valve for speed and direction of rotation of winch heads desired. Figure 21-17. Section through steam windlass reversing valve WINDLASSES, CAPSTANS, AND GYPSIES 21-31 To drop either anchor by steam power: 1. 2. 3. 4. 5. Make certain the clutch P on the intermediate shaft is engaged. Apply brakes CC on both wildcats. Engage block keys from the idle wildcat. Disengage block keys from the idle wildcat. Remove chain stopper pawl from chain-holding position on chain of anchor to be dropped. 6. Release brake CC on wildcat to be used and drop anchor by operating reverse valve control lever. 7. Apply brake CC on wildcat and place chain stopper pawl in chain-holding position after anchor has been dropped as desired. To heave in both anchors Figure 21-18. Side view of band brake To drop both anchors mechanical brakes: simultaneously or independently by using the 1. Pull up on anchor chain sufficiently to remove chain stopper pawl. 2. Tighten brakes CC on wildcats and disengage block keys from wildcats using bar in locking rings X. (Warning: Remove bar from locking ring before further operation.) 3. Release brakes CC (control the speed ofthe chains paying out by the use of the brake handwheels). 4. Apply the brakes on the wildcats when the anchors are dropped to the desired depth and place the chain stopper pawls in the chain-holding position. To drop both anchors simultaneously simultaneously: 1. Make certain the clutch P on the intermediate shaft is engaged. Tighten brakes CC on the wildcats. 2. Engage block keys in wildcats, using bar in locking rings X. 3. Open throttle valve between main steam supply and steam engine reverse valve D. 4. Release brakes CC on the wildcats and heave in anchors by operating reverse valve control lever. 5. Apply brakes on wildcats when anchors are raised, shut down engine, and place chain stopper pawls in chain-holding position. To heave in either anchor: 1. 2. 3. 4. 5. Make certain the clutch P on the intermediate shaft is engaged. Apply brakes CC on both wildcats. Engage block keys in the wildcat to be used. Disengage block keys from the idle wildcat. Open throttle valve, release brake on wildcat to be used, and heave in anchor by operating lever of steam reverse valve D. 6. Tighten brake on wildcat when the anchor is raised. Place chain stopper pawl in chain-holding position. by steam power: w i n d l a s s c o n s t r u c t i o n . On modern vessels it is common practice to select a hydraulic powered anchor windlass. Figure 21-19 is a photograph of a hydraulic powered anchor windlass for 4V2-inch chain. Typically, this windlass would be capable of lifting 190 fathoms of chain. A typical hauling rate is 30 feet per minute. Figure 21-20 is the assembly and arrangement drawing of the anchor windlass. The windlass base shown in the drawing is a steel fabrication which provides bearing supports for all windlass shafts and an oil-tight enclosure for the speed reduction gearing. A machined surface 49 between the gear cover and the windlass base is designed to provide oil tightness. M odern anchor 1. Make certain the clutch P on the intermediate shaft is engaged. 2. Engage block keys in both wildcats, using bar in locking rings X, and tighten brakes on both wildcats. 3. Remove chain stopper pawls from chain-holding positions. 4. Open throttle valve between main steam supply and steam engine reverse valve D. 5. Release brakes CC on wildcats and drop anchors by operating reverse valve control lever (steam power opposing pull of anchor). 6. Apply brakes on wildcats after anchors are dropped as desired, shut down engine, and place chain stopper pawls in the chain-holding position. Figure 21-19. Installed view of a typical hydraulic powered anchor windlass for a 41/2-inchchain Because the hydraulic motors or electric motors selected for powering a windlass have very high speed, a four-step speed reduction gear is employed to provide the slow speed high tension necessary to raise an anchor. Figure 21-21 is a section view of the windlass reduction gear. The four spur-type gears are keyed to steel shafts which are supported by oil lubricated antifriction bearings. The wildcat's shaft, which is manufactured from alloy steel, is supported by bronze bushings which are grease lubricated. The anchor windlass is fitted with a brake assembly shown in Figure 21-22. This brake must be capable of controlling the running anchor during "letting go" operations. A typical cable speed during "letting go" is 20 feet per second. The anchor should not be allowed to drop freely for more than 15 fathoms without applying the brake to control descent. Twobrake bands are mounted side by side on the wildcat. The dead end of each band is anchored to the base by a pin. The live ends are controlled by the handwheel through linkage. W i n d l a s s m a i n t e n a n c e . A windlass can be expected to operate with little maintenance if the machinery is subjected to continuous application of proper lubricants, both oil and grease. Periodic inspection for worn parts Figure 21-22. Anchor windlass brake assembly such as bearings, bushings, and gears and their replacement is required. It is essential that the gear cover and bearing covers be carefully replaced to assure that salt water is excluded. C a p sta n The capstan is used primarily for warping, i.e., the act of changing the vessel's position with respect to a wharf, pier, or other ship by the manipulation of a common line attached to both objects. A modern 30-inch head capstan suitable for continuous hauling of 81,000 pounds is shown in Figure 21-23. In the arrangement, the electric motor is mounted below the deck to avoid problems with sealing against the salt environment. W IN C H E S Winches are employed aboard ship for multiple purposes involving the handling of lines, handling cargo, warping ship, and handling boats. In general, a winch is a large cable-winding drum mounted on a horizontal shaft with an additional head called a gypsy mounted on the end of the shaft. Power for winches is commonly supplied through a reduction gear train from a hydraulic or an electric motor. Steam engines and diesel engines are sometimes used to furnish power. Figure 21-24 is the installa- 21-36 HULL MACHINERY tion of a hydraulic powered mooring winch with a clutch connection to a traction drive for hauling lines. The special traction drive is used in first-line-ashore applications. W in c h C o n s tr u c tio n The mooring winch shown in Figure 21-24 has a fabricated steel bedplate. The horizontal drum shaft is fitted with a clutched drum which is driven through a jaw-type clutch. The gypsy head and the first-line-ashore winch may be driven independently of the winch drum. A handwheel-operated band brake is mounted on the rope drum. A hydraulically operated disc brake capable ofholding about 70,000 pounds is mounted on the high speed shaft for automatic braking of the winch. Figure 21-24. Mooring winch with single gypsy and traction drive ACKNOW LEDGM ENTS The editors are indebted to the following companies for the material and illustrations in this chapter: Lake Shore Inc., Iron Mountain, Michigan Brown Brothers & Co. Ltd., Edinburgh, Scotland Transmission Technology Company, Fairfield, New Jersey CHAPTER 22 M a r in e E le c t r ic a l S y s t e m s CONRAD C. YOUNGREN S H IP B O A R D E L E C T R IC A L D IS T R IB U T IO N SYSTEM S I n tr o d u c tio n R EGULATIONS for electrical installations on shipboard are promulgated by governmental regulatory agencies (e.g., U.S. Coast Guard), the Intergovernmental Maritime Organization (IMO), and classification societies (e.g., American Bureau of Shipping). In making any modifications to the existing electrical installation, the ship's engineer should always make reference to those regulations that may be applicable, such as Title 46 (Shipping) ofthe C o d e o f F e d e r a l R e g u la tio n s , Subchapter J, which in turn references sections ofNFPA 70 (National Electrical Code) and IEEE Standard No. 45 (Recommended Practice for Electrical Installations on Shipboard). With respect to applicable regulations, the U.S. Coast Guard has divided vessels into the five following groups: (1) oceangoing vessels that navigate on any ocean or the Gulf of Mexico more than 20 miles offshore; (2) ocean-going vessels that navigate on any ocean or the Gulf of Mexico but 20 miles or less offshore; (3) vessels navigating on the Great Lakes only; (4) vessels navigating on bays, sounds, and lakes other than the Great Lakes; and (5) vessels navigating on rivers only. S y s te m V o lta g e s The following voltages are recognized as standard: A lte r n a tin g C urrent ( v o lts ) D ir e c t C urrent Generation 120-208-230-240 450-480-2400-4160 120-240 Power 115-200-220-230 440-460-2300-4000 115-230 115 115 Lighting 22-1 ( v o lts ) 22-2 MARINE ELECTRICAL SYSTEMS The following systems of distribution are recognized as standard with a frequency of 60 Hz standard for all AC lighting and power systems: (1) two-wire (single-phase AC or DC); (2) three-wire (single-phase AC or DC); (3) three-phase, three-wire AC; and (4) three-phase, four-wire AC. On small vessels with little power apparatus, 120V three-phase alternators may be utilized. Power of 115V and lighting distribution may consist of three-phase feeders. Lighting distribution may consist of 115V single- phase distribution balanced at the main switchboard to provide approx-imately equal load on each phase. On DC vessels of comparable size, 120V generators with 115V power and lighting distribution systems would be utilized. On intermediate size vessels with considerable power apparatus, 240V (or 230V) three-phase generators supplying three-phase power loads at the 230V (or 220V) utilization level and 115V three- or single-phase lighting distribution may be utilized. Four-wire 2 0 8 Y /1 2 0 volt (or 2 0 0 Y /1 1 5 ) systems are another possibility for this application. Typical of today's large merchant ships is the installation of 450V (or 480V) three-phase generators to supply 440V (or 460V) power consumers and 115V, three-phase lighting distribution via a bank of step-down transformers. Four-wire, 2 0 8 Y /1 2 0 lighting distribution, though rare, is another possibility. Although any ofthe above AC systems may be grounded or ungrounded, most 450V, three-phase shipboard systems are ungrounded. Consequently, while four-wire 4 8 0 Y /2 7 7 volt distribution (277V lighting) is a common shoreside industrial distribution scheme, it is not a common shipboard system. Certain vessels with very large electrical systems may require generation at higher voltages (4160V or 2400V) with some power utilization at 4000V or 2300V, three-phase, and the remainder of the system at 440V (power) and 115V (lighting). The lower voltages are provided through three-phase transformer banks. Occasionally 440V or 460V floodlights may be used. S y ste m A r r a n g e m e n ts S h i p ' s service d i s t r i b u t i o n . Electrical power is generally routed through the ship's electrical service system in an arrangement known as a r a d i a l system. That is, from any consumer there is only one continuous path back to the source of electrical power. Other arrangements utilized in shoreside commercial and industrial applications, such as primary loop, primary selective, secondary selective, and secondary network, are generally not found on commercial vessels. This is not to say that certain specified equipment cannot be energized from an alternative source in an emergency, but that typically only one protective device or disconnect is necessary to isolate a piece of equipment or section of the system. SHIPBOARD ELECTRICAL DISTRIBUTION SYSTEMS 22-3 Figure 22-1 is a schematic overview of a typical cargo ship's power and lighting circuit distribution system. Under normal operating conditions all power emanates from the generator section of the main switchboard. In this example it is assumed that the ship's service generators are independently driven by diesel or steam turbine prime movers. Main engine- 22-4 MARINE ELECTRICAL SYSTEMS driven generators will be addressed in a later section of this chapter. Individual consumers may be connected directly to the main switchboard, or to distribution panels that are in turn connected to the main switchboard. Only two voltage levels are shown, although additional transformer banks could provide 240V service if it were required. Isolation transformers are shown segregating a refrigeration container power supply panel from the rest of the system. This is often done so that unintentional grounds in the container equipment do not affect the rest of the system. On ships where there is an exceptional lighting load, additional lighting transformers and lighting load centers may be included and located remote from the machinery spaces. A detailed map ofthe electrical power and lighting system is known as the "electrical one-line diagram" or sometimes the "electrical general arrangement." This drawing is a schematic layout of the electrical plant circuitry, with a one-line route signifying a possible path of electrical power flow regardless of the number of conductors actually carrying the current. Other information found on the one-line diagram includes the generator rating, voltage frequency, and number of phases; circuit number or name including cable designator and connected load (amperes) or other basis for sizing cable; circuit protective devices with frame size, continuous rating, trip settings, and any special features such as interlocks, undervoltage trips, etc.; motor ratings and starter data such as NEMA (National Electrical Manufacturers Association) size, full voltage or reduced voltage, LVR or LVP, etc.; transformer primary and secondary voltages, continuous KVA ratings, and winding connection; panel board identification with circuits supplied and rating of consumers served; and ground protection/detection equipment. Figures 22-2 and 22-3 show the level of detail available on a typical electrical one-line diagram. d is tr ib u tio n . To ensure a dependable independent power source with the capacity to supply all those services necessary for the safety of passengers, crew, and other persons in an emergency, an emergency power distribution system is incorporated in all vessels equipped with an electric plant. For all cargo vessels over 1,600 tons and certain passenger vessels with restricted operations, an automatically connected storage battery bank or automatically started generator is required as a final emergency power source. For passenger vessels operating on oceans or the Great Lakes, on international or coastwise voyages, a temporary emergency source of limited capacity, that can carry for a short time selected emergency loads, is required in addition to the final source described above. Figure 22-1 shows an emergency bus tie connecting the 440V section of the main switchboard to the 440V section of the emergency switchboard through an automatic bus transfer (ABT) switch. Voltage failure at the Emergency power Figure 22-2. One-line diagram, power circuits main switchboard would automatically start the emergency generator and simultaneously connect the emergency bus to the final emergency power source. Notice that the arrangement ofthe ABT prohibits the paralleling of emergency and ship's service sources. A feedback breaker is fitted to temporarily bypass the ABT for operations such as connecting to the main switchboard for dead ship start-up. The feedback breaker is designed to open automatically upon overload of the emergency power source before the emergency generator is tripped off the line from overload. Where a temporary emergency source is required, it must be arranged to supply the following: SHIPBOARD ELECTRICAL DISTRIBUTION SYSTEMS 22- 7 2. Electric communications systems that do not have a dedicated storage battery source; emergency loudspeaker systems. 3. Power-operated watertight doors and fire screen door release systems. 4. Smoke detector and fire detection systems. 5. Each daylight signaling light. 6. Each electrically controlled or powered ship's whistle. The final emergency source, in turn, must be arranged to service the following: 1. The entire temporary load. 2. Each elevator on a passenger vessel. 3. Charging panels for temporary emergency batteries, emergency generator starting batteries, and general alarm batteries. 4. Where required by regulation, one bilge pump, one fire pump, a sprinkling system pump, lube oil pump for each propulsion turbine and ship's service generator requiring external lubrication, one steering gear feeder. 5. Each rudder angle indicator and steering gear failure alarm. 6. Navigation equipment such as radio direction finders, radars, Loran, gyrocompass, and depth sounders. Figure 22-4(a) illustrates a typical arrangement of finaVtemporary source transfer. Figure 22-4(b) details suggested arrangements for AC/DC source transfer. Where equipment requiring emergency supply operates on power that has characteristics other than those available from the emergency sources, the motor-generators, converters, rectifiers, or other apparatus supplying this equipment must automatically start and assume the load upon establishment of emergency supply. A navigating and signal light panel installed in the wheelhouse combines an automatic or semiautomatic telltale navigating light section for audible and visible alarm and control of the masthead, range, side, and stern lights, and a signal light section for control of anchor, not-under-command, and towing lights. The navigating light section (Figure 22-5) is arranged to indicate failure of each primary lamp or filament and is provided with a switch for transfer to the secondary lamp or filament. Figure 22-3. One-line diagram, lighting circuits 1. Emergency lighting, including navigation lights; sufficient illumination in machinery spaces for operations essential to restoring normal power; illumination of passageways, stairwells, lifeboat and life raft assembly, embarkation, and launching points, and watertight doors; at least one lamp in each public and working space. Main engine-driven auxiliaries. The concept of generating electricity by utilizing the main engine as a prime mover is not new. When DC installations were prevalent aboard ships, the shaft generator was quite common, as voltage control was easily obtained. With the transition to AC-driven auxiliaries, however, because of the problem of maintaining system frequency with varying main engine RPM, and the lack of economic justification due to low fuel costs, the shaft generator disappeared from ships in favor of independently driven alternators. Today, economic pressures favor installations that are fuel efficient. Since the production of Figure 22-5. Semiautomatic navigation light panel circuit Figure 22-4. Final and temporary emergency power distribution electricity is, next to propulsion, the largest consumer of energy sources, attention again has focused on main engine-driven generators. This enables the production of electricity to be based on the consumption of heavy fuel used in the main engines. Four arrangements involving the main engine in the production of electrical power are discussed in the following paragraphs. C o n s t a n t s p e e d e n g i n e s . The use of a controllable pitch (CP) propeller theoretically allows a power take-off (PTO) generator to operate at constant speed and therefore constant frequency. While this arrangement is simple, there are losses in propulsion efficiency and the inevitable frequency variations (especially in heavy weather) that prohibit parallel operation with diesel- or turbine-driven alternators. Figure 22-6 is a schematic of the electrical generation plant aboard a 125,000 DWT North Sea shuttle tanker. Two PTO generators are associated with each propulsion engine. One ofthe main engines drives two 2200 kW, 1200 RPM shaft generators which service nonfrequency-sensitive loads. During cargo loading, the propeller is uncoupled and the main engine is used to control cargo pump output. The other main engine drives a similar 2200 kW generator and a 960 kW generator with a frequency converter at speeds ranging from 850 to 1200 RPM. A diesel generator is incorporated in the plant for in-port operation only. Figure 22-6. Constant speed PTO Figure 22-7. Variable speed PTO with solid-state frequency converter With the main engine driving a fixed pitch (FP) propeller, a frequency converting device is required to supply ship's service mains with constant frequency at varying engine speeds. These frequency converters may be of the static type that employs a solid-state rectifier and inverter, or of the rotating type that utilizes a DC PTO generator and DC-to-ACmotor-generator set. The electrical plant of a 65,000 DWT product carrier is illustrated in Figure 22-7. Here a 500 kW exhaust gas-generated steam turbine supplies the base load, augmented by an integral shaft-driven alternator. The shaft generator (S/G) can supply 800 kW between 70 and 123 RPM and reduced output in proportion to speed down to 35 RPM. The DC-link converter comprises a three-phase bridge rectifier utilizing uncontrolled silicon diodes, a thyristor switch in the intermediate DC circuit to protect the rectifier and inverter, and the solid-state, AC mains-commutated inverter consisting of a thyristor bridge. The inverter can only supply active power to the ship's mains, and thus the reactive load must be supplied by a synchronous compensator. The compensator, a standard brushless synchronous machine, is accelerated to near-synchronous speed via a squirrelcage starting motor flanged to the generator. At synchronous speed it operates at a power factor near zero, supplying the reactive power to the ship's mains and also the power required by the inverter for proper commutation. In addition, the compensator provides the almost sinusoidal form of the system output voltage and the short circuit current required for selective tripping of the system's protective devices. Maintenance in this type of system is minimal, as there are no moving parts in the power V a r ia b le s p e e d d r iv e s w ith fr e q u e n c y c o n v e r te r s . circuitry, and the converter equipment may be installed anywhere in the engine spaces. The schematic of a rotating converter or generator-motor-alternator (G-M-A)system is shown in Figure 22-8. The DC generator runs at engine speed and is typically designed to provide constant output with speed variations of ±15 percent. Speeds outside this range would initiate the automatic start-up and takeover by selected diesel-driven auxiliary generating sets. A conventional shunt-wound DC motor drives a synchronous alternator to provide ship's service power. Although the maintenance costs are slightly higher than for static converters, the efficiency of rotary converters is higher and the technology is more familiar to ship's personnel. No compensator is required since the alternator is capable of supplying both real and reactive power. V a r i a b l e r a t i o m e c h a n i c a l c o u p l i n g . The use of a variable ratio epicyclic gear train to provide constant speed generator drive (CSGD) from a variable speed main engine PTO is another arrangement allowing the main engine energy source to be utilized in generating electricity. In the British Shipbuilders' version, shown diagramatically in Figure 22-9, the CSGD consists of an input clutch coupling attached to the PTO shaft, a single-stage epicyclic train with fixed displacement hydraulic units, the generator, and the variable displacement hydraulic unit in coaxial configuration. The variable displacement hydraulic unit is driven at constant speed (1800 RPM) by the output (sun gear) side of the train. If the input speed is nominally 1050 RPM, the planet carrier is held stationary by the SHIPBOARD ELECTRICAL DISTRIBUTION SYSTEMS 22-13 units, which in turn act as motors to turn the planetary carrier against its developed torque. Conversely, ifthe input rises above 1050 RPM, the fixed displacement units extract power from the planet carrier, which is allowed to rotate under its developed torque. The variable displacement unit then absorbs the power as a motor driven by the fixed displacement pumps, and delivers it to the generator. A u g m e n t e d g e n e r a t o r d r i v e . A PTO attached to a variable speed main engine may be used to augment the mechanical power required to drive a ship's service alternator. In the arrangement shown in Figure 22-10, a geared turbogenerator, supplied with steam from an exhaust gas boiler, is mounted in tandem with a DC motor. The DC motor in turn is fed through a three-phase silicon diode bridge from a variable frequency PTO alternator. The supplementary supply has to provide the difference between the required electrical load and the available turbine power after priority steam consumers with fluctuating loads have been supplied from the exhaust gas boiler. Figure 22-8. Variable speed PTO with rotary converter hydraulic units and the system operates as a fixed ratio drive. If the input falls below 1050 RPM, power is developed by the variable displacement unit which, acting as pump, transmits power to the fixed displacement 22-14 MARINE ELECTRICAL SYSTEMS Hazardous Areas Commercial practice classifies hazardous areas into three zones: Z o n e 0 , in which an explosive gas-air mixture is continuously present or present for long periods. Interior spaces of cargo oil tanks would be an example of a Zone 0 area. Z o n e 1 areas would include enclosed or semi-enclosed spaces on the deck of a tanker, battery rooms, and in general any space where an explosive gas-air mixture is likely to occur during normal operation. Z o n e 2 areas, such as open spaces on the deck of a tanker, are those where an explosive gas-air mixture is not likely to occur during normal operation or, if it occurs will be of short duration. Areas not classified as Zone 0, 1, or 2 are considered to be nonhazardous or "safe" areas. Current maritime practice is to simply designate areas as "dangerous spaces" or "normally safe" as determined by the nature of the flammable cargo of the tanker; however, electrical equipment is manufactured on the basis of such zones. Tankers are classified as Type A, B, C, or D and for each classification specific areas are designated as dangerous spaces. T y p e A tankers carry non boiling oil cargoes having a flash point of 60° or less. T y p e B tankers transport nonboiling oil cargoes with a flash point in excess of 60°C. T y p e s C a n d D tankers are LPG or LNG carriers and chemical carriers respectively. Flammable atmospheres in which electrical equipment may have to operate are grouped according to the spark energy required to ignite the gas. Gasses associated with the mining industry are classified as G r o u p I ; all other industrial gasses are described as G r o u p I I with I I A , I I B , and I I C as subcategories. Equipment certified for use in a Group IIC (hydrogen) environment may be used in Group IIB and IIA atmospheres as well, but Group IA certified equipment may not be used in any other group. Although the terms "flameproof' and "explosion-proof' have been used for some time, and the construction techniques associated with flameproof enclosures, for example, are well established, more recent developments in electronics and materials have led to an internationally agreed code for identifying explosion protective equipment. The following is a list of the types of protection and brief explanations. The types are designated by the symbol "Ex" followed by a code letter. A flameproof enclosure will withstand, without damage, the ignition of the flammable gas within itself, and prevent transmission of any flame that could ignite the gas in the surrounding atmosphere. E x i : I n t r i n s i c a l l y S a f e . These are circuits from which no spark or any thermal effect produced under a prescribed test is capable of causing ignition of a given explosive atmosphere. Generally this means limiting the circuit conditions to less than 30V and 50 mA. Cable for intrinsically safe circuits E x d : F la m e p r o o f E n c lo s u r e . SYSTEM COMPONENTS 22-15 aboard ships should be separated from power cables to prevent magnetic induction from compromising the intrinsically safe circuit, and metallic sheaths surrounding intrinsically safe cabling should be grounded at the supply end only. E x e : I n c r e a s e d S a f e t y . This was originally a German construction specification calling for tight tolerances on the control of surface temperatures and the elimination of "open sparking" at relay and switch contacts, for example. E x n : N o n - S p a r k i n g . This is the British version of Exe, but less rigorous and therefore more restricted in its application. E x o : O i l I m m e r s e d . Equipment is placed in oil-filled tanks to isolate any arc from the surrounding atmosphere. E x p : P r e s s u r i z e d E n c l o s u r e . Air or an inert gas is supplied to the equipment at slightly above atmospheric pressure to prevent entry of a hazardous atmosphere. E x q : S a n d F i l l e d . This is similar to the oil immersion concept, except it uses quartz crystals (sand), as in some fuses. E x s : S p e c i a l P r o t e c t i o n . Precautions other than those described above must be taken to prevent an explosion in a given environment. Only Exi and Exs (when specifically certified) may be used in Zone 0 areas. Exd, Exi, Exe, Exp, and Exs may be used in Zone 1 and Zone 2 areas. Exn, Exo, and Exq may only be used in Zone 2 areas. For example, a fixture marked "Exd IIC T4" (flameproof, certified for a hydrogen atmosphere, maximum surface temperature 135 C) is acceptable in a battery room. The United States has no governmental certifying organization. However, Underwriters Laboratory (UL) and Factory Mutual Research Corporation (FM) test equipment and issue listings of approvals based on their published standards. In the United Kingdom, BASEEF A, in Canada, C.S.A., and in Germany, P.T.B. are the national testing and certification authorities. 0 SYSTEM COMPONENTS Switchboards and Distribution Panels Switchboard construction/requirements. Ship's service switchboards are located so as to be accessible front and rear, with the space in front of the switchboard convenient for operation, maintenance, and removal of equipment, and the space in the rear enclosed if it would be accessible to unauthorized personnel. An insulating mat or grating is provided on the deck in the front and rear, a nonconducting handrail is attached to the front ofthe switchboard, and guardrail protection is provided at the rear. Switchboards are installed in a dry location, away from steam, water, and oil pipes, and fitted with drip-proof covers for the top and protective covers for the ends of the structure. 22-16 MARINE ELECTRICAL SYSTEMS Electrically, buses and primary connections are arranged so that, for three-phase AC, the phase sequence is A, B, C (and for DC switchboards, positive, neutral, negative), from front to back, left to right, and top to bottom, as viewed from the front of the switchboard. The following equipment is required for AC generator switchboards: 1. A draw-out or plug-in generator circuit breaker for each generator. Low-volt- age, trip-free circuit breakers must include both short-time delay and longtime overcurrent trip units. Medium-voltage circuit breakers need not include a short-time feature if the long-time pickup characteristic provides sufficient protection. When th r e e or more generators are capable of being operated in parallel, "generator breakers should also include instantaneous trips set in excess ofthe maximum short circuit contribution ofthe individual generator. When t w o or more generators are to operate in parallel, each machine must be protected against reverse power flow. 2. An ammeter for each generator with selector switch to read the current in each phase. 3. A voltmeter for each generator with a selector switch to read each phase voltage on the generator and one phase on the bus. One of the voltmeter switches should be capable of providing a reading for each phase of voltage at the shore connection. 4. An indicating wattmeter for each generator. 5. A teII1pJrature detection instrument with a selector switch for each generator rated 400 kW or above. 6. A pilot lamp permanently connected to the source side of each generator circuit breaker. 7. Control for each prime mover speed. 8. A synchroscope and synchronizing lamps with selector switch to provide for paralleling in any combination. These must be located so as to be readily visible from the position from which the operator controls the incoming generator during paralleling. 9. A voltage regulator (unless internal with the exciter), complete with all accessories for each generator: a switch for cutting out the regulator and transferring from automatic to manual control mode for each generator, an automatic control rheostat for each generator, and a manual control rheostat for each generator. Voltage adjusting rheostats must be in the vicinity oftheir respective voltmeters. 10. Ground detection means for each bus of the switchboard: for ungrounded systems, three ground lamps and a closed spring return-to-normal switch or push button in the ground connection; for grounded systems, a ground current ammeter, ammeter switch, and current transformer if required, in the ground connection circuit. G r o u p c o n t r o l panels. Grouped motor control, or motor control centers, provide a means for centralizing motor control and related control equipment. Made up of standardized vertical sections that house vertical and main horizontal bus, vertical wiring troughs, and compartmentized control SYSTEM COMPONENTS 22-17 units (Figure 22-11), the sections are bolted together to form a single line-up. Each section may contain all controller units, all distribution units, or a combination of both. Controller units contain a circuit breaker or fused load switch, main and auxiliary contactors for across-the-line, reduced voltage, reversing, and/or multispeed starter types, overload relays, indicating lamps, ammeter, push buttons, etc. Distribution units contain a circuit breaker and possibly interlocking relays, and metering and other miscellaneous devices. The distribution/control groups may form an integral part of the main switchboard or a separate group starter board, and sectionalizing may be obtained with circuit breakers, load switches, or disconnect links. Circuit breakers are normally used when operation with a separate busbar system, such as a diesel generator and a shaft generator busbar system, is desired; Figure 22-11. Group-control section 22-18 MARINE ELECTRICAL SYSTEMS or when large motors are to be run solely on one generator; or for the purpose of reducing the available short circuit current. Load switches, often motor operated, are used in automated preferential tripping schemes, where a number of nonessential loads may be dumped simultaneously in the event of generator overload. Disconnectors are typically used in the horizontal bus bar system for isolating one part of the switchboard when it is desired to service it under no-voltage conditions. The National Electrical Manufacturers Association (NEMA) classifies control centers by the degree of control system engineering required. NEMA Class I motor control centers are essentially a mechanical grouping of combination motor control, feeder tap, and/or other units arranged in a convenient assembly. They include connections from the common horizontal bus to the units but not interwiring or interlocking between units or to remotely mounted devices. Only diagrams of the individual units are required to be supplied by the manufacturer. Class II motor control centers consist of a grouping of units to form a complete control system. They include the necessary interwiring and interlocking between units and interlocking provisions to remotely mounted devices. The control manufacturer is required to supply a suitable diagram to illustrate operation ofthe control associated with the motor control center. for distribution to motor, appliance, lighting (including emergency), receptacle, or other branch circuits are fitted with switches that have a rated capacity of not less than 30 amps, with one pole that includes a fuse or other overload device per conductor. The number of branch circuits controlled by a single panelboard should not exceed 18 for three-phase branch circuits or 26 for single-phase or two-wire DC service. Figure 22-12 shows a typical arrangement of single- and three-phase branch circuits in a three-wire, three-phase panel. Figures 22-13 and 22-14 show typical arrangements in four-wire, three-phase and three-wire, singlephase panelboards respectively. As with switchboards, buses and primary connections should be arranged so that for three-phase assembled distribution panels and panelboards the phase sequence is A, B, C, from front to back, top to bottom, and left to right as viewed from the front ofthe panel. Panelboards not exposed to moisture, and particularly flush-mounted panelboards in way of joiner work, may be of UL standard construction with cables entering the bottom and sides through bushings or clamps and through terminal tubes on the top. Elsewhere, panelboards must be of drip-proof or watertight construction as required by location. P a n e lb o a r d s M a r in e C a b lin g A variety of cable constructions, designed to meet the special environmental, installation, and reliability N o m e n c la tu r e . recognized Figure 22-12. Three-wire, three-phase 120V panelboard requirements of marine service, are found on shipboard. The ship's engineer should be familiar with the various conductor insulations, jacket types, ampacities, and designation schemes when making repairs and installing new equipment. In general, marine cabling is identified by a series of letters and numbers signifying the type of service, number of conductors and conductor size, and types of insulation, moisture-resistant jacket, and armor as follows: Cable type (service) S D T F C TP TPS Single conductor-distribution Double conductor-distribution Three conductor-distribution Four conductor-distribution Control cable Signal (twisted pair) Signal (twisted pair, shielded) 22-20 MARINE ELECTRICAL SYSTEMS Insulation E X T GTV M S Ethylene propylene rubber Cross-linked polyethylene Polyvinyl chloride (PVC) Impregnated glass varnished cloth Mineral Silicone Jacket I Moisture-resistant jacket (type T: thermoplastic PVC) (type N: thermosetting polychloroprene) (type CP: thermosetting chlorosulfonated polyethylene) Armor A B Aluminum Bronze Unarmored c a b l e s of600V are recommended for the supply power equipment and lighting circuits up to their voltage rating. Occasionally these are also used in communication, control, and electronic circuits as well. I n D is tr ib u tio n Figure 22-14. Three-wire, single-phase 240/120V panel board general, multiple-conductor cable is installed for all AC lighting and power circuits (packaging all current-carrying conductors of any circuit in a single cable tends to neutralize inductive effects). Where the rating of a circuit exceeds the rated capacity of anyone multiple-conductor cable (see Table 22-1), two or more multiple-conductor cables may be connected in parallel (banked). I n this arrangement one conductor of each phase should be contained in each cable. For relatively short runs of very high capacity circuits, single-conductor cables may be installed utilizing one or more cables per phase. This is often the case in the run from generator to switchboard. The use of armored cable is typical, although unarmored cable may be used provided special precautions are taken during installation. Excessive pulling force, sharp bends, abrasion against metal surfaces or other cables in wireways may cause damage, particularly to the watertight jacket. Armor (or metal sheath in the case of mineral-insulated, metal-sheathed cable) should be continuous from outlet to outlet and grounded at each end. Final subcircuits, which may be grounded at the supply end only, are an exception. Although splicing is permitted under certain conditions per SYSTEM COMPONENTS 22-23 regulatory body restrictions, all cables should be continuous between terminations. The designation of distribution cables includes, in addition to cable symbol, the conductor size in thousandths of circular mils (MCM). Some examples: TXIA-66: AWG #2 (66,400 CM), three-conductor, X-linked polyethylene insulated, moisture-resistant jacket, with aluminum armor. (6)SEIB-300: six (two per phase), 300,000 CM, single-conductor, ethylene propylene rubber-insulated, moisture-resistant jacket, with bronze armor. For "medium voltage" cables (2000V or 5000V) a "-2kV"or "-5kV" suffix is added to the designation (e.g., TXIB-5kV-83). One further note: On many "one-line diagrams" found among the available ship's blueprints, a note may be found stating "... all cables type XIB unless otherwise indicated." In this case cable designations may be abbreviated T-16 or D-4, indicating number of conductors and cable size only. It is advisable to avoid using terms like "number four cable" as it may lead to confusion between 4110 CM (AWG #14) and AWG #4 (41,700 CM) cables. Control and signal cable. Rated at 600V (see Table 22-2), c o n t r o l c a b l e s are used for control, indicating, communication, electronic, and similar circuits where multiple parallel conductor cables are required. Particular attention should be paid to voltage drop considerations when selecting control cabling. S i g n a l c a b l e , rated at 300V, is used for signal transmission where twisted pairs of conductors are desired. Twisted pair signal cable comes shielded or unshielded (see Table 22-3) and the shielded variety includes a drain (ground) wire for each twisted pair. In designating control or signal cable, the conductor size (in AWG#, not MCM) is embedded in the cable symbol following the service designator. The suffix is the number of conductors (for control cable) or number of conductor pairs (for signal cable). Hence: C14TIB-19: AWG #14, 19-conductor control cable, thermoplastic insulated, with moisture-resistant jacket and bronze armor. TPS18TIA-22: AWG #18 signal cable, 22-twisted pairs (shielded), thermoplastic insulated, with moisture-resistant jacket and aluminum armor. Designation as to application of cabling in signal and communications systems is accomplished by utilizing the system designators (per Table 22-4) as well as cable type designations. In both control and signal cable, the individual conductors are color coded (see Table 22-5). "Navy" Cable. Where the above cable designations, dimensions, and ampacities are often referred to as "IEEE" cable, occasionally cable manufactured and tested in accordance with MIL C-915-19 is installed. Use of such "Navy" cable leads to designators such as: TSGA-60:60,090 CM, three-conductor,shipboard, general use, (aluminum) armored cable. Navy cable is silicon insulated and contains an impervious sheath. In addition, Navy cable sizes do not match IEEE sizes exactly (e.g., T-60 versus T-66) and are generally somewhat smaller and lighter per unit length for the same ampacity. Consequently, it is fairly common to find D-9 Navy cable utilized in lieu of D-I0 IEEE cable in certain lighting branch circuits. Adding to the alphabet soup is the occasional use of special-purpose Navy cable; for example: TTRSA-12:(Twelve)twisted-pair,telephone,radio, shielded,armoredcable. *Reprinted from ANSI/IEEE Std 45-1983, © 1983, by The Institute of Electrical and Electronics Engineers, Inc., with permission of the IEEE Standards Department. 22-26 MARINE ELECTRICAL SYSTEMS MHOF-24: Multiple-conductor, heat- and oil-resistant, flexible cable (24 conductors). SHFS-2.5: Single-conductor, heat- and flame-resistant, switchboard cable (2613 CM). TABLE S ig n a l a n d C o m m u n ic a tio n 2 2 -4 S y s te m s D e s ig n a tio n s C ode S y s te m A AC AN BL CE CO D DA DE Call bells Autocombustion control Nurse's call light indicator Boiler remote water level Electric clocks C02 release alarm Docking telegraph Docking announcing Electric door control (other than watertight doors) Sound-powered telephone and voice tube call Lubricating oil low pressure alarm Lubricating oil low level alarm Echo depth sound Diesel circulating water high temperature alarm Fire alarm Fuel oil high level alarm Fog alarm Fire door release Fuel oil filling signal system Resistance thermometers Feedwater low level alarm General alarm Burglar alarm Anchor telegraph Wind direction indicator Wind intensity indicator Telephone, manual or automatic Telephone, sound powered, ship control Telephone, sound powered, engineer's Telephone, sound powered, miscellaneous Shaft revolution Steering telegraph E EC EL ES EW F FA FB FD FS FT FW G GB HB HD HE J 1JV 2JV 3JV K L SYSTEM COMPONENTS T A B L E 2 2 -4 (c o n tin u e d ) C ode S y s te m LA Steering gear alarm Gyrocompass Automatic pilot Engine order telegraph Emergency announcing Boiler telegraph Motion pictures Turbogenerator telegraph Rudder angle indicator Pyrometer Radio Refrigerator alarm (stores) Radio broadcast receiving antenna distribution Radio broadcast receiving and entertainment Radar navigation Refrigerator alarm (cargo) Salinity indicator Smoke indicator Sewage tank alarm Wet and dry bulb indicator Voice tubes Whistle operator LC LP MB MC ME MP MT N PB R RA RB RE R-ER RH SB SM ST T V W WD X Y 22-27 Watertight door control Tank level indicator Underwater log S y s te m P r o te c tio n Reliable circuit protection is automatic protection that minimizes the danger of smoke and fire, not only in the equipment, but in the cabling leading to and from the equipment. Secondly, besides protecting the cabling and equipment, the protector must isolate the fault from the power source so that nonfaulted circuits can continue functioning in a normal manner. These objectives may not always be achieved by a single protective device, and indeed coordination between protective devices, correct sizing of wire, and safe routing of circuits all contribute to a safe circuit protection philosophy. I n tr o d u c tio n . 22-28 MARINE ELECTRICAL SYSTEMS The entire electrical system, including power source, distribution cabling, switching devices, and equipment, must be adequately protected. Generally a circuit-protecting device is located at any point in the system where the conductor size is reduced, unless the immediate upstream TABLE 22-5* C o lo r C o d e : T w is t e d - P a ir C o n t r o l a n d S ig n a l C a b le s *Reprinted from ANSVIEEE Std 45-1983, © 1983 by The Institute of Electrical and Electronics Engineers, Inc., with permission of the IEEE Standards Department. SYSTEM COMPONENTS 22-29 protector provides adequate protection for the smaller wire. Consideration must also be given to the fact that circuit components such as motors, transformers, rectifiers, and electronic circuits have significantly different overload-withstanding characteristics than those of wire and cable. For example, many electronic circuit components require extremely fast clearing devices to provide adequate protection from thermal damage. Fuses. Cartridge fuses of the nonrenewable type are permitted for shipboard use. Although typically utilized in control and electronic circuitry, they may be used as motor overload protection up to 200A (see Table 22-6). Where fuses may often offer an adequate and low-cost method ofprotection, there are some troublesome characteristics that mitigate their use in power circuits on U.S. merchant ships. For one thing, the replaceable nature of fuses and the ease of overfusing may result in inadequate protection. Within certain minimum and maximum ratings, fuse dimensions are usually the same; and it is possible to substitute a 20ampere fuse where a 5- or lO-ampere fuse should be. Single-phasing may be another problem although today some fuse designs incorporate a pin or "target" that is expelled when the fuse blows, actuating a switch trip mechanism to open all phases. A less well known but still troublesome characteristic is fuse element deterioration, caused by physical and chemical stresses produced in the fuse element during repeated short-duration overloads. During acceleration, motor inrush current, for example, often exceeds the trip rating ofthe fuse. The inrush current is not present long enough to blow a correctly sized fuse; however, deterioration of the element, resulting from repeated motor starting, often causes mysterious fuse failures. When semiconductor devices, such as SCRs and power diodes, are sub~ ected to high current overloads, thermal damage occurs proportional to 1 T. As a result, fast-blow, silver-link fuses have been developed with performance characteristics that are similarly 12 T dependent. Power matching offuse to semiconductor results in very effective protection. The high speed action of "current limiting" semiconductor fuses comes from a silver link with a small link section joining a substantial size sheet of silver. The silver provides a maximum of thermal conductivity, and short circuit protection is provided when the rate of rise ofheat in the small link exceeds the rate of thermal conduction away from the link. As the link melts, the voltage across it rises and arcing begins. Arc quenching is aided by silica sand crystals, which effectively lengthen the arc path. The "clearing time," which includes the melting and arcing periods, and the "peak let-through current" are the most significant parameters ofcurrent-limiting fuse action (see Figure 22-15). The voltage rating of the fuse is also important. Sometimes an operator is tempted to apply 250V fuses in 120V circuits in the mistaken belief that this will achieve greater safety. Voltage ratings should 22-32 MARINE ELECTRICAL SYSTEMS breakers may be of the adjustable or nonadjustable type. Adjustable breakers are generally factory set, however, in accordance with an overall protection and coordination scheme, and should not be adjusted by the plant operator without careful consideration of the effects on the entire system. The overcurrent and tripping means of molded-case circuit breakers may be ofthe thermal type, the magnetic type, or a combination ofboth. In the following example (see Figure 22-16), the overcurrent and tripping mechanisms of a thermal-magnetic, molded-case circuit breaker are described. Thermal circuit breaker elements (in this case a bimetallic strip) are dependent for actuation upon temperature rise in the sensing element. Deflection of the thermal sensing element causes the circuit to open by rotating the trip latch counterclockwise, activating the common tripper bar. In turn this releases the mechanism latch that separates the contacts. Sufficient deflection to operate the breaker occurs when a predetermined calibration temperature is reached. Temperature rise in the sensing element is caused principally by load current 12 R heating. The thermal element also integrates heating or cooling effects from external sources, and tends to derate or uprate with ambient temperature. Some models include an ambient temperature compensating element (usually electrically isolated from and independent of the current-carrying thermal trip SYSTEM COMPONENTS 22-33 element), which acts only when a change in ambient temperature occurs. To protect components from high fault level currents, an electromagnet mechanism is added to cause faster tripping. During a short circuit fault, 22-34 MARINE ELECTRICAL current passing through the circuit causes the electromagnet to attract its armature, the common tripper bar being rotated in this instance by the upward movement of the armature plunger. The "instantaneous" (magnetic) trip levels are typically 10 times (1000 percent) the nominal breaker rating and often 2000 percent for breakers rated at 20 A or less. Some molded-case breakers include a magnetic time-delay mechanism in lieu ofthe thermal apparatus. In these units a movable solenoid core is held with a spring in a tube, and damped with a fluid. As the magnetic field of a series coil moves the core toward a pole piece, the reluctance of the magnetic circuit containing the armature is reduced. The armature is then attracted, causing the mechanisms to trip and open the contacts on an overload condition. Typically the minimum current that will provide a reliable trip of the breaker is 115 percent of the rating, independent of ambient temperature. The instantaneous trip current is that value of current required to trip the circuit breaker without causing the core to move inside the tube. This is possible because excessive leakage flux in the magnetic circuit, caused by extreme overloads or faults, will attract the armature and trip the breaker immediately. The instantaneous trip point is also independent of ambient temperature. Instantaneous trip circuit breakers have no intentional time delay, and may be thought of as the thermal-magnetic type described above with the thermal sensing element removed. These are utilized where overload protection is available in another form (such as overload relays incorporated in motor control circuits) and so is not required in the circuit breaker. Steering gear motor supply feeders, for example, are required by U.S.C.G. regulations to be protected by instantaneous trip elements (set at 200 percent ofthe motor locked rotor current) only. Note that in steering gear applications, overcurrent relays trigger an alarm, but do not trip the breaker. Solid-state molded-case circuit breakers are available today, and do not contain a bimetallic strip or electromagnetic apparatus. Instead they employ a current transformer and electronic circuitry that compares sensed current load with a preprogrammed time-inverse characteristic. switchgear. Circuit breaker protection involves four major components: (1) A sensing device, such as a current transformer, which monitors the quantity to be controlled; (2) a relay that interprets the sensing device signal and initiates power circuit breaker response; (3) an actuating mechanism to open and close contacts; and (4)a main interrupter, including arc controlling. A metal-enclosed low voltage air circuit breaker switchgear, as the name indicates, is a metal-enclosed assembly containing air circuit breakers, together with bare buses, connections, control wiring, and accessory devices. M e ta l-c la d SYSTEM COMPONENTS SYSTEMS 22-35 In shipboard use, metal-clad air circuit breakers are reserved for larger size (amperage) applications and are likely to have separate relays and sensing devices, adding to their flexibility. They may have independently adjustable long-time delay (LTD), short-time delay (STD), and instantaneous trip settings to customize the time-current characteristics of a particular breaker for coordination purposes. Breaker accessories may also include under-voltage relays, phase-balance relays, reverse-power relays, mechanical-electrical interlocks, and load-shedding relays. Air circuit breakers may be operated manually or electrically (remote operation) and are mounted in individual metal compartments. They may be of the stationary type (rigidly mounted on bases within the enclosure) or the draw-out type. Draw-out type switchgear contains breakers so arranged that they may be easily withdrawn from or inserted into their housing. As breakers are removed or inserted, they are automatically disconnected from or connected to their power and control circuits by means of selfn aligning contacts. Interlocking is provided so that breakers cannot be removed or inserted when they are closed. U.S.C.G. regulations require that all shipboard circuit breakers be of the "trip-free" variety, that is, breakers whose poles cannot be maintained closed when carrying overload current that would normally trip the breaker to the open position. T r a n sfo r m e r s a n d o p e r a t i o n . A simple means for providing systems with multiple voltage levels is one of the great advantages of AC distribution. The device that facilitates this arrangement is the transformer. Containing no moving parts, transformers require little maintenance and the larger capacity units operate at efficiencies in the 98 percent range at full load. Two-winding transformers consist of two coils, electrically isolated, wound around a common laminated iron core. The winding connected to an alternating voltage source is called the p r im a r y . The winding connected to the load circuit is called the s e c o n d a r y . When the primary winding is energized, an alternating magnetic flux is created that circulates in the core and cuts the conductors ofboth the primary and the secondary windings. Since the turns ofboth windings are cut by the same flux, the emf produced in each turn ofthe primary and secondary windings is the same. The ratio ofprimary to secondary voltages, therefore, is equal to the ratio of primary to secondary coilturns. That is: C o n s tr u c tio n 22-36 MARINE ELECTRICAL SYSTEMS It follows that when the winding with the greater number of turns is connected as the primary, a lower voltage is produced at the secondary terminals. In this mode of operation the transformer is called a s te p - d o w n transformer. With the fewer-turns winding connected to the supply voltage source, the voltage produced at the secondary terminals is greater than the primary voltage. Such a connection is termed a s te p - u p transformer. The standard transformer terminal markings are HI, H2 ... for the high voltage windings and Xl, X2 ... for the low voltage windings. Any transformer may be connected as either a step-up or a step-down transformer, depending on the application, as long as the voltage ratings are consistent with the application. In shipboard systems, the generated voltage is typically the highest voltage level, and thus step-down applications predominate. Under no-load (secondary circuit open circuited) conditions, only a small amount of ("magnetizing") current flows in the primary winding. This is because the self-induced voltage, or "counter-emf' in the primary winding is practically equal to the applied voltage. Transformer heating under no load is primarily due to hysteresis and eddy-current phenomena in the core; thus the small amount of power consumed under no-load conditions is called c o r e lo s s . When a load is connected to the secondary windings, a secondary current is produced that is inversely proportional to the load impedance. The secondary coil current, in accordance with L e n z 's la w , opposes the action that caused the (secondary) emf to be produced, and tends to reduce the flux circulating in the transformer core. However, the resultant decrease in counter-emf allows more primary current to flow, restoring the core flux to the original value. Since in the magnetic circuit the primary ampere turns (hNl) are (approximately) equal to the secondary ampere turns (IzN2), it follows that the ratio of primary to secondary currents is inversely proportional to the turns ratio. So, since it would seem that the volt-ampere output of the transformer is equal to the volt-ampere input. Indeed, this is a very close approximation, and useful in determining primary/secondary voltage and current relationships in practice. For example, the output rating or capacity of a transformer is given in kilovolt-amperes (kVA). This value, along with the voltage ratings of the transformer, may be used to determine the maximum primary and secondary currents the transformer can carry without exceeding the allowable temperature rise (at a given frequency). A 480V/120V, 30 kVA transformer, when operating at full load, will carry 62.5 A SYSTEM COMPONENTS 22-37 (30,000/480) in the high voltage winding and 250 A (30,000/120) in the low voltage winding. Note that the power factor of the load determines the k ilo w a tt output of the transformer at any percent of (kVA) load. In actual transformers, the relationships differ slightly from the above ideal characteristics. As already mentioned, the presence of magnetizing current leads to a reduction of efficiency to below 100 percent by introducing core losses. Also, the resistance of the conductors making up the primary and secondary windings gives rise to copper losses. Transformer efficiency is determined by dividing output kilowatts by input kilowatts. The input is the sum ofthe output plus the total losses (core plus copper). When operated at or near rated voltage, the core losses are constant (independent ofload); the copper losses, however, vary with the square of the load. Hence transformer efficiency varies with percent load as well as load power factor according to the following formula: "Percent impedance," which incorporates the effects of flux leakage (leakage reactance) as well as winding resistance, is found among the nameplate data on transformers. It is a value determined by the manufacturer by operating the transformer with the secondary shorted and the primary energized with just enough voltage to drive rated current through the transformer windings. The ratio ofthis "short circuit voltage" to the rated current is the winding impedance (in ohms). It is usually expressed, however, as a percent of some standard, or "base," impedance. This base value is: To reduce the number of transformer designs (differing only in voltage rating), manufacturers may divide the high and/or low voltage windings into two identical sections as shown in Figures 22-17(a), (b), and (c). The possibility of connecting the windings in either series or parallel enables 22-38 MARINE ELECTRICAL SYSTEMS the user to obtain either of two optional voltages from the split secondary winding, and thus the design could be suitable for three-wire, single-phase service. If the transformer has a center-tapped secondary as shown in Figure 22-17(a), the secondary is specified as "whole-half' voltage (240120V, for example). If the secondary coils are physically separated they will work satisfactorily in series or parallel, but not when used in threewire service, as unbalanced loading will lead to unbalanced voltages at the terminals. For three-wire service the coils must be interweaved with one another, and thus are distributed over approximately the same portion of the magnetic circuit. Transformers specifically designed for three-wire service are slightly more .expensive than those intended for only series or parallel connection and are designated in the form "parallel-series" volts (e.g., 120-240V). Transformers not intended for three-wire service are designated using the "parallel x series" format (e.g., 240 x 480V). Two optional supply voltages are made possible by splitting the primary winding. The transformer illustrated in Figure 22-17(c)may be suitable for up to six single-phase arrangements. Standard numbering of dual voltage transformer terminals requires that the lower numbers be simultaneously positive in all coils. This is important when making the series or parallel connections. Since the available supply voltage does not always match the nameplate rating ofthe primary, many transformers have taps arranged on the high voltage winding so that rated secondary voltage may be obtained when the primary voltage is less than the nominal rating. Standard distribution transformers have taps arranged in 2.5 percent steps so that rated secondary voltage can be obtained when the primary supply is 0, 2.5, 5, or 7.5 percent below the nominal primary rating. A primary winding with three BNFC (below normal full capacity) taps is shown in Figure 22-17(d). Ifthe primary were rated at 480V, then connecting terminals HI and H2 (7.5 percent BNFC) to a 444V supply would yield a secondary voltage of 120V at full load. Three-phase connections. Three two-winding, single-phase transformers may be connected in four possible arrangements to make a three-phase bank. These are the delta-delta, delta-wye, wye-wye, and wye-delta connections. In shipboard use, with ungrounded systems, the delta-delta is the most common arrangement. If connected as shown in Figure 22-18(a), the primary and secondary voltages will be in phase with each other. If a center tap is present on one transformer secondary, the bank may be used to supply a four-wire delta system (e.g., a 240-120V three-wire singlephase, two two-wire 240V single-phase, and a 240V three-phase load). Most importantly with a delta-delta connection, if one transformer fails in service, the remaining two can be operated as an open delta (Figure 22-18(b)) at 57.7 percent of the original capacity of the bank. If a spare SYSTEM COMPONENTS 22-39 transformer is available, the defective phase can be replaced and the capacity restored. In a complete three-transformer delta-delta bank, transformers tend to share the load in inverse proportion to their internal impedances, thus identical transformers should be used. With medium voltage generation, the 4160V primary line voltage can be stepped down to 480V using standard 2400/480V single-phase transformers connected in a wye-delta (Figure 22-18(c)).However, the secondary line voltages are always shifted 30 degrees from the nearest primary, which is of importance if banks oftransformers are to be paralleled. Also, the presence or absence of a primary neutral makes for significantly SYSTEM COMPONENTS 22-41 delta arrangement, shown in Figure 22-18(d), however, will be capable of supplying only 57.7 percent ofthe original bank. Furthermore, wye-delta connected units tend to share load inversely with their internal impedance, requiring the use of identical transformers. A delta-wye transformer bank, illustrated in Figure 22-18(e), can supply a four-wire, three-phase (208-120V)secondary system like those used on some ferries and passenger liners. These systems typically incorporate a grounded neutral, thus a ground fault occurring in the secondary will immediately trip the faulty circuit's protective device. This interruption ofsupply leads to rapid identification of the faulty branch circuit. The delta-wye connection also introduces a 30-degree phase shift (either leading or lagging) between primary and secondary line-to-line voltages. Wye-wye connections, illustrated in Figure 22-18(f), are common in high voltage transmission and distribution networks, but rarely found aboard ship. Three-phase transformer assemblies, with a common magnetic core to all three winding pairs, offer economy of size, weight, and cost per kVA. The first two ofthese advantages are of particular importance in aircraft. However, the inability to take one phase out of service and maintain a three-phase secondary supply (open delta), prevents their use on shipboard. often described as "single-winding" transformers, actually only require that primary and secondary circuits share a common terminal. This may be accomplished by use of a single tapped winding (Figure 22-19(a)), with the intermediate tap connected to the primary for step-up (boost) operation or to the secondary for step-down operation. But conventional two-winding transformers may be connected as shown in Figure 22-19(b) to obtain the desired secondary voltage. In the illustration, a 480-120V two-winding transformer is connected to provide 600V service from typical 480V mains. Since the load may still receive rated secondary winding current at this higher voltage, the kVA which may be delivered to the load is [1 + (Nl!N2)] times the original rating. (In this case a 30 kVA transformer, used as an autotransformer, can deliver 150 kVA to a 600V load without overheating.) Autotransformers cost less, have better voltage regulation, are smaller and lighter in weight, and are more efficient than two-winding transformers ofthe same kVA capacity. However, they have two major disadvantages. The autotransformer does not isolate the secondary from the primary circuit, which may be unsuitable for grounding purposes. Also, in the event the common winding becomes open, the primary voltage can still feed through the extension winding to the load. With a step-down transformer, this could mean a serious over-voltage condition for the secondary equipment and a safety hazard, particularly if the step-down ratio is high. Occasionally autotransformers are used on A u to tr a n sfo r m e r s, Figure 22-18. Three-phase transformer connections different operating characteristics. Without a primary neutral, this transformer arrangement does not require identical transformers, as differences in magnetizing currents are offset by a small circulating current around the delta. No appreciable voltage imbalance appears and the transformers uniformly share a balanced load even ifthe units do not have equal internal impedance. However, if one primary line becomes open (e.g., because of a blown line fuse), three-phase motors operating on the secondary side will continue to run, drawing normal current in two lines, but about twice the normal current in the third. Three overload devices are required to adequately protect motors operating on this type of system. With a primary neutral, the transformer bank can still deliver three-phase power with one transformer entirely disconnected from the other two. This open-wye/open- ELECTRIC PROPULSION 22-43 of potential transformers are typically rated at 120V and thus may be utilized in conjunction with 120V(or 150V)rated instruments. The burden on these transformers is very low (a 50,000 ohm 120V voltmeter will draw less than 3 mA, for example) and hence ratings of 200, 600, and 1000 VA are common. Operating on exactly the same principles as the power transformer, a 5000/120V potential transformer produces 100V at the secondary with a primary line voltage of 4160V. Thus a 120V full-scale voltmeter, with a 0-5000V scale behind the movement, will indicate the actual line voltage to the operator. The other type of instrument transformer is the c u r r e n t tr a n s fo r m e r . As the name implies, a "CT" delivers a current-proportional line current to a current-monitoring instrument (usually rated at 5 A). Stepping down current, of course, means stepping up voltage. Thus a 1000/5 A current transformer will step up voltage 200:11 However the primary winding consists of few windings of very low impedance and the primary current is a function ofthe load in the primary circuit rather than the load on the CT secondary. Thus a primary voltage of perhaps a fraction of a millivolt is stepped up to a few dozen millivolts in the secondary which is compatible with, say, a 50mV meter movement. Under normal operating conditions, the magnetomotive force (mmD established by the current in the secondary circuit opposes the primary mmf and limits the flux density in the transformer core. This demagnetizing mmf would disappear should the secondary circuit be opened while the primary circuit is still energized, resulting in a very high core flux density and dangerously high voltages induced in the secondary winding. Most current transformers are provided with a device for short-circuiting the secondary winding while an instrument is being removed or inserted in the secondary circuit. Thus anyone working with the secondary ofa current transformer should always make certain that the short-circuiting device is closed before the secondary winding is disturbed. ships as series line boosters for exceptionally long runs (large bow thruster feeder, for example), but primarily autotransformer use is reserved for fluorescent lamp ballasts and reduced-voltage motor starters (discussed in Chapter 23). Instruments, meters, and control apparatus are rarely connected directly to high voltage or high current circuits. Instrument transformers are utilized (1) to act as r a tio d e v ic e s , making possible the use of standardized low voltage and low current instrumentation; and (2) to act as is o la tin g d e v ic e s , to protect apparatus and personnel from direct contact with high voltage or high current mains. P o te n tia l tr a n s fo r m e r s supply a voltage-directly proportional to the line voltage-to the terminals of potential monitoring instruments. Secondaries In str u m e n t tr a n sfo r m e r s. E L E C T R IC P R O P U L S IO N O v e r v ie w Electric drive propulsion systems offer 0 ) adaptability to multiple use of prime mover generator sets such as cargo pumps or dredge operations as well as propulsion; (2) the use of multiple prime movers to power the propeller to meet varying torque requirements such as in ice-breaking; (3) flexibility of arrangement, as the prime mover-generator set is mechanically independent ofthe propulsion motor-propeller combination; and (4) ease of control, which may be effected from a number of different locations, and which may easily incorporate time lags or other limits to inhibit abusive operation of propulsion machinery. Electric drive is there- 22-44 MARINE ELECTRICAL SYSTEMS fore most often applied on vessels requiring a high degree ofmaneuverability, large amounts of special-purpose power, the utilization of non-reversing, high speed, multiple prime movers, or combinations thereof. Considering the use of turbines (either steam or gas) or diesel engines as prime movers, and either AC or DC machines as propulsion motors, four basic categories of electric drive propulsion plants are possible. Examples of each will be discussed in the following paragraphs. Traditionally, however, the use of AC drives has been associated with turbine prime movers, as conventional DC machines are incompatible with these high speed machines and reduction gears must be employed. Likewise, the frequency changing methods of motor speed control appropriate to AC propulsion motors do not match the torque characteristics of diesel engines at reduced speeds; thus there is the risk of stalling the engines during critical maneu-vering if sophisticated control restrictions are not employed. A C T u r b o e le c t r ic D r iv e The classic T-2 tanker of World War II utilized this type of drive, but at 6000 hp those plants were well below the 10,000 to 60,000 hp per shaft deemed appropriate for this arrangement today. The typical system consists of a single turbine driving a directly connected high speed alternator, which supplies power to a single low speed synchronous motor directly coupled to the propeller shaft. With a two-pole synchronous alternator turning at 3600 RPM supplying 60 Hz, three-phase AC to a 60-pole synchronous motor, a propeller speed of 120 RPM is obtained. The pole ratio of motor to generator is in effect a fixed speed ratio of the prime mover to propeller similar to a geared turbine drive. Varying the prime mover speed from 900 to 4500 RPM will therefore produce propeller speeds over the 30 to 150 RPM range. Figure 22-20 shows the arrangement of a typical AC turboelectric drive with a two-lever control station. To illustrate the operation of this plant consider the following operational sequence and plant response: 1. Reversing lever at S to p , throttle in M a n e u v e r position. (Turbine is idling at minimum speed, excitation is at zero volts with M-G set running, contacts F, R, and G2 open, D and G1 closed.) 2. Reversing lever moved to A h e a d position. (F contacts close, propulsion regulator increases generator excitation to maintain rated volts/Hz or pre-determined maximum level, propulsion motor accelerates as an induction motor, contacts E close and contact D opens via an automatic synchronizing system designed to apply motor field at proper slip and phase angle to obtain maximum pull-in torque.) 3. Throttle moved to desired speed setting. (Turbine governor set point resets to level consistent with throttle lever position, prime mover power increases until speed reaches governor set point.) ELECTRIC PROPULSION 22-45 To reverse from full ahead to full astern: 1. Throttle moved to M a n e u v e r position. (Turbine governor set point resets to minimum speed, synchronous motor decelerates.) 2. Reversing lever moved from A h e a d to A s te r n . (Propulsion regulator drops excitation to zero, E contacts open, D contact closes, F contacts open, R contacts close, propulsion regulator increases generator excitation to maintain rated volts/Hz or to predetermined maximum level, propulsion motor stops, reverses, and accelerates as an induction motor, contacts E close and contact D opens via automatic synchronizing system.) 3. Throttle is moved to desired propeller speed. (Turbine governor set point resets to level consistent with throttle lever position, prime mover power increases until speed reaches governor set point.) D C T u r b o e le c t r ic D r iv e As mentioned earlier, the use of a reduction gear is required to match the characteristics of a high speed, nonreversing steam turbine to speeds compatible with DC machinery. Often twin or double armatures are utilized to take advantage of the higher speed at which the smaller units may be run, and, running at constant speed, the propulsion turbine may also be used to drive a ship's service generator. An example of a twin-screw turboelectric DC drive employing exactly this arrangement is the cable- 22-46 MARINE ELECTRICAL SYSTEMS laying shi p L o n g L in e s . Varying the magnitude and polarity of the double armature 900 RPM propulsion generator through excitation control to regulate the speed and direction of a single armature direct drive DC propulsion motor provides the excellent maneuvering capabilities required during cable-laying operations. The propulsion motors, capable of developing 4,250 hp at 135 RPM, can be operated from the bow, the stern, three locations on the bridge, and the engine room. Employing high capacity silicon controlled rectifier circuits is another possibility for matching a high speed turboelectric generator with DC propulsion. Known as AC/DC drive, or rectified DC drive, this system is illustrated in Figure 22-21. Losses between prime mover and propeller in an AC rectified system are less than in a DC drive system because of the higher efficiency of the high speed AC generator. In this arrangement, the speed of the propulsion motor is controlled by varying the AC generator excitation; the direction of rotation is determined by the polarity of the motor field. Some alternatives include connecting 6-phase or 12-phase rectifiers directly to the generator and/or inserting a transformer between AC generator and rectifier. Unlike conventional DC drives, however, the rectifier blocks the transfer of power back to the generator during engine reversal; therefore the armature-rectifier circuit must be capable of absorbing the pump-back power. In some cases resistors are switched into the armature circuit to absorb this energy and limit armature currents. D C D ie s e l- E le c t r ic D r iv e A schematic diagram of a two-generator, two-motor, DC diesel-electric drive system is shown in Figure 22-22. Control of the system is accomplished by varying the voltage and polarity of the main generators. Considerable redundancy is built into this arrangement, which is currently employed aboard a 3,000 hp oceanographic survey ship whose specialpurpose mission demands extraordinary maneuverability. Switching to operate with only one generator in the DC loop is easily accomplished via the setup contacts (Gl and SI for main engine 1); the gas turbine can be switched into the DC loop via the Al contacts if both diesels needed to be shut down; and either motor armature can be removed from service in an emergency via movable links (not shown). The gas turbine is normally used to power a 350 hp bow thruster, but an auxiliary diesel can also perform this service. The vessel also contains a 250 hp AC propulsive rudder electrically connected to a 300 kW AC generator on either the gas turbine or the auxiliary diesel shaft. A C D ie s e l- E le c t r ic D r iv e Though the fundamental principles of speed control and reversal are the same as for synchronous turboelectric drive, the inherent problems of producing sufficient motor starting torque (which is maximized at low Figure 22-21. Rectified DC turboelectric drive frequency and hence low prime mover speed) have rendered this combination of prime mover and electric drive rare. However, when there is a large demand for auxiliary power concurrent with little or no demand for propulsive power, the arrangement may be applicable. One example of this application is the plant aboard some U.S. Navy submarine tenders (Figure 22-23). This propulsion system consists of six 2000 kVA, 70.8 Hz, 3300V, diesel generator sets connected to a 15,000 hp, 157 RPM, 3300V synchronous propulsion motor. From three to six engines may be lined up to the propulsion motor to obtain desired ship speed and power. A single exciter supplies field power to both the propulsion motor and the propulsion generators. Up to three ofthe generator sets can be devoted to ship's service power or to an alongside submarine. In this mode they are operated at 720 RPM, producing a rated 1875 kVA at 2800V and 60 Hz. Three 2000 kVA transformer banks step down the 2800V to 450V for ship's service. Above 35 percent rated speed (55 RPM) the generator and propulsion motor are synchronized and there is a fixed ratio between engine and propeller speed. During starting and reversing, however, the motor, fitted with a heavy amortisseur winding, runs as an induction machine. With three generators, the system torque is sufficient to accelerate the propeller Figure 22-22. DC diesel-electric drive to synchronous speed at about 40 percent of rated speed and then to increase to about 70 percent of rated speed. Reversing from full ahead, however, requires considerably more torque. In fact, the system torque with six generators running is insufficient to reverse the propeller until ship headway is reduced to about 65 percent. By the use of dynamic braking (the braking energy absorbed in a resistor, not the motor windings), the propeller speed can quickly be reduced to a few RPM ahead. With the brake applied, the ship will lose headway until the available system torque is capable of pulling the motor into synchronism astern. With the increased availability of reliable and economically viable solid-state frequency converters, the operation of AC propulsion motors off a fixed frequency bus (and therefore constant RPM prime movers) becomes more feasible. An example of a large plant utilizing this technique is illustrated in Figure 22-24. Nine 10.5 MW diesel generator sets in various combinations provide propulsion and ship's service power at 10 kV and 60 Hz. On this twin-screw vessel, four generator sets and one 44 MW synchronous propulsion motor are located in one engine room, the remaining five generator sets and propulsion motor in the other. The 10 kV buses in the separate engine rooms are electrically connected through a bus tie which may be broken in either machinery space. At speeds above 18 knots, the propulsion motors are powered through 60 Hz mains and ship speed is controlled via propeller pitch. Under 18 knots, propeller pitch is fixed 22-50 MARINE ELECTRICAL CIRCUIT CALCULATIONS SYSTEMS motors therefore rotate at 144 R P M above 18 knots and down to 72 R P M under converter supply. Cross-connection is provided so that either motor 22-51 may be supplied through either converter. Excitation is provided through silicon controlled rectifiers fed from the 3.3 kV bus. To segregate ship's service supply from propulsion supply, the outer generator sets may be electrically disconnected from the propulsion bus. This may be advantageous in rough weather where severe torque variations on the propeller may result in frequency disturbances on the propulsion bus. C IR C U IT C A L C U L A T IO N S C ir c u it E le m e n ts Circuit elements may be classified as a c tiv e or p a s s iv e . Active elements have the capacity to produce electrical energy and may be idealized as sources of either voltage or current. Some examples of voltage sources (not necessarily ideal) are batteries and generators. Some electronic circuits can behave essentially as current sources as can exciters under the control of an automatic voltage regulator (AVR). A c tiv e e le m e n ts . e l e m e n t s . Passive elements absorb electrical energy and are classified according to the manner in which the energy is absorbed. R e s is to r s dissipate energy in the form of heat, in d u c to r s store energy in the form of a magnetic field, and c a p a c ito r s store energy in the form of an electric field. The voltage across a resistor is proportional to the current through it. This is true for DC, AC, and any other form of voltage and current. Expressed mathematically: P a s s iv e 22-56 MARINE ELECTRICAL SYSTEMS Using these parallel and series combination techniques sequentially can result in significant simplification of the circuit under inspection (Figure 22-25(b)). In series or parallel combinations only, resistors may be combined with resistors, inductors with inductors, etc., although the o r d e r in which they appear in the original circuit may be rearranged (see Figure 22-25(c)). In AC circuits, resistances (R) and reactances (XL or XC) are combined geometrically or as v e c to r s . Resistances are represented as horizontal segments whose lengths are proportional to their ohmic value. Reactances are represented as vertical segments oriented either upward (XL) or downward (XC). Mathematically, the symbol} is used to designate "up" and -} to designate d o w n ; thus an impedance described as (40 + }30) ohms consists of 40 ohms resistance and 30 ohms (inductive) reactance. These components are combined into a single impedance value using the geometric relationships of the im p e d a n c e tr ia n g le (Figure 22-26). The length of the impedance side of the triangle (hypotenuse) is found using the Pythagorean theorem (Figure 22-26(a)). Impedances are also described by documenting the total impedance value, Z, and the "phase angle," 8. Thus (40 + }30) and (50 L 3 6 .9 ') describe the same impedance. Note that since 8 indicates the phase difference between voltage and current, positive phase angle (]X)implies that the current la g s the voltage, which is of course true in an inductive circuit. When 8 is negative (Figure 22-26(b)), the reactive component is capacitive (-JX) and the current leads the voltage. Impedances are combined in series by adding their resistive and reactive components separately (Figure 22-26(c)),and n o t the overall impedance values. Figure 22-27. AC power triangle calculations illustrated in Figure 22-27(a). The phase angle is often denoted indirectly as cos 8 or p o w e r fa c to r . Power factor (pf) may be thought of as that fraction ofthe apparent power actually doing work, since it is the ratio oftrue power to apparent power (e.g., kW/kVA). Since the cosines of30' and -30' are the same value, power factors are identified as la g g in g (8) 0') or le a d in g (8 < 0') to differentiate between the presence of positive or negative vars. In three-phase networks, true, reactive, and apparent powers are calculated using lin e voltages and currents and a factor of >/3 or 1.73. Again, note that a p p a r e n t p o w e r times pf equals tr u e p o w e r (Figure 22-27(b)). 22-58 MARINE ELECTRICAL SYSTEMS AC loads therefore may be described by identifying (1) any two sides of the power triangle; (2) one side and the phase angle; or (3) one side and the power factor. To illustrate this, consider Figure 22-28. Shown is a 440V, three-phase distribution panel with four loads described by various schemes. The power triangle for each load is shown, and the total power that must be supplied to the panel is determined. Note that the total kVA supplied is not equal to the sum ofthe individual load kVA (902 kVA versus 1341 kVA) but considerably less. This is because the leading power factor of Load 2 has canceled out much of the lagging reactive power ofthe other CIRCUIT CALCULATIONS 22-59 loads. Leading pfloads such as capacitor banks, synchronous condensers, or overexcited synchronous motors are often used in this manner to improve (increase) the power factor of the overall plant. The distribution panel power factor (880/902 = .976) in this example is considerably higher than any of the individual loads. The feeder current is then calculated based on the panel kVA, not the sum ofthe individual load currents. V o lta g e D r o p C a lc u la tio n s Although sized initially by ampacity ratings, classification society and regulatory body requirements specify the minimum voltage drop that is permitted in a given service (3 percent in lighting circuits, for example, 5 percent in motor circuits, and 1 percent in bus ties). These requirements may dictate use of cabling larger than that which simply meets ampacity requirements at full load. The voltage drop in percent of nominal circuit voltage is calculated as follows: (1) Two-conductor circuit (DC or singlephase AC) This might suffice if none of the branch circuits had a voltage drop of greater than 1.1 percent. G LO SSARY All the definitions in the glossary are taken from IEEE Standard 45-1983. G eneral T erm s Those spaces primarily used for machinery of any type, or for equipment for the control of such machinery, as boiler, engine, ship's service generator, and evaporator rooms. D ir e c t c u r r e n t ( D C ) . A unidirectional current in which the changes in value are zero or so small that they may be ignored. A lte r n a tin g c u r r e n t ( A C ) . A periodic current, the average value of which over the period is zero. C y c le . The complete series of values of a periodic quantity that occurs during the period. F r e q u e n c y . Of a periodic quantity, the number of cycles completed in a unit of time. H e r tz . The unit offrequency; one cycle per second equals one hertz (Hz). S in g le - p h a s e c ir c u it. A circuit energized by a single alternating electromotive force. T h r e e - p h a s e c ir c u it. A combination of circuits energized by alternating electromagnetic forces that differ in phase by one-third of a cycle (120 degrees). S lip . In an induction machine, the difference between its synchronous speed and its operating speed. It may be expressed in the following forms: (1) as a percent of synchronous speed, (2) as a decimal fraction of synchronous speed, or (3) directly in RPM. E m b e d d e d te m p e r a tu r e d e te c to r . A resistance thermometer or thermocouple built into a machine for the purpose of measuring the temperature. C a p a c ito r . A device, the primary purpose of which is to introduce capacitance into an electric circuit. Capacitors are usually classified, according to their dielectrics, as air capacitors, mica capacitors, paper capacitors, etc. C a p a c ita n c e ( c a p a c ity ) . That property of a system of conductors and dielectrics that permits the storage of electricity when potential differences exist between the conductors. Its value is expressed as the ratio of a M a c h in e r y s p a c e s . 22-62 MARINE ELECTRICAL SYSTEMS quantity of electricity to a potential difference. A capacitance value is always positive. R e a c to r . A device, the primary purpose of which is to introduce reactance into a circuit. A reactor introduces reactance into a circuit for purposes such as starting motors, paralleling transformers, and controlling current. R e a c ta n c e . The reactance of a portion of a circuit for a sinusoidal current and potential difference of the same frequency is the product of the sine of the angular phase difference between the current and potential differences times the ratio of the effective potential difference to the effective current, there being no source of power in the portion of the circuit under consideration. D ir e c t c u r r e n t c o m m u ta tin g m a c h in e . A machine that comprises a magnetic field excited from a DC source or formed of permanent magnets, an armature, and a commutator connected thereto. Specific types of direct current commutating machines are DC generators, motors, synchronous converters, boosters, balancers, and dynamotors. S y n c h r o n o u s m a c h in e . A machine in which the average speed of normal operation is exactly proportional to the frequency ofthe system to which it is connected. A s y n c h r o n o u s m a c h in e . A machine in which the speed of operation is not proportional to the frequency ofthe system to which it is connected. I n d u c tio n m a c h in e . An asynchronous AC machine that comprises a magnetic circuit interlinked with two electric circuits, or sets of circuits, rotating with respect to each other, and in which power is transferred from one circuit to another by electromagnetic induction. Examples of induction machines are induction generators, induction motors, and certain types of frequency converters and phase converters. C o r r o s io n - r e s is tin g m a te r ia ls . Silver, corrosion-resisting steel, copper, brass, bronze, copper-nickel, certain nickel-copper alloys, and certain aluminum alloys are considered satisfactory corrosion-resisting materials within the intent of the foregoing. C o r r o s io n - r e s is ta n t tr e a tm e n ts . Treatments that, when properly done and of a sufficiently heavy coating, are considered satisfactory corrosionresistant treatments, within the intent of the foregoing, and that include electroplating of cadmium, chromium, copper, nickel, silver, and zinc; sherardizing; galvanizing; and dipping and painting (phosphate or suitable cleaning, followed by the application of zinc chromate primer or equivalent). F la m e - r e ta r d in g . Flame-retarding materials and structures should have such fire-resisting properties that they will neither convey flame nor continue to burn for longer times than specified in the appropriate flame test. GLOSSARY 22-63 A device consisting of multiple nonmetallic cable seals assembled in a surrounding metal frame for insertion in openings in decks, bulkheads, or equipment enclosures and through which cables may be passed to penetrate decks or bulkheads or to enter equipment without impairing their original fire or watertight integrity. L u g . A wire connector device to which the electrical conductor is attached by mechanical pressure or solder. M u ltic a b le p e n e tr a to r . G e n e r a to r C la s s ific a tio n s E le c tr ic g e n e r a to r . A machine that transforms mechanical power into electric power. g e n e r a to r . A DC generator in which ordinarily the entire field excitation is derived from one winding consisting of many turns with a relatively high resistance. This one winding is connected in parallel with the armature circuit for a self-excited generator, and to the load side of another generator or other source of direct current for a separately excited generator. S ta b iliz e d s h u n t- w o u n d g e n e r a to r . A stabilized shunt-wound generator is the same as the shunt-wound type, except that a series field winding is added, of such proportion as not to require equalizers for satisfactory parallel operation. The voltage regulation ofthis type ofgenerator must comply with that given for shunt-wound generators. C o m p o u n d - w o u n d g e n e r a to r . A DC generator that has two separate field windings: one of them, supplying the predominating excitation, is connected in parallel with the armature circuit, and the other, supplying only partial excitation, is connected in series with the armature circuit and is of such proportion as to require an equalizer connection for satisfactory parallel operation. M a g n e to e le c tr ic g e n e r a to r . An electric generator in which the magnetic flux is provided by one or more permanent magnets. E x c ite r . The source ofall or part ofthe field current used to excite an electric machine. P ilo t e x c ite r . The source of all or part of the field current used to excite anothfr exciter. S y n c h r o n o u s g e n e r a to r . A synchronous AC machine that transforms mechanical power into electrical power. (A synchronous machine is one in which the average speed of normal operation is exactly proportional to the frequency of the system to which it is connected.) I n d u c tio n g e n e r a to r . An induction machine driven above synchronous speed by an external source of mechanical power. B r u s h le s s e x c ite r . An AC (rotating armature type) exciter whose output is rectified by a semiconductor device to provide excitation to an electric S h u n t- w o u n d 22-64 MARINE ELECTRICAL SYSTEMS machine. The semiconductor device would be mounted on and rotate with the AC exciter armature. B r u s h le s s s y n c h r o n o u s m a c h in e . A synchronous machine having a brushless exciter with its rotating armature and semiconductor devices on a common shaft with the field of the main machine. This type of machine with its exciter has no collector, commutator, or brushes. M o to r C la s s ific a tio n E le c tr ic m o to r . A machine that transforms electric power into mechanical power. m o to r . A series-wound or compensated series-wound motor de-signed to operate at approximately the same speed and output on either direct or single-phase alternating current of a frequency not greater than 60 Hz and of approximately the same root-mean-square voltage. S h u n t- w o u n d m o to r . A DC motor in which the field circuit is connected either in parallel with the armature circuit or to a separate source of excitation voltage. S ta b iliz e d s h u n t- w o u n d m o to r . A shunt-wound motor that has a light series winding added to prevent a rise in speed (or to obtain a slight reduction in speed) with increase ofload. S e r ie s - w o u n d m o to r . A commutator motor in which the field circuit and the armature circuit are connected in series. (It operates at a much higher speed at light load than at full load.) C o m p o u n d - w o u n d m o to r . A DC motor that has two separate field windings: the first, usually the predominating field, is connected in parallel with the armature circuit, and the second is connected in series with the armature circuit. (The characteristics regarding speed and torque are intermediate between those of shunt and series motors.) I n d u c tio n m o to r . An AC motor in which a primary winding on one member (usually the stator) is connected to the power source, and a polyphase or squirrel-cage secondary winding on the other member (usually the rotor) carries induced current. S q u ir r e l- c a g e in d u c tio n m o to r . A motor in which the secondary circuit consists of a squirrel-cage winding suitably disposed in slots in the secondary core. W o u n d - r o to r in d u c tio n m o to r . An induction motor in which the secondary circuit consists of polyphase winding or coils whose terminals are either short-circuited or closed through suitable circuits. (When provided with collector or slip rings, it is also known as a slip ring induction motor.) Synchronous m o to r . A synchronous machine that transforms electric power into mechanical power. Note: Unless otherwise stated, it is U n iv e r s a l GLOSSARY 22-65 generally understood that a synchronous generator (or motor) has field magnets excited with direct current. A m o r tis s e u r . A permanently short-circuited winding consisting of conductors embedded in the pole shoes of a synchronous machine and connected together at the ends of the poles, but not necessarily connected between poles. This winding, when used in salient-pole machines, sometimes includes bars that do not pass through the pole shoes, but are supported in the interpole spaces between the pole tips. S q u ir r e l- c a g e w in d in g . A permanently short-circuited winding, usually uninsulated, that is chiefly used in induction machines, and has its conductors uniformly distributed around the periphery ofthe machine and joined by continuous end rings. C o n v e r te r s A form of converter that combines both motor and generator action, with one magnetic field and with two armatures or with one armature having separate windings. M o to r - g e n e r a to r s e t. A machine consisting of one or more motors mechanically coupled to one or more generators. S y n c h r o n o u s c o n v e r te r . A converter that combines both motor and generator action in one armature winding and is excited by one magnetic field. It is normally used to change AC power to DC power. R e c tifie r . A converter unit in which the direction of average power flow is from the DC circuit to the AC circuit. S ta tic c o n v e r te r . A unit that employs static rectifier devices such as semiconductor rectifiers or controlled rectifiers (thyristors), transistors, electron tubes, or magnetic amplifiers to change AC power to DC power or vice versa. D y n a m o to r . M o to r S p e e d C la s s ific a tio n m o to r . A motor whose speed of normal operation is constant or practically constant-for example, a synchronous motor, an induction motor with small slip, or an ordinary DC shunt-wound motor. M u ltis p e e d m o to r . A motor that can be operated at anyone of two or more definite speeds, each being practically independent of the load-for example, a DC motor with two armature windings, or an induction motor with windings capable of various pole groupings. A d ju s ta b le - s p e e d m o to r . A motor whose speed can be varied gradually over a considerable range, but when once adjusted remains practically unaffected by the load, such as a DC shunt-wound motor with field resistance control designed for considerable range of speed adjustment. C o n s ta n t- s p e e d 22-66 MARINE ELECTRICAL The lowest speed obtained at rated load and rated voltage at the temperature rise specified in the rating. V a r y in g - s p e e d m o to r . A motor whose speed varies with the load, ordinarily decreasing when the load increases, such as a series-wound or repulsion motor. A d ju s ta b le v a r y in g - s p e e d m o to r . A motor whose speed can be adjusted gradually, but when once adjusted for a given load will vary in considerable degree with change in load; an example is a DC compound-wound motor adjusted by field control or a wound-rotor induction motor with rheostatic speed control. B a s e s p e e d o f a n a d ju s ta b le - s p e e d V e n tila tio n m o to r . of Machines A machine having ventilating openings that permit passage of external cooling air over and around the windings. S e lf- v e n tila te d m a c h in e . A machine having its ventilating air circulated by means integral with the machine. S e p a r a te ly v e n tila te d m a c h in e . A machine that has its ventilating air supplied by an independent fan or blower external to the machine. E n c lo s e d s e lf- v e n tila te d m a c h in e . A machine having openings for the admission and discharge of the ventilating air, which is circulated by means integral with the machine, the machine being otherwise totally enclosed. These openings are so arranged that inlet and outlet ducts or pipes may be connected to them. E n c lo s e d s e p a r a te ly v e n tila te d m a c h in e . A machine having openings for the admission and discharge ofthe ventilating air, which is circulated by means external to and not a part of the machine, the machine being otherwise totally enclosed. These openings are so arranged that inlet and outlet duct pipes may be connected to them. T o ta lly e n c lo s e d m a c h in e . A machine so enclosed as to prevent the free exchange of air between the inside and outside of the case, but not sufficiently enclosed to be termed airtight. T o ta lly e n c lo s e d fa n - c o o le d m a c h in e . A totally enclosed machine equipped for exterior cooling by means of a fan or fans integral with the machine but external to the enclosing parts. O p e n m a c h in e . E n c lo s u r e o f E q u ip m e n t An enclosure that primarily protects against accidental contact and slight indirect splashing but is neither drip-proof nor splash-proof. S e m ig u a r d e d e n c lo s u r e . An enclosure in which all ofthe openings, usually in the top half, are protected (as in the case of a guarded enclosure) but the others are left open. G e n e r a l- p u r p o s e e n c lo s u r e . GLOSSARY SYSTEMS 22-67 An enclosure in which all openings giving direct access to live or rotating parts (except smooth rotating surfaces) are limited in size by the structural parts or by screens, bames, grilles, expanded metal, or other means to prevent accidental contact with hazardous parts. The openings in the enclosure shall be such that they will not permit the passage of a rod larger than a half-inch in diameter, except where the distance of exposed live parts from the guard is more than four inches, in which case the openings may be of such shape as not to permit the passage of a rod larger than three-quarters of an inch in diameter. D u s tp r o o f e n c lo s u r e . An enclosure so constructed or protected that any accumulation of dust that may occur within the enclosure will not prevent the successful operation of, or cause damage to, the enclosed equipment. D r ip - p r o o f e n c lo s u r e . An enclosure in which the openings are so constructed that drops of liquid or solid particles falling on the enclosure at any angle not greater than 15 degrees from the vertical either cannot enter the enclosure, or if they do enter the enclosure, will not prevent the successful operation of,or cause damage to, the enclosed equipment. S p la s h - p r o o f e n c lo s u r e . An enclosure in which the openings are so constructed that drops of liquid or solid particles falling on the enclosure or coming toward it in a straight line at any angle not greater than 100 degrees from the vertical cannot enter the enclosure either directly or by striking and running along a surface. W a te r p r o o f e n c lo s u r e . An enclosure constructed so that any moisture or water leakage that may occur into the enclosure will not interfere with its successful operation. In the case of motor or generator enclosures, leakage that occurs around the shaft may be considered permissible provided it is prevented from entering the oil reservoir and provision is made for automatically draining the motor or generator enclosure. O il- p r o o f e n c lo s u r e . An enclosure constructed so that oil vapors, or free oil not under pressure, that may accumulate within the enclosure will not prevent successful operation of, or cause damage to, the enclosed equipment. E x p lo s io n - p r o o fe n c lo s u r e . An enclosure designed and constructed to withstand an explosion of a specified gas or vapor which may occur within it, and to prevent the ignition ofthe specified gas or vapor surrounding the enclosure by sparks, flashes, or explosions of the specified gas or vapor that may occur within the enclosure. Note: An explosion-proof apparatus should bear Underwriters Laboratory approval ratings of the proper class and group consonant with the spaces in which flammable volatile liquids or highly flammable gases, mixtures, or substances may be present. G u a r d e d e n c lo s u r e . 22-68 MARINE ELECTRICAL SYSTEMS An enclosure constructed so that dust cannot enter the enclosing case. W a te r tig h t e n c lo s u r e . An enclosure constructed so that a stream of water from a hose not less than 1 inch in diameter under a head of 35 feet from a distance of 10 feet can be played on the enclosure from any direction for a period of 15 minutes without leakage. The hose nozzle shall have a uniform inside diameter of one inch. O il- tig h t e n c lo s u r e . An enclosure constructed so that oil vapors or free oil not under pressure that may be present in the surrounding atmosphere cannot enter the enclosure. S u b m e r s ib le e n c lo s u r e . An enclosure constructed so that the equipment within it will operate successfully when submerged in water under specified conditions of pressure and time. D u s t- tig h t e n c lo s u r e . T o r q u e C la s s ific a tio n The minimum torque that a motor will develop at rest for all angular positions ofthe rotor, with rated voltage applied at rated frequency. P u ll- o u t to r q u e . The maximum sustained torque a synchronous motor will develop at synchronous speed with rated voltage applied at rated frequency and with normal excitation. B r e a k d o w n to r q u e . The maximum torque a motor will develop with rated voltage applied at rated frequency, without an abrupt drop in speed. T o r q u e m a r g in . The increase in torque above rated torque to which a propulsion system may be subjected without the motor pulling out of step with the generator. L o c k e d r o to r to r q u e . D u ty C la s s ific a tio n A requirement of service that demands operation at a substantially constant load for an indefinitely long time. I n te r m itte n t d u ty . A requirement of service that demands operation for alternate periods of (1) load and no load, (2) load and rest, or (3) load, no load, and rest, such alternate intervals being definitely specified. C o n tin u o u s d u ty . C o n tr o l A p p a r a tu s a n d S w itc h g e a r A device, or group of devices, that serves to govern, in some predetermined manner, the electric power delivered to the apparatus to which it is connected. F u ll m a g n e tic c o n tr o lle r . An electric controller having all of its basic functions performed by devices that are operated by electromagnets. S e m im a g n e tic c o n tr o lle r . An electric controller having only part of its basic functions performed by devices that are operated by electromagnets. M a n u a l c o n tr o lle r . An electric controller having all of its basic functions performed by devices that are operated by hand. E le c tr ic c o n tr o lle r . GLOSSARY 22-69 An electric controller that utilizes a drum switch as the main switching element. S ta r te r . An electric controller for accelerating a motor from rest to normal speed, and for stopping the motor. (A device designed for starting a motor in either direction of rotation includes the additional function of reversing and should be designated a controller.) A u to m a tic s ta r te r . A starter in which the influence directing its performance is automatic. A c r o s s - th e - lin e s ta r te r . A device that connects the motor to the supply without the use of a resistance or autotransformer to reduce the voltage. It may consist of a manually operated switch or a master switch, which energizes an electromagnetically operated contactor. M a s te r s w itc h . A switch that dominates the operation of contactors, relays, or other remotely operated devices. R e s is to r . A device the primary purpose of which is to introduce resistance into an electric circuit. A resistor, as used in electric circuits for purposes of operation, protection, or control, commonly consists of an aggregation of units. (Resistors, as commonly supplied, consist of wire, metal, ribbon, cast metal, or carbon compounds supported by or embedded in an insulating medium. The insulating medium may enclose and support the resistance material as in the case of the porcelain tube type, or the insulation may be provided only at the points of support as in the case of heavy-duty ribbon or cast iron grids mounted in metal frames,) O v e r c u r r e n t p r o te c tio n ( o v e r lo a d p r o te c tio n ) . The effect of a device operative on excessive current (but not necessarily on short circuit) to cause and maintain the interruption of power in the main circuit. U n d e r v o lta g e o r lo w v o lta g e r e le a s e . The effect of a device, operative on the reduction or failure of voltage, to cause the interruption of power to the main circuit, but not to prevent the reestablishment of the main circuit on return of voltage. C o n s ta n t to r q u e r e s is to r . A resistor for use in the armature or rotor circuit of a motor in which the current remains practically constant throughout the entire speed range. F a n d u ty r e s is to r . A resistor for use in the armature or rotor circuit of a motor in which the current is approximately proportional to the speed of the motor. O v e r lo a d r e la y . An overcurrent relay that functions at a predetermined value of overcurrent to cause the disconnection of the power supply. Note: An overload relay is intended to protect the motor or controller and does not necessarily protect itself. S te p - b a c k r e la y . A relay which operates to limit the current peaks of a motor when the armature or line current increases. A step-back relay D r u m c o n tr o lle r . 22-70 MARINE ELECTRICAL may, in addition, operate to remove such limitations when the cause of the high current has been removed. T e m p e r a tu r e - c o m p e n s a te d o v e r lo a d r e la y . A device that functions at any current in excess of a predetermined value essentially independent of the ambient temperature. M o ld e d - c a s e c ir c u it b r e a k e r . A circuit breaker assembled as an integral unit in a supporting and enclosing housing of insulating material, the overcurrent and tripping means being ofthe thermal type, the magnetic type, or a combination of both. R a te d c o n tin u o u s c u r r e n t. The designed limit in root-mean-square (rms) amperes or DC amperes, which a switch or circuit breaker will carry continuously without exceeding the limit of observable temperature rIse. C ir c u it b r e a k e r in te r r u p tin g r a tin g ( r a te d in te r r u p tin g c u r r e n t) . The highest rIDScurrent at a specified operating voltage that a circuit breaker is required to interrupt under the operating duty specified and with a normal frequency recovery voltage equal to the specified operating voltage. C o n tr o l c ir c u it. The circuit that carries the electric signals of a control apparatus or system directing the performance of the controller but does not carry the main power circuit. I n d ic a tin g c ir c u it. That portion of the control circuit of a control apparatus or system that carries intelligence to visible or audible devices which indicate the state ofthe apparatus controlled. T y p e s o f C ir c u its a n d T e r m s A circuit supplying energy to lighting outlets only. (Lighting branch circuits also may supply portable desk or bracket fans, small heating appliances, motors of one-quarter horsepower (1861;2 W) and less, and other portable apparatus of not over 600 W each.) A p p lia n c e b r a n c h c ir c u it. A circuit supplying energy to one or more outlets to which appliances are to be connected; such circuits are to have no permanent connected lighting fixtures not a part of an appliance. M o to r b r a n c h c ir c u it. A branch circuit supplying energy only to one or more motors and associated motor controllers. C o m m u n ic a tio n c ir c u its . Circuits used for audible and visible signals and communication of information from one place to another aboard the vessel. F e e d e r . A set of conductors originating at a main distribution center and supplying one or more secondary distribution centers, one or more branch circuit distribution centers, or any combination of these two types of equipment. (But the circuits between generator and distribution switchboards, including those between main and emergency switchboards, are not considered feeders.) L ig h tin g b r a n c h c ir c u it. GLOSSARY SYSTEMS 22- 71 That portion of a wiring system extending beyond the final overcurrent device that protects the circuit. O u tle t. A point on the wiring system at which current is taken to supply utilization equipment. L ig h tin g o u tle t. An outlet intended for the direct connection of a lampholder or a lighting fixture. R e c e p ta c le . A device installed in a receptacle outlet to accommodate an attachment plug. R e c e p ta c le o u tle t. An outlet where one or more receptacles are installed. A tta c h m e n t p lu g . A device which, by insertion in a receptacle, establishes connection between the conductors of the attached cord and the conductors connected permanently to the receptacle. D is tr ib u tio n c e n te r . A point at which is located equipment generally consisting of automatic overload protective devices connected to buses, the principal functions of which are subdivision of supply and the control and protection of feeders, subfeeders, or branch circuits, or any combination of feeders, subfeeders, or branch circuits. S w itc h b o a r d s . A device that receives energy from the generating plant and distributes it directly or indirectly to all equipment supplied by the generating plant. A subdistribution switchboard is essentially a section of the generator and distribution switchboard (connected thereto by a bus feeder and remotely located for reasons of convenience or economy) which distributes energy for lighting, heating, and power circuits in a certain section of the vessel. D is tr ib u tio n p a n e l. A piece of equipment that receives energy from a distribution or subdistribution switchboard and distributes that energy to energy-consuming devices or other distribution panels or panelboards. P a n e lb o a r d . A distribution panel enclosed in a metal cabinet. D e m a n d fa c to r . The ratio of the maximum demand of a system or part of a system to the total connected load of the system or of the part ofthe system under consideration. V ita l s e r v ic e s . Those services required for the safety of the ship and its passengers and crew. These may include propulsion, steering, navigation, firefighting, emergency lighting, and communications functions. Since the specific identification ofvital services is influenced by the type of vessel and its intended service, this matter should be specified by the design agent for the particular vessel under consideration. B r a n c h c ir c u it. 23-2 ELECTRICAL MACHINERY In synchronous alternators, as in all electromagnetic devices, voltage is determined by relative motion between conductors and lines ofmagnetic flux. In small units the armature (load carrying conductors) may be rotated in way of a magnetic field. In the majority of synchronous machines, and in all of the larger units, magnetic flux is produced in the rotor poles and swept across stationary armature windings. These armature windings consist of coils embedded in slots on the periphery of the stator. The number of slots spanned by an armature coil is called the coil pitch. The pole pitch is the total number of stator slots divided by the number of machine poles. The coil pitch is usually slightly less than the pole pitch and is in units of e le c tr i.c a l d e g r e e s , since by definition poles are separated by 180 degrees: C o n s tr u c tio n S t a t o r w i n d i n g s . Figure 23-1 shows a developed view of a two-layer, three-phase stator winding. In this view the cross section of the cylindrical stator is laid flat; in the actual stator, slot 18 is adjacent to slot 1. In a two-layer winding each slot holds two coil sides (the "left" side of one coil and the "right" side of another) and, since each coil has two sides, the number of stator slots is equal to the number of stator coils. Each phase has the same number of coils (18/3 = 6), and each phase has groups of coils corresponding to the number of poles (e.g., the "A-group"and the "a-group" for phase A). Therefore the number of coils per group equals the number of "slots per pole per phase." In the example winding, each coil spans 8 slots (A-group coil #1: slots 1 and 9; coil #2: 2 and 10; coil #3: 3 and 11, for example) and the center of each group of a particular phase is separated by 180 degrees (A-group is centered at slot 6, a-group is centered at slot 15-9 slots away). Thus when peak positive voltage is induced in the A-group, peak negative voltage is induced in the a-group. The senses ofthe A-group and a-group windings are opposite, so that with the groups connected in series the A-phase sinusoidal voltage is twice the group Figure 23-1. Synchronous machine stator construction voltage. The B-phase windings are centered at slots 12 and 3, 1 2 0 d e g r e e s from the centers of the A-phase groups; hence peak values will be reached in these groups one-third of a cycle after the A-phase groups have peaked. Similarly, the C-phase groups are centered at slots 18 and 9, 120 degrees after the B-phase and 120 degrees before the A-phase groups. This physical distribution of the phase windings results in the production of three sinusoidal voltages (of equal amplitude and frequency) out of phase with one another by exactly one-third cycle. The phase winding leads may be connected in either a delta or a wye, yielding line voltages equal to phase voltages (delta) or 1.73 times the phase voltage (wye). Wye connected windings must be sized to carry full line current, and delta connected windings carry 86.6 percent (1/1.73) ofline current under balanced conditions. Wye connected machines allow for four-wire, three-phase service as aw ay 23-4 ELECTRICAL MACHINERY the neutral terminal is accessible. Embedded temperature detectors are generally provided in the stator windings of machines rated over 500 kVA. R o t o r w i n d i n g s . Rotor windings on synchronous machines are "concentrated" windings (as opposed to the "distributed" windings that cover the periphery ofthe stator), which carry DC current in an appropriate direction so as to determine the polarity of each individual pole. On high speed machines, typically 3600 RPM or 1800 RPM turbine-driven alternators (turbogenerators or TG), the cylindrical construction pictured in Figure 23-2(a) is utilized. This compact construction is necessary because the high centrifugal stresses make the bulkier salient-pole construction (Figure 23-2(b)) difficult to design. On lower speed machines, such as the 10-pole (720 RPM) rotor shown in Figure 23-2(c), rotor designs have projecting laminated pole pieces mounted on the rim of a steel spider. The spider is in turn keyed to the rotor shaft. The field coils are fitted over the pole pieces and connected so that adjacent poles have opposite polarity when energized by the DC excitation current. Amortisseur or dampening bars are embedded in the pole faces and connected to one another by conducting straps that serve to dampen rotor oscillations during abrupt load changes. systems. Regardless of rotor type, cylindrical or salient pole, a source of DC current is needed to excite the field of the alternator. For many machines, field power is supplied by a DC generator called an e x c ite r , which itself may be separately excited by a p ilo t e x c ite r . These exciters may be driven by separate prime movers or their rotors may be on the same shaft as the field poles ofthe alternator, as shown in Figure 23-3. In either case, DC power must be extracted from the armature ofthe exciter through a commutator and brushes and injected into the alternator field circuit through a set of slip rings and brushes. Note that Figure 23-3(a) shows three sets of sliding contacts: (1) amplidyne (pilot exciter) armature to exciter field connection; (2) exciter armature to stationary (exciter) terminal connection; and (3) stationary terminal to alternator field connection. Alternator output voltage and current are monitored by potential and current transformers respectively. The voltage regulator serves to maintain the alternator line voltage at the desired level by compensating for the effects of generator loading. By adjusting the pilot exciter's field, the exciter voltage is altered, and control of DC excitation to the alternator rotor is thereby accomplished. As brush/commutator/slip ring arrangements tend to be a significant maintenance burden, many modern alternators are fitted with a brushless excitation system as illustrated in Figure 23-3(b). Here the pilot exciter is a small permanent magnet alternator whose output voltage, together with a feedback voltage from the synchronous alternator armature terminals, E x c ita tio n Figure 23-2. Synchronous machine rotor construction is fed into a magnetic amplifier. The output of the amplifier supplies the exciter field, which is stationary. The exciter armature (rotor) output is fed through an uncontrolled rectifier that is mounted on, and rotates with, the shaft common to pilot exciter permanent magnet, exciter armature, and alternator field poles. The rectifier supplies the alternator field with its DC excitation. Control is accomplished through regulating the magnetic amplifier output. Another version of this arrangement includes controlled rectification of the alternator's output directly to the (stationary) exciter field, eliminating the pilot exciter. SYNCHRONOUS ALTERNATORS 23-7 is also a function of plant status (i.e., total load, number of machines paralleled, load share, etc.). Exciter output, for a given speed, determines no-load voltage. Again the system voltage is a function of plant status as well. In the a u to m a tic mode, adjustment ofthe automatic voltage regulator (AVR) set point accomplishes this control. In the m a n u a l mode, direct control of exciter field current is available. Manipulation of both governor and AVR set points may have different purposes depending on whether the affected alternator is operating alone or in parallel. These cases will be treated separately in the following paragraphs. o p e r a t i o n . When an alternator is operating alone, its load is determined by the aggregate demand of all plant equipment on line. Power factor is a function of the reactive/resistive characteristics of the load items and cannot be adjusted by manipulating the alternator control devices. In this mode the governor set point control has a direct effect on generator speed and therefore bus frequency. Thus any deviation from the nominal system frequency may be corrected by raising or lowering the governor set point. With the exciter output under the control of the AVR, any slight speed adjustment will have a negligible effect on system voltage. System voltage may be adjusted by raising or lowering the AVR set point. System voltage does have a slight effect on load characteristics, as does frequency. However, since the goal is to maintain rated voltage and frequency (e.g., 450 volts, 60 Hz), it is not the normal function of the governor or AVR to manipulate load characteristics. The following procedure may be used as a guide for single alternator operation: S in g le a lte r n a to r Figure 23-3. Exciter arrangements O p e r a tio n Control of synchronous alternator output is accomplished through the manipulation of (1) prime mover speed (or more precisely, torque) and (2) exciter output (and thereby rotor field strength). The first of these is normally performed by adjusting the prime mover governor set point. While this action determines the no-load speed ofthe prime mover, the speed (and hence frequency) of the alternator under load A lte r n a to r c o n tr o l. 1. Make a visual inspection to insure that all repairs have been completed, and that unit is free of tools or other debris and in general is in a condition satisfactory for operation. 2. Insure that disconnect links are in place. (Note: Disconnect links should be opened or closed with an insulated wrench and always with the alternator circuit breaker open.) 3. Start prime mover. 4. Switch voltage regulator to A u to m a tic . 5. Adjust frequency to rated value with governor set point control. 6. Use AVR set point control to obtain rated voltage. Check A-B, B-C, and C-A voltages (they should be equal), and line-to-ground voltages where possible. 7. Close circuit breaker. (Note: In some installations where parallel operation is possible, the synchroscope switch is interlocked with the circuit breaker switch so that even under single generator operation the synchroscope switch must be on G e n . N o . 1 in order to close the circuit breaker on generator number 1. The synchroscope should be turned off once the alternator is on line.) 23-8 ELECTRICAL MACHINERY P a r a lle l o p e r a tio n S y n c h r o n iz in g . Offline, the governor and AVR set point controls have the identical functions as described under single alternator operation. Thus the procedure for parallel operation is the same as above u p th r o u g h s te p 6. For connection in parallel the goal is to have the incoming generator output be in phase with and identical to (or slightly greater than) the bus voltage in amplitude and frequency. Here is a guide to complete the paralleling procedure: 7. Adjust incoming generator voltage to slightly above bus voltage. It is preferred to utilize the same voltmeter (i.e., a calibrating voltmeter) to compare voltages rather than two different meters on the on-line and incoming machines. 8. Switch the synchroscope to the incoming machine and adjust frequency until the synchroscope pointer revolves slowly in the fa s t direction. Pointer position indicates relative phase ofincoming machine and bus voltages: 12 o'clock identifies a zero phase difference and 6 o'clock identifies a 180-degree phase difference. The r a te of rotation indicates the frequency differential, with 1 RPM (the speed of a clock's second hand) identifying a difference of 1 1 6 0 Hz. 9. Check synchroscope indication against synchronizing lamps to insure proper operation of synchroscope. Indicating lamps should be alternating bright and dark in unison and at a rate equal to synchroscope rotation, with 12 o'clock corresponding to the middle of the dark period, and 6 o'clock corresponding to the brightest condition of the lamps. (Note: If lamps are not in unison, phase rotation of oncoming machine is opposite that of b u s - D o n o t p a r a lle l m a c h in e s .) 10. Close circuit breaker at a few degrees before 0 degrees. (Because of a slight time lag-15 to 30 cycles-for the closing mechanism to complete its operation, it is good practice to anticipate closing by a few degrees so that actual closing occurs at 0 degrees.) Closing ofthe breaker with voltage out of phase by more than 15 degrees will cause dangerous crosscurrents between the machines paralleled. Out-of-phase closing at 180 degrees is identical to short-circuiting both machines! A few additional comments on step 9 are required. Any discrepancy between synchronizing lamps and synchroscope usually indicates a defective synchroscope. This assumes proper lamp connection and use of identical wattage lamps with the specified voltage rating. L o a d s h a r in g . The total plant load is the sum of the true power (kW) and reactive power (kvar) load on each machine operating in parallel. The load is balanced when the kW and kvar loads are distributed in proportion to the ratings ofthe machines paralleled. For purposes of this illustration, it is assumed that two machines of equal capacity (rating) are supplying a common load. SYNCHRONOUS ALTERNATORS 23-9 The prime mover governor position has a direct effect on the steam flow to a turbine-driven machine (fuel flow to a diesel generator). Considering a turbogenerator example, the aggregate steam flow to both machines, operating at nominal bus voltage and frequency, is determined by the plant kW load. The d iv is io n of kW load between generators is in approximate proportion to the fraction of total steam flow to each unit. Increasing the governor set point of, say, generator #1 while s im u lta n e o u s ly decreasing the governor set point on generator #2 will shift some of the kW load to generator #1 from #2. Since the total steam flow has not changed, the bus frequency remains unchanged. It should be noted that generator #1 speed is not increased nor is #2 speed decreased, but rather at a constant speed (frequency) prime mover #1 is producing more torque (torque x RPM = horsepower) and #2 less. If the governor set point of #1 were increased without a corresponding decrease of the #2 setting, generator #1 would take on an increased share ofthe kW load; but since the total steam flow had been increased, b o th machines would run faster and the bus frequency would rise. Increasing the governor set points of both machines simultaneously would similarly raise the bus frequency without changing the kW load share percentage of either machine. Corresponding arguments could be made when decreasing the governor set points of either (or both) machines. The aggregate excitation (which may be thought of in terms of either total flux or total rotor field current) determines the system voltage for a given load demand. Reactive (kvar) loads are shared between machines in approximate proportion to the percentage of total excitation supplied to each machine by its AVR (assuming kW loads are equal). Therefore, increasing the AVR set point of generator #1, while s im u lta n e o u s ly decreasing the AVR set point of#2, shifts kvar load to #1 from #2. And, since the total excitation has not changed, the bus voltage remains constant. Increasing the excitation to #1 only will increase its share of reactive load; but since the total excitation has been increased, the system voltage will ris~ . When balancing kvar load, remember that increasing the kvar load d e c r e a s e s the power factor ofthat machine. Since most ship's switchboards do not include varmeters, equal power factor meter readings indicate reactive power balance (assuming kW load has already been balanced). Another indication ofequalized reactive power division is amperage ofeach machine. With the kW load balanced (say, equally), equal reactive load will be realized when the amperage of both machines is the same. In the following numerical example, two machines of equal capacity are operating in parallel, with generator #1 having just been put on the line. The status in Figure 23-4(a) is that unit #1 is supplying 40 kW and 40 kvar, #2, 760kW and 560 kvar (total kW load is 800, total kvar load is 600). Note that there is not a great disparity in power factor, yet the amperages differ widely. Also the total amperage is 73 plus 1211-1284 amps. SYNCHRONOUS 23-11 ALTERNATORS load, generator #1 is now grossly underexcited (higher pf, lower amperage) while #2 is overexcited (low, lagging pf, higher amperage). The total amperage is now 513 plus 925-1438 amps (Figure 23-4(b». Finally, the AVR set point of generator #1 is increased while simultaneously the set point is lowered on unit #2, maintaining the system at 450 volts while shifting 260 kvars to #1 generator. The result is equal kW, equal kvar, equal amps, and equal pf(Figure 23-4(c». Total current in the balanced condition is 641 plus 641-1282 amps, which is the minimum value for any distribution of an 800 kw, 600 kvar system load. The above is an oversimplification of a complicated set of phenomena. I n particular, increasing kW load on a synchronous alternator without increasing field strength causes some shedding of kvar to the other machine and may result in a leading power factor for the machine gaining kW. An even greater over-simplification, though occasionally useful, is the statement: "The governor controls watts, the exciter controls amps." M a in te n a n c e I n t r o d u c t io n . Electrical maladies in AC machinery generally consist of either opens (discontinuity where there should be continuity), shorts (low resistance paths bypassing circuits/devices intended to be energized), and grounds (low resistance paths to ground or component frame or housing). Controller malfunctions, of course, can be a maintenance problem, as can mechanical failures of bearings, couplings, ventilation, etc. The following sections, however, will deal with locating opened, shorted, and grounded windings in the stator and rotor of synchronous machines. A breakdown ofthe winding insulation is a root cause of any ofthese, and excessive heat, moisture, or mechanical abrasion will lead to insulation deterioration. Keeping machinery clean and dry is an obvious preventive maintenance priority, and periodic monitoring ofbearing and coupling performance will lessen the probability of mechanical damage due to reduced clearances. A synchronous alternator troubleshooting chart is presented as Table 23-l. TABLE Synchronous A lte r n a to r 2 3 -1 T r o u b le s h o o tin g C hart P r o b le m P o s s ib le C a u s e R em edy No output voltage Defective voltmeter Check with properly operating voltmeter and replace if defective. Check potential transformer (PT) primary and secondary voltages; replace PT if defective. Test and replace defective coil(s). Test and repair (see Table 23-5). Disconnect field coils and check slip ring insulation resistance with megger. Test and replace if defective. Check fuses, connections, instrument transducers. Open/shorted PT Open field coil(s) Exciter failure Shorted slip rings Blown diodes Defective regulator 23-12 ELECTRICAL MACHINERY P r o b le m P o s s ib le Low output voltage Low speed Insufficient excitation Shorted/grounded field coil(s) Shorted/grounded armature coil(s) Check frequency; adjust governor set point. Check regulator output; adjusUrepair as necessary; troubleshoot exciter (see Table 23-5). Test and repair. High speed Overexcitation Check frequency; adjust governor set point. Check regulator output; adjusUrepair as necessary; troubleshoot exciter (see Table 23-5). Repair/replace defective armature coil(s). High output voltage C ause Open armature winding (one leg of delta c(mnection) Fluctuating voltage Poor brush/slip ring contact Faulty regulator Irregular Fluctuating quency fre- speed Faulty governor Irregular excitation Prime mover performance R em edy Test and repair. Clean slip rings and brushes; replace brushes. Check regulator output; regulator sensing transducers, loose connections. Check frequency; adjust governor if required; check prime mover operation. Adjust governor; check lube oil pressure for fluctuation. Check regulator performance; check exciter connections. Check fuel system (DG); steam supply, back pressure (TG). Overheating Overload Phase imbalance (extreme) Insufficient ventilation Bearings Uneven air gap Shorted armature winding Shorted field coil Reduce load. Check for open phase connection; open/shorted/ grounded armature winding. Remove restrictions; clean windings; check C.W. supply. See "Noisy operation" below. See "Noisy operation" below. Test and repair/replace. Noisy operation Bearings Check for worn, loose, dry, or overlubricated bearings; replace worn or loose bearings; lubricate dry bearings; relieve overlubricated bearings. Check for bent shaft, bearing condition, gear alignment. Check alignment; re-align and/or tighten coupling. Tighten bolts; dip in varnish and bake. Uneven air gap Coupling Loose laminations Test and repair/replace. S ta to r fa u lts S h o r te d s ta to r c o ils . A higher than normal current, particularly in one phase, is an indication of a shorted stator coil. Visual inspection of the stator windings for evidence of burning or overheating may be all that is necessary to identify the offending coil. When discoloration is not evident, a "growler" may be used to locate the shorted coil. A growler consists of a SYNCHRONOUS ALTERNATORS 23-13 coil wound on an iron core. It is connected to an AC source and placed against the armature core. Either a flexible feeler built into some portable growlers, or a separate feeler fashioned from a hacksaw blade, will vibrate (causing a buzzing sound) when located over a slot containing a shorted coil. The alternating flux produced by the growler induces a voltage in each armature coil as the feeler is moved from slot to slot around the periphery of the stator. If a coil is shorted, the completed circuit allows current to flow in that coil and the resulting magnetic field encircling the shorted coil attracts and repels the feeler, causing the telltale buzzing (growling). G r o u n d e d s ta to r c o ils . "Megging" between stator terminals and the machine frame will indicate whether or not there is a grounded stator coil, but will not identify the particular coil. Often the path to ground is through minute cracks in the insulation that have become filled with conducting contaminants; and moisture absorbed in the windings or condensed on the insulation surface will result in low insulation resistance readings. Thus cleaning and drying of the stator windings will often remedy the problem. For trending purposes it is advisable to take periodic insulation resistance readings at regular intervals and under similar conditions, usually immediately after shutdown when the windings are still warm and, presumably, dry. On large machines, however, good insulation has considerable capacitive effect between conductors and frame, and a large static charge can be built up during normal operation of the machine. Thus the windings of large machines should be grounded for about 15 minutes prior to taking "megger" readings. The response of the megohmmeter to good insulation is an initial downward dip, followed by a gradual climb toward the true resistance value. Slight downscale dips during this climbing period are indicative of dirty insulation, thus careful observation of the pointer movement during the test is important. Visual inspection may identify the problem coil. As an aid in spotting the grounded coil, the somewhat sarcastically named "smoke test" is employed. After first insulating the stator from ground with wood blocks or other heavy insulating material, a reduced voltage is applied between stator winding and machine frame as shown in Figure 23-5. The current limiting rheostat should be adjusted to allow sufficient current to circulate through the defective coil to frame to cause overheating and perhaps smoking at the grounded location. O p e n s ta to r c o ils . An open in a single-circuit stator (one conducting path per phase) will prohibit a three-phase motor from starting and result in severe phase imbalance in a running motor or generator. If the open occurred under load conditions the resulting arcing may have caused visible damage to the insulation in the vicinity of the open. If not, the following sequence of tests is a guide to determine the exact location of the defective coil: 1. Determine if winding is wye or delta. Meg between each pair of terminal leads; three zero readings indicate a delta, one zero and two high readings indicate a wye with the offending phase identifiable by the lead common to the two high readings (Figure 23-6(a)). 2. If delta, isolate phase. Disconnect and carefully tag for identification the nine leads (two per phase plus three line leads). By megging each pair, the two leads to the phase containing the defective coil can be identified. In Figure 23-6(b), an infinity reading will result when megging between al and any other terminal, and a similar result will occur with a2. With the other four phase leads, however, there will be one other lead to which there is continuity (bl-b2, c1-c2). 3. Pinpoint fault. With the offending phase leads isolated, one lead of the megohmmeter is clamped to one ofthe phase leads and a needlepoint probe on the other megger lead is moved from coil junction to coiljunction until a high reading is indicated. The faulty coil is located between junctions that yield zero and high readings, as illustrated in Figure 23-7(a). Large machines may have multiple parallel paths in each phase, in which case each path must be isolated, tagged, and tested as in step 2 above before the fault can be pinpointed as in step 3. E m e r g e n c y r e p a ir s . Operating a machine with one coil out of service may have serious effects on a machine with only a few coils per phase. Figure 23-6. Identifying open stator phase Large machines, however, may operate satisfactorily at reduced loads with a number of coils cut out. An emergency repair-cutting a defective coil in half and jumping adjacent coils to complete the stator circuit (Figure 23-7(b))-may be appropriate depending on the number of stator coils in the machine. R o to r fa u lts S h o r te d fie ld c o ils . By applying 120 or 240V AC (60 Hz) to the collector rings of a salient-pole synchronous alternator (or motor), and measuring the voltage drop across each individual field coil, it is possible to determine SYNCHRONOUS ALTERNATORS 23-17 G r o u n d e d fie ld c o ils . A grounded coil may be identified with a megohmmeter after disconnecting each coil from the others (Figure 23-9). Each coil is then tested coillead-to-shaft, a low (zero) reading indicating a grounded coil. On brushless machines care should be taken to isolate the shaftmounted rectifier before using a megger on the rotor windings, as the high voltage generated by the megger could damage the diodes. O p e n fie ld c o ils . Open field coils may be identified by applying rated (DC) voltage or less to the field circuit. A voltmeter is connected from one field lead to successive coil junctions (Figure 23-10). The voltmeter will read zero until the defective coil is bridged, at which time full voltage will be indicated. Alternatively a megohmmeter could be used to locate the discontinuity. Open or grounded field coils should be replaced; shorted field coils should eventually be replaced too, but in an emergency may be left and the machine operated, provided overheating and/or vibration is not present. R e v e r s e d fie ld c o ils . Since adjacent field poles should be of opposite polarity, a simple way of testing for correct polarity is with a compass. Alternatively two bolts may be put in contact with the faces of adjacent poles and an attempt made to bridge the gap between poles by bringing the bolt heads together. Strong attraction of the bolt heads indicates opposite polarity of the adjacent poles; repulsion between the bolts indicates identical polarity and, hence, a reversed pole. the defective coil (Figure 23-8). If all the coils were electrically identical (same impedance), the voltage drop across each should be 240 divided by the number of poles. On the presumption that a shorted coil will exhibit a lower impedance, simple voltage division dictates that the shorted coil will yield a smaller voltage drop than the rest. On brushless machines the diode circuitry must be first isolated from the field circuit and the AC voltage applied to the terminals ofthe field circuit. SYNCHRONOUS MOTORS 23-19 R o ta tin g M a g n e tic F ie ld The key to understanding the operation of polyphase motors (both synchronous and induction machines) is comprehension of the production of a magnetic field, rotating at synchronous speed (fx 120/P), by the stationary windings of the machines. The stator (armature) windings, as described previously, consist of three independent (phase) circuits distributed around the periphery of the stator such that each of the phase windings is separated from the other two by 120 degrees (electrical). When each Figure 23-9. Identifying grounded field coil SYNCHRONOUS MOTORS D e s c r ip tio n The description of the synchronous machine in the previous section is appropriate to both motor and generator operation. In fact, there are operations where the same machine is used for both applications: a synchronous machine as a motor driving a DC generator to charge a bank of batteries which, in turn, could supply the DC machine, acting as a prime mover for the synchronous machine in the role of an alternator; and a synchronous motor driving a pump in a large pump-storage facility during off-peak hours, acting as a generator driven by the pump/water turbine to supply peak electrical power. The fundamental difference between the two modes of operation is, of course, that as an alternator the machine is spun at synchronous speed by a source of mechanical power (prime mover), is supplied with DC excitation, and delivers electrical power in the form of three-phase AC. As a motor the stator is energized with three-phase AC, the rotor again is DC excited, and mechanical power (torque at synchronous speed) is produced. Figure 23-10. Identifying open field coil winding is fully energized (at intervals separated by one-third of a cycle) the magnetic field produced by the armature windings will be oriented along the axis of that particular phase winding. Thus the direction of the stator-produced magnet field shifts around the stator, returning to its original orientation in one full cycleofarmature current (see Figure 23-11). In a two-pole machine (two coil groups per phase) this constitutes one complete revolution in 1/60 second (60 rps = 3600 RPM). In a four-pole machine, although the initial magnetic orientation is repeated after one cycle, in following a particular pole around the stator, it takes tw o cycles for that pole to return to its original position (4 1 6 0 sec/rev yields 30 rps, which equals 1800 RPM). In summation: a north pole returns to its original position f x 60 times per minute no matter how many north poles are SYNCHRONOUS 23-21 MOTORS O p e r a tio n S y n c h r o n o u s s p e e d . A synchronous motor rotates at synchronous speed because the rotor electromagnet is attracted by the rotating stator field. The rotor and stator magnetic fields are not perfectly aligned, however. The rotor lags the stator-produced rotating magnetic field by a few degrees. This is not to be confused with "slip"; rotor and stator fields are revolving at the same speed, but not at exactly the same position. The stronger the rotor electromagnet, the closer the two fields are in alignment. With constant excitation, the greater the mechanical load on the motor, the greater the degree of lag between rotor and stator fields. For a given mechanical load, this lag can be decreased by increasing the excitation to the rotor. There is a point at which, for a given level of excitation, an increase in mechanical load will cause the rotor to fall out of synchronism with the stator field. This phenomenon is called "slipping a pole" and results in severe vibration and oscillation in stator current. This is a situation to be avoided and may be remedied by decreasing the mechanical load or increasing the level of excitation. Exciter failure will, of course, cause this condition, requiring that the machine be shut down. This and other symptoms of maloperation are outlined in Table 23-2. TABLE Synchronous M o to r T r o u b le s h o o tin g C hart P r o b le m P o s s ib le C a u s e R em edy Fails to start Open line(s) Check fuses/circuit breaker. Note: Will not start with one line open. Test armature windings for open. Check controller relay coils and contacts. Note: Field should not be excited until rotor is spinning at near-synchronous speed. Relieve mechanical load on motor; check for bearing seizure or other mechanical restriction. Check supply. Open phase Field excited Overload Low voltage Overheating Overload Insufficient ventilation Insufficient excitation Excessive excitation High voltage Uneven air gap Figure 23-11. Rotating magnetic field oduced by the stator windings; but, since there are P /2 north poles, ? original north pole is back at its original position in P/2 cycles. Thus ~ synchronous speed formula-Ns = f x 60 I (P/2) or f x 120/Pscribes the rate of rotation of the stator magnetic field. 2 3 -2 Open/shorted/grounded stator coil(s) Open/shorted/grounded field coil(s) Check status against nameplate ratings; relieve mechanical load. Remove restrictions; clean windings; check C. W. supply. Check pf; if too lagging, increase excitation. Check pf; if too leading, reduce excitation. Check supply. Check gap clearances; check for evidence of rotor/ stator contact; realign if necessary. Test and repair/replace as required. Test and repair/replace Runs fast Frequency high Check supply. Runs slow Frequency low Check supply. as required. 23-22 ELECTRICAL P r o b le m P o s s ib le Pulls out of synchronism Overload C ause Insufficient Will not synchronize excitation MACHINERY R em edy Check status against nameplate ratings; relieve mechanical load. Check pf; increase excitation; check field coils; check exciter output, exciter field rheostat. Controller failure Exciter failure Check controller relay coils and contacts. Check exciter output, slip ringlbrush assembly; check for blown diodes (brushless types) (see Table Overload Check status against nameplate ratings; relieve mechanical load. Test and repair/replace as required. 23-5). Open/shorted/grounded field coil(s) Noisyoperation Out of synchronism Bearings Uneven air gap Coupling Loose laminations See above. Check for worn, loose, dry, or overlubricated bearings; replace worn or loose bearings; lubricate dry bearings; relieve overlubricated bearings. Check for bent shaft, bearing condition, gear alignment. Check alignment; re-align and/or tighten coupling. Tighten bolts; dip in varnish and bake. P o w e r f a c t o r c o r r e c t i o n . From an electrical point of view the relationship between load and excitation is indicated by the machine's power factor. An underexcited machine manifests a la g g in g power factor. Excessive lagging may cause the machine to slip a pole. For lightly loaded machines this level may be as low as a .4 or .5 lagging power factor; at rated loads the rotor may fall out of synchronism at power factors as high as .8 lagging. Increasing excitation will bring the power factor to unity, and overexciting the rotor yields a leading power factor. If optimizing the synchronous motor's operation were the only consideration, tuning the excitation to produce unity power factor would be desirable. At unity power factor the stator current draw is minimized for a given power output. Over- or underexciting the rotor results in more current to produce the same horsepower. At unity power factor the motor consumes kW proportional to the mechanical load, but no kvar; underexcited, the motor consumes lagging (positive) kvar; overexcited, the motor consumes leading (negative) kvar. Synchronous propulsion motors are typically the only load on the propulsion generator and thus operating at unity power factor is advantageous to both motor and generator. In a plant containing a synchronous motor and other inductive loads (induction motors, for example), the o v e r a ll power factor of the plant may be improved by operating the synchronous machine in the overexcited state. The negative kvar consumed by the synchronous machine would cancel out some, or all, of the lagging kvar demanded by the inductive loads. A synchronous motor not loaded mechanically (and thus consuming only a small amount ofkW) and deliberately overexcited to consume leading kvar (which may be thought SYNCHRONOUS MOTORS 23-23 of as s u p p ly in g lagging kvar to those plant loads demanding them) is called a s y n c h r o n o u s c o n d e n s e r . Banks of capacitors can accomplish the same purpose. S t a r t i n g . Synchronous motors do not develop any starting torque. This is because, although the stator-produced rotating magnetic field is instantly rotating at synchronous speed when the stator is energized, the rotor must a c c e le r a te to synchronous speed. At the instant of starting, the rotor poles are alternately attracted and repelled by the rotating stator poles, north poles followedby south poles 1/120 second later, resulting in zero net torque. It is necessary, therefore, to have the rotor spinning at near synchronous speed b e fo r e energizing the rotor field. On some very large machines this is accomplished with an auxiliary starting motor. In most applications, however, the amortisseur windings serve as the starting device. These damper bars, imbedded in the rotor pole faces, form a squirrel-cage rotor, and the synchronous rotor is thereby accelerated as an induction machine. When the rotor is at near synchronous speed (approximately 1 percent slip or less) the field poles are excited with DC current and the rotor locks into synchronism. At synchronous speed no currents are induced in the starting (amortisseur) windings. The rotor windings may be energized manually by the operator upon witnessing the rotor speed approaching synchronous speed and the concurrent drop in stator current or it may be accomplished automatically. The DC source of excitation may be a separate source, from the AC mains via a rectifier or a separate DC generator, or an exciter mounted on the motor shaft. To provide application ofDC excitation at the proper moment, automatic synchronous motor starters typically incorporate a p o la r iz e d fr e q u e n c y r e la y (PFR). The PFR illustrated in Figure 23-12 includes a static core supporting two windings, one DC and one AC. The direction of the flux produced by the DC coil is constant, but that of the AC coil alternates. If a voltage decaying in amplitude and frequency is applied to the AC coil, at some point the flux in the armature is no longer adequate to hold it closed. The threshold at which this occurs is on the negative portion of the cycle. Thus both magnitude and phase govern the timing of the relay closure. The PFR is appropriate for synchronous motor starting because, as the motor accelerates, voltages induced in the r o to r windings decay in magnitude and frequency. Thus, applying the induced rotor voltage to the AC coil of the PFR makes the relay sensitive to both rotor speed and the relative position of the rotor field coils to the synchronous stator flux. In the starting circuit schematic illustrated in Figure 23-13, the AC coil of the PFR is shunted by a reactor that acts as a short at very low frequencies. Upon depressing the start button, the M contacts close, energizing that stator and inducing stator-frequency AC in the rotor Figure 23-13. Synchronous motor starter Figure 23-12. Polarized frequency relay circuit. The PFR contacts are held open, preventing energization ofthe FC coil. As the rotor accelerates and the frequency drops, the reactor takes on a greater percentage ofthe induced rotor current. The PFR contact will fall back closed when the rotor nears synchronous speed, allowing the FC contacts to close and introducing the DC excitation current to the rotor field windings. The o u t- o r - s te p r e la y (OSR) is included in the field circuit during acceleration so that if synchronization does not occur within a preset time (5 to 10 seconds), the stator will be de-energized and the motor shut down. T H R E E -P H A S E IN D U C T IO N P r in c ip le s M OTORS o f O p e r a tio n Synchronous motors and, for that matter, DC motors require external sources of electrical power to energize both their stator and rotor windings 23-26 ELECTRICAL MACHINERY to produce torque. The stator of an induction machine is identical to the synchronous machine stator, producing a magnetic field that rotates at synchronous speed (Ns = fx 120/P) when energized from a three-phase AC supply. The rotor of an induction machine, however, is not connected to any external power source. As the name implies, the principle of electromagnetic induction is responsible for rotor current. This is the same action that produces secondary voltage in a transformer; accordingly, stator and rotor circuits of induction machines are known as p r im a r y and s e c o n d a r y windings respectively. Two types or rotor construction differentiate classes of induction motor: the squirrel-cage rotor and the wound rotor. S q u ir r e l-C a g e M o to r s The squirrel-cage induction motor is the most widely used industrial motor in the world. A rugged, easily constructed, economical, and highly effective rotor, the s q u ir r e l c a g e is illustrated in Figure 23-14. Laminations, punched from magnetic steel sheets and including slot cutouts, are sandwiched on the rotor shaft. Conducting bars of copper or aluminum are inserted in the slots and cast end rings form a low-resistance short circuit. Rotor bar cross section will determine, to some extent, the operating characteristics of the motor as will be explained later. M o to r c o n s tr u c tio n . M o to r c h a r a c te r is tic s S lip . The stator-produced rotating magnetic field induces voltages in the rotor bars as it sweeps across the cage. Currents circulate between bars under north and south poles with a frequency and amplitude proportional to the rate at which the stator-produced rotating field is moving relative to the bars ofthe cage. These circulating currents produce a rotor field that imparts torque to the rotor. As the rotor accelerates, the magnitude and frequency ofthe induced rotor currents decrease as the relative velocity of rotating magnetic field (Ns) and rotating squirrel cage (Nr) decreases. If the rotor were to spin at synchronous speed (Ns), no rotor currents would be induced and no rotor torque developed. Thus under normal (torque producing) conditions, induction motor speed (Nr) is a lw a y s le s s th a n synchronous speed (Ns). The difference (Ns - Nr) is called the "slip speed" expressed in RPM. More often this difference is expressed as a decimal fraction (or percent) of synchronous speed called "slip" (or "percent slip"). Figure 23-14. Squirrel-cage rotor For example, a four-pole induction motor (Ns = 1800 RPM) spinning at 1746 RPM has a slip speed of 54 RPM and a slip of .03 (3 percent). T o r q u e . Induced rotor voltage and frequency are proportional to slip: Er = s x Es and fr = s x fs. Thus at near synchronous speed, rotor current (and therefore developed torque) is small, although almost in phase with rotor voltage. At large slips (zero RPM is 100percent slip) the induced rotor currents are greatest, but because of the greater rotor frequencies, the rotor reactance is higher and therefore power factor is lower. Since developed torque is also proportional to rotor power factor, m a x im u m developed 23-28 ELECTRICAL MACHINERY torque is neither at the lowest nor highest slips, but somewhere in between. Mathematically it can be shown that "somewhere in between" is the slip at which rotor resistance and reactance are equal. A generic squirrel-cage motor torque/speed characteristic is illustrated in Figure 23-15. The points of particular interest are (1) rated torque and speed (the operating range of the motor being between this point and synchronous speed), (2) maximum or "pull-out" torque (the motor will stall ifthe load demand exceeds the pull-out torque), (3) starting torque, and (4) the "pull-in" torque (minimum torque during acceleration). The operating speed of the motor is determined by the intersection of the motor characteristic and the load or system curve. The area between the two curves to the left of the operating point determines the acceleration during starting. The dotted line in Figure 23-15 illustrates what the characteristic would be ifit were not for displacement currents. In deep bar rotors, leakage flux surrounding THREE-PHASE INDUCTION MOTORS 23-29 the current-carrying bars produces a pattern illustrated in Figure 23-14. The inner end of the bar, looped by the most flux, will have the greater inductance. At low frequencies (i.e., low slip) this is not of much significance. However, at higher frequencies the inductive reactance of the inner portion of the rotor bar tends to choke off current at that end ofthe bar, displacing the current to the outer end of the bar. This action raises the effective value of rotor resistance which, simultaneously, raises starting torque while decreasing starting current. The inductance of a squirrel-cage rotor is determined primarily by the depth at which it is buried in the rotor iron. The resistance is determined by the cross-sectional area and the resistivity of the material used for the bars and end rings. Thus a variety of rotor bar shapes, sizes, and materials can be used to massage the generic characteristic into one suitable for a particular application. Double squirrel-cage rotors, for example, consist of a high resistance, low inductance outer cage and a low resistance, high (by virtue of its deeper position) inductance inner cage. At high slip, most of the current flows in the outer cage, providing good starting torque; at low slip, most ofthe current will be in the low resistance inner bar, providing high efficiency and low slip at rated power. For standard induction motors there are four lettered NEMA design classes: A, B, C, and D. Although characteristics within a design class vary with power and speed ratings, typical torque/speed characteristics based on NEMA MG-l are shown in Figure 23-16. The classes are distinguished by their starting torque and current, and slip at rated load. Classes A, B, and C produce rated output at 5 percent slip or less and have pretty much the same characteristics within the operating range. Locked rotor currents for Class A motors, however, are approximately 8 x FLA, where 5 to 6 x FLA is more typical for Classes Band C. Class B, usually incorporating a deep bar rotor, is the general-purpose machine used for most centrifugal pumps and ventilation blowers. Class C is a double cage type utilized where its increased starting torque is applicable: compressors without unloaders, hydraulic machinery, conveyers started under loaded conditions, etc. Class D motors, employing high resistance rotor bars (sometimes of brass), provide a very high starting torque but sacrifice speed at full load (typically 12 percent to 15 percent slip at rated torque). Applications of Class D motors include hoists, punch presses, and machines with large flywheels. D u a l- v o lta g e m o to r s . If a motor is reconnected to get twice as many parallel paths in the stator winding, it will have precisely the same operating characteristics it had before the change, providing only half the line voltage is applied. The current per winding will remain the same but the line current will double in the low voltage mode. A dual-voltage motor has the necessary winding leads accessible at the motor terminal box. Manufacturers commonly build motors suitable for both 460 and 230 volt supplies since both these levels are commonly available. Terminal mark- THREE-PHASE INDUCTION MOTORS 23-31 operate at a speed which decreases as load increases, as do DC series motors and some Class D squirrel-cage machines. A d ju s ta b le s p e e d m o to r s ' RPM may be adjusted over a continuous range for a given load but, once adjusted, may vary with subsequent load changes. Field rheostat control of a DC compound motor or secondary rheostat control of a wound-rotor induction motor fall in this category. M u ltis p e e d m o to r s operate at one oftwo or more speeds which may be selected, and run at essentially the selected speed regardless of load changes. Squirrel-cage motors with provisions for changing the number of stator poles fall in this category. V a r y in g s p e e d m o to r s S p e e d r e g u la tio n . Ofthe above categories, the first two describe a motor characteristic rather than a means of control. The term s p e e d r e g u la tio n is used to describe the difference between no-load and full-load speed (speed "droop") as a percent of rated speed. Since the full-load speed is by definition the rated speed, these two terms are related by the formula: Figure 23-16. NEMA design class characteristics ings T1 through T6 are utilized by NEMA to standardize the connection of wye and delta wound motors for dual-voltage service. The diagrams illustrated in Figure 23-17(a) or (b) are typically incorporated on the nameplate of dual-voltage motors. The connections are not to be confused with those for part-winding starters or consequent-pole, two-speed machines, which will be discussed in the following section. S p e e d v a r ia tio n C la s s ific a tio n . Motors are often classified speed in the following categories: based on the variability of rotate at constant, or practically constant, speed over the normal load range. Classes A, B, and C squirrel-cage motors, DC shunt motors, and of course synchronous motors fall in this category. C o n s ta n t s p e e d m o to r s 23-32 ELECTRICAL MACHINERY A d ju s ta b le s p e e d c o n tr o l. The latter ofthe two above categories actually describes means for controlling speed. Considering the relations hi p among rotor speed, synchronous speed, and slip (Nr = [1 - s] x N s), it is obvious that changing the synchronous speed or altering the shape of the torque! speed characteristic of the motor will effect a speed change of the motor. A d ju s ta b le s p e e d c o n tr o l of induction motors may be accomplished by varying the stator frequency (hence synchronous speed) over some, possibly wide, range. Varying the stator voltage alters the torque characteristic by changing the torque produced at any speed in proportion to the voltage squared. This effectively changes the slip at which a given load will be met. This method is often utilized in electronic controllers (Figure 23-29). M u ltis p e e d c o n tr o l. Multispeed (pole changing) controllers may be of two types: consequent-pole (single winding) or multiple winding stators. The ability to change the number of poles with a single winding is illustrated for a single-phase stator in Figure 23-18. In shifting from conventional to consequent-pole operation it is only possible to double the number of poles, hence halve the synchronous speed. Thus two-speed, single winding (2S1W) motors must have speed ratios of 2:1. There are three types of consequent-pole motors: run with the windings delta connected for highspeed operation and two parallel wye connections for low speed. With rated power available at both speeds, the torque at the lower speed is twice that at the higher speed. Figure 23-19 shows the NEMA standard winding terminal markings and connections. C o n s ta n t to r q u e m o to r s have the same full-load torque available at both speeds, therefore the horsepower ratings are proportional to the synchronous speeds produced by each winding arrangement. These run with the windings connected parallel wye for high speed and series delta for low speed (Figure 23-20). V a r ia b le to r q u e m o to r s produce a full-load torque which is proportional to the synchronous speed of the two winding arrangements and thus the power is proportional to synchronous speed s q u a r e d . This is accomplished by connecting the windings parallel wye for high speed and series wye for low speed (Figure 23-21). C o n s ta n t h o r s e p o w e r m o to r s If a motor is provided with two stator windings that are wound for a different number of poles (not necessarily in the ratio of 2:1), two-speed operation can be obtained by energizing either of the windings separately (2S2W). For three-speed operation, generally one consequent-pole and one single-spaced winding are utilized (3S2W); for four-speed machines, two consequent-pole windings are fitted (4S2W). More than two windings, and therefore four speeds, are generally not practical. By properly designing the second winding, two-winding machines may be constant horsepower, constant torque, or variable torque. Also, whenever two stator windings Figure 23-18. Consequent-pole windings are installed, voltages will be induced by the rotating magnetic field in the unenergized winding. If the unused winding has closed loops (e.g., parallel paths or a closed delta connection) currents will circulate in the winding, causing loss of power and rapid overheating. For this reason windings must be series wye or broken delta. Two wye connected windings are preferred in 2S2W machines because extra contacts in the controller are necessary S ta r tin g Figure 23-19. Constant-horsepower drive to break the delta. The purpose of the T7 terminal in Figure 23-22 is to break the closed loop when the second winding is energized. A c r o s s - th e - lin e s ta r tin g . Unlike DC machines, where only the resistance of the motor windings is available to oppose current inrush during a full voltage start, induction motor impedance is sufficient to limit the starting current to 400 percent to 600 percent of rated stator current when started at full voltage. Thus, where system capacity permits, squirrel-cage Figure 23-21. Variable-torque drive Figure 23-22. Three-speed, two-winding, constant-torque drive motors may be started by simply energizing the main contactor coil (Figure 23-23). The control circuit is generally at a lower voltage than the power circuit, is supplied via a control transformer, and is often grounded. The energizing mechanism may be (1) a two-point starter (manual select switch or automatic pressure, temperature, or float switch) that allows restart upon resumption ofvoltage after voltage failure (low voltage release, LVR); or (2) a three-point starter that utilizes momentary contact push buttons and a sealing contact to provide low voltage protection (LVP). Full-voltage starters may be reversing or non-reversing, include plugging (energizing in one direction while rotating in the other) or anti plugging features, include a jogging circuit, and may be used in multispeed applications Figure 23-23. Full voltage starter (Figure 23-24). On larger machines the main contactor may be energized indirectly via a control relay. R e d u c e d v o lta g e s ta r te r s . Although squirrel-cage motors of almost any size can themselves withstand starting inrush currents, starting large machines across the line may have a debilitating effect on the rest of the system. Taking into consideration the capacity of the system supply, the size of the motor to be started, the motor load already on line, and response time/capacity of the voltage regulator/exciter systems, the amount of voltage dip upon starting a large motor may be determined. If the dip is severe enough to possibly drop out voltage-sensitive equipment (e.g., contactors in LVP starters) then a means of reducing the inrush current to the motor in question must be provided. One of several common methods may be employed. In ( p r im a r y ) r e s is ta n c e s ta r te r s , starting current is reduced by inserting resistances (or reactances) in series with the stator windings, thus bolstering the starting impedance ofthe motor. In the starter illustrated in Figure 23-25, the normally closed starting contacts (IA) are energized prior to the main contacts (M), inserting the starting resistances. The starting contactor has a built-in time delay on de-energization (TDODE) so, although de-energized by the M contactor, the IA contacts will fall back to their normally closed position after a predetermined accelerating interval. Re- Figure 23-24. Reversing controllers sistance starters are, in a sense, self-regulating since the reduced voltage at the motor terminals is a result of the IR drop across the resistors. As the motor accelerates and the inrush current abates, the motor terminal THREE-PHASE Figure 23-25. Primary resistance start voltage rises, assuring smooth starting. Starting resistors are sized to produce a specific (reduced) voltage at the motor terminal at the instant of maximum inrush. At 80 percent of normal line voltage, for example, the motor will draw 80 percent of the starting current it would draw under full-voltage starting conditions. Other typical values are 65 percent and 50 percent ofline voltage. Unfortunately, the starting torque, proportional to the applied voltage s q u a r e d , is reduced to 64 percent (.8 x .8), 42 percent, and 25 percent of the full-voltage starting torque with 80 percent, 65 INDUCTION MOTORS 23-41 percent, and 50 percent reduced voltage starters respectively. The goal in choosing a reduced voltage starter is to limit the inrush current to a level that does not adversely effect the rest of the system, yet provides the motor being started with sufficient torque to accelerate to operating speed promptly. Similar to primary resistance starters in sequence of operation and current and torque reductions, p r im a r y r e a c to r s ta r te r s are sometimes employed with large motors where heat dissipation from starting resistors is a problem. The reduction in voltage dip, however, is not as great as the reduction in starting current, since reactance starters tend to reduce motor power factor (which is already low) on starting. The autotransformer starter is an even more common method of reduced voltage starting. This type of starter may utilize either three wyeconnected or two open delta-connected autotransformers (compensators) to provide reduced voltages (typically 80 percent, 65 percent, or 50 percent) at the motor terminals. The voltage remains constant across the motor throughout the starting period (and so motor torque is reduced to 64 percent, 42 percent, or 25 percent of full voltage start) and full voltage is applied after a set delay determined by the accelerating time required of the particular application. While the motor starting current is always proportional to the voltage tap used, the turns ratio of the transformer reduces the starting lin e current even further. Thus on the load side ofthe autotransformer starter operating on the 80 percent voltage tap, the motor draws 80 percent of the starting current it would draw under across-theline conditions; but the system need only supply 64 percent of the full voltage inrush current. Autotransformer starters and wye-delta starters are the ones that reduce current and starting torque in the same proportion. With all other types, the reduction in starting torque is greater than the reduction in inrush current. The starter illustrated in Figure 23-26(a), the starting contacts (S) are opened b e fo r e the run contacts (R) are closed, briefly leaving the motor unenergized. This arrangement is called o p e n tr a n s itio n and may cause a voltage flicker at changeover, particularly when the motor is driving a high friction, low inertia load. This effect may be overcome with a c lo s e d tr a n s itio n arrangement illustrated in Figure 23-26(b) known as a Korndorffer starter. In this scheme contactors Sand 1A pick up, and, after a suitable accelerating interval, control relay CR releases 1A and R picks up in rapid succession. Autotransformers are usually sized for short-time duty only and may overheat if the motor is started too frequently. The starters described above may be applied to any appropriately sized motor independent of the particular stator winding arrangement. A p a r tw in d in g s ta r te r , however, requires a motor designed specifically for this purpose. The stator must include two parallel paths (either wye or delta) with separate leads accessible for each path. As illustrated by the starter Figure 23-27. Part-winding starter Figure 23-26. Autotransformer starters in Figure 23-27, the S contactors pick up first, energizing half the stator winding. After a suitable interval contactor R picks up, including the second parallel path in the stator circuit. This type of starter is inherently closed transition. The two parallel paths in the stator winding are generally identical (but not necessarily so) and it might be expected, therefore, that the inrush current be halved. However, the mutual inductance between the parallel windings yields an additional impedance not present when either winding is energized alone. Typically the starting current is reduced to 65 percent of the full-voltage value with the accompanying 42 percent torque reduction. Most part-winding starters have a separate set of overload relays for each half of the motor winding (not shown in Figure 23-27), as a single set of relays sized for parallel operation would not protect the motor against burnout if contactor R failed to pick up. Also note that the terminal markings in the example are identical to the wye connected dual-voltage motor. Indeed, it is easy to connect an ordinary dual-voltage motor to a part-winding starter on a 230V system. However, if the motor is not 23-44 ELECTRICAL MACHINERY designed specifically for part-winding start, it may have a severe torque dip at about half speed. This means that the motor will start to accelerate, then stall, then re-accelerate when the second winding is energized. This may be accompanied by excessive heating and noise upon starting. Not applicable tojust any three-phase motor, w y e - d e lta s ta r te r s require a motor designed to operate as a delta, with separate leads available at each end of each phase winding. The starter illustrated in Figure 23-28 is ofthe closed transition type, although open transition wye-delta starters are available also. In the example controller, contacts M and S1 pick up together, organizing the windings in a wye and starting the motor. Mter a suitable delay supervised by timing relay CR, S2 picks up, S1 releases, THREE-PHASE INDUCTION MOTORS 23-45 and R picks up, in that order and in rapid succession. Since the equivalent impedance of a wye connected load is three times that of a delta arrangement, the inrush current (and torque) is 33 percent in the wye mode of what it would be in the running delta mode. This low torque makes the wye-delta starter inadequate for many applications. But for high inertia, low friction loads, such as variable pitch, motor-driven bow thrustersstarted at zero pitch-they provide suitable accelerating and current limiting service. S o lid - s ta te c o n tr o l. As previously mentioned, solid-state electronic devices can be used to alter the torque/speed characteristics of induction motors and thereby offer adjustable speed control. This may be accomplished by (1) stator voltage control, (2) frequency control, (3) stator current control, or (4) combinations of the first three. S ta to r v o lta g e c o n tr o l is effective since torque is proportional to voltage squared and proportional to slip in the operating range of most induction motors. Therefore, varying the voltage under a constant load torque will affect the slip. The range of control, however, depends on the slip at pull-out torque, and is narrow for most induction motors (Figure 23-29(a)). With this limited speed range requirement, AC voltage controllers (and, in rare instances, voltage-fed DC link inverters or pulse-width modulated [PWM] inverters) are used to provide the voltage control. The AC voltage controllers are simple, but the harmonic contents are high and the input power factor of the controller is low. These are used in low power applications such as small fans, blowers, and centrifugal pumps where the starting torque is low. They are occasionally used as reduced voltage starters for larger induction machines. If the voltage is held fixed at its rated value while the frequency is reduced below its rated value, the flux will increase quickly to the saturation level. At low frequencies the motor reactances decrease and stator currents become unacceptably high. This type of fr e q u e n c y c o n tr o l is seldom used. Increasing the frequency above the rated value weakens the flux and speeds up the motor but reduces the torque proportionally (Figure 23-29(b)).In this type of control, the motor is said to be in the "field-weakening mode" applicable to constant horsepower type loads. If the ratio of voltage to frequency is kept constant, the air gap flux remains constant as does the pull-out torque (Figure 23-29(c)). This type of v o lta g e a n d fr e q u e n c y c o n tr o l is known as "volts/Hertz" control. Three possible circuit arrangements for obtaining variable voltage and frequency are shown in Figure 23-30. In the first, the DC voltage remains constant and PWM techniques are applied to vary with the voltage and frequency in the inverter. Due to the diode rectifier, regeneration (dynamic braking) is not possible and the inverter generates harmonics into the AC supply. In the second, a chopper varies the DC voltage to the inverter and the inverter controls the frequency. Due to the chopper, harmonic injection into Figure 23-29. Solid-state control the AC supply is reduced. In the third arrangement, the DC voltage is varied by a dual converter (permitting regeneration) and frequency is controlled within the inverter. Input power factor to the converter is low, however, especially at low speeds. With stator current control, a constant current is supplied by threephase current source inverters. The current-fed inverter has the advantages of(1) fault current control and (2) that the current is less sensitive to motor parameter variations. However, the variations generate harmonics and torque pulsation. Two possible arrangements of current-fed inverter drives are shown in Figure 23-31. In the first, an inductor acts as Figure 23-30. Volts/Hertz control methods a current source and the controlled rectifier controls the current source. The input power factor of this configuration is very low. In the second arrangement, the chopper controls the current source and the power factor is higher. In either case to keep the air gap flux constant and avoid saturation due to high voltage, the motor is operated on the positive slope region of the torque/speed characteristic (Figure 23-31(c)) with voltage Figure 23-32. Voltage/current/frequency control stator current (and torque). Closed loop control is normally required to satisfy the steady state and transient performance specifications of these AC drives. W o u n d -R o to r control. As speed increases, the stator voltage rises and the torque increases. This is the unstable region of the characteristic and this closedloop control is appropriate. For the widest range of speed control, it is necessary to vary the voltage, frequency, and current to meet the torque/speed requirements illustrated in Figure 23-32. I n the first region (below base speed), the speed can be varied by voltage (or current) control, keeping the torque constant. I n the second region the motor is operated at constant current and the slip is varied. I n the third region the speed is controlled by frequency at a reduced M o to r s The second type ofrotor is the wound rotor, in which insulated conductor coils are inserted in rotor slots to form a polyphase winding, with the ends of each phase connected to a slip ring. The rotor must be wound for the same number of poles as the stator, but not necessarily the same number of phases (although it usually is). The number of rotor turns seldom matches the number of stator turns. Note that stator winding pole-changing methods are not appropriate for speed control of woundrotor motors. The wound-rotor motor operates under exactly the same principles as the squirrel-cage machine: a stator-produced rotating magnetic field induces currents in the rotor windings (as long as there is relative motion between rotating field and rotor conductors, i.e., slip). For similar motor parameters (rotor and stator resistances and reactances) the torque/speed D e s c r ip tio n . Figure 23-31. Current control ( S lip R in g ) I n d u c t io n 23-50 ELECTRICAL MACHINERY characteristics would be the same. However, the slip rings give access to the rotor circuit, thereby allowing control of the running characteristics by altering the rotor circuit impedance externally. If the slip rings are shorted, the motor runs essentially like a low slip squirrel-cage type. Connecting resistance between the slip rings, however, has the effect of increasing rotor resistance and "stretching" the torque/speed curve toward increased slip. The amount of maximum torque is not changed, but the slip at which it occurs is increased-and may be increased all the way to 1.0 (standstill) so that the maximum possible starting torque is obtained. The advantages of slip ring motors are (1) high starting and accelerating torque (and heat generation in the control resistors where it may be more readily dissipated); (2) with external resistance set for maximum starting torque, starting current is much less (1.5 x FLA) than squirrelcage motors; and (3)varying the external rotor resistance under load offers adjustable speed control over a fairly wide range. As for disadvantages: (1) since efficiency is approximately proportional to operating speed, by operating at slips near 0.5, efficiency is virtually cut in half; (2) since the controller cannot change the no-load speed significantly, speed regulation is poor for low speed settings; and (3) wound-rotor motors are bulkier and more expensive, and are a greater maintenance burden than squirrel-cage types. W o u n d -r o to r m o to r c o n tr o l M a n u a l c o n tr o l. By connecting a three-phase rheostat to the brushes that ride on the slip rings (Figure 23-33), the operator can dial in the appropriate resistance for maximum starting torque, and adjust the speed THREE-PHASE INDUCTION MOTORS 23-51 under load. Although adding rotor resistance does reduce starting current, adding too much will not obtain maximum torque (Figure 23-33(b)). Utilizing resistor banks and a drum controller to activate accelerating (and reversing) contacts, directly or through relays, is another method of manual control. Reversing is accomplished by interchanging any two stator leads; manipulation of rotor circuit resistance will yield the exact same torque/speed characteristic regardless of direction of rotation. This method usually results in multispeed, rather than adjustable speed, control and is applicable to winch drives. A u to m a tic c o n tr o l. The starting sequence may be automated through the use of time-delay relays. This is most common where increased rotor resistance is to be utilized as a starting device (secondary resistance start) and, once running, the motor is intended to operate at one speed setting. Definite (fixed) timers or compensated timers may be used to control acceleration. Definite timers, which may be pneumatic, dashpot, or electronic relays, are set for the highest load current and remain at the same setting regardless of load. The operation of a compensated timer is based on the applied load; the motor will be allowed to accelerate faster for a light load and slower for a heavy load. The frequency relay is one such type of timer and its application is illustrated in Figure 23-34. Two accelerating coils (AI and A2) are connected in parallel across one leg ofthe rotor circuit. A capacitor is connected in series with AI, which will block current in that branch ifthe frequency is low enough. At start the voltage and frequency in the rotor circuit are maximum and both Al and A2 pick up, inserting full resistance. As the rotor accelerates, the induced rotor voltages decrease in magnitude and frequency in proportion to the slip. At some point, the frequency is low enough to cause sufficient voltage division between the capacitor and the Al coil to drop out AI. Part of the rotor resistance is cut out and the motor continues to accelerate. When the slip and, hence, induced rotor voltages become low enough, A2 drops out, leaving minimum external rotor resistance. Because normally closed contacts are used in this example, the secondary cannot be totally shorted or else the accelerating contacts would never pick up upon starting. The sequence is totally dependent on the mechanical aspects of acceleration which, in turn, depend on the mechanicalload on the motor when started. S o lid - s ta te c o n tr o l. The three-phase external resistance can be replaced by a three-phase diode rectifier and chopper as illustrated in Figure 23-35. The gate turn-off thyristors (GTO) operate as the chopper, and the inductance as a current source. The effective external rotor resistance depends on chopper duty cycle, k, and is equal to R x (1 - k); thus the duty cycle may be used to control the speed ofthe motor. In large power applications where a limited range of speed control is required, other, more sophisticated, solid-state arrangements may be utilized. One such is called "static THREE-PHASE INDUCTION MOTORS 23-53 units (indicators) will chase the transmitter rotor until their positions are once again synchronized. Transmitter and indicator rotors only differ in that the transmitter rotor must be capable of being locked in place (e.g., engine order telegraph handle), or connected to something massive enough not to be turned by an out-of-synch indicator rotor (e.g., rudder angle indicator). Indicator rotors are generally connected to pointers, mounted on an appropriate scale, and equipped with mechanical dampening to prevent them from oscillating or spinning. Figure 23-34. Wound-rotor motor automatic control Kramer drive," in which the chopper described above is replaced by a three-phase full converter and transformer, delivering the slip power back to the AC mains. Another version, called "static Scherbius drive," connects the rotor circuit directly back to the stator three-phase supply through three, three-phase dual converters (or cycloconverters), permitting bidirectional flow of slip power. This arrangement is sometimes called a "doubly fed" machine, as a rotor current at frequency fr is being fed to the rotor to produce a slip equal to fr /fs . S e lf-S y n c h r o n iz in g (Selsyn) M o to r s Another use of the three-phase wound rotor is in self-synchronizing devices. In these applications the rotor windings of two (or more) machines are connected together via their slip rings and brushes. Currents will circulate between rotor windings if unequal voltages (in either magnitude or phase) are induced in them. This occurs if the rotor windings differ in position (or rate of change in position, i.e., speed) given identical stator excitation. In a single-phase selsyn motor system, the stators of all machines in the system are energized by the same single-phase source. On one unit (the transmitter), the rotor is turned and locked in place; rotors on the other 23-54 ELECTRICAL MACHINERY Power selsyn motor stators are energized by three-phase AC producing the rotating magnetic field. For current not to circulate between rotors, the two rotors must be in step (same speed and same angular position). Thus two (wound-rotor) induction motors may be locked in step from start-up to full speed. Such an arrangement is applicable, for example, to large conveyers powered by more than one motor. Figure 23-36 shows the rotor and stator circuits for singe-phase and three-phase selsyn motor systems. M a in te n a n c e Stator maintenance. Since induction motor stators are virtually identical to those on synchronous machines, refer to the section on synchronous machine maintenance for procedures to identify stator opens, shorts, and grounds. THREE-PHASE INDUCTION MOTORS 23-55 maintenance. Broken squirrel-cage rotor bars will cause the machine to fail to come up to speed even at rated voltage and frequency and under normal load. Noisy operation is also a symptom. Rebrazing the defective rotor bar to the end ring is usually the effective emergency repair. In the event that the open bar is not readily visible, applying a (low) single-phase voltage to two motor terminals will allow determination of whether the rotor is defective. By turning the rotor slowly by hand and noting any variations in stator current, the number of open bars can be detected (number of variations per revolution = broken bars x number of poles per phase). If a wound-rotor motor fails to start, there may be a problem with the external circuit (open in rheostat, loose terminal connections, poor brushring contact, etc.). If shorting the slip rings will start the motor, the problem is in the external circuit (note that for such a low resistance start, the motor should be unloaded). If not, there are internal rotor (or stator) problems. R o to r Bearing maintenance. There are four types of bearings commonly used on motors: (1) sleeve bearings (made of steel, bronze, oil-impregnated bronze, or bronze with graphite-filled recesses); (2) ball bearings (either oil packed or grease packed); (3) needle bearings; and (4) thrust bearings (either of the needle type or, in smaller machines, steel, bronze, and even nylon thrust washers). Sleeve bearings are the most common, the least expensive, and the most likely to give trouble. The most common problem is lack of lubrication. A dry sleeve bearing will make a scraping or sliding sound when turned slowly and may squeal at operating speed. Oil-impregnated bronze and graphitized bearings are sometimes referred to as "selflubricating." However, all types of sleeve bearings generally require lubrication. Motors having sleeve bearings usually have felt packings on one end of each bearing. The packing is kept saturated with oil, which seeps into the bearing surfaces. Lubricating oil is either applied directly to the felt wick or, more commonly, through a small hole in, or tube protruding from, the motor housing. It is advisable to use 10-weight motor oil (20- or 30-weight will also suffice) and only enough to saturate the wick. Too much oil (and oil in the wrong places) is as bad as no oil; it impedes the dissipation of heat and accelerates the deterioration ofthe winding insulation. Detergent oils should not be used because the additives can attack the winding insulation. Ball bearings are more durable, less likely to seize, require less maintenance, and are more expensive than sleeve bearings. Grease-packed bearings are often sealed, do not require periodic maintenance, and are less noisy than oil-packed ball bearings. In cold weather, however, acceleration can be sluggish. Oil-packed bearings require periodic 1ubrication 23-56 ELECTRICAL as do needle-type bearings. The latter are used where severe space restrictions exist and, although noisier, provide about the same performance as oil-packed ball bearings. Worn balls or races in a ball bearing, or excessive clearances between shaft and inside of a sleeve bearing, require bearing replacement. Ball bearings often may be removed by simply tapping the motor end-bell until the bearing drops out. Sleeve bearings usually have to be pressed out or, using a cylindrical fitting, forced out by hammering the fitting. A new bearing can be pressed in the same way, with care taken not to deform or burr the new bearing. Occasionally, freezing the bearing overnight and preheating the bearing housing facilitates bearing insertion. Sometimes sleeve bearings must be reamed to size the bearing appropriately for free rotation of the motor shaft. In reassembling a motor with sleeve bearings, care should be taken in tightening the end-bells evenly to avoid binding and misalignment. A sleeve bearing operated dry for an extended period can actually melt and fuse to the shaft. "Frozen bearings" may occasionally be broken loose from the shaft by hammering. Ifnot, a propane torch may be necessary to remove the bearing from the shaft (using an oxyacetylene torch increases the chance of damaging the shaft). In any case, a scarred, pitted, or grooved shaft must be dressed before being reused. This can be accomplished by filing and polishing or, in the case of deep pits or grooves, building up shaft by welding or with steel epoxy, and turning down to size on a lathe. Finally, gummy residues often build up in motor bearings from combinations of oil and dust, improper oil and heat, or just prolonged unattended use. Gummy bearings should be washed with an appropriate solvent (e.g., lacquer thinner), and care taken to keep solvent offthe motor windings. See troubleshooting chart (Table 23-3) for induction motor diagnostic procedures. TABLE 23-3 Induction Motor Troubleshooting Chart P r o b le m P o s s ib le C ause Fails to start Open line(s) Open phase Controller failure Overload (wound-rotor motors) Low voltage Open in rotor circuit SINGLE-PHASE MACHINERY R em edy Check fuses/circuit breaker. Note: Will not start with one line open. Test stator windings for open. Check controller relay coils and contacts; repair/ replace as required. Relieve mechanical load on motor; check for bearing seizure or other mechanical restriction. Check supply. Check slip ringlbrush rigging; check external resistance/rheostat; test rotor windings and repair as required. P r o b le m P o s s ib le Overheating Overload C ause MOTORS 23-57 R em edy Insufficient ventilation High voltage Low voltage Low frequency Uneven air gap Check status against nameplate ratings; relieve mechanical load. Remove restrictions; clean windings; check C. W. supply. Check supply. Check supply. Check supply. Check gap clearances; check for evidence of rotor/ stator rubbing. Runs fast High frequency Check supply. Runs slow Low frequency Low voltage Overload Check supply. Check supply. Check status against nameplate ratings; relieve mechanical load. Test and repair/replace as required. (squirrel-cage motors) (wound-rotor motors) Open/shorted! grounded stator coil(s) Broken rotor bar Open in rotor circuit Secondary resistance high Noisy operation Bearings Uneven air gap Coupling Loose laminations Rebraze defective bar(s) to end ring. Note: Cast aluminum rotors can seldom be repaired. Check slip ringlbrush rigging; check external resistance/rheostat; test rotor windings and repair as required. Check rheostat position/rating; check contacts on machines with fIxed-resistance starters. Check for worn, loose, dry, or overlubricated bearings; replace worn or loose bearings; lubricate dry bearings; relieve overlubricated bearings. Check for bent shaft, bearing condition, gear alignment. Check alignment; re-align and!or tighten coupling. Tighten bolts; dip in varnish and bake. SINGLE-PHASE MOTORS Classification Sometimes referred to as "fractional horsepower" motors, these small (typically 10 W to 1 kW) machines are found in appliances, hand power tools, and a myriad of other applications. Single-phase motors may be loosely grouped in one of three categories: induction types, synchronous types, or commutator types. And, although many different varieties of these have been produced over the years, competition has effectively narrowed the field to squirrel-cage types, synchronous types, and the universal motor. Induction Types General description. Single-phase stator windings produce an o s c illa tin g magnetic field, as opposed to the r o ta tin g magnetic field produced by polyphase stators. This oscillating field will induce currents in a squirrel- 23-58 ELECTRICAL MACHINERY cage rotor, and these currents will produce a torque once the rotor is spinning. However, a single-phase stator winding will not, by itself, generate any torque at rotor standstill. Thus, the means of providing starting torque is the factor used to classify this category of single-phase motor. The totors ofsingle-phase squirrel-cage motors are essentially the same as the three-phase types described earlier. The stator core, with the exception of shaded pole motors, is also very similar. A single-phase stator can be wound for any even number of poles, with two, four, and six being the most common. However, where the slots of three-phase machines are all of the same size, single-phase stator cores may have two or three different size stator slots. The stator consists of two separate windings displaced from one another by 90 electrical degrees, with one usually having fewer turns of smaller wire than the other. In motors that operate with both windings energized, the heavier winding is known as the main winding and the other the auxiliary winding. If the motor operates with the auxiliary winding opened after starting, they are generally referred to as running and starting windings. The principle is to arrange for the current in the auxiliary winding to be out of phase with the main winding current-producing, in effect, a two-phase machine supplied from singlephase mains. Split-phase motor. The starting winding of these windings is about one quarter ofthe size ofthe main winding and thus has a lower X/Rratio. This results in a starting winding current which leads the main winding current by about 30 degrees, producing a rotating magnetic field. These motors normally have a centrifugally operated starting switch (Figure 23-37) that cuts out the starting winding when the motor reaches about 75 percent of synchronous speed (and recloses at about 25 percent). The starting winding in these motors heats up rapidly during starting. Ifthe load has too much inertia, or if the centrifugal switch malfunctions, the winding will quickly burn out. The rotational direction of a split-phase motor may be reversed by interchanging the starting winding connections. The torque/speed characteristics are illustrated in Figure 23-38. Capacitor start motor. In every sense capacitor start motors (and dual- capacitor motors and PSC motors) are split-phase machines. That actual term, however, has been reserved for the machine described above where the X/R ratio is lowered by boosting the resistance. Inserting a capacitor in series with the auxiliary winding achieves the same result by reducing the reactance (X).The greater phase differential achieved by using starting winding conductors half the size of the main winding conductors, plus an electrolytic capacitor, results in a higher starting torque and less starting current than split-phase motors. Figure 23-37. Single-phase squirrel-cage motors The electrolytic capacitor is generally rated for short time duty only, and starting winding burnout can still occur. However, the higher starting torque will accelerate higher inertial loads in sufficient time. Dual-capacitor motor. Also known as the "capacitor-start/capacitor-run" motor, this has an additional continuous-duty, oil-filledcapacitor (ofsomewhat smaller capacitance) in the auxiliary winding circuit (Figure 23-37). The motor continues to run with both windings energized after the electrolytic capacitor is cut out by the centrifugal switch at about 75 percent of synchronous speed. The advantages of this arrangement are (1) higher breakdown torque as seen in Figure 23-38, (2) quieter operation (less vibration) due to maintaining a rotating magnetic field during operation, and (3) improved power factor by virtue of the leading current in the auxiliary winding. capacitor (PSC) motor. The auxiliary winding of this machine usually has the same size conductors and as many turns as the main winding. A continuous-duty, oil-filledcapacitor is connected in series with the auxiliary winding (Figure 23-37), sized to cause a 90-degree phase Permanent-split Figure 23-38. Single-phase motor torque/speed characteristics differential in main and auxiliary currents and rated load. The actual phase difference varies with load, and, at start, the (small) capacitor size results in a low starting torque (Figure 23-38). A permanent-split capacitor motor operates quietly, with high power factor and low current, and, with no starting switch, is most suitable for wide-range speed control. m o t o r . The construction of the stator pole pieces (Figure 23-39) includes a closed copper loop enclosing an axial strip of about one quarter of the pole width. Currents induced in the loop oppose the main winding flux, causing the flux to sweep across the pole face. This effect is amplified by designing the pole piece so that the air gap is wider at the pole sides opposite the shading coil. Shaded-pole motors are very economical for low power applications and are compatible with wide-range speed control. However, they are not reversible, and their efficiency and power factor are low. S h a d e d -p o le Synchronous Types Obviously named for their constant (synchronous) speed operation, these usually very low power (15 W) motors are used wherever timing is essential, such as in clocks and tape drives. Most Figure 23-39. Shaded-pole and reluctance type single-phase motors single-phase synchronous motors use shaded-pole stators and rotors that can lock in step with the rotating magnetic field by becoming essentially a permanent magnet. There are two common types. m o t o r . The rotor on this type consists of a hollow cylinder of a hard ferromagnetic material with appreciable hysteresis. When operating below synchronous speed, the magnetizing of the rotor and the delay (due to hysteresis) in remagnetizing in the opposite direction develop torque in the rotor. Pulling into synchronism gradually, the induced rotor poles tend to remain permanently located (due to hysteresis) once synchronous speed is reached. H y s te r e s is G e n e r a l d e s c r ip tio n . m o t o r . The reluctance motor rotor is similar to squirrel-cage rotors with portions cut away (Figure 23-39), leaving projecting iron poles. R e lu c ta n c e 23-62 ELECTRICAL MACHINERY The motor starts as a squirrel-cage motor, but when it gets very close to synchronous speed, the rotor poles snap into synchronism with the stator poles. The acceleration is not as smooth as that ofthe hysteresis type and below synchronous speed the speed is unstable, but they are inexpensive to manufacture. C o m m u ta to r T y p e s G e n e r a l d e s c r i p t i o n . A wound rotor, with rotor coil terminals connected to commutator segment (essentially a DC armature), distinguish this category of single-phase motors consisting of the universal (AC series) motor and the repulsion family of motors. U n i v e r s a l m o t o r . Electrically identical to a DC series motor, the universal motor is specifically designed to run on DC or AC of equivalent voltage and any frequency up to 60 Hz. The only design changes are a minimal number of field turns and the laminating of the entire magnetic circuit. Some universal motor arrangements have the field poles distributed in stator slots as opposed to the concentrated winding shown in Figure 23-40, and some have distributed compensating windings also. The principle ofoperation is that, since the direction of armature current and field pole polarity are reversed at exactly the same time during each half-cycle, the torque remains in the same direction. To reverse the direction of rotation, the armature leads must be reversed. Although they are designed to run on either AC or DC, universal motors have characteristics that are not exactly the same on each type of voltage. I n both modes the starting torque is high, as is the no-load speed. With universal motors, direct coupling or geared drives are used to prevent runaway at no load. When operated on AC, saturation of the magnetic circuit during peak current tends to raise the motor speed. On the other hand, inductive reactance tends to choke motor current and reduce torque when the motor is operated on AC. The universal motor is generally designed to operate at very high speed (12,000 to 24,000 RPM), and speed control may be obtained by either varying the applied voltage or inserting resistance in series with the motor. Because they run at such high speeds, the motors are smaller and lighter for the same rated horsepower, which makes them particularly suitable for portable hand tools. m o t o r s . A repulsion-type motor consists of a split-phasetype stator with the rotor a typical lap-wound DC armature. There is no electrical connection, however, between rotor and stator. Instead, the brushes that contact the commutator are shorted together. The physical position of the brushes determines the axis of the rotor field and the direction of rotation, if any. There is no rotation ifthe brushes are aligned R e p u ls io n -ty p e Figure 23-40. Commutator type single-phase motors with the alternating magnetic field produced by the stator (called the h a r d and no torque is produced with the brush axis perpendicular to the stator field either ( s o ft n e u tr a l) . Maximum torque is produced at approximately 15 to 25 degrees on either side of hard neutral; the side n e u tr a l) 23-64 ELECTRICAL MACHINERY determines the direction of rotation as shown in Figure 23-40. The members of the repulsion family include (1) r e p u ls io n m o to r s , which operate on the repulsion principle described at start and under normal operation; ( 2 ) r e p u ls io n s ta r t m o to r s , in which a centrifugally operated mechanism shorts all the commutator bars, leaving the motor to operate as an induction machine (in some cases the repulsion brushes are lifted by the same centrifugal device that shorts the commutator); and (3) r e p u ls io n - in d u c tio n m o to r s , in which the rotor has two independent sets ofwindings-repulsion and conventional squirrel-cage-and both are continuously operative, producing a characteristic curve (Figure 23-40) in which the torque is the sum of the repulsion and induction torques. Repulsion-type motors, in particular repulsion-induction motors, have been made as large as 10 hp, but are expensive and very rare today. S in g le -P h a s e M o to r C o n tr o l Across-the-line starting is standard for single-phase motors. For those with starting windings, a mechanically activated centrifugal switch is typically responsible for opening the auxiliary circuit. Hall-effect sensors are also occasionally used to monitor RPM and open the starting winding at the appropriate time. Centrifugal switches have some drawbacks: Their maloperation is the most significant maintenance burden associated with single-phase machines; they inherently prohibit plug-stop operation (required for electric chain hoist operation); and, in hermetically sealed refrigeration compressors, the arc drawn in the switch can break down the refrigerant chemically. For those applications where centrifugal (or internal) relays are inappropriate, external starting relays may be employed. A thermistor in series with the starting winding, or a thermal starting relay whose heating element is designed to open the starting contact after about two seconds, is common in small air-conditioning units. They must be allowed to cool before they can reclose the starting circuit and are therefore not suitable for plugging or short-cycling types of operations. A current-actuated starting relay which picks up with main winding inrush current and releases when the current drops off at operating speed is another possible arrangement. These relays are suitable for plugging and repetitive starting. On capacitor start motors, a voltage-sensitive relay connected across the starting winding can detect the sharp rise in starting winding voltage as the motor approaches synchronous speed and thus can open the starting contacts. When the motor is running, the voltage generated in the unenergized starting windings keeps the relay open. This arrangement is not applicable to split-phase machines. S ta r tin g . S p e e d c o n tr o l V a r ia b le v o lta g e s p e e d c o n tr o l. In motors that do not employ a centrifugal switch (i.e., shaded-pole and PSC motors), a considerably wide range SINGLE-PHASE MOTORS 23-65 of speed control can be realized by varying the applied voltage. This may be accomplished by a tapped inductance in series with the motor winding, a tapped autotransformer, or electronic manipulation of the line voltage wave form. In the latter method a triac (triode AC switch) is often used to produce the varying voltage. In the circuit shown in Figure 23-41, a diac (diode AC switch) is used in the phase shifting (or "triggering") circuit, and the R-C time constant (determined by the variable-resistance/fixed-capacitor combination) provides the delay-time control. Note that some triac voltage controllers, designed to be used as dimmers for incandescent lamps, conduct on only one half of the cycle, producing, in effect, DC. These should not be used on inductive loads and can burn out single-phase stators. Universal motors may employ tapped series resistors or triac voltage controllers to vary speed. T w o - s p e e d c o n tr o l. Changing stator lead connections can also be used to control speed in discrete steps, often at a ratio of 2:1. Tapped stator windings on shaded-pole motors offer the possibility of altering the volts/ turn ratio (Figure 23-41) and hence torque. Since the actual speed depends on load, however, this method has no effect on no-load speed. Two-speed PSC motors are rare, but two-speed starting switch types of single-phase motors are fairly common. If the starting switch is centrifugally operated, the speed range seldom exceeds 2:1, suggesting the use of consequent-pole main winding connections. The starting winding is typically not ofthe pole-changing type but wound for the lo w e r number of poles. Thus the motor starts on its high speed winding and the starting switch changes the main winding to consequent-pole arrangement (if the low speed terminals have been energized) as well as cutting out the starting winding. Ifthe two speeds are not in the ratio of2:1, the motor will have two windings (usually four and six poles). Some ofthese motors have two starting windings (one for each speed), but often one starting winding is employed, working in conjunction with the high speed main winding. These machines are started on high and switched to the low speed winding either by the starting switch or manually. M a in te n a n c e Once it has been determined whether failure of a single-phase motor is due to problems within the starting winding or main winding, techniques to pinpoint the offending coil or bar are similar to those described for threephase machines. The problem, of course may also lie in the centrifugal switch mechanism or starting relay, or capacitor, if one is employed. By spinning the rotor rapidly by hand and closing the line switch, it is possible to isolate the problem to either winding. If the motor accelerates to normal speed, the trouble is with the starting circuit; if it cycles, accelerating and decelerating, the trouble is in the main winding. Capaci- SINGLE-PHASE 23-67 MOTORS should be momentarily shorted to the anode; the resistance reading should drop to a low value and remain there. With a triac (which is a back-to-back SCR) the same procedure is executed, but then the probes are reversed and the test is repeated. A diac can only be tested with an ohmmeter for shorts, as it requires 25 to 35 volts to trigger. Occasionally, a triac comes with an internal diac and, for the same reason, it can only be checked for shorts. To determine if you have an open triac or one with an internal diac, you must consult the information available from the manufacturer of replacement units or guides for selecting parts. For that matter, the same method must be used to determine whether a component is a transistor, an SCR, or a triac, as the three may have the same external appearance. A single-phase motor troubleshooting chart is displayed as Table 23-4 TABLE S in g le - P h a s e M o t o r T r o u b le s h o o t in g P r o b le m P o s s ib le Fails to start Blown fuse Open/shorted/ grounded stator coi](s) Defective centrifugal mechanism Open in starting winding (split-phase types) (commutator types) C ause Defective capacitor Brush contact Brush position Overheating (split-phase types) Figure 23-41. Single-phase motor speed control tors, starting relays, and centrifugal mechanisms should be checked if the starting circuit is at fault. If the speed control unit is at fault, the cause is often a defective SCR, triac, or diac. An SCR may be checked with an ohmmeter by connecting the positive probe to the anode and the negative to the cathode; an infinite resistance should be indicated. With these connections held fast, the gate (repulsion types) (series, reluctance, and shaded-pole types) Runs slower than normal 2 3 -4 Overload Voltage high/low Shorted/grounded stator coi](s) Starting winding not cut out Defective centrifugal mechanism Capacitor shorted Improper brush position Shorted/grounded field poles Voltage low Overload Starting winding not cut out Shorted/grounded stator coil(s) Open/shorted/grounded rotor coil(s) C hart R em edy Check and replace. Check and repair/jump defective coil(s). Disassemble, clean, inspect, adjust, repair, or replace parts. Check connections; check and repair/jump defective coil(s). Replace. Inspect; adjust brushes for proper grade, pressure, seating. Inspect; align brush rigging. Reduce load. Check supply and adjust voltage. Check and repair/jump defective coil(s). Inspect centrifugal switch. Disassemble, clean, inspect, adjust, repair, or replace parts. Check and replace capacitor (capacitor-run type motor). Inspect and align brush rigging. Check and repair/replace field poles. Check supply and adjust. Reduce load; check for mechanical binding. Check centrifugal mechanism. Check and repair/jump as required. Check and repair as required. 23-68 ELECTRICAL MACHINERY P r o b le m P o s s ib le C a u s e R em edy Noisy operation Bearings Check for worn, loose, dry, or overlubricated bearings; replace worn or loose bearings; lubricate dry bearings; relieve overlubricated bearings. Check for bent shaft, bearing condition, gear alignment. Check alignment; re-align and/or tighten coupling. Adjust end-play takeup screw or add thrust washers to shaft. Tighten all loose components. Uneven air gap Coupling Excessive end play Loose m o u n tin g s l accessories DC M A C H IN E R Y B a s ic P r in c ip le s The principal phenomenon in all DC machine operation is the behavior of a moving charged particle in a magnetic field. From the submicroscopic point of view, this takes the mathematical character ofthe expression: F = q (v x B) where F is the "induced" force, q is the amount of charge in motion, v is the velocity, and B is the magnetic field density. F, v, and B are v e c to r quantities, having both a magnitude and a direction assigned to them (i.e., the strength and the direction offorce F depend on the direction and speed ofv and the strength and orientation of B). There are more than a handful of "hand rules"-left-hand rules, right-hand rules, etc.-with bewildering sets of instructions as to when and where to use them. One of the most useful is the "right-hand rule," which indicates the relative directions ofF, v, and B. In Figure 23-42(a), particles ofcharge q are packed in a conductor. The conductor is being moved down in a magnetic field which is oriented into the page (i.e., a north pole is above the page, a south pole under the book). If the fingers of your right hand are first aligned with v, and then curled into B, your right thumb should point to the right-hand margin. (This mathematical operation is pronounced as "v c r o s s B.") The e le c tr o m a g n e tic force (em£)is the voltage produced in a generator, and your thumb is pointed toward the positive end of the conductor-the direction conventional current would flow if a closed external circuit permitted. If, instead of moving the c o n d u c to r with velocity v, the charges were pumped down the conductor (to the right) with velocity v, then "v cross B" yields an induced force in the upward direction, tending to move the conductor up. This is, of course, the principle of motor action ( e x a c tly the same as the phenomenon that produces voltage in a generator-the only difference being the orientation of v). A load on the generator (moving down and DC MACHINERY 23-69 producing an em£)will cause a current to flow to the right, inducing a force "up" which opposes the original motion of the conductors. This countertorque, which is proportional to load current, must be overcome by the prime mover to maintain the machine at its rated RPM. This is why the prime mover must produce a higher output (and consume more fuel or steam) at higher electrical loads. As a motor, the upward motion of the conductor, c a u s e d by pumping current to the right, in d u c e s an emf to the left. This counterelectromotive force (cem£) opposes the original applied voltage. Thus, on the submicroscopic scale, torque and countertorque, emf 23-70 ELECTRICAL DC MACHINERY MACHINERY and cemf, motors and generators, all are explainable in terms of "v cross B." On a slightly larger scale, instead of considering the induced forces on individual charges, the aggregate emf or force on a conductor of length I (oriented perpendicularly to the magnetic flux) is Blv or Bli respectively. Stepping back still further, and considering all the conductors present in a DC machine, the production of armature voltage (or counter-emD and rotor torque is described by the two fundamental relationships: E == K x <I> x RPM T == Kx <I> x 1a Here K is the machine constant and depends on the number and arrangement of armature windings; <I> is the flux per pole produced by the field circuit; and K<I> is common to both expressions-indicating that T x RPM (mechanical power) should equal E x Ia (electrical power). Of course there are mechanical losses so this T is not the torque at the coupling, and there are electrical losses so this E is not the terminal voltage, but somewhere in the middle of the machine-in the "air gap" between stator and rotorthe conversion between electrical and mechanical energy is accomplished. C o n s tr u c tio n Overview. A DC machine may be considered as an "inside-out" synchronous machine and is physically identical when utilized as a motor or as a generator. Its field circuit consists of coils mounted on stationary pole pieces similar to salient-pole rotors. The coils are connected in series with one another with the sense of winding reversed in adjacent coils to produce opposite polarity. The field circuit terminals are labeled F1 and F2. In some machines a heavier winding of a few turns is wound around the outside of each field coil (in the same direction as the inside coil), connected in series with one another, and the leads are brought out as Sl and S2. Smaller pole pieces, with heavy conductor and few turn windings, are found in between the main field poles of large machines. These are known as in te r p o le s or c o m m u ta tin g p o le s , the purpose of which will be described later. A DC rotor consists of a distributed winding of armature coils fitted in slots on the periphery of the rotor core. The ends of each coil are connected to commutator bars or segments mounted on the rotor shaft, and insulated from the shaft and each other by a sheath of mica. (With the commutator, the similarity between synchronous armature [stator] and DC armature [rotor] ends.) Each armature coil is connected to two bars, and, in most cases, each bar is connected to an end of two coils, so the number of commutator bars equals the number of armature coils. On two-layer armatures, with two coil sides per slot, the number of slots, coils, and bars 23- 71 is identical. There are several ways, however, in which armature coils may b e connected to one another, via a common commutator bar. A r m a t u r e w in d in g s C la s s ific a tio n . The number of commutator bars between connections to a particular armature coil is called the commutator pitch and may be expressed in bars or degrees (mechanical or electrical). It is the commutator pitch that defines the armature as lap- or wave-connected and the degree of multiplicity (m) and the degree ofreentrancy (r). L a p - w o u n d a r m a tu r e s . Lap windings have a rather small commutator pitch and, in their simplest form, result in the ends of a given armature coil being connected to adjacent commutator bars. This is known as a s im p le x winding and has a multiplicity of one. Lap windings with commutator pitches of two, three, or four bars are known as duplex, triplex, and quadruplex respectively (m == 2,3,4 respectively). Regardless of the number of poles or degree of multiplicity, a lap winding can be accomplished with any number of coils and commutator bars. The multiplicity is always equal to the commutator pitch, and the number of parallel paths is always m times P (the number of poles) as illustrated in Figure 23-43(a). The principal advantage oflap windings is the relatively large number of parallel paths that may be created. Since there are practical limits on the amount of current that can be carried by a single coil, generators with large current ratings generally require a large number of parallel paths. The drawback is that, in each parallel path, the coils in that path "see" only one pair of poles. Ifthe flux per pole is not identical for each pole (and air gap tolerance, pole piece reluctance, and field coil resistance inevitably contribute to inequalities in practical machines), circulating currents can develop among the paths, giving rise to additional heating and poor commutation. This effect can be remedied with e q u a liz e r c o n n e c tio n s . (Note: Equalizer connections in lap-wound armatures are not to be confused with equalizing buses required of compound generators operating in parallel, which will be discussed later.) An equalizer is a direct, low resistance path between points in the circuit that should be at the same potential. In the four-pole armature in Figure 23-43, coils 2 and 8 are identically situated from an electromagnetic point of view. An equalizer can therefore be installed between corresponding points of the two coils, either at the commutator end or the load end or at the rotor. If equalizers were connected between coils 1 and 7, 2 and 8, and so on, to 6 and 12, the armature would be said to be 100 percent (or fully) equalized. Due to space limitations, this cannot always be the case; if only half the possible connections were made (as symmetrically as possible) the armature would be 50 percent equalized. W a v e - w o u n d a r m a tu r e s . The wave winding avoids the problem of uneven parallel path currents by insuring that all coils in any path are DC MACHINERY 23-73 and returns to the commutator bar adjacent to where it "started." Ending two commutator bars away results in a duplex winding (m = 2), three commutator bars away, a triplex (m = 3). Wave-wound machines seldom have a degree of multiplicity greater than 3, and the number of parallel paths is always 2 x m, no matter how many poles. Figure 23-44(a) shows a duplex wave winding: Starting with commutator bar 1, the winding passes through coil 1-1', ending at bar 8, and continues through coil 8-8', ending at bar 3, two bars away from where it "started"; hence, m = 2. Note that the number of parallel paths is 4 (2 x m) and that the same voltage would be obtained between brushes B1 and B2 whether or not brushes B3 and B4 are present. The number of brush sets is normally equal to the number of poles and m u s t be so for nonequalized lap windings and frog-leg windings. Wavewound armatures and armatures with fully equalized lap windings can be operated with only tw o brush sets, one positive and one negative, located 180 electrical degrees apart. The term "reentrancy" refers to the number of closed loops formed by the completed armature circuit; it must always be at least one (singly reentrant: r = 1), but may be two (doubly reentrant: r = 2), or three (triply reentrant: r = 3), etc. Reentrancy is equal to the largest common factor between the number of elements and the degree of multiplicity, which usually equals m. This is significant during certain armature troubleshooting procedures. F r o g - le g w in d in g s . A frog-leg winding is a combination of a lap and a wave winding connected to the same commutator. It generally results in better commutation than either lap or highly multiplexed wave windings. With both lap and wave elements having the same number of turns and the same coil span, and each being equal in number to the number of commutator bars, the windings must have the same number of parallel paths to produce equal voltages. This means that if the lap winding is simplex (and it usually is), the wave winding must have a degree of multiplicity equal to P/2. Figure 23-44(b) shows a frog-Ieg-wound armature with 16 commutator bars, lap elements, and wave elements, a coil span of 3, and lap and wave commutator pitches of 1 and 7 respectively. r e a c t i o n . Under no-load conditions (no armature current) the only air gap flux present is that produced by the field poles. Under load, however, the armature generates a magnetic field that is not aligned with the stator field. The resultant skewed, or distorted, field will cause a problem as the magnetic neutral (an imaginary line through the center of rotation, perpendicular to the net flux) is shifted. The brushes are intended to be located "on the magnetic neutral" ( b r u s h n e u tr a l) so that the coil(s) shorted by a given brush are exactly those in which no emf is generated. Having the brushes "off neutral" results in poor commutation (sparking) A r m a tu r e Figure 23-43. Lap-wound armatures distributed under all the poles. Therefore a simplex wave winding has only two parallel paths regardless ofthe number ofpoles. The commutator pitch in wave winding is close to 360 electrical degrees, which on a two-pole machine is not fundamentally different from a lap winding. Two-pole wave windings are virtually nonexistent. A single simplex (m = 1) wave winding is connected to the commutator at each north pole (360 degrees electrical) Figure 23-45. Interpoles and armature reaction exacerbate the situation. In some small machines, only one interpole per main pole pair is installed. In some very large DC machines, c o m p e n s a tin g w in d in g s , buried in the main pole faces parallel to the armature conductors (and likewise connected in series with the armature) are utilized. Figure 23-44. Wave-wound and frog-leg armatures and less than full voltage produced by generators or less than full torque produced in motors. Shifting the brushes is a possible remedy, but the brush neutral changes whenever the load changes. Armature reaction cannot be eliminated, but its effects can be limited by canceling out the rotor-produced magnetic field with a third field. This is the responsibility of the in te r p o le s . Interpoles are electrically connected in series with the armature so that their strength varies approximately with the distorting armature field. Armature/interpole connections are usually made internally and the series combination brought out as leads Al and A2. Figure 23-45 shows the arrangement for either generator or motor operation. Note that for armature current to be in the same direction, generator and motor rotation must be opposite. Thus the polarity ofthe interpoles with respect to the direction of rotation in generators and motors is opposite. A reversed commutating pole, instead oflimiting the effects of armature reaction, will DC Generators Voltage characteristics. The voltage produced in a rotating armature, as described above, is proportional to the speed ofrotation (RPM) and the flux per pole (<1». The voltage available at the generator terminals, however, is less than this voltage due to (1) the internal IR drop in the armature circuit and (2) the effects of armature reaction. Thus: VL = Eg - laRa - AR where Eg is the product of K x <I> x RPM and AR is the armature reaction voltage drop. At no load, the terminal voltage VLis the internal voltage Eg. A plot ofno-load voltage (Eg) versus field current If(Figure 23- 46) is known as the no-load characteristic, input characteristic, magnetization curve, or saturation curve. Note in this characteristic that (1) with zero field current Figure 23-46. Magnetization curve there is some voltage generated due to r e s id u a l m a g n e tis m ; (2) there is a ofvoltages that are proportional to field current; and (3) there is a s a tu r a tio n r e g io n , where little increase in voltage results from increasing excitation. The characteristic is obtained at some constant base speed but the curve may be resealed for any RPM without changing its shape. lin e a r r a n g e Generator types S e p a r a te ly e x c ite d g e n e r a to r s . Generator types are designated by the arrangement of their flux-producing components (fields) relative to their voltage-producing component (armature). In a s e p a r a te ly e x c ite d g e n e r a to r , an independent DC source is connected to the field circuit. This results in Eg being constant throughout the load range of the generator. The voltage droop has a linear component (IR) and a nonlinear component (armature reaction) as shown in Figure 23-47 and Figure 23-48(a). Normally operated at constant speed, voltage control is accomplished by regulating field circuit supply voltage or field circuit current via a rheostat. Separately excited DC generators are used as exciters in synchronous machines; the (separate) exciter field current is supplied from the AC mains via a rectifier, or from the output of an amplidyne pilot-exciter. In fact the a m p lid y n e is an example of a special-purpose separately excited machine. Also known as a rotary amplifier, the amplidyne can Figure 23-47. Shunt generator input characteristics produce a ratio of output (armature) power to input (field) power as high as 20,000:1! This level is achieved by exploiting the effects of armature reaction. By s h o r tin g o u t the brushes, large armature currents can be made to circulate at low voltages (still in the linear range of the magnetic curve). These large currents produce an armature (reaction) field, and a second set of brushes on the direct axis taps the voltage induced due to this field. DC MACHINERY Figure 23-48. DC generator output characteristics By connecting a load to this second set of brushes (and carefully designing a compensating winding to eliminate a second armature field) the amplidyne produces voltage very sensitive to changes in field current and nearly constant over the load range. In fact, in lieu ofa single field winding, several independent windings may be placed on the field poles' cores. Each winding, in turn, can be excited by an electrical signal proportional to some 23- 79 physical quantity-torque, speed, acceleration, line voltage, load currentunder the control of the amplidyne. DC generators in which the electrical output is used to energize its own field windings are called s e lfe x c ite d g e n e r a to r s . These include the shunt, series, and compound types. S h u n t g e n e r a to r s . In a shunt generator the armature circuit and field circuit are connected in parallel, resulting in the armature terminal voltage being applied across the field circuit. Remember, the armature circuit consists of armature windings, commutator, bushes, and interpole windings. The field circuit consists of the main field windings and a field rheostat (or other control device). The no-load voltage produced by a shunt generator is easily obtained from the saturation curve if the field circuit resistance (coils plus rheostat) is known. Since under zero external load, Ia equals Ir and Eg equals Vr, the intersection of the saturation curve (Eg versus Ir) and the resistance line (Vr versus Ir) is the equilibrium point defining the current and voltage in both circuit branches. Decreasing the field resistance defines a new equilibrium point at a higher voltage. With the generator operating at rated speed and the field and armature circuits isolated, the armature terminal voltage is determined by the amount of residual magnetism. Connecting the field and armature circuits in parallel via a field switch suddenly impresses that (small) armature voltage across the field circuit. The resultant field current produces additional flux and a higher armature voltage which, in turn, causes an increase in field current. The process repeats itself, "climbing the stair" (Figure 23-47(a» until, after several seconds, the equilibrium point is reached. For a fixed field rheostat setting, the external voltage characteristic has a greater droop than the separately excited type. The additional droop component is due to the fact the Eg is not constant, but falls with reduced Ir. This can be shown on the input characteristic (Figure 23-47(b» where the dotted line indicates the difference between Eg and VL due to IR and armature reaction, similar to the separately excited machine. The intersection of the dotted line and the resistance line is the new equilibrium point under load. Point 1 is the no-load voltage; point 2 corresponds to Eg under load; point 3 is the terminal voltage of a separately excited machine under this load; and point 4 is the terminal voltage of the shunt machine, which is below point 2 by an amount corresponding to the difference between points 3 and 1. Shunt machines are used on some ships as service generators and are capable of being paralleled. S e r ie s g e n e r a to r . Field current and armature current are the same quantity in a machine that has a series connection between armature and field circuit. A series field winding would necessarily be wound with a few turns of a conductor sized to carry full load current; simply reconnecting the many turns of low ampacity shunt field windings would not suffice. Series generators have no practical application as DC power supplies, but 23-80 ELECTRICAL MACHINERY their r is in g c h a r a c te r is tic is important in terms of the more practical compound generators discussed later. Series generators are sometimes found as exciters on AC welding machines. The voltage characteristic of the series machine is virtually the same as the input characteristic, increasing with load (i.e., field) current. C o m p o u n d g e n e r a to r s . DC generators with both shunt field windings and series field windings are called compound machines. The actual field-armature connections may be either lo n g s h u n t or s h o r t s h u n t (Figures 23-49(d) and (e» depending on whether the series field current is to be armature current or load current CIa- If). It is, however, the relative strengths of the series and shunt fields (not the specific connection) that determine the voltage characteristic ofthe compound generator. The series field strength increases with load, tending to increase the generator output voltage. Thus compound generators inherently have a smaller voltage droop than shunt machines (Figure 23-48(b». If the series field is strong enough to produce a voltage at rated load greater than the no-load voltage, the machine is said to be o v e r - c o m p o u n d e d . If the full-load voltage is lower than the no-load voltage, the machine is u n d e r - c o m p o u n d e d . A fla t- c o m p o u n d e d machine has zero voltage regulation (droop divided by full load voltage), although the characteristic curve is not flat (Figure 23-48(b». Compound generators with a weak series field and a characteristic droop only slightly less than a shunt machine are referred to as s ta b iliz e d s h u n t generators, and the light series winding is referred to as the s ta b iliz in g fie ld . All of the above compound generator descriptions assume that the series field coils have the same sense of winding as the shunt field coils. That is, both coils, on the same pole piece, are producing magnetic fields of the same polarity. This is known as c u m u la tiv e c o m p o u n d in g and is assumed to be the case unless it is specifically stated that the machine is d iffe r e n tia lly c o m p o u n d e d , that is, has series field windings opposing shunt field windings in the production of pole flux. Differentially compounded generators have a huge voltage droop characteristic and are rare except for use in some DC welding machines. To provide three-wire DC for dual voltage systems, the DC generator apparatus must include a b a la n c e c o il. A balance coil is essentially an autotransformer with a central tap. The generator armature windings are tapped symmetrically (one tap per parallel path) and leads are brought out to a pair of slip rings. Each slip ring is connected to a group of armature taps at the same potential (there are two such groups). The resultant voltage between the two slip rings is alternating, with a peak value equal to the DC potential between positive and negative armature leads. Brushes riding on these slip rings are connected to the outer terminals of the balance coil; thus the center tap of the autotransformer is always at a potential halfway between slip rings (AC) and halfway between armature lead (DC) potential. The neutral is connected to the center tap of the Figure 23-49. DC generator winding connections. are compound generators. N o te : (d) and (e) balance coil as shown in Figure 23-50. The neutral current is the difference between positive and negative amps, which would be zero if there were only 240-volt loads or if the 120-volt loads were exactly the same on either side of the neutral. Unfortunately, exact balancing of positive-to-neutral and neutral-to-negative loads is hardly ever accomplished in a practical system. The balance coil then returns to the generator any excess of DC MACHINERY 23-83 positive over negative amperes, or supplies from the generator any deficiency if the negative-to-neutralload is greater. What the balance coil does n o t do is balance load between generators in parallel or balance (make equal) positive and negative amps. What it does is "balance the books"with respect to the total current entering and leaving a particular generator in a three-wire DC system. Balancer sets are a mechanical means of accomplishing the same thing using motor-generator (M-G) sets. These were once used aboard ships, but are no longer allowed. Any number of DC generating sets of similar (and drooping) voltage characteristics may be paralleled. This drooping requirement limits parallel operation to shunt (and stabilized shunt), under-compounded, and separately excited machines. The machines may be of the two-wire or three-wire type. Considering only self-excited machines supplying a three-wire system, Figures 23-51(a) and (b) show standard line connections for stabilized shunt and compound generators respectively. The most significant difference is the requirement for e q u a liz in g b u s e s when compound machines are to be operated in parallel. Equalizing buses connect together the terminals of the series fields of compound machines supplying a common load. In effect, one master series field is serving all of the paralleled compound generators. Without equalizing buses, the machines would not be stable in parallel. Rather, the machine with a slightly larger load would have a stronger series field and thus would take more of the load-further increasing its (series) field strength. Equalizing buses do n o t equalize (balance) the load distribution between generators, but they allow s ta b le load division, controlled by the relative s h u n t fie ld strengths ofthe paralleled machines. Equalizing buses are interconnections between generators only, and are not connected to any load. They are sized, however, no smaller than the main line leads. In the figures referenced above, the series fields are split and connected on either side of the armature. This arrangement requires tw o equalizing buses, and five-pole circuit breakers and disconnect switches (or links). Most three-wire systems are arranged this way. In two-wire systems, the entire series field is connected on one side of the armature (Figure 23-52(a)), requiring one equalizing bus and three-pole switchgear. An alternative three-wire arrangement is shown in Figure 23-52(b) that does not require algebraic (comparative) type reverse current and overload relays. Figure 23-53(a) shows the idealized voltage characteristics of two DC generators operating in parallel, with the load equally divided. The characteristic of generator #1 is shown in mirror image to demonstrate better the degree of load sharing. Note that the no-load voltages of the two identical machines were set equal. In Figure 23-53(b) the no-load voltage of generator #1 has been raised by decreasing s h u n t field resistance and thereby increasing If. The load has shifted to that machine, and the system P a r a lle l o p e r a tio n . DC MACHINERY 23-85 originally. Thus the kW load on DC generators operating in parallel may be divided by manipulating the shunt field current. The series field strengths are equalized at any division ofload by the equalizing buses. By simultaneously increasing excitation on one machine while decreasing excitation on the other(s), load may be shifted among generators while maintaining constant line voltage. Ideally, load division should be in proportion to the rating ofthe machines on line (e.g., two 600 and one 300 kW machines should share a common load 40 percent, 40 percent, and 20 percent respectively). The following procedure may be used as a guide for DC generator parallel operation: 1. Make a visual inspection to insure that all repairs have been completed, that the unit is free of tools or other debris, and in general is in a condition satisfactory for operation. 2. Insure that disconnect links are in place. (Note: Disconnect links should be opened or closed using an insulated wrench and always with the generator circuit breaker open.) 3. Start prime mover. 4. Close field switch. 5. Raise voltage by decreasing field rheostat resistance or, in A u to , increasing the voltage regulator set point to a few volts above line voltage (verify with calibrating voltmeter). 6. Check line-to-ground voltages to detect any grounds on oncoming generator. 7. Close circuit breaker; generator should pick up some load. 8. Balance loads appropriately with field rheostats or, in A u to , voltage regulator adjusting pots (potentiometers). DC Motors Speed characteristics. In DC motors Eg is the counter-emf (cern£)and is less than the line voltage by laRa. This fundamental relationship can be expressed in a number of different forms: Figure 23-51. Three-wire DC generator line connections. Reprinted :£rom ANSI/IEEE STD 45-1983, © 1983 by The Institute of Electrical and Electronics Engineers, Inc., with permission ofthe IEEE Standards Department. voltage has increased. In Figure 23-53(c) the field current (hence no-load voltage) of generator #2 has been lowered, shifting the loads further to the left and, in this case, lowering the system voltage back to where it was Figure 23-52.Two-wireDC generator line connections. Reprinted from A N S V IEEE STD 45-1983, © 1983 by The Institute of Electrical and Electronics Engineers, Inc., with permission of the IEEE Standards Department. speed; (3) increase in current with drop in cemf(current equation); and (4) increase in torque with current to meet increased load demand-if not, deceleration continues until equilibrium is met. Motor types S h u n t m o to r . DC motor and generator nomenclature is essentially the same, with motor classifications being derived from the manner of field Figure 23-53. DC generator load sharing armature connection. As with DC generators, the s h u n t arrangement is defined by connecting the main field circuit in parallel with the armature circuit. In fact the main field winding is usually referred to as the shunt field winding. Often the field circuit contains a rheostat. The result of this combination is a motor with constant field strength (<1» throughout the load 23-88 ELECTRICAL MACHINERY range, very little speed droop, and a torque characteristic that is proportional to armature current. The starting torque is relatively low. S e r ie s m o to r s . At the other end of the spectrum, the series motor produces a torque in proportion to nearly the square of the armature current (since Ia is responsible for <1». Thus, tripling the armature current yields almost nine times the original torque; a similar demand on a shunt motor would require the current to increase ninefold. However, the denominator in the speed equation varies with K x <1>, which is constant in a shunt machine but varies with Ia in a series machine. This yields a speed reduction, in the above example, in excess of 67 percent. At light loads, a series motor runs very fast and can run away if completely unloaded. Therefore series motors are always directly connected to their loads, either close coupled or via gear drives. For example, a series motor is never connected to a belt-driven load. C o m p o u n d m o to r . With careful design, cumulative compound motors can be built with characteristics that approach either the shunt machine or the series machine without the drawbacks of low starting torque or no-load runaway. The windings may be connected in the short-shunt or long-shunt fashion but, again, the motor characteristic depends more on the relative strengths of the series and shunt fields. Lightly compounded motors, with relatively weak series fields, behave in a manner similar to shunt machines. The series winding improves the starting torque and provides a definite droop, which is good for stability purposes. These general-purpose s ta b iliz e d s h u n t machines are suitable for centrifugal pumps, ventilation blowers, and other applications where the motor will not be started under appreciable load. Moderately compounded DC motors have a stronger series field, greater starting torque, and a greater speed droop. In general use, the term c o m p o u n d m o to r applies to this group, although stabilized shunt machines are technically compound motors. These motors are suitable for positive displacement pumps, compressors without unloaders, valve actuators, conveyors, and the like. W in c h or c r a n e m o to r s are heavily compounded, with the shunt field's role simply to guarantee an acceptable no-load speed. Their application is for those high-torque demand loads where a large speed droop is acceptable. DC motors may, theoretically, be d iffe r e n tia lly c o m p o u n d e d , but this is usually not practical. A motor connected with the series field reversed can be very dangerous. A sudden increase in load torque can cause the series field to cancel out the shunt field flux, resulting in runaway or even reversal of direction. DC motor torque/speed characteristics are summarized in Figure 23-54. S p e e d c o n tr o l F ie ld r e s is ta n c e c o n tr o l ( F R C ) . The disadvantages of DC machines are significant. They are larger and more costly for the equivalent horsepower Figure 23-54. DC motor torque/speed characteristics compared with induction motors; the brush and commutator arrangement adds considerably to the maintenance burden; and since DC power is not often readily available, special-purpose supplies are required to operate DC machinery. They have one major advantage, however, over most other types of electrical drives-the ease with which wide range adjustable speed control may be accomplished. From the speed equation, it is obvious that VL, Ra, and <1> are all candidate parameters to be manipulated in a speed control scheme. With the voltage across the shunt field circuit fixed (usually at nominal line voltage value), the field current, and hence <1>f, may be reduced by increasing field circuit resistance. Since speed is in v e r s e ly proportional to <1>, increasing shunt field resistance increases speed. The lowest speed possible under this method of control is at maximum If, which occurs with the field circuit resistance reduced to that of the field coils themselves. This may be an appreciable amount, from a few dozen ohms in large machines to several hundred ohms in smaller machines. Anyway, minimum field resistance corresponds to base speed and FRC can only increase RPM above base speed. Theoretically there is no upper limit: increasing field resistance decreases If, increasing RPM, ad infinitum. However, the prac- 23-90 ELECTRICAL MACHINERY tical limits are dictated by acceptable armature current. For a given load torque (K x <I> x Ia), Ia must increase to offset the lower <1>. These limits range from 2:1 for large machines to 8:1 for integral horsepower machines. The reciprocal relationship offlux and speed means that the cemf(K x <I> x RPM) remains relatively constant. Since at rated current, constant power (cemf x Ia) is developed over the entire speed range, this arrangement is known as constant hp drive. FRC is best suited for loads requiring the highest torque at lower speeds. B u c k / b o o s t fie ld c o n tr o l. If the series field terminals are not connected to the armature circuit, but instead brought out and connected to a separate variable DC supply, they may be used to make fine adjustments in the amount offield flux. This method is appropriate for DC/AC rotary inverters (M-G sets) where the bucklboost field is under the control of a governor. A r m a tu r e r e s is ta n c e c o n tr o l ( A R C ) . Increasing resistance in the armature circuit under load will decrease the numerator of the speed equation and reduce speed. Maximum speed is achieved with no additional armature circuit resistance and ARC can only reduce speed below the base speed. With the flux remaining constant, cemfis proportional to speed and torque is constant for a given load current regardless of speed. This is known as constant torque drive. Problems with this method include (1) virtually no speed control at light loads and (2) large armature IR2 losses, especially when running at low speeds for long periods of time. When combined with FRC, however, a very wide range of speed control can be obtained with (high) constant torque at low (below base) speeds and constant hp characteristics above base speed. At one time DC winches used such a control combination. A r m a tu r e v o lta g e c o n tr o l ( A V C ) . AVC requires two DC sources, one (fixed) field supply and one (variable) armature supply. Motor RPM is controlled by varying the VLin the speed equation. With <I> held constant by the fixed field supply, AVC, like ARC, is an example of constant torque drive. The Ward-Leonard system is an example ofthis type of motor control. Used originally to accommodate DC motor-driven winches in AC ship's service supply systems, the Ward-Leonard arrangement consists ofthree rotating machines (Figure 23-55(a)). An M-G set (three-phase induction motorlDC generator) supplies a variable DC voltage to the armature of a DC winch motor. The low slip induction motor provides nearly constant generator RPM and the separately excited DC generator output is controlled by varying the generator field current. At rated output (i.e., maximum generator field current) the generator supplies the winch motor with rated armature voltage. With the DC motor excited (separately) with maximum (rated) field current, and the armature supplied with rated voltage, the motor will run at b a s e s p e e d . Between minimum and base DCMACHINERY 23-91 .p e e d the motor field is energized at the maximum level while the armature voltage is increased by increasing generator excitation (AVC). The motor speed may be further increased by decreasing the motor field strength. This arrangement yields an extremely wide range of speed control with the characteristic of constant torque drive below base speed a n d constant horsepower drive above base speed (Figure 23-55(b)). S o lid - s ta te c o n tr o l. Replacing the M-G set described above with a solid-state converter and utilizing a second converter for the motor field supply will maintain the torque/speed characteristics ofthe Ward-Leonard drive. Figure 23-56 shows the circuit schematic for the four basic s in g le p h a s e converter drives: (a) half-wave converter drive, (b) semiconverter drive, (c) full converter drive, and (d) dual converter drive. For larger machines, three-phase converters may be used. The type of converter is determined by the size ofthe motor to be driven and the mode (quadrant) of expected operation. For example, only d u a l c o n v e r te r d r iv e arrangements are capable of four-quadrant operation: forward powering, forward braking (regeneration), reverse powering, and reverse braking. The field circuit in such an arrangement is a single- or three-phase full converter to allow field current reversal. Single-phase, dual converter drives may be used to power motors up to 15 kW, three-phase dual converter drives up to 1500 kW. Two-quadrant operation is possible w ith fu ll c o n v e r te r d r iv e s . Again the field circuit should be of the full converter type so that the field current may be reversed during regeneration. Power limitations of this type of drive are similar to that of the dual converter types. S e m ic o n v e r te r d r iv e s and s in g le - p h a s e , h a lf- w a v e c o n v e r te r d r iv e s are limited to one-quadrant operation only and applications up to 115 kW for three- phase semiconverter dri ves, or 15kW for single-phase semiconverter drives. A freewheeling diode is required across the armature circuit and semiconverters are required as the field circuit supply. Single-phase, half-wave converter drives typically require a large inductance in series with the armature to reduce the ripple in the current and are limited to applications up to 0.5 kW. Three-phase, half-wave converter drives are capable of two-quadrant operation and could be used for applications up to 40 kW. However, since DC components can be passed into the AC supply during regeneration, this arrangement is not generally used in industrial applications. With a fixed DC source available, c h o p p e r d r iv e s are often used to supply a variable voltage to the armature. In addition to AVC, chopper drives can provide regenerative braking by either returning power to the supply or to a resistor connected across the armature circuit. Actually the former method is technically regenerative and the latter rheostatic braking; often a combination of the two is utilized with chopper-driven DC motors. 23-92 D C m o to r ELECTRICAL MACHINERY starters The current equation dictates the need for external apparatus when starting all but the smallest DC motors. With only the O v e r v ie w . DC MACHINERY 23-93 armature winding resistance in the denominator (less than an ohm for even moderately sized machines, and milliohms in very large units), and no cemf upon starting, the current draw could be hundreds of times the rated current of the machine if started across the line. Ward-Leonard systems and electronic controllers solve the problem by reducing the applied armature circuit voltage at start. With a fixed DC voltage supply, however, the solution to limiting starting current to a reasonable amount 23-94 ELECTRICAL MACHINERY (two to four times the rated current) is by inserting s ta r tin g r e s is to r s in series with the armature circuit. This starting resistance may be removed when the cemf has built up enough to protect the armature circuit. The starting resistance may be removed all at once or partially removed in distinct steps. The ohmic size of the resistors is determined by the maximum allowable starting current, and the number of steps is dependent on the m in im u m allowable current (and, hence, minimum starting torque) during acceleration. For large machines, the difference between maximum and minimum currents during acceleration is relatively small and a large number of steps is required. In the examples below, three-step starters are illustrated where: in step 1 all starting resistance is in and armature current rises quickly to the maximum value, then decreases, and RPM and accompanying cemf increase to the minimum allowable value; in step 2, part of the starting resistance is removed (bypassed by an accelerating contactor) and the current rises up again to maximum, then decreases again as the motor continues to accelerate; and in step 3, the remaining resistance is removed (leaving only the winding resistance itself) and the current rises for the third time. The final current level is a function ofmotor load (Figure 23-57). Various methods may be utilized in determining the DC MACHINERY 23-95 rate at which the starting resistance is removed, including manual starters where the operator gradually removes the starting resistance as the motor accelerates. Three types of automatic DC starters are described below. D e fin ite - tim e s ta r te r s . Ifthe load characteristics are known and the load conditions will be practically identical whenever the motor is started (such as centrifugal pumps that are always started with no flow and shut-off head), then the time interval between steps may be precalculated. This enables the use of timing relays to close the accelerating contacts. The timing device may be pneumatic, hydraulic (dashpot), mechanical ratchet, or electronic. The control circuitry of Figure 23-58 is arranged so that the accelerating contacts, lA and 2A, are normally closed and are energized upon starting prior to energizing the main, M, contacts. A normally closed auxiliary M contact opens, de-energizing the lA coil. The accelerating contactors are TDODE (time delay on de-energization) and thus the contacts fall to their normal position after a set delay period. This action shorts part of the starting resistance (beginning the second step) and de-energizes coil 2A which will, after another delay period, bypass all the starting resistance. Another auxiliary M contact seals the start button. Replacing the momentary-contact push buttons with a maintain-contact selector switch will result in a controller with the LVR feature rather than LVP, but will not alter the starting sequence. C e m f s ta r te r . The problem with a definite-time starter is that if the motor fails to accelerate sufficiently in the time allotted, there will not be enough counter-emfto protect the armature from excessive currents when the starting resistance is removed. Starting under excessive load or mechanical binding of motor or pump shaft will cause such a problem. With a cemfstarter, successive steps do not take place until the cemfis sufficient to limit starting current to an acceptable level. This level is greater than the rating of the machine, of course; so eventually the machine will trip out on overload if sufficient acceleration does not take place. However, the timing of the steps is dependent on load and therefore cemf starters are suitable for applications where a variety of starting conditions may exist. In Figure 23-59 the normally open accelerating contacts lA and 2A insure sufficient starting resistance in the armature circuit until accelerating r e la y s lAR and 2AR pick up, energizing lA and 2A respectively. lAR and 2AR are designed to pick up at successively higher voltages, insuring the proper sequencing. The disadvantage of cemf starters is that line voltage fluctuations may cause the accelerating relays to release, inserting starting resistance during otherwise normal operation. S e r ie s lo c k - o u t s ta r te r . Sometimes called a current limiting starter, the series lock-out starter incorporates current-sensitive relays in the starting circuit that prevent (lock out) the accelerating contacts from picking up until the armature current falls below a preset value. In Figure 23-60, the initial inrush activates relay IS which will only drop out when the motor Figure 23-58. Definite time starter Figure 23-59. Cemf starter speed (and hence cern£)is sufficient to limit the current during the next step. Should the motor not build up speed, the process is held at that step, drawing the maximum allowable starting current, until the overload relay aborts the start. Like the counter-emf starter, lock-out starters are utilized in applications where a variety of starting conditions may be encountered. the procedures for identifying open, shorted, or grounded field (stator) coils in a DC machine are identical to those described earlier for salient-pole synchronous rotor coils and will not be repeated here. Also the use of a growler to identify shorted armature coils is apropos to DC machines. Identification of open or grounded armature coils is facilitated by the fact that, in DC machines, access to coil ends is available at the commutator. A troubleshooting chart for DC generators is given in Table 23-5, for DC motors in Table 23-6. D C M a c h in e M a in t e n a n c e The techniques for pinpointing defects in DC machinery are very similar to those appropriate for synchronous machines. In fact, T r o u b le s h o o tin g . 23-99 DC MACHINERY P r o b le m P o s s ib le C ause Low output voltage Low prime mover speed Poor commutation Shorted/grounded field pole(s) Shorted/grounded armature coil(s) Reversed field pole Faulty voltage regulator R em edy Check speed with tachometer; adjust governor setting as required. Replace defective brushes; insure proper seating and brush pressure; clean commutator; undercut mica; adjust brush rigging to brush neutral if necessary . Check and repair/replace as required. Check and repair/jump as required. Check and connect properly. Adjust or repair/replace as required. High output voltage High prime mover speed Faulty voltage regulator Check with tachometer and adjust governor setting. Adjust or repair/replace as required. Overheating Overload Insufficient ventilation Bearings Uneven air gap Shorted armature winding Shorted field coil Brushes off neutral High ambient temperature Reduce load. Remove restrictions; clean windings; check C. W. supply. See "Noisy operation" below. See "Noisy operation" below. Test and repair/replace. Dirty commutator Wrong brush grade Clean, stone, and burnish commutator as required. Check with manufacturer's guidelines; replace as required. Check brush pressure; check rigging alignment. Stone or cut on lathe as required. Undercut mica. Repair or jump coil as required. Replace/reconnect as required. Sparking at brushes Brush maladjustment Commutator eccentric High mica Open armature coil Shorted or reversed commutating pole(s) Vibration Noisy operation Bearings Uneven air gap Coupling Test and repair/replace. Adjust brush rigging as required. Check cooler operation; adjust as required. See "Noisy operation" below. Check for worn, loose, dry, or overlubricated bearings; replace worn or loose bearings; lubricate dry bearings; relieve overlubricated bearings. Check for bent shaft, bearing condition, gear alignment. Check alignment; re-align and/or tighten coupling. The arcing that occurs when a brush comes in contact with commutator bars connected to an open coil leaves a discoloration normally detectable by visual inspection. I n fact, the cause ofthe open may be failure of the soldered connection of coil lead to commutator bar riser, which also may be detected visually. If not, a low voltage may be applied to the armature through two strips of copper conductor taped to commutator bars one pole apart. The open coil may then be detected by measuring voltage between commutator bars. Bridging the open coilwould yield a reading equal to the applied voltage. An emergency repair may be made by bridging over the affected bars with a jumper of the same size as O p e n a r m a tu r e c o il. 23-100 ELECTRICAL MACHINERY the armature windings. Lap-wound armatures will have only one burned spot on the commutator for each open, regardless of the number of poles. 23-101 DC MACHINERY wave-wound machines of four poles or more, the number of discolored spots will be P /2 and anyone (but only one) ofthe spots should be bridged. In G r o u n d e d a r m a t u r e c o il. By connecting a low voltage source to the commutator as described above, grounded armature coils may be detected with a millivoltmeter in a bar-to-ground test. This assumes that megger readings have indicated that an armature ground does exist (all bar-toground voltages will be zero in an ungrounded armature). With one lead of the voltmeter connected to the rotor shaft, the other lead is moved around the commutator one bar at a time. Every bar that yields a zero or near zero reading should be marked with chalk. Because of the symmetry of the armature circuit, coils in equivalent positions to the affected coil in other parallel paths will also yield low (zero) readings. These are called phantom grounds. By shifting the voltage source connection points, the symmetry is changed and the phantom grounds will shift in location while the actual ground locations, of course, will remain the same (Figure 23-61). TABLE 2 3 ·6 D C M o t o r T r o u b le s h o o t in g P r o b le m P o s s ib le Fails to start Open in armature circuit C ause Open in field circuit Mechanical binding Controller malfunction Overheating Overload Insufficient ventilation Bearings Uneven air gap Shorted armature winding Shorted field coil Brushes off neutral Voltage too high or low Sparking at brushes Dirty commutator Wrong brush grade Brush maladjustment Commutator eccentric High mica Open armature coil Shorted or reversed commutating pole(s) Vibration C hart R em edy Check fuses; check armature circuit for loose connections, brush contact; check starting resistances, series field, and interpole windings for opens; reconnect, repair, or replace as required. Check field circuit for continuity; tighten connections; repair/replace open field coil or field rheostat. Check for seized bearings, pump impeller, etc. Check contactor and relay coils; replace if necessary, If tripped on overload, allow thermal element to cool, and reset. Reduce load. Remove restrictions; clean windings; check C. W. supply. See "Noisy operation" below. See "Noisy operation" below. Test and repair/replace. Test and repair/replace. Adjust brush rigging as required. Check supply and adjust as required. Clean, stone, and burnish commutator as required. Check with manufacturer's guidelines; replace as required. Check brush pressure; check rigging alignment. Stone or cut on lathe as required. Undercut mica. Repair or jump coil as required. Replace/reconnect as required. See "Noisy operation" below. 23-102 ELECTRICAL P r o b le m P o s s ib le Runs faster than normal Voltage high Reversed series field C ause Open/shorted/grounded field coil(s) High field rheostat resistance Runs slower than normal Low voltage Overload Short/grounded armature coWs)" Brushes off neutral Starting resistance inse rted Noisy operation Bearings Uneven air gap Coupling MACHINERY R em edy Check supply and adjust as required. Check connections and reverse Sl-S2leads ifnecessary. Check windings and repair/replace defective coils. Note: Motor will not start with open field coil, but a lightly loaded compound machine will run-fastand probably hot. Check rheostat setting and rating. CHAPTER 24 S h ip b o a r d C e n t r a l O p e r a t in g S y s t e m s Check supply and adjust as required. Reduce load. Check windings and repair/jump as required. AARON R. KRAMER Adjust brush rigging; check interpole integrity and polarity. Check accelerating relays for proper operation. Check for worn, loose, dry, or overlubricated bearings; replace worn or loose bearings; ]ubricate dry bearings; relieve overlubricated bearings. Check for bent shaft, bearing condition, gear alignment. Check alignment; re-align and/or tighten coupling. A H IS T O R Y HE operation of a ship's machinery space from a central location requires instrumentation hardware to transmit information for the operation and control of engine room equipment. The central control concept began in the late 1940s and early fifties with the introduction of the Bailey board, shown in Figure 24-1. Instrumentation hardware was based on relatively large case instruments. Each instrument was directly connected to a process sensing point. The operator could only interface with one control loop at a time, e.g., the combustion control system or the refrigeration system. With the development of pneumatic transmission techniques, centralized control became possible, gradually permitting more control hardware to be placed on a central control room panel. However, the receiving instrumentation was still fairly cumbersome and dedicated to the measurement and control of only one process variable at a time. By the late 1950s, miniaturization of the receiver instrumentation was another step towards central control. Although largely pneumatic, the individual central control room instrument size had decreased to 6 x 6 inches, then 3 x 6 inches, and finally the 2 x 6-inch standard was achieved. About this time, electronic instrumentation hardware based on transistor technology became available for the control applications. The development of electronic transmission made possible the centralization of many more instruments and functions on one control panel. During the early 1960s the use of digital computation was introduced to process control, adding peripheral hardware to the control room. New interface hardware, such as printers, typewriters, cathode ray tube (CRT) screens, and keyboards, made the control room somewhat complex, because this new hardware required backup by conventional analog equip- T 24-1 Figure 24-1. Typical Bailey board. Courtesy Bailey Controls Company, Wickliffe, Ohio ment. Thus, the operator had to learn new techniques, while using old familiar ones in emergencies. This was the state ofthe art of control panel design until the 1970s. The seventies produced a revolution in man-machine interface design with the introduction of systems based on microprocessor hardware. This new hardware digitized the usual analog signals and made applicable new methods of control and monitoring. The ease of communications between component parts ofthe system enabled an increase in flexibility of control and monitoring. All information on these new control consoles was visible and within easy access of the operator. M ic r o e le c tr o n ic s : T h e Im p act Some understanding of the revolution in microelectronics that has occurred over the past 15 to 20 years is also necessary for a full appreciation ofthe development of distributed control systems. A microcomputer on a single chip, comparable in performance to some of the early computers, is possible only because of the development of large-scale integrated circuit (LSD metal oxide semiconductor (MaS) technology. Figure 24-2. Evolution of control panel design Microelectronic development has progressed exponentially in very recent history. It evolved from the earlier development of the germanium transistor, then the basic silicon integrated circuit. The metal oxide semiconductor technology of the medium-scale integrated circuit (MSD progressed to the current LSI; most recently, the very large-scale integrated circuit (VLSD is the current state of the art. The growth of this technology has provided the basis for the modern central operating systems aboard vessels today and in the foreseeable future. Figure 24-2 shows this evolution of central operating systems (CaS) from 1950 through 1990. CENTRAL E N G IN E ROOM O P E R A T IN G SYSTEM COM PONENTS Any central operating system consists of the following components. S e n s in g D e v ic e s Sensing devices that measure temperature, pressure, flow, salinity, position, torque, speed, and level must be selected, installed, and maintained with great care. These are the instruments that are required to indicate the necessity for corrective action at the central operating system. (Figure 24-3 shows typical pressure, temperature, and flow devices.) 24-4 SHIPBOARD CENTRAL OPERATING SYSTEMS DESIGN CONSIDERATIONS 24-5 Variables that may require separate transmission and sensing devices are torque, salinity, pH, smoke, and crankcase vapor. Care must be taken to insure compatibility with the total COS system to insure proper and accurate transmission of information. Receiving Devices Receiving devices are those which convert the transmitted signal into useful information for observation by the operator. The devices that perform these functions are items such as bellows, pneumatic amplifiers, electric amplifiers, digital converters, and optical encoders (which convert light to a series of electrical impulses). These receiving devices are connected to observation mechanisms (pens, lights, pointers, etc.). Indicators and Recorders Figure 24-3. Typical pressure, temperature, and flow devices. Courtesy Bailey Meter Co., Cleveland, Ohio, a McDermott company. Indicators and recorders are devices that are manipulated by receivers so that operators may observe the information relating to the function of the engine spaces. These devices take the form oflights, strip indicators, chart recorders, gauges, CRTs, and teleprinters, and provide sound and sight information to the watch engineer and maintenance crew relating to plant operation. Control Devices Transmitting Devices Transmission devices convert the information sensed by the sensing devices installed in the line (i.e., steam line, shaft speed, etc.) into a physically realizable common low energy signal, so that it can be transmitted to receiving or control devices. Standard transmission signals and media are: 1. Pneumatic: 3 psi-15 psi; 3 psi-27 psi; 0-60 psi 2. Electric: 1 volt-5 volt, -10 volts-Ovolts-+10 volts 10 mA-50 mA, 4 mA-20 mA* 3. Digital transmission: Digital transmission varies as to type oftransmission and receiving devices used. A rate can be selected from 110 to 19,200 bits/second. Bits/second digital transmission is defined as a baud rate. 4. Fiber optics: This is a method of direct transmission oflight through cables. The optics can be incorporated within the sensing device and installed as a single unit. Some items that can be installed as a unit are differential pressure devices, temperature and pressure devices, flow devices, and level devices. " Becoming the industry standard Control devices regulate or maintain the proper operating condition ofthe systems in the engine spaces. Drum level, combustion control, engine RPM, refrigeration temperature, starting air systems, etc., all require control devices. Failure of this equipment generally requires a transfer from automatic to manual operation until the failure is corrected. Application of sensing, transmitting, receiving, indicating, and controlling devices requires a thorough understanding ofthe process in which these items are used. Proper engineering principles must be followedif the central operating system is to function as intended. DESIGN CONSIDERATIONS The design of a shipboard central operating system must consider the following requirements. System Environment 1. Operator (crew) capability to perform operation, maintenance, and repair activities 2. Operator (crew) availability (number, location, duties) 24-6 SHIPBOARD CENTRAL OPERATING SYSTEMS 3. Ambient conditions (temperature, humidity, cleanliness, corrosion, explosion hazards, etc.) 4. Relative location of control system components S y ste m 1. Interrelationship of controlled events a. Dependent and independent operation (safety interlock, etc.) b. Requirement for logic sequencing c. Requirement for computed outputs 2. Characteristics of controlled variables a. Number and characteristics of inputs b. System response (time factors, stability, etc.) c. Permissible deviation from ideal conditions (pressure, temperature, force limitations, etc.) 3. Effect offailure of C om ponent S e le c tio n Instrument selection requires consideration of the following areas.* S ta tic p e r fo r m a n c e 1. 2. 3. 4. 5. 6. 7. 8. 9. c h a r a c te r is tic s Ambient temperature effects Steady state error Hysteresis Friction Repeatability Reproducibility Resolution and threshold Error band Linearity D y n a m ic p e r fo r m a n c e 1. 2. 3. 4. 5. 6. c h a r a c te r is tic s P r o c e s s I n s tr u m e n ts Douglas M. Considine, New York: McGraw-Hill Book Co., 1985. a n d C o n tr o ls 1. 2. 3. 4. 5. o f in p u t v a r ia b le s tr a n sd u c e r in p u t/o u tp u t r e la tio n Accuracy Linearity Sensitivity Resolution Repeatability Reproducibility Friction Hysteresis Threshold, noise, etc. Zero drift Motion to th e sy ste m o f w h ic h th e tr a n sd u c e r is p a r t Output characteristics Size and weight Power requirements Needed accessories Mounting requirements Environment oftransducer Crosstalk Effect of presence oftransducer on measurand Need for corrections or modifications dependent on other transducers Need for corrections or modifications dependent on external information F a c to r s r e la tin g 1. Life cycle operational H andbook, 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. o n c h a r a c te r is tic s Range Overload protection Dynamic response (transient and resonant) Purpose ofthe measurement (observation and/or computation) F a c to r s a ffe c tin g 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. c h a r a c te r is tic s * An explanation of each of these terms may be found in 1. 2. 3. 4. F a c to r s r e la tin g Frequency response Response time Damping Temperature effects Motion effects Installation R e lia b ility 2. Overrange protection 3. Life storage F a c to r s th a t d e p e n d (M a c h in e ) C h a r a c te r is tic s E le m e n ts 24-7 DESIGN CONSIDERATIONS to m e a su r e m e n t r e lia b ility Ease and speed of calibration and testing Time available for calibration prior to and/or during use Mission time Stability against drift Vulnerability to sudden failure (mean time between failures, or MTBF) 24-8 SHIPBOARD CENTRAL OPERATING SYSTEMS CENTRAL OPERATING SYSTEM TYPES 24-9 6. Failure safety (Willtransducer failure represent or cause a system failure?) 7. Failure recognition (Will transducer failure be immediately apparent to system or observer?) 8. Level of technical competenceof all personnel expectedto handle, install, use, and/or servicethe device F a c to r s r e la tin g to p r o c u r e m e n t 1. "Off-the-shelf'availability(Is developmentworknecessaryforan operational transducer?) 2. Cost 3. Availabilityand delivery 4. Previous experiencewith vendor 5. Availabilityof spare parts 6. Availabilityofcalibration and test data from vendor 7. Operationalfamiliarity CENTRAL O P E R A T IN G SYSTEM TYPES Central operating systems are classified as direct connected, pneumatic transmission, electric/electronic, or digital. Figure 24-4. Direct connected panel E le c tr o m e c h a n ic a l D ir e c t C o n n e c te d S y s te m s A direct connected system brings sensing equipment to a common point, usually the operating floor. The sensing equipment, which is also the indicating device, is connected directly to the lines through which the operating medium is flowing (e.g., steam lines, flue gas, cylinder exhaust temperature, etc.). The direct connected system exposes the operator to engine room hazards if a failure occurs in any of the connecting lines to the indicators. Figure 24-4 shows a direct connected panel. P n e u m a tic S y ste m s Pneumatic systems utilize low pressure air to transmit a signal proportional to the measured variable. The receiver or indicator also functions with the same low pressure air. The sensing device is connected to the operating high pressure lines at a location remote from the operating personnel. Personnel are therefore not directly exposed to failure of operating lines. Pneumatic systems eliminate the hazards of electrical equipment when the operator is monitoring the variables associated with liquefied natural gas, liquefied petroleum gas, and other volatile chemicals. Figure 24-5 shows a pneumatic system and its associated central control panel. S y ste m s Electromechanical systems combine the essentials of mechanical sensing and electrical transmission devices with electric receiving and control devices. Electromechanical systems offer similar advantages to that ofthe pneumatic system in that multiple indicators and control functions may be utilized with the same transmission device. The advantages of electric transmission are the ease of install at ion, convenience of electric cable, and the speed of transmission of the measured signal. The significant disadvantage is the inherent danger of its application in explosive hazardous areas, such as aboard LNG, LPG, or volatile chemical transport vessels. Figure 24-6 shows a typical electromechanical system. D ig ita l S y s te m s All of the previously discussed systems were analog in nature. An analog system consists of a separate signal conditioner (transmitter) and a separate recording and indicating channel for each measured variable. The ease of conversion from the analog to the digital world makes possible the application of computers to the ship systems. Digital systems convert continuous analog signals from the sensing and transmission devices into discrete bits of information that can be understood only by a digital computer. These bits of information are then 24-10 SHIPBOARD CENTRAL OPERATING SYSTEMS DIGITAL SYSTEM COMPONENTS 24-11 Figure 24-6. Typical electromechanical system panel. Courtesy General Regulator Division of Forney Engineering, Carrollton, Texas. digital information (switches, pulses, contacts, etc.). The conversion of this information to a common form, understandable to the central operating digital computer, requires components called the input/output section or "front end." A general configuration for each component connection is shown in Figure 24-7. The components are: Figure 24-5. Pneumatic system schematic. Courtesy General Regulator Division of Forney Engineering, Carrollton, Texas. translated into useful functions by various computer languages for further processing by the system. Clarification of analog and digital concepts may be provided by envisioning two common musical instruments. A piano is a digital device, because there are only 88 keys related to 88 distinct, discrete notes. A violin has only 4 strings but the fingers can be positioned on these strings in an infinite number of continuous (analog) positions. A Simpson voltmeter is an analog device. D IG IT A L SYSTEM COM PONENTS S y s te m C o m m u n ic a tio n s A digital centralized control and monitoring system requires communication with the analog information (flow, temperature, pressure, etc.) and 1. 2. 3. 4. 5. 6. 7. Multiplexer Filters Analog to digital converter The computer Digital to analog converter Man-machine interface Memory (RAM, ROM, PROM, EPROM, E2 pROM) ( m u l t i p l e x i n g ) . A multiplexer is a high speed solid-state switching device that has the ability to scan a large number of sensors in a short period oftime (20,000-30,000 points/second). It is controlled by the computer in a programmed manner so that when the sensing point is addressed, all other necessary connections operate on that specific point. Examples of these operations may be switching a light, reading the variable on the CRT, or storing the information for future processing. The multiplexer may be installed in the same location as the computer or in M u ltip le x e r DIGITAL SYSTEM COMPONENTS 24-13 be converted into a series of discrete bits of information for processing by the computer by means of an analog to digital (AID) converter. The speed with which the AID unit processes the signal makes the information appear continuous to any observer. Figure 24-8 shows how a continuous signal is digitized. After processing by the AID converter and the computer, the information appears as a number, or a group of numbers, for future use. Figure 24-7. Digital system components some remote location connected to the central computer, each arrangement having advantages depending on application. Digital and analog systems, pneumatic or electronic, function as carriers of information signals with respect to the process they monitor. These signals consist oftwo parts: the actual representation ofthe variable being monitored and the unwanted noise or information that is totally unrelated to the process. The unwanted noise serves no useful purpose. It masks the true picture of the measured variable and in many instances can cause serious problems in the operation of a system. A filter, specifically designed to attenuate the most common sources of noise, is a necessary component in a digital system. Different filters are selected to remove noise generated from 60-cyclecurrent, momentary high voltage spikes due to ground loops, and radio frequency transmission. Any of these interferences may cause system operational problems if they are not filtered out of the system prior to entering the central operating system computer. F ilte r s . The computer section of the central operating system functions in a digital format only; therefore, all analog signals must A n a lo g to d ig ita l c o n v e r te r . T h e c o m p u te r . A computer is composed of five basic parts: 1. Internal storage-the memory bank used as the residence for instructions, computations, and other functions that determine the characteristics of the system. 2. Internal control-the unit that interprets the instructions in a specified order and acts like a "traffic cop" to insure that the instructions do not conflict. 3. Arithmetic unit-a component that gives the computer the ability to add, subtract, multiply, divide, and do logical comparisons. 4. Input registers-units to provide a path into the internal storage for the instructions or data upon which the computer operates as a system. 24-14 SHIPBOARD CENTRAL OPERATING DIGITAL SYSTEM COMPONENTS SYSTEMS 5. Output registers-providers of the path for the results of any. operation directed by the system to be output to the observer or the central system. 24-15 5. Implement proper emergency procedures. 6. Implement proper start-up and shutdown procedures. 7. Check control and monitoring system calibration and performance. The central operating system requires additional comments: 1. 2. 3. 4. A real-time clock An external interrupt for operator communication with the system A man-machine interface such as a keyboard, keypad, or typewriter Control system input and output so that continuous and digital parameters may enter and leave the system 5. Intra- and intersystem communications 6. Memory protection in event of electrical failure 7. Dual redundancy of critical components t o a n a l o g c o n v e r t e r . In many central operating systems digital information must be communicated to the outside world in analog form. An electric motorized analog valve may be driven directly from the digital computer. The information represented by these digital bits is converted to an analog format and the motor sees a continuous rather than a discrete signal. Figure 24-9 shows the conversion process. D ig ita l Figure 24-9. D/A conversion process i n t e r f a c e . The engineering officer on watch in the central control room is required to perform the following duties: The format in which the information is presented must be carefully engineered to enable the officer to perform his duties. A 30- to 40-foot panelboard is no longer required for operation of engine spaces on new complicated vessels. Microprocessor-based digital control and monitoring systems have reduced these panels to small desk size consoles containing computer-generated CRT displays in detail limited only by system capacity. Figure 24-10(a) is a typical console for a central control system of the 1960s and 1970s; Figure 24-10(b) shows a console for the 1980s. Presentations of the engineering functions can be programmed to allow the operating engineer to select dynamically active overviews of the entire plant or finite components of every subsystem in the operating spaces. Figure 24-10(a). Typical 1960s control panel. Courtesy General Regulator Division of Forney Engineering, Carrollton, Texas. M a n -m a c h in e Data, control, and information processing require storage in the computer for further processing. The storage takes place in banks located either in the computer or in an externally controlled memory device accessed by the computer. Memory can be stored on paper tape, magnetic tape, disk, or electronic components. Ofthe four mentioned, central operat- M em ory. 1. 2. 3. 4. Monitor for malfunction of operating equipment. Monitor key operating variables for deviation from the norm. Detect and interpret alarms. Obtain prompt access, display, and control of systems during malfunctions. 24-16 SHIPBOARD CENTRAL OPERATING SYSTEMS DIGITAL SYSTEMS DIGITAL 24-17 SYSTEMS Configurations Digital centralized control and monitoring systems can be configured as: 1. 2. 3. 4. Figure 24-10(b). Typical 1980s control panel. Courtesy Bailey Meter Co., Cleveland, Ohio, a McDermott company. ing systems generally rely on electronic memory and disk. Here are some definitions: A plastic, iron-impregnated wafer that can store information and programs in a specified order, to be later accessed by the computer. R a n d o m a c c e s s m e m o r y ( R A M ) . An electronic method of storing information and retrieving it in any order as directed by the computer. R e a d o n ly m e m o r y ( R O M ) . An electronic method ofretrieving information from the memory, but not a two-way communication device. P r o g r a m m a b le r e a d o n ly m e m o r y ( P R O M ) . A programmable electronic device in which information can be processed on a one-time basis during an initial phase and permanently stored, to be accessed later by the computer. E r a s a b le r e a d o n ly m e m o r y ( E P R O M ) . Same as a PROM except that the storage disk can be erased (by exposing the unit to an ultraviolet light) and then reused. E le c tr ic a lly e r a s a b le p r o g r a m m a b le r e a d o n ly m e m o r y ( E 2P R O M ) . Same as EPROM except that the memory can be erased by electrical means and re-programmed. D is k . System components can be configured in several different ways to suit the requirements of central operating and control design objectives. Supervisory monitoring and control advisory Set point manipulation and monitoring Direct digital monitoring and control Distributed control and monitoring Supervisory monitoring and control advisory. Supervisory monitoring and control advisory is a digital computer system that accepts key operating parameters of the engine spaces, external parameters that may affect the operation of the vessel, and manual inputs by authorized personnel concerning some other variables. All the inputs are processed by the computer; the results are advisory outputs to the operating engineer and selected other authorized personnel, who determine control set points, speeds, course, draft, etc. Any variation in operation is manually controlled by the engineer. Figure 24-11 shows such a system. The computer can be programmed to calculate best speed for vessel structural loading, best speed for minimum fuel consumption, and offers optimum operating conditions. These computations were impossible to calculate before digital computers. Set point manipulation and control. The basic objective of a vessel's operation is to optimize the financial return on investment. The economic return on an operation depends upon a number of factors, especially the day-to-day operating strategy. It is frequently not clear to the operating personnel what is optimum. A ship's plant is a complex, interacting entity, and the optimum operating strategy can only be ascertained after considering the combined effects of many different options. The digital computer can be used to perform the analysis, select the optimum, and implement it with analog control systems. A process model is needed to relate all of the various factors to the economic return on the operation. Although the computer application software determines the optimum operating strategy, the analog control system still implements the decisions. In many cases the control computer simply provides the set points to the analog control loops. The computer system does not replace Figure 24-11. Advisory control Figure 24-12. Supervisory control any analog hardware. The system backup problem is not critical since, if there is computer failure, the set points simply remain at their last settings. Figure 24-12 shows the component configuration of a set point manipulation system with analog backup, i.e., supervisory control. c o n t r o l a n d m o n i t o r i n g system. In direct control (DDC) the computer calculates the values of the manipulated variables (e.g., valve positions) directly from the values of the set points, controlled variables, and other measurements on the process. The decisions ofthe computer are applied directly to the process, hence the name direct digital control. This control arrangement is illustrated in Figure 24-13. All manipulated variables, control functions, set points, and other conditions necessary for the ship's operation are preprogrammed in the computer. These programs are called algorithms. An early incentive for using DDC systems was economic savings. The fundamental premise was that, since one computer could provide the same functions as several analog controllers, there must be some economic point at which the cost of these several analog controllers would equal the cost of the digital system. Two problems were encountered: D ir e c t d ig ita l 1. Programming costs. With no prior experience and without DDC software packages or proven monitors, the programming effort far exceeded that which was anticipated. This problem no longer exists since most manufacturers now provide the required software packages. 2. Backup hardware. This problem stems from the fact that operating personnel must be able to exercise effective control over the plant in the event of total computer failure. In many cases this backup was a complete analog system, thus eliminating any hardware savings. With the use of larger integrated circuits (IC) and better system configuration, the present reliability of computer systems is greater than that ofthe instrumentation system. Most manufacturers also can provide various degrees of backup at a reasonable cost, thereby eliminating the above problem. An important justification for DDC is the application of control techniques which are either impractical or impossible to realize with analog hardware. New control methods can be easily implemented, tried out, and changed at any time in a digital system. Figure 24-13. Direct digital control c o n t r o l . All distributed control systems designs (see Figure 24-14) have the following generic features in common: D is tr ib u te d The heart of each control and monitoring system is located at the local machinery unit, in the engine space, close to the sensing point of the primary variable being measured. All of the local controllers' house microprocessor-based hardware is capable of "predigesting" and, thus, converting control signal and set point information into digital pulses acceptable to a communications system. The information is then placed on a redundant communications link that is run throughout the ship's spaces. Much of the primary signal and control wiring is multiplexed; that is, it is run through a local switching system prior to placement on this communications link, so that only two wires are run around the plant. Conventional control board size in the central control room is also drastically reduced. CRT screens are used for flow sheet, process value, set point, and alarm information. These screens, plus host computer memory, enable storage ofcurrent and past ship's operating conditions by the operator or by management. Figure 24-14. Distributed system with peripheral equipment. Courtesy Bailey Controls Co., Cleveland, Ohio, a McDermott company. I n summary, distributed control is appearing in both computer and instrumentation systems designs. The development ofthe microprocessor has made possible the location ofmicrocomputers closer to the point offinal control and the redesign of the conventional analog instrumentation control and monitoring boards. Variations in both are beginning to appear in the marine marketplace. Sophisticated controls, not previously practical, are now possible. Plant optimization, on-line, and managerial decisionmaking information will become commonplace in the industry if applications are properly implemented. 24-22 SHIPBOARD IN S T A L L A T IO N AND CENTRAL OPERATING M A IN T E N A N C E INSTALLATION AND MAINTENANCE SYSTEMS O F D IG IT A L SYSTEM S TABLE 2 4 -1 S u m m ary of T ran sd u cers O u tp u t T y p ic a l A p p lic a tio n s I n te r fa c in g M e th o d C a b le Contact Alarms, status, binary DigitaVmultiplexer Twisted pair, overall shield High level analog Flow, level, pressure, etc. Analog/multiplexer Twisted pair, overall shield Low level analog Temperature, smoke Analog/multiplexer Twisted pair, individual shield and overall shield 24-23 O u tp u t T y p ic a l A p p lic a tio n s I n te r fa c in g M e th o d C a b le Pneumatic Flow, level, pressure, etc. p/r converter, P/D converter, pneumatic/multiplexer Not applicable Digital Flow, level, pressure, temperature, RPM, torque Parallel, serial As required by communications channel Digital interface, analog/multiplexer Digital-as required by communications channel; Analogtwisted pair, overall shield H is to r y When the application of computer systems began in the 1960s, the interfacing of the sensing devices with the digital system created many problems. These were caused by the use of improper signal cable, improper grounding techniques, poor cable routing, and poor layout of termination cabinets. In general, the methods for interfacing devices with computers were not fully developed. These early failures showed the importance of adhering to proper interfacing techniques. Reliable measurements were essential, especially when the computers were intended for control. Signal noise could not be tolerated and therefore it was imperative to find ways to eliminate this along with other problems. Today these methods have been developed and are in practice. For example, high and low level signals are not mixed in the same cable tray. Shielded cable is used and the shield is grounded only at one end. In addition to the basic techniques, signal muItiplexingis employed to reduce the cost of transmitting signals. New ways ofinterfacing pneumatic signals with digital devices are being utilized. The emergence ofthe microprocessor into the instrument environment has renewed interest in interfacing with digital devices. Initially, most of the sensing devices will send analog or contact signals to a microprocessor-based receiving device. Eventually, the sensors themselves will use microprocessors, thus allowing sensor measurements to be sent over a digital communications channel. The types oftransducers discussed herein are shown in Table 24-l. OF DIGITAL SYSTEMS Discontinu- Composition analous ysis (salinity, dissolved oxygen, etc.) D ig ita l C o n ta c ts Relay contacts must be interfaced with a computer. (Consider a computer to be any digital device, such as a microcomputer, data logger, programmable logic computer, minicomputer, etc., which has an analog or digital multiplexer associated with it.) Contacts are used to represent binary information, such as the on/off status of a motor or the presence of an alarm. A group of contacts can be used together to represent digital information being sent in parallel. A contact should be isolated from ground and all other signals, except those from the computer. This is sometimes referred to as a dry contact. A typical connection for a contact computer input is shown in Figure 24-15. The contact input, or digital input, is connected to a digital multiplexer within 24-24 SHIPBOARD CENTRAL OPERATING SYSTEMS the computer system. Note that the computer sends out a signal (a ground in this example) and senses if the signal is returned. If the computer were to be disconnected from the contact, it would be completely isolated. It is imperative that the contact is rated for the voltage and current of the computer sensing signal, although, in general, these voltages and currents are not very high. Analog Signals Analog signals are divided into two categories, high level and low level. While there is not a universally agreed-upon break point for these two categories, signals below 200 millivolts are considered to be high level. For example, a 4-20 mA signal working across a 250 ohm impedance has a voltage drop of 1 to 5 volts and is considered to be a high level signal, while thermocouple signals are classified as low level. Analog signals are connected to an analog multiplexer. High level. A typical hookup of a 4-20 mA sensor in a plant with electronic analog panelboard instrumentation is shown in Figure 24-16. Resistors R1 INSTALLATION AND MAINTENANCE OF DIGITAL SYSTEMS 24-25 and R2 should be located in a terminal board cabinet, where they will not be disturbed during equipment maintenance. In this way, the computer analog input card can be pulled from its slot for servicing and the measurement circuit will not open up. Similarly, the panel board receiver instrument, such as a controller or indicator, can be disconnected without affecting the computer reading. Also, a short across R1 or R2 will not affect the other reading. It is imperative that this circuit be grounded in only one place, i.e., at a connection to signal ground. Signal ground should be a separate ground connection running into the plant grid or several feet into the ground. The wire connecting the signal to earth ground at the equipment should be large enough to present a low impedance. (In applications where intrinsic safety is required, barriers should be employed between the field transmitter and the control room/computer room equipment.) Low level. Low level analog signals present more problems than high level signals because the low level signals are more susceptible to noise. Noise can be a combination of: 1. Electrostatic coupling 2. Inductive pickup 3. Common mode noise Electrostatic Noise This type of noise is the result of coupling the electric field with the signal wires because of the capacitance between the signal conductors and between the conductors and ground. Since the capacitance between the conductors increases as the length of the cable increases, electrostatic coupling is more of a problem in long signal cables. To eliminate this noise (also referred to as capacitive coupling or static noise), shielded cable should be used. One end of the shield must be grounded. The use of aluminum-polyester tape and a drain wire as the shield is becoming popular. To achieve a higher degree of noise rejection, a cable should be used which provides a separate shield for each pair of wires and a shield overall. Inductive Pickup Noise Power lines, motors, transformers, etc., generate electromagnetic fields that can be picked up inductively. (This is also called electromagnetic noise.) The use oftwisted pairs greatly reduces inductive pickup. The more twists per foot, the greater the noise reduction. Six twists per foot should be sufficient. In situations where a low level signal cable is in close proximity to high voltage cables, the low level cable can be run in conduit. 24-26 SHIPBOARD CENTRAL OPERATING SYSTEMS C o m m o n M o d e N o is e A common mode voltage is a signal that appears simultaneously at both input terminals with respect to a common reference point. This common reference point is usually the electrical ground. The common voltage contains no useful information. The useful signal is the voltage that is present across the input terminals, i.e., the normal mode voltage. The analog input multiplexers used in computer systems are designed to reject common mode noise. Almost all common mode noise is at power line frequency, 60 Hertz. Typically, these analog front ends provide 120 dB attenuation of60 Hz cQmmonmode voltage. Common mode noise can be caused by inductive coupling, capacitive coupling, and by improper grounding procedures which result in ground loops. G round L oops A signal circuit should be grounded in one place only. If one signal lead is grounded at the analog input multiplexer and also in the field at the thermocouple junction, and these two grounds are several hundred feet apart, there may be a considerable potential difference between the two grounds. This potential difference causes a current to circulate through both signal conductors. This current combines with the signal current, causing a common mode voltage to be present at the analog input terminals. The correct connection is shown in Figure 24-17. No ground loops are present. Note that if the cable shield were also grounded at the multiplexer end, a ground loop would result. Therefore, the cable shield should be grounded in one place only. INSTALLATION AND MAINTENANCE OF DIGITAL SYSTEMS 24-27 are separate from trays containing power wiring. Where signal cable must cross power wiring, it should be done at right angles, with as much separation as is practical. W h ic h E n d t o G r o u n d ? We have established that both the signal circuit and the cable shield should be grounded only in one place. However, there is some question whether this grounding should be done at field end or at the computer end of the cable. For example, thermocouples may be either grounded or ungrounded in the field. The grounded variety provides a connection between the tip of the thermocouple and the sheath. This contact with the sheath provides a quicker recognition of temperature changes at the measuring point. However, as shown in Figure 24-18, any accidental grounding ofthe signal cable will result in common mode noise. Therefore, there is no clear-cut answer to whether grounded or ungrounded thermocouples are preferable. In cases where thermocouples are being used for control, the quicker response time is very important and the grounded type may be preferred. Often millivoltto-current transmitters are installed in the field for temperature control loops. This allows the temperature measurement to be sent as a 4-20 mA high level signal, thereby reducing the noise considerations. Figure 24-18. Common mode noise caused by ground loop A convention should be established for an installation as to whether cable shields are to be grounded at the computer terminal boards or at the sensor in the field. This convention must be strictly followed. Figure 24-17. Correct connection for a low level signal P n e u m a t ic C a b lin g P r o c e d u r e s As much as is practical, cables carrying different types of signals should be segregated into separate trays. Signal cables must be run in trays that In str u m e n ts A generation ago, many plants used pneumatic instrumentation. Today some new ships are still being built which employ pneumatic instruments, but the vast majority are electronic. Many ofthe older pneumatic-operated 24-28 SHIPBOARD CENTRAL OPERATING SYSTEMS ships are candidates for the addition of new computer systems and, therefore, the subject of interfacing pneumatic transducers with digital systems is important today. The use of a P /I (pneumatic to current) converter is one solution. This device receives a 3 to 15 psig signal and converts it to a 4-20 mA electronic signal. The 4-20 mA signal can then be connected to a digital device in the same manner as any other analog input. This requires one P /I converter for each pneumatic signal. In a situation where there are many pneumatic signals, this may be costly. Also, the maintenance work needed to keep a large number of converters in calibration should be considered. In existing plants, the space to mount racks of P /I converters may be too difficult to obtain. While this solution might not be suitable for many pneumatic signals, it might be the best solution for interfacing a small number of signals with a computer. Another solution is to use P/D (pneumatic to digital) converters. One of these devices accepts a 3 to 15 psig pneumatic signal and converts it to digital signals which can be sent over a communications channel (see "Digital Transducers" below). This method is particularly applicable to a situation where the digital system does not have the capability of receiving analog signals. As with the use of PI I converters, this solution becomes less practical as the number of signals increases. A pneumatic multiplexer is often used in situations where there are many pneumatic signals to be interfaced with a computer. This device senses the pneumatic signals one at a time and converts each signal to analog. Some of these devices provide digital output. The switching is slow (only a few points per second), but it is much less expensive than purchasing individual P /I converters, and the maintenance time is less. If needed because of speed requirements, several units can be operated simultaneously. D ig ita l T r a n s d u c e r s An increasing number oftransducers now provide digital outputs. The fact that a device has a digital output does not necessarily make it easier to interface to a computer than an analog output device. Often special programs must be written for the computer and these programs can be expensive. If these development costs can be amortized over several applications, a digital interface will often be less expensive. Digital interfaces can be divided into two categories: parallel and serial. Figure 24-19 shows a typical parallel interface for 16 bits of information. The data to be transferred to the computer is held in a register. The 16 bits of data each have dedicated signal paths. During the period when the register is being updated, the data ready signal will be false. When the register contains valid data, the data ready signal will be true. The computer should be programmed to read the data whenever the data ready signal makes a transition from false to true. This will allow the computer to retrieve information as soon as it is available. The computer should disregard any information read when the data ready signal is false. Sometimes a parity check bit is included. A parity check is the simplest but lease secure method of error checking. This involves adding an extra bit, called a parity bit, to the message. This bit is set or reset so that the total number of binary ones in the message is always odd. (This is referred to as "odd parity." Similarly, some systems use "even parity," where the number of binary ones in a message is always even.) A serial interface uses one communication path as opposed to the parallel interface, where a separate path is needed for each bit. In the serial interface, the message is sent a bit at a time, with the bits following in order. The parallel interface is, of course, much faster. An entire block of data (17 bits in this example) can be sent in the same time as one bit can be sent over a serial interface. But the parallel interface requires a separate communications channel for each bit and is therefore more expensive. For communications over long distances, the serial interface is preferred, because transmission time becomes expensive. Parallel interfaces are used for short distances where high speed is essential. Serial interfaces are gaining in popularity even for short distances because data rates of 1 24-30 SHIPBOARD CENTRAL OPERATING SYSTEMS million bits per second can be achieved today over a communications channel. This is more than adequate to meet the interfacing requirements of any transducer. Also, using a serial interface allows the use ofthe same software for interfacing with a digital transducer, regardless of how far from the computer it may be. For long distances, greater than a few hundred feet, digital data are normally transmitted over the communications channel as modulated signals. At the receiving end, the signals are demodulated back into digital signals. The transformation of digital logic to modulated signals and the transformation from modulated signals to digital logic is performed by a modulator-demodulator, commonly referred to as a modem or data set. A modem is not required for short distances. When the system is selected and the central operating philosophy established, detailed installation engineering of system interfacing should be done to insure a successful project. USE O F D IG IT A L CENTRAL O P E R A T IN G , M O N IT O R IN G , AND CONTROL SYSTEMS The state-of-the-art digital technology can provide, at present, the following typical functions for use aboard ships: 1. Main engine condition monitoring, comparing with trial results or company standards. 2. Individual cylinder operating conditions, comparing with engine mean values. 3. Hull performance related to main engine performance: a. Evaluating hull condition, e.g., detection of hull fouling. b. Evaluating and comparing hull coatings. c. Determining optimum periods between drydocking, various trade routes. 4. Fuel efficiency (relates to Items 1, 2, and 3 above). 5. Fuel quality evaluation (including chemical treatment application). 6. Lube oil and cylinder oil inventory control. Establish and control planned ordering and storing cycles. 7. Lube oil analysis control. 8. Shipboard spare part management, inventory control, cost evaluation and control. 9. Overtime distribution, i.e., separating operating overtime from M&R overtime. 10. Assistance with crew payroll computations and payoffs. 11. Personnel records. 12. Training and instruction manuals. 13. Inventory control, requisition printing. 14. Routine maintenance planning and record keeping. 15. Machinery running hours record with maintenance projection. ACKNOWLEDGMENTS 24-31 16. Auxiliary machinery performance analysis. 17. Maintenance of shipyard and drydock records, and assistance in planning offuture drydock work. 18. Storing and printing drydock specifications. 19. Part incorporation of existing preventive maintenance reportable items. 20. Trend analysis of machinery performance, voyage reports, etc. 21. American Bureau of Shipping survey planning, record keeping, and projection. 22. U.S. Coast Guard inspection/reinspection, work lists/record keeping. 23. Company directives. 24. Training programs. 25. Safety programs. 26. Miscellaneous record keeping. 27. Ballast control and trim. The engine room data logger, chief engineer's officeevent recorder, and bridge bell recorder should interface with the shipboard management computer so that information/data in memory of the former can be communicated to the latter and stored in a logical format or in a reliable storage medium. These stored data are then available for local display and printout as well as transmission ashore by satellite and/or modem to company headquarters. The shipboard management computer should be capable of receiving transmitted data via satellite. Future applications may include (1) remote operation vessels, totally unmanned and supervised by a host ship, and (2) automatic cargo loading and unloading. The implementation of a digital central operating system and its uses are limited only by the imagination of the engineer and the desires of the owner. The potential of these systems is virtually untapped! ACKNOW LEDGM ENTS Grateful acknowledgment is made to Mrs. Margaret Hoering and Lillian Ferrari of the State University ofNew York Maritime College Engineering Department who suffered through the many typing drafts ofthis work and to my wonderful wife Iris who made literary sense out of my scrambled thoughts. CHAPTER 25 S h ip b o a r d V ib r a t io n A n a ly s is EVERETT C. HUNT IN T R O D U C T IO N T HERE are many ways in which to determine that operating machinery is in a satisfactory condition, including (a) measurement of temperatures, pressures, and flows; (b) observation of noise; and (c) the observation of the machinery response to changing demands. Engineers have always included the observation of vibration characteristics among the methods of determining the condition of machinery. A crude but useful test used by marine engineers has been to place a coin on the bearing cap of a rotating machine. Ifthe coin remained on the cap, the vibration level was judged to be satisfactorily low. Modern vibration analysis techniques and instruments have greatly refined and extended the engineer's ability to judge the condition of operating machinery. This chapter will introduce the basic methods and various equipment associated with vibration analySIS. V ib r a tio n A rotating or reciprocating machine is subject to the influence ofoscillatory forces. These oscillatory forces repeat themselves at equal time intervals, hence, they are periodic. For example, a small weight attached to a perfectly balanced rotating cylinder will cause the cylinder to vibrate periodically. That is, the period or time elapsed between maximum vibration forces will be equal to the time for one revolution of the cylinder. The reciprocal of the period is the frequency of the vibration or the number of vibrations which occur per unit time. Frequency is measured in cycles per second, or cycles per minute, where the cycle is the motion completed during the period. Vibrations may be divided into two general types, free and forced. 25-1 25-2 SHIPBOARD VIBRATION ANALYSIS F r e e V ib r a tio n If an elastic system, such as a weight connected to a fixed position by a spring, is moved so that the spring is compressed and then released, with the assumption that there is no friction in the subsequent movement ofthe weights, the weights will adopt an oscillating motion. This free vibration will occur at the natural frequency of the system. The natural frequency is a result of the physical properties of the spring and the weight. Figure 25-1 is a schematic of a simple free vibration system. INTRODUCTION 25-3 damped by physical features of the rotor and the internal friction of the lube oil in the bearings. Figure 25-2 shows a simple damped system with forced vibrations. Figure 25-2. A simple damped system with forced vibrations V ib r a tio n Figure 25-1. A simple free vibration system F o r c e d V ib r a tio n When an oscillatory external force is applied so that the system vibrates at the frequency of this exciting force, the system is said to have a forced vibration. Ifthe frequency of the external exciting force is the same as one of the natural frequencies of the vibrating system, then a condition of resonance exists. I n resonance, the external force reinforces the natural vibration during each cycle so that the amplitude or range of motion increases with the ultimate possibility of machinery failure. D a m p e d V ib r a tio n Actual vibrating systems, such as rotating machinery, have internal forces due to friction which act to reduce the forces of excitation and prevent the amplitude ofmotion from becoming excessive during resonance conditions. For example, a steam turbine passing through its first critical speed has exciting forces at the same frequency as the natural rotor frequency. Nevertheless, damage is prevented because the vibration amplitude is M easu rem en t There are three common measurements to describe the periodic motion of vibration: displacement, velocity, and acceleration. These three measurements are related to each other by mathematical formulations. For example, the periodic motion may be represented by a unit vector rotating at 0) radians per second. A radian per second equals 21t times revolutions per second. I n this case, the vibratory motion or amplitude represented by x is x = sin w t and the velocity, v, is the rate of change of x or v = w CDS wt and the acceleration, a, is the rate of change of the velocity or a = _w 2 sin w t Plotting these values on a time scale as shown in Figure 25-3 illustrates the relationship among these vibration measurements. The velocity leads the displacement by 9 0 The acceleration leads the displacement by 1 8 0 0 0 • • Figure 25-3. Relationship among displacement, velocity, and acceleration in a vibration system is the distance the mass travels in a simple vibrating system. In a complex system such as the bearing cap of a boiler feed pump, displacement would be the actual distance the part moves in a selected direction such as vertical or horizontal. Displacement is commonly measured in fractions of an inch, such as the mil or .001 inch. Displacement measurements have many uses but are the least satisfactory for measuring the severity of vibration. The proximity pickup is used to measure displacement. This pickup is a noncontact device which establishes an electric field in the vicinity ofthe moving part that generates eddy currents inversely proportional to the distance between the pickup and the moving part. The eddy currents modulate the amplitude of the carrier frequency which is converted to distance or displacement by the circuits of the analyzer. Figure 25-4 illustrates the operations principle of an eddy current pickup. D is p la c e m e n t is movement per unit time ofthe vibrating part during its periodic or oscillatory motion. Velocity is the most useful measurement of the severity of vibration. This measurement is normally used in the frequency range of 400 cycles per minute to 60,000 cycles per minute. Below this range displacement is most effective while acceleration is commonly used for higher frequency vibration. V e lo c ity Figure 25-4. Operating principle of a reluctance pickup used to measure displacement A typical velocity sensor consists of a spring-held coil of wire which is surrounded by a permanent magnet moving at the velocity ofthe vibrating part. This device produces a voltage which is proportional to inches per unit time movement of the oscillating part. Figure 25-5 illustrates the operating principle of the velocity pickup. is the rate of change of the velocity of movement of the vibrating part. Since the part is oscillating, the acceleration is constantly changing and reversing direction. The part slows down to approach the maximum displacement and reverses direction to accelerate in the opposite direction. Acceleration reaches its maximum when the part has traveled the maximum displacement of its periodic motion. The acceleration sensor or accelerometer consists of a mass resting on a piezoelectric crystal. The force of the mass acting on the crystal due to change in acceleration produces an electric voltage proportional to the acceleration. This voltage is converted to an acceleration indication by the output device or meter. Figure 25-6 is a cross section illustrating the operating principle of an accelerometer. A c c e le r a tio n DESIGN ENGINEER'S APPROACH 25-7 Analyzers are fitted with strobe lights to provide a phase angle measurement. By tuning the analyzer filter to a machine's shaft rotational frequency and directing the strobe light at a reference mark placed on the shaft, the mark will appear stationary at a certain angular position. Should the phase of the vibration change, the mark will shift by an angular amount equal to the phase shift of the vibration. D E S IG N E N G I N E E R 'S M a th e m a tic a l Figure 25-5. Operating principle of velocity pickup is the angle between the instantaneous position of a vibrating part and a reference position which is the fractional part of the vibration cycle through which the part has advanced relative to the reference position. The phase angle between two vibrating parts is the angular difference between the two when both are compared with the same reference position. P h a s e a n g le APPROACH M o d e ls During the design phase of machinery or ship systems, the design engineer calculates the natural vibration frequencies ofthe machine or system. The natural vibration frequencies are compared to the anticipated periodic exciting forces to determine the cases where a resonant condition can exist. For example, a turbine designer must ensure that the turbine blades are not excited by the periodic force of the blades passing the nozzles. The ship designer must assure that structural arrangements do not have natural frequencies which will resonate with the periodic forces generated by the propeller. When the design engineer determines that an external vibratory force may excite resonant conditions in machinery or structure, he modifies the design to (a) tune the natural frequency of the machine or structure away from the frequency of the external force; (b) change the frequency of the external vibrating force (for example, changing the blades of a propeller will modify the frequency of propeller blade forces acting on the hull); or (c) add damping characteristics to the machine or structure so that it can operate successfully in a resonant condition. M a c h in e r y B a la n c in g Machinery design engineers also specify the accuracy of rotating balance which a machine must have to operate smoothly without vibration due to unbalanced weights. Manufacturers utilize dynamic balancing machinery in their shops to achieve the specified level of balance before delivery ofthe machinery. Figure 25-7 shows a typical balancing machine used by a machinery manufacturer. I f repairs, wear, or accidents cause a machinery rotor to become unbalanced, it may be necessary to remove it from the ship for rebalancing in a dynamic balancing machine. It is often possible to correct unbalance aboard ship by using portable equipment with the machinery operating in its own bearings. Machinery unbalance manifests itself as a one-perrevolution vibration. SHIPBOARD APPROACH TO VIBRATION ANALYSIS 25-9 has become unbalanced so that it can be repaired before the problem leads to complete failure. I n v e s tig a tio n o f S u sp e c te d P r o b le m s The source of malfunction of any rotating or reciprocating machine may be uncovered as a result of vibration analysis. It is often possible to determine that an external oscillatory force is causing a problem in a machine. A hot bearing may raise questions which can be answered by measuring the severity and frequency of vibration in the machine. E v a lu a tio n o f M a c h in e r y R e p a ir s A frequent shipboard maintenance task requires the removal ofrotors from pumps, turbines, fans, etc., for rebalance in a shoreside repair facility. Vibration analysis provides the ship's engineering officers with a tool to evaluate the repair and rebalance job which the repair facility completed. Such evaluation may save them from returning to sea with an unsatisfactory piece of machinery. B a s e lin e Figure 25-7. A typical balancing machine V ib r a tio n S ig n a tu r e A very valuable technique of vibration analysis is the vibration signature. This procedure requires that the amplitude of vibration over a range of frequency be recorded when the machinery is placed in service. During the subsequent life ofthe machine this "signature" is recorded at intervals and compared to past vibration amplitudes and frequencies. Changes may be interpreted to provide an understanding of the current condition of machinery. E c o n o m ic B e n e fits S H IP B O A R D APPROACH T O V IB R A T IO N A N A L Y S IS During the life of machinery, the engineering officer can use portable instruments and analyzers for monitoring conditions and solving problems which manifest themselves as vibrations. Vibration analysis involves the measurement of the frequency of vibration which in turn permits identification of the source of excitation. Once the source is discovered, shipboard personnel can take corrective action. I n s p e c tio n o f N e w M a c h in e r y a n d P r e v e n tiv e M a in te n a n c e Vibration analysis of new machinery installed aboard ship provides evidence that the manufacturer has met the specifications and the normal standards for smooth running machinery. A planned program of vibration analysis can also be a key to preventive maintenance. Levels, or intensities, and frequency ofvibration taken on a regular basis reveal the deterioration of antifriction bearings and facilitate replacement prior to failure. Preventive maintenance based on vibration analysis identifies machinery that of Vibration A n a ly s is Vibration analysis results in reduced machinery downtime. Downtime can cause expensive delays and disruptions in sailing schedules. Vibration analysis helps avoid costly machinery repairs caused by unexpected major breakdowns. Furthermore, early correction of faults means lower repair bills and longer machinery life. Finally, the classification societies, such as the American Bureau of Shipping, have recognized the value of vibration analysis in their rules to permit an extension of the period between required surveys of machinery. This extension reduces the cost of machinery survey during the life of a vessel. I n 1971, the American Bureau of Shipping added the following statement to the "Rules for Building and Classing Steel Vessels": In addition to the foregoing requirements, turbine blading and rotors, cylinders, pistons, valves, condensors and such other parts of main and auxiliary machinery as may be considered necessary, are to be opened up for examination. At Special Periodical No.1 only, for vessels having more than one main propulsion 25-10 SHIPBOARD VIBRATION ANALYSIS ahead turbine with emergency steam crossover arrangements, the turbine casings need not be opened provided vibration indicators and rotor position indicators are fitted and that the operating records are considered satisfactory by the Surveyor. An operational test of the turbines may be required if considered necessary by the Surveyor. Shipboard Applications of Vibration Analysis With some particular types of machines there are specific tasks which are notable because they can be accomplished with vibration analysis equipment. These machines and tasks include the following: General propulsion systems. For geared-turbine propulsion drives, fore and aft vibrational displacements and frequencies are measured in the machinery during trials to determine whether excessive motions exist in the system. Vibration displacement and frequency are also an important measure ofthe stiffness ofthe main thrust bearing, the thrust foundation, and the supporting structure of the ship's inner bottom. Propulsion turbine-gear unit spline and coupling motions may be determined and evaluated by measuring the relative and absolute vibratory motion of casings and shafts of the turbines, reduction gears, and thrust bearing units. Vibration analysis provides an effective measurement of excessive athwartship vibration of diesel engine and reciprocating steam engine propulsion systems. Continuous monitoring of steam or gas turbine propulsion systems permits analysis which provides early identification of impending trouble. Shafts and propellers. Where outboard shafting is a part of a propulsion system, vibration measurements may be used to analyze structural rigidity and the effect ofadding struts or other shaft support members. In the same manner, propellors, main shafting, and other rotating parts of the propulsion train can be checked for balance, indications of misalignment, and excessive clearance in bearings, all of which may contribute to unsatisfactory operations. Air-handling equipment. Fans and blowers driven by motors or turbines are easily balanced in place after reblading or other repair using vibration analysis equipments. Fan or blower aerodynamic vibrations, misalignment, and wear may be identified with vibration analysis. Pumps and compressors are similar high speed machines for which vibration analysis helps to prolong service life and avoid operating problems. Motors, generators, and similar rotating machinery. AIl electric motors and generators aboard ship can be analyzed and compared against established standards for permissible vibrations to obtain an evaluation of MACHINERY VIBRATION ACCEPTABLE LIMITS 25-11 the general condition. Unbalance, electrically caused vibrations, bearing wear, and machine misalignment are readily detected. For large critical electrical machines, vibration signature data should be accumulated overtime for preventive maintenance programs. Marine gas and diesel engines. On ships with industrial or aircraft-type gas turbines, vibration measurement determines the presence ofexcessive vibration levels. Causes of vibration in gas turbines may be determined. For marine diesel engines, dynamic balancing of engine assemblies and components such as flywheels and turbochargers is feasible. Gears and gear drives. Vibration analysis will reveal the condition of gear tooth contact, tooth geometry, tooth spacing, gear concentricity, and rotor balance. Bent shaft and gear box misalignment are also revealed through vibration analysis. This can be accomplished without the expensive disassembly and physical measurements which are normally required to detect these conditions in gears. MACHINERY VIBRATION ACCEPTABLE LIMITS An essential part of any vibration analysis program is the establishment of acceptable and unacceptable limits of vibration for the machines in the program. Such a program is only useful when there is confidence that certain vibration levels accurately indicate when a machine is in good mechanical condition, or conversely that the levels are too high and the machine must be shut down to effect repairs. Establishing such limits cannot be done casually and must draw on a number of sources of inform ation. These include manufacturers' recommendations, vibration severity guideline charts, vibration measurements ofmachines in the program, and increases in vibration levels of machines in the program. Manufacturers' recommendations on vibration limits are usually established on the basis of experience. For example, certain limits on design tolerances, balance levels, and general vibration, which insured long machine life, have evolved into recommended limits for specific classes of machines. Vibration severity guideline charts are indirectly related to the manufacturers' recommendations. Basically, they represent the experience of those who operate, maintain, inspect, and approve machinery. These data have been collected, evaluated, and reduced to guideline format for ease of use. Some of the guideline charts cover specific types of machines. Others are generalized guidelines. Figure 25-8 is an example ofa general vibration chart prepared by IRD Mechanalysis, Inc., of Columbus, Ohio. If vibration measurements of a number of similar machines in a program can be obtained, these, in themselves, form a good representative VIBRATION MEASURING EQUIPMENT 25-13 to insure that the sample includes only machines in good mechanical condition. Those which have defects are usually readily detected because their vibration levels are much higher than the average. High vibration levels in a machine, which are later found to have been caused by a specific defect, provide useful vibration limit data. Such levels can be used as a vibration limit guideline to warn of a similar defect when seen in other machines. Still another method which assists in establishing vibration limits makes use of the fact that increases in vibration levels of machines in a program indicate machinery defects and hence a tentative upper limit. When a machine's vibration level remains unchanged over an extended operating period (i.e., on the order of six months), it is reasonable to assume that the machine is in good mechanical condition and its vibration levels indicate acceptable limits. On the other hand, vibration levels which are changing generally indicate a deterioration of machine conditions. While vibration limits which must be met by some machinery are used as a simple "go" or "no-go"tool, more often the limits are set up as a series of graded levels. One of these graded guidelines, "Vibration Velocity Standards for Shipboard Machinery," is tabulated below: P e a k V e lo c ity S e v e r ity S e v e r ity in c h / s e c o n d R a tin g C la s s ific a tio n Above .6 Very rough .4 to .6 Rough .2 to .4 .1 to .2 Slightly rough Fair D to .1 Good High vibration level, considered potentially unsafe. Detailed vibration analysis required to identify trouble. Excessive vibration may cause oil-film breakdown. Consider shutdown . Considered hazardous. Detailed vibration analysis needed to identify trouble. Rapid wear expected. More frequent periodic vibration checks needed to detect further increases. Schedule for repair . Faults likely. Detailed analysis needed. Continue periodic checks. Schedule repair, if needed . Minor faults. Continue periodic checks to detect increase. Typical of well-balanced, well-aligned equipment. Acceptable vibration levels for speed reduction gears have been published by the American Gear Manufacturer's Association. Figure 25-9 shows these levels in mils peak to peak amplitude versus frequency in Herz or cycles per second. Figure 25-8. General machinery vibration severity chart vibration sample which can be used to establish levels for machines in good operating condition. Of course, some analysis and judgment are required VIBRATION MEASURING EQUIPMENT Some of the equipment used for measuring vibrations include the pickup devices, the operation principles which are illustrated in the introduction 25-14 SHIPBOARD VIBRATION ANALYSIS VIBRATION MEASURING EQUIPMENT 25-15 of600 to 600,000 CPM (cycles per minute). Displacement in the range of 0 to 100 mils peak to peak and velocity of vibration in the range of 0 to 100 in/sec (inches per second) peak may be displayed by selecting from the overlapping ranges of the instrument. Little training is required to use such an instrument in a vibration monitoring program. Model 308 Vibration Level Meter shown in Figure 25-10 is typical of portable vibration meters. FREOUENCY - Ha Figure 25-9. Acceptable vibration levels for reduction gears. Extracted from AGMA Standard Specification for Measurement of Lateral Vibration on High Speed Helical & Herringbone Gear Units (AGMA 426.01), with the permission of the publisher, The American Gear Manufacturers Association, 1330 Massachusetts Avenue, N.W., Washington, D. C. 20005. Figure 25-10. Typical portable vibration meter to this chapter, and electronic devices which display and analyze the pickup signals. These electronic "black boxes" range from simple hand-held vibration meters, which display the displacement and velocity of vibration, to complicated systems which provide an analysis in real time of the complete spectrum of vibration frequencies and the amplitudes in each of the frequency bands. Devices of primary interest to the shipboard marine engineer include the basic vibration meter, the vibration analyzer, and the permanently installed vibration monitors. V ib r a t io n M e te r s The portable vibration meter and self-generating velocity pickup is a basic vibration measuring device. Such devices typically cover a frequency range V ib r a t io n A n a ly z e r An analyzer permits the trained shipboard engineering officer to measure frequency of vibration in addition to the velocity and displacement. It is the frequency and phase which normally reveal the source of a vibration problem. A tunable filter allows the user to measure the frequency. A typical range of frequency measurement is 50 to 500,000 CPM. Analyzers are normally fitted with stroboscopic lights which permit visual identification of the vibrating part for which a displacement, velocity, or frequency measurement is being made. A plotter may be connected to the analyzer to record vibration amplitude versus frequency. This plot is called a 25-16 SHIPBOARD PROGRAMS FOR PREVENTIVE MAINTENANCE VIBRATION ANALYSIS 25-17 acceptable limits. Figure 25-12 is a schematic of a typical four-pickup vibration monitor arrangement and Figure 25-13 shows a typical permanently installed vibration monitor. "vibration signature" and it provides the information used for comparison of changes in machinery conditions over a period of time. The Model 350 Vibration Analyzer with a plotter attached, as shown in Figure 25-11, is typical of this type of instrument. This equipment can be used with the stroboscopic light for in-place balancing of rotating machinery. I•• Figure 25-11. Typical vibration analyzer with plotter accessory M o n ito r in g E q u ip m e n t Main propulsion units may have permanently installed vibration monitoring equipment as specified by the propulsion plant designer. Acceleration transducers and noncontact transducers, mounted on the turbine bearing housings, sense vibration and axial turbine shaft positions. The monitor receives the signals from the transducers and compares the vibration levels and rotor position to quantities specified by the machinery manufacturer. I f the vibration level or axial position exceeds predetermined quantities, the monitor sounds a warning alarm providing the engineer the opportunity to correct the problem before machinery is damaged. The monitor also provides for display of the vibration level and the axial position of the rotor. I n some installations, the monitor provides a signal which shuts down the machinery if vibration or axial position varies excessively from Figure 25-12. Permanently installed vibration monitor arrangement used on main propulsion machinery. I R D vibration monitor with automatic scanner provides intermittent sampling of four vibration pickups. PROGRAM S FOR P R E V E N T IV E S im p le P r o g r a m M A IN T E N A N C E U s in g P o r ta b le M e te r The portable meter which indicates overall vibration level in mils displacement or in in/sec velocity may be employed to establish a simple but useful program of vibration monitoring. Following installation or overhaul of a machine, the overall vibration levels are recorded for selected points on the bearing caps. These points, usually in the vertical, horizontal, and axial directions, are recorded so that identical location readings may be taken on a regular basis, e.g., biweekly. The vibration levels are compared to general severity data to provide an indication of the condition of the machinery. A tabulation of these readings for a period of time provides trend information which is most valuable. A sudden or gradual change in Figure 25-13. Typical vibration monitor display and control cabinet identically recorded vibration levels indicates a change in the internal condition of a machine which should be investigated to correct the cause before damage occurs. Figure 25-14 shows a typical set of vibration readings taken in the athwartship direction for a one-year period. Eventual high vibration readings led to an investigation and correction of a problem. Figure 25-15 illustrates the use of the hand-held vibration meter while recording an axial overall vibration level of a refrigeration compressor. A M a in te n a n c e P rogram U s in g a V ib r a tio n A n a ly z e r When measurements are limited to overall vibration levels, the ship's engineering officer does not have all the information needed to identify the source of the problem. The ability to indicate vibration levels and the frequency at which they occur is necessary for complete understanding of the machinery condition. A vibration analyzer provides this information. 25-20 SHIPBOARD VIBRATION ANALYSIS Such information may be recorded in tabular form by measuring the amplitude and frequency of the machine's major vibration peaks. If an X-Y plotter is available as an accessory to the vibration analyzer, a continuous plot of vibration level versus frequency can be prepared as a vibration signature. Figure 25-16 shows a typical plotted vibration velocity peak measurement versus the frequency. Figure 25-16. A vibration signature plotted by an X-Y plotter accessory to the vibration analyzer E s ta b lis h in g a S h ip b o a r d B a s e lin e P rogram A preventive maintenance baseline program may be established aboard ship utilizing vibration analyzer trend information to determine the condition of machinery. Eight steps are required to set up the program. 1. Select the shipboard machinery to be included in the program. This selection must be made on the engineering officer's judgment on the importance of the machine to the operation of the vessel and the avoidance of costly repairs. Certainly all the turbines, pumps, fans, and compressors on the main and auxiliary machinery lists would be candidates for a baseline program. INTERPRETING RESULTS OF VIBRATION MEASUREMENTS 25-21 2. Establish and record on appropriate forms the individual measurement points on each machine. Typically, the points will include horizontal, vertical, and axial measurement locations on each ofthe machine's bearings. 3. Select the type of measurements to be taken at each of the points. For typical machines, the measurements should include amplitude (velocity in/sec) versus frequency in a tabulation form or a plot if the plotter accessory is available. It is also desirable to record the overall vibration level by switching offthe analyzer's tunable filter. Figure 25-17 is a comparison ofvibration signatures for the same pump bearing using three different measurements -displacement (mils), velocity (mils/sec), and acceleration (in/sec/sec)-at identical operating conditions. 4. Select the interval at which measurements will be recorded. Aboard ship, a biweekly schedule of overall vibration measurement will be adequate. The vibration signature should be recorded at quarterly intervals or when an overall noise measurement suggests a developing problem. 5. Establish and record the standard operating conditions at which vibration levels will be measured. It is essential that the machine be in the same generating mode for each measurement. Depending on the machine, speed, pressure, temperature, flow, and load should be considered. 6. Make sure the machine's foundation, piping, and associated piping are not changed during the intervals between measurements. Changes in these structural elements may introduce variations in the vibration characteristics. Ifrepairs or modifications result in structural changes, it is necessary to establish new baseline data. 7. Compare the original baseline data for each machine with the manufacturer's vibration data or the industry standards for vibration severity to ensure that the machine is in good condition at the start of the monitoring program. 8. Maintain an effective record keeping system. This is essential to the success of a baseline monitoring program. With forms and plotter sheets available from instrument manufacturers, individual files can be kept for each machine. IN T E R P R E T IN G RESULTS O F V IB R A T IO N M EASUREM ENTS One ofthe reasons that vibration analysis has proved to be such a powerful tool is the fact that small changes in a machine's mechanical condition produce significant changes in the machine's vibration level. Overall vibration level increases on the order of a factor of ten are not uncommon as a machine changes from smooth to very rough running. Even the smoothest running machine has characteristic vibrations which can be measured. These are caused by minute variations in the dimensions that are inherent in the machine's manufacture. In fact, the machine's design, and particularly its tolerances (i.e., the allowable variations in its dimensions), determine the absolute at which the machine will charac- INTERPRETING RESULTS OF VIBRATION MEASUREMENTS 25-23 teristically vibrate. When the amplitude of these vibrations increases, it is an indication that the machine has developed a defect. Overall vibration is particularly valuable in vibration analysis for a number of reasons. First, its measurement produces a single number that includes all the different defects which may be present in a machine (e.g., unbalance, misalignment, bad bearings, etc.), thereby providing a total assessment of the general mechanical condition of the machine. Second, for record keeping purposes overall amplitude (vibration) provides a very compact data record. Third, because it requires only a few seconds to make a measurement, it is a very rapid method of analyzing a machine. Fourth, when the overall amplitude is compared with the sum of the individual frequency peaks which have been analyzed, it provides a check as to whether or not all the individual frequency peaks have been measured. It is also a remarkable fact that the vast majority of rotating machines generally have similar overall vibration levels which indicate their mechanical condition. For example, an overall level of .1 inlsec-pk (peak) generally indicates a smooth operating machine with no defects, while a level of .6 inlsec-pk indicates a machine with severe defects. Of course, there are many exceptions, but these levels do provide a valuable guideline. No less important than a specific measurement of the overall vibration level is the pattern-or trend-indicated by a series of such measurements over a period of time. It is almost invariably true that if a vibration level of a machine remains essentially constant over a period of time (e.g., six months or one year), the machine is in good mechanical condition. In contrast, a machine with developing defects will show a rising trend in overall vibration. The use of the trend is valuable because it lessens the dependence on the absolute vibration level as an indication of mechanical condition. F r e q u e n c y A n a ly s is Frequency analysis is the method utilized to pinpoint the source of a vibration and thereby detect the defective component within a machine. This is accomplished by relating the measured discrete frequencies to the characteristic frequencies of the machine. It is possible to calculate most of the characteristic frequencies. For example, a 5-vane impeller centrifugal pump which is rotating at 1,800 RPM (revolutions per minute) will have a vibration at 1,800 CPM that is indicative of imbalance, misalignment, or a bent shaft. A vibration at twice rotational speed, 3,600 CPM, is characteristic of looseness in the pump, while vibration at five times rotational speed, 9,000 CPM, is caused by the vanes of the pump impeller passing the pump cutwater and creating a hydraulic pulsation. In similar manner gear tooth vibrations can be calculated by counting the number ofteeth and multiplying by the rotational speed. 25-24 SHIPBOARD VIBRATION ANALYSIS Some frequencies which appear in a vibration signature cannot be readily calculated and are identifiable on the basis of experience. Typically, ball bearings have vibrations in the 10,000 to 60,000 CPM range and also at very high ultrasonic frequencies. When in its incipient stage, vibration caused by cavitation can appear at high frequencies (i.e., above 60,000 CPM) and will tend to produce a broad peak of frequencies, rather than a single peak. As vibration caused by cavitation becomes more severe, it tends to shift down to lower frequencies. Natural frequencies of the machine and its supporting structure can also appear in a machine's vibration signature. These can sometimes be identified by striking the structure while the machine is shut down. For this to be successful the structure must be relatively undamped. If the natural frequency is pronounced, it will be momentarily registered on the analyzer frequency meter. Machine critical speeds can be identified in many cases by a similar technique. As the machine coasts down, the amplitude of vibration will momentarily increase (when it passes through the critical speed) and the meter will register the frequency ofthe critical speed. An excellent way of separating and identifying natural frequencies, criticals, and various types of shaft whirling vibrations is by changing machine speed. Those vibrations such as unbalance, etc., which are related to machine speed will change in frequency when the machine speed changes. However, natural frequencies, criticals, and shaft whirl will tend to remain at the same frequencies, although their amplitudes may change as the forces which are exciting them change. Of course, there are a few exceptions to this rule. In certain types of aerodynamic and hydrodynamic pulsations which involve resonance, there may be a small change in frequency. Most frequencies observed tend to have individual, discrete peaks. These are the vibrations caused by unbalance, looseness, misalignment, etc. Some vibrations, however, cover a broad range of frequencies. Cavitation-caused vibration, which was previously mentioned, is one of these. Steam, liquid, and gas flow through pipes, valves, and ducts also produce a broad band of frequencies. In fact, most of the broad band vibrations measured can be related to some type of flow mechanism. On occasion, these broad band vibrations will excite the natural frequencies of the piping and other structures with which they come in contact. When this occurs these vibrations tend to take on a discrete frequency appearance, rather than a broad band. If the amplitudes ofthese peaks are observed, you will note that they tend to vary in a random manner. This provides a clue to the source ofthe vibration since vibrations caused by unbalance, misalignment, impeller vane passing, etc., tend to have fairly steady amplitudes. Impulse-type vibrations occur in reciprocating machines, or when one component in a machine strikes another. These vibrations are a harmonic series of discrete frequencies which occur as multiples of the impulse INTERPRETING RESULTS OF VIBRATION MEASUREMENTS 25-25 repetition frequency. Generally, the fundamental impulse repetition frequency has the highest amplitude, with the higher harmonics decreasing in amplitude. In some cases, however, one of the higher harmonics may coincide with a natural frequency of the structure and consequently have a larger amplitude than adjacent peaks. As many as seven or more harmonics can sometimes be measured. Beat frequency vibration occurs when two vibrations are present in a machine at almost the same frequency. In most cases the analyzer filter bandwidth is not sufficiently narrow to separate these two vibrations into their individual peaks. Instead, the vibration signal appears to be a single vibration whose amplitude is changing in a periodic or random manner. Two machines running at almost the same speed on a common foundation are often the cause of such a beat frequency. A beat frequency in itself is not usually an indication of any mechanical defects, but it may be mistaken for an amplitude modulation vibration which can be an indicator of mechanical problems. Amplitude modulation vibration occurs when the amplitude of a relatively high frequency vibration is caused to vary in a periodic manner. For example, when a 24-tooth gear turning at 1,800 RPM has some eccentricity in the gear wheel, the amplitude of the tooth contact vibration (43,200 CPM) will vary at the rotational frequency 0,800 CPM) of the machine. P h a s e A n g le A n a ly s is Phase can be very useful in diagnosing specific machinery defects. For example, the vibration measurement of a motor-driven overhung fan may reveal a relatively high vibration amplitude at rotational frequency in the axial direction. This indicates that a mechanical defect is present which could be anyone of the following: unbalance of the fan rotor, coupling misalignment, bearing misalignment, or a bent shaft. It is phase analysis which aids in determining which of these is the actual problem. To diagnose the problem, phase measurements are made in the axial direction at four or more equally spaced positions around the shaft on each bearing housing, as shown in Figure 25-18. Iflarge phase differences (i.e., on the order of 180') are seen for different positions on bearing housing, it indicates that a severely bent shaft (small shaft bow may not cause a phase difference), a cocked ball, or roller bearing is the cause. Should the phase, however, be approximately the same (i.e., on the order of 0') at each position around the shaft at all bearing housings, a further phase comparison must be made among all the bearing housings to pinpoint the problem. Ifthe phase at all the bearing housings is the same (i.e., approximately 0'), it is an indication of unbalance or possibly an axial resonance of the foundation. The latter can be ascertained by tests for resonance. If, however, the two motor bearings have a large phase difference between CHARACTERISTICS C H A R A C T E R IS T IC S OF SPECIFIC MACHINERY DEFECTS O F S P E C IF IC M A C H IN E R Y 25-27 DEFECTS Some of the more common machinery problems and their associated characteristics of amplitude, frequency, and phase are described in the following paragraphs: M e c h a n ic a l U n b a la n c e This is the most common cause of vibration. Mechanical unbalance can be recognized by the fact that the vibration occurs at rotational frequency and the amplitude in the radial direction is more than twice the axial amplitude. It can often be separated from electrical defects by cutting the power and noting whether the amplitude drops off gradually or abruptly. A gradual drop indicates mechanical unbalance; an abrupt drop indicates electrical unbalance. The strobe phase measurement shows a single steady reference mark. M is a lig n m e n t (S h a fts , C o u p lin g s , B e a r in g s ) This is the second largest cause of vibration even when self-aligning bearings and flexible couplings are used. Vibration occurs at shaft rotational frequency and can also occur at two and three times the rotational frequency. Amplitude in the axial direction is 50 percent or more of the radial direction. Phase shows a single reference mark. Phase readings in the axial direction taken around the shaft on the bearing housing are approximately constant. Misaligned bearings, as well as shafts misaligned at couplings, can cause these vibration characteristics. B e n t S h a ft Figure 25-18. Measurement locations for vibration phase on bearing housing using strobe light and vibration analyzer them, it indicates a bent shaft or severely misaligned bearings in the motor. Furthermore, if the two motor bearings are in phase with each other and the two fan bearings are in phase with each other, but there is a large phase difference between the motor and the fan, then the coupling is most probably misaligned. Note: I n recording phase measurements, care must be exercised to subtract 180" from the measured phase at each position where the transducer direction for measurement is opposite from the first position measured. This problem may show the same vibration characteristics as misalignment. The phase, however, when measured axially around the shaft will vary through approximately 180", particularly when the bend is close to the bearing. Misaligned ball and roller bearings also show these characteristics. A further indication of a bent shaft is an approximate 180" phase difference between axial vibration at opposite ends ofthe machine. D e fe c tiv e B a ll a n d R o lle r B e a r in g s This vibration is characterized by a number of high frequency vibration peaks, plus frequency regions of fluctuating vibration levels. The lower frequencies are usually not multiples of shaft rotation frequency but can be roughly calculated from bearing dimensions and shaft speed. The high frequencies emitted by the bearings cannot be readily calculated. Phase is unsteady. 25-28 SHIPBOARD VIBRATION D e fe c tiv e MARINE VIBRATION CASE HISTORIES ANALYSIS S u m m ary of M easu rem en t G e a r T e e th Vibration occurs at shaft rotational frequency times the number of gear teeth. Side band frequencies may also occur at frequencies equal to the tooth contact frequency plus and minus the gear rotational speed. Phase shows multiple reference marks and is unsteady. L oosen ess (S le e v e B e a r in g s , B e a r in g F o u n d a tio n s , Vibration occurs at twice shaft rotational frequency. Phase shows two reference marks and is usually somewhat unsteady. Change in balance and alignment generally affects this vibration. B a d D r iv e B e lts Vibration occurs at three and four times, as well as twice, the rotational frequency of the belts. Frequencies and amplitudes are somewhat unsteady because of belt slippage. Phase shows one or two reference marks which are also unsteady. P u ls a tio n Vibration occurs at shaft rotational frequency times the number of blades or impeller vanes. Amplitude is generally low, except when amplified by resonance. R u b b in g Vibration can occur at many frequencies, each with fluctuating amplitudes (i.e., random vibration) simultaneously, when rub is continuous. It may also occur at one and two times shaft rotational frequency when rub is intermittent. Phase is unsteady. R eson an ce Vibration occurs at a single "natural" frequency of the part which is vibrating. This is a frequency at which the part "likes" to vibrate. For example, a resonance occurs when the shaft RPM reaches the shaft critical frequency. To check for resonance, let the machine coast down from operating speed. Watch the analyzer amplitude and frequency meter. If the amplitude drops off at first, then rises, and then drops off again, the frequency at which the rise occurs is a natural frequency. A resonance will occur if the machine is operated at a speed equal to the natural frequency. Phase will shift by a significant amount, as much as 180 as machine speed moves through a resonant frequency. 0 , V IB R A T IO N CASE H IS T O R IE S P e d e s ta ls , E t c .) A e r o d y n a m ic lH y d r a u lic I n te r p r e ta tio n Figure 25-19 is a vibration identification chart which shows the amplitude, frequency, and phase characteristics of the most common machinery defects. M A R IN E M e c h a n ic a l 25-29 Some case histories of shipboard vibration analysis will suggest the range of application and the value of vibration monitoring. Severe vibration of a fan was noted aboard a ship. The engineering officers believed that severe imbalance was the result of bad bearings and recommended removal of the fan for bearing replacement. However, vibration analysis indicated that it was not bad bearings but vibration caused by the fan running in a partially stalled condition due to a dirty intake filter. Cleaning ofthe intake filter eliminated the vibration problem. In another case, apparent severe vibrations of a fan were assumed to be caused by unbalance in the fan rotating parts. The shipboard engineers recommended removal of the fan for balancing. Vibration analysis showed that it was not unbalance but a resonance in the fan mounts. The vibration was eliminated by adjustment ofthe mounts. Following the installation of a new bearing in a feed pump motor, ship's engineers conducted a vibration analysis of the pump to determine that the repair was satisfactory. The analysis showed that the repair was not satisfactory since severe vibrations existed. The new bearing was removed and a metallic chip was found under the bearing, causing improper seating of the bearing. Vibration was eliminated with the correct seating of the bearing. Vibration analysis was conducted on two diesel generators. One of the generators had significantly increased vibration levels over a period of several months. The engine was secured and dismantled for inspection. The flexible coupling between the engine and the generator was worn and a complete overhaul of the coupling was required. A water pump had a defective upper antifriction bearing which caused high vibration levels in the frequency range of 60,000 CPM. An adjacent pump was found to have an overall vibration level higher than the pump with the defective bearing. The adjacent pump was found to have a locked-up flexible coupling which caused the high axial vibration at twice the pump fundamental frequency in the 63 CPS (cycles per second) band. Correction of vibration level was achieved by properly fitting the flexible coupling. Figure 25-20 shows vibration signature analysis comparison of a machine with and without defective drive belts. This signature, taken at the CHAPTER 26 In e r t G a s S y ste m s a n d C r u d e O il W a s h in g M a c h in e r y EVERETT c . HUNT AND JAMES MERCANTI IN T R O D U C T IO N AND BACKGROUND NERTgas is a gas, or a mixture of gases, such as flue gas, containing oxygen to support the combustion of hydrocarbons. It can be obtained from a ship's main or auxiliary boiler, from the exhaust of an internal combustion engine, from a separately fired generator, or from independent storage tanks. An inert gas system consists ofthe means of distributing inert gas from a specific source to cargo tanks. It includes all the equipment specially fitted to supply, cool, clean, pressurize, monitor, and control delivery of inert gas to the cargo-tank system. It also includes the necessary piping, valves, and associated fittings to properly distribute the inert gas to the cargo tanks. The primary purpose of an inert gas system is to provide protection against a tank explosion, achieved by introducing inert gas into the tank to bring the oxygen content far below the flammable range. Furthermore, by allowing the atmosphere in the tank to be expelled and replaced by inert gas, the hydrocarbon gases as well as the oxygen are reduced to safe levels. In addition to its primary purpose, the inert gas system is often used as a common vent for venting gases from the cargo tanks to the atmosphere, and also as a common manifold for protection of cargo tanks against excessive pressure or vacuum. Inert gas systems have been used on oil tankers for over fifty-five years. Between 1925 and 1927, the Standard Oil Company of California installed a combination flue gas, tank-cleaning, and venting system on twelve of their seagoing tankers. An application for a patent covering the system was filed in 1928, and a patent was granted in 1932. In 1932, after a serious casualty during gas-freeing operations, Sun Oil Company installed an inert gas system on all of its oceangoing tankers. Although over thirty Iinsufficient 26-1 26-2 INERT GAS SYSTEMS AND CRUDE OIL WASHING MACHINERY companies installed the system during that period, the use of flue gas to protect cargo tanks never became widespread until the advent ofthe very large crude carriers (VLCCs). In 1969, a series of explosions aboard the VLCCs M a c tr a , M a r p e s s a , and K in g H a a k o n V I I caused grave concern about the safety of such ships. The common factor in all of the casualties was that at the time the explosions occurred, tank-cleaning operations were being carried out. A formal board of inquiry investigating the M a c tr a explosion recommended that VLCCs, including ships with cargo tanks exceeding 10,000 cubic meters in capacity, be fitted with an inert gas system. In 1972, the Intergovernmental Maritime Consultative Organization (lMCO, now IMO) advocated the use of inert gas systems on all new tankers, and in 1973, IMCO's resolution was adopted by the international marine community, requiring inert gas systems on all new tankers of 100,000 DWT and over, and on combination carriers of50,000 DWT and over. During the winter of 1976-77, several tanker casualties occurred in or near United States waters which caused great public concern about the risks associated with the transportation of oil by tankers, and resulted in demands for the United States government to take steps toward the improvement of tanker safety and the prevention of pollution. In 1978, the Port and Tanker Safety Act of 1978 (PTSA) became law, and mandated that inert gas systems be included on certain new vessels and retrofitted on certain existing tankers. In 1979, the U.S. Coast Guard issued its final rules concerning the requirements for installation of inert gas systems. Existing crude carriers over 20,000 DWT must be fitted with inert gas systems by June 1, 1983, in order to enter a United States port. Crude carriers and product carriers between 20,000 DWT and 40,000 DWT not fitted with high capacity tank-washing machines may be granted an exemption by the office of merchant marine safety, if the owner can show that compliance would be unreasonable and impractical within the vessel's design characteristics. Existing crude carriers and product carriers above 70,000 DWT were required to have inert gas systems installed by June 1, 1981. The law provides that any vessel in noncompliance can be barred from United States waters. Furthermore, civil and criminal penalties can be imposed by the Coast Guard against the owner, operator, or any individual crew member for violating certain aspects ofthis law. In the early 1980s, there has been a recurrence of major explosions aboard tankers. Of particular concern is the fact that a number of these casualties has occurred on ships fitted with inert gas systems. One extensive investigation by an independent regulatory association revealed that an alarming percentage of inert gas system equipped ships had systems inoperative because of various design and material problems. Simultaneously, the improper use of the system may jeopardize its effectiveness, while at the same time bring a false sense of security to those PRINCIPLES OF INERT GAS SYSTEMS 26-3 involved. Should the inert gas system fail, for whatever reason, or be used improperly, problems could arise that would not occur in its absence. Therefore, it is imperative that the system be designed, installed, and operated properly. P R IN C IP L E S O F IN E R T GAS SYSTEM S An explosion within a cargo tank can only occur when these three components are all present at the same time: hydrocarbon gas, oxygen, and a source of ignition. A mixture of hydrocarbon gas and oxygen can only ignite if its composition falls within a certain range, known as the "flammable range." The hydrocarbon gas and the oxygen contents are expressed in percentages by volume. Although flammable limits vary for different types of hydrocarbon gases, they are generally assumed to fall within a range of 1 to 10 percent for oil cargoes carried in tankers. The flammable limits for oxygen fall within approximately 11 and 21 percent (the 21 percent representing fresh air). Possible sources of ignition are smoking, welding, metal-to-metal sparking, spontaneous combustion, static electricity, and lightning. As shown in Figure 26-1, any point on the flammability triangle diagram represents a hydrocarbon gas-air-inert gas mixture, specified in terms of its hyrocarbon and oxygen content. Hydrocarbon-air mixtures 26-4 INERT GAS SYSTEMS AND CRUDE OIL WASHING MACHINERY without inert gas are on the line AB, the slope of which shows the reduction in oxygen content as the hydrocarbon content increases. Points to the left of line AB represent mixtures with their oxygen content further reduced by the addition of inert gas. It is evident from the diagram that as inert gas is added to hydrocarbon-air mixtures, the flammable range progressively decreases until the oxygen content reaches a level generally taken to be about 11 percent by volume, at which level no mixture can burn. Therefore, an inert gas system, with a design point of 5 percent and maximum allowable operating point of 8 percent oxygen, allows a good margin of safety. The lower and upper flammable limit mixtures for hydrocarbon gas in air are represented by points C and D in the diagram. As the inert gas content increases, the flammable-limit mixtures change. This is indicated by lines CE and DE, which converge at point E. Only those mixtures represented by points in the shaded area within the loop CED are capable of burning. Changes of composition, due to the addition of either air or inert gas, are represented by movements along straight lines. These lines are directed either towards point A, representing pure air, or towards a point on the oxygen content axis corresponding to the composition ofthe added inert gas. Such lines are shown for the gas mixture represented by point F. When an inert mixture, such as that represented by point F, is diluted by air, its composition moves along line FA and therefore enters the shaded area of flammable mixtures. This means that all inert mixtures in the region above line GA, the "critical dilution" line, pass through a flammable condition as they are mixed with air, for example, during a gas-freeing operation. Those below line GA, represented by point H, do not become flammable upon dilution. It is possible to move from a mixture, as represented by point F, to one represented by point H, by dilution with additional inert gas, i.e., purging. M e th o d s o f Gas R e p la c e m e n t In order to properly understand the inerting process, the various operations which involve replacement of gas in cargo tanks-namely, i n e r t i n g , p u r g i n g , and g a s - f r e e i n g - n e e d to be discussed. In each of these replacement operations, one of two processes can predominate: d i l u t i o n , which is a mixing process; and d i s p l a c e m e n t , which is a layering process. These two processes have a marked effect on the method of monitoring the tank atmosphere and the interpretation of the results. Figures 26-2 and 26-3 show that an understanding ofthe nature of the gas replacement process actually taking place within the tank is necessary for the correct interpretation of the reading shown on the appropriate gas-sampling instrument. Figure 26-2. Dilution concept The dilution theory illustrated in Figure 26-2 assumes that the incoming gas mixes with the original gases to form a homogeneous mixture throughout the tank. The result is that the concentration of the original gas decreases exponentially. In practice the actual rate of gas replacement depends upon the volume flow of the incoming gas, its entry velocity, and the dimensions of the tank. For complete gas replacement it is important that the entry velocity of the incoming gas be high enough for the jet to reach the bottom ofthe tank. It is therefore important to confirm the ability of every installation using this principle to achieve the required degree of gas replacement throughout the tank. D ilu tio n . Displacement. The displacement theory, illustrated in Figure 26-3, is based on expelling the heavier gases only. Ideal replacement requires a stable horizontal interface between the lighter gas entering at the top of the tank and the heavier gas being displaced from the bottom of the tank through some suitable piping arrangement. This method requires a relatively low entry velocity of gas and in practice more than one volume change is necessary. It is therefore important to confirm the ability of every installation using this principle to achieve the required degree of gas replacement throughout the tank. Figure 26-3 shows an inlet and outlet configuration for the displacement process, and indicates the interface between the incoming and outgoing gases. Also shown are typical curves of gas concentration against time for three different sampling levels within the cargo tank. TYPES OF INERT GAS SYSTEMS 26- 7 General Policy of Cargo Tank Atmosphere Control Tankers fitted with an inert gas system must have their cargo tanks kept in a nonflammable condition at all times. Therefore, tanks should be kept in the inert condition whenever they contain cargo residues or ballast. The oxygen content should be kept at 8 percent or less by volume with a positive gas pressure in all the cargo tanks. The atmosphere within the tank should make the transition from the inert condition to the gas-free condition without passing through the flammable condition. In practice this means that before any tank is gas freed, it should be purged with inert gas until the hydrocarbon content of the tank atmosphere is below the critical dilution line as shown in Figure 26-l. When a ship is in a gas-free condition before arrival at a loading port, tanks should be inerted prior to loading. In order to maintain cargo tanks in a nonflammable condition, the inert gas plant will be required to inert empty cargo tanks; to be operated during cargo discharge, deballasting, and necessary in-tank operations; to purge tanks prior to gas freeing; and to top up pressure in the cargo tanks when necessary, during other stages of the voyage. TYPES OF INERT GAS SYSTEMS Flue Gas System Most marine inert gas systems utilize flue gases from the main or auxiliary boilers. A typical composition of such flue gas, based on burning bunker C fuel, is (by volume): Oxygen (02) Carbon dioxide (C02) Sulfur dioxide (S02) Nitrogen (N2) Water vapor (H20) Solids (soot) 4.2% 13.5% 0.25% 77.0% 5.0% 3 250 mg/m Before being distributed to the cargo tanks, this gas must first be cooled and cleaned. This is normally accomplished by the scrubber which, if properly designed and operated, will produce a gas free of corrosive components and solid particles. The cooling effect, using seawater for scrubbing, should yield a gas temperature that is within 5°C above the seawater temperature. Independent Generator If there are no main or auxiliary boilers, or if there is a need for special quality inert gas, such as is required on LNG tankers, a separately fired 2 6 -8 INERT GAS SYSTEMS AND CRUDE OIL WASHING MACHINERY inert gas generator can be used. This system generates its own gas through independent combustion. The fuel most often used is low sulfur diesel oil in order to obtain cleaner combustion and less corrosion. Some independent generator systems are able to burn heavier fuels such as bunker C, but with sacrifice to gas quality. E x h a u st G a s G e n e r a to r Although little has been done to develop the potential of this concept, there are a few installations aboard tankers utilizing a gas turbine driving an electric generator. Because the exhaust gases are normally too high in oxygen content, an afterburner is used to reduce the oxygen level to 5 percent by volume. Diesel engines have been used in similar fashion, using a separate afterburner section to burn off excess oxygen. A serious problem is the difficulty in maintaining constant load on the prime mover to ensure stable combustion. Because of its questionable suitability for this application, no further reference is made to this system. In d ep en d en t S to r a g e T a n k s Where small quantities of inert gas are required, such as in small size LPG tankers, storage tanks of an inert gas, such as nitrogen, are installed aboard ship, and used to fill the void areas surrounding the tanks with inert gas. Although extremely simple in concept, the major disadvantage of this system is limited capacity. Furthermore, refilling facilities are not easily available on a worldwide basis. D E S IG N O F IN E R T G AS SYSTEM S This section deals generally with inert flue gas systems. The design of systems other than inert flue gas systems should take into account, whenever applicable, the general design principles outlined in this section. G e n e r a l D e s c r ip tio n A typical arrangement for an inert flue gas system is shown in Figure 26-4. It consists of flue gas isolating valves located at the boiler uptake through which pass hot, dirty gases to the scrubber. Here the gas is cooled and cleaned before being piped to the blowers which deliver the gas through the deck water seal, the nonreturn valve, and the deck-isolating valve, thence to the cargo tanks. A gas pressure regulating valve is fitted downstream of the blowers to regulate the flow of gases to the cargo tank. A liquid-filled pressure-vacuum breaker is fitted to prevent excessive pres- 26-10 INERT GAS SYSTEMS AND CRUDE OIL WASHING MACHINERY sure or vacuum from causing structural damage to the cargo tanks. A vent is sometimes fitted between the deck water seal and the gas pressure regulating valve to vent any leakage when the plant is shut down. For delivering inert gas to the cargo tanks during cargo discharge, deballasting, tank cleaning, and for topping up the pressure of gas in the tanks during other phases of the voyage, an inert gas deck main runs forward from the deck-isolating valve for the length of the cargo deck. From this inert gas main, inert gas branch lines lead to the top of each cargo tank, or to each cargo tank hatch. Scrubber Function. The purpose of the scrubber is to cool the flue gas and remove most of the sulphur dioxide and particulate soot. This is achieved by direct contact between the flue gas and large quantities of seawater. Before entering the bottom of the scrubbing tower, the gas is cooled by being passed through a water spray. Within the scrubbing tower, the gas moves upwards through downward-flowing water. For maximum contact between gas and water, several layers made up of one or more of the following arrangements may be fitted: spray nozzles; trays of "packed" stones or plastic shapes; perforated "impingement" plates; or venturi nozzles and slots. At the top, or downstream, of the scrubbing tower, water droplets are removed by a demister which may be a polypropylene pad or a cyclone separator. Figure 26-5 illustrates the principles of inert gas scrubber operation. Design considerations. The scrubber should be capable of cooling the flue gas to within 9'F (5'C) of the cooling water. The performance of the scrubber at all gas flows should be such as to remove at least 90 percent of sulphur dioxide and to remove solids effectively at the designed pressure differential ofthe system. In product carriers more stringent requirements may be needed for product quality. The internal parts of the scrubber should be constructed in corrosion-resistant materials because of the corrosive effect of the gas. Alternatively, the internal parts may be lined with rubber, fiberglass epoxy resin, or other equivalent material, in which case the flue gases may require cooling before they are introduced into the lined sections of the scrubber. Adequate openings and sight glasses should be provided in the scrubber shell for inspection and cleaning. The sight glasses should be reinforced to withstand impact and be suitable for the temperatures encountered. The design should be such that under normal conditions oftrim and list the scrubber efficiency will not drop by more than 3 percent; and the temperature rise at the gas outlet not exceed the designed gas outlet Figure 26-5. Gas scrubber section drawing temperature by more than 5'F (3'C). The location of the scrubber above the load waterline should be such that the drainage of the effiuent is not impaired when the ship is in the fully loaded condition. Inert Gas Blowers Function blowers are used to deliver the scrubbed flue gas to the cargo tanks. Regulations require that at least two blowers be provided which together will be capable of delivering inert gas to the cargo tanks at a rate of at least 125 percent of the maximum rate of cargo discharge capacity of the ship expressed as a volume. In practice, installations vary from those which have one large blower and one small blower, whose combined total capacity meets the regulations, to those in which each blower can meet this requirement. The advantage claimed for the former is that it is convenient to use a small DESIGN OF INERT GAS SYSTEMS Figure 26-6. Typical gas scrubber capacity blower when topping up the gas pressure in the cargo tanks at sea. The advantage claimed for the latter is that if either blower is defective, the other one is capable of maintaining a positive gas pressure in the cargo tanks without extending the duration of the cargo discharge. c o n s i d e r a t i o n s . The blower casing should be constructed of corrosion resistant material or alternatively of mild steel lined with rubber, fiberglass epoxy resin, or other equivalent material to protect it from the corrosive effect of the gas. The impellers should be manufactured in a corrosion-resistant material having high tensile strength. Inconel impellers are excellent for this service. Aluminum-bronze impellers should be tested by overspeeding to 20 percent above the design running speed of 26-13 the electric motor or 10 percent above the speed at which the overspeed trip of the turbine would operate, whichever is applicable. Large-sized drains, fitted with adequate water seals, should be provided for each casing to prevent damage by an accumulation of water and soot. Means should be provided for freshwater washing of the impellers to remove buildup of deposits which would cause vibration during blower operation. The casing should be adequately ribbed to prevent panting and should be so designed and arranged as to facilitate the removal ofthe rotor without disturbing major parts of the inlet and outlet gas piping. Sufficient openings in the casing should be provided to permit inspection. Where separate shafts are provided for the prime mover and the blower, a flexible coupling with protective cover between the shafts should be installed. When roller or ball bearings are used, proper attention should be paid to the problem ofbrinelling and the method oflubrication. The type oflubrication chosen, i.e., oil or grease, should take into account the diameter and rotational speed ofthe shaft. If sleeve bearings are fitted, resilient mountings are not recommended. The blower performance characteristics should be matched to the maximum system requirements. The characteristics should be such that in the event of the discharge of any combination of cargo tanks at the maximum discharge rate, a minimum pressure of 200 millimeters water gauge is maintained in any cargo tank after allowing for pressure losses due to any of the following: the scrubber tower and demister; the hot gas piping from the stack to the scrubbing tower; the cold gas distribution piping downstream of the scrubber; the deck water seal; all valves and fittings; and the length and diameter ofthe inert gas distribution system. When the blowers are not of equal capacity, the pressure-volume characteristics and inlet and outlet piping should be so matched that if both blowers can be run in parallel, they are able to develop their designed outputs. The arrangements should be such as to prevent the blower on load from motorizing the blower that is stopped or tripped out. Figure 26-7 is a sectional view ofthe blower. Figure 26-8 is a photograph ofa typical blower. The prime mover should be of sufficient power to ensure that it will not be overloaded under all possible operating conditions of the blower. The overload power requirement should be based on the blower inlet conditions of 23°F (-5°C) at -400 millimeters water gauge, outlet conditions of 32°F (O°C),and atmospheric pressure. D e s ig n N o n r e tu r n D e v ic e s F u n c t i o n . The deck water seal and mechanical nonreturn valve together form the means of automatically preventing the backflow of cargo gases from the cargo tanks to the machinery space or other safe area in which the inert plant is located. DESIGN OF INERT GAS SYSTEMS 26-15 deck main but prevents any backflow of cargo gas even when the inert gas plant is shut down. It is vital that a supply of water is maintained at the seal at all times, particularly when the inert gas plant is shut down. In addition, drains should be led through a vented loop-seal directly overboard, and should not pass through the machinery spaces. There are different designs but one ofthree principal types of deck water seals may b e adopted. The three types are the following: 1. Wet type, which is the simplest type of water seal. When the inert gas plant is operating, the gas bubbles go through the water from the submerged inert gas inlet pipe, but if the tank pressure exceeds the pressure in the inert gas inlet line, the water is pressed up into this inlet pipe and thus prevents backflow. The drawback of this type of water seal is that water droplets may be carried over with the inert gas which, although it does not impair the quality of the inert gas, could increase corrosion. Therefore, a demister is usually fitted in the gas outlet from the water seal to reduce carryover. Figure 26-9 shows an example ofthis type. Figure 26-9. Wet type of deck water seal Figure 26-8. Typical gas fan assembly Deck water seal. This is the primary barrier between the safe area and the hazardous area. For safety, it is the most important device in the inert gas system. A water seal is fitted which permits inert gas to be delivered to the 2. Semidry type, in which instead of bubbling through the water trap, the inert gas flow draws the sealing water into a separate holding chamber by venturi action, thus avoiding or at least reducing the amount of water droplets being carried over. Otherwise, it is functionally the same as the wet type. Figure 26-10 shows an example of this type. 3. The dry type, in which the water is drained when the inert gas plant is in operation (gas flowing to the tanks) and filled with water when the inert gas plant is either shut down or the tank pressure exceeds the inert gas blower discharge pressure. Filling and drainage are performed by automatically operated pneumatic valves controlled by the levels in the water seal and drop tank, and by the operating state of the blowers. The advantage of this type is that water carryover is prevented. The drawback could be the risk offailure 26-16 INERT GAS SYSTEMS AND CRUDE OIL WASHING MACHINERY of the automatically controlled valves which may render the water seal ineffective. Figure 26-11 shows an example of this type. DESIGN OF INERT GAS SYSTEMS 26-17 nonreturn devices. The separate deck isolation valve has the advantage of enabling maintenance work to be carried out on the nonreturn valve. Design considerations. The material used in the construction of the nonreturn devices should be resistant to fire and to the corrosive attack from acids formed by the gas. Alternatively, low carbon steel protected by a rubber lining, fiberglass epoxy resin, or equivalent material may be used. Particular attention should be paid to the gas inlet pipe to the water seal, which is most susceptible to corrosion, because it is constantly immersed in salt water. The deck water seal should present a resistance to backflow of not less than the pressure setting of the pressure-vacuum breaker on the inert gas distribution system and should be so designed as to prevent the backflow of gases under any foreseeable operating conditions. The water in the deck seal should be maintained by a regulating flow of clean seawater through the deck seal reservoir. Sight glasses and inspection openings should be provided on the deck seal to permit satisfactory observation of the water level. The sight glasses should be reinforced to withstand impact. P ip in g a n d Valves Function. The inert gas distribution system, together with the cargo tank venting system, where applicable, must provide the means of delivering inert gas to the cargo tanks during discharge, deballasting, and tank-cleaning operations, and for topping up the pressure of gas in the tanks; venting tank gases to atmosphere during cargo loading and ballasting, including an additional inlet or outlet points for inerting, purging, and gas freeing; isolating individual tanks from the inert gas main for gas freeing; and protecting tanks from excessive pressure and vacuum. Figure 26-11. Dry type of deck water seal Nonreturn and isolation valves. As an additional precaution to avoid any backflow of gas from the cargo tanks, and to prevent any backflow ofliquid which may enter the inert gas main if the cargo tanks are overfilled, regulations require a mechanical nonreturn valve, or equivalent, which should operate automatically at all times. This valve should be provided with a positive means of closure or, alternatively, a separate deck isolation valve fitted forward of the nonreturn valve, so that the inert gas deck main may be isolated from the Design considerations. The flue gas uptake point should be selected so that the gas is not too hot for the scrubber, or that it causes hard deposits on the flue gas isolating valves. It should not be too close to the uptake outlet so that air is drawn into the system. When boilers are fitted with rotary air heaters, the offtake point should be before the air heater inlet. The materials used for flue gas isolating valves should take into account the temperature of gas at the offtake. Cast iron is questionable even for temperatures below 428"F (220"C). Valves exposed to a temperature exceeding 220"C should be made from a material not only compatible with the temperature but also resistant to the corrosive effect of stagnant flue gases laden with sulfuric and sulfurous acids. Flue gas isolating valves should be provided with facilities to keep the seatings clear of soot unless the valve is designed to close with a seat-cleaning action. Flue gas isolating valves should also be provided with air-sealing arrangements, usually fed from the forced draft blowers. If expansion 26-18 INERT GAS SYSTEMS AND CRUDE OIL WASHING MACHINERY bellows are considered necessary, they should have a smooth internal sleeve and preferably be mounted so that the gas flow through them is vertical. They should be constructed of material resistant to sulfuric and sulfurous acids. The pipework between the flue gas isolating valve and the scrubber should be made from Corten steel or heavy gauge steel resistant to corrosion and so arranged to prevent the accumulation of damp acidic soot by the avoidance of unnecessary bends and branches. The inlet piping to the scrubber should be arranged to permit positive isolation from the flue gases prior to gas freeing the scrubber for entry for maintenance purposes. This may be accomplished by the removal of a suitable length of pipe section and blanking, or a spectacle flange. The gas outlet piping from the scrubber to the blowers and recirculating lines should be made from steel suitably coated internally. Suitable isolating arrangements should be incorporated in the inlet and outlet of each blower to permit overhaul and maintenance of one blower safely while the other blower is in operation. The gas pressure regulating valve should be provided with means to indicate whether the valve is open or shut. Where the valve is used to regulate the flow of inert gas, it should be controlled by the inert gas pressure sensed between the deck-isolating valve and the cargo tanks. Inert gas deck lines should be of steel and arranged to be self-draining. They should be firmly attached to the ship's structure with suitable arrangements to take into account movement due to heavy weather, thermal expansion, and flexing of the ship. The diameter ofthe inert gas main, valves, and branch pipes should be sized to avoid excessive pressure drop. The inert gas velocity should not exceed 40 meters per second in any section ofthe distribution system when the inert gas system is operating at its maximum capacity. If the inert gas main is used for venting during loading, other factors may need to be taken into consideration. All pressure and vacuum relief openings should be fitted with U.S. Coast Guard approved flame screens, with easy access for cleaning and renewal. The flame screens should be at the inlets and the exits of any relief device and be of robust construction sufficient to withstand the pressure of gas generated at maximum loading and ballasting operations while presenting minimum resistance. G as P ressu re R e g u la tin g V a lv e Pressure control arrangements should be fitted, fulfilling two functions: (1) to prevent automatically any backflow of gas in the event of either a failure ofthe inert gas blower, scrubber pump, etc., or when the inert gas plant is operating correctly but the deck water seal and mechanical nonreturn valve have failed and the pressure of gas in the tank exceeds the blower discharge pressure, e.g., during simultaneous stripping and DESIGN OF INERT GAS SYSTEMS 26-19 ballasting operations; and (2) to regulate the flow of inert gas to the inert gas deck main. A typical arrangement by which gas regulation can be achieved is when a system with automatic pressure control and a gas recirculating line permits control of inert gas pressure in the deck main without requiring adjustment of the inert gas blower speed. Gas not required in the cargo tanks is recirculated to the scrubber or vented to the atmosphere. Gas pressure regulating valves are fitted in both the main and recirculating lines. One is controlled by a gas pressure transmitter and regulator, while the other may be controlled either in a similar manner or by a weightoperated valve. The pressure sensor is located downstream of the deck isolation valve which enables a positive pressure to be maintained in the cargo tanks during discharge. (Figure 26-12 shows a typical automatic pressure control system.) Figure 26-12. Automatic pressure control system P -V (P r e ssu r e -V a c u u m ) B reaker One or more liquid-filled pressure-vacuum breakers should be fitted, unless pressure-vacuum valves have the capacity to prevent excessive pressure or vacuum. These devices required little maintenance. However, they will only operate at the required pressure if they are filled to the correct level with liquid of the correct density. Either a suitable oil or a freshwater-glycol mixture should be used to avoid freezing in cold weather. Evaporation, ingress of seawater, condensation, and corrosion should be taken into consideration and adequately compensated for. In heavy weather, the pressure surge caused by the motion ofliquid in the cargo tanks 26-20 INERT GAS SYSTEMS AND CRUDE OIL WASHING MACHINERY may cause the liquid of the pressure-vacuum breaker to be blown out. (Figure 26-13 illustrates the principles of a liquid-filled pressure-vacuum breaker.) Figure 26-13. Liquid-filled pressure-vacuum breaker The characteristics of the deck water seal, pressure-vacuum breakers, and pressure-vacuum valve, and the pressure settings of the high and low inert gas deck-pressure alarms must be compatible. It is also desirable to check that all pressure-vacuum devices are operating at their designed pressure settings .. H ig h V e lo c ity V e n t V a lv e Also referred to as the mast riser (vent) valve, this assembly consists of a hydraulically actuated or manual butterfly valve, a high velocity cone and mixing chamber, and a flame safety screen. The assembly is normally connected to the forward section of the inert gas main, and is used to discharge the inert gas at high velocity into the atmosphere to reduce its toxic effect on personnel on deck. According to regulatory requirements, the outlet of the high velocity vent valve should be at least 2 meters above deck level. Numerous installations have the gas outlet located on top of the mast (thereby being previously referred to as a "mast riser" vent), but this arrangement has been under question following an accident aboard an American flag tanker. I n s tr u m e n ta tio n a n d A la r m s Certain fixed and portable instruments are required for the safe and effective operation of an inert gas system. It is desirable that all instruments be graduated to a consistent system of units. Clear instructions provide for operating, calibrating, and testing all instruments and alarms, including suitable calibration facilities. All instrumentation and alarm equipment required should be suitably designed to withstand supply DESIGN OF INERT GAS SYSTEMS 26-21 voltage variation, ambient temperature changes, vibration, humidity, shock, impact, and corrosion normally encountered on board ships. To fulfill regulation requirements the arrangement of scrubber instrumentation and alarm equipment should be set up as follows: With the water flow to the scrubber monitored either by a flowmeter or by pressure gauges, an alarm will be initiated when the water flow drops below the designed flow requirements by a predetermined amount, and the inert gas blowers are stopped automatically in the event of a further reduction in the flow. The precise setting of the alarm and shutdown limits are related to individual scrubber designs and materials. The water level in the scrubber is monitored by a high water level alarm. This alarm will be activated when predetermined limits are reached, and the scrubber pump will be shut down when the level rises above set limits. These limits will be set with regard to the scrubber design as well as with regard to the flooding of the scrubber inlet piping from the boiler uptakes. The inert gas temperature at the discharge side of the gas blowers is monitored and an alarm given when the temperature reaches 149°F (65°C). Automatic shutdown of the inert gas blowers will occur if the temperature reaches 16TF (75°C). If a precooler is necessary at the scrubber inlet to protect coating materials in the scrubber, the arrangements for giving an alarm will also apply to the outlet temperature from the precooler. To monitor the scrubber efficiency,it is desirable that the cooler water inlet and outlet temperatures and the scrubber differential pressures be indicated. All sensing probes, floats, and sensors required to be in contact with the water and gas in the scrubber are made from materials resistant to acidic attack. For the deck water seal, an alarm should be given when the water level falls to a predetermined level, but before the seal is rendered ineffective. For certain types of deck water seals, such as the dry type, the water level alarm may need to be suppressed when inert gas is being supplied to the inert gas distribution system. The pressure of the inert gas in the inert gas main should be monitored, and an alarm given when the pressure reaches the set limit. The pressure limit should be set with regard to the design of cargo tanks, the mechanical non return valve, and the deck water seal. The arrangement of the oxygen analyzer, recorder, and indicating instruments should meet the following specifications. The gas-sampling connection must be located in the pipe at a point where turbulent flow conditions prevail for all possible blower operating conditions. The sampling point, located after the blower and before the gas regulating valve, must be easily accessible and have air- or steam-cleaning connections. The probe used for sampling must be fitted with a dust filter. Both the probe and the filter are designed to be removed for cleaning or replacement. The sensing pipe from the probe to the analyzer is designed to drain condensate INERT GAS SYSTEMS AND CRUDE OIL WASHING MACHINERY DESIGN OF INERT GAS SYSTEMS to preclude interruption of the gas flow to the analyzer. Ingress of air is minimized by limiting the number ofjoints in the sampling pipe. The position of the analyzer is chosen so that it is protected from heat and adverse ambient conditions since it is placed as close as practicable to the sampling point in order to reduce to a minimum the time between the extraction of a sample and its analysis. The oxygen analyzer should have an accuracy of ±1 percent of the full-scale deflection of the indicator. Between the automatic gas pressure regulating valve and the deck water seal a sampling point is provided for use with portable instruments. The inert gas pressure sensor and recorder obtains the signal from a point in the inert gas main between the deck isolation nonreturn valve and the cargo tanks. When the pressure in the inert gas main forward of the nonreturn devices falls below 50 millimeters on the water gauge, a means is provided to shut down the main cargo pumps automatically, or to give an audible alarm on the navigating bridge and in the machinery space. Portable instruments provide for measuring oxygen and flammable concentration. Hydrocarbon vapor meters working on the catalytic filament principle are unsuitable for measuring hydrocarbons concentration in oxygen deficient atmospheres. Furthermore, meters using this principle cannot measure concentrations of hydrocarbon vapors above the lower flammable limit. It is, therefore, advisable to use meters utilizing a principle which is not affected by oxygen deficiency, and is capable of measuring hydrocarbon concentration in and above the flammable range. For measuring below the lower flammable limit, provided sufficient oxygen is present, the catalytic filament meter is suitable. All metal parts of portable instruments and sampling tubes requiring to be introduced into tanks should be securely grounded to the ship structure while the instruments and sampling tubes are being used. These portable instruments should be of an intrinsically safe type approved by the V . S . Coast Guard. Sufficient tubing in cargo tanks should be provided to enable a fully representative sampling of a cargo tank atmosphere to be obtained as well as suitable openings to enable fully representative samples to be taken from each tank. Where tanks are subdivided by complete or partial wash bulkheads, additional openings will be needed for each subdivision. Piping made in glass-reinforced plastic of acceptable manufacture, substantial thickness, pressure tested, and adequately supported may be acceptable for effluent piping from scrubbers when effluent lines are, as far as possible, led through cofferdams or ballast tanks and are in accordance with the load line regulations in force. Glass-reinforced plastic is acceptable also where effluent lines are led through machinery spaces in which the arrangements include the following: 2 6 -2 2 2 6 -2 3 1. A valve fitted to a stub piece at the shell, actuated both from inside and outside the machinery space by pneumatic or hydraulic means led through steep piping, which will close automatically in the event of failure of the operating media. The valve should have a position indicator and is to be closed at all times when the plant is not in operation as well as in the event of a fire in the machinery space. Suitable instructions to this effect are to be given to the master. 2. A flap-type nonreturn valve. 3. A short length of steel pipe, or spool piece, lined internally and fitted between the valve referred to in 1 above and the nonreturn valve referred to in 2 above fitted with a 12.5-millimeter diameter-flanged drain branch pipe and valve. 4. A further spool piece fitted inboard of, and adjacent to, the non return valve referred to in 2 above, similarly fitted with a drain. (Note: The purpose of this arrangement is to enable the tightness of the valves and nonreturn valves referred to in 1 and 2 above to be checked and to facilitate the removal of the nonreturn valve for examination and replacement. 5. A means outside the machinery space for stopping the scrubber pump. (A suitable arrangement is illustrated in Figure 26-14.) A water seal in the shape of a V-bend at least 2 meters (6.5 feet) in depth is fitted at least 2 meters below the equipment to be drained and a means of draining the lowest point of the bend must be provided. I n addition the seal should be adequately vented to a point above the water level in the scrubber or deck water seal. The diameter of the effluent and drainpipes should be adequate for the duties intended, and the pipe run should be self-draining from the water seal. S e a w a te r S e r v ic e E fflu e n t a n d D r a in p ip in g The effluent piping from scrubbers and deck water seal drainpipes, where fitted, are of corrosion-resistant material, or of carbon steel suitably protected internally against the corrosive nature of the fluid. The scrubber effluent pipe and deck water seal drainpipe, where fitted, are not to be led to a common drainpipe, and the deck seal drain is to be led clear of the engine room and any other gas-safe space. It is advisable that the main supply of water to the inert gas scrubber be from an independent pump. The alternative source of supply of water may be from another pump, such as the sanitary, fire, bilge, and ballast pumps, provided that the quantity of water required by the inert gas scrubber is readily available, and the requirements of other essential services are not thereby impaired. The requirement for two separate pumps to be capable OPERATION OF THE INERT GAS SYSTEM 26-25 S ta r t-U p Start-up procedures involve the following steps: 1. Ensure boiler is producing flue gas with an oxygen content of 5 percent by volume or less (for existing ships, a maximum of 8 percent by volume and, whenever practicable, less). 2. Ensure that power is available for all control, alarm, and automatic shutdown operations. 3. Ensure that the quantity of water required by the scrubber and deck water seal is being maintained satisfactorily by the pump(s) selected for this duty. 4. Test operation of the alarm and shutdown features ofthe system dependent upon the throughput of water in the scrubber and deck seal. 5. Check that the gas-freeing fresh air inlet valve where fitted is shut and the blank is securely in position. 6. Open the flue gas isolating valve. Ensure that the air for sealing is shut off when the valve is open. 7. Open the selected blower suction valve. Ensure that the other blower suction and discharge valves are shut unless it is intended to use both blowers Figure 26-14. Scrubber effluent arrangement of supplying water to the deck water seal can be met by any of the pumps referred to above under alternative source of supply. The pumps supplying water to the scrubber and the deck water seal should be such as to provide the required throughput of water at light draft conditions. The quantity of water at all other draft conditions should not flood the scrubber or increase the gas flow resistance excessively. A loop seal with vent should be provided in the supply water piping to the deck seal to prevent the backflow of hydrocarbon vapor or inert gas and should be positioned outside the machinery space, suitably protected against freezing. With reference to the deck water seal arrangement, provisions should be made to prevent any pneumatically controlled system from freezing. simultaneously. 8. Start the blower. 9. Test blower "failure" alarm. 10. Open the blower discharge valve (this should be done without delay to prevent overheating within the blower casing). 11. Ensure that the blower recirculating valve opens automatically to enable plant to stabilize. 12. Open the inert gas regulating valve. 13. Check that oxygen content in the cold inert gas is 5 percent by volume or less (for existing ships 8 percent by volume or, wherever practicable, less). (Note: Some oxygen analyzers require as much as two hours to stabilize before accurate readings can be obtained.) The inert gas system is now ready to deliver gas to the cargo tanks. Continue the start-up steps as follows; 14. Open main deck isolation valve. 15. Open individual cargo tank branch valves if fitted. S h u td o w n O P E R A T IO N OF THE IN E R T GAS SYSTEM Though inert gas systems differ in detail, certain fundamentals remain the same: (1) starting up the inert gas plant; (2) shutting down the inert gas plant; and (3) utilizing safety checks when the inert gas plant is shut down. In all cases manufacturer's detailed instructions should be followed. Shutdown procedures should be as follows: When all tank atmospheres have been checked for an oxygen level of not more than 8 percent and the required in-tank pressure has been obtained, 1. Shut the deck isolation valve and the gas pressure regulating valve. 2. Shut down the inert gas blower. 26- 26 INERT GAS SYSTEMS AND CRUDE OIL WASHING MACHINERY 3. Close the blower suction and discharge valve; check that the drains are clear; open the water-washing system on the blower while it is still rotating with the power off the driving motor, unless otherwise recommended by the manufacturer; and shut down the water-washing system after a suitable period. 4. Close the flue gas isolating valve and ensure the air sealing system functions. 5. Keep the full water supply on the scrubber tower in accordance with the manufacturer's recommendation; then flush with fresh water. 6. Ensure that the water supply to the deck water seal is running satisfactorily, that an adequate water seal is retained, and that the alarm arrangements for it are in order. Safety Checks When an inert gas plant is shut down, safety checks are in operation as follows: 1. The water supply and water level in the deck seal are ascertained at regular intervals of at least once per day depending on weather conditions. 2. The water level in loop seals installed in pipework for gas, water, or pressure transducers is checked to prevent the backflow of hydrocarbon gases into gas-safe areas. 3. In cold weather arrangements are ensured to prevent the freezing of water in the deck seal, pressure vacuum breaker, etc. 4. Inerted cargo tanks are repressurized with inert gas before their pressure drops to 100 millimeters. Failure Symptoms Among the possible causes for failure of an inert gas system is high oxygen content, which may be caused or indicated by the following conditions: 1. Poor combustion control at the boiler, especially under low load conditions; 2. Air drawn down the boiler uptake when gas output is less than the inert gas blower demand, especially under low load conditions; 3. Air leaks between the inert gas blower and the boiler uptake as well as faulty operation or calibration of the oxygen analyzer; 4. Operating in the recirculation mode; and/or 5. Entry of air into the inert gas main through the pressure-vacuum valve, pressure-vacuum breaker, or mast riser valve, due to misoperation. If the inert gas plant is delivering inert gas at an oxygen content of more than 5 percent, the fault is to be traced and repaired. All cargo tank operations shall be suspended if the oxygen content exceeds 8 percent unless the quality of the gas is improved. Another cause for failure is the inability of the system to maintain positive pressure during cargo discharge or deballasting operations. This may be caused by: OPERATION OF THE INERT GAS SYSTEM 2 6 -2 7 1. Inadvertent closure ofthe inert gas valves; 2. Faulty operation ofthe automatic pressure control system; 3. Improper opening of the recirculating valve; 4. Inadequate blower pressure; or 5. A cargo discharge rate in excess of the blower output. The cargo discharge or deballasting should be stopped or reduced depending on whether or not the positive pressure in the tanks can be maintained while the fault is rectified. Standard Procedures The inert gas system should be used during the full cycle of tanker operation as described in this section. Inerting of tanks. Tanks that have been cleaned and gas freed should be reinerted preferably during the ballast voyage to allow the inert gas system to be tested fully prior to cargo handling. Purge pipes/vents should be opened to atmosphere. When the oxygen concentration of the atmosphere in the tank has fallen below 8 percent, the purge pipes/vents should be closed and the tank pressurized with inert gas. During the reinerting of a tank following a breakdown and repair of the inert gas system, nongas-free and noninerted tanks should be reinerted. Duringinerting, no ullaging, dipping, sampling, or other equipment should be inserted unless it has been established that the tank is inert. This should be done by monitoring the effiux gas from the tank being inerted until the oxygen content is less than 8 percent by volume and for such a period of time as determined by previous test records when inerting gas-free tanks to ensure that the effiux gas is fully representative of the atmosphere within the tank. When all tanks have been inerted, they should be kept common with the inert gas main and maintained at a positive pressure in excess of 100 millimeters water gauge during the rest of the cycle of operation. Discharging water ballast. Before the discharge of cargo tank ballast is undertaken, the following conditions are checked as indicated: 1. All cargo tanks are connected up to the inert gas system and all branch isolation valves (if fitted) in the deck pipework are locked open. 2. All other cargo tank openings are shut. 3. The valve isolating the high velocity vent from the inert gas system is shut. 4. The inert gas plant is producing gas of an acceptable quality. 5. The deck isolation valve is open. 26-28 INERT GAS SYSTEMS AND CRUDE OIL WASHING MACHINERY OPERATION OF THE INERT GAS SYSTEM 26-29 During the deballasting operation, the oxygen content of the gas and its pressure in the inert gas main should be continuously recorded. Descriptions of crude oil washing and regulations governing it will be presented at the end of this chapter. Loading cargo. When loading cargo, the deck isolation valve is closed. The inert gas plant may be shut down unless other cargo tanks are being deballasted simultaneously. All openings to the cargo tanks except the connections to the high velocity vent arrangement are to be kept closed to minimize flammable vapor on deck. Before loading commences the flame screens in the high velocity venting arrangement should be inspected, and any stop valves isolating the cargo tanks from the inert gas main locked in the open position. Ballasting cargo tanks. The conditions for ballasting of cargo tanks are the same as those for loading. When, however, simultaneous discharge and ballasting is adopted, a close watch must be kept on the inert gas main pressure. Loaded passage. During the loaded passage a positive pressure of inert gas of at least 100 millimeters water gauge should be maintained in the cargo tanks and topping up of the pressure may be necessary. When topping up the inert gas of pressure in the cargo tanks, particular attention should be paid to obtaining an oxygen concentration of 5 percent or less in the inert gas supply before introducing the gas into the cargo tanks. On motor tankers, the auxiliary boiler loading may have to be increased in order that the low oxygen concentration in the inert gas supply can be achieved. It may also be necessary to restrict the output of the inert gas blowers to prevent air being drawn down the uptake during the topping up operation. If by these means inert gas of sufficient quality cannot be achieved, then inert gas from an alternative source of supply, such as an inert gas generator, might have to be used. Discharging cargo. It may be necessary to relieve the inert gas pressure in the cargo tanks on arrival to permit manual measurement before cargo is discharged. If this is done, no cargo or ballasting operation is to be undertaken and a minimum number of small tank openings are to be uncovered for as short a time as necessary to enable these measurements to be completed. The tanks should then be repressurized before cargo discharge commences. Cargo discharge should not be commenced until all the conditions have been checked and are in order. During discharge the oxygen content and pressure of the inert gas in the inert gas main should be continuously recorded. Crude oil washing. Before each tank is crude oil washed, the oxygen level shall be determined at a point 1 meter below the deck and at the middle region of the ullage space, and neither of these determinations should exceed 8 percent by volume. Where tanks have complete or partial wash bulkhead, the determination should be taken from similar levels in each section of the tank. The oxygen content and pressure ofthe inert gas being delivered during the washing process should be continuously recorded. Ballast voyage. During a ballast voyage, tanks other than those required to be gas free for necessary tank entry should be kept inerted with the cargo tank atmosphere at a positive pressure of not less than 100 millimeters water gauge, and an oxygen level not exceeding 8 percent by volume especially during tank cleaning. Before any inert gas is introduced into cargo tanks to maintain a positive pressure, it should be established that the inert gas contains not more than 5 percent by volume of oxygen. Tank cleaning. Cargo tanks should be washed in the inert condition and under a positive pressure. The procedures adopted for tank cleaning with water should follow those for crude oil washing. Purging prior to gas freeing. When it is desired to gas free a tank after washing, reduce the concentration of hydrocarbon vapor by purging the inerted cargo tank with inert gas. Open purge pipes/vents to atmosphere, introduce inert gas into the tank until the hydrocarbon vapor concentration measured in the efflux gas has been reduced to 2 percent by volume and until such time as determined by previous tests on cargo tanks has elapsed to be confident that readings have stabilized and the efflux gas is representative of the atmosphere within the tank. Gas freeing. Gas freeing of cargo tanks is only to be carried out when tank entry is necessary (e.g., for essential repairs). It should not be started until it is established that a flammable atmosphere in the tank will not be created as a result. Hydrocarbon gases are to be purged from the tank. Gas freeing may be effected by pneumatically, hydraulically, or steamdriven portable blowers, or by fixed equipment. In either case it is necessary to isolate the appropriate tanks to avoid contamination from the inert gas main. Gas freeing should continue until the entire tank has an oxygen content of21 percent by volume and until a reading ofless than 1 percent of the lower flammable limit is obtained on a combustible gas indicator. Care must be taken to avoid the leakage of air into inerted tanks, or inert gas into tanks which are being gas freed. Preparation for tank entry. The entry of personnel into the cargo tank should only be carried out under the close supervision of a responsible 26-30 INERT GAS SYSTEMS AND CRUDE OIL WASHING MACHINERY ship's officer and in accordance with all applicable safety rules. Some safety precautions to observe are listed as follows: 1. Secure inert gas branch line gas valves and blank(s) in position or, if gas freeing with the inert gas blower, isolate the scrubber from the flue gases. 2. Close all drain lines entering the tank from the inert gas main and secure relevant cargo line valves or controls in the closed position. 3. Keep the inert gas deck pressure on the remainder of the cargo tank system at a low positive pressure such as 200 millimeters water gauge. (This minimizes the possible leakage of inert hydrocarbon gas from other tanks through possible bulkhead cracks, cargo lines, valves, etc.) Clean sample lines well into the lower regions of the tank in at least two locations. Keep these locations away from both the inlet and outlet openings used for gas freeing. M t e r it has been ascertained that a true bottom sample is being obtained, require the following readings: 21 percent on a portable oxygen analyzer; and less than 1 percent lower flammable limit on a combustible gas indicator. 4. Observe use of breathing apparatus whenever there is any doubt about the tank being gas free, e.g., in tanks where it is not possible to sample remote locations. (This practice should be continued until all areas, including the bottom structure, have been thoroughly checked.) 5. Ventilate continuously and sample regularly the tank atmosphere whenever personnel are in the tank. 6. Observe carefully normal regulations for tank entry. Reinerting after Tank Entry When all personnel have left the tank and the equipment has been removed, the inert gas branch line blank, if fitted, should be removed, the hatch closed, and the main deck isolation valve reopened. As soon as a gas-free tank is reconnected to the inert gas main, it should be reinerted to prevent transfer of air to other tanks. Special Precautions for Product Carriers The basic principles ofinerting are exactly the same on a product carrier as on a crude oil tanker. However, there are differences in operation of these vessels as outlined below. Product carriers transport products having a flash point exceeding 140°F (60°C) (closed cup test) as determined by an approved flash point apparatus. Product carriers may carry bitumens, lubricating oils, heavy fuel oils, high flash point jet fuels, some diesel fuels, gas oils, and special boiling point liquids without inert gas systems having to be fitted, or, if fitted, without tanks containing such cargoes having to be kept in the inert condition. If cargoes with a flash point over 60°C are carried heated at temperatures near to or above their flash point (some OPERATION OF THE INERT GAS SYSTEM 26-31 bitumen cut-backs and fuel oils), a flammable atmosphere can occur. When cargoes with a flash point above 140°F (60°C) are carried at a temperature higher than gOF(5°C) below their flash point, they should be carried in an inerted condition. When a nonvolatile cargo is carried in a tank that has not been previously gas freed, then that tank shall be maintained in an inert condition. Product contamination. Contamination of a product may affect its odor, acidity, or flash point specifications, and may occur in several ways. Those ways which are relevant to ships with an inert gas main connecting all cargo tanks are contamination by other cargo, liquid contamination due to overfilling a tank, and vapor contamination through the inert gas main. The latter is largely a problem of preventing vapor of low flash cargoes (typically gasolines) from contaminating the various high flash cargoes, aviation gasolines, and most hydrocarbon solvents. This problem can be overcome by removing vapors of low flash point cargoes prior to loading and by preventing ingress of vapors of low flash point cargoes during loading and during the loaded voyage. When carrying hydrocarbon solvents where quality specifications are stringent and where it is necessary to keep individual tanks positively isolated from the inert gas main after a cargo has been loaded, pressure sensors should be fitted so that the pressure in each such tank can be monitored. When it is necessary to top up the relevant tanks, the inert gas main should first be purged of cargo vapor. For a well-designed and -operated flue gas system, experience suggests that petroleum cargoes traditionally carried on product tankers do not suffer contamination from the flue gas itself, as opposed to contamination from other cargoes. However, unacceptable contamination from the flue gas may be encountered if proper control is not exercised over fuel quality, efficiency of combustion, scrubbing, and filtering. The more critical petrochemical cargoes which may be carried by product carriers can be contaminated by flue gas. All lubricating oils and jet fuels are acutely water critical. Current practice requires full line draining and mopping up of any water in tanks before loading. Water contamination may occur on inerted ships because of water carryover from the scrubber or deck water seals due to inadequacies in design or maintenance of the various drying arrangements and from condensation of water from warm, fully saturated flue gas delivered to the tanks. Requirements for additional purging and gas freeing. Gas freeing is required on noninerted product carriers more frequently than on crude carriers because of the greater need both for tank entry and inspection, especially in port, and for venting vapors of previous cargoes. On inerted 26-32 INERT GAS SYSTEMS AND CRUDE OIL WASHING MACHINERY product carriers any gas-freeing operation has to be preceded by a purging operation, but gas freeing for purely quality reasons may be replaced by purging only. It should be recognized that there are increased risks of air leaking into inert tanks and of inert gas leaking into a tank being entered; that purging is not a prerequisite of gas freeing when the hydrocarbon gas content of a tank is below 2 percent by volume; and that the operation of gas freeing for product purity and where tank entry is not contemplated does not require the atmosphere to contain an oxygen content of21 percent by volume. Special Precautions for Combination Carriers The basic principles of inerting are exactly the same on a combination carrier as on a tanker. However, there are differences in the design and operation of these vessels, and in this respect, considerations are outlined below. Static electricity in slack holds. It is particularly important for combination carriers to have their holds inerted because whenever a hold in an OBO (oil-bulk-ore) carrier (which could extend the full breadth of the ship) is partially filled with clean or oily ballast, water agitation of this ballast can occur at small angles of roll and this can result in the generation of static electricity. The agitation is sometimes referred to as sloshing, and it can happen whenever the ullage of the liquid content of the hold is more than 10 percent of the depth of the hold, measured from the underside of the deck. (See Figure 26-15 for remedy condition.) Figure 26-15. Cargo hold ofOBO carrier Leakage through hatches. To ensure that leakage of tank gas, particularly through the hatch centerline joints, is eliminated or minimized, it is essential that the hatch covers are inspected frequently to determine the state of their seals, their alignment, etc. When the hatch covers have been OPERATION OF THE INERT GAS SYSTEM 26-33 opened, particularly after the ship has been carrying a dry bulk cargo, the seals and trackways should be inspected and cleaned of any foreign matter. Ballast and void spaces. The cargo holds of combination carriers are adjacent to ballast and void spaces. Leakages may occur in pipelines or ducts in these spaces, or by a fracture in the boundary plating. In this event there is a possibility that oil, inert gas, and hydrocarbon gas may leak into the ballast and void spaces. As a consequence gas pockets may form, and difficulty with gas freeing could be anticipated due to the considerable steelwork, in the form of stiffening, which is characteristic ofthese spaces. Therefore, personnel need be alerted to the hazard. Inert gas distribution system. Due to the special construction of combination carriers, the vent line from the cargo hatchway coaming is situated very close to the level of the cargo surface. In many cases, the inert gas main line passing along the main deck may be below the oil level in the hold. During rough weather, oil or water may enter these lines and completely block the opening, preventing an adequate supply of inert gas during either tank cleaning or discharge. Vent lines should therefore have drains fitted at their lowest point, and these should always be checked before any operation takes place within the cargo hold. Applications carrying oil and nonoil cargo. On combination carriers the inert gas system should be utilized whenever the ship is engaged exclusively in the carriage of oil. When a combination carrier is carrying a cargo other than oil, it should be considered as a tanker. When cargoes other than oil are intended to be carried it is fundamental that all holds of cargo tanks other than slop tanks be emptied of oil and oil residues, and cleaned and ventilated to such a degree that the tanks are completely gas free and internally inspected. The pump room, cargo pumps, pipelines, duct keel, and other void spaces are to be checked to ensure that they are free of oil and hydrocarbon gas. Where holds are required to carry cargo other than oil, they should be isolated from the inert gas main and oil cargo pipeline by means of blanks which should remain in position at all times cargoes other than oil are being handled or carried. During the loading and discharging of solid cargoes and throughout the intervening periods all holds of cargo tanks other than slop tanks, cargo pump rooms, cofferdams, duct keels, and other adjacent void spaces should be kept in a gas-free condition and checked periodically at intervals of not more than two days to ensure that: 1. There has been no generation of hydrocarbon gas or leakage of hydrocarbon gas from the slop tanks. If concentrations of more than 20 percent of lower flammable limit are detected, the compartment(s) should be ventilated. 26-34 INERT GAS SYSTEMS AND CRUDE OIL WASHING MACHINERY 2. There is no deficiency of oxygen which could be attributable inert gas from another compartment. to leakage of As an alternative to this procedure, those cargo tanks which are empty of cargo may be reinerted provided they are subsequently maintained in the inert condition and at a minimum pressure of 100 millimeters water gauge at all times, and provided that they are checked at intervals of not more than two days to ensure that any generation of hydrocarbon gas does not exceed one percent by volume. If such a concentration is detected the compartment(s) should be purged with inert gas. Slops should be contained in a properly constituted slop tank and should be: 1. Discharged ashore, and the slop tanks cleaned and ventilated to such a degree that the tanks are completely gas free and then inerted; or 2. Retained on board for not more than one voyage when, unless the vessel reverts to carrying oil, the slop tank should be inerted; or 3. Ifretained on board for more than one voyage because reception facilities for oily residues are not available, the slop tank should be inerted. When slop tanks have not been discharged, it is required that they be isolated from other tanks by blank flanges which will remain in position at all times when cargoes other than oil are being carried. On combination carriers where there are also empty cargo tanks which are not required to be isolated from the inert gas main, the arrangement for isolating the slop tanks from these tanks should be such as to: Figure 26-16. Bypass arrangement for topping cargo tanks E m ergency P rocedures In the event of total failure of the inert gas system to deliver the quality and quantity of inert gas to maintain a positive pressure in the cargo tanks and slop tanks, action should be taken immediately to prevent any air being drawn into the tank. All cargo tank operations should be stopped, and the deck isolating If it is assessed to inert gas system to pressure in the cargo be resumed provided valve should be closed. be totally impractical to effect a repair to enable the deliver the quality of gas and maintain a positive tanks, cargo discharge and deb alia sting should only that the following precautions are taken: 1. The flame screens are checked to ensure that they are in a satisfactory 1. Prevent the passage of hydrocarbon gas from the slop tanks to the empty tanks; and 2. Facilitate monitoring of and, if necessary, topping up of the pressure in slop tanks and any empty cargo tanks if the latter are being kept in the inert condition. (A suggested arrangement is shown in Figure 26-16.) In addition, all cargo pipelines to or from the slop tanks should be blanked off. The above requirements need not apply and instead carrier may be operated as a bulk carrier without having gas system: a combination to use its inert 1. If it has never carried a cargo of oil; 2. If, after its last cargo of oil, all of its cargo tanks, including slop tanks, the pump room, cargo pumps, pipelines, cofferdams, duct keel, and other void spaces are emptied of oil and oil residues, cleaned, and made completely gas free and the tanks and void spaces internally inspected to that effect; or 3. If, in addition, the monitoring for hydrocarbon gas is continued until it has been established that generation of hydrocarbon gas has ceased. condition. 2. The valve on the high velocity vent is opened. 3. No free-fall of water or slops is permitted. 4. No dipping, ullaging, sampling, or other equipment should be introduced into the tank unless essential for the safety of the operation. If such equipment must be introduced into the tank, it should only be done after at least 3 0 minutes have elapsed since the injection of inert gas has ceased. All metal components of equipment to be introduced into the tank should be securely grounded. This restriction should not be applied until a period of five hours has elapsed since injection of inert gas had ceased. If it is essential to clean tanks following system, and satisfactory inerted conditions following precautions the failure of the inert cannot be maintained, gas the are necessary: 1. Tank washing should only be carried out one tank at a time. 2. The tank should be isolated from other tanks and from any common venting system or the inert gas main and maximum ventilation output should be 2 6 -3 6 INERT GAS SYSTEMS AND CRUDE OIL WASHING MACHINERY concentrated on that tank both before and during the washing process. Ventilation should provide as far as possible a free flow of air from one end ofthe tank to the other. 3. The tank bottom should be flushed with water and stripped. The piping system including cargo pumps, crossovers, and discharge lines should also be flushed with water. 4. Washing should not commence until tests have been made at various levels to establish that the vapor content in any part of the tank is below 10 percent of the lower flammable limit. 5. Testing the tank atmosphere should continue during the washing process. 6. If the vapor level rises to within 50 percent of the lower flammable limit, washing should be discontinued until vapor level has fallen to 20 percent of the lower flammable limit or less. 7. If washing machines with individual capacities exceeding 60 cu. meters/hr are to be used, only one such machine shall be used at anyone time on the ship. Ifportable machines are used, all hose connections should be made up and bonding cables tested for continuity before the machines are introduced into the tank, and should not be broken until after the machines have been removed from the tank. 8. The tank should be kept drained during washing. If buildup of wash water occurs, washing should be stopped until the water has been cleared. 9. Only clean, cold seawater should be used. Recirculated water should not be used. 10. Chemical additives should not be used. 11. All deck openings, except those necessary for washing and designed venting arrangements, should be kept closed during the washing process. 12. The regulations of the local port authorities shall take precedence over any of the foregoing emergency procedures during cargo operations in port. Safety Considerations Backflow of cargo gases. To prevent the return of cargo gases or cargo from the tanks to the machinery spaces and boiler uptake, it is essential that an effective barrier is always present between these two areas. In addition to a nonreturn valve, a water seal and deck isolation valve should be fitted on the deck main. It is of prime importance that these devices are properly maintained and correctly operated at all times. OPERATION OF THE INERT GAS SYSTEM 2 6 -3 7 body. It is therefore necessary to ventilate thoroughly so no pockets of oxygen-deficient atmosphere remain. When testing for man entry, a steady reading of21 percent oxygen by volume is required. T o x i c i t y o f h y d r o c a r b o n v a p o r s . Inert gas does not affect the toxicity of hydrocarbon gases, and the problem is no different from that of ships without an inert gas system. Because of possible gas pockets, regeneration, etc., gas freeing must continue until the entire compartment shows a zero reading with a reliable combustible gas indicator or equivalent, or a 1 percent lower flammable limit reading should the instrument have a sensitivity scale on which a zero reading is impractical. T o x i c i t y o f f l u e g a s . The presence of toxic gases such as sulphur dioxide, carbon monoxide, and oxides of nitrogen can only be ascertained by measurement. However, provided that the hydrocarbon gas content of an inerted tank exceeds about 2 percent by volume before gas freeing is started, the dilution of the toxic components of flue gas during the subsequent gas freeing can be correlated with the readings of an approved combustible gas indicator or equivalent. If, by ventilating the compartment, a reading of 1 percent lower flammable limit or less is obtained in conjunction with an oxygen reading of21 percent by volume, the toxic trace gases will be diluted to concentrations at which it will be safe to enter. Alternatively, and irrespective of initial hydrocarbon gas content, ventilation should be continued until a steady oxygen reading of 21 percent by volume is obtained. Inert gas is asphyxiating. Great care must be exercised when work on the plant is undertaken. Although the worker may be in the fresh air, inert gas leaking from the plant could render him unconscious very quickly. Before opening up any equipment, therefore, it is recommended that the inert gas plant is completely gas freed. If any unit (e.g., the inert gas scrubber) is to be examined internally, the standard recommendations for entering enclosed spaces must be followed. Blind flanges should be fitted where applicable, or the plant should be completely isolated. Health hazards Tank pressure. When an inerted cargo tank is maintained at a positive pressure, personnel should be advised of the practical hazards of this pressure. Such pressure must be adequately reduced before opening any tank lids, ullage plugs, or tank washing openings. O x y g e n d e f i c i e n c y . Exposure to an atmosphere with a low concentration of oxygen does not necessarily produce any recognizable symptom before unconsciousness occurs (at which time the onset of brain damage and risk of death can follow within a few minutes). If the oxygen deficiency is not sufficient to cause unconsciousness, the mind is liable to become apathetic and complacent, and even if these symptoms are noticed, and escape is attempted, physical exertion will aggravate the weakness of both mind and Electrostatic hazards. Small particulate matter carried in flue gas can be electrostatically charged. The level of charge is usually small, but levels have been observed well above those encountered with water mists formed during tank washing. Because cargo tanks are normally in an inerted condition, the possibility of electrostatic ignition has to be considered only if the oxygen content of the tank atmosphere rises as a result of an ingress 2 6 -3 8 INERT GAS SYSTEMS AND CRUDE OIL WASHING MACHINERY MAINTENANCE of air or if it is necessary to inert a tank which already has a flammable atmosphere. M A IN T E N A N C E AND T E S T IN G The safety arrangements are an integral part of the inert gas system and it is important for the ship's staff to give special attention to them during any inspection. Inspection routines for some ofthe main components are dealt with in this section. In ert G as S cru b b er Inspection may be made through the manholes. Checks should be made for corrosion attacks, fouling, and damage to: AND TESTING D e c k W a te r S e a l This unit performs an important function and must be maintained in good condition. Corroded inlet pipes and damage to flat control valves and level indicators are not uncommon. The overboard drain line and connection are also possible sources of trouble. An inspection ofthe deck water seal should include opening for internal inspection to check for: 1. Blockage of the venturi lines in semidry-type water seals. 2. Corrosion of inlet pipes and housing. 3. Corrosion of heating coils. 4. Corroded or sticking floats for water drain and supply valves and level monitoring. Appropriate testing for the function of the deck water seal should include automatic filling and draining (check with a local level gauge if possible) and presence of water carryover (open drain cocks on inert gas main line) during operation. 1. Scrubber shell and bottom. 2. Cooling water pipes and spray nozzles (fouling). 3. Float switches and temperature sensors. 4. Other internals such as trays, plates, and demister filters. N o n r e tu r n Checks should be made for damage to nonmetallic internal lines, demisters, and packed beds. 2 6 -3 9 parts such as I n e r t G a s B lo w e r s To a limited degree, internal visual inspection will reveal damage at an early stage. Diagnostic monitoring systems should be used as they greatly assist in maintaining the effectiveness of the equipment. By fitting two equal-sized blowers or, alternatively, supplying and retaining on board a spare impeller with a shaft for each blower, an acceptable level of availability is ensured. Visual inspection through the available openings in the blower casing are adequate for this purpose. An inspection of inert gas blowers should include: 1. Internal inspection of the blower casing for soot deposits or signs of corrosive attack. 2. Examination of fixed or portable washing system. 3. Inspection of the functioning ofthe freshwater flushing arrangements, where fitted. 4. Inspection of the drain lines from the blower casing to ensure that they are clear and operative. 5. Observation of the blower under running conditions for signs of excessive vibration, indicating too large an imbalance. V a lv e The nonreturn valve should be opened for inspection to check for corrosion and also to check the condition of the valve seat. The functioning of the valve should be tested in operation. Scrubber E fflu e n t L in e The scrubber effiuent line cannot normally be inspected internally except when the ship is in dry dock. The ship side stub piece and the overboard discharge valve should be inspected at each dry-docking period. T e s tin g o f O th e r U n its a n d A la r m s A method should be devised to test the correct functioning of all units and alarms, and it may be necessary to simulate certain conditions to carry out an effective testing program. An effective testing program should include checking for: 1. All alarm and safety functions. 2. The functioning of the flue gas isolating valves. 3. The operation of all remotely or automatically controlled valves. 4. The functioning of the water seal and nonreturn valve (with a backflow pressure test). 5. The vibration level of the inert gas blowers. 6. Leakages (in systems four years old or more, deck lines should be examined for gas leakage). 7. The interlocking ofthe soot blowers. 26-40 INERT GAS SYSTEMS AND CRUDE OIL WASHING MACHINERY 8. Oxygen-measuring equipment, both portable and fixed, for accuracy by means of both air and a suitable calibration gas. C om ponent P r e v e n tiv e M a in te n a n c e Flue gas isolating valves Operate the valve. M a in te n a n c e Clean with compressed air or steam. Dismantle for inspection and cleaning. In te r v a l Before start-up and one week Before operating valve Boiler shutdown Flue gas scrubber Water flush. Clean demister. Dismantle level regulators and temperature probes for inspection. Open for full internal inspection. Overboard pipes and valve from flue gas scrubber Flush with scrubber water and pump for about one hour. Dismantle the valve for overhaul; inspect pipeline and overboard end. After each use Blowers Check vibration. Flush. Inspect internally through hatches. During operation Mter each use Mter flushing and six months Two years or more frequently if required or at dry-docking Dismantle for full overhaul of bearings, shaft tightenings, and other necessary work. Deck water seal Mter each use Three months Six months Dry-docking Dry-docking repair period Dismantle level regulatorslfloat valves for inspection. Open for total internal inspection. Overhaul automatic valves. Six months One year One year Deck mechanical nonreturn valve Move and lubricate the valve ifnecessary. Open for internal inspection. One week and before start One year to 18 months Pressure-vacuum valves Operate and lubricate the valves. Open for full overhaul and inspection. Six months One year Deck isolating valve Open for overhaul. One year Gas pressure regulating system Remove condensation in instrument, air supply. Open gas pressure regulating valves for overhaul. Before start Liquid filled pressure- Check liquid level when system is at atvacuum breaker mospheric pressure. IN S T R U C T IO N CRUDE OIL WASHING 26-41 slop tank; gas scrubber; scrubber cooling water pump and pipework up to the effluent discharge overboard; blowers including the suction and discharge valves; recirculation or other arrangements to stabilize the inert gas plant operation; fresh air inlets; automatic gas pressure regulating stop valve; deck water seal and water supply; heating and overflow arrangements; deck nonreturn stop valve; water traps in any supply, vent, drain, and sensing pipework; cargo tank isolation arrangement; purge pipes/vents; pressurevacuum valves on tanks; pressure-vacuum breakers on the inert gas main; permanent recorders and instruments and the takeoff points for their use; arrangements for using portable instruments, complete and partial wash bulkheads, mast risers, mast riser isolating valves; high velocity vents; and manual and remote controls. 2. A description ofthe system and a listing of procedures for checking that each item of the equipment is working properly during the full cycle of tanker operation. This includes a listing of the parameters to be monitored such as inert gas main pressure, oxygen concentration in the delivery main, oxygen concentration in the cargo tanks, temperature at the scrubber outlet and blower outlet, blower running current or power, scrubber pump running current or power, deck seal level during inert gas discharge to cargo tanks at maximum rate, deck seal level at nil discharge, etc. Established values for these parameters during acceptance trials should be included, where relevant. 3. Detailed requirements for conducting all operations particular to the installation of the ship such as times to inert, purge, and gas free each tank; sequence and number of tanks to be inerted, purged, and gas freed; sequence and number of purge pipes/vents to be opened or closed during such operations, etc. 4. Precautions to be taken (relating to the particular construction and equipment on board) to prevent leakage of inert gas and hydrocarbon vapors. 5. Precautions to be taken to prevent dangers arising from cargo over- and underpressure during various stages in the cycle of tanker operation. 6. Precautions to be taken to prevent dangers relating to the particular construction ofthe vessel or equipment on board. As appropriate CRUDE When opportunity permits and every six months M ANUALS Instruction manuals are required to be provided on board by regulations, and should contain the following information and operational instructions: 1. A line drawing of the inert gas system showing the positions of the inert gas pipework from the boiler or gas generator uptakes to each cargo tank and O IL W A S H IN G The pumping out operation for crude oil tank ships has been historically complicated by asphaltic compounds, waxes, and petroleum sands which form a difficult-to-remove sediment in the tanks. The buildup of this sediment over a number of voyages reduced the cargo-carrying capacity of the vessel, and proved to be expensive and difficult to remove when it was necessary to gas free and clean the tanks for shipyard entry. The International Convention for the Prevention of Pollution from Ships, 1973 (MARPOL),provided further incentive for crude oil washing by requiring that crude oil carriers over 40,000 DWT be equipped with segregated ballast tanks or be fitted with crude oil washing systems to be used instead of the traditional water washing system which contributes to pollution. INERT GAS SYSTEMS AND CRUDE OIL WASHING MACHINERY CRUDE OIL WASHING Since crude oil washing techniques require inert gas systems, it is appropriate to consider the important design and operating characteristics of crude oil washing machinery in this chapter on inert gas systems. main components: the gearbox unit, the drop pipe, and the wash head! nozzle assembly. The general arrangement of this machine is shown in Figure 26-17. Figure 26-18 is a photograph ofthe machine. 26-42 T e c h n iq u e s of Crude O il W a s h in g For reliable details on crude oil washing, refer to "Revised Specifications for the Design, Operation and Control of Crude Oil Washing Systems" published by IMO. The appendix to this chapter includes extracts of text from the "Annex to Tanker Safety And Pollution Prevention 1978 Protocol." Since crude oil is a good solvent and carrier of sediment, it is an effective medium for the removal of the voyage accumulation of sediment from the cargo tanks. With an inert gas system and fixed-in-place tank-cleaning machines similar to those described in the following paragraphs, it is possible to use the crude oil being discharged to clean the cargo tanks. As the cargo tank is discharged, crude oil is pumped under pressure to the tank-cleaning machines where it is directed from the nozzle at high velocity to the tank walls. The jet of crude oil removes sediments clinging to the tank's top, walls, and bottom. The washing machines are programmed to effectively cover the entire tank area. The sediments are mixed with the cargo oil and discharged to the shoreside storage tanks. Crude oil washing may only be undertaken in tanks fitted with an inert gas system which is operational and capable of maintaining the tank atmosphere at not more than 8 percent oxygen content and a positive pressure. Because the shipboard engineer officer is not usually involved in crude oil washing operations, it is not necessary to provide a complete description of the complicated procedure required for safe operation of the crude oil washing system. T he C rude O il W a s h in g S y ste m Crude oil washing systems include permanently fixed-in-place tank-washing machines with fluid supplied from the main cargo pumps through permanent pipework. A typical modern installation will have single nozzle, high capacity, programmable, self-powered washing machines which are deck mounted. For difficult-to-c1ean areas, submersible machines supplied from permanent pipework are located within the tank. Tank-cleaning machine nozzles normally have nozzle diameters from 1 5 /s to V I 4 inch which establish the flow of the cleaning jet. The submersible machines located in the bottom of the tank are inaccessible except during yard periods. Therefore, they must be highly reliable and rugged. The submersible machines are nonprogrammable. A T y p ic a l W a s h in g M a c h in e Typical of the deck-mounted machines in use on modern crude oil carriers is the Lavomatic SA tank washing machine. This equi pment includes three 2 6 -4 3 CRUDE OIL WASHING Figure 26-18. Lavomatic SA tank washing machine The gearbox unit shown in Figure 26-19 houses the impeller assembly which converts energy in the fluid stream to rotary power to drive the machine. The drive end of the impeller assembly is supported in the gearbox casing by a static bearing sleeve and seal housing. The non-drive end of the impeller shaft is supported in a bushing located in an end cap fitted on the gearbox casing. The cross shaft is located above the idler shaft, from which the drive to the cross shaft is transmitted via a spur gear on 26-45 the cross shaft. The cross shaft assembly, together with the top and bottom nozzle angle adjusters, controls the total angular movement ofthe nozzle, the selected arc angles, and the automatic reversing of nozzle movement as programmed. A preselect facility is built into the assembly to enable the next new top angle setting to be selected during a wash cycle. The cross shaft assembly, through a dog clutch mechanism, drives a bevel gear keyed to the top of the drive shaft that turns the wash head assembly mounted on the bottom of the drop pipe. Where the drive shaft and the impeller penetrate the gearbox, double seals are provided. Any washing fluid seeping past the impeller seal or drive shaft a-rings drains into the tank and will not contaminate the gearbox. The rack end of the cross shaft is fitted with a shaft steady which is clamped to the gearbox casing by four screws. The two bevel pinion gears on the cross shaft are fitted with ball bearings. The drive for the main shaft is transmitted from the rotating bevel pinions on the cross shaft assembly to the bevel pinion keyed to the top of the main shaft assembly. In addition to the key, a castellated nut and split pin are also fitted for security. A robust thrust bearing is fitted below the bevel pinion to take the weight ofthe revolving wash head/nozzle assembly. Where the main shaft penetrates the gearbox, double a-ring seals are provided in a flanged plain bearing bush fitted to the top of the main shaft. The extension ofthe separate shaft protrudes through the top of the gearbox lid casing, and has a squared end to enable the machine to be wound manually. This shaft is supported in two self-lubricating bushes. Shaft sealing is provided by an a-ring fitted to a groove machined on the shaft. Manual drive is transmitted to the main drive via a bevel pinion keyed to the upper main shaft extension. As shown in Figure 26-20 the drop pipe assembly basically comprises a long tube and a solid drive shaft. The outer tube (i.e., the drop pipe) extends from the main deck level into the cargo tank. Inside the drop pipe is the drive shaft (which provides the rotary wash head movement) and the vertical nozzle movement by means of spur and worm gearing fitted on the shaft. The gearbox unit is bolted directly to the drop pipe. The drive shaft provides the nozzle vertical movement and the rotary movement of the wash head assembly. Gear teeth cut on the bottom of the main inlet casting teeth mesh with a double pinion gear mounted on an end cap. The end cap is keyed to the drive shaft. The upper end of the shaft passes up through the gearbox; the bottom end passes through the wash head, both secured by a castellated nut and split pin. The wash head/nozzle assembly is situated at the lower end of the drop pipe assembly inside the cargo tank. This assembly is illustrated in Figure 26-21. The assembly comprises three main parts: the main housing of gun metal, the flanged inlet housing of aluminum bronze, and the nozzle tube assembly. The main housing incorporates the worm and gear drive together with the nozzle tube assembly. The nozzle tube assembly primary 2 6 -4 8 INERT GAS SYSTEMS AND CRUDE OIL WASHING MACHINERY bend is secured to the main housing by means of a retaining plate fitted over the tube flange and secured in position by eight hexagonal headed bolts. An a-ring seal fitted between the flange and the main housing prevents leakage. An extension of the primary bend carries the worm wheel. This extension has four cast ports which allow the washing fluid to enter the nozzle tube assembly. Two a-ring seals are fitted to the extension to prevent leakage. c o n s i d e r a t i o n s . At three-month intervals, check the washing machine for signs of leakage and ensure that the gearbox lubricating oil is at the proper level. Check all nuts for tightness. Periodically manually rotate the cross shaft assembly to recoat gearbox internals with protective lubricating oil. At six-month intervals the gearbox should be checked for signs of water or crude oil, an indication ofleaking a-ring seals. Fill lubricating oil to the proper level during this check. W a s h in g m a c h in e m a in te n a n c e A P P E N D IX : EXTRACTS C O N C E R N IN G FROM CRUDE IM O O IL R E G U L A T IO N S W A S H IN G P ip in g 1. The COW (crude oil washing) system shall consist of permanent pipe work and be independent of the fire mains. (Flexible hoses can be used to connect the COW system to tank cleaning machines in cargo tank hatch covers.) 2. Pipes and valves shall be of steel or equivalent. The supply piping must have a pressure release device. 3. Hydrant valves must be blanked off. 4. The piping system shall be of such a diameter that the greatest number of tank-cleaning machines required can work simultaneously at the designed pressure and throughput. 5. The piping shall be tested to 11;2 times normal working pressure once installed. 6. No part of the COW system shall enter the machinery space. T a n k -C le a n in g M a c h in e s 1. Machines must be permanently mounted and must have their own valve to allow isolation. 2. Where separate drive units are used, there must be enough units on board to ensure no unit has to be moved more than twice from its original position to enable the washing cycle to be completed. 3. The number and location of machines in each cargo tank shall be such that all horizontal and vertical areas are washed by direct impingement or effectively by splash back or deflection of the impinging jet. In assessing an APPENDIX: EXTRACTS FROM IMO REGULATIONS 2 6 -4 9 acceptable degree of jet deflection and splashing, particular attention shall be paid to the washing of upward-facing horizontal areas and the following parameters shall be used: (a) For h o r i z o n t a l a r e a s of a tank bottom and the upper surfaces of a tank's stringers and other large primary structural members, the total area shielded from direct impingement by deck or bottom transverses, main girders, stringers or similar large primary structural members s h a l l n o t e x c e e d 10 percent of the total horizontal area of the tank bottom, the upper surfaces of stringers and other large primary structural members. (b) For v e r t i c a l a r e a s ofthe sides of tanks, the total area ofthe tanks' sides shielded from direct impingement by deck or bottom transverses, main girders, stringers or similar large primary structural members s h a l l n o t e x c e e d 15 percent of the total area of the tanks' sides. 4. To confirm the cleanliness ofthe tank, a visual inspection will be made after COW and prior to water rinsing. 5. To verify the effectiveness of the stripping and drainage arrangements, a measurement will be made of the amount of free oil floating on top of the d e p a r t u r e b a l l a s t . The ratio of the volume of oil (on top of the d e p a r t u r e b a l l a s t ) to the volume of tanks that contain this water, shall not exceed .00085. Note: This is 8.5 tons oil per 10000 tons water. So, for a 250,000- DWT tanker, this represents 212 tons of oil. 6. To verify the design, installation and operation of the system, the a r r i v a l b a l l a s t will be discharged through an oil-monitoring system and the oil content of the effluent m u s t n o t e x c e e d 15 p a r t s p e r m i l l i o n . 7. The d e c k - m o u n t e d t a n k - c l e a n i n g m a c h i n e s must have an external indicator to indicate the rotation and arc of movement of the machine. 8. The s u b m e r g e d m a c h i n e s must be non programmable to verify their operation. (a) They can have an external indicator. (b) Their characteristic sound pattern can be checked during operation (i.e., noise of the jet on deck structure). If more than one machine is mounted on the same supply line, they must have separate valves to verify operation of all the machines. (c) Their operation can be visually checked during the ballast passage. This check must be carried out after a maximum of six usages or at least once a year. Pum ps 1. The capacity of the pumps must be sufficient to provide the required throughput and the pressure for the maximum number of machines that will run simultaneously. T h i s m u s t b e m e t w i t h o n e p u m p i n o p e r a t i v e . 2. Where the back pressure presented by the shore terminal is below the required pressure for COW, provision must be made to ensure the pressure can be maintained to run all the machines (again with o n e p u m p i n o p e r a t i v e ) . If minimum supply pressure condition (usually 8kg/cm 2 ) cannot be met, c r u d e o il w a s h in g o p e r a tio n s s h a ll n o t b e c a r r ie d o u t. 26-50 INERT GAS SYSTEMS AND CRUDE OIL WASHING MACHINERY ACKNOWLEDGMENTS 26-51 Other nominated personnel must have had s i x m o n t h s ' experience on S tr ip p in g S y ste m 1. The stripping system must be able to remove oil at a rate of 1.25 times the total throughput of all the machines to be operated simultaneously when washing the bottom ofthe tank. 2. Verification of the stripping system wilI be by level gauges and "hand-dipping." Suitable arrangements for "hand-dipping" will be in the aftermost position of the tank as well as three other positions. 3. The line-and-pump draining must be discharged ashore through a special small diameter line connected outboard ofthe ship's manifold valve. This line shall not exceed 10 percent of the main cargo line for new vessels, or 25 percent for existing vessels already having fitted the line. O p e r a tio n 1. The number oftanks that have to be crude oil washed wilI be checked: (a) to ensure the vessel can comply with minimum draught and trim requiremen ts; (b) to ensure that any additional ballast water required during the voyage is not put into tanks that have not been crude oil washed; (c) in addition, approximately one quarter of the remaining tanks must be crude oil washed for sludge control. No tank for sludge control need be washed more than once every four months. 2. Crude oil washing c a n n o t be done during the ballast voyage. 3. Crude oil washing can o n l y be done at sea when the vessel is between multiple discharge ports. 4. The i n e r t g a s s y s t e m must be in proper use and prior to crude oil washing the oxygen level at a point o n e m e t r e from the deck and at the middle of ullage space m u s t n o t e x c e e d 8 percent by volume. 5. Crude oil washing m u s t b e s t o p p e d if: (a) Oxygen level of inert gas being delivered exceeds 8 percent by volume, or (b) The pressure of the atmosphere in the tank is no longer positive. (c) To avoid excessive electrostatic generation in washing due to the presence of water, the content of any tank to be used as a source of crude oil washing fluid must be discharged by at least one metre. Q u a lific a tio n of P erson n el Where a person, such as the Master, Chief Officer or Cargo Control Officer, assumes overall command of the COW operation, he must: 1. Have at least one year's experience on an oil tanker where his duties have included the discharge of cargo and associated crude oil washing; 2. Have participated in crude oil washing programmes t w i c e - < > n c e o n t h a t p a r tic u la r s h ip . o il tankers, where they had been involved in cargo discharge. ACKNOW LEDGM ENTS The editors are pleased to acknowledge and thank the following organizations for the material used in this chapter: Camar Corp. Inc., Worcester, Massachusetts Butterworth Systems Inc., Florham Park, New Jersey CHAPTER 27 C o a l B u r n in g T e c h n o lo g y EVERETT C. HUNT IN T R O D U C T IO N E c o n o m ic a n d P o litic a l F a c to r s VENTS in the decade 1970 to 1980 caused a rapid escalation of world costs beyond what anyone imagined possible. The economic impact and political implications of this fact have fostered an environment in which the historic trend toward use ofliquid fuels aboard ship is being reconsidered for certain applications. Even the original diesel engine was designed to burn coal dust. However, the discovery ofliquid hydrocarbons and the development oflow cost production techniques caused the marine industry to adopt residual fuels for both steam and diesel engine-propelled vessels. A rise in residual fuel cost and concern about their future availability are factors that can cause shipowners to consider coal once again as an energy source for ship propulsion. E fuel S p e c ia l C o n s id e r a tio n s D e s ig n in th e O p e r a tio n o f C o a l B u r n in g P r o p u ls io n and P la n ts The use of coal is, of course, conditioned by regulations both local and international, and by ship construction rules. Codes governing the building and operation of coal burning propulsion plants come from such sources as classification societies, the International Maritime Organization (IMO), the U.S. Coast Guard, and the U.S. Occupational Safety and Health Administration (OSHA). Coal availability is an important consideration in the selection of coal fired boilers. The first ships to change to coal will undoubtedly be those that follow trade routes with easy access to this fuel. Other conversions will follow as the logistical problems of worldwide coal distribution are solved under pressure of economic need. It will require an analysis to determine the types of coals available. This in turn will be a factor in the selection of particular coals and in the choice "'7 1 27-2 COAL BURNING TECHNOLOGY of firing methods. Due to the variety of coals which a ship may be required to use, the overfed stoker with traveling grate which can handle a wide range of coal characteristics will be the most common marine coal firing system. Other coal firing processes such as pulverized coal and fluid bed systems have serious disadvantages for shipboard application. The means of handling the coal and ashes will influence the operation and design of a coal burning vessel. In loading coal aboard the vessel, methods for controlling noise and dust must be provided while assuring rapid movement of the fuel. A choice must be made between mechanical and pneumatic systems of conveying coal to the day bunker and to the boilers. Ash transf~ r , storage, and disposal methods must be selected to meet economic and regulatory requirements. The owner and designer must give careful attention to the general arrangement ofthe ship for the economic utilization of coal. Considerations such as loss of cargo space, round trip bunker capacity, bunker geometry, need for cofferdams, inert gas systems, corrosion of bunkers, and watertight integrity are all important in providing a suitable general arrangement. Combustion control systems for induced draft and forced draft combinations will bring about an additional complication for coal fired vessels. Due to the slower response rate inherent in stoker fired systems, it will be necessary to have a steam dump connected to the main condenser to avoid lifting safety valves during routine maneuvers. The main condenser must be specially designed to accept the steam dump load. The steam power plant cycle will be influenced by the selection of coal fuel. Because of the increasing cost of oil fuels, the steam plant design trend has been toward higher efficiency with a consequent increase in complexity. Coal fired steam plants will have lower initial steam conditions and fewer feedwater heaters since these plant features cannot be economically justified with the lower cost coal. Figure 27-1 shows a typical proposed steam plant cycle for a 20,000 SHP coal fired vessel. COAL FUEL 27-3 age and degree of coalification. Bituminous coal is relatively low in moisture and high in heating value. Subbituminous and lignite are progressively higher in moisture content and lower in heating value. Although the classification system provides some information relative to a coal's application for steam generation, it does not provide sufficient information for the boiler designer or operator who needs a complete analysis to design and operate correctly all ofthe boiler components and to be able to predict boiler performance when coals of different characteristics are burned. Coal Analysis For marine applications, coal will often be acquired from more than one source and will vary in characteristics. This will require consideration of each of the several different coals to be used in order to design a boiler which will perform satisfactorily with each. It is important that analysis provided to the boiler designer describe the specific coals selected for use and that it not represent average figures derived from the analysis of a large number of coals. Using average results could lead to the adoption of a boiler larger than required or one which will not operate satisfactorily with the available coals. There are a large number of analyses available to the designer, some routine and others very specialized, all of which may be important in the design and operation of a boiler. Figure 27-3 shows an example of an analysis for normal design requirements. The American Society for Testing Materials Standards (Part 26) describes the procedures used for measuring the physical and chemical properties of coal. The significant properties are (a) total moisture content; (b) volatile matter, fixed carbon, and ash determined by proximate analysis; (c) carbon, hydrogen, nitrogen, sulfur, and oxygen determined by ultimate analysis; (d) BTU per pound measured by heating value; and (e) ash fusion temperatures. Effect of Coal Properties on Design and Operation COAL FUEL Physical and Chemical Characteristics In order to use coal effectively its physical and chemical characteristics must be known and the relationship of these characteristics to boiler operation and design understood. Marine engineers who specify and operate coal fired marine boilers must understand the methods a boiler designer uses to relate coal analysis to boiler design. Moisture Content and Heating Value Coals are identified or ranked by their moisture content and heating value (mineral matter free) as shown in Figure 27-2. This method is based on Coal moisture can effect boiler design and performance in many ways. Moisture tends to reduce flame temperature and increases the specific heat of the flue gases. Large changes in the moisture content can alter the heat transfer characteristics in the furnace and thus the steam temperature. Moisture will increase the volume of flue gas which in turn will affect induced draft fan size, boiler draft loss, particulate collection equipment size, and boiler efficiency. The extent of volatile matter in coal is an indication of the ease of its ignition, the higher the volatile matter in coal, the easier the coal is to ignite. Various types of firing equipment will require different limits of volatile matter for proper operation. Figure 27-2. Coal classification by moisture content and heating value. Courtesy Babcock & Wilcox. Fixed carbon is the combustible residue remaining after volatile matter has been driven off and represents the portion ofthe fuel which is burned in a solid form. Again, some types of firing equipment will require limitations of the amount of fixed carbon in the coal. The ash in coal has to be paid for, transported, conveyed, heated, collected, and disposed of, but adds nothing useful to the operation of the boiler. The amount of ash is used to determine the dust loading to particulate control equipment and to determine the capacity of the ash removal and storage systems, and it may influence the design of the soot blowing system. The chemical characteristics ofthe ash are important and will be described herein. The heating value of the coal will determine the amount to be stored, handled, and burned to provide the steam output required. It will directly affect the size ofthe burning and handling equipment. The ultimate analysis breaks down the coal composition into its elemental components in addition to moisture and ash. Carbon, hydrogen, and oxygen are used, together with BTU per pound and moisture, to calculate the combustion air requirements, flue gas weight and composition, and boiler efficiency. Sulfur content is important because the acid dew point of the flue gas is related to the percent of sulfur in the coal. Reduction of the flue gas temperature below the dew point can cause condensation on air heater or COAL FUEL 2 7 -9 The second type of deposition is called f o u l i n g . These high temperature bonded deposits are caused by constituents ofthe ash which are volatilized during combustion and condense on fly ash particles, tube surfaces, and existing deposits where the temperatures are such that the constituents remain liquid. These constituents react chemically with fly ash, other deposits, and flue gas to form the bonded deposit. The severity of fouling is related to sintering strength ofthe ash as determined in the laboratory. S la g g in g a n d F o u lin g Figure 2 7 - 5 shows the areas of a boiler which may be subject to slagging or fouling. In order to design a boiler for reliable operation it is necessary to minimize the tendency for slagging and fouling. By using a large number of coal analyses and comparing ash properties in operating boilers with those produced in laboratory combustors it has been possible to develop slagging and fouling indices. These indices reflect the severity of each type of deposition and are used by the boiler designer in a number of ways. C h a r a c te r iz a tio n Sulphur In fuel, percent b y weight (as fired) Figure 27-4. Tube metal temperatures to minimize corrosion. Courtesy Babcock & Wilcox. reducing conditions in a fuel bed and oxidizing atmospheres at the top of the furnace since both conditions may exist in a boiler. These temperatures are used to determine the applicability of a coal to a specific type of firing, the size ofthe furnace, and the slagging tendency of the ash. The softening temperature is the temperature used by the designer to set the furnace exit temperature. The ash spectrographic analysis provides an elemental analysis ofthe ash reported as the oxide. This information is the basic data needed to characterize the ash relative to its tendency to deposit on boiler heat transfer surfaces. Deposition is a major concern to the boiler designer and operator. Proper application of the data derived from this analysis is necessary to insure reliable operation of the boiler. There are two major types of ash deposition in addition to the low temperature deposits associated with flue gas dew point temperature previously described. The first of these, s l a g g i n g , is caused primarily by the physical transport of molten or partially fused ash particles entrained in the flue gas stream. When these particles strike a furnace wall or tube surface, they are chilled and solidify on the surface. The strength of their attachment is influenced by the temperature and physical contour of the surface, the direction and force of impact of the ash, and its melting characteristics. As a general rule, coals with low ash fusion temperatures have a high slagging potential. o f S la g g in g The first step in characterizing the slagging and fouling potentials of any coal ash is to determine whether the coal is a b i t u m i n o u s or l i g n i t e type since different tests for slagging and fouling potential for each type of ash have been found necessary and have been developed. A lignite ash is defined as one in which the total of CaO and MgO in the ash is greater than the amount of Fe203. When the amount of Fe203 is greater than the total of CaO and MgO, the ash is considered a bituminous type. This is shown in Figure 27-6. Figure 27-14. Sintering strength versus time. Courtesy Babcock & Wilcox. The boiler designer will relate the appropriate fouling potential data to boiler design in several ways. A coal producing an ash with a severe fouling potential as compared to one with low fouling potential will require: (a) lower furnace exit gas temperature; (b) fewer tubes in the direction of flow; (c) increased open area between tubes; and (d) additional soot blowers. Figures 27 -15 and 27 -16 illustrate two applications of the fouling index. Figure 27 -15 shows the relationship of superheater tube spacing to fouling potentials for two different gas temperatures. Figure 27-16 indicates the need for low soot blower cleaning radii for high fouling ashes, particularly at high gas temperatures. The goal of the boiler designer is to prevent slagging and fouling from occurring. Therefore, the operator must be sure that he uses coals for which the boiler was designed. S p e c ia l C o a l A n a ly s is There are several special analyses which are of interest particularly when the coal in question is unknown to the designer or for which there is no burning experience in the type of installation under consideration. The first of these is the b u r n i n g p r o f i l e . This test employs a derivative thermogravimetric technique in which a sample of the fuel is oxidized under controlled conditions. The burning profile, shown in Figure 27 -17, is a plot Figure 27-17. Burning profile for typical coals. Courtesy Babcock & Wilcox. The second test is the v o la tile r e le a s e p r o file which is also a plot of the rate of weight loss of a coal sample versus the furnace temperature. However, in this case the sample is heated in an inert atmosphere rather than oxidized. The principle function ofthis test is to indicate the ignition characteristics of the fuel. It has been found that fuels having the same amount ofvolatile matter do not always release it at the same temperature and therefore have different ignition characteristics. By comparing the profile of an unknown fuel with those of fuels with known ignition characteristics the boiler designer can gain valuable information. One property of coal which does not usually appear on a standard analysis, but which can be very important in shipboard applications of coal firing, is bulk density. This is the density of coal as handled naturally or stored with no compaction. Bulk density depends on a number of factors, such as ash content, moisture content, and sizing, and is used to size bunkering and handling equipment. Im p o r ta n c e o f C o r r e c t F u e l A n a ly s is From the preceding discussion it is apparent that the boiler designer has a number of tools at his disposal which make it possible for him to design an economical and reliable boiler. However, the design will only be as good as the fuel analysis he has to work from. Unlike an oil fired boiler where the changes in the characteristics of that fuel rarely require major changes in boiler configuration, major changes in boiler design can be brought about from utilization of coal. It is 27-18 COAL BURNING TECHNOLOGY obvious then that the boiler designer needs the best information about the coal or coals to be used to design the best boiler for all specified conditions. Similarly, the boiler operator must have the analysis of all coals loaded in the ship's bunkers and analysis must comply with the range of acceptable coal for which the boiler has been designed and which the manufacturer specifies in the operating instructions. Otherwise, the boiler operator may unwittingly defeat the purpose of the designer. C O M B U S T IO N COAL F IR E D E N G IN E E R IN G M A R IN E (C -E ) B O IL E R S G eneral The following sections describe modern marine coal fired boilers. For information on marine boilers in general including description, operation, and maintenance, see Chapters 10 and 11 of M o d e r n M a r i n e E n g i n e e r 's M a n u a l , Volume 1. C -E V 2 M 9 S B o ile r The C-E V2M9S boiler is similar to an oil fired unit except that it has been suitably modified to fire coal and is physically larger for any given horsepower than its oil fired counterpart. The cross section ofthe V2M9S boiler is shown in Figure 27-18. The V2M9S boiler design has a dropped furnace and is supported at the midpoint by the sidewall header and the water drum saddles. As compared to a bottom or top supported unit this midsupport feature reduces the effects ofthe forces imposed on the boiler as a result of the vessel's motion. It also minimizes the effects of thermal expansion. The furnace consists of welded wall panels in way of the roof, front, rear, and side walls, and this construction continues on the sidewalls to the beginning of the generating bank. A single row of 2-inch OD screen tubes on 41;2-inchcenters originating from the screen header and terminating in the steam drum is utilized to reduce the radiation heat input entering the superheater. One row of screen tubes is satisfactory for these units as compared to oil fired units because the gas temperature leaving the furnace is considerably lower due to the significantly lower furnace release rates utilized when firing coal. The superheater has a primary and a secondary stage, and each is of the vertical in-line, multipass type. The primary superheater consists of a series of 4-loop elements; each element is 2-inch OD tubing, on 41;2-inch centers. This spacing is the same as that utilized for the screen thereby providing clear gas paths through the screen and first stage of the superheater. Tube spacing is wider than normally utilized for an oil fired unit to preclude ash buildup. 27-20 COAL BURNING TECHNOLOGY The secondary superheater consists of a series of 4-100p assemblies, each element manufactured from 2-inch OD tubing on 3-Y2-inchcenters. Because some of the heavier cinders will drop out into the reinjection hopper between the stages due to the reduction in velocity as the gases pass through the primary superheater, assemblies are placed somewhat closer together in the secondary superheater than they are in the primary superheater. The superheater floor is sloped to direct the cinders which collect in this area into the hopper for reinjection. The steam generating bank consists of in-line tubes of l-Y2-inchOD on 2-Y2-inchcenters. An economizer is installed either at the boiler outlet or is separately supported by the ship's structure. It is of the conventional counterflow multibank type utilizing extended surface elements of spiral finned design. Elements are installed in-line, and fin spacing is considerably greater than for an oil fired unit. Each element is manufactured from 2-inch OD tubing utilizing a spiral steel fin which is 3/4-inch high by .105-inch thick with l-Y2fins per inch. A balanced draft system is utilized for each boiler which requires fans for forced draft (FD) and for induced draft (lD). Because this system results in a slightly negative pressure in the furnace and eliminates gas and soot leakage, a single casing construction is permitted for the boiler. In the furnace area, the welded wall panels act as the casing and in all other areas a single layer of reinforced sheet steel forms the casing. In addition to the FD and ID fans, a small overfire fan is required. In providing air which is injected above the fuel bed through overfire air nozzles located in the rear wall of the furnace and below the distributors, additional turbulence in the furnace is afforded. A suitable number of soot blowers are installed to ensure adequate cleaning of all external surfaces from the superheater through the economizer. As shown in Figure 27-18 two retractable soot blowers are installed before the screen to clean the leading edge ofthe primary superheater. Two additional ones are also installed in the access space between the primary and secondary superheater stages to ensure adequate cleaning ofthe rear portion of the primary stage and the leading edge of the secondary stage. Rotary blowers are installed in the generating bank and in the economizer. From the screen tubes through the generating bank and in the economizer, the in-line tube configuration, as well as the wide tube spacing, assists the soot blowers to ensure adequate cleaning of all external surfaces. C -E C o n tin u o u s D is c h a r g e Spreader S to k e r Coal is fired on a C-E continuous discharge spreader stoker. Across section of this equipment is shown in Figure 27-19. The traveling grate portion 27-22 COAL BURNING TECHNOLOGY comprises carrier bars and keys and, depending upon the size and application of the equipment, is driven from the drive shaft through either a hydraulic cylinder activated ratchet device or an electric motor powered gear. The idler shaft end is fitted with Belleville spring devices which are used to maintain optimum chain tension and to permit compensation for chain elongation during operation. Return rails are located at the underside of the grate and serve to support and guide the grate during operation. The combination of the tensioning device, the return rails, and the use of thrust collars on the drive and idler shafts provides for excellent resistance to the forces caused by ship motion during operation. The speed of the grate is adjusted to maintain a constant thickness of ash at the discharge end, usually 2 to 3 inches. The maximum grate speed is about 15 to 20 feet per hour with a 6 to 1 reduction to provide for reduced ratings. The standard direction of grate travel is toward the distributors with the ash discharging at the stoker front into a hopper located below. A thermocouple monitoring system is provided at the top ofthe grate supports just beneath the chain to warn of abnormally high grate metal temperatures. The coal spreading units are located at the front end ofthe grate directly above the drive shaft. Two or more of these units are fitted depending upon boiler capacity and the grate dimensions. A constant head of coal is maintained at the spreader inlet by the coal forwarding equipment. The spreader contains a feeder and a distributor. The coal feeder is a variable speed device regulated by the combustion control system to admit coal to the distributor rotor located directly below it, in response to boiler pressure. This device performs the same function as a fuel oil control valve. The function of the distributor is to properly distribute the coal supplied by the feeder onto the grate. The larger coal particles are thrown to the rear ofthe grate where they will have the maximum time to burn and the smaller particles which require less burning time are thrown to the front of the grate. Depending upon the type of coal and the mixture of particle sizes, the speed ofthe distributor is varied manually to obtain an optimal coal spread. This adjustment is necessary when new bunkers are received. Even distribution of coal on the grate is important in obtaining smokeless combustion at reasonable excess air levels. In order to obtain an even distribution, a good mixture of fines and coarse particles must be present in the coal fired. For optimum results, the following coal sizing criteria are recommended: (a) top size 11;2inches; (b) through a 1f4-inchround hole screen 95 percent; and (c) through a 1f4-inchround hole screen no more than 50 percent. In most cases this consistency can be achieved by use of a coal crusher without the necessity of screening out excessive fines. Actually a certain percentage of fines is beneficial in fully utilizing the furnace of a spreader stoker fired boiler. As the coal is thrown out from the COMBUSTION ENGINEERING (C-E) 27-23 distributor, the fines ignite and burn in suspension while the larger particles fall and are burned on the grate. The volatile matter, which in most stoker grade coal ranges from 25 percent to 35 percent, is distilled off from the larger particles and is burned above the grate as a gas. Therefore, one can readily observe that an excessive trend toward coarse or fine particles could upset one or more of the simultaneous combustion processes, resulting in a loss of combustion efficiency. Approximately 80 percent of the combustion air is admitted to the interior section ofthe grate by means of wind boxes located on each side of the grate. Perforations and channels in the grate keys allow the air to pass through the grate thereby cooling it as well as providing air for the combustion process. Zone air control dampers are located in the grate interior to allow air flow adjustments to be made in order to accommodate wind box design variances as well as to allow for adjustments to obtain good combustion at reduced ratings. The air to this section is provided by a forced draft blower(s) and is proportioned by the combustion control system to the amount of coal fired. This air may be preheated to a maximum temperature of 400°F, depending upon the cycle selected. C -E O v e r fir e A ir S y s te m Approximately 20 percent ofthe combustion air is supplied by overfire air nozzles which are located in the furnace waterwall above the grate. These nozzles are supplied by a separate fan which is not controlled by the combustion control system. This air is supplied at approximately 25 inches water gauge static pressure and is not preheated. The purpose of this air is to provide agitation above the grate to assure good mixing of the fines and gases which are burning above the grate with the combustion air. C in d e r R e in je c tio n S y ste m The overfire air system additionally provides an air source which is used to reinject a portion of the coal ash into the furnace for reburning. This arrangement is shown in Figure 27 -20. The ash hopper located below the superheater cavity collects the larger unburned coal particles which exit the furnace. This area is a logical place for large particle dropout due to the slowing ofthe gas velocity as it exits the front superheater stage prior to entering the second superheater stage. At the base ofthe cinder reinjection ash hopper, a rotary air lock valve is fitted to prevent combustion gas from being circulated in this system. The rotary valve feeds the cinders into the reinjection piping where a portion of the overfire air then propels them into the furnace for further burning. Boiler efficiency is improved approximately 4 percent by use of this system. COMBUSTION ENGINEERING (C-E) 27-25 A s h H a n d lin g The coal used for stoker firing, depending upon origin, may have an ash content as high as 20 or 25 percent. The boiler must be designed with provisions to remove and store the ash for its eventual disposal. There are four locations for the removal of ash from the boiler system. Most of the ash (approximately 70 percent) falls off the end of the traveling grate at a point just below the spreader unit, as shown in Figure 27-21. This ash will be hot and may still contain some glowing embers. Therefore, the ash collection hopper must be refractory lined. A small amount of ash sifts through the stoker keys and falls into the interior of the grate. This ash falls out of the grate as it turns on the idler shaft and drops into the siftings hoppers. A third ash collection point is located below the exit ofthe boiler main bank which catches particles which drop out of the gas stream as it turns to exit the boiler. The final ash collection point is located at the base ofthe mechanical dust collector. The ash collection system is designed for a 24-hour capacity assuming full power operation with coal containing approximately 15 percent ash. I n most cases, this would require ash disposal once per day and would allow for several days in port without the necessity of handling ash. The ash handling system selected is a pneumatic system. A steam eductor utilizing 150 psi auxiliary steam creates the vacuum in the ash handling system piping. Valves or gates are then opened sequentially at the base of each ash collection hopper. A vacuum switch located at each hopper signals the ash handling system that a particular hopper is empty before the system empties the next hopper. Clinker grinders are fitted at the base of the main grate discharge hoppers to insure that large ash particles do not enter and jam the system. The other hoppers will contain only fine ash which does not require further processing prior to removal. Depending upon trade route and possible regulatory body or governmental regulations, the ash removed from the boiler ash hoppers may be discharged directly overboard or stored onboard the vessel for shore-based disposal. Some combination of the two may be used. A ir a n d G a s S y s te m s Combustion air is supplied by two systems: the forced draft system, which supplies 80 percent of the air required, and an overfire air fan, which supplies 20 percent of the total. The overfire air is supplied at ambient temperature and is not preheated. The combustion air does not require preheating. However, where cycle economics dictate, preheating up to a limit of about 400°F is possible depending upon the steam cycle utilized. The combustion air is supplied at a positive pressure up to the point of its introduction into the furnace. The furnace itself is maintained at a negative pressure of approximately 0.1 inch or 0.2 inch water gauge by use COMBUSTION ENGINEERING (C-E) 27-27 lofan induced draft fan which is located at the base ofthe ship's stack. The Intire boiler with its attendant heat recovery equipment and dust collector il therefore protected against gas and soot leakage under normal circumstances. The forced draft and overfire air fans are typical marine fan designs and require no special consideration. The forced draft fan contains a two-speed motor and is inlet vane controlled. The overfire air fan has a lingle speed motor with inlet vane control. Due to its elevated operating temperature and the erosive and corrosive nature of the flue gas which it handles, the induced draft fan requires special consideration. Typically, a mechanical dust collector would be fitted prior to the induced draft fan inlet. Depending upon the efficiency of the dust collector, 95 percent or more of the dust present in the flue gas may be removed. If a low efficiency collector is fitted (collection efficiency 85 percent) the induced draft fan is limited to 900 RPM maximum to avoid excessive blade erosion. If a high efficiency collector is fitted (collection efficiency 95 percent or greater) then the maximum fan speed may be raised to 1,200 RPM. Three-speed fans are utilized. Maximum fan RPM is only necessary at boiler overload. The medium speed is selected for boiler full power or normal load, whichever is the prevalent operating rate. Low speed is for use in port only. By selecting these speed ranges, fan blade life may be increased significantly. The induced draft fans contain replaceable fan blade wear surfaces to avoid the entire fan rotor having to be pulled for worn blades. The induced draft fan flow is controlled by inlet vane dampers which are actuated by the combustion control system in response to furnace pressure and rate changes. Dust Collectors Due to the environmental restrictions on stack opacity which are in effect for potential ports of call, coal fired vessels are fitted with some form of mechanical or other type of dust collector. Depending upon collector efficiency, most visible flue gas particles will be removed. The dust collector is located after the economizer and immediately prior to the induced draft fan. Depending upon the type of coal fired and the economizer surface selected, it may be desirable in some cases to locate the dust collector immediately after the boiler bank rather than after the economizer. Because of the increased gas temperature, this alternate location will significantly increase the size and cost of the collector. A centrifugal-type dust collector is shown in Figure 27-22. This device removes dust by creating a cyclone-type motion as the dust-laden gas enters the annular space around the collector tubes. The dust then drops into the hopper below and the cleaned gas moves up the center of the cyclone vortex through the center ofthe collector tube. This device has no moving parts and, other than infrequent water washing, requires little or no maintenance. Depending upon the diameter and type of precipitator COMBUSTION ENGINEERING (C-E) 27-29 is then brought up to pressure by feeding coal to the grate. As required by various regulatory agencies, the ignitor has a fan and flame scanner. The fan also supplies cooling air to the ignitor after it is secured. S u p p le m e n ta r y O il F ir in g Depending upon the operating requirements of the vessel and projected trade route, it may be desirable to have a supplementary oil firing capability. This can be readily accommodated by installing a conventional oil fired burner register(s) in the upper furnace section. As long as a 2- or 3-inch protective layer of ash remains on the grate, and a small amount of cooling air is supplied to the underside of the grate (supplied by the forced draft blower), oil firing may be continued indefinitely. The combustion air is supplied by the same forced draft blower that supplies air to the grate for coal firing. The induced draft fan is also utilized during oil firing; however, the overfire air system is secured. Supplementary oil firing capability substantially increases the complexity of the boiler control system by requiring the installation of a burner management system as well as increasing the complexity of the combustion air ductwork. C -E B o ile r P e r fo r m a n c e Table 27-1 shows typical performance for a single coal fired C-E boiler applied to a 24,000 SHP plant. TABLE Figure 27-22. Mechanical dust collector. tubes used in the mechanical dust collector, collection efficiency of 95 percent may be achieved. If a higher collection efficiency is desired, two cyclone separators located in series may be utilized. Collection efficiencies in excess of 98 percent are then possible. C o a l B e d I g n itio n A coal fired boiler is initially started by building a wood fire on top of the grate and then spreading coal which quickly ignites. For a unit which must be restarted at infrequent intervals this method is quite simple and adequate. However, if frequent boiler shutdown is required, a light oil ignitor in the lower furnace section should be installed and sized to fire number 2 oil at a rate of 200 to 250 pounds of oil per hour. The light-off sequence involves lighting off the ignitor and preheating the grate to approximately 700"F (measured by the grate thermocouple monitoring system) then starting the distributors and depositing a layer of coal on the grate. M t e r the deposited coal ignites, the ignitor is secured, and the boiler T y p ic a l P e r fo r m a n c e 2 7 -1 o f a C -E V 2 M 9 S C o a l F ir e d B o ile r Number of boilers Normal power Operating condition Rating HHV fuel Superheater outlet pressure Feedwater teperature to economizer Steam pressure to air heater Superheater steam flow Steam temperature Efficiency Fuel firing rate Air temperature leaving air heater Air flow at 100°F Gas flow Total draft loss Excess air Furnace release rate 1 24,000 SHP Normal 100% 9,387 BTU/lb 870 psig 302°F 55 psig 165,6001b/hr 920°F 82.7% 24,5751b/hr 278°F 55,397 CFM 253,943 Ib/hr 7 inch WG 30% 3 25,009 BTU/hr-ft 27 -30 COAL BURNING TABLE TECHNOLOGY 2 7 -1 (c o n tin u e d ) 7,038 BTU/hr-ft2 10.861b coal/hr-ft2 1,900°F Absorption rate/RHAS Firing rate/RHAS Furnace exit temperature Coal fired boilers have lower efficiency than comparable capacity oil fired boilers. The difference in efficiency is due to carbon loss which is unburned in the ash and the loss of moisture inherent in the content of coal. These losses will vary depending on the analysis of the purchased coal. O p e r a tin g P rocedure O il B u r n e r d u r in g FOSTER WHEELER (F -W) COAL FIRED BOILER 27 -31 2. Purge furnace (FD and ID fans running). Set fans to go to low fire after purge. 3. Light coal ignitor, monitor grate temperature. Spread layer of coal on grate when grate temperature reaches 700°F (371 °C). 4. Start grate once fire is established, and secure coal ignitor (ignitor fan will remain on to provide cooling for ignitor throat). 5. Start overfire air fan. 6. Bring boiler up to pressure at low firing rate. 7. Close superheater drains upon reaching 3.5 kg/cm 2 (50 psi). 8. Put boiler on line and close superheater vents when boiler is up to pressure and steam flow can be established. 9. Firing rate minimum will be approximately 25 percent of maximum load. Steam dump will be in operation below this point. L ig h t-O ff w ith a n d C o a l I g n ito r FO STER W HEELER (F -W ) COAL F IR E D B O IL E R 1. Open superheater drains and vents. 2. Purge furnace (FD and ID fans running). Set fans to go to low fire after purge. Establish fire with coal ignitor. 3. Spread layer of coal on furnace when grate reaches 700°F. 4. Allow enough air under stoker to burn coal. 5. Establish 50-75 mm (2-3 inches) of ash on stoker grate. Monitor grate temperature and keep below 1,000°F by adjusting air flow under grate. 6. Bring boiler up to pressure at low oil fire. When pressure reaches 3.5 kg/cm2 (50 psi), close superheater drains. 7. Establish steam flow when boiler is up to pressure. Put boiler on line, and close superheater vents. 8. Spread coal on grate, and establish stoker fire with coal ignitor on leaving port. 9. Shut down coal ignitor once fire is established. Keep ignitor fan on in order to cool ignitor throat. 10. Start grate and overfire air fan. Bring coal rate up to hold boiler load. 11. Shut down oil burner (some leakage will be required to cool oil burner throat). O p e r a tin g P rocedure C o m in g in to P o r t 1. Reduce coal rate. 2. Establish oil fire. 3. Shut off coal. Keep a layer of ash on grate. 4. Take load on oil. Shut down grate. 5. Monitor grate temperature damper. to hold under 1,000°F by adjusting stoker air 6. Shut down overfire air fan and coal ignitor fan. O p e r a tin g P rocedure d u r in g L ig h t-O ff w ith C o a l I g n ito r 1. Open all superheater vents and drains. G e n e r a l D e s c r ip tio n The Foster Wheeler (F-W) stoker coal fired boiler is a marine "D"-type that is supported at the lower drum and from the furnace walls just above the stoker grate. Figures 27-23 (a) and (b) are sectional views of a F-W stoker coal fired marine boiler. The pressure vessel sections of this boiler, including steam drum, water drum, headers, tubes, superheater, and economizers, are all very similar to the oil fired boiler. Arrangements to accommodate and remove ash deposits on the heating surfaces are the only significant differences from the oil fired boiler design in practice. This is also designed for oil firing for in-port or maneuvering operations of the ship. A ir a n d F lu e G a s F lo w Air for combustion is delivered by a forced draft fan and heated by a steam coil air heater. The warmed air then enters the plenum chamber under the continuous ash discharge grate. The air is evenly distributed through the grate via orifices and grate clearances. The air mixes with the coal particles on the grate and in suspension to complete the combustion process. The flue gas then passes through the screen tubes and superheater banks and continues through the generating tube section. The flue gas enters a mechanical dust collector to remove the fly ash particles before entering the economizer. The cleaned flue gas then enters the induced draft fan and flows out through the stack. The coal fired boilers are designed for balanced draft operation, achieved by maintaining a slightly negative pressure (.1 inch water gauge) in the furnace. The induced draft fan controls the furnace pressure, and the forced draft fan controls the air flow to insure proper combustion in the furnace. Figure 27-23 (a). F-W stoker fired boiler. Courtesy Foster Wheeler Boiler Corp. Figure 27-23 (b). Typical arrangement of single boiler. Courtesy Foster Wheeler Boiler Corp. C oal F eed A higher pressure overfire air fan is used to blow air through nozzles over the coal bed on the front and rear walls to optimize combustion condition in the freeboard volume above the grate. The stoker size coal (11/4 inch) with fines not more than 40 percent is supplied to the boiler coal day bunkers from the main bunkers by a mechanical or pneumatic transport system. 2 7 -3 4 COAL BURNING TECHNOLOGY The coal day bunkers have multiple outlets to feed each mechanical spreader stoker for the unit. The number of spreaders depends on the size of the boiler, but will range from two to six per boiler. A variable speed mechanism feeds the coal to the rotating paddle wheel spreader. The spreader paddle wheels evenly distribute the heavier coal particles on the continuous ash discharge stoker grate. A small portion of the coal fines are burned in suspension before they reach the grate. The stoker grate is moving forward continuously to dump the ash formed on the grate during the burning of the coal. A minimum depth of ash is maintained on the stoker grate to protect the moving parts from overheating. The unburned carbon and fly ash are carried in the flue gas and collected in the boiler outlet hopper and a mechanical dust collector. This mixture of fly ash and unburned carbon is reinjected into the furnace just above the stoker grate to improve the boiler combustion efficiency. F ly A s h S y s te m The ash from the various hoppers is pneumatically transported overboard or to ash holding bins. The holding bins can be unloaded by the same methods upon reaching port with a facility for receiving the fly ash. This system normally operates under vacuum produced by a steam exhauster or mechanical exhauster. The ash from the various collection points is pneumatically conveyed to the ash holding bin. The system cycles through each ash collection point on a timed basis to keep the hoppers from getting overfilled. The system can be designed to allow ash transfer to the land side equipment or possibly pumped overboard when at sea. Figure 27-24 is a schematic diagram of the ash system. O p e r a tio n a l C o n s id e r a tio n s Some units are furnished with load carrying oil burners for low load and in-port operation. These burners are located far enough above the stoker grate to prevent overheating of the grate. It is normally recommended that the stoker grate remain covered with ash during oil firing periods. This provides additional protection to the stoker grate system. The coal firing can be established manually by igniting some easily combustible material such as wood or rags. Newer units may be equipped with oil ignitors close to the stoker grate to establish the coal bed burning. Once the coal ignition is stabilized, the ignitors are removed from service. Close operator observation of the furnace combustion conditions is recommended to maintain satisfactory combustion over the load range. The supervision of the operating equipment for coal and fly ash handling is very important to maintain continuous, reliable operation of the boilers. Figure 27-24. Arrangement of ash system. Courtesy Foster Wheeler Boiler Corp. P e r fo r m a n c e Tables 27-2 and 27-3 show performance comparisons between coal fired and oil fired boilers for typical single boiler and two boiler shipboard arrangements. 27-36 COALBURNINGTECHNOLOGY TABLE C o m p a r is o n 2 7 -2 o f C o a l F ir e d B o ile r w ith F u ll L o a d O il F ir in g (Single Boiler Arrangement) Boiler capacity, lb/hr Design pressure, psi Superheater outlet pressure, psi Superheater outlet temperature, OF Feedwater temperature, of Stack temperature, of Boiler efficiency, % Fuel rate, lb/hr* Furnace volume, cu ft Furnace ratings: Liberation, BTU/cu ft Firing rate, lb/hr sq ft RHAS WeighUship (based on single boiler arrangement) Boiler dry, lb Stoker,lb Total operating, lb Basic dimensions, per boiler Furnace width Overall boiler width (fore and aft) Furnace depth Overall boiler depth Height between drums at centerline Height, furnace floor of stoker to top of drum TABLE C o m p a r is o n O il F ir e d C o a l F ir e d 162,800 1,100 870 950 284 330 87.8 12,319 10,790 162,800 1,100 870 935 284 330 83.9 17,959 10,790 21,770 4.87 21,927 7.11 Liberation, BTU/cu f t Firing rate, lb/hr sq ft RHAS WeighUship (based on two boiler arrangement) Boiler dry, lb Stoker,lb Total operating, lb Basic dimensions, per boiler Furnace width Overall boiler width (fore and aft) Furnace depth Overall boiler depth Height between drums at centerline Height, furnace floor of stoker to top of drum M A R IN E COAL S h ore B ased 618,000 808,000 618,000 190,000 808,000 18'- 3W' 26'- 0" 18'- 0" 41'- 0" 14'- 3" 43'- 2" 18'- 3W' 26'- 0" 18'- 0" 41'- 0" 14'- 3" 43'- 2" - 27-37 MARINECOALHANDLINGSYSTEM H A N D L IN G O il F ir e d C o a l F ir e d 26,809 3.32 28,056 5.36 650,000 750,000 650,000 100,000 750,000 8'- 0" 16'- 0" 12'- 0" 32'- 6" 14'- 3" 35'- 4" 8'- 0" 16'- 0" 12'- 0" 32'- 6" 14'- 3" 35'- 4" - SYSTEM C o a l P r e p a r a tio n Proper preparation of coal prior to loading aboard ship will minimize most coal handling problems. The land based coaling station must perform a series offunctions to prepare coal for shipboard use including the following: 1. Crush coal to optimal size. 2. Wash coal to remove clay. 3. Screen coal to remove oversize pieces, noncombustibles, and trash. 4. Dry coal to reduce surface moisture to a point where it does not cake and will not be dusty (5 to 8 percent surface moisture for most coals). 5. Remove iron from coal by passing it over a magnetized head. 2 7 -3 o f C o a l F ir e d B o ile r w ith F u ll L o a d O il F ir in g (Two Boiler Arrangement) Boiler capacity, lb/hr Design pressure, psi Superheater outlet pressure, psi Superheater outlet temperature, of Feedwater temperature, of Stack temperature, OF Boiler efficiency, % Fuel rate, lb/hr* Furnace volume, cu f t Furnace ratings: *Fuel oil: 18,500BTU/lb;stoker coal: 12,500BTU/lb. O il F ir e d C o a l F ir e d 47,925 1,100 870 900 297 330 87.8 3,362 2,413 47,925 1,100 870 900 297 330 83.9 5,406 2,413 S h ip b o a r d C o a l H a n d lin g S y ste m (S to c k ) The Stock Equipment Company has proposed the typical coal handling system for shipboard use shown in Figure 27-25. The coaling station (or lighter) should be equipped with a hammerhead crane mounting 1 or 2 belt conveyors which carry the coal to a hatch on the fore and aft centerline of the vessel. The belt conveyors place the coal on 1 or 2 shuttle conveyors. These are belt conveyors mounted on carriages which allow them to be moved back and forth over the bunker for layer loading of the coal. The shuttle conveyors are reversible so that port and starboard sides will be filled alike. A movable inlet chute is required for this mode of operation. Operation of the shuttle conveyors is manually controlled to provide the highest reliability. A gallery at the upper part ofthe boiler room allows the operator to control the operation. Bull's-eyes along the bunker provide for visual inspection. These are hinged-type portholes for cleaning of the 2 7 -3 8 COAL BURNING TECHNOLOGY bunker side when necessary. Bull's-eyes must be in the closed position at all times when underway in order to maintain watertightness of the bulkhead. The coal bunker runs the full usable width of the ship. It is as high as possible consistent with ship stability considerations. The bunker length varies according to the BTU ofthe coal anticipated and the cruising range desired. The greater the number of bunker outlets provided, the more coal can be stored in a given bunker volume. The fewer the number of bunker outlets, the lower the investment cost for coal handling machinery. These factors are carefully weighed in each design. There are 16 outlets at the bottom of the bunker shown in Figure 27-25. Each outlet is 18 inches square. The bunker pockets are designed with a slope angle of not less than 50'. The lower half of each bunker outlet pocket is made of type 304 stainless steel. No roughness or ledges are permitted in this area since these may initiate stoppage of coal flow. Even a horizontal welding bead may cause coal flow stoppage in a hopper. The coal valves are equipped with motor operators to make the system automatic. The outlet is somewhat larger to provide a minimum face to face dimension without creating space under the gate for tramp coal to collect. With the arrangement shown, tramp coal falls directly down into the drag conveyor. A drag conveyor is a heavy malleable iron chain with stainless steel pins and cotters, running in a trough. When the chain moves, coal moves with it. In this design, two 80-foot drag conveyors operate simultaneously in parallel with one another. One conveyor runs from starboard to port and the second from port to starboard. This arrangement loads the gravity discharge conveyor elevator without permitting coal segregation. Both drag conveyors are always run at the same time. There is only one coal valve open on the port side and one coal valve open on the starboard side when the conveyors are operating. The gravity discharge conveyor elevator consists of two chains, one on each side connected by a V-shaped bucket. This bucket acts as a flight to push the coal along horizontal runs and to carry the coal in the bucket vertically in the vertical runs. At the end of a horizontal run there is a chain sprocket that allows the chain and bucket to go upward. The flight which has pushed the coal horizontally now carries the coal vertically. In this design, there is a length of the gravity discharge conveyor elevator operating horizontally underneath the drag conveyors. This turns upward to carry coal the full height of the bulkhead before ending in another short length ofthe horizontal conveyor that moves the coal to the day bunker. As shown, the forward and aft sides ofthe bunker continue down to the bottom ofthe vessel and make a watertight bulkhead. In such an arrangement, watertight access doors are required for access to the conveying machinery under the bunker. 27-40 COAL BURNING TECHNOLOGY The day bunker receives coal from the gravity discharge conveyor elevator. This bunker will have sufficient capacity for between 30 minutes and 2 hours of full power steaming. This should be adequate if fed by a reliable conveying system under automatic control. To achieve automatic operation, a paddle-type stop switch in the upper part of the day bunker stops the gravity discharge conveyor elevator. A timer on a 20-minute or half-hour cycle restarts the coal conveyors. The automatic controls will open and close the coal valves so that coal is withdrawn from each bunker outlet equally and therefore all bunkers will become empty at approximately the same time. At the bottom of the day bunker, there is a manually operated coal valve. Normally, this valve is used to shut off the flow of coal when the ship is reaching port, allowing the conical non segregating coal distributor and the stoker hopper to be emptied if desired. The conical nonsegregating coal distributor shown has a conical bottom and top plate. Therefore, coal is placed into the stoker hopper without segregation of the coarse and fine pieces. The tops of the stoker hopper are closed with dust-tight slip joints, making a final dust-tight enclosure for the entire system. I n front of the slip joints is an access door for inspection of the coal flow if desired and a drive gate to shut off the flow of coal to any particular stoker hopper. O p e r a tio n a l C o n s id e r a tio n s Bunker fires are not a problem with the system described unless the type of coal used is subject to spontaneous combustion. The small day bunker permits the bunker to be emptied at frequent intervals, reducing the bunker coal temperature and the chance of fire. The entire coal handling system below the bunkers is dust-tight, eliminating the air ingress and the consequent risk of bunker fires. Provision for the introduction of C02 into the bunkers is another means of controlling bunker fires. Coal flow problems will be minimized if proper preparation of the coal ashore reduces the surface moisture to acceptable levels. The shore removal of oversized coal, foreign material, tramp iron, and clay are important to the free flow of coal in the shipboard system. The traveling shuttle conveyors provide for layer loading of the bunker to reduce bunker coal segregation. Dual drag conveyors loading the gravity discharge conveyor elevator from opposite directions should eliminate segregation at that point. The gravity discharge conveyor elevator loads the day bunker along a line perpendicular to the boiler front. I f loaded parallel to the boiler front, coal segregation in the day bunker would go through the conical distributor, and uniform firing would be difficult to maintain. STOKER SYSTEM (DETROIT ROTOGRATE) 27-41 Since the coal conveying system is designed to be dust-tight, it is possible to introduce a slight vacuum in the bunker during coaling operations. The coal-dusty air is discharged through filter bags to control dust during loading. This typical automatic mechanical coal handling method is the system used in many shore-based power plants. Other coal handling systems such as a pneumatic system are available and may be applied to shipboard use. STO K ER SYSTEM (D E T R O IT ROTOGRATE) I n tr o d u c tio n The typical automatic stoker system (the Detroit RotoGrate) is illustrated in Figures 27-26 and 27-27. The system includes fuel receiving hoppers from which the fuel is picked up by revolving rotors and distributed into the furnace. The rotor blades are curved in a manner which causes fuel to be distributed uniformly on the grate. The fine particles offuel are burned rapidly before they fall to the traveling gate. High pressure overfire air jets are located around the furnace wall above the grate to provide mixing of the fuel and air needed for complete combustion. The heavier fuel particles are spread evenly on the grate to form a thin, fast-burning fuel bed. The traveling grate discharges ash continuously over the front end ofthe grate into the ash receiver. The grate is a hinged bar design which permits the individual grate bars to open at the lower portion of the catenary during the return to the rear of the furnace as illustrated in Figure 27-27. This facilitates air admission to the fuel bed and permits discharge of any ash which drops through the grate. Figures 27-27(a) and 27-27(b) show the self-adjusting air seal arrangements at the front and rear ends ofthe traveling grate. The bearings for the coal feeder drums are water cooled to prevent failure from overheating. Shearing devices are installed in the grate drive assemblies to prevent damage to the system from foreign material in the coal. L ig h tin g -O ff P r o c e d u r e a marine boiler, the normal operating procedure will be to raise steam and maneuver the ship from port using the oil fired operating mode. During this time the grate must be covered with about 2 inches of ash to protect it from overheating. When ready for coal firing, the coal hoppers are filled and the stoker drive motor is started. The stoker is operated manually by occasionally engaging the feed latches (4 in Figures 27-28 and 27-29) while adding coal a little at a time until all the coal on the grates is ignited and burning freely. The oil burners must be turned down as necessary to maintain drum pressure. The feed latches may be engaged and coal fed continuously at In Figure 27-27(a). Front end of chain grate assembly. Courtesy Detroit Stoker Co. Figure 27-26. Detroit stoker system. Courtesy Detroit Stoker Co. this time. The arms positioning crosshead (5) should now be raised to the top notch of the link for feed control (7). The handwheel (6) is backed off to prevent overfeeding coal. Once the fire has been started and the fuel feed and distribution adjusted, the grate drive can be started. . In adjusting the speed of the grate drive, the governing factors are the load being carried and the thickness of the ash coming off the front end of the grate. Normally the ash should be 3 to 4 inches deep where it comes off the grate. If the ash is too thin, the grate is moving too fast. The grate speed may be reduced by turning the handwheel on the safety release link to variable speed reducing. (The variable speed reducing unit must always be running when making this speed change adjustment.) If the ash is too thick, it will tend to form clinkers and carry unburned coke forward on top of the clinkers. If this condition develops, the grate speed should be increased by turning the handwheel on the safety release link in a clockwise direction. If the ash bed is too thin, as little as two inches or less, and forms clinkers with unburned coke riding forward on top of the clinkers, it indicates that the fuel bed is too heavy and the C02 is too high. This condition will often cause smoke to be emitted from the stack. To correct this condition the coal feed is reduced by turning the handwheels (shown as 6 in Figures 27-28 and 27-29) counterclockwise one-quarter turn a time until the desired results are obtained. C o m b u s tio n C o n tr o l A d ju stm e n ts The most satisfactory method of adjusting the combustion control is to start at the minimum or closed position, take gas samples to determine the percentage of C02, adjust the coal feed and fan damper to produce the correct coal-to-air ratio, then move the power jack or piston about 2 inches, Courtesy Detroit Stoker Co. and again check the C02 at the furnace outlet. Ifsatisfactory C02 readings are obtained at the low position and low C02 readings at the open position, it will be necessary to increase the arc of travel of the coal feed control shaft. When making this adjustment the connecting arms to notched links ( 1 2 ) on the coal feed control shaft ( 1 3 ) must be reset so that the crossheads will be at the top ends ofthe slotted arms ( 2 ) when the master regulator is at the closed or minimum position. Low C02 can also be corrected by reducing the air flow. This can best be done by moving the pin in the fan damper arm to a longer radius, thereby reducing the travel as well as the maximum opening ofthe fan damper. In making this change the closed position of the fan damper will have to be reset when the master regulator power unit is in the closed or minimum position. The link to the fan damper must be lengthened to bring the damper back to the original setting at the closed position. The air pressure under the grates should increase about in proportion to the square of the load on the unit. 1 F e e d p la t e 10 I n n e r a r m o p e r a t in gf e e d p la t e 2 S lo t t e d a r m 11 R o t o r h o u s in g ,lo w e r 3 L in k lo c a t in gc r o s s h e a d 12 C o n n e c t in ga r m t o n o t c h e dlin k 4 L a tc h fo r c o a l fe e d 13 S h a ftfo r c o n tro l 5 A r m lo c a t in gc r o s s h e a d 14 C r o s sh e a d 6 A d ju s t in gh a n d w h e e l 15 O u t e r a r m o p e r a t in gf e e d p la t e 7 N o t c h e dlin k f o r c o a l f e e d c o n t r o l 16 S h o u ld e rs c r e w f o r a r m lo c a t in gc r o s s h e a d 8 S p illin g p la t e 18 B e a r in gf o r c o n t r o ls h a f t 9 S c r e w f o r s p illin g p la t e 24 S if t in g p a n u n d e rf e e d e r Figure 27 -28. Coal feed assembly. Courtesy Detroit Stoker Co. By continuing this process of combustion control adjustment, it is possible to establish the proper relation between the coal feed and the air supply so that a given setting ofthe coal feed adjusting handwheels (6) and the crossheads (14) will give a rising characteristic of C02 leaving the furnace as the control opens. This total rise should be approximately 3 or 4 percent to change C02 in the furnace from about 9 or 10 percent to about STOKER SYSTEM (DETROIT ROTOGRATE) 27-47 13 to 14 percent C02 at the open or maximum position of the master regulator. C oal F eed 2 S lo t t e d a r m 15 3 L in k lo c a t in g c r o s s h e a d 16 O u t e r a r m o p e r a t in g f e e d p la t e S h o u ld e rs c r e w f o r a r m lo c a t in g c r o s s h e a d 4 L a tc h fo r c o a l fe e d 17 E c c e n t r iclin k a s s e m b ly B e a r in g f o r c o n t r o l s h a f t 5 A r m lo c a t in g c r o s s h e a d 18 6 A d ju s t in g h a n d w h e e l 19 P in f o r n o t c h e d lin k 7 N o t c h e d lin k f o r c o a l f e e d c o n t r o l 20 S c r e w f o r f e e d la t c h 12 C o n n e c t in ga r m t o n o t c h e d lin k 21 P in f o r a r m a n d lin k 13 S h a ft fo r c o n tro l 22 S c r e w & s p r in g f o r h a n d w h e e l 14 C r o s sh e a d 23 P in f o r c r o s s h e a d Figure 27-29. Coal feed control. Courtesy Detroit Stoker Co. C o n tr o l The coal feed is automatically controlled by the master regulator of the combustion control through a system of arms or links or a special power jack connected to the stoker control shaft and from there to the crossheads (14) located in the slotted arms ( 2 ) . Raising the crossheads in the slotted arms decreases the coal feed and lowering the crossheads in the slotted arms increases the coal feed. The crossheads are positioned by using one of the several notches in the links (7). The proper notch to be used depends on the amount of coal required, the size ofthe coal, and the amount of moisture in the coal. The crosshead position should be lower for coarse or wet coal than for fine or dry coal. If it is found that proper coal feed cannot be obtained with the adjustments outlined above, the travel of the feed plates (1) can be increased or decreased by changing the location of the pin, bushing, and block assembly in arms ( 1 0 ) thereby increasing or decreasing the coal feed. Placing the pins and blocks farther from the shaft increases the travel of the feed plate thereby increasing the coal feed and vice versa. (There are three locations for the pin and block, and feeders are shipped with the pins in the center holes. Therefore, if making a change in block location, all feeders of the stoker must be changed to correspond.) In case too much coal is being fed to the grates when the crosshead (14) is carried in the upper notch of the links (7), the adjusting hand wheels (6) should be turned counterclockwise one-quarter or one-half turn at a time until the desired coal feed is obtained. Where not enough coal is being fed when the crossheads are being carried in the upper notch oflinks (7) and the handwheels are turned up tight, the crosshead should be lowered to the second notch. If this setting feeds too much coal the handwheel can be turned one-quarter turn at a time counterclockwise until the desired amount of coal is obtained. The crossheads should always be carried as high as possible. One-half turn lost motion of the handwheels (6) is normally equivalent to one notch in links (7). In other words, if the crossheads are carried in the second notch with the handwheels backed off one-halfturn or more, the crosshead should be raised to the top notch and the handwheel readjusted for the desired condition. The notches in the links are the coarse adjustment and the handwheels are the fine adjustment of the coal feed. The adjustments should be kept the same on all feeders of the stoker provided the coal in all hoppers is of uniform size and moisture content. M te r the coal feed has been properly adjusted, the master regulator will raise and lower the crossheads in the slotted arms in proportion to the load requirements, as indicated by 2 7 -4 8 COAL BURNING TECHNOLOGY changes in steam pressure. It will likewise increase or decrease the air supply and pressure under the grates and change stoker grate speed. F urnace D r a ft R e g u la to r A d ju stm e n t The furnace draft regulator of the combustion control system is provided to maintain the furnace draft, or draft over the fire, at a predetermined point. It is desirable to hold the furnace draft somewhere between negative 0.05 and 0.10 inches water column. The exact setting will have to be determined to suit the shipboard installation. Best results can be obtained by holding the furnace draft as near zero as possible without either causing gas to escape from around the doors or leak through the setting or overheating ofthe stoker parts exposed to the furnace. The furnace walls and the ash must also be considered in respect to amount of draft, for if held too low, the walls may slag and the ash may fuse into clinkers. C o a l D is tr ib u tio n Strictly speaking, there is no fuel bed with the Detroit RotoGrate Stoker shown in Figure 27-30. The depth ofthe burning coal on the grates or on top of the ashes will vary from practically nothing to about 1.0 or 1.5 inches, depending on the size and characteristics ofthe fuel. It is of importance to prevent the burning coal from reaching a greater thickness than this because of the tendency to smoke and the possibility of forming clinkers. The fuel bed is kept level by maintaining the proper adjustments ofthe rotor speed and the proper position of the spilling plate. Normally it is recommended that the spilling plate (8) be moved forward until the center rib bears against the end ofthe adjusting screws (9) by turning a screw (9) clockwise, and then controlling the fuel distribution by adjusting the rotor speed. To feed the coal towards the rear end of the grate, speed up the rotors by turning the handwheel on variable speed drive adjusting base to decrease centers between the motor and the driven pulley. To feed the fuel forward on the grate or towards the front end of the stoker, turn the handwheel on variable speed drive adjustable base to increase distance between centers of the motor and the driven pulley. For normal operation turn the handwheel about one revolution at a time and wait for the fuel bed to readjust itself. Ifmechanical coal handling equipment is used to fill the stoker hopper, and one rotor section of a hopper consistently receives either coarser or finer coal than another section, this inequality can be compensated for in fuel size by adjusting the spilling plate (8) using the adjusting screw (9). To feed the coal toward the rear of the stoker, move the spilling plate (8) forward by turning the adjusting screws (9) clockwise. To feed the coal Figure 27-30. Coal feed assembly installation. Courtesy Detroit Stoker Co. toward the front end of the grate, move the spilling plate rearward by turning the adjusting screws (9) counterclockwise. Coarse, nut, and dry coal will feed farther back for the same setting than will fine or wet coal. For coarse or dry coal, screws (9) should be turned counterclockwise. For fine or wet coal, screws (9) should be turned clockwise. The fuel distribution should be adjusted so that the coal will build up on the grate just at the forward end ofthe rear tuyeres. lfthe coal piles up on top ofthe rear tuyeres, the coal is being fed back too far; if the coal does not feed back to the rear tuyeres, it is not being fed back far enough. 27-50 COAL BURNING TECHNOLOGY G r a te S p e e d A d ju stm e n t The grate speed is determined to a large extent by the load being carried, the amount of coal being burned, and the percentage and characteristics of the ash in the fuel. Normally the ash should be maintained at a thickness of 3 to 4 inches where it comes off the front end of the grate. With some fuels this may be too thick due to the clinkering characteristics of the ash. When the ash fuses into sheet clinkers with unburned coke riding forward on top of the clinker, it indicates that the grate speed is too slow. The grate speed should be increased by turning the handwheel on the safety release link to variable speed reducing unit to lengthen the safety release link. The above is based on the assumption that the coal feed control is properly adjusted and that too much fuel is not being fed. Coal feed control can be determined by observing the fire, the gases leaving the stack, and the carbon dioxide (C02). If the fuel feed is too heavy, the furnace will be smokey, the carbon dioxide (C02) will be too high, and the stack will be smoking. A change in the grate speed will also have some effect on the distribution adjustment. I f the grate speed is increased, it may be necessary to increase the rotor speed in order to keep the fuel back to the forward edge of the rear tuyeres. And, if the grate speed is reduced, the rotor speed will need to be reduced to keep the coal from piling on top of the rear tuyeres. The safety release link for the variable speed reducing unit disengages if the control shaft is rotated while the grate drive variable speed reducing unit is idle or stopped. I n order to synchronize the relation between the grate speed and the coal feed, a handwheel is provided on the end of the safety release link, to increase or decrease its length. Increasing the length of this safety link assembly proportionally increases the speed of the grate for any given coal feed adjustment and vice versa. To increase the length of the assembly the locknut is backed off, the handwheel is turned clockwise, and the locknut is retightened. Should the plunger assembly disengage when the grate drive variable speed reducing unit is operating, the plunger is reengaged. To prevent repetition ofthis difficulty the tension on the spring is increased by tightening the pipe cap. S a fe ty D e v ic e s Safety shear pins are provided on the drive shaft for protection of each of the feeder units. If foreign materials such as tramp iron, wood, etc., lodge in the feeder mechanism, the small pin in the safety device will shear and stop the feeder before breakage occurs. Very often when the feeder is stopped, the foreign material will dislodge itself, but if this does not happen and the cause of stoppage is not readily removed, it may be necessary to remove the lower part of the rotor housing (11). This is accomplished by taking out 2 or 3 bolts at the upper front edge of the lower section of housing STOKER SYSTEM (DETROIT ROTOGRATE) 27-51 allowing this section to be dropped and slipped forward for inspection of the rotor drum and blades. It is suggested that when the feeder is open, a check be made to insure no accumulation of grease and coal dust around the ends ofthe rotor drum. Mter the feeder is cleaned, the cover should be replaced and bolted securely. Before replacing the shear pin the clutch must be disengaged. The coal feed latch (4) should also be disengaged, and after the pin is in place, the shear pin assembly is rotated by hand to insure that the feeder is entirely free. Ifthe shear pin assembly and rotor shaft turn freely, the clutch may be reengaged, putting the feeder back in service. Safety devices or shear pins are provided for the protection ofthe grate driving mechanism. Two shear pins are located in the large sprocket mounted on the end of the front grate drive shaft. When these pins shear, it is very important that the grates and grate supporting chains and mechanisms be inspected. This is done by entering the wind box or air chamber through the access door in the bottom of the air chamber. It is also necessary to inspect the underside of the grates by entering the air chamber through the access door in the side support. When these pins shear, some foreign material has probably been wedged at some point. Mter the obstruction has been removed, new pins can be inserted in the shear pin bushings of the large sprocket. It will be necessary to start the grate drive and rotate the large sprocket until the holes of the shear pin assembly are in line. O v e r fir e A ir a n d C in d e r R e tu r n S y ste m The high pressure fan of the system serves to return the cinders and fly ash from the hoppers under the boiler mud drum, economizer, air heater, dust collector, or other traps to the furnace in the case of pneumatic systems and to the intake manifold of cyclone assembly in the case of pneumatic gravity flow systems. The same fan also serves in supplying air to create furnace turbulence~ The cinder return fan should be started when the fire is started and remain in operation as long as there is fire in the furnace. All settling hoppers should be thoroughly cleaned out after erection as any foreign matter such as pieces of brick, mortar, etc., will obstruct the openings of the cinder return pickup assemblies. Hoppers should be kept empty down to the level of the pickup assemblies and should also be inspected periodically to make sure that the cinders are being picked up. For pneumatic gravity flow systems the cyclone vanes and tubes should be inspected whenever there is a boiler outage to make certain there are no obstructions. To check the operation of each cinder carrier jet, look through the fly ash observer to see whether cinders are passing through the carrier pipes. 2 7 -5 2 COAL BURNING TECHNOLOGY Required air pressures will have to be determined in the field as each installation has its own peculiarities of layout and operating conditions which make adjustment necessary. The character offuel burned must also be taken into consideration. In the case of the pneumatic gravity systems the setting of the slide gate in the exhaust duct from the gravity cinder return system manifold should be determined by taking pressure readings in the fly ash settling hopper of the system. This is done by setting the slide gate for a pressure in the settling hopper which will always be on the plus side of the negative pressure, or draft, found at the rear of the furnace and at the stoker grate line to eliminate any possibility of flue gases short-circuiting up through the discharge pipes of the fly ash settling hopper ofthe gravity flow system. It is very important that the entire cinder return system be maintained in good operating condition and kept in operation at all times when the stoker is carrying a fire. If the high pressure fan is shut down while there is a fire in the furnace, the hot gases and flame may short-circuit through the cinder return piping and cause serious damage to the system. If the straight gravity flow cinder return system is used, the high pressure fan will serve only for overfire air and turbulence purposes. I n s p e c tio n fo r C lin k e r s Periodic inspection should be made of the fuel bed by observing the fire through the front fire and access doors and the side wall observation doors or ports. Ash or small clinkers may have a tendency to build up at the front end of the grates at either side next to the sidewalls. If this condition develops, these clinkers should be removed by using a hook through the front door. If they are not removed, they will probably grow in size and impede the normal progress of the ash to the front end discharge point. A pron T uyeres The apron tuyeres are designed to admit air into the furnace over the fire and under the coal stream leaving the feeders. These tuyeres are connected to the overfire air duct mounted below the feeders at the stoker front. The dampers should be set below each feeder so that the air pressure at the apron tuyeres is just sufficient to keep them cool and free from buildup. The apron tuyeres should be examined every time the unit is shut down for inspection. If the tuyeres should become plugged and the flow of air shut off, they will heat up and, in time, burn out if this condition is not corrected. When the apron tuyeres become overheated, the coal sticks to the top surface and obstructs the rotor opening. This affects the fuel distribution. Fuel piles up near the front end of the grate which overheats the front wall and causes considerable damage. STOKER SYSTEM (DETROIT ROTOGRATE) C oal F eeder 2 7 -5 3 o r R o to r O p e r a tio n The stoker should not be operated with one feeder or rotor shut down while the grate drive is in operation because the grate section back of the idle feeder would soon become bare and be exposed to high furnace temperature. This would probably result in grate bar warpage or other damage. If for any reason it is found necessary to stop one feeder for a short period, the grates should also be stopped, so that they remain covered with ash and thus are protected from the radiant heat ofthe fire. A s h P it C le a n in g The ash pit should be cleaned periodically long before it becomes nearly filled. If the fire should get out of control for a short time and it is found necessary to run hot clinkers, ash, and coke over into the ash pit, the ash pit should be cleaned promptly. If this refuse is allowed to remain there, it may fuse into a solid mass of clinkers. S to k e r O u t o f S e r v ic e Whenever the boiler and stoker are taken out of service for cleaning and inspection or repairs, the coal hoppers should be emptied before letting the fire go out. If the stoker is stopped with coal in the hoppers, the coal may ignite and cause serious damage to the rotor or feeder assemblies. The grate drive should be stopped as soon as the hoppers are empty so that the ash will remain on the grates and serve as an insulator against the high furnace temperature. The feeder rotors should be kept rotating, with feed latches disengaged so that rotor drum temperatures will be kept uniform on all sides and prevent warping. Three to four hours after the fire has gone out, the furnace temperature should be sufficiently lowered to permit starting the grate drive and discharging the ashes into the ash pit leaving the grates uncovered for inspection. Im p o r ta n t O p e r a tin g C o n s id e r a tio n s Keep reasonably clean all settling chambers in the boiler, breeching, and heat traps where cinders and ash can accumulate. Keep the ash pits clean. Keep the combustion control system in proper adjustment and working freely. Always keep cooling water flowing through water cooled bearing casings when stoker is in operation. Always keep the cinder return fan running when stoker is in operation. Grease all bearings at least every eight hours. Check oil level of mechanical drive gear case daily. Check oil level of enclosed variable speed reducing units daily and check chain tension after every 1,000 hours of operation. 2 7 -5 4 COAL BURNING TECHNOLOGY Clean and oil all silent chain drives at regular intervals. Clean fans and fan blades and repair any blades that may have become loose or damaged. Clean and check motors and starters periodically. Examine grates frequently to be certain that all air holes are open. Keep coal and air properly proportioned so that fire burns clean and without smoke. Do not allow coal to accumulate on top of the deflector tuyeres, back of the coal hoppers, and above each feeder opening. Check all moving parts frequently and replace worn or burned parts. COMBUSTION CONTROL SYSTEM Introduction Figure 27-31 is the logic diagram for a combustion control system developed by the Forney Engineering Company. A combustion control system regulates the amount of air and fuel to maintain a preset steam pressure for any boiler load and for any rate of change in boiler load. In doing so, a predetermined ratio is maintained between the fuel and the air. A pneumatic two-element combustion control system is selected for this application, whereby the complete range from maximum overload to minimum load can be obtained by firing oil only, coal only, or a coal and oil combination. The system has 3 modes of operation: 1. Automatic control (through pneumatic control functions and solid-state burner management interface protection); 2. Remote manual control (operated from the manual/auto stations in the control room console); and 3. Local manual control (operated at the boiler, by means of mechanical devices on the final control elements). As shown in Figure 27-31 the combustion control system consists of seven subloops (plant master controller, steam flow feed forward, FD fan controller, ID controller, coal fire control, oil fire control, and burner management interface), each of which is briefly described in the following text. Plant Master Controller A steam pressure transmitter senses the pressure in the common superheated steam header. The output signal from the transmitter is compared with the set point and through proportional and integral action of the master controller, a load demand signal is forwarded to both boilers. High limit and low limit relays are included in the controller output line, with 2 7 -5 6 COAL BURNING TECHNOLOGY a feedback to prevent the controller from winding out of control range. A master manual/auto station for each boiler with a built-in adjustable bias function provides the capability to divide the load share between the boilers. The plant master controller is common to the two-boiler plant. All other subloops are provided in duplicate, one for each boiler. S te a m F lo w F eed F orw ard The steam flow is measured by means of a flow nozzle in the saturated steam line. The output signal of the steam flow transmitter is introduced as a rate of change action signal in the impulse relay (d/dt). As a result, whenever the steam flow changes, the control signal receives a temporary impulse to commence a firing rate change which is proportional to the rate of load change. This control action anticipates the change in steam pressure that will result in a firing rate change of the master controller. The result is a faster response from the combustion control system. The same impulse signal from the steam flow is used in the FD fan and the ID fan control loop. F D (F o r c e d D r a ft) F a n C o n tr o lle r The function of the FD fan control loop is to provide a constant combustion air pressure in the FD fan discharge ducting. An air pressure transmitter senses the pressure on the discharge side of the FD fan. The output signal from the transmitter is compared with the set point and, through proportional and integral action of the FD fan controller, a demand signal is forwarded to the inlet vane damper actuator. High and low limit relays with controller feedback are included to prevent the controller from winding out of control range. A manual/auto station is included for remote manual operation of the inlet vane damper actuator. ID (In d u c e d D r a ft) F a n C o n tr o lle r The function of the ID fan control loop is to provide a constant furnace pressure. A furnace pressure transmitter senses the pressure in the furnace. The output signal from the transmitter is compared with the set point and, through proportional and integral action of the ID fan controller, a demand signal is forwarded to the ID damper actuator. High and low limit relays with controller feedback are included to prevent the controller from winding out of control range. A manual/auto station is included for remote manual operation of the ID damper actuator. C o a l F ir e C o n tr o l The coal fire control loop controls the amount of coal and corresponding combustion air to satisfy the plant master demand. When the coal firing mode is selected, the load demand signal from the plant master controller COMBUSTION CONTROL SYSTEM 2 7 -5 7 is introduced into a proportional plus reset airflow controller which directs the undergrate damper actuator to the desired position. An undergrate airflow transmitter is installed, measuring the differential pressure across an air duct resistance as a function of under fire airflow. Via a square root extractor and a coal/air ratio relay, the transmitter output signal becomes the feedback signal to the airflow controller, thus closing the airflow control loop. The output signal from the coal/air ratio relay is also introduced into a characterizing relay which is preprogrammed to correct the signal for any nonlinearity of the stoker feeder. The programing also includes an additional amount of predetermined excess air at lower firing conditions. The output signal from the characterizing relay directs the stoker feeder actuator to the desired position. The grate speed is linked directly to the stoker feeder so that the traveling speed ofthe grate is directly proportional to the amount of coal fed onto the bed. When firing coal, an overfire air fan provides a fixed amount of combustion air which is blown into the furnace across the surface of the coal bed. Approximately 15 percent of the full load airflow is thus introduced and compensated for by the coal characterizing relay. Manual/auto stations are provided for remote manual operation of both the undergrate damper actuator and the stoker feeder actuator. O il F ir e C o n tr o l The oil fire control loop controls the amount of fuel oil and corresponding combustion air to satisfy the plant master demand. When the oil firing mode is selected, the load demand signal from the plant master controller is introduced into a proportional plus reset airflow controller which moves the air damper actuator on the entry ofthe oil firing wind box to the desired position. An airflow transmitter measures the differential between wind box pressure and furnace pressure. Via a square root extraction and an oil/air ratio relay, the transmitter output signal becomes the feedback signal to the airflow controller, thus closing the airflow control loop. The boiler load demand signal is introduced into a minimum selector relay. The output signal from the oil/air ratio relay is also introduced into this minimum selector relay, where the lower of the two input signals (actual airflow and boiler demand) is passed on as a fuel demand signal. The oil/air ratio relay is adjustable and actually subtracts the desired percentage of excess combustion air from the total amount of air available in the wind box. Thus the relay output signal becomes the stoichiometric combustion air signal and the corresponding fuel oil signal. The fuel oil demand signal is entered into a characterizing relay which is preprogrammed to correct the signal for any nonlinearity of the fuel oil control valve. The programming also includes an additional amount of predetermined excess air for low fire conditions. The output signal from the characterizing relay moves the fuel 2 7 -5 8 COAL BURNING TECHNOLOGY oil pressure control valve to the desired position. Manual/auto stations are provided for remote manual operation of both the air damper actuator and the fuel oil control valve. Burner Management Interface The burner management interface instrumentation has the function of assuring that protective conditions are satisfied prior to lighting the oil burners or the coal bed. The burner assures management interface that the fuel supply to the boiler is limited or shut off whenever a condition exceeds preset limits. The burner management interface accomplishes these functions by overriding the regular combustion control signals. The burner management interface consists of a set of solenoids and pressure regulators which can be divided into the following functions. Boiler prepurge. The prepurge section has the function of assuring that the furnace is thoroughly purged with combustion air prior to allowing the initial fire. Via solenoids, a maximum air signal is introduced to the undergrate damper actuator, the wind box air damper actuator, and the inlet vane damper actuator which forces these dampers to the full open position. Upon completion of the purge function, the solenoids are released, and the dampers return to a position which corresponds to the combustion control demand signals. Oil light-off and low fire. Prior to lighting off the first oil burner, low fire control signals are introduced to the air damper actuator and to the fuel oil control valve. These signals force the air damper and the fuel oil control valve to a position corresponding with the required amount of air and oil for lighting off a burner. As soon as the oil flame is established (proven by the flame scanner), the oil solenoid is released back to the combustion control. The airflow solenoid is released when the flame is established, the drum pressure is satisfactory, and the coal/oil selector switches are set properly. Coal light-off and low fire. Prior to introducing coal to the stoker, a low fire control signal is introduced to the undergrate damper actuator. This signal forces the undergrate damper to a position corresponding to the minimum airflow through the grate. As a result of the combustion control system, the stoker feeder will be in the minimum feed position. Positioning the coal/oil selector switches in the desired mode will release the coal light-off solenoid and will return the operation back to the combustion control. COMBUSTION CONTROL SYSTEM 2 7 -5 9 Coal/oil select. A two-position coal selector switch introduces the master demand signal to the coal firing control loop. In the same way, a two-position oil selector switch introduces the master demand signal to the oil firing control loop. Before the first fire is introduced into the furnace, the operator turns the selected fuel switch to the "on" position. This will open the corresponding solenoid, and the control signal can enter the subloop. If oil firing is selected and the oil firing switch is therefore in the "on" mode, the coal switch must be in the "oft" mode. When the flame is established and the drum pressure is satisfactory, the oil light-off solenoid will be released, and the combustion control will command the oil and the corresponding amount of combustion air. To initiate coal firing, the coal selector switch must be turned to the "on" position. The coal fire will then be initiated, but will remain in the low fire position as long as the oil burner is operated and controlled by the combustion control. To increase the base coal fire load, the manual/auto station for undergrate air and/or the manual/auto station for the stoker feeder can be switched to manual. The fuel oil demand will automatically decrease proportionally as the coal firing is manually increased. When the coal fire is established, the oil firing can be stopped by switching the oil selector switch to the "oft" position. This results in shutting down the oil flame while at the same time it releases the coal firing from the light-off mode, so that the combustion control takes over control of the coal and undergrate air, proportional to the steam pressure demand. The next time oil firing is desired while coal fire is on, the oil selector switch must be switched to the "on" position. A fifteen-second wind box purge will be initiated to remove any coal dust from the wind box. Then the oil burner will be ignited, but the oil will be held in low fire position as long as the coal firing is modulated by the combustion control. To increase the base oil fire load, the manual/auto station for the air damper and/or the manual/auto station for the fuel oil valve can be switched to manual. The coal demand will automatically decrease proportionally. To summarize, in the automatic mode, the first fuel (coal or oil) switched in the "on" position will modulate automatically in response to the combustion control demand. The other fuel can be either shut down (switch in "oft" position) or in base load condition (switch in "on" position). This base load can be increased by switching the corresponding manual/auto station into manual mode and the other fuel demand will decrease automatically. 27 -60 COAL BURNING TECHNOLOGY ACKNOWLEDGMENTS CHAPTER 28 The editors are indebted to the following companies for permission to publish the information included in Chapter 27. Babcock & Wilcox, New York, New York Combustion Engineering, Inc., Windsor, Connecticut Detroit Stoker Company, Monroe, Michigan Forney Engineering Company, Addison, Texas Foster Wheeler Boiler Corp., Livingston, New Jersey Stock Equipment Company, Cleveland, Ohio Waste Disposal Systems EVERETT C. HUNT INTRODUCTION HIP designers and ship operators have always found it necessary to deal with trash, garbage, and sewage resulting from the ship's operation. In the past when environmental concerns had not yet reached a high priority, waste materials were usually dumped in the oceans. Untreated sewage was often flushed in the world's harbors while the vessels were in port. Since the early 1970s, a complicated set of national and international regulations, laws, protocols, and agreements have placed restrictions on the discharge of waste into the sea, and have required ship designers to incorporate new systems to handle the waste. As a result, the shipboard engineer officer has additional systems to operate and maintain. S General Restrictions on Discharge into the Sea Garbage and trash. The Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter at Sea (LDC 1972) prohibits discharge of all plastics. Dunnage and similar floating materials may be discharged within twenty-five nautical miles ofland. Food waste and other garbage may be discharged within 12 nautical miles of land. Current trends in international agreements suggest that complete prohibition of the discharge of garbage and trash into the oceans will be required in the future. Sewage and wastewater. No ship is permitted to discharge untreated sewage in any U.S. controlled waters. There is no restriction, however, on the discharge of gray waste, i.e., sink and shower drains. Oil and oily mixtures. A series of international agreements place complicated restrictions on the discharge of oil or oily mixtures from any vessel. Some of these restrictions are discussed in Chapter 26. • . .n .• 1 2 8 -2 WASTE DISPOSAL SYSTEMS SEWAGE TREATMENT The methods now in use on ships for the treatment of sewage include the following: collection and retention; maceration and chlorination; physical separation; physical/chemical treatment; and biological treatment. Collection and Retention Collection and retention systems involve, as the name implies, no more than simple holding tanks. These tanks must be capable of retaining the collected waste until it can be discharged ashore, or in unrestricted waters that are acceptable. Where there are no shore receiving facilities, it may be necessary to retain the waste for a considerable period. Under such circumstances measures must be taken to prevent septic conditions occurring which generate toxic, corrosive, and inflammable gases. Maceration And Chlorination Macerator/chlorinators are simple and relatively inexpensive systems. They include a macerator which discharges the sewage into a small holding tar.k in which the waste is treated with a metered dose of disinfectant (usually a sodium hypochlorite) and held for approximately 30 minutes before it is discharged overboard. However, this type of unit is not usually capable of discharging an effiuent product acceptable to the legislating authorities. Physical Separation A number of systems utilize physical separation of sewage. This generally involves the use of some form of filter, and in some instances includes a settling tank. The separated liquid effiuent is disinfected before being discharged overboard. Sludge accumulated by filtration and settling must be stored or incinerated. If stored, measures have to be taken to prevent the onset of septic conditions. DESIGN OF SEWAGE TREATMENT SYSTEMS 2 8 -3 of these processes have been applied to the treatment onboard ships, and have become, for the most part, the preferred systems. The basis of all these treatment variations is nature's own purification process in which microorganisms use the waste as food material. Two variations of the biological method found in modern marine sewage treatment units are extended aeration and the trickling filter. In the extended aeration process the sewage is held in an aeration tank for approximately 24 hours, and air is continuously bubbled through the liquid (mixed liquor, as it is called). Mixed liquor is then displaced by incoming sewage into a settling tank where the biological floc is formed and allowed to settle under quiescent conditions. The settled material (activated sludge) is continuously withdrawn from the bottom of the settling chamber and recycled to mix with the waste material entering the aeration tank. Clear effiuent from the top of the settling tank is discharged after disinfection. The trickling filter process utilizes the ability of certain aggregate materials to hold large communities ofmicroorganisms, allowing the waste to trickle over the material and the organisms to remove the substrate. The effiuent draining from the bottom of the filter then settles and any entrained floc is removed before it is disinfected and discharged overboard. Variations of this process utilize submerged filters with air bubbling through the material to provide the oxygen required by the microorgamsms. Assessment of the advantages and disadvantages of these processes must include such factors as ease of maintenance, degree of supervision required, running costs, capital costs, and, where installation in existing vessels is being considered, the cost of installation. The designer of shipboard equipment must take into consideration all of the advantages and disadvantages ofthese processes. In addition, he must examine his design with respect to manufacturing techniques, because the manufacturing of the unit has a significant bearing on its final cost. Finally, the equipment must, of course, comply with the requirements established by the legislating authorities and must be approved by the relevant local authority. Physical/Chemical Treatment Physical/chemical treatment systems use flocculating agents to aid the physical separation and precipitation of solids. These systems may be started in a very short period of time and can discharge good effiuents almost immediately. Like physical separation units they produce sludge that must be continuously removed and eliminated. Biological Method There are a number of different processes in the biological treatment of sewage universally used for the management of municipal wastes. Several DESIGN OF SEWAGE TREATMENT SYSTEMS Almost all of the early sewage treatment plant designs were of the biological type, and many of these used the anaerobic process similar to septic tanks found ashore. Although they required the absolute minimum of supervision and had only one moving component-a discharge pumpthese anaerobic plants were not capable of producing effiuents of the quality proposed by IMO and other legislating bodies. Consequently, anaerobic plants were superseded by equipment using other biological 2 8 -4 WASTE DISPOSAL DESIGN OF SEWAGE TREATMENT SYSTEMS SYSTEMS processes that were capable of meeting the proposed standards for discharges. Aerobic biological plants, which superseded the earlier anaerobic units, are based on the extended aeration process of waste treatment. As may be expected the designers of early units drew on experience gained from the operation of industrial equipment, and while the process remained the same as in the earlier anaerobic plants, the practical problems associated with shipboard use were not insignificant. The evolution ofthe present-day unit-the extended aeration method-has been along a path of trial and error where the need to make it as simple as possible has been a prime objective. Since the plants had to be small, compact, and suitable for installation in the limited spaces available onboard ship, the biological oxygen demand per volume of aeration space (BOD/vol.)imposed on them was considerably higher than those accepted ashore and in some cases was more than double. This deviation from land practice means that there is a need for fairly frequent, periodic desludging. For example, a shipboard unit needs to be desludged at intervals of2 to 3 months, whereas an industrial unit for an equivalent population on shore would be expected to operate for 10 to 12 months without desludging. Sludge built up in an extended aeration unit consists ofbiological solids together with any nondegradable material that may be present in the waste. The nondegradable material increases the solid content ofthe liquid in the aeration chamber until a point is reached where settling in the final sedimentation chamber is impaired, and a reduction in the solids level becomes necessary. The reduction in the solids content of the mixed liquor is achieved by desludging, an operation consisting of pumping out 90 percent of the contents ofthe aeration tank and refilling with clear water. Although the sludge, 99 percent water and handled entirely by the discharge pump, may not be dumped into waters covered by regulations, it may be discharged at sea well clear of the protected areas. Ifthe vessel is in port when desludging becomes necessary, the sludge will have to be retained on board or discharged to a shore receiving facility. Where ships are fitted with incinerators, small quantities of sludge may be incinerated daily with other wastes having higher caloric value. The size of a waste treatment plant is determined by the biological load, or material to be digested per unit time, and the hydraulic load, or liquids to be physically separated per unit time. While it is possible to increase biological load, the limitation is established by the hydraulic load on the final settling chamber where the minimum retention time of fluids is 4 to 5 hours to achieve the effluent condition required. The size of the extended aeration sewage treatment unit for a given population equivalent will depend on a number of parameters. For example, the separation of black and gray wastes in the shipboard systems is of prime importance because 2 8 -5 if both wastes are introduced into the sewage treatment plant, the size of the plant would be unacceptably large. Units using flocculating agents to aid settling can usually accept higher hydraulic loads or be reduced in size. The sludge storage or disposal unit is independent ofthe sewage unit and therefore is sized for the material it must handle for the assumed time period. Aeration of the mixed liquor is a fundamental requirement of aerobic treatment systems. The air supplied is necessary as a source for the oxygen required by the bacteria for metabolism. It also provides the means of creating the turbulence needed to promote mixing ofthe bacterial population with the waste material and to prevent settling in the aeration tank. A number of different aerating methods have been used on marine installations, including: (a) coarse bubble; (b)venturi tube; and (c)fine bubble aeration system. Each of these methods has unique advantages and disadvantages. C o a r s e B u b b le S y ste m The coarse bubble system has a simple perforated tube with relatively large holes, not easily blocked, and not requiring particularly clean air. Since the oxygen transfer efficiency is not high because oflarge bubble size, air requirement is greater than in other methods. However, the large bubble size creates good mixing. V e n tu r i I n je c tio n S y ste m The venturi injection system has high oxygen transfer. No air compressor is necessary because the power source for the injector can be a simple centrifugal pump used to pump the recirculated, activated sludge. The venturi nozzle is prone to blockage, and the minute air bubbles coming out of solution in the aeration tank adhere to sludge floc, causing the sludge floc to have a tendency to rise. F in e B u b b le A e r a tio n S y ste m The fine bubble aeration system, provided by aerator diffusers, has a relatively high oxygen transfer efficiency, and consequently the quantity of air required for bacterial metabolism is low (the quantity of air approximates 100 m 3Jkg of BOD). The aerator diffusers are invariably of a porous material and relatively clean air is necessary to prevent diffuser blockages. This system, in most common use at the present time, requires additional air to be supplied to power air lifts for recycling sludge and for skimming settling tanks. The air is supplied in most instances by small compressors of either the rotary vane or lobe type. Since the air quantity is not large, the pressure necessary is that which is needed to overcome the depth of submersion of the aerator diffuser. 28-6 WASTE DISPOSAL SYSTEMS Figures 28-1 through 28-3 show the stages of development ofthe marine extended aeration sewage treatment unit. Figure 28-1 depicts an early design utilizing an aeration system ofthe coarse bubble type. Figure 28-2 shows a similar tank design with an aeration system of the venturi type. The unit illustrated in Figure 28-3 is of a design in which the arrangement Figure 28-3(a). Advanced model of an extended aeration sewage system ofprocess compartments provides more compact configuration and permits the discharge pump to be built onto the unit, and is a good example of the fine bubble aeration type. Design for the settling tank of the fine bubble aeration system has required more effort than any other section of any treatment plant. This is because the floc formed in the biological plants is much lighter than that formed in physical/chemical plants where the flocculating agents are added. For this reason, the settling time is longer with the tank requirement being proportionately larger. The retention time desirable in the settling tank of a biological unit is on the order of 4 to 5 hours with, ideally, a mean upward velocity of the effiuent of approximately 1 meter per hour (3.28 ft./hr.). Unfortunately, the restrictions imposed by the need to build marine units as compact as possible have made this objective difficult to achieve. As a result the final design has invariably been a compromise. To reduce the effect of ship motion on the settling process, the position of the settling tank has been changed from time to time and later designs have incorporated the tank in the center of the complete plant with the aeration tank wrapped around it in a U form. Several designs have used settling tanks which are slightly pressurized and which have no free surface. Disinfection of the final effiuent before discharge overboard is necessary to reduce coliform to a level acceptable to legislating authorities. In most cases the reduction is brought about by treating the effiuent with sodium or calcium hypochlorite. The Saint Lawrence Seaway and Great Lakes authorities impose severe limitations on the quantities of residual disinfectants that may be discharged in an effiuent. These limitations have Figure 28-4. Arrangement of ultraviolet disinfection unit Figure 28-3(b). Cutaway view of an advanced extended aeration sewage system stimulated interest in disinfection by means of ultraviolet light. This method is now being widely used and disinfection is achieved by passing the clean effiuent through tubes fitted with ultraviolet lamps. The effect of the ultraviolet radiation at a wavelength of 254 n.m. is that it breaks down the outer membrane of the microorganism and destroys the nucleus. Ultraviolet disinfection has no residual effect and is therefore suitable for treating effiuents being discharged into confined receiving waters. A typical ultraviolet module is shown in Figure 28-4. Most manufacturers use steel tanks in their sewage treatment plants and these must be suitably protected from the corrosive effects of their contents, although it should be noted that where aerobic conditions prevail, these effects are likely to be no worse than those encountered in seawater ballast tanks. Where holding tanks are in use and anaerobic conditions can occur, it is then probable that the space above the liquid level would be subjected to considerable corrosive conditions created by the conversion of hydrogen sulphide to sulphuric acid. Therefore, it is essential that care is taken to ensure that adequate plate protection is provided. From experience with sewage plants already in service and from tests carried out on development units, it has been found that the coal tar epoxy compounds provide one of the most effective coatings available, and in terms of cost about the most economical. Some manufacturers use fiberglass for tank materials, but it seems that cost and legislation will limit further use of this material. O P E R A T IO N O F A SEW AG E TREATM ENT PLANT Figure 28-3(b) is a cutaway view of a typical modern sewage treatment plant for shipboard use. This type of unit is manufactured in sizes for daily sewage flow range of 250 to 3,830 gallons suitable to support 9 to 127 persons per day at the anticipated rate of 30 gallons per day per person. The unit is divided into 3 compartments: the aeration section (11); settling 28-10 chamber (12); and the chlorine contact tank adjacent to the pumps and compressors. Sewage enters the aeration compartment (11), via the soil inlet (1), and is retained for approximately 24 hours. During this period, it is mixed and aerated by the compressed air delivered to the bottom of the chamber by the aerator (10). The aerobic bacteria and microorganisms break down the organic waste material into carbon dioxide, water, and inert organic material. The mixture passes through a coarse screen into the settling compartment (12). The settling compartment is designed to precipitate all solid material to the bottom ofthe hopper as sludge. The sludge is returned by pneumatic lift (2) to the aeration compartment, where it mixes with the incoming raw sewage. The clear liquid in the settling compartment is displaced into the chlorination compartment via a flow-through type chlorinator. A combined surface skimmer and outlet weir (5), located in the center of the settling compartment surface, removes any floating debris and controls the flow of clear liquid into the chlorinator. The chlorinator compartment provides residence time for the chlorine to kill any remaining bacteria. The discharge ofthe chlorinated effluent is controlled by a float-switch operated pump. DISCHARGE OF OILY WATER In recent years the oil pollution of the seas has probably been discussed oftener and at greater length than almost any other maritime topic. In addition to the many attempts over the years to analyze the sources ofthis pollution, there have been an equal number of efforts made to tighten up regulations controlling the discharge of oil-contaminated wastes. Table 28-1 gives the reader an indication of the varying quantities of oil that are being deposited in the world's oceans, and although the actual total figure may be subject to some disagreement among authorities, very few will question the validity of the relative percentages. As indicated, the marine transportation industry is the greatest offender, and tankers contribute the greatest quantity. TABLE 28-1 Sources of Oil Input to the Oceans Source Marine transportation L.O.T. tankers Non-L.O.T. tankers Dry docking Terminal operations Bilges bunkering A n n u a l In p u t, 28-11 DISCHARGE OF OILY WATER WASTE DISPOSAL SYSTEMS 1973 ( m i l l i o n 0.31 0.77 0.25 0.003 0.50 to n s ) TABLE 28·1 (continued) Source Tanker accidents Nontanker accidents Subtotal Offshore oil production Coastal oil refineries Industrial waste Municipal waste Urban runoff River runoff (including input from recreational boating) Natural seeps Atmospheric rainout Total A n n u a l In p u t, 1973 ( m i l l i o n 0.2 0.1 (2.133) 0.08 0.2 0.3 0.3 0.3 to n s ) 1.6 0.6 0.6 6.133 As discussed in Chapter 26, there has been a gradual strengthening of regulations controlling the discharge of oil-polluted water. An IMO convention sets out the "requirements for the control of pollution." The convention covered not only the problem oftanker discharges, but also discharges from general cargo vessels. To achieve the quality of overboard discharge required by the regulations, it has now become essential to install an efficient oily water separator as a standard item of ship's equipment. In the past the regulations only required the maintenance of a logbook detailing the position of the vessel when discharges took place. The new requirements calling for the oily water separator are in addition to the inclusion of a monitoring system to upgrade the quality of overboard discharge. In order to meet these new requirements, most oily water separators consist of three stages. The first, a gravimetric stage, involves a simple chamber where the velocity of the mixture is reduced to permit large oil globules to rise to the surface and heavier debris to settle. The second stage is usually an inclined plate coalescer in which a series of plates, either corrugated or flat, are set at an angle of approximately 40/50 slope in the oily water stream. The plates can be either in line with the flow or across the flow and collect the small oil droplets on the underside of one plate, allowing the droplets to coalesce with the larger globules and then escape at the top ofthe plate pack, while the solid debris particles fall and collect on the upper surface ofthe lower plates, gradually sliding to the bottom of the separator. Dependent upon flow rate, pump type, etc., the plate type separator can produce very good effluents, certainly below 100 ppm. However, very few are capable of performing to the 15 ppm standard and to meet this higher quality an additional polisher coalescer is fitted. Figure 28-5 shows a typical separator/coalescer. DISCHARGE OF OILY WATER 28-13 The third stage of an oily water separator consists of a polisher coalescer usually made of a fibrous or knitted material. This material, formed in either blocks, cartridges, or socks, has the properties to attract oil (oleophilic) or to attract water (hydrophilic). In the oleophilic coalescer the minute particles of oil remaining after the plate separator stage collect on the fibers of the coalescer material where they form larger droplets which rise relatively easily to the surface of the water and are removed. Unfortunately, the very nature of the material ofthe polisher coalescer makes i t an extremely good filter for any other debris which may come through a plate separator and will in time cause the final coalescer to block, necessitating replacement of the coalescing material. In this respect the essential requirements of a streamlined flow with settling velocities in excess of the hydraulic velocity will be seen to have a tremendous influence on the life of the final coalescer. Coupled with the improved design of the separator, there has been a greater realization of the effect that the pump supplying the separator has on the performance of the separator. For instance, a centrifigal pump is a very good mixer, and to use a unit ofthis type with an oily water separator is unwise. Similarly, a reciprocating pump with plate- type valves produces a very homogeneous mixture which is difficult to separate. Ideally, a diaphragm pump is the most suitable for use with an oily water separator. However, such a pump is not particularly suited to shipboard application and as a reasonably good alternative a screw pump can be used. Figure 28-6 shows curves depicting the effect of pump type on oil droplet size. Figure 28-6. Oily water separator pump droplet size It is also important that the separator's proportions be such that the settling velocity of both oil droplets and/or debris particles not be exceeded by the hydraulic velocities in critical areas ofthe separator. Stokes' law is DESIGN FEATURES OF INCINERATION SYSTEMS 28-15 Once the various materials are identified, it is necessary to obtain estimates ofthe quantities of such materials. An accepted estimate is 2 to 2.5 kg (4.4 to 5.5 pounds) per man per day. The quantity of solid garbage covers both categories (a) and (b) above. In steamships the accumulation of oily wastes is almost negligible, whereas the amounts obtained for motor ships vary from 112 to 2 percent ofthe daily fuel consumption, and include lubricating and fuel oil sludges. Sewage sludge quantities vary with the type oftreatment process. For example, a biological treatment plant may be desludged at sea before entering waters subject to legislation and will then not require desludging for another 2 or 3 months. On the other hand, if daily desludging is preferred, the quantity to be incinerated will be approximately equivalent to 2.5 liters (.6 gallons) per man per day. A physical or physical/chemical system will produce approximately 5 liters (1.3 gallons) of sludge per man per day. The heating values for these materials are: IN C IN E R A T IO N O F O IL W ASTE AND GARBAGE The disposal of solid and liquid wastes other than sewage has become the subject of investigation following the publication of the 1973 IMCO proposal. Garbage can be contained and compacted, but storage is necessary and not always available. Packaging will inevitably include a large quantity of plastics which must be retained on board since discharging overboard at sea will not be permitted regardless of distance offshore. Liquid wastes, mainly oil sludges, must be retained on board and, along with garbage, will necessitate the provision of storage space. Incineration of these materials is without doubt the most effective means of dealing with the storage problem because the resultant residues are extremely small in volume and easily disposed of. Before deciding on the type of incinerator to be adopted, an assessment of storage requirements are necessary. Equally important in this assessment is the categorization of the waste to be consumed. These categories are: (a) galley waste including food scraps, bones, cans, bottles, plastics, etc.; (b) accommodation wastes such as paper, cardboard, cans, plastics, textiles; (c) oils/sludge including bilge, oil purifiers, lubricating oils, etc.; and (d) sewage sludges. From this estimated heat release rate, the size of a suitable incinerator can be calculated on the basis of normal daily usage of 8 hours. The hourly heat release rate will then be approximately 518,000 kcal (2.055 x 106 BTU). D E S IG N FEATURES O F IN C IN E R A T IO N SYSTEM S The requirements for a successful incinerator are: (a) The operating temperature must be high enough to sterilize all residues and ensure that all discharges are innocuous. (b) Gas residence time must be as long as possible. (c) The temperature must not be high enough to melt glass or metal. (d) Auxiliary fuel usagE;)m ust be at a minimum. (e) Simplicity of operation consistent with safety. (f) Capital cost as low as possible. (g) Number of man-hours required in daily attendance must be kept to a minimum. (h) Ash removal must be as easy as possible. 28-16 WASTE DISPOSAL SYSTEMS Various designs of incinerators are available to the shipowner. These are mostly of the single chamber, multiple chamber, or vortex types. All the vortex types are of the vertical form while the single and multiple chamber units can be either vertical or horizontal. Figures 28-8, 28-9, and 28-10 show three typical units. FEEDING AND CONTROL OF INCINERATORS 28-17 types, but the latter are invariably somewhat higher, a factor which may prove to be a disadvantage where installation between decks is necessary. Figure 28-10. Typical marine incinerator arrangement Figure 28-9. Multiple chamber incinerator While single and multiple chamber units can be used for incinerating liquid sludges, the vortex types available at present are capable of higher throughputs and, with the tightening of rules covering the discharge of oil, this aspect of incineration is very important. The physical dimensions of any incinerator are also important because when available space is allocated invariably the incinerator comes close to the bottom in the list of priorities. As a general rule, the single and multi chamber incinerators will occupy more deck area than the vortex FEEDING AND CONTROL O F IN C IN E R A T O R S Various methods of automatic garbage feeding have been tried on marine incinerators, including chute, screw feed, and ram feed, and the problems that have been encountered with all of these have been significant. I f the limitation of time spent on maintenance is of great importance, then there is no doubt that hand loading utilizing paper or plastic sacks is the most reliable. Segregation of waste is not practical, and the incinerator must be able to accept anything that is put into a sack. Accordingly, the loading door must be of dimensions that are big enough to accept a fully loaded sack of approximately 450 mm (17.7 inches) diameter by 750 mm (29.5 28-18 inches) long. The average contents of one of the sacks will weigh approximately 6 kg (13 lb) and on the basis of the combustion rate mentioned earlier, it will be noted that a crew of 40 producing approximately 100 kg (220 lb) of garbage per day will require a daily attendance of 20 to 30 minutes for manual loading for the incinerator. Liquid wastes and sludges are easier to handle and store although it must be borne in mind that sludges can contain solids of 6 to 8 mm (.2 to .3 inches) diameter as well as abrasive materials from fuel oil separators. Consequently, the pump or pumps designated for handling these media should be properly selected. Because of the possibility of encountering relatively large solids, restrictions in transfer pipework should be avoided if blockages are to be prevented. The previously mentioned points must also be considered when the type of burner used is being appraised. Sludge and liquid waste burners at present are of the rotary cup type, low pressure weir type, or combustion chamber gutter type. Jet burners should be avoided unless there is no chance of the entrainment of solids. The preparation of liquid wastes is important, and provision must be made for heating and mixing these wastes before incineration. A good homogeneous mixture is necessary where oil and wastewater sludges are being burned because separation of the constituents will result in combustion at a steady rate, becoming impossible to contain. Evaporation of water in wastes is most economically achieved by using the available heat in waste oils. If this cannot be done, it is necessary to use a supporting oil burner with its attendant cost. In practice, it has been found that with efficient mixing, a combination of oil and water sludges containing 50 to 55 percent water can be consumed without the aid ofa support burner. The control of an incinerator must be such that all aspects of safety are covered. Exhaust gas temperatures must be controlled to ensure that they are high enough to destroy odors, but not so high as to melt glass. Temperature of exhaust gases can also be used to control the operation of the auxiliary or support burner so that, whenever possible, the use of additional fuel is avoided. All the usual interlocks and lockouts associated with an oil-fired device must be fitted, and include flame failure, forced draft fan failure, and high exhaust gas temperature. Since incinerator exhaust gas temperatures are considerably higher than those from heat exchange devices such as boilers, it is necessary to reduce them to more acceptable levels. This can best be achieved by introducing cold-diluted air into the exhaust stream at a point as close to the incinerator discharge as possible, and before the exhaust trunking passes through spaces adjacent to spaces where people are accommodated. The temperature of the exhaust after dilution should be no more than 662°F (350°C). Some classification societies require this exhaust gas tem- 28-19 GLOSSARY WASTE DISPOSAL SYSTEMS perature to be monitored and excessively high temperatures to instigate shutdown procedures. O P E R A T IO N O F A M A R IN E IN C IN E R A T O R A cutaway view of a typical modern incinerator is shown in Figure 28-11. This incinerator is a vertical cyclone type with a rotating arm device (6) to improve combustion and remove ash and noncombustibles from the furnace. A sludge burner (4) is incorporated to dispose of sewage sludge and waste oil. The auxiliary oil burner (12) is fitted to ignite the refuse. Automatic controls secure the ignitor when the refuse burns without it. Combustion air is furnished by a forced draft fan (8). The loading door (11) for the refuse is pneumatically operated and is interlocked with the burner and forced draft fan. Solid waste is fed into the incinerator in standard plastic bags, about three at one time. A single "close door" button initiates automatic functioning of the incinerator. A charge is incinerated in 5 to 10 minutes. Liquid waste may be introduced through the liquid waste burner when the refractory is hot. The sludge-burning cycle is fully automatic. Following use, the incinerator is allowed to cool, and ash and noncombustibles are removed by pulling out the ash slide door in the hearth. The rotating arm will scrape all the solid residue into the ash box from which it may be disposed. G LO SSARY The process or method of bringing about intimate contact between air and liquid to dissolve oxygen into the liquid. A e r o b i c . In the presence of air. A e r o b i c D i g e s t i o n . A waste degradation process utilizing microorganisms requiring oxygen for their metabolism and growth. A n a e r o b i c . In the absence of air. A n a e r o b ic D i g e s t i o n . A waste degradation process utilizing microorganisms which do not require oxygen for their metabolism and growth. B io c h e m ic a l O xygen D em and ( B O D ) . The amount of oxygen utilized by microorganisms in the stabilization of organic matter. C h lo r in e , R e s id u a l. The total amount of chlorine (combined and free available chlorine) remaining in water, sewage, or industrial waste at the end of a specified contact period following chlorination. C o l l o i d a l S o l i d s . Extremely finely divided suspended materials which will A e r e a tio n . not settle. C o m m in u to r . D e n itr ific a tio n . Sewage grinder or shredder. The reduction of nitrates in solution by biochemical action. 28-20 WASTE DISPOSAL SYSTEMS GLOSSARY A device through which air is forced and divided into small bubbles for diffusion in liquids. D i s i n f e c t i o n . The killing of the larger portion (but not necessarily all) of harmful and objectionable microorganisms. E f f l u e n t . Sewage, water, or other liquid, partially or completely treated or in its natural state, flowing out of a reservoir, basin, or treatment plant. F l o c . Small gelatinous masses, formed in a liquid by the addition of coagulants, through biochemical processes or by agglomeration. F l o c c u l a t i o n . The process of raising suspended matter to the surface of the liquid and the subsequent removal of the matter by skimming. H y p o c h l o r i t e . Compound of chlorine in which the radical (OCD is present. H y d r o g e n C o n c e n t r a t i o n . A measure of the acidity or alkalinity of a solution (See pH). I m h o f f C o n e . A conically shaped graduated glass vessel used to measure the volume of settled solids. I n f l u e n t . Sewage, water, or other liquid, partially or completely treated, or in its natural state, flowing into a reservoir, basin, or treatment plant. M i c r o o r g a n i s m . A minute organism either plant or animal, invisible or barely visible to the naked eye. M i x e d L i q u o r . A mixture of activated sludge and sewage in the aeration tank. N i t r i f i c a t i o n . The conversion of nitrogenous matter into nitrates through biochemical action. O x y g e n , A v a i l a b l e . The quantity of uncombined or free oxygen dissolved in a liquid. O x y g e n , D i s s o l v e d . Usually designated as DO. The oxygen dissolved in sewage, water, or other liquid usually expressed in parts per million. O x y g e n D e f i c i e n c y . The additional quantity of oxygen required to satisfy the biochemical oxygen demand in a given liquid. Usually expressed in parts per million. O x y g e n D e m a n d . Oxygen required for oxidation of unstable matter in sewage. P a r t s p e r M i l l i o n ( p p m ) . A ratio of pounds per million pounds, grams per million grams (approximately equal to milligrams per liter), expressing the concentration of a specified component. P a t h o g e n s . Disease-causing organisms. p H V a l u e . The logarithm to the base 10 of the reciprocal of the concentration of hydrogen ions in a solution. If the pH is below 7.0, the solution is acidic, above 7.0, the solution is alkaline. P r o c e s s , A c t i v a t e d S l u d g e . A biological sewage treatment process in which a mixture of sewage and activated sludge is agitated and aerated. The activated sludge is subsequently separated from the treated sewage (mixed liquor) by sedimentation, and wasted or returned to the process D iffu s e r . 1 A ir d ilu te r 2 E x h a u s t g a s d e fle c to r 8 F o rc e d d ra ft fa n 9 C o n tro l p a n e l 3 R o ta tin g ra b b le a rm s h a ft 4 L iq u id w a s te b u rn e r 5 C o m b u s tio n a ir in le t 6 R a b b le b la d e s 13 A s h s lid e 7 R a b b le s h a ft d riv e 14 Ash hopper 1 0 S ig h t g la s s 11 P n e u m a tic a lly o p e ra te d g a rb a g e d o o r 12 A u x ilia ry b u rn e r Figure 28-11. Cutaway view of marine incinerator 28-21 2 8 -2 2 ACKNOWLEDGMENTS WASTE DISPOSAL SYSTEMS as needed. The treated sewage overflows the weir of the settling tank in which separation from the sludge takes place. P r o c e s s , B i o l o g i c a l . The process by which the life activities of bacteria and other microorganisms break down complex organic materials into simple, more stable substances. Self-purification of the sewage-polluted streams, sludge digestion, and all so-called secondary sewage treatments result from this process. Also called biochemical process. P u t r e f a c t i o n . Biological decomposition of organic matter with the production of ill-smelling products, associated with anaerobic conditions. S e d im e n ta tio n . The process by gravity of settling and deposition of suspended matter carried by water, sewage, or other liquids. It is usually accomplished by reducing the velocity of the liquid below the point where it can transport the suspended material. Also called settling. S e p t i c S e w a g e . Sewage undergoing putrefaction in the absence of oxygen. S k im m in g . The process of removing floating grease or scum from the surface of a tank. S l u d g e , A c t i v a t e d . Sludge produced in raw settled sewage by the growth of Zoogloea (bacteria) and other organisms in the presence of dissolved oxygen and accumulated in sufficient concentration by returning floc previously formed. S l u d g e B u l k i n g . A phenomenon that occurs in activated sludge plants in which the sludge occupies excessive volumes and will not concentrate readily. S l u d g e D i g e s t i o n . The process by which organic or volatile matter in sludge is gasified, liquefied, mineralized, or converted into more stable organic matter, through the activities ofliving organisms. S o l i d s , D i s s o l v e d . Solids which are in solution. S o l i d s , S e t t l e a b l e . Suspended solids which will subside in quiescent water, sewage, or other liquid within a reasonable period. Such period is commonly, though arbitrarily, taken as one hour. Also called settling solids. S o l i d s , S u s p e n d e d . Solids physically suspended in sewage which can be removed by proper laboratory filtering. S o l i d s , T o t a l . The total amount of solids in solution and suspension. S o l i d s , V o l a t i l e . The quantity of solids, in water, sewage, or other liquid, lost on ignition of the total solids. S t e r i l i z a t i o n . The destruction of all living organisms, ordinarily through the agency of heat or some chemical. T r e a t m e n t , P r i m a r y . The first major (sometimes the only) treatment, usually sedimentation, in a sewage works. The removal of a high percentage of suspended matter but little or no colloidal and dissolved matter. T r e a t m e n t , S e c o n d a r y . The treatment of sewage by biological methods. ( 2 8 -2 3 Water which does not contain objectionable pollution, contamination, minerals, or infection, and is considered satisfactory for domestic consumption. W a te r , P o ta b le . ACKNOW LEDGM ENTS The editors are indebted to Hamworthy Engineering Limited, Dorset, England, for the material included in this chapter. CHAPTER P r o p e lle r s 29 a n d P r o p u ls io n * WILLIAM B. MORGAN IN T R O D U C T IO N A PROPELLER is a device which transforms energy from a rotating shaft into kinetic energy in the slipstream. Thus, the propeller adds momentum to the fluid passing through it, and an axial force is generated on the shaft which pushes the ship. This energy is transferred through the blades which act physically like wings. However, to cope with the combination of rotational velocity and axial velocity, the blades are manufactured in a general helical shape. Because of this general helical shape of each blade, similar to a screw thread, the name "screw" is often used interchangeably with "propeller." The analogy is often made that each blade is like a part of a screw thread and that the pitch of the propeller blade is like the pitch of a screw. The propeller advances a distance through the water in one revolution equal to its pitch minus a slippage, called slip, which is equivalent to the axial velocity induced by the propeller. This analogy is crude and will not be carried further since physically the blades are analogous to airplane wings and not to screw threads. There are many definitions associated with propellers, and these tend to change somewhat over the years as new terms are added and old terms are dropped. More recently, an effort has been made to standardize terminology internationally. In 1975, the International Towing Tank Conference adopted a set of terms for propulsion and propellers, and some of these definitions are reproduced in the glossary. There are also a number of propulsor types included in the definitions. In general, these propulsor types operate along the same principle as the screw propeller, and they will also be discussed in this chapter. S H IP R E S IS T A N C E AND P R O P U L S IO N R e s is ta n c e When a ship moves through the water, the viscosity ofthe water gives rise to shear forces on the surface of a ship and her appendages. These shear *This chapter reflects the opinion of the author and not the official opinion of the U.S. Navy. 29-1 SHIP RESISTANCE 29-2 PROPELLERS AND PROPULSION 29-3 AND PROPULSION forces cause the hull to have a d r a g , called frictional resistance or skin friction drag. In addition to the shear forces, the boundary layer modifies the pressure distribution around the ship giving rise to a pressure or form drag. If this pressure distribution is adverse enough, the boundary layer will completely separate from the ship's stern. When this occurs, the pressure drag greatly increases. These two components, frictional resistance and pressure or form drag, compose the total v i s c o u s d r a g ofthe ship. Since a ship operates in a free surface, there is another component of drag which arises, and that is the w a v e d r a g caused by the disturbance of the surface (wave formation) by the ship. There may be other components of drag caused by operating near other boundaries, shallow water, etc. In any case, the propeller must overcome all these drag components. Another effect ofthe viscosity ofthe water is that there is a wake formed behind the ship. (The wave component may also affect the flow behind the ship.) The ship, as it moves through the water, actually drags some ofthe water with it, the velocity of which depends on how close a particular particle of water is to the hull. In fact, a method used for determining the ship's viscous resistance is to survey the wake downstream of the ship and to determine the velocity of the water being dragged along. The energy in the water behind the ship is representative of the viscous resistance. Ship resistance is normally predicted from model tests where the model is towed through the water and the drag measured. The model must be towed through the water at a speed corresponding to the square root of the linear ratio of the ship to model length. This is called the Froude-scaled speed. At this speed, the wave formation is the same for the model as for the ship. An example is that for a 500-foot ship with a 20-foot model, the model speed would be one-fifth ofthe ship speed. The viscous part of the model drag is calculated by assuming that the viscous drag corresponds to the drag of a flat plate the same length as the model and with the same wetted surface. In actuality, therefore, only the frictional resistance part of the viscous drag is considered by the flat plate drag. This frictional resistance is subtracted from the total drag of the model, and the resulting drag, called r e s i d u a r y r e s i s t a n c e , is essentially that of the wave drag and pressure or form drag. The residuary resistance of the model is nondimensionalized by the dynamic head (:J,2pV2) times the area ofthe model-wetted surface. It is assumed that the ship coefficient of residuary resistance has the same value as the model coefficient, and hence the ship residuary resistance is calculated from this coefficient using the ship speed and the area of the ship's wetted surface. As for the model, the viscous drag (actually frictional resistance) of the ship is calculated by assuming that it is equal to the drag of a flat plate of the same length as the ship and with the same wetted surface. Curves have been developed for the drag coefficient of a flat plate. This drag coefficient is a function of a dimensionless parameter defined as the velocity times the length divided by the kinematic viscosity: VxL v This parameter is called the Reynolds number. The flat plate drag coefficient decreases as the Reynolds number increases. In addition to these two drag components, an additional resistance must be added to the smooth ship prediction to complete the model-ship balance. This additional resistance has been called the roughness allowance, supposedly to account for the additional drag of a rough ship versus a smooth model. Actually the roughness allowance covers more factors such as differences in flow separation between model and ship, air resistance, etc. Therefore, the term r o u g h n e s s a l l o w a n c e has been changed to c o r r e l a t i o n a l l o w a n c e . Drag due to wind and waves in rough weather can also be added to the list of drag components. Other techniques for scaling model resistance to ship resistance have been proposed or are in use. For instance, the use of the drag of an equivalent body of revolution the same length as the model or ship and with the same wetted surface distribution has been proposed. And form factors to account for the ship form in scaling the various drag components are in use. Series data from model tests have been developed so that the resistance can be predicted for many ship types. Two series of note are the Taylor Standard Series (see Taylor, 1933) and David Taylor Model Basin Series 60 (see Todd, 1963). There are also series for high speed craft (see Yeh,1965). It should be noted that the ship resistance increases significantly as the hull becomes fouled. Hull fouling increases shaft torque for a given ship speed and decreases ship speed for a given shaft torque. The net effect is an increase in fuel cost for a given ship speed. The increase in drag over the time a vessel is out of dry dock is a function of the amount of time the ship is at sea and the time in port and the location of the port since ships in warm waters foul much faster than ships in cold waters. The resistance can increase as much as 0.25 percent per day in temperate waters and 0.50 percent per day in tropical waters and will cause significant increases in fuel costs. Special paints to inhibit fouling and periodic hull cleaning should be considered. F ittin g th e P r o p e lle r A propulsor must be added to the ship to overcome the total resistance of the ship. Only propellers will be considered and not sails or other propulsion devices. The propeller is usually located at the stern of the ship although most positions have been investigated. The reason for this location is that when 2 9 -4 PROPELLERS AND PROPULSION the propeller operates in the wake of the ship, there is improved performance since the propeller "fills in the wake defect" to some extent. Also, if the propeller is in front ofthe hull, the induced velocity from the propeller increases the drag of the ship. Furthermore, there is improved protection of the propeller with a stern location, as well as some payoff with regard to control by locating the propeller at the stern with a rudder behind. For whatever reason, then, the propeller is located at the stern ofthe ship and normally within the viscous wake of the ship. The propeller accelerates the flow in the wake relative to the ship's motion in such a way that the wake behind the ship becomes essentially momentumless on an average. W ake The wake is the term used to describe the motion imparted to the water by the passage of the ship. On the other hand, the velocity field ofthe water is usually measured with the measuring apparatus moving with the ship (or model). Thus, the velocity field measured is the ship speed minus the wake. There is a tendency to use these terms interchangeably in discussion ofthe interaction between a ship and a propeller, but when actual data are presented, it is made clear which is being used. The propeller operates behind the ship in the wake caused by the viscosity of the water and the wave action. If we make a survey of the velocity field behind a single-screw ship, a pattern something like Figure 29-1 is obtained. The two components of velocity shown are the axial, Vx, and tangential, Vt. There is also the radial component of velocity, Vr, which must be considered. As it rotates, a blade section operating in this velocity field encounters a velocity which fluctuates both in magnitude and direction. Thus, the propeller operates in an unsteady flow field, and the propeller blades produce an unsteady force. For design purposes, the velocity is analyzed by a harmonic analysis; i.e., at each radius r a harmonic analysis is carried out with respect to the angular position e on the wake components: Figure 29-1. Representative velocity distribution in place of the propeller for a single-screw ship which are the nth harmonic amplitudes of the velocity components give rise to the unsteady forces on the propeller. It is assumed in the design process of the propeller that the circumferential average velocity and the harmonic amplitudes of the velocity can be considered separately; i.e., the propeller can be designed to meet the conditions of power, speed, and RPM independent of the harmonic content ofthe velocity. It will be shown later that the harmonic amplitudes ofthe velocity V n must be considered when designing the propeller for good cavitation performance and minimum vibration. H u ll-P r o p e lle r I n te r a c tio n You should not infer that the only interaction between the ship and its propeller is in the requirement that the propeller operate in the wake of the ship. Since it must accelerate a mass of fluid in order to produce thrust, the propeller accelerates part ofthe fluid over the stern ofthe ship as well as over the rudder. This acceleration of the fluid changes the pressure distribution over the stern and has two effects: (1) there is a change in the drag, and (2) the boundary layer is affected. This change in resistance or drag is called the thrust deduction, t, and is defined as SHIPRESISTANCE ANDPROPULSION 29-7 be made. Data are available from series of model tests to estimate the Taylor wake fraction, e.g., Taylor Standard Series and DTMB Series 60. These data show that, for single-screw ships, the Taylor wake fraction increases with increasing ship block coefficient. (Note that increase in the wake fraction implies that velocity at the propeller decreases.) For low block coefficient single-screw ships (CB<0.5), the wake fraction can be as low as 0.2 and for high block coefficient single-screw ships (CB>0.85), the wake fraction can be as high as 0.5. For twin-screw ships, especially with propellers supported by struts, the Taylor wake fraction is much lower and can approach 0.1 or less. For high speed military-type ships with struts, the wake fraction usually lies between -0.02 and +0.02. The distribution ofthe velocity at the propeller is important for cavitation and vibration considerations. As mentioned above, the harmonics in the wake are obtained from a velocity survey in the propeller plane. Hadler and Cheng (1965) have given data for a large number of twin-screw and single-screw ships. Their data consist ofthe mean velocity, the distribution of the circumferential average velocity, and the wake harmonics. Thus, one can obtain a reasonable estimate ofthe nominal wake field behind the ship. However, one should be aware that the data are for a model and that the wake behind a full-sized ship is somewhat thinner. Thrust deduction can also be estimated from model series data. For single-screw ships, the thrust deduction is expressed as a percentage of the Taylor wake fraction. For instance, for ships equipped with streamlined rudders or contra-rudders, the thrust deduction varies between 0.5 and 0.7 of the Taylor wake fraction. For reasons of economy, it is said to be desirable to propel the ship with the minimum possible power. Ideally, what is desired is the minimum fuel consumption, and not minimum power. However, this requires knowledge of engine fuel rate, and so, when considering the propeller independent of the main engine, it is sufficient to consider minimum power. Thus, when designing the propeller without knowing the main engine fuel rate, a propeller should be selected which uses the minimum power. Because of the hull-propeller interaction, it may be that the propeller with the maximum efficiency may not use the minimum power. For a ship with a given speed, the power required to overcome the resistance is the resistance times the ship speed. This power is called the effective power PE. The delivered power at the propeller, PD, is 21ttimes the shaft torque times the RPM. For efficient propulsion, it is desirable to maximize the ratio of the delivered power to the effective power. This ratio is known as the propulsive efficiency, llD: PROPELLER The relative rotative efficiency is the ratio ofthe propeller efficiency behind the hull and in open water. The difference in the behind and open water efficiency is that in the behind condition the velocity to the propeller varies in both the radial and circumferential directions, and in open water the inflow is uniform. The relative rotative efficiency usually lies between 1.0 and 1.05 for most merchant ship forms. This means that the propeller is slightly more efficient in the behind condition than in open water. PROPELLER D E S IG N T heory For many years, propellers were designed using design charts developed from model tests with a series of propellers. A number of series have been developed, the most extensive of which is the Wageningen B-Series, or Troost Series, published by the Netherlands Ship Model Basin (see Wageningen Series, 1969). The series extends from 2 through 7 blades with a variation of a number of other parameters. Table 29-1 lists the blade area ratios and range of pitch ratios tested for each number of blades. Starting in the early 1960s, significant advances were made in the design of marine propellers. These advances were related to the practical application of theory and the use of the computer in propeller design. Even where propeller series are used, e.g., for preliminary calculations, they have been 29-9 DESIGN computerized. Today it is practical to design propellers theoretically, and the use of charts is becoming a thing of the past. Since the theories are very complicated and computers are necessary for execution of the calculations, a description ofthe present-day design theories will not be given. It suffices to say only that propeller theory stems from wing and airfoil theory but instead ofthe blade being in a plane as a wing, the blade lies along a helix. This geometry greatly complicates the theory and the interference between the blades. The mathematical representation of the blades in the design theory is normally in two parts. One part, called lifting line theory, is where each blade is represented by a single line with the slipstream shed behind each line. Lifting line theory is used to predict the performance of a propeller and to select the pitch of the shed helical vortices. The other part, called lifting surface theory, is where each blade is represented mathematically both in blade camber and in blade thickness. Lifting surface theory is used to give the blade distortion in both camber and pitch to achieve the performance predicted from lifting line theory. Although the complete design can be carried out with lifting surface theory, for practical computational reasons, and with no loss in generality, lifting line theory is used. A further discussion of the theoretical design of propellers would include a rather complete discussion of the vortex theories entailed in aerodynamics and hydrodynamics. However, a good understanding of propeller action can be obtained from momentum theory. TABLE Sum m ary B la d e 2 9 -1 o f th e W a g e n in g e n B la d e A rea B -S e r ie s R a tio P itc h N um ber 2 3 4 5 6 7 R a tio 0.30 0.38 0.35 0,50 0.40 0.65 0.55 0.45 0.50 0.80 0.70 0.60 0.65 0.55 0.85 1.00 1.00 0.75 0.80 0.70 0.85 0.6-1.4 0.5-1.4 0.5-1.4 0.5-1.4 0.6-1.4 0.6-1.4 I n momentum theory, the propeller is replaced by a hypothetical disk called the actuator disk. I n practice, this is equivalent to a propeller with an infinite number of blades and no blade area and no thickness. Despite being far away geometrically from an actual propeller, the theory does give a good idea of the velocity induced by the propeller and the shape of the slipstream. The propeller will be assumed to be at rest with a steady flow of water past the propeller. To calculate the propeller action, three laws of hydrodynamics are required: This equation gives an upper bound to the propeller efficiency since it does not include the rotational energy losses in the slipstream, the frictional losses on the blades, or the effect of a finite number of blades, etc. The equation does show that as the load coefficient, C T h i increases, the efficiency decreases. A general momentum theory can be derived which takes into account the rotational losses in the slipstream and the actual radial load distribution. To take the viscous losses on the blades into account, the blade area and pitch angle, besides the viscosity of the fluid, must be known. Approximations for this relationship have been derived. From momentum theory, the following conclusions can be drawn: 1. The axial velocity induced by the propeller increases from zero forward of the propeller to a maximum constant value downstream. The induced velocities 29-12 PROPELLERS AND PROPULSION at the propeller are half their value far downstream (axial, tangential, and radial components). 2. The increase in axial velocity through the propeller causes the slipstream diameter to reduce and is a function of the thrust loading. 3. The propeller efficiency decreases with increasing loading. S e r ie s D a ta Propeller series data are very useful for checking operation of propellers and in determining optimum RPM and diameter. In principle, theoretical series of performance could be derived and have been for supercavitating propellers, but for conventional propellers over the full range of performance, experimental series are preferred. The series data are plotted in a number of various forms. Figures 29-3 to 29-7 show data from the Wageningen 4-bladed series with a blade area ratio of 0.55. Figure 29-3 shows data in the standard K T - K Q - J system ( J is defined as the advance coefficient). This system presents the complete performance of the propeller when the propeller is moving ahead through the water and rotating PROPELLER DESIGN 29-19 constant ambient temperature. Cavitation occurs in the low pressure region on the propeller blade surfaces and in the low pressure regions of the hub and tip vortices. Surface cavitation, when it covers a significant part of the blade, can cause thrust breakdown. This results in the requirement for increased power and RPM to maintain a given speed. In other words, the performance of the propeller can significantly decrease when compared to a noncavitating condition. The majority of problems with cavitation are with blade erosion and increased vibration. Performance breakdown occurs for high speed ships, and cavitation is a product of high speed. The following formula indicates this. The cavitation number: where The series are all for propellers in the unfouled condition. Fouling can change the propeller performance significantly. Efficiency can be reduced by 25 to 50 percent, torque is increased significantly, and thrust is reduced somewhat. The overall effect on a ship is a significant increase in power required for a given speed and an increase in RPM. This means that the shaft torque goes up significantly. Propeller cleaning should be done often since it is a small area to clean relative to the hull area, and the power reduction can be significant. One-fourth of the power increase for a fouled ship can be due to the propeller fouling. C a v ita tio n Cavitation is the process in which the formation of the vapor phase ofthe liquid takes place as the liquid is subjected to reduced pressure at a g = acceleration due to gravity H = absolute static head at the place cavitation occurs minus the head due to the vapor pressure of the liquid. For a propeller this is the head due to the atmospheric pressure plus depth of submergence minus the head due to vapor pressure. Oftentimes the depth of submergence is taken to be the shaft centerline in order to give an overall cavitation number for a propeller. V = local velocity at the point on the propeller where the cavitation number is calculated. For convenience, the velocity is often taken as the speed of advance ofthe propeller or the ship speed to give a cavitation number that is representative of the whole propeller. Propeller series have been developed where the propellers have varying amounts of cavitation (Gawn-Burrill Series, 1957, and for the Wageningen B-Series, 1969) so that the cavitating performance of propellers can be predicted. Theoretical series for supercavitating or ventilated propellers have been developed by Caster, 1963. These propellers operate with a complete cavity on the blades extending beyond the trailing edge of the blades. These propellers are for very high speed craft. They are not subject to large decreases in efficiency as conventional propellers under severe cavitation, and the blades are not subject to erosion since the cavities collapse beyond the blade trailing edge. Figure 29-8 gives an indication of when supercavitating propellers should be used. The ordinate ofthis figure is the absolute pressure (includes atmospheric pressure) in feet of water at the propeller divided by the speed of advance in knots, and the abscissa is the propeller advance coefficient. Region I is practical for supercavitating propellers, and Region III is best for subcavitating propellers. Region II between Regions I and III indicates where partial cavitation is likely to occur on either type of propeller. Region N for J less than 0.4 indicates a region oflow efficiency. The regions on this figure should be used only as a guide to what type of propeller to design. PROPELLER DESIGN 29-21 If cavitation is suspected to be a problem, cavitation tests are conducted on a model propeller, and the ship propeller performance under cavitating conditions is predicted from these tests. The cavitation tests are normally conducted in a cavitation tunnel which is a large circulating flow facility where the velocity is controlled by a pump in the circuit. It is necessary to control the pressure so that tests can be carried out at the full-scale cavitation number. Many tunnels do not have sufficient velocity to test the propeller at full-scale speeds. Also, many model propellers are made ofmaterial not as strong as the material used for full-scale propellers and since full-scale speeds on a model would imply, approximately, fullscale stresses, it may be necessary to test the models at less than full-scale speed. Some tunnels are large enough to take 7-meter ship models so that the wake at the propeller can be reproduced, while in others the wake is simulated by partial hulls or screens. There are a few cavitation facilities which are towing tanks where the pressure can be controlled. In these facilities, the model is tested as in a standard towing tank but at reduced pressure. Cavitation can be caused by fouling on the propeller and by nicks on the blades near the leading edge and the tip. The propeller should be inspected for these defects and cleaning and repairs made. S tr e n g th A propeller blade must contain enough material to keep the stresses within the blade below a certain level. This level depends both on the steady state and fatigue strength of the material and on both the mean and unsteady blade loadings. The material selected controls the allowable stress level, and the blade chord and thickness are the main parameters which control the blade stress for a given blade loading. For highly skewed propellers, the skew is also an important propeller geometric parameter influencing the stress level. The maximum stress for a propeller normally occurs near the point of maximum blade thickness unless the blade has an unusual configuration, such as high skew. Without consideration of the centrifugal forces and blade rake and skew, the required maximum blade thickness/chord, t i c , at a given propeller radius can be approximated by 29-24 PROPELLERS PROPELLER AND PROPULSION and 0.9), C c is a coefficient dependent on the chordwise camber distribution (for standard sections, assume it is equal to one), and c is the section chord. All propellers should be designed considering both mean and unsteady loads acting on the blade. These must be considered together since the allowable unsteady stress decreases as mean stress increases. A simple equation for allowable stress can be derived from the Goodman diagram for both mean and unsteady loads. This equation is known as the equation for the Goodman safe region. difficult problem. Table 29-2 shows the fatigue strength in salt water of some of the more common propeller materials. Which material to select depends not only on the fatigue strength, but its other properties such as cost and ease to manufacture and repair. TABLE 29-2 8 Fatigue Strenghts (psi) in Seawater, 10 cycles A B S * Type M a te r ia l where Rs is the ratio of minimum stress to maximum st{ess during a cycle, is the fatigue strength for a given number of cycles, single amplitude, and ( J r n a x is the maximum stress for a given number of cycles. The maximum value of unsteady stress for single screw cargo ships is considerably greater than the mean stress. The difficulty in making strength predictions of propeller blades is the lack of knowledge of the unsteady bending movements on the propeller blade. A few measurements have been made which show that the unsteady load for a single screw cargo ship can be as much as 50 percent larger, and even higher, than the mean value. For design purposes, unsteady propeller theory must be relied upon for predicting the unsteady blade loads. In practice, the American Bureau of Shipping publishes criteria for blade thickness which standard propellers must meet before they can be certified for use on a particular ship. These criteria depend on the propeller geometry, the horsepower, and the RPM. All criteria are for the steady state condition, but it is obvious that unsteady loading is accounted for in the factor of safety used. Many factors determine the choice ofthe structural material for marine propellers. These are weight, material and fabrication costs, yield strength, fatigue strength, castability, machinability, weldability for repair, corrosion resistance, impact resistance, and erosion resistance. The propeller operates in the hostile environment of salt water. Because of the corrosive nature of salt water on most materials and the substantial decrease in fatigue strength of most materials in salt water, and considering the other factors for selecting propeller materials, only a few materials are suitable for use in propellers. Most common materials are some ofthe bronzes and some types of stainless steels. Cast iron was used some years ago but is not common today. Also, propellers have been built of Inconel, titanium, and glass reinforced plastics, but these materials are not yet used for large size propellers. It is quite probable that composite materials such as glass reinforced plastics will find more common usage. The more exotic materials such as Inconel and titanium have been used for special application to high speed craft where cavitation erosion and strength are a particularly 29-25 DESIGN Manganese bronze Nickel manganese bronze Nickel aluminum bronze CF-4 stainless steel 2 3 4 (J f (J a 6,000 6,000 12,500 6,500 (much higher with cathodic protection) Vibration and Noise The propeller can induce ship vibration in two ways: (1) unsteady forces transmitted through the shaft bearings and (2) pressure forces which act on the hull plating and appendages in the vicinity of the propeller. The unsteady forces come from the blades operating in the spatially varying wake field with unsteady forces being introduced through the shaft. Bearing forces are little affected by cavitation. They can be minimized by selecting the blade number to avoid strong wake harmonics at blade number and blade number ±1. The wake harmonics at blade number give rise to fluctuating axial force and torque on the shaft while the wake harmonics at blade number ±1 give rise to side forces and movements on the shaft. Harmonic analysis of the wake at the propeller will provide excellent guidance to the selection of number of blades. Also, in the selection of the number of blades, it is essential to avoid exciting hull, superstructure, and machinery resonances at normal operating speeds. By changing blade number, it is often possible to alter the blade frequency so that hull and machinery resonances are avoided. The propeller pressure forces are dependent on the clearances between the hull and the propeller, the blade thickness, and the blade load distribution. The clearance should be selected with care. Blade cavitation can greatly enhance the propeller pressure forces especially if the unsteadiness in the cavitation coincides with the blade frequency which, of course, is the usual case. Blade rake and skew can be used to minimize vibration. Blade skew, i.e., differential displacement of the blade chords along the helical reference lines, permits a more gradual entry of the blade into a wake region. The idea is to mismatch the wake with the blade shape. Highly skewed * American Bureau of Shipping 29-26 PROPELLERS AND PROPULSION blades can reduce the bearing forces to one-fourth the value for unskewed blades, and the pressure forces can be reduced by one-half. Figure 29-10 shows a drawing of a highly skewed propeller and Figure 29-11 is a picture of a highly skewed propeller. Recommendations have been developed by most all of the classification societies for propeller clearances, and theoretical and experimental techniques have been developed for predicting unsteady bearing forces. However, the main source of propeller induced vibration, especially on high speed ships, is the unsteady pressure force caused by a cavitating propeller. This problem is being widely investigated, and no clear criteria have yet been established. The cavitation problem is very severe if the cavitation is particularly unsteady. The cavitation can also cause noise problems as well as vibration problems within the hull and, if the noise is severe enough, can cause hearing problems and be extremely uncomfortable. When this happens, the problem is usually associated with a hull form which gives rise to an extremely sharp wake peak at the upper part of the propeller aperture. Oftentimes, it is necessary to modify the hull form, such as by putting fins on the hull, to alleviate the noise and vibration problems. Another propeller noise is singing. This is intense discrete frequency sound radiating from the propeller and is much like a steady bell tone. This noise is generally associated with vortices shedding from the blade trailing edges in such a way that the whole trailing edge is at resonant frequency. Sometimes singing is associated with a damaged blade, excitation from the shaft, or cavitation. However, the self-excitation from vortex shedding is the most common cause of singing. It can generally be controlled by sharpening the blade trailing edge so that the included angle between the blade face and back is 25 degrees or less. TYPES OF PROPELLERS There are a number of pro puIs or types used for propelling ships and many of these are defined in the glossary. Many more tend to be one of a kind, have limited usage, or are inventor's names for variations in otherwise common propulsors. The following discussion includes the most common types, except for oars and sails, but is not meant to be all-inclusive. Where the description is adequate under the definition, in the glossary, only the advantages and disadvantages will be given here. In all cases, these propellers are more expensive than the standard screw propeller. C o n tr o lla b le P itc h P r o p e lle r Capable of giving reverse thrust without reversing shaft and allows full power to be absorbed for all loading conditions. A d v a n ta g e s : Figure 29-11. Photograph of a highly skewed propeller. Courtesy Ferguson Propeller and Reconditioning, Ltd. D i s a d v a n t a g e s : More complicated mechanically and subject to failure. Less efficient at design condition (for standard single-screw hull forms, the efficiency loss is on the order of2 percent and for shaft-strut systems where the shaft is large and struts must be large to support the extra weight of the propeller and shaft, the net efficiency loss can be on the order of6 to 8 percent). (See Figure 29-12 for hub details.) C y c lo id a l P r o p e lle r Thrust is produced in any direction normal to axis of rotation. Axis of rotation would usually be vertical so that thrust is produced in any horizontal direction. This propeller is used where excellent maneuverability, as for ferries and tugs, is required. D i s a d v a n t a g e s : Very complicated mechanism and more subject to failure. Efficiency is about 25 percent less than standard propeller. A d v a n ta g e s : Figure 29-12. Hub details of a controllable pitch propeller. Courtesy Bird-Johnson Co. D i s a d v a n t a g e s : Less efficient when the propeller is lightly loaded. Actual efficiency difference (can be more or less) depends on loading and can be several percent. Types of ships which use Kort nozzles, or some form thereof, are ships which typically have high thrust and low speed, such as tugboats, trawlers, and full form tankers. Gains in efficiency can be on the order of 10 percent. On the other hand, for fast cargo ships, the loss in efficiency can be on the order of 10 percent. The decelerating pump jet types are not commonly used except for torpedoes and some military ships. F u lly C a v ita tin g More efficient under fully cavitating conditions. Applicable only to high speeds (40 knots or above), same for Propeller Types, Interface Propellers (see the glossary), except speed can be somewhat lower. A d v a n ta g e s : D u c te d P r o p e lle r More efficient when propeller is heavily loaded, more bollard pull, better protection ofthe propeller. A d v a n ta g e s : P r o p e lle r PROPELLERS 2 9 -3 0 AND PROPULSION Water Jet No exposed propulsor parts, good for shallow water operation. By rotating, exit nozzle can control thrust in any direction. D is a d v a n ta g e s : More complicated, heavier, and less efficient. Efficiency can be on the order of 25 percent less than a standard propeller. Often used for small, high speed craft. A d v a n ta g e s : M A IN T E N A N C E AND R E P A IR OF PROPELLERS The construction of propellers must be carried out according to the rules ofthe classification society which certifies a propeller for a particular ship. These rules consist mainly of the material specifications and specification ofthe blade thickness. Also, the protection of the steel shaft from corrosion due to action of the water is specified. The International Organization for Standardization (ISO) has recommendations for the manufacturing tolerances for casting and finishing of ship propellers. These recommendations discuss methods of mea suring propellers and tolerances on pitch, radius, thickness, blade section lengths, location of design median lines, longitudinal position, and surface finish of the propeller blade. The tolerances are broken into four classes. For instance, Class I for the majority of merchant vessels gives tolerances on pitch for total pitch of the blades of ±O. 75 percent with any local pitch on the blade being ±2 percent. Tolerances on thickness are an upper deviation of +3 percent of the maximum thickness and a lower deviation of -1.5 percent of the maximum thickness. The classification societies require the stamping of certain data on the propeller hub in a conspicuous place. The following data are usually stamped on the propeller hub: Name of Ship Builder Owner's approval Plan number Hull number Diameter Pitch (at 0.7 radius) Classification society approval Manufacturer's Heat Number Finished Weight I n s p e c tio n o f th e P r o p e lle r The propeller should be inspected whenever the opportunity arises for bent blades, erosion on the blades, nicks and gouges to the blades, and fouling. MAINTENANCE AND REPAIR OF PROPELLERS 29-31 Nicks and gouges, particularly near the leading edges and blade tip, should be particularly noted since they can give rise to cavitation as well as increase the blade drag. Damages should be repaired by qualified personnel. Repair of propellers will be discussed in a subsequent section. Marine fouling should be removed from the blades and hub in all cases. As explained in the previous section, marine fouling can greatly increase the blade frictional resistance resulting in a significant decrease in propeller efficiency. Also, it can give rise to cavitation. Marine fouling can usually be removed with a wooden scraper or a wire brush. When a wire brush or a more abrasive tool is used, great care must be taken to insure that the propeller surface is not damaged. Underwater inspection for fouling should be considered before ship operation when there is a possibility that the propeller is fouled, such as when the ship has been inactive for a long time or has been in warm waters. The propeller can be cleaned underwater by divers by the methods just described. Commercial firms also are available for this. P r o p e lle r M easu rem en t Although methods are under development for accurately measuring propellers while on the ship, the techniques have not yet been verified as giving accurate results. If measurements are deemed necessary, the propeller should be removed and taken to a shop with the proper facilities. There are cases where it is desirable to know about certain aspects of the propeller, e.g., the shape ofthe leading and trailing edges and the blade pitch. Some idea ofthe local leading and trailing edge shapes, as compared to design, can be obtained by using leading and trailing edge templates. Female templates can be cut from stock material using the propeller drawing, but care must be used in applying them at the correct propeller radius. A crude estimate of the blade pitch at a particular radius can be made by the following technique. Turn the propeller blade so that the radius section R at which the pitch is to be measured is horizontal. Drop plumb bobs from the leading edge and trailing edge of the propeller blade at this section to a level surface, making certain that the distances from the shaft centerline to both plumb bobs are the same. Measure the distance between the two plumb bobs, A, between the level surface and the blade leading edge, B, and between the level surface and the blade trailing edge, C. The tangent of the pitch angle is given by the formula GLOSSARY The absolute sign about B - C means only that the distance should be taken as positive whether the leading edge is up or down. Errors in this procedure occur in making the linear measurements, locating the propeller section horizontally, determining how level the surface is, finding the radius of the leading and trailing edges (the radius affects where the cord on the plumb bob leaves the blade, which may not be precisely the location of the pitch line), and locating the leading and trailing edges at the same propeller radius. It is estimated that the accuracy of the pitch measured in this way is no better than ±2 percent. R e p a ir o f P r o p e lle r s In this section, the repair of bronze propellers only will be discussed since this is the material used for most propellers. The repair of stainless steel propellers or propellers of other material should be carried out according to the special considerations for those propellers. Repair of bronze propellers should be carried out only by experienced personnel with the proper equipment. Some of the bronzes, e.g., manganese (Mn) bronze, nickel manganese (Ni Mn) bronze, and manganese nickel aluminum (Mn Ni AI) bronze, are subject to cracking due to stress corrosion, and any welding or bending must be followed by stress relieving to assure that the residual stresses have been reduced to safe levels. There have been numerous instances where blades and tips ofblades have broken due to fatigue failure where the original crack arose from stress corrosion. Nickel aluminum (Ni AI) bronze is not subject to stress corrosion cracking and stress relief is not required. For actual repair of bronze propellers, one should refer to the publication by the American Bureau of Shipping entitled G u i d a n c e M a n u a l f o r M a k i n g B r o n z e P r o p e l l e r R e p a i r s . The section below will give a brief summary of important points in the manual. There are major and minor repairs. Minor repairs are defined as those repairs to the edges on the outer one-third of the propeller blade and limited to the repair of sections under one and one-fourth inches thick. Propellers of Mn or Ni Mn bronze must be stress relieved for both minor and major repairs; Mn Ni Al bronze needs to be stress relieved for only major repairs; and Ni Al bronze need not be stress relieved for either minor or major repairs. In general, welding for cosmetic purposes should not be done on the blade. A critical area ofthe blade is defined as the area on the pressure side of the blade between the fillet and 0.4 radius and starting at the 29-33 leading edge and extending to 80 percent of the chord length. No repairs in the critical area should be undertaken without prior approval of the American Bureau of Shipping surveyor. Special preheat procedures are recommended before undertaking welding repairs. These procedures depend on the welding process and the welding material used. Welding should be done only by a qualified welding operator. Straightening can be carried out while the propeller is cold by means of dynamic loads, but this procedure should be restricted to minor repairs. Hot straightening by means of dynamic loads, or slowly applied loads, can be carried out for major repairs. The area of the propeller being repaired should be kept within the recommended temperature range during the whole straightening process, e.g., the temperature range for major straightening for Ni Al bronze is 760-954 degrees centigrade. As previously stated, Mn bronze, Ni Mn bronze, and Mn Ni Al bronze should all be stress relieved after all welding or straightening repair to prevent stress corrosion cracking. Furnace stress relief is recommended wherever possible. However, local stress relief can be carried out ifnecessary. G LO SSARY Except where stated, the entries refer generally to screw propellers. c o e f f i c i e n t (J). A parameter relating the speed of advance of the propeller, VA,to the rate of rotation, n, given by J = VAlnD, where D is the propeller diameter. The advance coefficient may also be defined in terms of ship speed, V, in which case it is given by: A dvance ( A E ) . An approximation to the surface area ofthe propeller equal to the area enclosed by an outline of a blade times the number of blades. The outline of a blade is constructed by laying off at each radius r the chord length along a straight line. The outline is formed by the locus of the end points ofthe chord lines laid out in the above manner. A r e a , p r o j e c t e d ( A p). The area enclosed by the outline ofthe propeller blades outside the hub projected onto a plane normal to the shaft axis. The outline is constructed by laying off, along each radius r, the extremities of each section as determined in a view along the shaft axis. The locus of the end points of the chord lines laid out in the above manner is the required outline. B a c k ( o f b l a d e ) . The side of a propeller blade which faces generally in the direction of ahead motion. This side of the blade is also known as the suction side of the blade because the average pressure there is lower than the pressure on the face ofthe blade during normal ahead operation. This side of the blade corresponds to the upper surface of an airfoil or wing. B l a d e a r e a r a t i o . A term used to denote the ratio of either the developed or expanded area of the blades to the disc area. The terms e x p a n d e d A rea, expanded PROPELLERS 2 9 -3 6 AND PROPULSION GLOSSARY a r e a r a tio or d e v e lo p e d a r e a r a tio are recommended in order to avoid ambiguity. Most commonly taken to mean the shape of a propeller blade at any radius when cut by a circular cylinder whose axis coincides with the shaft axis. B la d e s e c tio n . If the maximum thickness of the propeller blades varies linearly with radius, this variation of thickness of the blades may be imagined to extend to the axis of rotation. The hypothetical thickness at the axis of rotation, to, divided by the diameter is known as the blade thickness fraction or blade thickness ratio. If the thickness does not vary linearly with radius, then the blade thickness fraction is not uniquely defined. B la d e th ic k n e s s fr a c tio n . o f r e v o l u t i o n . A symmetrical body having the form described by rotating a plane curve about an axis in its plane. B o l l a r d p u l l . The pull force exerted by a ship at zero ship speed. It is the sum of the propeller thrust and the interaction force on the hull. B o u n d a r y l a y e r . The region of fluid close to a solid body where, due to viscosity, transverse gradients of velocity are large as compared with longitudinal variations, and shear stress is significant. The boundary layer may be laminar, turbulent, or transitional. C a m b e r . The maximum separation of the mean line and the nose-tail line. C a m b e r r a t i o . The camber divided by the chord length, f/c. C a p , p r o p e l l e r . See C o n e , p r o p e l l e r . Body In most engineering contexts, cavitation is defined as the process offormation of the vapor phase of a liquid when it is subjected to reduced pressure at constant ambient temperature. In general, a liquid is said to cavitate when vapor bubbles are observed to form and grow as a consequence of pressure reduction. The following definitions refer to cavitation. C a v ita tio n . B a c k c a v ita tio n . blade. Cavitation occurring on the suction side (back) of a propeller f l o w . A two-phase flow composed of a liquid and its vapor is called a cavitating flow when the phase transition is a result of hydrodynamic pressure change. C a v ita tin g d a m a g e . Deformation and/or erosion of materials in cavitated regions, associated primarily with the high pressures developed during cavity collapse. C a v ita tio n n u m b e r (cr). The ratio of the difference between absolute ambient pressure p and cavity pressure pc to the free stream dynamic pressure q; C a v ita tio n 2 9 -3 7 Cavitation occurring on the pressure side (face) of a propeller blade. It is generally a result of operation in which the local blade angle of attack is excessively negative. F u l l y d e v e l o p e d c a v i t y . A cavity formed on a body which terminates sufficiently far downstream so that the flow at the downstream region does not influence the body itself. For example, the cavity is fully developed when the re-entrant jet formed at the downstream end of the cavity is dissipated without impinging on the body. H u b v o r t e x c a v i t a t i o n . Cavitation in the vortex produced by the blades of a propeller at the hub. O r a n g e p e e l s u r f a c e a p p e a r a n c e . Description of a surface moderately damaged by cavitation in which the appearance is that of the surface of the Jaffa or California orange. P i t t e d s u r f a c e a p p e a r a n c e . Description of a surface damaged by cavitation in which pits are formed either by craterlike deformation without loss of material or by actual loss of material following work hardening or fatigue. P r o p e l l e r - h u l l v o r t e x c a v i t a t i o n . Propeller tip vortex cavitation that extends intermittently to the surface of the hull. R o o t c a v i t a t i o n . Cavitation in the low pressure region of the blade roots on a marine propeller. S p o n g y s u r f a c e a p p e a r a n c e . Description of a surface badly damaged by cavitation in which erosion has taken place to a considerable depth and has the appearance of a sponge. This description is particularly characteristic of brittle materials and other materials after long exposure. S u p e r c a v i t a t i n g f l o w s . Cavity flows in which attached, fully developed cavities extend beyond the trailing edge ofthe body about which the cavity is formed. T i p c a v i t a t i o n . Surface cavitation which occurs near the tip of a propeller blade. T i p v o r t e x c a v i t a t i o n . Cavitation occurring in the low pressure core of the tip vortex of a propeller. F a c e c a v ita tio n . The straight line connecting the extremities of the mean line. The length ofthis line is called the chord length or simply the chord. It passes through, or nearly through, the fore and aft extremities of the section. Synonymous with nose-tail line. C o n e , p r o p e l l e r . The conical cover placed over the after end ofthe propeller shaft for the purpose of protecting the nut and forming a hydrodynamic fairing for the hub. Also known as a propeller fairwater or a propeller cap. C o r r e l a t i o n a l l o w a n c e , m o d e l s h i p ( R A Y . This is the addition which has to be made to the resistance of the "smooth" ship, as predicted from the model results, to bring it into agreement with the actual ship performance determined from full-scale trial or service results. The correlation allowance depends upon the method used to extrapolate the model results to the "smooth" ship, the ship length and type, the basic shell roughness ofthe newly painted ship, fouling, weather conditions at the time the ship measurements were taken, scale effects on the factors making up the model, and ship propulsive coefficients. C h o r d lin e . The vertical distance from the top of the propeller tip circle to the at-rest water surface when the tips are exposed. E x p a n d e d a r e a r a t i o . The ratio of the expanded area of the blades to the disc area. F a c e ( o f b l a d e ) . The side of the propeller blade which faces downstream during ahead motion. This side of the blade is also known as the pressure side because the average pressure on the face of the blade is higher than the average pressure on the back of the blade during normal operation. The face corresponds to the lower surface of an airfoil or wing. F i l l e t . The transition region (fairing) between the propeller hub and the blades at the blade root. F l o w , s e p a r a t e d . The detachment ofthe main fluid flow from a solid surface due to an adverse longitudinal pressure gradient sometimes caused by a sudden change of the direction or the curvature of the surface. The fluid in the separated flow contains eddies, and may be nearly static or may contain a region of reversed flow. F l o w , t u r b u l e n t . A flow in which there are rapid and apparently random fluctuations both in the magnitude and in the direction of velocity. The velocity fluctuations may also be described by a random spectrum of vortices of varying size and strength. Turbulent resistance is higher than that in laminar flow at the same Reynolds number because of the high momentum exchange by transverse fluctuations. F l o w , v i s c o u s . The flow of a fluid where the flow characteristics include the effects ofthe shear forces acting on the fluid, and within it. E m e r g e n c e , tip . where r is the radius in question, is the pitch angle ofthe chord line at radius r (geometric pitch), and Z is the number of blades. G e n e r a t o r l i n e . The line formed by the intersection of the pitch helices and the plane containing the shaft axis and the propeller reference line. The distance from the propeller plane to the generator line in the direction of the shaft axis is called the rake. The generator line, the blade reference line, and the propeller reference line each intersects the shaft axis at the same point when extended thereto. Because of ambiguities which can arise in so extending the generator line and blade reference line when nonlinear distributions of rake and skew angle are used, it is recommended that each of these lines be defined as originating at the reference point of the root section (see Figure 29-14). The rake and skew angle of the root section will thus be defined as zero, and the propeller plane will pass through the reference point of the root section. H u b . The central portion of a screw propeller to which the blades are attached and through which the driving shaft is fitted. Also known as the boss. H u b d i a m e t e r ( d ) . The diameter ofthe hub where it intersects the generator line. H u b r a t i o . The ratio of the diameter ofthe hub to the maximum diameter of the propeller, dID. H y d r o f o i l . A structure externally similar to an airplane wing designed to produce lift and which operates in water. H y d r o f o i l s e c t i o n . The cross-sectional shape of a hydrofoil. Im m e r s io n ( h ) . The depth of submergence of the propeller measured vertically from the shaft axis to the free surface. <1> Figure 29-14(a). View of unrolled cylindrical sections at blade root and at any radius r of a right-handed propeller (looking down) showing recommended location of propeller plane r o t a t i o n . A propeller which is not located on the centerline of the ship is said to have inboard rotation if the blades move toward the centerline as they pass the upper vertical position. The opposite direction of rotation is called outboard rotation. Also called inward and outward rotation respectively. I n d u c e d v e l o c i t y , a x i a l ( U A J . The change in the velocity component in the direction parallel to the propeller axis due to the presence of the propeller but not including any change in the wake field due to propeller-hull interactions. Positive upstream. I n d u c e d v e l o c i t y , r a d i a l ( U R ) . The change in the velocity component in the radial direction due to the presence of the propeller but not including any change in the wake field due to propeller-hull interactions. Positive outward. I n d u c e d v e l o c i t y , t a n g e n t i a l ( U T ) . The change in the velocity component in the tangential direction due to the presence of the propeller but not including any change in the wake field due to propeller-hull interactions. Positive clockwise looking forward. L e f t - h a n d e d p r o p e l l e r . A propeller which rotates in the counterclockwise direction when viewed from astern. L i f t ( L ) . The fluid force acting on a body in a direction perpendicular to the motion ofthe body relative to the fluid. M e a n l i n e . The mean line is the locus of the midpoint between the upper and lower surface of an airfoil or hydrofoil section. The thickness is In b o a rd Figure 29-14(b). Diagram showing recommended reference lines (looking to port) generally measured in the direction normal to the chord rather than normal to the mean line. The maximum distance between the mean line and the chord line, measured normal to the chord line, is called the camber. The term camber line is often used synonymously with mean line. w i d t h r a t i o . Mean expanded or developed chord of one blade divided by the propeller diameter. Equal to the inverse of one-half the aspect ratio for a wing. N o s e - t a i l l i n e . Synonymous with chord line. N o z z l e . The duct portion of a ducted propeller. Synonymous with duct or shroud. M ean A type of an airfoil or hydrofoil section having a straight face, a circular arc or parabolic back, maximum thickness at the midchord, and relatively sharp leading and trailing edges. O u t b o a r d r o t a t i o n . A propeller not located on the centerline of the ship is said to have outboard rotation if the blades move away from the centerline as they pass the upper vertical position. The opposite direcO g iu a l s e c tio n . GLOSSARY 29-47 ( i s ) . The amount of axial displacement (rake) of a blade section which results when skewback is used (see Figure 29-14). It is the distance, measured in the direction of the shaft axis, between the generator line and the blade reference line and is given by res tan<\>, where r is the local radius, 8s is the local skew angle, and <\> is the local pitch angle. It is positive when the generator line is forward ofthe blade reference line. R a k e , t o t a l ( i T ) . The sum of the rake and the skew-induced rake. (See Figure 29-14.) R e f e r e n c e l i n e , b l a d e . The locus ofthe reference points ofthe blade sections (see Figure 29-14). Sometimes used synonymously with generator line. R e f e r e n c e l i n e , p r o p e l l e r . The straight line, normal to the shaft axis, which passes through the reference point ofthe root section (see Figure 29-14). It lies in the plane containing the shaft axis and the generator line. R e s i s t a n c e ( R ) . The fluid force acting on a moving body in such a way as to oppose its motion; the component of the fluid forces acting parallel to the axis of motion of a body. Resistance is the preferred term in ship hydrodynamics, while drag is generally used in aerodynamics and for submerged bodies. Total resistance is denoted by RT and various (not mutually exclusive) components of resistance are defined below. See also D r a g . R a k e , s k e w -in d u c e d r e s i s t a n c e ( R A P ) . The increase in resistance relative to that of the naked, or bare, hull resistance, caused by appendages such as bilge keels, rudders, bossings, struts, etc. F r i c t i o n a l r e s i s t a n c e ( R F ) . The component of resistance obtained by integrating the tangential stresses over the surface of a body in the direction of motion. P r e s s u r e r e s i s t a n c e ( R p ) . The component of resistance obtained by integrating the normal stresses over the surface of a body in the direction of motion. R e s i s t a n c e c o e f f i c i e n t ( C F , C R , C s , C r , Cy, C w , e t c . ) . The non dimensional ratio of any specific component of resistance per unit area to the dynamic pressure far ahead of the body. R e s is ta n c e i n w a v e s , m e a n i n c r e a s e i n ( R A W ) . The mean increase in resistance in wind and waves as compared with the still water resistance at the same mean speed. R e s id u a r y r e s i s t a n c e ( R R J . A quantity obtained by subtracting from the total resistance of a hull a calculated friction resistance obtained by any specific formulation. R o u g h n e s s r e s i s t a n c e ( R A R J . The increase in resistance relative to the resistance of a hydraulically smooth hull due to the effect of roughness. The hull roughness may be of different types, such as structural roughness caused by the method of hull construction, waviness of plating, scoops, valve openings, etc. S p r a y r e s i s t a n c e ( R s ) . The component of resistance associated with the expenditure of energy in generating spray. V i s c o u s r e s i s t a n c e ( R y ) . The component of resistance associated with the expenditure of energy in viscous effects. A ppendage REFERENCES 29-53 Todd, F. H. 1963. "Series 60, Methodical Experiments with Models of Single-Screw Ships." D a v i d T a y l o r M o d e l B a s i n R e p o r t 1712, July. [Wageningen B-Series] Van Lammeren, S. P. S., Van Manen, J. D., and Oosterveld, M. W. C. 1969. "The Wageningen B-Screw Series." T r a n s a c t i o n o f t h e S o c i e t y o f N a v a l A r c h i t e c t s a n d M a r i n e E n g i n e e r s , Vol. 77, 269-317. Yeh, H.Y.H.1965."Series 64-Resistance Experiments on High-Speed Displacement Forms." M a r i n e T e c h n o l o g y , Vol. 2, No.3. A form of propulsion in which water is taken into the hull ofthe ship by means of ducting, and energy is imparted to the water with a pump. The water is then ejected astern through a nozzle. W i n d m i l l i n g . The rotation of a propeller caused by flow past the propeller without power being applied to the propeller shaft. This action may take place while the ship is moving under its own momentum, while it is being towed, or while it is being propelled by other means of propulsion. W a te r je t. REFERENCES Caster, E. B. 1963. "TMB 2, 3 and 4-Bladed Supercavitating Propeller Series." D a v i d T a y l o r M o d e l B a s i n R e p o r t 1637, January. [Gawn-Burrill Series] Gawn, R. W. L., and Burrill, L. C. 1957. "Effect of Cavitation on the Performance of a Series of 16-inch Model Propellers." T r a n s a c t i o n s , R o y a l I n s t i t u t e o f N a v a l A r c h i t e c t u r e , Vol. 99, 690-728. Hadler, J. B., and Cheng, H. M. 1965. "Analysis of Experimental Wake Data in Way of Propeller Plane of Single and Twin-Screw Ship Models." T r a n s a c tio n o f th e S o c ie ty o f N a v a l A r c h ite c ts a n d M a r in e E n g in e e r s , Vol. 73,287-414. Taylor, D. W. 1933. "Speed and Power of Ships." Published by the Department of Commerce, Washington, D. C. See also Gertler, M., 1954. "A Reanalysis of the Original Test Data for the Taylor Standard Series." D a v i d T a y l o r M o d e l B a s i n R e p o r t 8 0 6 , March. CHAPTER M a c h in e r y 30 T e s ts a n d T r ia ls EVERETT C. HUNT IN T R O D U C T IO N MODERN ship consists of complex machinery, piping and electrical systems, and systems for control, data display, and internal communication. Tests and trials are required to demonstrate the operation, performance, and endurance of these various kinds of equipment. In addition to good engineering practice, regulatory agencies and statutory law require certain tests and trials to assure the safety of personnel and the seaworthiness ofthe ship. This chapter provides an outline of the types of tests and trials with which the engineer officer should be familiar. A TESTS A T M A N U F A C T U R E R 'S PLANT Classification agencies, governmental requirements, and contractual specifications require tests of some machinery components at the manufacturer's plant. These tests include verification of the physical and chemical properties of materials used in the manufacture of critical parts such as rotor forgings and steam drums, hydrostatic testing of pressure vessels, and equipment operational demonstrations usually at considerably less than maximum rating. For example, most steam turbines and reduction gears are spin tested without load at the manufacturing facility. Component tests are specified in the machinery purchase contract and in the shipbuilding contract. Equipment for which manufacturer's operational tests are normally required include: Air compressors Refrigeration and air-conditioning units Deck machinery 30-1 MACHINERY TESTS AND TRIALS 3 0 -2 Diesel engines Large pumps Evaporators Generators Oil purifiers Large motors Turbines and gears Controls DOCK TRIALS Trials at dockside are conducted by a shipbuilder to verify that the newly installed machinery and systems are operable and ready for the more rigorous sea trials. Dock trials may reveal piping system leaks, misalignment of equipment, electrical problems, and control and instrument adjustment needs. During dock trials, the ship's lines may be doubled up for a part load test of the main propulsion equipment. This is frequently the first time that the equipment has operated under load. Instrumentation accuracy and reliability are essential for a successful sea trial, and therefore the dock trial provides the opportunity to assure the builder that instruments are properly installed, calibrated, and operable. Final adjustments are made to many machinery components during the dock trial. Since the equipment manufacturers are in attendance, dock trials also present an opportunity to train the crew for the trial at sea. SEA TRIALS A Preliminary Builder's Trial A builder's trial is sometimes conducted to provide the builder with an opportunity to make final adjustments to machinery and other systems, before the contract sea trial, which must demonstrate conformance to contractual requirements. Crew training for the contract sea trial is also undertaken on a builder's trial. But, because ofthe high cost ofmaintaining a ship at sea during these trials, most shipbuilders have abandoned the practice of builder's trials for commercial vessels. Contract Sea Trial The contractual sea trial is a formal and highly organized series of events which typically lasts for three days. There are many reasons for conducting the sea trial, and there are many parties interested in this trial and its results. The trial extends to every characteristic ofthe ship, including hull, machinery, anchor, and navigational equipment characteristics. The operational aspects ofthe trial are planned in order to demonstrate to all interested parties the proper operation of the vessel and its machin- SEA TRIALS 3 0 -3 ery and electrical systems. A deep sea environment is essential for such demonstrations. Performance tests are scheduled to demonstrate that the vessel meets the design (and/or contractual) levels for speed, power, and propeller RPM (PRPM). The performance is predicted by model testing during the ship design phase, and the sea trial performance is compared to the model predictions. The endurance portion of the trial demonstrates the ability of all ofthe ship's systems to function for a prescribed extended period oftime at maximum speed and power without the failure of the components. Economy tests are included to demonstrate that the design (and/or contractual) fuel consumption is achieved under the conditions predicted by heat balance calculations corrected for actual shipboard conditions during the trial. The sea trial presents the owner with the opportunity to collect operating data while the ship is in the as-built condition. These data can be very valuable to the owner for future trend analysis and other baseline comparisons. The use of modern data collection systems and microprocessors on some new vessels makes trend analysis a useful tool for the ship's engineer officer and the owner's technical staff ashore. Sea trials are used to collect such ship characteristic data as turning, stopping, and backing ability. This information is useful to the ship's officers in maneuvering the ship and may be required as evidence in a legal action involving the ship. Finally, sea trials provide the opportunity to make the required demonstration of safety features, equipment, and systems to the proper classification agencies and governmental regulatory organizations. Typical Trial Specifications All dock and sea trials are required to be scheduled and completed to the satisfaction of the shipowner and must be conducted in accordance with the Society ofN aval Architects and Marine Engineers (SNAME)Technical and Research Bulletin, C o d e f o r S e a T r i a l s . If conflicts or contradictions exist between the SNAME codes and the owner's detailed trial specifications, the owner's specifications take precedence. The shipbuilder must prepare and install all maneuvering information in the pilothouse, as required by the U.S. Coast Guard. This information will be based on the maneuvering trials performed during the sea trial. When a sea trial requires more than 24 hours of continuous testing, a recuperation period of6 hours is required at the end ofeach 18-hour period during which all testing will be suspended. Shaft calibration is required for vessels which are scheduled for progressive speed trials or are contractually subject to penalty for exceeding the specified fuel consumption rate. When the shaft is not calibrated, use of a torsion meter to measure torque will be based on a standard modulus of rigidity of 82 x 199 Pa (11.9 x 106 psi) for Grade 2 ABS steel shafting. SEA TRIAL MAIN ENGINE MACHINERY TESTS AND TRIALS 3 0 -4 A vibration survey is sometimes required to be conducted in accordance with SNAME's Technical and Research Bulletin, C o d e f o r S h i p b o a r d V i b r a t i o n M e a s u r e m e n t . Collection ofvibration data provides useful design and baseline data for future comparison to determine changes in vibration characteristics during operation. Finally, the trials are not considered complete until they have been documented in a comprehensive report issued by the builder. T im e 0030 0230 0300 0415 T y p ic a l T r ia l R e p o r t A typical formal trial report issued by the shipbuilder will include the following information: 1500 Hull description Machinery description Log of trial events List of personnel in attendance Draft, displacement, and ballast data Anchor windlass test results Steering gear test results Machinery test results Distiller test results Fuel rate test results Water rate test results Model basin curves Standardization data and curves "Z" maneuvering test results Crash stop test results Right turning circle Left turning circle Supporting data, such as heat balance design correction factors 2200 2230 0230 0245 0300 0315 0345 0430 0500 0645 0715 0845 T y p ic a l T r ia l S c h e d u le The schedule of events for a typical sea trial for a steam vessel is as folIo\'; T im e 1230 1330 1430 1530 1600 1630 E vent 0945 1100 1900 U H U 30-5 E vent Second Day (In test area) Start distilling plant. Calibrate direction finder. Adjust magnetic compass Adjust main propulsion plant. Test drag shaft. Test zero torque meter. Increase speed for economy trials. Commence economy trials. 4 hours at rated power 2 hours at 110 percent rated power 2 hours at maximum power Commence nonextraction steam rate tests of main turbine, 1 hour each at 110 percent power, 100 percent power, 75 percent power, 50 percent power. Test drag shaft and zero torsion meter. Conduct nonscheduled events. Third Day Conduct emergency steering test. Conduct ahead steering test. Conduct "crash astern" test. Conduct "crash ahead" test. Conduct astern full power test for 1 hour. Conduct "astern steering" test during last 15 minutes of astern full power test. Conduct "boiler overload" test on 1 boiler. Conduct "crash astern" test from maximum power. Test additional boiler. Proceed to area with 100 fathoms water depth. Conduct anchor han dling tests. Test combustion controls with different firing arrangements. Demonstrate emergency circulation. Arrive at shipyard. S J ):A First Day Energize gyro and electronic equipments 4 hours prior to departure. Sound all tanks. Light off 1 boiler, necessary auxiliaries, and turbogenerator. Light off second boiler 2 hours prior to departure. Make preparations for getting underway. (The sea trial director ally has prepared a detailed check-off list for these preparations). Test navigation gear. Depart shipyard and proceed to test area. TESTING T R IA L M A IN A head E N G IN E E ndurance T E S T IN G T est The ahead endurance is a 4-hour test ofthe main propulsion machinery at maximum design capability. During this test it is desirable to record power developed with a high accuracy. Normally, power will be determined with a torque meter which measures torsional deflection of the line shaft and an RPM measurement. If a calibrated meter is not installed, the measurement of power developed is limited to an estimate. Other pertinent power 3 0 -6 MACHINERY TESTS AND TRIALS plant parameters are included such as temperatures, pressures, and flows in the cycle and should be recorded during the endurance trial run to provide data for subsequent analysis in the event that the trial proves unsatisfactory. In all trials, every effort must be made to prevent fluctuation of the power plant parameters during the run period. Fluctuations greater than 5 percent of the average value of power plant parameters require consideration of a test rerun. The ahead endurance trial demonstrates the ability of every power plant component to reach a capability level consistent with the maximum power of the system and to sustain acceptable operation at that level for a time period of sufficient duration to suggest the capability of the plant to operate indefinitely at the maximum power level. All aspects of the main machinery must operate satisfactorily during this period, that is, the thermal, mechanical, and electrical controls, etc. Since this is frequently the first time the reduction gear carries full torque, the gear is usually inspected for contact by using red and blue dye checking techniques. A ste r n E ndurance T est The astern endurance is a one-half-hour test of the main propulsion machinery at maximum design astern power. The critical measurement during this period is the torque in addition to propeller RPM (PRPM) developed. All significant power plant parameters must also be measured. The temperature rise in the ahead turbine elements and the differential growth of the turbine casings and rotors are carefully monitored during this test to demonstrate the adequacy of design and installation. E conom y T est The economy test is run to demonstrate that the power plant overall fuel rate, corrected for differences between the guarantee and/or design heat balance, and the actual sea trial conditions meet the contractual requirements. The economy test is usually a 4-hour run at service power conditions. Critical data measured during this test are the power and the fuel consumption. In addition, all thermodynamic measurements are recorded at 15-minute intervals. Useful data on fuel consumption versus shaft horsepower are generated during this test. S te a m R a te T e st This I-hour test is sometimes conducted to demonstrate the nonextracting guarantee steam rate for the propulsion turbine. Usually the test is less than satisfactory due to the practice of not installing an accurate steam or condensate flow measuring device for sea trials. SPECIAL SHIPBOARD INSTRUMENTATION B o ile r O v e r lo a d 3 0 -7 T est The boiler overload test is normally conducted at the maximum design fuel flow to the boiler to demonstrate the overload capacity ofthe boiler and its associated auxiliaries. This test will reveal any temperature problems, casing leaks, control problems, etc. All boiler operating parameters should be adjusted to the manufacturer's recommended values during this test. To achieve the overload steam flow capacity, a plan is required to include operation of major auxiliary equipment to provide the power plant steam flow demand required. P r o p u ls io n C o n tr o l S y ste m T est This operational test demonstrates the ability ofthe central control system to function properly under transient conditions in all operating modes including maneuvering, emergency, and steady running of the main propulsion unit. If bridge control is furnished, operation under that mode must also be demonstrated. S P E C IA L S H IP B O A R D IN S T R U M E N T A T IO N Until the 1970s, when fuel cost became a dominant factor in ship operating expense, most commercial vessels had relatively simple instrumentation generally limited to pressure gauges, thermometers, displacement-type fuel oil flowmeters, and shaft revolution indicators. Now trials require the installation of special instrumentation and the calibration of other instruments to permit the measurement ofvital performance characteristics such as torque and condensate flow. It is now apparent that shipowners have an interest in and an economicjustification for the installation ofhigh accuracy and more complicated instrumentation for the measurement of shipboard operating parameters including condensate flow, steam flow, main propeller shaft thrust and torque, fuel flow, and selected temperatures and pressures. Some owners even provide a vessel with Lorantype equipment which can be used by the ship's force to accurately meas ure vessel speed relative to the bottom. Such equipment if installed and maintained in calibration can be used during the initial sea trial and in periodic subsequent mini sea trials to provide comparisons between the performance of the new ship and its current performance. Such information is necessary for economic decisions on dry dock scheduling, machinery overhaul, boiler cleaning, etc. The engineer officer on a properly instrumented ship has the opportunity to playa key role in the control of operating costs for fuel, maintenance, and dry docking of the vessel. 30-8 MACHINERY TESTS AND TRIALS S h a ft H o r se p o w e r M e te r A practical and reasonably accurate method of measuring the power transmitted by the line shaft is to measure the angle of shaft deflection (i.e., twist) due to torsional loading. The amount of deflection of the shaft is a function of the applied torque, the length of shaft over which the deflection is measured, and the stiffness of the shaft. If the deflection of the line shaft for a fixed distance can be measured and the modulus of shear and the RPM are known, the horsepower transmitted may be calculated. A typical device suitable for providing a torque measurement with one percent accuracy is the McNab Mark II Shaft Horsepower Meter shown in Figure 30-1. This type of meter is a mechanical electric-electronic device which performs the measurement of the shaft angular deflection and computes the data collected to provide shaft torque, RPM, and horsepower. Figure 30-2. Torque meter husk assembly one another. Relative torsional movement ofthe two rings is deflected and measured by a differential transformer (i.e., transducer) which produces a direct current output signal proportional to the shaft angular displacement. The DC signal is transmitted via slip rings to the panel where it is modified by a series of operational amplifiers which perform the functions required to solve the torque formula. Since the angular data from the husk is not linear, one operational amplifier incorporates a nonlinear gain to provide a linear angle output signal. An adjustment for the shaft modulus value is made by selecting taps of a resistive voltage divider at the input of one amplifier. The basic modulus data is built into the unit during manufacture at 11.89 x 106 psi. The output of the last amplifier is a DC signal directly proportional to the torque of the shaft. This signal is used by the shaft horsepower circuits to calculate power and may be displayed on the panel digital meter as shown in Figure 30-3. The husk assembly also consists of two cylindrical sections, each with an internal circular knife edge that clamps to the shaft. Leaf springs run axially inside the husk to connect the two cylindrical sections maintaining longitudinal alignment without interfering with the torsional displacement of the shaft. Attached to one section of the husk are pole pieces of a Figure 30-3. Torque power meter panel differential signal transformer and attached to the other section is the armature of the transformer. The transformer pole pieces have a primary winding that is excited with AC current from a power supply. There are two secondary windings, the outputs from which are rectified separately and connected series-opposing. Under no-torque conditions, the transformer armature is centered between the pole pieces, resulting in rectified secondary voltages that are equal, providing a zero output signal. Torque loading produces a shaft deflection which moves the transformer armature away from one pole and closer to the other pole. This movement unbalances the secondary rectified voltages to provide a DC output signal corresponding to the direction and amplitude of shaft deflection. C a lib r a tio n D e m o n s tr a tio n Since the shaft horsepower meter will be used to verify contractual requirements and, if permanently installed, will be used to monitor and correct power plant performance, it is essential that the metering system be F u e l C o n s u m p tio n M easu rem en t Since fuel oil is a major factor in the cost of ship operations, it is desirable to have an accurate and reliable system to determine fuel consumption rates. In fact, two metering systems are justified: a conventional nutating disk-type meter for general hourly and daily measurement, and a second alternative turbine meter located in a bypass for use during periodic high accuracy performance tests such as sea trials and similar tests conducted by the ship's crew. flowmeter. This potentially very accurate meter is simply a miniature turbine wheel suspended in a length of pipe. The flow of fluid through the meter rotates the bearing-supported axial turbine at a speed which is directly proportional to the velocity ofthe fluid. Since the diameter of the pipe is fixed, the turbine rotational speed is proportional to the volumetric flow rate of the fluid. External electronic devices count the rotations of the turbine rotor and convert the pulses to gallons per minute or any other desired volumetric flow rate. Figures 30-4 (a) and (b) illustrate a typical turbine flowmeter manufactured by Flow Technology, Inc. When used with a liquid it is possible to achieve a calibration accuracy of±0.05 percent. T u r b in e S H IP B O A R D USE OF T R IA L S T A N D A R D IZ A T IO N DATA Data collected during the trial for shaft horsepower, ship speed, and propeller RPM are used for the analysis of the combined performance of the hull, propeller, and propulsion machinery. Ship speed in knots is plotted versus shaft horsepower and propeller RPM for the model prediction performance and for the corrected trial performance to provide comparisons between the anticipated and the actual performance. Figure 30-6 shows the plots of typical data used in trial analysis. The type data in Figure 30-6 plotted for his ship should be of interest to the merchant marine engineer officer, since it provides a benchmark to compare the original new condition ship performance to a current speed versus horsepower estimate for the ship. As time passes, the engineer officer should continue to collect operating data on ship speed and power at a draft condition and with sea and wind conditions similar to the trial conditions. These data plotted on the trial analysis curve will provide current indications of the hull performance. In analyzing ship trial performance, it is common to calculate and plot the propeller characteristic curves. One of the curves plotted is the speed coefficient versus the torque coefficient shown in Figure 30-7. This information can be valuable to the engineer officer in monitoring the future combined performance of the hull, the propeller, and the power plant. IN D E X Amplifiers, control system, 24:5 Analyzer, vibration, 25:15-16 Apron tuyeres, coal-fired boiler, 27:52 Armature reaction, 23:73-75 Aromaticity index, 16:18 Ash coal fuel, 27:6 analysis, 27:7 deposition, 27:7-9 fusion temperature, 27:7 handling, C-E coal-fired boiler, 27:25 lignitic, coal-fired boiler, 27:10 pit cleaning, 27:53 system arrangement, F-W coalfired boiler, 27:35 Aspiration, natural, diesel engine, 16:2 Atkinson cycle, 16:4 Atmosphere control, cargo tank, 26:7 Accelerometer, 25:5 AC controllers anti-plugging, 23:37 autotransformer starter, 23:41 jogging, 23:37 part-winding starter, 23:41-44 plugging, 23:37 primary reactance starter, 23:41 primary resistance starter, 23:38-41 reversing, 23:37 single-phase speed control, 23:64-65 single-phase starters, 23:64 solid state, 23:45-49, 51-52, 64-65 wye-delta starter, 23:44 Acid attack, sulfur, diesel engine, 16:19, 72, 108, 112 AC motors capacitor start, 23:58-59 dual-capacitor, 23:59 hysteresis, 23:61 permanent-split capacitor, 23:59 reluctance, 23:61 repulsion type, 23:62-64 selsyn, 23:52-54 shaded-pole, 23:60 single-phase, 23:57-68 split-phase, 23:58 squirrel-cage, 23:26-49 synchronous, 23:18-25 three-phase induction, 23:25-57 synchronous, 23: 18-25 universal, 23:62 wound-rotor, 23:49-52 A-frame, diesel engine, 17:9, 10 Air cooler, diesel engine, 17:26, 28, 62 Air systems C-E coal-fired boiler, 27:25-27 F-W coal-fired boiler, 27:31 Air valve, diesel engine, 17:51 Air-conditioning, 18:12, 19:1 Air injection, diesel, 16:3 Alternator control of, 23:6-7 excitation systems, 23:4-6 frequency of, 23:1 load sharing, 23:8-11 parallel operation, 23:8 rotor windings, 23:4 single machine operation, 23:7 stator windings, 23:2-4 synchronizing, 23:8 Amortisseur windings, 23:4, 23 Bailey board, 24:1, 2 Balance coil, 23:80 Balancing diesel engine, 17:26-28 machinery, 25:7 Ballast, inert gas system cargo tanks, 26:29 spaces, 26:33 voyage, 26:29 Barred speed, diesel, 16:112, 63, 69 Base/acid ratio, coal-fired boiler, 27:10 Baseplate, diesel engine, 17:42 Bearing antifriction, defect detection, 25:27 diesel engine, 17:11-13, 42-43, 50 Bedplate, diesel engine, 17:9-10 Belt drives, defect detection, 25:28 Bernoulli equation, propeller, 29:10 Blade, propeller area, 29:20 face, 29:39 rake, 29:25 ratio of area, 29:35 section, 29:36 skew, 29:25 strength, 29:21-25 stress, 29:21 thickness, 29:21 thickness fraction, 29:36 Block coefficient, hull, 29:7 Blowby, diesel, 16:116 Blowdown pulse, diesel, 16:40 Blowers, inert gas, 26:11 design considerations, 26:12 1 2 INDEX Blowers, inert gas (cont.) performance, 26: 13 Boiler areas subject to slagging, 27:9 C-E V2M9S, 27:18-31 coal-fired, prepurge, 27:58 dual fuel (coaVoil),procedure, 27:58-59 installation, C-E V2M9S, 27:24 overload test, 30:7 Bollard pull, propeller, 29:36 Bolt, hold-down, diesel engine, 17:9 Boost blower diesel, 16:37, 112 diesel engine, 17:26 Boost ratio, diesel, 16:25 Brake power, diesel, defined, 16:12 Brine, refrigeration, 18:14-15 Buffer tank, 16:89, 17:28 Builder's trial, 30:2 Bunker, day, coal, 27:40 Burner management system, dualfired boiler, 27:58 Cabling control, 22:23 medium voltage, 22:23 Navy,22:25 nomenclature, 22:18-20 power and lighting, 22:20 resistance of, 22:51 signal, 22:23 voltage drop in, 22:59 Camshaft diesel, 16:6, 8, 46-47, 83, 111, 119 diesel engine, 17:20 drive, 17:10,20,45 Capacitance, 22:52-54 Capstan, electric 21:33 Carbon residue, fuel, 16:18 Cargo, refrigerated, 18:13 Catalytic fines, fuel, 16:16, 19 Cathode ray tube, controls, 24:1, 5, 11, 15 Cavitation erosion, diesel, 16:47 propeller, 29:18-21,29-30,36 Central operating system, 24:1, 2, 5, 6, 8, 9, 10,12,18,24,26 Cetane number, diesel fuel, 16:18,43 C-E V2M9S boiler, internal arrangement, 27:19 Chain grate assembly, coal-fired boiler, 27:43 Chlorine, residual, sewage system, 28:19 Chock, diesel engine, 17:9 Chopper drives, 23:91 Chord line, propeller, 29:37 Cinder rejectionsystem,C-Ecoal-firedboiler,27:23 return system, Detroit stoker, 27:51-52 Circuit breakers draw-out, 22:35 metal-clad, 22:34, 35 molded-case, 22:31-34 trip-free, 22:35 Clinker inspection, coal-fired boiler, 27:52 Coal analysis, 27:7 burning profile, 27:15 special, 27:15-17 bunkers, 27:38 burning economic and political factors, 27:1 technology, 27:1-59 classification, moisture content, 27:6 conveyor, 27:38 distribution, Detroit stoker, 27:48-49 fire control, combustion control, 27:56 handling system, 27:37-41 properties, 27:3-7 valves, 27:38 Coal feed assembly, Detroit stoker, 27:45 control, 27:46, 47-48 F-W coal-fired boiler, 27:33-34,40-41 operation, 27:53 Coal fuel analysis, 27:3 analysis, importance, 27:17-18 characteristics, 27:2-18 fixed carbon, 27:6 gross heating value, 27:7 moisture content, 27:2 proximate analysis, 27:7 ultimate analysis, 27:6 volatile release profile, 27: 17 Coil pitch, 23:2 Colloidal solids, sewage, 28:19 Combustion control, coal-fired boiler, 27:5459 diesel, 16:48-49 engineering, 27:18-31 forced-draft, coal, 27:56 induced-draft, coal, 27:56 Combustion cycle, dual, IC engine, 16:2, 3 Comminutor, sewage, 28:19 Common rail, diesel, 16:43 Communications, control system, 24:10 Compressed air system, diesel, 16:81-85 Compression ignition, IC engine, 16:1, 4 ratio, diesel, 16:7,11,25 Compressors, refrigeration, 18:16-29 Condenser, refrigeration, 18:30 Condition monitoring, diesel, 16:115 Conductance, 22:52 Connecting rod INDEX Connecting rod (cont.) diesel, 16:4, 120-123 diesel engine, 17:11, 50-51 Containers, refrigerated, 18:15-16 Continuity equation, propeller, 29:10 Continuous rating, maximum, diesel engine, 16:17,70 Control, diesel, 17:30-32,58-61 air, 16:83 system test, propulsion, 30:7 Controllable pitch propeller, 29:26-28 Controllers, loop, direct, 24:1, 2, 4, 8, 10, 13-17,18,20,27,30 Control systems algorithms, 24:18 analog, 24:10, 11, 12, 13, 14,17, 18, 19, 21-28 baud rate, 24:4 binary, 24:22, 23, 29 capacitance, 24:25 central, 24:1,8, 10, 11, 15 components, 24:6, 10, 11, 12, 15, 16, 18 computer, 24:11-15, 16, 17-21,23-30 configuration, 24: 11 damping, 24:6 demodulated,24:30 differential, 24:4 digitized, 24:2 direct-connected systems, 24:8 discrete, 24:13, 14 distributed systems, 24:21 dynamic performance, 24:6 electromagnetic, 24:25 electromechanical systems, 24:9 electronic, 24:4, 5, 8, 9, 12, 15, 24-28 electrostatic, 24:25 fiber optics, 24:4 filter, 24:12 indicators, 24:5 inductive, 24:25 integrated, 24:3,19 interface, 24:1, 2,11, 14,28-31 interlock, 24:6 microcomputer, 24:2, 21, 23 microelectronic, 24:2, 3 microprocessor, 24:2, 15,20,21,22 miniaturization, 24:1 misalignment, machinery vibration, 25:27 mission time, performance, 24:7 mode, 24:25, 26, 27 modem, 24:24-30, 31 modulated, 24:30 MOS, 24:2 MSI, 24:3 MTBF, performance, 24:7 multiplexer, 24:11,20,22,23,24,26,28 optical encoder, 24:5 3 pneumatic, 24:4, 8, 10, 12, 23, 27 silicon, 24:3 stability, 24:7 static performance, 24:6 supervisory, 24:19 monitoring, 24:17 transmission, 24:1, 8, 22 Convectors, 19:22-24 Converter drives dual, 23:91 full, 23:91 half-wave, 23:91 Cooling bore, diesel engine 17:13 central, diesel, 16:100 coils, HVAC, 19:22-23 system, seawater, diesel, 16:100-101 Correlation allowance, 29:3, 37 Corrosion cold end, diesel, 16:19,72,108,112 control system, 24:6 Counter-rotating shaft, diesel, 16:66 Counter-EMF, 23:69,85 Counterweight, diesel engine, 17:27,44 Coupling, control system, 24:25, 26 Crane, 16:120-123 Crank pin, diesel, 16:4 Crankcase, diesel engine, 17:9, 10, 41-42 explosion, 16:93, 114, 123 overpressure, diesel, 16:93 Crankpin, diesel engine, 17:11 Crankshaft, diesel engine, 17:11, 43-44 deflection, 16:119 Critical dilution, 26:4 Critical speed, diesel, 16:68 Crosshead, diesel, 16:2,92-93, 113, 119, 120 engine, 17:1, 40 guide, 17:9 pin, 17:13 Crosstalk, control system, 24:7 Crude oil washing, 26:41-50 drop pipe, 26:46 inert gas system, 26:28 piping regulations, 26:48 stripping regulations, 26:50 system, 26:42 techniques, 26:42 typical machine, 26:42-48 Cryogenics, 20:1 Cycle simple refrigeration, 18:9 vapor-compression, 18:7 Cycloidal propeller, 29:28 Cylinder, diesel bore, 16:4 block, 16:4, 17:10, 11 engine, head, liner, 17:13, 16, 17, 18,4748,51 4 Cylinder, diesel ( c o n t . ) head, 16:4, 118, 120-123 liner, 16:4, 120-123 lubricator, 16:93, 110, 113, 117, 118 oil, 16:19, 93-95,113, 118 INDEX forces, 16:50-69 foundation bolts, 16:119 four-stroke engine, 16:2, 6,14,37,41,42, 66,67 margin, 16:75 matching, 16:74-81 Damper, diesel engine, 17:27, 45 peak pressure, 16:13 David Taylor Model Basin, 29:3 performance, 16:70-73 DC controllers preventive maintenance, 16:115 armature, resistance and voltage conrating, 16:6, 17:8 trol,23:90-91 speed, critical, 16:68 buck-boost control, 23:90 Diesel fuel counter-EMF starter, 23:95 heating value, 16: 17 definite-time starter, 23:95 incompatibility, 16:21 field resistance control, 23:88-90 rack, 16:44, 110, 111, 115, 116 generators, s e e Generators system, 16:85-90 series lock-out starter, 23:95-96 Diffuser, 28:20 solid state control, 23:91 Digital computation, converters, conWard-Leonard drive, 23:90 troIs, 24:1-5, 9-17,19,25-30 DC machines Displacement, diesel, 16:6 armature, 23:90-91 Distribution, electrical, 22:2 basic principles, 23:68-70 one-line diagram, 22:4 construction, 23:70-73 power circuits, 22:5 field windings, 23:70 protection of, 22:27-35 grounded coil, 23:100 Dock trials, 30:2 open coil, 23:99 Double-acting, diesel, 16:2 windings, 23:70-73 Doxford, diesel engine, 16:3, 43 DC motors, 23:86-88 Draft regulator, coal-fired boiler, Detroit speed characteristics, 23:85 stoker, 27:48 Deck machinery, 21:23-35 Drag, ship, 29:2 Deck water seal(s) Drum, control system, 24:5 inert gas system, 26: 14-15 Dust collectors inspection, 26:39 C-E coal-fired boiler, 27:27 Dehumidification system, cargo hold, 19:13mechanical, B&W coal-fired 15 boiler, 27:28 Degrees, electrical, 23:2 Economy test, main engine, 30:6 Dehydrator, refrigeration, 18:41-42 Efficiency Derating, diesel engine, 17:8 propulsive, diesel engine, 29:7 Detroit Rotograte, coal-fired boiler, 27:41-48 relative rotative, 29:8 Diesel engine Effiuent, sewage, 28:21 air-injection, 16:3 Electric drive blowby, 16:116 AC blowdown pulse, 16:40 diesel,22:46-51 boost blower, 16:37, 112 turbo, 22:44-45 boost ratio, 16:25 DC brake power, 16:12 diesel,22:46 camshaft, 16:6, 8, 46-47, 83,111,119 turbo, 22:45-46 combustion, 16:48-49 Electrohydraulic steering compression ratio, 16:7,11,25 gear, regulations 21:16-17 condition monitoring, 16:115 pumping system, 21:6-10 control air, 16:83 system maintenance, 21:9-10 corrosion, cold end, 16:19,72, 108, 112 Electromagnetic force, 23:68 crank pin, 16:4 Emergency power cylinder, 16:4, 118, 120-123 distribution, 22:4 displacement, 16:6 final source, 22:7 efficiency, mechanical, 16:13 temporary source, 22:6 engine rating, 17:8 Endurance test, main engine, 30:5-6 firing pressure, 16:13 Engine testing, sea trial, 30:5 INDEX Enthalpy, 19:4 EfROM, memory, control system, 24:16 E PROM, memory, control system, 24:11, 13,14,15,16,20 Equalizing bus, 23:83 Evaporator, refrigeration, 18:31 Exciter, 23:4-5 Exhaust gas turbine, 16:37 Exhaust valves, diesel, 16:18, 20, 72, 108, 112, 115, 116, 119, 120-123 Expanded area, propeller, 29:35 Expansion tank, 16:97-99 Expansion valve, thermostatic, 18:31-34 Fairbanks Morse diesel engine, 16:3, 17:41-66 Fan HVAC, 19:15 centrifugal, 19:17 propeller, 19:16 vane-axial, 19:16 inert gas, 26:14 Fan-coil units, 19:24-25 Fillet, propeller, 29:39 Filters control system, 24:10, 12 HVAC, 19:19-20 Firing pressure, diesel, 16: 13 Firing, supplementary, C-E coal-fired boiler, 27:29 Flame screens, inert gas system, 26:18 Flammability triangle, 26:3 Flammable limits, 26:3 tankers, 26:4 Flash point, fuel, 16:20, 87, 89, 116 Flocculation, sewage, 28:21 Flow, instrumentation, control system, 24:3,4, 10,22,23 Flowmeter, fuel measurement, 30:12 Flue gas composition, 26:7 system(s) F-W coal-fired boiler, 27:31 isolating valves, 26: 17 toxicity, inert gas system, 26:37 Fluid pulsation, analysis, 25:28 Fluorocarbons, 18:4 Fly ash system, F-W coal-fired boiler, 27:34 Foster Wheeler coal-fired boiler, 27:31-37 installation, 27:33 Fouling coal-fired boiler, 27:8, 13-15 control system, 24:30 propeller, 29:21 Foundation bolts, diesel, 16:119 Four-stroke diesel, 18:2, 6, 14,37,41,42, 66,67, 17:41-66 Frame, diesel engine, 17:41-43 Frequency analysis, vibration, 25:23-25 Frequency response, performance, control, 24:6 Freshwater system, diesel, 16:97-102 Frog-leg windings, 23:73 Froude number, 29:40 Fuel(s) aromaticity index, 16:18 consumption measurement, 30:12 diesel,16:15-22 analysis, 16:21 injection, 16:3, 7,10,42-47 rack, 16:44, 110, 111, 115, 116 system, 16:85-90 total base number, 16:19, 92, 93, 95, 116 diesel engine injection, 17:23,54 quality, 17:8, 62-63 incompatibility, 16:21 rate calculation, 30:14-15 Fuses, 22:29-31 Garbage disposal, restrictions, 28:1 Gases, hydrocarbon, 26:1 Gawn-Burrill series, propeller, 29:19 Gear teeth, defect detection, 25:28 Generator, AC, s e e a l s o Alternator amplidyne, 23:76 attached, diesel engine, 17:32 compound,23:80-83 electric, diesel engine, 17:32 parallel operation, 23:83-85 separately excited, 23:76-79 series, 23:79 shunt, 23:79 voltage characteristics, 23:75-76 Germanium, control system, 24:3 Gland, piston rod, diesel engine, 17:17 Glow plug, IC engine, 16:11 Goodman stress criteria, propeller, 29:24 Grate speed, Detroit stoker, coal-fired boiler, 27:50 Gudgeon pin, diesel, 16:4 Guide force, diesel, 16:54, 56 Gypsy head, 21:34 Harmonics analysis, propeller, 29:4, 25 wake, 29:7 Hatch leakage, inert gas system, 26:32 Hazardous areas classification of, 22:14 equipment protection in, 22:14 Heat balance, coal-fired plant, 27:4-5 latent, 18:2 recovery, waste, diesel engine, 16:41, I' ~ 6 Heat INDEX (cont.) 42,98, 101-109 sensible, 18:2 transfer, conduction, and radiation, 18:1, 2 Heaters, unit, 19:24-25 Heat exchanger, liquid-suction, 18:31 Heating, 19:2 and cooling loads, 19:4 coils, HVAC, 19:20-21 value coal fuel, 27:2-3 diesel fuel, 16: 17 Hele-Shaw pump, principle, 21:8-9 Helix control, diesel, 16:44 Holzer tabulation, diesel, 16:68 Hot bulb, IC engine, 16:11 Hull efficiency, 29:8 fouling, 29:3 interaction with propeller, 29:5 machinery, 21:1, 2 Humidity, instrumentation, control system, 24:6 Humidity, ratio and relative, 19:4 HVAC system(s) control, 19:26-34 actuators, 19:31-33 relays and controllers, 19:26-34 sensors, 19:27-28 dual duct, 19:9 multiple zone, 19:8 single zone, 19:7-8 terminal reheat, 19:8-9 testing and balancing, 19:35-39 unitary, 19:11-14 variable air volume, 19:10-11 water, 19:10-11 Hydrocarbon toxicity, inert gas system, 26:37 Hydrofoil, 29:40 Hypochlorite, 28:21 Hysteresis, control system, 24:6, 7 Ignition coal bed, coal-fired boiler, 27:28-29 delay, IC engine, 16:7, 10, 18,48 sources, 26:3 spark, IC engine, 16:4 IMCO,26:2 Imhoff cone, sewage, 28:21 IMO regulations, crude oil washing, 26:48-50 Impedance, 22:56 performance, control, 24:24, 25 Incineration systems design considerations, 28: 15-17 garbage, 28:14-19 oil waste, 28:14 Incinerator arrangement, 28: 17 chamber, 28: 16-17 cutaway view, 28:20 operation, 28:17-19 Indicator card, diesel, 16:12, 113, 116 Inductance, 22:52-54 Inert gas system, 26:1,3,7,8-40 alarms, 26:20, 39 backflow of cargo gases, 26:36 blowers, inspection, 26:38 C-E coal-fired boiler, 27:25-27 combination carriers, 26:33 precautions, 26:32 dilution, displacement, 26:5 discharging, 26:26,27-28 distribution, 26:33 drainpiping,26:22 effiuent,26:22 electrostatic hazards, 26:37 emergency procedures, 26:35-36 failure symptoms, 26:26 fan, 26:14 flue, 26:7 freeing, 26:29-31 generator exhaust, 26:8 independent, 26:8 health hazards, 26:36 high velocity vent valve, 26:20 instruction manuals, 26:40-41 instrumentation, 26:20 maintenance and testing, 26:38-41 materials, 26:17 nonreturn devices, 26: 13-17 operation, 26:24-30 piping, 26:17 pressure regulator, 26:18 product contamination, 26:31 regulation, 26:19 replacement, 26:4 safety checks, 26:26 scrubber, 26:11 inspection, 26:38 shutdown, 26:25 special precautions, product carriers,26:30-31 standard procedures, 26:27 start-up, 26:25 valves, 26:17 void spaces, 26:33 Inerting of tanks, inert gas system, 26:27 Injection, diesel lag, 16:46 pump, 16:43-44 secondary, 16:44, 47 Injector, diesel engine, 16:45-46, 98, 119,17:18,23,57,62 INDEX Instrumentation, 24:21, 24, 27 calibration, 24:7, 8,15,28 inputs, 24:6,7, 11, 13, 14, 17, 23-28 level, 24:3, 4, 5, 22, 23, 25 position, control system, 24:3 shipboard, tests, 30:7 test, 30:10-11 Intake air, direct vs. external, diesel, 16:39 Interpoles, 23:70,74 Isolation valves, 26:16,17 Kramer drive, 23:52 Lanchester balancer, diesel, 16:60, 62 Lap-wound armatures, 23:71 Lavomatic SA tank -washing machine, 26:43-44 Lighting circuits, electrical, 22:6 Lighting-off, coal-fired boiler, 27:30-31, 58 procedure, stoker system, 27:41-43 Linearity, performance, control system, 24:6, 7 Liquefaction processes, 20:2-4 Liquefied gases, properties, 20:1 Liquid indicator, refrigeration, 18:40 LNG, 20:1 cargo operations, 20:21-23 tank(s),20:21-24 cargo systems, 20:10-20 cargo tanks, 20:5 Conch, 20:6 Gas Transport, 20:8, 11 Kvaerner-Moss,20:8,12-13 Technigaz, 20:7, 9-10 Loaded passage, cargo, inert gas system, 26:28 Low speed diesel engine, 17:1-40 Low voltage protection, 23:37 release, 23:36 LPG, 20:1 LSI, control system, 24:2, 3 Lubricating oil system, diesel, 16:90-97 Lubrication, diesel engine cylinder, 17:16, hydrostatic, 17:13, 28 Machinery defects, vibration characteristics, 25:27-29 tests, 30:1-18 tests, manufacturer's, 30:1-2 Magnetic field, rotating, 23:19-20 Magnetism, residual, 23:76 Magnetization curve, 23:76 Maintenance AC stator, 23:11-15 bearings, 23:55-56 crude oil washing machine, 26:48 DC machines, 23:96-102 7 diesel engine, 17:35-40, 63-66 inert gas system, 26:38-41 preventive baseline program, vibration, 25:17,20-21 diesel engine, 16:115 schedule, diesel, 16:118-120 single-phase motors, 23:65-68 squirrel-cage rotors, 23:55 synchronous rotor, 23:15-18 windlass, 21:33 wound-rotors, 23:55 Maneuvering, diesel engine, 16:111-113 Margin diesel engine, 16:75 sea, diesel, 16:75 Master controller, combustion control, coal, 27:54-56 Materials cryogenic, 20:4 propeller, 29:24 MCR, diesel engine, 17:8 Medium speed diesel engine, 17:41-66 Meter(s) portable, vibration, 25:17-18 shaft horsepower, 30:8 vibration, 25:14-15 Microorganism, sewage, 28:21 Miller cycle, 16:4 Misalignment, machinery vibration, 25:27 Moments, diesel engine, 16:50-69 Momentum, propeller, 29:9,10 Monitoring control system, 24:2,10,14,15,17,19,20 diesel engines, 17:30-32, 58-61 equipment, vibration, 25:16-17 Mounting, resilient, diesel, 16:51 Navigation light and signal panel, 22:7 Nitrification, sewage, 28:21 Noise, propeller, 29:25 Nonreturn valves design considerations, 26: 17 inert gas system, 26:16 inspection, 26:39 Nozzle, propeller, 29:42 Ogival section, propeller, 29:42 Oil disposal, restrictions, 28: 1 fire control, combustion control, coal-fired boiler, 27:57 firing, coal-fired boiler, 27:58 mist detection, diesel engine 16:93 Oily water discharge, 28:10 regulations, 28:11 separators, 28:11-14 8 INDEX Operation/operating procedure, coal-fired boiler, 27:30-31 regulations, crude oil washing, 26:50 Opposed-piston diesel engine, 16:3, 10 Oscillating motion, 25:2 Otto cycle, IC engine, 16:2, 3-4 Outputs, instrumentation, control system, 24:6,7,11,14,17,22,23,28 Overfire air C-E coal-fired boiler, 27:23 Detroit stoker, coal-fired boiler, 27:51-52 Oxygen, inert gas system analyzer, 26:22 deficiency, 26:36 Panelboards, 22:18, 19,20 Pathogens, sewage, 28:21 Performance C-E coal-fired boiler, 27:29-30 F-W coal-fired boiler, 27:35-37 Personnel qualifications, crude oil washing, 26:50 pH value, sewage, 28:21 Phase angle analysis, vibration, 25:25 Pielstick, diesel engine, 17:41-66 Piezoelectric crystal, 25:5 Pinch point, waste heat boiler, 16:108 Piston, diesel engine, 16:4, 120-123, 17: 13, 16, 48-50 oil-cooled, 17:8, 17, 28, 48-50 pin, 16:4, 17:49 rod, 17:13,17 trunk, 16:2, 90, 92, 93, 95, 119 water-cooled, 17:8, 11-13, 17,28 Plastic piping, inert gas system, 26:23 Polarized frequency relay, 23:23 Pole pitch, 23:2 Potable water, 28:23 Pour point, fuel, diesel, 16:21 Power AC, 22:56-59 apparent, 22:56 coefficient, 29:44 propeller, 29:14 DC, 22:56 delivered, 29:44 effective, 29:7,44 factor, 22:57 correction, 23:22 loading coefficient, 29:45 reactive, 22:56 shaft, 29:44 take-off, augmented drive, 22:13 constant speed engines, 22:9 diesel engine, 17:32, 33, 43, 45, 62 frequency converters, 22:10 variableratio mechanicalcoupling,22:11 test code, diesel engine, 16:15 three-phase, 22:57 thrust, 29:44 triangle, 22:56 true, 22:56 Precombustion chamber, IC engine, 16:3 Pressure maximum, diesel engine, 16:13, 14, 113, 116 mean effective and indicated, diesel engine, 16:14 line, propeller, 29:41 width ratio, propeller, 29:42 Pressure control instrumentation, 24:3, 4, 6, 10, 22, 2 ratio, IC engine, 16:22, 25 side, propeller, 29:45 system, inert gas system, 26:19 Pressure vacuum breaker, liquidfilled, 26:20 Printers, control system, 24:1 PROM, memory, control system, 24:16 Prony brake, 16:13 Propeller(s), s e e a l s o Blade, propeller advance angle, 29:33 advance coefficient, 29:12 allowable stress, 29:24 angle of attack, 29:35 back of blade, 29:35 bending moment, 29:21 body of revolution, 29:36 boundary layer, 29:36 camber, 29:36 cone, 29:37 data, 29:30 definition, 29:1 design, 29:8-25 developed area, 29:35 disc area, 29:35 ducted,29:28-29 efficiency, 29:8, 38-39 fitting, 29:3-4 flow angle, hydrodynamic, 29:34 generator line, 29:40 hub,29:40 immersion, 29:40 inboard rotation, 29:41 induced velocity, 29:41 inspection, 29:30-31 interaction with hull, 29:5 left-handed,29:41 lift, 29:41 lifting line theory, 29:9 maintenance, 29:30 mean line, 29:41 measurement, 29:31-32 open water test, 29:12, 13 optimum diameter, 29:17-18 INDEX Propeller(s) (cant.) pitch,29:43-44 plane, 29:45 projected area, 29:35 propulsion, ship, 29:1-3 race, 29:46 rake, 29:46 repair, 29:30, 32-33 rotation, outboard, 29:42 singing, 29:26 skewed, 29:27 span, 29:49 supercavitating,29:19-20 theory, 29:8-12 types, 29:26-30, 45 width ratio, 29:42 Psychrometric chart, 19:2-3 PTSA, 26:2 Pulse converter, 16:41 Pulse turbocharger, diesel engine, 16:40, 41 Pulses, instrumentation, control system, 24:11 Pump diesel engine attached, 17:45, 62 injection, 17:23, 54-55 Hele-Shaw, steering gear, 21:6-9 regulations, crude oil washing, 26:49 Purging, inert gas system, 26:29, 31 Purifier, oil, 16:87, 95-97, 113, 114 Push rod, diesel engine, 16:6, 17:53 Putrefaction, sewage, 28:22 Radiators, 19:22 RAM, memory, control system, 24:16 Rapson slide, steering gear, 21:5 Ratios slip, real, 29:49 stroke-to-bore, diesel, 16:3, 10, 17:2, 8 Reactance, 22:52, 53, 56 Receiver, refrigeration, 18:30 Receivers, instrumentation, control system, 24:5 Recorders, instrumentation, control system, 24:5 Reference line, propeller, 29:42 Refrigerants, saturation properties 18:4, 5, 6 Refrigeration applications, 18:9-15 definition, 18:1 ice, 18:3 systems charging, 18:46 compressor oil, 18:50-51 compressor overhaul, 18:51-55 control method, 18:10 defrosting, 18:45-46 evacuation and dehydration, 18:48-49 9 leak testing, 18:47-48 noncondensable gases, 18:49-50 normal operation, 18:45 preventive maintenance, 18:51-55 removing refrigerant, 18:47 safety precautions, 18:43-44 securing, 18:45 starting, 18:44-45 switches, 18:34-38 troubleshooting, 18:57-63 Relay out-of-step, 23:24 polarized frequency, 23:23 Report, sea trial, 30:4 Resistance, 22:51-54 ship, 29:1-3 definitions, 29:47 Resonance, vibration, machinery, 25:28, Resonant speed, diesel engine, 16:68 Response system, 24:6, 7 Reversing, diesel engine, 17:22, 30-32, 45, 58-61, Reynolds number, 29:3, 48 Rocker arm, diesel engine, 17:53 ROM, memory, control system, 24:16, Rotocap, diesel engine, 17:51-53 Roughness allowance, 29:3, RPM, instrumentation, control system, 24:5, 23 Rubbing, machinery, analyis, 25:28, Rudder, active, 29:48 Rudder torque, 21:5, Rudderstock, steering gear, 21:4 Running gear, diesel engine, 16:2 Sabathe cycle, 16:2, 3 Safety considerations, inert gas system, 26:36-38 devices, Detroit stoker, coal-fired boiler, 27:50-51, Salinity, instrumentation, control system, 24:3, 5 Sampling, diesel, 16:21, 118, 119 Scale effect, 29:48, Scavenging, diesel engine, 16:2, 8, 9 loop, 16:90-97, 17:13, pulse, 16:40 uniflow, 17:2 Scherbius drive, 23:52, Scrubber flue gas, 26: 10 inert gas system, effluent line, 26:24 inspection, 26:39, Sea margin, diesel, 16:75, Sea trial{s), 30:2-7 code for, 30:3 10 Seawater service, inert gas system, 26:23 Sedimentation, sewage, 28:22 Self-synchronizing motors, 23:52-54 Semiconductor, control system, 24:3 Semiconverter drives, 23:91 SEMT,17:41-66 Sensing, instrumentation, control system, 24:1,3,4,8,9,22,24 Sensitivity, performance, control, 24:7 Sensor, inert gas pressure, 26:22 Separated flow, propeller, 29:39 Separators oily water, 28:12 Series data, propeller, 29:12 Series 60, ship resistance, 29:7 Service margin, diesel, 16:75, 79,80 Setback, propeller, 29:48 Set point manipulation, control system, 24: 17 Sewage, 28:1-10 activated sludge, 28:22 process, 28:21 aerobic treatment systems, 28:5, 19 aeration, 28:7, 9-10, 19 anaerobic digestion, 28:19 available oxygen in, 28:21 biochemical oxygen demand, 28:19 coarse bubble system, 28:5 denitrification, 28: 19 disinfection, 28:21 dissolved oxygen in, 28:21 dissolved solids in, 28:22 effluent, 28:21 fine bubble aeration, 28:5 hydrogen concentration, 28:21 influent, 28:21 mixed liquor, 28:21 oxygen deficiency in, 28:21 primary treatment, 28:22 secondary treatment, 28:22 septic, 28:22, settleable solids, 28:22 skimming, 28:22 sludge, 28:22 suspended solids, 28:22 volatile solids, 28:22 Shaft angle, propeller, 29:34 Shaft horsepower, calculation, 30:11-12 Ship's stores, refrigerated, 18:11 Shroud, propeller, 29:48 Signals, control system, 24:2, 4, 5, 8, 9, 12,13,22 Sintering strength, coal-fired boiler, 27:14 Skew angle, skewback, propeller, 29:49 Slagging, coal-fired boiler, 27:8, 9-13 bituminous, 27:10 Slip, 23:26 Sodium, fuel contaminant, diesel, 16:20 INDEX Soot blower, coal-fired boiler cleaning radius, 27:16 spacing, 27:13 Spark arrestor, 16:40 Specifications, sea trial, 30:3 Specific gravity, fuel, diesel, 16:17, 87 Speed advance, 29:6 control system, 24:3, 7 diesel engine, 16:1 regulation, 23:31 Squirrel-cage motors adjustable speed control, 23:32, 45-49 characteristics, 23:26-30 construction, 23:26 dual-voltage, 23:29 multispeed control, 23:32-35 NEMA design class, 23:29-30 starting, 23:35-49 Stack gas inerting system, 26:9 Starter closed transition, 23:41 open transition, 23:41 Starting, diesel engine, 16:81-83, 109-111, 17:28, 30-32, 53, 58-61 Static electricity, inert gas system, 26:32 Stator winding, 23:12-15 Steam flow, combustion control, coal, 27:56 Steam rate test, trial, 30:6 Steam system, diesel plant, 16:101-109 Steering engine, electrohydraulic, 21:3-10 Steering gear, 21:3-24 communications requirements, 21:19 control requirements, 21:18 electromechanical, 21:11-14 floating lever control, 21:4 four ram, 21:4 hydraulic, 21:5,6 pressure, 21:17 inspections, 21:9-10 operating requirements, 21:22 power units, 21:18 regulations, 21:15-16 amended, 21:17-22 reliability, 21:12 relief valves, 21:17 rotary vane, 21:10-11 rudder stock, 21:17 requirements, 21:20 SCR controller, 21:14 SOLAS requirements, 21:16-23 tanker requirements, 21:20 testing and drills, 21:23 types, 21:3 variable stroke pump, 21:6-9 Sterilization, sewage, 28:22 Stock coal-handling system, 27:37-40 Stoker, coal-fired boiler INDEX Stoker, coal-fired boiler (cont.) C-E continuous discharge, 27:20-23 operation, 27:53, 54 out of service, 27:53 system, 27:41-48 Stopping, diesel engine, 16:114 Strainer, refrigeration, 18:42-43 Strip indicators, control system, 24:5 Stroke, diesel engine, 16:4 -to-bore ratio, 16:3, 10 Sulfur, fuel contaminant, diesel, 16:19, 72 Sulfur content, coal fuel, 27:6 Sulzer, diesel engine, 17:1-40 Sun Oil Company, 26:1 Switchboards, 22:15-16 Synchronizing lamps, 23:8 Synchronous machine alternator, 23:1-18 field coils grounded, 23:17 maintenance, 23:11-18 motor, 23:18-25 open, 23:17 reversed, 23: 17 shorted, 23:15-16 troubleshooting, 23:11-12 Synchronous motor description, 23:18-20 operation, 23:21-25 power factor correction, 23:22 starting, 23:23-25 troubleshooting, 23:21 Synchronous speed alternators, 23:1 motors, 23:21 Synchroscope, 23:8 Tank cleaning, inert gas system, 26:29 cleaning machine regulation, crude oil entry inerting after entry, 26:30 preparation, inert gas system, 26:29, mixing, diesel, 16:89, 112, 17:28, 54 pressure, inert gas system, 26:37 washing, 26:48-49 washing machine, gear box, 26:44 Lavomatic SA, 26:43 Taylor series, 29:3 Teleprinters, control system, 24:5 Temperature condensing, 18:9 dew point, 19:4 dry bulb, 19:2 evaporator, 18:7 instrumentation, control system, 24:3, 4,6,8,10,22,23 1 1 saturation, 18:3 wet bulb, 19:2 Terminals, HVAC, 19:26 Test, machinery, 30:1-18 Thermocouple, instrumentation, control system, 24:24, 26, 27 Thrust bearing, diesel engine, 17:9, 43 breakdown, 29:50 coefficient, 29:50 propeller, 29:14 deduction, 29:5-6, 50 loading coefficient, 29:50 propeller, 29:22-23 Thruster, 29:50 Tie bolt, diesel engine, 17:9-10 Tie rod, diesel engine, 17:11 Tiller crosshead, steering gear, 21:4 Tip emergence, propeller, 29:39 Topping cargo tanks, inert gas system, 26:35 Torque, 29:51 diesel engine reaction, 16:56, 61, 65, 17:27 variation, 16:68 instrumentation, control system, 24:3,5, 23 meter, McNab, 30:8-9 Torsional vibration, diesel engine, 17:20, 27, 45 Transducers, control system, 24:7, 8, 22, 27, 28,30 Transformer auto-,22:41-42 current, 22:43 efficiency, 2 2 :3 7 magnetizing current, 22:36 percent impedance, 22:37 potential, 22:42 rating of, 22:36 step-down, step-up, 22:36 terminal markings of, 22:38 three-phase connections, 22:38-41 Transmission, control system, 24:4, 5, 9, 12, 29,31 Transmission, electronic, control system, 24: 1 Transmitter, control system, 24:24, 25, 30 Trash disposal, restrictions, 28:1 Trend analysis, diesel engine, 16:115 Trial data use, 30:15 Trials, sea, 30:2-7 Troost series, propeller, 29:8 Troubleshooting DC generators, 23:98-99 DC motors, 23:101-102 induction motors, 23:56-57 single-phase motors, 23:67-68 synchronous, alternator, 23:11-12 12 INDEX INDEX Troubleshooting (cant.) motor, 23:21-22 Trunk piston, diesel engine, 17:41-66 Tube temperature, coal-fired boiler, 27:8 Turbine, exhaust gas, diesel engine, 17:33-35 Turbocharger characteristics, 16:26 diesel engine, 17:23-26, 33, 57-58 washing, 17:35, 58 efficiency, 16:15,25,36 matching, 16:29-37 surge, 16:26, 38, 72, 77 two-stage, 16:4, 22, 38 uncooled, 16:22 water wash, 16:37, 113, 118 Turbocharging, 16:2, 22 diesel, 16:22-23, 92, 120 constant pressure, 16:41 Turbulent flow, propeller, 29:39 Two-stroke engine, diesel, 17:1-40 Unbalance, mechanical, machinery vibration, 25:27 Ultraviolet disinfection unit, sewage system, 28:9 Uniflow, diesel, 16:8, 9, 10 Valve diesel engine, 17:18, 22, 51-53 fuel circulation, 17:23, 28 refrigeration, 18:38-41 solenoid, 18:37 Vanadium, fuel contaminant, diesel, 16:20 Vapor crankcase, control system, 24:5 meters, inert gas system, 26:22 Velocity distribution, propeller, 29:5 Ventilation, criteria, 19:1, 6-7 Venturi injection, sewage system, 28:5 Vibration(s),21:1 ABS rules, 25:9 acceleration, 25:5 acceptable limits, 25:11-13 air-handling equipment, 25:10 amplitude, 25:2 modulation, 25:25 analysis diesel engine, 16:117 economic benefits, 25:9 new machinery, 25:8 shipboard, 25:8,10-11 analyzer, maintenance program, 25:1820 beat frequency, 25:25 bent shaft, machinery, 25:27 case histories, 25:29-31 damped, 25:2 diesel engine, 16:50-69, 114, 117,118,17:26-28,25:11 displacement, 25:4 forced, 24:2 free 21:2 frequency, 25:2 gas turbines, 25:11 gear standards, 25:11, 13-14 investigation of problems 25:9 machinery repairs, 25:9 mathematical models, 25:7 measurement, 25:3 equipment, 25:13-17 interpretation, 25:21-26,30 motors and generators, 25:10-11 phase angle, 25:6 propulsion systems, shaft and propellers, 25:10 signature, 25:9 systems, 25:2 torsional, diesel, 16:68-69 velocity, 25:4 velocity standards, 25: 13 Viscosimeter, fuel, 16:90 Viscosity fuel, diesel, 16:16, 17,87,89,93, 116 kinematic, 29:3 temperature relation, coal-fired boiler, 27:12 water, 29:1 Voltage, standard, 22:1 Voltage dip, 23:38 Voltmeter, control system, 24:10 VoltslHertz control, 23:45 Volume, specific, 19:4 VSLI, control system, 24:3 Wageningen series, propeller, 29:8 Wake, 29:4 fraction, 29:6, propeller, 29:51 Ward-Leonard drive, 23:90 Wash head assembly, crude oil washing, 26:47 Waste disposal in sea, restrictions, 28:1 heat recovery, diesel engine, 17:28, 62 Water jet, 29:52 seal(s), inert gas dry, semi-dry, wet, 26:15 inspection, 26:39 Wave-wound armatures, 23:71 Winches, 21:33 construction, 2 1 :34 Windings compensating, 23:75 consequent-pole, 23:33 Windlass, 21:24 band brake, 21:28 hydraulic, 21:30-33 maintenance, 21:33 reduction gear, 21 :21-30 steam, 2 1 :24-30 13 Windmilling, propeller, 29:52 Wound-rotor motors, 23:49-52 Wrist pin, diesel, 16:4, 120 Zero drift, performance, control system, 24:7