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.
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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