June 2010 Hazardous Fluid Piping Design www.che.com r, PAGE 28 Focus on Computer Modeling Corrosion Monitoring Systems Containing Fugitive Emissions Process Cooling Improvements Facts at Your Fingertips: Distillation Tray Design Special Advertising Section: Sealing elligence Pressure Vessel Design Bridging the gap between users' and manufacturers' responsibilities for the ASME pressure vessel code Keith Kachelhofer, Jedson Engineering Inc. T racing its origins to 1915, the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (the code) [1] has become the established safety standard governing the design, fabrication and inspection of boilers and pressure vessels, as well as nuclear power plant components during construction. Section VIII, Division I of the code addresses pressure vessels operating at either internal or external pressures exceeding 15 psig. Despite the prevalence of pressure vessels in the chemical process industries (CPI), a clear understanding of the basis-of-design responsibilities involved in designing, fabricating and repairing such a device remains elusive. Vessel users are responsible for providing all necessary data to ensure the manufacturer can design and fabricate a pressure vessel in full compliance with the code. The lack of clear understanding can result in a disconnect between users and manufacturers during pressure vessel specification. The disconnect is often magnified because, although Section U-2(a) of the ASME Code clearly defines the responsibilities for establishing the 28 basis of design, users often lack access to the code language and its associated interpretations. Basis of design refers to well-defined information that could form the foundation for inspection and test acceptance criteria. While engineering specifications often provide sufficient data for a manufacturer in certain basic areas — such as internal and external pressure, temperature, vessel orientation, material of construction, corrosion allowance and vessel contents — pressure vessel fabricators usually receive insufficient information from users in areas such as wind, seismic and external loadings. The incomplete specification information makes a proper and complete vessel design difficult and can lead to inaccurate price quotes. Providing complete information will help avoid cost overruns and change-orders. The intent of this article is to clarify those areas of pressure vessel specification where information is commonly omitted and areas where further clarification is required. Further, this article is intended to improve understanding of which responsibilities are shouldered by vessel users and which by manufacturers. By providing a more CHEMICAL ENGINEERING WWW.CHE.COM JUNE 2010 comprehensive basis of design for a vessel, users and manufacturers can save money and formulate specifications with public safety in mind. All 50 U.S. states, all Canadian provinces and many local jurisdictions and territories have formally adopted the ASME Code as a safety standard for boilers and pressure vessels. Each jurisdiction employs a chief inspector who is a member of the National Board of Boiler and Pressure Vessel Inspectors. Meanwhile, the code is frequently a prevailing basis in other countries throughout the world. VESSEL DESIGN Design versus operating T and P In engineering specifications, often no distinction is made between the design pressure and operating pressure. Section UG-21 of the code recommends a suitable design pressure above the operating pressure of the vessel at which the vessel will normally operate. The operating pressure should represent the most severe exposure of pressure and temperature the vessel is expected to experience under normal operating conditions, whereas the design pressure should allow for potential pres- I •• B TABLE 1 . SURFACE FINISH COMPARISON Grit finish Ra (microinch) RMS (Mm) 36 142 4.06 60 87 2.49 80 71 2.03 120 52 1.47 that is the case, clearly indicate it in the specification. A material specification for 42 150 1.20 appurtenances is commonly 180 30 0.86 not given. Items such as lift220 19 0.53 ing lugs, support lugs, skirts 15 240 0.43 and support legs can often be specified with a different grade 12 0.36 320 material than that of pressure 400 9 FIGURE 1. The pickling process re0.25 retaining items. The user may moves the heat tint produced during Mirror +/-4 0.13 have a vessel where all of the welding (left = before; right = after) pressure-retaining items and sure surges up to the setting of the strictions for material-grade, post-weld wetted surfaces are 316L stainless pressure-relief device. The design tem- heat treatment, and allowable hard- steel, but the lifting lugs and support perature should account for the lowest ness of the weld and heat-affected zone; ring may be fabricated from 304 stainand highest operating temperature, in all of which will impact the manufac- less steel. Depending on the user and addition to operational upsets, atmo- turer's cost for fabrication. The user the service, a stainless-steel vessel spheric temperature and other sources should identify dangerous compounds with carbon-steel legs might be acceptof cooling. The design and operating in its process and address the dangers able, provided there is a "poison pad" conditions should be established in a in a process safety review meeting. between the two materials such that process safety review meeting within The user should provide the specific the pressure-retaining items are not the user's organization. gravity of the process fluid, since the at risk of carbon contamination. Often Based upon the material of construc- manufacturer must account for the ad- manufacturers will specify an altertion, the nominal plate thickness and ditional static pressure due to the static nate material grade for appurtenances the minimum design temperature, the head of the liquid, per section UG-22(b). in order to minimize fabrication cost. If manufacturer will have to determine alternate material grades are unacceptthe requirements for welding. For car- Materials of construction able for appurtenances, that should be bon steel and low-alloy vessels, the re- While the material of construction is stated in the specification, particularly quirement for Charpy impact testing commonly included in equipment speci- when manufacturers are competitively can be determined in Section UCS-66 fications, clarification is often required bidding for the contract. of the code. For high-alloy vessels, as to the impact the material specifisuch as those fabricated of austenitic cation has on the fabrication and the Stainless-steel surface finish stainless steel, the manufacturer will quote. Information in the specification For users in the food-and-beverage and refer to Section UHA-51 of the code. should allow the manufacturer to deter- pharmaceutical industries, there are The manufacturer will determine if mine whether or not its qualified weld often requirements for special interimpact testing is required and if the procedures and qualified welders are nal- and external-surface finishes. Amshop has a qualified weld procedure to sufficient for the alloy specified. In cases biguity and different interpretations meet the requirements of the Code. where the manufacturer has to qualify about user expectations and manufaca procedure for an alloy not commonly turer capabilities can arise when the Vessel contents welded in the shop, the cost impact mechanical finish is specified as one With regard to vessel contents, the should be evaluated. of the following: satin, polished, bright, key phrase for the fabricator is "lethal The material specification and the dull or mirror. Parameters such as conservice." Vessels are considered lethal grade designation should also be pro- tact time, material feedrate, abrasive service if the contents, whether mixed vided to the manufacturer. For ex- pressure and application of lubrication with air or alone, are dangerous to life ample, if the vessel is fabricated from will have an impact on the finished when inhaled. Lethal service imposes austenitic stainless plate, then indi- product. Special finishes supplied by mandatory Code-compliance require- cate the ASTM International (ASTM) the manufacturer are not published by ments on the manufacturer, such as specification and grade, such as: A240- ASTM International. 100% radiography of all welds. These 316L. For the nozzles the specificaPolishing and grinding involve the requirements can substantially in- tion and grade will be A312-316L. If removal of metal from a surface with crease the vessel's fabrication cost. the process safety review determines an abrasive, resulting in surface direcIf the process involves hydrogen that seamless pipe is required for the tional marks. There is no definition of sulfide, where there is a risk of sulfide nozzles, then this should be clarified, an abrasive grit size that differentiates stress cracking, then the manufacturer since it will impact the vessel's cost. grinding from polishing. As a guide, needs to be advised of the require- For flanges, the specification and grade however, grit sizes of 80 and coarser can ments of the National Association of will be A182F-316L. Depending on the be associated with grinding, whereas Corrosion Engineers (NACE) standard process, some users may prefer using a grit sizes of 120 and finer can be asRP0472 and NACE publication 8X194. carbon-stoel backing flange in conjunc- sociated with polishing. Nevertheless, Hydrogen sulfide service will have re- tion with a stainless-steel stub-end. If simply specifying the grit size cannot CHEMICAL ENGINEERING WWW.CHE.COM JUNE 2010 29 TABLE 2. GUIDELINE FOR HEADTHINNING DURING FORMING Cover Story be equated to a specific surface finish. Buffing is not intended to remove metal from the surface. It is intended to brighten and smooth the existing surface with cotton- or felt-based media and with the application of lubricants to the buffing wheel. For precise and consistent results, it is recommended that the surface finish be specified in a range of minimum and maximum level of roughness average (Ra). This can be expressed in microinches or micrometers (Table 1). The Specialty Steel Industry of North America (SSINA) publishes a designer handbook of specialty finishes for stainless steel, which provides detailed descriptions and sample photographs. The handbook can be downloaded free of charge at www. ssina.com. Photographs for comparison of certain standard finishes (Nos. 1, 2B, 2D, 2BA, 3, 4, 6, 7 and 8) for sheets or various nominal thicknesses can also be found at the website. Allowable thinning during forming 0.032 in. 12-gauge, up to and including 0.25-in. plate 5/16-in. nominal thicknesses up to and including 0.5-in. plate 0.062 in. 9/16-in nominal thickness up to and including 1.0-in. plate 15% Head thickness range in. For heads 54 in. and larger, a 3-in. straight flange can be provided with a minimum plate thickness of 0.25 in. When specifying a torispherical head for a pressure vessel, it is important for the user to clearly define an ASME flanged and dished (torispherical) head. Standard flanged and dished heads are manufactured, but do not meet the code requirement of a minimum 6% inside-crown radius for the knuckle region. As a result, the standard flanged and dished heads provide a higher stress concentration factor and discontinuity in the knuckle region. Some manufacturers offer an ASME 80-10 head where the dish radius is 80% of the head diameter and the knuckle radius is 10% of the head diameter. The advantage of an ASME 80-10 head is that it is thinner (-66% of the thickness of an ASME torispherical head), which reVessel heads sults in a smaller blank size and reSome of the most common heads in duced labor cost. service are as follows: ASME flanged A third option for a torispherical head and dished (torispherical), 2:1 elliptical is an ASME high-crown head, where the flanged and dished (ellipsoidal), conical, dish radius is 80% of the head diameter toriconical, hemispherical and flat. and the knuckle radius is a minimum of Heads are formed based upon out- 6% of the head diameter. side vessel diameter, with the excep- Ellipsoidal (2:1) head. A 2:1 elliptical tion of elliptical and hemispherical flanged and dished head provides a dish heads, which are formed to the inside radius that is approximately 90% of the diameter. When ordering the head, the inside head diameter and a knuckle vessel manufacturer will provide the that is approximately 17.3% of the inhead manufacturer with the minimum side head diameter. The geometry of the permitted thickness that is required ellipsoidal head is provided in Section based upon the calculations. Thinning UG-32(d) of the ASME Code. of the vessel head takes place primar- The decision of whether to specify and ily at the knuckle regions and the cen- use a torispherical head versus an elter of the dish (Table 2). lipsoidal head is mainly an issue of Torispherical heads. Torispherical head clearance. Users should decide heads have dish radii equal to the di- which head better suits their needs. ameter of the head or vessel shell, and If a dished head requires a boltthe knuckle is 6% of the head inside- ing flange, then the manufacturer crown radius as required by Section must design the head and flange in UG-32(e) of the ASME Code (Figure accordance with the code's Appendix 2). The straight flange (skirt) is a 1 (1-6). The cost of adding a bolting standard 1.5 in. for heads formed from flange is significant. 3/16-in. plate and heavier. Straight Toriconical heads. The transition flanges up to 2 in. can be provided geometry of a toriconical head is typiupon request. For some head manu- cally limited to a maximum half-apex facturers, a 3-in. straight flange can angle of 30 deg (Figure 3). The knuckle be provided for head diameters rang- cannot be less than 6% of the outside ing from 36 to 54 in. as long as there diameter of the head skirt or lees than is a minimum plate thickness of 3/16 three times the calculated knuckle 30 CHEMICAL ENGINEERING WWW.CHE.COM JUNE 2010 thickness as outlined in UG-32(h). Toriconical heads or transitions may be used when the half-apex angle is greater than 30 deg and further requires the design to be in compliance with the mandatory Appendix 1 of the code. A conical head or transition does not have a knuckle. Therefore a reinforcing ring is required by Appendix 1—5(d) and (e). Half-apex angles greater than 30 deg for conical heads and transitions shall be in accordance with Appendix l-5(g) of the code. Un-stayed flat heads. These can be incorporated into the design, but have limitations in pressure and temperature due to their geometry (Figure 4). Section UG-34 of the code provides the design requirements for un-stayed flat heads and covers. This includes bolted blind flanges, flat plates with retaining rings, and threaded covers. The section provides nineteen examples of un-stayed flat heads that can be used, but clarifies that other designs, which meet the requirements of UG-34, are acceptable. The user may have an un-stayed flat head design that is to be incorporated. If so, users should provide a sketch of what is desired and allow the manufacturer to bring the proposed design into compliance with the code. Details are needed when specifying closure heads on a pressure vessel. When specifying the vessel shell length, reference it from the tangent line of one head to the tangent line of the opposite head. The tangent line is an accepted datum for most shops. Nozzle schedule Most users generally provide a nozzle schedule, but significant information is inherently omitted. When providing a nozzle schedule, the manufacturer is focused on size, type and quantity. The physical placement of the nozzles, and their projections can be addressed during the drawing review process (Figures 5 and 6). The user needs to be clear on the types of flanges required — raised-face slip-on flanges, raisedface weld-neck flanges or lap-joint flanges with stub-ends. When stubends are considered, be sure to clarify Straight flange Telltale holes are not permitted in vessels intended for lethal service. Outside diameter (O.D.) Knuckle radius 10% (O.D.)—'i Outside diameter (O.D.) Inside knuckle radius 173% (I.D.) Inside diameter (I.D.) FIGURE 2. The geometry of an ASME 80-10 torispherical head is such that the dish radius is 80% of the head diameter and the knuckle radius is 10% of the head diameter, while an ellipsoidal head has a dish radius that is 90% of the inside head diameter and a knuckle 17.3% of the inside head diameter Type A or B stubs ends. For the nozzle necks and any internal piping, specify electric-resistance-welded (ERW) pipe or seamless pipe (SMLS). Seamless pipe cost is considerably higher and will increase the vessel's cost. The manufacturer is responsible for determining if the nozzle requires a reinforcement pad. Users should also define the nozzles used for inspection and overpressure protection. If there is not a safety relief device attached directly to the vessel, most users will not identify an overpressure protection nozzle. If the safety relief device is not directly attached to the vessel, identify the nozzle connected to the piping system containing the safety relief device. If the vessel has a manway opening then clarify whether or not a hinge or davit arm is required. If the vessel requires a stud pad or sight glass, then specify it. A manufacturer and model number for the sight glass should be provided with the vessel specification so the manufacturer can obtain a quote. For stud pads, be sure to specify the size and flange rating for the pad. SAFETY AND TESTING Corrosion allowance The user should also specify a corrosion allowance for the vessel according to Section UG-25 of the pressure vessel code. The only situation for which a corrosion allowance is not required in the specification is when experience in "like service" has proven corrosion did not occur or the corrosion is superficial. Both the internal and external surfaces of the vessel should be considered. If the vessel is subject to internal corrosion, then the design should incorporate a drain nozzle at the lowest point or a pipe extending into the vessel from any other location to within 0.25 in. of the lowest point. Depending on the service, the user may elect to have telltale holes drilled part of the way into the pressure retaining items. The code requires the holes to be 1/16-in. to 3/16-in. dia. The holes' depth must be greater than 80% of the thickness required for a seamless shell of like dimensions. The holes should be on the surface opposite where the deterioration is expected. Non-destructive examination A common oversight is the specification of the degree of non-destructive examination (NDE) testing that is required. Radiographic examination is the most common method of NDE and is incorporated into the code to establish joint efficiencies for the weld seams. All radiographic examination should be in accordance with Section VIII, Division I, UW-51 and with Article 2, Section V of the code. Section UW-52 provides the minimum extent of spot radiography, as well as procedural standards, evaluation and retesting. When specifying NDE, be sure to clarify what level is required. Radiography will increase the cost of the vessel. Inspection openings Inspection openings are important for routine inspections of the vessel for safety and life expectancy. Elliptical manhole openings are permitted by the code provided the opening is not less than 11 in. by 15 in. or 10 in. by 16 in. and a circular manhole is not less than 15-in. inside diameter (ID). For hand-holes, the minimum size restriction is 2 in. by 3 in., except for vessels over 36 in. dia. where the minimum size handhole is 4 in. by 6 in. and is used in place of a manhole. If the vessel is less than 18 in. ID but over 12 in. ID, the code requires the vessel to have at least two handholes, or two plugged and threaded inspection openings, no smaller than 1.5 in. nominal pipe size (NPS). For vessels with ID between 18 and 36 in., the code requires either a manway, two handholes or two plugged, threaded inspection openings not less than 2 in. NPS. For vessels with IDs in excess of 36 in., the code requires one manway opening, with the exception that two 4- by 6-in. handholes can be used if the vessel geometry does not permit a manway. Nozzles attached to piping or instrumentation can be used for inspection openings, provided the openings meet the required size and are located to afford an equal view of the interior of the vessel. It is the user's responsibility to identify the CHEMICAL ENGINEERING WWW.CHE.COM JUNE 2010 31 TABLE 3. EXAMPLES OF COMBINED LOADINGS 1 Cover Story Design Pressure in Annulus, psig Pressure Used for Design of Inner Vessel, psig Pressure Used for Design of Jacket, psig +50 +250 and -50 +50 -15 +200 -215 +200 +100 and -15 +150 +100and-165 +150 iLTilittMll psig +250 inspection openings on the vessel prior to design and fabrication. Overpressure protection removal of iron compounds from the surface of stainless steel by means of a chemical dissolution. This is accomplished with an acid solution that will not etch the surface or have significant effects on the material. Some methods involve cleaning the vessel with a 20-25 vol.% nitric acid solution at 120°F for 30 min. The nitric acid solution removes contaminants and oxidizes nickel on the surface to form a chromium-oxide film on the surface and thus prevent further corrosion and oxidation. Citric-acid treatment is the least hazardous and most environmenStainless-steel surface treatment tally safe method for removal of free Users requiring a stainless-steel vessel iron and other metal and light suroften do not provide specifications for face contamination. Citric acid is cleaning the vessel prior to shipment. preferred with most manufacturers, Stainless-steel surfaces and welds re- since no special handling equipment quire special surface treatments in or safety devices are required; no order to remove light surface contami- NOx fumes are released and no corronation. During fabrication, a vessel may sion occurs in nearby equipment that be exposed to shop dirt, carbon-steel might come in contact with the soluparticles, permanent marker, crayon tion. Typical citric acid solutions are marker, oil and grease — all contami- 4-10 wt.%. Spraying the solution and nants that can accelerate corrosion. lightly scrubbing the surface with a Carbon steel particles and iron can be- soft brush is the preferred method for come embedded in the plate and heads cleaning large vessels. due to routine shop handling, forming The user must be aware that a pasrolls, layout tables, cutting tables and sivation treatment includes degreascarbon-steel grinding operations. ing, immersion and rinsing. The deThe most complete resource for greasing process is crucial since air cleaning stainless steel is ASTM cannot form a protective film when International A380-06 (Standard grease or oil is present on the surface. Practice for Cleaning, Descaling and Therefore, it is important that the Passivation of Stainless Steel Parts, manufacturer clean the vessel with a Equipment and Systems) [2]. The commercial-grade degreaser prior to standard recommends the user pre- passivation. After treating the vessel cisely define the intended meaning with nitric or citric acid, a thorough of passivation since there are several rinse with clean water should follow distinct operations. without allowing the surface to dry The first of these operations is de- between steps. fined in Paragraph 1.1.1 of ASTM The pickling process removes the Standard 380, where passivation is heat tint produced during welding a process by which a stainless-steel operations (Figure 1). Since nitric surface, when exposed to air or other and citric acids do not remove surface oxygen-containing environments, will layers, pickling removes the protecspontaneously produce a chemically tive oxide layer between 0.001 and inactive surface. It is now accepted 0.0015 in. of the substrate layer. Pickthat this film will develop in an ox- ling paste, nitric-hydroflouric acid ygen-containing environment pro- (HNOg-HF), is typically applied with vided the surface has been thoroughly a nylon brush and can only be left in All vessels are required to have overpressure protection in accordance with Section UG-125 of the code. The relief device can be located directly on the vessel or installed within a process or utility pipeline connected to the vessel. In either case, authorized inspectors may require identification of the nozzle that will be connected to the safety relief device. The identification of the nozzle for safety relief is the responsibility of the user and should be discussed internally during the user's process safety review. cleaned and descaled. For most users, passivation is the 32 contact for 15—30 min before excessive corrosion is initiated. Similar CHEMICAL ENGINEERING WWW.CHE.COM JUNE 2010 to the passivation treatments with nitric acid, shop personnel will need to wear the proper protective clothing and receive proper training for handling the product. ASTM standard A380 also addresses mechanical cleaning, including such processes as power brushing, sanding, chipping, grinding and abrasive blasting. For removal of localized areas of scale, grinding is typically the most effective. To avoid the risk of contaminating the stainless steel, grinding operations have to be carefully monitored to ensure the grinding wheels being used have not been previously used on carbon steel plate. However, the standard does not recommend abrasive blasting with silica, since it is nearly impossible to remove the embedded silica from the surface of the material. Walnut shells or glass beads are the preferred media for abrasive blasting. EXTERNAL LOADINGS Section UG-22 of the code provides a short list of various external loading conditions that need consideration, including the following: wind, snow, seismic loadings, as well as superimposed static and dynamic reactions from attached equipment, such as machinery, piping and insulation. While most specifications issued to fabricators cover the bare necessities for sizing a new vessel, many exclude external loadings. Some specification sheets are incomplete, such as those requesting consideration for wind and seismic loadings. Specifications typically will reference the required code for wind and seismic loadings, such as American Society of Civil Engineers (ASCE) Standard 7-05 Minimum Design Loads for Buildings and Other Structures [3]. However, they often do not provide specific information on the vessel's geographical location, the wind exposure category, the elevation of the vessel from grade or the importance factor. Without these details, the wind and seismic loadings provide an inaccurate picture as to what the vessel might see in an upset condition. The ASME Inside diameter (I.D.) —;— - nside Straight flange Tangent -= termining wind speed, importance factor, exposure category, topographical factor and gust factor. The wind speed is obtained from ASCE 7-05., which provides a map of the U.S. with basic wind speeds for various locations, including special wind areas at the hurricane coastlines of the following regions: the west coast of Mexico, the eastern part of the Gulf of MexFIGURE 3. Toriconical heads and transition ico and the Southeast U.S. and geometries are limited to a maximum half-apex the Mid- and North Atlantic. angle of 30 deg It is important to understand that it is assumed that the wind could come from any horizontal direction. Where there is mountainous terrain, gorges or other K^_<£ Vessel — special wind regions, there can be an adjustment made to the FIGURE 4. Requirements of un-stayed flat values in Figure 6-1 of the code heads and covers include bolted blind flanges, flat plates with retaining rings and threaded cov- to account for higher local wind speeds. This adjustment shall ers. Here are a few examples be based on local meteorologiBoiler and Pressure Code, Section VIII cal information. is referenced in ASCE 7-05. The occupancy category is provided Seismic loadings. Section 15.7.2 (c) in Table 1-1 of ASCE 7-05. The occuof ASCE 7-05 requires hydrodynamic pancy category is based upon the navertical, lateral and hoop forces to be ture of occupancy during upset condiconsidered in cylindrical tank and ves- tions involving excessively high winds sel walls. These forces shall be evalu- or earthquakes. The categories for ocated to determine the increase in hy- cupancy range from Occupancy Catdrostatic pressure and hoop stress. egory I (buildings with low hazard to In addition, Section 15.7.3 requires human life in the event of catastrophic the evaluation of all structural com- failure) to Occupancy Category IV (esponents that are an integral part of sential facilities, such as hospitals). the lateral support system. The evalu- Facilities that manufacture or process ation should ensure that connections hazardous fuels, hazardous chemiand attachments for anchorage and cals, hazardous waste or explosives other lateral-force-resisting compo- are considered to be an Occupancy nents, as well as nozzle penetrations Category III. If the hazardous mateand openings are designed to main- rial exceeds a threshold quantity estain structural stability and integrity tablished by a local jurisdiction, then of the shell. Vessel stiffness in rela- the vessel is classified as Occupancy tion to the support system should be Category IV. The importance factor used to determine the forces on the for wind loadings is provided in Table vessel. If the vessel is oriented hori- 6-1 of ASCE 7-05 and is based upon zontally, then analysis is required the occupancy category. at the saddle supports per Section Pressure vessels are placed into one 15.7.14.3. The combination of these of three exposure categories (B, C or loads should be used to establish the D), which depend on ground surface maximum allowable working pres- roughness. To derive the exposure catsure of the vessel as outlined in Code egory, surface roughness must first be Interpretation VIII-1-01-03. defined. Surface roughness is deterWind loadings. Wind loadings are mined by natural topography, vegetacovered in Section 6.5 of ASCE 7-05. tion and nearby buildings and strucThe design procedure for wind load- tures. Surface roughness category B is ings on pressure vessels requires de- defined as suburban areas and wooded areas with numerous, closely spaced obstructions the size of a single-family house or larger. Surface roughness C is defined as open terrain with scattered obstructions of heights less than 30 ft. This category encompasses flat open country, grasslands and water surface areas in hurricane-prone regions. Surface roughness D is characterized by flat, unobstructed areas outside of the hurricane-prone regions. This includes salt flats, mud flats and unbroken ice. A vessel's surface-roughness category is then used to determine its exposure category. Exposure category B is defined by surface roughness B with the wind prevailing in the upwind direction for a distance of at least 2,600 ft, or 20 times the height of the building, whichever is greater. Exposure category C is used for cases where Exposures B or D are not applicable. Exposure D applies when the surface roughness is defined as surface roughness D, where the prevailing wind direction is upwind for a distance of 5,000 ft or greater. Exposure D can also apply to surface roughness B or C for a distance of 600 ft, or 20 times the height of the structure, whichever is greater. If the site under consideration is located in a transition zone between exposure categories, then the largest wind forces apply. Topographical effects only need to be considered for increased wind speed over hills and ridges. Section 6.5.7 of ASCE 7-02 provides detailed procedures for calculating topographical effects. The gust effect factor for rigid structures should be 0.85, per Section 6.5.8.1, or should be calculated. If the vessel is dynamically sensitive, then Section 6.5.8.2 of ASCE 7-05 provides the necessary steps to calculate the gust effect factor. These five constants and categories provide sufficient data for the fabricator to properly design the vessel for wind loadings. The ASME Code requires manufacturers to consider the combination of the vessel design pressure along with the secondary stresses from wind or seismic loads. External piping loads. External nozzle loadings are typically overlooked, especially those loadings imposed by high-temperature piping. It is the user's responsibility to notify the CHEMICAL ENGINEERING WWW.CHE.COM JUNE 2010 33 Cover Story V1 fabricator if the piping may impose excessive loadings on the nozzles. This includes excessive loadings Nozzel identiduring normal operating conditions and during upset conditions. Sec- fication: Nozzel size: tion 15.7.4 of ASCE 7-05 requires P (Concentrated the analysis of the piping system radial load): Mi (External overconnected to the vessel during , in.-lb turning moment):. Nozzel identification: earthquake conditions. M2 (External overNozzel size: The piping system and supports turning moment):. .in.-lb . Ib P (Concentrated radial load): — (Concentrated ML (External overturning moment): in.-lb shall be designed such that there Jb shear load): Me (External overturning moment): in.-lb is no excessive loading on the vesV2 (Concentrated VL (Concentrated shear load): Ib sel wall. The assumption that a .Ib shear load):Vc (Concentrated shear load): Ib MT (Concentrated external nozzle is an anchor point for piping MT (Concentrated external .in.-lb torsional moment): torsional moment): in.-lb is poor practice. Stresses need to be considered in the shell/head at the FIGURE 6. Agitators and mixers are nozzle-to-shell juncture. Therefore FIGURE 5. It is the vessel user's responsibility to notify the fabricator as to sources of external nozzle loading. Users all external loads are considered to whether the piping imposes excessive should obtain the mixer reaction loads be acting simultaneously. All exter- loadings on the nozzles from their maker, and relate that to the nal loads, such as the longitudinal vessel manufacturer and circumferential shear loads and moment loads, have to be considered the vessel head needs to be increased. entirely to cylindrical shell with the radial and torsional loads in Some users have invested significant • Type 2 - Jacket covering part of the conjunction with the design pressure of expense to replace wrecked agitators cylindrical shell and one head the vessel. due to flexing nozzles. Calculating the • Type 3 - Jacket covering any portion These loads can be analyzed per the stress on the vessel shell and nozzle is of the head Welding Research Council (WRC) Bul- the same as those calculations for pip- • Type 4 - Jacketed with an added letin 107 and its supplement (Bulletin ing nozzles using the WRC 107 / WRC stay or equalizer rings to the cy297 — for cases where the stress is 297 procedures. lindrical shell portion to reduce evaluated in the shell only) [4,5]. Calthe effective length culating loads with WRC 107 and 297 JACKETED VESSELS • Type 5 - Jacket covering the cylinis time-consuming if performed manu- Jacketed vessels are addressed in Apdrical shell and any portion of eially, because numerous non-dimen- pendix 9 of the ASME Code. Appendix ther head sional geometric parameters have to 9 applies to the jacketed portion of the Half-pipe jackets are covered in a nonbe interpolated from multiple charts. vessel, which includes the wall of the mandatory appendix of the code, ApComputer software programs are avail- inner vessel, the wall of the jacket, pendix EE. The calculation procedure able to aid calculations. and the closure between the inner ves- in the code is for the conditions where sel and the jacket. The manufacturer there is positive pressure in the vesExternal equipment loads shall consider the combined loading sel shell or head and positive pressure Agitators and mixers are another of the vacuum/pressure on the jacket in the half pipe jacket. The code fursource of external nozzle loadings. Ob- wall along with the pressure/vacuum ther provides restrictions to half pipe tain the mixer reaction loads from the within the inner vessel wall and deter- jacket sizes of NFS 2, 3 and 4 with equipment manufacturer and relate mine which of these is greater than the vessel diameters ranging from 30 to these loads to the vessel manufac- individual loading (Table 3). 170 in. The code does permit jackets of turer. The mixer reaction loads have The code categorizes jacketed ves- other geometries, such as circular segan impact on the cost of the vessel if sels, which provides a convenient ments, channels or angles. a reinforcement pad, or gussets are method to assign closures. The vessel manufacturer should required, or if the plate thickness on • Type 1 - Jacket (any length) confined consider other combinations of pres- References 1. American Society of Mechanical Engineers (ASME). "ASME Boiler & Pressure Vessel Code, Section VIII, Division I," 2007 Edition, Addenda, 2007. 2. ASTM International. "Designation: A380-06, Standard Practice for Cleaning, Descaling, and Passivation of Stainless Steel Parts, Equipment, and Systems." 3. American Society of Civil Engineers. "ASCE 7-O5: Minimum Design Loads for Buildings and Other Structures." 34 CHEMICAL ENGINEERING WWW.CHE.COM 4. Wichman, K.R., Hopper, A.G., Mershon, J.L., "WRC Bulletin 107 / August 1965: Local Stresses in Spherical and Cylindrical Shells due to External Loadings", Welding Research Council, 1965. 5. Mershon, J. L. and others. "WRC Revised Bulletin 297: Local Stresses in Cylindrical Shells due to External Loadings on Nozzles - Supplement to WRC Bulletin No. 107 - (Revision 1)", Welding Research Council, 1987. 6. Tuthill, A. H. and Avery, R. E. "Specifying Stainless Steel Surface Treatments." Nickel JUNE 2010 Institute, http://www.nickelinstitute.org. Farr, J.R. and Jawad, M.H., "Guidebook for the Design of ASME Section VIII Pressure Vessels," ASME. 1998. Chuse, R., and others "Pressure Vessels: the ASME code simplified," 7th Edition, McGraw-Hill, Inc., 1993. Powell, C. and Jordan, D. "Fabricating Stainless Steels for the Water Industry," Nickel Institute, Reference Book Series No. 11 026, October 2005. sure loadings outside of what is provided in the rules of Appendix EE. These include the following: • Negative pressure inside the vessel Dry Screening Reaches New Heights and inside the jacket The APEX1" Screener from ROTEX Global, LLC • Negative pressure inside the shell and is the smart solution for dry screening. The presents positive pressure inside the jacket APEX delivers high productivity and low • Positive pressure inside the vessel and operating costs with the same efficiency and gyratory-reciprocating motion as negative pressure inside the jacket the ROTEX®Screener. Economically f • External nozzle loadings from piping designed to increase uptime, the connections APEX features side access • Cyclic loadings to any of the above doors that enable quick screen combinations changes and maintenance by When specifying the half pipe jacket be one person. sure to provide the manufacturer with To find out how the APEX™ Screener can the pipe size required, the pitch of the increase your productivity, go to coils, the design temperature and the rotex.com/apex, or call 1-800-453-2321. design pressure. An approximate location of the half-pipe inlet and outlet IB & c nozzles is beneficial for the manufacturer to determine potential interferences with other nozzles on the vessel. Dimpled and embossed jackets are another form of jacket assemblies and are addressed by Appendix 17, a ©2010 ROTEX Global. LLC mandatory appendix of the Code. If Circle 17 on p. 62 or go to adlinks.che.com/29251-17 the manufacturer is using a plate that has been dimpled or embossed prior Solid-Liquid Separation Solutions to welding, then a proof test shall be sub-micron to macro-molecules performed in accordance with Section UG-101 of the code. The proof test requires the use of a representative •^ VertkaI:::F.i>teri!ig::gfntnfygg§ - standard & custom panel, which is rectangular with at cGMP designs are economical and high capacity 4, least five pitches in each direction and (up to 35 ft') with small footprint not less than 24 in. in either direction. A proof test can add additional cost to the fabrication of a vessel. »• .High-Speed Tubular It is strongly recommended that if a up to 20.000 G-force for vessel requires any type of jacket, the separating sub-micron user should provide a supplemental partides from liquids and ,1? specification sheet with a drawing of liqutd-lsqutd separations the vessel and the required location of the jacket(s). The required surface area and heat transfer calculations are the responsibility of the user. • Edited by Scott Jenkins V Horizontal Peeler & Inyerting-Filter Centrifuges designed for very fine soWs, thin-cakes and other HE IX Author Keith Kachelhofer is the corporate mechanical group leader for Jedson Engineering Inc. (Park 50 Technecenter, 5300 Dupont Circle, Milford, OH 45150; Phone: 912-330-7777; E-mail: keith. kachelhofer@jedson.com) at its Georgia office. He holds a degree in mechanical engineering technology from Southern Polytechnic University in Marietta, Georgia. Kachelhofer has over fifteen years experience with ASME pressure vessels and is a licensed professional engineer (FB; In Georgia, North Carolina, Delaware, Maine, New York, Ohio and Utah. drfficua applications, Superior sanitation, cGMP design and tow partfcte breakage. > Rebuilding & Refurbishing Services > Large Parts inventory >• Fully Staffed Field Service Dept. r The Western States Machine Co. phone S13J63.4758 - www.wtsternstates.com Circle 23 on p. 62 or go to adlinks.che.com/29251-23 CHEMICAL ENGINEERING WWW.CHE.COM JUNE 2010 35