Pim200_piping Component Selection

200 Piping Component SelectionAbstract This section gives guidance on the selection and use of the mechanical components that are fabricated into piping systems. The dimensional and material standards for common components are described. Illustrations are provided of these components and of some special valves and fittings. Guidance is given on the selection and use of piping components in applications common to all types of installations. Contents Page 210 Introduction 200-4 211 Definition of Terms 212 Pressure Rating 213 Piping Standards 214 Joining Methods 220 Material Selection Considerations 200-7 221 Factors to Consider 222 Sources of Information 223 Service Conditions 224 Typical Material Selections 225 Materials Commonly Used for Piping 226 Materials Selection Summary 230 Pipe and Tubing 200-17 231 Recommended Materials for Pipe 232 Dimensional Standard for Pipe 233 Pressure Design of Pipe 234 Economics of Pipe Wall Selection 235 Tubing 240 Fittings 200-28 241 Materials for Fittings November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-1 200 Piping Component Selection Piping Manual 242 Dimensional Standards for Fittings 243 Company Practice for Fitting Use 244 Straight Connections 245 Direction Changes 246 Branch Connections 247 Reducing and End Closure Fittings 250 Flanges, Blanks, and Blinds 200-46 251 Flanges 252 Flange Facing 253 Flange Attachment to Pipe 254 Commonly Used Flanges 255 Special Purpose Flanges 256 Flange Covers 257 Blinds and Blanks 258 Thickness Calculation for Blanks 260 Nonflange Connections 200-52 261 Grayloc Connector 262 Cameron, Securamax, G-CON, and OTECO Hub and Clamp Fittings 263 Victaulic Coupling 264 Dresser Coupling 270 Valves 200-53 271 Factors to Consider 272 Types of Valves 273 Specifying Valve Parts 274 Valve Operators 275 Valves for General Service 276 Special Purpose Valves 277 Valves for Sour Service 278 Valves for Saltwater Service 280 Bolts and Gaskets 200-84 281 Bolts 282 Gaskets 200-2 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection 290 Miscellaneous Engineered Equipment 200-89 291 Strainers 292 Flame Arresters 293 Expansion Joints 294 Swivel Joints November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-3 200 Piping Component Selection Piping Manual 210 Introduction The general category of piping includes not only pipe, but also tubing, fittings (elbows, tees, flanges, reducers), valves, and specialty piping components such as expansion joints. This section defines frequently used terms, discusses pressure rating and standards for manufacture of piping components, and lists methods of joining pipe. The rest of Section 200 describes piping components in this order: 1) pipe and tubing; 2) fittings; 3) flanges, blinds and blanks; 4) valves; 5) bolts and gaskets; and 6) miscellaneous engineered equipment. The discussion includes recommended methods for use of these components. 211 Definition of Terms Class. This word refers to ANSI class. ASME/ANSI B16.5 and B16.34 define the pressure/temperature relationships for steel flanges and fittings. Pressure ranges are defined by the classes. The class number (150, 300, 600, 900, 1500, 2500 for flanges) has no direct relationship to the actual pressure capability of the piping component. Flanges and valves have specific pressure ratings. Steel pipe and welding fittings do not. Although socket welding and threaded fittings are called out by class (2000, 3000, 6000), their pressure capability is defined by the pipe they connect. Corporate Piping Classes. Classes that apply to piping systems with specific combinations of pressure rating, valve type, material, joint connections, etc. The Corporation Piping Specification (See Volume 2 of this manual) contains about 30 of these classes, which themselves are sometimes referred to as specifications. Design Operating Temperature. This term is used by process designers to mean the desired process temperature. The term is not useful for piping designers and should not be confused with design temperature or operating temperature, defined below. Design Temperature. Maximum temperature the system is designed to withstand. This is usually the maximum process temperature plus a safety factor. EFW Pipe. Pipe commonly produced from plate with a longitudinal butt weld. EFW stands for electric fusion welded. Welding of carbon and low alloy steels normally is accomplished by submerged arc welding (SAW). In submerged arc welding an automatic electric arc process is used, with the filler metal coming from the electrode supplying the electric arc. The weld is shielded by granular or fusible flux. Mechanical pressure is not required. Typical specifications are API 5L and ASTM A671, A672, and A691. Electric Flash Weld Pipe. Pipe produced by a principle similar to that used for electric resistance welded pipe. The heat for welding is obtained from the resistance to the flow of the electric current between the butted edges of a single pipe length simultaneously. A typical specification for electric flash welded pipe is API 5L Grade A25. This low quality pipe is no longer made in the U.S.A. Spiral welded electric flash weld pipe is also available, but is also low quality. 200-4 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection ERW Pipe. Pipe produced from a continuous coiled strip (skelp). ERW stands for electric resistance welded. Unlike furnace welded pipe, in which the whole width of the strip is heated, only the edges of ERW pipe are heated to welding temperature. The edges are heated by their resistance to the flow of an electric current. At the appropriate welding temperature, rollers force the edges together. Excess molten metal is forced out from the weld to the outside and inside of the pipe. Because of this ejection of weld metal and the significantly higher welding temperatures as compared to the furnace welded pipe, ERW pipe is allowed a higher weld joint factor than furnace welded pipe. Typical specifications are ASTM A53 Type E, and API 5L. Furnace Continuous Welded Pipe. Pipe (furnace welded, butt welded) made by running a steel strip through a furnace and then forcing the edges of the strip together under pressure while at furnace temperatures to form the welded pipe. This is a low quality pipe and should not be used without consideration of the reduced reliability. Typical specifications are ASTM A120 and A53 Type F. Grade. A subclassification that defines the chemical and mechanical properties of a specific material within a material specification. For example, WPB and WPC are grades of wrought steel for fittings produced to ASTM A234. Nominal Pipe Size (NPS). A number to represent the outside diameter of pipe. This number equals the outside diameter for pipe sizes 14 inches and larger. For smaller- diameter pipe, up to and including NPS 12, the outside diameter is a fraction of an inch larger than the NPS number. Appendix D gives tables of the nominal pipe size and outside diameter of pipe. Malleable Iron. Cast iron made from pig iron by long, high temperature heating and slow cooling. The result is a very strong, malleable metal. Operating Temperature. Normal temperature at which the system will operate. SMLS (seamless) Pipe. Pipe produced by a sequence of hot extrusion or hot piercing, followed by sizing operations that form the pipe’s desired final dimen- sions. Typical specifications are ASTM A106, A53 Type S, and API 5L. Schedule. A pipe’s wall thickness. See Wall Thickness. Size. Outside diameter (O.D.) of pipe. See Nominal Pipe Size. Wall Thickness. Given as Weight (standard, extra strong, double extra strong), Schedule Number (40, 80, 160), or actual thickness. Appendix D of this manual gives the weight and schedule numbers for pipe. The appendix also shows that Weight and Schedule are not always equivalent. For example: • Standard Weight (ST) equals Schedule 40 for pipe sizes NPS 1/8 to NPS 10 • Extra Strong (XS) equals Schedule 80 for sizes NPS 1/8 to NPS 8 • Double Extra Strong (XX) and Schedule 160 are not equal for any line size. Weight. A pipe’s wall thickness. See Wall Thickness. November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-5 200 Piping Component Selection Piping Manual Welded Austenitic Stainless Pipe. Pipe made by fusion welding—either submerged arc, heliarc or gas shielded tungsten arc. The typical specification is ASTM A358. Wrought Steel. Steel items produced by rolling, extruding, or forging from hot or cold shapes. 212 Pressure Rating Piping must be designed to satisfy the pressure and temperature requirements of the intended service. Some piping components have a specific pressure rating and others do not: Pipe has no specific pressure rating. It must be calculated on the basis of wall thickness and allowable stress value. An example of the calculation is given in Section 230. Fittings have no specific pressure rating, but it is generally the same as the pressure rating of straight pipe of the same material and schedule. Exceptions to this rule will be discussed under the appropriate fitting classes. See Section 240. Flanges and valves have specific pressure ratings, which are listed in ASME/ANSI B16.5 and B16.34 respectively. 213 Piping Standards The most commonly used standards for piping components are: ASME/ANSI B16.5 Pipe Flanges and Flanged Fittings ASME/ANSI B16.9 Factory-Made Wrought Steel Buttwelding Fittings ASME/ANSI B16.11 Forged Steel Fittings, Socket Welding and Threaded ASME/ANSI B16.34 Valves—Flanged, Threaded and Welding End ASME/ANSI B36.10M Welded and Seamless Wrought Steel Pipe ASME/ANSI B36.19M Stainless Steel Pipe For a complete list of standards applicable to piping components see Table 326.1 in ASME/ANSI B31.3, Chemical Plant and Petroleum Refinery Piping. See also Section 100 of this manual. 214 Joining Methods The common joining methods for steel pipe are: Butt Weld. For pipe NPS 2 and larger Socket Weld or Threaded. For pipe smaller than NPS 2 except for high pressure and temperature services 200-6 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection Flanged. Generally for pipe-to-valve or pipe-to-equipment connections NPS 2 and larger, but also frequently used for equipment and instrument connections NPS ¾ and up Hub-and-clamp. For high pressure sweet hydrocarbon gas and liquid and water service. See Appendix B Clamp. For low pressure special services 220 Material Selection Considerations This section presents an overview of how to select materials for a piping system. It is intended to provide enough information so that issues are known but steers the reader to other sources when detailed information is needed. The section gives a list of factors to consider, reviews sources containing more information, discusses the service conditions which affect materials selection and discusses some pertinent materials properties. 221 Factors to Consider Selection of materials for piping involves many factors. The following list high- lights the primary considerations. Further details are available in the Corrosion Prevention and Metallurgy Manual and from specialists, such as those in ETC’s Materials and Equipment Engineering Unit. Maintenance Versus New Construction Maintenance is usually done on a “replace in kind” basis unless corrosion or oper- ating problems justify a change. New construction will frequently require more thought about whether existing piping classes are appropriate. Revisions may be necessary due to new information. For instance, the Company’s practice for stress relief of lines in MEA and DEA service changed in 1987 due to failures in these services. Code Requirements Local, state and federal codes and regulations need to be considered. Safety and Reliability It is important to assess the likelihood and consequences of failure. Doing so may lead to different choices of materials for seemingly similar piping systems. Consid- eration should be given to the following: • Reliability – Past history in the same or similar services – Onstream inspection as a means to predict failures – Shutdown frequency in similar services November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-7 200 Piping Component Selection Piping Manual • Consequences of Failure – Personnel safety. Determine if hazardous materials are involved (acids, caustics, H2S, HF) – Fire hazards (LPG, high pressure H2, proximity to a furnace) – Extent of lost production if the piping fails – Ease of repair or replacement – Availability of expert craftsmen and replacement material in the field – Leakage. Determine effect on plant performance, such as of catalyst poisoning – Plant shutdown. May result from leakage unless equipment can be bypassed – Shutdown of related plants. May result from plant shutdown Service Environment The process environment may require special materials or postweld heat treatment. Velocity limitations may be required to limit corrosion or erosion-corrosion. Operating Temperature and Pressure Operating temperature and pressure may be critical in the selection of materials. Cyclical pressure or temperature may require additional design considerations. Design Life The Company normally designs piping for a ten-year minimum life. It should be determined if this is appropriate. Design Corrosion Allowance The expected corrosion rate may be constant or change with time. External corro- sion (atmospheric corrosion or chlorides from insulation) may be a factor. The Company usually specifies a 1/16-inch minimum corrosion allowance for carbon steel and low alloy steel piping if the corrosion rate can be predicted accu- rately and is less than 3 mils per year (mpy). For stainless steel, the Company normally specifies 1/32-inch minimum corrosion allowance. If more than 3/16-inch corrosion allowance is needed the Company normally uses a more corrosion resis- tant material. Cost, Availability and Delivery The first choice may or may not be the economic choice in terms of life-cycle cost. It should be determined if the material is available for construction or maintenance and if delivery time is acceptable. Environmental Regulations Leakage may cause environmental problems such as pollution of navigable waters and unacceptable emissions. 200-8 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection Standardization Making a new piping class may not be economical; standardization may lower overall facility costs. Constructability The material must be readily fabricated, available and amenable to inspection. 222 Sources of Information Answering the concerns raised in Section 221 involves a lot of research. Common places to look for information are: Inspection Records If the Company has a similar installation, inspectors and their inspection records are the best starting points for learning what works and where mistakes can be avoided. Look for common failure locations and types of failures. For example, elbows may fail first due to velocity, and corrosion failures may be because of general, pitting or localized (stress corrosion cracking) corrosion. Piping Class Sheets See what others have chosen, and whether the Corporate Piping Class sheets have changed since the last similar project. Piping classes in different operating locations may differ due to different design considerations. Operators Operators should be consulted about existing plants; the piping class sheets may be out of date with current maintenance practices. Previous Materials Selections For many large facilities materials selection is formalized on a report written by materials engineers, such as ETC’s Materials and Equipment Engineering Unit specialists. This is a good source for construction materials and for notes on mate- rials considerations in that process environment. Other Owners If the Company does not have a similar process, another company may. Most will provide informal information if asked. If the Company is licensing a process, the licensor will provide information. Other licensees should be visited, with the licensor’s assistance, for a first-hand account of their experience. Designers Industry standards are usually well known by design firms, but designers commonly lack a feedback mechanism for their designs unless they also operate the process. The result can be that designers learn more slowly from failures and tend to be at the industry norm rather than on the leading edge. November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-9 200 Piping Component Selection Piping Manual Other Other sources of corrosion data include the Corrosion Prevention and Metallurgy Manual, laboratory tests, and published data. Volume 2 of the Corrosion Prevention and Metallurgy Manual gives good general material selection guidelines for several specific types of plants. Contact a ETC Materials and Equipment Engineering Unit specialist for additional information and specific recommendations. 223 Service Conditions This section presents general principles of material selection (introduced in the preceding sections) needed to prevent deterioration in the service environment. Service Environment Materials are selected to limit corrosion to acceptable rates in a given service envi- ronment. The service environment comprises the contents of the piping, its tempera- ture and pressure, contaminants, physical state, and, sometimes, velocity. Materials selections need to consider both corrosion rates and other deterioration mechanisms. Certain environmental conditions may cause deterioration mechanisms such as stress corrosion cracking, sulfide stress cracking, and hydrogen attack. The Corro- sion Prevention and Metallurgy Manual describes these mechanisms and, in the sections dealing with specific plants, highlights potential deterioration mechanisms. Operating Temperature and Pressure Operating temperature and pressure may limit the choice of materials and can have a significant influence on corrosion rates. Temperature can limit the choice of mate- rials based on considerations of strength, metallurgy, and corrosion. For example, carbon steel is limited to a maximum design temperature of 800°F. Above 800°F, the strength decreases significantly and graphitization may cause the steel to embrittle. Corrosion rates frequently increase with temperature. In sour hydro- carbon services, corrosion of bare carbon steel accelerates at temperatures above 550°F. Operating pressure can also influence the stability of a material in a given service environment, as with hydrogen attack of steels in high pressure hydrogen at elevated temperatures. Low temperature can also limit materials selection. Design Life and Corrosion Allowance The design life typically used for piping is ten years. Corrosion allowances are spec- ified to achieve the design life and are based on the expected corrosion rate. Corro- sion allowances are discussed in more detail in the Corrosion Prevention and Metallurgy Manual, but are summarized in Figure 200-1. 200-10 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection Fig. 200-1 Minimum Corrosion Allowance Carbon and low alloy steel(1), (2) 1/16 in. (3), (4) Stainless Steel and high alloy steel 1/32 in. (1) Low alloy steels include chrome-moly steels such as 2¼ Cr 1 Mo and 9 Cr 1 Mo steels. (2) 1/16-inch is usually used, assuming available corrosion data clearly shows a corrosion rate less than 3 mpy. If the corro- sion rate is inconclusive or greater than 3 mpy, then a minimum of 1/8-inch is recommended. A corrosion allowance greater than ¼-inch usually justifies a change to a more corrosion resistant alloy. (3) High alloy steels include alloys with more than about 10 percent chromium (such as 12 Cr, Alloy 20 and Monel). (4) The break point for upgrading stainless steels and high alloys is not defined precisely as it is in note 2. This is because by the time you are selecting stainless, you are already fine-tuning the selection for a grade of stainless that gives a reason- able economic corrosion allowance. Cost One of the objectives of materials selection is to select the most economical mate- rial. This usually leads to the use of carbon or low alloy steels rather than stainless and highly alloyed materials. For some aqueous services—up to about 200°F—nonmetallic thin film coatings can be applied to reduce corrosion rates and the need for alloy material. 224 Typical Material Selections Figure 200-2 shows typical materials for some common environments. Data in this table is illustrative only and is not intended for design. Perhaps too often metallic piping is the only kind considered. Nonmetallic piping should also be considered; the Company has had successful experience in many services. Section 400 gives detailed information on various types of nonmetallic and nonmetallic-lined pipe. A few examples of successful service are given in Figure 200-3. Consult a materials specialist for more help. Fig. 200-2 Common Piping Materials Selection Service Typical Materials Comments Produced fluids Carbon steel with coating Corrosivity of produced fluids varies containing water widely. Consult a corrosion specialist Hydrocarbons—sweet Carbon steel Sensitive to trace H2S above 550°F Hydrocarbons—sour Carbon steel Limited to 550°F Hydrogen—sweet Carbon steel, 1¼ Cr ½ Mo, and 2¼ Cr 1 Mo Choice depends on temperature and steels hydrogen partial pressure(1) Hydrogen—sour Carbon steel, 1¼ Cr ½ Mo, and 2¼ Cr 1 Mo Choice depends on temperature and steel, and Type 321 and 347 stainless steels hydrogen partial pressure(1) Steam Carbon steel Superheated steam may require low alloy steel Steam condensate Carbon steel CO2 corrosion may require stainless steel in a condensing service Acids Plastic-lined steel Temperature limited Salt water Fiberglass reinforced plastic Joint type is important November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-11 200 Piping Component Selection Piping Manual Fig. 200-3 Nonmetallic Pipe Uses Material Service High density polyethylene (HDPE) Produced water Polyvinyl chloride (PVC) Acids, caustics, demineralized water Fiberglass reinforced plastic (FRP) Seawater, brine, wet gas Plastic-lined steel Acid/base mixing (1) See API Recommended Practice 941. 225 Materials Commonly Used for Piping The following discussion of the characteristics of common piping materials is an abbreviated version of information contained in the Corrosion Prevention and Metallurgy Manual. Consult that manual and a ETC Materials and Equipment Engi- neering Unit specialist on specific questions. Carbon Steel Carbon steel with a 1/16-inch to 3/16-inch corrosion allowance is the economic material selection for a large percentage of piping in refinery, chemical plant, and producing applications. Carbon steels have a nominal composition of iron with about 1% manganese and up to 0.35% carbon. Higher carbon results in poor weldability. Carbon steels are easily fabricated. Some limitations of carbon steels are as follows: Brittle Fracture. Carbon steels may be susceptible to brittle fracture at normal ambient temperatures. Brittle fracture can be prevented by choosing the right mate- rial and minimum pressurizing temperatures. Refer to the Pressure Vessel Manual for information on prevention of brittle fracture. Hydrogen Attack. Carbon steel will suffer hydrogen attack at elevated temperature in high pressure hydrogen. Material selection should be based on the Nelson Curves, shown in API Recommended Practice 941 in the Corrosion Prevention and Metallurgy Manual. Graphitization. Welded carbon steel must be limited to 800°F maximum to prevent graphitization. Graphitization is the formation of graphite, primarily in weld heat affected zones, from the decomposition of iron carbides. Graphitized steel can fail under small loads or strains. Stress Corrosion Cracking. As-welded or cold-worked carbon steel is susceptible to stress corrosion cracking in caustic, nitrate, carbonate, and amine solutions and in anhydrous ammonia. Stress relief is required to prevent failures. More information is given in the Corrosion Prevention and Metallurgy Manual. Consult a ETC Mate- rials and Equipment Engineering Unit specialist for specific applications. 200-12 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection Sulfide Stress Cracking. High strength steel and hard welds in carbon steel in aqueous solutions containing H2S are susceptible to sudden failures called sulfide stress cracking. Controlling maximum strength and hardness is generally sufficient to prevent cracking. The Company’s piping specifications limit steel strength and weld hardness to prevent cracking. Postweld heat treatment may also be beneficial to prevent cracking. Hydrogen-induced Cracking. Some low strength carbon steels may be susceptible to hydrogen-induced cracking (HIC) in wet services containing H2S. Blistering is one example of this type of cracking. Refer to the Corrosion Prevention and Metal- lurgy Manual for additional details. Steel makers offer steels made with very low sulfur contents and calcium treated for inclusion shape control to resist HIC. The Company generally has not specified these steels except in some pipelines and pres- sure vessels. Postweld heat treatment may also be beneficial to prevent cracking. Carbon-Moly Steels Carbon-moly steel is similar to carbon steel but with 0.5% molybdenum, added. The molybdenum addition improves the steel’s high temperature strength and graphitiza- tion resistance. The corrosion resistance is the same as for carbon steel. Limitations of carbon-moly steels are as follows: Brittle Fracture. Unless made to fine-grain practice and normalized, carbon-moly steels may have poor resistance to brittle fracture. Hydrogen Attack. Carbon-moly is no better than carbon steel in resisting hydrogen attack. Carbon-moly should not be specified for hydrogen attack resistance. Refer to the Corrosion Prevention and Metallurgy Manual and API Recommended Practice 941. Graphitization. Carbon-moly will graphitize similarly to carbon steel, but is resis- tant up to 850°F. Stress Corrosion Cracking. Same as for carbon steel. Sulfide Stress Cracking. Same as for carbon steel. Chrome-Moly Steels Chrome-moly low alloy steels are similar to carbon steel but with chromium and molybdenum added. Typical grades are 1¼ Cr ½ Mo, and 2¼ Cr 1 Mo. The general corrosion resistance of these grades is about equal to that of carbon steel. Chrome- moly steels have better resistance to hydrogen attack than carbon steel and have better high temperature strength. They do not graphitize. Chrome-moly steels are somewhat more difficult to fabricate; they require control of preheat for welding and postweld heat treatment for all welded construction. Limitations of chrome- moly steels are as follows: Brittle Fracture. Like the carbon steels, chrome-moly steels undergo a ductile-to- brittle transition at low temperatures and become susceptible to brittle fracture. In addition, chrome-moly steels in service above about 650°F embrittle in service. The 2¼ Cr 1 Mo steels are particularly susceptible, but 1 Cr ½ Mo and 1-¼ Cr ½ Mo can also be susceptible. Consult a Materials and Equipment Engineering Unit specialist for specific applications. November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-13 200 Piping Component Selection Piping Manual Hydrogen Attack. Resistance to hydrogen attack is dependent on the chromium and molybdenum contents in the steel. Resistance improves with increased alloy content. Refer to the Corrosion Prevention and Metallurgy Manual and API Recom- mended Practice 941. Stress Corrosion Cracking and Sulfide Stress Cracking. Same limitations as for carbon steel. Stainless Steels Stainless steels are alloys of iron and chromium, typically with at least 12% chro- mium. Series 300 stainless steels also contain nickel. A term commonly used for Type 304 stainless steel is 18-8, designating its 18% chromium and 8% nickel alloy content. Other alloying elements such as molybdenum, titanium, and niobium can be added for specific purposes. Stainless steels are classified as ferritic, martensitic, austenitic, or duplex depending on their microstructure. • Austenitic. Examples are Type 304, 304L, 316, 321 and 347 stainless steels. Austenitic stainless steels will not harden with heat treatment. They are nonmagnetic. Austenitic stainless steels are generally readily weldable. • Martensitic. Type 410 stainless is the most common example. Martensitic stainless steels can be hardened with heat treatment. They are magnetic. Use of martensitic material in a piping system is prohibited due to poor weldability. • Ferritic. Examples are Types 405 and 429, AL 29-4 and Sea-Cure. Ferritic stainless steels will not harden with heat treatment. They are magnetic and usually don’t contain nickel. Use of ferritic stainless material in a piping system is prohibited due to poor weldability. • Duplex. Examples are Avesta 254SMo, Type 329, Sandvik SAF2205, and Ferralium 255. Duplex stainless steels have structures of roughly 50% auste- nite and 50% ferrite. They are nonhardenable by heat treatment. They currently are not widely used. They have corrosion properties similar to the austenitics but are higher strength. They share some of the limitations of both the ferritics and austenitics. Limitations of the stainless steels are as follows: Austenitic Stainless Steels in Chloride Solutions. Chloride stress-corrosion cracking of austenitic stainless steels can occur in dilute chloride solutions containing as low as 5 ppm chloride ions at temperatures in the 140°F to 200°F range. Cracking is most severe where the chloride ion concentration is high, the solution is hot, the pH is neutral or low, and especially where evaporation builds up deposits on the stainless steel. Stainless equipment hydrostatically tested with sea water has failed due to the residual sodium chloride film left behind. Other failures have been traced to chlo- rides leaching out of wet insulation. Many failures have resulted from not protecting stainless equipment from chlorides during shutdowns. There can be an incubation period of several hours or many weeks before cracking occurs in certain 200-14 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection environments. Cracking can be greatly reduced by stress relieving the stainless equipment. However, complete freedom from chloride stress corrosion cracking can be assured only by protecting austenitic stainless steels from any chloride ions or by using the more expensive super stainless grades with 30-45% nickel, such as Inconel 625. Duplex stainless steels have improved resistance to chloride stress corrosion cracking. Austenitic Stainless Steels in Sulfur-derived Acids. Sulfurous or polythionic acids can cause stress corrosion cracking of austenitic stainless steels. Unlike chloride stress corrosion cracking, the austenitic stainless steel must be sensitized with chro- mium carbide precipitates along the grain boundaries before polythionic acid stress corrosion cracking can occur. Sensitization occurs by heating above 750°F, such as occurs by welding or heat treatment. Several grades of stainless are designed to have increased resistance to sensitization. Regular grades of austenitic stainless steel (Types 304, 316, etc.) sensitize easily. The extra low carbon grades of stainless steel (Types 304L, 316L, etc.) normally do not sensitize during welding. However, they will sensitize if held too long at temperatures above about 750°F. Some austenitic stainless steels (Types 321 and 347) are chemically stabilized to minimize sensitization. However, they too can sensitize during prolonged exposures to temperatures above about 850°F. Neither sulfurous nor polythionic acids are normally found in process units during operation. However, these acids commonly develop during shutdowns by the oxida- tion of iron sulfide scale in the presence of moisture and oxygen. Chromium Stainless Steels in 750°F to 900°F Service. Ferritic and martensitic stainless steels containing 13% or more chromium can embrittle during exposure to temperatures in the 750°F to 900°F range. This phenomenon is known as 885°F embrittlement. Some of these stainless steels are so sensitive to 885°F embrittle- ment that even slow cooling through this temperature range will cause embrittlement. Duplex stainless steels are also susceptible to 885°F embrittlement. Stainless Steels Above 1000°F. At elevated temperatures, all stainless steels with high chromium contents will develop a constituent called sigma phase which causes embrittlement at lower temperatures. Sigma phase is a very hard, nonmagnetic, brittle phase. The straight chromium ferritic and martensitic stainless steels containing 13% and more chromium are very susceptible to extensive sigma phase formation at tempera- tures above about 1000°F. The austenitic stainless steels are not as susceptible because of their high nickel content, but they can develop damaging amounts of sigma phase when held between about 1000°F to 1550°F for long periods of time. Certain highly susceptible austenitic alloys, such as castings and welds, may develop serious embrittlement in a few hours at temperatures of 1200°F to 1300°F. Duplex stainless steels are also very susceptible to sigma embrittlement. Sigma phase normally does not affect the steel’s elevated temperature properties but may make it so brittle at lower temperatures that failures will occur during startup or shutdown. November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-15 200 Piping Component Selection Piping Manual Sulfide Stress Cracking. The martensitic stainless steels are especially susceptible to sulfide stress cracking. Welds are difficult to soften with heat treatment and are, therefore, susceptible to cracking. Low carbon grades, like Type 410S, are used to limit weld zone hardness. Nickel Alloys Nickel alloys such as Monel, Inconel alloys, Incoloy alloys, and Hastelloy alloys are usually very expensive, and are used only for specialized applications. Some nickel alloys have good resistance to chloride solutions where stainless steels are poor. Fabricating and weldability are generally good with proper precautions. Titanium Alloys These are rarely used for piping. Welding is difficult, requiring very clean condi- tions. Field repairs are not practical. Castings Various types of castings are used in piping. Figure 200-4 lists some. Fig. 200-4 Castings Used in Piping Type Brittleness Common Specifications Grey cast iron Very brittle ASTM A126 Gr.B Malleable cast iron Near steel Ductile or nodular cast iron Nearest to steel ASTM A395 Cast steel Can equal steel ASTM A216 Gr.WCB 226 Materials Selection Summary Figure 200-5 summarizes some limitations of the commonly used piping materials. Data in this table is illustrative only and is not intended for design. Fig. 200-5 Piping Material Temperature Limits Maximum Use Temperature, °F (1 of 2) 1¼ Cr - 2¼ Cr - 5 Cr - 18 Cr - Carbon Steel C-½ Mo ½ Mo 1 Mo ½ Mo 8 Ni (304) Allowable stress (3000 psi) (per 900 1081 1095 1150 1096 1345 ASME/ANSI B31.3) Oxidation (10 mpy loss) 1050 1050 1100 1175 1200 1500 Graphitization (welded only) 800 850 Temper embrittlement 700-1050 700-1050 Sigma embrittlement 1100-1700 Hardening on cooling 1330 1330 1375 1425 1450 Carbide precipitation 750-1500 (sensitization) 200-16 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection Fig. 200-5 Piping Material Temperature Limits Maximum Use Temperature, °F (2 of 2) Hydrogen attack (750 psia 460 460 955 1000 1080 H2 P)(1) H2S corrosion (10 mpy loss at 590 590 630 630 670 ≈1000 800 ppm H2S) H2/H2S corrosion (10 mpy loss 485 485 485 485 485 875 at 10 psia H2S P) Caustic embrittlement stress 140 140 140 140 140 140 corrosion cracking(2) Amine stress corrosion cracking MEA(3) X X X X X X (2) DEA 100 100 100 100 100 100 Chloride stress corrosion 140 cracking(2) Sulfide stress(4) corrosion X X X X X cracking (1) P indicates Partial Pressure (2) Not susceptible if stress relieved. (3) X = Susceptible at ambient temperature when not stress relieved. (4) X = Susceptible at ambient temperature when tensile strength exceeds 90 ksi or hardness exceeds Rockwell C22. 230 Pipe and Tubing This section discusses the selection of pipe materials and dimensional standards for pipe, and it gives pressure design calculations for pipe. Recommendations are also given for tubing materials and connections in various services, and pressure rating of tubing is discussed. 231 Recommended Materials for Pipe This section will help with engineering decisions regarding materials selection. Selection of both material and fabrication method is dictated by the piping class (see the Corporation Piping Specification). Carbon Steel Pipe There are many methods of manufacturing carbon steel pipe, and several levels of quality. In general, seamless pipe is specified for critical services and for large processing facilities. Whether of carbon steel or alloy, pipe without a longitudinal seam provides maximum fire safety and overall reliability. Welded seam pipe, however, has many applications and can result in significant savings on large jobs. Whether seamless or welded pipe is used, Grade B is specified rather than Grade A because of its higher allowable stress. Grade B pipe costs the same and is usually more readily available than Grade A. Grade A can be used in any service but, because of the lower allowable stresses, wall thicknesses will be greater. Grade A is rarely used and is being phased out of piping codes. November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-17 200 Piping Component Selection Piping Manual The upper temperature limit for all plain carbon steel pipe is 800°F, although flange ratings and gasket material may well dictate a lower limit. Above 800°F strength is reduced by graphitization, and alloy piping is specified. Named below are the common fabrication methods for carbon steel pipe and services, in general, for which they can be considered. Only the first three methods produce acceptable pipe for Company use. Additional information on specifications and uses is available from the ETC Materials and Equipment Engineering Unit. For descriptions of these fabrication methods, see Section 210. Seamless Pipe (SMLS). Seamless pipe is preferred for most on-plot services in both large plants and producing areas. It is available in sizes up to about NPS 20. Electric Fusion Welded (EFW) Pipe. For process facilities, EFW pipe is an acceptable substitute for seamless pipe if produced by the submerged arc process and if additional mill inspections are performed. See Model Specification PPL-MS-1050, Line Pipe. Carbon steel EFW pipe is normally available only in sizes NPS 16 and larger. EFW pipe is usually less expensive than seamless, but more expensive than electric resistance welded (ERW) pipe. Electric Resistance Welded (ERW) Pipe. ERW pipe is acceptable for pipeline and off-plot service. It also can be considered for utility services in process facilities. In the above cases, additional mill inspections and vendor approval should be obtained as outlined in Specification PPL-MS-1050. Some operating centers prefer ERW over EFW for on-plot services because of the lower cost. See Section 700. Electric Flash Welded, Furnace Lap Welded and Furnace Butt Welded Pipe. These are all of lower quality than EFW or ERW and are seldom used. They should be considered only for noncritical, nonhazardous services where reduced reliability can be accepted, such as low pressure concrete-lined pipe, off-plot vent lines, stand- pipes, and submerged outfall lines. Electric flash welded and lap welded pipe are no longer made in the United States. Spiral Welded Pipe. Historically, spiral welded pipe has shown poor quality and is not recommended. The same applications and restrictions apply as for furnace welded pipe. X Grades of API 5L Pipe. The X grades of API SPEC 5L pipe are not used in process plants. These grades have a higher yield strength of 42,000 to 65,000 psi. They are more susceptible to welding problems and hydrogen embrittlement, espe- cially at the higher strengths. ASME/ANSI B31.3 limits their use to a maximum of 400°F, and little or no advantage can be taken of the higher yield strength in pres- sure calculations. The X grades are used extensively for pipelines. Special Service Pipe Plain carbon steel piping is adequate for the majority of services encountered in industry. However, for many applications, service conditions or economics require other materials. Most materials and related services are considered in the Corporation Piping Specification. For more information consult the Corrosion Prevention and Metallurgy Manual or contact the ETC Materials and Equipment Engineering Unit. 200-18 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection The following items comprise an incomplete compilation of materials and special services in Company facilities: Carbon Steel in Low Temperature Service. ASME/ANSI B31.3 allows carbon steel piping to be used down to minus 20°F without toughness testing. Based upon toughness data for typical grades of carbon steel piping, such as ASTM A53 and A106, Company practice has been more restrictive than this, typically requiring impact testing below about 10°F. Current Company practice is to use the impact exemption curves shown in the ASME Boiler and Pressure Vessel Code Section VIII, Division 1, Section UCS-66. This is essentially the same as the Company’s past practice. For colder temperatures, impact tested grades, such as ASTM A-333 Grades 1 or 6 are used. Carbon—½% Moly. Resists graphitization and retains strength to 850°F. Has limited application. See Model Specification PIM-MS-4772 for fabrication require- ments. Low Chrome (1 to 3%). Resists hydrogen attack above 400°F. Used also for high- temperature strength. See the Corrosion Prevention and Metallurgy Manual for details. See PIM-MS-4772 for fabrication requirements. High Chrome (5 to 9%). Resists H2S corrosion in stocks above 500°F in the absence of hydrogen. A 12% chrome alloy is not recommended because of diffi- culty in making reliable welds. See PIM-MS-4772 for applications and limitations. Stainless Steel. Used where both hydrogen and H2S are present in stocks above 500°F. Austenitic types 304L, 316L, 321 and 347, which do not sensitize during welding or require postweld heat treatment, are generally specified. Also used in rich MEA/DEA, sour water, and some acid services. Common in lube oil and seal oil systems, where contaminants cannot be tolerated. See PIM-MS-4770 for fabrica- tion requirements. Alloy 20. Used with dilute (<80% concentration) or concentrated sulfuric acid at elevated temperatures or high velocity. See the Corrosion Prevention and Metal- lurgy Manual. PIM-MS-4770 covers fabrication requirements. Cast Iron. Used in brine and saltwater systems with flanged connections, and sewers and drains with bell and spigot connections. See the Corrosion Prevention and Metallurgy Manual and Civil and Structural Manual. Copper/Brass. Used in drinking water and instrument air service. Common in salt- water service and plumbing systems within buildings. Unsuitable in atmospheres corrosive to copper. See the Corrosion Prevention and Metallurgy Manual. Galvanized Steel. Used primarily in instrument air and drinking water systems. Replaces tubing in areas subject to mechanical damage. Plastic. Used extensively at low pressures and temperatures for chemical and water treating services, but not with hydrocarbons. See Section 400. Concrete. Primarily used in saltwater, drainage, and wastewater service. See Section 400 and the Civil and Structural Manual. November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-19 200 Piping Component Selection Piping Manual Lined Pipe. Carbon steel pipe coated with a protective liner is used with corrosive waters, brines, and many chemicals. Liners include Teflon®, polypropylene, epoxy and glass. See Section 400. Cement mortar is used in water service; see Specification PPL-MS-1632 in the Pipeline Manual, and see the Corrosion Preven- tion and Metallurgy Manual. Double-Wall Pipe. Generally fabricated of carbon steel. Provides a steam jacket for high pour point stock. Generally used in molten sulfur service. 232 Dimensional Standard for Pipe The dimensional standards for steel and stainless steel pipe are ASME/ANSI B36.10M and ASME/ANSI B36.19 respectively. Pipe is designated by size and wall thickness. The manufacturing process and tooling used dictates that the O.D. of the pipe be kept standard and any change in wall thickness be made by changing the inside diameter (I.D.) of the pipe. See Appendix D for tables of sizes and wall thickness. The current dimensional standard is the result of consolidating the old Manufac- turers Standard, in which the wall thicknesses were designated by weight. The designations “standard weight,” “extra strong” and “double extra strong” were carried over to the present standard, which now uses both weight designations and schedule numbers, and some wall thicknesses without schedule number or weight designation. 233 Pressure Design of Pipe Pipe has no specific tabulated pressure rating. The rating is calculated using the formula from the applicable section of the ASME/ANSI B31 Code for Pressure Piping. Pressure rating calculations are usually carried out in one of two ways: • Find the thinnest wall pipe that will satisfy a given design pressure and temper- ature with a given corrosion allowance • Find the maximum design pressure at a given temperature for a known wall thickness and corrosion allowance Difference between Piping and Pipeline Formulae Piping Formula. This manual is concerned only with plant piping covered by ASME/ANSI B31.1, Power Piping, and B31.3, Chemical Plant and Petroleum Refinery Piping. Both use the same design formula: PD t m = ----------------------------- + c 2 ( SE + PY ) (Eq. 200-1) 200-20 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection Pipeline Formulae. Equations 200-2 and 200-3 (from ASME/ANSI B31.4 and B31.8) are for pipeline design only and are shown here only for comparison. The ASME/ANSI B31.4 design formula is: PD t = -------- 2S (Eq. 200-2) The ASME/ANSI B31.8 design formula is: P = ⎛ ----------⎞ F E T 2ST ⎝ D ⎠ (Eq. 200-3) These simpler formulae should not be used for the design of piping systems. The allowable stresses and various design factors for the three formulae are defined differently, depending on the type of construction, area classification and pipe used. Consult the Pipeline Manual for guidance. Because the design formulae for piping and pipelines are different, be careful to select the correct code and apply the correct design factors. As an example, the maximum design pressure for NPS 6 Sch. 40 ASTM A106 Gr B or API 5L Gr B pipe is tabulated in Figure 200-6 to show the difference between the ratings allowed by the various Code sections. Fig. 200-6 Example Pressure Ratings(1) of NPS 6 SCH 40 ASTM A53 GR.B Seamless and ERW Pipe According to Different Code Sections Seamless Pipe ERW Pipe(2) Allowable Stress at Maximum Design Maximum Design Code Section 100°F, psi Pressure, psig Pressure, psig B31.1 15,000 1143 975 B31.3 20,000 1524 1295 B31.4 S = (0.72)(35,000) 2130 2130 B31.8 S = 35,000 2130 2130 F = 0.72 T=1 (1) These ratings apply to all fittings that have no specific pressure rating and to all that are rated by comparison to equivalent pipe. (2) Joint efficiency for ERW pipe: B31.1 and B31.3E = 0.85 B31.4 and B31.8E = 1.0 Design Pressure Formula for Piping A design pressure and temperature must be established prior to any specific pipe wall thickness calculations. This may be the service pressure and temperature plus a safety factor, or the pressure/ temperature limitations of the flanges, if the flange class has been established (flange ratings are tabulated in ASME B16.5). November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-21 200 Piping Component Selection Piping Manual ASME/ANSI B31.3 is the Company-wide basis for development of most piping designs and specifications. The following wall thickness calculations are based on the 1984 edition, Paragraph 304. Standard Form PIM-EF-539, Wall Thickness Calculations, can be used in conjunction with the ASME/ANSI B31.3 procedure outlined below for documentation of calculated pipe wall thicknesses. The required thickness of straight sections of pipe shall be determined in accor- dance with the following equation: tm = t + c (Eq. 200-4) where: tm = required minimum pipe wall thickness, including pressure design thickness, mechanical allowance, and corrosion/erosion allow- ance, in. The selected nominal thickness must be equal to or larger than tm t = pressure design thickness, in. c = additional wall thickness for mechanical allowances (thread or groove depth) plus corrosion/erosion allowances, in. Pressure Design Thickness (t). Pressure design thickness, t, is calculated as follows: PD t = ----------------------------- 2 ( SE + PY ) (Eq. 200-5) where: P = internal design pressure, psig D = outside diameter of pipe, in. t = pressure design thickness, in. (t must be less than D/6) Y = coefficient based on pipe material. Use 0.4 for carbon steel at 900°F temperature and below. (See ASME/ANSI B31.3 for other materials and temperatures.) S = basic allowable stress for material from ASME/ANSI B31.1 or B31.3 Table A-1, psi E = quality factor from Table A1-A or A1-B of ASME/ANSI B31.3 (This is a function of how the pipe is manufactured. For example, E=1.0 for seamless pipe, and 0.85 for ERW.) Additional Wall Thickness (c). The additional wall thickness required for mechan- ical and corrosion/erosion allowances should be as follows: • Pipe threads Nominal Pipe Size Thread Depth, in ½ and ¾ 0.057 1 through 2 0.070 200-22 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection • Groove depths. As specified. Victaulic couplings, for example, require the addition of the manufacturer’s specified groove depth to the pressure design thickness. Note If t, based on the sum of the pressure and mechanical allowances, is less than the “throwaway” thickness on PIM-EF-539, then the throwaway figure should be used in the remaining calculations. The throwaway thickness provides some structural strength for self-support. Some OPCOs prefer other throwaway thicknesses. • Corrosion/erosion allowance. Varies from zero (no corrosion or erosion) to 3/16 inch (severe corrosion or erosion). The actual amount should be deter- mined with the help of materials engineers; 1/16 to 1/8 inch is typical. Nominal Wall Thickness. The total required minimum wall thickness, tm, is arrived at by totalling pressure design thickness, and mechanical and corro- sion/erosion allowances. A further allowance is made for mill tolerances (12½% for seamless pipe), and the nearest larger commercially available nominal thickness is specified. Therefore: tm nominal thickness ≥ ------------- 0.875 (Eq. 200-6) and PD t m = ----------------------------- + c 2 ( SE + PY ) (Equation 200-1) Nominal wall thickness of pipe can be obtained from pipe manufacturers’ hand- books or ASME/ANSI B36.10M, Welded and Seamless Wrought Steel Pipe. Not all nominal wall thicknesses made are always commercially available. Pipe wall thicknesses calculated as shown here do not specifically provide for unusual thrusts or bending moments. Most sizes have sufficient metal to withstand moderate displacement strains from normal thermal expansion or terminal move- ments, but this should bechecked as described in Section 330, and should adhere to code requirements for expansion and flexibility. 234 Economics of Pipe Wall Selection Thin Wall Pipe Pipe wall thicknesses should be selected with care. In a given size, thin wall pipe can be more expensive to use than standard wall pipe. Costs of piping materials for several plants were analyzed to compare (1) thin wall pipe and fittings, (2) thin wall pipe and taper-bored standard weight fittings, and (3) standard weight pipe and fittings. The results were incorporated into the Corporation Piping Specification November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-23 200 Piping Component Selection Piping Manual (GB135169), but may be stated in general terms as follows. For NPS 2 through NPS 24, Standard Weight pipe and Standard Weight fittings are less costly than Schedule 20. On long, straight runs of pipe without many fittings or when ERW pipe is used, thinner-than-standard walls can frequently save money. However, where a number of fittings are involved, standard weight pipe and fittings are normally less costly than special thin wall fittings or taper-boring Standard Weight fittings. Also, availability of thin wall fittings and taper-bored fittings and flanges is often a problem. Future Maintenance Standard wall thicknesses will lead to simpler and less expensive maintenance. Flange Rating versus Service Conditions Where flanges have a higher rating than required by the design pressure and temper- ature (for example, Class 300 flanges used for H2S or LPG service when actual pressure and temperature conditions require only Class 150 flanges), savings might be realized for large quantities of pipe if the actual pressure and temperature of the system are used for pipe design. Any extra cost to taper bore the flanges or fittings for good weld fit-up to the thinner wall pipe must be considered. Future Capacity Anomalies in pipe wall thicknesses may allow economical use of deviations from customary economics by switching from some pipe sizes to the nearest larger sizes (for example, NPS 1½ Schedule 80 to NPS 2 Schedule 40, which has approxi- mately the same weight per foot). Runs of the larger pipe not having many valves or fittings can be equal or nearly equal in cost. 235 Tubing It is often necessary to choose between tubing and small diameter piping. Where mechanical damage is not a hazard, tubing offers several advantages. It is faster and less expensive to form and install tubing than to fit up threaded or socket weld piping; tubing has less mass and therefore undergoes less stress at connections in vibrating service; and in most systems it has fewer connections, and therefore fewer potential leaks. Tubing is not recommended for process service. The most common services are instrument air, utilities, instrument process leads, and steam tracing. One-quarter-inch through ½-inch diameter tubing is in common use in almost all facilities. Use of sizes over ½ inch is discouraged because of concern for tight connections, greater possibility of kinking and, in the case of stainless steel, diffi- culty in handling; ½-inch diameter, 0.065-inch wall SS tubing is the upper practical limit for manual field bending. Materials for Tubing Materials specified are typically (1) seamless annealed copper with brass fittings for nonhazardous low pressure services to its upper limit of 400°F, (2) 304 stainless steel with 316 SS fittings for corrosive service, hazardous service or higher pres- sures/temperatures, and (3) plastic for instrument air in certain areas. 200-24 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection Seamless annealed copper tubing with brass fittings per ASTM B88 Type K, Water Tubing, is recommended for utilities and steam tracing. Instrument air tubing should conform to ASTM B280 (DHP), Refrigeration Tubing. Copper tubing has an upper temperature limit of 400°F. Note that ASTM B88 tubing is specified with a nominal diameter 1/8 inch less than the actual outside diameter, while other copper tubing is specified by actual outside diameters. This must be considered when matching tubing with compression fittings, which are sized by actual outside diameters of the tubing. Stainless steel is always used for hydrocarbons and in hazardous or critical services in fire risk areas. Company practice is to specify solution-heat-treated and pickled Type 304 stainless steel per ASTM A269, with 316SS fittings in corrosive service. This material is included in ASME/ANSI B31.3. It is available in seamless and welded seam types and in several wall thicknesses for each diameter. A maximum wall thickness of 0.065 inch is recommended; above this there can be problems obtaining a proper seal with compression fittings. Seamless is preferred because of the higher allowable stress and concern for tight connections. Carbon steel is not recommended because internal corrosion and scaling can plug the tubing and damage instruments and equipment. Aluminum tubing was formerly used for instrument air in chemical (ammonia, H2S, etc.) plants, but is no longer recommended. It proved susceptible to work-hard- ening and frequent failure at the tubing connections, resulting from the normal minor vibration of process piping. Plastic tubing is used for instrument air in certain limited areas, such as within compressor panels. Connections for Tubing The preferred connections for tubing are compression fittings, and there are many variations. Compression fittings have one or more ferrules that grip the tubing within a compression nut. Care must be taken to avoid a reversal of ferrules during installation. Compression fitting suppliers with proven success in Company facilities are Craw- ford Swagelok, Hoke Gyrolok and Parker CPI. Other types of fittings, including flared tubing fittings, are considered poorer choices. Operating centers should stan- dardize the fittings used within their facilities to avoid mixing fitting components and thereby risking leaks. Because a compression fitting actually deforms the tube to obtain a proper seal, the tube wall thickness and hardness should always be checked against the fitting manu- facturer’s maximum wall thickness and hardness limitations. Also, the tubing surface condition is important for proper sealing; scratches or dents can result in leaks, especially in gas service. Pressure Rating of Tubing Allowable working pressures for given tubing sizes are available from tables supplied by most tubing and tubing fitting manufacturers. However, varying pipe code interpretations are applied in these tables, and the information is inconsistent. November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-25 200 Piping Component Selection Piping Manual For hazardous and critical services, allowable working pressures for stainless steel should be checked using Equations 200-1 and 200-4 through 200-6. If welded seam tubing is used, a quality factor E from Table A-1B in ASME/ANSI B31.3 must be used. Also, ASTM A-269 allows a ±15% variation in wall thickness for tubing less than ½-inch O.D. and ±10% for tubing ½-inch O.D. or larger. This is similar to mill tolerance for pipe. Normally no corrosion/erosion allowance is applied to tubing. If corrosion is suspected, consult the Materials and Equipment Engineering Unit of ETC. Figure 200-7 gives maximum allowable working pressures for seamless stainless steel tubing per ASTM A-269, in the most common sizes and wall thicknesses. Figure 200-8 is a table of de-rated allowable pressures for ½-inch O.D. stainless steel tubing at elevated temperatures; and an example calculation. Fig. 200-7 ASTM A269 Seamless 316 Stainless Steel Tubing Maximum Allowable Working Pressures, psig at 100 to 300°F Tube Wall Thickness, in. Tube OD, in. 0.028 0.035 0.049 0.065 0.083 0.095 0.109 0.120 0.2500 4000 5000 7100 9400 0.3125 4000 5600 7500 0.3750 3400 4700 6200 0.5000 2500 3500 4700 6000 0.6250 2800 3700 4800 5500 0.7500 2400 3100 4000 4600 5200 0.8750 2000 2700 3400 3900 4500 1.0000 2300 3000 3400 3900 4300 Pressure ratings are calculated by formula: 2tSE P = ------------ D (Eq. 200-7) where: P = design pressure, psig t = minimum wall thickness = nominal wall thickness minus tolerance (tolerance for less than ½-in. OD is 15%) (tolerance for ½-in. and larger OD is 10%) S = allowable stress at temperature, psi E = weld efficiency (for welded ASTM A269, E = 0.8) D = outside diameter, in. The calculated values are rounded to the nearest 100 psi per ASTM A450. 200-26 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection Fig. 200-8 Temperature-derated Allowable Pressures for ½-inch O.D. Stainless Steel Tubing ASTM A269 Stainless Steel Tubing—0.5 in. OD Derated Allowable Pressures at Elevated Temperatures, psig Welded Tubing Seamless Tubing Temp, °F 0.5-in. OD, 0.065-in. Wall 0.5-in. OD, 0.065-in. Wall 304 SS 316 SS 304 SS 316 SS 100 3700 3700 4700 4700 300 3700 3700 4700 4700 600 3100 3200 3800 4000 850 2800 2900 3500 3700 Pressure ratings are calculated by formula: 2tSE P = ------------ D (Eq. 200-8) where: P = design pressure, psig t = minimum wall thickness = nominal wall thickness minus tolerance (tolerance for ½-in. OD is 10%) S = allowable stress at temperature, psi E = weld efficiency (for welded ASTM A269, E = 0.8) D = outside diameter, in. The calculated values are rounded to the nearest 100 psi per ASTM A450. Example: ½-in. OD, 0.065-in. wall, 316 SS welded ASTM A269 tube, 100°F: 2 × 0.065 × 0.9 × 20, 000 × 0.8 )- = 3744 psig ------------ = (--------------------------------------------------------------------------------- P = 2tSE D 0.5 After rounding, P = 3700 psig In the midrange of wall thicknesses, compression fittings for each tubing size are generally pressure-rated higher than the connected tubing. For critical and hazardous services, and particularly with heavier wall thicknesses, this should be confirmed with the vendor. Copper Tubing Applications One-quarter-inch O.D. instrument air is the most common copper tubing applica- tion in almost all facilities. Tubing is available with a PVC jacket for use in atmo- spheres corrosive to copper. See the Instrumentation and Control Manual. Another application is 3/8-inch and 1/2-inch O.D. steam tracing beneath thermal insulation on piping and equipment. To guarantee leak-tight intermediate connec- tions in long runs of insulated pipe, soldered joints in place of connectors can be used, or the connectors brought outside the insulation. November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-27 200 Piping Component Selection Piping Manual Preinsulated copper tubing is used for steam supply and condensate return lines connected to steam tracing. This may be more economical than insulated pipe at some locations. See the Utilities Manual. Low-temperature cooling water connections and lube oil connections on rotating equipment use copper in noncritical off-plot service, as do drinking water systems, where mechanical strength for support is not a concern. Stainless Steel Tubing Applications Instrument connections downstream of the first block valve on process lines use usually ½-inch O.D. stainless steel tubing with 0.065-inch wall thickness for mechanical strength. Presteam-traced and preinsulated tubing bundles are available for instrument process leads. Stainless steel instrument tubing is discussed in Section 340 of this manual and in the Instrumentation and Control Manual. Self-contained cooling water and lube oil systems on hot or critical rotating equip- ment normally use ½-inch O.D. stainless steel with 0.065-inch wall thickness. Refer to the relevant equipment manual or the General Machinery Manual. Centrifugal pump mechanical seal flush connections use stainless steel with a ½-inch O.D. and 0.065-inch wall. Stainless steel replaces copper tubing for instrument air and steam tracing in corro- sive atmospheres such as chemical, ammonia, sulfur, and H2S removal plants. Plastic Tubing Applications Polyethylene tubing, singly or in color-coded bundles, is available for instrument air service. It is normally installed underground, or in control houses and similarly fire- protected areas. In shutdown systems, polyethylene tubing is used as a failsafe device which initiates shutdown when burned through by fire. See the Instrumenta- tion and Control Manual. 240 Fittings Fittings are used for: • Joining straight pieces of pipe • Changing direction • Making branch connections • Changing the size of pipe This section discusses materials for fittings, including the industry standards that cover pressure rating of fittings fabricated from these materials, and recommended materials for various applications. The section then describes various fittings. 200-28 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection 241 Materials for Fittings Fittings are fabricated of forged steel, wrought steel, and malleable iron. Forged steel socket welding and threaded fittings are manufactured in sizes from NPS 1/8 to NPS 4. Corporate Piping Classes (defined in the Corporation Piping Specification in Volume 2 of this manual) generally specify forged steel socket welding or threaded fittings for the NPS ¾ through NPS 1½ size range and allow their use for NPS 2 fittings in special cases. Malleable iron threaded fittings are manufactured in sizes from NPS ½ through NPS 6. Corporate Piping Classes allow them for instrument air and cooling water service only. Wrought steel butt welding fittings are manufactured in sizes from NPS ½ to NPS 48. Corporate Piping Classes generally specify them for NPS 2 and larger piping. In exceptional cases, where crevice corrosion is of concern or in lube oil systems where it is difficult to clean dirt and slag from the crevices, butt welding fittings may be specified for NPS ¾ and larger piping. Butt welding of pipe smaller than NPS 2 is more expensive than other types of joints. Wrought steel butt welding short radius elbows and returns are a special class of fittings manufactured for use in close quarters. It is significant that these fittings are rated only for 80% of the pressure rating calculated for seamless pipe of the same size and nominal thickness. The pressure rating method for each of the above classes of fittings is somewhat different. Refer to Section 242. 242 Dimensional Standards for Fittings The four ASME/ANSI standards for fittings are discussed next. ASME/ANSI B16.11, Forged Steel Fittings, Socket Welding and Threaded Fittings covered by this standard include: 45- and 90-degree elbows, tees, crosses, couplings, caps, plugs, and bushings. Forged steel fittings are designated by pressure classes. The pressure classes and the schedule of pipe corresponding to each are given in Figure 200-9. Fig. 200-9 Forged Steel Fitting Pressure Classes, per ASME/ANSI B16.11 Class Threaded 2000(1) 3000(1) 6000 Socket Welding 3000 6000 9000 Corresponding Pipe Sch 80 Sch 160 XXS Schedules (1) Company practice for both threaded and socket welding systems is to use Class 3000 fittings with Schedule 80 pipe and Class 6000 fittings with Schedule 160 pipe. November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-29 200 Piping Component Selection Piping Manual Pressure Ratings. The maximum allowable pressure of the fitting is that computed for straight seamless pipe of equivalent material and corresponding pipe schedule (from Figure 200-9). The wall thickness used in the computation is that tabulated in ASME/ANSI B36.10M for the size and applicable schedule of pipe, reduced by manufacturing tolerances and other allowances (corrosion and thread depth). Marking. Fittings are marked with: • Manufacturer’s name or trademark • Material identification (such as A105) • Suffix “WP”, indicating conformance to B16.11 • Pressure class • Size (NPS) Material. Forged carbon steel fittings conform to ASTM A105. There is only one grade designated A105. Forged alloy steel fittings conform to ASTM A182 and come in many grades, according to composition. For example, ASTM A182 Gr F11 is 1¼ Cr-½ Mo; ASTM A182 Gr F304L is type 304L stainless steel. For further details refer to ASTM A182. Forged carbon and low alloy steel fittings intended for low temperature service conform to ASTM A350. Chemical composition is specified by grade. Tensile properties are specified by Class designations. For example, ASTM A350 Gr LF3 Class 2 is 3% Ni. For further details refer to ASTM A350. ASME/ANSI B16.3, Malleable Iron Threaded Fittings This standard covers malleable iron threaded fittings in Class 150 for sizes NPS 1/8 through NPS 6 and Class 300 for sizes NPS 1/4 through NPS 6. Fittings include: 45- and 90-degree elbows, Tees, crosses, Y-branches, couplings (straight and reducing), street tees and elbows, caps, and return bends. Pressure Ratings. The pressure ratings are as listed in Figure 200-10. Marking. Class 150 fittings are marked only with the manufacturer’s name or trademark. Class 300 fittings are marked with the: • Manufacturer’s name or trademark • Numeral 300 • Letters MI to designate malleable iron • Size (NPS) Material. The material conforms to ASTM A197. 200-30 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection Fig. 200-10 Pressure Ratings for Malleable Iron Threaded Fittings, per ASME/ANSI B16.3 Class 150 Class 300 Temperature, °F All Sizes Sizes ¼ to 1 Sizes 1¼ to 2 Sizes 2½ to 3 -20 to 150 300 2000 1500 1000 200 265 1785 1350 910 250 225 1575 1200 825 300 185 1360 1050 735 (1) 350 150 1150 900 650 400 935 750 560 450 726 600 475 500 510 450 385 550 300 300 300 (1) Permissible for service temperature up to 366°F, reflecting the temperature of saturated steam at 150 psig. ASME/ANSI B16.9, Factory-Made Wrought Steel Buttwelding Fittings This standard covers the following fittings in sizes NPS ½ through NPS 48. Fittings include: long radius elbows (90-degree and 45-degree), long radius reducing elbows, long radius returns (180-degree return bends), straight tees and crosses, reducing outlet tees and crosses, lap joint stub ends, caps, and reducers (concentric and eccentric). Butt welding fittings are designated by weight or schedule of the associated pipe. The commonly stocked weights are standard weight, extra strong, Schedule 160, and double extra strong. Pressure Ratings. The allowable pressure ratings for fittings designed in accor- dance with this standard are calculated as for straight seamless pipe in accordance with ASME/ANSI B31. Marking. Fittings are marked with the following: • Manufacturer’s name or trademark • Material identification • Prefix WP, indicating conformance to B16.9 • Schedule number or wall thickness designation Material. Materials for wrought carbon and alloy steel fittings conform to ASTM A234. Chemical composition is designated by grades. Low carbon wrought steel is designated by ASTM A234 Gr WPB; 1¼ Cr-½ Mo is designated by ASTM A234 Gr WP11. Wrought austenitic stainless steel fittings conform to ASTM A403. This standard covers two general classes, WP and CR. Fittings designated WP must withstand a test pressure equal to that prescribed for the specified matching pipe. Pressure requirements for Class CR fittings are based on MSS SP-43 and are less rigorous. November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-31 200 Piping Component Selection Piping Manual The Company does not recommend use of Class CR fittings. Class WP fittings are subdivided into three subclasses: WP-S Seamless Construction WP-W Welded Construction; X-ray of manufacturers’ welds only WP-WX Welded Construction; full X-ray of all welds. Company practice is to specify WP-S or WP-WX. The type of stainless steel is indi- cated in the grade symbol. For example: ASTM A403 Gr WP-S 304L is a 304L seamless stainless steel fitting. Wrought carbon and alloy steel fittings intended for low temperature service conform to ASTM A420. Chemical composition and tensile requirements are desig- nated by the grade symbol. The standard lists 4 grades: Gr WPL6, Gr WPL3, Gr WPL8, Gr WPL9. ASME/ANSI B16.28, Wrought Steel Buttwelding Short Radius Elbows and Returns This standard covers short radius elbows and returns. Pressure Ratings. The pressure ratings are 80% of those calculated for seamless pipe of the same size, material, and nominal thickness. Marking. Fittings are marked with: • Manufacturer’s name or trademark • Material identification • Prefix WP, indicating conformance to B16.28 • Schedule number or nominal wall thickness designation Material. Material references are the same as given for ASME/ANSI B16.9. Note Use of short radius fittings is discouraged because of the higher pressure drop and lower pressure rating. Generally in a Class 150 flanged system with stan- dard weight pipe and sizes up to 12 inches the pressure rating of the short radius elbows will be satisfactory, but it has to be checked. 243 Company Practice for Fitting Use Company practice is to use butt weld fittings for piping NPS 2 and larger and threaded or socket weld fittings for most NPS 1½ and smaller piping. Except for galvanized pipe in air and drinking water service, threaded piping larger than NPS 1½ is rarely used today. This discussion is limited to carbon and alloy steel fittings. Cast iron pressure fittings are used almost exclusively in saltwater service. Small diameter piping is discussed in Section 340 of this manual. Weld fittings should be wrought carbon or alloy steel (ASTM A105, A182, A234, A350, and A420), not cast steel, which has proven less reliable. The metallurgy of the fittings should always match that of the pipe. Consult the Materials and Equip- ment Engineering Unit of ETC if this is not possible. 200-32 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection To facilitate fit-up and welding, the bore of the fitting should be aligned as accu- rately as possible with the bore of the pipe; for details see Standard Drawing GC-L34496. In some cases, standard weight butt weld fittings do not fit well enough with thinner wall pipe to obtain proper ID alignment for welding. When the wall thicknesses of the fitting and pipe differ by more than 1/16 inch, ASME/ANSI B31.3 requires the thicker component to be taper-bored, or back- beveled, so that the inside diameter mismatch does not exceed 1/16 inch at the weld joint. This will permit better quality welds in the root pass, and the more gradual change in cross-section will reduce the likelihood of stress-related failure. Carbon steel welds on pipe and fittings with 3/4-inch or greater wall thickness and most alloy welds require heat treatment. These requirements are covered in the Welding Manual. 244 Straight Connections These fittings—couplings, unions, and flanges—are shown in Figure 200-11. Flanges are discussed in detail in Section 250. November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-33 200 Piping Component Selection Piping Manual Fig. 200-11 Straight Connection Fittings Courtesy of Taylor Forge and Bonney Forge (1 of 3) Slip-on Flange Threaded Flange Lap Joint Flange Welding Neck Flange 200-34 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection Fig. 200-11 Straight Connection Fittings Courtesy of Taylor Forge and Bonney Forge (2 of 3) Socket Welding Flange Threaded Union Full Coupling Socket Welding Union November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-35 200 Piping Component Selection Piping Manual Fig. 200-11 Straight Connection Fittings Courtesy of Taylor Forge and Bonney Forge (3 of 3) 200-36 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection 245 Direction Changes The three fittings that change direction are elbows, bends and miters. See Figure 200-12. Elbows Wrought steel elbows (or ells) of 90 degrees and 45 degrees are readily available. Returns of 180 degrees are also available, but are seldom used other than in fired heater coils and heat exchangers. For angles other than 45 and 90 degrees, an ell is usually trimmed to fit. Long radius elbows are preferred to provide smooth flow and reduce erosion. Short radius ells are allowed when space restrictions require their use.  Caution Short radius ells and return bends are pressure rated at only 80% of connected piping with the same wall thickness and material. Normally, only standard weight (ST), extra strong (XS), and double extra strong (XX) wall thickness ells are available off-the-shelf. When matching thin wall pipe, it is usually less expensive to purchase heavier wall ells and back-bevel the ends (on a lathe) to match the pipe than to special-order the ells. Section 230 discusses the economics of using thin wall pipe. Although not in common use, long tangent ells can be economical for mounting slip-on flanges in close quarters, and 90-degree reducing ells for combining direc- tion change with line size change in close quarters. Bends Common practice in off-plot and remote locations (including offshore platforms) is to install pipe bends instead of fittings. They have also replaced special heavy wall fittings in high pressure process plants. Typical bends are made to a radius of five times the pipe diameter and can be done in the shop or the field. Care must be taken not to reduce either (1) the pipe wall thickness to less than allowable by ASME/ ANSI B31.3 or (2) the flow area. Induction bends, cold bends to a radius less than five times the diameter and some alloys require postweld heat treating, depending on the material (see paragraph 332.4 of ASME/ANSI B31.3). Piping fabrication with bends is discussed in Model Specifications PIM-MS-2505, PIM-MS-4770, PIM-MS-4772. Model Specification PPL-MS-4737, Induction Bending (in the Pipeline Manual), specifies procedures for induction bends. See also Section 300 of the Pipeline Manual. Miters Once common in all services, 45- and 90-degree miters have been abandoned because of their higher labor cost and greater flow resistance. Welded miters have high stress intensification factors and are good candidates for fatigue failure, espe- cially in hot service. No miters should be buried. Miters may be safe to use for very large piping in low pressure service (below 200 psi). Design calculations for miter bends are included in ASME/ANSI B31.3. November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-37 200 Piping Component Selection Piping Manual Fig. 200-12 Change of Direction Fittings Courtesy of Canvil and Taylor Forge (1 of 2) 90° Long-Radius Elbow 90° Reducing Elbow 45° Long-Radius Elbow 90° Short-Radius Elbow 180° Long-Radius Return 180° Short-Radius Return 90° Three Piece Miter Elbow Elbows and Returns 200-38 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection Fig. 200-12 Change of Direction Fittings Courtesy of Canvil and Taylor Forge (2 of 2) Socket Welding Elbows Threaded Elbows November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-39 200 Piping Component Selection Piping Manual 246 Branch Connections The proper selection of branch reinforcement for given service conditions depends on ASME/ANSI B31.3 requirements and material and fabrication costs. Figure 200-13 illustrates branch connections. Tees Wrought steel tees make the strongest and safest branch connections and are recom- mended for most applications. They have great resistance to fatigue in vibrating service and are used almost exclusively in hydrocarbon service when the branch size and run size are the same. Wrought steel reducing tees and straight tees used with weld reducers are equal in quality. Reinforced Branches Weldolets, sweepolets, sockolets, thredolets, elbolets, and stub-ins with saddles and reinforcing pads are acceptable. Unreinforced stub-ins are allowed only for low- pressure, noncritical utilities. For significant differences in branch size and run size, integrally reinforced weldo- lets are generally preferable to reinforced stub-ins. Their welding is more consistent and easier to inspect, and calculation is not required for code compliance. See the Corporation Piping Specification for recommended branch reinforcing tables. These tables must be used in conjunction with the appropriate piping class sheet. Sweepolets are normally used in high-pressure pipeline service. They offer strength with low residual metal stress, and the attachment welds are easily radiographed. Saddles, reinforcing pads, weldolets, and stub-ins should be installed per ASME/ANSI B31.3 and Standard Drawing GC-L34496. Weld bosses are no longer recommended for piping. They have been used in place of thredolets and the like, but they are custom-made and costly. (The rationale for their use was: they provided weld reinforcement while allowing enough length for damaged threads to be cut off and re-tapped.) Although ASME/ANSI B31.3 allows threaded and socket weld couplings or half-couplings, the potential for poor attach- ment welds makes their use unadvisable. Other Branch connections at other than 90 degrees are not recommended and seldom encountered; however, ASME/ANSI B31.3 covers calculation of reinforcement requirements for such connections. They should be considered only for utility services. Prefabricated 45-degree lateral outlets are available and integrally rein- forced latrolets are recommended for small diameter piping connections. Welding crosses are acceptable but seldom used. Almost all connection needs are met with tees of some type. 200-40 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection Fig. 200-13 Branch Connection Fittings Courtesy of Bonney Forge, Canvil, and Taylor Forge (1 of 3) Straight Reducing Threaded Tees Socket Welding Tee Straight Reducing Butt Welding Tees November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-41 200 Piping Component Selection Piping Manual Fig. 200-13 Branch Connection Fittings Courtesy of Bonney Forge, Canvil, and Taylor Forge (2 of 3) Weldolet® Socket Welding Elbolet® Butt Welding Elbolet® Threaded Elbolet® Elbolet® 200-42 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection Fig. 200-13 Branch Connection Fittings Courtesy of Bonney Forge, Canvil, and Taylor Forge (3 of 3) Butt-Weld Threaded Socket-Weld For 45° Lateral Connections Latrolet® Caution Before choosing thred- olet in cyclic service, check the branch tables in the Piping Specifica- tion to be sure it is allowed. Thredolet® Sockolet® November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-43 200 Piping Component Selection Piping Manual 247 Reducing and End Closure Fittings These fittings (Figure 200-14) are used wherever pipe changes size and where piping ends. Fig. 200-14 Reducers and End Closure Fittings Courtesy of Bonny Forge, Taylor Forge and Standard Fittings (1 of 2) Concentric Reducer Eccentric Reducer Butt Welding Reducers Butt Welding Cap Concentric Eccentric Swaged Nipple Swaged Nipple Swaged Nipples 200-44 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection Fig. 200-14 Reducers and End Closure Fittings Courtesy of Bonny Forge, Taylor Forge and Standard Fittings (2 of 2) Eccentric Swaged Nipple, Beveled Large End - Plain Small End (BLE-PSE) Concentric Swaged Nipple, Threaded Both Ends (TBE) Eccentric Swaged Nipple, Beveled Large End - Threaded Small End (BLE-TSE) Steel Barstock Plug (Solid) November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-45 200 Piping Component Selection Piping Manual Changes in Pipe Size Changes in pipe size are normally done with concentric or eccentric reducers and infrequently with reducing elbows, which are not readily available. Changes in pipe wall thickness between sizes sometimes makes it necessary to back-bevel one end of the reducer or pipe to meet fit-up requirements. Unless care is taken in the selec- tion and installation of reducers, pockets of liquid or vapor can be trapped, requiring additional venting or draining. Standard Drawing GB-L88267 provides dimensional information for the fabrica- tion of welding tapers. These are used when large line size transitions are not commercially available or when minimum system pressure loss is desired. Line Terminations Pipe ends not connected to equipment, for instance, at the end of pipeway headers, are normally capped with wrought steel weld caps. If the line must be cleaned peri- odically or if it may be extended in the future, a flange with a blind flange is used. See Section 320 regarding the use of ells with dummy extensions instead of capped deadlegs. 250 Flanges, Blanks, and Blinds 251 Flanges Flanges provide a bolted, separable joint in piping. Most piping connections to equipment are flanged for ease of installation and maintenance. Most valves in sizes NPS 2 and larger are flanged. There are two categories of flange joints: (1) unconfined gasket that can blow out under excess pressure or with poor makeup and (2) confined gasket that may leak but cannot blow out. When to Use Flanges The use of flanges and nonflange connections (see Section 260) should be limited to locations where there is a clear need for removal of valves or equipment, for access or maintenance, or for blinding. Because all connections, including flanges, are potential leak sources, their use should be kept to the minimum needed for safe and reasonably convenient operation and maintenance. Other than welding, the preferred means of connecting NPS 2 and larger piping in most services is the flange. Flanges are also an option with small piping. This is discussed in Section 340. Flanges should be used in place of threaded unions for small piping in systems subject to thermal shock and in steam services between 150 psig and about 600 psig. At 600 psig the use of flanges and other connections in all services is further limited because of the high potential for leaks. In LPG and hydrogen service flanges should also be used in small piping and have a minimum rating of Class 300. See Section 1100 for a detailed discussion. 200-46 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection Industry Standards ASME/ANSI B16.5, Pipe Flanges and Flanged Fittings. This standard covers the pressure-temperature ratings, materials, dimensions and testing for flanges in sizes NPS ½ to NPS 24. ANSE B16.5 flanges are divided into Classes 150, 300, 400, 600, 900, 1500, and 2500. The flange dimensions are standardized. Pressure-temperature ratings are given for the most commonly used materials. These ratings are applicable to the flanged joint if the gaskets and bolting specified in the standard are used. The standard also gives a method of pressure rating for materials not included in the tables. ASME/ANSI B31 also allows ratings for flanges to be calculated using the proce- dure in the ASME Boiler and Pressure Vessel Code, Section VIII, Division 1, Appendix 2. This calculation is now available as a PC program. For certain flange and gasket combinations the calculated ratings can be higher than the ratings tabu- lated in B16.5. Other Flange Standards. ASME/ANSI B16.5 only covers flanges up to NPS 24. ASME/ANSI B16.47 includes dimensional standards for flanges up to NPS 60. Temperature Limitations of Class 150 Flanges Current Company practice is to limit Class 150 flanges to a maximum temperature of 450°F because of the potential for leakage caused by warping and the subsequent fire hazard. Nevertheless, several Company installations have many years of good experience with Class 150 flanges at temperatures up to 700°F. Materials Recommendations Flanges are available in as many materials as is pipe. They are selected by type of connection to the pipe, flange face, and ASME/ANSI B16.5 Class designation. Although available in forged or cast steel, forged is specified for its greater reli- ability. ASME/ANSI B16.5 has four groups of materials within the general cate- gory of carbon steel (groups 1.1, 1.2, 1.3 and 1.4). The Company avoids using carbon steels from group 1.2 because they can produce excessively hard welds. 252 Flange Facing The common flange facings are flat (plain) face, raised face, and ring joint. Tongue- and-groove facings may still be found in older installations, and may still be used on some equipment, but are no longer used on piping flanges. Flat facing is normally used only on cast iron flanges or steel flanges that mate to cast iron flanges, and requires a full face gasket. Raised face is the most common flange facing. The ring joint flange facing is used for high pressure, high tempera- ture services such as steam and hydrogen above 600 psig, and hydrocarbons above Class 600. November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-47 200 Piping Component Selection Piping Manual Raised Face Flanges These flanges are used with both 1/16-inch composition gaskets and spiral-wound gaskets. When used with composition gaskets, the joint is considered unconfined. However, when used with spiral-wound gaskets with a centering ring, raised face flange connections are considered the equivalent of confined gasket connections in most services. With spiral-wound gaskets, the flange face must not be excessively rough or the gasket will not seat properly. For this application, flanges should comply with paragraph 6.4.4.3 of ASME/ANSI B16.5: “Either a serrated concentric or serrated spiral finish having a resultant surface finish from 125 μin. to 250 μin. average roughness shall be furnished. The cutting tool employed should have an approximate 0.06 in. or larger radius, and there should be from 45 grooves/in. through 55 grooves/in.” Composition gaskets can be used with flange faces rougher than 250-microinch, but it is usually impractical to segregate and control the use of multiple flange facing standards, both during construction and for maintenance. The aforementioned 125 to 250-microinch roughness standard can be used with both composition and spiral- wound gaskets. Flat Face Flanges Flat face cast iron flanges can break from the moment induced by bolting against a raised face. To avoid this problem, steel flat face (or plain face) flanges are speci- fied for flanging against Class 125 cast iron valves or equipment. Full face composi- tion gaskets are required. Class 125 flat face cast iron flanges are dimensionally compatible with Class 150 raised face steel flanges, (except for the raised face) and the two can be bolted together. Machining the raised face steel flange to a flat face is often more expe- dient than ordering a special flange. Cast iron flange ratings above Class 125 are seldom encountered. ORJ Flanges Octagonal or Oval Ring Joint (ORJ)—also called Ring Type Joint (RTJ)—flanges are more secure than raised face flanges with spiral-wound gaskets. These confined gasket flanges are specified for high-temperature/pressure and hazardous services. Company practice is to use ORJ flanges with all hydrogen systems and any hydro- carbon systems that contain appreciable amounts of hydrogen. They should be considered for all services with Class 900 flanges or higher. ORJ flanges are more expensive than raised face flanges, and each application should be individually reviewed. Factors to consider are the exacting fit-up require- ments and the additional flexibility of the connected piping needed for gasket installation and removal. The flange groove and ring gasket are also extremely susceptible to mechanical damage during handling and storing. A small scratch on either surface can result in a leak. Small scratches and nicks can be removed from the groove by lapping a ring into the groove. This is expensive, especially when done in the field. See also Section 280. 200-48 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection Tongue-and-Groove Flanges Once in common use by the Company in high-temperature/pressure services because of the confined gasket feature, T & G flanges are no longer specified. They are subject to corrosion in the groove and have the added problem of matching male and female flanges at each installation. 253 Flange Attachment to Pipe Flange attachment to pipe is by weld neck, socket weld, threaded, slip-on or lap joint stub. Figure 200-11 in Section 240 illustrates these types. 254 Commonly Used Flanges Weld neck flanges are preferred for most services because of the strength of its full penetration butt weld, ease of radiographing, and a hub that tapers from the pipe connection to the flange. It is generally purchased bored to the same ID as the matching pipe. If the inside diameters differ by more than 1/16-inch, back-beveling is required per Standard Drawing GC-L34496. The socket welding flange is commonly used for NPS ¾ to NPS 1½ pipe. To prevent cracking of the fillet weld it is important to provide a 1/16-inch gap between the end of the pipe and the bottom of the socket before starting the weld. See also Section 340 and 650. Socket welding may not be acceptable for services where crevice corrosion can develop or in oil systems where minute solid contaminants cannot be tolerated. The slip-on flange is acceptable for undemanding services. Slip-on flanges shall be fillet welded inside and out. They are less desirable than the weld neck flange because of the fillet weld attachment rather than a full penetration butt weld and radiography of fillet welds are not effective. Material cost is less than for a weld neck flange, but the need for fillet welds inside and out makes the installed cost about the same. Slip-on flanges are not acceptable for high pressure/temperature hydrocarbon service, hazardous services, H2 service or wet sour service where H2 can be generated as a corrosion product; the trapped space between welds is subject to H2 blister distortion. See the Corrosion Prevention and Metallurgy Manual. The threaded flange resembles a slip-on flange but has a threaded bore. It is seldom used. Although it can be fitted and installed in areas where welding is not allowed. Prefabricated spools welded elsewhere and bolted into place are preferred. Threaded flanges have low fatigue resistance and are suitable only for low pressure/tempera- ture, nonhazardous, nonvibrating service. The lap joint (Van Stone) flange uses a lap joint stub end on the pipe and a flange that rotates freely. This allows easy field alignment of the bolt holes, such as is required to accommodate rotation at tanks that are settling. An advantage in corro- sive chemical services is that while the pipe and the stub end may be a corrosion resistant alloy, the flange, which is not in contact with the fluid, can be lower-priced carbon steel. However, lap joint flange connections have a very low fatigue resis- tance and are not suitable for cyclic or vibrating services. November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-49 200 Piping Component Selection Piping Manual 255 Special Purpose Flanges Orifice Flanges Used to meter fluid flow. In larger sizes it is purchased as an assembly of flanges, bolts, nuts, and jack screws. Orifice flanges are available as weld neck, slip-on, and threaded; however, Company practice is to specify weld neck in sizes through NPS 10. The flanges must be bored to the same diameter as the connecting piping. Above NPS 10 regular flanges are specified with the orifice “throat” tap connec- tions mounted on the connected piping. The proper installation of orifice flanges is discussed in the Instrumentation and Control Manual. Reducing Flanges This flange (threaded or slip-on) reduces the line to a smaller pipe size. It can be used in place of a concentric reducer where: a size change is desired, flow resis- tance is not a problem, the line drain and flush requirements are considered, and a flanged connection is needed. Reducing flanges are rarely used, are difficult to obtain and are not normally recommended. Blind Flanges These flanges are used to close off vessel nozzles or the ends of piping where frequent cleaning or inspection is necessary or if the piping may be extended in the future. See “Blinds and Blanks”, below. 256 Flange Covers In hazardous chemical services (sulfuric acid, caustics, etc.) flange covers can be used to protect personnel from leakage. Milsheff plastic cloth covers, metal sheets, and plastic tape have been used. The need to periodically inspect for leaks and the ease of replacement after maintenance should be considered when choosing covers. 257 Blinds and Blanks A blind is flat flange with no hole through the center that bolts to the flanged end of a run of pipe or to a flanged equipment nozzle. Blinds are purchased ready-made for a particular pressure class. A blank is a circular metal plate bolted between two pipe flanges. Blanks The safest and most effective way to isolate a line or connected equipment is to install a plate blank. A plate blank should always be used in hydrocarbon and hazardous services where positive isolation must be guaranteed. Closed valves without a plate blank have the potential for leakage past the seat. One kind of blank, called a figure-eight (or spectacle) blank is useful where piping is blocked off frequently and the piping is not easily sprung apart or pulled together. One of the flat circles of plate that comprise this type of blank has a hole through the center; the other is solid. 200-50 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection Design Information for Blanks The Company has the following standard drawings for blanks: GC-L31452 Blanks for raised face flanges GF-L14298 Blanks for ring joint flanges In these installations jack screws may be needed to separate the flanges to change blanks, as shown on Standard Drawing GD-L1050. Dimensional information on ORJ and flat plate blanks is also provided by API Stan- dard 590, Steel Line Blanks. Included are material specifications and other informa- tion for direct purchase from vendors. Alternative to Blanks Acceptable substitutes for plate blanks and flanges in low pressure services are commercially manufactured three-bolt line blinds—sometimes called line blind valves. These are more expensive than plate blanks with flanges but are more quickly and easily operated. Because of their lower strength, these quick-acting blinds are not recommended for high pressures. They should be specified only where frequent swinging of blinds is necessary. Care should be taken in selecting line blinds, because they have O-ring seals; some models are not fire safe. Models with secondary metal-to-metal seating should be specified. Acceptable manufacturers include Hindle-Hamer and Greenwood (Vernon Tool Co.). 258 Thickness Calculation for Blanks The design formula for the required thickness of plate blanks is given in ASME/ANSI B31.3 paragraph 304.5.3 as: 3P 1 ⁄ 2 t = d g ⎛ -------------⎞ +c ⎝ 16SE⎠ (Eq. 200-9) where: dg = inside diameter of gasket, in. S = allowable stress, psi P = design gage pressure, psig c = corrosion allowance, in. E = quality factor = 1.0 for one-piece plate November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-51 200 Piping Component Selection Piping Manual 260 Nonflange Connections Where flanges are not suitable because they are too large, too heavy, or of inade- quate pressure rating, nonflange connectors are available. Several brands are discussed below. Be careful not to mix brands. 261 Grayloc Connector Initially developed for production drilling and wellhead service, Grayloc connec- tors provide a metal-to-metal seal with a steel ring clamped between two machined hubs. They are a good alternative to flanges in low temperature, high pressure, noncorrosive service. The seal rings have a coating of Teflon or molydisulphide to aid assembly. As with ORJ gaskets and flanges, alignment requirements are exacting; the rings and hubs are susceptible to mechanical damage, and seal ring insertion and removal require sufficient piping flexibility. The design and fabrica- tion of this connector is proprietary and not covered by industry piping codes. Grayloc connections generally are not economical below Class 900 in carbon steel, Class 600 in stainless steel, or in some smaller sizes. See Appendix B. They often have longer delivery times than high-pressure flanges. Company experience has been excellent up to 400°F in services above 1000 psig. One pilot plant has used Grayloc connectors up to 850°F and 3000 psig for 3 years with excellent results. Advantages of Grayloc connectors include: • Cost less than flanges in the higher pressures and larger sizes • Have less mass than equivalent flanges (This can be important in high pressure services near vibrating equipment such as compressors) • Require less space than flanges • Assemble more easily since there are no bolt holes to align • Can withstand greater pipe bending stresses than equivalent flanges See Appendix B for a discussion of field installation practices for these connectors. 262 Cameron, Securamax, G-CON, and OTECO Hub and Clamp Fittings These fittings are very similar to the Grayloc fitting. They have essentially the same features. The Company has less experience with these manufacturers. See Appendix B. 263 Victaulic Coupling The Victaulic coupling is a semiflexible, quick-connect, clamp-type coupling with a soft seal. It has a circumferential groove near the end of each pipe to be joined. The coupling consists of a split ring with rims at both ends that fit into the grooves and a resilient 200-52 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection cylindrical gasket that is compressed between the ring and the pipe ends to prevent leakage. The fit of the split ring is sloppy to permit a small amount of angular flexibility. The machined groove pipe end is preferred over the optional rolled groove. The groove must be considered when calculating allowable pressures and wall thicknesses. Suggested Applications Normally the Victaulic coupling is not suitable for hydrocarbon service. Typically it is used for low pressure/temperature and nonhazardous service in fire-safe areas. Used with aluminum or light steel pipe, it is convenient for quick installation of temporary piping where welding is not allowed. It is also useful in isolating vibra- tion and adjusting for minor piping misalignment, and is used extensively in commercial buildings on permanent utility water piping. 264 Dresser Coupling The Dresser coupling is a semiflexible, compression-type coupling with soft seals. It is used with plain-end pipe to remedy small initial misalignment and to provide a limited amount of angular and longitudinal flexibility. The flexibility is provided by resilient gaskets at each end of a ring which is centered over the pipe ends. Dresser couplings are suitable for low pressure/temperature service—typically cooling tower pump suction lines—where they provide a tight joint at moderate cost. Dresser couplings have no fire resistance and must not be used in hydrocarbon service. The Dresser coupling requires the addition of some means of restraining longitu- dinal pipe movement, because the joint is a slip fit and can separate. Dresser couplings must not be used in systems subject to hydraulic shock. 270 Valves Valve pressure classes and ratings are given in ASME/ANSI B16.34, while end-to- end dimensions are given in ASME/ANSI B16.10. This section of the manual lists items to consider when choosing the type of valve to use; describes the various valve types; discusses design choices for valve parts; lists valve operators; and recommends valves for specific applications. 271 Factors to Consider Valve selection is one of the most important phases of piping specification develop- ment. The primary considerations for selection of the proper valve are material selection, pressure class selection, and process function. Additionally, refer to Section 1400, “Reducing Fugitive Emissions from Valves”.” November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-53 200 Piping Component Selection Piping Manual Material Selection Material selection depends on the fluid handled by the valve and the operating temperature, and includes the material for the body, trim and packing. The trim includes the working parts of a valve and the seat rings. For more detail about trim see under the individual valve types, following. The Materials and Equipment Engineering Unit of CTRC has extensive files on material performances for most process applications and their recommendation should be sought for any new installation. Pressure Class Selection Pressure class selection is a part of piping specification development and is deter- mined by the design pressure and temperature. Flanged and butt welded valves are manufactured to pressure Classes 150, 300, 600, 900, 1500, and 2500. With the valve open, the body pressure-temperature ratings are the same as for the corresponding flange classes; however, some trim configura- tions may be rated lower than the body. Threaded and socket weld gate valves sized from NPS ¾ to NPS 1½ are manufac- tured to API Standard 602, Compact Carbon Steel Gate Valves. These compact valves are rated Class 800, and the pressure-temperature rating tables are published in API STD 602. Process Function Selection The process function of a valve can be on-off, throttling, or backflow prevention. Some small valves combine these functions. Valves for on-off operation do not control flow rate; they are either fully open or completely closed. Typical examples are suction and discharge valves on pumps, and valve manifolds. Operators usually refer to these valves as block valves. Any valve type can be used as a block valve except straight body globe valves, which are usually not considered suitable because of their high pressure drop. However, in high pressure, high temperature hydrocracking services Y-body globe valves are the preferred choice for block valves. Valves for throttling are used to manually control the flow rate. The most frequent application is the control valve bypass, which gives flow control when the control valve is removed for maintenance. Most control valves are throttling valves. A throttling valve is generally either a globe or a butterfly valve. In some cases it may be a plug or ball valve with a special port configuration for throttling. Check valves are used to prevent flow reversals in lines. Their most frequent use is on centrifugal pump discharge lines. Drain and vent valves are normally used during maintenance to vent, drain or steam out a line before it is opened for inspection. Drain and vent valve selection is the same as on-off valve selection. 200-54 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection Instrument connection root valves are the same as block valves, except usually limited to ¾- to 1½-inch size. Root valves are either gate valves or Y-body globe valves. Instrument valves and instruments downstream of root valves are consid- ered part of instrumentation. Selection Rules of Thumb Rules of thumb have limited validity in valve selections. Most simple rules have many exceptions and are valid only for a narrow size range. For example, “solid stainless steel wedge” is a sound specification for a carbon steel gate valve in sizes up to 2 inches or even 3 inches, but it is a waste for a 12-inch or larger valve. Only the seating area of the wedge has to have high corrosion resistance, and while it is simpler to make a small wedge from solid stainless it is much more economical to use a stainless-steel-overlayed carbon steel wedge in the larger sizes. The relative costs of different valves are changeable. Caution is called for because the cost of stainless or special alloy valves versus carbon steel can change quickly. Be sure your data are up-to-date. Worth noting is the cost of cast iron valves. Histor- ically, they cost about half as much as cast steel valves. Today (1989) the cost differential has shrunk to 5% or 10%, and there is little economic justification for using cast iron valves. 272 Types of Valves Gate Valves Gate valves generally impose a smaller pressure drop than other valves. When fully opened, they allow straight-through flow in a passage that is essentially the same diameter as the associated piping. See Figures 200-15 and 200-16. The plain solid wedge, inclined seat gate valve is the most common type. However, at high temper- atures and moment loads resulting from thermal expansion, this type of gate valve may experience galling of the solid wedge against its inclined seat. A flexible wedge tends to accommodate misalignment, minimize galling and improve sealing. Unfortunately, in dirty service the groove machined into the wedge to provide the flexibility may become plugged and lose effectiveness. Split wedge designs were also developed to accommodate seat misalignment, but the service life of hinged two-piece gates in dirty high temperature services is gener- ally unsatisfactory. The split wedge gate valve and through-conduit gate valves are designed to allow pigging operations in pipeline applications. November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-55 200 Piping Component Selection Piping Manual Fig. 200-15 Gate Valve, Typical Courtesy of Velan Valve Fig. 200-16 Gate Valve, Stem and Wedge Types Rising stem, Non-rising stem, Rising stem, outside screw and yoke inside screw Inside screw Courtesy of Velan Courtesy of Walworth Courtesy of Walworth Courtesy of Pacific Valves 200-56 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection Plug Valves The basic components of the plug valve are the body, plug and cover. The plug can be either cylindrical or tapered. In the open position, the bore of the plug connects the inlet and outlet ends of the valve, providing straight-line flow. Generally the opening in the plug is oval. Plug valves with round openings are also available but generally weigh more. An important feature of the plug valve is its suitability for multiport construction. Three- and four-way plug valves are widely used for flow switching in lube oil systems and in batch operations. In smaller sizes the plug is rotated by a lever operator attached to the stem. The lever position indicates whether the valve is open or closed. There are two types of plug valves: lubricated and nonlubricated. The lubricated valves have metal-to-metal-seats and provide a good seal. In the former type, lubri- cant is injected between the plug and the body and helps to lift the conical plug. The main problem with lubricated plug valves is “freezing” of the plug. If the plug is not rotated for a long time the lubricant washes out and corrosion products may freeze the plug into the body. Lubricated plug valves perform well in applications where the fluid handled has good lubricating properties. Plug valves should not be used above about 350°F without careful analysis of: • Temperature limits of lubricant or, in nonlubricated valves, seal material • Thermal expansion between valve and plug, which can freeze the plug Like ball valves, plug valves are not recommended for throttling service and are subject to port erosion while being operated. However, where quarter-turn valves are required in abrasive service, such as sandy crude oil, plug valves are preferred. Nonlubricated plug valves usually have an elastomer sleeve that eliminates the need to lubricate the plug. See Figure 200-17. Application is limited by the temperature limit and fire resistance of the sleeve material. Their principal use is in corrosive chemical service at ambient to moderate temperatures. Fig. 200-17 Nonlubricated Plug Valve Courtesy of Xomox Corporation November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-57 200 Piping Component Selection Piping Manual Figure 200-18 shows the TruSeal® plug valve, which incorporates features of the split wedge gate valve into a plug valve. Fig. 200-18 TruSeal® Double Block and Bleed Plug Valve Courtesy of Orbit Valve Company Ball Valves The flow control element in the ball valve is a sphere with a hole connecting the inlet and outlet ports. Sealing is normally accomplished with resilient elastomer seat rings; metal seat rings have been developed for use in high-temperature and/or abra- sive service. Two basic types of ball valve construction are available, floating ball and trunnion- mounted ball. Floating Ball Valves. The line pressure pushes a freely floating ball into the down stream seat. As the pressure increases, the effectiveness of the seal also increases; however, leaks are likely at low differential pressures. To effect a seal at low differ- ential pressures, the resilient elastomer seats are precompressed during assembly and require a higher torque to turn. Precompression may cause scoring of the resil- ient seats when abrasive particles are caught between the ball and the seat rings. The operating torque in floating ball valves increases with the size of the valve and with the pressure differential. See Figure 200-19. Ball valves are available in full port, reduced port and venturi port patterns. See Figure 200-19. Full port valves have the least pressure drop but the largest bodies and, consequently, the greatest weight. The reduced port has the highest pressure drop and least weight. The venturi port design lowers the pressure drop by incorpo- rating a gradual transition from the full pipe cross-section to the reduced port in the ball itself. 200-58 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection Fig. 200-19 Floating Ball Valve Configurations Courtesy of Marpac a. Full Port b. Reduced Port c. Venturi Port Body styles include top entry, split body, end entry, and completely welded. See Figure 200-20. Top entry is much like opening a bonnet on a gate valve, and is the most convenient for maintenance because the ball can be pulled out without disturbing the piping. Top entry ball valves are usually available in smaller sizes. End entry ball valves must be pulled from the line for disassembly. Welded body valves are usually repairable only by factory maintenance services. The advantage of the welded body style is reduced weight, especially in higher pressure classes. Ball valves compare favorably against gate valves in weight and dimensions. Offshore production platforms particularly favor ball valves. Figure 200-21 shows a metal seated floating ball valve that has an integral metal seat with a spring loaded washer/guide that forces the ball against the integral seat. The preferred flow direction is to have the pressure pushing the ball against the inte- gral seat. Trunnion-Mounted Ball Valves. The position of the ball is fixed by top and bottom trunnions mounted in bearings. The seats are moved against the ball by line pres- sure. Each seat moves independently, and most designs incorporate spring loading to effect sealing at low differential pressures. Additionally, the seat ring periphery requires sealing against the body, usually with elastomer O-rings. See Figure 200-22. November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-59 200 Piping Component Selection Piping Manual Fig. 200-20 Ball Valve Body Styles Courtesy of Marpac Valve and Cooper Cameron a. Three Piece b. End Entry c. Top Entry d. Welded body ball valve (not for refinery use) Figure 200-23 shows the Orbit ball valve, which is a rising stem ball valve with a single seat ring. The stem both rises and rotates. The rotation is effected by a stationary stem guide, located on the bonnet extension, which engages a spiral slot cut into the stem. During an opening cycle the stem first moves straight up and simultaneously backs the ball away from the resilient seat ring. The stem then rotates 90 degrees to open the flow path through the valve. The ball has no contact with the seat ring during the rotation, which saves the seat ring from scuffs. Seat selection includes the traditional bubble tight Teflon TFE seal with a secondary metal seat (Type H) for temperatures below 450°F or an all-metal stainless steel seat (Type H8) for temperatures above 450°F. Orbit valves should be used in liquid, gas or vapor services that do not contain grit or fines and they should be installed with the pressure pushing the ball into the seat. 200-60 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection Fig. 200-21 Metal Seated Ball Valve Courtesy of Valvtron Ball Valve with Integral Seat and Spring Loaded Washer/Guide Globe Valves Globe valves generally have a tortuous flow path, often with two 90-degree changes in the direction of flow inside the valve body. See Figure 200-24. Most designs are unidirectional, with the flow brought in under the disc. The flow control characteris- tics of globe valves are determined by the shape of the disc or plug. A flat disc provides quick opening, while a needle-type plug provides a very gradual opening of the flow path. Pressure drop in globe valves is usually high. The pressure loss can be reduced by Y-pattern or angle-pattern bodies. Globe valves are used where throttling of the flow is required or as block valves in high temperature, high pressure services. The disc of a globe valve is pushed onto the seat ring perpendicularly and so has freedom of angular adjustment. It will not bind even under high moment loads. However, globe valves are heavier and more expensive than gate valves. November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-61 200 Piping Component Selection Piping Manual Fig. 200-22 Trunnion Mounted Ball Valve Courtesy of Cooper Cameron Valves, a Division of Cooper Cameron Corporation Butterfly Valves There are three types of butterfly valves: a rubber-lined valve in which the center- line of the stem runs along a radius of the disk, a high performance valve in which the disc is offset from the centerline and has a spherical seating surface, and a metal seated triple offset valve in which the center of rotation is double offset and there is also an offset cone shape to the seating surface. Rubber-Lined Butterfly Valves. Rubber-lined butterfly valves are typically made of a cast iron body with a heavy rubber liner which overlaps the flanges and serves as a gasket. This type of butterfly valve is low priced but can only meet the require- ments for cooling water service. See Figure 200-25. High-Performance Butterfly Valves. High performance butterfly valves have an elastomer seat ring - backed up with a secondary metal seat mounted on the body. The disc is attached to the stem in a double offset position. One offset is by moving the stem from the centerline to the side and the other is to move away from the plane of the disc. High performance butterfly valves are no longer recommended because of the temperature limit of elastomers in hydrocarbon service and metal fatigue of the thin back-up seat. If stress corrosion is present it will reduce the cycle life of these valves. When reconditioning of an existing high performance butterfly valve becomes necessary, replacement with a metal seated triple offset design is recommended. 200-62 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection Fig. 200-23 Orbit Ball Valve and Seat Selection Courtesy of Orbit Valve Company November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-63 200 Piping Component Selection Piping Manual Fig. 200-24 Globe Valves a. Plug Type b. Y-Pattern Courtesy of Pacific Valves Courtesy of Edward Valves, Inc. Metal Seated Triple Offset Butterfly Valves. These valves are a later develop- ment, but by now are well proven and in use at the refineries. The distinguishing feature of these valves, now made by several manufacturers, is the offset cone- shaped seating surface. The placement of the center of rotation coupled with the shape of the seat results in a disc being lifted off the seating surface in just a few degrees of rotation. Contact between the disc and the body seat is only in the fully closed position. There is no friction or wear between the disc and seat which results in minimum leakage rates. See Figure 200-25. Designs vary somewhat from one manufacturer to another but the body seat style most commonly used is solid metal. The body seat can be a hard metal overlay, a pressed-in seat ring or a bolted-in seat ring. The mating seat, bolted to the disc, is a flexible laminated design built of several layers of stainless steel with graphite sand- wiched in between the layers. There are also designs where the disc is solid metal and the flexible laminated seat ring is bolted into the body. Metal seated butterfly valves are available in a variety of face-to-face dimensions. The triple offset metal seated butterfly valves are superior to elastomer seated fire safe high performance butterfly valves. Any refinery service previously utilizing a high performance butterfly valve with an elastomer seat can be more efficiently handled with the metal seated triple offset design. 200-64 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection Fig. 200-25 Metal-Seated Triple Offset Butterfly Valve Courtesy of Orbit Valve International Incorporated TRIPLE AXIS DESIGN Single axis offset moves the shaft Double axis offset moves the disc off the sealing edge allowing pivot-point off the valve center- complete, uninterrupted seating line, producing cam-closure for contact around the entire rim. positive torque seating. Triple axis offset moves the seat cone angle off-axis, to achieve compressive sealing. FRICTION FREE SEALING Disc and seat surfaces meet at a chosen contact angle for frictionless compression. All points of the seal ring contact the seat at the same instant and rotation stops. The seat contour ensures compres- sive sealing around the entire seal ring. Further torque applied to the valve shaft loads the seal ring equally at all points, with radial force, to assure zero leakage. November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-65 200 Piping Component Selection Piping Manual Check Valves Check valves are designed to prevent reversal of flow in lines. The swing check valve is the most commonly used. It can be installed in lines with horizontal or upward vertical flow. It is usually used with low fluid velocities where flow reversals are infrequent. Pressure drop is moderate. The lift check valve comes in two types: lift disc and ball check. When flow reverses, the disk or ball is reseated by gravity. The gravity action means these valves must be installed in correct orientation. The construction resembles that of a globe valve and the pressure drop is higher than in swing check designs. These valves can handle frequent liquid flow reversals. Piston check valves are generally used in gas service such as compressor discharge. The downstream side of the valve is connected back to the space above the piston and the pressure increase helps to close the valve. Most piston check valves can be spring-assisted. They are designed to handle frequent gas flow reversals. Pressure drop is comparable to the pressure drop of globe valves. Tilting-disc check valves are similar to swing checks. However, the disc is lighter weight and is pivoted close to the center of gravity. It takes a lower pressure differ- ential to open the disc, and it reacts more quickly to flow reversals and minimizes slamming. The main use is in gas flow lines. Split-disc or butterfly check valves consist of two semicircular discs mounted on a common hinge pin, with springs to help close the discs. The body design is usually a wafer style. Manufacturers claim that it is suitable for operation even with vertical downflow. In larger sizes the pressure drop is low; in smaller sizes the butterfly obstructs a larger portion of the flow area. Stop check valves combine the function of a block valve and a check valve. The construction is similar to the lift check, with a stem added above the disk allowing it to be held down onto the seat. When the stem is lifted the valve functions as a check valve. With the stem lowered, the valve functions as a globe valve. It is mainly used on boiler steam outlet lines. Figure 200-26 shows five types of check valves. 200-66 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection Fig. 200-26 Check Valve Types Courtesy of Edward Valves, Inc. a. Swing b. Lift Disc c. Lift Ball d. Tilting Disc e. Stop Check November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-67 200 Piping Component Selection Piping Manual 273 Specifying Valve Parts Fire Safety Because they require moveable parts, packing, flanges (in most cases), and human operator involvement, valves can almost be considered “equipment” from the fire safety standpoint. Fire safety in designs and practices is discussed in detail in the Fire Protection Manual. However, general requirements regarding valve selection are outlined here. The Company requires that elastomer seated ball, plug, and butterfly valves for hydrocarbon service pass the API 607 or API 6FA fire tests. Gate and globe valves are normally not tested for fire resistance because they have metal-to-metal seats. Soft-seated valves in hydrocarbon service must be fire-resistant, with metal-to-metal backup seating to limit fluid leakage when the seat or stem sealing material is lost in a fire. See this section under Valve Packing and Seals. Double block-and-bleed valves in liquid service should have relief valves or drains to relieve pressure from the body cavity. See Section 276, under Tight Shutoff Valves. Steel valves are used on tanks and vessels, in areas exposed to mechanical hazards, and in lines carrying petroleum products, especially in or near operating units or valuable equipment. Cast iron valves should be limited to brine and low-pressure, noncritical utilities. Steel valves are used for utility lines in areas where failure during a fire would impede firefighting. Spring-actuated, self-closing (deadman) valves should be considered in congested areas where failure to close a valve could result in danger to personnel or permit flow of oil or gas into hazardous areas. Examples are chemical draws, vents, bleeders, drains, sample draws, level cocks, and tank truck and barrel filling operations. Thermal-closing valves actuated by loss of a fusible link, and remotely operated valves are justified in areas that would be inaccessible for manual closing of critical valves once a fire had started. Examples are LPG storage tanks, bulk storage plants contained within tank yard walls, and confined storage areas near high value prop- erty. Thermal valves are described in detail in the Tank Manual. Seat Selection Valves can be divided into two groups according to their seating surface construc- tion: metal-seated valves and elastomer-seated valves. The fundamental difference is that elastomer-seated valves have a limited operating temperature determined by the elastomer used. Elastomer seats are subject to destruction if the valve is exposed to a fire. The traditional metal-seated valves are: • Gate valves • Plug valves (lubricated) • Globe valves • Check valves 200-68 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection The traditional elastomer-seated valves are: • Ball valves • Plug valves (nonlubricated) • Butterfly valves Special metal-seated valves are: • Ball valves • Butterfly valves All metal-seated valve types can also be made with elastomer inserts for so-called bubble-tight shutoff. Ball valves and butterfly valves also can be made with all metal seats, but these are special, relatively expensive valves. Some elastomer-seated valves are available with firesafe construction. In these valves the seat is constructed so that when the primary elastomer seat is destroyed the ball or butterfly is displaced by line pressure and brought in contact with a secondary metal seat. Although the shutoff may no longer be bubble-tight, the through-leakage is slight and there is no leak to the outside. In oil refineries metal-seated valves are generally suitable for all services, while elastomer-seated valves are suitable only for selected nonflammable fluid services and lower operating temperatures. In oilfield production applications, operating temperatures are generally lower, and ball valves are much more common than in refineries. Their lower weight, lower cost and quarter-turn operation (with the valve handle serving as indicator) account for the attractiveness of ball and butterfly valves, but only within certain size limits. In larger sizes gear operators are required, and the quick quarter-turn operation is lost. This, however, turns out to be for the best because in larger sizes a quick valve shutoff could induce a pressure surge (water hammer). Choosing Bonnet Design Not all valves have bonnets. Both gate and globe valves do, and some people call the top closure of a check valve a “bonnet”. The bonnet designs for gate and globe valves and for check valve top closures are: (1) union, (2) screwed, (3) screwed and seal welded, (4) welded, (5) bolted, and (6) pressure seal. The choice depends on the service and on the size and pressure rating of the valve. For example, only the latter 3 designs are acceptable for steel valves in hydrocarbon service. Choice of bonnet design is discussed below in three ways: by service, by valve type, and by bonnet type. Low Pressure Utility Service (NPS ½ to 1½). Screwed and union bonnet designs are compact and satisfactory for low pressure utility services, water, utility and instrument air. The size range is ½ inch to 1½ inch. Hydrocarbon Service (NPS ½ to 1½). Small valves for hydrocarbon and process fluid services require screwed and seal-welded, bolted, or welded bonnet designs. The bolted design increases valve weight but provides access for seat maintenance. November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-69 200 Piping Component Selection Piping Manual The welded bonnet-type valve is lightweight. However, except for replacement of packing, welded bonnet valves cannot be easily repaired. They are useful in vibrating services where low mass at root valve connections is desired. The screwed and seal-welded valve is the middle ground, offering high-pressure leak-free perfor- mance at moderate weight and cost. Any Service (NPS 2). The 2-inch gate and globe valves are generally of the bolted bonnet design. Any Service (≥NPS 3). Gate and globe valves 3 inches and larger are either bolted bonnet or pressure seal bonnet design. The pressure seal bonnet reduces the weight and is predominantly used in larger sizes and higher pressure classes. Pressure seal designs vary from manufacturer to manufacturer. The Company requires a design with a segmented locking ring, which assures that the cover can not pull out of the body. Globe and Gate Valves. Regardless of bonnet type, steel gate and globe valves should be specified as outside-screw-and-yoke (OS&Y). This preferred design isolates the stem threads from the fluid and reduces galling and thread corrosion. The rising stem also indicates the valve position. Check Valves. NPS 2 and larger steel check valves are acceptable with bolted or pressure seal bonnet. Steel NPS 1½ and smaller check valves are acceptable with bolted or screwed bonnet with seal weld. Bonnet Types. Welded bonnets are normally used on NPS 1½ and smaller valves. Except for replacement of packing, welded bonnet valves cannot be easily repaired. They are useful in vibrating services where low mass at root valve connections is desired. Bolted bonnets are available in all valve sizes and are the most common with NPS 2 and larger. The bonnet is flanged and should be specified with a confined gasket joint. Pressure seal bonnets are used in Class 600 or higher valves. The adequacy of the bonnet retention design should be reviewed. Pressure seal bonnets with secondary mechanical retention features are recommended; thermal distortion of the valve body during fire exposure can result in loss of the bonnet seal. Screwed-in bonnets are potentially dangerous because the bonnet can work loose in vibrating service or normal repetitive valve operation. Screwed-in bonnets are acceptable when the body-to-bonnet joint has been seal welded. High-pressure instrument valves (NPS ¾ and smaller) with union bonnets are acceptable only with a locking pin, strap, or tack weld that prevents the loosening of the bonnet. Union bonnets on block valves are acceptable only on brass or bronze valves in low pressure utility service such as air and water. They are less expensive with the inside screw pattern; however, the rising stem version indicates valve posi- tion and is preferred. 200-70 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection Stem Retention The valve stem/shaft must be designed to be blowout-proof. Retention by packing friction alone is not an acceptable design. A positive mechanical retention feature is necessary and the design shall be such that removal of the stem seal retainer (e.g. gland) alone will not allow the stem to be removed. Additionally, the stem/shaft shall be designed so that if failure of the stem/shaft-to-disc connection or internal failure of the stem/shaft occurs, no portion of the stem/shaft can be ejected from the valve as a result of internal pressure. Port Size Port size can be full port, reduced port, or Venturi port. Reduced port is some- times called “regular” or “conventional” port. Reduced ports are usually 0.7 times the diameter of the full port. Venturi port, usually offered in ball valves, is 0.5 times the diameter of the full port. The reduction in port size generally reduces the weight and cost of the valve, and reduced port valves are often the economic choice. Full port valves are required in some applications, for example, at hot taps or in pipeline applications where pigging of the lines is required. Port size was illustrated in Figure 200-19. Valve Materials Valve Body Materials. For valves with bonnets, bonnet material is the same as the valve body material. Carbon steel is the most common valve body material for valves in hydrocarbon and critical services. Experience with both forged and cast steel valves has generally been acceptable. Cast iron materials are less expensive but should be limited to saltwater and offplot utilities because they can fracture from overbolting or line expansion stress, and can crack in a fire, especially if struck with water while hot. Repair by welding is not practical. Malleable cast iron and ductile iron (also called nodular cast iron) are preferred over gray cast iron (ASTM A126) because they will tolerate some bending loads. However, their cracking resistance in a fire may be no better than regular cast iron. In the U.S.A., OSHA places limitations on valves carrying flammable or combus- tible liquids (See Section 100). Brass and bronze valve bodies are suitable for water and low pressure air. They have good corrosion resistance with water and brines. However, these materials melt at low temperatures and should not be used in hydrocarbon service. They should not be used at locations with atmospheres corrosive to copper (i.e., containing H2S or NH3). Stainless steel and chrome alloys are rarely justified except for serious corrosion problems or extreme temperatures. Stainless valves should be purchased solution- annealed and pickled to provide the best corrosion resistant properties. Section 200 gives some guidance on stainless steel and alloy applications. The Materials Unit of ETC should be consulted for specific applications. November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-71 200 Piping Component Selection Piping Manual Valve Trim. Trim includes stems, seats, discs, yoke bushings, and other internal parts. Stems, Seats, and Discs. Hardened 12% chromium steel is the most widely used valve trim material for general services. Stellite trim is available at additional cost and performs better with steam than 12% chrome. Stellite resists wire drawing, increasing valve life, and is specified for steam above 200 psig. Small socket welded and screwed valves can sometimes be obtained with Stellite seat rings at little or no additional cost. Precipitation-hardened stainless steels, such as 17-4PH and 17-7PH, are used for stem materials by some manufacturers. Use of these materials with aqueous chlo- rides or sulfides should be carefully reviewed because of their susceptibility to cracking failure. Material selection for sour service is discussed later in this section. Yoke Bushings. Steel valves in hydrocarbon service should have yoke bushings, sleeves, drive nuts, sleeve nuts, and gland followers of material with a melting point of 1750°F or higher for fire resistance. This precludes most bronze or brass mate- rials normally supplied as manufacturers’ standards. Valve Packing and Seals A brief discussion of packing and seals follows. Fugitive Emissions. These emissions include leaks from valves, especially around the stem packing. Some jurisdictions are beginning to control fugitive emissions. Some vendors claim to have special graphite packings that reduce emissions, but the benefits are unproven. The Company believes that use of good packing, in accor- dance with Standard Drawing GC-L99771, and adequate periodic maintenance will yield satisfactory results in almost all services. For services where fugitive emis- sions cannot be tolerated, special packing or bellows seal valves may be considered. Packing. Steel valves should be specified with a packing good for any service for which the valve might be installed. For most applications flexible graphite with anti- extrusion-end rings of braided graphite fiber is recommended. Teflon packing should not be used for general purpose applications in hydrocarbon service because it will be lost in a fire. Standard Drawing GC-L99771 provides guidance for selection of valve stem packing for hydrocarbons and most other services. Asbestos has been almost completely phased out by U.S. manufacturers. It has limited availability but is still the first choice for phosphoric and sulfuric acids. Fluorocarbon Seals. Teflon is a fluorocarbon plastic. It is limited to 450°F or lower, depending on the grade and valve design. Unconfined Teflon can “cold flow” and should be limited to maximum temperatures of 250°F to 350°F, depending on valve design. Teflon is chemically resistant to almost all common services. However, Teflon seals are not as resilient as the elastomers and are subject to cold flow. Teflon should be limited to applications where its superior chemical resistance or low friction is 200-72 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection required. In flammable fluid service, fire resistant design with metal-to-metal backup seating is required. Elastomer Seals. Most elastomer seals are manufactured in the “O” ring form. • Viton is a synthetic rubber limited to about 350°F. It is used mainly for its proven resistance to aromatic hydrocarbons. Viton should not be used with ammonia or amines. • Nitrile (Buna-N) and Neoprene are common elastomer materials used mainly for water and hydrocarbon services low in aromatics. These materials are limited to maximum temperatures of about 200°F to 300°F, depending upon the specific service. Seal material must suit the service fluid. If it does not, the material can swell, reducing strength and life of the seal. For additional guidance on the selection and limitations of elastomers and fluorocarbons consult the Materials and Equipment Engineering Unit of ETC. Valve End Connections Following is a brief discussion of various types of end connections. Flange Connections. There are as many types of flanged valve connections as there are flange faces: flat face with cast iron valves, raised face, and ORJ (ring joint) with steel and alloy valves. See also Section 250. Valves of sizes NPS 2 and larger should normally be flanged. Butt Weld Connections. Normally used on valves of ANSI Class 600 and higher to minimize the number of connections that can leak. Examples are hydrogen service and high pressure steam. The valve construction should allow in-line repairs by removing the bonnet. The bore of butt weld valves should match the inside diameter of the connecting pipe. If postweld heat treating of the pipe-to-valve weld is necessary the vendor should be consulted regarding temperatures and technique. During welding and heat-treating some disassembly of the valve may be required to prevent destruction of elastomer seal materials or warping of metal seals. Threaded Connections. Typically used with NPS 1½ and smaller valves in low pressure service although NPS 2 sizes are commonly used on producing wellheads. In vibrating service steel valves should be seal- or bridge-welded to eliminate the notch effect of the pipe threads. Valve bodies must be specified with weldable body material. Free machining leaded steels are not acceptable. See Section 340 for small piping design. Socket Weld Connections. An alternative to screwed valves. Eliminates the need to thread pipe. Recommended for high pressure service. Socket welding small valves at times requires dismantling the valve or other precautions to protect the seats. Refer to Section 340 for small piping design. If postweld heat treating of the pipe-to-socket weld is necessary, the valves should be furnished with 6-inch stubs welded into the sockets and heat treated by the manu- facturer prior to finish machining. November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-73 200 Piping Component Selection Piping Manual Combination Connections. API 602 Standard Extended-Body valves with an inte- gral nipple on one end and female connection on the other (threaded or socket welded) saves one field weld. The additional body length must be considered for drain valve installations. Wafer or Wafer-Lug Connections. Primarily used with butterfly, check or slide- gate valves for sandwiching between flanges. The wafer-lug valve can be furnished with tapped holes that allow pipe removal from one side of the valve while containing pressure on the other. 274 Valve Operators Following is a discussion of manual gear, chain wheel, electric motor, and pneu- matic and hydraulic operators. Manual Gear Operators Large valves and valves with high pressure differentials across them are normally supplied with a geared operator. Several options are available for configuration of the hand-wheel with respect to the valve body. Recommended requirements for operators follow. In doubtful situations the manufacturer’s guidelines should be used: • Gate and globe valves NPS 12 and larger, ANSI Class 125 through 300 • Gate and globe valves NPS 6 and larger, ANSI Class 400 and higher • Quarter-turn valves NPS 8 and larger, ANSI Class 150 through 400; NPS 6 and larger, ANSI Class 600 and 900; NPS 4 and larger, ANSI Class 1500, and NPS 3 and larger, ANSI Class 2500 • Torque— above 70 foot-pounds, an operator of some sort is generally required. Chain Wheel Operators Flanged gate and globe valves located overhead and out of reach can be mounted horizontally and furnished with a chain wheel operator. These are generally a nuisance and sometimes a hazard and should be considered only when operation is infrequent and the valve cannot be relocated. Electric Motor Operators Very large valves that require an excessive number of turns to open or close are often equipped with a motor driven gear and are called motor operated valves (MOVs). Typical applications for MOVs of all sizes are tank field operations and emergency shutdown where the valves are operated from a remote station. Pneumatic and Hydraulic Operators Quarter-turn ball or plug valves can be equipped with remotely actuated pneumatic operators for on-off operation. These are typical in emergency shutdown systems in self-contained production facilities. Similar pressure actuated hydraulic operators are common at wellheads both onshore and offshore. These systems are discussed in Sections 700 and 800 of this manual and the Instrumentation and Control Manual. 200-74 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection 275 Valves for General Service Gate Valves Gate valves are the workhorse of the industry. Except where ball or plug valves are used for frequent or rapid operation, they are preferred for general use because they are rugged, simple, self-draining, and have low pressure-drop characteristics. Gate valves are not suitable for throttling, and should be used only for on-off applica- tions. If kept partially open, the bottom of the gate becomes eroded. Gate valves are available with solid, flexible solid, and split wedge gate (disc) designs. Although the split wedge design can better accommodate thermal distor- tion, the plain solid or flexible solid wedge is preferred. The split wedge is suscep- tible to additional wear and corrosion, and the discs tend to foul and plug in dirty service. An acceptable expanding wedge design is discussed in Section 276 under Tight Shutoff Valves. Gate valves subjected to large pressure differentials when closed can be supplied with a valved NPS 1 bypass line around the gate. This can be welded into the body of the valve by the manufacturer, if specified. It allows partial pressure equalization prior to opening the valve and, in the case of high temperature service, preheating of the low pressure side. Bypass lines are typical in NPS 6 and larger valves in 600 psig and higher steam service. Full port configuration is required when (1) pressure loss is important, (2) complete line drainage is required, or (3) hot tapping is practiced. Most gate valves NPS 6 and larger are manufactured as full port valves. Reduced port valves are typically available in NPS 4 and smaller. Their use is urged for general applications since they cost about 30% less than full port valves. Although most full port small valves provide greater corrosion allowance and a deeper stuffing box, the additional cost is rarely justified unless: • A full port is needed for rodding-out in lines prone to scale buildup or plugging • Higher-than-normal corrosion is anticipated • A full port is needed for hot tapping or for inserting probes such as corrosion probes The Corporation Piping Specification shows reduced port gate valves for all NPS 1½ and smaller sizes, but the engineer can substitute full port valves if required for a particular service. Ball Valves Ball valves are typified by quick and easy quarter-turn operation and tight shutoff. They are favored in producing operations where frequent valve operation and ease of actuation are necessary. In larger sizes, ball valves are generally more expensive than other valves. Ball valves are not recommended for throttling service. Ball valves are manufactured in two basic configurations: floating ball and trunnion- mounted ball. The floating ball type moves to the seat, and in the trunnion-mounted type, the seats move to the ball. The ball in both types of valves is normally held between soft seats. However, the trunnion-mounted ball requires less torque to turn November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-75 200 Piping Component Selection Piping Manual at high pressures. Both valves can be provided with resilient seats and a metal-to- metal backup seat in a fire-safe design. Some acceptable manufacturers are Cooper- WKM, Dresser-TK, Neles-Jamesbury, and Cooper-Orbit. The maximum service temperature is usually limited by the elastomeric or fluoro- carbon seat material to between 250°F and 450°F. Ball valves with butt welded ends should receive special attention to be certain that welding heat does not damage the seats. One way is to have nipples welded to the end connections at the factory before the seats are installed. Another approach is to use low heat-input welding for the field welds such as short-arc MIG welding. See the Welding Manual. Bronze ball valves with double seats of elastomer or fluorocarbon materials may be considered for water and air services where Class 125 or Class 150 brass gate valves are customarily used. Carbon steel ball valves have also been used in dry air or gas piping such as around furnaces. Fire-safe ball valves provide good shutoff even after fire destroys the primary elas- tomeric seat because they have a secondary metal retaining seat ring that is machined to the contour of the ball. With the floating ball design the ball-to-stem connection is slotted, and after the destruction of the primary seal, line pressure forces the ball into the closed position against the metal retaining seat ring. The blow-out proof stem design and packing selection should prevent stem leakage even after fire exposure. Approved fire-safe ball valve designs normally have stainless steel balls and trim as a minimum requirement. This requirement evolved because of several failures caused by holidays or leafing of the coating or plating on carbon steel balls. Cooper-Orbit Company trunnion ball valves with a rising stem are widely accepted and are available with either a full or reduced port design. They use a ball that is forced by the camming action of the stem against a TFE seal confined in a metal seat ring. In addition to fire resistance, these valves have remained tight and oper- able in high-pressure service over long periods of time. Satisfactory experience includes LPG, hydrocarbon, hydrogen, and ammonia services. They are generally more expensive than other valves. Cooper-Orbit valves are limited to 450°F by the Teflon (TFE) seal. Although the valve seals in both directions, for the highest reliability it should be installed in the preferred direction as indicated on the valve body. Cooper-Orbit valves are also available with an all-metal seat and special high-temperature packing for service temperatures above 450°F. Chevron has some experience with their all-metal seat. Globe Valves Globe valves are preferred for throttling, flow-control services, and tight shut-off. The circular configuration of plug and seat allows more precise fabrication and a tighter seal than the gate valve. Globe valves are also discussed later in this section under Tight Shut-off Valves. Pressure drop through globe valves is much higher than through gate valves. Although most globe valves are vertical or upright stem, the Y-pattern is selected 200-76 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection when reducing pressure loss is important. Angle valves, with piping connections at right angles, incur less pressure loss. However, they are subjected to the same stresses as piping elbows and are seldom used in processes. One common applica- tion, though, is bronze angle valves with composition disks at fire hose stations, where repeated tight closure is required and infrequent use is expected. Check Valves Check valves are used to prevent reversal of flow and are available in several configurations, with lift check and swing check the most common. Check valves should never be relied on for tight shutoff under pressure, regardless of vendors’ claims (see the following discussion of stop check valves below). Metal-to-metal seats or soft seats with metal-to-metal backup should always be specified in hydro- carbon service. Lift Checks. Most often these valves are used for NPS 1½ pipe and smaller with threaded or socket weld ends. They have a ball or guided piston plug that is lifted by vertical fluid flow and reseated by reverse flow and gravity. They are available in a Y-pattern for reduced pressure drop. Lift checks tend not to slam shut and are good in applications with irregular flow or frequent reversals of flow. They should be used in horizontal lines only. The Y-pattern can be considered in vertical lines with upward flow. Swing Checks. These valves are more frequently used. They have less resistance to flow and are normally used in NPS 2 and larger pipe. They tend to slam, and frequent flow reversals can produce “chatter,” though the tilting disc version is slightly less susceptible in this regard. Swing checks cannot operate in vertical lines with downward flow. Swing Checks with Snubbers, Dashpots or Counterweights. In some services, slamming or flow reversal can be a significant problem; such as at the discharge of large cooling water pumps when they are shut down. Very high forces are involved and the valves can be damaged unless the action of the disc can be controlled. This can be done by installing valves with external pneumatic snubbers, dashpots or counterweights connected to the extended shaft. Positive retention of the extended shaft is required. This is accomplished by a step change in the diameter of the shaft and designing it so that removal of the stem seal (e.g., gland) retainer alone will not allow the shaft to be removed. Shaft retention by dowel pins between the clapper and the extended shaft is not acceptable. Stop Checks. These valves are a combination of a lift check valve and block valve. The stem can be used to hold the disc closed, ensuring tight shutoff. They are gener- ally used on boiler outlet steam lines when boilers operate in parallel to guarantee boiler isolation on shutdowns, and in similar applications in high pressure process plants. Normally specified in the Y-pattern, they are used in horizontal lines or vertical lines with upward flow. Their dual function reduces valve costs in high pressure services. Dual Disc Checks. These valves are normally of the wafer type installed between matching flanges. The spring-actuated closing feature makes them suitable for use November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-77 200 Piping Component Selection Piping Manual in any position. They are not as reliable as swing or lift checks because of the possi- bility of spring failure, and are not recommended for pulsating flow or in corrosive or critical service. Care must always be taken in spring material selection. In hori- zontal lines the valves should be installed with the hinge pin in the vertical position. Although available with metal-to-metal seats, they are normally specified with an elastomer liner that also acts as the gasket against the flanges. See the discussion of butterfly valves, following in this section, for the limitations of wafer-type valves. Plug Valves Like ball valves, plug valves should be used where rapid operation and (for lubri- cated valves) tight shutoff are required. Lubricated Plug Valves. These valves have a special fitting that allows lubricant to be pumped into the valve between the body and the plug. The lubricant also improves the seal. A systematic lubrication program is essential. The lubricated plug valve has been used extensively in NPS 6 and smaller sizes in marketing plants and terminals because of its rapid operation, positive shutoff, and ready indication of valve posi- tion. However, sticking problems with valves that were not regularly lubricated has led to replacement with ball valves in many locations. Nonlubricated Plug Valves. These valves have a solid metal plug that rides against a low-friction seating face of Teflon or an elastomer for easy valve operation. Nonlubricated plug valves are especially attractive where a plug valve is desirable but a systematic lubrication program cannot be maintained. The seating material normally limits the operating temperature to 350°F or less. Nonlubricated plug valves for corrosive services are fully lined, and the fluid has no contact with the metal parts. These valves can leak if the liner is destroyed by fire and are not recommended for flammable fluids. Eccentric Plug Valves. These valves, such as DeZurik rubber-lined plug valves, provide reliable service in utility air as a final valve on tool air connections, and in saltwater service. They hold against pressure only in one direction and are not recommended for vacuum service or flammable fluids. Butterfly Valves There are three types of butterfly valves: a rubber-lined valve, a high performance fire-safe valve, and a metal seated triple offset valve. Rubber-lined Butterfly Valves. These valves with wafer or wafer-lug bodies are less expensive than gate valves and are frequently used in fresh water, saltwater, and (in the past) carefully selected locations in hydrocarbon services. They are also used to throttle flow where pressure drop is not excessive. Elastomer-lined butterfly valves are available as flanged valves or wafer valves mounted between matching flanges. The elastomer seat or liner material limits the valve operating temperature to 350°F or less and can allow substantial flange and seat leakage in case of fire. 200-78 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection Wafer valves are available in three styles: plain wafer, wafer-lug with holes to pass the flange stud bolts, and wafer-lug with tapped holes. They are less expensive than flanged valves. Wafer valves also have the following limitations: • The long bolts used with plain wafer valves will expand when exposed to flame and allow leakage. Wafer valves are not recommended for critical hydrocarbon service. In noncritical hydrocarbon services, the use of lug-type bodies or the installation of fire shields is required. See Standard Drawing GB-L1110. • Wafer valves require room in the piping on either side to allow proper opera- tion. If blinding is expected at wafer valves, tapped lugs with capscrews are required • Overbolting can be a problem with wafer butterfly valves whose liner extends over both faces and acts as both gasket and seat. If overbolted or subject to line movement, the liner can bulge into the valve cavity and make the valve diffi- cult or impossible to operate • With wafer valves, a flange misalignment can cause a leak. Their use is not recommended unless the pipe flanges can be kept aligned High Performance Fire-Safe Butterfly Valves. High-performance butterfly valves are fire resistant. They have a secondary metal-to-metal seat ring that provides shutoff if the primary elastomeric seat ring is destroyed in a fire. Fire resistant butterfly valves with a lug-type body are not acceptable for critical locations such as the first valves connecting to a tank or pressure vessel, emergency shutdown valves, depressuring valves, or valves required for safe facility shut- downs during an emergency. Metal seated butterfly valves are now recommended as a replacement when reconditioning is required. Metal Seated Triple Offset Butterfly Valves. The placement of the center of rota- tion coupled with a cone-shaped metal seating surface are the distinguishing features of these valves. There is no friction or wear between the disc and the seat because the offset design results in the disc being lifted off the seating surface in just a few degrees of rotation. The body seat most commonly used is solid metal and the mating seat, bolted to the disc, is a flexible laminated design built of several layers of stainless steel with graphite sandwiched between the layers. There are also designs where the disc is solid metal and the flexible laminated seat ring is bolted to the body. Metal seated butterfly valves are available in a variety of face-to-face dimensions. These valves are in use in Chevron refineries and have demonstrated reliability with minimal leakage in service. November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-79 200 Piping Component Selection Piping Manual 276 Special Purpose Valves Tight Shutoff Valves In certain services a tight shutoff or a double block-and-bleed valve is required for product containment or segregation. Several special valves are available at extra cost for these applications. The Orbit valve is a good single seated ball valve with tight shutoff. It uses a soft seat backed up with a metal seat-ring. Y-pattern globe valves with metal-to-metal seats are also excellent single block valves in high pressure, high temperature service. Edwards, Invensys-valves are often used in high pressure process plants and steam systems. Double block-and-bleed valves generally have soft seals that are backed up with metal seats. The three generic types, with acceptable vendors, are: • Modified plug valve (General Valve Twin Seal, Cooper Industries TruSeal, Emerson-Daniel Dan-Ex) • Modified gate valve (Crane Valve-Pacific HIS) • Full conduit slab gate valve (Cooper Industries-WKM, Dresser-Grove) All of these valves use fluorocarbons or elastomers that impose a limit on the oper- ating temperature. General performance of these valves has been good. The following discussion covers the most commonly used block-and-bleed valves. Liquids will become trapped in the body cavity of any block-and-bleed valve. Thermal expansion of the liquid caused by ambient temperature or fire exposure may damage the valve or make it impossible to open. Block-and-bleed valves there- fore require a relief or drain valve for the body cavity. The relief valve can be installed at the factory and connected internally to the low-pressure side of the valve. Modified Plug Valves. TruSeal, Twin Seal, and Dan-Ex valves have plugs or slips that rotate 90 degrees and wedge into position when the valve closes. The body can be fitted with a bleed valve venting the space between the slips. The slips have elas- tomer seats attached to each face. The camming action of the valve frees the seat before it is rotated and thus minimizes damage to the soft seals. Because the elas- tomer is narrower than the seat ring recess, metal-to-metal seating develops when the elastomer is damaged by fire. Company experience with these valves has generally been good. However, the elas- tomer has a tendency to stick if the valve is left in one position for several months. Seat material generally limits maximum temperatures to between 250°F and 350°F, depending on the particular seat material. The elastomer must be compatible with the stock. Viton or other special material should be used with reformates. Modified Gate Valves. The Crane Valve-Pacific HIS gate valve has pressure ener- gized resilient seals with metal-to-metal backup on both sides of the gate. Service and temperature limitations are the same as with modified plug valves. The body can be fitted with a drain valve. These valves have given excellent long term block- and-bleed service in product loading manifolds. 200-80 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection Metal Seated Ball Valves. Tyco-Valvtron, Mogas and Valvtechnologies Compa- nies are advancing metal seated floating ball valve designs. The valve utilizes a split body design with a spring-loaded washer/guide that forces the ball against an inte- gral hard-faced body seat. They were developed for critical high-temperature and abrasive applications. Full Conduit, Expanding-Gate Valves. Cooper Industries-WKM Pow-R-Seal full conduit bolted bonnet gate valves have generally given excellent service at ambient temperature where long term tight shutoff performance is mandatory. It is designed to pass line scrapers (pigs) and has been used extensively on pipelines and in oil field applications. The Pow-R-Seal offers a Teflon seal with a parallel expanding split gate and metal-to-metal seats. The wedge gate design requires a large body cavity and has a “preferred flow direction” stamped on the valve end connection. Because the body cavity is sealed off from the piping in both the open and closed positions, a body relief valve is mandatory. Full Conduit, Slab-Gate Valves. Dresser-Grove G4 and Cooper Industries-WKM “Saf-T-Seal” models are full conduit slab gate valves with a one piece gate. They depend on spring-loaded rubber O-ring seats and line pressure to maintain a seal and are limited to a maximum of 250°F. They are less expensive than Pow-R-Seal, and the Company has had good experience with these valves in pipeline service. The construction materials should be reviewed when using any of these valves in services subject to sulfide cracking. Metering Valves Needle valves are small globe valves normally used to manually control pressure or flow. Although available in smaller sizes, NPS ½ and ¾ are normally specified. They are typically machined from stainless steel with threaded connections and rated at ANSI Class 2500 or more. They have a screwed bonnet and should be purchased with a locking device to prevent the bonnet from backing out. Their main use is with instrumentation piping and continuous sampling systems. The proper applications and installation are covered in the Instrumentation and Control Manual. Flow chokes, also called flow beans, are used primarily by Producing Departments in very high pressure systems and to control steady flow while absorbing very high pressure drops. Generally supplied in the angle valve configuration, they are typi- cally available from NPS 1 to NPS 6, with threaded, flanged, or butt welded connec- tions through ANSI Class 2500 and API Class 10,000. Normally fitted with a manual handwheel and a position indicator, they can also have pneumatic or hydraulic operators. Typical applications are reducing wellhead system pressures at gathering stations and balancing flow in injection lines at steam flood manifolds (see Section 700). Slide Gate Valves Also called knife gates, have a fabricated body with a wafer design and tapped lugs for bolting between matching ANSI Class 150 flanges. They are used primarily in low pressure, large diameter relief or wastewater lines, where full line size is required and a conventional gate valve would be too heavy. November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-81 200 Piping Component Selection Piping Manual Slide gates normally have soft seats against the knife gate. Bubble tight service is advertised but cannot be expected in most cases. Minimal leakage must be accept- able, especially in the larger sizes. 277 Valves for Sour Service The prime concerns when handling sour fluids are corrosion and cracking, odor, and the H2S hazard. Handling H2S is discussed in Section 1000 of this manual. Valve Trim Materials Good service is obtained with conventionally hard 12% chrome seats and discs in most sour services. However, 12% chrome, carbon steel, alloy steels, and precipita- tion-hardened (PH) stainless steels are susceptible to sulfide cracking in sour liquids and wet sour gas if their hardness is above Rockwell C22. The tendency to crack varies widely depending on trace impurities. Acceptable replacement materials not subject to sulfide cracking include iron base super alloys like ASTM A638 Grade 660 cobalt, nickel-base hard-facing material such as Stellite, 300 series stainless steels, or ferritic steels like carbon steel and 12% chrome tempered to hardnesses less than Rockwell C22. The Materials and Equipment Engineering Unit of ETC should be consulted if there is doubt about selection. Valve Design Where gate or globe valves are necessary, grease seal valves with extra-deep stuffing boxes, a lantern ring and sealant injection fitting are desirable. These are considerably more expensive than standard valves. Orbit ball valves are effective where tight shutoff is required. Standard Orbit sour trims do not resist sulfide cracking and should not be used in high H2S service. Instead, Orbit’s T7 Special Chevron (SC) modified trim should be specified. Consult the Materials and Equipment Engineering Unit of ETC. 278 Valves for Saltwater Service In saltwater service where valves can be easily replaced, cast iron or carbon steel valves are generally used in large sizes, and bronze valves in small sizes. Where replacement is difficult, the more expensive bronze valves may be preferable since they resist corrosion in saltwater if velocities are not excessive. Very large cast iron and steel valves are sometimes epoxy-lined. The 300 series stainless steels are subject to pitting and crevice corrosion in stagnant or slow-moving (less than 2 to 3 fps) saltwater. Monel performs satisfactorily in flowing saltwater and is usually less subject to pitting and crevice corrosion. However, Monel is 5 to 10 times more expensive than steel or cast iron. 200-82 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection 280 Bolts and Gaskets Bolts and gaskets are used at flanged connections in piping. 281 Bolts Bolt Material Bolting materials are divided into the four groups below by ASME/ANSI B16.5. Choice of bolting material is governed by service fluid (sweet, sour), its tempera- ture, and bolt style. • High strength • Medium strength • Low strength • Nickel and special alloy The most commonly used bolts for flanges in refinery piping are the ASTM A193 Gr B7 stud bolts, which fall into the high strength group. The temperature range for these bolts is from -20°F to 750°F. See Figure 200-27. Fig. 200-27 Machine Bolt with Hex Nut and Stud Bolt with Hex Nuts Courtesy of Crane Valves The medium strength ASTM A193 Gr B7M studs are required in sour services to avoid sulfide stress corrosion cracking. Sour service here means continuous H2S exposure, such as in heat exchangers or possibly with insulated flanges. Grade B7 studs should be used for flanges in open atmospheres. Low carbon machine bolts per ASTM A307 Gr B are in the low strength group. They should be used for all cast iron flanges (to prevent overstressing the flange) and for Class 150 flanges up to 16 inches. Low carbon bolts are limited to a bolt temperature of 400°F. High strength ASTM A193 Gr B16 bolts are used for temperatures between 750°F and 950°F. Bolt Size Selection of proper stud bolt and machine bolt sizes, including length, is given in ASME/ANSI B16.5, Pipe Flanges and Flanged Fittings. The calculation of proper bolt length always assumes the use of heavy pattern hex nuts per ASME/ANSI B18.2, Square and Hex Nuts. Heavy pattern nuts are desir- able because of additional strength and the larger area on the flats that helps avoid rounding corners. November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-83 200 Piping Component Selection Piping Manual Certain pipe fittings and valves, such as the wafer valves discussed in Section 270, have extra long stud bolts. Such installations are a concern in fire hazard areas. Long bolts are more susceptible to expansion when subjected to flame impinge- ment and are a potential leak source. If long bolts are used in such areas, they should have flame impingement shields as shown on Standard Drawing GB-L1110. Threading. All machine bolts, stud bolts, and nuts should be specified with threads as described in ASTM A193 Paragraph 15. Bolt Applications Proper material selection of bolts and nuts for various applications is discussed below, and summarized in Figure 200-28. Fig. 200-28 Recommended Bolting Materials Service Flanges Pipe Dia. Bolt Dia. Temp Range °F Bolt Spec Nut Spec General Cast Iron All All -20 to 400 ASTM A307 ASME/ANSI Grade B B18.2 C.S. ≤ 16 ≤1 -20 to 400 ASTM A307 ASME/ANSI Class 150 Grade B B18.2 C.S. All All -20 to 750 ASTM A193 ASTM A194 All Classes Grade B7 Grade 2H General C.S. All All -20 to 850 ASTM A193 ASTM A194 Sour All Classes Grade B7M Grade 2HM C.S. All All -20 to 850 ASTM A193 ASTM A194 All Classes Grade B7M Grade 2HM High Temp C.S. All All -20 to 950 ASTM A193 ASTM A194 All Classes Grade B16 Grade 2 C.S. All All Select bolts to match service conditions All Classes Low Temp C.S. All All ≤ -20 ASTM A320 ASTM A194 All Classes Grade L7 Grade 2H Low Temp Sour S.S. All All ≤ -20 ASTM A320 ASTM A194 All Classes Grade L7M Grade 2HM General Service. ASTM A307 Grade B low carbon steel machine bolts with square heads and heavy hex nuts per ASME/ANSI B18.2 are specified for cast iron flanges and are suitable for Class 150 carbon steel flanges up to NPS 16 where bolt diam- eter is one inch or less. Although low carbon machine bolts are acceptable for Class 150 steel flanges, common practice is to use B7 stud bolts (see following) on all steel flanges. Use of the low carbon steel machine bolts is limited to the -20°F to 400°F bolt metal temperature range by ASME/ANSI B31.3. 200-84 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection ASTM A193 Grade B7 alloy stud bolts are high-strength stud bolts. They are the most widely used bolts and should be specified for: • ANSI Class 300 and higher flange ratings • All bolt metal temperatures from -20°F to 750°F • Bolt diameters ≥ 1-1/8 inch in any ANSI class ASTM A194 Grade 2H heavy hex nuts are specified for B7 alloy studs. B7 studs are subject to sulfide cracking failure if exposed to stock containing aqueous H2S (e.g., on internal vessel piping). If sulfide cracking is likely, ASTM A193 Grade B7M studs tempered to less than 225 Brinnell hardness should be considered, with ASTM A194 Grade 2HM nuts. ASTM A193 Grade B7M studs have lower strength levels, and the bolt strength and flange design should be reviewed for suitability in pressure-temperature conditions. ASME/ANSI B31.3 lists allowable bolt stresses, and the calculation method is in Appendix 2 of ASME Boiler and Pressure Vessel Code Section VIII, Division 1. Bolting of internal parts is not covered by codes and engineering judgment must be used. High Temperature Service. ASTM A193 Grade B16 alloy stud bolts with ASTM A194 Grade 2 nuts should be used between 750°F and 950°F for most services. Also see Stainless Steel Stud Bolts, following. Low Temperature Service. ASTM A320 Grade L7 alloy stud bolts with ASTM A194 Grade 2H nuts should be specified where ambient or service temperatures can drop below -20°F. In sour services with low temperatures, ASTM A320 Grade L7M studs should be specified, with ASTM A194 Grade 2HM nuts. Stainless Steel Stud Bolts. Stainless steel stud bolts should be used with stainless steel flanges where high operating temperatures and thermal cycling can cause flange leakage because of differential thermal expansion between low alloy bolt material (B7 or B16) and the stainless steel flange. Stainless steel bolts should also be considered where leaking line fluid can cause severe corrosion of the bolts. Selection of the proper bolt material depends on the fluid being handled. The Materials and Equipment Engineering Unit of ETC should be consulted on the application and selection of stainless steel bolts. The design of flanged joints using stainless steel bolting should be reviewed to determine if the joint is suitable for the needed pressure-temperature rating with the lower allowable bolt stresses. See ASME/ANSI B31.3 for allowable bolt stresses and ASME Boiler and Pressure Vessel Code Section VIII, Division I for the calcula- tion procedure. Bolting-up Practices. Good practice for tightening bolted flanges is discussed in Section 600 of this manual. Bolt tightening instructions ensure that the gasket is uniformly compressed, but they usually cannot remedy leaking flanged connections that are improperly aligned, have the wrong gasket, have too much thermal stress or have excessive moment loads on the joint. November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-85 200 Piping Component Selection Piping Manual 282 Gaskets Two styles of gasket are commonly used with pipe flanges: Nonmetallic (ASME B16.21) and Metallic (ASME B16.20). Refer to Standard Drawing GD-L1264 “Gasket Specifications and Acceptable Brands” and form PIM-EF-743 “Flange Gasket Limitations” for Chevrons’ recommendations on gaskets and their limita- tions in various services. Class 150 carbon steel raised-face flanges use non-metallic flat-ring gaskets. Class 125 cast iron flat-face flanges use non-metallic full-face gaskets. Class 150 carbon steel raised-face flanges in non-flammable fluid service to 365°F (particularly cooling water service) use aramid fiber composition gaskets. In flam- mable service to 450°F Class 150 carbon steel raised-face flanges use graphite coated corrugated gaskets (not recommended for cooling water service because of galvanic corrosion concerns). Class 300 and Class 600 flanges use spiral-wound or ring-joint gaskets depending on the fluid service. Both of these gaskets are metallic. Flange classes higher than Class 600 in refinery process services use only ring-joint gaskets. In upstream production and in pipeline service the piping at pumping and compressor stations may use spiral wound gaskets at higher than Class 600 rating. Nonmetallic Flat Gaskets for Pipe Flanges (ASME Standard B16.21) Nonmetallic flat gaskets for pipe flanges are made to the ASME Standard B16.21. These gaskets have relatively lower tensile strength than metallic gaskets and may be subject to blowout (See Section 250). Chevron has limited their use to ASME Class 150 flanges and maximum service temperatures between 300°F-450°F depending on the material. Chevron does not recommend the use of Class 150 flanges above 450°F; for services above 450°F Class 300 flanges and spiral-wound gaskets are specified. Historically flat ring composition gaskets were cut from sheets made with asbestos fibers, some inert fillers and bound with an elastomer binder. In the old piping spec- ifications the asbestos sheet gaskets were limited to 450°F because of the elasto- meric binder. (Asbestos ropes were used in reactor internals up to 900°F because they contained no binders). Non-asbestos composition sheet gaskets are now made with synthetic fibers (replacing the asbestos fiber) and an elastomer binder, they are limited to 365°F in non-flammable service and to Class 150 flanges. Company experience with most reinforced flexible graphite sheet gaskets has been poor because of the lack of elastic recovery in flexible graphite material. However, the flat ring corrugated gaskets (stainless sheet metal core bonded with a flexible graphite coating) have performed well and are considered the exception. The coated corrugated gasket is still classified as nonmetallic despite the metal reinforcement. Chevron recommends the coated corrugated gasket for use with Class 150 flanges to 450°F. (Note: The 450°F limit is not the limit for flexible graphite it is the limit for the Class 150 flange). Special care and handling are necessary with gaskets that have flexible graphite as a coating or as a filler (spiral-wound or jacketed) because they are easily damaged. 200-86 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection The low resistance of graphite to concentrated acids such as H2SO4 and HNO3 makes it unsuitable for these and other highly oxidizing services. Spiral-Wound gaskets with a PTFE filler are recommended. Appropriate metal winding material is Alloy 20 for H2SO4 and Type 304 for HNO3. Surface Finish for Flanges The surface finish of the gasket contact area on flanges should comply with the latest edition of ASME B16.5 “Pipe Flanges and Flanged Fittings.” Refer to Section 250 for additional information. Metallic Gaskets for Pipe Flanges - (ASME Standard B16.20) Three styles of metallic gasket are covered in the ASME Standard B16.20: Ring- Joint, Spiral-Wound and Jacketed. They are suitable for use with flanges described in ASME B16.5, ASME B16.47 and API-6A Standards. Ring-Joint Gaskets (ORJ, RTJ). Type “R” Octagonal or Oval Ring-Joint (ORJ), also called Ring Type Joint (RTJ), gaskets have either an octagonal or an oval cross- section. They are used in high-pressure, high-temperature and/or critical services and are considered more secure than Spiral-Wound style gaskets. The octagonal ring-joint is preferred because of the greater seating surface. They should be used in all hydrogen systems and considered for all services with ASME Class 900 (or higher) flange rating. Material selection must be suitable for the service conditions. Common materials and their maximum allowable hardness are listed in ASME B16.20. See Section 250 for additional discussion on the use of Ring-Joint flanges. Spiral-Wound Gaskets. Spiral-Wound gaskets are fabricated from a thin V-shaped metal strip (winding) and a strip of gasket material (filler) wound together into a ring. (See Figure 200-29.) The metal strip is usually Type 304 stainless steel but can be of other materials as required for corrosion resistance. The ends of the strip are spot welded to prevent unraveling of the spiral. Filler material is either flexible graphite (good for high temperatures) or PTFE (recommended for oxidizing acids). Spiral-Wound gaskets for pipe flanges have a 1/8-inch thick outer metal ring that has a dual function: it centers the gasket and prevents over-compression. Considered a confined gasket, spiral-wound gaskets are recommended up to 750°F for general hydrocarbon, LPG and steam services. Spiral-wound gaskets are not recommended for hydrogen service. Gaskets NPS 12 and larger with a graphite filler and all NPS sizes with a PTFE filler should have both an outer carbon steel compression centering ring and an inner ring made of the same material as the winding. Additionally, ASME B16.20 requires outer and inner rings for NPS 12 and larger in ASME Class 1500, and NPS 4 and larger in Class 2500. Double-Jacketed Gaskets. Double-Jacketed metal gaskets consist of a soft resil- ient core encased in a soft iron jacket. The principal use for these gaskets was for high pressure, high temperature services with raised face or tongue-and-groove flanges. Spiral-Wound gaskets have largely replaced these gaskets. November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-87 200 Piping Component Selection Piping Manual Fig. 200-29 Spiral-wound Gasket Courtesy of The Flexitallic Group 290 Miscellaneous Engineered Equipment Some piping equipment must be specifically engineered (either by design or selec- tion) for certain tasks. 291 Strainers Strainers may be of either the permanent or temporary type. Permanent strainers are used to protect sensitive equipment such as turbine meters, close clearance compres- sors and other rotating equipment from particulate matter in the process fluids. Temporary strainers are used at startup to protect sensitive equipment, mainly pumps, from debris in the lines. See Figure 200-30 for illustrations of temporary and permanent strainers. 200-88 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection Fig. 200-30 Strainers Courtesy of Mack Iron Works Company a. Permanent Basket Strainer b. Temporary Cone Strainer c. Permanent Tee Strainer d. Permanent Tee Strainer - High Pressure Drop (not recommended) Strainer Selection and Use Strainers should be used for pump, compressor, and PD meter suctions during initial startup and where line or product debris is common. The issue of permanent versus temporary installation must be based on intended service, maintainability, and cost on a case-by-case basis. Tee, Basket, Conical Strainers Compared. Tee strainers are preferred. Basket strainers should be used only when continuous cleaning is required. Conical strainers, although initially less expensive, should be used only if the conical strainer’s disadvantages can be justified based on expected machinery reliability, startup costs, and maintenance costs. Tee and basket strainers have the following advantages. They: • Eliminate the need to realign machinery drivelines, since piping spool pieces are not disassembled • Reduce strainer cleanout time • Are serviceable by operators without assistance from maintenance crews November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-89 200 Piping Component Selection Piping Manual • Reduce startup costs when multiple cleanings are required • Have more robust strainer elements and are less likely to fail and damage machinery Tee and basket strainers have these disadvantages: • Higher initial cost • Extra gasketed joint that can leak • Requirement for extra piping space in some instances Tee Strainers. Tee strainers should be used wherever disassembly of piping is not practical because of the cost of machinery realignment or the piping configuration. Tee strainers can be used for continuous removal of debris from a line. Cleanout is much simpler and quicker than for conical strainers. Tee strainers are meant for permanent installation, although the strainer elements may be removed if no longer needed. They are commercially available as flanged, threaded, or (rarely) weld-end, for both in-line tee and Y-pattern styles. The style chosen depends on the line size, contaminants to be removed, and piping configura- tion. The strainer elements are bathtub-shaped. The screen size, mesh size, and basket size should be established with the vendor. Suppliers include Aitken, Inc., Houston, TX; Mac Iron; and others. Strainer bodies, internals, connected blowdown valves and drain piping should be fabricated of steel, as determined by the applicable pipe code and pipe material. Strainer element materials should be chosen with consideration for the service fluid and expected life. Tee strainers are more expensive than conical strainers. However, the higher cost is offset by lower maintenance costs and elimination of the need for a removable pipe spool. Figure 200-31 shows typical installations for tee strainers for a pump. Fig. 200-31 In-Line Tee Strainers for a Pump Not Recommended Preferred (High Pressure Drop) 200-90 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection Basket Strainers. Basket strainers should be used to continuously remove debris from a line. Applications include removal of corrosion scale in steam turbine suction lines and debris in compressor suction lines, and installation upstream of displacement meters and in pump suction lines in dirty service, such as crude lift- ings from tankers. Basket strainers are designed for permanent installation. They generally have cast iron or steel bodies with flanged connections. The strainer can be lifted out through a cover flange. Bolted cover flanges are recommended; clamp styles usually do not match the piping flange pressure rating. The screen size, mesh size and basket size should be established with the vendor. Basket strainers are as expensive as valves. Dual strainers with associated valving may be required for some services. Conical Strainers. Conical strainers should only be used in turbine, compressor and pump suction lines to catch debris during startup of new systems and newly modi- fied systems. They are typically installed at a pair of flanges close to the machine’s inlet. Installation of long conical strainers at large diameter suctions can present problems, especially where space is limited. An extra pair of flanges or removable spool is often required. Conical strainers are commercially available in many screen sizes, and for use with all ANSI flange faces, sizes and ratings. For general service, they should be fabri- cated of a minimum of 16 BWG carbon steel sheet with 1/8-inch diameter holes on 3/16-inch straight centers. Above NPS 20 the manufacturer should be consulted. Consider stainless steel in corrosive services or where the strainer will be in use for extended periods. Installation of Conical Strainers. It is a common misconception that these strainers should point upstream. Conical strainers should be installed with the pointed end downstream. This position retains more flow area as debris collects, is structurally stronger, and retains debris when the strainer is removed. When woven wire mesh is used to cover the perforated sheet, it should be placed on the upstream side. Company-designed Conical Strainers. In areas where acceptable conical screens are not commercially available, Standard Drawing GB-L88612 provides fabrication details. Because of the cost of fabricating screens to our own design, commercial screens should be used when possible. Some commercially manufactured strainer elements are fabricated of light gage steel and have been known to collapse when plugged with scale or grit, or as a result of vibration-induced metal fatigue. Strainers for Specific Services Pump Strainers. For services where solids finer than 1/8-inch cannot be tolerated, wire mesh can be mounted over the perforated screen. Sizing should be coordinated with the pump vendor and strainer manufacturer. The mesh should be installed on the upstream side of the screen. November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-91 200 Piping Component Selection Piping Manual Strainers should have an open area of at least 120% of the pipe flow area. A greater area should be specified when significant fouling is expected or when pump suction pressure is important (refer to the Pump Manual for information on NPSH). Centrifugal Compressor Strainers. Centrifugal compressors are more tolerant of small particles than reciprocating machines but are sensitive to larger objects that can damage the high-speed rotors. In critical services, fine-mesh startup strainer elements are sometimes replaced after startup with permanent “bolt catcher” strainer elements with heavier construction and larger openings. Conical strainers for centrifugal compressors should be of 11 BWG carbon steel plate with a support ring of 1/4-inch steel. The open area should be equal to 200% of the pipe flow area, with 1/8-inch diameter holes punched on a 3/16-inch trian- gular pitch. Longitudinal weld quality is critical for conical elements. A wire mesh overlay should not be used; it can plug too quickly. Instead, the suction piping should be mechanically cleaned sufficiently to remove particles large enough to damage the machine. Reciprocating Compressor Strainers. These machines are sensitive to fine scale and grit, which can damage the valves and cylinders. Larger objects are of less concern because they can be stopped when passing through suction bottles or at the suction valve guards. Suction system piping may need to be chemically cleaned before the compressor is commissioned. See Model Specification PIM-MS-2411. Strainer elements are usually fabricated of woven stainless steel wire mesh installed over perforated carbon steel plate. The strainer plate and support ring should be fabricated with the same thickness steel and perforation dimensions as for centrif- ugal machines. Particles larger than 140 microns should be removed. The open area of the perforated plate should be approximately 300% of the pipe flow area to compensate for the wire mesh overlay, which will reduce the open area. Strainers for Water Well Pumps. Water well discharge piping presents special problems because sand may be present during pump starts or may be continuously produced. The sand can quickly plug off fine weave mesh elements. It is difficult to remove sand from the element in service—especially angular sand. Where contin- uous sand removal is needed, strainers with automatic scrapers (for the strainers) may be satisfactory, but they are expensive. 292 Flame Arresters Flame arresters are installed to prevent flame propagation in vent piping. Their purpose is to slow, cool, and extinguish a flame front before detonation can occur. An explanation of how they function and proper installation procedures are presented in the Fire Protection Manual. The operation of flame arresters is described in the Flame Arrester Guide, by J. G. Seebold, available from ETC’s Environmental Services Unit. See also the Tank Manual and the Fire Protection Manual. 200-92 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection Common Applications The most common application is to prevent spark- or lightning-ignited flame at atmospheric vent discharges from traveling upstream. They are also installed in waste gas lines to furnace fireboxes, tank field and marine vapor recovery systems, and long exhaust lines of large engines. In the latter instance they prevent flame propagation downstream in the hot gas. Flame arresters should be steel-fabricated and must be strong enough to withstand detonation. Kemp flame arresters are currently acceptable for in-line use, and others are being tested. Maintenance is required because the minute passages in the device are susceptible to fouling with corrosion products and stock contaminants. To remain effective, they must be periodically inspected and cleaned. 293 Expansion Joints When a welded expansion loop to accommodate pipe expansion is not practical, for instance, in congested areas or when line drainage is important, an expansion joint may be considered. Expansion joints are not recommended for hydrocarbon service and should be the last resort. Expansion joints provide flexibility either by axial movement, angular rotation, or a combination of the two. Expansion joints for axial movement may be either packless or packed. Packed Joints The packed slip joint consists of a length of pipe, generally machined, that slides inside a packed sleeve. This joint is useful when large amounts of axial expansion must be absorbed. It must be carefully guided to avoid angular moments that might cause binding. Selection and installation should be reviewed with the vendor. Slip joints require restraint of movement, either with external bolts or an internal collar. These are normally supplied by the vendor. This joint is not recommended for flam- mable fluid service both because it is difficult to keep tight and because the packing may burn out in a fire. Slip joints are recommended for off-plot use only. Dresser couplings are sometimes considered to be a packed expansion joint. For information on this coupling, See Section 260. Packless Joints The preferred packless expansion joint is the bellows expansion joint. The bellows material must be carefully selected to avoid failures from fatigue and stress corro- sion cracking. To prevent accumulation of sediment in the convolutions, it is prefer- able to locate bellows expansion joints in vertical runs of pipe. Bellows can be designed to absorb axial, lateral, and angular pipe movement. They can be considered for on-plot noncritical steam and condensate. Fatigue failure is a concern; normal cyclic operation and exposure to pulsing or vibrating service must be considered. Proper material selection, sizing, and installation are critical and should be reviewed with the vendor. Fire resistance of bellows is low. Section 330 recommends that, if feasible, flexibility be provided by pipe bends, but it may be necessary to use packless corrugated bellows expansion joints for refinery service. Expansion joints are most commonly used in large lines where space does November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-93 200 Piping Component Selection Piping Manual not permit pipe bends, or immediately adjacent to rotating equipment to provide angular flexibility without high moments that might distort the rotating equipment casings and cause binding and misalignment. All of the joints discussed earlier, including the bellows expansion joint, slip joint, and Dresser coupling, may be blown apart by longitudinal hydrostatic pressure unless special precautions are taken to anchor the pipe on each side of the joint. The Victaulic coupling is sometimes considered to be a packless expansion joint. For information on this coupling, see Section 260. Hinge expansion joints are used to absorb expansion by the angular motion of the joint. This is achieved using two hinged expansion joints spaced a distance apart and located so that the major expansion movement is normal to the axis through the joints. To best utilize the hinge system the distance between the hinges should be as large as possible. The advantage of hinge joints is that they absorb the pressure thrust forces and require a minimum of guiding and intermediate anchors. See Figure 200-32. Fig. 200-32 Expansion Joints—Hinge and Gimbal Courtesy of Pathway Bellows a. Hinge Expansion Joint b. Gimbal Expansion Joint Note: HEJ = Hinge Expansion Joint GEJ = Gimbal Expansion Joint IA = Intermediate Action PG = Planer Guide The gimbal expansion joint incorporates a pair of hinges connected to a common floating gimbal ring. This construction provides for close control of the movement imposed upon the bellows and at the same time supports the dead weight of the system and absorbs the pressure thrust. Gimbal expansion joints are used in pairs to absorb multiplane motion in a piping system. See Figure 200-32. 200-94 © 2001 Chevron USA Inc. All rights reserved. November 2001 Piping Manual 200 Piping Component Selection 294 Swivel Joints Swivel joints are used off-plot, primarily in tanker loading booms, tank truck loading systems, and with floating roof tank internal roof drains where articulated piping is required. Swivel joints can be sealed with a variety of packing. The ball bearings require lubrication. Swivel joints are not considered fire resistant. November 2001 © 2001 Chevron USA Inc. All rights reserved. 200-95 200 Piping Component Selection Piping Manual 200-96 © 2001 Chevron USA Inc. All rights reserved. November 2001