Piping Lines and Fittings Introduction Fluids are transported under pressure through hollow materials called lines. The common three types of fluid lines are pipes, tubes, and hoses. These lines must provide leak proof passage at the required operating pressure of the system. Standard pressure rating of pressure system varies from 125 psig to 6000 psig. Test pressures are normally higher than the operating pressures. The operating pressure is greatly influenced by the operating temperature. Generally, the mechanical strength of materials decreases with increasing temperature, so operating pressures are determined by the operating temperatures. Selecting a proper line is an important function in piping system design. Pipes Pipes are rigid hollow cylinders used for transporting fluids and sometimes, solids such as pellets, powders, etc. Pipes are made of different materials such as steel, cast iron, plastic, etc. ASME B36 codes specify dimensional requirements of pipes. For example, ASME B36.10M: Welded and Seamless Wrought Steel Pipe; ASME B36.19M: Stainless Steel Pipe. Pipe Materials Pipes may be made from metals, plastic, concrete, and glass. Cast iron and ductile iron pipes are popular for underground water, gas and sewer applications. Steel is perhaps the most popular pipe material. Steel pipes are strong, durable, weldable, machinable, and can stand high temperature. They are often used to handle water, oil, petroleum and steam. Carbon steel pipes are the most popular, being cheaper than other materials. They are the natural choice if they can meet application requirements of pressure, temperature, corrosion resistance, and hygeine. Stainless steel pipes are used in corrosive environments and food processing facilities. Copper and copper alloys are corrosion resistant and are commonly used for instrument lines, food processing, and heat transfer equipment, like heat exchangers, steam, air and oil lines. However, stainless steel pipes are increasingly being used for these applications. Plastic pipes are used extensively in piping systems today, especially in plumbing. They have high resistance to corrosion and chemical degradation but cannot withstand high pressures and temperatures. Plastic pipes are used especially for handling corrosive or hazardous gases and dilute mineral acids. Concrete pipes are pre-cast and are used mainly for underground application. Glass pipes are popular in the food, beverage, chemical, and pharmaceutical plants. Their application is limited to temperatures of 450 o F (230 o C). Lined Pipe Lined pipes are made of internal surfaces different from the main pipe material. For example, a carbon steel pipe may be lined with Teflon to enhance corrosion resistance. Lined pipes and fittings are joined by flanges. Other lining materials include glass, asphalt, zinc, concrete, and different types of plastics, e.g. epoxy. Pipe Manufacture Pipes are manufactured by two main methods: seamless and welding methods. Seamless pipe production involves piercing a solid near-molten steel rod called a billet with a mandrel. The mandrel size determines the inside diameter of the pipe. Welded pipes are made from rolled steel plates and may be butt-welded or spiral-welded. Fig. 1 shows pipes of seamless, butt-welded or spiral-welded type. a) Seamless b) Rolled c) Spiral-welded Fig. 1: Pipe manufacturing methods 1
In butt-welded pipes, the steel plate is heated and fed through special rollers that bend and join the ends of the plate into a pipe. Spiral welded pipes are made from twisted strips of metals and welded in spiral form. This is the least common of the pipe making methods. ASME B31.1.0 assigns strength factors of 100%, 85% and 60% to seamless, butt- and spiral-welded pipes, respectively. Pipe Sizes The American National Standards Institute (ANSI) has standardized pipe sizes. Pipes are specified by the nominal pipe size (NPS) in English units. It is used as a reference or designation for a pipe size. The metric system assigns DN (Nominal Diameter) sizes to pipes. Table 1 gives pipe sizes in common use. The normal range of pipe sizes for process pipe is 1/8 to 48. However, stock size range is from ½ to 24. Sizes outside the normal ranges may be obtained by special order. Pipe sizes of 1/8 to 1½ are usually for service and instrument lines. The common process pipe sizes are in the range of 4 and 48 in diameter. English (NPS: in) Metric (DN: mm) English (NPS: in) Metric (DN: mm) English (NPS: in) Metric (DN: mm) 1/8 6 6 150 30 750 ¼ 8 8 200 32 800 3/8 10 10 250 36 900 ½ 15 12 300 40 1000 ¾ 20 14 350 42 1100 1 25 16 400 48 1200 1 ¼* 32 18 450 54 1400 1 ½ 40 20 500 60 1500 2 50 22 550 64 1600 2 ½* 65 24 600 72 1800 3 80 26 650 80 2000 4 100 28 700 88 2200 Table 1: Common pipe sizes (*For special applications. Not used in new designs) NPS of 1/8 to 12 is designation for a size only. It is not equal to the outside or inside diameter of the pipe, but is close enough to the inside diameter for most calculations. NPS above of 14 and above refers to the outside diameter of the pipe. The outside diameter of a pipe is it s inside diameter plus twice its wall thickness. The NPS helps in knowing the outside diameter of pipe; usually by referring to a table of pipe sizes. The inside diameter of a pipe is the outside diameter minus twice its wall thickness. Pipe wall thickness is standardized in ANSI B36.10M-1985 with schedule numbers from 5 to 160. Thickness varies with weight and size of pipe and higher schedule number represents thicker pipes. The outside diameter of all pipe sizes is fixed (Fig. 2) but the wall thickness can vary depending on the schedule number. Pipe thickness is also described as standard, strong and extra strong. Standard thickness corresponds roughly to schedule 40 and extra-strong thickness is about schedule 80. a) Standard b) Extra Strong (XS) c) Double extra strong (XXS) Fig. 2: Pipe thickness and schedules 2
OD = Outside diameter; ID = Inside diameter; T = wall thickness OD = ID + 2T ID = OD - 2T T = 0.5(OD ID) The pressure rating of pipes is an important parameter in their specification. Pressure standards range from 125 psig (0.862 MPa) to 4500 psig (31 MPa). Allowed pressure is greatly influenced by operating temperature. Thicker pipes are normally required for higher pressure applications. The commonly used schedule numbers are 40, 80, and 160. Schedule 40 or standard (SD) seamless pipes are usually used for low pressure applications. Schedule 80 or extra strong (XS) seamless pipes are designed for medium pressure applications; while schedule160 pipes or double extra strong (XXS) pipes are designed for high pressure applications. SCHEDULE NUMBER NPS OD 10 20 30 40 60 80 (in.) Wall Thickness (in.) 100 120 140 160 1/2 0.840 0.083 0.109 0.147 0.188 3/4 1.030 0.083 0.113 0.154 0.219 1 1.315 0.109 0.133 0.179 0.250 1-1/4 1.660 0.109 0.140 0.191 0.250 1-1/2 1.900 0.109 0.145 0.200 0.281 2 2.375 0.109 0.154 0.218 0.344 2-1/2 2.875 0.120 0.203 0.276 0.375 3 3.500 0.120 0.216 0.300 0.438 3-1/2 4.000 0.120 0.226 0.318 4 4.500 0.120 0.237 0.337 0.438 0.531 5 5.563 0.134 0.258 0.375 0.500 0.625 6 6.625 0.134 0.280 0.432 0.562 0.719 8 8.625 0.148 0.250 0.277 0.322 0.406 0.500 0.594 0.719 0.812 0.906 10 10.750 0.165 0.250 0.307 0.365 0.500 0.594 0.719 0.844 1.000 1.125 12 12.750 0.180 0.250 0.330 0.406 0.562 0.688 0.844 1.000 1.125 1.312 14 14.000 0.250 0.312 0.375 0.438 0.594 0.750 0.938 1.094 1.250 1.406 16 16.000 0.250 0.312 0.375 0.500 0.656 0.844 1.031 1.219 1.438 1.594 18 18.000 0.250 0.312 0.438 0.562 0.750 0.938 1.156 1.375 1.562 1.781 20 20.000 0.250 0.375 0.500 0.594 0.812 1.031 1.281 1.500 1.750 1.969 24 24.000 0.250 0.375 0.562 0.688 0.969 1.219 1.531 1.812 2.062 2.344 30 30.000 0.312 0.500 0.625 36 36.000 0.312 0.500 0.625 0.750 Table 2: Common pipe sizes and schedules Pipe Representations Single or double lines are used to represent pipes in piping diagrams and drawings. Single line representation uses a single thick line to represent the centerline of a pipe. This technique is easy and fast to create. Double line representation use double lines to represent the nominal pipe size with a center line at the middle. Double line representations are more realistic and are found in piping drawings but these can be obtained from 3D models by projection techniques. Fig. 3 shows single and double line representations of pipe joints and connections. 3
a) Single line b) Double line Fig. 3: Pipe representations Fig. 4 shows correct and incorrect layout of reducers at vertical and horizontal bends. The correct layouts are shown in Fig. 4a and Fig. 4b. Reducer must be placed such that air bubbles are not trapped in the pipe. At control stations, eccentric reducers are generally required and placed flat on bottom (FOB) at drain points for easy drain pipe attachment. At pump suctions, eccentric reducers are placed flat on top (FOT) to prevent air from being sucked into the pump. Correct Incorrect Correct Incorrect a) Vertical b) Horizontal Fig. 4: layout of reducers in pipe runs Tubes Tubes are hollow semi-rigid cylinders used for transporting fluids. They have outside diameters less than 4, but usually 2 and less. The nominal size of tubes refers to the outside diameter of the tube. Tubes are more flexible than pipes and may be made from steel, copper, aluminum and plastics. Steel tubes are made from low-carbon ductile steels with a minimum elongation of 30%. Copper tubes are normally restricted to hydraulic service due to their tendency to work-harden when flared and because copper is an oil-oxidizing catalyst. They are easier to bend thus reducing the need for fittings such as elbows. Aluminum tubes have good flaring and bending characteristics but are suitable only for low pressure applications. Plastic tubes are commonly made from nylon, polyvinyl chloride (PVC), polyethylene, and polypropylene. Nylon tubes are used for pressures up to 250 psig and in the temperature range of -100 o F to 225 o F. Polyvinyl tubes may be used for pressures up to 125 psig in temperatures not exceeding 100 o F on continuous bases and up to 160 o F, intermittently. Polyethylene tubes are ideal for pneumatic service and are also good for low-pressure applications. Polyethylene has great dimensional stability, resists most chemicals and solvents and can be manufactured in different colors (color coding). Polypropylene tubes are suitable in the temperature range of -20 o F to 280 o F. Polypropylene has good abrasive resistance. Hoses Hoses are flexible tubes. They have an inner lining that prevents fluid from leaking out, a reinforced thickness that determines its strength, and an outer covering that resists abrasion, heat, and weather. The inner lining is either an oil resistant synthetic rubber (e,g. neoprene) or an oil resistant thermoplastic. The reinforcement materials are fiber braids or spiral steel wires. Strong reinforcement means less flexible hose. The outer protective covering is made of similar material as the inside lining. The nominal size of hoses refers to the inside diameter of the hose. Hoses are suitable in situations where one end of a fluid conduit moves independently of the other. They are simple to route, have little or no thermal expansion problems, and can withstand vibrations. Hoses must have appropriate end fittings for proper connections. Working 4
pressures could range form 300 psig to 12,000 psig. Generally, fiber reinforced hose is used for low pressure, single and double wire braids for medium pressure, and 4 to 6 spiral wire braids for high pressure situations. Line Specifications Basic: Line number-pressure-class-nps Example: 104-A15-6 Full: Unit Number-Zone Number-Line number-pressure Class-NPS-Service Code Example: 02-08-104-A15-6 -ST(2-1/4 ) Fig. 5: Line specification Service Code Heat tracing symbols: ET-Electric Traced; SJ-Steam Jacketed; ST-Steam with tracers (Number-Size) Insulation symbols: IC-Cold; IH-Hot; IS-Safety; PP-Personal Protection Balloon diagrams are used to indicate pipe specifications in piping diagrams and drawings. Balloons are usually 0.25 thick and 1 long. Fig. 5 shows some methods of indicating pipe run specification. Piping Fittings Introduction Pipe fittings are standard or custom components that are attached to pipes so as to ensure proper connections between different segments of a pipe run. Standard components are parts that are made to standard specifications and are purchased by the user. A pipe run is a series of pipe segments and fittings between two equipment or between equipment and a pipe branch. Fittings allow a pipe run to change direction, pipe size or branches. There are two main types of fittings: connectors and joints. Connectors allow two segments of a pipe run to be joined into one unit while joints allows two or more segments to form detachable or permanent unit. When fittings are directly connected to each other without a pipe segment between, the assembly is called Fitting Make-Up (or FMU). When FMU is not the case, the minimum length for a pipe segment between fittings is limited to one pipe nominal size for pipe sizes of 100 mm (4 ) and above and 75 mm (3 ) for smaller pipes. Welding is the principal method of assembling pipe runs in the industry today. Screwed joints are more commonly used for smaller diameter pipe assembling. Screw and socket-weld fittings are generally used for pipe sizes less that 100 mm (4 ). They are available in cast iron, malleable cast iron, and forged steel materials. Cast iron fittings are normally used on low pressure and low temperature lines such as air, water and condensate. Forged steel fittings are used on high pressure and high temperature lines that are subjected to movement and vibrations. Forged steel screw and socket-weld fittings are made in the two pressure classes of 3000 psi and 6000 psi. Socket-weld fittings offer greater strength than screw fittings, so fabricators prefer the former. They are also easier to hold and weld compared to butt-weld fittings. Connection Fittings Connection fittings or connectors are used to join two segments of a pipe run. They can also be used to change the flow direction of fluid in a pipe, enlarge or reduce the pipe size, or provide means of attachment 5
for instrumentation. The two types of fitting connectors commonly used in industrial piping are welded and screwed fittings. Connectors may be welded or screwed to pipes. Weldolets (simply Olets) are small connectors of welds and screws. Among weld connectors are elbows, Tees, reducers, nipples, and swages. Elbows are designed with centerline radius of 1.5 times the NPS for NPS greater than 0.75. These are called large radius (LR) elbows. Short radius (SR) elbows have a centerline radius of 1NPS. LR elbow is assumed, except stated otherwise. Fig 1 shows some weld connectors. Common welded fittings are 90 o elbow, 90 o reducing elbow, 45 o elbow, straight tee (ST), reducing tee (RT), cross, concentric reducer, eccentric reducer, cap, straight lateral, and reducing lateral. Elbows change pipe run direction by the angle in its name. A 90 o elbow effects a 90 o change of direction in a pipe run while a 45 o elbow effects a 45 o change in direction in a pipe run. A 90 o reducing elbow changes a pipe run direction and reduces the pipe size also. A reversal of direction is achieved by using a 180 o elbow. Fig. 1: Some weld connectors. Mitered elbows are made by cutting and welding straight pipe pieces. They can have two, three, or more welded joints. Metered elbows produce more turbulence than standard elbows, so they are not often used in industrial piping. However, they are popular in ventilation duck works. The tees are used to create branches in pipe runs. The straight tee has equal pipe size in all three branches and the reducing tee has a different pipe size on the branch line. The straight portion of the Tee is called the header. A cross has four branches. Reducers create a change in pipe size. A concentric reducer tapers equally about the pipe central axis while an eccentric reducer tapers with an offset about the pipe central axis. Either the top or bottom or an eccentric reducer is flushed with the larger diameter side. A lateral fitting provides a 45 o branch to the main pipe central axis and may be straight or reducing. A Y-lateral has two branches inclined at 45 o to the central axis of the third branch. It has the appearance of a Y. Fig. 2 shows some applications of some weld fittings. Fig. 2: Some applications of some weld fittings 6
Screw Fittings Forged screw fittings are used mainly in 3000 psig and 6000 psig pressure classes. Threads on screw fittings are made to API (American Piping Institute) standards. Fig. 3 shows several screw connectors. Most screw fittings have internal threads or female threads. There are different types of crew fittings that include union, half coupling, street elbow, bushing, and plug. A union has two threaded sleeves and a union ring that is used to create a joint on a straight portion of a pipe run. A union creates a detachable joint in a pipe run and so allows a pipe run to be dis-assembled for inspection, repair or replacement. It is made for application with screw or socket-weld fittings. Unions should be located close to critical line devices such that there is easy access to it. A half union is welded to pipes on the unthreaded end and used mainly for instrument connections. It can thus be used for pipe branching with screw and socket-weld fittings for instruments. The instrument is screwed to the threaded end. A coupling has internal threads at both ends (TBE: threaded at both ends) and is used to join two segments of a pipe run. A half coupling is threaded at one end (TOE). Fig. 3: Some screw fittings A street elbow is a 90 o elbow with internal threads at one end and external threads at the other end. It eliminates the need for a nipple (a short piece of threaded pipe). A bushing is a reducer used to connect a small pipe to a larger fitting. It has external and internal threads. The external thread is screwed on the larger pipe while the smaller pipe or fitting screws on the internal thread. A plug has external (male) threads on one end and is used to seal off a pipe run. Nipples are short pipe segments that allow screw and socket-weld fittings to be joined. Normally this would be impossible in FMU since screw fittings have internal threads and socket-weld fittings have internal sockets. A common minimum nipple length is 75 mm (3 ) or 100 mm (4 ). Swages are reducers with different end preparations designed for butt-weld, screw and socket-weld fittings. Butt-weld swages have both ends beveled (BBE), screw swages have both ends treaded (TBE), while socket-weld swages have both ends plain (PBE). Screw swages have external threads and so do not need a nipple for use. Both concentric and eccentric swages are available. Olets When small diameter branch pipes are to be connected to much larger diameter header pipes, olets are used. Olets are standard fittings used to directly connect smaller pipe with the header pipe with the end on the header pipe shaped to its contour. They are commonly used for connecting instruments to pipes and equipment. There are different types of olets such as weldolet, elbolet, latrolet sockolet, brazolet, threadolet, coupolet, etc. Various kinds of olets are shown in Fig. 4. Weldolets are butt-welded to both the header and branch pipes. Weldolet connects a header pipe to a branch pipe at 90 o. Elbolet allows a tangential branch connection and latrolet allows a 45 o branch off from a header pipe. Threadolets are welded to the header pipe but provide a screw joint on the branch pipe. Brazeolets are brazed to both the header and branch pipes but this joint type is not common anymore. Sockolets are used to make socket-weld fitting connections. Olets come in various sizes up to 75 mm (3 ) but 150 mm (6 ) olets may be found in some installations. 7
Fig. 4: Some olets Stub-In Connection A stub-in connection is used as an alternative to a reducing Tee when it is cheaper or when standard fittings are not available. It is a welded connection between a header pipe and a branch pipe without standard fitting. For standard pipe sizes, the branch pipe normally has a diameter of two nominal sizes or more less than the header pipe but not small enough for an olet connection. The branch pipe is directly welded on the header with a cutout first created on the latter. The cutout bore on the header pipe may be made equal to the OD or ID of the branch pipe. The branch pipe is fitted and welded to the header pipe. The cutout weakens the header pipe, so some analysis should be done to ensure the strength integrity of the joint. Sometimes reinforcement pad (repad) and or reinforcement saddle (reddle?) are needed to ensure structural integrity of the joint. Repads and reddles are shaped to conform to the curvature of the header pipe. Stub-in connections are done on pipe sizes 50 mm (2 ) and above. Piping Joints Pipe joints ensure proper connection between two or more segments of pipes. They form the interface between the ends of pipe segments and connectors. Three common pipe joints are flanged, welded and screwed. Figs. 5 and 6 show samples of these joints. Joint Types Welded joints are used in situations of high pressures and temperatures or when permanent joints are preferred. Two types of welded joints: butt-weld and socket-weld joints are used. Butt-welding with beveled end is the most common method for joining pipes. The weld is strong, leak-proof, and needs little or no maintenance. Socket-weld joints are normally used in high pressure applications and for smaller pipe sizes. The pipe end is inserted into a recess and welded. Welded joints use smaller spaces compared to the other types. Screwed joints are used in applications with 2.5 pipe diameter and preferred in lower temperatures and pressures environment. It is the least leak-proof joint compared to the others. The threads are usually coated with a special lubricant to ease the connection process and seal the joint. Socket-weld fittings offer greater strength than screw fittings, so fabricators prefer the former. They are also easier to hold and weld compared to butt-weld fittings. a) Butt-welded joint b) Socket-welded joint Fig. 5: Types of permanent joints 8
a) Flanged joint b) Screw joint Fig. 6: Types of detachable joints Flanged joints are used when occasional disassembling and re-assembling of components are required. The flanges are often bolted or glued together. The outside diameter of a flange is greater than the pipe diameter. Most flanges are forged steel, cast iron and iron and have evenly spaced bolt-holes. They occupy the most space compared to other types of joints. A gasket is placed between the two faces of the flanges. It acts as a sealant and provides resilience in the joint for accommodating minute misalignments. Flange Types There are several designs in flanges and the list includes weldneck, slip-on, lap-joint, reducer or expander, screw, socket-weld, blind, etc. The weldneck and slip-on flanges are very popular. Weld neck flange (Fig. 7) is a disk with a long hub or neck. The disk has holes for bolting. The hub is welded to pipe. The hub inside diameter is the same as inside diameter of pipe. Nozzles on tanks and vessels are special types of weld neck flanges. Weld neck flanges are the most reliable flanges and are employed where bending loads are expected. They are placed on pipes at locations of fittings and instruments. It is used in severe service such as high temperatures and pressures or cryogenic conditions. Fig. 7: Weld neck flange Slip-on Flange: Has a disk with a very short hub. The disk has holes for bolting. The flange slips over the pipe outside diameter during construction. It is used mostly for mounting valves in lines. Reducer/Expander Flange: This type of flange functions as flange and reducer or expander. There is need to be sure of specification before application. Threaded Flange: This is similar to slip-on flange but has a threaded bore, so it can be assembled without welding. Threaded flanges are easy to work with. They can be used in conditions where welding may create hazard. Seal weld is sometimes used on the threads to minimize leakage. Socket Weld Flange: This is also similar to slip-on flange, but has a bore or recess for welding it to pipes. It is used in high pressure, small diameter size (4 or less) situations. It may be welded in the inside but must be ground smooth to minimize turbulence. Lap-joint Flange: This consists of a disk and a stub hub. The disk is similar to slip-on flange and can be made of carbon steel. The stub hub is made of stainless steel. Lap flange is used as joint in mainly stainless steel pipes. 9
Blind Flange: This is a disk without a central hole that is used as a temporary seal for a pipe. It is used where future expansion is anticipated. Orifice Flange: This is a special flange used with an orifice plate to create an orifice flow meter. An orifice meter has two orifice flanges. Each orifice flange has two drilled and tapped hole (0.5 diameter for NPS of 4 or larger) either at 90 o or 180 o apart. During assembling, the two orifice flanges are bolted together with after the orifice plate and gaskets are placed between the flanges. Flange Face Styles Flanges have different face designs or styles such as flat, raised, or ring-joint face (Fig. 8). Flat face flanges have flat connecting surfaces. These are commonly found in 150# and 300# forged steel flanges. They are used to connect with 125# and 250# cast iron flanges to ensure full surface contact, thus minimizing the cracking of brittle cast iron flanges. Raised face flanges have a raised face within the bolt circle diameter. Contact in assembled joint occurs over the raised face which is 1/16 thick for 150# and 300# flanges and ¼ for 400# and above. The raised faced flange is the most common type of flange face design. The raised face thickness for 400# and above must be added to flange dimensions. This is not necessary in the smaller size flanges. The ring-type joint is similar to the raised face joint but has a groove that accommodates a metallic ring gasket. This is considered to be the most efficient joint seal in piping systems. a) Flat face (FF) flange b) Raised face (RF) flange c) Ring joint face (RJF) flange d) Flange assembly elements Fig. 8: Some types of flanges 10
Nozzle Projections Nozzles are special weld-neck like flanges that provide points of connection between equipment and fluid lines. They are integral components of equipment and consist of a flange and a hub. The flange usually has a number of bolt holes that allow a fluid line to be joined by bolting. A gasket is always placed between the two flanges in the joint. The projection of nozzle is the perpendicular distance between the flange face and the body of the equipment. The projection is influenced by nozzle size but is normally the same value for all small nozzles and another fixed value for all large nozzles on any equipment. For example, all maintenance holes (nozzles) would have the same projection while other smaller nozzles have a smaller but the same projection. Table 1 gives some suggestions. Nozzle Size (in) < 8 8-12 > 12 Projection (in) 6 9 12 Table 1: Suggested nozzles projections Accessory Fittings Miscellaneous fittings include gaskets, studs, bolts and nuts, vent and drain connectors. Gaskets are required between flanges, studs and nuts or bolts and nuts are needed in flange assemblies, while vents and drain help keep piping systems safe and clean. Gaskets: Gaskets are materials used primarily to ensure proper sealing of flanged joints. They can compensate for minute misalignment too. Gaskets are made of materials softer than the other joint materials. They may be made of copper, lead, asbestos, rubber, Teflon, or neoprene. The three types of gaskets common in piping systems are full face, flat ring, and metal ring. Full face gaskets are used on flat face flanges, flat ring gaskets are used on raised face flanges, and metal ring gaskets are used in ring-type joint flanges. Gaskets may be made of various materials and thickness, but the 1/16 and 1/8 thick gaskets are quite popular in process piping. Vents and drains Piping systems need high point vents and low point drains, especially if they are to be hydrotested. Vents permit trapped air to escape into the atmosphere. Without vents, trapped air causes pressure level fluctuations during testing. Drains allow lines to be cleaned and maintained. It also allows lines to be emptied of commodity and be filled with test fluids. Strainers and filters Strainers and filters are used to remove dirt, undesirable or unwanted particles from line products. Strainers are coarse filters and are employed for particle size of 75 microns (about 200 mesh). and above while filters are used for fine particle size of less than 75 microns. The average human eye detects about 50 to 70 microns and most people cannot see particle size smaller than 45 microns (about 325 mesh). It is more beneficial to install a strainer before a filter because the strainer protects the filter from being quickly clogged with large particles. The filter then, does not need frequent cleaning. A Y-strainer is common on discharge lines while a basket strainer is common in suction lines and can screen particle size of 0.25 mm (0.001 in.) or larger. Generally, strainers are selected based on purpose, operating and maintenance philosophies, commodity, entrainments, space, and cost. Threaded Fasteners: Bolts and studs are the common threaded fasteners used in flanged pipe joints. Piping joint stud is a headless fastener with cylindrical shank that is threaded. Nuts are used on both ends in assembling a stud and is commonly used in piping systems with high pressure rating. A bolt is a fastener with a cylindrical shank that has a head and is threaded on the other end. An unthreaded portion lies between the bolt head and the threaded end. Studs and bolts in piping systems are available in two grades: A-193-B7 and A-193-B16. A-193-B7 grade bolts are used for temperatures up to 1000 o F and A-193-B16 are used when temperatures are above 1000 o F. Bolt holes in flanges are in even numbers: 4, 8, 12, 16, etc. Bolt circle diameter and bolt sizes of flanges of the same pressure rating are designed to match. ANSI standards require 11
all flanges to have bolt aligned vertically or horizontally with center lines of pipes or equipment, except otherwise noted on drawings. A thread specification provides necessary information about the thread for manufacture or purchase. Threads may be specified in basic or detailed format. Fig. 9a shows a basic specification of a Metric thread while Fig. 9b shows a detail specification. Fig. 10a and 10b show the basic and detail specification of threads respectively in the English units. Table 2 gives the interpretations of the thread elements shown in Fig. 9 while Table 3 gives the interpretation of the thread specifications interpretations of the thread elements shown in Fig. 9. The threads per inch (TPI) element of English thread, is the reciprocal of the thread pitch. a) Basic specification b) Detail specification Fig. 9: Metric thread specifications ITEM Description ITEM Description 1 Metric thread identifier 5 Major diameter tolerance specification 2 Major diameter (mm) 6 Minor diameter tolerance specification 3 Separator 4 Pitch (mm) Table 2: Interpreting Metric thread specification a) Basic specification b) Full specification Fig. 10: English thread specifications ITEM Description ITEM Description 1 Major diameter or Number reference 7 Left hand thread (RH = Right hand thread) 2 Threads per inch (TPI) 8 Number of starts 3 Unified National 9 Separator 4 Coarse (Series identifier) 10 Length value 5 Class 11 Length identifier 6 External thread (B = Internal thread) Table 3: Interpreting English thread specification (Fig. 13) 12
Fig. 11 shows a basic specification of a metric bolt. Fig. 11: Bolt Summary Pipe fittings are components that can be used to extend pipe runs and add branches and instruments. They include connection fittings, joint fittings and other miscellaneous fittings. Connection fittings or connectors are used to join two segments of a pie run. They can also be used to change the flow direction of fluid in a pipe, enlarge or reduce the pipe size. Connectors may be butt-weld fittings, screw fittings or olets. Among butt-weld connectors are elbows, Tees, reducers, nipples, and swages. A stub-in connection is used as an alternative to a reducing Tee when it is cheaper or the pipe sizes are not standard. Forged screw fittings are used mainly in 3000 psig and 6000 psig pressure classes and include union, half coupling, street elbow, bushing, and plug. Olets are standard fittings used to directly connect a small branch pipe to a header pipe with the end on the header pipe shaped to its contour. They are commonly used for connecting instruments to pipes and equipment. Branching in pipes can be achieved with a Tee, olet or stub-in connection. Pipe joints ensure proper connection between two or more segments of pipes. They form the interface between the ends of pipe segment and connectors. Three common pipe joints are flanged, welded and screwed. There are different types of flanges such as weldneck, slip-on, lap-joint, etc. The orifice flange is a special type of flange used in the assembly of an orifice meter, an instrument used in measuring the flow rate of fluids in a pipe. A gasket is required between a pair of flanges. Fitting Make-Up or FMU is a fitting assembly with all the fittings directly connected to each other without a pipe segment between. When FMU is not the case, the minimum length for a pipe segment between fittings is limited to one pipe nominal size for pipe sizes of 100 mm (4 ) and above and 75 mm (3 ) for smaller pipes. Welding is the principal method of assembling pipe runs in the industry today. Screwed joints are more commonly used for smaller diameter pipe assembling. Socket-weld fittings need extra pipe length than butt-weld fittings because some portions of the pipe must engage with socket holes on fittings. Similarly, screw fittings need extra pipe length than butt-weld fittings because some portions of the pipe must engage with threads in screw fittings. Accessory fittings include gaskets, bolts and nuts, vents and drains. A pair of flanges needs a gasket for assembly. Flanged joints are assembled with bolts and nuts or studs and nuts. Studs are used for joints with high pressure rating. Vents provide outlets for trapped gasses or for pressure relieving devices. Drains allow flushing a piping lines as well make maintenance easier. Forged steel fittings can withstand high pressure and high temperature in service. They offer the most flexibility in sizes and applications. Cast iron fittings are designed for low pressure and lower temperature applications, especially gravity-flow systems. There is a large number of assortments of cast iron fittings and they can be laid out in several ways. Plastic fittings are also available: nominal sizes of 100 mm (4 ) and below are made for screw and socket assembly, while sizes 150 mm (6 ) to 250 mm (100 ) are made for butt weld assembly. 13