1.5 Pipe and Tube Manufacturing Terminology
1.5.1 Continuous Weld (CW) or Butt Weld (BW)
1.5.2 Electric Resistance Weld (ERW)
1.5.3 Double Submerged Arc Weld (DSAW)
1.5.4 Spiral Weld (SPRLW)
1.5.5 Seamless (SMLS)
1 PIPE AND FITTINGS
1.1 A Brief History
Piping has been discovered in civilizations dating back many thousands of years. Initially pipe materials were limited to locally available natural substances, such as wood, stone and clay. Wooden pipes made from logs have been uncovered in many parts of the world and clay pipes dating back 6000 years have been discovered in ancient ruins such as Babylon. Sewage pipeline systems made from bored logs were laid in Boston, Philadelphia and New York as late as the 18th century. With the ability to smelt metals, society also turned to iron, brass, copper, and lead. The Greeks and Romans both experimented with lead and a lead piping system has been uncovered in the ruins of Pompeii, complete with bronze plug valves, just as it was when covered by ash in 87 BC.
The invention of the cast iron cannon brought about the adaptation to cast iron pipe. It isn’t surprising therefore that the first pipe manufacturers were the same that had the processes to manufacture cannons and muskets. Cast iron pipe was used in Germany as early as the 15th century, and by the French in the 17th century to supply water to the fountains at Versailles.
The first strong but economical pipe was made in 1825 when a method of manufacturing pipe from long strips of hot metal was devised. As the Industrial Revolution gained momentum and the efficiency of the steam engine was improved, the need for stronger piping materials that could contain higher pressures and temperatures became necessary. The discovery of the Bessemer process in 1855 and the open-hearth process in 1861 were important steps that made steel pipe widely available.
In the early part of the 19th century industry recognized the need for standardization, and the Iron Pipe Size (IPS) system was established. This system standardized an outside diameter and a wall thickness for each pipe size, and designated that pipe be referred to as 2″, 3”, 4″, 6″, and so on, according to the approximate inside diameter in inches, a practice that continues to this day. The one wall thickness per pipe size became known as standard weight (STD WT) with the introduction of two further heavier wall thicknesses required to satisfy the growing industrial demands for the handling of higher pressure fluids. These heavier walls, known as extra strong weight (XS WT) and double extra strong weight (XXS WT), were accomplished by reducing the pipe inside diameter while the standardized outside diameter remained unchanged.
In the post-World War 1 U.S. the power and process industries began to expand greatly, with the eventual need for even higher-pressure and higher-temperature materials and joining methods. Advances in welding technology were made, stronger more corrosion resistant materials were developed and manufacturing and testing methods of pipe and fittings were improved. For example, seamless pipe formed from solid billets of steel became common. The three IPS wall thicknesses no longer sufficed for all applications, and as a result a new method of specifying pipe size and wall thickness became necessary. In 1927 the American Standards Association (ASA), later to become the American National Standards Institute (ANSI), started the development of the Nominal Pipe Size (NPS) system to replace IPS. This system incorporated a wider range of wall thicknesses than the previous IPS system. In this system pipe is specified by two dimensionless designators: a nominal pipe size (NPS) for the diameter and a schedule number (Sch.) for the wall thickness.
Piping systems include pipe, flanges, fittings, bolting, gaskets, valves, and other pressure-containing components such as expansion joints and instruments. In today’s society piping systems transport products and waste of all types of fluids, and in all types of industries: residential, commercial, and industrial. They are designed to specific codes and standards for numerous applications. The design, construction, operation, and maintenance of piping systems involves an intimate understanding of piping principals, materials, generic and specific design considerations, fabrication and installation, examination, testing and inspection requirements, and application of local, provincial and federal regulations.
1.2 Codes and Standards
The assurance that the design and construction of a piping system will meet prescribed pressure-integrity requirements is achieved through the use of published codes and standards. Codes usually set forth requirements for design, materials, fabrication, erection, testing, and inspection of piping systems, whereas standards contain design and construction rules and requirements for materials and individual piping components such as elbows, tees, flanges and valves. Compliance to standards is normally required by the rules of the applicable code or the purchaser’s specifications. Compliance to codes is at the least generally imposed by insurance companies and owners’ requirements, but most often is mandated by government legislation enforced by regulatory agencies to ensure the safety of workers and the general public.
Two such legislations in Alberta are the Safety Codes Act and the Pressure Equipment Safety Regulation, enforced by ABSA the pressure equipment safety authority for Alberta. Two codes mandated in the legislation are: CSA B51, Boiler, pressure vessel, and pressure piping code, and CSA Z662, Oil and gas pipeline systems.
Codes and standards relating to piping systems and piping components are published by various organizations. These organizations have committees made up of representatives from industry associations, manufacturers, professional groups, government agencies, insurance companies, and other interested parties. The committees are responsible for maintaining, updating and revising the codes and standards in view of technological developments, research, experience feedback, problems, and changes in referenced codes, standards, specifications, and regulations. The following describes several of the organizations most often encountered:
The American Society for Testing Materials, now called ASTM International, is an organization that develops and publishes standards on the characteristics and performance of materials, products, systems, and services. The standards include test procedures for determining or verifying characteristics, such as chemical composition, and measuring performance, such as tensile strength and bending properties. The standards cover refined materials, such as steel and plastic, and basic products manufacturing methods and tolerances, such as for pipes and tubes.
The American National Standards Institute was earlier known as the American Standards Association (ASA) and for a short period of time, from 1967 to 1969, as the United States of America Standards Institute (USASI). ANSI provides a forum for development or obtaining a consensus for approval of standards having national impact and serves as a focal point for distribution of national and other standards, including those developed and issued by the International Organization for Standardization (ISO) and foreign governments. Development and approval functions are performed by the various committees. Many of the committees are chaired or sponsored by engineering societies such as ASME. Safety is the basic objective of the engineering design and construction requirements contained in ANSI standards. The standards include prohibition for practices considered unsafe and cautions where advisory warnings, instead of prohibitions, are deemed necessary. Many ANSI codes and standards have become ASME codes and standards. For example: the original ANSI B31, Pressure Piping Code has become ASME B31, Pressure Piping Code, and ANSI B16.5, Pipe Flanges and Flanged Fittings has become ASME B16.5 Pipe Flanges and Flanged Fittings. However, they are subjected to the approval of ANSI.
The American Society of Mechanical Engineers is one of the leading organizations in the world that develops and publishes codes and standards. Notably, the ASME established a committee in 1911 to formulate rules for the construction of steam boilers and other pressure vessels. This committee is responsible for the ASME Boiler and Pressure Vessel Code, an 11-section code covering the materials, construction and testing of piping, boilers, and pressure vessels in many different industries, including the nuclear industry. Other committees are responsible for the sections of the ASME B31, Pressure Piping Code.
The American Petroleum Institute publishes specifications, bulletins, recommended practices, standards and other publications as an aid to procurement of standardized equipment and materials. These publications are primarily intended for use in petroleum production and transmission facilities, specifically oil and gas pipelines. Examples of API standards are API 650, Welded Tanks for Oil Storage, and API 5L, Specification for Line Pipe.
The Manufacturers Standardization Society of the Valve and Fittings Industry publishes Standard Practices, which provide a basis for common practice by manufacturers, the user, and the general public. The MSS is represented on the committees of other standardization groups, such as ANSI and ASME. The existence of a MSS Standard Practice does not preclude the manufacture, sale, or use of products not conforming to the Standard Practice. Mandatory conformance is established only by reference in a code, specification, sales contract or public law, as applicable.
A MSS Standard Practice will often fill gaps not covered by other organizations and may lead to the development of other standards. For instance, MSS SP-44, Steel Pipeline Flanges, was originally written to provide a standard for flange sizes larger than NPS 24 not covered by ASME in the B16.5 standard, and later formed the basis for the ASME standard B16.47, Large Diameter Steel Flanges NPS 26 through NPS 60. Many of the ASME B16 series standards were originally developed as MSS Standard Practices.
The Canadian Standards Association publishes codes and standards to enhance the safety and health of the Canadian public. For instance, the CSA Z662, Oil and Gas Pipeline Systems, specifically deals with the design, construction, operation, and maintenance of oil and gas industry pipeline systems in Canada.
1.3.7 Other Organizations
While the above may be some of the more commonly encountered organizations, there are many other organizations to be aware of that make their own significant contributions to industry, such as:
• The National Association of Corrosion Engineers, now called NACE International.
• The Tubular Exchanger Manufacturers Association, Inc. (TEMA).
• The National Fire Protection Association (NFPA).
• The American Water Works Association (AWWA).
• The American Welding Society (AWS).
1.4 Pipe and Tube Definitions and Dimensions
Pipes and round tubes are separate, although quite similar, products. The term tubular product, which infers a long hollow cylinder, is applied to both, and both are fabricated of materials and to dimensions governed by ASTM and ASME standards. The main differences between the two are the end uses and that pipe is specified by nominal diameter and schedule for wall thickness, while round tube is specified by outside diameter and gauge for wall thickness.
A tube with a round cross section conforming to the dimensional requirements for nominal pipe size as tabulated in ASME B36.10M, Welded and Seamless Wrought Steel Pipe, and ASME B36.19M, Stainless Steel Pipe, and materials per the appropriate ASTM or API specifications. Products called pipe include conduit pipe, piling pipe, transmission (line) pipe, and pressure pipe.
An initial standardization between manufactures was the Iron Pipe Size (IPS) system which established an outside and an inside diameter for each of the various pipe sizes. Pipe was referred to by its approximate inside diameter in inches, e.g. 2″, 3″, 4″, and so on. Later, two heavier wall thicknesses were added, making three wall thicknesses per pipe size available: the existing standard weight (STD), plus the designations of extra strong (XS) and double extra strong (XXS). Extra strong and double extra strong are sometimes referred to as extra heavy (XH) and double extra heavy (XXH). Eventually the demand grew for an even broader range of wall thicknesses and the Nominal Pipe Size (NPS) system was introduced. The outside diameters (OD) established in the IPS system were retained, but in this system the correct nomenclature for pipe size became NPS followed by a dimensionless designator, e.g. NPS 2, NPS 3, NPS 4, and so on. Wall thicknesses (WT) are expressed as a schedule number (Sch. 5, 10, 20, 30, 40, 60, 80, 100, 120, 140, or 160). While it was intended that the three IPS wall thickness designations would be phased out, these were retained and correspond to certain schedule numbers: STD corresponds to Sch. 40 in pipe sizes NPS 10 and smaller; XS corresponds to Sch. 80 in pipe sizes NPS 8 and smaller.
In both the IPS and NPS systems the outside diameter of a given pipe size remains constant and the inside diameter (ID) decreases as the wall thickness increases. Refer to Figure 1-1 Pipe Dimensions. For pipe sizes NPS 12 and smaller the actual OD is larger than the nominal OD. For NPS 14 and above the actual OD and the nominal OD are the same, e.g. a NPS 12 pipe has an actual OD of 12.75″, and a NPS 14 pipe has an actual OD of 14″. Be aware that, although available, certain NPS sizes are not commonly used: 1¼, 2½, 3½, and 5.
The original intent of the NPS system was to establish a dimensionless system of schedule numbers for wall thickness and pipe size combinations having an approximately
uniform relationship equal to the expression 1000 x P/S, where P is the internal working pressure (psig) and S is the allowable stress (psi) for the pipe material at temperature, derived from the pipe wall thickness formula used at the time. However, while many texts still state this relationship, the official ASME position is that the numbers deviated appreciably from the common wall thicknesses in use and this original intent could not be accomplished, rendering the schedule numbers as a strictly convenient nominal number for use in ordering. (Note that the B31 code requires the use of a more complex hoop stress calculation to determine the actual pipe wall thickness to be used. This calculation takes into account mill tolerance, corrosion allowance, thread allowance, and seam weld strength.)
Stainless steel pipes, which were coming into more common use in the mid 20th century, permitted the use of thinner pipe walls with much less risk of failure due to corrosion. By 1949 thinner schedules 5S and 10S, which were based on the pressure requirements modified to the nearest Birmingham Wire Gauge (BWG) number, had been created, and other S schedule sizes followed (20S, 30S, 40S, and 80S). Schedule numbers followed by the letter S
are primarily intended for use with stainless steel pipe, and may or may not be the same wall thickness as that specified for a schedule number without the letter S. Note that due to their thin walls, the smaller S sizes cannot be threaded together according to ASME code, but must be fusion welded.
Tube is defined as: a hollow product of round or any other cross-section having a continuous periphery. Round tube size is specified with respect to outside diameter and wall thickness. Dimensions and materials are specified in the appropriate ASME or ASTM specifications. Pressure tubes are used in externally fired applications while carrying pressurized fluids. Structural tubing is used for general structural purposes related to the construction industry. Mechanical tubing is produced to meet particular dimensional, chemical, and mechanical property and finish specifications, which are a function of the end use, such as machinery and automotive parts. The biggest use of tubing in the process piping industry is instrument tubing, heat tracing, and heat exchanger and boiler tubes.
Any tubular products not built to ASME B36.10 or B36.19 dimensions are called tubing. Round tubing is referred to by its actual OD and WT. 1″ tubing is actually 1″ OD, 3″ tubing is actually 3″ OD, etc. For each tube size a variety of wall thicknesses are available, so the WT must also be specified. WT is often specified using Birmingham Wire Gauge.
1.5 Pipe and Tube Manufacturing Terminology
There are many methods of manufacturing pipe and tube, but basically there are two types of pipe and tube forming processes, namely welded and seamless. Seamless pipe or tube does not have a welded seam along the length of the pipe. The seam on welded pipe is a source of potential weakness and the allowable stresses for the pipe wall are reduced by a weld joint quality factor (E) ranging from 0.6 for furnace butt welded to 1.0 for seamless pipe (60% to 100% of the allowable stress value (S) for the material) in the B31 code wall thickness formula. The weld joint quality factor is set based on manufacturing process and Non-Destructive Examination (NDE) used. A weld joint strength reduction factor (W) ranging from 0.5 to 1.0 was added to the wall thickness formula in 2004. This factor is to account for the loss of pressure retaining strength of weld metal operating at creep range temperatures above 427°C (800°F). In the revised wall thickness formula, SE is multiplied by W.
Although longer lengths may be special ordered for pipeline use, steel pipe is commonly manufactured in random lengths, approximately 20′ (6m) long, and double random lengths, approximately 40′ (12m) long. The lengths vary due to the removal of damaged ends. The wall thickness of pipe is also subject to an allowable mill tolerance of -12.5% that must be taken into account when doing wall thickness calculations.
There are three end finishes available: beveled end (BE), plain end (PE), and threaded and coupled (T&C) to match the three most commonly used joining methods.
Some common manufacturing methods follow:-
1.5.1 Continuous Weld (CW) or Butt Weld (BW)
To produce this product a continuous flat strip of steel called a skelp is fed into a furnace and heated to approximately 2500°F. The hot strip is then shaped by a series of rollers into a round. The edges are superheated by a blast of air and the rolls provide sufficient pressure to form a fusion weld. This pipe is manufactured in size ranges of NPS 1/8 to NPS 3.
1.5.2 Electric Resistance Weld (ERW)
There are several electric welding methods employed in this type of manufacture: flash welding, low-frequency resistance-welding, high-frequency induction-welding or high-frequency resistance-welding. All processes begin with the forming of the cylinder with the longitudinal seam butt edges ready for welding. A skelp is gradually formed into a cylindrical shape by a series of rollers. As the two edges come together they are fed into a electric arc welder. The welding operation consists of copper electrodes making contact on each side of the open seam. A combination of the high temperature caused by the weld current passing from one copper electrode to the other, and the pressure exerted by the rolls, results in a fusion of the strip edges. No weld material is added. This type of pipe is manufactured in the size ranges up to NPS 42.
1.5.3 Double Submerged Arc Weld (DSAW)
A set of rollers pre-forms the edges of a steel plate with a curve. Pre-forming of edges ensures the proper curvature results in the O-Press forming operation. The plate moves into position in the U-Press where a U-Die pushes the plate down between the breaker rolls. At the same time the breaker rolls move together. The U-Die is retracted leaving a “U” shaped product. The “U” shaped plate moves into the O-Press, which consists of two semi-circular dies. The top semi-circular die moves down to form the “U” into an “O”.
A series of belts surround the pipe at intervals to hold the free edges tightly together. The edges are tack welded to secure the “O” form. A boom containing a welding head is inserted through the length of the pipe to create a continuous internal weld. An external weld is performed at a different welding station. This type of pipe is manufactured in the size ranges of NPS 20 to NPS 42.
1.5.4 Spiral Weld Pipe (SPRLW)
In this process, narrow sheets of steel sheet are helically wound into cylinders. The edges of the strip can either be butting or overlapping and are welded by any of several electric arc-welding processes. Spiral weld pipe is suited to larger diameter and is manufactured in size ranges NPS 36 to NPS 60.
1.5.5 Seamless (SMLS)
This is a hot formed product. Seamless tube and pipe are manufactured by first producing a hollow tube which is larger in diameter and thickness than the final product. Various piercing methods are employed dependent on pipe size range and material, but all follow a similar manufacturing process. Billets of steel are heated in a furnace, and then each billet is conditioned by a machine similar to a lathe. In the piercing mill, the billet is gripped by rolls that rotate and advance it over the piercing point, forming a hole through the length. Large sizes must go through more than once. From the piercing mill, the pierced billet is put through a plug-rolling mill where the billet is rolled over a mandrel to reduce the diameter and wall thickness and to increase the length of the pipe. A rotary mill is used for NPS 16 and over, the diameter is enlarged, and the wall thickness is reduced to approximate finished dimensions. From there the pipe passes through the sizing mill on a series of rolls that create a true round shape of the required diameter and wall thickness. As no weld exists the weld joint quality factor is rated as 1.0.
Pipe and piping components are made from many different materials. One of the most common materials is carbon steel, although steels made with other alloying elements such as manganese and chromium are also widely used. The choice of material is based on the following requirements:
• Service conditions – pressure, temperature and type of fluid.
• Corrosion and erosion resistance.
• Ease of welding or bending.
Each material is manufactured to an ASTM standard. The most common piping materials are specified by ASTM with the following prefixes:
• A – Ferrous metals.
• B – Non-ferrous metals.
• C – Ceramics and concrete.
• D – Plastics.
Some common ASTM standards for steel pipe are A106 Gr. B (carbon steel for high temperature service), A333 Gr.6 (carbon steel for low temperature service) and A312 TP316/316L (stainless steel for corrosion resistant service). Figure 1-3 Typical ASTM Specifications for Materials lists other common standards.
The standards are further subdivided into various grades, e.g. Grade A, B, C and Grade 1, 2. The grades indicate different chemistry of the same basic steel, such as a higher carbon content, which increases the tensile strength but tends to make the steel more brittle (less ductile). Note: ASME add an S prefix to the ASTM code for piping materials approved for use in boilers and pressure vessels, e.g. SA-106.
1.7 Fittings and Joining Methods
Fittings come in a range of materials, wall thicknesses and end connections to join with pipe. There are three basic types of end connections for the three basic pipe-to-pipe and pipe-to-fitting joining methods: threaded (Thrd), socket-weld (SW), and beveled end (BE). Fittings with threaded or socket-weld ends, although available in all pipe sizes up to and including NPS 4, are, generally speaking, only used in piping systems NPS 1½ and smaller, often referred to as small-bore piping. Fittings with beveled ends, although available in all pipe
sizes down to and including NPS 1/8, are, again generally speaking, only used in piping systems NPS 2 and larger, often referred to as large-bore piping.
1.7.1 Threaded (Thrd/Scrd)
Threaded fittings, also known as screwed (Scrd) fittings, are manufactured according to ASME B16.11, Forged Fittings, Socket-Welding and Threaded. These threaded fittings come in Service Classes 2000, 3000 and 6000, with threads that conform to the American National Standard Taper Pipe Threads (NPT). The service classes correlate to pipe schedule/wall thickness: 2000 equals Sch. 80/XS, 3000 equals Sch. 160, and 6000 equals XXS. The fittings have female threaded end connections, while the pipe has a male threaded end which must be threaded into the fitting a minimum distance; this is referred to as a thread engagement length. Refer to Figure 1-4 Socket-weld and Threaded Insertions. Piping systems made of this type of joint are economical and can be rapidly assembled, however the threads are prone to damage, and leakage.
1.7.2 Socket-weld (SW)
Socket-weld fittings are also manufactured according to ASME B16.11 and come in Service Classes 3000, 6000 and 9000. As with threaded fittings, the service classes correlate to pipe schedule/wall thickness: 3000 equals Sch. 80/XS, 6000 equals Sch. 160, and 9000 equals XXS. These socket-weld fittings have female connections whose ID is very slightly larger than the pipe OD in order to accept the insertion of plain end (PE) pipe. The distance that the pipe is inserted into the fitting is referred to as the insertion depth. Refer to Figure 1-4 Socket-weld and Threaded Insertions. An approximately 1/16” gap is left between the shoulder of the socket and the end of the pipe to allow for expansion during welding and operation. A full 360O fillet weld holds the pipe into the fitting. Piping systems made of this type of joint are economical in the smaller size ranges, can be rapidly assembled, can accept a certain amount of misalignment, and are strong and leak free; however, the gaps can be a source of sludge build-up and corrosion.
1.7.3 Butt-weld (BW)
Fittings with beveled ends are referred to as welded fittings and are manufactured according to ASME B16.9, Factory-Made Wrought Steel Buttwelding Fittings. This standard requires that the butt-weld fittings must be at least equal to seamless pipe in strength.
The end preparation of butt-welded pipe and fittings is known as a beveled end (BE) because the pipe or fitting end has an angled bevel. The companion bevels (pipe-to-pipe, pipe-to-fitting, or fitting-to-fitting) are welded together by a full 360° circumferential butt weld, also called a full penetration weld because its depth is fully to the inside of the pipe. Piping systems made of this type of joint are strong and leak proof, and, because the weld can be x-rayed to prove integrity, are well suited to severe service, such as vibration. However, they require accurate alignment, and weld icicles inside the pipe can cause sludge build-up.
1.7.4 Other Joining Methods
As stated, the three joining methods discussed above are the most common used for pipe and fittings, but it is worth mentioning that there are other joining methods. One of these, often found in lower temperature and pressure water service applications, is the grooved joint method. Made famous by the Victaulic Company, a 360° groove is placed a short distance from the end of the pipe or fitting. The two ends of pipe and or fittings are held together by means of a two bolt clamp that includes a gasket that fits into the grooves.
Another joining method is a flanged joint. Flanges are most often attached to pipe and fittings by one of the three common joining methods, but, as will be discussed in more detail later, the flanged joint is also a common joining method unto itself for connecting to valves, instruments and equipment. Also, fittings with flanged ends are available for use in piping systems where welding is impractical, such as is the case with lined piping systems.
1.8 Types of Fittings
A fitting is a piping system component used for: branching; reducing pipe size; changing direction; joining of pipe and piping components; and pipe end closure.
1.8.1 Branch Fittings
Branch fittings allow branches to be made on pipes for splitting or combining fluid flow, or for instrument connections. Common branch fittings are tees, branch-reinforced fittings, and laterals.
• Tees are available in straight or reducing configurations. Straight tees get their name from the fact that the run and the branch are the same NPS size. Reducing tees have a smaller NPS branch size than the run. Straight tees are referred to by the run and the branch size as one, e.g. an 8 inch tee has an NPS 8 run and NPS 8 branch. Reducing tees are referred to by the run size times the branch size, e.g. an 8 x 6 tee is one with an NPS 8 run and a NPS 6 branch.
• Pioneered by the Bonney Forge Corporation in the nineteen-thirties and manufactured to MSS SP-97, Integrally Reinforced Forged Branch Outlet Fittings – Socket Welding, Threaded, and Buttwelding Ends, branch reinforced fittings are commonly referred to as Olets and primarily consist of thredolets (TOL), sockolets (SOL) and weldolets (WOL). These are welded directly to the pipe, and are referred to by the pipe size they are welded to times the branch size, e.g. a 10 x 2 WOL is one where the pipe run is NPS 10 and the branch pipe size to be matched is NPS 2.
• Laterals are similar to tees with the exception that the branch is at a 45° angle to the run.
1.8.2 Reducing Fittings
Reducing fittings allow for a change of pipe size in-line. They are available in concentric and eccentric configurations.
• Reducers are used in large-bore piping and are referred to by the large end times the small end, e.g. a 10 x 8 reducer is one that reduces the pipe size from NPS 10 to NPS 8. They come with beveled ends for butt-welding.
• Swages are used in small-bore piping and differ from reducers in that they have a tangent portion for insertion into the female threaded or socket-weld fitting. They are available in all combinations of end connection types, which must be noted when ordering, e.g. Plain Large End (PLE) x Threaded Small End (TSE), or Threaded Both Ends (TBE).
1.8.3 Change of Direction Fittings
Change of direction fittings are elbows (ELLs) fabricated to accommodate a required angle change in the piping configuration.
• Large-bore elbows come with beveled ends and are available in 45°, 90° and 180° configurations. The 90° and 180° elbows are available in Long Radius (LR) and Short Radius (SR) configurations, whereas the 45° elbows are only available in a long radius configuration. Long radius elbows have a radius equal to 1½ times the NPS, e.g. a NPS 6 LR ELL has a radius of 9”. This is also referred to as 1½ D. Short radius elbows have a radius equal to 1 times the NPS, e.g. a NPS 6 SR ELL has a radius of 6”. Longer radius elbows can be created by bending pipe. In pipelines it is quite common to specify 3D or 5D elbows.
• Small-bore elbows come with threaded or socket-weld ends and are only available in 45° or 90° configurations, with one radius for any given NPS pipe size.
1.8.4 Joining Fittings
Joining fittings are used in piping systems to connect lengths of pipe, and to connect pipe to instruments, valves, and equipment.
• Couplings are used to join lengths of pipe in small-bore pipe configurations. They are available in threaded or socket-weld end types.
• Unions are also used in small-bore piping configurations, but are used in strategic locations around instruments, equipment and valves to allow for ease of assembly and disassembly. They too are available in threaded or socket-weld end types.
• Flanges are a stud-bolted, gasketed joint used to connect to instruments, equipment and valves. They come in a variety of facings, ratings and end types to address pressure, temperature and commodity requirements. They are found commonly in large-bore piping configurations, but are sometimes also used in small-bore piping configurations in place of unions.
1.8.5 Closure Fittings
Sometimes it is required to Blind-off or Cap-off an open-ended line. An example would be where piping has been designed with future expansion in mind, or equipment has been removed for repair.
• Plugs are used in small-bore lines, in socket-weld and threaded configurations. They are of a male configuration, and are inserted into the female fitting.
• Caps come in socket-weld, threaded and butt-weld configurations. The socket-weld and threaded caps are female types that the pipe inserts into. The butt-weld caps are welded to the pipe, as per other butt-weld fittings.
• Blind flanges bolt directly to a flange, and are of equal facing and class rating.
1.9 Flanges, Gaskets, Bolting
Flanges – a stud-bolted, gasketed joint – are one of the most common methods used to join piping systems. Flanges provide a ready method of assembly and disassembly of the piping components: in-line instruments, equipment, and valves, for maintenance and repair. They come in a variety of ratings, facings, end types and materials to address pressure, temperature and commodity requirements.
Steel flanges NPS 1/8 to NPS 24 conform to ASME B16.5, Pipe Flanges and Flanged Fittings. Steel flanges NPS 26 to NPS 60 conform to ASME B16.47, Large Diameter Steel Flanges. Caution: care must be taken, by the designer, when using flanges covered in B16.47, to ensure that the bolt hole drilling pattern will match the equipment to which it will be attached. Use Series A for previous installations with MSS SP-44, Steel Pipeline Flanges, and Series B for flanges mating to API 605 (now withdrawn), Large-Diameter Carbon Steel Flanges. Orifice flanges NPS 1 to NPS 24 conform to ASME B16.36, Orifice Flanges. Cast-iron flanges NPS 1 to NPS 96 conform to ASME B16.1, Cast Iron Pipe Flanges and Flanged Fittings.
Classes (Pressure – Temperature Ratings):
Steel flanges come in seven classes of pressure – temperature ratings: 150, 300, 400, 600, 900, 1500, and 2500. Common vernacular is to make reference to flange ratings in “pounds”: 150 lb, 300 lb, etc. This is because previously, in the older ASA B16.5, reference was in those “nominal” terms. To avoid confusion, as pounds generally infers either pressure containment (psi) or weight (lbs), further revisions to the standard dropped this practice in favour of Classes. The correct terminology is now Class 150, Class 300, etc. Note: Class 400 is not commonly specified for use in the process piping industry, but may show up on some equipment.
As the class rating goes up, so does the ability to contain higher pressure – temperature combinations. Referring to the attached extract from B16.5, Tables 1A and 2-1.1, it can be seen that a Class 150 flange of Material Group 1.1 can contain 285 psig between design temperatures of -20 to 100°F. Whereas a similar flange of Class 2500 can contain 6170 psig. The higher the rating the more material (thicker the flange), and the greater the number of bolts. Bolting is equally spaced around the flange on the bolt circle diameter (BCD). 360° divided by the number of bolts gives the bolt spacing, e.g. 4 bolts will be spaced 90° apart. The number of bolts increases by groups of 4: 4, 8, 12, 16, 20, etc.
Cast-iron flanges come in four classes of pressure – temperature ratings: 25, 125, 250, and 800. Classes 125, 250, and 800, have the same number of bolt holes and BCD as per Classes 150, 300, and 600 respectively, and can be bolted directly to those flanges of the same NPS size. Because of the brittle nature of cast-iron, and therefore the risk of cracking the flanges, the mating steel flange must be flat faced with a full face gasket.
Flanges are available in different types to match the piping systems:
• Welding Neck (WN)
• Lap Joint (no abbreviation)
• Slip-On (SO)
• Socket-weld (SW)
• Threaded (Thr’d)
Welding neck, lap joint, and slip-on are primarily used in large-bore(NPS 2 and larger), butt-weld piping systems. Socket-weld and threaded are primarily used in small-bore (NPS 11/2 and smaller) piping systems. The most common type, in the petro-chemical industry , is the welding neck flange with a raised face (RFWN).
Facings and Finishes:
It is important to have a leak-free joint between the mating faces of a pair of flanges. To ensure this a variety of flange facings and gasket materials are available. The choice of facing and gasket (see next section “Gaskets”) are closely linked, and are decided based on the pressure, temperature and commodity of the piping system. There are five basic types of flange facings commonly found:
• Raised Face (RF)
• Male and Female
• Tongue and Groove
• Ring Joint (RJ)
• Flat Face (FF)
Note: ASME B.16.5 uses the nomenclature of Ring Joint and RJ. While it is quite common to hear reference to Ring Type Joint and RTJ, this should not be used.
The raised face is the most common facing employed. This face type allows the use of a wide combination of gasket designs. The purpose of the raised face is to concentrate more pressure on a smaller gasket area and thereby increase the pressure containment capability of the joint. With RJ flanges, the force on the gasket is even greater, making them suitable for use at very high pressures.
In order for the gasket to seal it is required that the flange face “bite” into the gasket material. Therefore, flange facings are machined with grooves, the heights of which are measured in microinches (in) or micrometres (m), and are stated as Arithmetic Average Roughness Height (AARH) or Root Mean Square (RMS). AARH states the average height of the grooves. RMS averages the squared heights of the grooves, and takes the square root of that number to get an average height. RMS gives a more accurate result, as squaring the height accentuates the higher spots. The facing of raised face and flat face flanges are machined with a phonographic (most common) or concentric grooved finish, the industry standard being 125 to 500 in or 3.2 to 12.5 m. Most gasket manufacturers recommend 125 to 250 in (3.2 to 6.3 m) for spiral wound, and 250 to 500 in (6.3 to 12.5 m) for sheet gaskets. See diagram – flange facing finish.
Gaskets are covered under ASME B.16.20 or B16.21, depending on the style. They are placed between mating flanges to prevent leakage. The gasket material is softer than the flange material and will fill-in the grooves of the mating faces creating a seal. Gaskets are chosen based on several factors:
• Mechanical integrity – the ability to provide sufficient strength against crushing and in-service blowouts.
• Leak tightness – the ability to be “springy” and maintain a seal at times when the flange bolts may relax.
• Fluid service compatibility – the ability to tolerate the chemical concentration in the expected temperature range, providing resistance to degradation.
• Fire safety – the ability to maintain leak tightness, for piping in flammable services, when exposed to fire.
Gasket materials include graphite, rubber, fabric, steel, aluminum, copper, brass, and monel, and are constructed in the following ways: flat, composition, corrugated, grooved, jacketed, spiral wound, and oval ring. Prior to falling out of favour, asbestos-based gaskets represented about 95 % of all the gaskets used in process piping, due to the unique properties of chemical inertness, thermal behaviour, and excellent sealability. In recent years, flexible graphite has become the leading gasket material to replace asbestos in the petro-chemical industry .
The most common gasket styles used in the petro-chemical industry are the spiral wound, in lower to medium pressures, and the oval ring (made of soft iron or stainless steel) in higher pressures.
1.9.3 Bolts and Nuts
Mating flanges are held together with bolts. Two types of bolts are common – square head machine bolts (conform to B18.2.1, Square and Hex Bolts and Screws), and stud bolts (conform to B1.1, Unified Inch Screw Threads). Machine bolts are common with cast-iron flanges, while stud bolts are commonly used to join steel flanges. Stud bolts have several advantages:
• More easily removed if corroded.
• Not easily confused with other types of bolts.
• Easier to use in confined areas.
• Can be manufactured in the field by threading compatible bar stock.
In the petro-chemical industry alloy-steel is the common bolting material, because of its improved strength and toughness over carbon-steel. Various grades of steel are available to suit the design requirements. The vast majority of studs, used in regular service, are ASTM A193 Gr.B7 with ASTM A194 Gr.2H nuts; ASTM A193 Gr.B7M studs are used in sour service; ASTM A320 Gr.L7 studs are used in cold service.
Valves are mechanical devices whose purpose is to control the flow of fluids in piping systems. A valve performs this control function by placing an obstruction in the path of the fluid. The obstruction is referred to as the flow control element. The design of the flow control element determines the type of valve and the form of control for which the valve is suited.
There are many different valves, conforming to many standards written by various organizations. API and ASME are two of the principal organizations that publish standards for the petro-chemical industry. Two of the most frequently used standards in process piping are, ASME B16.34, Valves – Flanged, Threaded, and Welding End, and ASME B16.10, Face-to-Face and End-to-End Dimensions of Valves.
Dimensions for valves are listed in B16.10 . Note that many manufacturers do not follow these standard dimensions for ball, plug, and check valves. For dimensions of these valves always refer to the manufacturers’ catalog.
Valves are manufactured in pressure classes, e.g. Classes 150, 300, etc., to match the piping system in which they are connected. Pressure classes are often referred to as pounds, e.g. a Class 600 valve is often called a 600 pound valve. This can be misleading, as it has nothing to do with weight (lbs), and a Class 600 valve can contain internal pressures of more than or less than 600 psi, dependent on temperature. See attached extract from B16.34, Table 2-1.1. Note that the working pressures match those listed in the extract from B16.5, Table 2-1.1, for flanges. The allowable stresses on a metal decreases as the temperature increases, therefore, working pressure goes down as temperature goes up (a valve shell is assumed to have the same temperature as the fluid passing through it).
The body of the valve is marked to indicate the pressure class. The way in which this is marked varies, depending on shell material and intended service. Steel valves are marked with the class number. On bronze and iron valves there are two markings: S, SP, or SWP (for Steam, Steam Pressure, or Steam Working Pressure respectively), and WOG (Water, Oil, Gas) or CWP (Cold Working Pressure). Valves marked with WOG or CWP rating are primarily intended for use in services in which the fluid is maintained at ambient temperatures, such as, but not limited to, water, oil, and gas. WOG and CWP are sometimes referred to as cold rating.
The S, SP, or SWP rating represents the maximum allowable working pressure of the valve when used with steam (provided the steam temperature does not exceed the maximum temperature for the shell material). Manufacturers list temperature restrictions in their catalogs.
The WOG or CWP rating represents the maximum allowable working pressure at ambient temperatures,
-20oF to 100oF (refer to B16.34, Table 2 series).
As an example, a valve marked 125 S – 200 WOG is rated to 125 psig in steam service (higher temperatures), and 200 psig when used in services at ambient temperatures.
Note that the pressure rating of soft-seated valves, e.g. ball and butterfly valves, falls much faster as the temperature rises than that of metal seated valves, e.g. gates and globes. This is because the soft seats are made of materials such as teflon or nylon which softens quickly as temperature increases.
End (Connection) Types:
Valve bodies are available in all end connection types to match the piping system:
• Threaded (Thr’d)
• Socket-weld (SW)
• Flanged (Flg’d)
• Butt-weld (BW)
The end connections match the class ratings of fittings and flanges. Commonly threaded and socket-weld valves are used in small-bore (NPS 11/2 and smaller). Flanged and butt-weld are used in large-bore (NPS 2 and larger). Certain valves types, primarily butterfly, some checks, and some gates, have a wafer body style and are sandwiched between flanges.
Parts of a Valve:
There are four basic parts to a valve:
The “Body” – the part of a valve that houses the flow control element, contains seating surfaces, retains fluid pressure, and has ends for connecting into the piping system. The internal opening through the valve body is called the “Port”.
The “Bonnet” – the part of a gate, globe, or diaphragm valve, that is fastened to the body to complete the pressure containing shell. It has an opening for the stem to pass through. Depending on the valve stem
design, it may also have a yoke attachment. On other types of valve design, such as check valves, this may just be a “Cover” plate.
The “Trim” – the parts of a valve inside the shell that are “wetted” by the fluid. Components of the trim include:
• Disc – movable part which affects flow, the flow control element.
• Seat – causes seal when valve is closed.
• Stem – controls the position of the disc.
• Packing – seal between bonnet and stem.
• Gland – holds packing in place.
The “Operator” – the mechanism that moves the stem and opens or closes the valve. Operator types include:
• Handwheel – used on gate, globe, and diaphragm valves.
• Lever or wrench – quick open and closure (90° or quarter turn) on plug, ball, and butterfly valves.
• Chain operator – attached to the handwheel or lever for manual operation from grade, of a valve located too high to reach.
• Stem extension – used for valves below grade or floor level, to raise the handwheel to a convenient operating height for the plant personnel.
• Gear operators – used when it is necessary to reduce the force to manually operate the valve; such as is the case with large valves and valves operating against high fluid pressure.
• Electric, hydraulic or pneumatic actuators – used for automatic or remote operation of valves. Control valves are commonly operated by a pneumatic (diaphragm) actuator.
Also see section “Miscellaneous Information”.
Valves are available in many materials, including: carbon steel, stainless steel, iron, brass, and bronze. Carbon steel valves are the primary choice of the process piping industry. When a valve is called bronze or steel reference is being made to the shell material. The shell material is selected based on pressure containment, and, unless the valve is lined, the ability to withstand the corrosive and erosive affects of the fluid. The body of the valve is marked to indicate the material, but this marking may be an abbreviation. For instance, A216 Gr. WCB may be abbreviated as WCB, A352 Gr. LCB may be abbreviated LCB.
Trim materials can be categorized as metallic or non-metallic. Metallic groups include copper alloys, stainless steels, and exotic alloys. Non-metallic groups include plastics and elastomers. Trim materials are selected based primarily on resistance to erosion and corrosion, and mechanical wear resistance. Mechanical wear resistance is particularly important for the disc and seats, as it is the tight mating of these surfaces that creates a leak-free seal when the valve is closed.
Often the materials used to make the shell do not have the desired properties to be used for the trim.
Additionally to the ASME standards, NACE International publishes standards for materials, mechanical properties, and heat treatments for metals used in sour service (exposure to Hydrogen Sulfide).
The major valve functions, or service, are as follows:
• On-off – starting or stopping flow, fully open or fully closed.
• Throttling or regulating – varying the rate of flow.
• Checking – permitting flow in one direction only.
• Switching – diverting the flow direction.
• Discharging – discharging fluid from a system, or limiting fluid pressure.
There are hundreds of combinations of valve types, designs, materials, etc. Selecting the right valve for an application is based on several criteria:
• Fluid type – gas, liquid, two-phase, slurry.
• Fluid characteristics – corrosive, erosive, toxic.
• Operating conditions – pressure and temperature.
• Service requirements – on-off, throttling, checking, switching, discharging; degree of shutoff; slow or quick opening.
• Availability – cost vs. delivery.
When a valve is first opened (the obstruction to flow is removed) fluid begins flowing through it. Further movement of the flow control element allows more fluid to flow, until the valve is fully open and maximum flow rate has been achieved. The flow characteristic of a valve is the relationship between the position of flow control element and the rate of fluid flow through the valve. As the flow control element moves through its travel, the shape and size of the opening are changed. Different flow characteristics are achieved with differing size and shapes of flow control elements. Tests determine the inherent flow characteristics of a valve design, and these results are plotted on a graph. The curve shows the position of the flow control element and the flow rate as percentages of their maximums. There are three common valve flow characteristics: quick-opening, linear, and equal percentage.
Refer to the attached graph “Valve flow characteristic curves”:
• Quick-opening. It can be seen that the quick-opening characteristic produces large changes in flow rate at the start of flow control element travel, and then progressively smaller changes until the valve is completely open. The intake and exhaust valve of the internal combustion engine are examples of quick opening valves, and are characterized by their ability to provide full flow with relatively small stem movement.
• Linear. The linear characteristic produces changes in flow rate that are directly proportional to the position of the flow control element over the full range of flow control element travel. Stated another way, when the flow control element is 25% from the seat the flow rate through the valve will be 25% of maximum, when it is 50% flow rate will be 50% of maximum, and when the flow control element has reached the limit of its travel the flow rate will be 100%.
• Equal percentage. With the equal percentage characteristics, equal increments of flow control element travel produce equal percentage changes in flow rate. To illustrate this, follow the equal percentage flow characteristics curve. The curve shows that changing the flow rate control element position from 40% open to 60% open (a 20% increment) produces a change in flow rate from 12% of maximum to 24% of maximum – a 100% increase. Changing the flow control element from 60% open to 80% open (also a 20% increment) produces a change in flow rate from 24% of maximum to 48%
of maximum – again a 100% increase.
Valves are designed not to leak internally or externally. Internally, when fully closed, there should be no flow. Externally, fluid cannot be allowed to leak into the environment. Manufacturers conduct leak tests, as per set standards such as API 598, to determine leak tightness. A valve is termed “bubble tight” when the upstream side of the valve is pressurized with air and the downstream side is filled with water and no air bubbles are detected on the downstream side of the valve with the valve in the fully closed position.
The most common types of valves used in process piping are listed below. Descriptions for each are of a generic nature. The basic designs come in variations designed to suit specific applications.
Gate valves are used in on-off service. Advantages are:
• Good shutoff characteristics.
• Minimal pressure drop.
• Not quick opening or closing. Full stem travel requires many turns of the handwheel or an actuator.
• Require lots of room for installation, operation, and maintenance.
• The slow movement of the disc near to full-closed position results in high-fluid velocities, causing erosion of the seats. This is referred to as “wire draw”.
• Susceptible to thermal and/or pressure binding.
• Difficult to repair in-situ.
• Not suitable for slurry applications.
Globe valves are used in throttling service. Advantages are:
• Good shut-off capability.
• Good throttling capability.
• Shorter stroke (compared to gate valve).
• Available in various configurations, each offering unique capabilities.
• Easy to machine or resurface the seats.
• High pressure drop (compared to gate valve) due to changes in fluid flow direction.
• Requires high force to seat the valve when the pressure is under the seat.
• Practical size limitation is NPS 12 due to stem thrust.
• Suitable for on-off and throttling services.
• Compact design requires considerably less space than gate and globe valves.
• Light weight.
• Quick to open and close. Quarter turn.
• Available in all size ranges.
• Low pressure drop.
• Provides bubble tight service.
• Can be used in slurry applications.
• Only good for throttling service involving low differential pressures, as in cooling water or air supply systems.
• Throttling is limited to a 30° to 80° disc opening.
• Cavitation and choked flow are two potential concerns.
• The disc movement is unguided and affected by flow turbulence.
• Suitable for on-off and throttling services.
• Offer good chemical resistance due to a variety of available linings.
• Stem leakage is eliminated.
• Provides bubble tight service.
• Does not have pockets to trap commodities, such as slurries. Used extensively in industries where contamination can be a problem, such as pharmaceuticals, and food processing.
• The weir can prevent drainage of piping.
• Working pressures and temperatures are limited by the diaphragm material.
• The diaphragm may experience erosion when used extensively in severe throttling service containing impurities.
• Only available in limited sizes. Usually NPS 1/2 to NPS 12.
Check valves are used to prevent backflow. Advantages are:
• Fast acting and self-actuated, requiring no external means to open or close.
• All moving parts are enclosed so it is difficult to assess the condition of the internals, and whether the valve is open or closed (disc can stick open).
• Limitations on the installation configuration.
Ball valves are used in on-off service. Advantages are:
• Provides bubble-tight service.
• Quick to open and close. Quarter turn.
• Smaller and lighter than a gate valve.
• Available in multi-port configurations.
• Force required to operate the valve is smaller than that required for a gate or a globe.
• Minimal pressure drop.
• Not suitable for throttling applications.
• Not suitable for slurry applications.
Plug valves are used in on-off service. Advantages are:
• Simple design with few parts.
• Quick to open and close. Quarter turn.
• Can be serviced in place.
• Minimal pressure drop.
• Provides reliable leek tight service.
• Available in multi-port configurations.
• Can be used in slurry applications.
• Requires greater force to open/close due to high friction. NPS 4 and larger require actuators.
• Reduced port due to tapered plug.
• Typically cost more than ball valves.
Needle valves are used in throttling service. They are primarily used in instrumentation. Advantages are:
• Very accurate throttling.
• Can be used in high pressure/temperature applications.
• Available only in small sizes to NPS 1.
2.2 Valve Operators
2.2.1 Hand-wheels & Levers
Manually operated valves require a hand-wheel or lever to be opened or closed. Ball, plug, and butterfly valves (quarter turn valves) have a lever, while gate, globe, and diaphragm valves have hand-wheels. There are various stem combinations available (see attached “Variations in Stem Operation”). Inside Screw, Outside Screw, Raising Stem, and Non-Rising Stem.
External and internal corrosion considerations determine which design will be used. For example, it is desirable to have the threads outside of a corrosive fluid that could damage them. Likewise, in a corrosive atmosphere, such as offshore, it may be less damaging to have internal threads.
Generally speaking, small-bore valves have an internal screw and a rising stem. Large-bore globe and diaphragm valves come with an outside screw and a rising stem. Large-bore gate valves come in two variations: Outside Screw and Yoke (OS&Y), where the handwheel is attached to the yoke and draws the stem through itself. And an internal screw with non-rising stem arrangement, where the disc has internal threads and is drawn up the stem.
2.2.2 Gear Operators
Gear operators assist in the opening of large valves and/or valves with a high-pressure differential across the flow control element by providing reduction gearing to minimize the manual effort of opening or closing the valve
The term “control valve” could likely be applied to all valves, as all valves perform a fluid control function. However, this term has a specific meaning. The Instrument Society of America (ISA) defines a control valve as “a final controlling element, through which a fluid passes, which adjusts the size of flow passage as directed by a signal from a controller to modify the rate of flow of the fluid.” From this definition it is apparent that the regulation of the flow is automatic, and that a control valve is part of an automated system. Systems of this nature are referred to as ”closed loop”, and by regulating the flow, control valves can be used to regulate all four process variables: level, pressure, temperature, and flow.
For the valve to operate automatically, it must be equipped to do so. The equipment which achieves this is called the actuator. Actuators are attached to the valve bonnet and stem, and operate by either pneumatic, hydraulic, or electric signals.
2.2.4 Pressure Safety/Relief
Piping systems must be protected from overpressure. This is accomplished by discharging some of the fluid. There are three types of valves for discharging:
Pressure Safety Valve (PSV): an automatic pressure relieving device actuated by the static pressure upstream of the valve, and characterized by rapid, full opening as a pop action. A PSV reseats when the fluid pressure has been reduced to a level below the pressure at which the valve began to open. Used for steam, gas or vapour services (compressible fluids).
Pressure Relief Valve (PRV): an automatic pressure relieving device actuated by the static pressure upstream of the valve, which opens in proportion to the increase in pressure over the opening pressure. A PRV will close when pressures have been restored to normal. It is used primarily for liquid service (non-compressible fluids).
Pressure Safety-Relief Valve: an automatic pressure relieving device suitable for use as either PSV or PRV, depending on its application.
The contents of this section were kindly provided by Richard Beale, co-author of The Planning Guide to Piping Design
Other References are
• Piping Handbook – Seventh Edition, by Mohinder L. Nayyar. McGraw-Hill, ISBN 0-07-047106-1
• Piping Systems – Drafting and Design, by Louis Gary Lamit. Prentice Hall, ISBN 0-13-676445-2
• The Valve Primer, by Brent T. Stojkov. Industrial Press Inc., ISBN 0-8311-3077-6
• Process Piping Drafting, by Rip Weaver. Gulf Publishing Company, ISBN 0-87201-761-3
• ENGD 320 – Process Piping Drafting 1, Construction Department, Southern Alberta Institute of Technology
• Welding Fittings and Forged Flanges, Crane catalog 61
• Piping Equipment (handbook), Trouvay & Cauvin
Various websites and applicable ASTM and ASME standards where also referenced for the compilation of this material.