AN INTRODUCTION TO PIPING
Piping systems are like arteries and veins. They carry the lifeblood of modern civilization. In a modern city they transport water from the sources of water supply to the points of distribution; convey waste from residential and commercial buildings and other civic facilities to the treatment facility or the point of discharge. Similarly, pipelines carry crude oil from oil wells to tank farms for storage or to refineries for processing. The natural gas transportation and distribution lines convey natural gas from the source and storage tank forms to points of utilization, such as power
plants, industrial facilities, and commercial and residential communities. In chemical plants, paper mills, food processing plants, and other similar industrial establishments, the piping systems are utilized to carry liquids, chemicals, mixtures, gases, vapors, and solids from one location to another.
The fire protection piping networks in residential, commercial, industrial, and other buildings carry fire suppression fluids, such as water, gases, and chemicals to provide protection of life and property. The piping systems in thermal power plants convey high-pressure and high-temperature steam to generate electricity. Other piping systems in a power plant transport high- and low-pressure water, chemicals, low-pressure steam, and condensate. Sophisticated piping systems are used to process
and carry hazardous and toxic substances. The storm and wastewater piping systems transport large quantities of water away from towns, cities, and industrial and similar establishments to safeguard life, property, and essential facilities.
In health facilities, piping systems are used to transport gases and fluids for medical purposes. The piping systems in laboratories carry gases, chemicals, vapors, and other fluids that are critical for conducting research and development. In short,
the piping systems are an essential and integral part of our modern civilization just as arteries and veins are essential to the human body.
The design, construction, operation, and maintenance of various piping systems involve understanding of piping fundamentals, materials, generic and specific design considerations, fabrication and installation, examinations, and testing and inspection requirements, in addition to the local, state and federal regulations.
PIPING
Piping includes pipe, flanges, fittings, bolting, gaskets, valves, and the pressure containing
portions of other piping components. It also includes pipe hangers and supports and other items necessary to prevent over pressurization and over stressing of the pressure-containing components. It is evident that pipe is one element or a part of piping. Therefore, pipe sections when joined with fittings, valves, and other mechanical equipment and properly supported by hangers and supports, are called piping.
Pipe
Pipe is a tube with round cross section conforming to the dimensional requirements
of
● ASME B36.10M Welded and Seamless Wrought Steel Pipe
● ASME B36.19M Stainless Steel Pipe
Pipe Size
Initially a system known as iron pipe size (IPS) was established to designate the pipe size. The size represented the approximate inside diameter of the pipe in inches. An IPS 6 pipe is one whose inside diameter is approximately 6 inches (in). Users started to call the pipe as 2-in, 4-in, 6-in pipe and so on. To begin, each pipe size was produced to have one thickness, which later was termed as standard (STD) or standard weight (STD.WT.). The outside diameter of the pipe was standardized. As the industrial requirements demanded the handling of higher-pressure fluids, pipes were produced having thicker walls, which came to be known as extra strong (XS) or extra heavy (XH). The higher pressure requirements increased further, requiring thicker wall pipes. Accordingly, pipes were manufactured with double extra strong (XXS) or double extra heavy (XXH) walls while the standardized
outside diameters are unchanged. With the development of stronger and corrosion-resistant piping materials, the need for thinner wall pipe resulted in a new method of specifying pipe size and
wall thickness. The designation known as nominal pipe size (NPS) replaced IPS, and the term schedule (SCH) was invented to specify the nominal wall thicknessof pipe.
Nominal pipe size (NPS) is a dimensionless designator of pipe size. It indicates standard pipe size when followed by the specific size designation number without an inch symbol. For example, NPS 2 indicates a pipe whose outside diameter is 2.375 in. The NPS 12 and smaller pipe has outside diameter greater than the size designator (say, 2, 4, 6, . . .). However, the outside diameter of NPS 14 and larger pipe is the same as the size designator in inches. For example, NPS 14 pipe has an outside diameter equal to 14 in. The inside diameter will depend upon the pipe wall thickness specified by the schedule number. Refer to ASME B36.10M or ASME B36.19M.
Diameter nominal (DN) is also a dimensionless designator of pipe size in the metric unit system, developed by the International Standards Organization (ISO). It indicates standard pipe size when followed by the specific size designation number without a millimeter symbol. For example, DN 50 is the equivalent designation of NPS 2. Refer to Table 1 for NPS and DN pipe size equivalents.
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| table 1 |
Pipe Wall Thickness
Schedule is expressed in numbers (5, 5S, 10, 10S, 20, 20S, 30, 40, 40S, 60, 80, 80S, 100, 120, 140, 160). A schedule number indicates the approximate value of the expression 1000 P/S, where P is the service pressure and S is the allowable stress, both expressed in pounds per square inch (psi). The higher the schedule number, the thicker the pipe is. The outside diameter of each pipe size is standardized. Therefore, a particular nominal pipe size will have a different inside diameter
depending upon the schedule number specified.
Note that the original pipe wall thickness designations of STD, XS, and XXS have been retained; however, they correspond to a certain schedule number depending upon the nominal pipe size. The nominal wall thickness of NPS 10 and smaller schedule 40 pipe is same as that of STD.WT. pipe. Also, NPS 8 and smaller schedule 80 pipe has the same wall thickness as XS pipe.
The schedule numbers followed by the letter S are per ASME B36.19M, and they are primarily intended for use with stainless steel pipe. The pipe wall thickness specified by a schedule number followed by the letter S may or may not be the same as that specified by a schedule number without the letter S. Refer to ASME B36.19M and ASME B36.10M.10,11 ASMEB36.19M does not cover all pipe sizes. Therefore, the dimensional requirements of ASME B36.10M apply to stainless steel pipe of the sizes and schedulesnot covered by ASME B36.19M.
PIPING CLASSIFICATION
It is usual industry practice to classify the pipe in accordance with the pressure temperature
rating system used for classifying flanges. However, it is not essential
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| table 2 |
that piping be classified as Class 150, 300, 400, 600, 900, 1500, and 2500. The piping rating must be governed by the pressure-temperature rating of the weakest pressure containing item in the piping. The weakest item in a piping system may be a fitting made of weaker material or rated lower due to design and other considerations.
Table.2 lists the standard pipe class ratings based on ASME B16.5 along with corresponding pression nominal (PN) rating designators. Pression nominal is the French equivalent of pressure nominal.
In addition, the piping may be classified by class ratings covered by other ASME standards, such as ASME B16.1, B16.3, B16.24, and B16.42. A piping system may be rated for a unique set of pressures and temperatures not covered by any standard.
Pression nominal (PN) is the rating designator followed by a designation number, which indicates the approximate pressure rating in bars. The bar is the unit of pressure, and 1 bar is equal to 14.5 psi or 100 kilopascals (kPa). Table .2 provides a cross-reference of the ASME class ratings to PN rating designators. It is evident that the PN ratings do not provide a proportional relationship between different PN numbers, whereas the class numbers do. Therefore, it is recommended that class numbers be used to designate the ratings.
PIPE AND TUBE PRODUCTS
Pressure pipe and tube products are manufactured to a variety of standard specifications of varying designs, employing different manufacturing practices and using a wide variety of materials. The end user of these products must apply the leastcost product suitable for the specified service conditions. Typically, steel and alloy pressure piping is available in cast, wrought, and seam-welded forms. Welded and seamless wrought steel pipe is supplied in standard sizes and wall thickness conforming
to ASME B36.10M. Stainless-steel pipe is supplied in standard sizes and wall thickness conforming to ASME B36.19M.
Pressure Tubing
Pressure-tube applications commonly involve external heat applications, as in boilers or superheaters. Pressure tubing is produced to the actual outside diameter and minimum or average wall thickness specified by the purchaser. Pressure tubing may be hot- or cold-finished. The wall thickness is normally given in decimal parts of an inch rather than as a fraction or gauge number. When gauge numbers are given without reference to a system, Birmingham wire gauge (BWG) is implied.
Pressure tubing is usually made from steel produced by the open-hearth, basic oxygen, or electric furnace processes. Seamless pressure tubing may be either hotfinished or cold-drawn. Cold-drawn steel tubing is frequently process-annealed at temperatures above 1200 F (650 C). To ensure quality, maximum hardness values are frequently specified. Hot-finished or cold-drawn seamless low-alloy steel tubes generally are process-annealed at temperatures between 1200 F (650 C) and 1350 F
(730 C). Austenitic stainless-steel tubes are usually annealed at temperatures between 1800 F (980 C) and 2100 F (1150 C), with specific temperatures varying somewhat with each grade. This is generally followed by pickling, unless bright annealing was done.
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| table 3 |
Pipe Fittings
The major piping materials are also produced in the form of standard fittings. Among the more widely used materials are ductile or cast iron, malleable iron, brass, copper, cast steel, forged steel, and wrought steel. Other major nonferrous piping materials are also produced in the form of cast and wrought fittings. Ductile and cast-iron fittings are made by conventional foundry methods for a variety of joints including bell-and-spigot, push-on flanged, and mechanical (gland-type) or
other proprietary designs.
Steam Traps
The function of a steam trap is to discharge condensate from steam piping or steam heating equipment without permitting live steam to escape. Some principal types of steam traps are:
● Float
● Thermostatic
● Thermodynamic
● Inverted bucket
Strainers
are used in piping systems to protect equipment sensitive to dirt and other particles that may be carried by the fluid. During system start-up and flushing, strainers may be placed upstream of pumps to protect them from construction debris that may have been left in the pipe.
EXPANSION JOINTS
Expansion joints are used in piping systems to absorb thermal expansion where the use of expansion loops is undesirable or impractical. Expansion joints are available in slip, ball, metal bellows, and rubber bellows configurations.
Flange Types
Flanges differ in method of attachment to the pipe, i.e., whether they are screwed, welded, or lapped. Contact surface facings may be plain, serrated, grooved for ring joints, seal-welded, or ground and lapped for metal-to-metal contact
Gaskets
Since it is expensive to grind and lap joint faces to obtain fluid-tight joints, a gasket of some softer material is usually inserted between contact faces. Tightening the bolts causes the gasket material to flow into the minor machining imperfections, resulting in a fluid-tight seal. A considerable variety of gasket types are in common use. Soft gaskets, such as cork, rubber, vegetable fiber, graphite, or asbestos, are usually plain with a relatively smooth surface. The semimetallic design combines
metal and a soft material, the metal to withstand the pressure, temperature, and attack of the confined fluid and the soft material to impart resilience. Variousdesigns involving corrugations, strip-on-edge, metal jackets, etc., are available. Inaddition to the plain, solid, and flat-surface metal gaskets, various modified designs and cross-sectional shapes of the profile, corrugated, serrated, and other types areused. The object in general has been to retain the advantage of the metal gasket but to reduce the contact area to secure a seal without excessive bolting load. Effective gasket widths are given in various sections of the ASME Boiler and Pressure Vessel Code.
valves
The piping designer must be familiar with proper application of all types of valves including gate, globe, plug, butterfly, ball, angle, diaphragm, check, pressure relief, and control valves and their methods of operation including manual, chain, gear,air, hydraulic, or motor. The following general guidelines should be applied when locating valves in any piping system
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