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Pipe (material)
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A pipe is a tube or hollow cylinder used to convey materials or as a structural component. The terms pipe and tube are almost interchangeable. A pipe is generally specified by the internal diameter (ID) whereas a tube is usually defined by the outside diameter (OD) but may be specified by any combination of dimensions (OD, ID, wall thickness).
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A pipe is a tube or hollow cylinder used to convey materials or as a structural component. The terms pipe and tube are almost interchangeable. A pipe is generally specified by the internal diameter (ID) whereas a tube is usually defined by the outside diameter (OD) but may be specified by any combination of dimensions (OD, ID, wall thickness). A tube is often made to custom sizes and may often have more specific sizes and tolerances than pipe. Also, the term tubing can be applied to non-cylindrical shapes (i.e. square tubing). The term tube is more widely used in the United States, whereas pipe is more common elsewhere in the world.
Both pipe and tube imply a level of rigidity and permanence, whereas a hose is usually portable and flexible. Pipe may be specified by standard pipe size designations, such as nominal pipe size (in the United States), or by nominal, outside, or inside diameter and wall thickness. Many industrial and government standards exist for the production of pipe and tubing.
Uses
- Domestic water systems
- Pipelines containing high pressure gas or fluid
- Scaffolding
- Structural steel
- As components in mechanical systems such as:
- Rollers in conveyor belts
- Compactors (Eg: steam rollers)
- Bearing casing
- Casing for concrete pilings used in construction projects
- High temperature or pressure manufacturing processes
- The petroleum industry:
- Oil well casing
- Oil refinery equipment
- The construction of high pressure storage vessels
Manufacture
There are three processes for metallic pipe manufacture. Seamless pipe is formed by drawing a solid billet over a piercing rod to create the hollow shell. Seamless pipe provides the most reliable pressure retaining characteristics, and is often more easily available than welded pipe. Welded pipe is formed by rolling plate and welding the seam. The weld flash can be removed from the outside or inside surfaces using a scarfing blade. The weld zone can also be heat treated, so the seam is less visible. Welded pipe often has tighter dimensional tolerances than seamless, and can be cheaper if manufactured in the same quantities. Cast pipe is no longer very common, but still exists. Pipe is sometimes cast in a centrifuge.
Tubing, either metal or plastic, is generally extruded.
Materials
The manufacturing of pipe uses many materials including ceramic, metal, concrete, and plastic.
Pipe may be made from a variety of materials. In the past, materials have included wood and lead (Latin plumbum, from which we get the word plumbing).
Metal pipes are commonly made from unfinished, black (lacquer), or galvanized steel, brass, and ductile iron. Copper tubing is popular for plumbing systems.
Plastic tubing is widely used for its light weight, chemical resistance, non-corrosive properties, and ease of making connections. Plastic materials include polyvinyl chloride (PVC), chlorinated polyvinyl chloride (CPVC), polyethylene (PE), cross-linked high-density polyethylene (PEX), polybutylene (PB), and acrylonitrile butadiene styrene (ABS), for example. In the United States and Canada, PVC pipes account for the largest majority of pipe materials used in buried municipal applications for drinking water distribution and wastewater mains.
Pipe may also be made from concrete or ceramic. These pipes are usually used for low pressure applications such as gravity flow or drainage.
Most concrete pressure pipe for water and sewer lines is manufactured in accordance with the American Water Works Association (AWWA) () standards C301 and C303. C303 pipe, usually known as Bar-Wrapped Pipe (BWP), has round bar stock helically wound, under mild axial tension, around the exterior of a steel cylinder. Steel joint rings, a bell ring on one end and a spigot ring on the other, are welded to the ends of the steel cylinder. The spigot ring is shaped with a groove near the nose that accommodates a rubber o-ring sealing gasket. The steel cylinder and joint ring assembly form the watertight membrane for the pipe. The bar stock and steel cylinder are made from mild, low carbon steel. Bar stock diameters range from 7/32" up to 3/4". The steel cylinder thickness can range from 12 gauge (0.1046" thickness) to 3/8" plate. For corrosion protection, the interior and exterior surfaces are coated with portland cement-rich mortar or concrete. C303 pipe can be manufactured in diameters from 10" through 72". Nominal laying lengths of BWP can range from 20 feet to as much as 35 feet. The design of BWP is covered in the AWWA M9 Manual for Concrete Pressure Pipe. The AWWA C301 standard covers the manufacturing of prestressed concrete steel cylinder pipe. This pipe type consists of a relatively thin steel cylinder and formed steel joint rings (a bell ring and a spigot ring) which serve as the watertight membrane. The joint configuration for PCCP is very similar to that used on BWP. A concrete structural core is then cast and cured. This assembly is then helically-wrapped with a high tensile strength steel wire under controlled tension and spacing which induces a circumferential compressive stress in the core. After wrapping, the exterior surfaces receive a portland-cement rich mortar coating for corrosion protection. Two types of prestressed concrete cylinder pipe (PCCP) are currently manufactured. The concrete structural core for prestressed Lined Cylinder Pipe (LCP) is placed using a centrifugal method. LCP is normally manufactured in nominal diameters 16" up to and including 60". The concrete structural core for prestressed Embedded Cylinder Pipe (ECP) is placed in the more traditional method in a vertical position using a steel inside and outside form. ECP is normally supplied in diameters 54" and larger. ECP in diameters up to and including 144" is available. The design of PCCP is governed by the AWWA C304 standard. Both PCCP and BWP can be used for internal working pressures up to 300 psi although most water and sewer systems will operate at pressures of 150 psi or less.
Manufacturers of PCCP and BWP can offer a wide range of steel fittings such as elbows, tees, and branch wyes to complement the straight lengths of pipe. Special adapter fittings can also be designed and built to facilitate connections between the concrete pressure pipe and other pipe types.
Reinforced concrete can be used for large diameter concrete pipes. This pipe material can be used in many types of construction and is often used in the transport of storm water under gravity flow conditions. Usually such pipe will have a receiving bell or a stepped fitting, with various sealing methods applied at installation.
Sizes
Pipe sizes are documented by a number of international standards, including API 5L, ANSI/ASME B36.10M and B36.19M in the US, BS 1600 and BS EN 10255 in the United Kingdom and Europe, and ISO 65 internationally.
There are two common methods for designating pipe outside diameter (OD). The North American method is called NPS ("Nominal Pipe Size") and is based on inches (also frequently referred to as NB ("Nominal Bore")). The European version is called DN ("Diametre Nominal" / "Nominal Diameter") and is based on millimetres. Designating the outside diameter allows pipes of the same size to be fit together no matter what the wall thickness.
- For pipe sizes less than NPS 14 inch (DN 350), both methods give a nominal value for the OD that is rounded off and is not the same as the actual OD. For example, NPS 2 inch and DN 50 are the same pipe, but the actual OD is 2.375 inch, or 60.325 mm. The only way to obtain the actual OD is to look it up in a reference table.
- For pipe sizes of NPS 14 inch (DN 350) and greater the NPS size is the actual diameter in inches and the DN size is equal to NPS times 25 rounded to a convenient multiple of 50. For example, NPS 14 has an OD of 14 inches, or 366.5 mm, and is equivalent to DN 350.
Since the outside diameter is fixed for a given pipe size, the inside diameter will vary depending on the wall thickness of the pipe. For example, 2" Schedule 80 pipe has thicker walls and therefore a smaller inside diameter than 2" Schedule 40 pipe.
Steel pipe has been produced for about 150 years. The pipe sizes that are in use today in PVC and galvanized were originally designed years ago for steel pipe. The number system, like Sch 40, 80, 160, were set long ago and seem a little odd. For example, Sch 1120 pipe is even thinner than Sch 40, but same OD. And while these pipes are based on old steel pipe sizes, there is other pipe, like gold-flow cpvc for heated water, that uses pipe sizes, inside and out, based on old copper pipe size standards instead of steel.
Many different standards exist for pipe sizes, and their prevalence varies depending on industry and geographical area. The pipe size designation generally includes two numbers; one that indicates the outside (OD) or nominal diameter, and the other that indicates the wall thickness. In the early twentieth century, American pipe was sized by inside diameter. This practice was abandoned to improve compatibility with pipe fittings that must usually fit the OD of the pipe, but it has had a lasting impact on modern standards around the world.
In North America and the UK, pressure piping is usually specified by Nominal Pipe Size (NPS) and schedule (SCH). Pipe sizes are documented by a number of standards, including API 5L, ANSI/ASME B36.10M (Table 1) in the US, and BS 1600 and BS 1387 in the United Kingdom. Typically the pipe wall thickness is the controlled variable, and the Inside Diameter (I.D.) is allowed to vary. The pipe wall thickness has a variance of approximately 12.5 percent.
In Europe, pressure piping uses the same pipe IDs and wall thicknesses as Nominal Pipe Size, but labels them with a metric Diameter Nominal (DN) instead of the imperial NPS. For NPS larger than 14, the DN is equal to the NPS multiplied by 25. (Not 25.4) This is documented by EN 10255 (formerly DIN 2448 and BS 1387) and ISO 65, and it is often called DIN or ISO pipe.
Japan has its own set of standard pipe sizes, often called JIS pipe.
The Iron pipe size (IPS) is an older system still used by some manufacturers and legacy drawings and equipment. The IPS number is the same as the NPS number, but the schedules were limited to Standard Wall (STD), Extra Strong (XS), and Double Extra Strong (XXS). STD is identical to SCH 40 for NPS 1/8 to NPS 10, inclusive, and indicates .375" wall thickness for NPS 12 and larger. XS is identical to SCH 80 for NPS 1/8 to NPS 8, inclusive, and indicates .500" wall thickness for NPS 8 and larger. Different definitions exist for XXS, but it is generally thicker than schedule 160.
Another old system is the Ductile Iron Pipe Size (DIPS), which generally has larger ODs than IPS.
Copper plumbing tube for residential plumbing follows an entirely different size system, often called Copper Tube Size (CTS); see domestic water system. Its nominal size is neither the inside nor outside diameter. Plastic tubing, such as PVC and CPVC, for plumbing applications also has different sizing standards.
Agricultural applications use PIP sizes, which stands for Plastic Irrigation Pipe. PIP comes in pressure ratings of 22 psi, 50 psi, 80 psi, 100 psi, and 125 psi and is generally available in diameters of 6", 8", 10", 12", 15", 18", 21", and 24".
Standards
The manufacture and installation of pressure piping is tightly regulated by the ASME Boiler and Pressure Vessel Code. This code has the force of law in Canada and the USA. Europe has an equivalent system of codes. Pressure piping is generally pipe that must carry pressures greater than 10 to 25 atmospheres, although definitions vary. To ensure safe operation of the system, the manufacture, storage, welding, testing, etc. of pressure piping must meet stringent quality standards.
Manufacturing standards for pipes commonly require a test of chemical composition and a series of mechanical strength tests for each heat of pipe. A heat of pipe is all forged from the same cast ingot, and therefore had the same chemical composition. Mechanical tests may be associated to a lot of pipe, which would be all from the same heat and have been through the same heat treatment processes. The manufacturer performs these tests and reports the composition in a mill traceability report and the mechanical tests in a material test report, both of which are referred to by the acronym MTR. Material with these associated test reports is called traceable. For critical applications, third party verification of these tests may be required; in this case an independent lab will produce a certified material test report, and the material will be called certified.
Maintaining the traceability between the material and this paperwork is an important quality assurance issue. QA often requires the heat number to be written on the pipe. Precautions must also be taken to prevent the introduction of counterfeit materials.
Some widely used pipe standards are:
- The API range. Eg: API 5L Grade B
- ASME SA106 Grade B (Seamless carbon steel pipe for high temperature service)
- ASTM A312 (Seamless and welded austenitic stainless steel pipe)
- ASTM C76 (Concrete Pipe)
- ASTM D3033/3034 (PVC Pipe)
- ASTM D2239 (Polyethylene Pipe)
API 5L was changed in the second half of 2008 to edition 44 from edition 43. It is important to note that the change has created the requirement that sour service, ERW pipe, pass a hydrogen induced cracking (HIC) test per NACE TM0284 in order to be used for sour service.
- ACPA [American Concrete Pipe Association]
- AWWA [American Water Works Association]
See for more specification summaries.
Installation
Pipe installation is often more expensive than the material and a variety of specialized tools, techniques, and parts have been developed to assist this. An example tool is the pipe wrench.
Joining
Pipes are commonly joined by welding or by using pipe threads. The most common pipe thread in North America is the National Pipe Thread (NPT) or the Dryseal (NPTF) version. Other pipe threads include the British standard pipe thread (BSPT), the garden hose thread (GHT), and the fire hose coupling (NST).
Copper pipes are typically joined by soldering, brazing, compression fittings, flaring, or crimping.
Plastic pipes may be joined by solvent welding, heat fusion, or elastomeric sealing.
If frequent disconnection will be required, gasketed pipe flanges or union fittings provide better reliability than threads. Some thin-walled pipes of ductile material, such as the smaller copper or flexible plastic water pipes found in homes for ice makers and humidifiers, for example, may be joined with compression fittings.
Underground pipe typically uses a "push-on" gasket style of pipe that compresses a gasket into a space formed between the two adjoining pieces. Push-on joints are available on most types of pipe. Under buried conditions, gasket-joint pipes allow for lateral movement due to soil shifting as well as expansion/contraction due to temperature differentials.
Large above ground pipe typically uses a flanged joint, which is generally available in ductile iron pipe and some others. It is a gasket style where the flanges of the adjoining pipes are bolted together, compressing the gasket into a space between the pipe.
Mechanical grooved couplings or Victaulic joints are also frequently used for frequent disassembly & assembly. Developed in the 1920s, these mechanical grooved couplings can operate up to 1,200psi working pressures and available in materials to match the pipe grade.
Fittings and valves
Fittings are also used to split or join a number of pipes together, and for other purposes. A broad variety of standardized pipe fittings are available. Valves control fluid flow. The piping and plumbing fittings and valves articles discuss them further.
Hangers
Pipes are hung from devices called pipe hangers, which may incorporate springs and/or dampers to compensate for thermal expansion or to provide vibration isolation
Cleaning
The inside of pipes can be cleaned with the tube cleaning process, if they are contaminated with debris or fouling.
See also
Bibliography
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External links
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