Pipe (material)

Pipe is a hollow cylinder of material (metal or plastic). It differs from tubing in the way it is dimensioned. Pipe is specified by standard pipe size designations, such as Nominal Pipe Size, whereas tubing is specified by outside diameter (OD) and wall thickness. Pipe usually has thicker walls than tubing and cannot be bent without damage, but this depends on the application. Many industrial and government standards exist for the production of pipe.

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 is removed from the outside and inside surfaces and the weld zone is heat treated, so the seam is generally not visible. Welded pipe often has lower 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.

Plastic pipe is generally extruded.

Materials

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 may be made from black or galvanized steel, copper, and ductile iron.

Plastic pipes are also widely used for their light weight, chemical resistance, and non-corrosive properties. Plastic pipe materials include PVC, polyethylene, and ABS.

Pipe may also be made from concrete or clay. These pipes are usually used for low pressure applications such as gravity flow or drainage.

Concrete pressure pipe for water lines can be made in accordance with American Water Works Association (AWWA) standards of C-303 or L-301. C-303 pipe has a round bar helically wound around the steel cylinder and all surfaces are then coated with cement mortar. C-301 is a prestressed pipe and is made using two different techniques. The first method has a steel cylinder which has been lined with concrete and after the concrete cures the prestressing wire is wound directly on the steel cylinder and the exterior is then coated with cement mortar. The second method encases the steel cylinder in concrete which is then prestessed after the concrete cures and the exterior is then coated with cement mortar. C-303 pipe can be manufactured in from 10" through 72" and C-301 goes from 36" through 144". Working pressure is generally less than 300 psi.

Sizes

Many different standards exist for pipe sizes, and their prevalance varies depending on industry and geographical area. The pipe size designation generally includes two numbers; one that indicates the outside 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 in the US, and BS 1600 and BS 1387 in the United Kingdom.

In Europe, pressure piping uses the same pipe ID's and wall thicknesses as Nominal Pipe Size, but labels them with a metric Diametre 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 DIN 2448, 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 OD's than IPS.

Copper plumbing tube for residential plumbing follows an entirely different size system, often called Copper Tube Size (CTS); see domestic water system. PVC is available in two forms: PVC, which is made in NPS sizes, and CPVC, which is the same material but made in CTS sizes.

Agricultural applications use PIP sizes, which stands for Plastic Irrigation Pipe. PIP comes in pressure ratings of 22 psi, 50 psi, 80 psi, and 100 psi and is generally available in 6", 8", 10", 12", and 15" sizes.

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)

See this site 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). Other pipe threads include Nominal Pipe Size (NPS), Dryseal (NPTF), and British standard pipe thread (BSP).

Copper pipes are typically joined by soldering. Plastic pipes may be joined by solvent welding, heat fusion, or elastomeric sealing.

If frequent disconnection will be required, gasketed pipe flanges provid better reliability than threads. Some thin-walled pipes of ductile material, such as the smaller copper water pipes found in homes, may be joined with compression fittings.

Fittings

Since pipe cannot be bent or swaged as easily as tubing, changes in direction and size are usually accomplished with fittings. Fittings are also used to split or join a number of pipes together. A variety of standardized pipe fittings are available.

Elbows

Buttweld pipe elbows are available in short radius or long radius types. The short radius elbows have a centre to end distance equal to the NPS in inches, while the long radius is 1.5 times the NPS in inches.

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.

References

Piping | Plumbing Irrigation

Rohr (Technik) | Tuyau | Tubo

Gourt Home::Gourt :: Pipe (material)