pipe and tubes

A pipe can be defined as a tube made of metal, plastic, wood, concrete or fiberglass. Pipes are used to carry liquids, gases, slurries, or fine particles. A piping system is generally considered to include the complete interconnection of pipes, including in-line components such as pipe fittings and flanges. Pumps, heat exchanges, valves and tanks are also considered part of piping system. Piping systems are the arteries of our industrial processes and the contribution of piping systems are essential in an industrialized society.

Fig. 1 illustrates the magnitude of piping required in a typical chemical process plant. Piping systems accounts for a significant portion of the total plant cost, at times as much as one-third of the total investment. Piping systems arranged within a very confined area can be a added challenge to piping and support engineers.

The initial design of a piping system is established by the functional requirements of piping a fluid from one point to another. The detailed design is decided by criteria such as type of fluid being transported, allowable pressure drop or energy loss, desired velocity, space limitations, process requirements like free drain or requirement of straight run, stress analysis, temperature of fluid, etc. The supporting of piping systems requires a significant engineering, design, fabrication and erection effort. In some cases, special structures (like structural T or inverted L, cantilevers, U portals, pedestals, etc) must be built solely for the purpose of supporting piping systems.

Piping Material

The material to be used for pipe manufacture must be chosen to suit the operating conditions of the piping system. Guidance of selecting the correct material can be obtained from standard piping codes. As an example, the ASME Code for Pressure Piping contains sections on Power Piping, Industrial Gas and Air Piping, Refinery and Oil Piping, and Refrigeration
Piping Systems. The objective being to ensure that the material used is entirely safe under the operating conditions of pressure, temperature, corrosion, and erosion expected. Some of the materials most commonly used for power plant piping are discussed in the following sections.

• Steel – Steel is the most frequently used material for piping. Forged steel is extensively used for fittings while cast steel is primarily used for special applications. Pipe is manufactured in two main categories – seamless and welded.

• Cast Iron – Cast iron has a high resistance to corrosion and to abrasion and is used for ash handling systems, sewage lines and underground water lines. It is, however, very brittle and is not suitable for most power plant services. It is made in different grades such as gray cast iron, malleable cast iron and ductile cast iron.
• Brass and Copper – Non-ferrous material such as copper and copper alloys are used in power plants in instrumentation and water services where temperature is not a prime factor.

Commercial Pipe Sizes

Commercial pipe is made in standard sizes each having several different wall thicknesses or weights. Up to and including 304.8 mm (12 inch)  pipe, the size is expressed as nominal (approximate) inside diameter. Above 304.8 mm, the size is given as the actual outside diameter. All classes of pipe of a given size have the same outside diameter, with the extra thickness
for different weights on the inside. For example, if a pipe was designated as 152.4 mm size this would mean that it has a nominal or approximate inside diameter of 152.4 mm. The outside diameter is 168.28 mm. This is a constant value no matter what the wall thickness is. The actual inside diameter of the pipe will depend upon its wall thickness. For a standard wall
thickness, the actual inside diameter of 152.4 mm pipe is 154.06 mm. For an extra strong wall thickness, the actual inside diameter is 146.34 mm.

There are two systems used to designate the various wall thicknesses of different sizes of pipe. The older method lists pipe as standard (S), extra strong (XS) and double extra strong (XXS). The newer method, which is superseding the older method, uses schedule numbers to designate wall thickness. These numbers are 10, 20, 30, 40, 60, 80, 100, 120, 140 and 160. In most sizes of pipe, schedule 40 corresponds to standard and schedule 80 corresponds to extra strong.

Dimensions and the mass in kg/m of different sizes of steel pipe with varying wall thicknesses is shown in Fig.

Pipe Fittings

A fitting is used in pipe systems to connect straight pipe sections, adapt to different sizes or shapes and for other purposes, such as regulating (or measuring) fluid flow. Pipe Fittings (especially uncommon types) require money, time, materials and tools to install, and are an important part of piping and plumbing systems. Valves are technically fittings, but are usually discussed separately. The purposes of the fittings, shown in Fig. 3 may be generally stated as follows:

• Elbows – for making angle turns in piping.
• Nipples – for making close connections. They are threaded on both ends with the close nipple threaded for its entire length.
• Couplings – for connecting two pieces of pipe of the same size in a straight line.
• Unions – for providing an easy method for dismantling piping.
• Tees and Crosses – for making branch line connections at 90º.
• Y-bends – for making branch line connections at 45º.
• Return Bends – for reversing direction of a pipe run.
• Plugs and Caps – for closing off open pipe ends or fittings.
• Bushings – for connecting pipes of different sizes. The male end fits into a coupling and the smaller pipe is then screwed into the female end. The smaller connection may be tapped eccentrically to permit free drainage of water.
• Reducers – for reducing pipe size. Has two female connections into which the different sized pipes fit. May also be made with one connection eccentric for free drainage of water.