General 
A pipe or a tube is hollow longitudinal product. ‘A tube’ is general term used for hollow product having circular, elliptical or square cross-section or for that matter cross section of any closed perimeter.
A pipe is tubular product of circular cross-section that has specific sizes and thicknesses governed by particular dimensional standard. Tubes can be ordered for any OD or ID and thicknesses, pipes are ordered on basis of nominal sizes.
Classification:
Pipes can be classified based on methods of manufacture or end use.
Methods of Manufacture:
Seamless Pipes are manufactured by drawing or extrusion process. ERW Pipes (Electric Resistance Welding pipes) are formed from a strip which is longitudinally welded along its length. Welding may be by Electric resistance, high frequency or induction welding. ERW pipes can also be drawn for obtaining required dimensions and tolerances.
Classification Based on End Use:
Pipes are also classified as:
- Pressure pipes or Process pipes
- Line Pipes
- Structural Pipes
1. Pressure pipes are those which are subjected to motive pressure and system pressure and or temperatures. Fluid pressure in generally internal pressure due to fluid being conveyed or may be external pressure (e.g. jacked piping) and are mainly used as plant piping.
2. Line pipes are mainly used for long distance conveying of the fluids and are subjected to motive pressures. These are generally not subjected to high temperatures.
3. Structural pipes are not used for conveying fluids and therefore not subjected to fluid pressures or temperatures. They are used as structural components (e.g. handrails, columns, sleeves etc.) and are subjected to static loads only;
Pipes Dimensional Standards:
A. Diameters: Pipe are designated by. Nominal size, starting from 1/8" Nominal size and increasing in steps up to 36 inches
For the Nominal size upto and including 12", there is one unique O.D. (different from nominal size) and 1.0. would vary depending on schedule nuniber. For Nominal sizes 14" and above, 0.0. is same as Nominal size.
Thickness:
Pipe thicknesses are designated by schedule number (which determine internal pressure) or weight designation like Std. (S), Extra Strong (XS) and Double Extra Strong (XXS). Pipe schedule number S is defined as:
Sch. No. S = 1000 P/S
Where P = Internal Pressure (PSI)
S = Allowable tensile strength of material used.
Common pipe schedules are Sch 40, Sch 80, Sch 120, Sch 160, for larger pipe sizes intermediate schedule numbers (Sch 20 Sch 30 etc.) are also employed (Ref. pipe dimension Chart).
For Carbon steel, Pipe wall thickness tolerance is ± 12 1/2% i.e. Pipe wall thickness can, vary 12 1/2% from thickness obtained from dimension chart.
For stainless steels schedule numbers are designated by su~Tix S i.e; lOS, 40S, 80S etc.
Length:
Pipes are manufactured in ‘random length’ which is 20’+ -2.5 and in double random length 40’ + - 5.0.
Pipe Fittings:
Pipe fittings are the components which tie together pipe lines, valves, and other parts of a piping system. They are used in “making up” a pipe line. Fittings may come in screwed, welded, soldered, or flanged varieties and are used to change the size of the line or its direction and to join together the various parts that make up a piping system.
The majority of pipe fittings are specified by the nominal pipe size, type, material and the name of the fitting. Besides the end connections as above (screwed, welded, soldered, flanged) it is also possible to order bell and spigot fittings, which are usually cast iron and used for low pressure service.
In general, a fitting is any component in piping system that changes its direction, alters its function, or simply makes end connections. A fitting is joined to the system by bolting, welding or screwing, depending on many variables in the system.
1. Butt-Welded Fittings
Welded fittings are used primarily in systems meant to be permanent. They have the same wall thickness as the mating pipe. Among the many advantages of butt welded systems are the following:
• They have a smooth inner surface and offer gradual direction change with minimum turbulence.
• They require less space for constructing and hanging the pipe system.
• They form leak-proof constructions.
• They are almost maintenance free.
• They have a higher temperature and pressure limit.
• They form a self-contained system.
• They are easy to insulate
• They offer a uniform wall thickness through-out the system.
One of the major disadvantages of butt-welded systems is that are not easy to dismantle. Therefore, it is often advisable to provide the system with enough flanged joints so that it can be broken down at intervals. (One of the main uses of the butt-welded system, is for steam lines, which are usually in high temperature/ high-pressure service).
2. Socket Welded Fittings
Socket welded fittings have certain advantages over butt-welded fittings. They are easier to use on small-size pipelines and the ends of the pipes need not be beveled since the pipe end slips into the socket of the joint. With socket welded fittings there is no danger of the weld protruding into the pipeline and
restricting flow or creating turbulence. Thus, the advantages of the socket welded system are:
• The pipe does not need to be beveled.
• No tack welding is necessary for alignment since joint and the pipe are self -aligning.
• Weld a material can not extend into the pipeline.
• It can be used in place of threaded fittings, therefore, reducing the likelihood of leaks, which usually accompany the use of threaded fittings.
• It is less expensive and easier to construct than other welded systems.
One of the major disadvantages of this type of fitting is the possibility of a mismatch inside the fitt~ng where improperly aligned or mated parts may create a recess where corrosion could start.
Socket-welded fittings have the same inside diameter as standard (Schedule 4O), extra strong (Schedule 80), and double extra strong (Schedule 160) pipe, depending. on the weight of the fitting and mating pipe. Socket-welded fittings rare covered in ASA 816.11. They are drilled to match the internal diameter of schedule 40 or schedule 80 pipe.
3. Flanged Fittings
Flanged connections are found on piping systems throughout the petrochemical and power generation fields on pipelines that are a minimum of 2 in.(5.08 cm ) in diameter. The majority of flanged fittings are made of cast steel or cast iron.
Flanged steel fittings are used in place of cast iron where the system is subjected to shock or high-temperature/ high-pressure situations where the danger of fire is prevalent, because cast iron has a tendency to c rack or rupture under certain stresses. A flange may be cast or forged onto the ends of the fitting or valve and bolted to a connecting flange which is screwed or welded onto the pipeline, thereby providing a tight joint. An assortment of facings, ring joint grooves, and connections are available in flange variations.
One advantage of flanged systems is that, they are easily dismantled and assembled. One of the disadvantages is that they are considerably than an equally rated butt-welded system, because of the large amount of metal that go into making up joints and flanges. Moreover, flanged fittings occupy far
more space than the butt-welded or screwed equivalents. Because of this higher weight load, a flanged system becomes far more expensive to support or hang from the existing structure.
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