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Statutory Regulations

Chapter IIReview of Piping Fundamentals

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 apd

are subjected to motive pressures. These are generally not subjected tohigh 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, star ting 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.

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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/SWhere 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 thesystem 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.

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Statutory Regulations

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 enoughflanged 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 amismatch 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.

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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.