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LOADING PIPELINE DESIGN 2011

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Pipe line mechanical design

Introduction:

We calculated the pressure needed to

transport a given volume of gas through apipeline. The internal pressure in a pipe

causes the pipe wall to be stressed, and if

allowed to reach the yield strength of the

pipe material, it could cause permanent

deformation of the pipe and ultimate

failure. Obviously, the pipe should have

sufficient strength to handle the internal

pressure safely. In addition to the internal

pressure due to gas flowing through the

pipe, the pipe might also be subjected to

external pressure.

External pressure can result from the

weight of the soil above the pipe in a

buried pipeline and also by the loads

transmitted from vehicular traffic in areas

where the pipeline is located below roads,

highways, and railroads. The deeper the

pipe is buried, the higher will be the soil

load on the pipe. However, the pressure

transmitted to the pipe due to vehicles

above ground will diminish with thedepth of the pipe below the ground

surface. Thus, the external pressure due to

vehicular loads on a buried pipeline that

is 6 ft below ground will be less than that

on a pipeline that is at a depth of 4 ft. In

most cases involving buried pipelines

transporting gas and other compressible

fluids, the effect of the internal pressure is

more than that of external loads.

Therefore, the necessary minimum wall

thickness will be dictated by the internalpressure in a gas pipeline.

Purpose of mechanical

design:

The minimum wall thickness required to

withstand the internal pressure in a gas

pipeline will depend upon the pressure,pipe diameter, and pipe material. The

larger the pressure or diameter, the larger

would be the wall thickness required.

Higher strength steel pipes will require

less wall thickness to withstand the given

pressure compared to low-strength

materials.Piping design formula:

BARLOWS EQUATION

When a circular pipe is subject to internal

pressure, the pipe material at any point

will have two stress components at right

angles to each other. The larger of the two

stresses is known as the hoop stress and

acts along the circumferential direction.

Hence, it is also called the circumferential

stress.

The other stress is the longitudinal stress,

also known as the axial stress, which acts

in a direction parallel to the pipe axis.

Figure shows a cross section of a pipe

subject to internal pressure. An element

of the pipe wall material is shown with

the two stresses and inperpendicular directions.

Both stresses will increase as the internal

pressure is increased. As will be shown

shortly, the hoop stress is the larger ofthe two stresses and, hence, will govern

the minimum wall thickness required for

a given internal pressure.


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Figure 6.1 Stress in pipe subject to

internal pressure.

Where:

hoop or circumferentialstress in pipe material

P = internal pressure D = pipe outside diameter T = pipe wall thickness

Similar to Equation 6.1, the axial (or

longitudinal) stress,

, is given by the following equation:

Barlows equation is valid only for thin-

walled cylindrical pipes. Most pipelines

transporting gases and liquids generally

fall in this category. There are instances

in which pipes carrying gases and

petroleum liquids, subject to high external

loads, such as deep submarine pipelines,

may be classified as thick-walled pipes.

The governing equations for such thick-

walled pipes are different and more

complex.

THICK-WALLED PIPES

Consider a thick-walled pipe with an

outside diameter and inside diameterof, subject to an internal pressure of P.The greatest stress in the pipe wall will be

found to occur in the circumferential

direction near the inner surface of the

pipe.

This stress can be calculated from the

following equation:

The pipe wall thickness is

Rewriting Equation 6.3 in terms of

outside diameter and wall thickness, we

get


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Simplifying further,

( ) (

)

( )

In the limiting case, a thin-walled pipe is

one in which the wall thickness is very

small compared to the diameter. In thiscase is small compared to 1 andtherefore, can be neglected in Equation

6.5. Therefore, the approximation for thin

walled pipes from Equation 6.5 becomes

Which is the same as Barlows Equation

6.1 for hoop stress.

INTERNAL DESIGN

PRESSURE EQUATION

We indicated earlier in this chapter that

Barlows equation, in a modified form, is

used in designing gas pipelines. The

following form of Barlows equation is

used in design codes for petroleum

transportation systems to calculate wall

thickness based upon given the allowable

internal pressure in a pipeline, diameter,

and pipe material.

P = internal pipe design pressure D = pipe outside diameter T = pipe wall thickness S = specified minimum yieldstrength (SMYS) of pipe material

E = seam joint factor, 1.0 forseamless and submerged arc

welded (SAW) pipes.

F = design factor, usually 0.72 forcross-country gas pipelines, but

can be as low as 0.4, depending onclass location and type of

construction

T = temperature derating factor =1.00 for temperatures below

250F


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Table 6.2 Pipe Seam Joint

Factors

The seam joint factor E used in Equation

(6.8) varies with the type of pipe material

and welding employed. Seam joint factors

are given in Table 6.2 for the mostcommonly used pipe and joint types.

The internal design pressure calculated

from Equation is known as the maximum

allowable operating pressure (MAOP) of

the pipeline. This term has been shortened

to maximum operating pressure (MOP) in

recent years. Throughout this book we

will use MOP and MAOP

interchangeably. The design factor F has

values ranging from 0.4 to 0.72, as

mentioned earlier. Table 6.3 lists the

values of the design factor based upon

class locations. The class locations, in

turn, depend on the population density in

the vicinity of the pipeline.


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Table 6.3 Design Factors for Steel

CLASS LOCATION

Class 1 Offshore gas pipelines are Class 1locations. For onshore pipelines, any class

location unit that has 10 or fewer

buildings intended for human occupancy

is termed Class 1.

Class 2 This is any class location unit that

has more than 10 but fewer than 46

buildings intended for human occupancy.

Class 3 This is any class location unit that

has 46 or more buildings intended for

human occupancy or an area where the

pipeline is within 100 yards of a building

or a playground, recreation area, outdoor

theatre, or other place of public assembly

that is occupied by 20 or more people at

least 5 days a week for 10 weeks in any

12-month period. The days and weeks

need not be consecutive.

Class 4 This is any class location unit

where buildings with four or more stories

above ground exist.

The temperature deration factor T is equal

to 1.00 up to gas temperature 250F, as

indicated in Table 6.4.

Table 6.4 Temperature Deration

Factors


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Miters:

A miter is two or more straight

sections of pipe matched to

produce a change in direction.

Most are familiar with the miter as

the twin 45 cut that produces the

square corner in a picture frame.

One technique for designing

miters, where they are allowed,

is as. Figure shows the diagram

of a miter and labels the

symbols.

Where:

: Effective radius of miter T: minimum thickness of

miter pipe wall

: angle of miter cut : angle of change in

direction=2

: mean radius of pipe usingnominal wall to calculate.

E: efficiency

C: corrosion and mechanicalallowances

D: pipe OD

M: minimum distance frominside crotch to end of miter

There are three equations to utilize

in the design process. Equation 3 is

only applicable to single miters

where the angle q is greater than

22.5. Equation 2 is to be used for

single miters where the angle q is

not greater than 22.5. When onewants to use multiple miters, the

angle q must not be greater than

22.5 and one must use equations 1

and 2.

The lesser value computed with

those two equations is the

maximum internal pressure allowedby the code. Then the length M

must be calculated and applied to

the end sections. Those equations

are given in Table E.2.


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TABLE E.2 Equations Utilized

in the Design of Miters

The value of R1 should meet some

minimum for these miters to be in

compliance with the code. There

are two formulas for that value. The

more general formula is found in

B31.9 and is given as:

Code B31.3 has a more rigorous

requirement, giving the minimum

value of R1 as a function of the

thickness. This refinement has the

effect of requiring R1 to be larger

for thicker materials. The general

formula is the same but substitutes

a variable expression A for the 1 in

the B31.9 formula. It is

where A has an empirical value per

Table E.3.

TABLE E.3 Empirical Value of A


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Bends

For many years the code

requirement for the wall thickness

of bends was simply that thethickness shall be the same as that

required for straight pipe of the

particular code. Given the general

methods of bending that were

prevalent, and often are still used

today, this usually meant that one

needed to start with a wall thicker

than needed.

Assuming that one starts with a

straight piece of pipe, for the bend

there will be different lengths for

the different edges of the bend.

These edges have names. Figure

E.1 shows the net effect. One can

see that the extrados will be longerand that the intrados will be shorter

than the beginning length, which is

the length of the centerline of the

straight pipe.

Since there is no new material

added by the bending process and

no transfer of material from one

part of the pipe to the other, the netresult is that the extrados gets

thinner and the intrados gets

thicker. This is a fortunate

circumstance as the demands of

pressure in the bend were found to

need the thicker material at that

intrados.

A more unfortunate result is that the

extrados gets thinner. This requires

starting with thicker wall pipe to

meet the same requirement as for

straight pipe. But the question

became: How much thicker? One

result is that two of the codes give a

recommendation to the reader of

how much thicker one needs to start

with, depending on the bend radius

required. The shorter the radius

desired, the greater the thickness.

It was found that the extrados need

not have the same thickness as that

of straight pipe. The pressure

requirements are not as great at that

position. In addition, bending

techniques improved to the point

where there might be less thinning.

Figure E.1 Diagram showing

bend terminology.

It was also found that some

techniques did not thicken the

intrados enough to guarantee the

margins that the codes required.


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And a need developed to have a

quantitative measure for both walls

intrados and extrados.

A mathematical technique to definethose required wall thicknesses

existed. Code B31.1, Code B31.3,

and some of the bending standards

have included it in their books.

It involves the inclusion of a factor,

one each for intrados and extrados,

and including that factor in the

straight wall thickness equation.

Those factors are shown in Table

E.1.

TABLE E.1 Bend Factor

To calculate the wall thickness for

either the intrados or extrados, use

the appropriate factor in the

following modified straight wall

equation.

t = calculated required wall(note: allowances must be

added) P = pressure

S = allowable stress E = efficiency factor I = appropriate intrados or

extrados factor y = factor from table

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