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Piping Stress Related Design Factors

There are five basic factors that influence piping and thereforepiping stress in the process plant. There is temperature, pressure,

weight, force and vibration. These factors will come in many forms

and at different times. Stress problems become all the more complex

because two or more of these will exist at the same time in the same

piping system. The main obective of the focus when dealing with

problems related to piping systems is not normally the pipe itself.

!n a very high percentage of the time it is not the pipe that is the

wea"est lin". #ote this$ the pipe is normally stronger and%or less

vulnerable to damage than what the pipe is connected to. Pumps are

ust one examples of e&uipment to which pipes are routinely

connected. 'isalignment problems caused by expansion (or contraction)

in a poorly designed system can result in maor e&uipment failure.

*&uipment failures can lead to the potential for fire, plant shutdown

and loss of revenue. +t this point it should be emphasied that the

success (or failure) of the plant-s operation, years down the road

can and will depend on what is done up front by all the members of

the design team during the design stage. +n important point to

remember, /hile analysis cannot create a good design, it can

confirm a good design0 (!mproved Pump 1oad *valuation,0 2ydrocarbonProcessing, +pril 3445, 6y$ David /. Diehl, 78+D* *ngineering

Software, !nc 2ouston, T9). 8n the other hand, proper analysis will

identify bad design and potential problems in a piping system design.

Stress Related Design Factors

Temperatures in piping systems may range from well over 3:::oF

(;?:: oF (>3?5.5 7) on the low

side. *ach extreme on the temperature scale and everything in between

brings its own problems. There will also be times when both high andlow temperatures can occur in the same piping system. +n example of

this would be in piping that is installed in an arctic environment.

The piping is installed outdoors where it is subected to >3:: oF (>

=

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they cannot prevent the action caused by heat or cold. *xpansion or

contraction in a piping system it self is not so much a problem. +s

we all "now if a bare pipe was ust lying on the ground in the middle

of a dry barren desert it will absorb a lot of heat from ust solar

radiation. !n the hot sun piece of pipe can reached 3;:o

F (@;.; 7).The pipe will expand and with both ends loose it would not be a

problem. 2owever, when you connect the pipe to something, even if

only one end is connected you may begin to have expansion related

problems. /hen the pipe is anchored or connected to something at both

ends you absolutely will have expansion induced problems. *xpansion

induced problems in a piping system is stress. There are a number of

ways to handle expansion in piping systems. Flexible routing is the

first and by far the cheapest and safest method for handling

expansion in piping systems. The other way is the use of higher cost

and less reliable flexible elements such as expansion oints.

Stresswill exist in every piping system. !f not identified and the

proper action ta"en, stress will cause failure to e&uipment or

elements in the piping system itself. Stress results in forces at

e&uipment noles and at anchor pipe supports. Two piping

configurations with the same pipe sie, shape, dimensions,

temperature and material but with different wall schedules (sch. A:

vs. sch. 3@:) will not generate the same stress.

Forcein piping systems is not independent of the other factors.

Primarily, force (as related to piping systems) is the result of

expansion (temperature) and%or pressure acting on a piping

configuration that is too stiff. This may cause the failure of a pipe

support system or it may cause the damage or failure of a piece of

e&uipment. Force, and the expansion that causes it, is best handled

by a more flexible routing of the piping. Some people suggest that

force can be reduced by the use of expansion oints. 2owever we must

remember that for an expansion oint to wor" there must be an

opposite and e&ual force at both ends to ma"e the element wor". Thistends to compound the problem rather than lessen it.

ressurein piping systems also range from the very high to the very

low. Piping systems with pressure as high as

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affecting forces. !ncreasing the wall thic"ness of the pipe is the

primary method of compensating for increases in pressure. 8ther ways,

depending on many factors include changing to a different material.

/ith low or vacuum systems there are also other ways to prevent the

collapse of the pipe wall. +mong these the primary method is theaddition of stiffening rings. Stiffing rings may be added internally

or externally depending on the commodity type and the conditions.

/eightin a piping system is expressed normally as dead load. The

weight of a piping system at any given point is made up of many

elements. These include the weight of the pipe, the fittings, the

valves, any attachments, and the insulation. There is also the test

media (e. g. hydrotest water) or the process commodity whichever has

the greater specific gravity. Piping systems are heavy, period.

*verybody involved in the proect needs to understand this and be

aware that this weight exists and it needs to be supported. #inety>

nine times out of a hundred this weight will be supported from a

structural pipe support (primary pipe support system) of some "ind.

2owever there are times when the piping (weight) is supported from a

vessel or other type of e&uipment.

Bibrationswill also occur in piping systems and come in two types.

There is the basic mechanical vibration caused by the machines that

the piping is connected to. Then, there is acoustic (or harmonic)vibration caused by the characteristics of the system itself.

Typically the only place severe vibrations will be found is in piping

connected to e&uipment such as positive displacement reciprocating

pumps or high pressure multi>stage reciprocating compressors and

where there is very high velocity gas flows.

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