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RE: Pipe Stress Analysis - Compressor Piping

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Rajendran,

I don't have extensive experience in pipe stress analysis, but I do dabble occasionally. You say "the nozzle locations are considered as rigid anchors". Is this a code requirement, standard practice, or simply a software default? As a practical engineering matter, the stiffer the anchor, the more load, and therefore stress, it will attract. So if you are required to design the system to limit the stress at the nozzles to the allowable, the conservative approach would be to model the nozzles as rigid anchors. Just because the stresses are higher than allowable, does not mean the analysis is wrong. It more likely means that the piping system requires redesign. Most steam or hot oil piping I have dealt with has required expansion loops, even in tight spaces. The competing requirements for flexibility for temperature and rigidity for seismic restraint can often be very difficult to satisfy.

Dmitri


From: Padmanabhan Rajendran <prajendran(--nospam--at)ymail.com>
Subject: Pipe Stress Analysis - Compressor Piping
To: seaint(--nospam--at)seaint.org

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One of the requirements in pipe stress analysis of a compressor piping is t=
o ensure that the loads imposed by the piping on the compressor nozzles are=
less than the allowable loads. In the oil industry, the allowable loads ar=
e generally based on API 617 Appendix E. The imposed loads arise from tempe=
rature changes and pressure in the pipe, gravity loads etc.

Apart from the need to keep the nozzle loads at each of the nozzles below t=
he allowables, there are additional criteria to be met which are combinatio=
ns of=A0 forces and bending moments not only at the individual nozzles but =
also at the nozzle with the largest diameter where the resolved loads from =
all the other nozzles are included. These are given in Appendix E 2.b of AP=
I 617.

In modeling the piping, the nozzle locations are considered as rigid anchor=
s which means, in theory, there should be no displacement or rotations in t=
he six degrees of freedom. A typical pipe stress software may assume 1E+11 =
as the stiffness at a rigid anchor. In my experience, such assumption would=
not allow satisfying the nozzle load criteria described above. In other wo=
rds, some smaller stiffness value is required to be assumed at each of the =
six degrees of freedom to satisfy even the criteria at individual nozzles. =
Unfortunately, the compressor vendor does not provide the nozzle stiffnesse=
s.=A0 The nozzle/compressor configuration is so complicated that the nozzle=
stiffnesses cannot be quickly determined.=A0 I have come across some analy=
sts who lower the stiffnesses arbitrarily until the criteria are satisfied.=
Are there any pipe stress analysts in this list who could offer some guida=
nce regarding translational and rotational nozzle stiffnesses?

I also find that it is very difficult to satisfy all the equations of Appen=
dix E 2.b, particularly without introducing some pipe loops. Unfortunately,=
the industry trend seems to stay away from pipe loops. In most projects, t=
he configuration is so tight that there is no possibility for loops. Additi=
onal complication arise when the client asks for clamped supports along the=
pipe. Fundamentals of mechanics teach us that a structural element which i=
s subject to temperature change should be allowed to move in order to keep =
the stresses and reactions low in the system. How do other engineers handle=
these constraints?=20

So far, I have not found a good reference material on the subject where the=
above issues are discussed in a sound manner.=20

Rajendran


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