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RE: Pipe Stress Analysis - Compressor Piping

• To: <seaint(--nospam--at)seaint.org>
• Subject: RE: Pipe Stress Analysis - Compressor Piping
• From: Harold Sprague <spraguehope(--nospam--at)hotmail.com>
• Date: Tue, 14 Dec 2010 17:22:21 +0000

Here are a couple of good guides regarding the fundamentals on nozzle flexibility assumptions.
http://www.sstusa.com/pdfs/basic_pipe_stress_tutorial.pdf
http://www.osti.gov/bridge/servlets/purl/46579-dFMW3n/webviewable/46579.pdf
Any deeper than this and I call some of my piper specialist friends.

Regards, Harold Sprague

Date: Tue, 14 Dec 2010 07:03:12 -0800
From: prajendran(--nospam--at)ymail.com
Subject: Pipe Stress Analysis - Compressor Piping
To: seaint(--nospam--at)seaint.org

 One of the requirements in pipe stress analysis of a compressor piping is to 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 are generally based on API 617 Appendix E. The imposed loads arise from temperature changes and pressure in the pipe, gravity loads etc.Apart from the need to keep the nozzle loads at each of the nozzles below the allowables, there are additional criteria to be met which are combinations of  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 API 617.In modeling the piping, the nozzle locations are considered as rigid anchors which means, in theory, there should be no displacement or rotations in the 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 words, 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 stiffnesses.  The nozzle/compressor configuration is so complicated that the nozzle stiffnesses cannot be quickly determined.  I have come across some analysts who lower the stiffnesses arbitrarily until the criteria are satisfied. Are there any pipe stress analysts in this list who could offer some guidance regarding translational and rotational nozzle stiffnesses?I also find that it is very difficult to satisfy all the equations of Appendix E 2.b, particularly without introducing some pipe loops. Unfortunately, the industry trend seems to stay away from pipe loops. In most projects, the configuration is so tight that there is no possibility for loops. Additional complication arise when the client asks for clamped supports along the pipe. Fundamentals of mechanics teach us that a structural element which is 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? So far, I have not found a good reference material on the subject where the above issues are discussed in a sound manner. Rajendran