Dear Mr. Viswan,
Theory can help in designing a system only upto a certain extent. For example in your case, deriving the forcing function resulting from a fluid flow thru a pipeline using first principle would lead you in a n th order differential equation with unknown number of variables. Further you need to have an in-depth knowledge of gas dynamics and should have correct physical properties of the gas thru pipeline. And still you would never be able to match reality with your calculation. Although I am not sure what causes the vibration in the GAS pipeline (hope that there is no impulsive flow), I suggest you get a response spectrum for the system. There are many agencies who would do this job for you. Once you get the response spectrum of the system, you may proceed with your analysis in Caesar.
Getting back to your problem. You have rightly identified that the 200m U/G piping between compressor to the point of interest is sufficient to arrest any compressor induced vibration (damping effect of soil). You have to investigate further, try to identify the cause of this vibration and then try to solve the problem by eliminating the cause. Possible sources could be vibration transmitted from pipe supporting structure to piping due to a rotating machine placed in the same foundation or even due to magnification resulting from superimposition of natural frequency of piping and pipe supporting structure. A possible theoretical solution can be derived for the last case by calculating the natural frequency of pipeline and pipe supporting structure separately and then comparing the results. Please remember: it is not always possible to arrest flow induced vibration using only shock absorbers and vibration dampers.
Regards,
Pankaj Mandal.
-----Original Message-----
From: Sajit Viswan [mailto:sviswan@tebodin.co.om]
Sent: 21 December 2003 13:41
To: PipingDesign. com (E-mail)
Subject: [PipingDesign] Impressed vibration
Dear Group,
A problem has come up about the vibration of a 48", 600# LNG export pipeline and a connected 24" line near the pig launcher.
I would like to investigate the impressed frequency caused due to the flow and compare it with the natural frequency of the system.
I am aware that the natural frequency of the system may be determined using CAESAR. I wish to know how it is possible to determine the impressed frequency due to the gas flow.
The compressor driving the flow is about 200m away and is connected to this system by underground lines. Therefore, I think the transmission of the compressor induced vibration is not a contributing factor towards the vibration problem.
Sajit
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