Dear Raj,
Modal Analysis does not have any criterion for any minimum acceptable frequency. In the Modal analysis the natural frequency of the system is calculated and the modes are extracted. There is no external forcing function in this analysis. It is just a simple calculation of system natural frequency for the given support configuration. For the correct modal analysis, you need to have a correct mass model of the system. That means, all the masses and concentrated weights are specifically model in the modal analysis input. Also make sure that the fluid weight is included in the input. Ensure that the modulus of elasticity used for the analysis is the hot modulus (as a code requirement, we take cold modulus of elasticity for flexibility analysis) and density of the fluid is at hot (operating temperature). This is required since your system will be in operating condition for the max time.
The output of the modal analysis will give only natural frequencies of the system and mode shape associated with that frequency. You can check the active boundry conditions also.
I am again repeating that there is no forcing function in modal analysis, it is just to calculate the natural frequencies of the piping system for a given layout configuration. So there are no acceptable value of the frequency. Dpending on the results you can decide if you want your system to be more stiff or flexible (stiff is better), and again there are no set practices for this.
In general, refinery piping systems with static conditions should have a fundamental natural frequency of 4Hz.
Regards,
Kaustubh
Received on Tue Jul 16 23:24:00 2002
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