Dear Dave Diehl,

I am doing a stress analysis for the flare piping system 24" & 18" and associated small branch connections from the pressure relief valves in a single system. I need to maintain the system natural frequency above 4Hz. I am using the friction stiffness factor as 1000 in the dynamic modal analysis. Is this value is acceptable. Please clarify your suggestions.

Mr. Dave,

I am waiting for your reply.......

That's the number I would use but I cannot point to any law of physics or industry standard to support that value.
Setting a stiffness factor for friction is quite effective in tuning up the model to match real world results but of course this assumes the system is built and in operation and you have an idea of the true system response.

Last time I attended Caesar seminar in Singapore. Please find the Presentation regarding dynamic analysis very useful one by Dave

Mr. Dave,

Thanks for your response.

Mr. Kumar,

Thanks for your attachment (Very useful).

Mr Dave,

In Autopipe does not exist the friction factor stiffnes so the friction is ignored. (and it´s not possible to simulate it).

I like to simulate the friction because in the real world this value exist. I supose if the vertical component of the excitation is very important so the friction must be reduced or ignored. But i supose it´s not easy simulate any reduction. (Solve non-linear situation it´s not easy and the software need a linear system to solve).

what´s your opinion?

My opinion?
It is difficult to consider (model) friction in any case but dynamics even more so.
Where important, I would bound the possible results high and low.
Where really important, I would replace sliding friction supports with more dependable, fixed supports.