Thanks Michael for your opinion. Actually in my previous experience our senior Stress Engineer requires us to do a Modal Analysis where the 4 Hz is used as the minimum fundamental frequency to almost all critical lines in order to determine if the piping system is well restrained and well guided. But it's not an absolute requirement especially for high temperature lines where flexibility is required rather than a rigid piping system. Just a quick tool to determine the stability of the system against environmental effects.

Anyway, vibration at the "Design Stage" cannot be guaranteed 100% can be solved but good design practices that were developed through the years can be used as a countermeasure.

Cheers...