You can find useful information on two phase flow regimes at the following link. Slug flow is the main cause of piping vibrations. I cannot give you a minimum frequency as that varies with piping geometry and slug length.
The following article has PDF file with an example of modeling slug flow in AutoPIPE using time history. You can also model using static loading by applying the loads in USER case(s). Static analysis assumes no resonance in the system and dynamic amplification factor of 2 may not be enough in the case of slug flow, but can give you a quick way to see if your system has a chance of working.