For seismic case-33hz is the cut off frequency as per REGULATORY GUIDE 1.60.
want to know the basis for cut off frequency value to be used for other systems while performing dynamic analysis for:
1. slug analysis
2. water hammer analysis.
3. PSV systems

Your modal analysis for these dynamic loads should extend at least to the frequency which produces the largest modal response for that event.
The Response Spectrum generator is a good tool for this purpose as it converts the time history of the event into a response spectrum. Go to a frequency beyond the RS peak.
Of course there may be other criteria as well.

Thanks dave for your reply.

what will be the "other critieria" as mentioned by you in above reply?

One example - You may have a requirement that no more than 10% of the total system mass may be allowed as a static load add on for high frequency contribution. I have seen this specification with seismic evaluation.