Adiabatic Depressuring using HYSYS

My friend, I dont understand the reference you made to a "relief valve", but regarding your question I suggest you to proceed as follows: if you dont have a RO downstream your blowdown valve (BDV), then the only restriction that limits the flow rate is such BDV. BDVs are normally ball valves, either regular (reduced) port or full port.
To calculate the peak flow you need to know the Cv of your BDV, which can be obtained from vendors literature (be carefull: full port valves have a higher cv than regular port valves).
Once you have the Cv of your BDV you may calculate the peak flow using the following equation (for critical flow conditions P2 < 0,53 x P1):
q = 660 x Cv x P1 x sqrt (1/(SG x T1)), where q [=] SCFH, P1 [=] psia, T1 [=] &#186;R.
After obtaining the peak flow rate through your BDV, go to Hysys and modify the orifice area in so as to obtain the same peak flow rate.
In this way, the depressuring time calculated by Hysys will be reasonably realiable.
Hope this helps.
Good luck!
Mart&#237;n