high back pressure relief valves
high back pressure relief valves
(OP)
Recently a colleague asseted that PSVs with high set pressures (about 500 psig plus) and sonic velocity in the tailpipe should have a higher flange rating than 150# on the outlet because the pressure at that point would exceed the lower flange rating. Scanning a vendor catalogue shows most valves have 150# flange ratings. Should PSVs in this situation have 300# flanges?
RE: high back pressure relief valves
There can be any of a number of things that could affect the determination of the pressure at the outlet flange of a relief valve. Just to name a few - type of relief valve, superimposed backpressure, amount of overpressure used, etc. The bottom line is you need to determine what the pressure is and select the appropriate equipment.
If I were to assume for your example that you had a conventional type valve, discharging to atmosphere and sized for 10% overpressure, then the maximum allowed backpressure at the valve outlet can be only 50 psig regardless if the flow was sonic at the atmospheric end. So 150# flanges should be sufficient.
RE: high back pressure relief valves
If you have sonic velocity downstream it could be that you dont have critical flow in your psv.
If its sized for critical flow and you dont have that - the the flowrate will be much lower - hence not sonic velocity an so on.
I dont think its good to have such high velocity in the tail pipe.
Best regards
Morten
RE: high back pressure relief valves
Remember that the SRV is there to be the last defense between you and a crater with a plaque that reads "Site of the Great Plant Disaster".
If you routinely overpressure your system, you should have active valves and/or controls to mitigate the situation. SRVs are for when everything else fails. And although it is customary for SRVs to have a larger discharge than the inlet conection, it's not a bad idea to size the downstream pipe large enough so that the flow will be subsonic (<0.7M) anyway.
RE: high back pressure relief valves
RE: high back pressure relief valves
Best regards
Morten
RE: high back pressure relief valves