Sizing relief valves based on regulator failure modes
Sizing relief valves based on regulator failure modes
(OP)
If one is sizing a downstream relief valve based on pressure regulator failure conditions, can anyone tell me what flowrate/worst downstream pressure to assume for the failed regulator case? Haven't seen any real information on this. Am I to assume the worst a regulator can do is go full open to its mas downstream outlet pressure?
Lucas
Lucas





RE: Sizing relief valves based on regulator failure modes
RE: Sizing relief valves based on regulator failure modes
RE: Sizing relief valves based on regulator failure modes
The regulator failures I've witnessed have been many and have happened as follows: the operator attempts to adjust the setpoint. The regulator internals "stick", such that outlet pressure no longer responds to changes in setpoint. In frustration, the operator whacks the regulator with the wrench, un-sticking the regulator. The regulator now has a setpoint greatly higher than intended, and initially at least it dumps gas or liquid into the system through its fully open orifice (i.e. it is wide open with flow determined only by Cv). A relief valve to protect against that event would seem to need to be able to keep up with the regulator stuck wide open, at least for a time.
RE: Sizing relief valves based on regulator failure modes
it makes no sense to ignore that you might have 100 feet of 0.5" piping on the inlet to a regulator with a 0.25" orifice followed by 25 feet of downstream piping.
i have seen folks take the 0.25" orifice and treat it as if that is the only restriction in the system. If it was a 2" line feeding the 0.25" orifice and a 4" line leaving it, maybe but not relatively small piping where the dP in the system in a "fail open" case results in significant line loss......
RE: Sizing relief valves based on regulator failure modes
The flowrate calculated thru the fully open valve will be very high and so the pressure drop that it would be generated in the d/s system (ie piping between control valve outlet and PSV inlet) will be higher than the actual one.
So a trial-and-error calculation is needed to obtain the real flow.
This kind of problem can also be managed thru a dynamic simulation.
RE: Sizing relief valves based on regulator failure modes
What condition should be used to determine the relief flow rate to size the PSV?
Someone suggested that the relieving flowrate should be the total throughput from the valve. Is the condition right?
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RE: Sizing relief valves based on regulator failure modes
Basically, the PSV needs to pass enough fluid that the resulting head added by the pump minus the line losses do not result in more pressure than allowed by the applicable piping codes you are designing this system to.
RE: Sizing relief valves based on regulator failure modes
RE: Sizing relief valves based on regulator failure modes
SO your scenario could be: Outlet valve (or ESD) is closed (as a part of your operation). Incident: inlet valve fails to close. Here assume that the valve not even fails to close but opens fully. However, deduct the the pressure in the vessel is higher than during normal operation (as mentioned before ST+10%)
Best regards
Morten