Hello Everyone,

I am a university student doing an internship at a refinery. I have been working on relief valves/flare hydraulics for a few weeks now and there are still a lot of things that are unclear to me. One thing I would like to know is what happens in the event of an overpressure and the subsequent lifting of a PSV.
To take an example, let's say the MAWP of my vessel is 30 psig and the PSV (balanced bellows) is set at 30 psig. The superimposed backpressure is 0 psig. The required relief rate is 59,025 lb/hr and the capacity is 65,821 lb/hr. Now, let's say the vessel pressure up to 30 psig and the PSV opens. Does the PSV relieve at its capacity?

Thanks in advance,

Nate

At close to the set pressure, a spring loaded PSV will pop open and begin to relieve. But the system is designed so that the rated (stamped) capacity of the PSV will not be achieved until the pressure in the vessel reaches its relieving pressure, set pressure plus allowable overpressure (10% of set pressure for anything but a fire; 21% of set pressure for fire).

Generally for a gas, the PSV will relieve the rated (stamped) capacity of the PSV, at least initially. For a liquid, generally it will only relieve whatever the amount is that is being generated.

The object of the balanced bellows is to minimize the effects of built-up backpressure. It has nothing to do with the rated capacity of the valve.

This is the quick and dirty explaination.