Backpressure correction for balanced bellows
Backpressure correction for balanced bellows
(OP)
We have 2 identical balanced bellows type PSVs. PSV A is set at 9.5 barg and PSV B 10 barg. They both protect the same equipment which has a 10 barg design pressure.
PSV A has a 16% overpressure, PSV B has 10% overpressure. Each PSV has a backpressure of 4 barg.
When I refer to API 520, Figure 30, Kb factors for A and B are 0.95 and 0.87 respectively.
Thus, even though A and B are identical valves with same relieving pressure (11 barg), the de-rating due to backpressure is less for valve A. Therefore, A is predicted to have greater capacity than B.
This doesn't make sense to me. Can anyone confirm whether this logic is correct?





RE: Backpressure correction for balanced bellows
RE: Backpressure correction for balanced bellows
RE: Backpressure correction for balanced bellows
The 16% overpressure valve with the lower set point has around 8-10% more capacity which I don't understand. This difference results in valve A having adequate capacity, but B is undersized. The answer to which appears to be reduce the set point of B to 9.5 barg.
RE: Backpressure correction for balanced bellows
RE: Backpressure correction for balanced bellows
The PRV B may not be undersized if it can take half the relieving flow.
JoeWong
Chemical & Process Technology