Backpressure correction for balanced bellows

[steveb1111]

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?

 

[CJKruger]

steveb1111, I did not check the numbers, but think your logic is correct.

 

[lizking]

I assume your backpressure is built-up plus superimposed. The extra capacity for valve A will result in a higher backpressure and thus a lower Kb. It is somewhat of a trial-and-error procedure. (I'm not saying that they will equal out, but the Kb's should get close to each other.)

 

[steveb1111]

It's the the builtup plus superimposed backpressure. We don't find that much of a balancing out effect. Valve A has slightly higher backpressure, but this is more than compensted for by the difference in Kb.

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.

 

[lizking]

If your backpressures are approximately equal, I guess the capacities are different. It may have something to do with the mechanical design of the valve internals. I'd give it the old "consult valve manufacturer" routine.

 

[JoeWong88]

PRV A with lower set pressure will have lower spring force. When PRV A lifted, with same relieving pressure, it tends to open slightly more to allow larger flow.

The PRV B may not be undersized if it can take half the relieving flow.

JoeWong
Chemical & Process Technology