Richard,

I agree with you that it is the momentum eqn.My question is , the expression mentioned in my post, in what way it can be a substitute for Pressure differential at exit times exit area and how is it derived in that form?

Dear Stress Engineer,

My question is not about where to get the PSV reaction force from . I also know that it is there in API 520. My question is in the previous paragraph.If you have answer to this specific question , only then respond.

Regards