Sam,

Indeed, here the problem is that the knowledge is still rudimentary. Maybe in 10 or 20 years will be easy to have in seconds a FEA fluid simulation of PSV opening and to activate a time history analysis. At that time will be a matter of Truth, Knowledge and Ethics.

Today we imagine that the PSV opening generates a plane wave- something like a piston in a tube. We cannot describe in math how the PSV's disk moving on vertical generates a plane wave in horizontal (and fact is some other models imagine there a jet instead a piston effect) and this failure of knowledge forces us to look around desperately for a math expression just to generate an input. This math expression is the API formula- F1 force- that describes a "free jet".
We are not able to say why this expression is linked with wave amplitude or to describe the wave propagation in terms of gas-dynamics. We just pretend we know the amplitude, shape, speed, etc of the wave based on F1 force, opening time, speed of sound, etc. Here is nothing related to Truth, Knowledge and Ethics, it is rather a dummy procedure that may give an acceptable design validated by experience. The problem here is that one can say that just applying F1 horizontally in PSV gives also an acceptable design validated by experience and I cannot argue based on Knowledge and Ethics...