Refer to attachment . As shown in attachment there is a vertical force on PSV body at the point where inlet section and outlet section of PSV body join ( it is just an imaginary point).
Some organisations put value of force at this point even at pop up condition in static PSV analysis. To make matter worse this value is taken same as one calculated using 31.1 Non mandatory appendix II.
Some organisations superimpose force in case corresponding to normal flow at PSV outlet , post PSV pop up ( steady flow).But the value is taken 1/2 of that used in pop up case.That is also arbitrary.
I feel that this force is not to be superimposed in pop up case. In the other case of steady flow post pop up, can someone tell the value of force to be considered ?Any formula/ basis ?
regards,
bag_piper

There are as many explanations how this works as there exist PSVs, it would seem, and there are many PSV designs out there.

Take example 1:
A manhole cover of mass M, sitting on a sewer manhole of area A. From time = 0 to time t, it is exerting a force equal to mass M. At time t, P = M/A, and the manhole cover still exerts force M onto the fluid.

As time > t, the manhole cover still exerts the same force over the spot, but you also have the jet force, aka dynamic pressure... 1/2 density times velocity squared times area.

As time >> t, the cover blows away and you now have maximized dynamic pressure.

Take example 2:
Same as example 1, but you put a shroud around the manhole cover. The shroud captures the manhole cover, but has a vent hole of some other arbitrary size.

The manhole lifts up, strikes the shroud, and the shroud itself has its own steady state conditions as it vents.

Take example 3:
Same as example 2, but you connect a spring of constant k from the shroud to the manhole cover so it doesn't strike your shroud as hard.

Take example 4:
Same as example 2, but you route a line from the sewer into the shroud that pressurizes the shroud when the line reaches some pressure P1, to a pressure of P2 by the time the line reaches a pressure of P, so that the manhole cover doesn't strike your shroud as hard.

How can we have one equation to govern all these cases, especially when the particulars are largely unknown to the analyst and guarded by the PSV manufacturers?

It seems the only rational way the industry ought to proceed is to create a PSV standard that says that popping forces shall not exceed F1, F2, F3, ... and Fx for PSV types 1, 2, 3, ... and other.

However, my experience with PSV vendors is they don't even take the time to understand what their own product does to piping, but rather use the same conjectured assumptions the rest of us do.