I was looking to the picture of PSV (Caesar Help)that is showing the orifice at the exit of the PSV. But only few PSV have this design. Majority has the orifice on vertical. Now.... does this change your perception in what is happening in the PSV? Are now POP forces more likely to exist? What are the implications?
Which is more critical now, the pop forces or those in steady flow?

As always, the answer is "it depends."

There are three stages to the relief as it affects stress.

1) "Pop" forces. The seal is broken and the the disc is lifted, compressing the "spring." If you have a modulated valve, this is 100% negligible as it is slow-opening. If you have a pop-action valve, this is generally negligible compared to below forces. For pop-action to pose a significant threat, I think you'd have to have a large diameter yet low pressure system to find the sweet spot of "large mass" and "weak pipe."

2) A pressure wave develops down the line. For short, open systems (a la API 520 and your diagram), this is generally negligible. For closed systems, this is your primary source of forces and failures. For long, open systems (i.e. quasi-open or semi-closed), it's a combination of both.

3) Steady state conditions would have a jet force out the end of the pipe for short, open systems akin to API 520 calcs. For closed systems, you only have to consider momentum exchanges between fluids and bends, and this is normally negligible. For quasi-open/semi-closed, you have the jet force, but consider your pressure losses and expansion along the line, which can either help or hurt you. If your velocities go too high, you'll run into vibration problems.

I still investigate.

Huddling Chamber Area or the shroud must give some force at the opening.

Maybe you saw the sketches with the PSV opening reaction forces and their orientation (even for closed systems) 15 years ago it was quite common in some big companies to consider them. As a result, hold down supports and stops near the PSV were required.
I simply can not believe that nothing was real. Why they should consider something which is not real? I know what you say but I still wonder if I do not miss something.

Dan,

I think that nobody can deny the pop-up hydrodynamic force is involved when the disc lift off its seat and the spring transfer a force to the bonnet as well.
The question would be only the system of forces are internal forces into PSV body or the system of forces are transferred to the piping system having a resultant force acting as an external force.

Another question is how we evaluate the magnitude of pop-up force. The papers you mentioned used to calculate it with an absurd formula (in my opinion, of course).

Best regards.

Mario,
It is the only logic explanation that I received until now. It may be.

Regards,