Good day everyone,

API 520 does not specifically addressed how to calculate the value of the static pressure, P, within the outlet at the point of discharge of PSV(open system).

1. Is there any available formula for this one?

2. Is this P value related in any way to (P1-Pa)in ASME B31.1 Appendix II?

3. should we get same value for reaction force if we use API520, ASME B31.1 or CII relief load synthesizer?

Hope everyone is still up to these.
Thank you.

You will not obtain the same result from the CAESAR II Relief Load synthesizer. Those computations came from an ASME paper: "Relief Loads on Safety Vent Valves" by Braindmeyer.

Sir Richard,

Thank you for the reply.
So there's no way I can verify my manual calculations for the reaction force from API520/B31.1 using CAESAR II Relief Load synthesizer.

Now, do you have any suggestion on how can I validate my manual calculations for the reaction force from API520?
I have my preliminary vague idea but I want to confirm it from the experts here.

What is really "P" in the API520 formula and how is it determined?

See my opinion
http://65.57.255.42/ubbthreads/ubbthreads.php?ubb=showflat&Number=33633

Sir Mariog,

Thank you for bringing up this very old thread.
I still don't know anything about pipe stress analysis during the time you posted this.

Using API520 formula with "P" from the thread above seems a pretty good approach.
Although, reaction forces from this manual calculation vs. actual PSV vendor don't actually coincide I believe its fair enough compared to using set pressure as "P".

If it works for you with all your experiences, then I guess I'm in no position to question that.

Again thank you sir.

Dear jephunneh,

The paper I posted is just theory; the result can be checked versus Fluid Mechanics concepts.
The fact some B31.1 formulas appear as a particular case of this theory, well- this gives the confidence the result is correct (rather than my experience), isn't it?

About the possibility to consider that P equal to Ps- the PSV set pressure, this "revolutionary idea" appears in some companies specifications as the vertical force in PSV i.e. something as F2 = ( w^2 x R x T ) / ( g x Ps x A )

I commented this result in http://65.57.255.42/ubbthreads/ubbthreads.php?ubb=showflat&Number=44326.

My best regards.

Sir Mariog,

Thank you for the insights.
I'm not good in fluid mechanics and I admit I cannot follow your discussions perfectly. However, I really appreciate it.

Normally clients would just site in their specs regarding PSVs to refer to some particular code like API 520. The formula given is clear enough except for the static pressure. Of course I can use the formula and calculate the reaction force but the question is how accurate are the data I'm using and substituting in the formula? Using the code for me is at least understanding beyond calculations.

I think there is no specific answer to my question.


Thank you sir again for your time.

I submitted a request for interpretation to API.
You are right, API 520 should be clear enough when presents a formula.

Okay Sir Mariog,

Hope you can share it with us when API people give the interpretation.

Thank you.