real flow for a psv 2

[halizo]

a question for you guys:if i have a PSV installed (for whatever reason) and it opens ¿what is the flow trough this valve? ¿is the one calculated by a contingency (fire for example)? or ¿is the calculated by the actual orifice with the upstream pressure ?(assuming critical flow).

thanks for the help

 

[pleckner]

There could be some debate on the answer to this question and it does depend on the fluid state, vapor/gas or liquid. Since you mention the term "critical flow" as part of your question, I will assume you want to know about gas/vapor relief.

The only flow you are guaranteed of achieving is the flow stamped on the PSV. This is called the stamped or rated flow and is based on flow at a 10% over pressure; this of course implies the actual orifice. Again, there has been debate on whether you will ever really achieve this flow in a typical spring loaded, pop-open action relief valve but it has been reafirmed by API in interpretations that one must use the rated (stamped) flow in calculations for the PSV inlet and outlet piping.

 

[Latexman]

The actual flow through a PSV is determined by the inlet conditions; the fluid properties; the PSV set pressure, dimensions, and flow coefficient; and outlet conditions.


Good luck,
Latexman

 

[halizo]

thanks to all of you.
pleckner you are rigth, it is a gas /vapor relief (i forgot to mention it).

I asume that for a multiple valve scenario is the same for each valve: discharging the flow calculated in this way.

In the API 521 is not very clear if you can use the contingency flow or the actual flow. Obiously for design you use the contingency flow but for an evaluation of an existing system, should i use the actual flow?.

thanks again

 

[Latexman]

You determine the PSV nozzle size for the worst credible case for each possible flowing phase (gas/vapor, 2 phase, and liquid) using the scenario flows. Of these 3, pick the largest nozzle.

You determine the PSV inlet and outlet size, the inlet design (< 3% of set pressure), and the outlet design and PSV type (conventional, bellows, or pilot operated) using the actual flow of the overall worst condition.

Good luck,
Latexman

 

[pleckner]

The PSV tail pipe is to be sized based on the rated capacity (stamped flow rate) of the PSV, not the required relieving rate of the controlling scenario. If this is what Latexman means by "using the actual flow of the overall worst condition", then we are in agreement. If not, then I encourage anyone interested to review the criteria given in API RP521, Paragraph 5.4.1.3. However, for multiple relief valves discharging into a common header, one would still size each individual PSV tail pipe based on the rated capacity of the PSV but the header may be sized using the worst-case cumulative required capacities of all devices that may reasonably be expected to discharge simultaneously during a given relief scenario.

 

[halizo]

thanks a lot

 

[halizo]

The API 521 seem to let open the posibility to considerate another flow instead the required capacity, well it seem to me in my point of view. What is your opinion?


best regards

 

[pleckner]

You state that "The API 521 seem to let open the posibility to considerate another flow instead the required capacity..."

I already responded to what API RP521 has to say about which flow to use for the tail pipe (use rated capacity, NOT required capacity) and for the header (use rated capacity for a single discharging valve, use the worst-case cumulative REQUIRED capacities of ALL devices that may reasonably be expected to discharge simultaneously during a given relief scenario.) So to give you another opinion, you will need to indicate where in API RP521 you are referring to.

 

[halizo]

Ok i uderstand you, i was refering the paragraph that say:"simple rules cannot be expected to cover all installations. Good engineering judgment should be applied to select the flow basis most appropiate to each case.".

That is why a asked your opinion about use rated capacity in a multiple valve scenario.

best regard

 

[Latexman]

Yes, the actual flow of the PSV using the overall worst case conditions that determined the nozzle size, not the relief rate of this same scenario.

Good luck,
Latexman

 

[pleckner]

After rereading the section of API RP521 under discussion, I interpret the sentence in API RP521 Section 5.4.1.3 Design of Relief Device Discharging Piping, "Simple rules cannot be expected to cover all installations. Good engineering judgment should be applied to select the flow basis most appropriate to each case. (See the ASME
Code, Section VIII, Division I, Appendix M-8.)" as saying one needs to conisider that you should not be using the REQUIRED relieving rate of these simultaneously releasing PSVs but you may have to use the RATED flow for all valves relieving.

It still boils down to using either required relieving rates or the rated flows. A fire scenario or other external heating source would be an excellent example of when we should consider using the full RATED flow from each PSV and not the required relieving flows. Why would the valves NOT pass the rated flow in these scenarios? I can't think of any reason they would not.