13 replies to this topic

[phoenixmoca]

As we all know it is generally recommended that the pressure loss at the inlet of pressure safety valve is less than 3% of set pressure. But while conducting the pressure loss calculation which should be applied, the required capacity or rated capacity?

I agree with the API 520 Part II in which rated capacity is selected. So my question is how to persuade others to follow the requirement of API 520 Part II?

Thank you in advance.

[Pilesar]

As I understand the purpose of the 3% rule, it is designed to avoid relief valve rapidly opening and closing such that it destroys itself. The 3% limit was selected by committee as a guideline that was not originally intended as an inviolable rule for every fluid and relief condition. However, due to lawsuits and legal judgments, relief designs usually go to extraordinary lengths to comply with the rule. There may well be other basis for design that would be perfectly safe. The convincing argument for me is "how would I convince a court jury that I followed recommended industry practice?" This consideration guided me in so many engineering choices I have made. Even where my engineering judgment allowed less conservative selections, the financial risk to my company of lawsuit often dictated final design choices.

[latexman]

But while conducting the pressure loss calculation which should be applied, the required capacity or rated capacity?

I agree with the API 520 Part II in which rated capacity is selected. So my question is how to persuade others to follow the requirement of API 520 Part II?

 

Yes, definitely the PSV rated capacity, because that will be the flow when the PSV is activated.

 

If you are not in a position of authority over "others", then peer pressure.

[phoenixmoca]

Thank you all for the reply. It strengthen my understanding of the principle of pressure loss calculation and safety valve performance.

[breizh]

Hi,

Consider this resource to support your work

Breizh

Attached Files

•  Selection and sizing pressure relief valve.pdf   671.46KB   17 downloads

[phoenixmoca]

Thank you, breizh for your useful sharing.

[breizh]

other references

 

Breizh

Attached Files

•  solving-pressure-relief-valve-and-piping-capacity-problems-rev-1.pdf   280.64KB   11 downloads
•  Smith-Burgess.-Relief-Device-Inlet-Piping.pdf   387.71KB   9 downloads

[phoenixmoca]

Thanks a lot for valuable references.

If rupture disc is installed at the inlet of pressure safety valve and its resistance factor K is unknown should it consider its effect on pressure loss? Because while sizing safety valve the 0.9 of Kc factor of rupture disc has been considered.

[breizh]

Hi,

Yes, you should, in case the rupture disk is partially open. I've seen value of Kr about 1.2 -1.3

Breizh

[phoenixmoca]

Thank you, breizh.

It contributes a lot to the pressure loss calculation?

[breizh]

A few documents from Fike.

Any vendors of rupture disk should confirm Kr value for RD and PRV in serie.

Note : Contact them to get their feedback.

Good luck

Breizh

Attached Files

•  Fike_Page20.pdf   460.49KB   8 downloads
•  FIKE TB8100.pdf   242.4KB   7 downloads
•  fikerupture_disk_sizing_tb8102.pdf   201.46KB   8 downloads

[phoenixmoca]

So nice documents. Thank you,breizh.

[astro]

As we all know it is generally recommended that the pressure loss at the inlet of pressure safety valve is less than 3% of set pressure. But while conducting the pressure loss calculation which should be applied, the required capacity or rated capacity?

I agree with the API 520 Part II in which rated capacity is selected. So my question is how to persuade others to follow the requirement of API 520 Part II?

Thank you in advance.

 

As with many engineering questions, the answer to yours is, "it depends".

 

API STD 520 Part II (for info, I'm referring to 7th Ed) covers your question clearly at:
 

6.3 Backpressure Limitations and Sizing of Pipe 

6.3.1 General

7.3.7.5 Design Options to Address High Inlet Pressure Drop

 

Put simply, my read of the above sections is:

1. Use rated capacity for conventional spring-loaded, balanced spring-loaded, or pop-action pilot-operated PSVs;
2. Use required capacity for modulating pilot-operated PSV.

For 1, if the valve can accommodate the modification, restricted lift PSVs offer scope to apply a reduced rated capacity with the re-rating by the PSV vendor in accordance with API STD 526. Restricted lift offers a compromise between the mechanical simplicity of a conventional spring loaded valve and the capacity reduction of a modulating pilot PSV.

[astro]

As I understand the purpose of the 3% rule, it is designed to avoid relief valve rapidly opening and closing such that it destroys itself. The 3% limit was selected by committee as a guideline that was not originally intended as an inviolable rule for every fluid and relief condition. However, due to lawsuits and legal judgments, relief designs usually go to extraordinary lengths to comply with the rule. There may well be other basis for design that would be perfectly safe. The convincing argument for me is "how would I convince a court jury that I followed recommended industry practice?" This consideration guided me in so many engineering choices I have made. Even where my engineering judgment allowed less conservative selections, the financial risk to my company of lawsuit often dictated final design choices.

 

API STD 520 Part II 7th Ed at 7.3.5 Background on PRV Inlet Pressure Loss Criteria gives a short history on the "rule's" origin story.

 

My guess is that the question is contentious within the relevant technical committee at API with the line in the section cited above that aligns with your commentary, viz.:

 

Consequently, due to the complex nature of PRV instability behavior, further research is needed before changes to the inlet loss criteria in 7.3.4 can be justified. 

 

There's a lot of research referenced but no clear agreement / path forward, so the default is to stick with the view put forward in 1963. Not that inspiring from a technical advancement perspective but we've got what we've got for now, unless you want to delve into a force balance engineering analysis (cf. API STD 520 Part II, 7.3.6 Engineering Analysis).