Maximum operating pressure for pilot PSV

[amar122amar]

What may be the maximum operating pressure of a system containing pilot PSV. If PSV set point is x then what may be the maximum operating pressure as % of x. What do codes (API) state?

 

[Latexman]

95% of X. I don't think this is a Code thing, just a reality of the design.

Good luck,
Latexman

 

[amar122amar]

Thank u very much latexman. Is there any justification for this figure of 95 percent. I mean who provides this figure? How to determine it?

 

[MortenA]

API 520 figure 1 says that maximum expected operating pressure should be 90% of the maximum allowed working pressure=maximum allowed set point.

But the note then adds that the operating pressure may be higher (or lower naturally) than 90%.

This refers back to ASME sec VIII (i dont have that here).

Best regards

Morten

 

[JoeWong88]

For conventional spring loaded PSV, maximum 90% of set pressure. Some conventional type PSV can goes higher (i.e. 93%), however, need to confirm with PSV vendor.

Similarly, for Pilot operated PSV, maximum 95% of set pressure. Some conventional type PSV can goes higher (i.e. 97-98%), however, need to confirm with PSV vendor.

Both are experiences based rule of thumbs. If you use it, save some of your valuable time.


The statement "API 520 figure 1 says that maximum expected operating pressure should be 90% of the maximum allowed working pressure (MAWP) = maximum allowed set point." by MortenA is correct if the PSV set point same as MAWP. If the set pressure is lower than MAWP, the it should be 90% of set pressure.

JoeWong
Chemical & Process Technology

 

[don1980]

The system presure can get very close to the set pressure of a POSV (within ~97-98%) without causing the valve to open. That's one of the primary advantages of using a POSV.

When people ask me how close they can operate the system pressure, I caution them to first1) look at the variability of the operating pressure and (2) assess their tolerance for the relief device opening on this specific vessel.

In an ideal world, you could set your pressure controller at 95-96% of the set pressure of the POSV. In reality, all pressure controlled systems have some variability, and the consequences of the relief valve opening will vary from one vessel to another.

So, the bottom line is that you can operate about 5% higher using a POSV as compared to a PSV. But, look at each system individually to determine whether that mean 85%, 90% or 95% of set pressure.

 

[JoeWong88]

Sorry.

Reword for second paragraph...

Similarly, for Pilot operated PSV, maximum 95% of set pressure. Some Pilot operated PSV can goes higher (i.e. 97-98%), however, need to confirm with PSV vendor.

JoeWong
Chemical & Process Technology

 

[MortenA]

With regards to CODE as specifically asked in the question: I tried to quickly scan API 520 for any special allowance for pilot operates PSV's and could find that. Could you help point to a section (or reference to ASME) that actually OK's 95% or even higher?

Best regards

Morten

 

[amar122amar]

what about reseat pressure or blowdown? Can we operate system above the reseat pressure but lower than set pressure? Actually I have seen PSV that after reseating do not pop If we go above reseat pressure untill they reach set pressure.

Amarjeet Singh
Reliance Industries Limited

 

[greenche]

Morten, the applicable code is ASME Section VIII, Division I, Nonmandatory Appendix M, Section M-10. It gives general guidance for use unless a manufacturer recommendation allows higher.

For pressures between 71 and 1000 psig, 10% minimum differential between max operating pressure and set pressure is recommended. Above 1000 psig this is reduced to 7%. For 70 psig and below the minimum recommended differential is the greater of 10% or 5 psig. It also notes that pilot operated valves have shorter blowdown and so the operating margin may be reduced.

FWIW, JoeWong's guidelines are what I have seen used in practice.

 

[JoeWong88]

That is no specific clause in API stated that maximum operating pressure. This is more on what the PORV vendor guarantee and common acceptable range.

PORV normally has very low blowdown i.e 2-3%. The reseat pressure would be higher than maximum operating pressure. The pilot tapping should be directly from the protected equipment to get rid of any inlet line loss.



JoeWong
Chemical & Process Technology