What is the 3 rule for pressure relief valves?

Recent changes to the infamous 3% rule on inlet pressure drop to pressure relief valves (PRVs) have raised many questions. The ongoing efforts by API improve the current industry codes and standards for PRV instability.

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PRVs should be inspected upon installation and at frequencies sufficient to maintain reliability. By maintaining safety critical pressure relieving systems and developing practical mitigation solutions, you will meet risk management targets, compliance requirements, and business objectives.

Posted 07 September - 02:45 AM

Phil,
Thanks for knocking my head to think more...

QUOTE (pleckner @ Sep 6 , 05:42 AM) <{POST_SNAPBACK}>

You're reading too much into why the API Committee says to use set pressure for the 3% Rule. As a note, there is discussion about changing it to relief conditions instead.

You may be right that i read too much into it. However question raised by my young engineers really push me to read and think more into it so that my statement / judgment is convincing.

Moreover, your second statement already shows that API committee still in discussion on which conditions to be used.

QUOTE

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We are only talking about a 3% (+ or -) pressure loss, not a very significant change to vapor/gas desnisy in the scheme of things.

Most peoples aware that 3% rule is just a guideline to comply but it still can be deviated. I personally has deviated at least once (upto 4.5%) but selected a PSV with high blowdown in order to maintain the margin between opening & closing of PSV. So the 3% rule really is not strictly needs to be comply and one may exceed if one understand the background on the margin.

QUOTE

As a matter of fact, the 3% Rule also applies to liquid flow where there is not only no density change but you still aren't supposed to take into account static head; doesn't sound very conservative to me.

I remember in one of your post, you mentioned that 3% rule does NOT include the static head. I have not put much thought on this issue. I do agree (prelim) as margin loss on the line pressure drop during PSV opening, you will gain it back when the PSV is closing / reseating. I think i should put extra thought on this subject.

QUOTE

And your statment, "If you use rated flow (based on higher DP) will properties obtain from setpressure, then it will be conservative." confuses me. You are supposed to use the PSV rated (stamped) capacity in the calculation wheather you use the set pressure or not so there is no additional conservatism introduced.

In your brief review of the effect of pressure on flow with a selected orifice size, your Effect 1 is basically negligible and your Effect 2 to me is non-existent. It isn't less significant on critical flow; it is no significance on critical flow. And what driving force across the PSV?

Yes. This may create some confusion to certain level.

My starting point is ....a PSV will open at setpressure, pressure possible accumulate upto 110% of setpressure (non-fire case). Those the properties at 100% setpressure to 110% setpressure will be different (i agree it may be insignificant).

At 100% setpressure, density lower, higher pressure drop is expected with same mass flow.
At 110% setpressure, density increases, lower pressure drop is expected with same mass flow.
Thus lower density, higher volumetric flow, which probably result higher pressure drop.


At 100% setpressure, the driving force across a PSV (selected) orifice will be 100% minus backpressure (subcirtical flow) or Critical pressure (generally 50-55% of PSV inlet pressure)

At 110% setpressure, the driving force across a PSV (selected) orifice will be 110%* minus backpressure (subcirtical flow) or Critical pressure (generally 50-55% of PSV inlet pressure)

* to be exact we may need to minus inlet line loss

If we go in detail, we may find that driving force for 110% case is slightly higher which potential gives higher flow across PSV. High flow will cause higher inlet line loss.


Increasing pressure ==> increase density ==> increases volumetric flow ==> less pressure drop
Increasing pressure ==> increase driving force across PSV ==> increase flow passing PSV ==> higher pressure drop

Thus, maximum inlet line loss may/may not at setpressure nor Relieving conditions.

If one use properties at setpressure (lower density), calculate rated flow considering PSV inlet is at 110% set pressure (higher driving force), then probably the inlet line loss will be conservative. This scenario is not a feasible scenario but just built in the conservatism.



Write upto here...looks like a bit confuse...i would like some of you to point out mis-concept if any...

Anyway, i have done some studies years ago, the pressure drop is not so significant whether you use set pressure condition or relieving condition.

The consideration of "using ideal K for sizing PSV" is really give significant flow different as compare to using set pressure condition or relieving condition to determine the PSV rated flow.

Any of you who is reading this post, i would encourage you to read the findings in "Using the ideal gas specific heat ratio for Relief-valve Sizing", CE, Nov , by Aubry SHACKELFORD.


JoeWong

Thanks for knocking my head to think more...You may be right that i read too much into it. However question raised by my young engineers really push me to read and think more into it so that my statement / judgment is convincing.Moreover, your second statement already shows that API committee still in discussion on which conditions to be used.Most peoples aware that 3% rule is just a guideline to comply but it still can be deviated. I personally has deviated at least once (upto 4.5%) but selected a PSV with high blowdown in order to maintain the margin between opening & closing of PSV. So the 3% rule really is not strictly needs to be comply and one may exceed if one understand the background on the margin.I remember in one of your post, you mentioned that 3% rule does NOT include the static head. I have not put much thought on this issue. I do agree (prelim) as margin loss on the line pressure drop during PSV opening, you will gain it back when the PSV is closing / reseating. I think i should put extra thought on this subject.Yes. This may create some confusion to certain level.My starting point is ....a PSV will open at setpressure, pressure possible accumulate upto 110% of setpressure (non-fire case). Those the properties at 100% setpressure to 110% setpressure will be different (i agree it may be insignificant).At 100% setpressure, density lower, higher pressure drop is expected with same mass flow.At 110% setpressure, density increases, lower pressure drop is expected with same mass flow.Thus lower density, higher volumetric flow, which probably result higher pressure drop.At 100% setpressure, the driving force across a PSV (selected) orifice will be 100% minus backpressure (subcirtical flow) or Critical pressure (generally 50-55% of PSV inlet pressure)At 110% setpressure, the driving force across a PSV (selected) orifice will be 110%* minus backpressure (subcirtical flow) or Critical pressure (generally 50-55% of PSV inlet pressure)* to be exact we may need to minus inlet line lossIf we go in detail, we may find that driving force for 110% case is slightly higher which potential gives higher flow across PSV. High flow will cause higher inlet line loss.Increasing pressure ==> increase density ==> increases volumetric flow ==> less pressure dropIncreasing pressure ==> increase driving force across PSV ==> increase flow passing PSV ==> higher pressure dropThus, maximum inlet line loss may/may not at setpressure nor Relieving conditions.If one use properties at setpressure (lower density), calculate rated flow considering PSV inlet is at 110% set pressure (higher driving force), then probably the inlet line loss will be conservative. This scenario is not a feasible scenario but just built in the conservatism.Write upto here...looks like a bit confuse...i would like some of you to point out mis-concept if any...Anyway, i have done some studies years ago, the pressure drop is not so significant whether you use set pressure condition or relieving condition.The consideration of "using ideal K for sizing PSV" is really give significant flow different as compare to using set pressure condition or relieving condition to determine the PSV rated flow.Any of you who is reading this post, i would encourage you to read the findings in "Using the ideal gas specific heat ratio for Relief-valve Sizing", CE, Nov , by Aubry SHACKELFORD.JoeWong

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