Minimum PSV Set Pressure for API 520 Equations

[ChemEngSquirrel]

I'm calculating the capacity of an existing PSV with a set pressure of 0.8 barg. It is a conventional spring operated pressure relief valve. I'm using the PSV sizing equations from API 520 Part 1.

My concern is the validity of the API equations at the low set pressure of 0.8 barg. Is there a minimum set pressure at which the API 520 sizing equations can be applied?

Thanks.

 

[PaoloPemi]

API gives equations for both critical and subcritical flow,
select the correct set according your application,
the problem there could be the valve (make sure it works as expected)

 

[don1980]

Your application (0.8 barg set P) is technically outside the scope of API 520 Pt1. That document is applicable to PSVs that are set at 15 psig or higher.

Low set pressure applications create problems for PSVs. It's often impossible to size a PSV that has the necessary capacity while satisfying the inlet pressure drop limit.

I'm assuming you're protecting a pressure vessel and not a tank. If you can raise the set P to 1 barg or higher (the higher the better) you'll have more options. You could then use a remote sensing pilot operated PSV to eliminate the inlet loss problem. Check with a couple of suppliers to explore specific options.

 

[CMA010]

ISO 4126 is from 0.5 barg

 

[PaoloPemi]

from a technical point of view API (which has reference to 4126) gives equations for subcritical flow (probably the case if Pin ~1.8 bar.a Pout ~1 bar.a)
the problem for that application would not be the design but the selection of a suitable valve

 

[MortenA]

I think its API 2000 then?