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Two-phase relief. API 520 vs ISO 4126-10

Two-phase relief. API 520 vs ISO 4126-10

Two-phase relief. API 520 vs ISO 4126-10

Good day, forum

I have an issue concerning backpressure (pardon my assumption concerning discharge coefficient).

API 520-1-2014 stated in equations C.43 and C.44 that mass flux is calculated with upstream pressure and backpressure difference or backpressure ratio. By this way backpressure affects PRV reqired capacity.

ISO 4126-10-2010 stated in equations 35 that mass flux mass flux is calculated with only upstream pressure. By this way backpressure does not affect PRV required capacity.

In case of high back pressure ratio this situation leads to required capacity calculated with API 520-1-2014 is much less than ISO 4126-10-2010. Can anyone comment this?

RE: Two-phase relief. API 520 vs ISO 4126-10

Right. My take on this is the following. PS: I do hope that some other well informed contributors confirm or otherwise.

API Is a user standard and has always included sizing factors contributing/affecting fluid flow. While the subject of back pressure correction factors would take an age to agree between everybody, API has established some compromised factors which should be considered in sizing. At its most basic, back pressure does affect sizing (factors less than 1.0 increase calculated area to compensate).

ISO-4126-10 Is a fairly new, but very comprehensive standard. Without reading it all, it appears that it concentrates more of the methodology of calculation without correction factors of any kind. In much the same way as the ASME NB-18 Book publishes base flow formulae without any correction factors.

Whatever method is used to determine a back pressure correction fator, the factor should be applied to any sizing calculation. Note that manufacturers should have their own determined correction factors. This as I understand, is still an ongoing thing within ISO-4126 (Part 8 ? - anyone ?) regarding flow testing as no one can agree the method.

Hope that helps in the interim and I await other comments.

Per ISO, only the term Safety Valve is used regardless of application or design.

RE: Two-phase relief. API 520 vs ISO 4126-10

Equation 35 in ISO 4126-10 includes the backpressure as part of the mideal term as pseat from equation 32. It is not as explicitly stated as in the API documents. From the variable definitions following equation 34:

"pseat is the pressure in the seat cross-section of the safety valve, expressed in pascals

If a critical pressure ratio has been established in the valve, the pressure, pseat, in the narrowest cross-section is equal to the critical pressure, pcrit. Otherwise, it equals the back pressure, pb."

Note that pcrit is the fluid-dynamic critical pressure and not the same as pc, which is the thermodynamic critical pressure.

Bear in mind that API 520 Pt I equations C.43 & 44 are for subcooled liquids so the dP term uses either the backpressure or the saturation pressure because the fluid will choke at the phase boundary in the nozzle.

Hope this helps.

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