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Conventional Relief Valves and Constant Superimposed Back Pressure

Conventional Relief Valves and Constant Superimposed Back Pressure

Conventional Relief Valves and Constant Superimposed Back Pressure

Hi all,

I have come across a conventional relief valve that has challenged my understanding of API 520, so I am here to ask for some insight.

Set Pressure: 2500 kPag
Constant Superimposed Back Pressure: 2100 kPag
Variable Superimposed Back Pressure: 20 kPag
Built-Up Back Pressure: negligible
Sizing Case: Blocked flow pump discharge

This relief valve already exists in the plant I work at and relieves into the suction side of the pump it protects. The choice of a conventional PSV caught my attention given the high back pressure.

From API 520 Section the clauses strictly refer to built up back pressure not exceeding 10% of set pressure (21% for fire, etc).

Section states "Balanced valves can typically be applied where the total backpressure (superimposed plus built-up) does not exceed approximately 50 % of the set pressure.

I guess my questions are, is this a common installation for a conventional relief valve and am I correct in interpreting it as meeting the intent of API 520?


RE: Conventional Relief Valves and Constant Superimposed Back Pressure

You appear to have one of those rare PRV applications involving high constant back pressure.

If you read API-520 carefully towards the end, it states that for superimposed constant back pressure "...the spring load is reduced to compensate...". This means that for a Conventional Pressure Relief Valve, the shop Spring Setting is = Set Pressure - Constant Back Pressure.

Note that we are both referring to API-520 9th edition 2014 (New 2020 10th Edition exists - but I don't yet have).

If the fluid is a compressible gas/vapour then a flow correction factor is necessary if the flow is sub sonic. For Liquids no correction is made.

There is effectively no limit on constant back pressure within reason. The most extreme I have seen is 95% constant back pressure. For liquids the size of the valve is affected in the calculation which takes the pressure differential into account (DeltaP). For balanced bellows PRV's with variable back pressures, gas/vapours and liquids, API-520 states maximum back pressures at 50% but this is a guide only. Many manufacturers have possibilities to go higher and have their own corrections factors, but generally follow the established limitations.

In your case, I would think that the small variable superimposed back pressure was ignored as it represents only 0.8 % of set pressure. Most probably engineered away in 'set pressure tolerances'.

Your PRV nameplate should have a Spring Setting of Set Pressure 2500 - Constant Back Pressure 2100 = 400 kPag (it may be prefixed CDTP = Cold Differential Test Pressure and have a small temperature correction factor applied for ambient testing).

Your PRV should also have 300LB x 300LB fully rated inlet and outlet flanges. A 300LB outlet is non standard.

Hope that helps with your understanding.

Per ISO-4126, only the term Safety Valve is used regardless of application or design.

RE: Conventional Relief Valves and Constant Superimposed Back Pressure

Quote (FAC ENG)

"From API 520 Section the clauses strictly refer to built up back pressure not exceeding 10% of set pressure (21% for fire, etc)."

Note that this statement isn't precisely correct. The built-up backpressure is limited to the allowable overpressure. It's not strictly limited to 10%. When the PRV is set at the MAWP, and it's for a non-fire case, then the resulting limit (the allowable overpressure) is indeed 10%. But in cases where the allowable overpressure exceeds 10% (when the only scenario is a fire case, or if the set pressure is less than MAWP) then the built-up backpressure can be higher - as high as the amount of the allowable overpressure.

RE: Conventional Relief Valves and Constant Superimposed Back Pressure

With a conventional RV, this PSV will relieve at (2500x1.1)+2100+20=4880kpag.
A conventional PSV has no way of compensating for backpressure effects, it is merely a differential pressure relief device.

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