Relief Design 1

[BenjyMac]

I have two quick questions (or sanity checks).

1. Am I correct in thinking that pipework, silencers etc on the discharge side of a relief valve should be sized using the un-derated flow of the relief valve and NOT the derated flow?

2. When the relief case is from a single pressure reducing valve I would assume the valve to be in the fully open position. My downstream pressure would be the relief valve set point + 10% overpressure. Is the upstream pressure taken as the MAWP of the upstream system (or the upstream relief protection set-point if lower than MAWP). Does the API 520 or 521 give any detail (i.e. to use the RV setpoint upstream) on this point?

Many thanks in anticipation for your help.

Regards

Benjymac

 

[JimCasey]

Setpoint of the SRV should be MAWP.
Flowing pressure upstream of the SRV can be up to MAWP+10%, the allowable accumulation for an ASME secVIII valve. Downstream >should< be atmospheric, but you rightly point out that the silencers will create some backpressure. Use of a piloted or bellows-balanced SRV will ensure stable operation with the developed back pressure. A little back pressure will not reduce flow through the valve, but the effects of back pressure begin to be appreciable if the backpressure approaches the critical pressure drop...around half of the absolute inlet pressure.

 

[roker]

hello,

the calculations for sizing discharge line should be based on the rated flow of the relief valve, see API 520/521.

regards,
roker

 

[EGT01]

By saying derated flow, are you referring to the ASME practice of certifying valve capacity at 90% of the actual capacity observed during testing? If so, I believe in the USA the option is yours to use either the derated or un-derated flow. ASME code only requires that you size the inlet and outlet piping using the certified (nameplate) capacity. However, other countries may require the higher flow basis. You can find a good discussion about this in the Guidelines for Pressure Relief and Effluent Handling Systems (AICHE, CCPS publication). They site British Standard 6759 requiring the higher, un-derated flow.

As far as what pressure to use when determining the flow through a fully open pressure reducing valve, you should at least consider what the maximum expected operating pressure would be on your upstream side. In the absence of any other limiting factors, you may want to consider the minimum differential allowed for the type of relief device that is protecting your upstream system. For example, typically you would not want an operating pressure greater than 90% of a spring-loaded relief valve set pressure since the valve may start to simmer above that pressure.

As far as using a pressure greater than maximum operating pressure, API RP-521 suggests that the probability of two unrelated failures occurring simultaneous is remote and normally does not need to be considered. You will have to analyse your system to determine if you system meets that criteria. However, you may decide the consequences of overpressuring your downstream system are grave enough to warrant the use of a pressure greater than the upstream system maximum operating pressure.