Liquid flow through a vapor trim PSV?

[cdelan1]

Is it acceptable to use the method listed in API 520 (part 1) 8th edition section 5.9 "Sizing for Liquid Relief: PRVs Not Requiring Capacity Certification" to evaluate the liquid relief capacity of a PSV which has a vapor trim, stamped capacity in SCFM, and has been tested using air? What about vapor flow through a liquid trim PSV? The language in section 5.9 seems to be referring to PSV's that were sized prior to the code requiring capacity certification. So, I'm not sure if the aforementioned interpretation is accurate.

I spoke with a PSV vendor today who told me that a PSV with a vapor trim should not be used for liquid relief (and vice versa) because of chattering, poor reliability, etc. He also said that the manufacturer's Kd for liquid relief was only for liquid trim PSV's, and should not be used for liquid relieving through a vapor trim PSV.

My understanding of API 520 (part 1) is that PSV sizing equations using API discharge coefficients and API effective discharge areas are to be used only for initial sizing, and that the ASME effective area and discharge coefficient should be obtained from the PSV manufacturer. Is this correct, and if so, how can one evaluate the capacity of a relief valve that has both liquid and vapor relief scenarios?

 

[dcguffey]

Better to check out Consolidated dual service trim--just out.

http://www.dresserconsolidated.com/...detail/product/type_1900_universal_media_SRV/

It looks like a good option when you have a case where liquid and vapor are both credible.

 

[olcrazy1]

Farris Engineering 2600 Series Liquid trim valves are also due certified for liquid and vapor service. In fact Farris has had this for a while, Consolidated just released theirs.

 

[hniaboujemaa]

Hello,
"" Where liquid certified valves are specified, the sizing method for certified relief valves (refer to API Std 520 Part I) should be used. This ensures a more accurate size determination,
avoids oversizing and thereby minimises the probability of chatter. If chatter is a problem, the application of a friction damper may be considered where allowed by the relevant codes
and regulations. If a vapor trim valve is specified and there is a liquid relief scenario, then the non-certified valve equation from API Std 520 Part I shall be used to determine the required liquid relief capacity for the liquid relief scenario ""

 

[CJKruger]

hniaboujemaa, thanks for a very interesting reply. Two curiosity questions:

1) Do you know if your last sentence (non-cert liq eqns for vapor trim) is actually written down in a standard ?

2) What about liquid certified valves with a vapour relief case ? Are the normal vapour phase equations used for these valves ?

Thanks