Relief Valve Rated Flow definition

[Skeet1027]

Can anyone direct me to a definition for PSV rated flow? A free definition, that is (other than ASME PTC 25). I have scoured through API 520 and 521 and find many references to rated flow, but haven't yet come across an exact definition.

 

[EGT01]

Skeet1027,

Don't know about exact or free but the simplest definition for PSV rated flow is that flow the manufacturer guarantees and usually stamps on the PSV nameplate. As with any other equipment, the rated capacity is what the manufacturer says it is.

Now, if you want to know how that rated flow was determined, the most direct approach is to ask the manufacturer since they were the ones having rated the PSV. But, most likely what the manufacturer did was what your references in API RP-520 and RP-521 indicate...

f. The rated relieving capacity is the relieving capacity used as the basis for the application of a pressure relief device, code or regulation and is provided by the manufacturer.
Note: The capacity marked on the device is the rated capacity on steam, air, gas or water as required by the applicable code.


and

1.3.26 rated relieving capacity: That portion of the measured relieving capacity permitted by the applicable code or regulation to be used as a basis for the application of a pressure relief device.

So, if you wanted to know exactly what was involved with the rating process, what would you think you need to do?.....right, go look at the applicable code, which usually isn't free, but in some cases you can find excerpts for little or nothing....
thread1203-135368

For relief valves certified for ASME Section VIII, look at UG-131 (which references PTC-25) and Appendix 11.

 

[MortenA]

If you by free means that it dosnt cost money then take a look at the crossby engineering handbook at:

http://www.tycovalves-na.com/ld/CROMC-0296-US.pdf

Its (almost) everything thats in 520 (im not sure about 2 phase), its very excact and its free (and avaiable) at the web.

Best regards

Morten

 

[JimCasey]

Rated capacity of a PSV is measured flow at 10% overpressure. The ratings are for Air, Water, and steam. Other fluids' flows are mathematically predicted by considering differences in density and thermodynamic properties (k, z, Pcrit) Like Morten said, it's all in the Crosby handbook.

 

[EGT01]

Careful, all codes are not the same.

Rated capacity at 10% overpressure applies to ASME Section VIII.

ASME Section I requires rated capacity at 3% overpressure.

http://www.cwfc.com/farris/pdf/6400-6600.pdf

When in doubt, consult the manufacturer for the basis of their rating.

 

[JimCasey]

EGT101 is absolutely correct. Section 1 is for fired pressure vessels (boilers) and the limit is in fact 3% overpressure for Safety Valves.
Section VIII (unfired pressure vessels) Safety-relief valves are allowed 10% OP.
16% for multiple (staged) valves, and 21% for fire-sizing.