Relief Valve Capacity

Question

Relief Valve Capacity

TreeEng (Chemical)

(OP)

7 Aug 03 10:18

I am trying to calculate the capacity of a relief valve. The manufacturer gives capacity data in SCFM of air at 60F and the set pressure + 10% overpressure. My sizing case is a fire case, therefore the allowable overpressure is 21%. The data is given at 10% - how does this change the capacity for me? Also, how do I convert the capacity in SCFM air at the above conditions to lb/hr of the process fluid at relieving conditions?

Thanks for the help

jlapp (Chemical) · Answer 1 · 2003-08-07 12:48:07

TreeEng,

When sizing the PSV use the value 1.21(MAWP) + 14.7 for your P1 value, this will give you your 21% overpressure. You can also get sizing programs from most of the manufacturers. I believe you can download the Consolidated sizing software from the Dresser.com website.

J.

ilan7 (Mechanical) · Answer 2 · 2003-08-07 16:26:02

Dear TreeEng
Do not play with safety stuff uless you know exectly what you do or have an expert for a guidness.
The rulls are different for the nature of the application. Your best bet will be to consolidate with one of the reps. in your area. Give them all the details, ask for a quotation and for explanation about the calculations. Dresser, Crosby, Consolidateed, Tyco (Endress-Grenwood), SAPAGE (France), Laser (Germany) all have good manuals and helping program.

TD2K (Chemical) · Answer 3 · 2003-08-07 17:28:44

The capacity of a safety valve in gas service is directly related to the overpressure (assuming the compressibility factor doesn't vary over this pressure range).

So, scfm (@21% OP) = scfm (@10% OP) *(1.21*Pset +14.7)/(1.1*Pset + 14.7).

As suggested, I would talk to your vendor and get their assistance.  I don't work with valve ratings in terms of scfm of air so I've given one approach below BUT I know you can use that rating in scfm of air to evaluate other fluids at other conditions and they should be able to send you the relevant information.

Back to what I would do.  Take the valve's capacity at 10% overpressure with air and back calculate the effective orifice area.  The formulas will be in the vendor's catalog.

Then, using that orifice area, calculate the valve's capacity at the new conditions and fluid's properties.

As others have suggested, if you don't do this type of work, get some experienced to check it for you.  Safety valves are not something you want to make a mistake with.

EGT01 (Chemical) · Answer 4 · 2003-08-08 15:47:47

Relief valve capacity is generally certified according to ASME and the National Board.  ASME Sec VIII, Div 1, UG-133 gives guidelines how to prorate relieving capacity at relieving pressures greater than 10% overpressure.  This is the same as TD2K has indicated.  There is a limitation to that rule though, it does not apply to steam pressures above 1500 psig.  If you are interested in the details, take a look in ASME.

Also, in the same general ASME reference but Appendix 11, there are details of how to convert the capacity of relief valves from air or steam to process fluid.

Excerpts of ASME Sec VIII can be found in the Crosby Engineering Handbook.  But these excerpts are not the latest from ASME Sec VIII.
http://www.tycovalves-la.com/NA/CROMC-0296-US.pdf

Definitely seek assistance from an experienced colleague.  Relief valve vendors can help but they can only work with the info you give them.

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