Conversion of ACFM to SCFM for Vapor Relief

[RJB32482]

Hey Guys,
When calculated relief load for the fire case of a tank full of vapor, do you need to convert ACFM to SCFM (most valve manufacturers have their capacities in SCFM). Or can you convert the capacity in SCFM to lbs/hr and compare to your calculated load (API 521 calculation gives lbs/hr). So the relief situation is 800 Rankine and 150 PSIG.

Thanks.

 

[EGT01]

Refer to ASME Section VIII, Appendix 11 for converting the capacity of a safety relief valve in terms of a gas or vapor other than the medium for which the valve was officially rated.

If you don't have access to ASME Section VIII, you can find an older version of the referenced section in the Crosby Engineering Handbook.

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

 

[RJB32482]

Thanks for the info. My question is this, if I calculate the relief flow rate in lb/hr at the relief conditions (196 psia and 850R), can I just divide that by the specific volume (in lb/ft^3) of air at Standard Conditions to find SCFH of air relieved? Then just convert SCFH to SCFM.

Or should this be analyzed in another way?

Thanks.

 

[EGT01]

I repeat again, you need to follow the methods in ASME Appendix 11.

Example 4 worked backwards is very similar to what you need. This can easily be setup on a spreadsheet. Here's the results of Example 4 worked backwards...

Ws = 51.5 * (K * A * P) = 28,200 lb/h Pset same
(K * A * P) = Ws / 51.5 = 547.6


W = C * (K * A * P) * Sqrt(M/(Z*T))
C = 356
(K * A * P) = 547.6
M = 28.97 lb/lbmol
Z = 1.00
t = 60 F
T = 520 R
W = C * K * A * P * Sqrt(M/(Z*T)) = 46014 lb/h Pset same
0.0766 lb/scf
600705 scfh
10012 scfm

If your original relieving fluid is not steam, then substitute the general vapor equation for the steam equation and determine KAP the way the examples does for steam. Then proceed to the next step of using that KAP to determine the relieving capacity of air at standard conditions.

 

[mechprocess]

EGT01 is correct. In order to convert the relief flow rate of your service gas or vapor, you need to work backwards through the appropriate area sizing equations provided in the ASME Section VIII Appendix 11.

It can not be determined by an equivalent mass ballance since the area sizing equations are based on model test parametric relationships. You are essentially scaling backwards using certifide flow test data for the valve. The test fluid for a gas is 60°F air at relief pressures.

The parametric relationships provided in the code include test fluid density, pressure and temperature and ratio of specific heats and some use the molecular weight.