Convert PSV capacity from scfm air to scfm of gas 1

[elquesifilma]

I have a PSV valve report that gives me the calculated capacity of the PSV in scfm of air.

I need to know what the relief capacity is for another gas composition

help is greatly appreciated.

 

[Duwe6]

scfm = scfm

material is immaterial.

Sorry for the koans, couldn't resist.
A standard cubic foot of air is the same as a standard cu-ft of ammonia. benzene, or any other gas [fully evaporated liquid].

http://www.eng-tips.com/viewthread.cfm?qid=72789

 

[jrthomas]

elquesifilma,

I struggled with figuring out how to do the exact same type of capacity conversion not all that long ago. Of course, you should have no trouble converting from say, SCFM air to lb/min of air; this simply involves using the ideal gas equation, substituting in pressure (1 atm), volume flow rate (ft^3/min), and temperature (288.705 K), picking a gas constant with the right units (1.314 ft^3*atm*K^-1*lbmol^-1), solving for lbmol/min, and converting this to lb/min using a molar mass of 28.97.

Converting from volume or mass flow of one chemical to another is not as intuitive (at least not for me). Here are two ways (equally good) that you can perform this conversion:

1. Get a copy of ASME Code Section VIII, Division I, Appendix 11, Mandatory Capacity Conversions for Safety Valves. Most people will recommend this method. It sounds complicated, but it's really not. Read through this short appendix and you should be well on your way to converting both SCFM air and lb/hr steam to equivalent mass flows of any chemical you wish. FYI, I was able to locate a complimentary copy of this appendix at the end of my Farris Safety Valve binder.

2. Go directly to the source-the valve manufacturer. I would personally recommend this method. Every PSV manufacturer supplies equations (basically API equations with a few of their own special correction factors, etc. thrown in) for calculating the required valve orifice area for a given flow rate. In your case, you know the area of your valve (at least you should!), and you can work the equation backwards to solve for lb/hr of any chemical you wish. When using this method, all the properties you need (T, Z, C) are at the RELIEVING conditions for the "gas composition" you mention in your post. Also, be sure you use the RATED (ASME) coefficient of discharge (available from manufacturer) in combination with the ACTUAL (ASME) area. Or, you could use the API area of your valve in combination with the EFFECTIVE coefficient of discharge (available from manufacturer). Never mix ACTUAL area and EFFECTIVE coefficient of discharge or vice versa. The mass flow rate that you get using this method will be the rated capacity of your valve (the "relief capacity" you mention in your post).

I hope that this does not come across as too complicated (it's really not after you do it for a few valves), but it is the right way to do it. I'm sorry, but I don't believe that it's as simple as saying 1 SCFM of air is the same as 1 SCFM of any other chemical, and being done with it.

 

[TD2K]

As JRThomas said, a PSV with a given capacity for say 500 scfm of air does not mean it handles the 500 scfm of propane gas.

Check out a copy of a vendor's PSV manual/catalog. In addition to the standard sizing formulas, they have PSV capacities in terms of gpm of water, lbs/hr of steam and scfm of air and how to convert from one fluid to those standard fluids.