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Flow rate calculation for Gases:

Q = flow rate in SCFH (standard cubic feet per hour)
Note: that would be the flow at 68 °F and atmospheric pressure

SG = specific gravity of gas (air = 1)

p1 = absolute inlet pressure (gauge pressure in psi + 14.5)
Note: for a smaller pressure, the cV (and with it the valve orifice) need to be bigger to accommodate a given flow rate at a given pressure drop. As a consequence, and for this calculation only, you have to take the lowest possible pressure to not undersize the valve. The pressure rating of the valve still has to be done at the max. pressure, of course.

p2 = absolute outlet pressure (gauge pressure in psi + 14.5)

T = absolute temperature in Rankin (°F + 460)

Note: the gas takes the most volume at the higher temperature. So, here you have to take the maximum temperature

When the outlet pressure is greater than 0.5 x inlet pressure, use these formulae:

  963 x CV x ((p1 - p2) x (p1 + p2))½     Q x (SG x T)½ 
Q = -------------------------------------------   cV = -------------------------------------
  (SG x T)½     963 x ((p1 - p2) x (p1 + p2))½
         
         

When the outlet pressure is less than 0.5 x inlet pressure, use these formulae:

  838 x CV  x p1     Q x (SG x T)½
Q = -------------------------   cV = -----------------------
  (SG x T)½     838 x p1
         
         

To calculate, you may want to simply use our maximum flow and minimum CV calculator

                     Calculator    

Calculate the necessary cV first, then choose a valve from a manufacturers catalog that has a minimum cV of what you need. Ideally, the cV of the valve you choose, should be slightly bigger than what you need. A factor of more than 2-3 would be considered too big.

 

Going too big will result in using, say a needle valve, in only ½ out of 12 turns. This would make precise control and regulation of that flow nearly impossible.

Approximate specific gravities of some common gases are:

Air = 1.0 Ammonia = 0.6 Argon = 1.38 Butane = 2.1 CO2 = 1.53
Chlorine = 2.49 Ethylene = 0.97 Helium = 0.14 Hydrogen = 0.07 H2S = 1.19
Methane = 0.55 Nitrogen = 0.97 Oxygen = 1.15 Propane = 1.56 SO2 = 2.21

Notes:
Gas formulae are suitable for gas temperatures between 30 and 150°F.
All pressure drop calculations of this type are approximations. 
Fluid properties can be very significantly affected by pressure and temperature