Flow Calculation for Gases - Needle Valve

1 Flow Calculation for GasesThe coefficient of flow (Cv) is a formula which is used to determine a Valve s flows under various conditions and toselect the correct Valve for a flow application. The Cv was designed for use with liquid flows , it expresses the flow ingallons per minute of 60 F water with a pressure drop across the Valve of 1 psi. However, this same Cv value can beused to determine gas flows through a Valve . The formula becomes more intricate for Gases , as Gases are a compressiblefluids and are thus affected by temperature.

2 Furthermore, two formulas are required to accurately estimate flow. Whenthe upstream pressure equals or exceeds two times the downstream pressure, it is known as a choked flow calls for use of the Critical flow formula. If the upstream pressure is less than two times the downstream pressure,the Sub-Critical flow formula should be FlowWhen: P1 2 x P2 Sub - Critical FlowWhen: P1 < 2 x P2Cv = x T816 x PG 1Cv = Q962 ( x T)(P - P )G1222Q Cv 816 x x TG =1Q 962 x Cv(P - P )( x T)G =1222where:QG = Gas Flow in Standard Cubic Feet per HourP1 = Upstream (inlet) pressure in psiaT = Absolute temperature in R.

3 ( F + 460)P2 = Downstream (outlet) pressure in psiapsia = Absolute pressure. This is psig (gage pressure) plus (atmospheric pressure) = Specific Gravity of medium where air at 70 F and psia = :Determine which orifice size should be used for the following application. Upstream pressure is 100 psig, downstream toatmosphere. The medium is 70 F methane gas ( ) and the desired flow range is up to 600 = 600 x .554 x 530816 x 114. 7 = .1098 The Cv value at which 600 SCFH of methane will flow under the aboveconditions is.

4 1098. Upon examination of our Cv table (on previous page),you can see that this value is reached at approximately turn with our -3- (.094) orifice and at turn with our -4- (.125) orifice. The end user can choose between approximately or turnsof metering precision for this Data, Cv vs. No. of Turns 2 4 6 8 101214161820No. of Turns OpenCv -1- -2- -3- -4-Specific Gravities of GATEWAY DR. NE EAST GRAND FORKS, MN 56721 USA VOICE (218) 773-2455 FAX (218) 773-7665