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. 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.

2 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. ( 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.

3 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 .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.

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