I just read an article about control valve leakage calculation details below.Cv max = 182, Flow rate at 100% open = 50 MMScfd The valve is ANSI class IV specification. So design leakage will be 0.01 % at 50 psi(roughly).Leakage at 50 psi = 0.01/100 * 50 MMScfd = 0.005 MMScfdIf inlet pressure is 1800 psi.Then, the maximum leakage at 1800 psi differential = 0.005 * sqrt(1800/50)= 0.03 MMScfdFCI 70.2 (Was once called ANSI B16.104) Class IV specifies allowable leakage up to 0.01% of rated Cv when tested on air or water at nominally 50 psi differential to atmosphere. This is at ambient temperature. Extrapolating the leakrate to other pressures or other fluids is not meaningful. If a valve leaks 1 scfh at 50 psi on air, you can't predict what it will leak at, for example, 1000 psi on steam. 70.2 is a standard that allows comparison of fit and finish, but not much else.

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