Cv – Flow Coefficient = Index of Flow Capacity equivalent to the gallons per minute of water at standard temperature (60°F = 16°C) which will flow through a valve or fitting at a pressure differential across the valve of 1 psi.
FLOW COEFFICIENT, FLOW RATE, PRESSURE DROP EQUATIONS
TYPE OF FLUID
Cv = Valve flow coefficient, dimensionless
Q = Volumetric flow rate, gpm
q = Volumetric flow rate, scfh
Gf = Liquid specific gravity at upstream conditions [ratio of density of liquid at flowing temperature to density of water at 15.6°C (60°F)], dimensionless
Gg = Gas specific gravity (ratio of flowing gas to density of air with both at standard conditions, which is equal to the ratio of the molecular weight of gas to the molecular weight of air), dimensionless
DP = Pressure differential, psi
T1 = Absolute upstream temperatures (in °R)
P1 = Upstream absolute static pressure, psia
P2 = Downstream absolute static pressure, psia
Y = Expansion factor, ratio of flow coefficient for a gas to that for a liquid at the same Reynolds Number, dimensionless
(Y = 0.667 when P2 <= 0.5 times P1 for choked or critical flow, Y = 1.000 when P2 > 0.5 times P1 for very low pressure differential)
The above sizing equations are provided as courtesy and are derived from ISA-75.01 and ISA-75.02.
They have been simplified to include a piping geometry factor of 1.0.
Refer back to ISA-75 in case actual application differ from the assumptions taken in the herein.