Valve Cv Coefficient
Understand control valve sizing principles, calculations, and industry applications
The flow coefficient (Cv) is a relative measure of a valve's efficiency at allowing fluid flow. It is defined as the number of US gallons per minute (GPM) of 60°F water that will flow through a valve with a pressure drop of 1 psi.
For gas flow, Cv is calculated using the ISA-75.01 formula, which considers inlet pressure, pressure drop, gas gravity, and temperature. It also accounts for choked (critical) flow when the pressure drop exceeds a certain threshold.
Choked flow occurs when the fluid velocity at the narrowest point in the valve (vena contracta) reaches the speed of sound. At this point, further decreasing the outlet pressure will not increase the flow rate.