What is valve cv?

Cv is a capacity rating for a valve. A valve with a Cv = 1 will flow 1 gpm of water with a differential pressure of 1 psi. If the Cv was 100, it would flow 100 gpm of water with a 1 psi differential pressure.

For non-flashing/cavitating service, Q = Cv * (dP/SG)^0.5. Q is the flow rate in gpm (US gal), dP is the differential pressure in psi and SG is the liquid's specific gravity.