Valve Kv and Cv Calculator

What are Kv and Cv?

Kv and Cv are flow coefficients that tell how much flow a valve can pass. Kv is the flow of water through the valve at a pressure drop of 1 bar, in m³/h. Cv is the US customary counterpart: the flow of water at a pressure drop of 1 psi, in US gpm. The larger the coefficient, the more flow the valve passes for the same pressure drop.

Formula

For liquids Kv = Q × √(SG / Δp), with Q the flow rate (m³/h), Δp the pressure drop across the valve (bar) and SG the specific gravity relative to water. The same relation gives flow as Q = Kv × √(Δp / SG) and pressure drop as Δp = SG × (Q / Kv)². The two coefficients are linked by Kv = 0.865 × Cv and Cv = 1.156 × Kv.

Example

A liquid with a specific gravity of 1.2 flows at 25 cubic metres per hour and a drop of 0.4 bar is wanted across the valve. The required Kv = 25 × √(1.2 / 0.4) = 43.3, which is a Cv of 50.1. Pick the first catalogue valve above that. Going the other way, a valve with Kv 10 and a 4 bar drop on water passes 10 × √4 = 20 m³/h.

Limits

The relation is for liquids. Gases and steam need separate equations. Once cavitation or flashing starts in the valve, flow no longer rises with the square root of the pressure drop and the calculation stops being valid. Viscous liquids need a correction. Control valve selection is completed with manufacturer data and the relevant standard.