Control valve Cv for liquids

Required Cv and Kv for a liquid control valve, with the choked-flow check — water properties built in.

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How this calculator works

For liquids, Kv = Q·√(SG/ΔP) with Q in m³/h and ΔP in bar, and Cv = 1.156·Kv (US gpm at 1 psi). The calculator first checks for choked flow: when the pressure drop reaches ΔP_max = F_L²·(P₁ − F_F·P_v), with F_F = 0.96 − 0.28·√(P_v/P_c), more pressure drop gives no more flow, and the valve is sized on the choked drop instead.

The example is 100 m³/h of water at 25 °C, 10 bar a inlet and 1 bar drop through a globe valve (F_L 0.9): Kv 99.8, Cv 115.4 — choose a valve with a rated Cv of about 145 or more.

Choked is not the only danger. Cavitation damage often starts at well under the choked drop. For hot water, high pressure drops and liquids near their boiling point, have the valve maker check cavitation and noise.

Limits: liquids only, turbulent flow, valve the same size as the line (no reducers, F_P = 1). References: IEC 60534-2-1 / ISA-75.01.01 (method aligned with the published equations); Perry's §8; Ludwig Vol. 1.

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