Valve & Control Conversions — Cv, Kv, Valve Sizing

Cv (valve flow coefficient) is the US industry standard for sizing control valves. 1 Cv = 1 GPM of 60°F water at 1 psi pressure drop. It characterizes a valve's flow capacity independent of fluid and operating conditions. A properly sized valve operates at 30-80% travel under normal flow, providing good controllability and reserve capacity. For on-off valves, Cv determines if the valve can pass the required flow fully open.

Kv = 0.865 × Cv (Cv to Kv) and Cv = 1.156 × Kv (Kv to Cv). The difference arises from the unit definitions: Cv uses GPM and psi, Kv uses m³/h and bar. 1 bar = 14.5 psi and 1 m³/h = 4.403 GPM, yielding the 0.865 factor. For quick checks: a valve with Cv 50 has approximately Kv 43.25.

Choked (critical) flow occurs when the pressure drop across the valve reaches the vena contracta pressure limit regardless of further downstream pressure reduction. For liquids, choking occurs when the vena contracta pressure falls below the liquid vapor pressure, causing cavitation (harmful to the valve and piping). For gases, choking occurs when the downstream-to-upstream absolute pressure ratio falls below the critical ratio (typically 0.5-0.55). The ISA sizing equations include the liquid pressure recovery factor F_L and the gas pressure drop ratio factor x_T to account for these effects.

It's the opposite: ball valves typically have much higher Cv than globe valves of the same nominal size because a full-port ball valve has essentially zero flow restriction (Cv ≈ 30 × diameter²), while globe valves have tortuous flow paths with multiple direction changes (Cv ≈ 10 × diameter²). Higher Cv means lower pressure drop but typically worse controllability at low openings. For modulating service, globe valves are preferred for their linear characteristic; ball valves and butterfly valves are used where low pressure drop is the priority or for on-off service.