Sizing & Selection
Flow Coefficient (Cv, Kv)
The flow coefficient expresses a valve's capacity in one number. Kv is the flow of water, in m³/h, at a pressure drop of 1 bar (5–40 °C). Cv is the flow of water, in US gpm, at a pressure drop of 1 psi (60 °F). They are the same physical idea in two unit systems: Cv = 1.156 Kv (equivalently Kv = 0.865 Cv).
Cv is computed from process data — flow, density, pressures — using the equations of IEC 60534-2-1 / ISA-75.01.01, with corrections for viscosity, piping geometry and choking. When selecting, the usual margin practice is to size the valve so the required Cv lands at about 50–80 % of the valve's rated Cv at the normal operating point, preserving turndown and leaving growth room.
Caveats engineers forget: Cv is measured at full open unless stated as a "rated travel" value; the published figure assumes the standard test piping; and for choked liquid service the effective ΔP must be capped. Run the numbers in our liquid Cv calculator before trusting any shortcut.