Flow Coefficient Cv Calculator
Calculate the flow coefficient Cv (or Kv) for control valves, ball valves, and globe valves.
Size valves for liquid and gas service using the ISA S75.01 standard formula.
Flow Coefficient Cv Cv is defined as the flow rate of water (in US gallons per minute, 60°F) that creates a 1 psi pressure drop across the valve. It is the primary metric for sizing control valves and is dimensionless in concept but has specific units embedded.
Liquid Service (ISA S75.01) Cv = Q × √(SG / ΔP) Where: Q = flow rate (US gal/min) SG = specific gravity relative to water (water = 1.0) ΔP = pressure differential across valve (psi)
Metric equivalent (Kv) Kv = Q_m3h × √(SG / ΔP_bar) Conversion: Cv = 1.156 × Kv (or Kv = 0.865 × Cv)
Gas / Vapor Service (sub-critical flow) Cv = Q_scfh / (963 × P₁ × Y) × √(T × SG / ΔP) Where: Q_scfh = flow in standard cubic feet per hour P₁ = inlet absolute pressure (psia) Y = expansion factor (≈ 1 − ΔP/(3 × P₁) for subsonic) T = absolute temperature (Rankine = °F + 459.67)
Required vs Available Cv Required Cv (C_vreq) = calculated from flow requirements. Select a valve with available Cv ≥ C_vreq × 1.15–1.25 (safety margin). Operating point: valve should operate at 40–80% of maximum Cv for best control.
Valve Sizing Notes Over-sizing: valve operates near closed — poor control, instability, hunting. Under-sizing: insufficient flow — valve fully open, no control range. Always add 15–25% Cv margin above the calculated requirement.