Control Valve Sizing Theory
Flow Coefficient (Cv)
Cv is the flow coefficient that relates flow rate to pressure drop across a valve. It represents the flow in US gallons per minute of water at 60°F with 1 psi pressure drop.
Q = Cv × √(ΔP / Gf)
Liquid Sizing (ISA/IEC)
Cv = Q × √(Gf / ΔP)
Where Q is flow rate (GPM), Gf is specific gravity, and ΔP is pressure drop (psi). For choked flow, use ΔPmax = FL² × (P1 - FF × Pv).
Gas Sizing (ISA/IEC)
Cv = Q × √(MW × T × Z) / (N × P1 × Y × √x)
Where Y is the expansion factor and x = ΔP/P1 is the pressure drop ratio.
Choked Flow
Choked flow occurs when the flow velocity reaches sonic conditions (gases) or when vaporization occurs (liquids). Beyond this point, further pressure drop does not increase flow.
References
- IEC 60534 - Industrial-process control valves
- ISA-75.01 - Flow Equations for Sizing Control Valves
- Fisher Control Valve Handbook