What is Valve Flow Coefficient (Cv)?

The valve flow coefficient (Cv) is a standardized measure of a valve's flow capacity. It is defined as the flow rate of water in US gallons per minute (GPM) at 60F that will pass through a valve with a pressure drop of 1 psi.

For hydraulic directional control valves, Cv values are typically provided by the manufacturer and vary based on valve size (D03, D05, D07, D10) and spool position (A-T, B-T, P-A, P-B paths).

The Cv Flow Equation

For liquids, the relationship between flow rate, Cv, pressure drop, and specific gravity is:

Q = Cv x sqrt(dP / SG)

• Q: Flow rate in GPM (US gallons per minute)
• Cv: Valve flow coefficient (dimensionless)
• dP: Pressure drop across the valve in psi
• SG: Specific gravity of the fluid (water = 1.0, typical hydraulic oil = 0.85-0.90)

Rearranged Equations

To find required Cv:

Cv = Q x sqrt(SG / dP)

To find pressure drop:

dP = SG x (Q / Cv)^2

Metric Conversion

For metric units (LPM and bar), the equation becomes:

Q (LPM) = 22.67 x Cv x sqrt(dP (bar) / SG)

Or equivalently using Kv (metric flow coefficient):

Q (m3/h) = Kv x sqrt(dP (bar) / SG)

Conversion: Kv = Cv / 1.156

Hydraulic Valve Size Standards

• D03 (NG6): CETOP 3 / ISO 4401-03, typical Cv = 2-4
• D05 (NG10): CETOP 5 / ISO 4401-05, typical Cv = 6-12
• D07 (NG16): CETOP 7 / ISO 4401-07, typical Cv = 15-25
• D10 (NG25): CETOP 8 / ISO 4401-08, typical Cv = 30-60