Liquid control-valve Kv

Estimate a liquid valve flow coefficient for known flow, pressure drop and density.

How this calculation works

Kv expresses a valve's conventional water-flow capacity at a one-bar pressure difference. To estimate the required value for a liquid duty, enter design flow in liters per second, the differential pressure assigned to the valve and the liquid density. The calculator converts flow to cubic meters per hour and pressure drop to bar. It then multiplies flow by the square root of relative density divided by pressure drop. Relative density uses the stated reference of one thousand kilograms per cubic meter. This relationship assumes incompressible, turbulent, nonflashing flow. The resulting coefficient is a capacity requirement for those conditions, not a complete commercial valve selection. Viscosity, cavitation, choking, fittings, valve characteristic and the range of controllable flows require separate consideration with appropriate valve data.

Inputs and units

Design liquid flow (L/s)
Valve differential pressure (kPa)
Liquid density (kg/m³)

Method and formula

Kv = Qh sqrt(SG/Δpbar); Qh = 3.6 × Q in L/s, SG = ρ/1000, Δpbar = ΔpkPa/100. Kv is the conventional m³/h water coefficient at 1 bar.

Worked example

Example inputs

Design liquid flow: 1 L/s
Valve differential pressure: 10 kPa
Liquid density: 1000 kg/m³

Calculation steps

Flow = 1 × 3.6 = 3.6 m³/h; pressure drop = 10 / 100 = 0.1 bar; relative density = 1000 / 1000 = 1.
Required Kv = 3.6 × √(1 / 0.1) ≈ 11.384200 m³/h at 1 bar water.

Example results

Required Kv: 11.38419958 m³/h at 1 bar water
Liquid relative density: 1 fraction

Assumptions

Incompressible turbulent nonflashing liquid; density is relative to 1000 kg/m³ water.

Limitations

No correction for viscosity, fittings, choking, cavitation, valve characteristic or minimum controllable flow.
Kv alone does not select an acceptable commercial valve.
Preliminary educational check; verify inputs and equipment data before design or operation.

Sources

Spirax Sarco: Valve sizing: Page 5: liquid Kv = Q sqrt(relative density / differential pressure in bar)
Belimo: Globe valve project planning: Printed pages 10, 12 and 20: Kv relation and valve-authority definition