Cv and Kv relationship

The flow coefficient (Cv) is also known as the valve flow coefficient. It is an imperial measurement and is defined as the flow of water through a valve at 60°F in US gallons per minute (GPM) at a pressure drop of 1 lb/in².

Cv = Q * √ (SG/ΔP) (units in US GPM, psi)

Q = Flow in US GPM,

SG = Specific Gravity,

ΔP = Differential Pressure (psi).

Flow factor (Kv) is the metric equivalent of the flow coefficient. It is defined as the flow of water in cubic meters per hour (m³/hr) at a pressure drop of the bar.

Kv = Q * √ (SG/ΔP) (units in m3/hr, bar).

Q = Flow in m3/ hr,

SG = Specific Gravity,  

ΔP = Differential Pressure (bar)

This should not be misunderstood with the Discharge Coefficient (k) which is a non-dimensional factor of a valve used to calculate flow that discharges to the environment.

The Relationship between Cv and Kv is

Cv = 1.156 Kv OR 

Kv = 0.864 Cv.

Unit conversions from Imperial to Metric (vice versa):

1 US GPM = 0.227125 m3/hr.        1 m3/hr = 4.402868 US GPM

1 psi   = 0.06894757 bar     1 bar   = 14.503773773 psi

60 deg F = 15.5 deg C

Conversion of units of Cv to Kv:

Cv = 1US GPM * (1/1psi)

Cv = 0.227125 m3/hr * (1/0.06894757 bar)

Cv = 0.2271*3.818* m3/ hr * bar

Cv = 0.864 m3/ hr * bar

Cv = 0.864 Kv

Conversion of units of Kv to Cv:

Kv = 1 m3/hr * (1/1bar)

Kv = 4.4028812454 US GPM * 0.2625786985 *1/psi

Kv = 4.4028812454 US GPM * (1/14.503773773 psi)

Kv = 1.1561 US GPM / psi

Kv = 1.156 Cv

What is its importance?

  • Flow coefficient Cv and Flow Factor Kv are values used to describe the hydraulic capacity of a control valve.
  • Quickly compare different brands of valves and their flow capacities.
  • Help to select a valve that is the right size (and not oversized) for the application.
  • One can quickly understand how the valve you select will have an effect on the pressure in your system.