
Flow coefficient (Cv) is an imperial measurement and is defined as the flow of water through a valve at 60°F in US gallons/minute (gpm) at a pressure drop of 1lb/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 flow coefficient and is defined as the flow of water in meters cubed per hour (m³/hr) at a pressure drop of 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 confused with Discharge Coefficient (K) which is a non-dimensional factor of a valve used to calculate flow that discharges to the environment.
Relationship between 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 specify 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.
- Quickly understand how the valve you select will have an effect on the pressure in your system