The flow coefficient (Cv) is also additionally referred to as the valve flow coefficient. It is an imperial measurement system of unit 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 pound/square inch (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 system equivalent of the flow coefficient (Cv). Also referred to as International System of units (abbreviated as SI system).
It is defined as the flow of water in cubic meters per hour (m³/hr) at a pressure drop of one bar with temperature ranging from 5 to 30 degree C.
Kv = Q * √ (SG/ΔP) (units in m3/hr, bar).
Q = Flow in m3/ hr,
SG = Specific Gravity of water ( for water=1).
ΔP = Differential Pressure (bar).
The Flow Factor (Kv) should not be misunderstood with the Discharge Coefficient (k). Discharge Coefficient (k) is a characteristic non-dimensional factor of a valve, used to calculate flow that discharges from a tank 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 Kv to Cv:
Kv = 1US GPM * √ (1/1psi)
Kv = 0.227125 m3/hr. * √ (1/0.06894757 bar)
Kv = 0.2271*3.818* m3/ hr. * bar
Kv = 0.864 m3/ hr. * bar
Kv = 0.864 Cv
Conversion of units of Cv to Kv:
Cv = 1 m3/hr * √ (1/1bar)
Cv = 4.4028812454 US GPM * 0.2625786985 *1/psi
Cv = 4.4028812454 US GPM * √ (1/14.503773773 psi)
Cv = 1.1561 US GPM / psi
Cv = 1.156 Kv
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.