When flow goes through a valve or any other restricting device it loses some energy. The flow coefficient is a designing
factor which relates head drop (Δh) or pressure drop (ΔP) with the flow rate (Q).
(liquids)
Q: Flow rate
ΔP: Pressure Drop
Sg: Specific gravity (1 for water)
K: Flow coefficient Kv or Cv
Each valve has its own flow coefficient. This depends on how the valve has been designed to let the flow going through
the valve. Therefore, the main differences between different flow coefficients come from the type of valve, and
of course the opening position of the valve.
Flow coefficient is important in order to select the best valve for a specific application. If the valve is
going to be most of the time opened, probably there should be selected a valve with low head loss in order to save
energy. Or if it is needed a control valve, the range of coefficients
for the different opening positions of the valve should fit the requirements of the application.
At same flow rate, higher flow coefficient means lower drop pressure across the valve.
Depending of manufacturer, type of valve, application the flow coefficient can be expressed in several ways.
The coefficient can be non-dimensional or with units if parameters such as diameter or density are
considered inside the coefficient or just in the equation.
Most of valve industry have standardized the flow coefficient (K). It is referenced for water at a specific temperature, and
flow rate and drop pressure units.
Same model valve has different coefficient for each diameter.
Kv is the flow coefficient in metric units. It is defined as the flow rate in cubic meters per hour [m3/h]
of water at a temperature of 16º celsius with a pressure drop across the valve of 1 bar.
Cv is the flow coefficient in imperial units. It is defined as the flow rate in US Gallons per minute [gpm]
of water at a temperature of 60º fahrenheit with a pressure drop across the valve of 1 psi.
At valves which discharge the flow directly into the environment it is used the non-dimensional (C).
