Theory of flow through pipes, valves and fittings
Theory of flow through valves and fittings
Flow coefficient, control valve sizing
Flow in pipes and valves theory - full content
Selecting the correct valve size for a given application requires knowledge of process conditions that the valve will actually
see in service. In the industry of control valves it is practice to use flow coefficient and flow characteristics.
In the UK and in the USA coefficient Cv is used and it is defined as flow rate of water in gpm at 60OF that creates pressure
drop of 1 psi across the valve. Basic equation for valve sizing for liquid service is:
where is:
- Cv - flow coefficient [gpm]
- q - flow rate [gpm]
- &Delta p - pressure drop [bar]
- S - specific gravity (relative density) [ - ]
To aid in establishing uniform measurement of liquid flow coefficients Cv, standardized testing facility by
Fluid Control Institute (FCI) are used by manufacturers. The effect of viscosity of fluids other than water
should be considered when selecting the valve, as increased viscosity of fluid is reducing the valve capacity.
Another coefficient Kv is used in some countries, particularly in Europe and is defined as flow rate of water
in m3/h that creates pressure drop of 1kg/cm2 across the valve (1 kg/cm2 is equal to 0.980665 bar).
Control valve sizing is based on the calculation of flow coefficient for given pressure drop and flow rate.
Liquid flow capacity of a valve in metric units can be converted to Cv as:
where is:
- Cv - flow coefficient [gpm]
- qm - flow rate [l/m]
- &rho - density [kg/m3]
- &Delta p - pressure drop [bar]
Also, liquid flow capacity of a valve can be converted to Kv as:
where is:
- Kv - flow characteristic [m3/h]
- qh - flow rate [m3/h]
- S - specific gravity (relative density) [ - ]
- &Delta p - pressure drop [bar]
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