The flow coefficient of a device is a relative measure of its efficiency at allowing fluid flow. It describes the relationship between the

pressure drop Pressure drop is defined as the difference in total pressure between two points of a fluid carrying network. A pressure drop occurs when frictional forces, caused by the resistance to flow, act on a fluid as it flows through the tube. The main det ...

across an

orifice An orifice is any opening, mouth, hole or vent, as in a pipe, a plate, or a body * Body orifice, any opening in the body of a human or animal *Orifice plate, a restriction used to measure flow or to control pressure or flow, sometimes given speci ...

valve or other assembly and the corresponding flow rate. Mathematically the flow coefficient (or flow-capacity rating of valve) can be expressed as :

C_ = Q \sqrt

where: : is the rate of flow (expressed in US gallons per minute), : SG is the specific gravity of the fluid (for water = 1), : is the pressure drop across the valve (expressed in psi). In more practical terms, the ''flow coefficient'' is the volume (in US gallons) of water at that will flow per minute through a valve with a pressure drop of across the valve. The use of the flow coefficient offers a standard method of comparing valve capacities and sizing valves for specific applications that is widely accepted by industry. The general definition of the flow coefficient can be expanded into equations modeling the flow of liquids, gases and steam using the

discharge coefficient In a nozzle or other constriction, the discharge coefficient (also known as coefficient of discharge or efflux coefficient) is the ratio of the actual discharge to the ideal discharge,Sam Mannan, Frank P. Lee, ''Lee's Loss Prevention in the Process ...

. For gas flow in a pneumatic system the for the same assembly can be used with a more complex equation. Absolute pressures (psia) must be used for gas rather than simply differential pressure. For air flow at room temperature, when the outlet pressure is less than 1/2 the absolute inlet pressure, the flow becomes quite simple (although it reaches sonic velocity internally). With = 1.0 and 200 psia inlet pressure the flow is 100 standard cubic feet per minute (scfm). The flow is proportional to the absolute inlet pressure, so the flow in scfm would equal the flow coefficient if the inlet pressure were reduced to 2 psia and the outlet were connected to a vacuum with less than 1 psi absolute pressure (1.0 scfm when = 1.0, 2 psia input).

Flow factor

The metric equivalent flow factor () is calculated using metric units :

K_ = Q \sqrt

where : is the flow factor (expressed in m³·h⁻¹). : is the flowrate (expressed in cubic metres per hour m³·h⁻¹), : SG is the specific gravity of the fluid (for water = 1), : is the differential pressure across the device (expressed in bar). can be calculated from using the equation:

K_ = 0.865 \cdot C_

Flow factor

References

See also