The flow coefficient of a device is a relative measure of its efficiency at allowing

fluid In physics, a fluid is a liquid, gas, or other material that may continuously motion, move and Deformation (physics), deform (''flow'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are M ...

flow. It describes the relationship between the

pressure drop Pressure drop (often abbreviated as "dP" or "ΔP") 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 i ...

across an orifice valve or other assembly and the corresponding flow rate. A greater restriction in flow will create a larger pressure drop across a device and thus a smaller flow coefficient, conversely device with little restriction in flow will have a small pressure drop and a larger flow coefficient. For example, the flow coefficient of a 1"

ball valve A ball valve is a flow control device which uses a hollow, perforated, and pivoting ball to control fluid flowing through it. It is open when the hole through the middle of the ball is in line with the flow inlet and closed when it is pivoted ...

may be 80 while a similarly sized

globe valve A globe valve, different from ball valve, is a type of valve used for regulating Fluid dynamics, flow in a Piping, pipeline, consisting of a movable plug or disc element and a stationary ring seat in a generally spherical body. Globe valves are ...

in the same application may be 10. Mathematically the flow coefficient (or flow-capacity rating of valve) can be expressed as

C_\text = Q \sqrt,

where, : is the rate of flow (expressed in US gallons per minute), : SG is the

specific gravity Relative density, also called specific gravity, is a dimensionless quantity defined as the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for solids and liquids is nea ...

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. 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_\text = Q \sqrt,

where, : is the flow factor (expressed in m³/h), : is the flowrate (expressed in m³/h), : SG is the

of the fluid (for water = 1), : is the differential pressure across the device (expressed in bar). can be calculated from using the equation

C_ = 1.156 \cdot K_\text.

References

{{DEFAULTSORT:Flow Coefficient Fluid dynamics

Flow factor

See also

References