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 across an orifice valve or other assembly and the corresponding flow rate.
Mathematically the flow coefficient Cv (or flow-capacity rating of valve) can be expressed as
where
Q is the rate of flow (expressed in US gallons per minute),
ΔP is the pressure drop across the valve (expressed in psi).
In more practical terms, the flow coefficientCv is the volume (in US gallons) of water at 60 °F (16 °C) that will flow per minute through a valve with a pressure drop of 1 psi (6.9 kPa) 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 Cv for the same assembly can be used with a more complex equation.[1][2] 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 Cv = 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 Cv 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 Cv = 1.0, 2 psia input).
Flow factoredit
The metric equivalent flow factor (Kv) is calculated using metric units:
∆P is the differential pressure across the device (expressed in bar).
Kv can be calculated from Cv using the equation[4]
The kv factor or value as it is also called is defined in VDI/VDE Richtlinien No. 2173.[5] A simplified version of the definition is: The kv factor of a valve indicates "The water flow in m3/h, at a pressure drop across the valve of 1 kgf/cm2 when the valve is completely open. The complete definition also says that the flow medium must have a density of 1000 kg/m3 and a kinematic viscosity of 10−6 m2/s, e.g. water.[clarify]
Referencesedit
^"Valve Sizing" (PDF). Technical Bulletin. Swagelok. Retrieved 21 April 2020.
^"Cv Calculator". Generant. Retrieved 21 April 2020.
^"Control Valve Sizing". Control Valve Handbook(PDF) (5th ed.). Emerson Electric. September 2019. Retrieved 26 February 2022.
^Strömungstechnische Kenngrößen von Stellventilen und deren Bestimmung [Fluidic characteristic quantities of control valves and their determination] (PDF) (Standard). VDI, VDE. September 2007. 2173. Retrieved 17 April 2020.
flow, coefficient, flow, coefficient, device, relative, measure, efficiency, allowing, fluid, flow, describes, relationship, between, pressure, drop, across, orifice, valve, other, assembly, corresponding, flow, rate, mathematically, flow, coefficient, flow, c. 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 across an orifice valve or other assembly and the corresponding flow rate Mathematically the flow coefficient Cv or flow capacity rating of valve can be expressed asC v Q SG D P displaystyle C text v Q sqrt frac text SG Delta P where Q is the rate of flow expressed in US gallons per minute SG is the specific gravity of the fluid for water 1 DP is the pressure drop across the valve expressed in psi In more practical terms the flow coefficient Cv is the volume in US gallons of water at 60 F 16 C that will flow per minute through a valve with a pressure drop of 1 psi 6 9 kPa 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 Cv for the same assembly can be used with a more complex equation 1 2 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 Cv 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 Cv 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 Cv 1 0 2 psia input Flow factor editThe metric equivalent flow factor Kv is calculated using metric units K v Q SG D P displaystyle K text v Q sqrt frac text SG Delta P nbsp where 3 Kv is the flow factor expressed in m3 h Q is the flowrate expressed in m3 h SG is the specific gravity of the fluid for water 1 P is the differential pressure across the device expressed in bar Kv can be calculated from Cv using the equation 4 C v 1 156 K v displaystyle C text v 1 156 cdot K text v nbsp The kv factor or value as it is also called is defined in VDI VDE Richtlinien No 2173 5 A simplified version of the definition is The kv factor of a valve indicates The water flow in m3 h at a pressure drop across the valve of 1 kgf cm2 when the valve is completely open The complete definition also says that the flow medium must have a density of 1000 kg m3 and a kinematic viscosity of 10 6 m2 s e g water clarify References edit Valve Sizing PDF Technical Bulletin Swagelok Retrieved 21 April 2020 Cv Calculator Generant Retrieved 21 April 2020 Boysen Herman kV what why how whence PDF Technical paper Danfoss Retrieved 21 April 2020 Control Valve Sizing Control Valve Handbook PDF 5th ed Emerson Electric September 2019 Retrieved 26 February 2022 Stromungstechnische Kenngrossen von Stellventilen und deren Bestimmung Fluidic characteristic quantities of control valves and their determination PDF Standard VDI VDE September 2007 2173 Retrieved 17 April 2020 See also editDischarge coefficient Retrieved from https en wikipedia org w index php title Flow coefficient amp oldid 1207956340, wikipedia, wiki, book, books, library,
