For measuring the valve capacity of control valves the coefficient cv or its metric equivalent kv is adopted. However there is a problem with using this equation in pneumatic applications.
Cv value calculation a simple cv value formula.
Cv formula for control valve. The density of water at 15 c is 999 13 kg m 3. A control valve has a cv of 5. Please check your control valve selection with the control valve manufacturer.
However to be considered under the verification e cv easier we have a simple formula cv value can be calculated following them. 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. It applies to the factor of the head drop δh or pressure drop δp over a valve with the flow rate q.
What is the flow through the valve. C v 25 gpm 1 1 psi 1 2 25 flow coefficient c v for saturated steam since steam and gases are compressible fluids the formula must be altered to accommodate changes in. Q is the rate of flow expressed in us gallons per minute.
Case of the liquid. The flow coefficient for a control valve which in full open position passes 25 gallons per minute of water with a one pound per square inch pressure drop can be calculated as. Example flow coefficient liquid.
Mathematically the flow coefficient c v or flow capacity rating of valve can be expressed as. It is water at 50 c. It describes the relationship between the pressure drop across an orifice valve or other assembly and the corresponding flow rate.
Some control valve manufactures recommend that an allowance of 30 should be added to the calculated cv or kv flow coefficient to obtain the minimum full flow cv or kv flow coefficient rating which the selected valve should have when fully open. The instrumentation on the line show that the pressure drop through the valve is 0 1 bar. The valve coefficient cv in imperial unit the number of us gallon per minute of water at 60 f that will flow through a valve at specific.
I recommend using the control valve manufacturer has provided. Cv is based on the volume flow rate of water at 60 f through a device within one minute at a 1 psi differential. Calculation example of the flow through a valve of coefficient cv.
Control valve valve flow coefficient cv hence a valve with a specified opening giving cv 1 will pass 1us gallon of water at 60 degree fahrenheit per minute if 1psi pressure difference exists between the upstream and down stream points on each side of the valve. The coefficient cv iec standards are highly complex. The flow coefficient of a device is a relative measure of its efficiency at allowing fluid flow.
The formula will differ depending on the product the valve regulates such as liquid gas or steam. Cv is the flow coefficient in imperial units. How do we get value 1 156 conversion factor in formula cv 1 156kv or cv 1 156 q sqrt g delta p.
Because air is compressible cv must have additional factors built into the equations to compensate for that compressibility. Kv 0 865 cv cv 1 156 kv equivalence between flow coefficients kv and cv. Control valve cv refers to the flow coefficient of the valve.