A valve cv can be described as the number of gallons per minute gpm at 60of 15 5oc that will pass through a valve with an associated pressure drop of 1 psi. 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.
Q is the rate of flow expressed in us gallons per minute sg is the specific gravity of the fluid for water 1 δ p is the pressure drop across the valve.
Control valve cv rating. The flow coefficient c v for a water control valve can be estimated with the diagram below. They express coefficient cv kv as the flow rate of water in g p m. The flow rate for a liquid q cv sq.
When sizing calculating which size valve is needed for your application a control valve specialist needs to calculate the flow coefficient. Mathematically the flow coefficient cv or flow capacity rating of valve can be expressed as. Control valve cv refers to the flow coefficient of the valve.
Cv 1 156 kv. δp differential pressure. Cv 25 gpm 1 1 psi 1 2.
Root of δp sg q flow in us gpm. Example required c v for a water control valve. C v q sg δ p.
Cv flow coefficient of control valve. Download water c v diagram. A greater flow capacity value represents a less restrictive less resistive valve able to pass greater rates of flow for the same pressure drop.
Displaystyle c v q sqrt dfrac text sg delta p where. Most control valve manufactures recommend that you should select a valve where the required cv or kv value falls between 20 80 of the port opening. 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.
It is customary in control valve engineering to express the restrictiveness of any valve in terms of how much flow it will pass given a certain pressure drop and fluid specific gravity gf. With water flow 6 gpm and designed pressure drop 10 psi through a control valve the required flow coefficient c v can be estimated to 2 as indicated in the chart below. It applies to the factor of the head drop δh or pressure drop δp over a valve with the flow rate q.
For a specific control valve flow coefficient cv kv is determined experimentally by control valve manufacturers and you can found it in manufacturers technical specifications. This measure of valve performance is called flow capacity or flow coefficient symbolized as cv. Calculating control valve cv.
M3 h for a pressure drop of 1 psi 1 bar across a flow passage flow coefficient. Kv 0 864 cv. Sg specific gravity of fluid reference against water at 60 f now we discuss about how we get the conversion factor of 1 156 in the formula cv 1 156kv.
In simple terms a fully open control valve with a cv of 14 usgpm psi passes 14 usgpm of fluid with 1 psi pressure drop.