Cv is the capacity of the valve to deliver flow with an available differential pressure (∆P) across the valve. The Engineer of Record typically specifies the differential pressure that should be used when selecting the full flow Cv of the valve.
Cv by definition is the number of gallons per minute (GPM) a valve will flow with a 1 psi pressure drop across the valve. For example a valve with a Cv of 10 will flow 10 GPM with a 1 psi pressure drop. The formula used to select the valve Cv with the specified differential pressure is: Cv=GPM/((SQ RT(∆P)). If the specified ∆P is 4 psid and full flow is 10 GPM the calculated CV is 10/SQ RT of 4 = 5.
Factory available Cv's are limited and sometimes the valve Cv will be selected to deliver full flow with a ∆P that is less than specified and the valve could be considered oversized. Other times the valve Cv may be selected to deliver full flow with a ∆P that is great than specified and the valve could be considered undersized. An oversized valve will cause hunting, poor control and premature actuator wear due to excessive cycling. An undersized valve may not be able to provide enough flow to meet design specification if the actual available ∆P is inadequate.
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