Cv is the capacity of the valve to deliver flow with an available differential pressure (∆P) across the valve.
It specifies the differential pressure that should be used when selecting the full flow Cv of the valve.
Cv by definition is the number of US 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 pressure drop of 1 psi. 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 the full flow is 10 GPM, the Cv is 10 / (SQ.RT of 4) = 5.
Factory available Cv’s are limited and some times the valve will be selected to deliver full flow with a DP less than specified and valve could be over sized.
Other times, the valve Cv may be selected to deliver full flow with a delta P that is greater than specified and the valve could be considered as undersized.
Disadvantages with Oversized valve.
An over sized valve will cause hunting, poor control and premature actuator wear due to excessive cycling.
Disadvantages with Under sized valve.
An undersized valve may not be able to provide enough flow to meet design specification if actual available DP is not sufficient.