I always resist giving MortenA's example, Not that it is wrong, but it does seem to imply thatthe relationship between flow and Delta-P is linear. it's not.
If you get 1 gallon at 1 psi loss, you will get 1.414 gpm at 2 psi differential.
There are a family of Cv equations. Liquids, Gases, Steam, SUbcritical, Choked flow, etc. Most of the major control valve companies allow at least a trial version of their sizing software to be downloaded from their website. ANy decent program runs all the checks to determine the flowing conditions and applies the appropriate equation.
Cv is another way of expressing the amount of pressure drop, or in otherwords, flow resistance, you can expect through a valve or fitting. If the manufacturer cannot give you the value, you can calculate it using the data in Crane Technical Paper #410. However, I would be very surprised if the valve manufacturer does not have the data readily available for you.
