The flow coefficient also known as Cvcan be described as the volume (in US gallons) of water at 60¬∞ F that will flow per minute through a valve with a pressure drop of 1 psi across the valve. Cv gives us a method to compare flow capabilities of different valves. The flow coefficient allows us to determine what size valve is required for a given application.
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Flow Coefficient Formula
The general definition of the flow coefficient can be expanded into equations modeling the flow of liquids, gases and steam as follows:
= flow coefficient or flow capacity rating of valve
= specific gravity of fluid (where water = 1)
= pressure drop across valve (psi)
At low pressures, like gravity feed, Cv values typically are not accurate. In the extreme, if there is only one psi head pressure at the valve inlet, then theoretically, there will be no flow out of the valve. This is because 1 psi minus a 1 psi pressure drop across the valve equals zero psi or no flow.