What is Cv and how is it used in valve sizing?

Cv (valve coefficient or flow coefficient) is a measure of valve flow capacity - the number of US gallons per minute of water at 60 degrees F that will flow through the valve with a 1 psi pressure drop. Higher Cv means higher flow capacity. For liquids: Q = Cv * sqrt(delta P/SG). Cv is determined by valve size, trim design, and travel. Valve sizing calculates required Cv based on maximum flow and available pressure drop, then selects valve with appropriate Cv range. Kv is the metric equivalent (m^3/hr with 1 bar drop).