Friction area is proposed as an improvement over Cv for incompressible flow capacity. Nozzle area, experimentally determined, is proposed as an improvement over other parameters for choked flow of gases due to sonic velocity and choked flow of liquids due to cavitation. Choked flow problems occur in control valves and pressure regulators, system blowdown, and most recently in emergency isolation valves for nuclear steam and feedwater. Nozzle area also defines the expansion factor for non-choked compressible flow.

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