1Flow Capacity
1.1 Introduction
Valve flow capacity, also called flow coefficient or capacity index, is defined as the valve's capacity for a liquid or gas to flow through it. The flow coefficient is technically defined as the flow rate of water in US gallons per minute at the temperature of 60 °F with a pressure drop of 1 psi across the valve. Flow coefficient, as shown with parameter Cv, increases by opening the valve to the maximum value when the valve is 100% open. Pressure drop, also called differential pressure, is defined as the difference between the inlet and outlet of the valve; for a valve with a specific size, the greater the differential pressure, the greater the flow rate. These two parameters, flow rate and differential pressure, are brought together by a flow coefficient, which allows the performances of different valves regarding flow rates to be compared. The differential pressure across the valves is calculated by knowing the flow rates, and finally, the flow rates are determined for given differential pressure values. Furthermore, the flow coefficient is an essential parameter for sizing valves. The flow coefficient (Cv) for liquids is calculated from Eq. (1.1).
where:
- Cv: Flow coefficient (US gal per minute [gpm]/psi);
- Q: Flow rate (US gpm);
- SG: Fluid‐specific gravity (dimensionless) calculated ...
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