The rated Cv of a particular style can be included in the basic liquid flow equation to calculate flow (Q). When the flow rate and pressure conditions are known, the equation can be arranged to solve for the required Cv. Calculating the required Cv and selecting an appropriate valve is the essence of valve sizing.
Where:
Q is the liquid flow rate in GPM (gallons per minute).
Cv is the flow coefficient that describes the flow capacity of a valve.
ÆP is the pressure drop (P1-P2) across the valve in psid.
G is the specific gravity of the fluid at the valve inlet.
Determinants Of Cv
Although control valve sizing is the subject of another Module in this Course, the specifier should be generally aware of the following factors, which combine to influence the rated flow capacity (flow coefficient) of a control valve:
• The port size is obviously a major contributor to the flow coefficient.
• Flow capacity is directly related to the percentage of valve travel or valve opening. To prevent excessive fluid velocity at low flow, control valves are sized to pass the minimum required flow at 10-20 percent travel. To ensure reserve capacity, valves are sized to pass the maximum required flow at 80-90 percent travel.
• The flow geometry of a valve greatly influences its liquid flow capacity. The tortuous flow path of the globe valve limits liquid flow capacity, while the streamlined, flow paths of most rotary-shaft valves results in greater relative capacity for similarly sized valves. Refer to Figure 39. For gasses, flow is a function of the port area alone, and flow geometry has little impact on flow.
