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CONTROL VALVE SIZING EQUATIONS

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Control Valves Basics - Sizing & Selection
FMD3x00 98 DB Initialize (5), configuration-and-evaluation-software-pi-9045582-en-gb, configuration-and-evaluation-software-pi-9045582-en-gb, Document, colour present

CONTROL VALVE SIZING EQUATIONS
In order to determine the correct size of a valve fora specific system, many factors must be considered. The most important factor is the capacity parameter, Cv, or the flow coefficient. To determine the valve size needed for your system, you can estimate Cv with the following equations
Liquids (water, oil, etc
Because liquids are incompressible fluids, their flow rate depends only on the difference between the inlet and outlet pressures (P, pressure drop. The flow is the same whether the system pressure is low or high, so long as the difference between the inlet and outlet pressure is the same. The equation below shows the relationship Where,
• Cv = Valve flow coefficient, US GPM with P = 1 psi
• Q = Fluid flow, US GPM
• S = Specific gravity of fluid relative to water @ F
• P = Pressure drop (PP) at maximum flow, psi

Specific gravity correction is negligible for water below Fuse S = 1.0). Use actual specific gravity S of other liquids at actual flow temperature. Use the following equation for fluids with viscosity correction factor. Use actual specific gravity S for fluids at actual flow temperature. Where,
• K = Viscosity correction factor for fluids Estimating diameter of pipe Another important piece of information about sizing and specifications is what diameter pipe maybe used with a certain flow, seen in the following equation Where
• d = diameter of pipe (ft)
•
Q
max
= maximum flow through the valve (ft
3
/s)
• v = velocity of flow (ft./s)

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