Control Valve Sizing

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How to size a control valve

First of all, let's review some terminology.

The C_v of a valve is the flow rate of water (gal/min) at a temperature of 60°F, with a pressure drop of 1 psi when the valve is completely open.

Rangeability - Ratio of the maximum to minimum controllable flow: R = C_v / C_{min v}

R = (Q_max / Q_min) * (ΔP_min / ΔP_max)^0.5

Let's now size a control valve for liquid service. We will first determine the design C_vc with the following equation: C_vc = Q * (s / ΔP)^0.5 where s is the specific gravity of the liquid. In order to have a properly controlling valve the ratio of the valve coefficient at 100% open to the design coefficient is a range of 1.25 to 2 or C_v / C_vc = 1.25 to 2. Typically the specified pressure drop is 50% of the pressure drop in the line and a minimum of 5-10 psi.

Control Valve - Rangeability
Rangeability = 0

Specific Gravity of Fluid (i.e. 1.0 for water):
Flow Rate_min (gal/min):
Flow Rate_max (gal/min):
Delta P_min (psi):
Delta P_max(psi):

Control Valve - Design Data
Manufacturer's C_v of a 100% open valve = 0
If the design flow rate is 0 gal/min, with a ΔP of 0 and a specific gravity of 0 then the design C_v = 0
C_v / C_vc = 0
If the ratio is between 1.25 and 2 it is a good selection. If it is too high choose a valve with a lower C_v. For this application you would want a valve with a C_v between 0 and 0.

S.G. of Fluid (i.e. 1.0 for water):
Design Flow Rate (gal/min):
P_in(psi):
P_out(psi):
Mfg C_v:

Specific Gravity of Fluid (i.e. 1.0 for water):
Flow Rate_min (gal/min):
Flow Rate_max (gal/min):
Delta P_min (psi):
Delta P_max(psi):

Control Valve - Design Data

S.G. of Fluid (i.e. 1.0 for water):
Design Flow Rate (gal/min):
P_in(psi):
P_out(psi):
Mfg C_v: