Selecting the control valve for a given application depends on several factors, which are:
• Flow rate
• Pressure
• Temperature
• Corrosiveness or abrasion of a medium
• Requirements for tightness
• Application of specific industry norms (FDA, 3A, EPA...)
The capacity of a valve is given by the value of the flow rate coefficient, Cv or Kv, and shows the amount of water which flows through the valve under the defined conditions.
Cv – amount of water [U.S. gallon/min] when pressure drops 1 psi
Kv – amount of water [m3/h] when pressure drops 1 bar
Q ... volumetric flow rate, for calculation, Cv [U.S. gallon/min], for calculation, Kv [m3/h]
SG ... specific weight (for water is SG=1)
Δp ... pressure drop, for calculation, Cv [psi], for calculation, Kv [bar]
Simplified relationship between the coefficient Cv and Kv is:
Cv=1.15622 * Kv
Kv=0.86488 * Cv
The calculation of the flow rate coefficient for gases and steams:
P1 [bar] 
Pressure upstream of the valve 
P2 [bar] 
Downstream pressure 
T1 [K] 
Temperature upstream of the valve (°C +273.16) 
Δp [bar] 
Pressure drop across the valve (p1p2) 
Q [kg/h] 
Weight Gain 
Qn [m3/h] 
Volumetric flow rate of gas at standard conditions (T=0°C, P=1 bar) 
ρn [kg/m3] 
Gas density at standard conditions (T=0°C, P=1 bar) 
v2 [m3/kg] 
Specific volume of steam for p2 and T1 
v3 [m3/kg] 
Specific volume of steam for p1/2 and T1 
