Technical Consulting

Calculation of Cv, Kv

 

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 (p1-p2)
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

 

Technical Tables

 

Recommended output velocity of the valves and length of inlet and outlet pipes.