Flow Coefficient

Written by Jerry Ratzlaff on . Posted in Classical Mechanics

Flow coefficient, abbreviated as $$C_v$$, can be described as the volume (in US gallons) of water at 60°F that will flow per minute through a valve with a pressure drop of 1 psi across the valve. This gives us a method to compare flow capabilities of different valves. The flow coefficient allows us to determine what size valve is required for a given application.

Flow Coefficient is primarily used when sizing control valves.  However, it can be used to characterize other types of valves such as ball valves and butterfly valves.

Flow Coefficient Formula

(Eq. 1)  $$\large{ C_v = Q \; \sqrt {\frac{SG}{\Delta p} } }$$

(Eq. 2 by volume)  $$\large{ C_v = Q \; \left( {\frac{SG}{\Delta p} } \right) ^{\frac{1}{2} } }$$

(Eq. 3 by weight)  $$\large{ C_v = \frac {W}{500 \; \left( SG \; \Delta p \right) ^{\frac{1}{2}} } }$$

Where:

$$\large{ C_v }$$ = flow coefficient

$$\large{ Q }$$ = flow rate capacity

$$\large{ W }$$ = flow weight

$$\large{ \Delta p }$$ = pressure differential

$$\large{ SG }$$ = fluid specific gravity (water at 60°F = 1.0000)

Solve for:

$$\large{ Q = C_v \; \sqrt {\frac{SG} {\Delta p} } }$$

$$\large{ \Delta p = SG \; \left( {\frac{Q} {C_c} } \right) ^{\frac{1}{2} } }$$

$$\large{ C_v = 1.157 \; K_v }$$     (US units)

$$\large{ K_v = 0.8646 \; C_v }$$     (SI )units