What is Critical Flow Factor (Cf)?

Critical Flow Factor (Cf) is a coefficient that defines how pressure will recover after it drops to its lowest point (the vena contracta*) inside the control valve.  Each valve has one Critical Flow Factor, and this number doesn’t change due to percentage open, like the Flow Coefficient (Cv) does.

Cf is a factor that is needed in order to calculate Flow Coefficient (Cv) or Flow Rate for valve sizing purposes. The Cf for each Kimray valve can be found on its detail page or on the Sizing Page in the Valve Coefficients dropdown.

The Critical Flow Factor is also referred to as the Pressure Recovery Factor (FL).  Below is an illustration of how Critical Flow Factor is determined.

As the fluid or gas (shown here in light blue) enters a control valve, the pressure drops until it reaches the vena contracta, or its lowest point. Then it continues downstream and the pressure bounces back (or recovers). The coefficient for this recovery is the Critical Flow Factor. It is discovered by dividing the downstream pressure after vena contracta by the downstream pressure at vena contracta, then calculating the square root of that number.

*Vena contracta is the point in a flow stream where the diameter of the stream is smallest, and fluid velocity is at its maximum.