What is Critical Flow Factor (Cf)?

What is Critical Flow Factor (Cf)?

Critical Flow Factor, often referred to as “Cf” is a coefficient that defines how pressure will recover after it drops to its lowest point inside the control valve. This lowest point is also referred to as the vena contracta*.

All globe-style control valves have a Cf that stays consistent regardless of the trim position, unlike the Flow Coefficient (Cv).

What is Cf Used For?

Cf is a factor needed in order to calculate Flow Coefficient (Cv) or Flow Rate for valve sizing purposes. You can find the Cf for each Kimray valve on its product detail page or on the Sizing Page in the “Valve Coefficients” dropdown.

Note: The Pressure Recovery Factor (FL) also refers to the Critical Flow Factor in some literature.

How is Cf Calculated?

Below is an illustration of how Cf is determined.

First, fluid or gas (shown here in light blue) enters a control valve. This results in the pressure droping 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.

All globe-style control valves have a CF that stays consistent regardless of the stem travel. This is unlike the CV value, which increases as the valve opens.

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

If you have further questions regarding Cf or valve sizing, contact your local Kimray store or authorized distributor.