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).
Note: The Pressure Recovery Factor (FL) also refers to the Critical Flow Factor in some literature.
What is Cf Used For?
Cf is a factor needed in order to calculate Flow Coefficient (Cv) or Flow Rate for valve sizing purposes.
What is Valve Flow Coefficient (Cv)?
Valve Flow Coefficient (Cv) is a valve’s capacity for a liquid or gas to flow through it.
It is technically defined as “the volume of water at 60°F (in US gallons) that will flow through a valve per minute with a pressure drop of 1 psi across the valve.”
As the stem opens, the Cv increases.
The maximum Cv for the 2-inch regulator is 47, while the maximum for the 3-inch regulator is 117.
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