Equal Percentage Valves
Equal Percentage valves have a valve plug shaped so that each increment in valve percent open (lift) increases the flowrate by a certain percentage of the previous flow.
The relationship between valve lift and orifice size is not linear but logarithmic.
It is expressed mathematically as follows:
Where:

τ = valve rangeability (ratio of the maximum to minimum controllable flowrate)

H = valve percent open or lift (0 = closed; 1= fully open)

Cv,max = full open valve loss value