What is the difference between linear characteristic and equal percentage characteristic in controlling butterfly valves?

For “linear characteristic”, the flow rate is directly proportional to the amount of travel of butterfly disk. For example, at 25% open of butterfly disk, the flow rate of 25% of maximum flow.

For “equal percentage characteristic”, equal increment of opening of butterfly disk leads to equal percentage change in flow rate. For example, when butterfly disc open from 30% to 40%, it generates a change in flow rate of 50%. Therefore, when butterfly disc open from 40% to 50%, it also generates a change in flow rate of 50%. If the flow rate at 30% is 200m³/s, then the flow rate of 40% and 50% open are 300m³/s and 450m³/s respectively.

The use of different characteristics depends on the amount of pressure drop available to the butterfly value. Should more than 25% of system pressure drop is available to the butterfly value, then the employment of linear characteristic would provide the best results. On the contrary, if less than 25% of system pressure drop is available to the butterfly value, then the employment of equal percentage characteristic would be a better choice.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.