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.
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