'Any % change in the opening from its current existing value changes the flow by same percentage of its current value at current opening.'
Means if current opening is say 10% & u change it to 15% than the change in opening from its current value (of 10%) is 50%, so the flow will also increase by 50% of its existing value at 10% opening. This means if flow was say 20% of the total range (Or capacity of the valve) at 10% opening it will become 30% (50% higher compared to 20%) of the total range.
So how to calculate it????
Now you know the definition so you can generate an equation which gives you all the values where 0% opening or lift is 0% flow while 100% lift is 100% flow.
So the curve for these valves look like this.
Long back I generated an equation from a general curve for an equal percentage valve. Its fixed for all kinds of valves from any manufacturer generally. There may be minor variations in case of specially designed valves otherwise it is same.
The equation is.
% Flow is = .06 + 0.49 * X -.019 * X^2 +.0005 * X^3 - 0.0000502 * X^4 + 0.0000000242 * X^5
Where X is % Lift or opening , X = 60 for 60% opening & not 0.6
Hope it is useful for all.