I've been given some test data on a butterfly valve that I'm trying to validate. My validation information is in gallons per minute.  The acceptance criteria was given as being from 22 to 26 turns, and I was told that +/- 2 handwheel turns from the nominal 24 turns equated out to +353 gpm to -426 gpm.

Can I calculate a flowrate assuming a linear increase (i.e., 11*353=3883 gpm) or do I get a larger increase when the valve first opens?  What information would I need in order to figure out the flowrate?  The valve supposedly is an Enertech butterfly 15".

Large butterfly valves could have gears and such in the spindel but normally a butterfly valve will close in 90 deg - so unless the gear is really funny the closure of the valve will also be linear.

This does however not mean that the "resistance" introduced by closing the valve will increase linear. As i recall it the flow will only decrease slowly when close and then drop abruptly when the valve is almost fully closed (the last 20-30 % introduceses almost all of the pressuredrop).

Butterfly valves are meant for quick open and tight shut off only.And I agree with svanels about the Butterfly valve's function.
My thinking is,If The Handwheel is connected to a gearbox(It has to be!)Then The Disc's degree of opening depends on the module and pitch of the worm gear inside the gear box.
Now,The gear box is selected on the Basis of torque value of the valve.Each gear box has a unique value of Torque range of Operation beyond which It cannot be used.For eg.:-
consider a series of gear boxes called eng series then you name it as eng0-150,eng150-250,eng 250-350,eng350-450 etc.The numerical value being the torque range the paricular gear box can operate.Suppose you have a valve having a torque of 275kgm,the gear box selection is done by multipleing a factor of 1.3 to the torque which is 1.3*275=357.5kgm so then the eng350-450 gear box is selected for the valve. Sometimes for reasons of economy the gear box is selected with the minimal factor of safety then the eng250-350 gear box fits in.In the former case you have a larger gear box than neccessary but are assured of it's safe function;In the latter case the gearbox is much smaller and you are saving money too  but the flip side is the gear box is not design worthy for higher pressures.Kindly check with your supplier as to how the selection was made.
Also,Find out if the Handwheel is designed for the particular gear box only.

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