Butterfly valve torque

[JimCasey]

A long time ago in a job far away, I encountered an analytical method for calculating the torque of butterfly valves. That library is no longer accessible to me and probably no longer exists. I believe that it was for non-offset discs anyway.

Can someone supply or point me to an analytical method for calculating torque for a double/triple offset butterfly valve? (The torque difference between the two would only be at seating, anyway.)

Thanks in advance,