What kind of torque characteristics do butterfly valves demonstrate?
Butterfly valves demonstrate non-linear torque characteristics. The greatest amount of torque is required for opening the valve, and is called breakaway torque. The breakaway torque value is required when configuring semi-rotary actuators. As soon as breakaway torque has been overcome and the butterfly valve opens, the required amount of torque is reduced. This effect results from the continuously decreasing contact surface area between the flap and the seal, as well as pressure in the piping system that helps to open the valve. When rotary motion is ended, the required torque rises again to roughly 30% of the breakaway torque due to turbulence in the piping system. Due to the characteristic torque curve demonstrated by these valves, the actuator must be configured, as a rule, with 20% more starting torque than the butterfly valve requires in order to assure adequate reserves for opening and closing.
