What kind of torque characteristics do ball valves demonstrate?
Ball valves have non-linear torque characteristics for opening and closing. Ball valves always require more torque for opening than for closing. This value is known as break away torque and is very important when configuring a semi-rotary actuator. As soon as the breakaway torque has been overcome and the ball valve begins to open, the required amount of torque is reduced. This effect results from the continuously decreasing contact surface area between the ball and the sealing rings. When rotary motion is complete, the ball is once again seated in the sealing rings, resulting in increased friction. Torque is thus increased once more, but it does not reach breakaway torque due to existing kinetic energy. The required torque at the end of rotary motion is approximately 70% of the breakaway torque value. As a consequence, the actuator must be configured as a rule, with 20% more starting torque than the ball valve requires in order to assure adequate reserves for opening and closing.
