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- Aug 07, 2018 -

Torque rating determination

While choosing between a ball valve and a butterfly valve can involve more than a few considerations (such as pipe size, media type/pressure and valve cost), the steps for automating them are the same. The most critical detail when building an assembly is the torque rating. Proper sizing ensures that the actuator will be strong enough to open and close the process valve when needed. Determining an accurate torque rating begins with the breakaway torque (also known as the unseating torque). This is the rotational force required to open the valve from a fully closed position. Because the isolating elements are static and have the most contact with the seals in the closed position, the breakaway torque is higher than the force needed during the valve’s normal operation at any other position, usually referred to as running or action torque.

The media pressure can affect torque requirements. Valve suppliers typically offer breakaway torque relative to the nominal pressure of the valve. In the case of the butterfly valve and the large ball valve, it is also common to show breakaway torque relative to reduced media pressures because this will help properly size the actuator for the specific application.