Actuator Torque Sizing: Breakaway, Running, and End-of-Travel Torque Requirements

Correct actuator torque sizing is critical for reliable valve operation. An undersized actuator fails to open or close the valve under process conditions, causing process upsets and potential safety incidents. An oversized actuator adds unnecessary cost and weight, may damage the valve through excessive seat loading, and can cause rapid wear of valve components. Proper torque sizing requires understanding all of the torque components that the actuator must overcome at each point in the valve stroke, under the worst-case combination of process conditions.

Torque Components for Quarter-Turn Valves

Quarter-turn valves including ball valves, butterfly valves, and plug valves have torque requirements that vary significantly through the operating stroke. The dominant torque components are: breakaway torque, the torque required to initiate movement from the fully closed or fully open position; running torque, the torque required to sustain rotation through the midstroke position; and end-of-travel seating torque, the torque required to drive the closure element firmly against its seat at the end of the closing stroke. For soft-seated ball valves, the breakaway torque (unseating torque when opening) is typically the highest torque in the stroke.

• Breakaway torque: 1.5 to 2.5 times running torque for most ball valve designs

• Seating torque: closing torque required to achieve rated seat tightness class

• Running torque: lowest torque in the stroke, determines minimum actuator sizing

• Differential pressure torque: torque from process pressure acting on the closure element area

• Packing and bearing friction: typically 20 to 40% of total torque for properly lubricated valves

Valve Manufacturer Torque Data

Reputable valve manufacturers publish torque data in their product catalogs and technical data sheets. Torque values are typically given as a function of valve size, pressure class, temperature, and differential pressure. For control valve actuator sizing, the torque should be evaluated at the maximum differential pressure the valve will encounter, which may be at minimum flow (for pressure-controlled processes) or at valve shutoff condition. The actuator output torque at minimum supply pressure must exceed the valve torque requirement including a suitable safety factor, typically 25 to 50% above the calculated valve torque.

Safety Factors and Service Conditions

Actuator sizing practice includes applying safety factors to account for uncertainty in the valve torque data, degradation of valve and actuator performance over the service life, and the importance of the valve function. For on-off service in non-critical applications, a safety factor of 25% above the manufacturer's stated torque is commonly used. For emergency shutdown valves and other safety-critical applications, safety factors of 50% to 100% are applied to ensure reliable operation even with worn packing, contaminated process fluid, or actuator performance degradation. For modulating control service, the actuator must provide sufficient torque throughout the full control range with adequate margin for good positioner control.