Valve Torque Analysis: Breakout, Running, and Seating Torques

Accurate torque analysis is essential for selecting the right actuator and ensuring reliable valve operation. Underestimating torque leads to stalled actuators; overestimating leads to oversized actuators and increased valve wear.

Torque Components

Breakout (unseating) torque: force to initially unseat the disc or gate from the seatRunning torque: torque required to drive the valve through mid-travelSeating torque: force to push the seat to rated leakage class at full closeStem friction torque: component due to packing and stem guide friction

Trunnion Ball Valve Torque

Trunnion ball valves generate most torque during breakout (unseating) and seating. The seat contact force is controlled by the seat spring design, not system pressure. Running torque is significantly lower than breakout/seating torque. Actuators must be sized for the maximum of breakout or seating torque.

Butterfly Valve Torque

Butterfly valve torque varies sinusoidally with disc angle. Maximum torque typically occurs near mid-travel due to the hydrodynamic moment of the disc in the flow stream. For large-diameter butterfly valves, hydrodynamic torque at partial opening can exceed breakout torque.

Safety Factor for Actuator Sizing

Standard practice applies a safety factor of 1.25–1.5 over calculated valve torque when sizing actuators. This accounts for variability in packing friction, seat wear, pressure surge, and manufacturing tolerances. For ESD valves, higher safety factors (up to 2.0) are applied.

Obtain torque data from valve manufacturer at rated differential pressureConfirm torque at minimum, normal, and maximum operating pressureAccount for actuator torque loss at minimum supply pressureRe-evaluate actuator sizing after any seat or packing replacement that changes friction