Valve Actuator Close-Off Pressure and Torque

The differential a valve must close against is not the differential it sees at design flow, and that is the whole trap. As other valves on the system close, the pump rides up its curve and the pressure across the remaining valves rises toward the pump's shutoff head -- so the worst case for close-off is MINIMUM system flow, which is the condition a designer computing at design flow never looks at, and it is also the condition in which a valve is most likely to be commanded closed. On a system with many two-way valves and no differential pressure control, that rise can be large. The consequence of getting it wrong is subtle rather than dramatic, which is why it goes unfound for years. The valve strokes, the actuator reports closed, and a small flow continues past the seat -- so a coil stays warm, a zone overheats in the cooling season, and the problem reads as a control fault. Checking the actuator's close-off rating against the actual differential is what identifies it, and it is a nameplate comparison rather than a diagnosis. Spring-return actuators deserve their own line, which is why the spring rating is a separate input here. Their close-off in the SPRING direction is set by the spring rather than by the motor, and is often much lower than the driven rating -- so a valve can close reliably on command and leak on a power failure or a fire alarm shutdown, which is precisely the moment a closed valve matters. A single pass or fail on the driven rating hides that, so both are reported. Seat force is pressure times the EFFECTIVE seat area, which is entered from the valve manufacturer's data rather than computed from the nominal size; on a rotary valve, close-off is published as a torque and this force basis does not apply. This does not size the valve or its Cv, compute the differential at minimum flow (which needs the pump curve and the system), evaluate valve authority, or address cavitation and flashing. The valve and actuator manufacturer's close-off tables and the controls engineer of record govern.

seat force = differential pressure x effective seat area, evaluated at the differential at MINIMUM system flow rather than at design; the actuator's rated close-off is converted to a force the same way and compared; the spring-direction rating is compared separately.

Valve close-off as control-valve practice states it. The effective seat area and both actuator ratings are ENTERED from the manufacturer's tables; on a rotary valve close-off is published as a TORQUE and this force basis does not apply. It does not size the valve or its Cv, compute the minimum-flow differential (which needs the pump curve and the system), evaluate valve authority, or address cavitation and flashing.

One product and one comparison; no manufacturer table is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: seat_area_in2, design_differential_psi, minimum_flow_differential_psi, actuator_closeoff_psi, spring_closeoff_psi, is_spring_return, design_seat_force_lb, worst_case_seat_force_lb, actuator_force_lb, force_shortfall_lb, rise_ratio

• The effective seat area is entered it is not the nominal valve sizethe valve manufacturer's data
• Rotary valves use torque the force basis here does not apply to ball or butterfly valvesthe actuator manufacturer
• The minimum-flow differential is entered it needs the pump curve and the system to computethe controls engineer of record