Schiebel actuators: sizing for −40 °C winter starts
Breakaway torque at temperature is the number that matters, and it is rarely the number on the datasheet.
Actuator sizing is usually done against valve running torque with a safety factor applied. In a climate where an isolation valve can sit unmoved at minus forty for weeks, running torque is the wrong starting point. Breakaway torque is, and breakaway torque at temperature can be considerably higher than the ambient figure.
What drives the difference
Two things. Elastomer seats stiffen as temperature drops, which raises the force needed to break the seal. And grease in the valve stem and gearbox thickens, adding drag that does not exist in a workshop test. Neither effect appears in a standard sizing calculation unless you ask for it.
As a working rule for Mongolian installations we size against breakaway torque at the site design minimum, not at twenty degrees, and we specify low-temperature grease in the gearbox as standard rather than as an option. The actuator ends up a frame size larger than a naive calculation suggests. It also opens in January.
Heaters are not a substitute
Anti-condensation heaters keep the electronics dry. They do not warm the valve. An actuator specified for ambient conditions with a heater fitted will still stall against a cold seat, and the failure will present as an electrical fault rather than a sizing error — which is why it often takes two site visits to diagnose.
