An actuator is chosen from two things: the torque the valve needs, and what the process requires to happen when power or air is lost. Everything else follows from those.
Pneumatic, spring return
Compressed air drives the valve one way and a spring returns it the other. On loss of air the spring decides the position, which is what makes this the default for emergency shutdown duty.
The decision to make early is fail open or fail closed. It is not a detail to settle on site, because it determines what the plant does in its worst moment.
Pneumatic, double acting
Air drives the actuator in both directions, so it delivers more thrust than a spring return unit of the same size. There is no inherent fail position, so if one is required it has to be added with a lock up valve or an air accumulator.
Suits modulating duty and high cycle counts where the spring would be the limiting component.
Electric, motor operated
A motor and gear train position the valve, with limit and torque switches defining travel and cut off. Precise, and it does not need an instrument air supply, which is often the deciding factor on a site that has no reliable air.
Hydraulic
Where the thrust required is beyond what pneumatic can practically deliver. Hydraulic fluid gives high force in a compact envelope, which is why it turns up on large bore valves and high differential pressure service.
The mechanism matters too
A scotch yoke converts linear thrust to quarter turn rotation with a torque curve that peaks at each end of travel, which is exactly where a ball or butterfly valve needs it most. A rack and pinion gives a flat torque curve in a smaller, lighter housing, and suits modest torque in tight spaces.
Where there is no power at all, a worm or bevel gearbox reduces the effort enough for a large valve to be operated by hand.
Commission it as one assembly
Positioners, limit switch boxes, solenoids and manual overrides are part of the actuator decision, not an afterthought. Specified together, the assembly is commissioned once. Specified separately, it is commissioned twice.
The full range is on our actuators page, and units already in service can be repaired and recalibrated.
