Actuators are used for the automation of industrial valves and can be found in all kinds of technical process plants: they are used in waste water treatment plants, power plants and even refineries. This is where they play a major part in automating process control. The valves to be automated vary both in design and dimension. The diameters of the valves range from a few inches to a few meters.
Depending on their type of supply, the actuators may be classified as pneumatic, hydraulic or electric actuators. The classification indicates what is required to operate the valve. Pneumatic requires air, hydraulic requires a liquid (typically water/oil), and electric needs electricity.
Actuators are mounted onto the valve in lieu of a lever, gear operator or hand-wheel. Brackets are typically required that work to match up the bolting of the ISO pad (surface where operators are typically located) on the valve to the actuator. An adapter then fits inside the bracket to connect the stem of the valve to the mechanism of the actuator that opens/closes.
Actuators are set to be "fail-open" or "fail-close" which means to move the valve to open/close when certain conditions are met. Actuated valves can be used as ESD's (emergency shut-down valves) to close automatically when certain bad conditions are met or actuated valves can be used to automate processes (in lieu of manually opening/closing valves)
Actuators are sized according to the torque(amount of pressure) required to open/close a valve. Valves have a break torque (amount of pressure needed to open the valve), and a re-seat (amount of pressure needed to close the valve). Actuators are typically sized with a safety factor ensuring that an actuator strong enough to handle the torque is used.
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Forged valves usually cost more but you don't need to worry about bad castings i.e. porous castings.