Actuators are the devices which drive the valve stem. There are different types of valve actuators used in valves according to different purposes and based on actuating principle.
There are actuators such as:
Pneumatic actuators use compressed air as actuating force, with minimum 3psi for fully open and maximum 15psi pressure fully closed condition. Any signal between this range will hold the stem at an intermediate position to control the flow.
The above show is a diaphragm type pneumatic actuator. Compressed air displaces the diaphragm which leads the movement of the actuator
fig: piston type
Another type of pneumatic actuator is piston type, shown in above picture. Compressed air pressurize at the top of the piston which results in movement of the piston. Piston type is usually used to operate butterfly valves.
There are mainly two types of electrical actuators, Solenoid type and motor operated type
The above shown is a solenoid operated valves, usually used for on/off purpose. These type of valves are used in emergency shut down systems.
They are small in size.
Motor operated valves (MOV) are used in large diameter pipeline. This is an electro-hydraulic actuator electric signals drives the hydraulic fluid to open the valve. If the signal falls the valve closes.