Hydraulic Actuators:

Pneumatic actuators are generally used to control processes needs accurate and quick response, as they do not need a huge amount of motive force. Therefore, while a huge amount of force is needed to operate a valve (for example, the major steam system valves), hydraulic actuators are generally used. While hydraulic actuators come in several designs, piston categories are most general.

A classical piston-type hydraulic actuator is display in below Figure. It consists of a piston, cylinder, spring, hydraulic supply and returns line, and stem. A piston slides vertically within the cylinder and separates the cylinder into two chambers. An upper chamber holds the spring and the lower chamber holds hydraulic oil.

The hydraulic supply and return line is linked to the lower chamber and permits hydraulic fluid to flow to and from the lower chamber of the actuator. A stem transmits the motion of the piston to a valve.

Figure: Hydraulic Actuator

At the start, along with no hydraulic fluid pressure, the spring force contains the valve in the closed position. As fluid enters the lower chamber then pressure within the chamber raise. This pressure results in a force on the bottom of the piston opposite to the force caused by the spring. While the hydraulic force is greater than the spring force then the piston starts to move upward, the spring compresses, and the valve starts to open. As the hydraulic pressure increases then the valve continues to open. On the other hand, as hydraulic oil is drained from the cylinder then the hydraulic force becomes less than the spring force and the piston moves downward, and the valve closes. Through regulating amount of oil supplied or drained from the actuator, the valve could be positioned among fully open and fully closed.

The principles of operation of a hydraulic actuator are such as those of the pneumatic actuator. Every uses some motive force to overcome spring force to move the valve. Also, hydraulic actuators could be designed to fail-open or fail-closed to gives a fail-safe feature.