Actuator

Quite often is necessary to use an actuator to change the position of valves in process units. Such cases include large-size valves functioning against high-pressure fluids, emergencies, and accidents, remote control of valves, and 

• the need to change the valve position in the shortest time possible.
• The actuator is a device that turns input signals into circular or linear motion. The purpose of an actuator is to regulate the valve position to ensure the accurate control of process or service fluid. For effective function, an actuator has to be powerful enough to have a positive, accurate, and rapid response to the control signal. Thus, it is important to define the right type and size of the actuator so that the process needs, reliability and cost effectiveness can be fulfilled. Actuators are divided into three categories: electrical, hydraulic, and pneumatic each with its advantages and disadvantages.
• One of the important features of an actuator is its performance in case of motion force cut-off. This means that, for example, in case of interruption of the compressed air flow in a pneumatic actuator, in which position does the actuator hold the valve? This feature divides the actuator into four major categories: fail locked, fail as is, fail to close, and fail to open. Each of them explained:

• Fail to open: The actuator returns the valve to the open state when the driving force is cut off.
• Fail to close:  The actuator returns the valve to the closed state when the driving force is cut off.
• Fail as is: When the driving force is cut off, the actuator keeps the valve in the a position just like before force cut off.
• Fail locked: This mode is just like the Fail as is mode, except that in this mode, the position of the valve is maintained in the same position as it was before the driving force cut off. Meanwhile, in the fail as is mode, it is possible that the position of the valve may change slightly due to the force from the flow.

Sometimes the input command is different from the output command of the actuator, for example, the input command is an electric current and the output command is pneumatic. In such cases, a device called a transducer is used to convert the electric current input to pneumatic output.