Valve actuator is one of the essential part which is required for valve opening and closing in the event valve is required to be operated
automatically by a controller - PLC, DCS, PID controllers.
To my knowledge >90% of all valve actuators in industrial plants are pneumatical or electro-pneumatical, but the actual use still remains
the same. They can be considered a signal converter and PI controller to control the valve position. As they use the mechanical position
feedback in order to control the valve position. They can also be used to adapt the characteristics of a valve. Some valves have a linear position/flow relation, others a logarithmic,quadratic, inverse logarithmic or inverse quadratic relation . The positioner can be used to make sure that at 50% input, 50% of maximum flow is reached instead of 50% of valve position (which could be only 30% or 70% of flow).
A valve actuator is a primover of any kind of valve. They basically come in different types based on the operating power like Pneumatic, Hydraulic and electrical. Now based on your application and the torque reqirement of the
mechanical valve itself a suitable operating type is chosen. A actuator basically allows you to move your valve from a remote or any location to any desired poasition that you or the process may require. Different types of accessories are available to drive the actuators and to make sure that they position to the desired signal, like Valve positioners. The valve positioners have a Closed loop controller using the mechanical movement as the feed back to move the valve till it reaches the desired input signal(Position).
