In a process plant, the general function of a control valve is to restrict the opening of the valve so it affects the flow or pressure of the liquid or gas that is passing through it.

In any given application, an installed valve, has one fundamental variable – the position of the moving element, which could be a profiled ball, plug, or sleeve in the valve. That single moving element determines the exposed orifice that allows greater or lesser flow through the valve, which in turn provides the control of the process.

The valve itself may be extremely sophisticated with exotic body and seat material, or it may have complex flow patterns that allow for a high pressure drop or some other complex feature. However, the fundamental requirement to move the valve stem to position the control element remains the same regardless of whether it’s a simple or a sophisticated valve.

A control valve actuator is used to move the valve stem (which is attached to the internal control element) to the desired position and hold it in place. In addition to the act of moving and holding positions, there are many other parameters to that movement which determine the best type of actuator that should be used for every specific application. For example, other important considerations might include speed, repeatability, resolution, and stiffness.

Read more @ Valve-World