How does control valve work? 

how-control-valve-work

Control valves regulate and control the flow, pressure, the temperature of a flowing fluid, such as gas, steam, water, or the chemical mixture by opening or closing internal passages. They control the amount of flow in processes. The valve stem moves, altering the size of passage, and this increases, decrease or holds steady flow.

What is control valve positioner ?

rotary-positioner-bend-bracket

Valve positioners are the devices that are used to put a valve in the right positioner by increasing and decreasing the air load pressure on the actuator. The valve positioner senses the exact position of the valve and communicates to the actuator to drive the valve.