The last week of my training, I'd learned about he structure of the control valve. There are 2 types of valves in general: ON/OFF valve and Control valve. The difference between the two is that the ON/OFF valve can only be open or close, while for the control valve, the user can decide how many percent they want the valve to be opened.
The control valve is controlled by the power supply from the range of 4mA-20mA, which represents:
4mA - 0%
8mA - 25%
12mA - 50%
16mA - 75%
20mA - 100%
On a control valve, there were 2 inputs and 2 outputs. The 2 inputs refer to the pressure supply and the power supply. The 2 outputs is main to open and close the valve. 1 output is use to open the valve and another is used to close the valve. Below are the operation of a control valve.
A constant pressure inject to the positioner of the valve. The positioner will refer to the power supply (control variables) to it. If the supply is 4mA, then the positioner will gives an instruction and transfer the pressure into the output to close the valve. On the other hand, if the power supply received is 12mA, which gives an instruction to open the valve 50%, then the positioner will inject the pressure into the output where used to open the valve and stop opening the valve when it reach 50%.