Control loops on a PFD identify areas where variable must be properly maintained.
Main variables controlled in a process.
T - Temperature
P - Pressure
L - Level
F - Flow rate
Example of s simple control loop
What will be indicated
1.What kind of controller is being used
2.Where the controller is receiving its signals
3.How the controller is controlling the variable (valve,variable power supply)
Here Temperature controller is used. Pneumatic signal is used to give the controller output to final control element.