Examine this P&ID for a level control system in a vessel where two different fluids (Feed A and Feed B) are mixed together:

Determine the effect on the control system’s regulation of liquid level inside the vessel if an instrument technician accidently mis-configures the controller for the wrong type of action (e.g. direct action when it should be reverse, or vice-versa). Assume all other loop components are properly configured and that the controller is well-tuned.

Answer :

The liquid mixing vessel will either drain empty or overflow, depending on which side of setpoint the process variable was on at the time of the mis-configuration.