The level transmitter (LT) senses the distillate level in the condenser and transmits a corresponding 4-20 ma signal to the unit's PLC-based control system. The PLC distillate level logic (LIC) monitors this input signal and determines the error between the actual level in the condenser and the desired level. If the actual level is higher than the desired level, an output signal from the PLC will open the distillate control valve to increase the distillate flowrate from the condenser. If the actual level is lower than the desired level, an output signal from the PLC will close the distillate control valve to decrease the distillate flowrate from the condenser. The output signal from the PLC will take into consideration the magnitude of the error, the length of time that the error has existed, and the rate at which the error is changing to provide the most accurate control possible.

The 4-20 ma output signal from the PLC is sent to the current to pressure transducer (LY), which converts the electrical signal to a corresponding 3-15 psig pneumatic signal. The pneumatic signal is sensed by the distillate control valve positioner. The positioner also senses the position of the valve stem and maintains a valve position corresponding to the signal from the PLC, regardless of line pressure forces acting on the valve.