The level transmitter (LT) senses the feed water level in the evaporator and transmits a corresponding 4-20 ma signal to the unit's PLC-based control system. The PLC feed water level logic (LIC) monitors this input signal and determines the error between the actual level in the evaporator and the desired level. If the actual level is higher than the desired level, an output signal from the PLC will close the feed water control valve to reduce the feed water flowrate to the evaporator. If the actual level is lower than the desired level, an output signal from the PLC will open the feed water control valve to increase the feed water flowrate to the evaporator. 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 feed water 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.