The pressure transmitter (PT) senses the pure steam pressure in the evaporator and transmits a corresponding 4-20 ma signal to the unit's PLC-based control system. The PLC pure steam pressure logic (PIC) monitors this input signal and determines the error between the actual pressure in the evaporator and the desired pressure. If the actual pressure is higher than the desired pressure, an output signal from the PLC will close the plant steam control valve to descrease the plant steam flowrate to the evaporator. If the actual pressure is lower than the desired pressure, an output signal from the PLC will open the plant steam control valve to increase the plant steam 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 (PY), which converts the electrical signal to a corresponding 3-15 psig pneumatic signal. The pneumatic signal is sensed by the plant steam control valve.