The pressure of the pure steam produced by the MECO pure steam generator is maintained with a PID control loop. The proper pure steam pressure is maintained in the evaporator under all conditions. The PID control loop senses the actual pressure and adjusts the flow rate of plant steam up or down to maintain the pressure at the desired point.

Integrated into the generator, this system provides precise pressure control of the pure steam over the full range of pure steam demand. All loop parameters can be viewed from the unit's operator interface.

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 decrease 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.