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Proper boiler operation requires that the level of water be maintained within a certain band. A decrease in this level may uncover boiler tubes, allowing them to become overheated. An increase in this level may cause carry-over to the plant and a reduction in the steam space.
The 2704 high performance controller is ideal in applications where accurate control of the boiler level is required. This application uses a control loop configuration, which includes both cascade and feedforward. The main (master) process variable is the level with the secondary (slave)
process variable being the feedwater input flow. The feed-forward signal is derived from the output steam flow.
By using a cascade control configuration to control the level, the control loop will automatically compensate for variations in the upstream pressure of the feedwater supply system
The PV for the master loop is usually derived from a capacitance probe, from which a 4-20mA signal proportional to the measured water level is generated. This signal is fed into the 2704 and
scaled to suit the transmitter range. The Slave PV is derived from a flow transmitter that measures the feedwater-input rate. Differential pressure measurement using the orifice plate measurement principle or a vortex meter usually performs the flow measurement. Again a 4-20mA signal is generated which is proportional to the rate of flow. The control output of the slave loop provides a 4-20mA linear signal that is used to vary the feedwater inlet flow rate.
To compensate for variations in demand an output feedforward signal is applied to the master control output. Normally, a vortex meter measures the steam flow and this signal requires pressure compensation to provide a mass flow signal. Scaling of the feedforward signal is dependent on the particular application, but for example; a 4mA signal could decrease the master output by -10% and a 20mA signal could increase it by +10%.
