The Drum Waterlevel in the Multivariable Control System of a Steam Generator

The mutual effects on drum waterlevel and steam pressure due to sudden load changes of a boiler are to be studied by means of a suitable block diagram showing the steam pressure and waterlevel control loops and its mutual influence. Concerning normal load changes (¿N<20 percent No), these interactions are mostly neglected, and the dynamic characteristics of both control loops are considered autonomous. However, if there appear sudden load changes (¿N>20 percent No)¿for instance, heating changes in waste-heat boilers, load steps at industry boilers or at boiler-turbine units, operating on a separate electrical grid¿the mutual effects of the variables of both control loops are to be taken into account, too. The effects of those load changes on the time behavior of changes of both the waterlevel and the boiler pressure are to be investigated. The drop in pressure following the operation of the safety valves¿for instance, if the value of the exhausted steam flow is not measurable and thus cannot be used as a feed-forward signal for the feedwater controller¿and in consequence the amendments of the control diagram are to be taken into consideration.