A combined digital-analog mathematical model for the dynamic analysis of vertical U-tube natural-circulation steam generators is presented. The application of this model to the optimal design of a water-level controller for a steam generating unit is demonstrated. It is shown that a control system consisting of standard proportional and reset controls on water-level deviation from a desired set point and the difference between the steam and feedwater mass flow rates can be successfully employed for the control of water level in such a plant. The optimum values, as well as the range of the controller parameter sellings for which the steam generator exhibits a desired stable response, are determined.

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