The relationship between feed-water flow rate and drum level for the process studied in this module is
Assume that the valve transfer function is
the unit of time is minutes, and the input and output have been scaled so that the full range is –1 to 1, with a nominal value of 0. The constraints on the inputs and outputs are then −1 and 1.
Now consider process disturbances. The primary disturbance (header pressure) affects the drum level in the following fashion