Fig V.9 : Comparison of performance of two control Structures viz. simple PI control and a cascade control structure

V.2.4 : Few points to remember on Cascade controller:

•  The slave loop should be tuned before the master loop. After the slave loop is tuned and closed, the master loop should be designed based on the dynamics of inner loop.

•  There is little or no advantage to using cascade control if the secondary process is not significantly faster than the primary process dynamics. In particular, if there is long dead time in the secondary process, it is unlikely that the cascade controller will be better than the standard feedback control.

•  The most common cascade control loop involves flow controller (eg. TC/FC example in distillation column) as the inner loop. This loop easily rejects the disturbances in fluid steam pressure, either upstream or downstream of the valve.

The Fig V.10 presents the process and Instrumentation diagram of a distillation column on which a Cascade controller structure has been employed. Only the bottom part of the column has been shown in the figure.

Fig V.10 : Partial P & ID of a distillation column with cascade structure of temperature a flow control.