18.5. Common Control Strategies Used in Chemical Processes

There are many strategies used to control process variables in an operating plant. For more details, see Anderson [1] and Shinskey [2]. In this section, feedback, feed-forward, cascade, ratio, and split-range systems are considered.

18.5.1. Feedback Control and Regulation

The process variable to be controlled is measured and compared with its desired value (set-point value). If the process variable is not at the set-point value, then appropriate control action is taken.

Advantage. The cause of the change in the output variable need not be identified for corrective action to be taken. Corrections continue until the set-point value is achieved.

Disadvantage. No action is taken until after an error has propagated through the process and the error in the process variable has been measured. If there are large process lag times, then significant control problems may occur.