Process Control

AbstractThis article presents the theory and practice of automatic process control and instrumentation, outlining basic concepts of automatic control such as controlled variables, manipulated variables, and control strategies. A functional model of the process control hierarchy is described, as is the role played by modern (digital) control systems equipment. Because reliable process measurements form the foundation for automatic process control, the devices used to measure the basic process variables of flow, level, pressure, and temperature, as well as the practical guidelines for their selections and implementation, are discussed. On‐line process analyzers are also introduced because of their increasing use in process facilities for closed‐loop automatic control. A brief discussion of considerations in the selection and use of control valves is also included.Conceptual basis and practical aspects associated with both classical and modern (digital) control strategies are discussed. Both continuous and batch processes are considered. The analogue and digital implementations of the classical proportional–integral–derivative controller, cascade control as a technique to improve the overall control performance of a feedback control loop with multiple time constants, and various alternative schemes are presented, including guidelines for selection and design. Ration control is discussed as a form of a multiplicative feedforward control strategy. The use of a selective control strategy to prioritize control actions and to implement overrides and constraint control applications is highlighted. As the use of process integration and the availability of digital‐based control systems grow, more sophisticated control strategies are increasingly employed in process plants. Consequently, important considerations in the design and operation of multivariable control, model‐based predictive control, adaptive control, constraint and optimizing control, and batch sequence control strategies are presented. The economics associated with process instrumentation and control systems is discussed from a benefit–cost standpoint.