The adjustment of process variables such as pressure, flow rate, level, and temperature in a process plant at an intended or desired value. Processes are designed to operate safely and in a designed and intended manner to achieve product quality, energy efficiency, and waste minimization according to flowsheet conditions. However, all processes are dynamic and subject to disturbances such that they tend to deviate from the designed conditions. They therefore need to be controlled and returned back to the desired conditions of operation.

Processes may be controlled automatically or manually. Manual operation involves process operators who make adjustments to temperatures, pressures, flow, and levels. Automatic control is used to control complex processes, or for remote and hazardous processes, or where a high level of product quality is required. Computers are routinely used to operate and control processes.

A control system consists of several components including the process itself, a controller, and a valve. A good understanding is required to control the process. However, this may not always be possible since many processes are multivariable or non-linear in behaviour, or not well understood such as in the case of biological processes.

Feedback control and feedforward control are two forms of automatic process control. The former involves comparing a controlled signal with the desired value of a process variable and automatically making adjustments to minimize the difference. Feedforward control involves controlling a process in which the disturbance is detected before it enters the system. The controller calculates the required counteracting disturbance. Process disturbances are measured and compensated for without waiting for a change in the controlled variable to indicate that a disturbance has occurred. It is useful where the final controlled variable cannot be measured.