IV.1.1 Process

Any equipment that serves the targeted physical/chemical operation of the plant is termed as a process. Reactors, separators, exchangers, pressure vessels, tanks, etc. are examples of a process. Typically these processes are connected in a logical fashion and the output of one process becomes input to the other. Any disturbance/malfunction of one process may affect other processes in the downstream side (and upstream too, in case recycle streams are used). Detailed discussions on these processes are not within the scope of this course, however, the modeling techniques and related issues have already been discussed before. Process variables are primarily pressure, temperature, flow rate, level, composition, etc . From the process control perspective, it is crucial to study how the changes in one process variable affect the other, so that an educated measure of control action on one variable can be taken in order to maintain the other.

IV.1.2 Measuring Instruments or Sensors

The success of any feedback control operation depends largely on accurate measurement of process variables through appropriate sensors. There are a large number of commercial sensors available in the market. They differ in their measuring principle(s) and/or their construction characteristics. Module VII accounts of a few of such measuring instruments. Details of such devices may be found in technical booklets dedicated for those individual items.

IV.1.3 Controllers

A controller is basically a mathematical function block that reads the error between desired setpoint and the measured output and then computes the corrective action for the manipulated input that would steer process towards the desired setpoint. There are three basic types of feedback controllers which are widely used in the industry.

•  Proportional (P) controller

•  Proportional Integral (PI) controller

•  Proportional Integral Derivative (PID) controller

Let us study each one separately.