Ankur Neog - Process Variables and Process Control

1. Process Control and
Instrumentation
PROCESS CONTROL AND PROCESS VARIABLES

2. Process
A process denotes an operation or series of operations
on fluid or solid materials during which the materials are
placed in a more useful state

3. Objectives of process
The objective of a process is to convert certain raw
materials (input feedstock) into desired products (output)
using available sources of energy in the most
economical way.
Unit process may involve either a change of chemical
state or a change in physical state.
Many external and internal conditions affect the
performance of a process.
These conditions may be expressed in terms of process
variables such as temperature, pressure, flow, liquid
level, dimension, weight, volume etc.

4. Requirements of Process
A process must satisfy several requirements imposed by
its designers and the general technical, economic and
social conditions in the presence of ever changing
external influences (disturbances)

5. The Requirements
Safety of men and machine
Environmental regulations
Production specifications
 Operational constructions and economics

6. Control
The term control means methods to force parameters in
the environment to have specific values

7. Process Control for the 90’s Kids

8. Process Control for 2000’s kids

11. Process Control in old times

12. Process Control in Recent Times

13. What if Process Control fails?

14. Process variables
Control Variable
Set point
Steady state
Measured variable
Error
Manipulated variable
Load variable

15. Process variables
Controlled variable: it is the variable that needs to be maintained or controlled at some desired
value or range. Sometimes also referred to as process variable.
Set Point: it is the desired value of the controlled variable. Thus the job of a control system is to
maintain the controlled variable at its set point.
Manipulated variable: is the variable used to maintain the controlled variable at its set point.
Disturbance: any variable that causes the controlled variable to deviate from its set point. Also
referred to as upset.

16. Process Variables

17. Causes for deviation

18. Types of feedback system
Negative feedback system
When the measured variable is subtracted from the set point it is called as negative feedback
system
Positive feedback system
When the measured variable is added from the set point it is called as positive feedback
system

19. Positive Feedback loop

20. Negative Feedback loop

21. Reasons for positive feedback to stop
When the output reaches the saturation
Till the system breakdowns

22. Advantages of feedback system
Automatically reduce errors
Improve stability
Increase robustness against external disturbances
Reliable and repeatable

23. Disadvantages of feedback system
Action of the controller is post-facto