This document provides an introduction to process control. It defines a process as an operation that transforms raw materials into a more useful state. The objectives of process control are to produce desired outputs from inputs in the most economical way. Processes can be described by differential equations and are affected by various internal and external conditions. Effective process control requires maintaining safety, meeting production specifications, and optimizing economics while addressing changing external influences. Examples of processes include unit operations in chemical plants and manufacturing units. The document outlines the basic components of a process control system and loop.

1. Introduction to process
Control
9AEI-406.1 1

2. 9AEI-406.1 2
Define the 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. 9AEI-406.1 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.
• A process can be described either by an ordinary
differential equation ( lumped parameter system) or by
partial differential equation (distributed parameter
system).

4. 9AEI-406.1 4
• 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.

5. 9AEI-406.1 5
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).

6. 9AEI-406.1 6
The Requirements
• Safety of men and machine
• Environmental regulations
• Production specifications
• Operational constructions and economics

7. 9AEI-406.1 7
Examples of Process
By process it is meant either
• Unit process like an alkylation reactor
• Unit operation like evaporator, distillation column or
storage vessel.

8. 9AEI-406.1 8
Control
• The term control means methods to force parameters in
the environment to have specific values.

9. 9AEI-406.1 9
Example
• This can be as simple as making the temperature in a
room stay at 25 Degree Celsius or
• As complex as manufacturing an integrated circuit or
guiding a spacecraft to Jupiter.
• In general, all of the elements necessary to accomplish
the control objective are described by the term control
system.

10. 9AEI-406.1 10
Process Control
• The process may be controlled by measuring a variable
representing the desired state of the product and
automatically adjusting one of the other variables of the
process.
• In process control, the basic objective is to regulate the
value of some quantity.
• To regulate means to maintain that quantity at some
desired value (reference value or set point) regardless of
external influences.

11. 9AEI-406.2 11
Development of Process Control
• During the first industrial revolution the work done by
human muscle was gradually replaced by the power of
machine.
• During second industrial revaluation the routine functions
of human mind and need for continuous presence of
human observer was also taken care by Machine
Process.
• Control made optimization and there by beginning of
third industrial revolution.

12. 9AEI-406.2 12
Goal of third industrial revolution
Maximum quantity with
• Maximum quality
• Durability of product
Minimum consumption of
• Energy and raw materials
• Maximum recycling and reuse

13. 9AEI-406.2 13
Advantages of process control
• Increase the productivity
• Improve the quality
• Improve the consistence product dimension
• Increase stability
• Optimize the performance
• Meet the environmental regulations
• Minimize the influence of External Disturbance

14. 9AEI-406.2 14
Applications
• Process industries
• Goods manufacturing
• Transport system
• Power machines

15. 9AEI-406.2 15
Process Industries
• Petroleum
• Chemical
• Steel
• Power
• Food

16. 9AEI-406.2 16
Goods manufacturing
• Automobile parts
• Refrigerators
• Electronic equipments like T.V and Radio

17. 9AEI-406.2 17
Transport system
• Railways
• Airplanes
• Free missiles
• Ships

18. 9AEI-406.2 18
Power Machines
• Machine Tools
• Compressors and Pumps
• Prime movers
• Electrical power – Supply Units

19. 9AEI-406.3 19
• Process Control System is the
arrangement of Physical Components and
devices connected to regulate the
Quantity of interest at some desired value
regardless of external Influences.
Definition of Process Control

20. 9AEI-406.3 20
C F.C.E P
F.B
+
-
S.P
C.V
e m
Block Diagram of Process Control Loop

21. 9AEI-406.3 21
General Control Loop Block Diagram

22. 9AEI-406.3 22
Variables
S.P = Set Point or Reference Point
C.V = Control variable
e = error (Difference of S.P & C.V)
m = Manipulated Variable
Terms of Process Control Loop

23. 9AEI-406.3 23
Hardware Elements
C = Controller
F.C.E = Final Control Element
F.B = Feed Back Device
P = Process

24. 9AEI-406.3 24
Process
• The material equipments along with the
physical or chemical operation which
take place.
Examples
• Tanks
• Heat exchangers

25. 9AEI-406.3 25
Measuring system
• Transducer : Many measurements cannot
be used for control
until they are converted to voltage or
current etc. Transducers are used for this
purpose.
Example:
• R.T.D,

26. 9AEI-406.3 26
Controller
This is Hardware element that has intelligence. It receive
the information from the measuring device and decides
what action should be taken and error detector is a part
of the controller.
Example: Thermostat

27. 9AEI-406.3 27
Converts the control signal that is manipulated variable
into physical action on the process.
Example: Control Valve
Final Control Element

28. 9AEI-406.3 28
Transmission lines
• Used carry the measurement signal from
the sensor to the controller and control
signal from the control to the final control
element
These lines are
• Electrical
• Pneumatic

29. 9AEI-406.3 29
Example for the Process Control System
Human Body is the best Example :
standards of Human Body.
• Human Body standard temperature
98.4o
F or 37o
C.
• Blood Pressure = 120/80 mmHg
• Glucose level on fasting 100 -125mg

30. 9AEI-406.3 30
• Designer of the Human Body is the “GOD”.
• The above standards are fixed by the designer.
• Any deviation of the above standards due to external or
internal disturbances results the system out of control.
• Now it needs corrective action to bring back the standards to
the normal values to make the system under control.

31. 9AEI-406.3 31
Home Heating System
Example of Process Control System

32. 9AEI-406.3 32
Home Heating Control System
• The quantity to be controlled that is Process Variable i.e.,
temperature.
• The desired value is called Set point or Reference point
=71o
F.
• The present or current value of process variable measured
by feed back device(transducer) is called Controlled
variable = 69o
F.

33. 9AEI-406.3 33
• The difference of set point and controlled variable is called
error. e = 71-69 = 2o
F
• The variable which is selected for adjustment by controller to
maintain the controlled variable at desired values is called
manipulated variable. In home heating system the
manipulated variable is fuel.

34. 9AEI-406.4 34
Batch Process
• A process in which the materials or work are stationary
at one physical location while being treated is termed a
‘Batch Process’.

35. 9AEI-406.4 35
Example of batch process
• Batch processes are most often of the thermal type
where materials are placed in a vessel or furnace
• System is controlled for a cycle of temperatures under
controlled pressure for a period of time.

36. 9AEI-406.4 36
• Batch or Hood annealing of steel rolled coils
• Steel melting in Bessemer converters
• Coke making in coke ovens
• Furnaces in foundries, batch reactors in chemical plants
etc are some of the familiar batch processes.
• Idles making in kitchen is one of the simplest example of
batch process.

37. 9AEI-406.4 37
Batch continuous process

38. 9AEI-406.4 38

39. 9AEI-406.4 39
Characteristics of batch process
• Batch processes are nearly always defined by
temperature, pressure, or associated conditions such
as composition.
• The degrees of freedom are usually well defined.

40. 9AEI-406.4 40
• The purposes of such processes is to produce one or
more products at
(a) a given composition,
(b) a maximum amount,
(c) best economy (employing least materials, energy,
and time).
• In short production rate (quantity), quality and economics
are all to be taken care of.

41. 9AEI-406.4 41
Characteristics of batch process
• The product composition desired is that at the end of the
processing period and thus cannot be measured during
the process.
• Maximum production and best economy result when the
variables of the process are properly manipulated.
• A process computer may be used to insure a relationship
among variables providing best operation.

42. 9AEI-406.4 42
Continuous Process
• A process in which the materials or work flows more or
less continuously through a plant apparatus while being
manufactured or treated is termed a ‘continuous
process’.

43. 9AEI-406.4 43
Examples of continuous processes
• Heating and rolling of steel ingots or billets,
• Production of sinter,
• Continuous annealing of metal sheets,
• Production of steam and hence power,
• Continuously stirred tank reactors (CSTR) etc are some
of the familiar continuous processes.

44. 9AEI-406.4 44
Continuously stirred tank reactors (CSTR)

45. 9AEI-406.4 45
Characteristics of batch process
• Continuous processes possesses a number of degrees
of freedom given by the number of variables and defining
relations for the system.
• These variables are generally the temperature, pressure,
flow rate, and composition of each of the entering and
leaving materials.

46. 9AEI-406.4 46
• Usually the purpose of the process is to produce one or
more product at
(a) a given composition,
(b) a given or maximum flow rate,
(c) best economy (employing least materials, energy,
personnel time, and equipment).

47. 9AEI-406.4 47
Characteristics of batch process
• Product composition is best insured by measuring produce
composition and controlling it by manipulating one of the
degrees of freedom of the process fixed product flow rate
usually requires flow controllers at several points of entering
and leaving materials.
• Best economy is accomplished by maintaining all process
variables in a predetermined relation such that the highest
efficiency, least waste or some other criteria are satisfied.

48. 9AEI 406.5 to 6 48
Controlled Variable
• Controlled variable is the one that must be
maintained precisely as the set point.
Typically the variable choosing to represent
the state of the system is termed the
controlled variable.
(or)

49. 9AEI 406.5 to 6 49
Manipulated Variable
• Manipulated variable is the one that can be
changed in order to maintain the controlled
variable at the set point.
• In other words the variable chosen the
control system state is termed as
manipulated variable.

50. 9AEI 406.5 to 6 50

51. 9AEI 406.5 to 6 51

52. 9AEI 406.5 to 6 52
Set Point Variable
• Set point variable is the one that is set by
operator, master controller or computer
as a desired value for a controlled
variable.
• It is also called reference value.

53. 9AEI 406.5 to 6 53
Load variable
• Load variables are those variables that
cause disturbances in the process.
• They are also called as load
disturbance.
• The load variable may change either

54. 9AEI 406.5 to 6 54
Examples of the controlled variable
a) Temperature
b) Pressure
c) Flow rate
d) Level
e) Displacement
f) Concentraction
g) Density
h) Viscosity
i) PH
j) Humidity

55. 9AEI 406.5 to 6 55
Examples of manipulated variable
• Coolant flow
• Fuel flow
• Feed water flow
• Steam flow

56. 9AEI 406.5 to 6 56
Liquid level control system
m
cv
Fig. 1

57. 9AEI 406.5 to 6 57
Liquid level control system as shown in
figure 1.
• The process variable to be controlled is
liquid level.
• H is the set point that is desired liquid
level.

58. 9AEI 406.5 to 6 58
Hardware Elements
• Transducer : Plastic float
incorporated with
Potentiometer
• Controller : Electronic controller

59. 9AEI 406.5 to 6 59
Controlling mechanism
• If controlled variable is larger than the set
point, controller opens the valve a little to
let increase the flow out and thus level
(controlled variable) lowers towards the
set point.
• If the controlled variable is smaller than

60. 9AEI 406.5 to 6 60
Liquid Temperature control of the system
Fig. 2

61. 9AEI 406.5 to 6 61
Liquid Temperature control system as
shown in figure 2.
• The process variable to be controlled is
temperature of the liquid in the tank.
• TL is the controlled variable.
• Qs is manipulated variable.

62. 9AEI 406.5 to 6 62
• Qs = steam flow rate
• TA = ambient temperature
• TO = inlet fluid temperature
• TS = steam temperature

63. 9AEI 406.5 to 6 63
Hardware Elements
• Transducer : Thermocouple
• Controller : Computer
• Final Control Element : Control
valve

64. 9AEI 406.5 to 6 64
Controlling mechanism
• This process control loop regulates the
dynamic variable that is temperature of the
liquid.
• The controlled variable TL is the liquid
temperature may depend on many other
parameters and thus suffer changes from
many different sources.

65. AEI-406.7 65
Control System Evaluation
• A process control system is used to
regulate the value of some process
variable.
• The question of how well the control
system is working is answered by
• Assuring stability.

66. AEI-406.7 66
Ensure the Stability of a
Process
Stability
Fig : 1

67. AEI-406.7 67
• Consider the behavior of the variable x
shown in Figure1
• At time t = t0 the constant value of x is
disturbed by some external factors, but that
has time progresses the value of x returns
to its initial value and stays there.
• If x is a process variable such as
temperature, pressure, concentration, flow

68. AEI-406.7 68
Fig : 2

69. AEI-406.7 69
• In contrast the behavior described in figure
1, the variable y in figure 2 does not return
to its initial value after it is disturbed by
external influences.
• Processes whose variables follow the
pattern indicated by y in a figure 2 (curves

70. AEI-406.7 70
• In general, the term tuning is used to indicate
how a process control loop is existed to
provide the best control.
• Example :
• The explosion of a hydro carbon fuel with air is

71. AEI-406.7 71
Evaluating the response to set point changes and transient
effects against certain standard criteria.
Fig : 3 (a)
A system with over
damped response will
react to change in set
point or to a transient
as shown in (a) & (b).

72. AEI-406.7 72
Fig : (b)

73. AEI-406.7 73
• One type of criteria requires that the
controlled variable exhibit a response such
as that shown in Figures 3 (a & b) for
excitations of both set point changes and
transient effects.
• Note that the error is of only one polarity
(i.e., it never oscillates about the set point.
• The measures of quality are the tD of the
excursion and for the transient, the
maximum error emax for a given input.

74. AEI-406.7 74
Cyclic response
Fig : 4 (a)
A system with under
damped response will
react to change in set
point or to a transient
with oscillations as
shown in (a) & (b).

75. AEI-406.7 75
Cyclic response
Fig : (b)

76. AEI-406.7 76
• An other type of criteria applies to those
cases in which the response to a set point
change or transient is as shown in figure
4.
• The controlled variable oscillated about
the set point.
• Parameters of interest are maximum error,
eMAX and theduration tD also called settling

77. AEI-406.7 77
Two criteria for judging the quality of control
systems.
Fig : 5 (a)
Two criteria for judging
the quality of control
systems response are
the minimum area and
quarter amplitude.
Figure (a) & (b).

78. AEI-406.7 78
Fig : (b)

79. AEI-406.7 79
• The nature of the response is modified by
adjusting the control loop parameters,
which is called tuning.
• There may be a large maximum error but
short duration or long duration with small
maximum error and every thing in
between.
• A number of standard cycling criteria are
used.

80. AEI-406.7 80
• Figure 5 shows the area as a shaded part
of the curve., for the same degree of
excitation.
• A - ∫l e(t) l dt = minimum
• The quarter – amplitude criteria shown in
figure 5(b), specifies the amplitude of each

81. 9AEI 406.8 81
The Process Characterstics
The following are the Characterstics of a process
•Process equation
•Process load
•Process lag
•Self regulation

82. 9AEI 406.8 82
Process equation
Process equation can be explained by considering an
example of Control of liquid temperature in a tank as
shown in figure (1) In this example the liquid temperature
( TL) is the controlled variable

83. 9AEI 406.8 83
Controlling of temperature of the liquid
Fig.1

84. 9AEI 406.8 84
• The ( TL ) depends upon the following
• Ambient temperature ( TA )
• Inlet fluid temperature ( T0 )
• Steam temperature ( TS )
• Input flow rate ( QA )
• Out put flow rate ( Q B )
• Steam flow rate ( QS )

85. 9AEI 406.8 85
• If any one of the above parameter changes a change in
temperature.
• To bring the temperature back to the set point value the
steam flow rate is changed, That is heat input to the
process.
• The process is described by the process equation
• The liquid temperature is TL is a function
TL = F ( QA, QB, QS, TA,TS, TO )

86. 9AEI 406.8 86
Where
QA, Q B = Flow rates in pipe A and B
QS = Steam flow rate
TA = Ambient temperature
T0 = Inlet fluid temperature
TS = Steam temperature
The control loop adjust QS and there by regulate
TL regardless of how the other parameters in
equation.

87. 9AEI 406.8 87
Process load
• It is possible to identify a set of values for the process
parameters that result in controlled variable having the
set point values.
• The process load refers to this set of parameters
excluding the controlled variable.

88. 9AEI 406.8 88
• When all parameters have their nominal value, and is
called nominal load on the system.
• The parameter change from nominal value then than
process load change has occurred.
• The controlling variable is adjusted to compensate for
this load change and its effect on the dynamic variable to
bring it back to the set point.

89. 9AEI 406.8 89
Process lag
• A process load change or transient cause a change
controlled variable.
• A process loop respond to assure some finite time later
that the variable return to the set point value.
• Part of this time consumed by the process it self's and is
called process lag.
(or)
• A delay in reaction of the controller variable to a change
of load variable.

90. 9AEI 406.8 90
Example

91. 9AEI 406.8 91
• Inlet flow is suddenly doubled
• A large process load change radically change (reduce) the
liquid temperature
• The control loop respond by opening the steam inlet valve
to allow more steam
• And heat in put to bring the liquid temperature back to the
set point
• The loop itself react faster than the process
• The physical opening and of the control valve is the slowest
part of the loop

92. 9AEI 406.8 92
• Once steam flowing at the new rate the body of the liquid
must be heated by the steam before the set point value
reached again.
• This time delay or process lag is the heating function of
process and not the control system.
• No advantages in design of control system many times
faster than the process lag.

93. 9AEI 406.8 93
The tendency to adopt some value to controlled
variable for nominal load with no control action.
Self Regulation