Types of Process Control, Feedback control, feed-forward control loops, ratio control loop, split range control. How to read Piping and Instrumentation Diagram for Process Engineers

1. Process Control
Fundamentals
ENG. AHMED DEYAB
ADEYAB@ADEYAB.COM

3. Introduction to Process Control
•Chemical plants are never truly at steady state.
•Feed and environmental disturbances, heat exchanger
fouling, and catalytic degradation continuously upset
the conditions of a smooth-running process
•Process dynamics refer to an unsteady-state or
transient behavior.
Steady-state vs. unsteady-state behavior
i.Steady state: variables do not change with time

5. Control
◦To maintain desired conditions in a
physical system by adjusting selected
variables in the system.

6. What does a control system do?
Imagine you are sitting in a cabin in front of a small fire on a cold
winter evening. You feel uncomfortably cold, so you throw another
log on the fire.
This is an example of a control loop. In the control loop, a variable
(temperature) fell below the setpoint (your comfort level), and you
took action to bring the process back into the desired condition by
adding fuel to the fire. The control loop will now remain static until
the temperature again rises above or falls below your comfort level.

7. Why is Control necessary?
Control is necessary because during its operation, a chemical plant 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).

8. Importance of Process Control for
the Chemical Process Industries
Manufacturers control the production process for three reasons:
Reduce variability
Increase efficiency
Ensure safety
Process Control has a major impact on the profitability of a company in the CPI.

9. Safety and Reliability
The control system must provide
safe operation
Alarms, safety constraint control, start-up and
shutdown.
A control system must be able to
“absorb” a variety of disturbances
and keep the process in a good
operating region:
Thunderstorms, feed composition upsets,
temporary loss of utilities (e.g., steam supply),
day to night variation in the ambient
conditions

10. Safety
The safe operation of a chemical process is a primary
requirement for the well-being of the people in the plant and
for its continued contribution to the economic development.
Thus the operating pressures, temperatures, concentration of
chemicals and so on should always be within allowable limits.

11. Production specifications
A plant should produce the desired amounts and quality of the
final products.
For example, we may require the production of 2 million pounds
of ethylene per day, of 99.5% purity. Therefore, a control system
is needed to ensure that the production level and the purity
specifications are satisfied.

12. Environmental regulations
Various federal and state laws
may specify that the
temperatures, concentrations
of chemicals and flow rates of
the effluents from a plant be
within certain limits.
Such regulations exist for
example on the amounts of
SO2 that a plant can eject to
the atmosphere, and on the
quality of the water returned
to a river or lake.

13. Maximizing the Profit of a Plant
Many times involves controlling against constraints.
The closer that you are able to operate to these constraints, the
more profit you can make. For example, maximizing the product
production rate usually involving controlling the process against
one or more process constraints.

14. Constraint
Control
Example
Consider a reactor temperature control
example for which at excessively high
temperatures the reactor will experience a
temperature runaway and explode.
But the higher the temperature the greater
the product yield.
Therefore, better reactor temperature
control allows safe operation at a higher
reactor temperature and thus more profit.

15. Temperature Control – Heat Exchanger
TT
Condensate
Steam
Feed
TCProduct
Stream
Setpoint

16. How is control done
Control is accomplished through a rational arrangement of
equipment (measuring devices, valves, controllers, computers)
and human intervention (plant designers, plant operators),
which together constitute a control system.

17. THREE
TASKS
Control loops in the process control
industry work in the same way, requiring
three tasks to occur:
Measurement
Comparison
Adjustment

18. Loop Components

19. Important
terms
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.

20. Heat Exchanger Control
Controlled variable - Outlet temperature of product stream
Manipulated variable - Steam flow
Actuator - Control valve on steam line
Sensor - Thermocouple on product stream
Disturbance - Changes in the inlet feed temperature

21. Logic Flow Diagram for a
Feedback Control Loop
Controller Actuator Process
Sensor
CV
Setpoint
Disturbance
+-
uce

22. Feedback vs.
Feedforward
Feedback
Control action after an error exists
Feedforward
Reacting to the disturbance before the error
occurs

23. Feedback Control
The key feature of all feedback control loops is that the
measured value of the controlled variable is compared with
the set point and this difference is used to determine the
control action taken.

24. Feed forward Control:
25
Distinguishing feature: measure a disturbance variable
Advantage:
• Correct for disturbance
before it upsets the
process.
Disadvantage:
• Must be able to measure
the disturbance.
• No corrective action for
unmeasured
disturbances.

25. Manual Feedforward Control

26. Feed forward Model (Boiler Steam Drum)

28. Typical Process Supply and Demand

29. Temperature Control Loop

30. Simple Feedback Loop

31. Feedback Control Loop

32. Level control
The inlet flow comes from
an upstream process, and
may change with time
The level in the tank must
be kept constant in spite of
these changes
Flow in
Flow out

33. Level controller
The level controller (LC) looks at the
level (monitoring)
If the level starts to increase, the LC
sends a signal to the output valve to
vary the output flow (change)
This is the essence of feedback control
LT
LC
SP
Flow in
Flow out

34. Feedback control
It is the most important and widely used control strategy
It is a closed-loop control strategy
Block diagram
process
transmitter
controller
disturbance
comparator manipulated
variable
controlled
variable
+
– errorset-point
y
sp y

35. Back to level control
LT LC
SP
Flow in
Flow out
desired value
(set-point)
transmitter
controllercontrolled
variable
(measurement)
manipulated
variable
disturbance
process

36. Components
of Control
Loops
• Primary element/sensor
• Converter
• Transducer
• Transmitter
• Indicator
• Recorder
• Controller
• Correcting element/final control element
• Actuator

37. TRANSMITTERS
A transmitter is a device that converts a reading from a sensor into a
standard signal and transmits that signal to a monitor or controller.
Transmitter types include:
- Pressure transmitters
- Flow transmitters
- Temperature transmitters
- Level transmitters

38. SIGNALS
There are three kinds of signals that exist for the process
industry to transmit the process variable measurement from
the instrument to a centralized control system.
1. Pneumatic signal
2. Analog signal
3. Digital signal

39. INDICATORS
While most instruments are connected to a control system, operators
sometimes need to check a measurement on the factory floor at the
measurement point. An indictor makes this reading possible.
An indicator is a human-readable device that displays information about
the process. Indicators may be as simple as a pressure or temperature
gauge or more complex, such as a digital read-out device.
Some indicators simply display the measured variable, while others have
control buttons that enable operators to change settings in the field.

40. RECORDERS
A recorder is a device that records the output of a measurement
devices.
Many process manufacturers are required by law to provide a process
history to regulatory agencies, and manufacturers use recorders to help
meet these regulatory requirements.
In addition, manufacturers often use recorders to gather data for trend
analyses.
By recording the readings of critical measurement points and
comparing those readings over time with the results of the process, the
process can be improved.

41. Trend
Display

42. Trend Display

43. CONTROLLERS
A controller is a device that receives data from a
measurement instrument, compares that data to a
programmed setpoint, and, if necessary, signals a control
element to take corrective action.

44. DCS System Consoles

47. Final Control Element
Final control elements are typically used to increase or decrease fluid flow.
For example, a final control element may regulate the flow of fuel to a burner to control temperature, the
flow of a catalyst into a reactor to control a chemical reaction, or the flow of air into a boiler to control boiler
boiler combustion.
In any control loop, the speed with which a final control element reacts to correct a variable that is out of
setpoint is very important.
Many of the technological improvements in final control elements are related to improving their response
time.

48. Actuators
An actuator is the part of a final control device that causes a physical change in the final
control device when signaled to do so.
The most common example of an actuator is a valve actuator, which opens or closes a
valve in response to control signals from a controller.
Actuators are often powered pneumatically, hydraulically, or electrically.
Diaphragms, bellows, springs, gears, hydraulic pilot valves, pistons, or electric motors
are often parts of an actuator system

49. A disadvantage of feedback control
In conventional feedback control the corrective action for
disturbances does not begin until after the controlled variable
deviates from the set point
stack gas
cold oil
hot oil
fuel gas
TC
TT
If either the cold oil flow rate or
the cold oil temperature change,
the controller may do a good job in
keeping the hot oil temperature at
the setpoint
What if the pressure or flow of the fuel gas changes?

50. stack gas
cold oil
hot oil
fuel gas
TC
TT
PC
PT
Cascade control # 1
Two control loops are nested within each other: the master
controller and the slave controller
◦ the output signal of the master (primary) controller serves as the set point of
the slave (secondary) controller
The performance can be
improved because the fuel
control valve will be adjusted
as soon as the change in
supply pressure is detectedslave loop
master loop
set point

51. Simple
Cascade
Control
Loop

52. CASCADE EXAMPLE
• WOULD REQUIRE THAT IT CHANGE ACCORDING TO THE MASTER FEED
• THE EXTENDED LAG TIME WOULD MAKE THIS CONTROL INEFFECTIVE.
USING A FLOW CONTROL TO THE FLASH TANK
• PROVIDES A MORE IMMEDIATE RESPONSE TO CHANGES IN THE REACTOR
FEED
• THE GAIN WOULD NEED TO BE ADJUSTED TO ALLOW FOR THE CAPACITANCE
OF THE SYSTEM.
USING A FLASH FEED BASED ON LEVEL CONTROL

53. Summary
on cascade
control
 It is used to improve the dynamic response
of the process to load disturbances
 It is particularly useful when the
disturbances are associated with the
manipulated variable or when the final
control element exhibits nonlinear behavior
 The disturbances to be rejected must be
within the inner loop
 The inner loop must respond much more
quickly than the outer loop
 Two controllers must be tuned

54. Relative Loop Performance to
Supply Upsets

55. Typical
Split-Range
Control
Loop

57. Multiple use of an End Device

58. SAFETY & ALARM SYSTEMS
OPERATOR ALARMS AND INTERLOCK ALARMS
◦ (LO, LOLO, HI, HIHI)
◦ SHOULD BE ON LOOPS THAT ARE INDEPENDENT FROM
CONTROL LOOPS
RELIEF SYSTEMS NEED TO BE DIRECTED TO FACILITIES TO
SAFELY PROCESS THE RELEASE
SAFETY SYSTEMS SHOULD BE INTERLOCKED TO SHORT-
OR LONG-TERM SHUTDOWN LOOPS, AS APPROPRIATE.

59. Piping and Instrument
Diagram(P&ID)
Contains: plant construction information (piping, process,
instrumentation, and other diagrams)

60. ISA Symbology
The Instrumentation, Systems, and Automation Society (ISA)
is one of the leading process control trade and standards
organizations. The ISA has developed a set of symbols for
use in engineering drawings and designs of control loops

65. First Letter
First letter Parameters controlled
A Analysis
C Conductivity
D Density
E Voltage
F Flow Rate
I Current
L Level
M Moisture(Humidity)
P Pressure or Vacuum
T Temperature
V Viscosity

66. Next Letter
Next letter Control equipment type
A Alarm
C Controller
I Indicator
T Transmitter
V Control Valve
E Element
IC Indicator Controller
FC Ratio Controller
R Recorder
HS Hand Switch
HV Hand Valve
Q Totalizer
IQ Indicating Totalizer
XV Solenoid Valve
Y Calculation
FY Ratio Calculation
SL Switch Low
SH Switch High
AL Alarm Low
ALL Alarm Low Low
AH Alarm High
AHH Alarm High High

67. Legend

68. Abbreviations

70. Valves

71. Valves

72. Valves
"V" - D# - SQ
Where;
•HV or V - A literal and required part of all hand valve tags
•D# - last two digits of P&ID drawing number
•SQ - Sequence Number (01 to 99)
•V0001 - The first hand valve on P&ID D100
•V1205 - The fifth hand valve on P&ID D112

74. Piping &
Connection

75. Piping

76. Lines

77. Lines
1) Usage: For
example, process,
drain, nitrogen,
blowdown, etc.
2) Line Number:
The identification
number of the line
on the plant.
3) Size: Usually in
inches.
4) Piping Class:
The piping
specification, both
material and
pressure rating
5) Insulation Class

78. Lines

79. Lines

80. Instruments

81. Instruments

82. ISA Symbology
In a P&ID, a circle represents individual
measurement instruments, such as
transmitters, sensors, and detectors

83. A single horizontal line running across the center of the shape
indicates that the instrument or function is in a primary location
(e.g., a control room).
A double line indicates that the function is in an auxiliary
location (e.g., an instrument rack).
The absence of a line indicates that the function is field
mounted.
Dotted line indicates that the function or instrument is
inaccessible (e.g., located behind a panel board).

84. Board Mounted

85. Field Mounted

86. Not Accessible

87. A square with a circle inside
represents instruments that
both
- Display measurement
readings and
- Perform some control
function

88. A hexagon represents computer
functions, such as those carried
out by a controller

89. Exercise
2222
2222

91. THANK YOU

92. Process
Continuous Batch

93. Continuous Processes
147

94. Batch Processes
148

95. Important
terms
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
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.