1. Welcome to Instrumentation, Process Control
and Process Instrumentation & Diagram
By: Dr. Zin Eddine Dadach
Chemical Engineering department
ADMC
1
2. The first part of this course introduces the students
to the basics of electrical circuit theory followed by
the latest process instrumentation technology and
selection criteria.
This section explains the measurement of common
process variables such as temperature, pressure,
level and flow and describe their corresponding
sensors.
A lab experiment on calibrating a manometer.
PART I: Instrumentation
2
3. Define and explain the
various circuit
components and describe
the basic of electronic
theory.
L.O. #1
3
4. SENSORS, TRANSMITTERS AND CONVERTERS ARE
ELECTRICAL AND ELECTRONIC DEVICES THAT
TRANSFORM PHYSICAL PROPERTIES (PRESSURE,
PRESSURE DROP, DISPLACEMENT, HEAT..) INTO
ELECTRICAL CURRENT
IT IS THEREFORE NECESSARY TO STUDY SOME
BASIC THEORIES OF ELECTRICITY
4
THE NEED OF ELECTRONIC IN
INSTRUMENTATION
6. WHAT IS CURRENT?
Electrical current is the movement of charged particles in a specific
direction
The charged particle could be an electron ,a positive ion or a negative
ion
The charged particle is often referred to as a current carrier
In a solid, the current carrier is the electron
The symbol for current is I
6
7. An ammeter is a measuring instrument used
to measure the flow of electric current in a
circuit.
Electric currents are measured in amperes,
hence the name.
The word "ammeter" is commonly misspelled
or mispronounced as "ampmeter" by some.
7
AMMETER?
8. More modern ammeters are digital, and use
an analog to digital converter to measure the
voltage across the shunt resistor.
The current is read by a microcomputer that
performs the calculations to display the
current through the resistor.
8
MODERN AMMETERS
9. Direct current (DC) is the unidirectional
flow of electric charge. Direct current is
produced by sources such as batteries,
thermocouples, solar cells, and electric
machines of the dynamo type.
DC: Direct Current
9
10. In alternating current (AC), the movement of
electric charge periodically reverses direction.
While in direct current (DC), the flow of electric
charge is only in one direction.
Alternating Current
10
13. Voltage is the electric pressure OR
POTENTIAL that causes current to flow.
Voltage is also known as electromotive
force or emf or potential difference.
If there is no potential difference (V=0),
there will be no current (I=0)
13
VOLTAGE =POTENTIAL
14. We need a unit to indicate the potential energy
between two points such as terminals of a battery.
This unit must specify the energy available ( JOULE is
unit for energy) when a charge ( COULOMB is unit for
charge) is transported.
The unit of voltage is : Volt= joule/coulomb
14
UNIT OF VOLTAGE?
15. The moving coil galvanometer is one example of this
type of voltmeter. It employs a small coil of fine wire
suspended in a strong magnetic field.
When an electrical current is applied, the
galvanometer's indicator rotates and compresses a
small spring.
The angular rotation is proportional to the current
that is flowing through the coil
15
OHMMETER OR GALVANOMETER
17. The opposition a material offers to electrical current
is called resistance
All materials offer some resistance to current
Resistance converts electric energy into heat
The symbol for resistance is R
The unit of resistance is the Ohm (Ω)
17
WHAT IS ELECTRICAL RESISTANCE?
18. Conductance refers to the ability to conduct current.
It is symbolized by letter G
The base unit for conductance is the siemens or S
CONDUCTANCE IS THE EXACT OPPOSITE OF
RESISTANCE
R=1/G or G=1/R
18
WHAT IS CONDUCTANCE?
19. Materials with a big resistance are : INSULATORS
or RESISTORS
Examples of insulators : paper, wood, plastics,
rubber, glass and mica
Materials with a small resistance are :
CONDUCTORS
Examples of conductors : Copper, aluminum,
silver
19
CLASSIFICATION OF MATERIALS
20. OHM’LAW: The relationship between current (I),
voltage ( V) and resistance (R) was discovered by the
german Georg OHM
I= V/R
20
OHM‘S LAW?
21. How much current ( I) flows in a circuit where the
voltage is 2.8 V and there is a resistance of 1.4 Ω in the
circuit?
21
CLASS WORK #1:
22. How much voltage is required to cause 1.6 amperes in a device that
has 30 ohms of resistance?
The current flowing through a 10 kΩ resistor is 35mA. What is the
potential energy difference ( voltage) across the resistor?
A lamp has a resistance of 96 ohms . How much current flows
through the lamp when it is connected to 120 volts?
A manufacturer specifies that a certain lamp will allow 0.8 ampere of
current when 120 volts is applied to it. What is the resistance of the
lamp?
22
Home work #1
24. A great majority of electrical circuits operate
more than one load. Circuits which contain
two or more loads are called multiple-load
circuits.
A multiple-load circuit can be a series circuit, a
parallel circuit or a series-parallel circuit
24
INTRODUCTION
25. A series circuit is the simplest circuit.
The conductors, control and protection devices,
loads, and power source are connected with only one
path to ground for current flow.
The resistance of each device can be different.
The same amount of current will flow through each.
The voltage across each will be different.
If the path is broken, no current flows and no part of
the circuit works
25
A SERIES CIRCUIT
30. A parallel circuit has more than one path for current
flow.
The same voltage is applied across each branch.
If the load resistance in each branch is the same, the
current in each branch will be the same.
If the load resistance in each branch is different, the
current in each branch will be different.
If one branch is broken, current will continue flowing
to the other branches
30
PARALLEL CIRCUIT
34. 1) THREE RESISTANCES ( 35, 70 AND 45 OHMS) IN
SERIES WITH A VOLTAGE SOURCE OF 90V,
CALCULATE : IT,RT,VR1,VR2,VR3
2) WHAT IS THE TOTAL RESISTANCE OF A SERIE OF TWO
RESISTORS 20, 30 OHMS IN PARALLEL WITH A SECOND
SERIE OF RESISTORS 70, 80 OHMS?
LISTEN..LEARN..THINK..ENJOY YOURSELF 34
HOME WORK
35. Electrical energy is undoubtedly the primary source of energy
consumption in any modern household.
Most electrical energy is supplied by commercial power
generation plants like Tawillah
The most common power generation plants are fueled by : Fuel
gas or fuel oil
35
WHAT IS ELECTRICAL ENERGY?
36. When a current flows in a circuit with resistance, it does work.
Devices can be made that convert this work into heat (electric
heaters), light (light bulbs and neon lamps), or motion (electric
motors)
P=W/t
P is the power and the unit is watt,
W is energy in joules and t time in seconds
1 Watt = 1Joule/second.
36
GENERAL DEFINITION OF POWER (P)
37. Electric power, like mechanical power, is represented
by the letter P in electrical equations, and is measured
in units called watts (symbol W).
P = I .V
where
P = power in watts
I = current in amperes
V = potential difference in volts37
ELECTRICAL POWER
38. Joule's law can be combined with Ohm's law to produce
two more equations:
P= I2.R
and
P=V2/R
where
R = resistance in ohms.
For example:
(2 amperes)2 × 6 ohms = 24 watts
and
(12 volts)2 / 6 ohms = 24 watts
38
ELECTRICAL POWER: OTHER FORMULAS
39. What is the power input to an electrical heater
that draws 3 amperes from 120 volt outlet?
Find the power used by a resistor of 10 ohms
when a voltage of 1.5 v is applied
39
CLASS WORK #2:
40. How much power is dissipated when 0.2 ampere of
current flows through a 100 ohms resistor?
How much energy is taken from the battery by the
resistor ( 10 ohms) if the voltage is 1.5 V and the
switch is closed for 30 min?
What is the cost of operating a 100 watt lamp for 3
hours if the rate is 6 cents per kWh?
An electrical iron operates from 120 volts outlet and
draws 8 amperes of current. At 9 cents per kWh , how
much does it cost to operate the iron for 2 hours
40
HOMEWORK #2:
43. INSTRUMENTATION USE SENSORS LIKE
THERMOCOUPLES, PRESSURE AND
FLOW SENSORS TO MEASURE THE
DIFFERENT PARAMETERS IN THE PLANT.
INFORMATION IS SENT TO THE
CONTROLLER ( IN THE CONTROL ROOM)
TO TAKE APPROPRIATE ACTIONS.
.
43
DEFINITION OF INSTRUMENTATION
44. Measurements have got to be one of the most
important equipment in any processing plant.
Since successful process control requires
appropriate instrumentation, engineers
should understand the principles of common
instruments.
44
GOOD INFORMATION=GOOD CONTROL
45. Like human body uses nerves, Sensors are used for
process monitoring and for process control.
Sensors are essential elements of safe and profitable
plant operation.
This can be achieved only if the proper sensors are
selected and installed in the correct locations.
While sensors differ greatly in their physical
principles, their selection can be guided by the
analysis of a small set of issues .
45
INSTRUMENTATION USE SENSORS
47. Explain theory and apply the principles of
temperature measurement and select the
appropriate sensor for the application and
discuss their common operating and
troubleshooting problems.
L.O #2
47
49. The temperature is the most important variable in a chemical
process. Very often, the temperature should be controlled very
precisely like:
In a reactor where the reaction outcome depends on the
temperature’
For safety reasons where explosions can occur
Therefore, temperature need to be measured precisely with a
very accurate sensor.
49
INTRODUCTION
50. ITS-90 (International Temperature
Scale of 1990- used as a worldwide
practical temperature scale in
national metrology labs like NIST,
NPL et al).
50
INTERNATIONAL STANDARDS FOR
TEMPERATURE MEASUREMENTS
51. Fluids and solids are composed of atoms or molecules
These atoms or molecules vibrate, rotate and move in
general, the atoms have an average energy
When is cold, they move slowly and the energy is low
when it is hot, they move fast and the energy is high
51
WHAT IS TEMPERATURE?
52. SCALES ARE INTERNATIONAL STANDARDS USED
IN ALMOST ALL THE COUNTRIES
CELSIUS SCALE OR CENTIGRADE SCALE:
FROM 00C ( melting ice) TO 1000C ( boiling
water) at 1 atm.
KELVIN SCALE :
0 K = -2730C
T (K)= T(0C) + 273
52
SCALES FOR TEMPERATURE
53. AMERICAN SCALE:
RELATIONSHIP BETWEEN FAHRENHEIT AND CELSIUS
SCALES :
320F = 00C
2120F= 1000C
T(0F)= 1.8xT(0C) +32
53
FAHRENHEIT SCALE
55. Convert 1000C into :
K, 0F,0R
Convert 50 K into:
0C, 0F,0R
Convert -750F into:
0C, K, 0R
Convert 0 0R into:
0C,0F, K
55
HOME WORK
56. T = temperature
TI = Temperature Indicator ( in plant)
TT= Temperature Transmitter
TC= Temperature Controller
TRC= Temperature Recorder & Controller
TCV= Temperature Control Valve
TAG DESCRIPTORS FOR
TEMPERATURE
56
59. A Resistance Temperature Detector (RTD) is a device
with a significant temperature coefficient (that is, its
resistance varies with temperature).
It is used as a temperature measurement device,
usually by passing a low-level current through it and
measuring the voltage drop.
59
DEFINITION OF A RESISTANCE TEMPERATURE
DETECTOR
60. The relationship between the
resistance of a RTD and the
temperature of the medium is the
temperature coefficient α of the RTD .
coefficient α is also the sensitivity of
the RTD
60
TEMPERATURE COEFFICIENT α OF A RTD
61. α IS A LINEAR APPROXIMATION BETWEEN RTD RESISTANCE
AND THE TEMPERATURE :
R(T)= R(TO) { 1+ α.ΔT}
R(T)= approximation resistance at Temperature T
R(T0)= resistance of RTD at T0
ΔT = T-T0
α depends on R(T0) and α> 0 because
Metal resistance increases with temperature61
TEMPERATURE COEFFICIENT α OF A RTD
62. Platinum is very repeatable, quite sensitive and
very expensive
For Platinum, coefficient α is around 0.004/0C
Example: for PRTD of 100 Ω, if the temperature increases by 10C, R(T)
changes by 0.4 Ω
Nickel is not quite as repeatable, more sensitive and
less expensive
For Nickel, coefficient α is around 0.005/0C
Example: For RTD of 100 Ω, if the temperature increases by 10C, R(T)
changes by 0.5 Ω
62
SENSITIVITY α OF DIFFERENT METALS
63. RTD's are the best choice for
repeatability, and are the most stable
and accurate. However they have a slow
response time and because they require
a current source they do have a low
amount of self heating.
63
ADVANTAGES & DISATVANTAGES OF RTDs
64. RTDs work in a relatively small temperature domain,
compared to thermocouples, typically from about
-200 °C to a practical maximum of about 650 to 700 °C.
Some makers claim wider ranges and some construction
designs are limited to only a small portion of the usual range.
64
RANGE OF TEMPERATURES FOR RTD
65. A special set of RTD’s are called PRT’s because they
use platinum are a material
A special set of PRTs, called SPRTs, are used to
perform the interpolation in such labs over the ranges
13.8033 K (Triple point of Equilibrium Hydrogen) to the
Freezing point of Silver, 971.78 °C.
65
RANGE OF TEMPERATURE FOR PRT (
PLATINUM RESISTANCE TEMPERATURE)
67. Thermistors are temperature sensors that use semiconductor
materials not metals like RTD’s
R(T) = R(T0) {1+ α (T-T0)}
Semiconductors for temperature sensing have Negative
Temperature Coefficient (NTC) OR α< 0
Semiconductor becomes a better conductor of current.
Resistance decreases when the temperature increases.
67
DEFINITION OF THERMAL RESISTORS
68. The characteristics of these devices are very
different from those of RTD’s
Thermistors are the most sensitive and fastest
temperature measurement devices.
Thermistors can be used for small range of
temperatures
Thermistors are non-linear .
68
PROPERTIES OF THERMISTORS
69. Because the resistance become too high at
low temperature, the low limit is -1000C
Because the semiconductor can melt or be
deteriorated at high temperatures, the high
limit is 3000C
In most cases, the thermistor is encapsulated
in plastic , epoxy, Teflon or some other
material to protect the thermistor from the
environment
69
THERMISTOR’ S LIMITATIONS
70. Thermistors have a fast output and are
relatively inexpensive but are fragile and
have a limited range. They also require
a current source and do experience
more self heating than an RTD and are
nonlinear.
ADVANTAGES & DISADVANTAGES OF
THERMISTORS
70
72. When a pair of dissimilar metals are joined together
for the purpose of measuring temperature, the device
formed is called a thermocouple.
Thermocouples for instrumentation use metals of
high purity for an accurate temperature/voltage
relationship (as linear and as predictable as possible).
Thermocouples cover a range of temperatures from
-2620C to 27600C
72
DEFINITION OF THERMOCOUPLES
74. Thermocouples suffer from 2 major problems that
cause errors when using them
1) Small voltage generated
EX: 10C temperature change on a platinum
thermocouple results of an output change of 5.8 μV
2) the non-linearity that requires polynomial
conversion
74
PROBLEMS OF THERMOCOUPLES
75. The voltage (emf) produced by a heated junction of
two wires is directly proportional to the temperature.
This fairly linear relationship is called SEEBECK EFFECT
Thus, the Seebeck effect provides for us an electric
method of temperature measurement
RTD’S AND THERMISTORS USE RESISTANCES FOR
MEASUREMENT BUT THERMOCOUPLES USE VOLTAGE
75
SEEBECK EFFECT
76. ε = α. ( T2-T1)
WHERE:
ε= THE EMF
TYPES OF THERMOCOUPLES
α = SEEBECK COEFFICIENT
T2 ,T1= JUNCTION TEMPERATURE IN K
76
SEEBECK COEFFICIENT
77. K = Chromel-alumel
Temperatures : -190 to 13710C
Seebeck Coefficient= 40 μV/0C
J = Iron-constantan
Temperatures : -190 to 7600C
Seebeck Coefficient= 50 μV/0C
77
TYPES OF THERMOCOUPLES
78. T = Copper-constantan
Temperatures: -190 to 7600C
Seebeck coefficient : 50 μV/0C
E = Chromel-constantan
Temperatures : -190 to 14720C
Seebeck coefficient: 60 μV/0C
78
TYPES OF THERMOCOUPLES
79. S= Platinum- 10% Rhodium/Pt
Temperatures: 0 to 17600C
Seebeck Coefficient: 10 μV/0C
R = Platinum-13%Rhodium/Pt
Temperatures: 0 to 16700C
Seebeck coefficient : 11 μV/0C
79
TYPES OF THERMOCOUPLES
80. Thermocouples are inexpensive, rugged, and
have a fast response time but are less
accurate and the least stable and sensitive.
Thermocouples also read only relative
temperature difference between the tip and
the leads while RTD's and thermistors read
absolute temperature.
ADVANTAGES AND DISDVANTAGES
OF THERMOCOUPLES
80
81. Temperature
Measurement
Comparison Chart
Criteria Thermocouple RTD Thermistor
Temp Range -267°C to 2316°C -240°C to 649°C -100°C to 500°C
Accuracy Good Best Good
Linearity Better Best Good
Sensitivity Good Better Best
Cost Best Good Better
COMPARISON BETWEEN THE
DIFFERENT TEMPERATURE SENSORS
Temperature Measurement Comparison Chart
81
82. Find the seebeck emf (ε) for a
thermocouple J with α. = 50 μV/0C
if the junction temperatures are 20
and 1000C
82
CLASS WORK
83. Objective of the lab:
I) During the experiment: Reading of the temperature of the water being
heated and the corresponding values for the three temperature sensors.
II) After the lab, draw the three different calibration curves and find the
sensitivity factor α for each sensor using the corresponding formula.
RTD = Resistance vs. Temperature
Thermistors: Resistance vs. temperature
Thermocouples = Voltage vs. Temperature
III) Write a lab report
LAB #2
TEMPERATURE SENSORS
83
85. Explain theory and apply the principles of
pressure measurement and select the
appropriate sensor for the application and
discuss technical issues including
calibration.
L.O #3
85
87. Pressure is the second most important
measurement in process control
Pressure is controlled for process reason but
also for safety reason.
The most familiar device are manometers and
gauges but they require a manual operator
87
IMPORTANCE OF PRESSURE
88. DEFINITION OF PRESSURE
PRESSURE IS THE AMOUNT OF FORCE EXERTED ON
A UNIT AREA OF A SUBSTANCE:
A
F
P
88
90. SI UNITS:
1Pa = 1N/M2=1KG/S2.M
1ATM (ATMOSPHERIC PRESSURE)= 1.01x105 Pa
1 ATM= 101 kN/M2
1ATM= 760 MM. HG
US UNITS:
1PSIA = 1LBF/IN2
1PSIA = 6894.7 Pa
1ATM= 14.696 PSIA
90
UNITS OF PRESSURE
91. STATIC PRESSURE IS FOR A FLUID WITH IS NOT IN
MOTION
EX: FLUID IN A TANK
DYNAMIC PRESSURE IS FOR A FLUID IN MOTION IN
PIPES
91
STATIC VS DYNAMIC PRESSURE
93. THE PRESSURE OF A FLUID IN A PIPE IS MEASURED
BY A PRESSURE GAUGE.
FLOW CALCULATED BY BERNOUILLI EQUATION
93
DYNAMIC PRESSURE
94. IT IS EXTREMILY IMPORTANT TO MAKE THE
DIFFERENCE BETWEEN THE ABSOLUTE AND
RELATIVE PRESSURE
THE ABSOLUTE PRESSURE IS THE REAL
PRESSURE OF THE FLUID WHERE THE
RELATIVE PRESSURE IS THE PRESSURE WE
READ IN A PRESSURE INDICATOR WITH
REFERENCE THE ATMOSPHERIC PRESSURE
94
ABSOLUTE AND GAUGE PRESSURE
95. PA = PG + 1 ATM
EXAMPLE #1 :EXPRESS A PRESSURE GAUGE OF 155
KPa TO ABSOLUTE PRESSURE WHEN THE
ATMOSPHERIC PRESSURE IS 98 Kpa
EXAMPLE #2: WHICH PRESSURE DO YOU READ IN A
GAUGE MANOMETER FOR A PRESSURE OF 225 KPa (
ABSOLUTE ) WHEN ATMOSPHERIC PRESSURE IS 101
KPa
95
RELATIONSHIP BETWEEN PA AND PG
CLASS WORK
96. In many cases, gauge pressure is more
important than the absolute pressure
because we read gauge pressure in
manometers.
Pg= Pabs- Patm
96
GAUGE PRESSURE
98. PRESSURE IS USUALLY MEASURED FOR INDICATION
ONLY BY READING:
GAUGES
U TUBES
98
PRESSURE INDICATORS
99. A hard metal tube ( bronze or brass) is
flattened and one end is closed. Under
pressure, the tube is bent into a curve or arc.
The open end is attached to a header by
which the pressure can ne introduced inside
the tube
99
MANOMETER= GAUGE OR BOURDON TUBE
102. I) A tank open to atmosphere holds water with a
depth of 7 m. Density of water = 1000 kg/m3
a) What is the pressure in a gauge at the bottom of
the tank in Pa ?
b) Draw the figure showing the manometers
readings
102
CLASS WORK
103. in a closed tank under vacuum, the bottom pressure of an unknown
liquid at 1.2 m depth is 12.55 kPa (absolute).
1) Draw a figure showing the manometer readings
2) What is the density of the fluid?
A crude oil, in a tank at 60 kPa top absolute pressure, has a specific
gravity of 0.89 and a pressure of the bottom of 345 kPa ( gauge).
1) Draw a figure showing the manometer readings
2) What is the level of the oil in the tank ?
A fluid in a tank has a specific gravity of 0.76 and a absolute pressure
at the top 150 kPa and a gauge pressure at the bottom of 140 kPa.
1) Draw a figure showing the manometer readings
2) What is the level of liquid in the tank?
HOMEWORK
103
104. CALIBRATION OF A MANOMETER BY MEASURING THE
PRESSURE OF A GIVEN WEIGHT USING A HYDRAULIC
OIL
USE DIFFERENT WEIGHTS
READ THE PRESSURES IN THE MANOMETER
APPLY THE FORMULA (P=m.g/S)
COMPARE the reading with the calculated PRESSURE and
calculate the error
104
LAB #3 :
calibration of manometers
106. Explain theory and apply the principles of level
measurement and select the appropriate
sensor for the application instruments and
discuss technical problems including
calibration.
L.O #4
106
107. In any chemical plant, you will find tanks,
reservoirs, vessels and drums where liquids
are stored. These could be for:
The feed of the plant
Intermediate between sections
The products before selling them
Liquid capacities are also found in distillation
columns and reactors
107
LIQUID CAPACITIES IN A CHEMICAL PLANT
108. Level of liquid in a vessel should be maintained above the exit
pipe because if the vessel empties the exit flow will become
zero, a situation that could damage PUMPS.
A minimum level of liquid is then necessary to avoid
cavitation of the pump
This minimum should be known (measured) and respected
during the production
108
MINIMUM LEVEL
109. The level should also have a maximum value to:
not overflow an open vessel (safety for workers)
should not exit through a vapor line of a closed
vessel, which could disturb a process designed for
vapor ( safety for COMPRESSOR , TURBINES)
109
MAXIMUM LEVEL
110. L= Level
LI= Level Indicator
LT= Level Transmitter
LC= Level controller
LRC= Level Recorder & Controller
LCV= Level Control Valve
LLA and VLLA: Low level Alarm and Very…
HLA and VHLA: High Level Alarm and Very..
TAG DESCRIPTORS FOR LEVEL
110
111. Level measurement sensors are divided into two
categories:
point level switches for ALARMS
continuous level gauges for CONTROL
111
LEVEL MEASUREMENT SENSORS
112. Point level is used mostly for SAFETY.
Will operate when the liquid is above or below a
certain point.
Switches devices indicate when a vessel is full, empty
or at intermediate level
You will have LLA ( low level Alarm) and HLA ( high
level Alarm)
112
POINT LEVEL SWITCHES
113. Continuous level gauges provide information
about material level at all points in the vessel
Continuous level gauges are used for control
purpose
113
CONTINUOUS LEVEL GAUGES
117. The differential pressure is the most commonly used for
continuous level measurement of liquids.
a membrane is used where the value
H(Level)= ΔP/ρ.g
117
LEVEL MEASUREMENT BY HYDROSTATIC
PRESSURE
120. A tank open to atmosphere holds water. The pressure
at the bottom is 200 kPa ( absolute)
1) Draw the figure showing the tank and the
differential pressure ’s reading
2) What is the level in the tank ?( density of water =
1000 kg/m3)
In a closed tank under vacuum and containing crude oil
( ρ= 780 kg/m3) , the bottom pressure is 12.55 kPa
(absolute).
1) Draw a figure showing the tank and the differential
pressure ’s reading.
2) What is the level in the tank?120
Class Work
121. A crude oil, in a tank at 120 kPa top absolute pressure, has a
specific gravity of 0.80 and a gauge pressure of the bottom of
345 kPa .
1) Draw a figure showing the tank and the differential pressure s
reading.
2) What is the level in the tank?
A fluid in a tank has a specific gravity of 0.65 and a gauge
pressure at the top 150 kPa and a absolute pressure at the
bottom of 140 kPa.
1) Draw a figure showing the tank and the differential pressure
’s reading.
2) What is the level of liquid?
HOME WORK
121
123. Explain theory and apply the principles of
flow measurement and select the
appropriate sensor for the application and
discuss technical problems including
calibration.
L.O #5
123
124. Quantity of fluid flowing in a system by unit time.
This quantity can be expressed in three ways:
Volume Flow rate ( Q) :Bring a flask and a stop watch
to measure volumetric flow
Mass Flow rate ( M)
Weight Flow rate ( W)
124
WHAT IS FLOW?
125. F= Flow
FI= Flow Indicator
FT= Flow Transmitter
FC= Flow controller
FRC= Flow Recorder & Controller
FCV= Flow Control Valve
TAG DESCRIPTORS FOR LEVEL
125
126. If we know the volume flow rate Q, we can calculate
the mass flow rate by : M=ρ.Q
If we know the volume flow rate Q, we can calculate
the weight flow by : W=γ.Q
126
RELATIONSHIP BETWEEN FLOWS
127. The volume flow rate is the volume of fluid
flowing past a section per unit time
In a pipe, we can have the relation: Q=A .v
(where v is the average velocity of flow)
Units used:
SI : EX: v (m /s) Q (m3/s)
US : EX: v (ft /s) Q(ft3/s)
127
Volume flow rate Q
128. An average flow rate of water produced by a
plant is 11600 m3 /hr. Find the equivalent flow
rate in m3/s, mass flow rate in kg/s ( density of
water = 1000 kg/m3) and the weight flow rate (
Weight= Mass x gravity) and gravity = 9.8 m/s2
128
CLASS WORK (units)
129. A) MATERIAL BALANCE OF A PLANT: VERY
VERY IMPORTANT
Measure flow of feeds
Measure flow of products
We should have : IN=OUT in mass ( Otherwise
we have leaks in the plant)
B) FLOW IS A IMPORTANT VARIABLE FOR THE
SYSTEM ( EX:REACTOR)
WHEN YOU HAVE A RATIO CONTROL SYSTEM
129
WHY WE NEED TO MEASURE FLOWS
130. In the instrumentation market, we find two types of
flow-meters:
Energy-extractive Flow meters
Energy additive Flow meters
130
FLOW MEASUREMENT TECHNIQUES
131. Several sensors rely on the pressure drop or head
occurring as a fluid flows by a resistance.
131
THE PRINCIPLE OF FLOW SENSORS
132. ORIFICE
VENTURI TUBE
FLOW NOZZLE
ELBOW METER
PITOT TUBE
TURBINE
132
MOST IMPORTANT FLOW SENSORS
133. Bernouilli Equation
Old system : use low measurement devices that reduce the energy of the system.
The differential pressure is used to measure flow using Bernoulli equation:
Applying Continuity equation: QA=QB ( assuming constant density). Find the
relationship between flow ( You want to estimate) and ΔP ( your readings).
this relationship is used in Energy extractive flow meters as a conversion factor
22
2
1
2
1
BB
B
AA
A
v
g
z
p
v
g
z
p
133
134. From Bernouilli Equation:
𝑄 = ∆𝑃.
2( 𝐴1.
2 𝐴2
2 )
𝜌(𝐴1
2 −𝐴2
2)
Pressure drop in Pa
Area in m2
Density in kg/m3
Q in m3/s
Calculating volumetric flow rate Q
134
135. In a pipe of 0.3 diameter, water is flowing at
600C. We use a venturi tube to measure the
flow rate. The venturi tube has a diameter of
0.2 m and we observe a pressure drop of 50
pa
What is the volume flow rate and the
conversion factor?
What is the mass flow rate?
135
CLASS WORK
137. Define the terms used in chemical process control and discuss the role
and importance of process control systems in industrial plants.
Define P, PI and PID controllers
Explain feedback control and the dynamic behavior of this controller.
Apply the principles of feed-forward and show how this type of control
can be applied.
Describe how the principles of cascade control, ratio, the selective
control and split - range control are used in processes control.
Define the principles of computer control and distinguish between
direct digital control and supervisory control.
Do experiments and write laboratory reports in a professional manner.
PART II: PROCESS CONTROL
137
138. L.O #1
Define the terms used in
chemical process control and
discuss the role and importance
of process control systems in
industrial plants.
138
139. 139
THE SEVEN OBJECTIVES OF A CONTROL SYSTEM
• 1. Safety
• 2. Environmental Protection
• 3. Equipment protection
• 4. Smooth Operation and production rate
• 5. Product Quality
• 6. Profit
• 7. Monitoring and Diagnosis
140. Example
Heating up the temperature in the tank is a
process that has the specific, desired outcome to
reach and maintain a design value for the
temperature (e.g. 80°C), kept constant over time.
The desired temperature (80°C) is the set point.
The controller will manipulate the valve of hot
water to maintain the room temperature at 800C.
140
142. 142WHAT ARE THE DESIGN VALUES?
THE DESIGN ENGINEERS CALCULATE THE VALUES
OF SOME VERY IMPORTANT VARIABLES OF THE
PROCESS THAT SHOULD BE MAINTAINED
CONSTANT IN ORDER TO GIVE MAXIMUM
PROFITABILITY BY RESPECTING SAFETY AND
ENVIRONMENT ( OPTIMIZATION)
THESE CALCULATED VALUES ARE THEN
INTRODUCED AS SET POINTS ( VALUES TO BE
RESPECTED) IN THE CONTROLLER ONCE THE
PLANT IS BUILT .
143. 143
HOW ARE THE VALUES OF THE IMPORTANT VARIABLES (
SET POINTS) MADE CONSTANT?
ACTING ON SOME OTHER LESS IMPORTANT
VARIABLES OF THE PROCESS IN ORDER TO
SUPPRESS THE EFFECTS OF EXTERNAL
DISTURBANCES ON THE IMPORTANT
VARIABLES
145. Overview of Process Automation
The process is “that portion of an automation
operation which use energy measurable by some
quality such as pressure, temperature, level, flow,
(and many others) to produce changes in quality or
quantity of some material or energy.”
PROCESS
Some Quality or Quantity
of the
Material or Energy
Input
Energy
or
Material
Desired
Result
146. Example of a Temperature Process
Heating Element
Water Bath
Temperature
The objective of this process is to maintain a
constant water bath temperature.
147. Temperature Process Terminology
Heating Element
Water Bath
Temperature
This is a Temperature Process
The measuring means is the thermometer. (Temperature Indicator- TI)
The process temperature is maintained at a desired point (Set Point – SP)
Steam (Control Agent) is used to vary the temperature by opening and closing the
control valve (Final Control Element)
148. Level Process
Oil Stock
Level Indicator
Oil Feed to
next
process
The control objective is to maintain a constant liquid
level of oil inside the tank (e.g. 100 gallons +/- 20
gallons). The hand valve is opened and closed as
required to maintain the desired tank level.
149. Terminology used to describe the process
PROCESS: Level
CONTROLLED VARIABLE: Level by Head pressure at bottom of tank
CONTROL POINT: The level of oil in the tank (Set Point = 100 gallons)
MEASURING MEANS: Level Indicator (Head Pressure)
MANIPULATED AGENT: Volume of oil stock
MANIPULATED VARIABLE: Flow rate of oil (gpm)
Oil Stock
Level Indicator
Oil Feed to
next
process
150. Basic Model of a Process
The process is maintained at the desired point (SP) by
changing the FCE based on the value of the PV
Manipulated
Variable
Desired
Result
Control
Agent
PROCESS
(Temperature,
pressure, level, flow)
FINAL
CONTROL
ELELMENT
(valve)
Measuring
Means
(transmitter)
Process Variable (PV)
Controlled
Variable
Actuating
Input
pH, conductivity, humidity,
density, consistency, etc.
Process equilibrium (balance) is when the input energy maintains the
output at a constant “desired” point.
151. Basic Model of a Process
The measuring means provides the
standardized signal that represents the
condition of the process, i.e. is the process
at the desired point?
Manipulated
Variable
Desired
Result
Control
Agent
PROCESS
(Temperature,
pressure, level, flow)
FINAL
CONTROL
ELELMENT
(valve)
Measuring
Means
(transmitter)
Process Variable (PV)
Controlled
Variable
Actuating
Input
pH, conductivity, humidity,
density, consistency, etc.
152. Review of Measuring Means
Pressure
Level
Flow
Temperature
Thermocouples
RTDs / Thermistors
Filled Systems
Bi-metallic
Strain gauge
Piezo-electric
Capacitance
Bourdon Tube
Head meters
(orifice, venturi)
Coriolis, velocity,
Mass,
Mechanical Floats
Guided Wave
Weight (load cell)
Ultrasonic
Differential Pressure
Transmitters
Pressure Transmitter
Level Transmitter
Differential Pressure Cell
Flow Transmitter
Temperature Transmitter
Pneumatic
3-15 PSI
Electrical
Current
4 – 20 mA
0 – 20 mA
10 – 50 mA
Voltage
0 – 5 V
1 – 5 V
0 – 10 V
Digital
ON/OFF
Field Bus
ModBus
ProfiBus
HART
153. Manual Control
Open loop (or manual control) is used when very
little change occurs in the Process Variable (PV)
Manipulated
Variable
Desired
Result
Control
Agent
PROCESS
(Temperature,
pressure, level, flow)
FINAL
CONTROL
ELELMENT
(valve)
Measuring
Means
(transmitter)
Process Variable (PV)
Controlled
Variable
Actuating
Input
pH, conductivity, humidity,
density, consistency, etc.
Corrective action is provided by manual feedback
154. THE FOUR BASIC STEPS OF A PROCESS
CONTROL SYSTEM
HOW DOES IT WORK?
154
155. 155
THE FIRST STEP: TAKING THE
INFORMATION
IF WE DO NOT KNOW WHAT IS WRONG, HOW CAN WE CONTROL ?
TAKING INFORMATION OF THE IMPORTANT VARIABLES
( Design Values) OF THE PROCESS.
156. 156IN OUR CASE:
Temperature of the tank has to be controlled.
Temperature SHOULD FIRST BE MEASURED.
THE EQUIPMENT FOR temperature
MEASUREMENT IS : thermocouple
157. 157
THE SECOND STEP OF A PROCESS CONTROL SYSTEM:
TRANSMISSION OF THE INFORMATION
LINK BETWEEN THE PLANT AND THE CONTROL ROOM)
THE MEASUREMENT OF THE CONTROLLED VARIABLE IS SENT
TO THE CONTROLLER IN THE CONTROL ROOM.
THE EQUIPMENT FOR TRANSMISSION IS THE TRANSMITTER
Thermocouple is also a transmitter
158. IN OUR CASE:
THE ANALOG SIGNAL OF THE VALUE OF FB (
MEASURED VARIABLE) IS TRANSMITTED TO
A/D CONVERTER
THE RESULTING DIGITAL SIGNAL IS SENT TO
THE CONTROLLER (digital or computer
software)
WHY A/D CONVERTER?
158
159. 159
THE THIRD STEP :THE CONTROLLER MAKE
DECISION
THE THIRD STEP IS THE CONTROLLER IN THE CONTROL ROOM
THE CONTROLLER:
1) RECEIVE THE INFORMATION FROM THE PLANT
2) COMPARE IT WITH THE SET POINT
3) CALCULATE THE DIFFERENCE ε BETWEEN THE SET POINT AND
THE INFORMATION.
4) MAKE A DECISION FOR ACTION TO BE TAKEN IN THE PLANT.
160. IN OUR CASE:
THE CONTROLLER WILL FIRST COMPARE T (
MEASURED VARIABLE) TO ITS SET POINT TSP.
THE CONTROLLER WILL THEN CALCULATE
THEIR DIFFERENCE ε =( TSP-T)
THIS DIFFERENCE ε IS MULTIPLIED BY A
FACTOR K DEPENDING ON THE TYPE OF
CONTROLLER ( P,PI OR PID TO BE STUDIED
LATER)
160
161. 161
THE FOURTH STEP: ACTION ON A CONTROL VALVE OR
MOTOR IN THE PLANT
A SIGNAL FROM THE CONTROLLER, RELATED TO
THE DIFFERENCE ε IS SENT TO THE VALVE TO
MANIPULATE THE FLOWRATE OF STEAM WHICH
IS A LESS IMPORTANT VARIABLE
THE VALVE IS THE FOURTH AND LAST
EQUIPMENT OF THE PROCESS CONTROL SYSTEM
THE FLOW OF STEAM IS THE MANIPULATED
VARIABLE.
162. 162IN OUR CASE
TO ELIMINATE THE EFFECTS OF THE SURRONDINGS (
DISTURBANCES) ON THE IMPORTANT VARIABLE
TEMPERATURE WHICH IS MEASURED
TO BRING T AS CLOSE AS POSSIBLE TO ITS SET POINT
VALUE TSP THE CONTROLLER ACT ON ANOTHER
VARIABLE FA CALLED MANIPULATED VARIABLE
164. CLASS WORK
We want to produce ammonia from nitrogen and
hydrogen in a reactor where the temperature should
be maintained constant by a coolant in a jacket
around the reactor.
Draw the process
Draw the process control system
Show the FOUR steps of the control loop
164
169. 169A) MANUAL CONTROL
DURING START UP AND SHUT DOWN: OPERATOR CONTROL THE
PLANT OPERATIONS
LEVEL3
170. 170
AUTOMATIC CONTROL
DURING OPERATING CONDITIONS: THE CONTROLLER TAKES
ACTIONS
ON AND OFF CONTROLLER:
CONTROLLER TAKES ACTION ONLY WHEN THE MINIMUM AND THE MAXIMUM
OF THE LEVEL ARE REACHED
NOT USED VERY OFTEN ONLY IN SIMPLE SITUATIONS WHEN SAFETY AND
PRODUCTIVITY ARE NOT AFFECTED
LEVEL4
171. 171CONTINUOUS AUTOMATIC CONTROL:
THE MOST USED CONTROLLERS:
PROPORTIONAL ( P)
PROPORTIONAL- INTEGRAL ( PI)
PROPORTIONAL-INTEGRAL-DERIVATIVE ( PID)
173. THE DIFFERENT FUNCTIONS OF A PROCESS
CONTROL LOOP
Between the measuring device and the final control
element, we have different steps and each step has its
own function
THE SENSOR : the output ym(t) of the sensor is related to
the real value in the controlled variable y (t) by a transfer
function
THE TRANSMITTER : The value yt (t) entering the
controller is related to ym(t) by a transfer function ( we
have delay in the information)
LISTEN..LEARN..THINK..ENJOY YOURSELF
173
173
174. DIFFERENT FUNCTIONS
THE CONTROLLER : after comparing to the set point ySP ,
the input to the controller is then ε (t) = ySP- ym(t). The
output c(t) is related to
ε (t) by a transfer function of the controller (P,PI,PID)
The way c(t) and ε (t) are related depends on the type of
controller ( TO BE STUDIED LATER)
THE VALVE: The output signal of the valve is related to
c(t) by a transfer function depending on the type of the
valve
LISTEN..LEARN..THINK..ENJOY YOURSELF
174
174
175. Lab #4:Demonstration lab
Demonstration lab for the pressure controller
including:
1) The four steps
2) Converters P/I , I/P for electronic Controllers
3) A/D and D/A converters for digital controllers
175
177. Closed Loop Control
Closed loop or feedback control provides a corrective
action based on the deviation between the PV and the
SP
Automatic
Controller Output
(3-15 psi, 4-20mA etc)
CONTROLLING
MEANS
Manipulated
Variable
Desired
Result
Control
Agent
PROCESS
(Temperature,
pressure, level, flow)
FINAL
CONTROL
ELELMENT
(valve)
Measuring
Means
(transmitter)
Controller Input (PV)
(3-15psi, 4-20mA etc)
Controlled
Variable
pH, conductivity, humidity,
density, consistency, etc.
Manual
SP
178. Controlling Means
Controllers provide the required control action
to position the FCE at a point necessary to
maintain the PV at the desired SP.
PID (single loop feedback controller)
DCS (distributed controllers)
PLC (programmable logic controllers)
179. Single Loop Feedback Control
1. Measuring Means
2. Controlling
Means
3. Final Control
Element
4. Temperature
Process
Temperature Controller and
Recorder
Sensing
Bulb
Temperature
Transmitter
Pneumatic
Control Valve
Heat Exchanger
Steam
2
3
4
1
The TT provides the signal (PV) that represents the condition
of the process being controlled. The TIC compares the PV to
the SP and opens and closes the FCE to maintain the process
at equilibrium.
180. Summary
Process automation makes use of instrumentation to maintain the
process at some desired condition.
Common instrumentation used in a process loop are the
measuring means (usually transmitters), the controlling means
(usually a PID controller), and the Final Control Element (usually
some type of valve)
The measuring means provides the feedback signal (PV) used in
the process loop. The controlling means operates the FCE based
on the difference between the PV and the SP.
Process equilibrium is maintained when the difference between
the PV and SP is zero or constant (offset?)
181. NEXT?
What are
transmitters?
What is PID? What are P&ID
symbols?
What types
of FCE are
there?
What am I
doing here?
How do I
measure?
Pressure
Level
Temperature
Flow
How do I
tune a loop?
What is
Integral
action?
What is a?
FIC
TT
LRC
PRV
Should I use a
3-15 psi or 4-
20 mA valve?
Check out
183. In the context of industrial process control, a
"transmitter" is a device that converts sensor
measured units into an electrical signal then directs
this data (via cabling or wirelessly) to be received by a
display or instrumentation control device within the
system.
183
Transmitters
184. Analog transmitters are the most commonly used
type in most industrial sectors. The transmitter is
connected to the rest of the system via 2 wires which
create something know as the 'current loop.'
The two wires can be used for both powering the unit
and for transmitting signals typically at a range of 4
mA to 20 mA
184
Analog Transmitter
185. In an increasing number of industrial situations wireless sensors
are an appropriate upgrade to classic industrial transmitters.
This is because current of generation sensors offer flexible
system solutions which are ideal for temporary installations and
in processes with moving parts/objects.
Such wireless sensor networks can be comprised of hundreds
or thousands of intelligent sensors. This allows for complex
network mapping that can provide advanced solutions to
today's processing environments.
185
Wireless transmitters
186. If the measuring device is pneumatic and the
controller is electronic: A P/I transducer is needed to
transform a physical movement into electrical
current.
The I/P transducer does the opposite direction but
not very used because most controllers are now
electronic or digital.
186
Transducers: P/I and I/P
187. If the controller is digital and the measuring device is pneumatic, we
need:
1) convert pneumatic into electrical by P/I transducer
2) convert electrical to digital using A/D converter.
At the exit of the digital controller we need:
1) D/A is the valve is electrical
2) D/A + I/P is the valve is pneumatic
187
Converters: A/D and D/A
188. Define P, PI and PID controllers
L.O #2: CONTROLLERS
188
190. P CONTROLLER IS PROPORTIONAL CONTROLLER
PI CONTROLLER IS PROPORTIONAL CONTROLLER
WITH INTREGRAL ACTION
PID CONTROLLER IS PROPORTIONAL CONTROLLER
WITH INTEGRAL ACTION AND DERIVATIVE ACTION.
DIFFERENT KINDS OF CONTROLLERS
190
191. PROPORTIONAL CONTROLLER
The proportional CONTROLER means that the
controller output c(t) is linearly related to the
error ε (t)
The proportional controller has a gain Kc or
Proportional Band (PB) related by the formula
(Kc= 100/PB)
LISTEN..LEARN..THINK..ENJOY YOURSELF
191
191
192. Chapter 15 - Process Control Methods 192
Proportional Band
Proportional band is defined as the percentage
change in the controlled variable that causes the
final correcting element to go through 100
percent of its range
PB =
Controlled Variable % Change
FinalCorrecting Element % Change
193. PROPORTIONAL ACTION
The proportional action means that the controller output
c(t) is linearly related to the error between set point (SP)
and measurement of process output ym (t) :
c(t) = Kc .ε(t) = Kc (SP – ym(t) )
The proportional gain Kc of a analog controller can be
adjusted by knob in the controller.
Direct or reverse actions ?
LISTEN..LEARN..THINK..ENJOY YOURSELF
193
194. SIGN OF THE GAIN KC
If he controller is direct acting the
gain K is positive.
When the controller is reverse acting
the gain K is negative
LISTEN..LEARN..THINK..ENJOY YOURSELF
194
195. PROPORTIONAL BAND
Proportional controllers are defined by their Proportional Band
(PB) or the proportional gain (Kc)with PB =100/Kc
For pneumatic valves, we define Kcp which is the output from the
controller to the valve. The range of the instrumentation pressure
for pneumatic valves is 3 -15 psia.
For electrical valves, we define Kce which is the output from the
controller to the valve. The range of the instrumentation current
for electrical valves is 4-20 mA.
LISTEN..LEARN..THINK..ENJOY YOURSELF
195
196. DIFFERENT SITUATIONS:
A) IF A FULL CHANGE IN THE CONTROLLED VARIABLE IS
ALSO A FULL RANGE FOR THE VALVE , WE WILL HAVE:
PB= 100%/100%= 1= 100% ,KC=1
IF WE ARE CONTROLLING TEMPERATURE FOR A RANGE
OF 60-100, WE WILL HAVE : Kcp = 0.3 PSIA/ DEGRE
LISTEN..LEARN..THINK..ENJOY YOURSELF
196
197. B) IF A 10% CHANGE IN THE CONTROLLED VARIABLE
GIVES A FULL RANGE IN THE 100% IN THE VALVE, WE WILL
HAVE PB= 10%/100% = 10%
IF WE CONTROL TEMPERATURE FOR THE SAME TOTAL
RANGE, 10% WILL BECOME 4 DEGRE AND WE WILL HAVE
Kcp= 3PSIA/DEGRE
THE CONTROLLER IS MORE SENSITIVE
LISTEN..LEARN..THINK..ENJOY YOURSELF
197
198. C) IF A 100% CHANGE IN THE CONTROLLED VARIABLE
GIVES A 20% RANGE IN THE VALVE, WE WILL HAVE PB=
100%/20% = 500%, KC=0.2
IF WE CONTROL THE SAME TEMPERATURE , WE WILL
HAVE Kcp= 0.06 PSIA/DEGRE
THE CONTROLLER IS LESS SENSITIVE
LISTEN..LEARN..THINK..ENJOY YOURSELF
198
199. EXAMPLE #1
Let’s consider a control system for a temperature in a process where the output of the
controller is a pressure signal to the final element or valve.
The controller is used to control temperature within the range of 600F to 1000F.
The controller is adjusted so that the output signal varies from 3 psi (valve fully open) to
15 psi (valve fully closed) as the controlled temperature (measured) varies from 710F to
750F.
Fpsi
FF
psipsip
Kcp
0
00
/3
)7175(
)315(
%10100.
)60100(
)7175(
00
00
FF
FF
PB
LISTEN..LEARN..THINK..ENJOY YOURSELF
199
200. EXAMPLE #2
Now, if we consider a PB of 75% for the same range of 600F to 1000F, what will
be the Gain Kc?
From the PB formula, we find ΔT ( the change of the measured variable)
From the Gain formula:
FFrangePBT 00
30)40.(75.0.
Fpsi
F
psipsi
Kcp
0
0
/4.0
30
)315(
LISTEN..LEARN..THINK..ENJOY YOURSELF
200
201. OFFSET OF PROPORTIONAL CONTROLLER
An important characteristic of a proportional controller is the
OFFSET
In a proportional controller, there is always a residual error of the
controlled variable.
It can be minimized by a large Kc which also reduce the PB
See figure 9-10 page 198
LISTEN..LEARN..THINK..ENJOY YOURSELF
201
202. EXAMPLES OF USES OF A PROPORTIONAL
CONTROLLER
Proportional controllers are mostly used for level control
where variations of the controlled variables carry no
economical and where others control modes can easily
destabilize the loop
It is actually recommended for controlling the level of a
surge tank when manipulating the flow of the feed to a
critical downstream process.
LISTEN..LEARN..THINK..ENJOY YOURSELF
202
203. CHARACTERISTICS OF
PROPORTIONAL CONTROLLER
Relationship between the output c(t) and error ε (t) is:
c(t) = Kc .ε(t) = Kc .ε(t)
Proportional Controller gives always an Off-Set, which is a
difference between the controlled variable and set point.
A proportional controller will have the effect of reducing
the rise time but never eliminate THE OFF SET
Increasing the gain or decreasing the PB will eliminate
decrease off set but gives fluctuations
203
204. We can reduce the off set by increasing the gain BUT if the gain
is too high, the controller become too sensitive and we will
experience fluctuations and instability.
GAIN AND OFF SET
204
205. Selecting the Right
Proportional band
or PB
That bit was the
“hard part” to
understand...
But it is not so difficult
to understand if we
take a look at what it
does in the actual
application... 205
206. PB too small
C°
(t)
SV
PB correct
C°
(t)
SV
PB too large
C°
(t)
SV
A Proportional Band that is too narrow causes
hunting! The TC will than behave like an ON/OFF
controller!
A correctly sized P-Band results in an Overshoot,
followed by an Undershoot and than Stabilization,
with a small offset near the Set Point.
With a (far) too large P-band the Setpoint temperature
will never be reached! (As the heater capacity will be reduced too much).
This will create a large offset from the Set Point!
P-Action.The right setting of PB is very important !
206
207. Lets have a look now
what will happen if
we add the PI
controller
That explains the P-Action so far...
The “Integral Action”
207
208. Chapter 15 - Process Control Methods 208
The need of an Integral Action
Because of the introduction of offset in a
control process, proportional control alone
is often used in conjunction with Integral
control.
Offset is the difference between set point
and the measured value after corrective
action has taken place
209. Chapter 15 - Process Control Methods 209
Integral or Reset Action
Integral control is also referred to as reset control
as the set point is continuously reset as long as an
error is present
Integral adjustments that affect the output are
labeled 3 ways:
Gain - expressed as a whole number
Reset - Expressed in repeats per minute
Integral Time - Expressed in minutes per reset
210. PI controller is a Proportional controller in which integral
action is added. It has then two constants:
A) PB
B) Integral time
An integral control will have the effect of eliminating the
OFF SET , but it may make the response more oscillatory
and needs longer to settle.
PI OR PROPORTIONAL INTEGRAL
CONTROLLER
210
211. The output of the controller is related to the
error ε (t) by the relationship:
c(t) = Kc { 1+ (1/τi.s) }. ε(t); τi is the integral
time.
Integral action eliminates the off set but the
response becomes more oscillatory and
needs longer to settle down.
CHARACTERISTICS OF PI
211
212. As explained: The I-Action eliminates the Offset, but influences
the whole process from the start ( fluctuations).
Making the Integral time shorter will give you more intense
control with a quicker response to eliminate the offset. But a
too short Integral time would result in “oscillation” (=hunting) !
Making the Integral time too long will reduce the possibility of
hunting but will slow down your overall Process response. So
the RIGHT setting is very important.
The right Integral Time
212
213. The setting of the right I-Time is very important !
0
20
40
60
80
100
120
140
°C
SP
PV @
I=80s
PV @
I=38s
PV @
I=20s
SV:
100o
C
The best way is to explain
with a real control graph :
A too long I-Time slows
down the whole Process
The RIGHT I-Time will
enable the TC to reach
the Setpoint quickly and
to eliminate the Offset
correctly.
Making the I-Time too
short creates a (large)
overshoot. Also takes
a long time to correct:
Example of
behaviour after a
disturbance
213
214. Lets have a look
now at the PID
controller
Well.. That explains the “P+I Action”...
The “Differential Action”
214
215. Chapter 15 - Process Control Methods 215
Derivative Action
For rapid load changes, the derivative mode is typically
used to prevent oscillation in a process system
The derivative mode responds to the rate of change of
the error signal rather than its amplitude
Derivative mode is never used by itself, but in
combination with other modes
Derivative action cannot remove offset
216. PID or Proportional Integral
Derivative Controller
PID controller is a PI controller in which the derivative
action is added. It has then three constants:
A) PB
B) Integral time : τi
C) Derivative : τd
A derivative action will have the effect of increasing
the stability of the system, reducing the overshoot, and
improving the transient response.
216
217. The relationship between the output of
the controler and the error ε(t) is c(t) =
Kc { 1+ (1/τi.s) + τd.s }. ε(t); τi is the
integral time and τd is the derivative
time
All design specifications can be reached.
CHARACTERISTICS OF PID
217
218. A too long D-Time leads to “excessive”
response!
Than we will Over- and Undershoot the setpoint.
(Far too long D-time will create oscillation, like ON/OFF Controller)
A correctly sized D-Time results in a fast return to
the Set Point. Could be followed by a small
overshoot and than return rapidly to the
Setpoint.
With a too short D-time the Process will behave like a
PI (only) controller, so will have a (too) slow response
to disturbances.
Note: With a setting of D-Time of 0 sec, we will have a PI Controller!
The right setting of the D-Action is also very important !
o
C
o
C
o
C
The value of the D-Time is usually around ¼ of the I-
Time. (For example: if the I-Time is 180sec., than the D-
Time will be 45sec.)
218
219. CONCLUSION:
These 3 actions combined:
* The “P-Action”
* The “I-Action”
* The “D-Action”
= PID controller.
That was a “tough part” to combine these 3 actions....
219
220. CL RESPONSE
RISE TIME-
First Time to
reach set
point
OVERSHOOT-
Highest
value/set point
value
SETTLING TIME-
Time to be inside 5%
of set point
OFF SET
Kp Decrease Increase Small Change Decrease
τi Decrease Increase Increase Eliminate
τd Small Change Decrease Decrease Small Change
EFFECTS OF PB, INTEGRAL TIME AND DERIVATIVE TIME ON THE PROCESS
220
221. Chapter 15 - Process Control Methods 221
Control Mode Summary
222. The following additional explanation can also help to
understand the actions of the PID-controller:
• The “P-Action” deals with the “present”
Depending on the deviation from the Setpoint:
more or less Output capacity will be given.
• The “I-Action” deals with the “past”
If we have been below setpoint: the Output will be increased.
If we have been above setpoint: the Output will be decreased.
• The “D-Action” deals with the “future”
If the controlled variable is going down: the Output will be increased.
If the controlled variable is going up: the Output will be decreased.
This “combination”, of “Present + Past + Future”,
makes it possible to control the application very well.
222
223. TUNING THE CONTROLLER
The task of controller tuning is usually left to an
instrument technician with experience in the cause and
effect of process reaction and controller adjustments.
223
224. 224
Usefulness of PID Controls
Most useful when a mathematical model of the plant is not
available
Many different PID tuning rules available
Sources
K. Ogata, Modern Control Engineering, Fourth Edition, Prentice Hall,
2002, Chapter 10
IEEE Control Systems Magazine, Feb. 2006, Special issue on PID
control
Proportional-integral-derivative (PID)
control framework is a method to control
uncertain systems
225. 225
Type A PID Control
Transfer function of PID controller
The three term control signal
sT
sT
K
sE
sU
sG d
i
pPID
1
1
ssEKsE
s
KsEKsU dip
1
227. Chapter 15 - Process Control Methods 227
Control Mode Summary
228. Chapter 15 - Process Control Methods 228
Tuning the Controller
Fine-tuning is the process to optimize the controller
operation by adjusting the following settings:
Gain setting (proportional mode)
Reset rate (integral mode)
Rate (derivative mode)
Three steps are taken when tuning a systems
Study the control loop
Obtain clearance for tuning procedures
Confirm the correction operation of the system
components
229. 229
PID Tuning
Controller tuning---the process of selecting the controller
parameters to meet given performance specifications
PID tuning rules---selecting controller parameter values
based on experimental step responses of the controlled
plant
The first PID tuning rules proposed by Ziegler and Nichols
in 1942
Other resource: K. Ogata, Modern Control Engineering,
Prentice Hall, Fourth Edition, 2002, Chapter 10
230. Chapter 15 - Process Control Methods 230
Trial-and-Error Tuning
Does not use mathematical methods, instead
a chart recorder is used and several bump
tests are made in the proportional and
integral modes
Trial-and-error tuning is very time consuming
and requires considerable experience on the
part of the technician or operator
234. 234
The S-Shaped Step Response
The S-shaped curve may be characterized by two
parameters: delay time L and time constant T
The transfer function of such a plant may be
approximated by a first-order system with a
transport delay
1
Ts
Ke
sU
sC Ls
238. Chapter 15 - Process Control Methods 238
Continuous Cycling Method
The continuous cycling method analyzes the
process by forcing the controlled variable to
oscillate in even, continuous cycles
The time duration of one cycle is called an
ultimate period. The proportional setting that
causes the cycling is called the ultimate
proportional value
These two values are then used in mathematical
formulas to calculate the controller settings
239. For a set point change : set the proportional band to high
value and reduce this value to the point where the system
becomes unstable
The proportional band that required causing continuous
oscillation is the ultimate value PBu.
The ultimate periodic time is Pu.
From these two values the optimum setting can be
calculated. 239
ULTIMATE PROPORTIONAL BAND
240. Chapter 15 - Process Control Methods 240
Continuous Cycle Calculations
Proportional only controller
Proportional Gain
Kc = Gu x 0.5
KC = proportional gain,
Gu= ultimate gain
Proportional Band
PB = Pbu x 2
PB = proportional band
PBu = ultimate proportional band
241. The frequency of continuous oscillation is the cross over
frequency ωco
Pu= 2Π/ωco
241
Pu = Ultimate period of sustained cycle
245. Chapter 15 - Process Control Methods 245
Ziegler-Nichols Reaction Curve Tuning
Method
This method avoids the forced oscillations that are found
in the continuous cycle tuning method
Cycling should be avoided if the process is hazardous or
critical
This method uses step changes and the rate at which the
process reacts is recorded
The graph produces three different values used in
mathematical calculations to determine the proper
controller settings
246. Chapter 15 - Process Control Methods 246
Reaction Curve Tuning Formulas
248. Final Control Elements
These are some devices
the controller operates:
Pneumatic valves,
solenoid valves,
rotary valves,
motors,
switches,
relays,
variable frequency drives.
249. Control valves are valves used to control
conditions such as flow, pressure, temperature,
and liquid level by fully or partially opening or
closing in response to signals received from
controllers that compare a "set-point" to a
"process control variable" whose value is
provided by sensors that monitor changes in
such conditions
249
Definition
250. The opening or closing of control valves is usually done
automatically by electrical, hydraulic or pneumatic actuators.
Positioners are used to control the opening or closing of the
actuator based on electric, or pneumatic signals.
These control signals, traditionally based on 3-15psi (Pneumatic
Valves), more common now are 4-20mA ( Electrical Valves) for
industry, 0-10V for HVAC systems.
The introduction of "Smart" systems, HART, Fieldbus
Foundation, and Profibus being the more common protocols.
250
Types of Control Valves
252. Control valves are used by automated systems to
adjust flow rates.
The adjustments are dependent on the controlling system's
setup. They can be automated based on sensor data and presets
or manually controlled by an operator at a remote workstation.
For pneumatic valves, an actuator changes the current from the
controller into pressure.
The relationship of current and pressure is calculated based on
the process specifications and the equipment used.
This system will be designed by control vendors or in-house
engineers in most cases.
252
ELECTRICAL OR PNEUMATIC
CONTROL VALVES?
253. When an issue develops in a manufacturing process, the control valve
will be designed to move into an open or closed position.
The safer option is dictated based on the process and the process
stream involved.
For this reason, valves that require energy to be open, are called:
Air or electricity to open
Fail-close
Reverse Acting
The valves that require energy to be closed, are called:
Air or electricity to close
Fail-open
Direct Acting
253
Fail-Open and Fail-Close Valves
255. Fail-open valves will open and continue to allow flow when the
control valve loses energy in a failure situation.
For example, a valve might fail open to avoid allowing pressure
of non-harmful gas to build up.
Cooling system control valves will usually fail open, since in
most cases overcooling a system will not harm the equipment.
When a failure causes energy to be lost, fail-close valves will close
to keep streams contained until they can be checked and fixed.
Toxic streams will almost always fail closed to prevent
contamination.
Reactor heating streams usually fail closed in order to avoid
feeding energy to runaway reactions.
255
Examples for Fail –open & Fail-close
Valves
256. 256
Flow Characteristics of the Control
Valve
The relationship between control valve capacity and valve stem
travel is known as the Flow Characteristic of the Control Valve.
Trim design of the valve affects how the control valve capacity
changes as the valve moves through its complete travel.
Because of the variation in trim design, many valves are not
linear in nature. Valve trims are instead designed, or
characterized, in order to meet the large variety of control
application needs.
Many control loops have inherent non linearity's, which may be
possible to compensate selecting the control valve trim.
257. The most common characteristics are shown in the next figure.
The percent of flow through the valve is plotted against valve
stem position. The curves shown are typical of those available
from valve manufacturers.
These curves are based on constant pressure drop across the
valve and are called inherent flow characteristics.
257
Flow Characteristics
259. When valves are installed with pumps, piping
and fittings, and other process equipment, the
pressure drop across the valve will vary as the
plug moves through its travel.
When the actual flow in a system is plotted
against valve opening, the curve is called the
Installed Flow Characteristic.
259
Installed Flow Characteristics
260. In most applications, when the valve opens, and the resistance
due to fluids flow decreases the pressure drop across the valve.
This moves the inherent characteristic:
•A linear inherent curve will in general resemble a quick
opening characteristic
•An equal percentage curve will in general resemble a linear
curve
260
Installed flow Characteristics
263. Ball Valve
Sphere with a port in a housing, rotate to
expose channel.
Applications: Flow control, pressure
control, shutoff, corrosive fluids, liquids,
gases, high temp.
Advantages – low pressure drop, low
leakage, small, rapid opening
Disadvantages – seat can wear if used for
throttling, quick open may cause hammer263
265. Gate Valve
Sliding disk, perpendicular to flow
Applications: Stop valves, (not throttling), high
pressure and temp, not for slurries, viscous
fluids
Advantages – low pressure drop when fully
open, tight seal when closed, free of
contamination buildup
Disadvantages – vibration when partially open,
slow response and large actuating force
265
267. Butterfly Valve
rotating disk on a shaft, in a housing
Low pressure, large diameter lines
where leakage is unimportant
Advantages – low pressure drop, small
and light weight
Disadvantages – high leakage, high
actuation forces so limited to low
pressures 267
269. Check Valves
allows flow in only one direction
Swing valve similar to butterfly except
hinged along one edge rather than rotate
about the diameter, used primarily for
check valves.
269
273. Servo & Regulator Problems
Two major problems could happen in any plant:
1) REGULATOR: The most common situation is when a disturbance
appears in the plant. The controller will make correction to bring the
controlled variable to set point.
2) SERVO: Very often, operators in the control room will have to
change the set point of some controlled variable. How the controller
will bring the controlled variable to the new set point.
Both situations will be investigated in the labs 5 & 6.
273
274. LABS #5 & 6
Controlling pressure in a tank using
digital P, PI and PID digital controllers
Tuning of a P, PI and PID controller to maintain
the pressure in a water tank constant during
servo or regulator situations:
Lab #5: The main objective of the lab is to
analyze and compare the graphs of the P,PI and
PID controllers.
Lab #6: Study constants of controllers to avoid
instability in the plant. 274
276. 276
Driving your car
Sense
Vehicle Speed
Compute
Control “Law”
Actuate
Gas Pedal
Goals
Stability: system maintains desired operating point (hold steady speed)
Performance: system responds rapidly to changes (accelerate to 65 mph)
Robustness: system tolerates perturbations in dynamics (mass, drag, etc)
277. Basic Feed back Control
House is too cold
Furnace
Thermostat Controller
recognized the house is too cold
sends signal to the furnace to turn on
and heat the house
furnace turns on
heats the housenatural
gas
house temperature
measured
is temperature
below setpoint?
Set-point = 200C
Controlled variable: temperature (desired output)
Input variable: temperature (measured by thermometer in thermostat)
Set-point: user-defined desired setting (temperature)
Manipulated variable: natural gas valve to furnace (subject to control)
277
278. Output of the system y(t) is fed back to the set-pint
r(t) through measurement of a sensor
Controller senses the difference between the set point
and the output and determines the error ε(t)
Controller changes the manipulated variable u to
Process to eliminate the error.
Feedback Control is a Single Loop
278
279. Example #2 for Feedback Control
Examples:
Room temperature control
Automatic cruise control
Steering an automobile
Supply and demand of chemical engineers
Controller
Transmitter
Set point
stream
Temp
sensor
Heat loss
condensate
280. Feedback Control-block diagram
Terminology:
Set point
Manipulated variable (MV)
Controlled variable (CV)
Disturbance or load (DV)
Process
controller
Σ Controller process
Sensor +
transmitter
+
-Set point
Measured value
error
Manipulated
variable
Controlled variable
disturbance
283. Feedback control is not predictive: Controlled variable has to be
affected before controller takes action
Requires management or operators to change set points to
optimize system:
- Changes can bring instability into system
- Optimization of many input and output variables
almost impossible
Limitations of Feedback Control
283
284. Apply the principles of feed-forward and show
how this type of control can be applied.
L.O #4
284
285. 285
FEED-FORWARD CONTROL
The feedback control can never achieve perfect control of a
chemical process
Why? Because the feedback control reacts only when it has
detected a deviation of the CONTROLLED VARIABLE from the
desired set point.
However, the feed-forward control measures the disturbance
directly and takes control action to eliminate its impact on the
CONTROLLED VARIABLE
Therefore Feed-forward controllers have the theoretical
potential to achieve perfect control
286. Feedforward Control
Window is open
Furnace
Feedforward
Recognize window is open and
house will get cold in the future:
Someone reacts and changes controller
setpoint to turn on the furnace preemptively.
furnace turns on
heats the housenatural
gas
house temperature
is currently OK
turn on furnace
Decrease
setpoint to turn
furnace on
Pre-emptive move
to prevent house from
getting cold
286
287. Feed-forward control avoids slowness of feedback control
Disturbances are measured and accounted for before they
have time to affect the system
In the house example, a feed-forward system measured the fact
that the window is opened
As a result, automatically turn on the heater before the house can
get too cold
Difficulty with feed-forward control: effects of
disturbances must be perfectly predicted
There must not be any surprise effects of
disturbances
Feed-forward is a single loop
287
288. 288
THE FEEDBACK AND FEED FORWARD
CONTROL
Both control involve a single loop with :
One measurement
One manipulated variable.
However:
In a feedback control, we measure the
controlled variable
In a feed-forward control, we measure the
disturbance
289. L.O #5
Describe how the principles of cascade
control, ratio, the selective control and
split - range control are used in
processes control.
289
291. 291
CONTROL SYSTEMS WITH MULTIPLE LOOPS
Other simples configurations which may use:
* More than one measurable variable and one
manipulated variable
* One measurable variable and more than one
manipulated variable
292. CASCADE CONTROL
In this configuration, we have :
More than one measurement
One manipulated variable
292
293. 293
CASCADE CONTROL LOOPS
Cascade control is two control loops using two different
measurements :
1) One measurement for the controlled variable
2) One measurement for the disturbance
3) One manipulated variable
The loop that measures the controlled variable is the
dominant or primary or master control loop
The loop that measures the disturbance is the
secondary or slave loop
296. Ratio Control is a special type of feed-
forward control
Two disturbances are measured and
held in a constant ratio
It is mostly used to control the ratio of
flow-rates of two streams
RATIO CONTROL :
296
299. We measure both flow-rates and take
their ratio
The ratio is compared to the desired ratio
The error is sent to the ratio controller
Strategy of ratio control:
299
300. 300
SELECTIVE CONTROL
In this kind of control, we
One manipulated variable
Several controlled output
Since with one manipulated variable, we can control only
one output, The selective control systems transfer control
action from one controlled output to another according
to need
we will discuss
* Override Control
* Auctioneering control
301. 301
SAFETY OF EQUIPMENTS: OVERRIDE
CONTROL
During the normal operation of a plant or during its startup or shutdown ,
it is possible that a dangerous situation may arise and may lead to
destruction of equipment.
In such cases, it is necessary to change from production control to safety
control in order to prevent a process variable from exceeding an
allowable upper or lower limit
This can be achieved by the use of switches: The switch is used to select
between the production controller and the safety controller.
The HSS ( high selector switch) is used whenever a variable should not exceed an
upper limit
The LSS ( low selector switch) is used whenever a variable should not exceed a lower
limit.
302. EXAMPLE OF OVERRIDE
The steam header must be maintained above a minimum
pressure (PC FOR SAFETY). Steam from the header is
used to heat water in a heat exchanger.
The temperature of the hot water is controlled by TIC-101
(PRODUCTION CONTROLLER)
SAFETY FIRST: t is more important that the header
pressure be above its minimum than that the water
temperature be at its set-point.
302
304. 304
SAFETY OF EQUIPMENTS:
AUCTIONEERING CONTROL
In this control system, among several
similar measurements, the one with the
highest value will feed the controller
This is a selective control between
several measured variables.
306. The split range control has
One measurement only ( Controlled variable)
More than one manipulated variable ( control valve)
If the valves are pneumatic: The instrumentation pressure range ( 3-15 psia) is
divided.
If the valves are electrical: The instrumentation current ( 4-20 mA) is divided.
Ex: If we have two pneumatic valves:
Valve #1 will operate between 3- 9 psia and Valve #2 will operate between 9 -
15 psia.
SPLIT RANGE CONTROL
306
307. Split Range Flow Control
In certain applications, a single flow control loop cannot
provide accurate flow metering over the full range of
operation.
Split range flow control uses two flow controllers (one
with a small control valve and one with a large control
valve) in parallel.
At low flow rates, the large valve is closed and the
small valve provides accurate flow control.
At large flow rates, both valve are open.
307
308. EX: Split Range Temperature
Control
TT
Cooling
Water
Steam
Split-Range
Temperature
Controller
TT TC
RSP
308
310. DIGITAL CONTROLLER
Digital control is a branch of control theory that uses
digital computers to act as system controllers.
Depending on the requirements, a digital control system
can take the form of a microcontroller to an ASIC to a
standard desktop computer.
Since a digital computer is a discrete system, the Laplace
310
312. DIGITAL CONTROLLER
Typically, a digital controller requires:
A/D conversion to convert analog inputs to
machine readable (digital) format
D/A conversion to convert digital outputs
to a form that can be input to a plant
(analog)
A program that relates the outputs to the
inputs
312
313. 313
Block diagram of a digital control
system
control:
difference
equations
D/A and
hold
sensor
1
r(t) u(kT) u(t)e(kT)
+
-
r(kT) plant
G(s)
y(t)
clock
A/D
T
T
y(kT)
digital controller
voltage → bit
bit → voltage
314. An Large Size PLC
The main module measures
19” x 20” x 14.5”.
have upto 10,000 I/O points
supports all functions
expansion slots to
accommodate PC and other
communication devices.
Allen-Bradley PLC-3
314
315. A Small Size PLC
Measures 4.72”x 3.15” x
1.57”.
32 I/O points
Standard RS 232 serial
communication port
Allen-Bradley MicroLogix 1000
315
316. PLC ARCHITECTURE
Programmable controllers replace most of the
relay panel wiring by software programming.
Processor
I/O
Modules
Memory
Power
Supply
Program
Loader
Printer
Cassette
Loader
EPROM
Loader
Switches
Machines
Peripherals External Devices
PC
A typical PLC316
317. PLC COMPONENTS
1. Processor Microprocessor based, may allow arithmetic
operations, logic operators, block memory moves,
computer interface, local area network, functions, etc.
2. Memory Measured in words.
ROM (Read Only Memory),
RAM (Random Access Memory),
PROM (Programmable Read Only Memory),
EEPROM (Electric Erasable Programmable ROM),
EPROM (Erasable Programmable Read Only Memory),
EAPROM (Electronically Alterable Programmable
Read Only Memory), and
Bubble Memory.
317
318. PLC COMPONENTS3. I/O Modular plug-in periphery
AC voltage input and output,
DC voltage input and output,
Low level analog input,
High level analog input and output,
Special purpose modules, e.g., high speed timers,
Stepping motor controllers, etc. PID, Motion
4. Power supply AC power
5. Peripheral hand-held programmer (HHP)
CRT programmer
operator console
printer
simulator
EPROM loader
graphics processor
network communication interface
modular PC
318
320. Distributed Control Systems
Collection of hardware and instrumentation necessary
for implementing control systems
Provide the infrastructure (platform) for implementing
advanced control algorithms
321. History of Control Hardware
Pneumatic Implementation:
Transmission: the signals transmitted pneumatically are slow
responding and susceptible to interference.
Calculation: Mechanical computation devices must be relatively
simple and tend to wear out quickly.
322. History (cont.)
Electron analog implementation:
Transmission: analog signals are susceptible to noise,
and signal quality degrades over long transmission
line.
Calculation: the type of computations possible with
electronic analog devices is still limited.
323. History (cont.)
Digital Implementation:
Transmission: Digital signals are far less sensitive to
noise.
Calculation: The computational devices are digital
computers.
324. Advantages of Digital System
Digital computers are more flexible because they are
programmable and no limitation to the complexity of
the computations it can carry out.
Digital systems are more precise.
Digital system cost less to install and maintain
Digital data in electronic files can be printed out,
displayed on color terminals, stored in highly
compressed form.
325. Computer Control Networks
1. PC Control:
Good for small
processes such as
laboratory prototype
or pilot plants, where
the number of control
loops is relatively
small
PROCESS
Final
control
element
Data
acquisition
Main
Computer
Display
326. Computer Control Networks
2. Programmable Logic Controllers:
specialized for non-continuous systems such as batch
processes.
It can be used when interlocks are required; e.g., a
flow control loop cannot be actuated unless a pump
has been turned on.
During startup or shutdown of continuous processes.
327. DCS: Computer Control Networks
Operator
Control
Panel
Main
Control
Computer
Operator
Control
Panel
Archival
Data
Storage
Supervisory (host)
Computer
PROCESS
Local
Computer
Local
Computer
Local
Computer
Local Display Local Display
Data highway
To other Processes To other Processes
Local data acquisition and
control computers
3. DCS
•Most
comprehensive
328. DCS Elements-1
Local Control Unit: This unit can handle 8 to 16 individual
PID loops.
Data Acquisition Unit: Digital (discrete) and analog I/O can
be handle.
Batch Sequencing Unit: This unit controls a timing
counters, arbitrary function generators, and internal logic.
Local Display: This device provides analog display stations,
and video display for readout.
Bulk Memory Unit: This unit is used to store and recall
process data.
329. DCS Elements-2
General Purpose Computer : This unit is programmed by a
customer or third party to perform optimization, advance
control, expert system, etc
Central Operator Display: This unit typically contain several
consoles for operator communication with the system, and
multiple video color graphics display units
Data Highway : A serial digital data transmission link
connecting all other components in the system. It allow for
redundant data highway to reduce the risk of data loss
Local area Network (LAN)
330. Advantages of DCS
Access a large amount of current information from the data
highway.
Monitoring trends of past process conditions.
Readily install new on-line measurements together with local
computers.
Alternate quickly among standard control strategies and
readjust controller parameters in software.
A sight full engineer can use the flexibility of the framework to
implement his latest controller design ideas on the host
computer.
331. Modes of Computer control
signals from digital
computer
Local PID
controller
Supervisory Control mode
Direct digital Control mode
valve setting
from computer
Flow measurement
to computer
1.Manual
2.Automatic
• PID with local set point
3.Supervisory
• PID with remote set
point (supervisory)
4.Advanced
332. Additional Advantage
Digital DCS systems are more
flexible. Control algorithms can
be changed and control
configuration can be modified
without having rewiring the
system.
333. Categories of process information
ExampleType
Relay, Switch
Solenoid valve
Motor drive
1. Digital
Alphanumerical displays2. Generalized digital
Turbine flow meter
Stepping motor
3. Pulse
Thermocouple or strain gauge (mill volt)
Process instrumentation (4-20 am)
Other sensors (0-5 Volt)
4. Analog
334. A/D and D/A converters or transducers are the
Interface between digital computer and analog
instruments
(A/D) Transducers convert analog
signals to digital signals.
(Sensor Computer)
(D/A) Transducers convert digital
signals to analog signals.
(Computer Valve)
335. Data resolution due to digitization
Accuracy depends on resolution.
Resolution depends on number of bits:
Resolution = signal range × 1/(2m -1)
m = number of bits used by the digitizer (A/D) to
represent the analog data
336. Data Resolution
Signal = 0 - 1 Volt, 3 bit digitizer:
Analog range
covered
Analog
equivalent
Digital
Equivalent
Binary
representation
0 to 1/14
1/14 to 3/14
3/14 to 5/14
5/14 to 7/14
7/14 to 9/14
9/14 to 11/14
11/14 to 13/14
13/14 to
14/14
0
1/7
2/7
3/7
4/7
5/7
6/7
1
0
1
2
3
4
5
6
7
0 0 0
0 0 1
0 1 0
0 1 1
1 0 0
1 0 1
1 1 0
1 1 1
337. Data Resolution
0 1/7 2/7 3/7 4/7 5/7 6/7 1
0
1
2
3
4
5
6
7
Analog data
Digitaldata
338. Utilization of DCS
DCS vendor job:
installation
Control Engineer Job:
Configuration
Built-in PID control:
How to Tune the PID control?
339. Utilization of DCS
Implementation of advanced control:
Developed software for control algorithms, DMC,
Aspen, etc.
Control-oriented programming language supplied by the
DCS vendors.
Self-developed programs using high-level programming
languages (Fortran, C++)
343. Process flow diagrams (PFDs) are used in chemical and
process engineering. These diagrams show the flow of
chemicals and the equipment involved in the process.
Generally, a Process Flow Diagram shows only the
major equipment and doesn't show details. PFDs are
used for visitor information and new employee
training.
DEFINITION OF PFD
343
344. A Process and Instrument Drawing (P&ID) includes more details
than a PFD. It includes major and minor flows, control loops and
instrumentation.
P&ID is sometimes referred to as a Piping and Instrumentation
Drawing. These diagrams are also called flow-sheets.
P&IDs are used by process technicians and instrument and
electrical, mechanical, safety, and engineering personnel.
DEFINITION OF PI&D
344
345. PFD & PI&D
In both diagrams arrows show the flow of
material and symbols show tanks, valves,
and other equipment. The symbols used
vary somewhat from organization to
organization. So you may see several
different symbols that all represent a
motor. 345
349. Piping and Instrumentation Diagrams or simply P&IDs are
the “schematics” used in the field of instrumentation and
control (Automation)
The P&ID is used to by field techs, engineers, and
operators to better understand the process and how the
instrumentation is inter connected.
INTRODUCTION
349
350. Most industries have standardized the symbols according to the
ISA Standard S5.1 Instrumentation Symbol Specification.
Piping & Instrumentation Drawing (original)
Process & Instrumentation Diagram (also used)
Process Flow Diagram – PFD (simplified version of the P&ID)
ISA Standard S5.1 Instrumentation
Symbol Specification
350
351. Building the P&ID using examples like
pressure or temperature control.
L.O #1
351
376. Other Symbols for PFD
Table 1.2 : Conventions Used for Identifying Process Equipment
Process Equipment General Format XX-YZZ A/B
XX are the identification letters for the equipment classification
C - Compressor or Turbine
E - Heat Exchanger
H - Fired Heater
P - Pump
R - Reactor
T - Tower
TK - Storage Tank
V - Vessel
Y designates an area within the plant
ZZ are the number designation for each item in an equipment class
A/B identifies parallel units or backup units not shown on a PFD
Supplemental
Information
Additional description of equipment given on top of PFD
376
377. Equipment Numbering
XX-YZZ A/B/…
XX represents a 1- or 2-letter
designation for the equipment (P =
pump)
Y is the 1 or 2 digit unit number (1-99)
ZZ designates the equipment number
for the unit (1-99)
A/B/… represents the presence of
spare equipment377
378. Examples
T-905 is the 5th tower in unit
nine hundred
P-301 A/B is the 1st Pump in unit
three hundred plus a spare
378
379. Equipment Information
Equipment are identified by number and a
label (name) positioned above the
equipment on the PFD
Basic data such as size and key data are
included in a separate table (Equipment
Summary Table).
379