process instrumentation & control
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Welcome to Instrumentation, Process Control and Process Instrumentation & Diagram
By: Dr. Zin Eddine Dadach
Chemical Engineering department
ADMC 1
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
Define and explain the various circuit components and describe the basic of electronic theory.
L.O. #1
3
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
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
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?
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
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
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
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
We need a unit to indicate the potential energybetween 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?
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
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?
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?
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
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?
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:
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
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
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
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
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
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?
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)
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 volts
37
ELECTRICAL POWER
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
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:
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:
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
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
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
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
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
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
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?
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
AMERICAN SCALE:
RELATIONSHIP BETWEEN FAHRENHEIT AND CELSIUS SCALES :
320F = 00C
2120F= 1000C
T(0F)= 1.8xT(0C) +32
53
FAHRENHEIT SCALE
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
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
RTD= RESISTANCE TEMPERATURE DETECTOR
THERMISTOR= THERMAL RESISTORS
THERMOCOUPLES
Radiation pyrometers
57
TEMPERATURE SENSORS USED FOR PROCESS CONTROL SYSTEMS
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
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
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 temperature
61
TEMPERATURE COEFFICIENT α OF A RTD
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
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
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
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)
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
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
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
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
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
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
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
ε = α. ( T2-T1)WHERE:
ε= THE EMF
TYPES OF THERMOCOUPLES
α = SEEBECK COEFFICIENT
T2 ,T1= JUNCTION TEMPERATURE IN K
76
SEEBECK COEFFICIENT
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
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
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
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
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
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
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 #2TEMPERATURE SENSORS
83
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
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
DEFINITION OF PRESSURE
PRESSURE IS THE AMOUNT OF FORCE EXERTED ON A UNIT AREA OF A SUBSTANCE:
A
FP
88
P= Pressure
PI= Pressure Indicator
PT= Pressure Transmitter
PC= Pressure controller
PRC= Pressure Recorder & Controller
PCV= Pressure Control Valve
PSV= Pressure Safety Valve
PRV= Pressure Relief Valve.
TAG DESCRIPTORS FOR PRESSURE
SI UNITS:1Pa = 1N/M2=1KG/S2.M1ATM (ATMOSPHERIC PRESSURE)= 1.01x105 Pa1 ATM= 101 kN/M2
1ATM= 760 MM. HG
US UNITS:1PSIA = 1LBF/IN2
1PSIA = 6894.7 Pa1ATM= 14.696 PSIA
90
UNITS OF PRESSURE
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
THE PRESSURE OF A FLUID IN A PIPE IS MEASURED BY A PRESSURE GAUGE.
FLOW CALCULATED BY BERNOUILLI EQUATION
93
DYNAMIC PRESSURE
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
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
In many cases, gauge pressure is more important than the absolute pressure because we read gauge pressure in manometers.
Pg= Pabs- Patm96
GAUGE PRESSURE
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
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
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
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
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
In any chemical plant, you will find tanks, reservoirs, vessels and drums where liquids are stored. These could be for:The feed of the plantIntermediate between sectionsThe products before selling themLiquid capacities are also found in distillation
columns and reactors
107
LIQUID CAPACITIES IN A CHEMICAL PLANT
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
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
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
Level measurement sensors are divided into two categories:
point level switches for ALARMS
continuous level gauges for CONTROL
111
LEVEL MEASUREMENT SENSORS
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
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
Float
Capacitance
Conductive level probes
Thermal & light beam
115
SENSORS FOR POINT LEVEL MEASUREMENT
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
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
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
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
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?
F= Flow
FI= Flow Indicator
FT= Flow Transmitter
FC= Flow controller
FRC= Flow Recorder & Controller
FCV= Flow Control Valve
TAG DESCRIPTORS FOR LEVEL
125
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
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
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)
A) MATERIAL BALANCE OF A PLANT: VERY VERY IMPORTANT Measure flow of feedsMeasure flow of productsWe 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
In the instrumentation market, we find two types of flow-meters:
Energy-extractive Flow meters
Energy additive Flow meters
130
FLOW MEASUREMENT TECHNIQUES
Several sensors rely on the pressure drop or headoccurring as a fluid flows by a resistance.
131
THE PRINCIPLE OF FLOW SENSORS
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
1BB
BAA
A vg
zp
vg
zp
133
From Bernouilli Equation:
𝑄 = ∆𝑃.2( 𝐴1.
2 𝐴22 )
𝜌(𝐴12 −𝐴2
2)
Pressure drop in PaArea in m2
Density in kg/m3
Q in m3/s
Calculating volumetric flow rate Q
134
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
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
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
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
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
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
HOW ARE THE VALUES OF THE IMPORTANT VARIABLES ( SET POINTS) MADE CONSTANT?
ACTING ON SOME OTHER LESS IMPORTANTVARIABLES OF THE PROCESS IN ORDER TO SUPPRESS THE EFFECTS OF EXTERNAL DISTURBANCES ON THE IMPORTANTVARIABLES
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.”
PROCESSSome Quality or Quantity
of theMaterial or Energy
Input Energy
or Material
DesiredResult
Example of a Temperature Process
Heating Element
Water BathTemperature
The objective of this process is to maintain a constant water bath temperature.
Temperature Process Terminology
Heating Element
Water BathTemperature
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)
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.
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
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
DesiredResult
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.
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
DesiredResult
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.
Review of Measuring Means
Pressure
Level
Flow
Temperature
ThermocouplesRTDs / ThermistorsFilled SystemsBi-metallic
Strain gaugePiezo-electricCapacitanceBourdon Tube
Head meters (orifice, venturi)Coriolis, velocity, Mass,
Mechanical FloatsGuided WaveWeight (load cell)UltrasonicDifferential Pressure
Transmitters
Pressure Transmitter
Level Transmitter
Differential Pressure Cell
Flow Transmitter
Temperature Transmitter
Pneumatic3-15 PSI
Electrical
Current4 – 20 mA0 – 20 mA10 – 50 mA
Voltage
0 – 5 V
1 – 5 V
0 – 10 V
Digital
ON/OFF
Field Bus
ModBus
ProfiBus
HART
Manual ControlOpen loop (or manual control) is used when very
little change occurs in the Process Variable (PV)
Manipulated Variable
DesiredResult
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
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.
156IN OUR CASE:
Temperature of the tank has to be controlled.
Temperature SHOULD FIRST BE MEASURED.
THE EQUIPMENT FOR temperature MEASUREMENT IS : thermocouple
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
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
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 PLANT2) 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.
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
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.
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
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
EXAMPLE OF OPEN LOOP SYSTEM : SYSTEM WITH NO CONTROL
level
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166
166
LEVEL WITH SET POINT BUT NO CONTROL
LEVEL2
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167
167
169A) MANUAL CONTROL DURING START UP AND SHUT DOWN: OPERATOR CONTROL THE
PLANT OPERATIONS
LEVEL3
170AUTOMATIC 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
171CONTINUOUS AUTOMATIC CONTROL:
THE MOST USED CONTROLLERS: PROPORTIONAL ( P)
PROPORTIONAL- INTEGRAL ( PI)
PROPORTIONAL-INTEGRAL-DERIVATIVE ( PID)
172CONTROL SYSTEM: P,PI,PID CHANGE THE SET POINT OF THE LEVEL AND OBSERVE THE
BEHAVIOR OF THE PROCESS
LEVEL5
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)
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173
173
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
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174
174
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
Closed Loop ControlClosed 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)
CONTROLLINGMEANS
Manipulated Variable
DesiredResult
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
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)
Single Loop Feedback Control
1. Measuring Means
2. Controlling Means
3. Final Control Element
4. Temperature Process
Temperature Controller and Recorder
SensingBulb
Temperature Transmitter
Pneumatic Control Valve
Heat Exchanger
Steam
2
34
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.
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?)
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
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
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
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
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
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
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
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)
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191
191
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
Final Correcting Element % Change
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 ?
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193
SIGN OF THE GAIN KC
If he controller is direct acting the gain K is positive.
When the controller is reverse actingthe gain K is negative
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194
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.
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195
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
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196
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
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197
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
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198
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.
FpsiFF
psipsipKcp
0
00/3
)7175(
)315(
%10100.)60100(
)7175(00
00
FF
FFPB
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199
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.
FpsiF
psipsiKcp
0
0/4.0
30
)315(
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200
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
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201
EXAMPLES OF USES OF A PROPORTIONAL CONTROLLER
Proportional controllers are mostly used for level controlwhere 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.
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202
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
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
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
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
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
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
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
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
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
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
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:
100oC
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
Lets have a look now at the PID controller
Well.. That explains the “P+I Action”...
The “Differential Action”
214
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
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
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
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 !oC
oC
oC
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
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
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 pointOFF 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
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
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
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
Type A PID Control
Transfer function of PID controller
The three term control signal
sT
sTK
sE
sUsG d
i
pPID
11
ssEKsEs
KsEKsU dip 1
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
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
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
231
Ziegler-Nichols Tuning Methods
Two formal procedures for tuning control loops:
Step response of plant
Continuous cycling method
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
237
Ziegler-Nichols PID Tuning---Second Method
Use the proportional controller to force sustained oscillations
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
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
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
The frequency of continuous oscillation is the cross over
frequency ωco
Pu= 2Π/ωco
241
Pu = Ultimate period of sustained cycle
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
Final Control Elements
These are some devices the controller operates:
Pneumatic valves, solenoid valves, rotary valves, motors, switches, relays, variable frequency drives.
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
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
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?
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
Control valves
Reverse acting: Fail-close or Air to Open
Direct Acting: Fail-Open or Air to close
254
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
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.
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
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
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
Ball ValveSphere 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
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 force265
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
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
Rupture Disk (not a valve – ruptures at a set
pressure)
271
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
LABS #5 & 6Controlling 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
Driving your car
SenseVehicle Speed
ComputeControl “Law”
ActuateGas 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)
Basic Feed back Control
House is too coldFurnace
Thermostat Controllerrecognized the house is too cold
sends signal to the furnace to turn on and heat the house
furnace turns onheats the housenatural
gas
house temperaturemeasured
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
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
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
sensorHeat loss
condensate
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
281
THE ELEMENTS OF A FFEDBACK PROCESS CONTROL SYSTEM
LEVEL6:
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
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
Feedforward Control
Window is openFurnace
FeedforwardRecognize 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 onheats the housenatural
gas
house temperatureis currently OK
turn on furnace
Decreasesetpoint to turnfurnace on
Pre-emptive moveto prevent house from getting cold
286
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
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
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
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
CASCADE CONTROL
In this configuration, we have :
More than one measurement
One manipulated variable
292
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
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
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
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 needwe will discuss
* Override Control* Auctioneering control
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.
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
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.
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
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
EX: Split Range Temperature Control
TT
Cooling
Water
Steam
Split-Range
Temperature
Controller
TT TC
RSP
308
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
PLC: Programmable Logic Controller
CPU
System
User LadderDiagram
Working memoryregisters
Input
Flag
Output
Input Module
OutputModule
311
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
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/DT
T
y(kT)
digital controller
voltage → bit
bit → voltage
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
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
PLC ARCHITECTUREProgrammable controllers replace most of the
relay panel wiring by software programming.
ProcessorI/O Modules
MemoryPower Supply
Program Loader
Printer
Cassette Loader
EPROM Loader
Switches
Machines
Peripherals External Devices
PC
A typical PLC316
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
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
Distributed Control Systems
Collection of hardware and instrumentation necessary for implementing control systems
Provide the infrastructure (platform) for implementing advanced control algorithms
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.
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.
History (cont.)
Digital Implementation:
Transmission: Digital signals are far less sensitive to noise.
Calculation: The computational devices are digital computers.
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.
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
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.
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 highwayTo other Processes To other Processes
Local data acquisition and
control computers
3. DCS
•Most comprehensive
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.
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)
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.
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
Additional Advantage
Digital DCS systems are more flexible. Control algorithms can be changed and control configuration can be modified without having rewiring the system.
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
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)
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
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
Utilization of DCS
DCS vendor job:
installation
Control Engineer Job:
Configuration
Built-in PID control:
How to Tune the PID control?
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++)
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
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
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
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
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
Learn about Shared Displays/Shared Control and draw a summary of instrument type & location
L.O #5
362
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 PFD376
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 equipment
377
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
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
Number of streams and information
Stream Number 1 2 3 4 5 6 7 8 9 10
Temperature (°C) 25 59 25 225 41 600 41 38 654 90
Pressure (bar) 1.90 25.8 25.5 25.2 25.5 25.0 25.5 23.9 24.0 2.6
Vapor Fraction 0.0 0.0 1.00 1.0 1.0 1.0 1.0 1.0 1.0 0.0
Mass Flow (tonne/h) 10.0 13.3 0.82 20.5 6.41 20.5 0.36 9.2 20.9 11.6
Mole Flow (kmol/h) 108.7 144.2 301.0 1204.4 758.8 1204.4 42.6 1100.8 1247.0 142.2
Component Mole Flow
(kmol/h)
Hydrogen 0.0 0.0 286.0 735.4 449.4 735.4 25.2 651.9 652.6 0.02
Methane 0.0 0.0 15.0 317.3 302.2 317.3 16.95 438.3 442.3 0.88
Benzene 0.0 1.0 0.0 7.6 6.6 7.6 0.37 9.55 116.0 106.3
Toluene 108.7 143.2 0.0 144.0 0.7 144.0 0.04 1.05 36.0 35.0
A Portion of Table 1.5
382
386
THE SECTIONS OF THE UNIT
The natural gas liquefaction ( NGL) is divided into 4 sections:
- The feed preparation Section
- The expansion and separation Section
- The recovery Section
- The propane refrigerant Section
387
THE FEED PREPARATION SECTION
The feed preparation section contains:
- Feed Gas Scrubber D-405
- Feed Gas separator D-401
- The feed gas/ gas off exchanger E-401
- The gas and liquid dehydrators
Purpose of the section: To remove any liquid from the inlet gas and to prepare the inlet feed gas for cooling and separation
388
EXPANSION AND SEPARATION SECTION
The expansion and separation section contains:- High Level Gas Chiller E-402- Low Level Gas Chiller E-404- Chiller Separator D-403- Expander Feed Separator D-404- Expander KT-100- Cold gas/ off gas exchanger E-405- Intermediate gas/off gas exchanger E-403
Purpose of the section: Cool the inlet feed gas in a series of heat exchangers and to provide several feeds at various temperatures to the demethanizer D-402
389THE SEPARATION SECTION
The separation section contains:
- Demethanizer Column D-402
- Demethanizer Reboiler E-406
- Compression Section K-100 and KT-100
- Two product booster pumps P-401A/B
390
REFRIGERATION SECTION
Propane refrigerant is used
Cooling takes place in the high Level Chiller E-402 and Low Level Chiller E-404
392
CONTROL OF THE FEED PREPARATION SECTION ( SCRUBBER)
TI-350: indicates inlet feed gas to gas Scrubber D-405 ( 0-650C)
HC-350: This is a manual valve to control the flow of inlet gas to the feed gas scrubber D-405
HS-400: This hand switch initiates the plant emergency shut down by closing the inlet feed valve HC-350
393
FI-405: Indicates the inlet feed gas vapor from the top of the scrubber D-405 ( 0 -1600 kNm3/d
FI-406: Indicates the liquid flow from the bottom of D-405 ( 0-50 m3/h)
LC-350: this controls the level of liquid in the scrubber D-405 by regulating the flow of methane to the D-402. Flow shown by FI-406
394
CONTROL OF THE FEED GAS SEPARATOR D-401
LC-411: controls the liquid level in the feed gas separator D-401 by regulating the flow of liquid from the bottom of D-401 to tray 20 of D-402
TI-400: Indicates the vapor temperature exiting the top of D-401
395
CONTROL OF HIGH LEVEL GAS CHILLER
LC-401:This controls the propane refrigerant level in the shell side of the High level gas chiller E-402
LAH-401: this alarm will start when the level of the liquid in the shell side of E-402 rises above 80% as read in LC-401
CONTROL OF HIGH LEVEL GAS CHILLER
PC-405: from 0 to 7 Barg , this controls the pressure in E-402 by regulating the flow of propane vapor from the chiller to the inlet of second stage of compressor ( not simulated)
PAH-405: This alarm when the pressure in the gas chiller E-402 rises above 2.94 as read in PC-405
396
397
CONTROL OF THE LOW LEVEL GAS CHILLER
LC-402:This controls the liquid propane in the shell side of E-404 by regulating the flow of propane from the propane tube side of E-409
LAHL-402: Alarm when the propane refrigerant level in shell side of E-404 rises above 80% or falls below 20%
PC-406 : from 0 to 3.5 barg, this controls the pressure in E-404 by regulating the flow of propane vapor from the chiller to first stage of the compressor ( not simulated)
CONTROL OF THE LOW LEVEL GAS CHILLER
PAH-406: Alarm when the pressure in E-404 rises above 0.62 Barg as read in PC-406
Ti-404: indicates the temperature of the effluent from the tube shell of E-404. Indicates also the temperature of the feed entering D-403
LC -403: Controls the level of D-403 by regulating the flow from the bottom of the separator to tray 14 of D-402
398
399
CONTROL OF THE EXPANSION SECTION
PC-401:from 0 to 50 Barg, this controls the pressure of the expander feed separator through a split-range control. This is also the pressure at the inlet to the expander section.
PAHL- 401: alarm when the pressure in D-404 rises above 48.0 Barg or falls below 44.13 Barg as read at PC-401
TI-405: Indicates the temperature of the feed gas as it enters D-404. this is also the temperature of the feed gas from the tube side of E-405
400
CONTROL OF THE EXPANSION SECTION
LC-404:this controls the liquid level in the chiller separator D-404 by regulating the flow of liquid from the bottom of separator to tray 8 of D-402
LAH-404: Alarm when the liquid level in D-404 rises above 80% as read at LC-404
HC-401: Manual control adjusts the position of the inlet louver vanes in expander4 section
HS-401:switch that determines witch controller HC-401 or PC-401 will regulate the position of the expander inlet vanes
401
CONTROL OF THE EXPANSION SECTION
FC-403: from 0 to 1600 kNm3/d, this regulates the inlet flow to the compressor section
FAL-403: Alarm when compressor recycle ( anti-surge) flow falls below 750.KNm3/d as read in FC-403
HS 100: this switch operates the expander section KT-100. When the switch in ON , the expander is in operation
402
DEMETHANIZER CONTROL SYSTEM
AAH-401 : Alarm when the concentration of methane in the bottom of the demethanizer D-402, rises above 2% as read in AC-401
AC-401: Controls the methane composition in the bottom of the demethanizer D-402 from 0 to 10% by sending a remote set-point signal to the demethanizer reboiler effluent temperature controller TC-403
403
DEMETHANIZER CONTROL SYSTEM
FI-408: this instrument indicates the flow of the methane product from the bottom of the demethanizer D-402 to the product pipeline . This flow is regulated by the Demethanizer level controller LC-400.
HS-401A-B : this are the product booster pumps. These pumps draw methane product from the bottom of the demethanizer D-402 and send it to the product line
404
DEMETHANIZER CONTROL SYSTEM
LAHL-400: This alarm fires when the level in the bottom of the demethanizer D-402 rises above 80% or below 20% as read at LC-400
LC-400 : this controls the level in the bottom of the demethanizer by regulating the flow of methane product from the bottom of D-402 as read in FI-408
405
DEMETHANIZER CONTROL SYSTEM
PAHL-402 : Alarm when the pressure in the top of D-402 rises above 13.48 Barg or below 10.69 Barg as read in PC-402
PC-402: Controls the pressure in the top of D-402 by regulating the flow of vapor from the discharge of the compression section K-100
406
DEMETHANIZER CONTROL SYSTEM
TAH-401 D-402 TRAY-1 Temperature:
This alarm is on when the temperature of TRAY-1 of D-402, rises above -950C as read in TC-401
TC-401( -1250C to -200C):
This controls the temperature of the feed of tray-1 of D-402 by regulating the amount of gas bypassing KT-100 and flows directly from the top of D-403
407
DEMETHANIZER CONTROL SYSTEM
TC-403: ( from -200C to 1000C):
This controls the effluent temperature from the D-402 re-boiler E-406 by regulating the flow of heating medium to the re-boiler. This controller can receive a signal control from the D-402 methane composition controller AC-401
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