PROCESS CONTROL FUNDAMENTALSPROCESS CONTROL FUNDAMENTALS
CONTROL LOOPCONTROL LOOP
© N.Bolf, Faculty of Chemical Engineering and Technology, University of Zagreb
CONTENTCONTENT
• Basics and short history of process control
CONTENTCONTENT
Basics and short history of process control
•• TheThe conceptconcept ofof controlcontrol looploop
• Controlled variable, manipulated variable and disturbance
•• FeedbackFeedback andand feedforwardfeedforward controlcontrol
Open and closed control loop• Open and closed control loop
•• ControlControl objectiveobjective andand benifitsbenifits
© N.Bolf, Faculty of Chemical Engineering and Technology, University of Zagreb
PROCESS CONTROLPROCESS CONTROL
Process controlProcess control
PROCESS CONTROLPROCESS CONTROL
Process controlProcess control
• Process dynamic behaviour analysis
• Development of dynamic process model
• Control loop and system optimizationControl loop and system optimization
• Before we integrate our engineering knowledge and skills to be able to design and analyze processes for the safe operation and g y p phigh quality products we need to master process control methods.
ForFor analysing and design of the process system we need analysing and design of the process system we need knowledge on control and process control skillsknowledge on control and process control skills!!
© N.Bolf, Faculty of Chemical Engineering and Technology, University of Zagreb
knowledge on control and process control skillsknowledge on control and process control skills!!
A LITTLE BIT OF HISTORYA LITTLE BIT OF HISTORYA LITTLE BIT OF HISTORY ...A LITTLE BIT OF HISTORY ...
The remains of tin mines in Cornwall (England) where the first steam engine was installed
1788 t if l f t ti t l f t ti d f t t bi
© N.Bolf, Faculty of Chemical Engineering and Technology, University of Zagreb
1788. – centrifugal governor for automatic control of rotating speed of steam turbine (mechanical feedback connection)
THE WATTS "GOVERNOR“THE WATTS "GOVERNOR“ 1st1st INDUSTRIAL CONTROLLERINDUSTRIAL CONTROLLERTHE WATTS GOVERNORTHE WATTS GOVERNOR 1st 1st INDUSTRIAL CONTROLLERINDUSTRIAL CONTROLLER
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HYSTORY OF AUTOMATIC CONTROLHYSTORY OF AUTOMATIC CONTROLHYSTORY OF AUTOMATIC CONTROLHYSTORY OF AUTOMATIC CONTROL
17881788 James Watt – automatic controller for steam engine(first modern control application)( pp )
19201920ss the begining of wider use of automatic control
19601960ss digital computers and microprocessors for controlbeginning of the modern automatic control era
19901990ss the methods of inteligent control (predictive control, expert system, neural networks, fuzzy logic)
T dT d i th i d t th 10% f i t t tToday Today in the process industry more than 10% of investments are spentfor instrumentation and control.
© N.Bolf, Faculty of Chemical Engineering and Technology, University of Zagreb
PROCESS ANALYSISPROCESS ANALYSIS –– SYSTEM VIEWSYSTEM VIEWPROCESS ANALYSIS PROCESS ANALYSIS –– SYSTEM VIEWSYSTEM VIEW
PROCESSβ
INPUTS OUTPUTS
Input variablesInput variables ((inputsinputs))Characterize material and energy flows which cause the change in processaccumulationDisturbances or manipulated variables
O t t i blO t t i bl (( t tt t ))Output variablesOutput variables ((outputsoutputs))Characterize process/system behaviour as the response on input changes.Key process variables.
ParametersParametersDefine the structure of the process/systemβ1 – parameters which characterize physical and chemical properties (e.g. λ, ρ, η, h)
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β1 p p y p p ( g , ρ, η, )β2 – parameters which characterize process equipment (e.g. dimensions, geometry)
CONTROL LOOPCONTROL LOOP
Disturbance variable
CONTROL LOOPCONTROL LOOP
PROCESS
Disturbance variable
Controlled Variable CVManipulated Variable DV
PROCESSMV
Final control
Process Variable
TransducerFinal control element
PV
ErroreOPOP
PV
ALGORITHMALGORITHMOutputOutput
ComparatorOPOP
© N.Bolf, Faculty of Chemical Engineering and Technology, University of Zagreb SP Set Point
CONTROLLER
CONTROL LOOPCONTROL LOOP
DisturbancesDisturbances
CONTROL LOOPCONTROL LOOP
ControlledControlled variablevariableManipulatedManipulated variablevariable
DisturbancesDisturbances
PlantPlant / Process/ Process SensorSensor
FinalFinal controlcontrolelement (element (ValveValve))
PressurePressureLevelLevelFlowFlow
ControllerController
element (element (ValveValve)) FlowFlowTemperatureTemperatureCompositionComposition
T dT dControllerController TransducerTransducer
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MeasuredMeasured variablevariableControlControl actionaction
CONTROLLED VARIABLECONTROLLED VARIABLECONTROLLED VARIABLECONTROLLED VARIABLE
CV - Controlled VariablePV - Process Variable
The variables that are monitored and controlled.Actual (current) value of the controlled variable.
Flow FITemperature TILevel LILevel LIPressure PITurbine rotatin speed SIProduct quality QI, AI
In some complex plant we can find more than 200
Product quality QI, AI
© N.Bolf, Faculty of Chemical Engineering and Technology, University of Zagreb
In some complex plant we can find more than 200 controlled variables and 100 manipulated variables.
MANIPULATED VARIABLEMANIPULATED VARIABLEMANIPULATED VARIABLEMANIPULATED VARIABLE
MV - Manipulated VariableOP - Output Variable
To keep controlled variable at the desired value (set point -SP) we must change (manipulate) some input variable(s).
Examples
Control valve position Flow control in a pipeControl valve position Flow control in a pipeVent valve position Pressure controlInlet/outlet flowrate Level control in a tankSteam flow in a reboiler Temperature control in a columnSteam flow in a reboiler Temperature control in a columnReflux flow Top product purity control
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DISTURBANCESDISTURBANCES
DV - Disturbance Variable
DISTURBANCESDISTURBANCES
The variables that affect the process, but we can not directly influence on it – it is out of our controlbut we can not directly influence on it it is out of our control.
Examples
Change in input stream composition influence distillation product properties.A bi t t t h ff t th l ’ t tAmbient temperature changes can affect the column’s temperature.Variation in calorific value of the fuel gas affects the furnace temperature.
DV is also input variable, but:• MV is under our influence!
© N.Bolf, Faculty of Chemical Engineering and Technology, University of Zagreb
• DV is not under our influence!
CONTROL LOOP EXAMPLECONTROL LOOP EXAMPLE
SP
CONTROL LOOP EXAMPLECONTROL LOOP EXAMPLE
FCOP
FT
PVProtok kroz cijev
Flow through the pipe
Regulacijskiventil
jthe pipeControlvalve
FT – flow transmitterFC flow controlFC – flow controlSP – set point (enter by operator)PV – actual measured value of controlled variableOP – calculated controller output
© N.Bolf, Faculty of Chemical Engineering and Technology, University of Zagreb
AUTOMATIC STABILIZATIONAUTOMATIC STABILIZATIONAUTOMATIC STABILIZATIONAUTOMATIC STABILIZATION
.SP konst
• Set point is constant and correspons to the desired value of the controlled variable;
• Controller acts on any deviation of controlled variable (PV) from desired value (SP);
• Most industrial control falls into automatic stabilization category;
• The purpose of automatic stabilization is to maintain• The purpose of automatic stabilization is to maintain material/energy accumulation close to the zero.
© N.Bolf, Faculty of Chemical Engineering and Technology, University of Zagreb
TRACKING CONTROLTRACKING CONTROL
SP k t
TRACKING CONTROLTRACKING CONTROL
.SP konst
• The setpoint is changing to follow a predefined trajectory;The setpoint is changing to follow a predefined trajectory;
• Controlled variable (PV) tracks down the changes of set point (SP);
ExampleExample
During the industrial furnace start-up heat supply is changed so that the furnace temperature gradually increases following the defined profile (eg. at an uniform rate).)
Applying tracking control on batch reactor the process is controlled to follow desired temperature profile (according to the recipe!).
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MANUAL FEEDBACK CONTROLMANUAL FEEDBACK CONTROLMANUAL FEEDBACK CONTROL MANUAL FEEDBACK CONTROL
Controlled variableManipulated flowPROCESS
• On the output stream an indicator providing to the operator with the actual value of the controlled variable (PV);actual value of the controlled variable (PV);
• The operator reads the indicator and adjusts the valve to achieve the desired value of the variable guided(SP);
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AUTOMATIC FEEDBACK COAUTOMATIC FEEDBACK CONTROLNTROLAUTOMATIC FEEDBACK COAUTOMATIC FEEDBACK CONTROLNTROL
Automatic feedback control
Disturbances
M i l d i blPROCESS
Controlled variable
Required value of
Manipulated variable
Measured value of controlled variable
Required value ofmanipulated variable
Algorithm
CONTROLLER Set Point
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MANUAL FEEDFORWARD CONTROLMANUAL FEEDFORWARD CONTROLMANUAL FEEDFORWARD CONTROLMANUAL FEEDFORWARD CONTROL
Disturbance
Controlled variableManipulatedflow
PROCESS
• The operator observes an indicator of disturbaces and adjusts the manipulated variable;
• The aim is to prevent any change or variation in the controlled variable caused by the disturbance;
© N.Bolf, Faculty of Chemical Engineering and Technology, University of Zagreb
• Feedback control operates to eliminate errors, but feedforward control operates to prevent errors.
OPEN / CLOSED CONTROL LOOPOPEN / CLOSED CONTROL LOOP
Open loopOpen loop
OPEN / CLOSED CONTROL LOOPOPEN / CLOSED CONTROL LOOP
• no continous feedback information;• manual controlmanual control;1
23
4
5
• the water temperature depends on power of the heater, water quantity and somewhat on ambient temperature and pressure;
Closed loopClosed loop
• feedback controlfeedback control;
t ti t l
120
140 180200
220 +
- • automatic control;
• for temperature control we needt t t
© N.Bolf, Faculty of Chemical Engineering and Technology, University of Zagreb
temperature measurement(controlled variable).Temperature measurement
AUTOMATIC FEEDAUTOMATIC FEEDFORWARD CONTROLFORWARD CONTROLAUTOMATIC FEEDAUTOMATIC FEEDFORWARD CONTROLFORWARD CONTROL
Set point Disturbance
TransducerFeedforwardcontroller
Manipulatedvariable
Controlledvariable
Proces
• Sensor measures the disturbance and sends to feedforward controller;
• Based on mathematical relationship feedforward controller computes the change of manipulated variable to compensate for variation in disturbance;
© N.Bolf, Faculty of Chemical Engineering and Technology, University of Zagreb
• It is common for a process to have combined feedforward and feedback control loops. Why?
CONTROL SYSTEMCONTROL SYSTEM DIAGRAMDIAGRAM
P & IP & I (Process and InstrumentationProcess and Instrumentation) diagramdiagram
CONTROL SYSTEM CONTROL SYSTEM DIAGRAMDIAGRAM
P & IP & I (Process and InstrumentationProcess and Instrumentation) diagramdiagram
Graphical description of control loops using standard symbols(ANSI/ISA S5 1 Instrumentation Symbols and Identification)(ANSI/ISA-S5.1 Instrumentation Symbols and Identification)
F – flowF flowL – levelT – temperatureP pressureP – pressureA – analyzerU – surge
C – control, regulation
© N.Bolf, Faculty of Chemical Engineering and Technology, University of Zagreb
CONTROL LOOP INSTRUMENTATIONCONTROL LOOP INSTRUMENTATIONCONTROL LOOP INSTRUMENTATIONCONTROL LOOP INSTRUMENTATIONPower
4 20 mA4 20 mA signal 4-20 mA 4-20 mA signal
ControllerInstrumentation
air 20 psigI/P
t d air 20 psig
Signal 3-15 psigtransducer
Pressure transmitter
Pneumaticvalve
transmitter
© N.Bolf, Faculty of Chemical Engineering and Technology, University of Zagreb
PROCESS CONTROL HIERARCHYPROCESS CONTROL HIERARCHYPROCESS CONTROL HIERARCHYPROCESS CONTROL HIERARCHY
PlanningPlanning & & SchedulingScheduling ((daysdays--monthsmonths))
RealReal--Time OptimisationTime Optimisation (hours(hours--days)days)
Multivariable & Constraint ControlMultivariable & Constraint Control (minutes(minutes--hours)hours)
RegulatoryRegulatory ControlControl ((secondsseconds--minutesminutes))RegulatoryRegulatory ControlControl ((secondsseconds--minutesminutes))
SafetySafety & & EnvironmentalEnvironmental / / EquipmentEquipment ProtectionProtection ((secondsseconds))Safety & Environmental / Equipment ProtectionSafety & Environmental / Equipment Protection (seconds)(seconds)
MeasurementMeasurement & & ActuationActuation ((secondsseconds))Measurement & ActuationMeasurement & Actuation (seconds)(seconds)
© N.Bolf, Faculty of Chemical Engineering and Technology, University of ZagrebPROCESSPROCESSPROCESSPROCESS
WHWHY IS GOOD CONTROL SO IMPORTANTY IS GOOD CONTROL SO IMPORTANT??WHWHY IS GOOD CONTROL SO IMPORTANTY IS GOOD CONTROL SO IMPORTANT??
• To maximize plant throughput;
• To minimize fuel and energy consumption;To minimize fuel and energy consumption;
T i d t lit• To increase product quality;
•• For smooth and stabil operationFor smooth and stabil operation;pp ;
• Optimal process performanceOptimal process performance..
© N.Bolf, Faculty of Chemical Engineering and Technology, University of Zagreb
THE ROLE AND THE AIM OF PROCESS CONTROLTHE ROLE AND THE AIM OF PROCESS CONTROLTHE ROLE AND THE AIM OF PROCESS CONTROLTHE ROLE AND THE AIM OF PROCESS CONTROL
TheThe role role andand thethe aimaim:
• safety• enviromental protection• equipment protection• smooth operation
product quality• product quality• profit• monitoring and diagnosticsmonitoring and diagnostics
© N.Bolf, Faculty of Chemical Engineering and Technology, University of Zagreb
EXAMPLE: SEPARATION COLUMNEXAMPLE: SEPARATION COLUMNEXAMPLE: SEPARATION COLUMNEXAMPLE: SEPARATION COLUMN
BakljaInput stream: C1-C5
Gas product: C1-C2
Gas product
TheThe role role ofofcontrolcontrol: PC
Gas product: C1 C2
Liquid product: C3-C5
• safety
• env. protectionInput stream
• equip. protection
• smooth operationLCFC
Liquidproduct
• smooth operation
• product quality
AC product
Lower key componentHot stream Steam• profit
• monitoring anddiagnostics
AC
UA
© N.Bolf, Faculty of Chemical Engineering and Technology, University of Zagreb
diagnostics
Time
BLOCK DIAGRAMSBLOCK DIAGRAMSBLOCK DIAGRAMSBLOCK DIAGRAMS
Consistent way of representing control systemConsistent way of representing control systemConsistent way of representing control system.Consistent way of representing control system.Simple, symbolic, graphical tool commonly used in automatic control.Simple, symbolic, graphical tool commonly used in automatic control.
Basic symbolsBasic symbols:Basic symbolsBasic symbols:
Circle - algebraic addition or subtraction;
Rectangle - multiplication or dividing; - block output is product of block content and block input;
Arrows - the flow of information - algebraic sign (+ or -)
© N.Bolf, Faculty of Chemical Engineering and Technology, University of Zagreb
QUESTION AND EXCERCISESQUESTION AND EXCERCISESQUESTION AND EXCERCISESQUESTION AND EXCERCISES
1. Consider an electric oven in a typical modern kitchen. Id tif th t ll d i bl th i l t d i bl d thIdentify the controlled variable, the manipulated variabl, and the disturbances!
2 Consider an automatic gas fired home hot water tank2. Consider an automatic gas-fired, home hot-water tank.Identify the controlled variable, the manipulated variable, and the Identify the controlled variable, the manipulated variable, and the disturbancesdisturbances!!
3. Imagine your own backyard swimming pool. Describe a manual control system to measure pH and to add an Describe a manual control system to measure pH and to add an acid solution to adjust pH! Define the controlled varialbe, the acid solution to adjust pH! Define the controlled varialbe, the
i l t d i bl d th di t b !i l t d i bl d th di t b !manipulated variable and the disturbances!manipulated variable and the disturbances!
4. Now automate the control of your swimming pool! Assume you have a tank of acid solution to pump into your pool to control pHhave a tank of acid solution to pump into your pool to control pH. Use feedback control.
© N.Bolf, Faculty of Chemical Engineering and Technology, University of Zagreb
LITERATURELITERATURELITERATURELITERATURE
• Seborg, D. E., T. F. Edgar & D. A. Mellichamp (2010). Process Dynamics Process Dynamics and Controland Control 2nd ed John Wiley & Sons; New Yorkand Controland Control, 2nd ed., John Wiley & Sons; New York
• Bequette B. W. (2003). Process Process ControlControl:: Modeling, Modeling, DesignDesign, and , and SimulationSimulation, Prentice Hall
• Marlin, T. E. (2000). Process Control, Design Processes and Control Process Control, Design Processes and Control System for Dynamic PerformanceSystem for Dynamic Performance, 2nd ed., McGraw-Hill
• Ogunnaike B. A., W. H. Ray (1994). Process Dynamics, Process Dynamics, ModelingModeling and and ControlControl, Oxford University Press, Ney York Oxford
© N.Bolf, Faculty of Chemical Engineering and Technology, University of Zagreb
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