pmc 02 03 control loop control methods print.ppt€¦ · steam flow in a reboilersteam flow in a...

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


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!!


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


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


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.


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)


β1 p p y p p ( g , ρ, η, )β2 – parameters which characterize process equipment (e.g. dimensions, geometry)


Disturbance variable


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


DisturbancesDisturbances


ControlledControlled variablevariableManipulatedManipulated variablevariable

DisturbancesDisturbances

PlantPlant / Process/ Process SensorSensor

FinalFinal controlcontrolelement (element (ValveValve))

PressurePressureLevelLevelFlowFlow

ControllerController

element (element (ValveValve)) FlowFlowTemperatureTemperatureCompositionComposition

T dT dControllerController TransducerTransducer


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


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


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!


• 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


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.


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!).


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);


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


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;


• 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


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;


• 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


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


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..


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


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


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 -)


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.


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


pmc 02 03 control loop control methods print.ppt€¦ · steam flow in a reboilersteam flow in a...

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