1-introduction to process control (1).ppt
TRANSCRIPT
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Chemical Process ControlChemical Process Control
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What if….
….you drive your car with your eyes shut?
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IMPORTANCE OF PROCESS CONTROL
SAFETY PRODUCT QUALITY PRODUCTION RATES REDUCE ENERGY
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**dynamicdynamic time-dependent behavior of a process. time-dependent behavior of a process.
Why we need to control a process?◦Because processes are dynamic – changes
always occurred
What does a control system do?◦Maintain certain variables within some limite.g temperature in a room
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There are many applications where part or all of a process has to be controlled. e.g.
temperature pressure flow level composition others
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Automatic Control◦Maintaining the process variables
(temperature, pressures, flows, compositions) at some desired operating value.
◦Systems that control the variables without requiring intervention from the operator.
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System
SYSTEM
input
output
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VARIABLES
Controlled variables – the variable that must be maintained at the desired value (set point)
Manipulated variables – the variable that need to be regulated in order to maintain the controlled variable at the set point
Disturbances – variables that tend to drive the controlled variable away from the set point
c. Uncontrolled variables. Variables in the process that are not controlled.
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An Everyday Example of An Everyday Example of Process ControlProcess Control
Control Objective (Setpoint): Maintain the temperature in the room (27oC)
Controlled variable:Temperature in the room
Manipulated variable:Speed of the fan
Disturbance:Weather
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An Everyday Example of Process An Everyday Example of Process ControlControlDriving a CarControl Objective (Setpoint): Maintain car in proper lane. Controlled variable- Location of car on the road Manipulated variable- Orientation of the front wheels Disturbance- Curve in road
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Exercise
Select one control objective from a simple process and determine the controlled variable, manipulated variable and disturbances of the process.
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HEAT EXCHANGERHEAT EXCHANGER
Heat Exchanger
Fluid in Fluid out
Steam in
Steam out
Ti T desired
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HEAT EXCHANGERHEAT EXCHANGER
Heat Exchanger
Fluid in Fluid out
Steam in
Steam out
Ti T desired
(Manipulated variables)
(Load disturbances )
(Uncontrolled variables)
(Controlled variables)
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THREE BASIC COMPONENT OF THREE BASIC COMPONENT OF CONTROL SYSTEMCONTROL SYSTEM
Measurement (M) ◦ measure the variable to be controlled.
sensor-transmitter combination
Decision (D) ◦ Controller decides what to do to maintain the variables
at its desired value feedback controller
Action (A)◦ System must take an action based on controller’s
decision control valve (final control element)
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Another important term
◦Set point (SP)-desired value of the control variable.
◦Open loop system - The behavior with no controllers in the system
◦Closed loop system– the controller is connected to the process, comparing the set point to the controlled variable and determining and taking corrective action
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Heat Exchanger
Fluid in Fluid out
Steam in
Steam out
Ti T desired
TT
TC
Measured = T desired (sensor transmitter)
Control = Steam in
Action = valve (final control element)
Measure
DecideAction
SP
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Heat Exchanger
Fluid in Fluid out
Steam in
Steam out
Ti T desired
(Manipulated variables)
(Controlled variables)
(Uncontrolled variables)
(Load disturbances )
TT
TC
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Question- How to ensure that T remains at or near the set
point? w1, T1
w, T
QM
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Process & Instrumentation Diagram (P&ID)
Purpose1. To indicate the instruments or control devices attached to the process.
2. To indicate the control system architecture associated with the process.
How it is done?Standard symbols and notations representing instruments or control devices
are placed to the pipings and vessels.Standard symbols and notations are
available from ISA-5.1(1984) standard.
Methodology?
Process piping and sub-piping
A thick straight line represent main process piping
A thin straight line represent process sub-piping either to instruments or by-pass process line.
Unit 1: Process & Instrumentation Diagram
PG
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Instruments / control devices:
A circle representing locally mounted instrument
A circle with horizontal linerepresenting control room panel mounted instrument.
A circle with horizontal line inside a square representing
its function in DCS.
Unit 1: Process & Instrumentation Diagram
Symbols
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Final control elements
Unit 1: Process & Instrumentation Diagram
Control valves =
Manual valve
=
Manual valve
=
Not darkened Always open
Darkened Always closed
OR
OR
Solenoid valve
=
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Naming rule
Unit 1: Process & Instrumentation Diagram
Examples
1 2 3 4
Measurement Device Device / Condition Condition
PTLFA
PressureTemperatureLevelFlowAnalysis
CTRIASG
ControllerTransmitterRecorderIndicatorAlarmSwitch / SafetyGauge
CTRHLV
ControllerTransmitterRecorderHighLowValve
HL
HighLow
No such devices
yet.
PC PIC LG FR TAL TALL TAHL
Pressure Controller Pressure Indicating Controller Level Gauge Flow Recorder Temperature Alarm Low Temperature Alarm Low Low Temperature Alarm High Low
PCC PTC PTR PIR TRR TRA TAV
Complete guide on P&ID symbols & notations are available from ISA 5.1 (R1984)
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Transmission signalTransmission signal3 principle type of signals in in in process
industries
Usually signal is in percent
Example 0-100% = 3-15 psig
Signal Range Symbol
Pneumatic 3 – 15 psig
Electrical 4 – 20 mA1 to 5 V
Digital/discrete
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Signal are used by devices – transmitters, controllers, final control element to communicate.
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Change type of signal using transducer / converter
Example of transducer Current to pneumatic (I/P) Digital-to-analog (D to A) Pneumatic to current (P/I)
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Heat Exchanger
Fluid in Fluid out
Steam in
Steam out
Ti T desired
(Manipulated variables)
(Controlled variables)
(Uncontrolled variables)
(Load disturbances )
TT
TC
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Unit 1: Process Control Loop
Process control loop
I/P
Process
Sensor
Transmitter
Controller
Transducer
Control valve
4-20 mA1-5 Vdc
PIDFuzzy logic
4-20 mA3-15 psig
dP cellCapacitanceRadar, SonicMagneticResistanceIR/Laser
PressureFlowLevelTemperaturepH
LinearEqual percentage
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Unit 1: Tutorial 1
Can you read the following P&ID.
FT
FCI/P TC
TT
SP
product
Fuel
Main process line
Process instrument piping
Temperature transmitter
Temperature controller
Flow controller
Current to pnuematic transducer
Control valve
Electrical signal
Pneumatic signal
Feed
TC
ASD
1112 Temperature controller DCS function
11 Automatic shut down PLC unit
12 Solenoid valve
02:03 AM
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Regulatory and servo controlRegulatory and servo control
For process that the controlled variable deviates from set point because of disturbance – regulatory control
For process that the most important disturbance is the set point itself – servo control