1-INTRODUCTION TO PROCESS CONTROL (1).ppt

Chemical Process ControlChemical Process Control

What if….

….you drive your car with your eyes shut?

IMPORTANCE OF PROCESS CONTROL

SAFETY PRODUCT QUALITY PRODUCTION RATES REDUCE ENERGY

**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

There are many applications where part or all of a process has to be controlled. e.g.

temperature pressure flow level composition others

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.

System

SYSTEM

input

output

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.

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

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

Exercise

Select one control objective from a simple process and determine the controlled variable, manipulated variable and disturbances of the process.

HEAT EXCHANGERHEAT EXCHANGER

Heat Exchanger

Fluid in Fluid out

Steam in

Steam out

Ti T desired

HEAT EXCHANGERHEAT EXCHANGER

Heat Exchanger

Fluid in Fluid out

Steam in

Steam out

Ti T desired

(Manipulated variables)

(Load disturbances )

(Uncontrolled variables)

(Controlled variables)

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)

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

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

Heat Exchanger

Fluid in Fluid out

Steam in

Steam out

Ti T desired





TT

TC

Question- How to ensure that T remains at or near the set

point? w1, T1

w, T

QM

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

© Abdul Aziz Ishak, Universiti Teknologi MARA Malaysia (2009)

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.


Symbols


Final control elements


Control valves =

Manual valve

=

Manual valve

=

Not darkened Always open

Darkened Always closed

OR

OR

Solenoid valve

=


Naming rule


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)


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

Signal are used by devices – transmitters, controllers, final control element to communicate.

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)

Heat Exchanger

Fluid in Fluid out

Steam in

Steam out

Ti T desired





TT

TC

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


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


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

1-INTRODUCTION TO PROCESS CONTROL (1).ppt

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