PC-Hardware of a Process Control System

8/10/2019 PC-Hardware of a Process Control System

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Hardware of a Process Control

System

Chapter Three of Stephanopoulos


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Hardware Elements for a Control System

Chemical Process

Measuring Device

Transducers Transmission Lines

Controller

Final Control Element Recording Elements


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Chemical Process

Represents the material equipment together with

the physical or chemical operations that occur there

Measuring Sensor

Used to measure the Disturbances, controlled output

variables or secondary output variables

Are the main source of information about what is

going on in the process

E.g. thermocouples, bellows, gas chromatographs etc


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Transducers

Many measurements cannot be used for control untilthey are converted to physical quantities like electric

or pneumatic signal which can be transmitted easily Transducers are the devices which convert the

measurement into some sort of signal which can betransmitted

E.g. strain gauge change their electric resistance

when they are subjected to mechanical strain, sothey can be used to convert a pressure signal to anelectric signal


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Transmission Lines

Used to carry the measurement signal from themeasuring signal from the measuring device to

the controller In past Pneumatic lines were used

Now transmission lines carry electric signals

Transmission lines are equipped with Amplifiers

which raise the level of signal. E.g. reading fromthe thermocouple is in milli volts, it is amplifiedto few volts before it is transmitted to controller


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Controller

This hardware element has intelligence

It receives information from the measuringdevice and decides what action should be taken

In past simple control laws could be

implemented by controller but now with digital

computers and enhanced machine intelligence,very complicated control laws can be

implemented


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Final Control Element

This is the hardware element which implements

in real life the decision taken by the controller

If the controller decides to increase the flow rate

of a stream in order to keep the level at set

point, it is the Control valve (final control

element) on the stream which is opened

The final control element is usually a Valve


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Hardware Elements for a Control System Recording Elements

Used to provide a visual demonstration of how a chemical processbehaves

Various types of recording element can be seen in the control roomfor various variables


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Generalized Process Control System


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Example: Water Heating System


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Example: Water Heating System


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Types of Computer Control

Rapid technological development of digitalcomputers coupled with reduced cost has made

these computers the central piece for the

development of a control system for a chemical

process Some types of Computer Control are:

Direct Digital Control (DDC)

Supervisory Computer Control

Scheduling Computer Control


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Advantages of Computer Control

All plant data is available at a central point

Alarm conditions are detected and reported quickly

Plant is run according to the plan set by

management, which may not be the case where

there is a lot of manual intervention

Displays and printouts of information are available

readily

Changes to the operating conditions or recipes are

made in smooth, repeatable and closely controlled

manner


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Written records of all alarms and operatoractions are available for post-incident or routineanalysis

Control room and control panel size is reduced

All batches of material are treated identically

Complex start-up and shut-down procedure canbe automated

Safety interlocks can be programmed into thesystem

Advantages of Computer Control


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Parts of a Computer Control


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Computer Control System


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Computer Monitoring System


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Direct Digital Control

Today, chemical industry is moving more and more

towards DDC due to ease of control of complex

processes which have hundreds of control loops

In this control the computer directly receives the

measurements form the process and based on the

control law which is already programmed in its

memory calculates the values for manipulated

variables

These decisions are directly implemented on the

process by computer through proper adjustment of

the final control element


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Direct Digital Control


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One of the incentives of process control is to

optimize the economic performance of plant

Due to complexity of process, many times the

human operator cannot find the best operating

policy for a plant which will minimize the operating

cost

In such cases the speed and programmed

intelligence of a digital computer is used to analyze

the process and suggest the best policy

This is done by coordination of computer with basic

DDC loops


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Scheduling Computer Control

Computer can used to schedule the operation of a

plant

This can be to increase the production rate to meet

market demands or decrease the production rate to

avoid overstocking

Digital computers can make these changes easily

and communicate them to Supervisory Controllers

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PC-Hardware of a Process Control System