difference between plc

8/9/2019 Difference Between PLC

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Difference between PLC,DCS & SCADA In the past the strength and weakness of both the automation technologies were well understood. So it was easy

for engineers to select the one that suits their application, but now due to the advent of microprocessors both the

technologies are merging.The requirements which are thought to be exclusive to one technology can be found in the other

one too.But still there are some things which are to be checked out before selecting the one that suits your application the

best.

ow a days you cannot really tell the difference between a !"# or a $#S. Since the !"# was integrated with %nalog I&' it

crosses the boundary of being (ust digital and crosses to the realm of $#S in handling %nalogs, Bus Systems, $istributed

I&' and etc. %lso, since the $#S now handles logics of $igital I&' it also crossed the boundary to the realm of !"#.

%s you know !"# as to its name !rogrammable "ogic #ontroller. Its main purpose is to replace the relay logic controls

which is )'n) or )'ff). %nd $#S )$istributed #ontrol Systems) its emphasis is *ast analog handling because of

communications through Bus systems, networking and etc.

Summari+ing all these, !"# $#S......B-Tstill there are basic differences as below

!"# is a programmable "ogic controller which is used mainly for interlocking different equipments.!"# using for

perticular mechine or production unit.!"# is economicaly low cost.!"#S can handle analog and digital I&' as earliear it

could handle only digital.!"#S are automatic controllers which is a substitute to hard wired controllers.they are

extensively used for automation.!"# is for stand alone sytem.!"# is used #ommonly with 'n/'ff 0$igital1 #ontrol and

may be expanded with %nalog I/' 2odules for %nalog #ontrol and used for a control task.

Programmable Logic Controller

$#S3 $IST4IB-T5$ #'T4'" S6ST52S3 T7IS S6ST52 $'5S 87%T %

!"# 8'-"$ $', B-T T75 $I**545#5 IS T7%T % $#S IS -S5$ I

2-#7 "%4954 %$ #'2!"5: %!!"I#%TI' 5.93 !'854 9554%TI',

S'25 #'"$ 4'""I9 2I""S etc.

$#S is the System in which controller are distributed geographically and integrated all the control hardwares .which is

connected from the various field devices .$#S having its own network, #ontroller and 72I etc;.7oneywell, 6okogawa,

Invensys, %BB, 5merson are the leading .$#S


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distributed geographically and integrated all the control hardwares which is connected from the various field devices.$#S

having its own network, #ontroller and 72I etc;7oneywell, 6okogawa, Invensys, %BB, 5merson are the leading$#S


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time before the $#S hardware free+e date1. ote that this package system is a process system using continuous

control, not discrete. Based on this, a !"# can never be larger than a $#S in terms of I&' count.

?. In large plants the $#S is king because most owners want a single source of hardware support and service, and

this mentality naturally denies the !"# a foothold. !ackage vendors are no longer required to provide !"# for their

system. 5verything is connected to the $#S.

S#%$%3 %S %"45%$6 5:!"%I5$, IT 'T7I9 B-T % S'*T8%45 -S5$

T' 9%T754 $%T% *4'2 T75 *I5"$S IST4-25TS 8IT7 T75 75"! '*6'-4 $#S '4 !"# $5!5$I9 ' T75 %!!"I#%TI'


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S#%$% is the Software tool in which you can able to

view&2onitorontrol your !rocess


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*or eg.,if there is some problem in the manufacturing process of a bottling plant, even if the process is stopped for a while,

resuming the process back to its normal operation is simple and not time tacking. providing redundancy will be costlier

than handling the downtime expenses so generally redundancy is not provided by this.

$#S3 $owntime results not only inlost productionbut also leads todangerous conditions or damages the process

lineand resuming the process is expensive and difficult too.

*or eg.,In steel making process if the process stops abruptly and the pipelines with process fluid contents may free+e and

damage the pipelines this would lead to more expenses.

$egree of customisation!"#3It delivers a tool@kit of functions and building blocks that can be easily customized.So it can be used for varied

applications by programming it according to the application.

$#S3 Its highest priority is high reliabilityso standard functions are baked in to provide a complete solution to a

particular application. So it can't accept many custom modifications.

-p@front cost

!"#3 in $$.

$#S3in $$$$A7owever its superior to PLC in operationand proprietary in general.

Si+e

!"#3 compact

$#S3large4eplacement

!"#3 replaced the conventional relays

$#S3 replaced thepneumatic & single loop controllers

$#S is $istributed #ontrol Systems which is used to control very big plants by using simple 9-I screens.!"#s are

interfaced with $#S for interlocking.

INTRODUCTION TO FLOW METERS

A flow meter is a device used to measure the flow rate or quantity of a gas or liquid passing through a pipe.

figure(1)

They are wide variety of flow meters using in the industrial applications. Some of them are,
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1) Coriolis low !eter.

") #orte$ low !eter.

%) &ifferential 'ressure low !eter.

) Turine low !eter.

*) +lectro !agnetic low !eter.

) -ltrasonic low !eter.

Selecting A Flow Meter:

The asis of flow meter selection is a clear understanding of the requirement of the particular application. Therefore time

should e invested in fully evaluating the nature of the process fluid and of the overall installation. ere are some /ey

questions which we need to answer efore selecting the flow meter.

1) 0hat is the fluid eing measured y the flow meter

") &o you require rate measurement and2or totali3ation from the flow meter

%) 4f the liquid is not water, Then what is the #iscosityof liquid

) 4s the fluid clean

*) 0hat is the minimum and ma$imum flow rate of the flow meter

) 0hat is the minimum and ma$imum process pressure

5) 0hat is the minimum and ma$imum process temperature

6) &o you require a local display on the flow meter or do you need a electronic signal output

7) 4s the fluid chemically compatile with the flow meter wetted parts

18) 4s this is a process application, what is the si3e of pipe
http://en.wikipedia.org/wiki/Viscosityhttp://en.wikipedia.org/wiki/Viscosity


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P=P1-P2
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An orifice plate installed in a line creates a differential pressure as the fluid flows

through it. This differential pressure is measured via impulse lines y a differential pressure transmitter which converts it intoan Analog or &igital signal which can e processed to provide a display of the instantaneous rate of flow. The relationship

etween the rate of flow and the differential pressure produced is very well understood and is fully covered ycomprehensive national standards. The relevant standards are 9S18" and 4S>*15.

A&'ntge:

1) -se on @iquid, as B Steam.") Suitale for e$treme temperature B pressure.%) igh pressure loss.) ?o moving parts.

*) @ow material costs.

) 4ne$pensive, 0hen compared to other flow meter.

Di&'ntge:1) @imited rangeaility.") Affected y changes in &ensity, 'ressure B #iscosity.%) +$pensive to install.) @ow accuracy

*) 'ressure drops can e large.) Contraction of flow reduces accuracy of downstream pressure measurement.

F$OW *O++$":
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low no33les have a smooth elliptical inlet leading to a throat section with a sharp outlet. This restriction in the fluid flowcauses a pressure drop which relates to the flow rate y applying 9ernoullis +quation.

&ue to its rigidity the flow no33le is more stale at higher temperature and flow rate than >rifice.The flow no33le is generally used for measuring the flow of steam, nonDviscous, erosive and high velocity media. low no33lesare typically used to measure steam, air, gas, vapour and chemical sustances and liquids with a small degree of suspendedsolids.

The design and manufacture of the flow no33le complies with the requirement of 4S>*15 standard. Thus an accuracy of EF(or) D1.8


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&oppler ultrasonic flow meter operate on the doppler effect. 4n &oppler effect, >ne transducer transmits an ultrasonic eam

of 8.*!h3 into the flow stream. @iquid flowing through the pipe must contain sonically reflected materials such as solid

particles or air ules. the movement of these materials alters the frequency of the wave reflected onto a second receiving

transducer. The frequency shift is linearly proportional to the rate of flow of materials flowing in the pipe.

The asic equations defining the doppler flow meter are,
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+quation(%) clearly shows that the low velocity is equal to the &oppler frequency shift. ?ow, ecause the diameter(&) of the

pipe is /nown, #olumetric flow rate(e.g alloons per minute) can e measured using the following e$pressions,

0here, H; a constant dependent on the units of low velocity and &.

>ne &oppler meter design mounts oth the transmitting and receiving transducers in the same case attached to one side of

the pipe. Geflectors in the flowing liquid return the transmitting signal to the receiver with a frequency shift proportional to

the flow velocity.

Advantages >f &oppler low !eter=

The main advantage of doppler flow meter is its nonDintrusive design. An acousticDcoupling compound is used on the surface

of the pipe and the sensors are simply held in place to ta/e a measurement or, for a more permanent installation, they are

strapped around the pipe. Some manufacturers offer a special clampDon proe which allows connection to smaller pipe si3es.

>ther advantages include=

1) +asy installation and removal.

") ?o process downtime during installation.

%) ?o moving parts to wear out.

) ?o process contamination.

*) 0or/s well with dirty or corrosive fluids.

) ?o lea/age potential.

5) 4nsensitive to liquid temperature, viscosity, density or pressure variations.

&isadvantages >f &oppler low !eter=

The main disadvantage of the doppler flow meter is the fact that the liquid stream must have particulates, ules, or other

types of solids in order to reflect the ultrasonic signal. This means that the doppler flow meter is not a good choice for the

water and very clean fluids.

The another disadvantage is that the accuracy can depend on particleDsi3e distriution and concentration and also on any

relative velocity that may e$ist etween the particulates and the fluid. if there are not enough particulates availale, the

repeataility will degrade.The other potential prolem of this technology is that it can have troule operating at very low flow velocities.

Applications >f &oppler low !eter=

9eing an nonDintrusive, &oppler flow meters have a wide variety of applications. Some of them include=

1) 4nfluent and effluent water flow.

") &igester feed control.

%) 0aste water.

) Crude oil flow.

*) !ining slurries.
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) Acids.

5) @iquified gases.

%rnit %ie Flow Meter:

Transmit time flow meter sends and receives the ultrasonic waves etween transducers in oth the upstream and

downstream directions in the pipe. At no flow conditions, 4t ta/es the same time to travel upstream and downstream

etween the transducers. -nder flowing conditions, The upstream wave will travel slower and ta/e more time than the

downstream wave. 0hen the fluid moves faster, The difference etween the upstream and downstream time increases. The

difference in transit times in the upstream and downstream directions(TuDTd) measured over the same path can e used to

calculate the flow passing through a pipe.

Transmit time ultrasonic flow meters are more accurate and economical than the doppler ultrasonic flow meters.

Advantages >f Transit Time low !eter=

The main advantage of the transit time flow meter is, that it wor/s nonDinvasively with clean and ultrapure fluids. This allows

the user to maintain the integrity of the fluid while still measuring the flow. Some of the other advantages include=1) +asy installationDtransducer set clamps onto pipe.

") ?o moving parts to wear out.

%) Iero pressure drop.

) 0or/s well with clean and ultrapure fluids.

*) ?o lea/age potential.

) 4nsensitive to liquid temperature, viscosity, density or pressure variations.

&isadvantages >f Transit Time low !eters=

The transit time flow meter perfomance can suffer from pipeDwall interference, accuracy and repeataility prolems can

result if there is any Space ap etween the fluid and pipe wall.

The transit time flow meters will not operate on dirty, uly, particulateDladden fluids. Sometimes, the purity of a fluid may

fluctuate so as to effect the accuracy of the flow measurement. or such cases, there are yrid meters onthe mar/et that

will access the fluid conditions within the pipeand automatically choose doppler or transit time meter operations where

needed.

Applications >f Transit Time low !eter=

Transit time flow meter have wide applicaility for flow measurement of clean or ultrapure streams. Some of the applications

include=

1) Clean water flowrate in water treatment plants.

") ot or cold water in power plants.
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%) 'ure and ultrapure fluids in pharmaceutical, food and everage industries.

) @iquid to medium crude oils in the petroleum refining industry.

*) 0ater distriution systems used in agriculture and irrigation.

4n my ne$t post i will go through #orte$ B +lectromagnetic flow meter.

4f i s/ipped any main points in this post, Then please let me /now through your valuale comments.

Than/s for reading,

ave a gud day =)

difference between plc

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