PROCESS INSTRUMENTATION PDF CHEMICAL ENGINEERING

PI

1. PROCESS
INSTRUMENTATION
Dr. Asma Iqbal
Assistant Professor
NIT, Srinagar

2. PROCESSINSTRUMENTATION
Raw material Costly Products
Commercial-scale operation
Refinery Control Room

3. PROCESSINSTRUMENTATION
Processinstrumentationisaboutmeasurementand
control.
Instrumentationengineeringistheengineering
speciallyfocusedonthedesignandconfigurationof
processsystems.
Instrumentsaredeviceswhichareusedinmeasuring
attributesofprocesssystems.
Instrumentationisthebasisforthecontrolofa
process.

4. PROCESSINSTRUMENTATION
Itprovidesthevariousindicationstotheoperatorfor
controllingtheprocess.
Insomecases,operatorrecordstheseindicationsfor
evaluatingthecurrentconditionoftheprocessandto
takeactionsiftheconditionsarenotasexpected.
Becauseofthecontinuousinteractivenatureofmost
oftheprocesses,manualcontrolisnotfeasibleandis
unreliable.
Withinstrumentation,automaticcontrolofsuch
processescanbeachieved.

5. INSTRUMENT
Aninstrumentcanbedefinedasadevicefordetermining
thevalueormagnitudeofaquantityorvariable.
Measuringinstruments,andformaltestmethodswhich
definetheinstrument’suse,arethemeansbywhichthe
variablesandtherelationsbetweenvariablesarefound.
Need of Measurement
In chemical process industry, we always want to know:
I.Temperature:Boiler, Reactor, furnace, etc.
II.Pressure:High pressure reactor vessel, etc.
III.Flow Rate:Process Fluid in a pipe, reflux stream in
the distillation column, etc.

6. NEEDOFMEASUREMENT
IV. Concentration:Reactor, etc.
V. Others: pH, moisture content, conductivity,
density of a sample, etc.
Thefundamental needofmeasurement in
industrialmanufacturingandprocessingisto
obtainanumericalvaluecorrespondingtothe
variablebeingmeasuredsothatwecandetermine
thequantityofaproduct(or)theefficiencyof
production.

7. CLASSIFICATION OFMEASURINGINSTRUMENTS

8. ThemechanicalinstrumentThefirstinstruments
weremechanicalinnature.Thisinstrumentissuitable
formeasuringthestaticandstableconditionbecausethe
instrumentisunabletogivetheresponsetothedynamic
condition,e.g.Scales,Calipersetc.Mechanical
instrumentssufferfromadisadvantagethattheyare
unabletorespondrapidlytomeasurementsofdynamic
andtransientconditions.Theseinstrumentshave
severalmovingpartsthatarerigid,heavyandbulkyand
consequentlyhavealargemass.
Theelectricalinstrumentisusedformeasuring
electricalquantitieslikescurrent,voltage,power,etc.
Theirresponseisfasterthanthemechanicalone.The
ammeter,voltmeter,wattmeteraretheexamplesofthe
electricalmeasuringinstrument.Thesearelightand
compactandprovidegreaterflexibilityinconstruction.
TheyConsumelesspower.

9. Theelectronicinstrumenthasquickresponse
time.Theinstrumentprovidesthequickresponse
ascomparedtotheelectricalandmechanical
instrument.Electronicinstrumentsaresteadily
becomingmorereliableonaccountof
improvementsindesignandmanufacturing
processes.

10. TYPESOFELECTRICALINSTRUMENT
1. Absolute instrument
Absoluteinstrumentmeasurestheprocessvariabledirectly
fromtheprocesswithouttheuseofconversion.
Suchinstrumentsdonotrequirecomparisonwithanyother
standard.
Thetangentgalvanometerisanexamplefortheabsolute
instrument.
Theseinstrumentsareusedasstandardsinlabsand
institution.
2.Secondaryinstrument
Theseinstrumentsaresoconstructedthatthedeflectionof
suchinstrumentsgivesthemagnitudeoftheelectrical
quantitytobemeasureddirectly.
Theseinstrumentsrequiredtocalibratedwithrespecttothe
standardinstrument.
Theseinstrumentsareusuallyusedinpractice.
Ithastwotypes:

11. Digital Instrument
The digital instrument gives the output in the numeric
form.
The instrument is more accurate as compared to the
analogue instrument because no human error occurs in
the reading.
Analog instrument
The instrument whose output varies continuously is
known as the analogue instrument.
The analogue instrument has the pointer which shows
the magnitude of the measurable quantities.
The analogue device classifies into two types:
Null Type Instrument
Deflection Type Instrument

12. Null Type Instrument
Inthisinstrument,thezeroornulldeflection
indicatesthemagnitudeofthemeasuredquantity.
Theinstrumenthashighaccuracyand
sensitivity.Innulldeflectioninstrument,theone
knownandoneunknownquantityuse.
Whenthevalueoftheknownandtheunknown
measuringquantitiesareequal,thepointershows
thezeroornulldeflection,e.g.WeighingBalance
Thenulldeflectioninstrumentisusedinthe
potentiometerandingalvanometerforobtaining
thenullpoint.

13. Deflection Type Instrument
The instrument in which the value of measuring
quantity is determined through the deflection of
the pointer is known as the deflection type
instrument.
The measuring quantity deflects the pointer of the
moving system of the instrument which is fixed on
the calibrated scale.
Thus, the magnitude of the measured quantity is
known.
a)Indicating instrument
b)Integrating instrument
c)Recording instrument

14. (a) Indicating instrument:
The instrument which indicates the magnitude of the
measured quantity is known as the indicating
instrument.
The indicating instrument has the dial which moves
on the graduated dial.
The voltmeter, ammeter, clock are the examples of
the indicating instrument.
(b) Integrating instrument:
The instrument which measures the total energy
supplied at a particular interval of time is known as
the integrating instrument.
The total energy measured by the instrument is the
product of the time and the measure of electrical
quantities.
The energy meter and watt-hour meter are the
examples of integrating instrument.

15. (c) Recording instrument:
Theinstrumentrecordsthecircuitconditionata
particularintervaloftimeisknownastherecording
instrument.
Themovingsystemoftherecordinginstrumentcarriesa
penwhichlightlytouchesonthepapersheet.
Themovementofthecoilistracedonthepapersheet.
Thecurvedrawnonthepapershowsthevariationinthe
measurementoftheelectricalquantities.
Inmanycasestheinstrumentmakesawrittenrecord,
usuallyonpaper,ofthevalueofthequantityunder
measurement.E.g.ECG,EEGetc.
Theresponsetimeoftheelectronicinstrumentisvery
highascomparedtotheelectricalandmechanical
device.

16. FUNCTION OFINSTRUMENTS
IndicatingFunction:
Thisfunctionincludessupplyinginformationconcerning
thevariablequantityundermeasurement.
Severaltypesofmethodscouldbeemployedinthe
instrumentsandsystemsforthispurpose.
Mostofthetime,thisinformationisobtainedasthe
deflectionofapointerofameasuringinstrument.
RecordingFunction:
Inmanycases,theinstrumentmakesawrittenrecord,
usuallyonpaper,ofthevalueofthequantityunder
measurementagainsttimeoragainstsomeother
variable.
Thisisarecordingfunctionperformedbythe
instrument.

17. SignalProcessing:
Thisfunctionisperformedtoprocessandmodify
themeasuredsignaltofacilitaterecording/
control.
ControllingFunction:
Thisisoneofthemostimportantfunctions,
especiallyinthefoodprocessingindustrieswhere
theprocessingoperationsarerequiredtobe
preciselycontrolled.
Inthiscase,theinformationisusedbythe
instrumentorthesystemstocontroltheoriginal
measuredvariableorquantity.

18. FUNCTIONALELEMENTS OFAMEASUREMENT SYSTEM

19. ELEMENTS OFMEASUREMENT
The purpose of analysis and synthesis, the instruments
are considered as systems.
The different components are called elements and they
perform certain definite and required steps in the act of
measurement.

21. FUNCTIONOFEACHELEMENT
1.PrimarySensingElement:
Anelementthatissensitivetothemeasured
variable.Thesensingelementsensethecondition,
stateorvalueoftheprocessvariable.
2.Variableconversionelement(TransducerElement):
Anelementthatconvertsthesignalfromone
physicalformintoanotherwithoutchangingthe
informationcontentsofthesignal.
Example:BourdonTube,Bellow,etc.

23. 3. Data Transmitter Element:
Anelementthattransmitsthesignalfromone
locationtoanotherwithoutchangingitsinformation
content.Datamaybetransmittedoverlong
distances.
Example: Shaft and gearing assembly, telemetry
system for transmitting signals from missiles to
ground equipment.
4. Variable Manipulation Element:
An element that operates on the signal according to
some mathematical rule without changing physical
nature of the variable.
Input x Constant = Output

26. CHARACTERISTICS OFINSTRUMENTS
Therearetwotypesofcharacteristicsofinstruments:-
1.Staticcharacteristics,
2.Dynamiccharacteristics.
1.StaticCharacteristics
Thestaticcharacteristicsofaninstrumentare
requiredtobeconsideredfortheinstrumentswhich
measureunvaryingprocessconditions.
Thestaticcharacteristicsaredefinedforthe
instrumentswhichmeasurequantitieswhichdonot
varywithtime.

27. THEMAINSTATICCHARACTERISTICS ARE:-
1. Accuracy
2. Sensitivity
3. Reproducibility
4. Drift
5. Static error
6. Dead zone
7. Precision
8. Threshold
9. Linearity
10. Stability
11. Range or Span
12. Bais
13. Tolerance
14. Hysteresis

28. 1. Accuracy
Itisthedegreeofclosenesswithwhichan
instrumentreadingapproachesthetruevalueof
thequantitybeingmeasured.
Theaccuracyofameasurementindicatesthe
nearnesstotheactual/truevalueofthequantity.
2.Sensitivity
Sensitivityistheratioofchangeinoutputofan
instrumenttothechangeininput.
Themanufacturesspecifysensitivityastheratio
ofmagnitudeofthemeasuredquantitytothe
magnitudeoftheresponse.
ThisratioiscalledasInversesensitivityor
deflectionfactor.
SensitivityMeter

29. 3.Reproducibility
Reproducibility is defined as the degree of closeness by
which a given value can be repeatedly measured.
The reproducibility is specified for a period of time.
Perfect reproducibility signifies that the given readings
that are taken for an input, do not vary with time.
4.Drift
The drift is defined as the gradual shift in the
indication over a period of time where in the input
variable does not change.
Drift may be caused because of environment factors
like stray electric fields, stray magnetic fields, thermal
e.m.fs, changes in temperature, mechanical vibrations
etc.

30. Drift is classified into three categories:
a. Zero drift
b. Span drift or sensitivity drift
c. Zonal drift
5. Static error
Itisthedeviationfromthetruevalueofthe
measuredvariable.
Itinvolvesthecomparisonofanunknownquantity
withanacceptedstandardquantity.
Thedegreetowhichaninstrumentapproachestoits
exceptedvalueisexpressedtermsoferrorof
measurement.

31. 6.Dead zone
It is the largest changes of input quantity for which there
is no output.
For e.g. the input that is applied to an instrument may
not be sufficient to overcome friction. It will only respond
when it overcomes the friction forces.
7.Precision
Itisameasureofthereproducibilityofthemeasurement
thatisgivenafixedvalueofvariable.
Forexampleconsideraninstrumentonwhichreadings
canbetakenupto1∕100
th
ofunit.Thisinstrumenthas
zeroadjustmenterrorandwetakereadings,then,we
say,theinstrumentishighlyprecise.Howeverasthe
instrumenthasazeroadjustmenterrorthereadings
obtainedareprecise,buttheyarenotaccurate.

32. 8.Threshold
Thresholdisthesmallestmeasurableinput,below
whichnooutputchangecanbeidentified.
Whilespecifyingthreshold,manufacturesgivethe
firstdetectableoutputchange.
9.Linearity
Linearityisdefinedastheabilityofaninstrumentto
reproduceitsinputlinearly.
Linearityissimplyameasureofthemaximum
deviationofthecalibrationpointsfromtheideal
straightline.
Linearityisdefinedas,
linearity=Maximum deviation of o/p from
idealized straight line ∕ Actual readings

33. 10.Stability
Theabilityofaninstrumenttoretainitsperformance
throughoutitsspecifiedstoragelifeandoperatinglifeis
calledasStability.
11.RangeorSpan
Theminimumandmaximumvaluesofaquantityfor
whichaninstrumentisdesignedtomeasureiscalledits
rangeorspan.
Sometimestheaccuracyisspecifiedintermsofrangeor
spanofaninstrument.
12.Bias
Theconstanterrorwhichexistsoverthefullrangeof
measurementofaninstrumentiscalledbias.
Suchabiascanbecompletelyeliminatedbycalibration.
Thezeroerrorisanexampleofbiaswhichcanbe
removedbycalibration.

34. 13.Tolerance
Itisthemaximumallowableerrorthatisspecifiedin
termsofcertainvaluewhilemeasurement,itiscalledas
tolerance.
Itspecifiesthemaximum allowabledeviationofa
manufactureddevicefromamentionedvalue.
14.Hysteresis
Hysteresisisaphenomenonwhichdepictsdifferentoutput
effectswhileloadingandunloading.
Hysteresistakesplaceduetothefactthatalltheenergy
putintothestressedpartswhenloadingisnotrecoverable
whileunloading.
Whentheinputofaninstrumentisvariedfromzerotoits
fullscaleandtheniftheinputisdecreasedfromitsfull
scalevaluetozero,theoutputvaries.
Theoutputattheparticularinputwhileincreasingand
decreasingvariesbecauseofinternalfrictionorhysteric
damping.

35. 2. DYNAMICS CHARACTERISTICS
Instrumentsrarelyrespondtotheinstantaneous
changesinthemeasuredvariables.
Theirresponseissloworsluggishduetomass,
thermal capacitance,electricalcapacitance,
inductanceetc.sometimes,eventheinstrumenthasto
waitforsometimetill,theresponseoccurs.
Thesetypeofinstrumentsarenormallyusedforthe
measurementofquantitiesthatfluctuatewithtime.
Thebehaviourofsuchasystem,whereastheinput
variesfrominstanttoinstant,theoutputalsovaries
frominstanttoinstantiscalledasdynamicresponse
ofthesystem.
Hence,thedynamicbehaviourofthesystemisalso
importantasthestaticbehaviour.

36. Thedynamicinputsareoftwotypes:
1.Transient
2.Steadystateperiodic.
Transientresponseisdefinedasthatpartofthe
responsewhichgoestozeroasthetimebecomeslarge.
Thesteadystateresponseistheresponsethathasa
definiteperiodiccycle.
Thevariationsintheinput,thatareusedpracticallyto
achievedynamicbehaviourare:
I.Stepinput:-Theinputissubjectedtoafiniteand
instantaneouschange.E.g.:closingofswitch.
II.Rampinput:-Theinputlinearlychangeswithrespect
totime.
III.Parabolicinput:-Theinputvariestothesquareof
time.Thisrepresentsconstantacceleration.
IV.Sinusoidalinput:-Theinputchangesinaccordance
withasinusoidalfunctionofconstantamplitude.

37. Thedynamiccharacteristicsofameasurementsystemare:
1)Speedofresponse
2)Fidelity
3)Lag
4)Dynamicerror
1)SpeedofResponse
Itisdefinedastherapiditywithwhichaninstrument,
respondstothechangesinthemeasuredquantity.
Itshowshowactiveandfastthesystemis.
2)Fidelity
Itisdefinedasthedegreetowhichameasurementsystem
iscapableoffaithfullyreproducingthechangesininput,
withoutanydynamicerror.
3)Lag
Everysystemrequiresitsowntimetorespondtothe
changesininput.Thistimeiscalledaslag.
Itisdefinedastheretardationordelay,intheresponseof
asystemtothechangesintheinput.

38. 4)Dynamic error
Itisthedifferencebetweenthetruevalueofthe
quantitythatistobemeasured,changingwithtime
andthemeasuredvalue,ifnostaticerrorisassumed.

39. NUMERICALS
A meter reads 127.5V and the true value of the
voltage is 127.43V. Determine
a) the static error
b)The static correction for this instrument
Sol: a) Static error (δa) = Am-At
= 127.5 –127.43
= 0.07V
b) Static correction (δc) = -δa
= -0.07V

40. Ameterreads555.50Vandthetruevalueofthe
voltageis555V.Findthestaticerrorofthe
instrument.
Sol: Static error (δa) = Am-At
= 555.5 –555
= 0.5V

41. Athermometeriscalibratedfrom150
o
Cto300
o
C.
Theaccuracyisspecifiedas±0.2%ofspan.What
isthemaximumstaticerror?
Sol: Span = Max value –Min value
= 300 –150
= 150
o
C
Static error = 0.2% of Span
= 0.2 % * 150
= ±0.3
o
C

42. Whentemperatureischangedfrom0
o
Cto50
o
C,
theresistanceinaPt100thermometerchanges
from100Ωto119.4Ω.Thesensitivityforthis
rangeis?
Sol: Sensitivity = change in output value/ change
in input true value
=(119.4–100)/(50-0)
=0.388Ω/
o
C

43. Thetruevalueofasteelbeamis6m.Three
repeatedreadingswithalasermeterindicatesa
lengthof6.01m,6.0095m,and6.015m.Comment
onaccuracyandcalibration.
Sol: Truevalue = 6m
Average /Mean of readings =
(6.01+6.0095+6.015)/ 3
= 6.01m