Principle of mass flow meter

Principles of Mass Flow meters

1. PRINCIPLE OF MASS FLOW METER
PRESENTED BY
PREM BABOO
Sr. Manager(Prod)
National Fertilizers Ltd. India
An Expert for www.ureaknowhow.com
FIE institution of Engineers India

2. Mass Flow Operating Principle:
Contents:
1.How does a Mass flow meter measure mass
flow?
2.Why do the tubes vibrate?
3.How does the sensor detect mass flow
measurement?
4.What is the flow calibration factor and Delta-
T and how do these relate to the mass flow
measurement?
5.How does a flow meter measure volume flow?

3. PRINCIPLE OF MASS FLOW METER
AMassFlowMeteroperatingonthe"Coriolisprinciple"containsa
vibratingtubeinwhichafluidflowcauseschangesinfrequency,
phaseshiftoramplitude.Thesensorsignalisfedintotheintegrally
mountedpc-board.Theresultingoutputsignalisstrictlyproportional
totherealmassflowrate,whereasthermalmassflowmetersare
dependentofthephysicalpropertiesofthefluid.Coriolismassflow
measurementisfastandveryaccurate.Amassflowmeter,alsoknownas
aninertialflowmeterisadevicethatmeasuresmassflowrateof
afluidtravelingthroughatube.Themassflowrateisthemassofthefluid
travelingpastafixedpointperunittime.
Themassflowmeterdoesnotmeasurethevolumeperunittime(e.g.,cubic
meterspersecond)passingthroughthedevice;itmeasuresthemassperunit
time(e.g.,kilogramspersecond)flowingthroughthedevice.Volumetricflow
rateisthemassflowratedividedbythefluiddensity.Ifthedensityisconstant,
thentherelationshipissimple.Ifthefluidhasvaryingdensity,thenthe
relationshipisnotsimple.Thedensityofthefluidmaychangewith
temperature,pressure,orcomposition,forexample.Thefluidmayalsobea
combinationofphasessuchasafluidwithentrainedbubbles.Actualdensity
canbedeterminedduetodependencyofsoundvelocityonthecontrolled
liquidconcentration.
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4. Ifthefluidhasvaryingdensity,thentherelationshipis
notsimple.Thedensityofthefluidmaychangewith
temperature,pressure,orcomposition,forexample.
Thefluidmayalsobeacombinationofphasessuchas
afluidwithentrainedbubbles.Actualdensitycanbe
determinedduetodependencyofsoundvelocityon
thecontrolledliquidconcentration.

5. AMMONIA MASS FLOW METER

7. InaCoriolismassflowmeter,the“swinging”isgeneratedby
vibratingthetube(s)inwhichthefluidflows.Theamountof
twistisproportionaltothemassflowrateoffluidpassing
throughthetube(s).SensorsandaCoriolismassflowmeter
transmitterareusedtomeasurethetwistandgeneratea
linearflowsignal.
Coriolismassflowmetersmeasurethemassflowof
liquids,suchasAmmonia,water,acids,caustic,
chemicals,andgases/vapors.Becausemassflowis
measured,themeasurementisnotaffectedbyfluid
densitychanges

8. Coriolismassflowmetersmeasuretheforceresulting
fromtheaccelerationcausedbymassmovingtoward
(orawayfrom)acentreofrotation.Thiseffectcanbe
experiencedwhenridingamerry-go-round,where
movingtowardthecentrewillcauseapersontohave
to“leaninto”therotationsoastomaintainbalance.
Asrelatedtoflowmeters,theeffectcanbe
demonstratedbyflowingwaterinaloopofflexible
hosethatis“swung”backandforthinfrontofthe
bodywithbothhands.Becausethewaterisflowing
towardandawayfromthehands,oppositeforcesare
generatedandcausethehosetotwist.

9. Therearetwobasicconfigurationsofcoriolisflowmeter:
thecurvedtubeflowmeterandthestraighttubeflowmeter.
Thisarticlediscussesthecurvedtubedesign.Theanimations
ontherightdonotrepresentanactuallyexistingcoriolisflow
meterdesign.Thepurposeoftheanimationsistoillustratethe
operatingprinciple,andtoshowtheconnectionwithrotation
Fluidisbeingpumpedthroughthemassflowmeter.Whenthere
ismassflow,thetubetwistsslightly.Thearmthroughwhich
fluidflowsawayfromtheaxisofrotationmustexertaforceon
thefluid,toincreaseitsangularmomentum,soitbends
backwards.Thearmthroughwhichfluidispushedbacktothe
axisofrotationmustexertaforceonthefluidtodecreasethe
fluid'sangularmomentumagain,hencethatarmwillbend
forward.
Inotherwords,theinletarmislaggingbehindtheoverall
rotation,andtheoutletarmleadstheoverallrotation.

10. Coriolistype Mass Flow
Meters

11. Theanimationontherightrepresentshow
curvedtubemassflowmetersaredesigned.
Whenthefluidisflowing,itisledthroughtwo
paralleltubes.Anactuator(notshown)induces
avibrationofthetubes.Thetwoparalleltubes
arecounter-vibrating,tomakethemeasuring
devicelesssensitivetooutsidevibrations.The
actualfrequencyofthevibrationdependson
thesizeofthemassflowmeter,andrangesfrom
80to1000vibrationspersecond.
Theamplitudeofthevibrationistoosmallto
beseen,butitcanbefeltbytouch.Whenno
fluidisflowing,thevibrationofthetwotubesis
symmetrical,asshownintheanimations.

13. Theanimationontherightrepresentswhathappensduring
massflow.Whenthereismassflow,thereissometwistingofthe
tubes.Thearmthroughwhichfluidflowsawayfromtheaxisof
rotationmustexertaforceonthefluidtoincreaseitsangular
momentum,soitislaggingbehindtheoverallvibration.The
armthroughwhichfluidispushedbacktowardstheaxisof
rotationmustexertaforceonthefluidtodecreasethefluid's
angularmomentumagain,hencethatarmleadstheoverall
vibration.
Theinletarmandtheoutletarmvibratewiththesame
frequencyastheoverallvibration,butwhenthereismassflow
thetwovibrationsareoutofsync:theinletarmisbehind,the
outletarmisahead.Thetwovibrationsareshiftedinphase
withrespecttoeachother,andthedegreeofphase-shiftisa
measurefortheamountofmassthatisflowingthroughthe
tubes.

15. Coriolisforces Fc are generated in oscillating systems when a
liquid or a gas moves away from or towards an axis of
oscillation.
A Coriolismeasuring system is of symmetrical design and
consists of one or two measuring tubes, either straight or
curved.Adriver sets the measuring tube (AB) into a uniform
fundamental oscillation mode.Whenthe flow velocity v = 0
m/s / 0 ft/s, the Coriolisforce Fc is also 0. At flowing
conditions, i. e. flow velocity v > 0 m/s / 0 ft/s, the fluid
particles in the product are accelerated between points AC and
decelerated between points CB.

16. TheCoriolisforceFcisgeneratedbytheinertia
ofthefluidparticlesacceleratedbetween
pointsACandofthosedeceleratedbetween
pointsCB.
Thisforcecausesanextremelyslightdistortion
ofthemeasuringtubethatissuperimposedon
thefundamentalcomponentandisdirectly
proportionaltothemassflowrate.
Thisdistortionispickedupbyspecialsensors.
Sincetheoscillatorycharacteristicsofthe
measuring tubearedependent on
temperature,thetemperatureismeasured
continuouslyandthemeasuredvalues
correctedaccordingly.

17. Density and volume measurements
The mass flow of a u-shaped coriolisflow meter
is given as:
whereK
uisthetemperaturedependentstiffnessofthe
tube,Kashape-dependentfactor,dthewidth,τthetime
lag,ωthevibrationfrequencyandI
utheinertiaofthetube.As
theinertiaofthetubedependonitscontents,knowledgeofthe
fluiddensityisneededforthecalculationofanaccuratemass
flowrate.Ifthedensitychangestoooftenformanual
calibrationtobesufficient,thecoriolisflowmetercanbe
adaptedtomeasurethedensityaswell.

18. Thenaturalvibrationfrequencyoftheflowtubesdependon
thecombinedmassofthetubeandthefluidcontainedinit.By
settingthetubeinmotionandmeasuringthenatural
frequency,
themassofthefluidcontainedinthetubecanbededuced.
Dividingthemassontheknownvolumeofthetubegivesus
thedensityofthefluid.
Aninstantaneousdensitymeasurementallowsthecalculation
offlowinvolumepertimebydividingmassflowwithdensity.

19. Calibration
Bothmassflowanddensitymeasurementsdependonthe
vibrationofthetube.Calibrationisaffectedbychangesinthe
rigidityoftheflowtubes.
Changesintemperatureandpressurewillcausethetube
rigiditytochange,butthesecanbecompensatedforthrough
pressureandtemperaturezeroandspancompensationfactors.
Additionaleffectsontuberigiditywillcauseshiftsinthe
calibrationfactorovertimeduetodegradationoftheflow
tubes.Theseeffectsincludepitting,cracking,coating,erosionor
corrosion.Itisnotpossibletocompensateforthesechanges
dynamically,buteffortstomonitortheeffectsmaybemade
throughregularmetercalibrationorverificationchecks.Ifa
changeisdeemedtohaveoccurred,butisconsideredtobe
acceptable,theoffsetmaybeaddedtotheexistingcalibration
factortoensurecontinuedaccuratemeasurement.

21. Application Cautions for CoriolisMass
Flow meters
Ifthepressuredropisacceptable,operateaCoriolismassflow
meterintheupperpartofitsflowrangebecauseoperationat
lowflowratescandegradeaccuracy.Notethathighviscosity
fluidsincreasethepressuredropacrosstheflowmeter.For
liquidflows,makesurethattheflowmeteriscompletelyfullof
liquid.Beespeciallycarefulwhenmeasuringgas/vaporflow
withCoriolismassflowmeters.Payspecialattentionto
installationbecausepipevibrationcancauseoperational
problems.