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Physical Properties Afecting the Fluids' Flow

The major actors afecting the ow o uids

through pipes are:

1) The velocity of the uid: is defned as theuid speed in the direction o ow. Fluid velocitydepends on the head pressure that is orcing theuid through the pipe. Greater the headpressures, aster the uid ow rate

2) Pipe size:The larger the pipe, the greater thepotential ow rate

3) Pipe Friction: reduces the ow rate throughthe pipe. Flow rate o the uid is slower near wallso the pipe than at the centre.

4) Fluid viscosity: its physical resistance to ow.

Higher the viscosity the uid, the slower uid ow.

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5) The specifc gravity o the uid: t any given operatingcondition, higher the uid!s specifc gravity, lower the

uid!s ow rate.

") Fluid #ondition: The condition o the uid $clean or

dirty) also li%itations in ow %easure%ent, so%e%easuring devices &eco%e &loc'ed(plugged or eroded i

dirty uids are used.

) *elocity +rofles: *elocity profles have %aor e-ect onthe accuracy and peror%ance o %ost ow %eters.

The shape o the velocity profle inside a pipe depends onthe %o%entu% or internal orces o the uid, that %ovesthe uid through the pipe, the viscous orces o the uid

that tend to slow the uid as passes near the pipe walls.

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!aminar or "treamlined: isdescri&ed as liuid owing througha pipeline, divisi&le into layers%oving parallel to each other.

Tur#ulent ow: is the %ost

co%%on type o ow patternound in pipes. Tur&ulent ow isthe ow pattern which has atransverse velocity $swirls, eddycurrent).

Transitional ow: which is

&etween the la%inar and tur&ulentow profles. /ts &ehaviour isdi0cult to predict and it %ayoscillate &etween the la%inar andtur&ulent ow profles.

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$ %&FF()T&A! P(""*(F!+-T("

1i-erential pressure type ow %etersprovide the &est results where the ow

conditions are tur&ulent. 2o%e o the%ost co%%on types o di-erentialpressure ow %eters are:

34/F/# 6T42.

*7T84/ 6T42

7399 6T42

+/T3T T8;2

The wor'ing principle or 1+ ow%etersis that so%ething %a'es the velocity othe uid change and this produces achange in the pressure so that a

di-erence

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+(&F&6 F!+-T("

The co%ponents o a typicalorifce ow%eter installation are:

3rifce plate and holder

3rifce taps

1i-erential pressure trans%itter

34/F/# +T2

o re %etal plates have an eualouter dia%eter o the pipeline.

These plates have an opening?orifce &ore@ s%aller than thepipe inner dia%eter.

o The typical orifce plate has aconcentric, sharp edged opening.;ecause o the s%aller area theuid velocity increases, causing acorresponding decrease inpressure.

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The concentric orifce plate has a sharp $suareA edged) concentric &ore thatprovides an al%ost pure line contact &etween the plate and the uid. The &eta$or dia%eter) ratios o concentric orifce plates range ro% B.C5 to B.5. The%aDi%u% velocity and %ini%u% static pressure occurs at so%e B.E5 to B.5pipe dia%eters downstrea% ro% the orifce plate.

G ccentric orifce plates are typically used or dirty liuids( gases. iuidscontaining vapour $&ore a&ove pipeline ow aDis). *apours containing liuid $&ore&elow pipeline ow aDis).

G 2eg%ental orifce plates are used or heavy uids, in preerence to eccentric&ore plates, &ecause it allows %ore drainage around the circu%erence o thepipe.

+ri7ce 8oldersThe orifce is inserted into the pipeline &etween the twoanges o an orifce union. This %ethod o installation is costAe-ective, &ut it callsor a process shutdown whenever the plate is re%oved or %aintenance orinspection.

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+ri7ce taps

There are co%%on arrange%ents o pressuretaps:

$Flange taps are located I inch ro% the orifceplate!s suraces. They are not reco%%ended or

use on pipelines under C inches in dia%eter. 9 ena contracta taps are located one pipe

dia%eter upstrea% ro% the plate, anddownstrea% at the point o vena contracta. Thislocation varies ro% B.E51 to B.1. The venacontracta taps provide the %aDi%u% pressuredi-erential, &ut also the %ost noise. 7or%ally are

used only in pipe siJes eDceeding " inches. ; 6orner taps are predominant or pipes

under 9 inches

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%P Flow -easurement

Khen a 1+ cell is used to trans%it a ow %easure%ent the output o thetrans%itter is not linear. To solve this pro&le% so%e or% o signal conditioningis needed to condition the signal or use with a linear scaled indicator.

(elationship #etween %iferential pressure and ow

Khen the di-erential pressure is o&tained eDperi%entally and plotted againstow, the resulting graph is a suare unction.

G / the suare root o di-erential pressure is plotted against ow, a straightline is o&tained showing that the rate o ow is in direct proportion to thesuare root o di-erential pressure.

Thereore, in %any ow %easure%ent installations a 2uare 4oot Dtractor is

ftted to the output o a di-erential pressure trans%itter. /n 2%art trans%ittersit is in&uilt with %icroprocessor progra%%ing. 3r it is done in the +# or 1#2progra%s

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%P Flowmeter &nstallations

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Ad>antages and %isad>antages o +ri7ce owmeters

Ad>antages

G They are easy to install.

G 3ne di-erential pressure trans%itter applies or any pipe siJe.

G 6any 1+ sensing %aterials are availa&le to %eet process

reuire%ents. G 3rifce plates have no %oving parts and have &een researched

eDtensivelyL thereore, application data well docu%ented $co%paredto other pri%ary di-erential pressure ele%ents).

%isad>antages%isad>antages

G The process uid is in the i%pulse lines to the di-erentialtrans%itter %ay reeJe or &loc'.

G Their accuracy is a-ected &y changes in density, viscosity, andte%perature.

G They reuire reuent cali&ration

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)T*(& T*?"

o *enturi tu&e consists o a section o pipe with a conical entrance, a short straight throat, and a conicaloutlet. The velocity increases and the pressure drops at the throat. The di-erential pressure is %easured&etween the inlet $upstrea% o the conical entrance) and the throat.

o *enturi tu&es are availa&le in siJes up to CM, and can pass C5 to 5BN %ore ow than an orifce with the

sa%e pressure drop. Further%ore, the total unrecovered head loss rarely eDceeds IBN o %easured d(p. Ad>antages and %isad>antages o )T*(& T*?"

Ad>antage

G /t can handle lowApressure applications

G /t can %easure C5 to 5BN %ore ow than a co%para&le orifce

G /t is less suscepti&le to wear and corrosion co%pared to orifce plates

G /t is suita&le or %easure%ent in very large water pipes and very large air(Oas ducts.

G +rovides &etter peror%ance than the orifce plate when there are solids in 2uspension.

%isad>antage G /t is the %ost eDpensive a%ong the di-erential pressure %eters

G /t is &ig and heavy or large siJes

G /ts has considera&le length

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93 A(&A?! A(A F!+-T("

G *aria&le area ow %eters are si%ple andversatile devices that operate at a relativelyconstant pressure drop and %easure the ow

o liuids, gases, and stea%. G There are two %ain types o this %eter

I.Float type $4ota%eter)

C. Tapered plug type.

Float Type 1(otameter3

The oat is inside a tapered tu&e. The uidows through the annular gap around the edgeo the oat. The restriction causes a pressuredrop over the oat and the pressure orces theoat upwards.

;ecause the tu&e is tapered, the restriction isdecreased as the oat %oves up. ventually alevel is reached where the restriction is ustright to produce a pressure orce thatcounteracts the weight o the oat.

The level o the oat indicates the ow rate.

/ the ow changes the oat %oves up or

down to fnd a new &alance position.

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Tapered Plug Type

/n this %eter a tapered plug is aligned inside a hole ororifce. spring holds it in place. The ow is restricted

as it passes through the gap and a orce is producedwhich %oves the plug. ;ecause it is tapered therestriction changes and the plug ta'es up a positionwhere the pressure orce ust &alances the spring orce.

The %ove%ent o the plug is trans%itted with a

%agnet to an indicator on the outside.

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;3 -68A)&6A! F!+-T("

G 6echanical ow %eters that %easureow using an arrange%ent o %oving

parts, either &y passing isolated 'nownvolu%es o a uid through a series ogears or cha%&ers $positi>e

displacement meters3 34 &y %eans o a spinning tur&ine or

rotor $Tur#ine Flowmeters3

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;93 T*(?&) F!+-T("

The tur&ine ow%eter is an accurate and relia&le ow%eter or&oth liuids ow%eter. /t consists o a %ultiA&laded rotor%ounted at right angles to the ow and suspended in the uid

strea% on a reeArunning &earing. The rotor speed o rotation isproportional to the volu%etric ow rate. Tur&ine rotation can&e detected &y solid state devices $inductance pic'Aups).

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olumetric Flow (ate @uation

The outputs o reluctance and inductive pic'Aup coils are continuous sine waves with thepulse train!s reuency proportional to the ow rate.

/n an electronic tur&ine ow%eter, volu%etric ow is directly proportional to pic'up coiloutput reuency. Ke %ay eDpress this relationship in the or% o an euation: / .

Khere, P Freuency o rate output signal $HJ, euivalent to pulses per second) = P *olu%etric ow rate $e.g. gallons per second)

' P Tur&ine %eter actor $e.g. pulses per gallon)

k Factor

G tur&ine ow%eterQs K factor is determined by the manufacturer by displacing a 'nownvolu%e o uid through the %eter and su%%ing the nu%&er o pulses generated &y the%eter.

Ad>antages and %isad>antages o the tur#ine metersAd>antages

The tur&ine %eter is easy to install and %aintain.

G re &i directional

G Have ast response

%isad>antages

G They are sensitive to dirt and cannot &e used or highly viscous uids.

G Flashing or slugs o vapour or gas in the liuid produce &lade wear and eDcessive&earing riction that can result in poor peror%ance and possi&le tur&ine da%age.

They are sensitive to the velocity profle to the presence o swirls at the inletL they reuirea unior% velocity profle $i.e. pipe straightness %ay have to &e used ).

2trainers %ay &e reuired upstrea% to %ini%ise particle conta%ination o the &earings.

o The trans%ission ca&le %ust &e well protected to avoid the e-ect o electrical noise.

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-AG)T&6 F!+-T("

?ase principle o magnetic owmeter

The %agnetic ow %eter design is &ased on FaradayQs law o %agnetic induction which states that:The>oltage induced across a conductor as it mo>es at right angles through a magnetic 7eldproportional to the >elocity o that conductorB

That is, i a conductor is %oving perpendicular to its length through a %agnetic feld, it will generate an

electrical potential &etween its two ends

/ ? C ! C > here:

; P the strength o the %agnetic feld $induction)

P the length o the conductor $distance o electrodes)

v P velocity o the conductor $average ow velocity)

-agmeter Flow @uation

/ a conductive uid ows through a pipe o dia%eter $1) through a %agnetic feld density $;) generated

&y the coils, the a%ount o voltage $) developed across the electrodes will &e proportional to thevelocity $*) o the liuid. ;ecause the %agnetic feld density and the pipe dia%eter are fDed values,they can &e co%&ined into a cali&ration actor $>) and the euation reduces to: P>*

6anuacturers deter%ine each %ag%eter!s > actor &y water cali&ration o each owtu&e. The > valuethus o&tained is valid or any other conductive liuid and is linear over the entire ow %eter range.

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dvantages and 1isadvantages o 6ag%eter

Ad>antages

G re &iAdirectional

G Have no ow o&struction

G re easy to reAspan

G /t can %easure pulsating and corrosive ow.

G /t can install vertically or horiJontally $the line %ust &e ull, however)and can &e used with uids with conductivity greater than CBB u%hos(c%.

G #hanges inconductivity value donot,a-ect the instru%ent peror%ance.

%isad>antages G /t!s a&ove average cost

G /t!s large siJe

G /ts need or a %ini%u% electrical conductivity o 5 to CB R%hos ( c%

G lectrical coating %ay cause cali&ration shits

G The line %ust &e ull and have no air &u&&les $air and gas &u&&les

entrained in the liuid will &e %etered as liuid, causing a %easure%enterror).

G /n so%e applications, appropriate %echanical protection or theelectrodes %ust &e provided.

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The %oppler "hit

o 1opplerAe-ect ow %eters use a trans%itter that proectsa continuous ultrasonic &ea% at a&out B."B 6HJ throughthe pipe wall into the owing strea%. +articles in the strea%

reect the ultrasonic radiation, which is detected &y thereceiver.

o The reuency reaching the receiver is shited in proportionto the strea% velocity.

o The reuency di-erence is a %easure o the ow rate.

o Thus, ow velocity * $t(sec) is directly proportionalto thechange in reuency. The ow $= in gp%) in a pipe having acertain inside dia%eter $/1 in inches) can &e o&tained &y:

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Ad>antages and %isad>antages o %oppler-eter

Ad>antage

G The co%%on cla%psAon versions are easilyinstalled without process shutdown.

G /t can &e installed &iAdirectional

GFlow %easure%ent is not a-ected due to change in

the viscosity o the process. G The %eter produces no ow o&struction

G /ts cost is independent o line siJe.

%isad>antage

G The sensor %ay detect so%e sound energytravelling in the causing intererence reading errors.

G The instru%ent reuires periodic re cali&ration

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Transit Time -easurement

o /n this design, the ti%e o ight o the ultrasonic signal is %easured &etweentwo transducersL one upstrea% and one downstrea%. The di-erence in elapsedti%e going with or against the ow deter%ines the uid velocity.

o Khen the ow is Jero, the ti%e or the signal to get toTC is the sa%e as thatreuired to get ro% TC to TI. Khen there is ow, the e-ect is to &oost thespeed o the signal in the downstrea% direction, while decreasing it in theupstrea% direction. The owing velocity $*) can &e deter%ined &y theollowing euation:

o where > is a cali&ration actor or the volu%e and ti%e units used, dt is the

ti%e di-erential &etween upstrea% and downstrea% transit ti%es, and T isthe JeroAow transit ti%e

o The speed o sound in the uid is a unction o &oth density and te%perature.Thereore, &oth have to &e co%pensated or.

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Ad>antages and %isad>antages o Transit -eter

Ad>antagesAd>antages

G /t does not cause any ow o&struction

G /t can &e installed &iAdirectionall

G /t is una-ected &y changes in the process te%perature

G /t is suita&le to handle corrosive uids and pulsating ows. G /t can &e installed &y cla%ping on the pipe and is generally suited or

%easure%ents in very large water pipes.

%isad>antages%isad>antages

G This type o %eters are highly dependent on the 4eynolds nu%&er $thevelocity profle)

G /t reuires nonporous pipe %aterial $cast iron, ce%ent and f&re glass shouldS

&e avoided)

G /t reuires periodic re cali&ration

.

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53 -A"" F!+-T("

Traditionally uid ow %easure%ent has &een %ade in ter%s othe volu%e o the %oving uid even though the %eter user %ay&e %ore interested in the weight $%ass) o the uid. *olu%etricow %eters also are su&ect to a%&ient and process changes,

such as density, which changes with te%perature and pressure. There are three ways to deter%ine %ass ow:

I. The application o %icroprocessor technology to conventionalvolu%etric %eters.

C. 8se o #oriolis ow %eters, which %easure %ass ow directly.

E. The use o ther%al %ass ow %eters that iner %ass ow &yway o %easuring heat dissipation &etween two points in thepipeline.

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-&6(+P(+6""+(D?A"% +!*-T(&6 F!+ -T("

with %icroprocessors it is relatively si%ple to co%pensate avolu%etric ow %eter or te%perature and pressure.

Kith relia&le co%position $density) inor%ation, this actor also can&e entered into a %icroprocessor to o&tain %ass ow readout.However, when density changes %ay occur with so%e reuencyand changes %ay occur with so%e reuency, and particularly wherethe owing uid is o high %onetary value $or eDa%ple, in custodytranser),precise density co%pensation $to achieve %ass) can&e

eDpensive.

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o For the precise %easure%ent o gas ow $stea%) at varying pressures andte%peratures, it is necessary to deter%ine the pressures and te%peratures, itis necessary to deter%ine the

density, which is pressure and te%perature dependent, and ro% this valueto calculate the actual ow. The use o a co%puter is essential to %easureow with changing pressure or te%perature.

o This unit will auto%atically correct or variations in pressure, te%perature,specifc gravity, and superAco%pressi&ility. The pressure di-erential $h)developed &y the ow ele%ent is %easured, and the %ass ow $K) can all &ecalculated using the ollowing generaliJed or%ulas:t he ollowing generaliJedor%ulas:

here:

' is the discharge coe0cient o the ele%ent $which also reects the units o%easure%ent),

is the crossAsectional area o the pipe!s opening, and

1 is the density o the owing uid.

;3 "" + "

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5;3 T8(-A! -A"" F!+-T("

o The power supply directs heat to the %idpoint o a sensor tu&e that carries a constant percentage o the ow.3n the sa%e tu&e at euidistant two te%perature ele%ents $4T1) are installed at upstrea% and downstrea% othe heat input.

o Kith no ow, the heat reaching each te%perature ele%ent $4T1) is eual.

o Kith increasing ow the ow strea% carries heat away ro% the upstrea% ele%ent TI and an increasinga%ount toward the downstrea% ele%ent TC. n increasing te%perature di-erence develops &etween the twoele%ents.

o This te%perature di-erence detected &y the te%perature ele%ents is proportional to the a%ount o gasowing, or the %ass ow rate.

o The pipe wall te%perature is highest near the heater $detected as Tw), while, so%e distance away, there is nodi-erence as &etween wall and uid te%perature.

o Thereore the te%perature o the unheated uid $T) can &e detected &y %easuring the wall te%perature atthis location urther away ro% the heater. This heat transer process is non linear, and the correspondingeuation di-ers ro% the one a&ove as ollows:

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o /n the directAheat version, a fDed a%ount o heat $) isadded &y an electric heater. s the process uid owsthrough the pipe, resistance te%perature detectors $4T1s)

%easure the te%perature rise while the a%ount o electricheat introduced is held constant.

3 The %ass ow $%) is calculated on the &asis o the%easured te%perature di-erence $TC A TI), the %etercoe0cient $>), the electric heat rate $), and the specifcheat o the uid $#p), as ollows: