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Module 10 :

Measurement of two phase flow parameters

Lecture 34 :

Parametric Measurement of Two Phase Flow

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In spite of the extensive volume of past research activity, two phase flow is not yet an area in which

theoretical prediction of flow parameters is generally possible. Indeed, this situation is likely to persist for

the foreseeable future. Thus, the role of experiment and parametric measurement is particularly important.

The techniques of measurement for single phase flow are well established. Based on these techniques,various meters and instruments have been developed which are successfully employed for industrial

measurement as well as for R&D activities. Unfortunately, these instruments cannot be directly used for

multiphase flow measurement. Most of the problems in multiphase flow measurements arise from the fact

that the parameters characterizing it are many times larger than those in single phase flows. In single

phase flow, the flow regimes encountered are laminar, turbulent and a transition region between them. In

multiphase flow, numerous flow regimes are possible depending on flow geometry (size and shape) and

orientation (vertical, horizontal and inclined), flow direction in vertical or inclined flows (up or down),

phase flow rates and properties (density, viscosity, surface tension) as discussed in Chapter-2. In addition

the slip between the phases causes a difference in the in-situ and inlet composition of the multiphase

mixture, As a result, the methods of flow measurement conventionally adopted for single phase flow are

grossly inadequate.

This has given rise to the development of a number of techniques especially suited for the

measurement of two phase flow parameters. In the limited scope of this discussion, it is not possible to

consider the principles of measurements of all the parameters. However, void fraction and flow pattern

are two parameters of unique importance. Information regarding these parameters is essential for the

design and optimization of the components, control and monitoring of the equipment, overall efficiency

of the process and safety of the plant. Knowledge of these two parameters is often used as the input for

the measurement of other variables. In this chapter, different techniques for measurement of void fraction

and flow pattern is described. The description is primarily based on gasliquid two phase flow though

reference to other types of two-phase flow is made from time to time. Prior to the discussion of

measurement of the aforementioned parameters, we would describe the challenges involved in measuring

pressure drop of two phase flow just to emphasise the complexities involved in measurement of even

simple parameters under multiphase flow situations.

10.1Measurementofpressuredrop

This parameter is of interest since it governs the pumping power required to circulate two phase fluids

through the system and it governs the circulation rate in case of natural circulation. It is also important in

several flow metering applications like venturimeters and orifice meter. In two phase flow, measurement

of pressure drop requires special considerations as has been discussed below.

The scheme of the measurement is explained in Figure 10.1.

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Fig. 10.1. Void fraction estimation by pressure drop measurement

Making a pressure balance at Section A one gets,

( ) ( ) ( ) mCC gzzgzzpgzzp 13342121 ++=+ (10.1)

Rearranging we have,

( ) ( ) ( ) CCm gzzgzzpp 241321 += (10.2)

If p1= p2, the manometric difference is

(10.3)

This indicates an offset in the manometer which depends on (a) distance between tappings (b) density

of the line fluid ( C ). Further in absence of flow through the tube, considering no acceleration

pressure drop

( ) ett hgzzgpp += 2421 (10.4)

eh is the head loss due to friction. t is the mixture density and is given in terms of volume average

void fraction,

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( )Gt

+=A

1 (10.5)

Equating equations (10.2) and (10.4), we get the manometric difference as:

( ) ( ) ( )

( ){ } ( ){ }eg

CCm

hzzg

zzgzz

++=

+

24

2413

1

A

(10.6)

Neglecting head loss due to friction

(10.7)

Which shows that for zero differential in the manometer, t = c or line fluid has the same density as the

fluid in the tube. While this is an expected situation in single phase flows, the case is not so simple when

two phase flow occurs in the pipeline because the lines, under these conditions, can always be filled with

a two phase mixture of unknown and variable composition and from eqn (10.2), it is very important to

know the composition and density of fluid within connection lines (c). Or it is mandatory to control the

manometer lines in such a way that they are filled by a single phase fluid and there is no ingress of the

second fluid in them. In case of gas-liquid flow, it is generally filled up with the fluid corresponding to

either gas or liquid phase flowing in the pipe. Usually it is the continuous phase which fills the lines.

Tosummarise,theadditionaldifficulties inmeasurementofpressuredrop intwophase flowareas

follows:

1. Possibleambiguitiesincontentoflinesjoiningtappingpointstomeasuringdevice2. Pressuredropfluctuationscanbequitelarge3. AddedproblemsinheatedsystemsparticularlywhentheyareJouleheated

Themethodsforpressuredropmeasurementaresameasthoseadoptedinsinglephaseflows,viz

1. Fluidfluidmanometers2. Subtractionofsignalsfromtwolocallymountedpressuretransducers3. Differentialpressuretransducers

For fluidfluidmanometers,watermercurymanometers or invertedwatermanometers are adopted

dependingonthepressurerange.Forgreatersensitivitywatercarbontetrachlorideorwaterkerosene

manometers are used. Using inverted liquidgasmanometerwith liquid filled tapping lines ismore

( ) ( ) ( ) ( )CtCm gzzgzz = 2413

( ) ( )( )

( )CmCtzzzz

= 2413

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accurateascomparedtoliquidmercurymanometer.Inspecialcaseswheregasfilledpressurelinescan

beemployed,inclinedmanometers/micromanometersareused.

Theproblemforanyfluidfluidmanometeristhatthecontentofthelinecanbetwophasebyavariety

ofmechanismsnamely,

1. Changes in pressure drop and consequent movement in manometer can cause two phasemixturetoenterintotappinglinesfromflowpassage.Thiscanhavedisastrousresultseg.Mercuryfrom

manometers enteringmetal flow system. To overcome this difficulty, large diameter catch pots are

introducedinthetappinglinesfortwoliquidstomeet.

2. Condensationorevaporationcanoccurinlinesparticularlyasaresultofrapidchangesinsystempressure, forexamplegenerationofvaporbubble in liquid filled lines followingdepressurization.For

liquidvaporsystemsandvaporfilledlines,anevaporatorjustdownstreamoftappingpointsevaporates

anyliquidenteringtheline.Thisisparticularlyusefulforlowlatentheatliquidslikecryogenicfluidsbut

thetechniqueisnotcommonsincetherateofevaporationisratherslow.Similarlyacondensercanbe

installedinliquidfilledlinesiflowlatentheatliquidsareusedasthetestfluids.

3. Pressurefluctuationscancauseapumpingactionleadingtogasingressintoliquidfilledlinesorviceversa.For example, if lines are filled up with liquid phase, gas/ vapor ingress can occur by (a)

Changesinpressuredropandmovementofmanometricfluidallowingtwophasemixturetoenterany

oneof thepressure tappings. (b)Flashing in the linesafter rapiddepressurization (changes insystem

pressure)(c)Pressurefluctuationcausingpumpingactionleadingtogasingressintotappings.Similarly

forgas/vapor filled lines, liquid ingress canoccurby (a)Changes inpressuredropandmovementof

manometricfluid(b)Pressurefluctuationleadingtoliquidpumpingintolines(c)Vaporcondensationin

lines.Further,forliquidfilledlines,theperformancecanbeimprovedbyusingabalancedliquidpurge

systemasshowninFig.10.2.Itmaybenotedthatthelineshavetobetransparenttocheckgaslocksif

any.Alternatively,compressibilityoffluidinlinecanbecheckedbyusingacousticmethods.4. Theadditionaldisadvantageofmanometersisthatitisnotsuitablefortransientmeasurements.Onehastousetransducersforthispurpose.

Althoughtheconsequencesofliquidingressaresameasgasingress,liquidingressismoresevereand

likelythangasingressduetothefollowingreasons:

1.Compressibilityoffluidingasfilledlinescausesmuchworsepumpingactionbypressurefluctuation

2.Tendencyofliquidphasetowetthechannelwallcausescapillaryeffectsatgasliquidinterfacewhere

lineenterschannel.Thisisparticularlysignificantforsmalldiameterlines.

Thusonthewhole,gasfilledlinesarelesssatisfactory. Theonlyadvantageisthelowoffsetvalueat

zero p (eqn10.3)andthusgreateraccuracyofpressuremeasurementspossible.

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Fig.10.2:Purgingsysteminpressuremeasurement

Analternative tomanometersparticularlywhena rapid response is required is touseapairofwall

mountedpressuretransducersmountedlocallyatthepointsbetweenwhichthepressuredropistobe

measured,thesignalsofwhichareelectronicallysubtractedtoobtaintherequiredpressuredrop.The

principleofmeasurementisshownschematicallyinFig.10.3.

Among thedifferent typesof transducersnamelypotentiometric,straingauge,capacitive, reluctance,

inductive, eddy current and piezoelectric, the capacitance and piezoelecric type are suited for

measurementsusingsignalsubtraction.Acomparativestudyof theperformanceof the two typesof

transducersisgiveninTable10.1.

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Table10.1:Typesoftransducersparticularlysuitableformeasurementusingsignalsubtraction

Characteristics CapacitanceType PiezoelectricType

Stability More Less

ResponseTime 20 secs 2 secs(veryfast)

Sensitivity Less(0.01%fullscale) More(0.001%fullscale)

MaximumOperating

Temperature

370C 160C(Lower)

Theadvantagesoftransducersare:

1. Fastresponse 2. Enablesstudyoffluctuationsinpressuredrop3. Avoidsambiguityinlinecontent

Thedisadvantagesare:

1.

Signals from two separate instruments are measured and subtracted and this obviouslyincreaseserrors. Inordertoavoidthis,differentialpressuretransducersareadopted.However,this is

unavoidableifrapidlyfluctuatingpressuredroparetobemeasured.Inthatcase,specialcareisrequired

tocalibrate transducersand toensure that theoutput isproperlyconverted totherequiredpressure

drop.

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2. Furthereralthough theamountof fluidbetween flowpassageand transducer is rathersmall,thevolumeofthetappinglineandthefluidadjacenttothediaphragmshouldbekeptataminimumin

ordertoavoidreductionsinfrequencyresponse.

3. Both capacitanceandpiezoelectric transducersare limitedas tooperating temperaturesandneedtobecooledforhigheroperatingtemperatures.

Fig.10.3

Fig.10.3:Mountingforabsolutepressuretransducersformeasuringtwophasepressuredrop