*affum, h. a. , mumuni, i.i., appiah, g.k., adzaklo, s.y

Journal of Ghana Science Association, Vol. 18, No. 2, December 2019

VALIDATION OF WATER FLOW METERS USING RADIOACTIVE TRACERS

1,2*Affum, H. A. , 1Mumuni, I.I., 1 Appiah, G.K., 1Adzaklo, S.Y. and 1Coleman, A.1Nuclear Applications Centre, National Nuclear Research Institute, Ghana Atomic Energy Commission, P. O. Box LG 80, Accra, Ghana

2Department of Nuclear Science and Applications, School of Nuclear and Allied Sciences, University of Ghana

AbstractRadioactive tracer transit time method was employed for the measurement of volumetric flow rate for three different water flowrigs (FRs). Technitium-99m with activities 136.9 MBq, 92.5 MBq and 155.4 MBq were injected as a pulse at suitable locationson selected piping on FRs 1, 2 and 3 respectively. Downstream of the injection point, the tracer pulses were measuredusing two sodium iodide (NaI) radiation detectors placed 7.5 m, 10.0 m and 2.8 m apart on FRs 1, 2 and 3 respectively,depending on the accessibility on the FRs. The mean residence time (MRT) between each pair of detectors was obtainedusing a residence time distribution (RTD) Software, after which the volumetric flow rates for the rigs were calculated. ForFR1, the experiment was carried out three times at different flow rates of 5.0, 11.0 and 15.0 litres per minute (l/min) and theexperiment was repeated for each flow rate. Flow rates of 10.0 l/min and 17.0 l/min were set for FR2 and FR3 respectivelyand the experiments were repeated for these flow rates as well. Experimental flow rates calculated from the transit timemethod were 4.02 ± 0.07 l/min, 9.72 ± 1.26 l/min and 15.97 ± 0.06 l/min for FR1; 11.26 ± 1.23 l/min for FR 2 and 15.96± 1.04 l/min for FR 3. The good agreement between the experimental flow rates and those recorded by the installed flowmeters proved that the radiotracer transit time method can be a useful tool for flow meter calibration.

Keywordsflow rate, radioactive tracer, transit time, flow meter, calibration, validation

IntroductionThe fluid dynamic properties such as flow rate, phase distribu-tions, flow pattern, velocity and turbulent parameters of flowsystems are needful for complete process understanding. It istherefore necessary to measure them to facilitate the processcontrol and optimization to achieve efficient management ofindustrial processes.Online and continuous flow measurement is often necessary,particularly in industry and are thus determined using tech-niques involving venturi pressure drop, Coriolis, electromag-netic, and cross-correlation flow meters, gamma-ray absorp-tion and gradio-manometer densitometers; and local electri-cal and fibre-optic sensors (Baker, 2005; Oddie and Pearson,2004). These methods require a measurement device whichstays in direct contact with the fluid resulting in the corrosionof the moving parts of the meter and causes wear on theircomponents, and hence affecting the accuracy of the measure-ments (Baker, 2005; Frenzel et al., 2011; Spitzer, 1990).The application of radioactive tracers for flow rate measure-ment for calibration of flow meters is well known. A ra-diotracer is any substance whose atomic or nuclear property,provide for the identification, observation and following ofthe behaviour of various physical, chemical or biological pro-cesses (dispersion, mixing, kinetics and dynamics), whichoccur either instantaneously or in a given lapse of time (Inter-national Atomic Energy Agency (IAEA), 2001, 2008; Interna-tional Standards Organization (ISO), 1974). The radiotracertechnique offers high detection sensitivity and online detec-tion, once the radiotracer’s physico-chemical compatibilitywith the flow system has been established (IAEA, 2008; ISO,1974; Kasban et al., 2010; Yelgaonkar et al., 2009). Moreover,

since techniques using radiotracers employ statistical func-tions in their analysis, they are recognized for their precisionand safety (Frenzel et al., 2009; IAEA, 2001). Flow rates ofliquids and gases are measured to an accuracy of 1–2% insituations where flow meters are either not installed or areunreliable due to deposits or corrosion. Tracer techniquesfor single phase flow measurements are recognized as ISOstandards (ISO, 1974).Radiotracer technology for flow rate measurement is catego-rized into two-transit time method (formerly called Allen ve-locity method) and the constant rate injection method. Techniti-um-99m (Tc-99m) in the form of sodium pertechnetate is aknown good aqueous phase tracer. It has reasonable energy, ashort half-life, thus making it safer (IAEA, 2008) and moreimportantly it is readily available and obtainable from thehospitals. In this study, the transit time method was used tomeasure flow rates of 3 different water flow rigs using Tc-99mas radiotracer. The objective of the study is to demonstrate thecapability and suitability of application of radiotracer technol-ogy in flow meter validation and calibration.

Theoretical FrameworkIn the transit time method, illustrated in Figure 1, the radioac-tive tracer is injected into the conduit and the time (meanresidence time) taken by the radioactive tracer to travel a spec-ified distance between two cross-sections is measured. Twodetectors are placed downstream where sufficient mixing ofthe radioactive tracer and the flow material would have beenachieved (IAEA, 2008).The mixing distance required for closed conduits is deter-mined as per the guidelines specified in the standard ISO

53

Validation of water flow meters using radioactive tracers Affum et al. — 54/57

Figure 1. Principle of transit time radioactive tracer methodused to measure fluid flow rate in closed circuits

2975. The volumetric flow rate is obtained by dividing thevolume of flow with the mean residence time (transit time)between the detectors as given by Equation 1 . This flow rateis compared with the flow rate indicated by the flow meter tobe calibrated. The radioactive tracer injection is repeated forthe same flow rate and the result is obtained as the mean valuefrom the repetitions.

Q =Vτ

(1)

where V is the volume of the conduit between the detectorpositions, m3; τ is the transit time of the labelled fluid particlesbetween the two detectors, s.The transit time of the tracer between detector positions maybe determined by suitable graphical constructions or directcomputation on concentration/time distributions, or their inte-grals, recorded simultaneously with accurate timing signalsfrom a suitable device as given by Equation 2 (IAEA, 2008).

τ =

∫∞

0 tiCi(t)dt∫∞

0 Ci(t)dt(2)

Materials and MethodsThe measurements were carried out by injecting the radioac-tive tracer into the pipelines of the flow rigs with two (2)detectors placed on the pipeline at some distances apart de-pending on the accessibility on the FRs. Figure 2 illustratesthe placement of the detectors on FR 3.The distance between D1 and D2 for FR 1 was 7.50 m whilesthe pipe diameter was 1.27 cm. Similarly, for FR 2, thedistance between the two detectors was 10.00 m whiles thepipe diameter was 1.91 cm. For FR 3, the distance betweenthe two detectors was 2.80 m with a pipe diameter of 3.81 cm.A multi-input data acquisition system (DAS), as shown inFigure 3a, integrated with a NaI(Tl) radiation detectors (2.54

Figure 2. Set up for experiment on FR 3 showing detectorlocations (D1 and D2)

cm × 2.54 cm) was used for online monitoring and visualdisplay (Figure 3b) of the radioactive tracer signals in thepipelines at the detector locations.

Figure 3. (a) Data acquisition system set up. (b) Online visualdisplay of tracer signals

The data acquisition system consists of a laptop computerwith data acquisition software (CEASAR) supplied by theinternational Atomic Energy Agency. It is a multipurposeequipment for flow and mixing studies and has been used insimilar studies by many investigators including Adzaklo et al.(2018) and Luabanya et al. (2020). The DAS was connectedto the PC through a USB port. The NaI(Tl) detector wasconnected to the DAS through 10–50 m long coaxial cablesdepending upon the requirement. Tc-99m was chosen as theradioactive tracer for this study mainly due to its availability,its suitability as a water-phase tracer and its relatively shorterhalf-life in comparison with other water-phase tracers. Thiswould help minimize any effect of contamination and radio-logical problems associated with the handling of the isotope.After collecting background counts for 10 min and achievinga steady flow rate of 5.0 l/min, 139.9 MBq of Tc-99m wasinjected at a suitable location upstream of the first detectoron FR 1 as illustrated in Figure 4a. Figure 4b shows the flowmeter reading at the time of the experiment. The injection wasdone as rapidly as possible, with no "tailing" of the injectedradioactive tracer from the injection tubes into the conduit(Kasban et al., 2010). In order to improve the representative-ness of the measurements (IAEA, 2001; Oddie and Pearson,2004). The experiment was repeated for each flow rate of 5.0l/min, 11.0 l/min and 15.0 l/min for FR1; and for flow rates of10.0 l/min and 17.0 l/min for FR2 and FR3 respectively usingthe same radioactive tracer of reasonable activity.The radiation signals from the detectors, were treated andused as the inputs to the Residence time distribution (RTD)



Figure 4. (a) Tc-99m injection into FR1. (b) Flow meterreading for FR 1 at the time of the experiment

software. The software calculates the response E*H(t) of themodel to a given signal E(t); H(t) being the impulse responseof the model and * the convolution operation. If the actualresponse of the system, S(t), has been measured, then optimi-sation of the parameters of the model will ensure that E*H(t)is as close as possible to S(t) (IAEA, 2008). The programcalculates the mean residence time (MRT) between the twodetectors which is then used to calculate the flow rate as afunction of the volume of flow between the two (2) detectorsusing Equation 2.

Results and DiscussionThe residence time distribution curves for the tracer signalswere generated from which the MRTs were obtained. TheMRT was then used to calculate the flow rate as a functionof the volume of flow between the two (2) detectors usingEquation 2.The RTD curves are shown in Figures 5,6, 7. The experimentaland the calculated parameters are presented in Table 1.Comparing the experimental flow rates measured in the flowrigs with their corresponding flow meter readings, there wasgenerally good agreement with marginal uncertainties. Therelative error associated with experimental flow rate obtainedfor FR1 (14.04 l/min) was the least (0.06) whiles that asso-ciated with the experimental flow rate on FR1 (9.27 l/min)was the largest (1.26). The primary source of uncertaintyin the transit time method is related to time measurements.These include the response time of the detectors and the timedifference between the two detectors. Good mixing providesa faster rise time as the flow passes the detectors, thus im-proving the precision of the time difference. Additionally, anynon-uniformity in the internal shape of the conduit, as was thecase in FR2 and FR3, negatively affects the time measurementand hence the experimental results (IAEA, 2008; ISO, 1974;ISO, 1976). The flow rate as read from the flow meter couldalso be a source of error as this can be very subjective. Fromthe reasons adduced above, the differences between the flowmeter reading and the experimentally determined flow ratescould be due to the errors and or a slight malfunctioning of

Figure 5. RTD plots for three (3) experiments and their repe-titions: (a) and (b) for a flow rate of 5 l/min, (c) and (d) for aflow rate of 11 l/min and (e) and (f) for a flow rate of 15 l/min,on FR1

Figure 6. RTD plots for one (1) flow rate experiment and itsrepetition for a flow rate of 10 l/min on FR 2

Figure 7. RTD plots for flow rate experiment and their repeti-tions: (a) and (b) for a flow rate of 17 l/min on FR3.

the flow meters.



Table 1. Experimental and calculated parameters

Flow meterreading, l/min

Exprt 1MRT, s

Exprt 2,MRT, s

AverageMRT, s

Experimentalflow rate, l/min

FLOW RIG1 5.0 14.16 14.20 14.18 4.02±0.0711.0 5.88 5.85 5.87 9.72±1.2615.0 4.04 4.07 4.06 14.04±0.06

d = 1.27 cm, L = 7.50 m

FLOW RIG2 10.0 15.29 15.23 15.26 11.26±1.23d = 1.91 cm, L = 10.00 m

FLOW RIG3 17.0 12.80 11.20 12.00 15.96±1.04d = 3.81 cm, L = 2.80 m

L = distance between two detectors; d = pipe internal diameter

ConclusionsThe transit time method for flow rate measurement usingradiotracer technology has been successfully applied for mea-surements of water flow rate in three flow rigs. The valuesof the flow rates measured in the three rigs were generally ingood agreement with the flow meter readings with marginal er-rors. In view of the error margins stated above, the techniqueproved to be sensitive and versatile for flow measurements. Itcould therefore be applied for accurate measurement of flowrates in similar systems where conventional methods cannotbe used as well as flow meter validation or calibration for suchflow systems as used in this study.

RecommendationsIn order to fully demonstrate the capability and suitability ofthe radiotracer technique in flow meter validation and calibra-tion, a larger amount of data need to be used. It is thereforerecommended that the study be carried out on similar rigs withlarger amount of experiments than in this study. Secondly,measurements were carried out on laboratory flow rigs whichare industrial process flow simulators. As such, the pipingsystems on the rigs are not as large as those used in industrialprocesses which handle larger volume of materials. It is there-fore recommended that the experiments be done on industrialprocess flow pipes to further establish the applicability of thetechnique in such flow situations.

AcknowledgmentsThe authors are grateful to the radiotherapy unit of the Korle-Bu Teaching hospital, Accra, Ghana, for providing the ra-dioactive tracer and the International Atomic Energy Agencyfor the technical support

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*affum, h. a. , mumuni, i.i., appiah, g.k., adzaklo, s.y

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