PERFORMANCE EVALUATION OF EIGHT ELECTROMAGNETIC FLOWMETERS WITH FINE SAND SLURRIES. N I Heywood and K B Mehta

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1 Paper presented at 14th International Conference on Slurry Handling and Pipeline Transport (Hydrotransport 14), Maastricht, The Netherlands September 1999, pp PERFORMANCE EVALUATION OF EIGHT ELECTROMAGNETIC FLOWMETERS WITH FINE SAND SLURRIES N I Heywood and K B Mehta AEA Technology, 404 Harwell, Didcot, Oxon, OX11 0RA, UK Synopsis This work follows on from the results reported at Hydrotransport 12 in 1993 when flocculated and deflocculated kaolin slurry was used as test material. Performance data in terms of accuracy (percent error) and repeatability have been obtained using sand slurry over a concentration range of 23.5% to 74.0% by weight. The d 50 particle size was 9.5 m. The flow properties of these slurries were measured using both pipeline and co-axial cylinder viscometers. The slurries exhibited both Newtonian and some shear-thickening behaviour, depending on the solids concentration/shear rate combination. Laminar, transitional and turbulent flow regimes were covered during the testwork. A total of eight commercial electromagnetic flowmeters were evaluated. These fall into four classes : AC, pulsed DC, capacitance and dual frequency. The accuracy data were obtained in terms of percent full-scale and flowmeter Reynolds number, while repeatability data were obtained as a function of percent full-scale only. All flowmeters were evaluated using tapwater before and after sand slurry tests, in order to compare water data with manufacturers specification and to determine how these specifications compare with the data obtained for slurry flow. Representative data will be given to illustrate that flowmeter accuracy levels are almost always dependent on the flow regime, and therefore the velocity profile of the slurry flowing in the flowmeter. In general, data obtained using the turbulent flow of sand slurry agreed reasonably well with water data, but accuracy levels deteriorated in the laminar regime when parabolic velocity profiles are present in the flowmeter, the largest errors occurring in the laminar region when some shear-thickening flow behaviour was evident. As has been found with other slurry types, the worst accuracy levels and the greatest departures from the manufacturers specifications occur when the sand slurry was in the transitional flow regime in the flowmeter. 1. INTRODUCTION There are many examples of commercially-available flowmeters for fluid flow in pipes. However, the choice is considerably restricted when the flow of industrial slurries is to be monitored. Because these materials can exhibit both high viscosity and non-newtonian properties and, in addition, abrade intrusive-type flowmeters, the choice is restricted to essentially obstructionless flowmeters, namely electromagnetic, Doppler ultrasonic and Coriolis

2 mass flowmeters (1-7). However, within these three main categories of flowmeter, there is a very wide variety of commercially-available design, and the user can be faced with a bewildering choice. Unfortunately, comparative performance data on which a choice can be based are limited for either Newtonian or non-newtonian slurries. Most instruments are supplied by manufacturers with a calibration based on the turbulent flow of water. It is common, however, in slurry applications for the flow to be in the laminar flow regime when viscosity levels are high, and this can have a pronounced effect on the velocity profile of the fluid in the flowmeter. This, in turn, appears to influence the flowmeter calibration (8-12). Electromagnetic flowmeters have even been used (13) to measure fluid rheological properties and to use those to infer a velocity profile in the flowmeter. There is very little information about the performance of commercially-available electromagnetic flowmeters in the open literature. Heywood et al (8) have previously presented performance data for different types of electromagnetic flowmeters used in conjunction with both flocculated and deflocculated kaolin slurries. Van Laak of DSM Research, the Netherlands (14) describes how the International Instruments Users Association (WIB) has co-operated with eight electromagnetic flowmeter manufacturers and the Dutch government in a fundamental applications study called the Low Conductivity Project. The aim of this project was to assess the effects on electromagnetic flowmeter performance of (and combinations of) such parameters as conductivity, viscosity, fluid and ambient temperatures, pipeline size and liner material. All the meters had a quoted accuracy of 1%. The present paper summarises data obtained at AEA Technology using several commercial electromagnetic flowmeters when used with relatively fine sand of a specific size distribution. The full results appear in WSH Report 56 (10), while WSH Reports 48 (9) and 57 (11) provide more data on the performance of electromagnetic flowmeters when used with sand suspended in drilling mud, and coarse sand respectively. Other WSH summary research reports provide performance data for other slurry types such as drilling mud, cellulose acetate and pulverised fuel ash. Full details of these research reports can be obtained from the authors. 2. FLOWMETER PERFORMANCE MEASURES Two performance measures were used in the evaluation of the flowmeters: accuracy (percent error) and repeatability. Accuracy is defined as Vm - Vs ac = x 100 (1) V s where V m is the mean slurry velocity (from 51 instantaneous mean flow velocities, V i, for a given pump speed), m/s and V s is the mean slurry velocity as measured by the flow loop calibrated dynamic weighing system, m/s. The manufacturers accuracy specifications were used to assess both the water and slurry data.

3 Repeatability is defined as σ R = m 1 (Vm - V i ) m -1 2 (2) The standard deviation (σ R ) calculated from the mean slurry velocity (V m ) and the instantaneous mean slurry velocities (V i ) is a measure of the flowmeter repeatability, which was measured, depending on the flowrate, over a period of 40 to 300s. The repeatability was based on 51 instantaneous mean slurry flow velocity measurements made over one weighing cycle. Flowmeter manufacturers often quote repeatability levels for their instruments but seldom, if ever, provide the definition of repeatability they use. Consequently it is difficult to make a direct comparison between repeatability values obtained in this study with manufacturers' specifications. 3. EXPERIMENTAL FACILITIES During the twelve-year work programme, two slurry flow loops were used to evaluate the flowmeters. The rig at Warren Spring Laboratory initially comprised of a long 72 mm horizontal pipe section with two 25 mm horizontal pipe sections, and was operated between 1987 and Both the rig and the operating procedures have been described previously by Heywood et al (8, 15), and are discussed further in WSH Reports 48, 56 and 57 (9-11). Figure 1 shows the rig design at AEA Technology. The rig has been in operation since 1995 and now comprises mainly of two 75 mm vertical pipe and one 25 mm vertical pipe section, with a shorter 72 mm horizontal U-loop. The operation of this rig is very similar to the Warren Spring rig. Sand slurry was recirculated using a Mono progressive cavity pump, which gave essentially non-pulsatile flow through the flowmeters. In all, a total of eight electromagnetic flowmeters were used. All flowmeters gave a 4 to 20mA output. Their details are summarised in Table FINE SAND SLURRIES USED AND EXPERIMENTAL DETAILS HPF6 silica sand supplied by Hepworth Minerals & Chemicals Ltd, UK, was used in all testwork reported here. The sand had a mean particle size, d 50, of 9.5 m and density of 2650 kgm -3. The particle size distribution of the sand is given in Figure 2.

4 Figure 1 Schematic Diagram of Slurry Flow Measurement Rig at AEA Technology Table 1: Details of Electromagnetric Flowmeters Tested with Water and Fine Sand Slurries ELECTROMAGNETIC FLOWMETER TYPE MANUFACTURER MODEL 72mm LINE-1 AC Fischer & Porter Mag-Sm 72mm LINE-2 Krohne 51C-TIV50/F ABB-Kent Veriflux Pulsed DC Krohne X-1000-T900 Endress+Hauser Promag33 Endress+Hauser Promag35S Capacitance Yokogawa Admag CA * Dual Frequency Yokogawa Admag * * The Yokogawa Admag capacitance flowmeter was replaced with the dual frequency Admag flowmeter after completing the 48.6% slurry tests.

5 4.1 Settling tests Figure 2 Particle Size Distribution of Sand used in Testwork Owing to a significant fraction of coarse particles (20% greater than 20 m), settling tests were undertaken to assess the likely degree of settling which may occur throughout the recirculating flow loop (see Figure 3). Figure 3 Settling Tests for Sand Slurry at Different Concentrations

6 Sand slurry was prepared in a beaker by adding the required amount of sand to the water. The mixture was mixed thoroughly and about 100 ml of slurry was added to the measuring cylinder. Settling rate was measured by noting either the build up of solids at the bottom of the cylinder or the movement of the solid/liquid interface. For all concentrations measured the slurry formed a solid/liquid interface (solid settlement may have occurred at the base of the measuring cylinder but it was not visible). Figure 3 shows the drop in solid/liquid interface level as function of time for seven sand concentrations (15.0, 25.0, 57.9, 65.3, 67.5, 73.0 and 73.6% by weight). The plot shows that there was only about 2 ml settling in the initial 100 minutes for slurry concentrations above 57.9%. Slurry up to 57.9% sand concentration re-suspended easily when the slurry was stirred with a spatula. The re-suspension of the other higher concentration slurries required several repeated procedures of inverting the cylinder and agitating its content. 4.2 Rheological tests All the rheological tests were carried out using a Contraves Rheomat 30 co-axial cylinder viscometer. For these tests the A-cup and the A4-bob were used, except in the case of the 76.2% tests when the B-cup and the B4-bob were used. At slurry concentrations below 57.9% solids, the torques generated were lower than the minimum torque reading of the instrument. Figures 4 and 5 show that Newtonian behaviour was exhibited at solids concentrations below about 65%, and above the 70.0% slurry concentration shear-thickening behaviour was exhibited, after a critical shear rate had been reached. Figure 4 Flow Curves for Sand Slurry at Different Concentrations using Contraves Rheomat 30 Co-axial Cylinder Viscometer

7 Figure 5 Viscosity of Sand Slurry as a Function of Shear Rate and Sand Concentration Figure 6 shows that the slurry flow was turbulent for the velocity range of 0.2 to 2.5 m/s for 72 mm lines, up to slurry concentrations of 55% by weight. Hence it was concluded that tests should be carried out at about 25 and 50% by weight fine sand for the turbulent flow regime tests. Further tests were carried out at 65 and 74% by weight for determining the performance of the flowmeters when the flow is laminar. Figure 6 Lower and Upper Limit for Laminar Flow of Sand Slurry in 72 mm ID Pipe

8 Figure 7 shows the wall shear stress/wall shear rate relationship obtained from the 72 mm i.d., m long pipe viscometer, indicating that the fine slurry is in the laminar flow regime for slurry concentrations above about 70%. Figure 7 Flow Curves for the Sand Slurry using Pipe Frictional Pressure Loss Data Table 2 gives the concentration, density and power law flow model parameters. It shows that lower values of the power law index (n) were obtained when using the Contraves laboratory viscometer, compared with those values obtained from the pipe viscometer tests. However, the values of the consistency coefficient, K, were found to be higher from the Contraves tests. It is apparent why there should be this discrepancy, but it is clear that at high sand concentrations, significant shear-thickening behaviour is exhibited Flow Loop Experiments The sand slurries were prepared in the mixing tank by adding the required amount of solids to 1000 kg of mains water. During the tests the slurry was continuously stirred in order to avoid settling of solids. Slurry concentrations were measured by evaporating several samples to constant weight at 110 C. Table 3 gives a summary of the testwork undertaken.

9 Concentration % by weight Table 2: Physical Properties of Silica Sand Slurry Density d kg/m 3 Contraves Viscometer Power Law Index(n) Pipe Viscometer Flow Curves Consistency Coefficient (K), Ns n /m 2 Contraves Viscometer Pipe Viscometer A 0.02 A A 0.02 A A A A 0.05 A A 1.48 A A A A 1.48 A A 0.6 A A d Test not done at this concentration The density of the slurry derived from values of the slurry concentration and from densities of fine sand (2650 kgm -3 ) and water. Slurry Concentration (% by weight solids) Table 3 : Slurry Concentrations used Pipeflow Regime LINE-1 (72 mm) LINE-2 (72 mm) 0 (Water) Turbulent 23.5 " 48.6 " 63.2 " 64.3 " 72.1 Laminar 73.3 " 74.0 "

10 5. PERFORMANCE RESULTS 5.1 Accuracy of Flowmeters Accuracy, percent error, is plotted either against percent full scale (0 to 100% full scale range equated to 0 to 2.5 m/s for all flowmeters), or Reynolds number. Reynolds number was calculated from n 2-n D V ρ 4n Re' = n-1 8 K 1+ 3n n (3) where D = pipe diameter V = velocity in the pipe ρ = slurry density K = consistency coefficient = τ/γ n n = power law index = (d ln τ)/(d ln γ) τ = shear stress γ = shear rate AC Meters The accuracy of the Fischer & Porter (F&P), Krohne and Kent Veriflux AC electromagnetic flowmeters was assessed in terms of both percent full-scale and Reynolds number. The F&P accuracy shown in Figure 8 was generally within the manufacturer s specification of 1% above 20% full scale, although it fell outside this specification at sand concentrations exceeding 70% where shear-thickening behaviour occurred. It also deteriorated to 5%, below 20% fullscale. The Reynolds number plot confirms the wide 4% to +2% spread at Reynolds numbers below 1000, suggesting a significant velocity profile and/or flow regime effect. However, the water data are fell within the 1% specification between 10% and 100% of full-scale, confirming the manufacturer s calibration for turbulent flow. The Krohne AC meter data (Figure 9) all fell within the manufacturer s specification, except for just one data point. The lower sand concentrations tended to result in an underreading of the meter, whereas the higher concentrations (laminar flow) gave an overreading. The Kent Veriflux consistently underread (Figure 10) with error levels ranging from 1 to 5% at above 20% full-scale, and a maximum error of 9% at 8% of full-scale. In summary, all three AC meters gave percent error data which lay below water data at low to intermediate sand concentrations (usually corresponding to the turbulent flow regime), but above the water data for high sand concentrations (laminar flow).

11 Figure 8 Accuracy of Fischer & Porter Mag-SM AC Flowmeter Figure 9 Accuracy of Krohne (51C-TIV50/F) AC Flowmeter

12 Figure 10 Accuracy of Kent Veriflux Flowmeter as a Function of Reynolds Number Pulsed DC Meters The accuracy of the Krohne, Caleray, and the two Endress+Hauser pulsed DC meters were also compared in terms of both percent full-scale and Reynolds number. The Krohne pulsed DC meter gave accuracy data (Figure 11) for water largely within the manufacturer s specification of 0.5%. However, the two highest sand concentrations corresponding to laminar flow with a sharp velocity profile gave significantly lower accuracy levels than the lower sand concentrations. This is demonstrated in Figure 12.

13 Figure 11 Accuracy of Krohne Pulsed DC Flowmeter (X-1000-T900) as a Function of Percent Full-Scale Figure 12 Accuracy of Krohne Pulsed DC Flowmeter (X-1000-T900) as a Function of Reynolds Number

14 Similar trends in the data were found for the Endress+Hauser Promag 33 (Figure 13), but the Promag 35S did not perform in this way. Figure 14 shows the highest sand concentrations resulted in meter overreading, instead of underreading. Figure 13 Accuracy of Endress+Hauser Promag 33 Pulsed DC Flowmeter Figure 14 Accuracy of Endress+Hauser Promag 35S Pulsed DC Flowmeter Capacitance and Dual Frequency Meters The Yokogawa (ADMAG CA) capacitance meter gave water data well within the manufacturer s specification of 0.5% above 10% of full-scale (Figure 15). In contrast, slurry

15 data corresponding to turbulent flow conditions resulted in the meter overreading by between 1 and 3.5% above 20% of full-scale. The Yokogawa dual frequency (ADMAG) meter gave errors at high sand concentrations (laminar flow) which largely fell outside the manufacturer s specification of 0.5% (Figure 16). Figure 15 Accuracy of Yokogawa Capacitance Flowmeter (ADMAG CA) Figure 16 Accuracy of Yokogawa Dual Frequency Flowmeter (ADMAG)

16 5.2 Repeatability of Flowmeters AC Meters Repeatability levels were in general very good for all three AC meters. The F&P and Krohne meters both gave standard deviations below 0.3% above 30% full-scale, while the Kent meter gave standard deviations below 0.15% above 30% of full-scale. Interestingly, repeatability often seemed to improve (standard deviation reduces) with increasing sand concentration. This can be seen in Figure 17 for the F&P meter Pulsed DC Meters The Pulsed DC meters gave variable results. The Krohne meter gave standard deviations below 0.2% above 30% of full-scale, but both Endress+Hauser meters gave poor values over much of the full-scale range. The Promag 33 gave good repeatability for water, confirming the manufacturer s specification, but poor repeatability for sand slurry above 50% full-scale (Figure 18). The Promag 35S provided very poor repeatability (up to 6%) over the whole fullscale range. It is unclear why this occurred. Figure 17 Repeatability of Fischer & Porter Mag-SM AC Flowmeter

17 Figure 18 Repeatability of Endress+Hauser Promag 33 Pulsed DC Flowmeter Capacitance and Dual Frequency Meters Both the Yokogawa capacitance and dual frequency meters gave acceptable repeatability levels, with standard deviations below 0.3% above 30% full-scale in both cases. 6. CONCLUDING REMARKS The Wet Solids Handling Project at AEA Technology has undertaken testwork over a 12-year period with a wide range of slurry types. Testwork over a wide range of solids concentration have resulted in flow through the commercial flowmeters in either the laminar, transitional or turbulent flow regimes. The slurry types tested include magnesium hydroxide, flocculated and deflocculated kaolin, drilling mud, sand, cellullose acetate, radioactive simulant, and pulverised fuel ash. Most, if not all, electromagnetic flowmeters are supplied by manufacturers with calibrations based on the turbulent flow of water through their meters. The overall evidence from the 12-year study suggests that the performance of electromagnetic flowmeters when used with slurries most closely conforms to manufacturers calibrations when the slurry flow is turbulent. Errors are significantly larger when the slurry is in laminar flow and can usually not be predicted. Errors are often at their greatest in the transitional flow regime. Ideally, all electromagnetic flowmeters should be recalibrated for a given slurry at a given solids concentration for the flowrate range to be used. If this is not readily carried out, or reliance is to be placed on the manufacturer s calibration, then operational conditions should ideally be designed to facilitate the slurry flow in the turbulent regime. Clearly, this is not always feasible

18 for high concentration slurries, so the slurry will often be in laminar flow in the meter. Steps should always be taken to avoid operating the meters with slurry in the transitional flow regime, as this tends to lead to the largest errors. To undertake this, the rheological properties need to be measured appropriately, a suitable Reynolds number defined for flow in the meter, and the values of the Reynolds number corresponding to the range of likely operating conditions calculated and compared with values predicted for the boundaries between laminar, transitional and turbulent flows. Even when these guidelines are followed, our work over many years suggests that any design of electromagnetic flowmeter should not be operated at less than 40% of full-scale when slurries are to be metered, in order to minimise errors and maximise repeatability. This compares with the often quoted minimum of 10% of full-scale, which appears to be appropriate more for water and other conducting liquids. 7. ACKNOWLEDGEMENTS All the performance data reported in this paper were obtained within a programme of work funded by member companies of the Wet Solids Handling Project. Many of the electromagnetic flowmeters used in this study were provided by manufacturers on a long-term loan basis. In particular, Fischer & Porter, Foxboro (GB), Krohne (Wellingborough, UK) and Yokogawa Europe bv have all been active supporters of the work. 8. NOMENCLATURE a c accuracy of flowmeter defined by Eqn (1) K consistency coefficient in power law model, Ns n /m 2 m number of data points Re Reynolds number in flow through meter, defined by Eqn (3) V i instantaneous mean slurry velocity, m/s V m mean of cross-sectional area averaged velocity, kg/s V s standard velocity as measured by the flow loop calibrated weighing system, m/s n flow behaviour index in power law model, Eqn (4) τ shear stress, N/m 2 shear rate, s -1 ρ slurry density, kg/m 3 σ R repeatability, defined by Eqn (2), % 9. REFERENCES 1. Robinson, C (1986). Obstructionless Flowmeters : Smooth Sailing for Some, Rough Passage for Others. InTech, 33(12), Heywood, N I & Mehta, K B (1988). A Survey of Non-invasive Flowmeters for Pipeline Flow of High Concentration Non-settling Slurries. In: Proc of Hydrotransport 11, 11th International Conference on the Hydraulic Conveying of Solids in Pipes, October, Stratford-on-Avon, UK. Paper C2, pp

19 3. WSH Report 18 (1992). Selection and Operation of Flowmeters for Dirty Liquids and Slurries, K B Mehta & R L Pirie. 2 nd Edition. AEA Technology, Oxford, UK. 4. WSH Report 49 (1995). A Survey of Commercially-Available On-line Slurry and Paste Flowmeters. AEA Technology, Oxford, UK. 5. Brown, N P & Heywood, N I (1991). Slurry Handling: Design of Solid/Liquid Systems, pp Kluwer Publications, Dordrecht, Netherlands. 6. Little, D (1991). Magnetic Flowmeters for Slurries and Liquids. Aust J Instrum Control, 6(3), 7p. 7. Matsunaga, Y, Goto, S, Kuromori, K & Ostling, H (1988). New Intelligent Magnetic Flowmeter with Dual Frequency Excitation. Advances in Instrumentation. Proc, 43, pat 3. Published by ISA, Research Triangle Pk, NC, USA, pp Heywood, N I, Mehta, K B & Poplar D (1993). Evaluation of Seven Commercially-Available Electromagnetic Flowmeters with Newtonian and non-newtonian China Clay Slurry in Pipeflow. In : Proc of Hydrotransport 12, September, Bruges, Belgium, pp WSH Report No. 48 (1994). Ninth Summary Research Report on Performance Evaluation of Volumetric and Mass Flowmeters, and On-line Density Meters, for Pumpable Slurries and Pastes (Sand in Drilling Mud), K B Mehta, AEA Technology, Oxford, UK. 10. WSH Report No. 56 (1996). Tenth Summary Research Report on Performance Evaluation of Volumetric and Mass Flowmeters, and On-line Density Meters, for Pumpable Slurries and Pastes (Fine Silica Sand Slurry), K B Mehta, AEA Technology, Oxford, UK. 11. WSH Report No. 57 (1996). Eleventh Summary Research Report on Performance Evaluation of Volumetric and Mass Flowmeters, and On-line Density Meters, for Pumpable Slurries and Pastes (Coarse Silica Sand Slurry), K B Mehta, AEA Technology, Oxford, UK. 12. Luntta, E & Halttunen, J (1989). Effect of Velocity Profile on Electromagnetic Flow Measurement. Sens Actuators, 16(4), April, Traechtler, A (1996). Electromagnetic Flowmeter for Measuring Rheological Parameters and Reconstructing the Flow Profile. IEE Coloquium (Digest), n 092. IEE, Stevenage, UK, pp 4/1-4/ Van Laak, F A L (1992). Application Limits of Electromagnetic Flowmeters : what do we really know about them? Advances in Instrumentation, Proc, 47, pt 2. Published by ISA Services Inc., Research Triangle Pk, NC, USA, pp Heywood, N I & Mehta, K B (1996). The Performance of Commercially-available Coriolis Mass Flowmeters applied to Industrial Slurries. In: Proc of Hydrotransport 13, 13th International Conference on Slurry Handling and Pipeline Transport, 3-5 September, Johannesburg, South Africa, pp