FLOW BEHAVIOUR OF COARSE-GRAINED SETTLING SLURRIES

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1 Twelfth International Water Technology Conference, IWTC1 8, Alexandria, Egypt FLOW BEHAVIOUR OF COARSE-GRAINED SETTLING SLURRIES Kamal El-Nahhas *, Magdy Abou Rayan **, Imam El-Sawaf *** and Nageh Gad El-Hak * * Ph.D., Suez Canal Authority, Egypt. k_elnahhas@yahoo.com ** Professor, Faculty of Engineering, Mansoura University, El-Mansoura, Egypt mrayan@mans.edu.eg *** Professor, Faculty of Engineering, Suez Canal University, Port Said, Egypt iaelsawf@hotmail.com ABSTRACT The properties of slurries mainly depend on the tendency of the particles to settle out from the carrying liquid. The motion of particles under the influence of gravity is characterized by initial acceleration after which descent continues at the free-fall or terminal velocity. The extent to which the particles will be free to settle in horizontal flow depends on the terminal velocity, the turbulence within the liquid tending to support the particles and particle-particle interaction. The aim of study is experimentally investigate the flow behaviour of coarse-particle settling slurries with an attention to energy consumption analysis. INTRODUCTION Dense slurries could be consisting of very fine particles uniformly distributed over the pipe cross-section. For many other cases of slurry flow, the solid particles are not distributed uniformly throughout the pipe. Particle concentrations are larger near the bottom of the pipe and smaller near the top indicative of settling behaviour. Such concentration gradients are often associated with frictional losses larger than those of homogeneous slurries, [1]. The extent to which the particles tend to settle in horizontal flow depends upon the relative extent of gravitational settling velocity, the turbulence within the liquid tending to support the particles and particle- particle interaction, []. Slurries that show a marked tendency to settle include slurries consisting of coarse coal, potash, sand, rock and aggregates. The effective design, control and safe operation of a hydraulic transport system require the successful prediction of slurry flow behaviour in the pipeline. For homogeneous slurry, the flow behaviour is related to the rheological properties that could be predicted with the aid of viscometric techniques.

2 Twelfth International Water Technology Conference, IWTC1 8, Alexandria, Egypt Viscometric techniques developed for single-phase fluids or homogeneous slurries are not applicable to settling slurries. Settling slurries are showing distinct two-phase behaviour which is strongly affected by mutual particle-particle and particle liquid interaction. The flow behaviour of settling slurries is affected not only by the solids properties (particle size, size distribution, particle density and particle shape) and solids concentration but also by the nature of the carrier fluid (Newtonian or non- Newtonian), [3]. Instead of flow regimes as in the case of non-settling slurries, the flow of settling slurries in horizontal pipes work could be distinguished by the so-called flow patterns. These show how the solids are distributed within the pipe, [4]. The flow features exhibited by settling slurry result from the complex processes of momentum interchange between the two phases. Flow features provide considerable guidance for the rational selection of techniques to predict hydraulic behaviour and for suitable operating conditions for pipelines. The settling slurries can exhibit a complex range of flow patterns, depending upon the physical properties of both carrier fluid and transported solids, the superficial velocity and concentration of slurry. At one extreme, the solids are present as a gravity bed. Such a flow regime is referred to as fully-segregated or two-layer (sliding bed/stationary bed). The other extreme, at very high velocities with favourable solid and liquid properties, the solids may approach a pseudo-homogeneous flow. True homogeneous flow, in which the solids are evenly distributed throughout the pipe, rarely occurs in settling slurries. Between the two extreme cases lie the saltation and heterogeneous flow regimes. The heterogeneous regime is the most complex as the characteristics of the extreme flow regimes are embodied in the flow; some particles may be present in the form of a bed, others supported by fluid turbulence, [5]. As the superficial velocity of settling slurry is reduced, a minimum in the hydraulic gradient, which is closely associated with the formation of a bed of solids in the pipe, occurs. The velocity at the minimum is sometimes referred to as the bedding velocity or the limit of deposition. The velocity occurring at the minimum is often casually referred to as the critical velocity. Important parameters for the design and operation of a slurry pipeline are those which provide information about the safety and economy of slurry pipeline operation. Mean velocity in a pipeline is a basic parameter characterizing pipeline flow. It is defined as the bulk velocity, v, of a matter (solids, liquids or mixtures) obtained from the volumetric flow rate, Q, of a matter passing a pipeline cross section of the area, A. the equation v = Q/A is for a circular pipe of an inner diameter D written as 4Q v = (1) πd

3 Twelfth International Water Technology Conference, IWTC1 8, Alexandria, Egypt The determination of an appropriate mean velocity of slurry is crucial to safe and economic pipeline operation. An area in which slurries require a more careful treatment than single-phase fluids is that of the friction gradient or energy gradient. In the form of the pressure gradient dp/dx, the friction loss associated with the flow of slurry in a pipe will be clear. However, the expression of pressure loss as a hydraulic gradient, i, (in m water per m length of pipe) is so common in slurry pipelining, [1]. The specific energy consumption, SEC, determines the energy required to move a given quantity of solids over a given distance in a pipeline. It is given as i g SEC =.778 S () s where SEC is in units of [kw.hr/tonne.km], i is the slurry hydraulic gradient, S s is the solids specific gravity, and is the slurry volumetric concentration. The aim of study is experimentally investigate the flow behaviour of coarse-particle slurries, which show a marked tendency to settle, with an attention to energy consumption analysis. Flow behaviour have been studied by measurements of three different sorts of sand solids of nearly uniform size distribution (d 5 =.,.7 and 1.4 mm), mixed with water at different concentration ( 4 to 33%). The recirculation pipe loop with smooth stainless steel pipes of the inner diameter D = 6.8 mm was used for measuring the slurry flow parameters. EXPERIMENTAL FACILITY AND TESTED MATERIALS The extended set of experiments was carried out at the hydraulic laboratory of the Institute of Hydrodynamics, Academy of Science of Czech Republic. Reference could be made to El-Nahhas [6] for more details. Experimental Facility Description and Design Considerations An open-loop recirculation pipeline system was employed for testing the slurry flow behaviour, as shown in Figure (1). A centrifugal pump (Warman ultra heavy-duty slurry pump type /3) was used for forcing the slurries. The pump has a variable speed drive to allow slurry testing over a wide range of flow rates. A stainless steel holding tank (6 mm diameter and 1 mm height) equipped with a funnel-shaped tube located inside its cone shaped bottom, to allow solids separation after finishing a set of measurements, was used. It was taken into considerations that the slurry level in the holding tank might be high enough to ensure that air was not

4 Twelfth International Water Technology Conference, IWTC1 8, Alexandria, Egypt entrained in the mixture entering the loop. The experiments with comparable results were made at same slurry level in the holding tank. A stainless steel pipe loop of internal diameter 6.8 mm, with entire length of about 18 m, was used for slurry parameters measurement. The operating temperature was controlled by pumping cooling water, in counter flow direction, through the annulus of a double pipe heat exchanger. The heat exchanger which located in the front branch of the pipeline test loop keeps the temperatures during the experiments in a very narrow range. The test section is located in the back (downstream) branch of the piping loop system. The length-to-diameter ratios of the test sections exceed 4 (according to design criteria, [7]). A transparent section was mounted at the end of the test section. Differential pressure measurements were obtained over two sections of pipe. Therefore, the test section has three pressure tapping points, which are located on the upper part of the pipe perimeter at distances so that fully developed flow exists between them. The pressure is transmitted from the tapping points to three inductive differential transducers through transmission lines and plexi-glass sedimentation vessels filled with pure water. The sedimentation vessels prevent the penetration of solid particles from the slurry pipe into the pressure transducers and enable to vent the system holding tank 8 three channels carrier frequency amplifier screw pump 9 analogue/ digital converter 3 double pipe heat exchanger 1 computer 4 test section 11 flow divider vessel 5 stirrer 1 measurement of discharge & density 6 sedimentation vessels 13 cooling water inlet 7 differential pressure transducers 14 cooling water exit Figure (1) Schematic diagram of the experimental pipeline test loop Measuring Techniques Inductive differential pressure transducers were used to measure the pressure losses between the pressure tapings, i.e. the losses through a certain length of the pipe. The transducer output signals, which is proportional to the differential pressure at the

5 Twelfth International Water Technology Conference, IWTC1 8, Alexandria, Egypt transducer were amplified and displayed as an analogue value (in volts) by voltmeters. Also, these analogue signals were converted to digital signal by an analogue/digital module. The digital data signal input to a computer, which is accessed with MATLAB software that enables for online supervisory, analysis and data acquisition. At the downstream end of the test pipes a box divider was mounted and allows discharge to be diverted to a plastic container and measured by weight. Since the divider arm was connected to an electric stopwatch, the mass flow rate was precisely measured. If the plastic container was replaced by a glass calibrated cylinder, the slurry density and hence the volumetric concentration could be determined. This arrangement allowed a check on the concentration in the pipe during each experimental run. These data are input to the computer software for processing to present the supervisory plots online during the measurements. All pressure transducers had been calibrated periodically against the standard device, U-tube manometers. Test Material For settling slurry investigations three sorts of the sand were used for the experiments; fine (d 5 =. mm), medium (d 5 =.7 mm) and coarse (d 5 = 1.4 mm). The particle size distribution curves of the sand sorts that are shown in Figure (), shows that the solids were narrowly graded. 1 8 Fine sand Medium sand Coarse sand 6 Percentage (%) Particle diameter (mm) Figure () Particle size distributions of the tested sand sorts

6 Twelfth International Water Technology Conference, IWTC1 8, Alexandria, Egypt RESULTS AND DISCUSSIONS Solids Concentration Effect Figures (3) to (5) present the effect of solids concentrations on the resistance curve plots of fine, medium and coarse-sand slurries. These figures illustrate different trends in the development of the hydraulic gradient, i, with an increasing the mean velocity, v. The curve shapes is observed to be different for different solids sizes. Also, the shape of the resistance curve of a certain solids type is found to be dependent on the solids concentration. For fine-sand slurries, Figure (3), as the solids concentration increases, the hydraulic gradient increases for the whole velocity range. The solids effect (i-i w ) for dense slurries increases as the mean velocity increases. Medium-sand slurries have different characteristic behaviour. Referring to Figure (4), as the solids concentration increases, the hydraulic gradient increases till the velocity reaches about v = 5 m/s. For higher flow velocities (v > 5 m/s), the solids concentrations have a little effect on the hydraulic gradient, i.e., the resistance curves for the slurries with the different concentrations are more or less the same. Also, as the mean velocity increases the solids effect (i-i w ) gradually decreases and reached a limit value for the higher mean velocities. The limit value is dependent on the solids concentration. The effect of the solids concentration on the solids effect parameter (ii w ) is very limited at higher mean velocities (v > 5 m/s). For coarse-sand slurry, Figure (5), as the solids concentration increases, the hydraulic gradient increases for the whole velocity range. The solids effect (i-i w ) for dense slurries decreases as the mean velocity increases with a manner different from the medium sand slurries. Hydraulic gradient, i, (m-water / m-pipe) 1 = 4 % = 1.5 % = 4 % =8.5 % Fine Sand (d 5 =. mm) Water Mean velocity, v (m/s) Figure (3) Resistance curves of fine-sand (d 5 =. mm) slurries

7 Twelfth International Water Technology Conference, IWTC1 8, Alexandria, Egypt Hydraulic gradient, i (m-water/m-pipe) 1 = 6 % =1.5 % = 18 % = 4 % = 33 % Water Medium Sand (d 5 =.7 mm) Mean velocity, v (m/s) Figure (4) Resistance curves of medium-sand (d 5 =.7 mm) slurries. = 4.8 % Hydraulic gradient, i (m-water/m-pipe) =1 % =18.7 % = 4 % = 31 % Coarse Sand (d 5 = 1.4 mm) Water Mean velocity, v (m/s) Figure (5) Resistance curves of coarse-sand (d 5 = 1.4 mm) slurries Figure (6) shows the effect of the solids concentration on the pipeline hydrotransport efficiency, evaluated by plots of specific energy consumption versus the solids throughputs, for the three considered sand slurries. It is obviously shown, for the three sand slurries, that as the solids concentration increases the specific energy consumption for the same solids throughput decreases, i.e. the pipeline hydrotransport efficiency increases. The low-concentrated sand slurries hydrotransport in pipelines has a very limited solids throughputs accompanying with considerably high specific energy consumption.

8 Twelfth International Water Technology Conference, IWTC1 8, Alexandria, Egypt 5 = 4.% SEC (kw.hr/tonne.km) Fine Sand = % = 1% = 3.3% Solids throughput (tonnes/hr) 5 = 6% SEC (kw.hr/tonne.km) 15 1 Medium Sand = 1% = 18% = 3.8% = 3.7% Solids throughput (tonnes/hr) 35 SEC (kw.hr/tonne.km) Coarse Sand = 4.8% = 1% = 18% = 4% = 31% Solids throughput (tonnes/hr) Figure (6) Effect of solids concentration on the specific energy consumption

9 Twelfth International Water Technology Conference, IWTC1 8, Alexandria, Egypt Particle Size Effect The effect of the solids particle size of the sand slurries on the flow behaviour could be seen obviously in Figures (7) to (9). These Figures compare the resistance curves of fine, medium and coarse-sand slurries at nearly the same solids concentrations. For relatively low-concentrated slurry ( = 1%), Figure (7), the resistance curves for both medium and coarse-sand slurries are nearly identical except at the medium velocity range at which the medium-sand slurry is slightly lower than that of coarsesand slurry. The fine-sand slurry has the lowest resistance curve at the lower velocity range and then (at v > 3.5 m/s) it gradually exceeds the medium-sand slurry and at higher velocities (v > 5.4 m/s) it also exceeds the coarse-sand slurry. For higher concentrations ( 3%), similar behaviour could be observed for the fine-sand slurry. It has the lowest resistance curve at low velocities, and with increasing the mean velocity its curve gradually exceeds the medium sand curve (for v > 4 m/s) and exceeds that of coarse sand with more increasing of the mean velocity (for v > 5. m/s). At concentration of = 18%, Figure (8), the medium-sand slurry resistance curve is slightly lower than that of coarse sand at low and high velocity ranges, however the difference between the two curves is more significant at the medium velocity range. For higher concentrations ( = 4% and 3%), Figure (9), the medium-sand slurry resistance curve is more or less the same as that of the coarse sand at low mean velocities (v < 4 m/s). With increasing the mean velocity the medium-sand slurry resistance curve is much lower than that of coarse sand with the remainder considered velocity range. It could be noticed that the solids particle size effect on the flow behaviour of sand slurries is more significant for denser slurries.. Hydraulic gradient, i (m-water/m-pipe) Coarse sand Medium sand Fine sand = 1 % Mean velocity, v (m/s) Figure (7) Effect of solids particle size on the resistance curves of sand slurries ( = 1%)

10 Twelfth International Water Technology Conference, IWTC1 8, Alexandria, Egypt. Hydraulic gradient, i (m-water/m-pipe) Coarse sand ( = 18%) Medium sand ( = 18%) Fine sand ( = %) Water Mean velocity, v (m/s) Figure (8) Effect of solids particle size on the resistance curves of sand slurries ( 18 %). Hydraulic gradient, i (m-water/m-pipe) Coarse sand ( =31%) Medium sand ( = 3%) Fine sand ( =3%) Water Mean velocity, v (m/s) Figure (9) Effect of solids particle size on the resistance curves of sand slurries ( 3%) Figure (1) presents the particle size effect on the specific energy consumption plots at certain solids concentration. It could be noticed that from the specific energy consumption point of view, the effect of particle size is also different according to the solids concentration. At relatively low solids concentration ( = 1%) the specific energy consumption of both medium and coarse sand slurries vary in the same manner and have narrow values through the considered solids throughput range.

11 Twelfth International Water Technology Conference, IWTC1 8, Alexandria, Egypt 16 SEC (kw.hr/tonne.km) Coarse sand Medium sand Fine sand = 1% Solids throughput (tonnes/hr) 1 1 Coarse sand ( = 18%) Medium sand ( = 18%) Fine sand ( = %) SEC (kw.hr/tonne.km) Solids throughput (tonnes/hr) 6 5 Coarse sand ( = 31%) Medium sand ( =3.7%) Fine sand ( = 3.3%) SEC (kw.hr/tonne.km) Solids throughput (tonnes/hr) Figure (1) Effect of solids particle size on the specific energy consumption of sand slurries

12 Twelfth International Water Technology Conference, IWTC1 8, Alexandria, Egypt The specific energy consumption of the fine sand slurry has a trend with higher slope than that of both medium and coarse sands, so it has lower values at low solids throughput ranges and higher values at high ranges. For solids concentration ( 18%) the specific energy consumption is higher for higher solids particle size. At high solids concentration ( 3%) the trend of specific energy consumption of medium sand has the lowest slope and that of fine sand is the highest one. Therefore, at lower solids throughput range the specific energy consumption of fine sand is the lowest and that of medium is the highest with that of coarse sand in between. At higher solids throughput range (nearly higher than 6 tonnes/hr) the vice versa is occurred, the medium sand has the lowest SEC. CONCLUSIONS The sand settling slurries have different trends in the development of the hydraulic gradient, i, when increasing the mean velocity, v. The general trend is that increasing the solids concentration of certain slurry increases the flow friction loss. However, the curve shapes could be observed to be different for different solids sizes. Also, the shape of the resistance curve of a certain solids type is found to be dependent on the solids concentration. The solids particle size effect on the flow behaviour of sand slurries is more significant with increasing the solids concentration. For the three different-size sand slurries (d 5 =.,.7 and 1.4 mm), as the solids concentration increases the specific energy consumption for the same solids throughput decreases, i.e. the pipeline hydrotransport efficiency increases. The lowconcentrated sand slurries hydrotransport in pipelines is available with very limited solids throughputs accompanying with considerably high specific energy consumption. ACKNOWLEDGEMENT The bilateral international co-operation between Academy of Science of the Czech Republic and the Egyptian Academy of Scientific Research and Technology via Suez Canal University, which allowed conducting experimental measurements at the Institute of Hydrodynamics of Academy of Science of the Czech Republic, is gratefully acknowledged. NOMENCLATURE A area [m ] volumetric concentration [-] D pipe internal diameter [m] d 5 mass median particle diameter [mm]

13 Twelfth International Water Technology Conference, IWTC1 8, Alexandria, Egypt i hydraulic gradient for slurry flow [-] i w hydraulic gradient for water flow [-] p pressure [Pa] Q volumetric flow rate [m 3 /h] SEC specific energy consumption [kw.hr/tonne.km] S s solids specific gravity [-] v mean velocity [m/s] x axial distance [m] REFERENCES 1. Wilson, K. C., Addie, G. R. and Clift, R., (199), Slurry Transport Using Centrifugal Pumps, Elsevier Applied Science, London.. Baker, P. J., Jacobs, B. E. A. and Bonnington, S. T., (1979), A Guide to Slurry Pipeline Systems, BHRA Fluid Engineering, Cranfield, Bedford, England. 3. Vlasák, P., Chára, Z. and Konfršt, J., (4), Conveying of Coarse Particle in Non-Newtonian Slurry, Engineering Mechanics National Conference with International Participation, Svratka, Czech Republic. 4. Heywood, N. I., (1999), Stop Your Slurries from Stirring up Trouble, Chemical Engineering Progress, American Institute of Chemical Engineers, AICHE. 5. Brown, N. P., (1991), The Settling Behaviour of Particles in Fluids, Chapter of Slurry Handling Design of Solid-Liquid Systems, Elsevier Applied Science, London. 6. El-Nahhas, K., (), Hydraulic Transport of Dense Fine-Grained Suspensions, Ph.D. Thesis, Faculty of Engineering at Port Said, Suez Canal University, Egypt. 7. Gillies, R. G., (1991), Flow Loop Studies, Chapter 1 of Slurry Handling Design of Solid-Liquid Systems, Elsevier Applied Science, London.

oe4625 Dredge Pumps and Slurry Transport Vaclav Matousek October 13, 2004

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