The performance of centrifugal pumps when pumping ultra ... The pump efficiency ... The performance of centrifugal pumps when pumping ultra-viscous paste ... The performance of centrifugal

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  • Introduction

    Centrifugal pumps are able to pump highlyviscous materials, but with a reducedperformance compared to when pumpingwater. Pump manufacturers provide clearwater pump performance data. This data needsto be adjusted for viscous slurries beforedesigning a pumping system. To do this, thepump head and efficiency ratios need to bedetermined for viscous slurries.

    Definitions

    The standard definitions for head (HR) andefficiency (ER) ratios are as follows:

    HR The pump head ratio (HR) is the ratioof slurry head produced to the headproduced for water at the pump best-efficiency point (BEP) measured forwater

    ER The pump efficiency ratio (ER) is theratio of slurry efficiency to theefficiency for water at the pump BEPmeasured for water.

    Pumping viscous slurries

    Centrifugal pumps are used to pump fine solidsin suspension in various mining applications(clay, cement, slimes disposal, thickenerunderflow, full plant metallurgical tailings,etc.). These fine solids settle very slowly or, atsome solids concentrations, not at all, and canbe treated as homogeneous mixtures. Govierand Charles (1961) state that generally, forsolids with settling velocities lower than0.61.5 mm/s, the slurry can be consideredhomogeneous. Using this definition, the uppersize at which clay, with a solids density of 2.6 t/m3 will form homogeneous mixtures, isapproximately 25 m to 39 m.

    As the solids concentration increases, theviscous characteristics of the slurry change.Most fine slurry mixtures have non-Newtonianrheological behaviour and the flow behaviouris interpreted by means of a flow curve, whichis a plot of shear stress versus shear rate. Awidely accepted model to use for mineralslurries is the three-parameterHerschelBulkley model (Brown and Heywood,1991). The model takes the following form:

    [1]

    The performance of centrifugal pumps whenpumping ultra-viscous paste slurriesby J. Crawford*, F. van Sittert, and M. van der Walt

    SynopsisSignificant advances have been made in the design of centrifugalslurry pumps that are used for pumping high-density viscous andpaste slurries. This paper presents the results of a series ofcentrifugal pump performance tests that were conducted using anFLSmidth 6 x 4 Krebs millMAX centrifugal pump. The pump wasfitted with a dry mechanical seal to prevent dilution of the slurryfrom gland service water, as the rheology of the slurry is sensitiveto small changes in slurry density.

    Tests were done using paste kaolin slurry with Bingham yieldstress values between 5781 120 Pa and plastic viscosities between728 and 1 491 mPa.s. The viscous properties of the paste duringeach pump test were measured using a rotational viscometer. Pumphead and efficiency de-rating were measured and compared to thecorrelations of Walker and Goulas (1984).

    The test data shows the pumps best-efficiency point (BEP)changes and shifts to the left of the performance curve. The pumpefficiency decreases as the slurry yield stress increases. The headde-rating data shows more head de-rating at the low flow ratescompared to the high flow rates for the high viscosity slurry, whichis in line with the findings of Walker and Goulas (1984). This paperdemonstrates that it is possible to pump very viscous, high yieldstress slurries with limited head and efficiency de-rating usingcentrifugal pumps, provided that positive suction conditions aremaintained at all times.

    Keywordscentrifugal pumps, viscous slurry, pumping efficiency.

    * FLSmidth Krebs, USA. Paterson & Cooke Consulting Engineering, South

    Africa. The Southern African Institute of Mining and

    Metallurgy, 2012.ISSN 2225-6253. This paper wasfirst presented at the, 15th International Seminaron Paste and Thickened Tailings (Paste) 2012,1619 April 2012, Sun City, South Africa.

    959The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 112 NOVEMBER 2012

  • The performance of centrifugal pumps when pumping ultra-viscous paste slurries

    where:0 = shear stress (Pa)y = yield stress determined from the intercept of the

    rheogram at zero shear rate (Pa)K = fluid consistency index (Pa.sn) = shear rate (s-1)n = flow behaviour index.

    Homogeneous mixtures with yield stress values are calledviscoplastic liquids. The HerschelBulkley model provides aconvenient way to approximate the behaviour of time-independent viscoplastic liquids common in the miningindustry. The HerschelBulkley model parameters forrheological characterization of time-independent liquids aresummarized in Table I, and illustrative flow curves of theseliquids are shown in Figure 1.

    A common model to use for viscoplastic liquid is theBingham plastic model. For a Bingham plastic model the flowbehaviour index n is equal to unity and Equation [1] reducesto:

    [2]

    where:0 = shear stress (Pa)y = Bingham yield stress (Pa)KBP = plastic viscosity (Pa.s) = shear rate (s-1).

    Generally, the Bingham yield stress and plastic viscosityshow an exponential increase with solids concentration.

    Walker and Goulas (1984) found that the pumpdeveloped head deviates significantly from the pumps watercharacteristics at lower flow rates, and that there is areduction in pump efficiencies at flow rates near the waterbest-efficiency point.

    Their results show that the pump performance isdependent on the slurrys rheological characteristics. Theyfound that pump performance correlation methods used forNewtonian fluids also gave good results for non-Newtonianfluids, given that a suitable value for the viscosity can bechosen.

    Cooke (2007) compared the Walker and Goulas (1984)data to a range of other data with yield stresses of up to 200 Pa, and concluded that the Walker and Goulas (1984)

    data provides a reasonable estimate for head de-rating valuesfor pump Reynolds numbers greater than 3 x 105, but thatefficiency de-rating values are generally conservative.

    Test equipment, methodology and calculations

    Test facility description

    Figure 2 shows a schematic of the test facility used for thecentrifugal pump performance tests. A conventional large-diameter slurry pipe test facility was modified for these tests,to accommodate the viscous pumping requirements.

    960 NOVEMBER 2012 VOLUME 112 The Journal of The Southern African Institute of Mining and Metallurgy

    Table I

    Parameters for the HerschelBulkley model,Equation [1]

    Liquid y K n

    Newtonian = 0 = Newtonian viscosity = 1Pseudoplastic (power law) = 0 = liquid consistency index < 1Dilatant = 0 = liquid consistency index > 1Yield-pseudoplastic > 0 = liquid consistency index < 1Bingham > 0 = plastic viscosity =1

    Figure 1Various time-independent liquids described by theHerschelBulkley model

    Figure 2Schematic diagram of the pump performance test facility

  • The pump performance test facility is fitted with anFLSmidth Krebs 6 x 4 millMAX centrifugal pump with a drymechanical seal. The pump has a four-vane, 400 mm impellerwith closed vanes. The pump is belt-driven using a 90 kWpremium efficiency motor matched to a variable-frequencydrive (VFD). A magnetic flow meter is used to measure flowrate. The pump head is determined using pressuretransmitters on the suction and discharge ends of the pump.

    The pressure transmitters are located two pipe diametersfrom the suction and discharge flanges respectively, inaccordance with the American Hydraulic Institute StandardsANSI/HI 1.62000 requirements. The flow meter is locatedfive pipe diameters downstream from the discharge flange;the recommended minimum distance.

    Slurry mass concentration is measured from slurrysamples taken during tests and oven dried in a laboratoryoven. The VFD provides an output for pump rotational speedand motor absorbed power.

    An Anton-Paar RheolabQC bob-and-cup rotationalviscometer with temperature control is used to obtain allrheological data. A handheld Hanna Instruments pH probe isused for measuring pH.

    Tested materialPaste slurry of kaolin clay was used for the tests as theviscous properties of kaolin are well documented in theliterature, and its flow behaviour can be approximated by theBingham plastic model. Figure 3 shows the measured particlesize distribution data for the kaolin slurry used for the tests.The dry kaolin solids density is 2.6 t/m3. The d90 and d50particle sizes are 34.5 m and 2.2 m respectively.

    Test methodologyThe centrifugal pump performance tests are done as follow:

    1. The clear water pump performance curves (head andefficiency versus flow rate) are measured at threefixed pump rotational speeds of 1 000 r/min (20.9 m/stip speed), 1 200 r/min (25.1 m/s tip speed), and 1 400 r/min (29.3 m/s tip speed) to validate the pumpmanufacturers performance curve and to provide thebaseline reference

    2. Kaolin paste is prepared to the required solids concen-tration and pumped into the pipe loop using aprogressive cavity pump

    3. The paste is circulated in the pipe loop to remove anyentrapped air from the slurry

    4. The test loop is pressurized using the progressivecavity pump to ensure there is no cavitation due toinsufficient NPSHa, which could compromise thecentrifugal pump head and efficiency

    5. The pump is run at a constant rotational speed (1 400 r/min). Suction pressure, discharge pressure,power supplied to motor, and flow rate are recordedfor a range of flow rates. The pump head, efficiency,and available net positive suction head for each flowrate are calculated at each of the recorded points. Theslurry flow rate is varied by throttling the pumpdischarge valve

    6. A sample is taken and the kaolin paste temperature,pH, solids concentration, and rheology are measured

    7. Steps 2 to 6 are repeated for a range of slurry solids