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The selection of an operating domain is complicated by the flat system and pump head curves, and additionally by the pump solids effect which lowers the clear water head and efficiency. For example, a reduction of the pump head curve by only a few percent can cause large changes in flow, as shown in Figure 1.It can be seen from Figure 1 that an error in estimating the solids effect could mean that the system might run ineffectively or not at all. Detailed information is needed on the effect of pump head and power consumption in order to achieve reliable and energy-efficient operation.The addition of clay has been found to be effective in reducing friction pipe) of up to 40% and 60% when a phosphate clay was added (sand to clay mass ratio of between 5:l and 6:l) giving a total solids concentration by volume of 25% and 35%, respectively. A further marginal increase of the clay content then resulted in increasing hydraulic gradients. These results illustrated how small variations in the clay content may cause large variations in the pipeline friction losses; thus, the pumping head requirement. Therefore, an important question is how the sand and sand-clay mixtures influence the pump head and efficiency.The objective is to present experimental pump performance results with a 0.15 by 0.1 m branch centrifugal pump for the three narrowly graded sands and mixtures with clay used by Scott (1997). The aim is also to compare the results to simple design criteria and to discuss the mechanism involved. Experimental StudyThe experimental work was carried out at the GIW Hydraulic Testing Laboratory, CIW Industries Inc., Crovetown, CA, U.S.A. A radial type 4-vane all-metal pump with impeller diameter 0.625 m was used in a 0.1 03 m pipeline loop system. Power input to the pump was measured with a torque bar. Solids concentrations were measured in a vertical U-loop and flow rates were obtained from a magnetic flowmeter. Readings of pump suction pressure and discharge pressure, flowrates, input 'Author to whom correspondence may be addressed. E-mail address: anders.sellgren@ sb.hrth.seThe addition of clay to sand slurries has been found to reduce the pipeline friction losses, thus lowering the pumping head and power consumption. Pump water heads and efficiencies are decreased by the presence of solid particles. Experimental results are presented for a centrifugal pump with an impeller diameter of 0.625 m for three narrowly graded sands with average particle sizes of 0.64, 1.27, and 2.2 mm. Reductions in head and efficiency of up to 30% were observed for sand slurries with volume concentrations of up to 35%. Head and efficiency were lowered by about onethird for sand-clay mixtures with sand to clay mass ratios between 4:l and 6:l. Comparisons are made with design criteria and a mechanistic model approach based on a prediction of the relative motion of the solids and water in the volute region of the pump. power, rotary speed, and solids concentration were used to calculate the total pump head developed and pump efficiency. A detaile...

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