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Meeting the challenge of lower TCOs in slurry pumping

first_imgThe cost of running a slurry pump system is determined by multiple factors that vary depending on the mining environment. GIW wants its customers get the lowest possible total cost of ownership (TCO) for the equipment and its service engineers work tirelessly to improve two important cost factors: pump wear and efficiency. In the last issue of The Flowsheet, GIW examined how optimising efficiency when pumping slurry is different from that of pumping straight liquids. Here, the experts look at some design elements that make GIW pumps work efficiently with slurry and explain how GIW partners with its customers to maximise slurry system efficiency in their individual environments.To get the lowest possible TCO, a solids pipeline system needs to operate at the highest practical concentration that the mining process allows. Graeme Addie, GIW’s VP of Engineering and R & D, explains that a centrifugal pump has a point at which the speed of the pump and the flow are optimised for peak performance: the best efficiency point quantity (BEPQ) or ‘sweet spot.’“A pump efficiency curve is an inverted parabola with zero efficiency at zero flow and a maximum efficiency at a flow determined by the hydraulic geometry of the pump,” says Addie. The efficiency curves and their peak values vary for different pumps. A lot of factors determine this maximum efficiency, but, in general, bigger, wider pumps have the highest BEPQ values at higher flows.Pump design can play a significant part in improving efficiency and reducing wear. For example, a correctly designed set of curved section vanes can increase efficiency overall as can certain types of collector shells.Other variables such as thicker vanes and deep-clearing vanes, however, reduce efficiency but provide worthwhile improvement to the wear life.Changing pump speed also changes the efficiency of a pump according to a principle called the affinity law. Addie explains: “When we change the speed of a pump, the flow at the same efficiency changes in the ratio of the speed change, and the head varies as the square. For example, doubling speed at a given percentage of the BEPQ provides double the flow and four times the head at approximately the same efficiency.”This so-called affinity law change is one of the main ways the performance of a pump can be varied to accommodate different head quality (H-Q) duties. (For an explanation and video showing how pump speed can affect wear, see the article Why Do Slurry Pumps Experience High Wear at Low Flow Rates? in GIW’s latest issue of The Flowsheet.)Different geometric hydraulic designs can achieve the same H-Q by operating at different speeds. For example, a large-diameter slow-running pump can achieve the same H-Q as a smaller-diameter pump running faster. The smaller, faster-running pump is initially less costly. But the large, slower-running pump typically has better suction-liner and impeller wear. Liner and impeller wear often determine when parts have to be changed, and if parts must be changed frequently, the costs of downtime can be significant.The best way to optimise the efficiency of a pump system is to select pumps that best match the process requirements and provide acceptable wear life while operating near their best efficiency point. For example, if a customer is seeing significant pump wear and downtime cost is a factor, then low specific speed pumps should be selected over faster-running, small-diameter, lower-cost alternatives. Even with an initially higher investment, the customer will see significantly reduced TCO over the life of the pump.Any time there is a change in the mining process, the duty flow and head should be re-evaluated and a speed change or wet-end change made. GIW’s service experts work closely with customers to evaluate their systems and provide continual process improvements. www.giwindustries.com.last_img

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