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Preliminary Design Memorandum <br /> Silt Pumping Plant Rehabilitation <br /> isolates the pumps from the vacuum system so that the vacuum system size can be <br /> minimized and only one pump can be primed at a time. <br /> Advantages: <br /> • Vacuum priming system can be left active all the time <br /> which will prevent any air from accumulating at the top of <br /> the horizontal pump casings thereby preventing a reduction <br /> in pump efficiency from air binding. <br /> • Vacuum priming system can be left active all the time <br /> thereby having all pumps ready to start at any time. <br /> • An undersized vacuum priming system will most likely <br /> achieve prime but will take longer to do so. <br /> • Duplex vacuum pumps can easily be provided to add a <br /> measure of redundancy. <br /> Disadvantages: <br /> • If large vacuum leaks occur,prime will not be achieved. <br /> • Vacuum leaks are harder to locate than positive pressure <br /> leaks. <br /> • Vacuum pump will eventually lose capacity and require <br /> maintenance. <br /> • Pump discharge valve seats and packing must be kept in <br /> good repair to prevent a large vacuum leak. <br /> Recommendation: If horizontal centrifugal pumps are selected,provide a robust, <br /> duplex vacuum priming system for the following reasons: <br /> • A vacuum priming system does not require access to the sump for regular <br /> maintenance of foot valves <br /> • A vacuum priming system does not impact the efficiency of the pumps. <br /> • A vacuum priming system is commercially available from companies that <br /> will guarantee performance. <br /> If vertical turbine pumps are selected, no priming system will be required. <br /> Variable Speed or Fixed-Speed Pumps <br /> Introduction <br /> The District estimates that on average,about 15 percent more water is pumped per <br /> 17 <br /> FOR OFFICIAL USE ONLY <br />