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Preliminary Design Memorandum <br /> • <br /> Silt Pumping Plant Rehabilitation <br /> Gravity fill from common storage tank. A gravity fill system was considered as <br /> a way to prime the horizontal centrifugal pumps. Foot valves would be required at <br /> the entrance of each pump suction to prevent the fill water from running out the <br /> ends of the pipes. The tank would contain a sufficient volume of water to fill the <br /> suction piping,the pump cases and the discharge piping to the closed discharge <br /> valves. The tank would be recharged once the system was primed and operating. <br /> Advantages: <br /> • Positive pressure filling allows leaks to be easily found and <br /> repaired. <br /> Disadvantages: <br /> • Storage tank would take up limited floor space inside the <br /> plant unless located outside. <br /> • Initial filling of storage tank must come from an alternate <br /> submersible pump or water source. <br /> • Sticks could prevent the foot valves from sealing and <br /> render the priming system unusable until cleared. <br /> • Stop logs must be installed to dewater the sump to maintain <br /> foot valves. <br /> Self-priming pumps. Self-priming pumps can be used for automatic priming for <br /> suction lifts up to 25 feet. Self-priming pumps have a large casing that completely <br /> surrounds the pump volute. The casing provides priming water to the pump <br /> impeller through an opening in the bottom of the volute. With the priming water <br /> provided by the casing the pump impeller develops suction to remove air from the <br /> suction piping and to lift water from the sump. Self-priming pumps recirculate <br /> water from the discharge back to the casing so that water is always available for <br /> priming. However,this action reduces the efficiency of the pump. Self-priming <br /> pumps are typically non-clog centrifugal pumps used in the wastewater industry. <br /> Advantages: <br /> • Automatic self-priming <br /> • Pump removes air from suction piping until prime is <br /> achieved. <br /> Disadvantages: <br /> • Self-priming pumps are less efficient than conventional <br /> centrifugal pumps because of the built-in recirculation from <br /> the volute discharge to the volute casing. Less efficiency <br /> 15 <br /> FOR OFFICIAL USE ONLY <br />