Silt Pumping Plant Rehabilitation Preliminary Design

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Preliminary Design Memorandum Silt Pumping Plant Rehabilitation Selection of Pump for Best Efficiency: When selecting a fixed-speed pump, one looks for a pump that will run close to the Best Efficiency Point(BEP) at the design head and flow rate. For a variable speed pump with a large static lift, one looks to select a pump with a curve that intersects the system curve to the right of the pump's best efficiency point. Then when the speed of the pump is reduced by the VFD and the operating point shifts to the left on the pump curve,the pump will run within a more efficient portion of its curve. See Figure 3 which shows large pump curves for 100, 95, 90 and 86% speeds,the system curve,the efficiency curve for 100% speed,the Minimum Continuous Stable Flow Curve and the Best Efficiency Curve. At 100%speed, the pump and system curves intersect to the right of the BEP. At 95%speed,the pump will run near the BEP. At 90 and 86% speeds,the pump will operate at less than an ideal efficiency. The minimum speed the pump can be run with this particular system is 86%. At speeds less than 86%,the pump output will drop below the Minimum Continuous Stable Flow which is not acceptable for continuous operation. At speeds about 85%and less,the large pump will not discharge any flow and continuous pump operation will cause the water in the pump casing to recirculate and overheat. Selection of pump casing seals,mechanical or packed: The existing pumps have packing boxes to seal the pump casing shaft penetrations. Since the pumps are dual suction split-case centrifugal pumps,two packing boxes per pump are required. The packing boxes can be mechanically sealed or can be sealed with braided packing. The existing pumps are sealed with braided packing. If mechanical seals are selected for the new pumps, solid shaft motors with rigid couplings may be required since the seals will not tolerate excessive shaft runout or eccentricity. Mechanical seals require a source of clean seal water for cooling which could be from domestic potable water or filtered canal water. Packed seals often can be lubricated and cooled with leakage of the pumped water if the water quality is adequate. Mechanical seals would allow less air infiltration during priming but mechanical seals require more maintenance and clean seal water. The original submittal drawings for the existing pumps show that mechanical seals were originally provided. Sometime during the life of the project,the mechanical seals were replaced with packing.No one at the District knows when the replacement happened or why. Pump control valve versus check valve: The existing pumping plant has hydraulically-actuated gate valves located on the discharge side of each pump that are used as pump control valves. Each pump is started against a closed valve and once the pump is up to speed,the control valve is opened in a timed operation. For a normal shutdown the pump control valve is closed in a timed operation before the pump motor is turned off. Pumps are started and stopped one pump at a time. The existing plant does not have check valves in the individual pump discharge lines but there is a flap valve that is vented at the end of the pipeline. In an emergency or loss of power event,the 19 FOR OFFICIAL USE ONLY