Silt Pumping Plant Rehabilitation Preliminary Design

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Preliminary Design Memorandum Silt Pumping Plant Rehabilitation season than the actual water user seasonal demand. Of this extra amount pumped, about a quarter is sold to the users. This over pumping is caused by the inability to match pump flow from the fixed-speed pumps to the irrigation system demands. The District has asked if variable speed pumping would better serve the District. On the electrical supply side of the project,Xcel Energy has informed the District that if the power is provided from the distribution power line adjacent to the pumping plant, all motors greater than 200 horsepower(hp)will require soft starts or a ramped VFD start. This requirement provides another reason to consider VFDs since a soft start capability is built into VFDs. Sizing of Pumps Capacity of System: The capacity of the inlet canal is 32 cfs at a minimum river flow of 1200 cfs. The design capacity of the pipeline canal is 32 cfs at normal water levels. The original pumps were selected to provide a slightly higher capacity, 36 cfs,to account for the minimum inlet canal level and for anticipated wear caused by pumping raw water. For purposes of this study,the capacity of the pumping plant was selected to be 30 cfs. The reduction in capacity is due to the District's recognition that the existing system has more capacity than needed and that future demands are likely to be less than current demands. The exact capacity of the new pumps needs to be agreed upon between the District and Reclamation before final design can start. Limitations of Existing Discharge Manifold: The existing discharge manifold pipe is encased in reinforced concrete. The discharge pipes from each pump are sized to limit the flow velocity to a reasonable value. Since the manifold is encased in reinforced concrete, changing the discharge pipe sizes and the size and number of the pumps is not considered economical. This study assumes that 4 pumps of similar size to the existing pumps will be provided in the final design. The two large pumps were reduced in capacity to 10.8 cfs,the medium pump was sized to produce 5.6 cfs and the small pump was selected to discharge 2.8 cfs at a design head of 223.8 feet. The pumps were selected to produce 30 cfs at the minimum inlet canal level.No allowance for wear was provided for this study but a 3%additional capacity for wear is anticipated for the final design. Development of the System Head versus Flow Rate Curve: The system head curve is a plot of Total Head versus Flow Rate for the entire range of flow(zero to design flow). Total Head is defined as the static lift(difference between pump canal elevation and sump elevation)plus the dynamic loss(friction plus minor losses) between the suction intake and the discharge flap gate at the end of the pipeline. The system head curve is plotted on top of the pump curve. See Figure 2 for a plot of the pump curves(small, medium, large pumps and all four pumps combined)and the system curve. The intersection of the pump curves and the system curve represents the head and flow rate that the pumped system would produce for that particular pump or combination of pumps. 18 FOR OFFICIAL USE ONLY