Silt Pumping Plant Rehabilitation Preliminary Design

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Preliminary Design Memorandum Silt Pumping Plant Rehabilitation pumps are abruptly shut down and the control valves are closed in a timed operation. The flap valve at the end of the pipeline closes as soon as forward flow erodes and prevents water from the pump canal from flowing back into the discharge pipeline. But before the pump control valves are closed,the flow in the discharge lines is reported to reverse and spin the pump backwards. The pumps and motors are designed to spin backwards without damage. Current Reclamation designs typically include a pump control valve (often a motorized butterfly valve) and a check valve. The check valve closes at or just before flow reversal and prevents backflow from spinning the pump in reverse. A hydraulic transient study will be conducted in the design process to see which method is better for this particular project. Available Flow Rates for Individual and Combinations of Fixed-Speed Pumps Fixed-speed pumping can produce the following flow rates: • One small pump 3.4 cfs See Figure 2 • One medium pump 6.3 cfs See Figure 2 • One small, one medium pump 9.6 cfs See Figure 4 • One large pump 11.6 cfs See Figure 2 • One small, one large pump 14.7 cfs See Figure 5 • One medium and one large pump 17.4 cfs See Figure 6 • One small, one medium, one large pump 20.4 cfs See Figure 7 • One small,two large pumps 25.2 cfs See Figure 8 • One medium,two large pumps 27.6 cfs See Figure 9 • One small, one medium,two large pumps 30.0 cfs See Figure 2 All flow rates in between the above points cannot be achieved with fixed-speed pumping. Therefore, an intermediate flow point would dictate the use of the next larger pump size or combination. Available Flow Ranges for Variable Speed Pumps Number of VFDs needed for reliability and range: At least two VFDs are needed to provide a measure of redundancy. Adding VFDs to the two large pumps would allow the pumping plant to operate from about 3.2 to 30 cfs without any flow gaps within the range. See Figure 3 for the lower end of the flow range, Figure 10 for the mid-range and Figure 11 for the high end of the flow range. Interaction with system curve and efficiency variation over range: When the pump speed is reduced with a VFD,the pump curve revises in accordance to the affinity laws and a new intersection point of the pump curve and the system curve occurs.Note that the relative position of the operating point on the pump curve moves from right to left as the pump speed is reduced as 20 FOR OFFICIAL USE ONLY