Silt Pumping Plant Rehabilitation Preliminary Design

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Preliminary Design Memorandum Silt Pumping Plant Rehabilitation shown on Figure 3.As the operating point on the pump curve shifts,the efficiency of the pump also changes. This change is pronounced for a system with a high percentage of static lift and a small percentage of dynamic loss(friction plus fitting losses). Pump efficiencies for the large pumps would range from 84.7%at 100%speed to about 64.9%at 86%speed. Therefore,running the large pumps at the lower speeds will result in less efficient pumping which could offset some of the savings of using VFDs or could even use more energy than fixed-speed pumping. Therefore an energy analysis is needed to determine if VFDs will really save the District energy. Energy Analysis Using 2013 Flow Data Fixed-speed pumping: Using the flow data collected during the 2013 irrigation season,the energy required to pump the flow using new fixed-speed pumps was calculated. A summary of the calculation is included in Table 1. The actual flow demand was assumed to be 15% less than the recorded average since the District has reported that a 15% surplus is pumped due to a mismatching of the pump output to the demand. Then the best combination of pumps was chosen to meet or exceed the actual flow demand. Variable speed pumping: Using the actual flow demand (15 percent reduction of the year 2013 recorded flow averages),the energy required to pump the flow using new pumps with variable speed drives on the two large pumps and the medium fixed-speed pump when required. A summary of the calculation is provided by Table 2. From the table, one can see that pump and motor efficiencies were very comparable to fixed-speed pumping but 3%of the energy was converted to heat by the VFDs.A 5%energy charge was assumed to provide cooling to the VFDs. The 3%heat loss and the 5%cooling load are recommended by the paper"Electric Motor Efficiency under Variable Frequencies and Loads" by Professor Charles M. Burt of the California Polytechnic State University. The assumed 5%cooling load could be a little high considering the elevation and mountain location of the pumping plant. A site-specific HVAC study would be required to further refine the assumed cooling load. Tables 1 and 2 show VFDs could have reduced the District's year 2013 pumping plant energy requirement by 5.4%. Advantages/Disadvantages Variable Frequency Drives Advantages • Can match pumped flow to demand. • Saves water and about 5.4%of the pumping plant energy • Soft starter functionality is incorporated in the VFD equipment 21 FOR OFFICIAL USE ONLY