Water Well Journal
Issue link: https://read.dmtmag.com/i/728580
selected pump is indicated on the pump curve in Figure 1 at the design point (84%) and at each 10% decline of efficiency across the curve. (80% at ~360 GPM; 70% at ~260 GPM; 60% at ~230 GPM; and 50% at ~160 GPM). The second pump curve (Figure 2) also shows the same full speed (60 Hz) conditions but includes a cross-hatched window and system head line to indicate the proposed area of pump performance from the maximum operating head of 260 feet down to the projected minimum system head of 190 feet (comprised of the minimum system pressure of 60 psi [140 feet] + the minimum well lift of 50 feet from static). This also corresponds to a pump speed range between a full speed of 1760 RPM down to the projected minimum pump rotation speed of ~1466 RPM with use on a VFD at ~50 Hz. Input horsepower is also shown on each frequency line to indicate the horsepower draw at each speed and performance data point. Figure 3 shows the full speed portion of the curve that indicates a potential total dynamic head (TDH) between the design head of 260 feet up to the maximum head of 350 feet that will be encountered during operation with a control valve, with the horsepower draw at selected points along the curve. Our well pump, whether on a VFD or with a CV, must be able to deliver any flow rate required within these multiple head conditions. To fully evaluate the best choice for our in- stallation, we used the life cycle cost method. This method is one of many universally used when an engineering analysis is needed to compare options based on various factors—differ- ent projected lives, different purchase costs, different mainte- nance and salvage values, and other different factors like input horsepower and pump efficiency. It directly compares the projected total costs for an appli- ance or device based on the total costs accrued over its ex- pected lifetime. The lowest total cost for the stated lifetime of the system or component is generally the preferred option and the choice usually selected. For our analysis we were able to compare the costs of oper- ating the well pump at a reduced speed vs. operation against a control valve. The fundamental difference between the two types of devices is the total head. When using a VFD, the performance of the pump closely mirrors the affinity laws which state the capacity varies directly with the speed, or Q = RPM 2 head varies with the square of the speed, or Head = RPM 2 2 and HP with the cube, or HP = RPM 2 3 RPM 1 RPM 1 So for our example on a VFD, if the original capacity at 1760 RPM was 500 GPM at 260 feet at 39 BHP (brake horse- power), lowering the pump and motor speed to 1400 RPM will produce: 1400 RPM × 500 GPM = 398 GPM 1760 RPM WATER WORKS from page 58 Figure 1. Full speed pump curve. K = 7 lb./ft Max. lateral = .75″ waterwelljournal.com 60 October 2016 WWJ