Water Well Journal

May 2016

Water Well Journal

Issue link: http://read.dmtmag.com/i/668983

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Page 33 of 77

W e began a discussion in the February issue on the interaction—or more accurately—the intersection of the pump performance curve (head-capacity curve) and the system head curve. Now we will graphically take a more in-depth look at how varying operating conditions and our resulting pump selections can lead to a successful installation or a source of frustration. As previously discussed, a system head curve is comprised of three head components: static head, pressure head, and fric- tion head. These are shown in Figure 1. Static head (in blue) is the total lift or change in elevation from the surface of the water during operation (pumping water level when pumping from a well), plus any change in elevation between the pump discharge head and the irrigation system. Pressure head (in green) is the system design pres- sure requirement. This is given in pounds per square inch (PSI) generally, but is easily converted to feet of head by mul- tiplying by 2.31. Friction head (red line) varies with flow and is a function of the resistance in the piping system, including the size and length of the piping plus any valves, fittings, or other appurtenances. Once the system head curve has been established, a pump is selected that will match its head-capacity curve with the system curve at the desired flow rate. This intersection, when calculated correctly, occurs at what we call the design point. It is important to recognize the design point as the intersec- tion of the two curves for several reasons. First, whether we realize it or not, there are always secondary operating points, even if only temporarily such as during startup and shutdown. Secondly, there are usually several different pump selections that will meet our design point requirement, but may present different problems when secondary operating constraints are considered. Pumps are designed to provide complete hydraulic cover- age within certain physical sizes. Without getting too deeply into a discussion on pump design, suffice it to say that within a specific size, some pump designs will be engineered to pro- duce high head/stage at a low flow, and other designs to pro- duce high flows at a low head/stage. Between these two extremes a range of designs will pro- vide a continuum of hydraulic coverage. These designs are defined numerically by their specific speed (N S ), a dimen- sionless expression of the impeller's geometry. Each of these designs has a characteristic pump head-capacity curve shape which rotates around the best efficiency point (BEP) of the curve. Pumps with higher specific speeds have steeper head- capacity curves, and pumps with lower specific speeds have flatter head-capacity curves (Figure 2). There are additional characteristics of curves with differing specific speed and they will be discussed in a future article. With the intersection of the two curves (design point) iden- tified, conventional industry practices tend to focus on the lowest initial cost, followed closely by the highest efficiency offered at the design point. This results in a tendency to look for a selection that will meet the design point with the fewest number of stages. Low specific speed pump selections with their high head/stage offer this. The trade-off for this selection is its characteristically flat curve. So is this the best choice? Let's look at an example: • Center pivot irrigation system requiring 800 GPM at 45 PSI operating pressure • Pumping water level (PWL) of 156 feet. (Note: Later in the irrigation season or during periods of drought, the pumping water level can drop an additional 20 feet.) MIKE ALLEN IRRIGATING OUR FUTURE MAKING THE RIGHT CHOICE One size does not fit all, so make sure you make the right choice for your customers. There is no one-size-fits-all approach to pump design. Each application requires us to ask probing questions. Figure 1. IRRIGATION continues on page 32 waterwelljournal.com 30 May 2016 WWJ

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