Water Well Journal
Issue link: http://read.dmtmag.com/i/668983
Our initial selection is a pump with a specific speed (N S ) of 1765. We can see the intersection of this head-capacity curve with our system head curve at 800 GPM in Figure 3. Note the starting point for the system head curve of 260 feet (156 feet static head + 104 feet pressure head) and its intersec- tion with the pump head-capacity curve at 270 feet (10 feet friction head at 800 GPM). Since this pump model is high head/stage, it will meet the design point using only three stages. An issue with the flat curve arises at the secondary design point. The addition of just 20 feet of drawdown during a late season/drought is shown by the blue system head curve line in Figure 3. The point of intersection between the two curves happens at just 600 GPM. The pump will only be able to deliver the required pressure at that lower flow. Irrigation sprinkler performance at the design flow would be severely reduced to the point of being inoperable, particularly the end gun if equipped. The pump curve selection in Figure 3 demonstrates the importance of another common industry practice: selecting a pump with a steep head-capacity curve. We see in many engi- neering specifications where not only is a steep curve re- quired, it is mandated the design point must be to the right of the best efficiency point (BEP). This is because head-capacity curves are the steepest to the right of BEP. Our second example is shown in Figure 4 and features a pump with a specific speed (N S ) of 2720. An identical system head curve is drawn (note the y-axis scale is different but the curve is the same) intersecting the head-capacity curve again at 800 GPM. The selection requires five stages since it is a higher specific speed and therefore will produce more flow, but at a lower head/stage. Here, when the secondary design point is taken into con- sideration (blue system head curve), the flow at desired pres- sure is better than 740 GPM. Sprinkler performance will be somewhat diminished, but the system will still operate. This makes the selection more expensive (five stages vs. three stages), but it will operate successfully across a broader set of conditions. It is easy to see why it is generally viewed steep head- capacity curves are superior to flatter curves. There are exceptions to this, though. I was asked recently by a Natural Resources Conservation Service (NRCS) technician, "Why can't you pump guys de- sign a pump with flatter head-capacity curve?" The technician was dealing with a center pivot irrigation system with a corner system. When the end tower retracts all the way, the flow re- quirement drops from 800 GPM to 600 GPM and the higher pressure at the reduced flow was causing the boots between the pivot towers to fail. We've already seen in Figure 3 the pressure on our initial low specific speed pump go up only about 15 feet or 7 PSI at 600 GPM. This would be an acceptable increase in pressure. In Figure 5, though, we see when using our second pump with a specific speed (N S ) of 2720, the pressure at 600 GPM goes up more than 47 feet (over 20 PSI). It's additional pressure such as this, from using a pump with a steep head-capacity curve, causing boot failure. Figure 3. Blue system head curve line represents addition of 20 feet of drawdown. Figure 4. A five-stage selection where blue system head curve flow is better than 740 gpm. Figure 2. Noting pumps with higher specific speeds have steeper curves. IRRIGATION from page 30 waterwelljournal.com 32 May 2016 WWJ