If the irrigation system is simple, an efficient pump can easily be selected.
If the irrigation system is simple (e.g., one center pivot, no end gun, no corner arm and flat topography) there is one system curve, and an efficient pump can easily be selected. With a complicated system (e.g., multiple center pivots with corner arms, a golf course with various sized zones, an irrigation district system serving many parcels, etc.) there are many system curves with a wide variety of flows, and choosing a pumping system that always works efficiently is more difficult.
Pump curves
Pumping station using pressure control valves in the Raymond Irrigation District, Alberta, Canada (pictured: Len Ring)
1 2 3
12 Irrigation TODAY | April 2018
All figures adapted from Cornell Pump Company pump curves.
Designing for varying flows – VFDs In recent years, variable frequency drives have become a popular choice for pumping systems with varying flows. They can replace pressure-reducing valves used to control discharge pressures. This makes the design process easier. A VFD may be used to reduce energy costs, but the owner may find that energy savings were not as large as expected. The other benefits of a VFD (i.e., smoother operation, better pressure control, less hydraulic transients/water hammer, less maintenance) may make up for the less than expected reduced energy.
Why didn’t I save more energy? Because VFDs make designing easier, a designer may pick pumps that can provide the maximum pressure and flow and then let the VFD look after the other operating points. To reduce capital cost, the designer may use the fewest number of pumps, each operating over a wide flow range. With a VFD the pump will still operate on its pump curve, and that point must be known in order to predict the energy consumption. It is important to also take into account the VFD’s efficiency. Typically, they are 95-97 percent, so a savings of 3-5 percent in energy is necessary — just to break even.
Which pump to choose? An operator wants a pump that will produce 750-1,250 gpm at a pressure of 54 psi (125 feet total dynamic head). Using pump curves 1 and 2 provided (left), the operator would initially pick pump 1 over pump 2, as the efficiency at 1,250 gpm is better (84 percent to 82 percent), but this would not be the best choice.
FLOW RATE {gpm}
As the flows drop, pump 1’s efficiency drops from 84 percent to 75 percent at 750 gpm. Pump 2’s efficiency is 82 percent at 1,250 gpm, but it increases to over 85
TOTAL DYNAMIC HEAD {ft}
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