Water-Lifting Devices – Pumps - Practices of Irrigation and On-farm Water Management - page 472

Each pump curve typically reflects a single model of pump made by the manu-
facturer. At the top right of the chart it gives the pump speed. Finding the proper
pump is just a matter of selecting a model and size that will produce both the head
and GPM that you need. To do this, you select the horsepower and impeller size that
will give you the desired performance.
The red color curved lines (the top one in the pump curve above is labeled
“6.00 in. dia.”) represent the various impeller sizes. The green color straight
lines represent the motor horsepower ratings available for this pump. Together the
impeller curves and horsepower lines represent the best performance the pump is
capable of, if that horsepower or impeller size is selected. Some pump curves do not
have horsepower lines, and some pump curves combine the horsepower and impeller
lines into one single line. This is usually because the pump only is available with
one motor, so you don t get to select the horsepower. The pump may also only come
with one size of impeller, so you will only see a single line on the entire pump
curve.
To use the curves, you select the pressure you want on the left and then move hor-
izontally across the chart to the vertical line that corresponds with the flow (GPM)
that you want. You then select an impeller size curve and horsepower line that are
above this point to determine the impeller size and horsepower you will need for
your pump.
For example, a farmer wants a pump that produces 120 ft of head while pumping
100 gal/min (GPM). For that, start at 120 ft head on the left of the pump curve. Now
move straight across the curve to the right until you reach the line that goes down to
100 GPM on the bottom of the curve. From the point where the two lines intersect
move up the chart to see what horsepower pump will be needed. In this case a 5
hp will be needed, as the next horsepower line above the intersection point is the 5
hp line. Similarly, the impeller curve must also be higher in the chart than our line
intersection, so a 6.00 in. diameter impeller will work.
For the case the curve is midway between the 5.00 in. curve and the 6.00 in.
curve, then you would need a 5.50 in. impeller.
On many pump curves, additional set of ellipses labeled “efficiency” or simply
with percentages labeled on them exists. These ellipses indicate the efficiency of
the pump. To use them, simply look for the smallest ellipse that your line inter-
section point is inside. This is the efficiency at which the pump will operate. A
high-efficiency pump uses less energy to operate than a low-efficiency pump. It is
better to avoid any pump that has an efficiency of less than 60%.
12.12.4.2 Pump Selection Based on Pump Characteristics and Well
Characteristics
Pump can be selected based on the well characteristics, which best suits for the
particular well. In this approach, the well-characteristic curve is drawn with the
same scale as that of pump characteristic curve, made a copy on tracing paper.
Then the well-characteristic curve is placed over the pump curve (supplied by the
manufacturer). From the matching (intersection), a pump is selected.