Pumps & Systems - September 2007

What we learn is that the power required per gallon pumped increases continuously as the wet well is evacuated. In our example it is 0.007-hp/gpm at the beginning of the cycle and 0.01-hp/gpm at the end – an increase of almost 43 percent! Now there is nothing wrong with this; it is normal for a typical pump down application. But it does indicate that there may be an opportunity to decrease power consumption.

Figure 3 shows the same pump operating under variable speed control.

The colored curves are the pump HQ curves from 45-hz to 60-hz in 5-hz increments, and the red horizontal line is the desired wet well level at a static head of 22.5-ft. The intersections of the HQ curves and wet well level line are the flow rates at that particular frequency. As I have mentioned in my previous variable speed columns, the typical VFD has a resolution of 0.01-hz and an accuracy of about 0.1-hz, so be aware that there are actually quite a few more operating points than shown here.

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Subdivisions and

municipal applications

with continuous,

predictable flows

during certain periods

of the day, industrial

processes and

municipal wastewater

treatment are all good

candidates for level

control installations.

The HP required at full flow is the same as before, but the data shown below each of the other major flow points are a result of reduced operating head and a leftward migration of hydraulic efficiency due to a reduction in speed. The pump can operate at flows as low as 2250-gpm (49-hz) and not exceed its full flow power requirement of 0.007- hp/gal. Even at 1500-gpm (47-hz), power increases to just 0.009-hp/gal or the 3750-gpm point in the pump down application. As flow decreases to 750- gpm (45-hz, 40 percent eff, 10.7-hp), the power required per gallon increases to 0.014-hp/gal.