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.
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.
So how much power savings could we expect if this pump is used in level control versus pump down? It depends on the pumping range you decide upon. If you can keep flow above 2250-gpm, pumping efficiency per gallon will remain at its highest level and power savings can be substantial. Although not entirely accurate, if you compute the average hp/gal using several points across the pump down cycle, the result is about 0.0088-hp/gal. At a constant 0.007-hp/gal, level control will consume about 21 percent less power per gallon pumped.
One of the advantages of VFD operation is that we do not have to achieve "perfect" level control and can therefore avoid those higher power areas. If inflow drops below some acceptable minimum, the drive can stop the pump, allow the level to increase slightly, and then restart the pump a short time later. The restart begins at a lower frequency (say 30-hz) and then ramps up to the desired pumping frequency. By employing this "soft start" technique, starting current will never exceed the rated full load current of the motor and the number of starts per unit of time can be increased substantially.
Not all applications will benefit from level control. Lift stations with just a few starts per day are typically better suited for pump down. Potential level control installations include subdivisions and other municipal applications with continuous, predictable flows during certain periods of the day. Industrial processes and municipal wastewater treatment are also good candidates.
Although this column has focused on potential energy savings, there are other advantages that can justify level control. Soft start and stop can prolong both pump and motor life and first costs can often be reduced due to a smaller wet well requirement. There is also an inexpensive variation on level control that can be applied to smaller pumps. Known as variable fixed speed (VFS) control, it utilizes float switches to vary pumping speed based on inflow. We will take a look at this particular application in a future column.