By Allan R. Budris
When considering possible changes or options to reduce pump system energy costs, care must be taken not to base the decision on insufficient or overly optimistic data. During an industrial energy audit which the author conducted several years ago, he was asked to look into a pump system that was previously evaluated by another consultant, who used an abbreviated costing method. This short-cut calculation predicted an annual savings of $50,000 (by replacing a control valve with a variable speed drive). However, a more thorough evaluation conducted by the writer (which included field testing), showed the annual savings to be closer to $5,000, not enough to justify the VFD.
Variable Frequency (Speed) Drives can yield substantial savings in the right application, and they almost always yield the lowest energy cost. However, these savings may not be enough to justify the total capital VFD investment in many applications.
Average Flow Rate
One of the best ways to begin the process of obtaining a reasonably accurate estimate of the current (or future) pump system energy costs (and potential savings), is to determine the average expected pump flow rate. The writer has found that using the average pump flow (and head), over the total annual pump hours of operation, yields input energy results that are very close to the energy costs determined by the more time consuming method of summing up the various individual pump head-capacity values, for the time spent at each of the individual conditions.
Often pump operating conditions (at least the flow rate) can be obtained from existing system records. If actual pump field head-capacity test results are not available, the pump factory H-Q and efficiency curves can be used, at the pump speed, for a reasonable first approximation.
Figure 1 shows the actual nine month historical flow rates for a pump the writer has previously analyzed, broken down between low, average and high flow rates, with Figure 2 summing up the average time spent at each of the flow rates. The maximum overall flow rate (1,600 gpm in this case) should determine the pump size, with the selected pump also being capable of satisfactory operation at the minimum flow rate (550 gpm). It is, however, the average flow rate (988 gpm) and the associated pump and system heads, which should be used for any initial reasonable energy usage calculation.