Calculating the Savings
To determine the savings resulting from variable-speed pumping, one has to plot the flow vs. energy consumption relationship for both the constant and the variable-speed operation (Figure 2). The difference between the two curves is the amount of energy that is unnecessarily used by the control valves during constant-speed pumping; that is, the amount of energy that can be saved by optimization.
Once the savings curve has been determined (Figure 2), the next step is to estimate the average pumping load. If the savings curve is as shown in Figure 2, and if the average flow is 80%, then the electricity saving will be 30%. For a more accurate determination, a chart, such as the one shown in Table 1 is prepared. Using this table, and knowing the total horsepower of the pumps plus the cost of electricity, makes it possible to calculate the both the yearly savings and the payback period if the pump station is an existing one and its optimization is being considered.
The energy savings are higher in mostly friction type pumping systems, such as pumping stations in the water supply, wastewater, district heating or agricultural applications. In general, the steeper the system curve (Figure 1), the higher will be the energy saving. If combined with surge-free pump starts and stops and with automatic maintenance scheduling, in addition to energy savings, these strategies will also improve the overall plant operation.
In large pumping stations, model- predictive optimization can also be used. The details of this strategy are described in Chapter 8.35 of Volume 2, of the Instrument Engineers' Handbook, 4th edition. These models are so configured that they automatically identify and implement the optimum pump combinations to meet any load, while consuming the least amount of power. Each implementation needs to be configured with specific pump curves and tuned for the specific application. However, this effort is insignificant compared to the initial cost of developing the program shell.
In the not too distant future, all unit operations, not just pumping, will be controlled by unit-operations controllers in which the setpoints will be efficiency and operating costs instead of flow, pressure etc. These unit operations controllers already will be supplied with the software necessary to optimize the unit operations, such as pumping stations, so the user will need only to insert the system curve and the characteristics of each pump to complete the system. Such unit operation software packages will also be used for reactors, distillation, compressors, boilers or renewable energy systems and will also include the logic, optimization, self-diagnostics, auto-start/stop and other features to fulfill the needs of the process.
It is high time we stopped controlling flows, pressures and temperatures and started controlling and optimizing unit operations, not only because we must reduce waste in this century, but also because it will bring recognition to our profession, which is the only field of engineering that can optimize the operation of the total process.