It's everybody's job to save energy. Simple solutions to help optimize your pumping systems, like those outlined in this article, often can pay off more than you might think.
The facts speak for themselves and they're not very encouraging for consumers across the country. The cost of energy is skyrocketing, and with it, the cost of electricity. As a result, countless organizations are putting special emphasis on efforts to optimize their equipment and processes—which, in turn, will reduce energy costs and increase reliability and uptime. If not, they need to be doing so. And the sooner the better.
For most operations, pumping systems can be one of the best places to begin looking for energy savings. In fact, the U.S. Department of Energy (DOE) has estimated these possible savings could exceed $6 billion annually for industrial applications, which includes municipal operations.
Municipal water and wastewater, one of the larger applications for pumps, is responsible for about 2% of the nation's electrical energy use. The good news is that an estimated 20% reduction of the energy use in the municipal sector seems quite feasible. Municipal pumping stations are generally designed following guidelines that take into account how many people live in the area, what the peak flow rates will be over a specified time period, etc. These peak flow rates depend on both the anticipated number of customers that eventually will be hooked up to the system and on weather-related additional flow rates.
Normally, a pump station is designed with multiple pumps so that it can handle peak flow rates even if one pump is down. It also is common that pumps run on/off—which means that peak flow-rates are produced as soon as the pumps are turned on. Unfortunately, this type of operation generates high energy losses through friction.
It is a given that a pump station will have to be able to cope with peak flow rates and have redundancy if a pump fails. This practice, however, leads to higher-thannecessary energy use. It has been demonstrated that substantial savings can be achieved by using smaller or speed-regulated pumps for average flow conditions. Fig. 1 shows a typical duration curve for a wastewater lift station. Each point on the curve shows how many hours per year the flow exceeds a certain value. For example, the inflow is larger than 1,000 gpm for about 1,000 hours per year. The rest of the time, it is lower. A typical pump configuration for a pump station with this inflow characteristic would be two installed pumps that can each handle close to 3,000 gpm. It is evident that such pumps are much larger than needed most of the time.
Improving the situation
One popular solution to the problem is to install variable speed drives (VSDs) so that the pump capacity can be matched to the inflow. Many times, a VSD can be an excellent solution, assuring that the pumped volume is never larger than needed.On the other hand, in many cases involving lift stations, this "solution" can actually lead to increased energy usage. The main reason for this is that pump efficiency can deteriorate rapidly in systems exhibiting high static head when the speed is lowered. (For more information on this topic, refer to Variable Speed Pumping: A Guide to Successful Applications, by Europump and the Hydraulic Institute.1)
When static head is low (a fixed percentage is hard to give, but it's usually lower than 30-50% of total head),VSDs typically can be used with good result. If the static head is higher, a thorough study should be conducted before VSDs are installed. In some cases, a simple impeller trimming might be a better way to adjust supply to demand.
Another possible solution could be to use pumps of different sizes. There are several ways this can be done. One method, described in a DOE report2, suggests using a "Pony pump" in parallel with larger full-size pumps. Fig. 2 shows
how the larger pump is run for a couple of hundred hours a year, while the smaller pump is operating over 5,000 hours. The areas under the respective curves represent volumes pumped. The sum of the areas in the rectangles is the same as the area under the duration curve. As can be seen most of the flow is pumped by the smaller pump. The savings come from operating at lower flow rates and heads (see Fig. 3). The energy usage per unit volume is proportional to the head. In this case the pumps are chosen so that the operating points are close to the best efficiency point (BEP) for each flow rate. In the project referenced in the previously mentioned DOE report, energy savings of close to 40% were achieved using this method.
It is important to know that pumps which operate close to BEP not only use less energy, they also have substantially lower maintenance costs than pumps that are not operated this way. Fig. 4 shows such data from DuPont.
It is of the utmost importance that pumps operate close to their BEP, both from an energy and maintenance point of view. One can say that if the energy usage is optimized, the maintenance savings come for free. Interestingly, in many cases, the maintenance savings can actually be greater than the energy savings.
Other possible improvements could be to split the peak flow rate on two pumps and have a third pump as standby.Most of the time, only one pump would run and the station would exhibit lower operating costs.
To determine the best solution for the problem, a life-cycle cost calculation can be done. A simple calculation of the difference in energy cost for a hypothetical situation is shown in the sidebar above.
Calculating the Difference in Energy Costs
Assume a lift station with a static head of 35'. The station is equipped with two pumps that alternate. At the duty point (6,000 gpm and 55' head) they are 74% efficient and require 112.6 horsepower. A 94.5% efficient motor draws 89 kW. The pumps operate 3,000 hours/year, pumping 18 million gallons while consuming 267,000 kWh. If a smaller pump operating at 2,500 gpm is added to the station, the big pumps only have to be run at peak flow conditions—let's say 250 hours/year. They would pump 1.5 million gallons during this time. The small pump would have to run an additional 6,600 hours/year to pump the same total amount as the two larger pumps (18 MG). Assume that the smaller pump operates at 66% combined motor/pump efficiency. The total energy used would then be 205,000 kWh per year, a reduction of 23%–or close to $5,000/ year at 8 cents/kWh. It is, of course, necessary to put in real values if you contemplate such a solution, but the example shows how to calculate the savings. DOE's PSAT (Pump System Assessment Tool) program is useful for this kind of calculation. You might also realize savings from lower demand charges and lower maintenance costs that should be included in your calculations. In the DOE project mentioned in this article, those savings were actually larger than the energy savings in dollars.
A call to action
There are many reasons for our current energy situation being what it is. Historically, energy costs have been low and organizations have put more emphasis on lower initial purchase costs for equipment than on lower overall life-cycle (cradle to grave) costs of that equipment. No matter the cause, higher energy costs are, in all likelihood, here to stay. Therefore, it behooves both end users and design engineers—across all industry sectors—to rethink how they design all of their equipment systems. In some cases, guidelines and regulations will have to be reevaluated in order to pave the road for more efficient engineered systems. In the meantime, we must all do our best to use energy more efficiently than in the past. It is vitally important now, and will be even more so in the future, to identify and implement successful energy-saving strategies and solutions throughout industry— both at home and around the globe.