Keeping a close eye on pump efficiency can lead to energy savings.
Last month, I discussed the Draw Down calculator and its role in determining the actual operating point of wastewater pumps. Like several of my previous articles, it focused on off-BEP (best-efficiency-point) operation and the damage that can occur due to unbalanced radial forces and recirculation. The premature failures that occur in these pumps increase downtime, and repeated repairs are costly. You will probably think that I am beating a dead horse as this month's article also addresses off-BEP operation. However, this time it is from a different perspective. Instead of the cost due to increased maintenance and repair, we will review how it can affect electrical power costs.
Lev Nelik's “Pumping Prescriptions” (July 2011 Pumps & Systems) discussed the energy savings that can be achieved by re-placing a pump impeller with one designed to operate at BEP. The example he used was a large pump (40,000 gallons per minute) operating at 50 percent of BEP flow (20,000 gallons per minute). The energy wasted annually was 3.1 million kWh. You may be surprised how much can be wasted by much smaller pumps operating at 80 to 85 percent of BEP.
One of the seminars that I present at water and wastewater conferences is “Determining the Long- & Short-Term Costs of Pump System Efficiency.” In it, I break down a pumping system (installed or proposed) into its components and evaluate their individual contribution to the overall system efficiency. Achieving higher efficiency in a pumping system usually increases the initial costs, but this can be offset by long-term savings. Breaking a system into individual components makes assessing short- and long-term costs easier.
Components That Affect Efficiency
Pump and motor efficiency are major contributors to overall system efficiency. Higher efficiency pumps will typically have a higher first cost because they are often designed and built to a higher standard. One or two efficiency points may not have much affect in some applications, but when the difference is 5 percent or more, the long-term return should be evaluated.
Installed pumps will lose efficiency over time due to wear and corrosion. My 2010, two-part series on restoration and coating showed how the Monroe County Water Authority in Rochester, N.Y., reduced electrical consumption substantially. Even though the procedures appeared expensive ($4,000 to $13,000), the payback period was often less than one year.
|Figure 1. Screenshot of the Off-BEP Cost Calculator|
With the Energy Independence and Security Act of 2007 (EISA), all three-phase motors from 1 to 500 horsepower manu-factured after December 19, 2010, must meet the premium efficiency standard. Therefore, new installations will take advantage of increased motor efficiency unless you have an old motor in your warehouse. EISA does not apply to motors purchased before its inception, and those motors can also be rewound and continue to be used. When evaluating existing pumping systems' efficiency, the cost of replacing older, lower efficiency motors should be evaluated.
The Piping System
One example I use in my seminar is an Excel calculator that compares the wire to water efficiency of two pump and motor com-binations. It was the subject of my March 2010 Pumps & Systems column. It provides a simple payback analysis for comparing the short- and long-term costs of two pump/motor combinations with different operating efficiencies.
Although the pump and motor affect the overall efficiency, piping systems can have a greater influence. Poorly designed piping systems often require increased head, which translates into increased horsepower. Older piping is also problematic. Friction increases over time due to corrosion and build up, and increased demand will over tax the original design capability. The starting point is always the system curve, which shows the required pump performance (head and flow). When pumps run to the left of BEP, they will run longer at a lower efficiency, and energy costs can increase.
Another seminar example is shown in Figure 1. This spreadsheet compares the same pump and motor when operating at and off BEP conditions. The example included shows a pump system that was designed for 5,000 gallons per minute at 130 feet. Due to a design error, friction was miscalculated. The actual flow is limited to a maximum of 4,000 gallons per minute at 147 feet. To meet the required daily flow, the pump must run two more hours each day. It also operates at a lower hydraulic efficiency, and the head required is higher. These last two factors increase energy consumption and result in an increased cost per 1,000 gallons pumped ($0.06 versus $0.049).
What are the options? The most obvious fix would be to modify the piping and get the pump to BEP flow. If this is not cost-effective, a smaller pump with a better efficiency should be selected. An increase in efficiency from 81 to 85 percent at 4,000 gallons per minute would reduce the cost per 1,000 gallons to $0.057 and save almost $2,500 per year in electrical costs.
A more efficient fix would be to select a pump with a BEP flow that reduces the system head to its original calculated val-ue—about 3,000 gallons per minute at 130 feet, for example. Although the pump would run several hours longer than the one pumping 4,000 gallons per minute, the energy costs would be close to the original 5,000 gallons per minute estimates.
Energy consumption for a given volume of pumped fluid can be a bit perplexing. It has nothing to do with the flow rate of the pump. It involves the wire to water efficiency and head required. An increased flow rate will increase the horsepower required, but horsepower is an indication of the work performed per unit of time. A higher flow rate does more work in a shorter period of time. For example, if a 500 gallon-per-minute pump and a 1,000-gallon-per-minute pump have the same wire to water efficiency and are pumping against the same head, the energy required to pump 1,000 gallons will be the same. If the head is increased or the efficiency is lowered, its energy consumption per given volume will increase.