Tips on improving pump efficiency and decreasing energy consumption
Did you know that 85 percent of the average pump’s life-cycle cost is spent on energy?
While capital cost considerations often rule the day, the up-front cost of a water distribution pump is actually only 5 percent of the total, with the remaining 10 percent going toward maintenance. A bit more maintenance, however, could greatly enhance energy efficiency.
The above breakdown of life-cycle costs was provided by American Water, the largest publicly traded U.S. water and wastewater utility company. But more than just parsing the numbers, American Water has identified the conditions that steal energy efficiency from pumps, and offers tips to regain that efficiency. Tending to these suggestions, which were presented during an American Water Works Association (AWWA) technical session by American Water’s Ralph Wawrzyniakowski, EIT, will reduce the energy use, carbon footprint, and operating expenses associated with pumps.
Find Poor-Performing Pumps
The first step in optimizing overall pump efficiency is to find those pumps that aren’t performing to minimum acceptable requirements. Wire-to-water testing is helpful in this regard, as it considers the combined overall efficiency of the pump and motor together. Technically, it is a measure of the pumping power produced by a unit of electrical power (wire-to-water efficiency = water HP / wire HP). Assuming 95 percent motor efficiency, a typical pump has a wire-to-water efficiency in the range of 79 percent to 85 percent. Pumps that test below that rating can therefore be targets for improvements.
Another useful metric is the energy unit index (EUI), which can act as a baseline for past and future pump performance. Mathematically, EUI = energy used (MWh) / millions of gallons of water processed (MG). A lower EUI represents better efficiency; for example:
Causes And Cures
There are myriad factors that can lead to the loss of pump efficiency, but many can be resolved with maintenance. Here are American Water’s recommendations to resolve some common problems.
Problem: Normal wear of pump and motor
Solution: Mechanical rehabilitation.
For the pump, replace the wear rings, seals, sleeves, gaskets, and bearings. For the motor, rewind the motor windings.
Problem: Tuberculation (small mounds of corrosion in pipes and pumps)
Solution: Sandblast and recoat
Tuberculation makes the inside of pipes rough, which reduces pressure and increases pumping costs. Sandblasting removes the tuberculation and reduces friction, while an epoxy coating helps maintain efficiency and longevity.
Problem: Inefficient/nonexistent variable speed drive (VSD)
Solution: Install VSD
The energy-saving benefits of VSDs have been clearly established. The U.S. Department of Energy (DOE) reports: "With rotodynamic pump installations, savings of between 30 percent and 50 percent have been achieved in many installations by installing VSDs. Where PD [positive displacement] pumps are used, energy consumption tends to be directly proportional to the volume pumped and savings are readily quantified."
DOE further states that speed adjustments are a more efficient means of flow control than throttling valves or bypass lines. American Water follows suit by recommending that operators consider replacing throttled valves with VSDs. Another recommendation is to vary the flow at set hydraulic conditions, acknowledging total dynamic head (TDH).
Solution: Trim or replace impeller
Cavitation occurs when there are rapid changes of pressure, causing vapor cavities, or voids, in low-pressure areas. It becomes a problem when these voids implode in the presence of higher pressure, creating repeated shockwaves that wear and damage the pump’s impellers.
The damage to the impellers is most often accompanied by vibration, noise, and loss of efficiency. Trimming or replacing the impeller can resolve the cavitation issue(s).
Problem: Conservative engineering
Solution: Correct the pump design
A design centered on the “worst case” scenario is the wrong approach; rather, it is far more important to focus on the highest pump efficiency, or best efficiency point (BEP), when operating at average daily flow (ADF). Wawrzyniakowski and American Water also suggest using multiple design points to easily adjust to changes in process conditions.
Problem: Persistent motor inefficiency/change in hydraulic conditions
Solution: Replace pump (and motor)
Despite the best rehabilitation efforts, some pumps and/or situations will dictate that current models will never reach the desired wire-to-water efficiency of 80 percent or better. If the pump cannot be refurbished to operate near BEP at ADF, if the TDH/flow requirements have changed, or if the cost of rehabilitation is greater than or equal to the cost of replacement, the pump(s) should be replaced.
A pump that merely “does the job” — if it does so inefficiently — is simply not good enough in today’s environment of tight municipal budgets. Take the time to analyze and repair or replace, and avoid the long-term cost of pump energy waste.
Image credit: "Pumps at a wastewater treatment plant Mahombekombe" Sustainable sanitation © 2011, used under an Attribution 2.0 Generic license: https://creativecommons.org/licenses/by/2.0/