Upgrading Pumps to Keep Rate Payers Happy
Operators call for help when a pump system starts making unusual sounds or shows other signs of impending failure. A major wastewater utility in eastern Massachusetts, on the other hand, chose to improve the efficiency and lower the energy cost of its plant water system.
Category: Blogs, PSM Newsletter December 14, 2021
By saving energy, this New England wastewater utility reduced operating costs and built in energy savings for decades to come.
Operators call for help when a pump system starts making unusual sounds or shows other signs of impending failure. A major wastewater utility in eastern Massachusetts, on the other hand, chose to improve the efficiency and lower the energy cost of its plant water system.
The ultimate fix involved replacing one pump and rebuilding two others as well as coating the interior of all three units with a low-friction surface. This reduced electricity costs by 20 percent and achieved an ROI within three years.
The evaluation was led by Jen Muir, president of JKMuir, a pump specialist in Rocky Hill, Conn. Muir is also one of the Hydraulic Institute volunteers on the examination committee that developed the questions used for the Pump System Assessment Professional (PSAP) certification test.
Although she started out designing wastewater facilities and pumping stations for a large civil engineering firm, Muir soon developed an interest in energy management. In water facilities, that usually means keeping pumps running at their best.
“The design firms I had worked for were exceptionally good at what they did, but I wanted to focus specifically on energy in the water sector,” she said. “Pump testing is one of the most practical methods for identifying efficiency and cost savings opportunities. I decided to go out on my own and hope the water sector continued to invest in energy efficiency.”
That was in 2008, the year the Great Recession began. Yet Muir’s business thrived because public pressure was growing for municipalities to invest in greener, more sustainable infrastructure.
“We’re protecting public health by providing safe drinking and managing wastewater, but it takes a lot of energy to do those things,” she said. “But in places like New England, New York, and California, where energy costs are high, there is additional pressure to make investments that will reduce long-term costs so utilities will not have to raise sewer or water rates to pay their electric bills.”
Background
The wastewater treatment facility, located about 50 miles east of Boston, is one of New England’s largest, serving seven communities. To reduce long-term operating costs, district management decided to take advantage of state and utility incentives to reduce energy use (and costs) at its facilities.
Muir’s team focused on the plant water system, which recycled treated water within the facility for such tasks as spraying (to reduce foaming), incinerator exhaust scrubbing, sludge dewatering, and diluting chemical additions.
The plant water system consisted of three 150-hp pumps that discharged into a common header. The system design called for one pump to maintain system pressure of 66 psi (153 feet). The other two pumps were on standby to accommodate seasonal fluctuations in flow and for backup in case one of the pumps failed.
That was not how the system was operating when Muir arrived. Instead, it took two pumps to maintain system pressure and head.
Diagnosis
Muir decided to take a closer look. In many facilities, experienced engineers and technicians can visually identify candidates for energy savings. “Pumping systems are different,” she said. “There’s not much you can find by looking. You really need field instruments to measure a pump’s performance so you can compare it to the original pump curve and design conditions.”
Using a portable digital flowmeter and pressure gauges she took measurements at the suction and discharge end of the pumps. A portable power quality analyzer let her gauge energy consumption.
Muir found that it took two pumps to maintain system pressure of 66 psi at 153 feet total head. At 92-93 percent speed, the flowrate for each of the two pumps was 1500 gpm under typical summer conditions and 1350 gpm in the winter. The OEM’s original data sheet called for each pump to run at 2850 gpm at 170 total head.
When she compared actual results with the OEM pump curve, she found one of the units was operating well below the pump curve. The other two were also below spec, but not as far below. All three appeared unable to generate nameplate pressure and flow and were using more electricity than they should.
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