Municipal Water Cavitation Fail

Most municipal water case studies do not begin in a lab, but this one does. Eric Dole, a certified Pump System Assessment Professional (PSAP) with two patents to his name, was conducting research on electrocoagulation in a lab at Red Rock Community College outside Denver. A supervisor at Consolidated Mutual Water, who also taught at the school, was curious about the test and introduced himself. 

Soon, Garver was running demonstrations and doing pump assessments at Consolidated’s ultrafiltration facility. When one of the plant’s five large pumps failed catastrophically due to cavitation, Consolidated asked Dole, a water and energy practice leader for multidisciplinary engineering firm Garver, to investigate. 

Background

The pump was one of five 350 hp vertical turbines that pump membrane-filtered water from a clearwell to 280-foot-high elevated storage tanks that feed the multiple distribution system. The pumps account for about 80 percent of the plant’s electrical load. 

Dole began his investigation by running a wire-to-water test to determine the efficiency of the remaining four pumps. The test measures the water horsepower output of the pump divided by the total power input to the line side of the motor.  The resultant decimal is the wire-to-water efficiency, an excellent surrogate to determine the fitness of a pump. 

The remaining four pumps were not very efficient. That did not surprise Dole, who has done nearly 200 wire-to-water tests. “For 90 percent of tests I completed, the pump efficiency was between 40 and 60 percent. One of them, controlled by a variable frequency drive that never got up to speed, was actually 0 percent,” he said. 

Optimizing Consolidated’s pump system promised to save energy and lower operating expenses. Also, the local utility, Xcel Energy, had a demand side management (DSM) rebate program in place that would subsidize the cost of an investment-level pump assessment as well as the cost of new pumps if the facility could demonstrate improvements in wire-to-water efficiency.

Diagnosis

While running the wire-to-water tests, Dole also conducted an autopsy on the failed pump to see what caused the cavitation failure. It came down to two issues:

Deadheading. The failed pump had a pressure pilot-controlled, globe check valve that was set to open too slowly. Every time the pump would energize, it would pump against a closed check valve for 3 to 5 minutes.

“The water would hit against this check valve and then it would just stay there until the speed control slowly opened the valve,” Dole said. “That would deadhead the pump. When this occurred, a microscopic jet of water recirculated back around to the suction at such a high velocity, it caused cavitation. Those bubbles were imploding for years and you could see the pitting on the tip of the impeller.” 

Suction head. Denver is the Mile High City. That means the barometric pressure is significantly less than what is available at sea level. The firm that originally installed those pumps did not take this into account. When they set the constant water level the pumps should maintain, they set it three feet below the net positive suction head required (NPSHR). This reduced the system’s wire-to-water efficiency by about 20 percentage points. 

Solutions

Dole proposed a no cost and a low cost way to address these problems, but they provided very different returns. 

No-cost. This solution was very straightforward. First, it sped up the check valves to operate faster and eliminate deadheading and cavitation. Second, it raised the minimum pumping water level to meet NPSHR requirements. Third, using wire-to-water data, it staged the pumps so the most efficient pumps would run the most and the least efficient pumps the least. This would yield energy savings of just under $12,000 per year at no cost to the owner.

Low-cost. This approach included all the no-cost adjustments and adds two new pumps to replace the failed pump and the facility’s least efficient unit. The new pumps were more efficient, delivering more than 30 percent greater flow while using 10 percent less energy. 

Results

Consolidated Mutual Water opted for the low-cost approach. Although the two new pumps cost $203,400, the Xcel Energy rebates paid a substantial percentage of the cost. Dole estimated that the new pumps would generate $47,630 in annual net energy savings (not including roughly $13,000 in rebates for lowering demand in during summer). This yielded a return on investment within four years. 

To see a more detailed version of this case study, visit the HI Utility Resources page.  You can also learn more about Pump System Assessment Professional (PSAP) certification, or contact Eric Dole at ejdole@garverusa.com.  

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