Change-out to N-Pumps stops fouling, saves energy

Oct. 1, 2002
With older, low velocity pumps at a pump station that fouled as often as once a week on rags and other debris, the Suffolk County, New York Department of Public Works finally turned to N-Pumps from ITT Industries' Flygt unit.

Oct. 1, 2002 -- With older, low velocity pumps at a pump station that fouled as often as once a week on rags and other debris, the Suffolk County, New York Department of Public Works finally turned to N-Pumps from ITT Industries' Flygt unit. The change-out to N-Pumps have proven to overcome the persistent maintenance issues as well as drastically lowering operating costs.

The pumps at the Stony Brook Lift Station used to clog so often that crews for Suffolk County (NY) Department of Public Works (SCDPW) anticipated an event as part of their weekly routine, recalls Ron Warren, Maintenance Director of the Division of Sanitation, Operations & Maintenance.

"The clogging became predictable," he says emphatically. "Every week to ten days we'd have to go out there and free an impeller that had become jammed with rags or some other debris."

The problem-prone facility is one of 87 pump stations and 24 wastewater treatment plants maintained by the Department. The 3-MGD facility was built to serve the State University of New York (SUNY) when the campus at Stony Brook was smaller and generated less flow.

The Stony Brook has become New York's comprehensive university center for the downstate metropolitan region. Starting with 9 buildings on a 480-acre site, Stony Brook has expanded to encompass 123 buildings on nearly 1,200 acres. The faculty has grown from about 175 to 1,682, the student body from 1,000 to 18,628, and the annual budget from about $3 million to more than $600 million.

The station now serves a much larger university that includes a major center and three outlying areas, Warren added. Although it would appear the station's three, 60-HP pumps had ample capacity; the size of these units and their variable speed drives contributed to the clogging problem during low flow periods (most of the day). It is a problem shared by many Departments responsible with stations equipped with large, variable speed pumps.

The station was designed with a 1500-gallon wet well and three 60-HP end-suction pumps mounted horizontally in the dry pit. Approximately two years ago, when the pumps clogged as often as twice a week, SCDPW requested G.A. Fleet Associates, Inc. to recommend a solution. SCDPW has its own engineering staff and crews capable of handling most retrofit projects.

"Clogging is a problem we frequently encounter with older, large pumps with variable speed drives," explained Mark Cavanaugh, with G.A. Fleet Associates. " It was common practice in the past to oversize pumps but a unit then runs at minimum speed most of the day. The low-velocity flow contributes to rags and other material getting entangled on conventional impellers and eventually clogging the pumps. An oversized pump also demands a wasteful amount of energy."

After a review of the original specifications and spending a few months monitoring the station's flow, G.A. Fleet Associates recommended replacing all three units with the new ITT Flygt N-Pumps.

The new generation N-Pumps have been engineered to provide advantages in efficiency and reliability, resulting in efficient, reliable and trouble-free pumping over long duty periods. By improving operational economy, the new N-Pump can have a dramatic effect on the total life costs of an installation.

Externally, the pump's smooth new shapes are easy to keep clean and easier to service. But it's inside where the difference really counts. At the heart of the new generation N-Pumps, the patented N-technique can be found. The unique, semi-open impeller, combined with the relief groove in the volute, has been proven to reduce the risk of clogging and maintain pumping efficiency, even under the worst of conditions including rags, high-fibrous material, grease or solids. In tests at other installations where fouled impellers had been a recurring problem, the N-pumps operated for extended periods without becoming clogged, while dramatically saving energy.

Operational serviceability has been taken even further by using class H insulated motors with improved cooling: less heat, less wear and tear. And there's a separate inspection chamber for rapid checking and maintenance.

G. A. Fleet further recommended changing the sizes of the three pumps to a combination of two, 60-HP ITT Flygt Model NZ3300 pumps to handle peak flows and a jockey pump to handle the low-flow conditions. Instead of a third 60-HP unit, however, a 20-HP Model NZ3152 pump was recommended to reduce the station's energy consumption. In addition, the scope of upgrades also installed included a Flygt MultiTrode(r) Liquid Level Control System, a reliable conductivity probe-based system that is fully compatible with SCDPW's new SCADA system that operates on Citex software.

"We plan to add more MultiTrode systems and to gradually network eight stations with the SCADA as our first step in a full blown, continuously monitored system," Warren foresees.

The $85,000 upgrade retained the horizontal alignment of the old pumps to save money on piping. The station recorded a fairly typical 2.1 MGD flow through the month of June. "Although the university has its own power grid, we can make some good assumptions about our improved energy use," Warren noted. "For the first 24 days the station was online with the SCADA, the 20-HP jockey pump operated 533 hours, compared to 25 and 17 hours each for the 60-HP units."

Equally important, he adds, there were no incidents of clogging at the once problem-prone station. That, alone, saved $240 in labor costs each time a two-man crew would have been dispatched to free up the old impellers.

The new N-Pumps, which have pumping capacities up to 550l/s, 8,700 gpm, have undergone extensive testing in the field. The results from these tests show considerably lower energy consumption and fewer running problems.

The highest efficiency value for a typical single-vane pump in a best-specific speed range is around 70% efficiency. By comparison, N-Pumps deliver 80% or better - equating to 15% less power consumption In several installations where clogging of the conventional pump was an issue, the power saving was as great as 50%.

The broad range of pumping capacities offered by the N-Pump, coupled with the self-cleaning advantages of the impeller and volute design, has opened up new possibilities for cost-effective operation in a wide variety of applications.

"You don't realize how many problems you had until reliable equipment is in place," Warren concluded.

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