Pumps & Systems, November 2008
Responsible for supplying an average of 60 million gallons of drinking water each day, the Monroe County Water Authority (MCWA) in Rochester, N.Y., depends on more than 100 centrifugal pumps with a total installed capacity of more than 35,000-hp. These pumps range in size from 5-hp to 1750-hp and consume an average 5-MW of power daily. In an era of conserving energy and increasing sustainability, the MCWA is attempting to reduce energy consumption, greenhouse gases and operations and maintenance costs.
Through computerized modeling of its water distribution system, the MCWA discovered many discrepancies in their pumps between the original manufacturer pump curve and the actual field pump curve. When inputting the pump curves into the model, they discovered that the model could not be calibrated using original manufacturer pump curve data.
The MCWA staff went into the field and physically measured each pump's performance (head, flow, kW and RPM) to develop a set of field pump curves for the water distribution model. They found that in virtually every pump tested, actual field pump curves were significantly below the manufacturers specified pump curve and field measured pump efficiencies were often 20 percent or more below the original manufacturer's stated pump efficiencies.
|Figure 1. Pump interior with severe tubercultation
The MCWA team selected a few of the biggest offenders for refurbishment, targeting the units that operated at the least efficient rates. The MCWA assumed that overall performance decline was due to wear of internal components, so they overhauled them with the needed mechanical repairs, replacing rings, sleeves, seals, gaskets and bearings. When the pumps were returned to service, their performance improved, but many of the pumps were still running significantly below the manufacturer's specifications for head, flow and efficiency. Consequently, the MCWA began to consider other factors associated with the internal pump condition that might be causing declines in performance and efficiency.
During the teardown for mechanical repair, MCWA noticed extensive tuberculation buildup on the interior casings of all pumps. Tuberculation, the knob-like mound byproducts of corrosion that build up on the interior walls of pumps and unlined piping systems, has long challenged engineers and maintenance professionals in the water industry. For more than 50 years, the industry standard for pipeline manufacturers has been to apply a cement lining to the inside of cast and ductile iron pipe used in municipal water distribution systems to prevent corrosion and tuberculation buildup.
The MCWA team knew the effects on flow and head loss caused by corrosion, tuberculation and rough interior surfaces in old unlined cast iron pipelines installed before cement lining became the standard. They wondered if this roughness on just a few feet of pump casing between the suction and the discharge flanges really made a significant difference in a pump's performance and efficiency.
The MCWA team began a pilot study to refurbish several interior casings of the mechanically rebuilt pumps to see if reducing interior pump roughness would have any positive impact on pump performance and efficiency. Since only sandblasting the interior pump casings would allow corrosion and tuberculation to return quickly, the team began looking for acceptable coatings that could be applied after sandblasting to protect the interior surface from future corrosion and perhaps even enhance the reduction of head loss/hydraulic drag through the pump.
Due to their availability, ease of application and relative low cost, the MCWA team focused their coating search on NSF-approved brushable type ceramic-filled epoxy coatings. While the manufacturing and chemical industries have long embraced ceramic-filled epoxy coatings for abrasion and corrosion resistance in pumps, the MCWA team found little research to support that coatings improve or stabilize pump performance and energy efficiency.
Although the MCWA team was skeptical on how much performance and efficiency could be gained, they decided to sandblast and coat three of the mechanically rebuilt pumps as a pilot study test case. Once this was completed and the pumps were reassembled, MCWA conducted field pump performance tests to measure the coating's impact on pump performance and efficiency.
|Figure 2. Pump interior after coating with brushable ceramic epoxy