FPL Energy experienced multiple failures of the thrust bearing in the single turbine-generating unit at its 6-MW Cataract plant in Maine. To solve the problem, FPL Energy installed a new eight-pad, spring-supported PTFE thrust bearing and a new thrust block. The retrofitted unit began operating in July 2006 and has been failure-free ever since.
By Paul J. Plante, Eric D. Soule, and Mike A. Dupuis
FPL Energy’s 6.65-MW Cataract project is a run-of-the-river hydro facility on the Saco River in Maine. The station has a single Kaplan turbine-generating unit that began operating in 1939. Between 1959 and 2005, the unit’s thrust bearing failed eight times, with half of the failures occurring between 2003 and 2005.
To deal with the situation, FPL Energy installed a low-profile, eight-pad, spring-supported thrust bearing and a new thrust block. This modification solved the problem – the unit has operated since July 2006 with no thrust bearing failures.
Problem with the thrust bearing
The spring-bed babbitt thrust bearing at Cataract is above the rotor in the upper bridge, which also houses the upper guide bearing. There is a lower guide bearing under the generator rotor and a water-lubricated turbine bearing in the head cover. In 1959, FPL Energy repaired the thrust bearing because it had suffered from eccentric wear over the initial 20-year operating period. The eccentric wear was believed to be associated with concrete growth at the station. The repair work included installing a sleeve on the thrust block. Since that repair, the thrust bearing failed eight times, with four of those failures occurring since 2003.
In 2004, FPL Energy took the unit out of service to repair an oil leak in the Kaplan head. When the unit was disassembled, personnel discovered two significant adverse conditions. First, the babbitt shoes on the thrust bearing were cracked. Second, misalignment of the powerhouse as a result of alkali-aggregate reactivity (AAR) had progressed to such a degree that the unit centerline needed to be reestablished. Work to correct these two problems took about ten months.
In June 2005, personnel began to start up the rehabilitated unit. Personnel conducted mechanical runs and then initiated an auto-start sequence. Within 30 minutes, the unit tripped as a result of high thrust bearing temperature. Personnel performed an inspection after the trip and discovered a severely wiped bearing with a babbitt-filled oil reservoir.
FPL Energy personnel then conducted an investigation to determine the cause of failure during start up. During disassembly of the failed bearing, personnel discovered that the round keys that hold the split thrust runner halves to each other were distressed. The two keys are held in place by set screws. Personnel found one ejected key in the thrust bearing oil reservoir; the other key was still in place. Both keys had sheared set screws. And, personnel noticed displacement of about 1/16 of an inch between the thrust runner halves. However, they were not able to target a conclusive root cause for the failure.
To recover from this failure, personnel first reengineered the thrust bearing components. They installed a new split half thrust runner that included a robust key set. Additionally, personnel were concerned that rebabbitting the original backing plate might result in warping. Instead, they decided to install a two-piece babbitt plate. Personnel reassembled the unit and prepared to restart it in September 2005.
During this start up, personnel developed a start-up procedure, intended to address potential issues from the June start-up failure. This included a program of progressive starts and stops consisting of mechanical runs at various speeds, speed-no load runs, and runs of varying duration. Personnel also conducted intermediate inspections and cleaning and scraping to check for damage.
The second start up progressed normally through a run that included flashing the field. The unit was then auto-started and synchronized. Thrust bearing temperatures started to climb dramatically and the unit tripped within three minutes. Upon disassembly of the unit, personnel discovered a preferential wipe in the babbitt that was so severe that the thrust bearing components would have to be either repaired or replaced. Personnel also noted displacement between the two halves of the thrust runner, despite the enhancements made to improve rigidity and stiffness of the keys.
Investigating solutions
At this time, personnel completely removed the thrust bearing from the unit. FPL Energy then assembled a ten-member multidisciplinary team to determine the root cause of the thrust bearing failures and the appropriate corrective actions.
The team worked on the problem for six months. They performed an exhaustive study, evaluated the failed components, and consulted with several thrust bearing performance experts. Eventually, the team came up with one potential root cause and six contributors that enhanced the likelihood of the root cause. The team determined the likely root cause of both failures was the marginal load capacities of the original bearing (subject of the initial start-up failure) and of the reengineered bearing (subject of the second start-up failure).
This thrust bearing, from the 6.6-MW Cataract plant, failed during start up of the rehabilitated unit in June 2005. The oil reservoir of the bearing was filled with babbitt as a result of severe wiping of the bearing.
It has been widely reported that two-piece babbitt bearings on spring beds in hydro service have lower load-bearing capacity than more modern independent pad bearings.1,2 In the case of the thrust bearing at Cataract, calculations indicate that the design load is within 10 percent of the limit for babbitt, which is generally accepted to be 400 pounds per square inch (psi).3 With such a small margin between the design load and the load limit for babbitt, along with other factors at the station – including the situation with AAR that will progressively increase the amount of misalignment – FPL Energy’s focus moved away from refurbishing the existing two-piece spring-bed bearing to retrofitting the unit by installing a higher-capacity bearing.
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