For several years, Grant County Public Utility District (PUD) struggled with high temperatures – and subsequent failures – of the thrust bearings of two units at its 907-MW Priest Rapids hydro project. To solve the problem, the utility installed pressure transducers to measure load on the bearings, then used the measurements to more accurately adjust the bearings’ position.
Project background
The Priest Rapids project is on the mid-Columbia River in Washington. The powerhouse is equipped with ten vertical Kaplan turbines. Each umbrella-style generator is rated at 95 MW. Unit commissioning started in 1959 and was completed in 1961.
English Electric designed and manufactured the thrust bearing assembly for each unit. This assembly consists of ten tilting pad shoes supported with an equalizing table. Each shoe is pre-loaded with an adjusting screw. The thrust runner is in two pieces bolted together, and then bolted to a thrust collar. The thrust collar is shrunk fit 0.05-inch onto the generator shaft.
During start up and commissioning, it was necessary to scrape a depression in the babbitt to prevent overheating of the bearings. The scrape patterns evolved through the years with minor bearing wipes, developing their own patterns of areas needing a depression scraped. Often, a bearing set would require a wear-in period and more than one scraping before the temperatures settled. As a result of frequent high temperatures in the thrust bearing assembly, Grant County PUD conducted an inspection and scraping every four years.
Modifying the thrust runner and bearing
In 1995, after several bearing failures, PUD engineers decided to investigate alternatives to hand scraping for solving the overheating problem. On two units they measured bearing movement relative to the support, pressure at the high lift port, and temperature distribution across the leading and trailing edges of the thrust bearing shoe. From an analysis of the test results, the engineers concluded the oil wedge between the thrust runner and thrust bearing (also referred to as an oil film) pressurized the split between the two thrust runner halves, causing it to open slightly. This opening – aided by centrifugal force – allowed oil to flow out the end of the split. The oil leak resulted in unequal load on the thrust bearing, allowing the outer radius of the bearing to move up and briefly make contact with the thrust runner. This contact caused the temperature to rise on the outer radius and across the top of the thrust bearing shoe. As a result of the temperature increase, the bearing deformed into a crowned shape; this caused the top of the bearing to wipe if a depression was not scraped into it.
PUD engineers conducted testing of the thrust bearings and thrust runner to resolve the overheating and failure issues. During data collection, a physical observation of the thrust runner split leak confirmed this unusual phenomena. PUD crews reported that, when the unit was rotated manually (with the high-pressure lift system energized), a stream of oil “squirted” out the split in the thrust runner. Fretting corrosion between the thrust runner and thrust collar near the runner split was attributed to the split becoming pressurized and moving the thrust runner slightly.
As a result of the analyses, the thrust runner on every unit was removed and the thrust runner split machined to achieve a tight fit. The connecting bolts on each thrust runner were shortened to provide more resistance to flexing. All the thrust bearing sets were machined flat without the scraped depression. Another modification involved replacement of a threaded plug for the high-pressure lift system port with a plug that was welded flush. Additionally, all the radial anchor grooves – which were intended to help hold the babbitt in place but can be a source of bonding problems – were machined flat.
From 1995 to 2004, as a result of the modifications to the thrust runner and bearing, PUD was able to discontinue the previously described four-year cycle of scraping, inspection, and maintenance on the thrust bearings. The number of forced outages caused by thrust bearing problems improved from an average of almost one unplanned outage a year to one unplanned outage every three years.
Recent failures
Then, in December 2004, a bearing failure occurred on Unit 1. Three of the ten bearing shoes had sections of babbitt completely removed. This failure was attributed to babbit bond failure on an older rebabbitting process. In the areas of babbitt removal, the bearing shoe had no tin left. The babbitt was previously bonded to the shoe with a trimetal copper process and anchor grooves.
In May 2005, the Unit 1 thrust bearing failed again. A visual inspection showed the babbitt contained fatigue cracking on all the bearings shoes. PUD staff discovered three of the eight bolts between the thrust runner and thrust collar had broken. They also found a crack in the thrust runner split joint, resulting from the additional stress caused by the broken bolts. Powertech Labs performed material failure analysis, consisting of micrographs and scanning electron micrograph pictures. This analysis showed that the three bolts failed in tension due to fatigue.
After this second failure, PUD staff disassembled the entire bearing support mechanism in Unit 1, including the equalizing table, to inspect it for wear or failures. On the surfaces of the thrust runner and thrust collar where they face each other, staff found excessive fretting corrosion. The corrosion caused the thrust collar to be out of tolerance.
To fix the problem, PUD staff would need to machine the thrust collar in place. However, staff concluded in-place machining would be too risky. It would require design and fabrication of a custom machine tool. The high original tolerances for flatness on the thrust collar would be difficult for a custom machine tool. If the flatness tolerance was degraded further during an in-place machine operation, complete disassembly would be required to repair the damage.
The next option – to dismantle the unit even further to machine the thrust collar using a standard available machine tool – was too costly. This option would result in lost power generation from the unit for about two months.
The PUD ultimately decided to replace the thrust bearing runner with a spare and put the unit back in service. The PUD decided the damaged and out-of-tolerance thrust collar would not be repaired at this time.
Before placing the unit back on line, the PUD added monitoring instrumentation to provide feedback on the operating characteristics. PUD personnel placed a resistance temperature detector (RTD) inside the six thrust bearings with no monitoring instrumentation. The PUD already used these RTDs on four of the bearings to measure temperature in the thrust bearing shoe.
Project background
The Priest Rapids project is on the mid-Columbia River in Washington. The powerhouse is equipped with ten vertical Kaplan turbines. Each umbrella-style generator is rated at 95 MW. Unit commissioning started in 1959 and was completed in 1961.
English Electric designed and manufactured the thrust bearing assembly for each unit. This assembly consists of ten tilting pad shoes supported with an equalizing table. Each shoe is pre-loaded with an adjusting screw. The thrust runner is in two pieces bolted together, and then bolted to a thrust collar. The thrust collar is shrunk fit 0.05-inch onto the generator shaft.
During start up and commissioning, it was necessary to scrape a depression in the babbitt to prevent overheating of the bearings. The scrape patterns evolved through the years with minor bearing wipes, developing their own patterns of areas needing a depression scraped. Often, a bearing set would require a wear-in period and more than one scraping before the temperatures settled. As a result of frequent high temperatures in the thrust bearing assembly, Grant County PUD conducted an inspection and scraping every four years.
Modifying the thrust runner and bearing
In 1995, after several bearing failures, PUD engineers decided to investigate alternatives to hand scraping for solving the overheating problem. On two units they measured bearing movement relative to the support, pressure at the high lift port, and temperature distribution across the leading and trailing edges of the thrust bearing shoe. From an analysis of the test results, the engineers concluded the oil wedge between the thrust runner and thrust bearing (also referred to as an oil film) pressurized the split between the two thrust runner halves, causing it to open slightly. This opening – aided by centrifugal force – allowed oil to flow out the end of the split. The oil leak resulted in unequal load on the thrust bearing, allowing the outer radius of the bearing to move up and briefly make contact with the thrust runner. This contact caused the temperature to rise on the outer radius and across the top of the thrust bearing shoe. As a result of the temperature increase, the bearing deformed into a crowned shape; this caused the top of the bearing to wipe if a depression was not scraped into it.
PUD engineers conducted testing of the thrust bearings and thrust runner to resolve the overheating and failure issues. During data collection, a physical observation of the thrust runner split leak confirmed this unusual phenomena. PUD crews reported that, when the unit was rotated manually (with the high-pressure lift system energized), a stream of oil “squirted” out the split in the thrust runner. Fretting corrosion between the thrust runner and thrust collar near the runner split was attributed to the split becoming pressurized and moving the thrust runner slightly.
As a result of the analyses, the thrust runner on every unit was removed and the thrust runner split machined to achieve a tight fit. The connecting bolts on each thrust runner were shortened to provide more resistance to flexing. All the thrust bearing sets were machined flat without the scraped depression. Another modification involved replacement of a threaded plug for the high-pressure lift system port with a plug that was welded flush. Additionally, all the radial anchor grooves – which were intended to help hold the babbitt in place but can be a source of bonding problems – were machined flat.
From 1995 to 2004, as a result of the modifications to the thrust runner and bearing, PUD was able to discontinue the previously described four-year cycle of scraping, inspection, and maintenance on the thrust bearings. The number of forced outages caused by thrust bearing problems improved from an average of almost one unplanned outage a year to one unplanned outage every three years.
Recent failures
Then, in December 2004, a bearing failure occurred on Unit 1. Three of the ten bearing shoes had sections of babbitt completely removed. This failure was attributed to babbit bond failure on an older rebabbitting process. In the areas of babbitt removal, the bearing shoe had no tin left. The babbitt was previously bonded to the shoe with a trimetal copper process and anchor grooves.
In May 2005, the Unit 1 thrust bearing failed again. A visual inspection showed the babbitt contained fatigue cracking on all the bearings shoes. PUD staff discovered three of the eight bolts between the thrust runner and thrust collar had broken. They also found a crack in the thrust runner split joint, resulting from the additional stress caused by the broken bolts. Powertech Labs performed material failure analysis, consisting of micrographs and scanning electron micrograph pictures. This analysis showed that the three bolts failed in tension due to fatigue.
After this second failure, PUD staff disassembled the entire bearing support mechanism in Unit 1, including the equalizing table, to inspect it for wear or failures. On the surfaces of the thrust runner and thrust collar where they face each other, staff found excessive fretting corrosion. The corrosion caused the thrust collar to be out of tolerance.
To fix the problem, PUD staff would need to machine the thrust collar in place. However, staff concluded in-place machining would be too risky. It would require design and fabrication of a custom machine tool. The high original tolerances for flatness on the thrust collar would be difficult for a custom machine tool. If the flatness tolerance was degraded further during an in-place machine operation, complete disassembly would be required to repair the damage.
The next option – to dismantle the unit even further to machine the thrust collar using a standard available machine tool – was too costly. This option would result in lost power generation from the unit for about two months.
The PUD ultimately decided to replace the thrust bearing runner with a spare and put the unit back in service. The PUD decided the damaged and out-of-tolerance thrust collar would not be repaired at this time.
Before placing the unit back on line, the PUD added monitoring instrumentation to provide feedback on the operating characteristics. PUD personnel placed a resistance temperature detector (RTD) inside the six thrust bearings with no monitoring instrumentation. The PUD already used these RTDs on four of the bearings to measure temperature in the thrust bearing shoe.
Figure 1: Grant County Public Utility District connected pressure transducers to each of ten bearings in Unit 1 at its Priest Rapids hydro project to measure oil pressure at the high-pressure lift port.
In addition, they connected a pressure transducer to each bearing to measure the oil pressure at the high-pressure lift port, as shown in Figure 1 on page 72. The end nut of the bearing was drilled and tapped to allow the pressure at the port of the high-pressure lift system to be measured while the unit was on line. This modification does carry the risk of developing leaks between the oil port and the pressure transducer. A significant leak in the tubing could lead to the loss of the oil wedge and result in a bearing wipe. The risk was minimized by careful installation and pressure testing of all the tubing.
Grant County PUD had one month to assemble the unit and put it into operation, to meet future power demand. The short time available prohibited the use of a less risky, more traditional method of measuring the pressure, such as load cells or submersible transducers installed close to (but not on) the bearing.
In September 2005, the PUD began placing Unit 1 back in service. First, the thrust bearings were pre-loaded with the adjustment screw that holds the thrust bearing shoe up against the thrust runner, according to standard torquing procedures. However, two of the bearings did not leak oil out of the edges of the bearing with the high-pressure lift system energized. If a bearing shoe does not leak oil, the load on that particular bearing is too high. In addition, the unit would not rotate manually.
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