When faced with the need for an efficient bearing solution for the 242-MW Osage hydropower plant, Ameren sought an option that would extend bearing life and found bearings made of lignum vitae wood fit the bill.
By Alan Sullivan and Phil Thompson
In 1939, the world was thrust into its second great war of the new century with the German invasion of Poland. World War II would eventually involve more than 30 countries and 100 million people. To equip the massive war machine, the world’s resources were tapped. Petroleum was used for fuel, steel for ships and guns, and lignum vitae for bearings on the ships’ propeller drive shafts.
While the natural lignum vitae material was replaced long ago by a new breed of synthetic bearing materials, those of us in the hydropower industry are familiar with the name and its history. So this story is sort of like the dash you see on a grave stone between the date of birth and date of death. It is during the dash that a life was lived and the story of someone’s life unfolded. Except this story is different. The dash does not represent the time from birth to death, it represents a little-known story from birth to rebirth, a new life for an old friend with a renewed purpose.
|Workers are seen here assembling bearing shoes by adding blocks of lignum vitae wood to the frame.|
Lignum vitae has various names, including “iron wood,” but this name actually means “tree of life” in Latin because its resin has been used to treat medical conditions from coughs to arthritis. The wood is very tough and is heavier than water, so it actually sinks. It has been used for making cricket balls, croquet mallets, electrical insulators and sheaves of blocks on ships. This material’s strength and toughness also made it an excellent choice for bearings on water-lubricated shafts driving the propellers on ships, and it was even used on the USS Nautilus, the first nuclear-powered submarine.
Of particular interest to Ameren Missouri for its 242-MW Osage Energy Center is the use of lignum vitae for water-lubricated guide bearings in hydro turbines. It was the bearing material used in the world’s first hydroelectric plant in 1882 on the Fox River in Appleton, Wisc., and when Union Electric Co. built Bagnell Dam and the 201-MW (at the time of construction) Osage plant in central Missouri (from 1929 to 1931), lignum vitae was the bearing material specified by its turbine machinery designer, Allis Chalmers. Until some time after World War II, lignum vitae was heavily used for turbine bearings in hydro plants across the U.S.
Heavy demand for the material during WWII depleted much of the resource of lignum vitae trees of sufficient size from which to manufacture these bearings. The American, Japanese, British, Italian and German Navies all searched the world for the wood to support their war effort. Because the trees are extremely slow-growing, a replenishment of the lost resource was not an option. Higher prices for the material, combined with the development of good synthetic materials, resulted in a shift by the hydro industry away from lignum vitae and toward synthetic materials for water-lubricated bearings.
Synthetic guide bearing materials have generally performed well in many applications, as was the case at the Osage Plant at the Lake of the Ozarks in Missouri. That is, until new turbines were installed with increased hydraulic and power capacity, beginning in about 2007.
The higher bearing loads related to these new turbines resulted in drastically shorter guide bearing life. Even with new guide bearings and shaft sleeves, bearing life was shortened from a normal lifespan of more than a decade to just a few months. Plant staff working with the turbine manufacturer performed many tests to determine the root cause of the shortened life and eventually narrowed in on a design deficiency that was difficult and expensive to fix. Fixing the root problem would have involved the complete disassembly of several units, resulting in lost production and revenue while the necessary modifications were made. To avoid these expenses, plant personnel made a decision in the spring of 2013 to treat the symptom of high bearing wear rates in lieu of treating the root cause by trying a new, tougher turbine guide bearing material.
In addition to the problems encountered at the Osage Plant with its new turbines, problems were beginning to develop on one of the 80-year-old turbines installed at the facility in the early 1930s. Osage Main Unit 4 is an original, 1931 Allis Chalmers Francis turbine. Synthetic bearing materials have been successfully used for more than 40 years in this unit. However, during the early months of 2013, as inflows increased and Unit 4 was used for power generation, a serious problem occurred. The turbine that had been operating fine for more than 10 years experienced rapidly increasing shaft runout after only an hour of operation. The unit was taken out of service and a bearing adjustment was made to tighten the clearance. When the unit was restarted, the rapid wear of the bearing was still obvious.
In an effort to determine the root cause of the bearing failure, plant engineers visually inspected the bearing and equipped the instrumentation for performance testing. The inspection showed evidence of a rough turbine bearing sleeve and moderate cavitation on the turbine. Osage maintenance workers used wire brushes to smooth the turbine bearing sleeve. Hand working removed some surface rust and corrosion, but it was deeply pitted and could not be brought back to a proper finish. In addition to the visual tests performed by plant staff, Norconsult was contracted to conduct performance testing.
Pressure measurements were taken behind the runner band, electronic runout measurements of the shaft, and vibration signatures using accelerometers in an attempt to determine the root cause .Pressure pulsations were measured and were correlated to the runout readings and vibration data. The testing indicated that there was a higher than normal force being applied to the bearings because of a pulsating force existing on the outside of the turbine band.
The inspections and performance testing led to two hypotheses. One theory was that the turbine sleeve was too rough and it was “grinding” away the bearing material. The other theory was that the band seal clearances were too large due to years of wear and cavitation, which was allowing leakage to flow behind the turbine band. It was believed that this leakage water was causing high variable bearing loads.
A rough shaft bearing sleeve had been observed previously on this unit, so engineers at the plant could not explain why there would be such a drastic step change in bearing wear in 2013 and they tended to dismiss the first theory. Additionally, replacement or machining of the turbine sleeve had a two-month lead time, so the decision was made to pursue the second theory and close up the turbine band clearances using epoxy. The epoxy was applied to the wear ring area at the top of the turbine band to bring the clearance to an acceptable level. No action was taken on the bearing sleeve at that time.
The unit was restarted with new synthetic bearings, and runouts increased from 3 mils to more than 100 mils total indicated runout (TIR) in the first 45 minutes of operation. The unit was again shut down for inspection, which showed that the epoxy was still in place. This eliminated the excessive seal clearance theory.
Now focusing again on the sleeve roughness theory, plant engineers considered replacement of the shaft sleeve, which was estimated to cost more than $250,000. With that much money on the line and the theory still not confirmed, the plant manager asked engineers to explain why there was such a step change in bearing performance starting in early 2013.
|A completely assembled bearing shoe with lignum vitae blocks already in place is shown above.|
A suspect found
Engineers examined operational records for Main Unit 4 and found there were almost no operational hours on the unit since the spring of 2012 due to severe drought conditions. It was longstanding practice at the plant to “roll the unit” every 72 hours. A “roll” consisted of starting the unit and bringing it up to 100% speed and then taking the unit back off line, which was thought to improve the life of the oil-lubricated thrust bearings after a unit had been sitting idle for long periods. Additionally, the drought created severe water quality issues during the summer and late into the fall. The dry, low flow conditions allowed the water in the reservoir to stratify and unusually high levels of hydrogen sulfide existed in the reservoir at the level of the turbine intakes. This chemical was so prevalent that when water was passed through the turbines, hydrogen sulfide off-gassed and corroded electrical contacts in the plant, which contributed to generator start-up failures. Ameren environmental specialists confirmed that hydrogen sulfide is very corrosive to steel and that the practice of rolling the unit had likely increased the corrosion rate.
With these operational records in hand, engineers postulated that the frequent wetting of the turbine sleeve with highly corrosive water and allowing it to dry had contributed to rapid corrosion of the turbine sleeve. It is well-understood that water is not the best lubricant, but it has worked well in hydro turbines for decades. So why such a step change in bearing performance at the Osage plant? It was Norconsult’s opinion that the surface of the shaft sleeve had finally exceeded the roughness threshold such that the lubrication property of the water was not sufficient to provide adequate lubrication. They had seen rapid step-change bearing deterioration at other locations with oil-lubricated babbitt bearings. At this point, replacement of the turbine shaft sleeve seemed inevitable.
A solution to both problems – The return of an old friend
While researching suitable replacement bearing materials for the new turbines, plant staff discussed different synthetic material grades with bearing and turbine manufacturers. Osage Plant Manager Phil Thompson had previously attended HydroVision International and picked up several business cards from bearing vendors. One of the cards was from a company called Lignum Vitae Inc.
Thompson called Bob Shortridge at Lignum Vitae and they discussed the problem with the bearings in the new units. Shortridge believed the natural wood bearing would solve the problems and provided references to several other utilities using Lignum Vitae bearings. Thompson contacted personnel at about a half-dozen plants to discuss their experience with the bearings. All of the utilities had a similar story; they had originally used lignum vitae bearings at their plants but had later transitioned to synthetic replacements, a change that ultimately shortened bearing life. Each had eventually transitioned back to lignum vitae.
To the surprise of plant management, the material cost for Lignum Vitae bearings was significantly higher than the synthetic bearing material that was currently in use. During discussions related to cost, Lignum Vitae offered an extended warranty that made use of the natural material attractive despite the higher cost. Ultimately, Ameren purchased bearing blocks for Main Unit 6, one of the new high-power turbines, and bearing replacement was scheduled for May 2013. These blocks were cut on-site to fit the dove-tail shape of the bearing housing. Unlike the synthetic material that was held in the housing with tamped lead, the Lignum Vitae blocks were cut to close tolerance and were kept wet until installation so the wood remained swollen and tight in the bearing housing.
At the same time, troubleshooting work was progressing with Main Unit 4, the old Allis Chalmers Turbine. While on site assisting with the bearing build in Unit 6, Shortridge inspected the shaft sleeve on Unit 4 and felt confident that Lignum Vitae bearings would successfully operate with the existing rough bearing sleeve on that unit as well. Plant personnel were skeptical that the natural material would work any better than the synthetic material, but faced with no other alternative except leaving the unit out of service for two months while a new sleeve was manufactured, they decided to try lignum vitae on this unit as well.
On May 21, 2013, the new bearings had been installed and Unit 4 was ready for startup. The shaft was equipped with dial indicators to monitor the runout and rate of bearing wear. The unit started at about 3 mils TIR but the synthetic bearings had done the same. Everyone, with the exception of Shortridge, expected runout to gradually increase over the course of an hour, but after an hour runout had actually decreased to 2 mils. Plant operators were instructed to monitor bearing performance every 15 minutes through the night and shut down the unit if runout reached 30 mils. Plant management went home expecting to come in the next morning and find the unit off line, but to their surprise the unit was online and still running at about 2 mils.
It has been more than a year since the new Lignum Vitae bearings were installed. Both Units 4 and 6 are still running at their designed normal runout of about 3 to 4 mils, which is well under the 20 to 30 mil excessive limit, and there has been no need for a bearing adjustment.
The purchase of the new shaft sleeve for Unit 4 was canceled, saving significant dollars. Osage plant engineers have converted two more units to the lignum vitate bearings and plan to replace the remaining synthetic bearings as they wear out in the future.
Welcome back, old friend!
Alan Sullivan is a consulting engineer with Ameren Missouri and Phil Thompson is manager of plant operations at Ameren Missouri’s 242-MW Osage Energy Center.