Hydro Tech was called to the Dolgeville Generating Station in January 2018 to review the history of seal failures that occurred from 1987 through 2017. The objective was to see if there was an engineering solution available to keep seals functioning for a reasonable time frame before needing to be replaced.
This paper outlines the process of improving a turbine shaft water seal to offer a solution that has eluded power plant owners for decades. Lignum Vitae proved to be a tough and reliable selection to solve marginal lubrication problems in horizontal Kaplan water turbines.
Read the full paper here.
By chloecox -9.1.2010
Innovative approaches to designing, installing, retrofitting, and operating two major components of a hydroelectric plant are saving facility owners time and money.
By Elizabeth A. Ingram and Russell W. Ray, Hydro Review magazine
Bearings and seals are two primary components of hydro turbine systems. While project owners and operators often take these components for granted, if a problem occurs, this can create significant costs and unit downtime.
Hydro Review recently went on a quest to find solutions to some common challenges plant owners and operators experience with bearings and seals. From our search, we found a number of alternative materials, design tools, and new types of components that are being used in hydroelectric plants.
The following examples are meant to be just that: examples. The intent is not to be comprehensive. In fact, we hope these examples prompt readers to share other innovations and good ideas with us!
Here are few examples of some of the bearing problems hydropower producers can face and the remedies that were used used to fix them.
Bearing lubrication: water over oil
Using water instead of oil as a lubrication for guide bearings provides obvious environmental benefits, including eliminating the risk of river pollution as a result of oil leakage. Throughout the world, more than 20 hydro units are equipped with hydrostatic water guide bearings, representing more than 124 years of cumulative operating time. These bearings use filtered, pressurized water from the penstock — instead of oil — to lubricate and cool the bearing.
Hydrostatic water guide bearings contribute to overall plant efficiency by reducing friction losses by about 50 percent compared with oil bearings, according to Alstom Hydro engineers Philippe Gilson, Stephane Roy, Jean Doyon and Emmanuel Godoc, who authored a technical paper written for the Waterpower XVI conference in Spokane, Wash., in July 2009.1
With regard to reducing operation and maintenance costs, the authors say hydrostatic bearings have a higher bearing stiffness and proximity to the runner, both of which reduce vibrations. This reduces labyrinth wear because the shaft movements are attenuated. Thus, maintenance is limited to the water supply system, the authors say.
The Waterpower paper provides examples of installations of this type of bearing, including two units at the 48-MW Lake Chelan hydro plant in Washington State (first application of this technology in the U.S.).2
PTFE: alternative to babbitt for thrust bearing facings
The use of polytetrafluoroethylene (PTFE) composite for the facing of a turbine thrust bearing, as an alternative to white metal (babbitt), continues to receive attention. This material is especially attractive for use in equipment subjected to severe operating conditions. Units with PTFE-faced thrust bearings have been in operation for more than 30 years at hydro plants in Europe and Asia. There are more than 1,000 PTFE thrust bearings installed throughout the world.
Users of the PTFE composite point to a number of advantages over babbitt: low coefficient of friction, broad temperature range, excellent anti-seizure properties, superior resistance to chemical attack and moisture, a thermal conductivity about 170 times lower than that of babbitt, increased thrust bearing load carrying capacity compared with babbitt, and improved tolerance to misalignment and distortion.3
A recent installation of PTFE-faced thrust bearings in Syria is described in a technical paper written by Sergei B. Glavatskih of Lulea University of Technology in Sweden for the Waterpower XVI conference.
The plant, an eight-unit, 800 MW facility, operates with frequent startups and shutdowns. This type of operation led to elevated oil bath and bearing temperatures and frequent thrust bearing failures. To solve the problem, the plant owner replaced the thrust bearing facings with a PTFE composite. Thermocouples were placed in the PTFE layer at the PTFE-oil film interface to measure oil film temperature. Tests carried out to commission the bearings indicated that the temperature of the PTFE pad was 42 C, compared with 71 C for another unit with a babbitt-faced bearing.
Installing composite bearings: tool for determining required clearance
Replacing traditional bearings that rely on grease for lubrication with “greaseless” composite bearings is an attractive alternative for many hydro project owners. Use of these bearings avoids environmental concerns related to leakage of oil-based lubricating fluids. However, one potential concern is the larger running clearance required for a composite bearing than for traditional bronze bearings.
Hydro Review’s editorial staff found a software program that can be used to determine the required clearance for bearings made using Orkot composite materials.4 The software is offered by Trelleborg Sealing Solutions in Trelleborg, Sweden.
Here’s how the software works: to determine the smallest running clearance for a given radial load, personnel enter dimensions of various unit components (shaft diameter, housing diameter, bearing length, radial load, and projected bearing pressure), as well as machining tolerances and design load. The software then produces two calculations: the minimum required clearance and an “optimized” clearance.
By providing accurate running clearances, this software program allows hydro project owners to optimize the clearance on an Orkot composite bearing for a specific application, said Peter Bakker with Trelleborg. This can include retrofitting an existing unit or equipping a newly designed turbine, Bakker said.
CIP Composites used to improve fish passage at John Day Dam
The Northwest Power and Conservation Council called on the U.S. Army Corps of Engineers to develop juvenile fish passage plans on the lower Snake and Columbia rivers. The council amended the program to call for dam-by-dam case studies to “determine the most efficient level of bypass spill to maximize passage efficiency and fish survival,” according to the council.
John Day Dam was included in the case studies. Officials determined a new fish ladder design would be necessary after discovering the fish at John Day were not moving through the ladder as fast as expected. In many instances, the fish would “fall back,” or turn back to the beginning and start again. The problem was isolated to the upper third of the ladder, which was redesigned.
The original design was a pool and weir system (a series of fixed concrete weirs). The fish had to jump over a series of small dams and pools to move upstream.
The new design, which was completed in February 2010, incorporated a new fish-friendly shape and vertical-slot fish passage. The new design included a narrow slot near the channel wall, allowing the fish to swim upstream without leaping over the fixed weirs. The weirs now have the capability to open and close by the control of actuators and regulate the water levels more efficiently throughout each season.
The weirs rotate by actuators connected to a shaft held in place by two or three bearings, depending on the length of the shaft in its section. The project specifications called for self lubricated, greaseless composite bearings, inferring the avoidance of grease near waterways. In addition, the project included a section of wear plates. The wear plates were required to have higher creep resistance and load capabilities than the previous design. Transco Inc., the subcontractor of the project, chose CIP Composites as the material to supply for the bearings and wear plates for the fish ladder.
Transco chose CIP Composites over other greaseless bearing materials because they had “good luck and past experience with CIP,” and they met all the required specifications for the project. CIP provided Transco with the finished, machined bearings and wear plates.
CIP Composites, manufactured in Eugene, Ore., by Columbia Industrial Products (CIP), are custom laminate composite materials made of polyester resins and polyester textiles incorporated with solid lubricants. For the fish ladder project, CIP provided Transco with their CIP 151A material which includes solid lubricants (moly and PTFE) in addition to their proprietary additive, Enhancement A.
Operating dry or submerged in water, the self lubricating bearing material does not require external lubrication. Along with its low coefficients of friction, it has high load capacities with capabilities to withstand high shock loading and edge loading.
This allows the material to be stable where any misalignment may become present and because it is a 100 percent bearing material with no fiberglass or metallic shell. The material will not corrode, is light weight and will not absorb water. The resistance to wear and life expectancy of the material allowed the subcontractor, Transco, to be certain it will meet and exceed the project specifications and requirements.
Using water-lubricated bearings
The use of biologically degradable lubrication in turbine main bearings is better for the environment. However, it can lead to operational breakdowns and reduced availability because these types of lubricants react hydrolytically in the presence of water.
Water lubrication is the best way to solve the ecological problem and the lubrication problem, according to Federal-Mogul Deva GmbH.
Deva.metal main bearings, a product of Federal-Mogul Deva, evoke a protective graphite film on the shaft through their self-lubricating characteristics. The graphite film avoids excessive wear during solid component friction.
Federal-Mogul Deva conducted calculations and test rig examination, which showed that reliable operation with water lubrication is possible. The application has been used in more than 100 power stations for more than 20 years.
The disadvantage of using water lubrication arises mainly from its low viscosity in comparison with other lubricants such as mineral oil. Only a relatively thin lubricating film exists, which is stripped away at high surface pressures and low speeds.
It is possible to compensate for this lubricating disadvantage by improving the self-lubricating characteristics of the materials used to make sliding bearings.
The Lignum Vitae Supply Project
Lignum vitae is a dense tropical wood with a unique combination of mechanical and physical properties that make it ideal for underwater bearing applications.
The history of its use and exploitation spans hundreds of years and includes ancient pumping devices, water wheels, the first Francis turbines and the water lubricated bearings for Thomas Edison’s first hydroelectric plant in 1882.
Lignum vitae is used in hundreds of older hydroelectric plants. However, demand for a slow growing, over-exploited tropical wood will one day exceed supply. Until then, there is the Lignum Vitae Supply Project, established by Bob Shortridge, a wood craftsman and founder of Dreaming Creek Timber Homes.
Lignum vitae was only available in small quantities from widely scattered sources. Shortridge has stockpiled the a quantity of military industrial grade lignum vitae, doing business as Lignum-Vitae.com.
“Lignum vitae is not just another commodity,” Shortridge said. “We all benefit when this wood is set aside for its highest and best use in hydroelectric and other core industries.”
About 30 hydroelectric plants are using blocks from Lignum-Vitae.com. The operators and engineers of these plants typically speak of 20 to 30 years of service from their lignum vitae bearings, depending on conditions.
Using self-lubricated bearings on radial gates
Choosing the proper bearing material and design for gates at hydro facilities can be challenging for several reasons.
First, these gates are subjected to high specific loads so the bearings must be of sufficient strength to withstand this load. Second, gates may have long periods with no operation, which makes them susceptible to corrosion. Third, use of an oil-based product to lubricate the gate’s bearings poses risks of leakage.
To address these challenges, Empresas Publicas de Medellin (EPM) decided to install DB spherical bearings manufactured by GGB of Thorofare, N.J., at its 660 MW Porce 3 plant in Colombia.
DB bearings are self-aligning spherical bearings that feature a stainless steel inner ring and an axially-split aluminum-bronze alloy outer ring embedded with a polytetrafluoroethylene (PTFE) lubricant.
The bearings have a low coefficient of friction, good wear resistance, long service life, and corrosion resistance, GGB said.
The dam at this project, which will be commissioned in 2010, features an open-channel spillway controlled by four radial gates.
These gates are 11 meters high by 14 meters wide.
For this application, the bearings will be subjected to radial loads as high as 12,000 kilo-Newtons.
When the gates open and close, the bearings will rotate 70 degrees in 90 minutes at operating temperatures of 0 C to 40 C.
The following are examples of some of the challenges involved in fixing a bad seal and the techniques that were used to prevent the problem from recurring:
Solving leakage problems on main shaft seals
A common challenge at many hydro plants — no matter the size or location — is water leaking from the seal around the turbine’s shaft. On a turbine with a shaft diameter greater than 1 meter, industry experts say it is nearly impossible for the main shaft seal to completely eliminate leakage. Instead, the seal functions more to control leakage to an acceptable amount.
Hydro Review took a look at what was being done at hydro plants to control leakage from turbine shaft seals and uncovered some innovative approaches.
One such approach is the use of a seal with a sealing face made of elastic polymer instead of carbon. Thordon Bearings Inc., headquartered in Burlington, Ontario, Canada, manufactures the seal, called a Thordon SXL seal. A technical paper written by Thordon and presented at the Waterpower XVI conference in Spokane, Wash., in July 2009, outlines the advantages of the elastic polymer material.5
According to the paper, the material can be machined to the required size, up to 4 meters in diameter. The elastic polymer exhibits good abrasive resistance. Solid particles trapped between wear surfaces do not imbed into the material. Instead, they deform the polymer surface locally and roll between sealing faces until they escape the seal.
In the paper, authors Dr. Guojun Ren and Ken Ogle share an example in which installation of an elastic polymer seal on the shaft of a large vertical Francis turbine significantly reduced leakage and wear.
Before the installation, the shaft featured a segmented carbon axial seal with an average diameter of 4 meters. Seal leakage was about 140 liters per minute, and the segments were suffering from severe abrasive wear. Thordon installed the new seal, and the turbine was restarted. Over the next year, plant personnel monitored cooling water pressure, water supply flow, leakage rate, and temperature increase.
All results were satisfactory. Leakage past this new seal ranged from 10 to 90 liters per minute — reducing leakage by more than a third. At the end of the monitoring period, personnel dismantled the turbine for inspection. The sealing face was clean, and measurements indicated only one small section of the seal had worn by 0.1 millimeter.
Fugesco provides solution to mechanical seal problem
A pumping storage plant in southern California had a problem. The plant’s waterways are exposed to the desert sand blowing into the canals.
This sand and silt end up in the pumps. As a result, the pumps’ spring-energized, conical wedged mechanical seals were being destroyed within three months of start up. As a result, the pit would flood out.
The existing seal was not the right solution for this particular application. Canadian seal manufacturer Fugesco, known as a leader in mechanical seal technology, was asked to go on site and find a solution for this ongoing situation.
The Fugesco crew analyzed the water conditions below the seal, the water supply to the seal, the pressures below the seal as well as the access to the seal area for assembly and disassembly of the seal unit without having to dismantle the entire pump turbine. A Fugesco axial type F5000 was installed.
But the design alone was not the cure. The material selection was key. This model has an innovative combination of face materials that can withstand severe applications. By blending a mix of conventional metals and composite materials, Fugesco was able to resolve the customer’s problem. The seal has been running for three years with no problems.
Choosing the right seal
Some faults in hydropower stations are obvious. Others remain hidden until the very last moment. But whatever the problem, there is often a direct or indirect link to some kind of seal.
The cause of these problems and a whole variety of others is the type of seal design that has normally been used, said Frank Knoefel, technical director at IDG-Dichtungstechnik GMBH. Many of the problems often lead to repair work or even a permanent overhaul, Knoefel said.
Plaited cords, elastomeric profile rings, molded seals, and simple O-Rings are commonplace. Such seals can withstand neither extreme contamination of the water nor the vibration. The seals and their mating surface wear each other away. Sometimes abrasion of the material leads to the total disappearance of the seal. Simply fitting a new seal will seldom be sufficient.
“At the very least, the surfaces will have to be refashioned or redressed or, at worst, whole components replaced,” Knoefel said. “Problems of this nature can be avoided by using seals that differ from conventional types in respect of their geometry and the materials employed.”
Knowing the problem is the first step. Informing the manufacture is the second step, Knoefel said. A technical designer of components should also be informed about the limit advantages and disadvantages of a seal. The designer could make a minor change to material or profile to get the seal functioning properly.
Sealing system reduces leakage around turbine shaft
To reduce water leakage within the turbine-generating unit of its 5-MW Foyers Falls hydro plant In Scotland, Scottish & Southern Energy PLC in Scotland retrofitted the 12-inch-diameter main shaft with a split-type HydroSele cartridge seal from James Walker & Co.
The seal – which features two elastomer-based rotary sealing elements working back-to-back with flush water introduced between them instead of a face sealing arrangement typically used by mechanical seals – solved the leakage problem.
When the Foyers Falls unit was constructed in 1968, the shaft was outfitted with a pair of internally mounted mechanical seals of the spring-retained segmented carbon ring type, plus an outboard labyrinth system. Over time, sand and peat in the water caused excessive wear to these seals. In addition, the turbine shaft was becoming eroded and scored around the labyrinth system. Water was spraying out of the seal housing toward the generator. The utility was replacing the carbon seals every 12 months.
By 1996, the utility decided to replace the mechanical seals with the HydroSele system. The feature of the system is the way its two sealing elements operate within their housings. The two elements work back-to-back, with filtered water flushed between them at 30 pounds per square inch above the water pressure at the sealing gland.
The cartridge is a bolt-on unit that incorporates the housings for the two sealing elements and the flush area between them. Because the system is modular, each component can be designed and precision manufactured to fit together around a specific turbine shaft.
Scottish & Southern chose the HydroSele system, in part, because its modular concept and custom design minimizes installation time. What’s more, by installing the seal system, the utility could reduce seal maintenance requirements to intervals greater than five years.
Two engineers with James Walker installed the sealing system at the Foyers Falls plant in less than two days without mechanical lifting gear. The new sealing system reduced leakage to three-quarters of a gallon per hour, thus eliminating the problem of water spraying toward the generator.
Retrofitting a seal without tearing down the unit
When retrofitting mechanical seals on turbine shafts, the time required to disassemble and/or modify the machine can be costly in terms of lost production and labor. Depending on the size of the unit and the amount of time it typically runs, disassembling the unit to replace a seal can involve hundreds of man-hours and several hundred thousand dollars in outage losses.
In looking for ways to reduce the time required to retrofit seals, Hydro Review’s editorial staff discovered a solution. The E.A.S.-S.E.E. Seal, offered by Sealogic Innovations Corporation, can be installed without disassembling or modifying the unit. This seal is a fully split mechanical face seal; it can be used as an external main shaft seal on horizontal or vertical units or as the internal submerged bearing seal upstream of the runner on S-type and bulb turbines.
The design of this seal incorporates two “floating” faces (not subjected to any mechanical stresses) that self-align into the plane that is precisely perpendicular to the shaft’s axis of rotation, says Kevin Drumm with Sealogic. This means the seal does not “flex,” or oscillate during revolutions, minimizing wear and spring fatigue, he says.
The seal can be designed to accommodate axial movements in both directions, as well as radial movements, of 0.25 inch. It also has minimal requirements for micro-filtered flushing water and can run dry.
Gilson, Philippe, Stephane Roy, Jean Doyon, and Emmanuel Godec, “Hydrostatic Water Guide Bearings: Making Environmental Technology Profitable!” Waterpower XVI Conference Proceedings CD-Rom, Pennwell Corporation, Tulsa, Okla., 2009.
Roy, Stephane, Jean Doyon, Vincent De Henau, and Steve Sembritzky, “A Rehabilitation Scheme for the Lake Chelan Hydroelectric Project Based on Schedule Reduction,” Waterpower XVI Conference Proceedings CD-Rom, Pennwell Corporation, Tulsa, Okla., 2009.
Glavatskih, Sergei B., and G.A. Paramonov, “PTFE-Faced Bearing Technology: Advantages and Practical Examples,” Waterpower XVI Conference Proceedings CD-Rom, Pennwell Corporation, Tulsa, Okla., 2009.
Bakker, Peter, “New Methods Optimise the Performance of Composite Bearings in Hydropower Applications,” Waterpower XVI Conference Proceedings CD-Rom, PennWell Corporation, Tulsa, Okla., 2009.
Ren, Guojun, and Ken Ogle, “Hydro-Turbine Main Shaft Axial Seals of Elastic Polymer – Principle and Practice,” Waterpower XVI Conference Proceedings CD-Rom, Pennwell Corporation, Tulsa, Okla., 2009.
This article originally appeared in the July 2010 issue of Hydro Review magazine. Elizabeth Ingram and Russell are associate editors at the magazine.
By BearingNews -August 10, 2013
Lignum Vitae North America has been awarded a contract to supply a water-lubricated main guide bearing for the 574.54-MW Conowingo hydropower project in Maryland.
Located on the Susquehanna River, Conowingo Dam was completed in 1928 and is one of the largest non-federal hydroelectric plants in the United States.
Lignum Vitae said the conversion to its water-lubricated main guide bearings will provide “maximum uptime and lower maintenance cost.”
Bob Shortridge and Phil Thompson talk about Lignum Vitae’s work at the Osage hydropower project, qualities that make the ‘its self- and water-lubricated bearings unique, and the advantages the company’s wooden bearings can offer over synthetics.
Lignum Vitae Bearings
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.
Staring down the shaft of yet another failed bearing at the worst possible time is nowhere an engineer wants to be.
So, what goes into the decision-making process to select the right water lubricated bearing or seal?Unfortunately, there is no chart available showing bearing wear rates to help guide operators to the most reliable option. The process can be complicated when considering cost, availability, shaft conditions, water conditions, alignment issues, thermal expansion, marine growth, and environmental impact.
When it comes to ships, operators have several options: Babbitt white metal that require seals and oil, both are problematic for leaking and spilling oil. Polymers, plastics, and composites all are prone to seizures, hydrolysis and wear from crustacean invasion that turns the shells into a calcium paste causing premature wear. Lignum vitae that operates on a mixed mode of lubrication allowing the shaft to bear directly on hard cold starts, then goes hydrodynamic as speed increases and requires no oil, seals, or possibility of wiping/smearing or melting onto the shaft. This is due to mother nature’s ability of building the wood molecule by molecule cell by cell leaving no voids vs. mixing a plastic or polymer that leaves voids of structure and lubricity leading to failure.
Shipping is a conservative business that considers how reliable the bearing will be in service. Currently there are approximately 20 different polymers in play, that begs the question; How long has the bearing been proven in service? Many manufactures point to recent tests in a laboratory environment with polished samples in de-ionized water as proof their bearing will perform. Does the test replicate the real world the bearing will work in and how does any laboratory test guarantee long term performance in warm water, cold water and abrasive water? Another question is how many excuses will you accept?
Lignum Vitae North America has engineered new cost effective, easy to install systems. Our staves and stern tubes range from 4”/100mm to 50”/1300mm and larger and require no freezing or special tooling. Lignum vitae conforms to any shaft condition due to tremendous tenacity of homogeneously woven fiber surrounded by guaiac resin that will wear itself into the current shaft/journal condition. Removing and turning a shaft will become a thing of the past.
Lignum vitae has secured its position as the world’s most dependable water lubricated bearing after 168 years of flawless operation in thousands of vessels worldwide. It has been in continuous service since the first rotating shafts replaced sail vessels. In fact lignum vitae is the only water lubricated bearing that has served in pre WWI ships, every WWII ship, including Nuclear submarines, Aircraft carriers, Victory ships and has recently been recertified by ABS to qualify for future Frigates, Destroyers and Icebreakers.
Its rich history of reliability was central to the Indian Navy’s decision to use lignum vitae to extend the service life of the Aircraft carrier INS Vikramaditya after receiving 33 years’ service with the previous set of lignum vitae bearings. LVNA supplied 16 bearings weighing 5,450 lbs. for the 4 shafts of the INS Vikramaditya from purchase order to shipment in just 7 days.
There is no number of slick ads, fast names, bright colors, or flavor of the day bearing that can replace tried and true lignum vitae that has performed for nearly 2 centuries. We are proud to be leading the way with a sustainable supply of renewable lignum vitae that exceeds every environmental standard, avoids all legal, EPA and sub chapter M requirements.
If the year 2020 could be summed up in one word, it would be “uncertainty”. This word has even found its way into the most robust shipping organizations worldwide for over a year. Fortunately for ship owners, uncertainty can be replaced with reliability by adding lignum vitae to critical components such as main guide bearings.
Lignum Vitae understands our customers do not have time for a trial-and-error approach to sourcing solutions for equipment maintenance and repair especially in an emergency. Lignum vitae bearings are consistently engineered with end grain blocks known for decades of reliability in harsh conditions.
Lignum vitae has quietly and continuously been in use for more than 100 years. Bearings made from lignum vitae are the only natural water-lubricated solution with decades of continuous, reliable performance.
At the beginning of World War II, the US Navy ordered all warships to convert from oil lubricated Babbitt type bearings to water lubricated for fear of an explosion nearby capable of blowing the oil and seal from the stern tube. This lack of lubrication would render the ship helpless and vulnerable to attack. This threat was solved by using lignum vitae as a water lubricated wood bearing with nothing but seawater needed to cool and lubricate. As an added benefit, this saved tons of brass, lead and grease that could go to other uses.
Fast forward to a new battleground. With EPA regulations and Subchapter M inspections, again lignum vitae is called on to remove all environmental concerns while offering reliability that ensures your mission will be completed and the ship returns safely to port. Recent maritime articles have discussed the state of pollution in our waters caused by operational oil escaping. This is not the case with lignum vitae water lubricated bearings, stern tubes, or seals.
Lignum vitae is an extremely slow wearing component that is not prone to sudden failure or shaft seizures. The lignum vitae components routinely see 20-30 years of service as common and expected.
Lignum vitae is available in easy to install staves, tubes, and rudder stock for immediate delivery.
Let us show you how lignum vitae is the bearing material that will outperform your expectations, in every application, every time.
It pays dividends to consider the most rugged and fault-tolerant material known when looking for a retrofit or newbuild seal option.
Lignum vitae is well known as the hydro industry’s original bearing material. This high-density wood has incredible compressive strength, self-healing properties and innate lubricity from a natural resin embedded in its structure. It is a proven solution for water-lubricated bearings on everything from aircraft carriers to Hydro applications – and now, it can be used for sealing as well.
In partnership with engineering consultancy Hydro Tech, Inc., Lignum Vitae North America (LVNA) has developed a line of seals to solve problems with premature failure of the standard resin-composite materials. The project was initiated for an Upstate New York-based utility, which had a horizontal water turbine that had seen numerous resin-composite seals fail prematurely due to overheating and wear. To solve these problems, LVNA manufactured a drop-in replacement seal of about 33 inches in diameter with a brass backing, designed to fit right into the existing seal housing.
The previous seal material was failing within as little as three days and six stop/start cycles. With the new lignum vitae seal, a break-in period of 1400 hours of operation and 20 stop/start cycles caused very little wear (less than 0.08 inches), and the remaining thickness of the seal did not change after break-in.
“Lignum vitae is an anomaly of nature and is the only known wood with no silica. It has an innate lubricity originating from guaiac resin imbued in every cell and a hardness like aluminum, based on its long-chain tenacious molecular cellulose structure. The mixture of the smooth resin bound up in an extremely dense cellulose structure yields a natural bearing material with a high lubricity and a massive compression strength,” says Bob Shortridge, founder and president of LVNA. “Bearings made from lignum vitae, simply put, are self-lubricating in water and conform by self-healing. The nature of the material is inherently able to adapt to extreme and dynamic operating environments with less than perfect shafts. It is startlingly that simple.”
No synthetic material can outcompete tried and true lignum vitae in real-world bearing performance. LVNA is proud to be leading the way with a sustainable supply of renewable lignum vitae that exceeds every environmental standard and solves EPA compliance requirements. Contact us to see how the Lignum Vitae North America personalized approach can lead to the solution for your needs.
Special thanks to Paul Benecki with Maritime Executive for providing article content.