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 associate editor of HRW
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.
HRW 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 throughout the world.
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!
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.1
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 in Spokane, Washington, USA, in July 2009.2
The plant, an eight-unit, 800-MW facility, operates with frequent start ups 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 degrees Celsius (C), compared with 71° C for another unit with a babbitt-faced bearing.
HRW's editors are looking for other innovative and cost-effective alternatives to babbitt as the facing material for thrust bearings. If you have applications to share, please e-mail them to: firstname.lastname@example.org.
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, Washington, USA, in July 2009.3
While project owners and operators often take proper operation of bearings and seals for granted, if a problem occurs, this can create significant costs and unit downtime.
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 on a 45-MW pump-turbine at the Le Truel plant in France (bearing has been operating for more than 20 years, with high reliability) and on two units at the 48-MW Lake Chelan hydro plant in Washington State in the USA (first application of this technology in the United States).4
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.
HRW magazine's editorial staff found a software program that can be used to determine the required clearance for bearings made using Orkot composite materials.5 The software is offered by Trelleborg Sealing Solutions in Trelleborg, Sweden.
|On these gate arms, the hydro project owner is using self-lubricated spherical bearings embedded with a polytetrafluoroethylene lubricant. These bearings are resistant to corrosion and do not present concerns regarding oil leakage into the water.|
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, says Peter Bakker with Trelleborg. This can include retrofitting an existing unit or equipping a newly designed turbine, Bakker says.
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., United States, 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 says.
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 degrees Celsius (C) to 40° C.
Solving leakage problems on turbine 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.
In a quest to discover what is being done at hydro plants to control leakage from turbine shaft seals, HRW magazine's editorial staff 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, Washington, USA, in July 2009, outlines the advantages of the elastic polymer material.6
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.
HRW's editors are looking for other innovative and cost-effective approaches to controlling leakage around seals on turbine shafts. If you have ideas to share, please e-mail them to: email@example.com.
Using CFD to design labyrinth seals
Regarding seal design, we found an interesting application of computational fluid dynamics (CFD) modeling to assess the design of a labyrinth seal for a vertical Francis turbine in a hydro plant in Iran. Engineering firm Farab was faced with the task of identifying how to decrease power loss in the unit. Much of the loss was being caused by unsatisfactory performance of the labyrinth seals.
The firm used CFD modeling to find a labyrinth seal design that would minimize leakage flow and shear stress.7 The model was precise, including the ideal number, shape, and position of the seal grooves and several non-dimensional ratios, such as height to seal positioning diameter on the runner. Farab modeled nine different seal geometries for this application. The plant owner can use the results of this modeling to improve the design of the labyrinth seal on this turbine.
According to Farab engineers Cyrus Raghaghi and Ramin Mirian, CFD modeling allows for precise modeling of many different seal design variations. The tool can be a cost-effective method of refining a specific design before installation.
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.
|A common challenge at many hydro plants — no matter the size or location — is water leaking from the seal around the turbine's shaft. One solution to this challenge is using a seal with a face made of elastic polymer instead of carbon.|
In looking for ways to reduce the time required to retrofit seals, HRW magazine's editorial staff discovered a solution. The E.A.S.-S.E.E. Seal, offered bySealogic 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.
1Glavatskih, Sergei B., Michal Wasilczuk, and Michel Fillon, “Unique Performance Aspects of PTFE-Lined Thrust Bearings,” HRW, Volume 13, No. 6, December 2005, pages 32-37.
2Glavatskih, 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.
3Gilson, 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.
4Roy, 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.
5Bakker, Peter, “New Methods Optimise the Performance of Composite Bearings in Hydropower Applications,” Waterpower XVI Conference Proceedings CD-Rom, Pennwell Corporation, Tulsa, Okla., 2009.
6Ren, 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.
7Raghaghi, Cyrus, and Ramin Mirian, “Geometry Characteristics Effects on Labyrinth Seals Performance,” Waterpower XVI Conference Proceedings CD-Rom, Pennwell Corporation, Tulsa, Okla., 2009.