[bearings and seals
BEARINGS AND SEALS
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!
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, 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 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.
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, says Peter Bakker with Trelleborg. This can include retrofitting an existing unit or equipping a newly designed turbine, Bakker says.
