Tech Note February 2013: Improved Journal Bearings for Nuclear Powered Turbine Generators
Improved Journal Bearings for Nuclear Powered Turbine-Generators.
Controlling Sub-Synchronous Vibration
TRI Journal Bearings use upgraded designs and materials to improve rotor vibration control, bearing performance, and bearing life for large, heavily loaded journal bearings in nuclear powered 1800 rpm turbine-generators, as well as in large fossil powered units.
TRI uses computer simulations to evaluate the parameters of heavy loaded bearings often found in nuclear powered turbine-generators. A vertically integrated process that goes from analysis and design to manufacturing and installation has resulted in an enviable experience record for both fixed bore bearings and tilt pad bearings. In the design process, TRI uses its proprietary computer programs to adjust the design in order to optimize oil film thicknesses, film pressures, bearing metal temperatures, and power losses, and at the same time, to suppress sub-synchronous rotor vibrations over the load range of the bearing application. For bearings from other manufacturers that have been damaged in service and are being repaired at TRI, we often propose design adjustments based on the results of TRI’s computer simulations.
To accommodate dynamic alignment issues, regardless of the cause, TRI generally prefers to use a 6 pad TRI Align-A-Pad® Bearing configuration for most loading conditions other than extremely heavy loading. For extremely heavily loaded bearings, particularly those with a vertically downward load angle, typical of most generators and LP Turbines, TRI prefers to use a 5pad design with load between pads. These TRI Bearings have proven to be extremely successful for resolving bearing end loading issues and rotor vibration.
TRI Bearings, both fixed bore and tilting pad, have been used in large nuclear power turbine-generators operating at 1800 rpm that initially were Westinghouse or GE, and these bearings have achieved an experience record that is extremely good and enviable.
While it is generally accepted in journal bearing design that the loading on a bearing is constant and that the alignment between the journal and bearing is fixed and uniform from end to end, these are not always good assumptions for the design of heavily loaded bearings in most large turbines and generators. Many applications involve alignment changes that occur during start-up, thermal transients, and/or other operational conditions. When alignment changes occur under heavy loads, end-loading can easily occur, perhaps only during shut-down or start-up, but when rubbing occurs, it can lead to wiping or smearing the Babbitt at an end. Sometimes the wiping is so severe when the journal penetrates the oil film and rubs the Babbitt surface that the entire Babbitt layer is over-heated and liquefied, so that a substantial amount of the Babbitt is blown away. This can permit the rotor to drop and rub the seals in the steam path.
TRI’s various bearing designs result in bearing lives that meet the typical customer’s objective of 50,000 hours between inspections.
For excessive loads, lift oil can be used for both 6-pad and 5-pad bearing configurations to provide adequate film thickness from zero to low speeds and/or to reduce the power required for turning gear operation. The addition of lift oil features to almost any bearing is not very complicated, though the details must be done correctly, and the lift oil must be highly filtered.
Many 1800 rpm nuclear powered HP Turbines have been known in the industry to exhibit sub-synchronous vibrations under certain operating and/or testing conditions. TRI Align-A-Pad ® Bearings have been installed and successfully used to suppress such sub-synchronous rotor vibrations. Today, there is no excuse for living with sub-synchronous rotor vibration because TRI understands the sub-sync vibration issues very well, and TRI designs and manufactures bearings that almost always successfully suppress this dangerous type of rotor vibration.
For decades, new TRI Bearings, both tilt pad and fixed bore, have been used in large fossil powered turbine generators that initially were Westinghouse, GE or Allis-Chalmers that operate at either 3600 rpm or 1800 rpm, and have extremely good and enviable operating records. In more recent years, a number of new TRI large diameter bearings, both tilt-pad and fixed bore, have been used in nuclear-powered 1800 rpm turbine-generators with equally good operating records. TRI Tech Notes have addressed many problematical subjects, including our interesting Tech Note on Cracks Found, and to be Found, in Westinghouse Cast Steel Bearings with Conclusions and Recommendations for Resolutions, May 2010.
The success record of TRI bearings is based substantially on the fact that TRI’s design and manufacturing operations are highly integrated vertically, which means that all features of the process from recognizing the design objectives to installing the finished products are controlled from a single point, TRI’s facility in Pennsylvania. TRI has been manufacturing new large-bore journal bearings suitable for any Large Steam Turbine/Generator now existing or contemplated in the US or elsewhere. We use our large Babbitting machine, known as “Gigantum”. TRI designed, manufactured, and built Gigantum completely internally to TRI to meet these specifications: to cast Babbitt linings in large journal bearings with sizes up to 70 inches outside diameter (which corresponds to about 40 inch bore diameter) and 40 inches long, and weighing up to 20,000 lbs. We can easily pour 500 lbs. of Babbitt, or more, in a single pour within a matter of seconds. A foremost requirement is to heat, cast, and then cool each bearing safely, and not to expose an employee to excessive heat, to liquid Babbitt, to parts that are rotating quickly, or to jetting steam. The Gigantum design has proven to meet these objectives.
The housings or backings for TRI’s new large diameter fixed bore (circular bore, elliptical bore or tilting pad bearings) are either carbon steel or alloy steel plate or forged rings, and not cast iron or cast steel.
The immediate and long term benefits of using new TRI Bearings in 1800 rpm nuclear powered turbine-generators, relative to the original bearings, include significant improvements in rotor vibration control, bearing performance, and operational life.
With the simplistic, yet very functional designs that TRI Align-A-Pad® Bearings have, TRI can offer these bearings at prices that are lower than the price of a two-part fixed bore bearing (inner bearing ring, spherical interface, and outer bearing ring). Typically, the price of a TRI Align-A-Pad® Bearing is slightly higher than a comparable fixed bore bearing, either circular bore or elliptical bore, but the TRI tilting pad bearings offer superior technical features such as dynamic alignment and rotor vibration control that are not usually available with fixed bore bearings.
TRI’s overseas markets for our American-made TRI Tilting Pad Bearings for large steam turbine-generators have expanded to Asia, and TRI’s user base has expanded to 35,000 MW of TRI Align-A-Pad® Bearings, & 15,000 MW fixed bore bearings.