It’s been a while since our last Tech Note, but TRI has released a new tech note. This installment explains how pressure dam and elliptical bore bearings are used to control sub-synchronous vibrations and then explains why TRI prefers the elliptical bore bearing to the pressure dam.
In the fall of 1998, Dr. Mel Giberson wrote an opinion piece for Turbo Machinery Magazine regarding the market deregulation of electric power generation. The points discussed nearly 20 years ago, still hold true today. The piece is available on the TRI Web site.
TRI has released a new video showing the assembly of a simple tilt pad bearing.
TRI has recently completed a pair of lift oil pumping skids. They will produce 5 gallons per minute at 3000 p.s.i. The system will reduce the wear on bearings when the unit is on turning gear. The lift oil will also reduce the load on the turning gear and turning motor.
Many rotating equipment manufacturers seat a bearing in an end wall of a bearing standard / pedestal and use the standard cover to hold the bearing in place. This design method is definitely simple and low cost, and it works when the air temperature surrounding the bearing standard / pedestal is “ambient”, i.e., not heated.
However, there are applications for which this clamping design is not very effective and even inappropriate. For applications where the wall of the standard/pedestal is heated by exposure to hot steam escaping from shaft seals of a turbine, or is exposed to the radiant heat from an adjacent hot turbine, the standard / pedestal wall grows due to thermal expansion. While the bearing inside is cooled with lube oil in the neighborhood of 130 to 160 deg F, the external heating may cause the temperature of the standard wall to increase to 250 deg F. For a fit diameter of 32 inches and a temperature differential of 100 deg F, a gap between the two grows by 0.020 inches (0.5 mm) so that the bearing gets quite loose in the fit, even if clamped with a slight interference when installed cold. Looseness of bearings contributes greatly to increased rotor and bearing vibrations, as well as fretting of the bearing seat, which is why TRI considers this design to be inappropriate for hot steam turbine applications.
Consequently, where possible, TRI prefers to use a bearing clamping design wherein the bearing top half has an integral “strongback”, and the ears of the top half are bolted directly to the horizontal joint, as shown on Page 3. In this case, the standard cover can get hot and expand, but the bearing remains tightly fastened to the lower half. In a number of retrofit cases of TRI journal bearings, bolt holes are drilled and tapped into the horizontal joint and the standard cover is milled way to provide space for the ears of the bearing top half to fit.
This design of a top half bearing with ears and hold down bolts into the horizontal joint is definitely more expensive than fitting a round bearing into a hole in a wall, but the long term benefits of vibration control for light weight, high speed, high power density turbine rotors cannot be matched any other way.
It is important to give proper credit for the origination of this design feature. GE Engineers in Schenectady, New York developed this design method in the 1930s for the very reasons cited above. It became a standard GE bearing design feature by approximately 1940.
Through many years of solving severe bearing damage problems and various difficult rotor vibration issues, TRI has developed a large repertoire of bearing designs that were “custom or special designs” at the time, but which over the years have become “TRI standard bearings”. Many are now relatively popular designs.
TRI continues to design and manufacture the journal bearings presented in this catalog, or similar bearings adapted to meet customer’s specific needs, or other designs to suit new applications.
TRI release a new Tech Note today. This month, Dr. Mel discusses fluid drive scoop tubes. Our experience has shown that these critical parts sometimes fail. Understanding the reason why they fail gave way to a better design for scoop tubes.