By Fred C. Wiesinger, Jr. Technical Services Manager
Pioneer Motor Bearing Company
Image Credit: Søren Niedziella/Flickr User albioneurope
More than 30 years ago, near the beginning of my career with fluid film thrust bearings, an early mentor remarked “There’s only about one teaspoon of oil on any of the big thrust shoes while they are operating, so leakage of the jacking oil system cannot be tolerated.” No truer words of advice were ever spoken.
In a properly designed fluid film thrust bearing operating at its design speed, a continuous hydrodynamic flow of oil completely separates the babbitt surface of each thrust shoe and the steel surface of the rotating collar and supports the applied loads through self- generated hydrodynamic pressure. With proper care for the cleanliness, viscosity, and temperature of the lubricant, in the ideal case the bearing surfaces never contact the steel collar, so long, as mentioned, the shaft rotates at its design speed.
Unfortunately, this is not the case, and high friction and wear can easily result, when the shaft is stationary or operates at less than design speed, such as during reoccurring start-stop cycles.
In an attempt to eliminate such wear in less than ideal design conditions, many pieces of industrial rotating equipment utilize a high pressure, low flow pumping system to inject oil between the rotor and the bearing during start up, shut down, and periods of slow speed operation such as turning gear or maintenance tasks.
High pressure jacking oil systems typically consist of motor driven hydraulic pumps, high pressure filters, and a distribution system that directs the oil into the portion of the bearing at the highest load point of each thrust shoe. But as with any critical system, great care must be taken to insure its integrity. As the saying goes, “Failure is Not an Option.” Any path that develops allowing pressure within the bearing to leak may not only prevent the system from accomplishing its intended primary purpose of reducing friction and preventing bearing wear, but it can also significantly reduce the load carrying capacity of the bearing during normal operation and, ironically, be the root cause of bearing damage and an unplanned shutdown.
Over the years, I have seen many instances of failure of components within high pressure jacking oil systems, such as: check valves that did not seat properly; ruptured hoses; fittings that have come loose; and even poorly designed oil tight connections. A failure of just any one of these items will result in an instantaneous drop in load capacity of the bearing.
If the remaining pressure sufficiently carries the load on the bearing, this is then typically seen as a step change in the bearing’s operating temperature that then stabilizes as the “new” bearing temperature – the “new normal”. Assuming that this “new” operating temperature is below the alarm temperature set for the machinery, the bearing may continue to run in a degraded state but may no longer be capable of supporting additional loading from different equipment operating conditions.
Not long ago, I received an e-mail report describing bearing damage to a thrust bearing in a large hydroelectric generator. The machine was being put back online following a maintenance outage. During that outage, a crew disassembled and rebuilt the entire thrust bearing. On the day of drama, the unit operated at full load for a number of hours when suddenly the thrust shoe temperature indicators climbed from around 70°C, the historical operating temperature, to a level of around 145°C. The operators then shut down the unit to investigate the problem. Upon disassembly, they discovered severe damage to the thrust bearing. The accompanying photos show the thrust shoes as observed at this point.
To the trained eye of a bearing specialist, the root cause of the damage was attributable to an overload condition, as the damage was centered opposite the pivot of the shoes. Apparent thermal distortion of the shoe in the regions along the inner and outer radii prevented this material from also being wiped by the rotating thrust collar. Based on these photographs and the fact that there was absolutely no change in loading during the operation of the unit, I suggested that the entire high pressure system be disassembled and each component inspected as I believed one or more of the parts must have failed.
In this particular design, the high pressure oil flows to each thrust shoe to a block that is bolted to the shoe on the outside diameter. The oil then travels down a steel manifold tube and through a check valve mounted at the end of the tube. Bonded copper washers seal the tube at both ends. The photo below shows this design concept.
A team disassembled all of the thrust shoes and saw that the bonded seal on one of the blocks clearly had indeed failed.
Closer inspection of the failed parts plainly showed an undesirable oil flow path on the sealing surface of the bolted block.
Machinists re-milled the sealing surfaces to insure the proper surface finish and clamping pressure. Maintenance personnel replaced all of the sealing washers and cleaned up all the thrust shoes to remove the damage and the unit was returned to service.
Shortly afterwards, I was scheduled to attend a meeting with a number of engineers responsible for hydro maintenance for a different utility on the east coast. Ironically, the meeting had to be re-scheduled because of a thrust bearing failure on one of their units. When we finally got together, although our meeting topic was on a different subject, they shared some photographs of their recent failure.
In another of those twists of irony that fate seems to continuously throw at us, just a few weeks ago, I was scheduled to attend a meeting with a number of engineers who are responsible for hydro maintenance for a completely different utility here on the east coast. The meeting had to be re-scheduled because of a recent thrust bearing failure on one of their units and when we finally got together, although our meeting topic was on a different subject, they shared some photograph of this recent failure.
The unit had been running well for quite a few years but recently had experienced a significant rise in thrust bearing temperatures. The engineers felt certain they had identified the damage mechanism as contamination of the lubrication system.
Armed with their own photographs and those in my mind from the previously discussed thrust bearing damage investigation, I suggested that they had more likely experienced a failure of one of the components of the jacking oil system. Perhaps motivated more from concern than from conviction, they nevertheless inspected all of the components of their oil jacking system. The accompanying photographs show they discovered that the sealing washers were the real culprits causing the unit showdown.
This unit has not yet been repaired but I would expect that restoring the thrust shoes and replacing all of the sealing washers will return this unit to a successful operating condition.
Based on my experience, my impression is that most – turbine-generator engineers take the high pressure jacking oil system for granted, as if it were “bullet proof”. They seldom take time to review its condition even when the operation of a thrust bearing inexplicably changes. Nor do they recognize the system as a potential cause of serious bearing damage.
On the contrary, remember that the delivery port on any high pressure jacking oil system is located at the most critical point on the bearing. Leakage of hydrodynamic oil, no matter how small, from this location will immediately impact the load carrying capacity of the bearing.
Quite often OEM’s employ a combination of check valves and orifices to minimize the potential of back flow through the system. Each of these should be inspected and checked for proper operation when disassembling any bearing with a high pressure jacking oil system. It is equally important to check the oil tight integrity of the fittings where they make their final connection to the bearing.
In short, a high pressure jacking oil system extends the life of a rotating machine with fluid film bearings and makes maintenance easier when properly installed and periodically inspected. Careful inspection and installation of the individual components is critical to keeping this valuable system from becoming the root cause of a bearing damage outage.