The high-pressure jacking oil system on a hydrodynamic bearing is an often-overlooked component that can cause bearing problems if not properly inspected and maintained.
By Fred C. Wiesinger, Jr.
Of the many mechanical inventions attributable to early mankind, arguably the most significant was the wheel. Undoubtedly, the first man to use a wheel discovered the need to reduce the resistance between the wheel and axel in motion. The first fluid film bearing was likely animal fat applied to a wooden axel to make a wheel spin with less friction.
Rudimentary application and observations of success and failure were the early “design guides” used until testing conducted by English inventor and railway engineer Beauchamp Tower in 1883. Osbourne Reynolds, an innovator in the understanding of fluid dynamics, took Tower’s experimental results, applied mathematics to model the results and created the first predictive analytical tools for hydrodynamic lubrication, which still form the basis of calculations in use today.
|Failure of the check valve assembly on this large horizontal generator resulted in damage to the shaft because it was not discovered until there was sufficient damage that the unit would not clear a permissive on the start-up sequence.|
German engineer Richard Stribeck and American engineer Mayo D. Hersey conducted further testing and documentation of the different regimens of lubrication within two lubricated surfaces. Stribeck first published the results of these experiments in 1901, leading to what we refer to today as the Stribeck Curve. This work was further refined by Hersey, leading engineers to refer to this as the Stribeck-Hersey Curve. This curve shows that friction and wear are at a maximum during relatively low-speed operation. Attentive end-users have observed this for years in thrust bearings in vertical hydroelectric generators, where the weight load of the rotating mass is ever-present.
In the early years of operation of these units, many methods were applied in an attempt to minimize the friction and damage during start-up conditions. In 1912, American engineer Albert Kingsbury, who invented a tilting pad thrust bearing, found that scraping the babbitt surface on the thrust bearings reduced the start-up friction. During this same time frame, many operators of hydroelectric generators would manually jack up these vertical units, allowing fresh oil to flow between the operating surfaces. The load was manually released and the unit was started quickly in an attempt to beat the time required for the rotor weight to squeeze out the oil. As individual unit capacities grew, engineers recognized the need for a more elegant solution to minimize start-up wear.
A reminder of the principles at work in these bearings is helpful. In a hydrodynamic bearing, internal pressure is developed as a result of the interaction of relative movement of the rotor and bearing, a converging clearance space, and a viscous fluid. This pressure then, acting over the area of the bearing, supports the load. In a hydrostatic bearing, oil is injected into a bearing at a high pressure, developed by an external pumping system. This pressure then, acting over the bearing area, supports the load. The logical solution was to develop a bearing that could start as a hydrostatic bearing and then convert to hydrodynamic operation once there was sufficient relative velocity (or more correctly, sufficient hydrodynamic pressure) to fully support the load. This is the model that is still in use today. The engineering challenge was and still is to create a high-pressure lift system that is as robust as the fluid film bearing it is intended to enhance.
|The shaft of this unit suffered damage, including a deep groove machined in the shaft, from the failed check valve assembly.|
When you consider the critical portion of the bearing where the hydrostatic oil is delivered and the relatively meager oil film of even a large thrust bearing, it is obvious that leakage of any oil from the high-pressure system will impact the load capacity of the bearing and adversely affect operation of the generator. With this in mind, there are a number of design elements to consider when installing a new high-pressure lift system and maintaining the elements of an existing system.
Mechanical design elements
The first element to think about is the path of the high-pressure oil through the bearing to the babbitt surface. Drilling connecting holes is the least expensive method; however, do not overlook the mechanical integrity of the bearing itself. Newly manufactured parts made from material of known composition and integrity can be drilled from the connection point to the oil groove in the babbitt surface. But if the quality of the base material is unknown or in question, it would be worth considering sleeving or otherwise keeping the oil from coming into contact with the base material.
|The thrust shoes on this unit, which was being put back on line after a maintenance outage, suffered severe damage after temperature indicators climbed to about 145 degrees Celsius.|
In addition, if the material displays characteristics that call into question the technique used or outcome of the process of babbitting the bearing, a nozzle or some other sleeve may be used to keep the high-pressure oil from entering the babbitt bond region. Some OEMs (original equipment manufacturers) always design with such a nozzle to eliminate concerns with the babbitt bond directly adjacent to the drilled hole for the high-pressure oil.
Hydraulic design elements
A cardinal issue to take into account with regard to a high-pressure lift system is the risk it poses to the proper operation and reliability of the bearing system. If you’re puzzled by the irony of such an “upgrade” feature putting the reliability of the bearing’s performance at risk, consider carefully the hydraulic components that make up (or ought to) the high-pressure lift system.
For example, take the application of a check valve to each thrust bearing shoe. This could be regarded as an unnecessary expense, but a properly installed and sealed check valve ensures that none of the hydrodynamic pressure will leak out of the bearing oil film in the event of a failure of any of the upstream elements. The check valve must provide long life and be installed in a way that practically eliminates leakage where it is connected to the bearing shoe.
In looking at the “other side” of the high-pressure system, the importance of pressure-compensated flow-control valves on the supply line for each bearing shoe cannot be understated. They will help keep damage to any one leg of the system from “stealing” all of the pressure and potentially will allow continued operation of the unit until a convenient outage can be scheduled to fix the offending part.
Even the selection and installation of a component as unremarkable as tubing must be done with care. The interconnecting tubing between the oil supply manifold and bearing shoes must be selected based on a number of factors and installed in such a way to allow the shoes to freely tilt and move in service. Because of this, and in consideration of electrical insulation properties, designers often specify non-metallic flexible tubing for this connection. However, when selecting tubing, examine how compatible it will be, both internally and externally, when it is fully immersed in oil in the bearing housing.
Even the route of flexible tubing is an issue. Within the oil pot, seek to avoid areas of high turbulence, such as the plane where the thrust runner meets the babbitt surface. Crossing a boundary such as this can shorten the life of a flexible tube. Recall, too, that flexible tubing tends to attempt to “straighten out” when pressurized, which has resulted in some high-pressure fittings actually loosening in service. This phenomenon should serve as a warning when routing and anchoring flexible tubing.
Finally, when selecting the other parts of the high-pressure lift system, remember the British expression about driving a car: “What you hope to gain on the straight-aways you can lose on the roundabouts.” You are not going to save any money if a cheap motor-driven pump fails prematurely and results in unscheduled down time for the generator. Be mindful of the quality and reliability of the smaller items in the high-pressure lift system.
|The pens indicate an undesirable flow path on the sealing surface of this block, which was bolted to the shoe on the outside diameter.|
The addition or existence of a high-pressure lift system must not be ignored when planning maintenance on a hydro generator. All of the elements of the high-pressure system should be cleaned and inspected. Proper operation of check valves and flow-control valves should not be assumed based on successful operation in the past. Even if the elements themselves are operational, all connections should be inspected and all gaskets, o-rings, and seals should be renewed.
The real world
I recently visited a hydro plant with a large horizontal generator. The rotor was heavy enough that the OEM had applied high-pressure jacking oil to reduce the friction and minimize the wear during start up of this large machine. The OEM had also included a combination check valve/nozzle to achieve the gains noted above, namely: eliminating the possibility of bleeding back hydrodynamic pressure and keeping the high-pressure oil away from the babbitt bond line of the bearing.
During a routine inspection, maintenance personnel discovered the check valve/nozzle assembly had come loose within the bearing. Fortunately for this customer, this had done just minimal damage to the journal. However, this is the kind of experience that gives rise to nightmares about how such a tiny portion of the high-pressure lift system could cause significant damage if things go wrong. This could have been a catastrophic event if the nozzle had turned slightly and wedged itself into the bearing.
One of our customers, the operator of yet another large horizontal generator, experienced a failure of the check valve assembly. When you consider where these check valves are located, the next stop for all of the parts is the bearing itself. In this case, you could see the internal parts extending out of the check valve. The spring and check assembly did a fair amount of damage to the shaft in this machine, as this failure was only discovered after there was sufficient damage that the unit would not lift and clear a permissive on the start-up sequence.
Not long ago, I received an e-mail report describing damage to a thrust bearing in a large hydroelectric generator. The machine was being put back on line after a maintenance outage that involved disassembling and rebuilding 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 about 70 degrees Celsius, the historical operating temperature, to about 145 C. The operators shut down the unit to investigate the problem. Upon disassembly, they discovered severe damage to the thrust bearing.
|A thrust bearing failure on one unit, which had experienced a significant rise in thrust bearing temperatures, was attributed to contamination of the lubrication system but actually arose from failure of the sealing washers in the high-pressure jacking oil system.|
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 kept the outside and inside radii of the thrust shoes away from the rotating thrust collar. Based on photographs taken and the fact that there was absolutely no change in loading during 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.
A team disassembled all 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 ensure the proper finish and clamping pressure. Maintenance personnel replaced all the sealing washers and cleaned all the thrust shoes to remove the damage, and the unit was returned to service.
|The sealing washers in the high-pressure oil jacking system of one unit were identified as the culprit causing a unit shutdown.|
Shortly afterwards, I was scheduled to attend a meeting with several engineers responsible for hydro maintenance for a different utility on the east coast. Ironically, the meeting had to be rescheduled because of a thrust bearing failure on one of their units. When we finally got together, they shared photographs of their 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 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. They discovered that the sealing washers were the real culprits causing the unit showdown.
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 “bulletproof.” 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 OEMs 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 at your plant.
Fred Wiesinger, B.S.M.E., is technical services manager with Pioneer Motor Bearing Co.