By Michael Dupuis and Tihomir Maricic
Thrust bearing failures at Ontario Power Generation’s Sir Adam Beck Pump Generating Station caused the utility to lose generation output, revenue and confidence in its systems. This article details how OPG diagnosed and solved the facility’s bearing failure problem.
The Sir Adam Beck complex – on the Niagara River in Ontario, Canada – consists of 437-MW Sir Adam Beck 1, 1,499-MW Sir Adam Beck 2, and the 174-MW Sir Adam Beck Pump Generating Station (PGS). Sir Adam Beck PGS, commissioned in 1958, experienced failures of the Babbitt-lined thrust bearings on all of its units, from commissioning into the early 1990s. At that time, OPG conducted an exhaustive review of its thrust bearings and bearing split runner plate design. This resulted in a number of changes to the design of the thrust bearing, assembly tolerances and high-pressure oil lift system.
The thrust bearings then operated reliably until 2002, when the Unit 4 bearing failed. Investigation identified quality issues with the post-overhaul refurbished thrust bearing and a glitch in the shutdown system that could, under some circumstances, allow the unit to rotate without oil lift.
OPG and Hydro Tech determined that a thrust bearing upgrade could be accomplished with the installation of polytetraflouroethylene (PTFE) thrust bearing pads to replace the Babbitted pads.
Directives and objectives
In 2013, OPG issued objectives and directives to guide studies, designs and solutions to the bearing failures.
Bi-directional bearings have several requirements in the initial design stage that include:
- Must be submerged in oil at all times;
- Enough surface speed must be available for the thrust runner plate (rotating ring) to pull oil onto the bearing pads;
- There must be enough bearing area (specific load) to support the weight of the rotating parts and the hydraulic thrust; and
- Bearing must be reliable for operation in both rotational directions.
OPG had several directives that had to be met with the upgraded thrust bearing:
- Design a more secure split runner plate, which could not separate or become loose during operation over the life of the bearing;
- Design a better way to control the step, or offset, at the runner plate split (in the vertical and horizontal directions);
- Eliminate the operational requirement for the bearing pad temperature to be normalized between stops and starts;
- Improve the bearing design to maintain the oil film between the runner plate and bearing pads; and
- Refurbish the remaining bearing parts to allow for another 30 years of operation.
OPG contracted Hydro Tech to correct all the thrust bearing problems, as well as to increase reliability of the bearing. They surmised that permissible thrust bearing loads would be greatly increased and bearing losses would be minimized.
In addition, a newly designed split runner plate with a more reliable bolting solution of clamping the plate halves together, while also improving the keys, would alleviate runner plate separations during operation and thus eliminate catastrophic bearing failures.
Hydro Tech used bearing design software from GENMAT to study the Babbitted pads. EnEnergo, based in Russia, supported Hydro Tech in studying the PTFE bearing pad. EnEnergo has an exclusive agreement with North American PTFE Bearing to supply PTFE bearing pads in North America.
Comparative tests and calculations performed on Babbitted and PTFE thrust bearings were:
- Temperatures and computer model test operations;
- Maximum safe load for bearings under the old parameters;
- Specific load on bearings; and
- Oil viscosity changes and the effects on bearing operation.
Computer testing revealed the following:
- Specific loading was within the acceptable range of a Babbitted bearing;
- Speed of the Babbitted bearing was too slow to maintain a reliable oil wedge using ISO 46 oil;
- Operating temperature of the bearing was within the Babbitted bearing range; and
- Changing to a thicker oil, such as ISO 68, dramatically increased the reliability of the bearing oil wedge.
Root cause of failures
The PGS thrust bearing runner plate design is a common design used in the hydro industry, but inherent design features cause several flaws to develop as the bearing size increases.
Of primary importance, the runner plate key was not as tight as it should be. In addition, the runner plate bolting cannot be torqued properly to ensure proper bolt stretch. Therefore, the runner plate split-bolts will sometimes loosen, allowing a step in the runner halves to develop, or break if over-tightened, allowing the runner plate to separate.
In the original PGS runner plate design, crude methods where required to separate the runner plate halves. The only process available uses wedges/spacers and technicians must hammer behind the connecting studs or on the fasteners.
Babbitted thrust bearing pads
Compared to steel, greater expansion from heat occurs with Babbitt metal. During operation, heat causes Babbitted bearing pads to develop a crown because the Babbitted top surface expands more than the steel bottom surface.
Each time a PGS generator is shut down, the bearing temperature must normalize to eliminate the crown. Prior to start up, if there was insufficient time allowed for the temperature to normalize, starting the generator will be a main factor that causes bearing failure. At the same time, allowing the temperature to normalize causes time delays between stops and starts or when reversing directions.
PTFE thrust bearing pads
PTFE material is reinforced with bronze wire mesh embedded into the PTFE below its running surface to reduce crowning.
PTFE lining features antifriction and anti-scratch properties, as well as high electric insulation. The dry friction coefficient between PTFE lining and the steel runner plate friction surface is 0.05 to 0.08, about 20% to 30% compared with the coefficient of Babbitted bearings.
Crowning, using a PTFE-lined thrust bearing, is dramatically reduced because the heat-conductivity coefficient for PTFE lining is only 0.05% of the heat-conductivity coefficient of a Babbitted/steel pad. Thus, operational errors related to restarting generators can be eliminated between shutdown and restart.
|A technician measures the runner plate’s flatness.|
PTFE is a petroleum product that holds oil better than metal, and with the specially machined tapers on the leading edges of the bearing pads, the oil wedge develops at lower speeds, practically from the moment of unit start-up. It is possible to start the generator just after stopping (hot start) because the PTFE bearing pads resist crowning. It is also possible to start the unit after a lengthy stoppage (up to a month) without jacking the rotor first.
Creation of the lower-speed oil wedge eliminates the need for a high-pressure oil injection system (HPOI).
It is widely known that if a HPOI system is designed improperly, or if it malfunctions during the life of a bearing, the system can be the root cause of bearing failures. By eliminating a HPOI system, costs are reduced to maintain breakers, motors, starters, pressure transducers, filters and other items, while adding reliability.
PTFE has an estimated 1.5 to 2 times increased specific load capacity compared to Babbitt lining. With the PGS bearing pad dimensions being equal to the original Babbitted pads, the new bearing has a substantially increased load capacity.
Babbitt vs PTFE
A Babbitted bearing works best with an offset of 15% to 25% pivot-point toward the trailing edge. This, however, creates an issue for a reversing bearing as there can be no offset due to the bearing rotating in both directions. The bearing pivot-point must be in the center of the bearing, which lowers the ability of the bearing to achieve full capacity for its size.
A PTFE bearing works best with an offset of 5% to 7% pivot-point toward the trailing edge. This means that when the pivot-point is moved to the center of the bearing, the Babbitted bearing pad loses much more capacity than the PTFE bearing pad. A Babbitted reversing bearing has more capacity than a standard bearing. The design parameters for a PTFE reversing bearing afford it even more capacity than a Babbitted reversing bearing, due to the proximity of the PTFE’s optimum pivot-point.
The bearing runner plate needed a bolting process that would not fatigue or loosen over time. A small, split runner plate is highly toleranced to prevent a step from developing. The larger the runner plate, the more rigid the key must be. Problems occur when the same design and tolerances are used for large and small bearings. As the runner plate gets larger, the step is also amplified by the same proportion in magnitude (i.e., double the runner plate size can mean double the developed step).
If a vertical step develops, the step will act as a scraper or an oil disruption, depending on the direction in which the step develops. In the PGS case, due to the runner plate being reversible, when a step develops in any direction, it will become a scraper.
The PGS runner plate serves as both thrust bearing runner plate and guide bearing journal. Not only must the vertical step be controlled, but also the outer diameter must have minimal tolerances to control the journal step. The PGS runner plate needed to be designed and manufactured in such a way that proper bolt stretch could be achieved. PGS needed a repeatable, measurable method of tensioning the split bolts.
With a tightly fitted key, drawing together the runner plate halves is only part of the problem. On disassembly, one must also be able to separate the plate halves. Ease of disassembly must be designed into the runner plate to prevent site personnel from having to damage the mounting or running surface finishes or split faces. For the new PGS runner plate, 20 to 30 tons of force is required to separate the two halves. Hydraulic pressure is used to achieve separating the halves.
In today’s open market environment, requiring that bearing pads cool between stopping and starting posed a major operational issue. Crowning had to be minimized to allow immediate response when switching from generating to pumping, or vice versa.
The PGS bearing diameter was too small for the generator RPM. With this situation, the runner plate surface speed was not fast enough to maintain a reliable oil wedge. At times, the oil film failed, causing metal-to-metal contact (wiping) that immediately galled the Babbitted bearing pads and wiped the bearing. There was no way to increase generator RPM because its speed is synchronized to standard for the North American grid. Additionally, increasing the diameter would increase the surface speed of the bearing and a major modification to the thrust bearing pot and main bracket would be cost-prohibitive.
Temperature was not an issue for the PGS bearing. During normal operation, the bearing temperatures were not excessive, therefore the oil viscosity was not thinning due to heat. However, the oil film needed to be thicker to allow the bearing to operate at a reliable safety factor. For the PGS bearing, calculations indicated ISO 68 oil would improve the oil film thickness for this slow-rotating generator.
Despite all of these modifications, there was still opportunity for momentary metal-to-metal contact due to loss of oil film. If the thrust runner (rotating ring) contacts the Babbitted bearing pads for even a split second, the bearing will wipe at every contact. Hydro Tech’s and EnEnergo’s bearing design software each showed this was still a possibility.
Engineers resolved thrust runner contact by redesigning bearing pads to use a PTFE surface instead of a Babbitted surface. In the rare event a runner plate-to-bearing pad contact occurred, by using a PTFE bearing pad, the runner plate would slide on the PTFE for a split second, regain its oil wedge and keep operating instead of wiping the bearing. In all likelihood, the equipment would not register runner plate-to-bearing pad contact occurred.
The best real-time indicator of how a bearing is functioning is the temperature at which it is operating. The PGS Babbitted bearing had one resistance temperature detector (RTD) in the center of the bearing pad. The centered location of the original Babbitted bearing’s RTD recorded only a best-fit temperature for both rotational directions.
The new PTFE bearing pad has two RTD holes to allow the temperature to be measured at the hottest spot in both rotational directions. Toward the trailing edge is where the warmest temperatures develop on a bearing pad. Unfortunately, during assembly, additional RTDs were not available for installation. Therefore, only one RTD was placed at the trailing edge of the generating side of the bearing pad; the other RTD hole was left empty. Because of this, a direct temperature comparison between the old and new pads cannot be made. However, the single RTD still registered a lower, therefore better temperature.
The bearing temperature leveled out at 58 degrees Celsius. Starting and stopping was smooth.
Bearing temperature leveled out at 55 C with the RTD placed in the leading edge, not in the hottest part of the bearing pad. However, calculations indicate that the temperature range between RTD placement and the actual hottest part of the bearing pad is within 5 C. Starting and stopping was smooth.
Refurbishing and upgrading PGS thrust bearings produced the following outcomes:
- The runner plate has a clamping value at the split faces of 100 tons total, with a perfect step of less than 0.0001 in on assembly at both the vertical and horizontal split joints;
- Thicker ISO 68 oil is providing a more reliable oil film while operating at a standard temperature of 58 C;
- PTFE bearing pads are providing a more dependable service life due to PTFE’s ability to carry higher loads and withstand a momentary oil film loss; and
- With identical surface dimensions as the old Babbitted bearing pads, a 50% or better load capacity has been achieved.
Operational limitations for stopping and starting have been eliminated. The generator is able to stop and immediately reverse directions.
The HPOI system has been removed, eliminating maintenance and a potential failure mode.
With the increase in load capacity of the PTFE thrust bearing, the turbine stay vane flaps were put back in service, allowing the full output of the turbine/pump and generator/motor.
Mike Dupuis is president and chief executive officer of Hydro Tech Inc. Tim Maricic, P.Eng., is a senior engineer specialist in plant engineering services at Ontario Power Generation.
More HR Archives Issue Articles