Installation of new runners at the 1,020 MW Akosombo project revealed the inability of the existing thrust bearings to handle the higher hydraulic thrust. Replacing these bearings with a new runner plate and polytetrafluoroethylene (PTFE) bearings significantly reduced unit operating temperatures and eliminated recurring cavitation damage.
By Kenneth Arthur and Michael A. Dupuis
In the mid-1990s, the Akosombo Generating Station, on the Volta River in Ghana, contained six turbine-generating units with a total capacity of 912 MW. The first four units, commissioned in 1965, had a capacity of 588 MW. The final two units were commissioned in 1972 and had a capacity of 324 MW.
After 30 years of operation, owners the Volta River Authority carried out a technical audit with the objective of modernizing the plant to operate for another 25 to 30 years. This audit indicated that the units could also be upgraded to increase power production at the facility by nearly 12%.
In 1999, Volta River Authority began a major overhaul of the generators and turbines that included installation of new runners supplied by GE of Canada and VA Tech of Austria. This upgrade increased station efficiency and capacity. Now, Akosombo has a capacity of 1,020 MW.
However, installing the new runners at the Akosombo plant caused some problems. The new design increased hydraulic thrust while operating at the same flows as the previous runner, contributing to rare recurring cavitation damage on the generator thrust bearings.
After the upgrade, thrust bearing operating temperatures increased to 86oC from 76oC. To rectify the situation, the original equipment manufacturer (OEM) recommended rearranging the thrust bearing support springs. This reduced the high temperatures to an average of 82oC, but a residual problem of recurring cavitation damage to the thrust bearing pads emerged.
After several unsuccessful attempts to remedy the problem, Volta River Authority contracted Hydro Tech Inc. of Canada to design a new thrust bearing featuring a modified highly-toleranced split rotating ring, keyed and bolted together to form one complete ring (with no gaps at the splits). This would solve the cavitation problem, but the bearing would still be overloaded. Thus, Hydro Tech decided to improve load capacity of the bearing pads by replacing them with polytetrafluoroethylene (PTFE) pads.
Installation of the new thrust bearing assembly was completed on Units 2 and 5 at Akosombo in November 2009. Operating temperature was reduced 7oC to 8oC (from 83oC to 76oC). Continual monitoring of thrust bearing temperatures indicates good operation of the units.
Discovering the thrust bearing problem
Before the new runners were installed at Akosombo, Volta River Authority and its consultant, Canada-based Hatch, raised concerns about the possibility of increased thrust load and the ability of the existing bearings to match this load. However, the OEM assured them that the thrust bearing was capable of supporting the new levels of thrust loading.
Unit 3, the first to be overhauled at Akosombo, operated with an average thrust bearing temperature of 76oC before the upgrade. During commissioning of the upgraded unit in July 2000, this temperature increased to an average of 86oC, confirming increased thrust load associated with installation of the new runner. Monitoring was carried out while the retrofit project was under way. The Unit 3 thrust bearings failed while the unit was operating in April 2002, after almost 20 months of service. Even though other circumstances were involved, such as irregular cooling water flows, this failure again raised concerns about the existing thrust bearings.
Volta River Authority contacted the bearing OEM to study the problem and offer an appropriate solution. The OEM carried out a bearing optimization study at its own cost and recommended a revised support spring arrangement to increase the oil film thickness, therefore providing more reliable bearing operation with reduced temperature. The OEM implemented this solution by removing 11 support springs, mainly from the leading edge end of the thrust bearing. This resulted in a total of 40 support springs as compared to the original 51 springs per bearing pad. After this modification, the average thrust bearing operating temperature for Unit 3 dropped to 82oC.
|The new polytetrafluoroethylene (PTFE) bearing pads being installed on the six units at the 1,020 MW Akosombo Generating Station have reduced unit operating temperatures to acceptable levels.|
Although thrust bearing operating temperatures were now reduced, a new problem of rather rapidly recurring cavitation damage became obvious on the thrust pad surfaces. This cavitation damage was first discovered during an inspection six weeks after installation. The damage was mostly concentrated within and around the high-pressure oil injection groove ring, toward the trailing end of the bearing pad.
Cavitation of thrust bearing pads is unusual and typically is caused by a design flaw. At Akosombo, the highest measured average bearing pressure is about 4.07 MPa, but there are higher and lower pressures on the bearing surface. Generally, the middle of the bearing pad surface toward the trailing edge, close to the high-pressure oil lift port, is a higher load area. This port is where the most significant pressure drop occurs because of the change in oil film thickness and/or pressure. This results from a step in the babbitted surface and a gap between the thrust bearing runner plate segments.
With the increased thrust, the oil film between the babbitt thrust bearing pads and runner plate decreases, while average pressure rises. This increases the pressure drop at the thrust bearing runner plate split and increases its effect on the babbitt due to close proximity, resulting in cavitation pitting of the pads.
During a thrust bearing inspection performed in August 2008, evidence of fatigue failure of the babbitt surfaces was discovered, likely due to thermal effects (a condition known as “thermal ratcheting”). Thermal ratcheting is caused by temperature changes on the bearing surface, which causes the babbitt bearing pad to act as a bi-metal strip, slightly bending the bearing pad and causing crowning. Crowning occurs because the babbitt expands and contracts at a different coefficient of expansion to steel, and the surface of the bearing pad is hotter than the steel backing plate to which the babbitt is bonded. (The thicker the babbitt, the stronger the thermal ratcheting effect.)
When bolted together, the new runner plate on this Akosombo unit features no gaps or splits, thus eliminating pressure disturbances that can cause cavitation damage to the bearings.
These unusual bearing pad damages became a greater concern to Volta River Authority and its contractors as cavitation and thermal ratcheting progressed steadily. The OEM experimented with several modifications by machining different tapers and shapes within the high-pressure oil lift groove ring, but the problem persisted. It became obvious that initial concerns about the ability of the bearings to safely support increased thrust load associated with the new replacement runners were valid.
Developing a solution
Volta River Authority, working with Hatch, met with the OEM for a permanent solution to the Akosombo thrust bearing problem. The OEM’s preferred solution was to install a new one-piece rotating ring, an approach that involved completely removing the generator rotor. But because of the prolonged outage time involved with this approach (at least three months) and the fact that the thrust load might still be above the safe load-carrying capacity of the existing babbitt thrust pads, Volta River Authority did not accept this solution.
Volta River Authority contracted Hydro Tech to modify the Akosombo generator thrust bearing assembly to deal with the recurring cavitation problem. All parties fundamentally attributed this problem to the increased hydraulic thrust load of the new turbine runner. However, the original bearing was already susceptible to cavitation due to gaps in the runner plate quadrants. Before the turbine upgrade, there was at least one incident of cavitation damage discovered on a bearing pad. At that time, it was considered to be a result of porosity in the babbitt.
Objectives of the Akosombo generator thrust bearin g retrofit were to:
- - Address the fact that the thrust load of the new runners was above the safe load limit of the existing thrust bearings;
- - Solve the design flaw that was causing the cavitation on the thrust pad surfaces; and,
- - Lower operating temperatures.
Increasing thrust load
Hydro Tech first measured thrust loads on the units to inform the design of the new PTFE thrust pads. Thrust was as high as 4.07 MPa at a headwater elevation of 247 ft (75 meters) above sea level (maximum headwater level at Akosombo is 278 ft (285 meters) above sea level). Actual maximum thrust load increases as the headwater level rises. Full head pond thrust measurements could not be taken because the pond does not completely fill each year. The results again indicated that total thrust on the units was above the safe operating limit of babbitt bearings.
PTFE bearing pads with higher load capacity were installed to replace the traditional babbitt bearings. The new PTFE thrust pads for Akosombo feature compound tapers at the leading edge to facilitate oil flow and wedge formation, as well as pad eccentricity to permit higher specific load capacity. Using the PTFE bearing pads also allowed Hydro Tech to eliminate the high-pressure oil lift system, removing another potential source for cavitation.
Solving the design flaw
The solution Hydro Tech provided to address sudden pressure fluctuations caused by gaps at the splits of the existing rotating rings was a modified design of highly-toleranced segmented rotating rings, which are keyed and bolted together to ensure no gaps. This type of joining was achieved during installation. The splits were lapped (stoned) to completely eliminate sharp ends, eventually forming a one-piece thrust bearing rotating ring that was bolted to the thrust block.
The new thrust bearing runner plate acts as a solid one-piece ring, providing constant pressure on the bearing pads over its entire surface.
Lowering the operating temperature
The PTFE bearing has one-fifth the coefficient of friction compared with the existing system, preventing excess heating of the thrust oil. This allows the thrust bearing temperature to be lowered with no additional cooling.
With installation of the new thrust bearings on Units 2 and 5, all problems that resulted in higher operating temperatures have been resolved. The bearing pads were inspected after several months of running time and appeared to be in good condition. Bearings on the remaining units will be changed as soon as an outage time is available.
Special thanks to bearing designer Hydro Tech Inc. and its team; the technical staff and tradespeople of Akosombo Generating Station; EnEnergo of Russia for its PTFE bearing pad design and supply; Horst Mielke of Hatch; Kjell Nyqvist of North American PTFE; and John Sanderson and the crew of Canadian Babbitt Bearing for manufacturing and machining the runner plates.
Ken Arthur, chief mechanical engineer at Volta River Authority’s Akosombo Generating Station, coordinated the work at the facility. Mike Dupuis, president of Hydro Tech Inc., was the lead mechanical designer for the new thrust bearing.