Upon discovery of corrosion fatigue cracking on the turbine shafts of the six units at 180-MW Jenpeg, Manitoba Hydro commissioned machining work. By removing the damaged areas on the shaft and installing a new seal, the utility was able to return the plant to reliable service.
By Paul Halipchuk, Kevin Penner and Scott Smith
All six units at the 180-MW Jenpeg Generating Station in Manitoba, Canada, experienced corrosion fatigue cracking of the turbine shaft at the fillet between the shaft body and runner coupling flange. The shafts were analyzed to determine the failure mechanism, then machined in-situ using a specialized machine to remove the cracks and improve the shaft geometry. Finally, the turbine shaft seal was redesigned to prevent shaft corrosion and facilitate future inspections of this critical area.
As of early August, the turbine shafts of all six units had been inspected and machined. Two of the units have had new shaft seals installed. All units have been returned to operation. The remaining four units will receive new shaft seals in the near future.
Understanding the problem
Manitoba Hydro owns and operates five hydroelectric stations on the Nelson River in northern Manitoba. Jenpeg, completed in 1979, is the first station on the Nelson River near the outlet of Lake Winnipeg, which forms its reservoir. Manitoba Hydro uses the Jenpeg station to generate power, maintain water levels and regulate the outflow of Lake Winnipeg.
Jenpeg is a concrete gravity type dam equipped with five spillway gates. The powerhouse contains six identical horizontal Kaplan bulb-type turbine-generators, each with a maximum capacity of 30 MW.
In the spring of 2010, Manitoba Hydro became aware of a hydroelectric plant in Europe that contains turbine-generators of the same design, built by the same manufacturer and within the same decade as the Jenpeg units. This plant had experienced catastrophic turbine shaft failures related to fatigue cracking. Manitoba Hydro decided to inspect the Jenpeg units and determine if a similar problem was occurring.
Shaft inspections on Unit 1 were completed in June 18, 2010, in conjunction with a planned annual maintenance outage already in progress. A commitment to extend this outage was made to accommodate the dewatering and disassembly needed to confirm that the unit had a problem with the turbine shaft. Use of magnetic particle examination in the fillet between the shaft body and runner flange revealed hundreds of non-continuous circumferential cracks about 12 mm deep. Subsequent inspections of the remaining units were completed over the course of the year, prior to mobilization of each machining operation. The other units had similar cracking at the same location.
Between June 18 and 26, Manitoba Hydro personnel evaluated the consequences of failure and performed engineering analyses to confirm the likelihood, timing and impact of failures compared to those experienced in Europe. If a turbine shaft at this hydro facility were to fail, the best case scenario would involve an uncontrolled release of the turbine oil into the Nelson River. The worst case scenario would involve a rupture of the turbine water passage, with corresponding damage to the entire plant and risk to personnel.
Based on these results, Manitoba Hydro made the decision to shut down all six units at Jenpeg, pending further investigation and repairs.
Performing an analysis
Personnel in the mechanical engineering department at Manitoba Hydro performed a full mechanical stress and fatigue analysis on the Jenpeg turbine shafts in June 2010. Finite element modeling was used to determine the stress levels in the shaft at the body-to-runner flange transition. The major loads considered during this analysis included: gravity, tension due to hydraulic thrust, torsion due to torque and buoyancy of the runner in water.
Fatigue cycles considered were unit start/stops and loads varying once per revolution. Figure 1 shows a typical finite element analysis plot for the turbine shaft transition fillet area at Jenpeg.
Results of the analysis indicated that the turbine shafts of the Jenpeg units have an acceptable factor of safety for infinite fatigue life, provided that the surface finish in the fillet area is uniform and smooth. However, this fatigue life is sensitive to stress concentrations, such as corrosion pitting. The shaft surfaces in the fillet area were found to be pitted by corrosion.
Further analysis was performed to determine if the cracked material could be machined from the shaft, leaving it fit for continued reliable service. Manitoba Hydro personnel concluded that up to 75 mm of material could be removed from the turbine shaft without compromising the fatigue life. This is possible because although the mean stress in the shaft increases as the cross section is reduced, the stress concentration is reduced as the fillet radius increases.
|Magnetic particle examination of the turbine shafts, in the fillet between the shaft body and runner flange, at the 180-MW Jenpeg facility revealed hundreds of non-continuous circumferential cracks about 12 mm deep.|
Performing the repair
Manitoba Hydro determined disassembly alone would require four to six months, and original estimates were at least one year to disassemble, reassemble and perform repairs. Manpower would have been on the order of 20 to 30 workers. To avoid this, Manitoba Hydro investigated methods of completing the repairs in-situ.
Options considered were in-situ weld repair and in-situ machining. The risk factors considered in the evaluation were the as-left operating stresses resulting from the new machined profile compared with a welding process that would only extend the life of the turbine shaft until a replacement could be procured.
In July 2010 Peak Hydro Services was awarded a contract to perform in-situ machining of two turbine shafts at Jenpeg. The remaining four units were awarded in October 2010.
Peak Hydro crews mobilized on site July 20, 2010. Mobilization of personnel and equipment took only 11 days. The solution Peak Hydro chose to perform the work quickly and get the units back on line involved using a “clam-shell” machine originally intended for machining weld preparations on large-diameter pipes. To ensure this solution would work, Peak Hydro completed a mock-up of the equipment and developed some tool slide designs that were sent to a local machine shop for manufacturing. The tool slides were manufactured over a weekend and prepared for shipping to Jenpeg.
Peak Hydro expected that further modifications would be required once the parts arrived at Jenpeg. Once all the equipment was at the facility and the tool slides attached, the company verified that changes were required to enable the machine to cut the required radii on the turbine shaft fillet and remove the cracked material.
These modifications were performed in the Jenpeg machine shop during two shifts. The machine track then was moved into the bulb of the first turbine unit in halves and clamped onto the shaft. A hydraulic drive was used to rotate the tool head around the track. The tool feed rate was automatic for one axis and manual for the other because the tool was not specifically designed for machining curved profiles. The maximum depth of cut necessary to remove all indications of cracks was about 14 mm.
Work was performed on one unit at a time because Jenpeg does not have independent intake gates for each unit. Rather, two sets of stoplogs are used to isolate and dewater a unit to complete the work. Once units were returned to service, one set of stoplogs had to remain available to install in an emergency, leaving only one set available for repairs. Machining on the first unit required 10 days to achieve an acceptable profile and surface finish, free of all indications. Machining durations improved on subsequent units, with the final unit taking five days.
Three issues encountered throughout the duration of the machining work at the Jenpeg hydro plant are:
- Seasonal conditions, with winter and icing creating challenges for plant staff when trying to handle and obtain a seal on the stoplogs. Divers were used to seal the stoplogs as quickly as possible;
- Machining operations occurring 24/7 over the course of several consecutive shaft repairs resulted in the occasional equipment breakdown, requiring work stoppage to address the issue and re-align; and
- Variation in crack depths from one unit to the next. Abnormally deep cracks relative to the surrounding indications higher up on the shaft flange face required adjustment of approach and radius profiling.
After the machining work was complete, Peak Hydro personnel hand-dressed the fillet area to a smooth surface finish.
As of August 2011, all of the units have been successfully machined and returned to service.
Adding shaft protection
Because the fatigue life of these turbine shafts is so sensitive to surface finish in the fillet area, a means of protecting them from corrosion was necessary. The old shaft seal was designed so that this area was constantly submerged, and it appears the sensitive area of the turbine shaft was originally protected with a coating. However, Manitoba Hydro had no record of the nature of this coating or its inspection and repair requirements, and the coating had deteriorated to the point where it no longer served its intended purpose. This led to the corrosion pitting of the shaft surface and ultimately the fatigue cracking.
Manitoba Hydro personnel then investigated four options to protect the turbine shaft.
Water submergence epoxy paint could be used to coat the sensitive shaft area. This was not selected because of doubt regarding the long-term ability of the paint to remain compliant and flex with the shaft as it rotates. Further, any defects in the coating would allow corrosion to concentrate in these areas, leading to rapid pitting of the shaft surface. The paint could be inspected periodically, but this would require complete disassembly of the shaft seal and an outage of several weeks.
A grease-filled metal or plastic cover clamped over the sensitive shaft area was considered. Given the critical nature of the protection, this solution also would require periodic inspection, with the same disassembly and outage requirements as the paint. Further, Manitoba Hydro is committed to reducing the risk of environmental contamination, making the addition of a grease-filled chamber in the water passage unacceptable.
A corrosion-resistant flame spray metalized coating was considered but not selected because of difficulties anticipated with in-situ application and similar concerns with inspection as the options mentioned above.
Thus, the chosen solution was to redesign the shaft seal so that the sensitive area is kept dry.
|A “clam-shell” machine was customized to perform machining work on the Jenpeg turbine shafts, which consisted of removing about 14 mm of cracked material from the shaft, to return it to a smooth finish.|
Shaft seal design
The existing shaft seal consisted of a rubber lip loaded by the hydraulic pressure in the water passage. This design worked well but allowed a large amount of leakage into the bulb, which was collected in a shroud and drained to the station sump.
The new shaft seal type selected was a packing box. Although carbon segment type seals were considered, Manitoba Hydro felt that a packing box could be designed and delivered faster and would be more robust and simpler to maintain.
Although the traditional turbine shaft packing box design is very simple, the Jenpeg seals required unique features that made the design more challenging. Because the primary objective was to keep the shaft fillet dry, a typical upstream shaft seal sleeve was not possible. A steel ring was designed to attach to the runner coupling flange and act as the running surface for the packing box. Also, because a packing box must leak to some degree for cooling and lubrication purposes, an overhung shroud was provided to capture leakage to prevent wetting of the shaft fillet.
After many iterations, the form of the seal was finalized. The new shaft seal was designed so that no disassembly is required to access the fillet, reducing inspection outage durations from weeks to less than one day.
Instrumentation was added to monitor the condition of the shaft seal. Two resistance temperature detectors (RTDs) were installed in the shaft seal follower to monitor seal temperature, and a capacitance probe was installed in the shroud to provide an alarm if leakage exceeded the drain capacity.
Manitoba Hydro personnel installed the new shaft seal. Once the new shaft seals were installed, the sensitive area of the turbine shaft was coated with an overseas shipping-grade rust inhibitor to protect against drips and humidity. This coating can be easily removed using solvent to perform nondestructive examination of the shaft fillet.
There have been some setbacks and lessons learned over the course of this work. For example:
- Every shaft cracking scenario has unique characteristics (depth, location, density) requiring an experienced machinist to modify and adapt tooling to effectively complete the work;
- Creativity is required when addressing unique machining applications when specific tools have not been designed to complete the job. In the case of Jenpeg, finding a machine application that has similar characteristics (pipeline weld machining) and modifying the concept eliminated the need to remove the turbine shaft to complete the repairs; and
- Removing material from a turbine shaft in a high-stress area does not always reduce the service life of the unit and, in some instances, can improve the safety factor.
Overall, the turbine shafts have been repaired and the new seals are performing as intended, keeping the shaft fillets dry and corrosion-free, thus ensuring a reliable service life in the future.
Paul Halipchuk, P.Eng, a senior mechanical design engineer with Canadian provincial utility Manitoba Hydro, was the lead design engineer for the Jenpeg repair. Kevin Penner, P.Eng., a maintenance engineer with Manitoba Hydro, coordinated the initial investigation and managed the unit rehabilitation project. Scott Smith is sales manager for PeakHydro Services Inc., the company that performed the machining work at Jenpeg.