When years of gross cavitation at the Pointe du Bois station prompted a turbine blade replacement as opposed to a repair, Manitoba Hydro elected to replace the Francis blades in situ to reduce cost.
By Steven R. Potter
Canadian provincial utility Manitoba Hydro owns and operates the 120-MW Pointe du Bois generating station on the Winnipeg River in southeastern Manitoba. The project, constructed in 1909, has 15 camelback double Francis turbine-generator units and one Straflo unit.
Over the course of years, several turbines experienced partial loss of blades and significant band damage due to severe cavitation. Manitoba Hydro determined that a business case could be made to repair and restore the turbines with a view to extending station life. In the end, the utility chose to replace the damaged blades in situ, reducing overall cost of the work.
|The Pointe du Bois generating station shown above is the oldest in Manitoba Hydro's system, and is being refurbished as part of a multi-year modernization project.|
Understanding the situation
The oldest generating station in Manitoba Hydro's system, Pointe du Bois began operating in 1911 on the Winnipeg River, 95 miles (150 kilometers) northeast of Winnipeg. City Hydro, later known as Winnipeg Hydro, built the project. With the change of ownership, Manitoba Hydro experienced operating and maintenance problems on several units. Manitoba Hydro brought new capital and management to the powerhouse to support the local maintenance group. A project engineering team assessed the units and for safety reasons took several units out of service for more extensive evaluation and repairs. Repairs were to be done internally where possible, with the exception of specialty contract services.
In 2007, Manitoba Hydro initiated plans for an $800 million modernization program to generate more power for the Pointe du Bois plant. The utility started by seeking regulatory approval from the province of Manitoba to replace the 78-MW powerhouse, dam and spillway with a 120-MW powerhouse. Construction was projected to take six years, with a planned in-service date of 2015. However, this plan was withdrawn in 2011.
Business case for repair
Following the decision to withdraw the planned replacement of the Pointe du Bois station, Manitoba Hydro developed a plan to refurbish and repair the failing units. Station refurbishment is a project encompassing mechanical, electrical and structural elements.
Units 15 and 16 had been retrofit with new runners in 2006 and, although they suffered only minor cavitation, they were inoperable due to runner-to-shaft coupling issues and were removed from service. Additional units had been withdrawn from service for varied lengths of time; some were partially repaired, others were returned to temporary and/or limited service pending further repair or refurbishment, and others still were deemed inoperable.
Runner damage was extensive on Unit 14, involving partial loss of the upstream runner band; this unit is currently subject to a separate repair scheme.
Unit 13 is a double runner horizontal shaft 4.69-MW Francis turbine. The turbine casing consists of two cast iron halves. Both Unit 13 turbines exhibited extensive and gross cavitation damage. The turbine runner bands were intact except for cavitation damage, making it the focus of the repair project.
The turbine is located in a pit fed from the Winnipeg River. Water enters the unit at both upstream and downstream distributors, passes through the runners and is discharged through a common centrally positioned draft tube. The turbine shaft penetrates the pit wall, connecting to the generator.
The Pointe du Bois station sees significant seasonal temperature changes, which contribute to substantial rise and fall of the structure elevations. Structural elevation changes cause turbine and generator alignment issues, which impart stresses on the shaft and casing.
Manitoba Hydro's experience with another Pointe du Bois unit, where repairs required the casing to be separated, resulted in a challenging realignment. As a result, Manitoba Hydro elected not to separate the casing on Unit 13 and instead to attempt the cavitation and other repairs in situ using in-house employees and internal procedures. Pointe du Bois has a full local maintenance staff supported by engineering and project management based in Winnipeg. Station staff partially disassembled the unit to provide access to the runners in 2010. Man doors on each side of each end of the casing provide access to the inside, with work platforms installed over the draft tube.
Cavitation repairs had been performed throughout the station's history. The Manitoba Hydro engineering team determined from initial repair efforts at Pointe du Bois that with minimal internal welding resources, repairs would take too long to complete. Manitoba Hydro reassessed the situation and developed a plan to contract repairs and to restore all (or as many as possible) units to service by a combination of repair and component replacement using hydropower original equipment manufacturers, specialty hydro repair service providers, in house labor, and engineering and project management teams. Unit 13 is typical of the design of all units at the plant and operates at 45 feet (13.7 meters) of head.
With the withdrawal of the station replacement plan, the engineering and project management teams of Manitoba Hydro developed a plan that called for a 20-year life extension of Pointe du Bois by repair of the existing equipment. By implementing this plan, Manitoba Hydro could prepare more extensive plans to replace the station in its entirety or replace some or all units, or otherwise, as part of the corporation's future resource management.
During this process, Manitoba Hydro determined that replacement of the units was the most cost-effective solution. However, Units 13 and 14 were already partially disassembled; Unit 12 is of the same type and would involve a similar repair. Units 15 and 16, having previously been equipped with new runners, were considered to be relatively easy to repair. Subject to cost considerations, the utility chose to contract the repairs of Units 12 through 16 while the economic study/design phase of unit replacements was ongoing and, depending on the success of the replacements, Manitoba Hydro may decide to replace Units 12 through 16 later.
Manitoba Hydro contracted Voith Hydro Canada in Mississauga, Ontario, to evaluate the viability of repairing Unit 13. They in turn contracted Peak Hydro Services (a wholly owned subsidiary of Voith Hydro) to assist in assessing repair potential.
Analysis of metallurgical samples from the runner crowns, blades and bands taken as early as 2007 revealed that the chemistry of these structures was similar to ASTM A27 Grade 70-36, with each area exhibiting differing carbon content (carbon content determines weldability and, in part, crack susceptibility). A site inspection was conducted in 2010 by Voith Hydro and Peak Hydro, as well as other independent consultants. The Voith Hydro and Peak Hydro assessment ultimately concluded that the downstream runner of Unit 13, while grossly cavitated, was salvageable and could be restored through judicious application of in situ welding. The upstream runner was in considerably worse condition (true of all upstream runners at the plant). Several blades exhibited such gross cavitation that there were multiple through holes, and cavitation was so deep in places that establishing preparation of clean sound parent metal was not possible.
Manitoba Hydro determined that a minimum of three blades (unit total of 13) would need to be replaced. Project repairs costs for in situ cavitation and blade replacements were estimated to be substantially less than the costs of replacement runners and about half the project outage time due to runner manufacturing lead time. Therefore, the schedule for returning the unit to service was a year shorter than the alternative. Overall work will go on for four to six years, and the unit turbine work is complete. Work on Unit 13 began in February 2011.
|The old and new welds were fused using flux core arc welding, a specific method that addresses defects in the metal.|
Technical aspects of the repair
Voith Hydro and Peak Hydro performed a joint inspection during a site visit with other potential suppliers at Pointe du Bois at the invitation of Manitoba Hydro. Mehrzad Shahouei of Voith Mississauga Canada led the engineering effort, while the author provided the welding determinations for Peak Hydro. Examination revealed that all of Unit 13's 13 runner blades on both upstream and downstream runners were severely cavitated, along with adjacent areas of the runner bands.
Each blade had, over its lifetime, been repaired by gouging, grinding and welding multiple times. Some areas of prior weld repairs were undermined (weld metal not adhering to base metal), revealing that the preparation prior to repair had not been sufficient in removing damaged cavitated metal. In some cases, "foreign" material such as rebar had been included as filler to assist the welder. In other areas, patches had been welded to blade edges. Major areas of cavitation were on the suction side (typical) of the blades from the bucket area to the band fillet. Runner bands exhibited cavitation adjacent to blade fillets.
Based on the metallurgical analysis Manitoba Hydro had obtained in prior years, and despite elevated carbon content, it was determined that the material (classified as medium carbon steel) was weldable. Voith Hydro's welding engineering department in York, Pa., provided appropriate welding procedure specifications. Three blades on the upstream runner were in such poor condition that it was doubtful weld repairs could be made, with blade areas exhibiting cavitation completely through the blade. It was determined that the most cost-effective and least risk alternative was to replace these three blades by casting replacements.
Several blades were identified as being in "reasonable" physical condition and could be used to provide baseline profiles for reverse engineering. These runner blades were three-dimensionally laser scanned and the data translated into a CAD model and drawings from which blades could be cast.
Welding of castings and repair welding
The Pointe du Bois runner castings contain gas pockets, pores, voids and casting dross (impurities). Water immersion of the components allows moisture to migrate throughout the casting, filling those voids and pores. Moisture in the substrate is not conducive to high-quality, defect-free welding product. Trapped water pockets will flash to steam at exposure to the extreme temperatures of fusion welding, causing gas to become entrapped in the weld metal. A greater concern with the presence of entrapped water is the probability of hydrogen embrittlement of the material. Moisture must therefore be removed. Techniques and methods have been developed to remove substrate moisture by extended periods of preheating and repair area preparations (gouging, grinding and thermal surfacing). Welding procedures have been progressively modified through a series of tests to find the combination of techniques that consistently yields the least defects and post-weld indication results.
Each work task was subject to deliberate and careful planning well in advance of the site start dates. Given the extremely cavitated condition of the runners and the amount of welding required, welding engineering considered efforts to minimize weld distortion, shrinkage and potential defects from both the preemptive and post facto perspectives. Planning in each phase required technical substantiation and safe work plan submissions, which were subject to customer review at multiple levels, often requiring several revisions before acceptance by Manitoba Hydro. This meticulous level of forethought, planning and review of documentation played a significant role in avoiding unforeseen circumstances and allowed the site team to receive and act upon directives with a high degree of outcome success assured.
The Unit 13 downstream runner cavitation was repaired while replacement blades for the upstream runner were being cast. Due to the unit size, available space and safety, efficiency dictated only two welders work on the runner at one time. All 13 downstream blades were repaired. Blade dimensions are about 46 inches long by 16 inches wide with thickness varying from about 0.75 inch at the leading and trailing edges to 1.25 inches at the middle sections. The area of each blade (pressure and suction side) presented 1,472 square inches of surface. On average, the area repaired for each blade was 160 square inches, with some exceeding 250 square inches. Cavitation generally exceeded 0.3 inch deep, in places greater than 1 inch; average deposited weld exceeded (60) cubic inches finished volume.
Each blade was mapped prior to repair (as found condition), and areas for repair were identified. Completed areas of repair (as left condition) were again mapped and photographed. The runner blade pairs were measured for vent opening before and after welding.
Repairs abutting prior repair zones required special consideration, as the prior repairs contained surface and subsurface discontinuities that would not meet the acceptance criteria established for the new repairs. The primary welding process for all areas was flux core arc welding, a derivative process of GMAW (Gas Metal Arc Welding)(MIG) welding. To address the defects that randomly occurred at the repair margins, the gas tungsten arc welding process was used. This allowed the welders to address defects and discontinuities with more precision and to "float" out underlying contamination (dross, gas pockets and inclusions) and ensure complete fusion of old and new welds.
The skills applied by the welders proved very successful in eliminating defects and providing acceptable repairs to Manitoba Hydro. In-process inspections were conducted using liquid dye penetrant. In some instances where cracks were evident, magnetic particle testing was used. All inspections were witnessed by Manitoba Hydro.
|The runner blades were removed in situ, which means without separating the unit casing and using in-house employees and procedures.|
Removing, replacing the blades
Upstream runner cavitation repairs were undertaken using the same procedures developed and honed for the downstream runner. All cavitation repair was performed before replacement of the blades. During those repairs, the site team determined that one of the blades originally identified for replacement was actually salvageable.
Peak Hydro determined that the best solution was to use butt weld preparations as opposed to the natural fillet conjunction of band, crown and the blades in the "as cast" configuration because these thicker fillet areas are normally areas of casting that exhibit the largest, deepest voids and pores. Further, post-weld inspection (using ultrasound) would detect anomalies inherent to the casting while not being able to distinguish between existing casting issues and the newly welded joints. Initially the butt weld joints were to be located 1 inch from the band and crown fillets. This was later moved further outward from the blade junctions to lower-stress zones. Cavitation and other defects were excavated from the four weld zones at the base of each blade's junction with the runner band and crown, with the original blades still in place to provide structural integrity to the runner and bracing to the band during repairs.
Chain blocks and restraining/locating plinths were welded to adjacent blades to ensure that when cut free, the blade would not slip out of position.
Each blade was then plasma cut at the band and crown and removed from the runner. New blades were compared to old blades for shape and position. New blades were trimmed to length and joint geometry cut and ground in preparation for installation.
The replacement blades were fitted in place and welded according to qualified weld procedures, then subjected to ultrasonic inspections by a third party inspection company. Three of the four welds passed the first inspection, and one of the four required repair before being accepted to ASME VIII standards on reinspection.
Welding repairs to both downstream and upstream runners have proven very successful, restoring structure integrity, blade thickness, profile, fairness and Hydraulic Balance. While still a blend of old and new repairs, the surface area of quality weld material has substantially increased, particularly in areas prone to cavitation. All major defects have been repaired, and all cavitation has been removed, while some small surface pits typical to castings remain.
The two blades replaced in situ were subjected to full non-destructive testing examination in addition to dimensional checks and found to be acceptable to ASME VIII standards.
Steve Potter is a welding engineer/sales manager for the western region with Peak Hydro Services Inc.