What began as a unit inspection at a vintage hydroelectric project turned into a complete mechanical rebuild, with several challenges and change orders amounting to $1.2 million.
By Jay Pickett, John Stender and Linda Fulsaas
The city of Centralia, Wash., sought a 15- to 20-year maintenance solution in early 2010 for its 12-MW Yelm Hydroelectric Project, a run-of-river facility on the Nisqually River about 20 miles east of Olympia, Wash. The city has routinely maintained this 1930s facility, which supplies nearly a third of the city's electrical needs.
The project consists of a 20-foot-high concrete gravity dam that diverts water to a downstream powerhouse via a 9.1-mile-long canal. There is no impoundment at the diversion dam, which has a hydraulic height of only 4 feet and during high stages is almost completely submerged. (The difference between headwater and tailwater is less than 1 foot.) The diversion dam, which includes a fish bypass, was constructed in 1930, expanded in 1955 and reconstructed in 1985.
The powerhouse contains three vertical Francis units that operate at 208 feet of head and are fed by two 7-foot-diameter penstocks. Units 1 and 2, both 3-MW machines built by Pelton Water Wheel, share a penstock. Unit 3, a 6-MW unit built by S. Morgan Smith, is fed by the other penstock. The first two units were installed in the 1930s, and Unit 3 was added during an expansion in 1950. Discharge from the powerhouse is returned to the Nisqually River several miles downstream from the diversion dam.
Through a competitive bid process, the city sought services to perform repairs for known conditions and inspections to evaluate the condition of Units 1 and 3. These vintage units, which were nearing end-of-life mechanically, were not operating well despite routine maintenance. Problems included noticeable vibration, rough operation (startup was not smooth) and output below nameplate capacity. Unit 1 had a history of erosion of the inner headcover, which required frequent inspections and repairs, and the rotor and stator windings were saturated with oil due to leaks in the bearing lubricating and high-pressure lift systems. Unit 3 was experiencing uncontrolled leakage at the turbine shaft seal and binding of the wicket gates during load changes when the wicket gates were positioned in the 50% to 60% open range. Unit 2 was not included in this work.
A thorough inspection and assessment of the two units was in order while repairs were being performed. The original scope of work called for:
Unit 1: Remove the rotor and clean the rotor and stator (including the sliprings) of oil and deposits, as well as inspect the headcover, runner and wear rings to evaluate work performed in 2007; and
Unit 3: Replace the carbon seal and carbon seal sleeve on the vertical shaft, identify and repair a wicket gate binding problem, inspect the headcover and runner and wear rings, perform electrical testing of the rotor and stator, and clean the rotor and stator winding.
Inspection uncovers significant issue
NAES Power Contractors of Issaquah, Wash., was awarded the contract to conduct a thorough on-site evaluation of the two units, including documenting all findings. NAES began the inspection of Unit 3 in August 2010. The team found that six of the wicket gates were making contact with the face plates, and the runner was significantly out of alignment. The runner seal ring clearance was 300% greater than the manufacturer design.
The team attempted to center the unit, adjusting the spider, thrust bearing bracket and stator to plumb the shaft. As expected, the turbine seal clearances were fairly centered when hand-measured using feeler gages. In addition, the shaft was nearly plumb and the air gap between the rotor and stator was out of center. What the team did not expect is that, despite the fact that feeler gage measurements indicated the critical fits were clear, the turbine was mechanically bound (metal on metal).
To determine the best course of action, NAES consulted with the city of Centralia. An engineering firm was then consulted to investigate the misalignment of the unit. The cause was determined to be settling of the portion of the powerhouse containing Unit 3, which had settled since it was built in 1950 and was tilted at an angle of about 1 degree. Cracks observed in the powerhouse wall suggested that the newer portion of the powerhouse had shifted away from the older portion of the building, which houses Units 1 and 2. Due to the shifting of the powerhouse foundation, the headcover of the turbine was found to be out of level by about 0.005 inch per foot.
The tilt in the building caused the embedded steel to be seated at an angle, which resulted in the turbine components binding during operation. With a mass of 30 to 40 tons rotating at 400 rpm, plus the downward force of water during operation, it hadn't taken much of a 'tilt' to cause significant damage to Unit 3. Critical surfaces within the turbine - including the faceplates, wicket gate bushings and wicket gates - had decades to slowly become damaged as the building shifted.
|The 12-MW Yelm powerhouse, on the Nisqually River in Washington State, houses three vertical shaft Francis turbines.|
Evaluating the realignment of Unit 3
Because it would be impractical to realign the powerhouse building, both structurally and from a cost perspective, the team decided to realign Unit 3. Three options were considered:
- Align the shaft to meet the current turbine angle, then run the unit as is;
- Align the center of gravity of the rotating mass with the earth (leave the shaft as is); and
- Re-machine the embedded steel at angles to match the tilt in the powerhouse building.
The first option required that about 0.026 inch of run-out be captured in the guide bearings, leaving about 0.006 inch of clearance per side and the thrust bearing carrying an unequal dynamic force (see Figure 1). The team re-babbitted and machined the thrust and turbine bearings to new dimensions but determined these bearings would potentially be damaged again once operation commenced. The top guide and lower guides for the generator were left as-is.
For the second option, full alignment of the rotating mass would align critical surfaces, align the center of gravity for smooth operation, and close the turbine seal clearances, yet it would remove run-out. The team experimented with moving the bridge and found that the turbine seal clearances could be maintained at about 0.020 inch after proper alignment.
The third option, which would be time-consuming (requiring several months) and complex, included reworking all the embedded critical parts to introduce an angle that matched the shift in the powerhouse. This option also required aligning the center of gravity of the rotating mass with the earth, essentially incorporating the efforts of the second option as well.
Realigning Unit 3
The team determined that a combination of these options would be the best method to deal with the shifting of the embedded parts. The team prepared for the realignment work by repositioning the bridge and stator. Several adjustments were required to compensate for the unlevel embedded parts.
Unfortunately, the team was unable to find the center of the unit. All stator pads were shifted and the stator pad bottom nuts were tightened. Subsequent attempts to move the stator did not move the pads. The stator pads were then placed in original position for repair. Measurements indicated that the turbine was not centered on the shaft but instead was 0.01 to 0.02 inch off center. The next day, when the team removed the headcover, they discovered a nick on the upper stationary wear ring, causing it to make contact with the runner wear ring. The team corrected this by stoning the damaged area.
Starting from the ground up, the team reverse-engineered several components - including the crown plates and curb plates - because component drawings were not available. New upper and lower stationary wear rings also were fabricated and machined to meet manufacturer design clearance.
Once the unit was finally aligned, the team still had to complete several items: replace the air housings; align and install the lower guide bearings; set the wicket gates, install packing and connect the shift ring and servo arms; and install and connect the excitation. The last step was to perform mechanical commissioning and electrical tests before releasing the unit to commercial operations.
In all, the additional work scope items for Unit 3 included:
- Refurbish wicket gate sealing surfaces, bearing journals and turbine bearing;
- Replace wicket gate bushings and stationary wear ring;
- Prepare and coat scroll case surface;
- Repair eroded stay vanes and runner erosion;
- Machine runner rotating wear rings and slip rings;
- Overhaul brake cylinder and replace pads;
- Realign unit;
- Design, fabricate, and install wicket gate thrust collars; and
- Modify thrust bearing shoes and install high-pressure lube system.
Unit 1 inspected and repaired
Unit 1 was operating during the inspection, evaluation and work on Unit 3, and although plant output was acceptable, it was well below nameplate capacity. As the city of Centralia needed to get the plant back to full operation once the river began to rise in the spring, work on Unit 1 began in December 2010.
During disassembly of Unit 1, the team discovered the shaft had made contact with the headcover, and the headcover was cracked at the contact location. The team undercut the contact area on the shaft packing sleeve and repaired it using chrome metal spray.
The team also found that the inner headcover was severely damaged by erosion and needed to be replaced. This same type of erosion damage had been discovered twice before during past maintenance activities. The current project team was asked to investigate the issue to determine the root cause.
The team determined the headcover erosion had not started to occur until a new runner was installed in the late 1990s. The team determined the embedded parts provided for a pad height of 7.5 inches and that the runner installed in the 1990s was designed for a pad height of 7 inches. To correct this issue, the team modified the runner to fit the 7.5-inch pad height.
During reassembly of the unit, the team discovered that the fit between the thrust collar and shaft was causing excessive run-out at the runner. Corrective action required machining the generator shaft and thrust collar.
In all, the additional work scope items for Unit 1 were:
- Refurbish wicket gate sealing surfaces, bearing journals and guide bearings;
- Replace wicket gate bushings, stationary wear ring and inner headcover;
- Modify runner vane opening height to resolve inner headcover erosion;
- Modify thrust bearing shoes and install high-pressure lube system;
- Machine thrust bearing collar and rotor shaft collar mating surface; and
- Realign unit.
|Once the alignment problem with Unit 3 was resolved, it was reassembled and returned to service in January 2011.|
What began as a simple unit inspection turned into a complete mechanical rebuild with significant challenges. Throughout the project, NAES coordinated with several vendors and suppliers to find solutions to every challenge that arose. In addition, the project team often had to make decisions with unclear or insufficient data, requiring expertise from all parties involved. A simple and routine inspection had instead delivered unexpected additional work, outage delays and project costs.
Early in the project, the team implemented a task/work order process to expedite the work and as part of a financial strategy to garner support from the city. City Council involvement was crucial for approving project expenditures all along the way. By the end of the project, six change orders had been written totaling $1.2 million, all approved by the council to ensure the facility would achieve the maximum benefit from dollars spent.
Unit 3 returned to commercial operation on January 17, 2011, and Unit 1 returned to commercial operation on June 16, 2011. Both units are running well today. The collaboration of all parties involved - the owner, contractor, various vendors and suppliers, and City Council - ensured a smooth and successful outcome to what turned out to be a challenging project.
Jay Pickett, MBA, generation and system operations manager, oversaw the Yelm rehab for Centralia City Light. John Stender, director of hydroelectric maintenance and construction, oversaw the work for NAES Power Contractors. Linda Fulsaas is senior technical writer for NAES Power Contractors.
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