Jay Pickett, John Stender and Linda Fulsaas
April 02, 2012 | 0 Comments
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:
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.
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:
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.
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