L-2 stage repair
Ultrasonic testing results for the LP-B rotor found many more indications than on the L-3 rows just discussed. Ninety-eight indications were dispersed on all the hooks and were distributed around the entire wheel with a depth ranging from 0.04 inch to 0.39 inch on the generator end. The LP-A rotor (turbine end) was found to be in a similar state, with 78 indications ranging in depth from 0.04 inch to 0.26 inch on all three hook fillets and on both sides of the wheel.
Bigger problems/bigger solutions. TVA expressed a strong desire to maintain its original outage schedule on the turbines and to minimize any reduction in power generation after the repairs were completed. TurboCare, in conjunction with Structural Integrity, investigated several repair options to achieve TVA’s goals. Collectively, the team determined the best solution was a longshank replacement design. However, the longshank repair would also require the longest repair time in an already compressed upcoming planned outage.
The L-2 stage was much more susceptible than the L-3 stage to SCC because of higher stresses in the root and higher moisture content in the steam. The L-2 disks had more extensive disk dovetail cracks, requiring a complete redesign of the blade, modification of the wheel rim, and the use of titanium at the notch area to reduce blade attachment stresses. The redesign also included L-2 blade frequency testing and optimum frequency tuning of blades with over/under shroud covers for vibration control.
Repair of the L-2 stage involved machining the wheel root form in undamaged material. The general approach has been to first remove all blades and then to grind out the deepest indications to determine the crack depth. The minimum distance required to reestablish the root form is determined by overlaying the excavations and the root form. This approach could have lengthened the outage.
Because of TurboCare’s experience with many other longshank blade projects, the amount of drop—and therefore the amount of material removal required to ensure all the cracks were removed—was engineered and implemented with minimal delay. Concurrently, the design and manufacture of the replacement blades was started before the original blades were removed from the existing wheel.
The longshank redesign process also allowed the attachment form to be improved from stock conditions. The dovetail was machined with modified fillet radii to reduce the peak stresses for two reasons: to offset the additional weight from the longshank modification and to reduce the stress concentration factor of the geometry, which contributes to SCC (Figure 9). The reduction in peak stress is typically 10% to 15% for this modification.
9. Add longer blades. The blade on the left side is nearly identical to the L-2 longshank design used on this project. To accommodate the longer blade, the rotor must be machined to a reduced diameter and a new dovetail added. Courtesy: TurboCare Inc.
Frequency and vibration management. An important element in this process was designing a replacement blade with natural frequencies away from the operating speed. The tuning of frequencies was required to compensate for the change in the root attachment location. Generally in the design process, several parameters are investigated to optimize frequencies such as vane scaling, shroud configuration, shank length, and blade count.
Despite the short lead time to engineer a repair, all design calculations were expedited to minimize any outage delays.
Another important feature of the design is the use of chain link or over/under shroud covers. This design replaces the original single shroud segment with a two-tiered shroud. The inner shroud is assembled with a clearance around the tenon, and the outer segment is rigidly connected to the upper section of the tenon (Figure 10). The inner and outer segments are circumferentially offset to provide a continuous coupling of the blade tips. This configuration provides a significant increase in vibration damping and also suppression of several fundamental vibration modes caused by steam path excitation. This design provides an additional vibration safety margin with the ability to supplement tuning of blade frequencies to avoid the impulse line (1X running speed) with both the five and six nodal diameters.
10. Lapped joints. A typical over/under shroud assembly was added to L-2 to continuously couple the blade tips. This design approach increases blade vibration damping. Courtesy: TurboCare Inc.
To decrease the likelihood of SCC reoccurring in the repair, the design included five titanium blades at the entrance notch. Experience has shown that SCC usually occurs at this location first because of the locking closure piece arrangement. Titanium reduces the centrifugal load of the blades on the wheel in this area because of the 43% reduction in material density. To minimize the potential mass imbalance on the rotor for the titanium blades at the notch, five titanium blades are assembled 180 degrees opposite the entrance notch group.
The complete treatment plan. The final design required removal of all the SCC-damaged material and replacement with a set of blades tuned away from resonance frequencies, superior damping for vibration control, and improved geometry to reduce the reoccurrence of SCC.
These repairs were supplemented with a low-speed rotor balance at the site for a smooth turbine restart.
—Bruce Gans (bgans@turbocare.com) is chief technical officer for TurboCare Inc. Darryl A. Rosario, PE (drosario@structint.com) is an associate with Structural Integrity Associates Inc. Jim Olson (jrolson@tva.gov) is a principal engineer and Jerry Best (hgbest@tva.gov) is manager of the steam cycle and generator systems department for Tennessee Valley Authority.
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