October 15, 2008

Repairing low-pressure rotors with cracked blade attachments

Bruce Gans, TurboCare Inc.; Darryl A. Rosario, Structural Integrity Associates Inc.; and Jim Olson and Jerry Best, Tennessee Valley Authority

An increasing number of low-pressure steam turbines—especially at supercritical fossil units—have experienced stress corrosion cracking in the blade attachment region of their low-pressure rotors. Approaches to solving this problem range from redesign of the attachment and blade replacement to in-situ weld repair. Regardless of the procedure selected, the solution must completely restore the turbine performance while minimizing outage duration.

Tennessee Valley Authority’s (TVA’s) Paradise Fossil Plant, located in western Kentucky, consists of three units that began commercial operation between 1963 and 1970 that have a total generating capacity of 2,273 MW (Figure 1). In 2007, there was trouble in Paradise at Unit 3.

Unit 3 is equipped with a Babcock & Wilcox supercritical steam generator operating at 3,500 psig with 1,000F main and reheat. The 1,150-MW steam turbine is a General Electric cross-compound design whose high-pressure (HP) and first reheat (IP1) turbine section are coupled to a 3,600-rpm generator. The double-flow IP (IP2) and the two low-pressure (LP) turbines are connected to a second generator, rotating at 1,800 rpm. Each LP turbine has a double-flow configuration with 52-inch last-stage blades (Figure 2).

TVA’s standard steam turbine inspection interval is approximately 10 years. The spring 2007 maintenance outage at Unit 3 included a standard nondestructive examination (NDE) rotor inspection with phased array ultrasonic test (UT) inspections of the L-2 and L-3 blade wheel attachments in the LP turbines. Test results showed multiple indicators of what was believed to be stress corrosion cracking (SCC) on both LP rotors. The indicators were confined to the L-2 and L-3 stages of each LP rotor, although the extent and severity of cracking in the dovetail attachments was different between the stages.

There are 127 blades plus 1 locking (notch) blade in the L-3 row. Also, the L-2 stage on the LP-B rotor shroud covers on a section of the stage appeared to have moved outward and contacted the stationary diaphragm (Figure 3). The shroud failures and disk root indications strongly suggested that an early, extended repair outage was necessary.

The problems experienced by Paradise Unit 3 may be expensive and time-consuming to repair, but they are not unusual. To assist others facing similar problems, this article reviews the repair options for each stage identified by the TVA, the decision criteria used, and the solutions selected.

L-3 stage repair

The vulnerability of the L-3 stage dovetails to SCC during normal operation is limited because minimal wetness is present at this location. The steam dry-to-wet phase transition zone in an LP turbine is typically the location of the worst SCC. In the case of Unit 3, SCC was identified in both LP turbine rotors. Severe cracking in the dovetail of a typical fossil LP turbine is illustrated in Figure 4.

For the LP-B rotor’s L-3 stage, TVA elected to remove the notch blades to confirm the UT inspection results as well as the location and extent of the indications. A magnetic particle test (MT) was used to confirm the indication depth and length. Only two of the eight indications reported by the UT were confirmed with MT, but another five indications were found by MT that had been overlooked by UT.

From a remaining life standpoint, the worst combination of indications were aligned circumferentially at the stage 2T upper and middle hooks near the notch entry after excavation (Figure 5).

Additional UT tests of the L-3 stage LP-A rotor were performed in April 2007. The most significant indications were reported in lower dovetail hooks of the L-3 stages. The worst indication was measured by MT after excavation as having a maximum depth of 0.56 inch and a length of 2.5 inches (Figure 6).

Evaluating the options. For the two rotors, both short-term and long-term mitigation strategies for dovetail SCC were considered. The good news was that the cracks found on each wheel were confined to the entrance notch area. Sorting through all the available short-term repair options produced a short list of strategies that would minimize the length of a repair outage:

• Do nothing.
• Reduce loading at crack locations adjacent to the notch by pinning the notch blade directly to the wheel
• Reduce blade load by using titanium blades, which are 43% lighter than steel, although replacing steel blades with titanium blades creates a mass imbalance on the rotor.

To determine which option to choose, TVA’s detailed analysis began with collecting the dovetail profile dimensional data and constructing a finite element model (FEM) of the L-3 dovetail. A plot of the rotor dovetail region of the FEM is shown in Figure 7, as are the calculated stresses normal to typical dovetail crack trajectories. These stress distributions are normal to the plane of cracking away from the notch entry, at rated speed (1,800 rpm). The blade root (dovetail) FEM section (Figure 7) was also included in the FEM with gap (contact) elements simulating load transfer from the blade to the disk at the top, middle, and bottom loading lands.

The stresses do not include the load increase at the notch entry. This is accounted for in LPRimLife software using a load scale factor. LPRimLife is a program that evaluates the remaining life of rotors with known or suspected blade-attachment cracking. Its development by Structural Integrity Associates was sponsored by the Electric Power Research Institute.

The first step in the evaluation was to estimate the stage operating temperature and wetness. Wetness is a prerequisite, as SCC is not expected for attachments that encounter superheated steam during steady-state operation but during transient start-up and shutdown conditions. However, wetness during operation, when the dovetail attachments are fully loaded, makes the attachments susceptible to SCC.

Three loading options at the notch were evaluated over the expected operating load range, at overspeed up to 110%, with two different rotor start-up temperature profiles, and the like, as required by the rotor fracture mechanics software code. The code also accounted for the crack depth and length in the upper, middle, and lower hooks and was based on the NDE and/or grindout confirmation values. A crack depth uncertainty of ±0.060 inch and length uncertainty of ±1.0 inch were simulated.

Also, unlike the dry-to-wet transition predicted to be just upstream of the L-2 stage, and confirmed with the extensive SCC discovered in the L-2 stage dovetails, the L-3 stage is not expected to run “wet” during normal operation. The less severe cracking in the L-3 stage dovetails, which was confined to the entrance notch area, is consistent with this prediction. To account for transient wetness, 1,750 hours per year of wet time were conservatively simulated in the L-3 dovetail evaluation.

Evaluating simulation results. The probabilistic simulation results from LPRimLife estimated the “cumulative probability of failure” versus operating time in years for each of the three repair options:

• The “do nothing” option returned very high failure probabilities for the notch after another year of operation.
• The option of pinning the blade to the wheel as an interim fix significantly reduced the failure probability to below 1% for five more years of operation.
• The option of replacing the banded group of blades at the entrance notch with titanium blades resulted in a prediction of 10 more years of operation with a failure probability below 0.01%, or 20 more years with a failure probability below 0.65% (Figure 8).

TVA selected the third option as the most effective interim repair for mitigating the risk of dovetail failure at the notch.

Rebalancing. To minimize repair cost and reduce repair time, TVA did not want to disturb the remaining blades on the wheel. But it knew that the difference in material density between the existing and the new, lighter-weight blades would create a significant mass imbalance on the disk that would adversely affect rotor vibration. The entrance slots to the two L-3 stages are oriented 180 degrees out of phase with respect to each other. This creates a dynamic couple imbalance on the rotor that requires a significant weight correction.

To address this concern an analysis was completed that determined the potential imbalance on the rotor and the expected change in rotor vibration. Because both L-3 stages on each rotor would be modified with the titanium blades, the total imbalance would be double that for one disk. The correction capacity for the existing balance planes on the L-1 and L-4 stages was insufficient to correct for the expected imbalance. The L-0 stages were not considered for the correction but were reserved for trim balancing in operation.

To correct for the imbalance, two vanes were removed from each disk at approximately the five and seven o’clock positions to counterbalance the mass reduction at the titanium blades. The removal of the blades required the shroud band groupings to be evaluated to ensure that there was not a significant change to blade natural frequencies. The LP-A rotor also presented the complication of a lacing (tie) wire in both disks that required adjustment with the removal of the two vanes.

Removal of the two vanes did not precisely balance out the titanium group, so each rotor was low-speed balanced prior to reassembly to minimize residual imbalance. Acceptable rotor vibration on both rotors was achieved running up through the critical speeds and at operation speed. No trim balance corrections were needed on either rotor.

The design, machining, and assembly of replacement blades for the L-2 stages were completed concurrently with repairs on the L-2 stages, discussed in the next section.

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