LProtor Repair

7/29/2019 LProtor Repair

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Repairing low-pressure rotors with cracked

blade attachments

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

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An increasing number of low-pressure steam turbinesespecially at supercritical fossil unitshave 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 Authoritys (TVAs) Paradise Fossil Plant, located in western Kentucky,

consists of three units that began commercial operation between 1963 and 1970 that have a totalgenerating capacity of 2,273 MW (Figure 1). In 2007, there was trouble in Paradise at Unit 3.

1. Many cracks found. Unit 3 at Tennessee Valley Authoritys Paradise Fossil Plant uses asupercritical steam generator operating at 3,500 psig. The steam turbine is configured as a cross-

compound four-flow with 52-inch last-stage blades (CC4F52). Rotor repairs were recently

completed on both low-pressure steam turbines to correct cracks found in blades and rotorscaused by stress corrosion cracking. Courtesy: TVA

Unit 3 is equipped with a Babcock & Wilcox supercritical steam generator operating at 3,500psig with 1,000F main and reheat. The 1,150-MW steam turbine is a General Electric cross-
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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-flowconfiguration with 52-inch last-stage blades (Figure 2).

2. Double rotor inspection. The low-pressure turbine is configured with two, double-expansion

turbines. This is the Unit 3 LP-B rotor chucked up in a lathe for machining. It weighs 308,275

pounds and is 17 feet from tip to tip of the largest blades. Courtesy: TVA

TVAs standard steam turbine inspection interval is approximately 10 years. The spring 2007

maintenanceoutage 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 wheelattachments 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 dovetailattachments 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 stronglysuggested that an early, extended repair outage was necessary.
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3. Shroud of turbine. The inspection also found that the LP-B rotor L-2 stage shrouds had failed.

Courtesy: TVA

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 repairoptions 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 LPturbine 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.


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4. Imminent failure. A typical low-pressure steam turbine blade dovetail damaged by stress

corrosion cracking, which can cause extensive damage if the condition is not quickly corrected.

Courtesy: TurboCare Inc.

For the LP-B rotors 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 indicationsreported 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).


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5. Many indications found. LP-B rotor critical indications numbers 8 and 10 on the L-3 stage

discharge side middle and upper hooks are shown after completing excavation. The indicationswere caused by stress corrosion cracking. Courtesy: TurboCare Inc.

Additional UT tests of the L-3 stage LP-A rotor were performed in April 2007. The mostsignificant 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).


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6. More problem indicators. LP-A rotor critical indication number 8 is illustrated on the L-3

discharge side lower hook. An excavation of 0.5625 inch x 2.5 inch was ground out for therepair. Courtesy: TurboCare Inc.

Evaluating the options. For the two rotors, both short-term and long-term mitigation strategiesfor 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, althoughreplacing steel blades with titanium blades creates a mass imbalance on the rotor.

To determine which option to choose, TVAs detailed analysisbegan with collecting the dovetailprofile 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 stressesnormal 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 loadtransfer from the blade to the disk at the top, middle, and bottom loading lands.

7. Stressed blades. A finite element model of the LP-A L-3 dovetail determined the level of

stress found normal to the cracks and included load transfer from the blade to the disk on thethree lands. Source: TurboCare Inc.


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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 byStructural 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 theattachments susceptible to SCC.

Three loading options at the notch were evaluated over the expected operating load range, atoverspeed 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, andconfirmed 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. Toaccount 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 afteranother year of operation.

The option of pinning the blade to the wheel as an interim fix significantly reduced thefailure probability to below 1% for five more years of operation.

The option of replacing the banded group of blades at the entrance notch with titaniumblades 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).


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8. Option 3 selected. The choice of repair strategy for LP-A rotor stage L-3 (stage 6) dovetailswas determined by a detailed probabilistic analysis by Structured Integrity Associates using the

LPRimLife life assessment computer program it developed for EPRI. The analysis results

showed that using titanium blades with notch blades pinned to the wheel, with three pins in theaxial to the wheel and four cross-pins, was predicted to deliver 10 more years of operation with a

failure probability below 0.01%, or 20 more years with a failure probability below 0.65%.

Source: TurboCare Inc.

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 thedisk 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 onthe rotor and the expected change in rotor vibration. Because both L-3 stages on each rotorwould be modified with the titanium blades, the total imbalance would be double that for onedisk. 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.


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To correct for the imbalance, two vanes were removed from each disk at approximately the five

and seven oclock 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 therewas 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 rotorvibration 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.

Repairing low-pressure rotors with crackedblade attachments

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

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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 TVAs goals. Collectively, the team determined the best solution was alongshank 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 diskdovetail 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
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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 isdetermined by overlaying the excavations and the root form. This approach could have

lengthened the outage.

Because of TurboCares experience with many other longshank blade projects, the amount of

dropand therefore the amount of material removal required to ensure all the cracks were

removedwas engineered and implemented with minimal delay. Concurrently, the design andmanufacture 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 fortwo 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 reductionin peak stress is typically 10% to 15% for this modification.


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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 vibrationmanagement.An important element in this process was designing a

replacement blade with natural frequencies away from the operating speed. The tuning offrequencies 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 isassembled 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
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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 withthe 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 continuouslycouple 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 theblades 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, superiordamping 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 turbinerestart.

Bruce Gans ([email protected]) is chief technical officer for TurboCare Inc. Darryl A.Rosario, PE ([email protected]) is an associate with Structural Integrity Associates Inc.

Jim Olson ([email protected]) is a principal engineer and Jerry Best ([email protected]) is manager

of the steam cycle and generator systems department for Tennessee Valley Authority.
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