STEAM TURBINE ROTOR STRAIGHTENING

USING ADVANCED WELDING TECHNIQUES

Robert E. Kilroy Jr. Director of Engineering Michael Jirinec Welding Engineer Damian Parham Steam Turbine Engineer Alstom Power Inc. 1200 Willis Rd. Richmond, Virginia 23237

R. Hooper Principle Engineer First Energy Corporation 4701 Bay Shore Road Oregon, OH 43616

Presented at the:

EPRI Conference & Exhibition June 2004 Sandstone, Florida

Printed by Alstom Power Inc., Turbine & Generator Repairs Engineering, Richmond, VA

STEAM TURBINE ROTOR STRAIGHTENING USING ADVANCED WELDING TECHNIQUES

M. Jirinec R. Kilroy D. Parham

ALSTOM Power, Inc. 1200 Willis Road

Richmond, VA 23237

R. Hooper First Energy Corporation

4701 Bay Shore Road Oregon, OH 43616

Abstract

During system operation, an HP/IP rotor sustained a system upset resulting in a water induction in the cold reheat section of the turbine. This transient produced a thermal bow in the rotor resulting in a hard rub and high vibration. Attempts to balance out the rotor eccentricity in the unit were unsuccessful. After considering several temporary repair options, a decision was made to thermally straighten the rotor utilizing advanced welding technology to remove the 24-mil TIR condition and return the rotor to service. The methodology utilized for thermal weld straightening is discussed, including a detailed rotor runout history for each thermal iteration, and the results of an at temperature low speed balance to assure final rotor thermal stability.

Unit Configuration

First Energy Bay Shore # 4 is a Westinghouse 200 MW HP/IP fossil unit. The unit was placed into commercial service in 1968. Main steam temperature is 1050°F at 2400psig. Reheat temperature is 1000°F. A view of the HP/IP rotor is shown in figure 1.

Figure 1. Bay Shore # 4 HP/IP Rotor

Rotor Damage

During unit operation on October 1, 2003, the unit experienced a trip. During the subsequent attempt to restart the unit, the system sustained a thermal upset resulting in a water induction into the Reheat section of the turbine. This resulted in a thermal transient causing a hard rub, compounded into a rotor bow. Attempts to balance out the eccentricity were unsuccessful, and the unit was removed from service. Alstom was asked to evaluate the rotor condition and present a repair scenario to return the rotor back to service.

Inspection Findings

Visual examination of the as-received rotor was performed and confirmed the rotor had been rubbed between the HP blade stages 1-10, resulting in smeared seals and tenon damage. Magnetic particle (MT) examination was also conducted and revealed heat cracking between the blade stages. The results of the visual and MT examination findings are shown in figures 2-3.

Figures 2 & 3. Incoming Inspection Damage on the HP/IP Rotor Incoming Brinell hardness testing was also conducted and revealed several bands of high hardness in between Rows 1-10 HP blade stages as shown in Table 1. The incoming data showed maximum hardness values in the range of HB 321 – 398 (32 – 41 Rc), located mainly at interstage areas 3 through 8 with an arc of approximately 135°. Figure 4 provides a radial plot of this hardness data in regards to radial location and interstage area. After completion of these initial inspections, ALSTOM performed skim cut machining operations in 1mm (0.040”) radial increments up to a 3mm (1/8”) radial depth to take additional hardness readings at interstage areas 5 and 7 only. This data showed an increase in hardness values to HB 390 – 441 (42 – 47 Rc) after completion of each 3mm radial depth cut as shown in Table 2 and figure 5. At this point it was clear that the material thermal damage was not just a surface phenomenon.

ORIGINAL INTERSTAGE SEAL GROOVE WITH SMEARED SEAL & CAULKING

INTERSTAGE MATERIAL HEAT CRACKING

SKIM CUT OF INTERSTAGE AREAS TO SUPPORT HARDNESS CHECKS

POLISHED TENON RIVIT HEADS TO SUPPORT HARDNESS CHECKS

TABLE 1 Incoming Brinell Hardness Data

Position IS 11 IS 10 IS 9 IS 8 IS 7 IS 6 IS 5 IS 4 IS 3 IS 2 IS 1 1 253 226 244 344 390 326 386 383 220 210 218 2 244 223 248 361 370 357 371 379 370 212 242 3 247 347 250 379 352 341 363 360 370 371 220 4 248 321 242 370 348 338 351 321 358 204 201 5 255 212 249 343 347 363 307 347 369 325 200 6 248 204 249 292 382 320 379 366 369 220 234 7 238 208 239 208 254 222 312 202 240 222 227 8 232 226 232 209 213 205 216 233 242 220 216 9 233 223 241 212 217 237 223 229 245 218 227 10 237 223 238 216 213 209 229 239 250 211 203 11 246 215 246 229 213 204 232 233 232 232 215 12 249 217 225 221 231 234 211 238 240 231 239 13 249 226 248 222 237 220 229 244 234 223 233 14 255 232 251 221 284 227 204 220 242 220 201 15 257 238 231 212 339 268 303 221 233 227 212 16 244 218 242 260 356 223 398 321 223 223 202

Figure 4. Incoming Brinell Hardness Data by Interstage Area

≥ Rc 40 Rc 30 - 39

TABLE 2 Incoming Brinell Hardness Data Summary

After Successive 1mm Increment Machining Operations Position IS 7

Inc IS 7 1st

IS 7 2nd

IS 7 3rd

IS 5 Inc

IS 5 1st

IS 5 2nd

IS 5 3rd

1 390 403 405 427 386 370 433 363 2 370 389 403 411 371 365 394 432 3 352 365 343 374 363 373 390 404 4 348 397 378 436 351 371 370 390 5 347 441 389 383 307 354 360 394 6 382 436 360 404 379 351 327 403 7 254 344 260 232 312 192 222 223 8 213 250 271 241 216 216 213 225 9 217 260 307 272 223 271 243 268 10 213 285 228 229 229 269 277 265 11 213 256 263 261 232 288 255 255 12 231 283 255 263 211 216 227 280 13 237 267 306 233 229 196 257 285 14 284 313 261 249 204 239 237 213 15 339 264 349 327 303 241 348 211 16 356 369 405 432 398 341 413 362

Figure 5. Brinell Hardness after Interstage Machining 1mm Increments After completion of the visual, MT and hardness examination, Alstom performed a detailed runout analysis to determine the amount of eccentricity in the rotor shaft and to determine the axial location of the bend(s) / kink(s). Measurements of the rotor runout at site revealed a 0.013” rotor runout. After receipt of the rotor at ALSTOM, the rotor

≥ Rc 40 Rc 30 - 39

runout was measured at 0.021”, see figure 6. It should be noted that due to the limited number of axial runout data points taken by First Energy (FE), the data showed a single bend / kink in the rotor located at the HP balance plane in between the Curtis stage and the first stage reaction HP row. Upon receipt of the rotor at ALSTOM, a more detailed runout check was performed and it was obvious that the HP/IP rotor had Qty (2) bends / kinks and not one.

Figures 6. Incoming Run out Condition Provided by FE and Recorded by Alstom

After collection and evaluation of the hardness and run out data, a coordinate plot showing the hardness data in relation to the run out data was plotted. Review of the data and the plot showed the high runout point was almost exactly 180° opposite from the hard rotor rub material, see figure 7.

Figure 7. Hardness Profile vrs Runout High Spot Location In support of a possible weld repair, Alstom performed chemical analysis, tensile and charpy impact test on the rotor base metal to determine the current properties of the rotor. The material analysis results provided a rotor material similar to ASTM A470 Class 8 material, see Tables 3 and 4. In addition, it was also recommended to perform replicas of the rotor material to determine if any creep damage was present in the areas to be repaired. The results of the replica examination on the Bay Shore #4 rotor revealed no evidence of advanced creep-dominated damage (i.e. oriented cavitation or microcracking) that could be attributed to service at any of the test sites examined.(1) The locations of the replicas and typical microstructural findings are shown in figure 8.

TABLE 3 Rotor Mechanical Property Data

Bay Shore #4 HP-IP Mechanical Property Actual Rotor ASTM A470 Class 8

Tensile (ksi) 109 105 - 125 Yield (ksi) 85 85 min.

% Elongation 14 14 – 17 RA% 24 38 – 43

Charpy Impact – Ft. lbs. 10 6

TABLE 4 Rotor Chemistry Data

Element (Wt. %) Actual Rotor ASTM A470 Class 8 Carbon 0.33 0.25 – 0.35

Manganese 0.78 1.00 max. Phosphorus 0.010 0.015 max.

Sulfur 0.005 0.018 max. Silicon 0.35 0.15 – 0.35 Nickel 0.21 0.75 max.

Chromium 0.98 0.90 – 1.50 Molybdenum 1.11 1.00 – 1.50

Vanadium 0.23 0.20 – 0.30

Figure 8. Typical Results from Replication Examination on HP/IP Rotor

Repair Options After review and evaluation of the current condition of the rotor shaft, a total of (3) alternative options were provided and reviewed. Alstom’s recommendations were to remove the hardened and thermally affected material by machining, straighten the rotor

Replica 1

Replicas 3 & 4

Replicas 5 & 6

Replicas 7 & 8 Replica 9

Replica 2

IP 2HPIP 1

Replica 5 - Base MetalDamage Classification: 1

shaft using a proprietary welding process, and then restore the shaft configuration using a weld buildup procedure. The remaining options were reviewed and dismissed due to the fact that they represented higher risk repair activities, they were considered short term solutions / repairs, and they had a low percentage chance for success. Ultimately the unit owner was looking for a long term permanent repair that would allow for unrestricted future operation and output of the unit. The multiple alternative scenarios are provided below and discussed. Machining and Thermal Option #1

This option involved the machining down to remove the hardened material such that the remaining ligament area between the blade grooves would still be sufficient to support operational loads. A thermal straightening operation would then be performed to return the rotor back to acceptable runout conditions. After straightening, a thermal stress relief operation would be performed to remove any remaining residual stress. This option was initially predicated on the premise that minimal material removal would be required in the interstage areas to obtain an acceptable material hardness for operation. Unfortunately, this option was dismissed during preliminary interstage skim cut machining due to remaining material hardness after a depth of 3mm was machined. Any further machining to remove this hardened material would risk failure of the rotor groove hooks in the areas of the minimal hook thickness during operation and blade centrifugal loading. Although a possible viable option for repair, this method was no longer a viable option after interstage machining and hardness checks were completed. Hot Spotting Option #2

It has long been known that rotor shafts can be straightened using the inherent tensile stresses set-up via thermal induction and/or hot spotting. This methodology involves the application of select heat to specified locations on the rotor body to effect thermal movement. After completion of the hot spotting activities, a stress relief operation is performed to remove the resultant residual stresses setup by the original straightening operation. This method is at best a short-term fix, as it has been Alstom’s experience that 25% to 50% of the original bow tends to return over time. In addition, due to the large rotor cross sectional area in the locations of the bends / kinks, it was not a guarantee that the hot spotting would move the rotor enough to straighten to an acceptable level for operation. Also, while this may have provided a short-term solution to the bending issue, it would not have solved the hardened rub areas, which would still remain after the straightening operation. Again, although a possible viable option for repair, this method was considered to have a low chance for success and was also not a long term solution.

Mechanical / Welding Option #3

Alstom’s experience with rotor straightening is based on numerous repairs utilizing specialized mechanical options combined with specialized welding technoques to permanently restore the straightness of the rotor shaft. This option involved the machining removal of all hardened thermally affected material, straightening of the rotor using mechanical / welding induced stresses, stress relief of the rotor to remove residual stresses, weld restoration of the removed material, and then a final stress relief. This proposed option provided a permanent long term solution for the rotor, would address the hardened interstage material areas on the rotor, as well as provide a high probability for success. The only unknown, was the required number of thermal straightening iterations that would be required to bring the rotor runout to an acceptable level of operation. After consideration of the risks, the permanency of each repair option, and the schedule for each repair option, First Energy concluded that the Mechanical / Welding Option was the best alternative. In response to the unit owners concerns over rotor operational vibration and thermal stability, ALSTOM committed to straighten the rotor to less than or equal to 0.004” runout and also to perform a rotor thermal stability test prior to final assembly and machining. Following are the results of the Mechanical / Weld straightening repair option chosen. Mechanical / Weld Straightening Concept

The concept behind this operation involves thermally moving the rotor centerline to a predetermined location, locking the rotor centerline in this position, and then performing a thermal stress relief. Once the thermal stress relief is complete, a percentage of rebound or spring back is expected and must be taken into account during the initial rotor centerline move. After completion of a single initial iteration, additional iterations can be performed until the rotor final runout is within acceptable limits. Although this operation is a standard concept in regards to thermally straightening metallic material, ALSTOM has modified this process to include a direct welding operation to move material when higher configuration stiffness and/or residual stresses are present. The welding is performed in such a manner as to take advantage of the shrinkage stresses associated with welding to reposition the rotor centerline. The amount of movement can be estimated much more precisely due to the exact amount of heat input controlled during the welding operations. Rebound factors are based on rotor material chemistry, material properties, material age, and previous operational history. In addition, the amount of preferential movement, the % of relative movement as compared to the original position, along with the stress calculations are utilized to determine % rebound. Figure 9 shows the location of the initial weld straightening groove machined into the HP/IP rotor.

Figure 9. Typical Groove Design and Location for Straightening

Weld Straightening Repair Execution

Blade Reinstallation Preparation

While the critical path rotor shaft repair was under way, ALSTOM utilized a risk adverse methodology and prepared a dummy drum shaft to facilitate blade preparation, set-up, and shroud milling and rolling. The mock-up dummy drum shaft allowed Alstom to machine mock up blade grooves, preset each blade row, mill all the tenon holes, roll all the shroud stock, and mill / fit the oversized blades off of critical path. This helped reduce the final critical path blade installation process as well as improve the scheduled completion of the overall project. The mock-up assembly showing the blade configuration is shown in figure 10.

Figure 10. Blade Mock-up Assembly

Rotor Preparation

Since the interstage hardened material required removal and replacement, in order to restore the rotor to its original geometry complete reverse engineering measurements of the rotor configuration in the damage area were obtained. The distance to all axial features from the end of the blade rows and balance plane were carefully measured and recorded. The cylindrical features of the damaged area were measured with micrometers and double-checked with pi tapes and the transition radii were measured with radius gauges. Finally the more complicated geometric features were measured with a portable coordinate measuring machine (CMM). Metallurgical replication of the rotor was performed prior to the commencement of any work to eliminate any concerns of material creep damage. Material samples from the rotor were removed and set to the laboratory to confirm both chemical and mechanical properties in support of defining welding and post weld heat treatment parameters. Hardness readings were taken after completion of each weld prep operation and each post weld heat treatment operation to be used to determine if any of the subsequent stress relief operations resulted in rotor softening. The locations of the groove welds were chosen based on a review of the run out data as well as the physical locations of the blade grooves and rotor transition radii. Project witness points and hold points were defined and agreed to prior to the start of the project to ensure customer review, approval, and satisfaction in the repair results obtained. First Weld Straightening Operation

The rotor underwent an initial weld prep operation to remove the damaged material resulting from the rub and judiciously located the heat-affected zones of the build-up weld in low stress areas. Once completed, the first weld straightening groove was cut into the rotor shaft. Rotor shaft runout readings were again taken to measure the amount of rotor centerline movement during removal of the thermally affected material, as well as confirm the original bend / kink locations. A selective groove welding process was then applied to thermally move the rotor centerline. Figure 11 shows the initial groove machining prior to any welding and the first groove machining after weld has been applied to the bottom of the groove. Once the amount and direction of rotor centerline change was achieved, the remaining groove was filled with weld material to lock the change into place. After completion of the first iteration of selected welding and stress relief, the rotor runout was changed by a -0.013” (+0.021” initial runout to a +0.008” current runout). Figure 12 provides a graphical representation of the rotor runout change after the first iteration of rotor weld straightening was performed. Prior to continuing on to another straightening iteration, the deposited weld was inspected by ultrasonic and magnetic particle examination. No weld subsurface defects and/or fusion line surface indications were noted. Hardness taken of the deposited weld as well as of the rotor base material also showed no concerns for rotor base material softening.

Figure 11. Weld Straightening of first weld Groove

Figure 12. Runout after First Straightening Iteration

Second Weld Straightening Operation

After completion of the non-destructive examination of the first straightening weld, additional run out data was recorded and the second groove was cut at a pre-determined location based on this data and physical restraints. The second straightening operation was similar to the first iteration with the exception of the groove location. The straightening operation was again completed successfully with the rotor being stress relieved and non-destructively tested. Again, no issues were observed. Figure 13 shows the inprocess welding of the second groove as well as a view of the rotor midspan and the location of the second weld straightening groove. Rotor runout readings were again taken and showed that the rotor runout was changed by a -0.002” (+0.008” runout to a +0.006” Runout). The data showed that the runout for groove position #2 was acceptable, however, the first groove location had a much greater movement and less spring back, see figure 14.

Figure 13. Weld Straightening Second Groove

Figure 14. Runout after Second Weld Straightening Iteration

Third Weld Straightening Operation

The selection of the third groove was again based on bend location, physical restraint, and location of the first weld straightening groove. After completion of the machining of the third groove run outs were again taken and the straightening operation proceeded. It should be noted that like all thermal straightening operations, material rebound or spring back was expected. ALSTOM had a predetermined location to put the rotor runout in support of a calculated and expected rebound and/or spring back not only for each weld straightening operation, but also a rebound / spring back for the final weld buildup and post weld heat treatment. The third iteration had provided a -0.015” runout change (+0.006” runout to a -0.009” runout). This –0.009” runout high point was 180° opposite from the initial rotor runout high point). Figure 15, shows the run out after the third straigtening iteration. The rotor was again stressed relieved and non-destructively tested. No issues were observed and the rotor was then released for final weld build-up.

Figure 15. Runout after Third Weld Straightening Iteration

Weld Build-up and Stress Relief Operation

After completion of the third weld straightening iteration, the rotor was released for the final weld buildup and post weld heat treatment to restore the damaged material removed. Once the deposition of over 3,000 lbs of filler material the rotor was put through its final stress relief / post weld heat treatment. ALSTOM has expected to see a +0.006” to +0.010” runout rebound from the final welding and post weld heat treatment. The actual runout change after the final welding and stress relief was a +0.005” change (-0.009” Runout to a -0.004” Runout). This final runout, shown in figure 16, was just within the agreed to final runout achievement of equal to or less than 0.004” rotor runout and considered to be acceptable.

Figure 16. Final Rotor Runout after Stress Relief Rotor Body Weld Build-up

Alstom already had mechanical property data and PQR’s for the base metal to filler material weld combination, therefore no mock-up testing was required for this project. The weld build-up consisted of a base layer of similar material composition, followed by an intermediate layer of different composition (5Cr) and final layers of a high temperature (12Cr) material. The weld was completed, stressed relieved and non-destructively tested. No issues were observed and the rotor was released for final machining and assembly. Figure 17 shows the weld build-up and stress relief operations, respectively.

Figure 17. Wheel Build-up and Typical PWHT Operation Thermal Stability Testing

Due to the amount of weld material deposited onto this rotor and the amount of thermal stresses imparted then relieved in this rotor, unit thermal stability as well as unit operational vibration were a major concern to the unit owner. Therefore, ALSTOM agreed to perform a final thermal stability test of this rotor at operational temperatures. Alstom reviewed this and initially considered performing a thermal stability similar to the process and requirement called out in ASTM A472 with the following clarifications. 1.) The final runout acceptance criteria prior to testing shall be equal to or less than 0.004” runout. 2.) The maximum allowed runout deviation during the test shall be no more than 0.002”. This test involved the heating of the rotor forging up to operational temperature in defined temperature increments while taking rotor runout data to confirm there was no thermal influence in rotor runout. However, after review of the lack of equipment and tooling available in the industry to perform this type of heat stability test, it was decided against performing the test per ASTM A472. It was then recommended that the rotor thermal stability testing be performed utilizing successive low speed balance runs at various temperatures up to operational temperature and back to room temperature. Any unbalance recorded could quickly be calculated and converted to rotor runout for evaluation and acceptance. This test was proposed to the customer and agreed to. The rotor was split into (3) major heating zones, Qty (2) end zones that would see 600°F maximum and Qty (1) middle zone that would see 900°F maximum. The actual testing process was as follows:

• Balance HP/IP rotor at room temperature. • Heat rotor and soak at 300°F for all three zones. • Perform a balance check of the rotor at 300°F. • Heat rotor and soak at 600°F for all three zones.

• Perform a balance check of the rotor at 600°F. • Maintain 600°F for both end zones and heat the rotor center zone 900°F and soak. • Perform a balance check of the rotor at 900°F for the center zone. • Cool rotor back to room temperature. • Perform a final balance check of the rotor at room temperature.

The test was completed and the final amount of balance change in the rotor at each temperature was less than 0.001”. Well below the required 0.002”. Figure 18 shows the rotor installed in the low speed balance machine with the top removed and installed.

Figure 18. Thermal Stability Test Oven Non-Destructive Testing

After completion of the PWHT operation the rotor was moved to a large lathe and the welded area was machined to a smooth surface. An ultrasonic inspection of the final weld configuration and adjacent areas from at least three angles was performed. No operational or life limiting indications were observed.

Final Machining of the Weldment

The rotor was restored to its original geometry by machining and milling. The turning operations were performed in a large lathe, while the lock milling operation was performed on a horizontal-boring mill. After all machining operations were complete, the surfaces were again inspected by magnetic particle examination, no issues were observed. Figure 19 shows the final groove machining configuration prior to blading.

Figure 19. Final Blade Groove Machining

Rotor Re-Assembly & High Speed Balance

Once the rotor was final machined and the locks cut into the blade grooves, the rotor was re-bladed, shroud covers installed and the tenons peened. After skim cutting for radial seal clearances and shroud band width, the rotor was installed in the under ground bunker, as shown in figure 20, and high speed balanced as well as overspeed tested to 110% of operational speed.

Figure 20. Rotor in High Speed Balance Pit

Unit Post Repair Operation

After straightening, weld repair and re-balding, the unit was installed in service in early March of 2004. The overall project repair duration was a total of (75) days. The duration included the straightening, weld restoration, blade supply, blade installation, and balance of the HP/IP rotor. Unit vibrational start up was very smooth with no issues noted with the repaired HP/IP rotor. To date the rotor is in service and operating normally in all aspects. Overall the application of a rotor straighten process utilizing advance welding techniques provided the best proven, predictable, and achievable long term solution for this major rotor eccentricity.

References

1. R.E. Kilroy, M. Morin, M. Radcliff, D. Sculley, J. Cable, & D. Shriver, “Shaft Foging Section Replacement By Join Welding of a Large Generator Rotating Field – A Case Study”, Fifth International EPRI Conference on Welding and Repair Technology for Power Plants, Point Clear, Alabama, June 2002.

2. Jeff Henry Report – LN-03L706 Replication Examination

3. R.E.Kilroy Jr. “Weld Repair of Rotating Power Generation Equipment,”presented at the Alstom Users Symposium, Chicago, IL (July 2001).

4. Alstom Power Project Reference Files “1340085” (2003)