Rotor Lifetime Extension

Frame 5N rotor returned to plant better than new following EOL inspection, repairs

Gas-turbine rotors have a finite lifetime. For GE frames, Technical Information Letter (TIL) 1576 mandates an end-of-life (EOL) inspection for safety reasons after 200,000 factored hours of operation or 5000 factored starts, whichever comes first. A user attending the CTOTF™ Fall Conference, September 8-12, in Coeur d’Alene, Idaho, recently had completed an EOL inspection on a Frame 5N rotor with more than 5000 starts at the Dresser-Rand Turbine Technology Services (D-R) shop in Houston and offered to share that experience with colleagues through CCJ ONsite.

Many owner/operators of legacy GE engines—such as Frame 5s, 6Bs, and 7B-EAs—will be planning and conducting lifetime evaluations in the next couple of years, but may be unsure about how to prepare for an EOL inspection. They also may be unfamiliar with the various shop activities associated with such an important overhaul. This article offers some perspective.

Additionally, some users remain skeptical regarding the need for an EOL inspection, having listened to several colleagues at user-group meetings discuss how their units were disassembled, inspected, and reassembled with no findings. But in the case profiled here, a significant crack was found in the first-stage turbine wheel, verifying the positive value of the process.

Rotor experts, such as D-R Engineering Manager Greg Snyder, can predict with reasonable accuracy what they would expect to find during EOL inspection based on information extracted from plant records. It is in your best interest to make PI and other data available to the shop selected well in advance of the inspection to facilitate planning and decision-making. These data include the following:

  • Location (ambient conditions).
  • Operating hours at base load and partial load.
  • Number of starts: fast, normal, slow.
  • Number of trips/load rejections.
  • Ramp rates.
  • Shutdown times between restarts.
  • Control parameter time-history data.
  • Fuel type.
  • Operational profile.
  • Maintenance history and previous inspection findings.
  • Upgrades and parts replacements since COD.

The 5N rotor shipped to D-R by the owner had an interesting history. It was assembled in 1970; TIL 471, issued later, advised of potential “forging discontinuities” created during manufacture of the first- and second-stage turbine wheels. While no indications were found—or at least reported—during manufacture, UT reports from 1981 and 1984 each identified three indications in the first-stage wheel. Experts believed these were “birth defects,” the accept/reject defect size used during manufacture likely being larger than the indications found during the 1980s inspections.

As part of the major outage in summer 1981, the rotor was shipped to a GE shop for grinding of the No. 1 journal; the rotor was not unstacked. Details of the three indications in the first-stage wheel found via straight- and angle-beam UT were archived and the OEM recommend re-inspection in four years or 500 fired starts, whichever came first.

During a combustion inspection in September 1984, three indications were in evidence once again. However, one of the 1981 indications was not found and the OEM chalked that up to a reporting error. The new third indication was identified with straight-beam UT. One of the remaining two indications was reported as having grown but obviously was still of minor concern because GE extended the interval for re-inspection to six years or 1200 fired starts, whichever came first.

A boresonic UT inspection performed in accordance with TIL 471-C, conducted during a hot-gas-path outage in September 1990, revealed no indications.

Discrepancies such as those described above should not come as a surprise for several reasons, including these:

  • Wheel and disc design and inspection tools of the 1970s and 1980s were rudimentary by today’s standards.
  • The type and critical size of indications for acceptance/rejection of rotor components following manufacture were not as well understood as they are now.
  • Data archiving typically was manual.
  • People make mistakes, especially inspectors when inexperienced and not properly trained. Regarding this point, users should be sure to check the qualifications of all technicians performing inspections on their rotors, monitor the inspection process, work closely with shop engineering personnel in the evaluation of inspection results, and participate in the repair/replace decision-making process.

After your rotor arrives at the shop, it will be inspected and then disassembled (Fig 1). A standard inspection of the assembled rotor (Fig 2) includes balance, run-out, and dimensional checks, and nondestructive examination (NDE). The owner said the nuts came right off the through-bolts during disassembly, which is not always the case.

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Detailed inspections of high-temperature rotor components—including all turbine discs and the last four compressor wheels—using advanced NDE methods are conducted after disassembly (Fig 3). The sensitivity of the tools and techniques used provides increased confidence that there are no “visible” issues with individual rotor components. Such higher-order inspections also provide a baseline assessment for comparison during future inspections. They include the following:

  • Volumetric UT using phased-array probe technology, advanced 3-D signal processing, and archiving of digital data.
  • Eddy current.
  • Fluorecent magnetic particle.
  • Visual.
  • Dimensional.
  • Metallurgical evaluation.
  • Hardness measurement.

Note that inspection of wheel dovetails (Fig 3B) requires removal of compressor blades and turbine buckets. The former effectively are destroyed in the process, so consider ordering new blades well in advance of the shop visit to gain an advantage in negotiations. In this case, the owner ordered new compressor blades but chose to use its spare set of turbine buckets. The compressor blades were later coated in the shop to protect against corrosion.

Rotor evaluation next. Inspection findings were reviewed and interpreted by an experienced engineering staff. This is one of the most important steps in the overhaul. Action taken on issues identified typically is one of the following: retire, repair, rejuvenate, and accept as is. Engineering assessments and detailed analyses guide repair and rejuvenation processes.

Areas of concern on the Frame 5N rotor identified by conventional inspection methods and the corrective actions taken are described below:

  • Compressor stage-16 disc, dovetail cracks, blended out.
  • Turbine stages 1 and 2, bucket rock, applied coating to dovetails.
  • Fretted rabbets, installed patch rings, 12 small and one large.
  • Compressor rotating blades, migration, shifted back into place and restaked.
  • R0 compressor blades, erosion and foreign object damage (FOD), replaced.
  • Through-bolts, nuts, and other hardware, wear and tear damage, reworked or replaced.

The most significant finding on this rotor was a 180-deg circumferential indication in the forward rabbet fillet of the first-stage turbine wheel (Fig 4). A crack like this could compromise the integrity of the component and militate against its continued use. More information was required for proper engineering disposition.

The depth of the indication cannot be determined with confidence from eddy current or other techniques, especially because it is located in a relatively small radius up under the rabbet surface. D-R engineers decided, with the owner’s consent, that the first step should be to remove up to150 mils of material in the area of interest, which was considered a reasonable depth based on inspection results, calculations, and experience.

The indication cleared at 135 mils, as confirmed by eddy-current inspection. An additional 10 mils was removed for added assurance in the event there were any remaining indications below the detection capability of the inspection tools. Next steps: Apply final contour, then polish and shot peen the surface. Note that the contour was laser-scanned before and after the repair to enable a stress comparison for the actual part.

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Finite-element models were constructed for both the original (OEM design) and repaired geometries (Fig 5). The primary goal was to show that the repaired configuration was as at least as good as the original. The mesh refinement in the model provided confidence that the local concentrated stresses in the fillet were acceptable. Through-bolt dead loading was considered important and was modeled with good fidelity. Low- and high-cycle fatigue and creep loading were factored into the analysis.

The mean-stress comparison in Fig 6 showed the new contour, at right, reduced the peak stress in the fillet area by about 70% compared to the original design (left). Also, the point of peak stress was moved to the side of the fillet. The combined section stresses (membrane plus bending) are slightly higher than the original, but the much lower stress-concentration values more than offset the slight nominal stress increase attributed to the smaller cross section.

In sum, the repair achieved the following:

  • Reduced substantially the stress in the fillet area compared to the original contour.
  • Improved fatigue durability for both steady-state and high-cycle loadings.
  • Enhanced component durability with additional surface treatments—polish and shot peen.

Final point: D-R engineers consider the repaired configuration superior to the original configuration in the region of the repair with respect to allowable number of starts and hours of operation.

Repairs complete, the rotor was reassembled, inspected, and returned to the plant (Fig 7). In-shop time from receipt to final assembly was three weeks. CCJ

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