Case study provides valuable perspective for your first GT rotor overhaul

After only a few years of experience, most gas-turbine (GT) users become familiar with the repair of hot-gas-path components, because these parts—the turbine blades (buckets), vanes (nozzles), and combustors— require maintenance at frequent intervals. Users typically are less familiar with the repair of GT rotors, because they usually operate for 100,000 hours or more with little attention required.

At some point, however, your GT rotor will need shop-level repairs, because of an unexpected failure or simply because it has reached its OEM-recommended service interval. When that time comes, users need to employ the best inspection and repair techniques available, along with trained and experienced personnel.

The recent repair of a GE Frame 7B rotor by ReGENco LLC, West Allis, Wisc, provides an excellent case study of today’s GT rotor overhaul techniques.

Incoming inspection

When a rotor first enters the repair shop, it must be thoroughly inspected, before wrenches start turning and disassembly begins. That may seem like a frustrating delay to the user, who “knows” what’s wrong and wants it fixed right away, but the conservative approach is best.

As its first step, ReGENco conducted a multi-point inspection of its customer’s (name withheld by request) incoming 7B rotor, to determine the rotor’s condition, assess the need for repairs, and determine what level of disassembly the rotor would require. Combustion Turbine Business Segment Manager Karl Mattes explains that trained technicians need to look closely at the sources of the abnormalities—in this case, it was high residual unbalance—before the tear-down begins. “We start with a visual inspection, and follow-up with baseline measurements—such as overall rotor length, bucket tip height, bucket-rock measurements, and balance readings,” Mattes says.

ReGENco technicians performed a low-speed balance on the rotor asreceived (Fig 1), and then removed the buckets stage-by-stage, to determine the contribution of the buckets to the overall residual unbalance. The technicians also performed a run-out check of the rotor body, measuring radial run-out at multiple axial locations along the shaft.

Once these steps were completed, the entire rotor was blast-cleaned—to remove dirt, scale, and other residue accumulated during years of operation— then was inspected using the wet magnetic-particle method. This non-destructive examination (NDE) technique can detect both surface and sub-surface flaws in ferromagnetic materials. Its application process can be conducted wet or dry; the wet method is preferred for finding finer cracks on relatively smooth surfaces. Following the magnetic-particle test, ReGENco technicians applied another NDE method—ultrasonic examination— to evaluate the condition of the rotor bolts (Fig 2).

According to Mattes, a thorough incoming inspection of a GT rotor consists of:

1. Incoming visual inspection for shipping damage.

2. Visual inspection for general condition of rotor and any noticeable findings.

3. Dimensional inspection of journals, seals, coupling fits, blade tip diameters, and overall rotor length.

4. Incoming balance, with and without buckets.

5. Run-out inspection.

6. Bucket removal.

7. Blast cleaning.

8. Magnetic-particle inspection of rotor body and ultrasonic inspection of rotor bolts.

9. Repeat bucket rock inspection.

10. Bucket hardware detailed inspection.

Results of the initial inspection, and ReGENco’s recommended repair tasks, were then summarized for the customer in a detailed report. Some key indications supporting the need for disassembly and extensive repairs included abnormal rotor runout, excessive bucket rock, loss of bucket seal pins, and a high residual unbalance of the rotor. In this case, ReGENco’s specific findings were these:

  • Excessive rubbing on the secondstage buckets (Fig 3), the forward stub, spacer wheel 1-2, and the No. 3 journal.
  • Severe foreign object damage on the third-stage buckets (Fig 4).
  • Galling and excessive run-out of the forward coupling.
  • The unbucketed rotor balance was at the high limit of correctable unbalance.
  • Severe bucket rock on all three turbine stages (Fig 5).
  • Seal pins missing or dislodged in several locations (Fig 6).
  • No reportable indications found during the NDE inspections.

With findings such as these, Mattes says, a variety of recommendations and repairs should be considered. The table summarizes typical findings during GT rotor repairs, and highlights several repair or additional inspection options that users should consider. On the subject Frame 7 rotor, ReGENco recommended the following specific repairs:

  • De-stack the rotor and replace turbine wheel 2; repair the first and third stages using metal-spray build-up of dovetails.
  • Repair or replace spacer 1-2 to reestablish the proper geometry for “hi-lo” teeth (Fig 7).
  • Perform additional inspection of individual components—specifically, measure fit diameters, inspect run-out, and test balance
  • Replace bucket hardware on first and second stages. Consider replacement of third-stage buckets.
  • Replace extended D-keys on firststage.
  • Replace twist locks on second and third stages.
  • Make patch-ring repair of the forward coupling fit.
  • Machine and polish the No. 3 journal to reduce rub marks.
  • Perform a computer-based re-stack to ensure that the component stack is optimized during reassembly.

 

De-stacking

ReGENco presented these findings and recommendations to the customer, after which the service company was directed to de-stack the rotor, and inspect in more detail the individual rotor components—including the rotor stub shafts, turbine wheels, spacer wheels, and rotor bolts. Mattes explains these additional inspections: “We measured the rabbet fits on all of the components, checked the general condition of the parts, and performed an individual component balance. We also checked the general condition of the rotor bolts, and performed a runout check on them, and we conducted more magnetic-particle testing on these individual components.”

After the rotor de-stack and more detailed inspection, ReGENco’s findings were:

  • Three bolts had excessive galling. Excessive run-out was recorded on many of the bolts. Only one of the 12 bolts was considered acceptable for further use.
  • The forward stub was removed with little effort. Follow-up dimensional inspection of the rabbet fits showed the forward stub male fit being undersized. The size was verified by micrometer measurements and with pi-tape.
  • The spacer wheel 1-2 had a bore fan bushing and keys assembled to it. Machinists noted that the bushing had shifted forward and was pressed tight to the first-stage turbine wheel.

With these findings in hand, the service company then recommended the following repairs:

  • Purchase a new or refurbished complete set of turbine bolts.
  • Remove the bore fan bushing to ensure it will not interfere with turbine wheel 1. An alternative is to machine back the bushing where it is making contact.
  • Metal-spray forward fits on turbine wheels 1 and 2.
  • Metal-spray forward stub shaft to correct for undersize fit diameter.
  • Face grind turbine wheels 1 and 2, and spacer 1-2.
  • Skim-cut the fit of spacer wheel 2- 3, to improve run-out. (The spacer wheel has a tight press fit allowing it to be machined without metal spraying.)

Repair, re-stack, restart

The customer authorized all recommended repairs, and ReGENco technicians began work. One of the capabilities that sets ReGENco apart from other turbine shops, says Mattes, is its large computer numerical control (CNC) vertical turret lathes. These provide a high degree of accuracy during the various repairs. For example, the wheels, spacers, and stubs of the 7B rotor were machined on these lathes (Fig 8).

Once all repairs were made, and all of the components had a balance and run-out check performed on them, a re-stacking computer model was made to optimize the actual component assembly. Mattes explains that a software program was specially developed by ReGENco engineers for use on numerous types and models of rotor. The program evaluates the run-out data, as well as other characteristics, of each individual component, then computes the optimum re-stack position of each component to minimize the eccentricity of the stacked rotor assembly.

Goal is to stack each component in its optimum position, thereby increasing the probability that the assembled rotor will exhibit low unbalance, as well as run-out conditions that are within OEM specifications. “Any time we repair individual components in a disassembled state,” Mattes says, “we recommend to customers that they consider a computer- based re-stack. This ensures that the as-repaired state of each component is considered in the reassembly process.”

Upon completion of the re-stack, the Frame 7 rotor was returned to the balance machine to verify that stacking was successful. Next, ReGENco placed the rotor into a precision horizontal lathe, for a complete set of run-out readings, journal polishing, and coupling face check, as well as a true-up if required (Fig 10). Results of this step are provided to the customer as soon as possible, because this information may be required to optimize the mating flanges of the coupling in the field, Mattes says.

The overhauled rotor then made a final return to the balance machine, where the buckets were installed row-by-row and a check of the residual unbalance was conducted to ensure that the vibrations of the fully assembled rotor were within the OEM’s specification.

“The job ain’t done ‘til the paperwork is finished,” the old saying goes, so assembling of a final written report—including signoffs by the shop foreman, quality inspector, and the customer—was the next step, before ReGENco carefully prepared the rotor for shipment back to the powerplant. ccj oh