The ability of gas turbines to meet expectations depends to a large degree on regular inspections by experienced technicians equipped with the diagnostic tools capable of finding and evaluating potential problems before they adversely impact engine operation. Equally important is guidance on what to inspect and how often, and exactly what to look for “under the hood.”

Users rely heavily on the OEM for timely alerts and advice on issues as they are experienced in the fleet. One way this is done is by release of a Technical Information Letter (TIL, GE terminology) describing a specific concern, how to inspect for it (when and how often thereafter), how to correct findings, etc.

User group meetings like the 2019 7F annual conference, May 20-24, at the Renaissance Schaumburg, a few miles from Chicago, provide valuable perspective on the development of an optimal inspection plan for your plant. Formal presentations and open discussions allow the opportunity to relive the experiences of plant colleagues and listen to what the leading inspection companies are finding in their field work.

Snippets of valuable inspection-related information are sprinkled throughout every 7F program. At the 2018 meeting, the steering committee tried to aggregate some of this information in an “NDE Roundtable” with participation by the OEM, Veracity Technology Solutions, and Advanced Turbine Support. Team 7F didn’t want to restrict the dialog and suggested the following as possible discussion topics:

• • Compressor findings—clashing, cracking rocking, twisting, etc.

  • NDE inspections on enhanced compressors.

  • Combustion findings—fuel-nozzle leaks, transition seals, etc.

  • Turbine findings—cracks, clogged cooling holes in buckets and vanes, etc.

  • AGP hardware findings.

  • Which NDE techniques to use and why.

  • New NDE techniques.

In addition, inspection experience was sought on the following TILs: 2069 (1-2 spacer rim inspection), 2006-R2 (airfoil distress on third-stage bucket), 1972-R2 (conical flat-slot bottom compressor wheel), 1937-R1 (turbine-wheel inspection), 1907-R1 (forward shaft dovetail crack), and 1769 (aft-stator rocking).

Obviously an ambitious program for the hour allocated for the NDE Roundtable. The session could have filled a productive half-day workshop—in the opinion of the editors. A few of the topics addressed follow.

Register now for the 2019 7F conference.  There will be plenty of opportunity to get your inspection-related questions answered at the Schaumburg meeting—at user-only sessions on the compressor (Tuesday) and turbine (Wednesday) sections, during GE Day on Thursday, at meals, in the hallways, and at the vendor fair. Regarding the exhibit hall, inspection experts from Advanced Turbine Support will be on hand in the exhibit hall Tuesday and Wednesday evening at Booth 57; Team Veracity will be at the vendor fair on Tuesday in Booth 73.

Plus, don’t forget to sit in on the first series of vendor presentations Wednesday afternoon and listen to what Veracity President Kevin McKinley has to say about “Advanced NDT Protocols Needed for End-of-Life Assessments of Gas Turbines.”

GE

Flat slot-bottom findings. The OEM reported that inspections have revealed indications requiring part replacement at around 2400 to 5000 actual starts on FA.02, .03, and upgraded .04 machines. The GER 3620 recommendation was said to take precedent at 5000 factored starts. Recommendation: Inspect at 1700-2200 starts to obtain indication size and determine operational safety.

Inspections completed to date have confirmed TIL 1972-R2 inspection recommendations and that they are necessary to determine true rotor capability.

Third-stage bucket distress. The 7F users were told that five third-stage bucket (S3B) liberations had occurred since mid-2013, each within 8000 hours of operation following repair and creep rejuvenation heat treatment. The affected units were 7F.01-.03 machines, plus a few Dot 04s running with second-tour buckets.

The liberation events initiated either at the leading edge of the airfoil at 40% to 50% of radial span, or at the trailing edge at 20% to 30% of radial span. Total hours on the failed buckets ranged from 25,000 to 55,000 hours. TIL 2006-R2, dated January 2018, provides corrective actions identified during the OEM’s RCA.

Note that TIL 2006 was first released in July 2016 to provide mitigation during the RCA process. Eddy-current inspection was added to the OEM’s repair process the following January and its creep rejuvenation cycle was updated in May 2017 to improve ductility and the tolerance of buckets to damage.

1-2 spacer wheel rim inspection. TIL 2069 applies to both 7F and 9F-class turbines. It was issued following the inspection of a 9FA that had tripped on high vibration. Technicians found a section of the spacer rim had liberated and caused significant collateral damage. Operating data provided no early warning of the incident: Vibration levels, wheelspace and exhaust temperatures, etc, all were normal.

Multiple initiation locations were observed on the inner surface of the spacer rim which then propagated radially and circumferentially outward through the spacer rim. The RCA identified the propagation mechanism as hold-time fatigue, an hours-based phenomenon.

Recommendation is that the inspection prescribed by TIL 2069 be performed along with the wheel inspection suggested in TIL 1937, during the first HGP or major after the spacer wheel completes 80,000 hours of operation. At a minimum, the turbine casing must be removed to provide the necessary access.

Veracity Technology Solutions

Tulsa-based Veracity discussed its experience fulfilling the requirements of TIL 1509, “F-Class Front-End (R0, S0, and R1) Compressor Inspections.” The company reported being called to a 7FA site for a routine TIL 1509 inspection in mid-2018. The document’s recommendations were said to be important, and “required” by the OEM. One reason: Engine availability and reliability can be compromised by front-end compressor rubs, which are conducive to metal liberation and downstream damage.

TIL 1509 was said to be a good example of an inspection that benefits from using multiple NDE techniques to deliver very reliable results. Veracity relies on both the eddy-current method and its proprietary ultrasonic phased-array (UTPA) technology, originally designed for the Dept of Defense. Use of these two methods, the company says, yields the highest probability of detection over alternatives because it can find cracks on both the pressure and suction sides of R0 and R1 blades in-situ.

Case in point: A tip crack on an R1 blade was detected. On its own, this does not sound like news of any sort, but it’s where the crack was located that may lend credence to the use of UPTA technology for this type of inspection. The 0.67-in. crack was found on the back (pressure) side of the blade, 3 in. from the trailing edge (photos).

Those familiar with the R1 blade’s anatomy know this area is a “meatier” part of the airfoil and this indication could not be seen from the suction (front) side. It also is an area not typically inspected because of limited access.

According to Veracity’s Scott Kennedy, the crack would not have been located using the typical penetrant (PT) or eddy-current (EC) techniques employed in the field. UTPA allows for simultaneous capturing of both sides of the blade, thereby providing a full-coverage inspection.

Summing up the advantages of UTPA for a TIL 1509 inspection:

• • Timely compared to PT.

  • Very sensitive to small flaws.

  • Can inspect both sides of the airfoil from either side.

  • Not necessary to scan the entire length.

  • Provides a permanent record.

  • Can size flaws.

Eddy current’s advantages include the following:

• • Extremely timely compared to PT.

  • Very sensitive to small flaws.

  • Can detect material flaws and hardness changes that are precursors to stress risers.

  • Known minimal detectable calibration.

For the case history described, after technicians discovered the indication with UTPA, they validated the indication with a near-field borescope and with a borescope-assisted EC inspection.

Advanced Turbine Support

Mike Hoogsteden opened the company’s brief presentation by reviewing the leading-edge inspection tools used by the company’s technicians—including phased-array ultrasonic, eddy-current array, and specialized surface inspection methods. While advanced NDE equipment allows the company to go above and beyond what the OEM typically asks for in its TILs, he mentioned there are instances where tried-and-true legacy methods may provide optimal results. He put visible dye penetrant in this group.

Hoogsteden stressed the company’s ability to provide 100% NDE coverage of all R0 and R1 airfoils. He added that Advanced Turbine Support has never missed an indication or crack in its R0 and R1 inspections. On the turbine end of the machine, Hoogsteden said the company’s eddy current array gear proved itself in a qualification run for a major utility to meet the requirements of TIL 2006 (see above) in evaluating the condition of third-stage buckets. Inspectors identified all of the surface and subsurface indications in the validation block.

Referring to TIL 1972-R2, he showed photos to illustrate the detection and crack-sizing information achieved for flat-slot-bottom compressor wheels when the surface condition allows.

Still more photos confirmed the considerable capabilities of advanced NDE in finding dovetail cracking in the forward shaft (TIL 1907), plus confirmation with liquid penetrant.