GE 7FA Engine Inspection

Gas-turbine inspection was the theme of two of the dozen vendor presentations invited by the 7F Users Group steering committee for the organization’s 24th annual conference at the Sheraton Denver Downtown, May 11-15, 2015. Kevin McKinley, president of Veracity Technology Solutions, used his podium time to tell attendees about the company’s high-resolution VTS 2400 phased-array ultrasonic transducer and the enhanced inspection protocols it developed to augment a condition-based maintenance program.

Mike Hoogsteden, field service manager for Advanced Turbine Support LLC, approached the speaking opportunity differently. He focused on what his company believes 7F owner/operators should pay particular attention to when developing a scope of work for borescope inspections—recommendations based on the results of thousands of inspections conducted by Advanced Turbine Support’s technicians.

New imaging technique promises higher probability of early anomaly detection

Better personal health is being advanced by more sophisticated diagnostic techniques. So too, is the health of today’s advanced gas turbines. In fact, medical imaging techniques and practices can be leveraged to achieve a higher probability of early detection of cracks and other anomalies. Nondestructive inspections, for example, help ensure that turbines continue to run as scheduled through, and often beyond, their operational life expectancy. Now, an advanced ultrasonic imaging system originally developed for aerospace and defense is being introduced to the power industry.

President Kevin McKinley of Veracity Technology Solutions described his company’s VTS 2400 as a custom-made phased-array ultrasonic transducer consisting of a 128-element linear array interlaced to provide a resolution of 0.007 in. per element. The clarity of the resulting images is comparable to the clarity of those taken during pregnancy through an ultrasound. VTS 2400, McKinley explained, initially was developed to detect multi-layer corrosion on military aircraft. More recently, the technique was adapted for GE frame 7E/EA S-1 compressor vanes after a 7EA user experienced catastrophic failure and cracking was detected.

The OEM’s response was to issue technical guidance to perform fluorescent penetrant inspections to detect cracking. However, penetrant inspections are limited because of accessibility to the affected part. As a result, small flaws are often missed. So the user was willing to participate in the development of an advanced inspection protocol to address the issue (Figs 1 and 2).

7F inspection Figs 1 2

OEMs typically recommend and conduct penetrant testing (PT), magnetic particle testing (MT), and ultrasonic testing (UT), which require minimal training and therefore are easily staffed, McKinley noted. Unfortunately, material issues grow as turbines age, and accelerate as machines approach end of expected life. Mature frames face additional challenges as the supply of spare parts dwindles. Inspection techniques need to keep up.

One essential advantage of VTS 2400, McKinley said, is that the entire volume of the component can be inspected, not just the surfaces. In other words, he continued, it “sees inside” the object. Another key advantage over other UT methods used by third parties and OEMs is that the VTS 2400 employs a higher number of elements and therefore demonstrates greater fidelity. Existing techniques create the image from 16-32 elements spaced 0.023 in. apart; VTS 2400 uses 128 elements spaced 0.07 in. apart. What is essentially a medical-grade imaging technique has been adapted for gas turbines. And the cost of the inspections is no higher than traditional methods.

For this user, the inspection protocol around the VTS 2400 quickly identified additional cracking on the ten 7Es inspected; 87 of the 600 stator vanes were identified to have cracks, with some units having only one cracked stator vane and others having as many as 22 cracked.

The military uses a standard called the 90/95 probability of detection, which means that a defect of a certain size can be found nine out of 10 times with a 95% confidence level. The VTS 2400 allows anomalies of smaller sizes to be detected within this probability band, McKinley said.

Following the success with the initial 7E stator vanes, use of the technology has expanded to other 7E stator vanes, 7F R-0 and R-1 dovetail inspections (described in TIL 1638), the rotor forward shaft dovetail (TIL 1907), as well as for general end-of-life inspections for all frames.

Inspecting for and identifying 7F ‘hot items’

Mike Hoogsteden, field service manager for Advanced Turbine Support LLC, set the stage for his presentation with these introductory remarks: There are many variables in gas turbines that can be monitored to warn of impending component failure—such as vibration, temperature spreads, etc. However, relying exclusively on operating data for early warning increases the likelihood that problems will go undetected until damage occurs. Routine inspections by technicians with detailed knowledge of engine internals and equipped with the latest non-destructive examination (NDE) tools are considered by many to be your first line of defense against catastrophic damage.

TILs 1796 and 1870-R1. What better place to begin than at the beginning—of the compressor, at R0. Forward migration of R0 blades has been a concern for several years. Undersized stake marks are the primary issue. Technical Information Letter (TIL) 1796 (Apr 25, 2011) offered guidance on how to determine if blades are properly staked and the steps necessary to correct if need be. TIL 1870-R1 (Mar 5, 2013) followed, requiring owner/operators to check for first-row blade migration on F-class compressors that received an R0 re-installation between January 2008 and January 2013. A small population of turbines reportedly suffered migration events related to improper blade installation.

Hoogsteden stopped at this point to explain why he was spending so much time reviewing “old stuff” and would continue to do so throughout his presentation. The simple fact is Advanced Turbine Support’s inspectors are still finding problems that have been discussed for years. Reasons for this include virtually continuous turnover in the staffing at some plants and in the ownership of those facilities. During such churn, TILs and previous inspection reports go missing and new people may not have the background required to specify an experience-based scope of work for upcoming borescope inspections.

Biscuit rotation was the next topic—logical because it typically is caused by improper staking. Guidance on inspection and steps to take if biscuit rotation is identified also is provided by TIL 1870-R1 referenced above. Hoogsteden showed attendees the way Advanced Turbine Support technicians can inspect biscuits from the air inlet side of the compressor by drilling a small hole (less than ¼ in. diam) in the rub ring to accommodate a borescope probe.

TIL 1907 was issued Oct 7, 2013 to alert 7F users to the risk of cracking in the dovetail region of the forward stub shaft and to define inspection scope and intervals for specific unflared and flared configurations. Cracks in the region of interest can be hidden from view and the risk of not conducting a proper in-situ UT inspection at least annually is that the disc theoretically could fail before a crack is seen. To date, Advanced Turbine Support inspectors have identified R0 rotor wheel cracks in five 7FAs. One confirmed unit with cracks exhibited crack growth of approximately one-half inch in a 12-month period. Hoogsteden presented a couple of pictures to show attendees what these cracks look like. Members of the 7F Users Group can access the presentation through the organization’s website.

TIL 1509-R3, “F-Class Front-End (R0, S0, and R1) Compressor Inspections,” covers a lot of territory. Here are the highlights:

      • R0 leading-edge distress, which can be caused by impact damage; root-area erosion by fogging and/or the introduction of corrosive elements into the compressor inlet.

      • R0 and R1 blade-tip cracking, which is caused most commonly by tip rubs against the case during operation.

      • S0 stator-vane trailing-edge cracks, which apparently are caused by vane “lock-up.” The speaker noted that his company’s technicians have identified, with eddy current, both S0 and S1 stator-vane cracks in the initiation phase that measured less than 0.08 in. These indications were not identified visually, even after being inspected with fluorescent dye.

Hoogsteden once again showed several photos of the different types of damage covered by this TIL. They are accessible by 7F owner/operators on the users group website.

TIL 1638 (May 13, 2009), “F-Class R0/R1 Platform Ultrasonic Testing,” applies to gas turbines with rotating Stage 0 blades that do not have the platform undercut feature. Its purpose is to advise users of in-situ R0 and case-off R1 testing recommendations to ensure against the presence of dovetail distress below the blade platform.

Hoogsteden said that since 2007, Advanced Turbine Support has completed in-situ more than 2200 inspections (P-cut, standard, and enhanced blades) and in the process has identified the following:

      • 55 cracked R0 blades adjacent to the P-Cut relief.

      • 57 cracked R0 blades in the suction-side mid-span dovetail fillet.

      • 3 cracked R0 blades on the suction-side mid-span dovetail sloped face.

      • 2 cracked R0 blades in the pressure-side dovetail fillet (both in the leading and trailing edges).

      • 3 cracked R1 rotor blades in flared units.

Clashing between R2/S2 and R3/S3 airfoils has been identified in both the upper and lower halves of 7FA compressors. Hoogsteden, who has closely followed clashing incidents in the 7EA fleet for years, often presenting on the subject, said he knows of nine cases of 7FA clashing; two of those units had damage so severe forced outages were necessary.

The OEM has not yet released a TIL on 7FA clashing.

Cracking in R17 wheel dovetail slots also was mentioned. The Advanced Turbine Support field service manager suggested that 7FA owner/operators specify annual inspections of the rotor dovetails at the trailing edge of R17 (TILs 1971 and 1972). If cracks are identified, safe operating practice suggests not to run the unit until an engineering disposition can be performed. If cracking in the dovetails goes undetected, a possible result is blade liberation and significant damage.

“F-Class Shrouded Stator 17 Inspection,” covered by TIL 1850 (Aug 20, 2012), advises owner/operators to monitor the shrouded S17 design and provides recommendations for mitigating any observed distress. The photos Hoogsteden showed—such as failure of mounting hardware, rotor contact caused by shroud movement, etc, were similar to the illustrations found in the TIL.

Hoogsteden wrapped up his presentation to the packed meeting room with a series of photos from recent inspections by Advanced Turbine Support technicians to familiarize 7FA plant personnel with some of the damage others have found and why it’s important to inspect gas turbines regularly. Here are some of the images you can view in the speaker’s PowerPoint, available to owner/operators at

      • Combustion inspection: cracks in mounting brackets for transition pieces, impingement sleeves, and effusion plates; damage to PM2 fuel nozzles and cross-fire tubes.

      • Turbine inspection: first-stage nozzle damage and measurements; first-stage bucket platform cracks and material loss as well as tip-cap damage and material loss; cracking of the second-stage bucket tip shroud; liberated flex-seal ring-pipe material and exposed flex seal and resulting third-stage bucket damage.

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