Issues identified during recent borescope inspections on GE and Siemens frames

The GE E/EA Roundtable at CTOTF™ meetings always is well attended by owner/operators. Fleet size is one reason: There are more than 1000 of these frames in service today. Another is the deep knowledge of the session’s leadership: Pierre Boehler and Ed Wong of NRG Energy Inc, Dan Giel of Duke Energy, and Dave Hollandsworth of Gainesville (Fla) Regional Utilities have decades of O&M experience with these engines. This assures a top-notch program and meaningful discussion.  

At the user organization’s spring meeting (April 12-16 in Fort Myers, Fla), Mike Hoogsteden, service manager, Advanced Turbine Support LLC, was invited to present a historical perspective on 7EA compressor clashing and blade distress based on the company’s inspection findings. The thorough review was of particular value to newcomers. But it also reminded experienced attendees that there’s never time to relax when you’re managing assets. Changing operating profiles and ageing equipment are two reasons damage mechanisms may crop up “later in life.” Recall that the 7E was introduced in the mid-1970s, the 7EA about 10 years later.

If you are unfamiliar with the compressor issues that have disturbed the sleep of many 7EA owner/operators for the last several years, read the following three articles and dig down into the links provided in each. They summarize the first two-thirds of Hoogsteden’s presentation. The recommended reading:

Within the last six weeks, Advanced Turbine Support personnel identified cracks in 7EA S1 vanes at two plants while performing in-situ eddy-current (EC) inspections. Multiple cracks were identified in the suction side of 30% of the vanes in one engine. The cracks ranged in size from approximately 0.350 in. long and 8 mils deep to greater than 1.2 in. long and 30 mils deep. Three of the 12 vanes identified with cracking were confirmed via visible liquid-penetrant inspection. With comparative findings and documentation, Hoogsteden offered proof that the results from the in-situ S1 EC technique developed by Advanced Turbine Support was superior to other in-situ NDE methods for detecting cracks.

This is important to users because the OEM’s Technical Information Letter 1884, “7EA R1/S1 Inspection Recommendations” (April 2013), recommends a penetrant inspection in the “area of interest.” Hoogsteden said, “Our recent findings support performing an in-situ EC inspection to the entire suction side of every S1 stator vane.” Undetected cracks could lead to stator-vane liberations. In fact, he continued, “Liberations and major compressor damage already have been documented in at least two units that had no evidence of clashing damage.” The service manager added, “If cracks are found, we recommend not operating the unit until an engineering disposition can be performed.”

More recently, the company’s inspectors, using EC, identified cracks in five S1 vanes of a 1990-vintage 7EA. This was a particularly interesting finding: One of the vanes had both pressure-side cracks in the platform area and two trailing-edge cracks near the tip of the airfoil.

Hoogsteden concluded his presentation with the following recommendation for a clashing inspection: EC the trailing edges of all R1 rotor-blade platforms, the leading-edge tips, and the entire suction side of every S1 stator vane each peak run season, or every six months.

CTOTF is unique in that the group supports all models of gas turbines larger than about 10 MW. Presentation on the 7EA over, the editors sat down with Hoogsteden to get the latest inspection findings for other popular engines—specifically the 7FA, 501F4, and LMS100. Here’s an overview of that discussion:    

      • 7FA. Hoogsteden mentioned two areas of concern based on recent findings: dovetail cracks in wheels at the aft end of the compressor rotor (Fig 1) and tearing of inlet guide vanes (IGVs, Fig 2). Regarding the first point, the service manager said during recent hot-gas-path and major inspections, company personnel have identified several early 7Fs with cracks in R16 and R17 rotor-blade dovetail slots. Undetected, he said, this condition could lead to blade liberation and significant damage.

Advanced Turbine Support’s recommendation: During all borescope intervals, perform in-situ inspections on the rotor dovetails at the trailing edge of R16 and the leading and trailing edges of R17. If cracks are identified, the safe operating practice is to not operate the unit until an engineering disposition can be performed.

ATS 7FA fIGS 1, 2

The finding of two torn IGVs on a 7FA (2300 starts and 65,000 hours of service) before parts broke loose and went downstream was “serendipitous” and reinforced the need for regular inspection of critical assets. The jury is still out on the cause of these airfoil failures, but a quick web search by the editors identified at least one similar incident a couple of years ago on a 7EA.

Onsite recommendation, in this case, was to replace the broken IGVs and perform full fluorescent penetrant inspections of the remaining airfoils to ensure they were fit for duty. Also recommended was preservation of the fracture surfaces on the failed parts to support a root-cause analysis.

ATS 501FD4 Figs 3, 4

      • SGT6-5000F(4). A recent inspection revealed cracks in the tip shrouds of several R4 turbine blades (Fig 3). Better perspective on crack location is provided in Fig 4, the photo taken by an inspector sitting in the exhaust end of the machine and looking into Row 4. OEM service engineering personnel said they were aware of cracking in this low-stress area of the blade on other units and had traced it to a coating issue. Likely, according to an OEM representative, such cracking would not result in operating restrictions.

      • LMS100. Hoogsteden mentioned that Advanced Turbine Support had recently performed a borescope inspection on its first LMS100; no issues identified. The company regularly inspects other engines in the LM fleet—LM6000s in particular. A call to the owner revealed that this LMS100, located at a single-unit site, typically operates daily for about 12 hours with one start; sometimes service hours are split between two starts. This engine is inspected annually following an off-line water wash with soap.

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