7EA state-of-the-engine report for 2014 focuses on compressor clashing, blade tip distress

A timely opening presentation by an engaging speaker is critical to the success of most meetings, at least based on the editors’ experience. If the audience connects well with the first presenter, adrenaline flows and the day’s discussion likely will be robust and the give-and-take at breaks and lunch productive. The steering committee of the 7EA Users Group knows this, which probably is why President Rod Shidler and Service Manager Mike Hoogsteden of Advanced Turbine Support LLC are invited to jump-start the organization’s annual meeting each year.

Their state-of-the-engine report is front-page news, zeroing in on the company’s findings compiled from more than 1000 annual gas-turbine inspections. While the 2014 report, presented on October 21, offered no surprises in terms of new damage mechanisms identified in the past year, it did point to a year-over-year increase in fleet-wide findings.

Hoogsteden focused his remarks on Technical Information Letter 1884, “7EA R1/S1 Inspection Recommendations,” April 2013, and TIL 1854, “Compressor Rotor Stages 2 and 3 Tip Loss,” August 2012—both of great importance to 7EA owner/operators.

The service manager opened the 1884 portion of his presentation with a slide that screamed, “Clashing woes continue,” then migrated to the timeline (below) of important events from when the mechanism was first identified. Given the high percentage of first-timers at the conference, Hoogsteden’s summary enabled those unfamiliar with the clashing phenomenon of great concern to 7EA users to come up to speed quickly.

The timeline:

      • 2006. Clashing—the word adopted to describe the contact that occurs when the tip of a stationary vane moves forward and contacts passing rotating blades near their platforms—was first identified in the first stages of multiple 7EA compressors. Years later it would be found in 7FA and 6B compressors as well.

      • 2008. Advanced Turbine Support began using visible dye in its inspections. Back then, relatively few in the industry, including the OEM, appeared concerned about clashing.

      • 2009-2010. There was a noticeable increase in the number of units affected by clashing. Also during this period, improvements in borescope technology allowed Advanced Turbine Support to add measurements to its documentation.

      • 2011-2012. Inspectors noted increased damage, year-over-year, to airfoils in some engines that had experienced clashing. Documentation was upgraded to include trending data, and eddy-current inspections were implemented because of the elevated level of risk.

      • 2013. The OEM’s first technical information letter addressing clashing was issued in April. TIL 1884 identified a so-called area of interest on the suction side of Row 1 stator blades that should be inspected for cracking in units having experienced clashing.

      • May 2014. Inspectors from Advanced Turbine Support found two cracked S1 vanes in a single unit in the area of interest while performing a dye penetrant inspection. Those cracks were verified with eddy current. Yet another vane with multiple cracks in the area of interest was identified during eddy current testing of the entire Row 1. Those cracks, which had a maximum depth of 0.022 in., were retested with red dye and yielded no indications. A check of unit records revealed that from the time clashing first occurred in 2006 until cracking was found in 2014, the 7EA had operated only 3500 fired hours with 850 fired starts (round numbers).

      • October 2014. Technicians from Advanced Turbine Support identified cracking in the area of interest of an S1 vane in a second machine while performing a dye penetrant inspection.

      • During the OEM presentations at the 2014 meeting, attendees learned that one of the 7EAs inspected by GE technicians also revealed an S1 vane crack in the area of interest. 

Hoogsteden continued with dramatic pictures of trailing-edge damage on first-stage rotor blades attributed to clashing. The severe damage had occurred in only two years from when clashing first occurred (a nominal 250 fired hours and 50 fired starts later), surprising many attendees. He closed out this portion of his presentation with recommendations for clashing inspections of 7EAs and the company’s clashing history up until the time of the 2014 meeting.

Inspection guidelines. The speaker recommended that owner/operators perform in-situ red dye penetrant inspections on all affected vanes and rotating blades each peak-run season, or every six months. He urged checking the trailing edges of all rotor-blade platforms, leading-edge tips, and the entire area of interest of S1 stator vanes that have contacted blade platforms. Suspect indications should be re-inspected with eddy current.

To sum up, Advanced Turbine Support inspectors have identified R1/S1 clashing in more than 100 7EAs and R2/S2 clashing in over 25 engines. They have found cracks in the area of interest in four vanes resident in two units and tears in the trailing edges of three R1 blades.

Early detection. Late on Day One, the steering committee’s Pat Myers, plant manager of AEP’s Ceredo Generating Station, reported on the results of user efforts to identify both the root cause of clashing and a method for warning of its occurrence. This is the focus of the lead article in next week’s CCJ ONsite Issue 2 from the 7EA meeting.

After his remarks regarding TIL 1884, Hoogsteden briefly mentioned some other compressor issues Advanced Turbine Support inspectors were finding regularly, including these:

      • S5 vane rock. One way users can mitigate this is by pinning vanes.

      • Multiple stages with stator-vane rolled metal.

      • Trailing-edge clashing damage on second-stage stator blades in 7E compressors; plus, leading-edge tip clashing damage on S2 airfoils. No TIL has been issued yet to guide owner/operators dealing with this issue.

Shifting to TIL 1854, the speaker said the purpose of this advisory is to inform users about the recommended R2 and R3 blade blending and tipping to mitigate the impact of tip loss on availability and reliability. He mentioned three things that concern him about TIL 1854:

      • It does not address R1 blade tips.

      • It does not recommend in-situ inspections.

      • It considers “tip losses low risk to unit operation and reliability. 

Tip distress on first-, second-, and third-stage rotor blades generally is caused by casing rubs during operation, Hoogsteden continued. Identifying characteristics include tip discoloration or a heat-affected zone, and rolled metal. Recommendations from Advanced Turbine Support, based on more than 600 in-situ 7EA R1-R3 inspections since 2009 that have identified more than 50 cracked rotor blades and found more than a dozen and a half tip liberations, include the following:

      • For R1 and R2 blades with signs of tip distress, conduct visible dye-penetrant inspections to determine if radial cracks have initiated. These should be done up to four times at 25-start intervals, followed by two inspections at 50-start intervals, and then after every 100 starts—or annually.

      • For R3 blades showing signs of tip distress, do a minimum 360-deg roll inspecting all blade tips close up and at the same intervals.

The take-away from this portion of the presentation: Rubs cause blade distress and may lead to metal liberation and collateral damage. In-situ inspections can point to corrective action required to mitigate damage.

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