Borescope inspections reveal 7EA users’ ongoing battle against clashing, IGV rubs, shim migration

If one were reconstructing the E F Hutton financial services ad for the gas-turbine generation sector of the electric power industry, it might go something like this: “When Rod Shidler talks, everyone listens.” That’s why the 7EA Users Group steering committee invited Shidler, a founder of Advanced Turbine Support Inc (ATS), Gainesville, Fla, the well-respected inspection and technology services firm, to open the organization’s 2011 conference two weeks ago in San Antonio.

A solid first presentation is critical to the success of any conference. Get attendees focused early and they will “carry” the meeting. Shidler speaks the users’ language and may know more about what’s going on inside 7EAs than any other independent source in the industry. ATS performs hundreds of borescope inspections on these and related engines annually. There isn’t much, if anything, that escapes the trained eyes of the company’s inspectors.

The steering committee members who developed the program for the San Antonio meeting were Pat Myers, plant manager, Ceredo Generating Station, American Electric Power Co; Ray Lathrop, maintenance supervisor at Corn Belt Power Co-op’s Earl F Wilson Power Plant; Jim Beveridge, plant manager, Nebo Power Station, Utah Associated Municipal Power Systems, Progress Energy’s Amy Alix, and Lane Watson of FM Global. If you are a 7EA owner or operator and not a member of the 7EA Users Group, join today at no cost (http://ge7ea.users-groups.com) to participate in the organization’s forums and get access to the presentations from this and previous meetings.

Shidler opened his presentation with a review of what he considers the OEM’s most important Technical Information Letters (TILs) regarding the 7EA, specifically:

* 1132-2R1, IGV (inlet guide vane) rubs.

* 1562, stator vane shim migration.

* 1090-2R1 R17 blade movement.

* 1744 Stage S17 and EGV (exit guide vane) stator ring rail and CDC (compressor discharge case) hookfit wear.

Next, he discussed the importance of performing inspections in accordance with TIL recommendations, or sooner, depending on unit condition. Shidler stressed the importance of proper documentation of findings to obtain the correct engineering deposition. Documentation also is important to condition trending, he continued, which helps plant personnel decide if a review of operating procedures is in order. Baseline inspections of unit condition after major overhauls should be followed-up with annual or biannual inspections to identify any damage caused by rubs, foreign objects in the gas stream, corrosion pitting, deposits, cracks, coating loss, and component wear, movement, or loss.

Typical IGV damage found during an inlet-condition assessment is shown in Figs 1 and 2. TIL 1132-2R1 speaks to IGV spring and thrust washers—including when these components should be replaced. X-gap clearances should be inspected annually. Rotor blade-to-compressor case rubs were next, with Shidler pointing to R1 rolled metal and R2 tip liberation found (Figs 3-6). Where casing rubs have been identified, he advised users to specify remote dye-penetrant testing of blade tips to check for cracks. If tip discoloration or rolled metal is found, those blades should be dye-penned every 25 starts. ATS has determined that this interval is conducive to identifying cracks prior to the liberation of material. He added that the interval is not affected by tip grinding. ATS regularly finds cracks on units that have been tip-ground.

More than a dozen photos of clashing seemed to send a shiver up user spines. The term is used to describe contact between rotating blades and stationary vanes in Frame 7 compressors. There have been no catastrophic failures to date. Clashing is relatively common and has been discussed at every 7EA meeting for the last several years. However, its cause remains unknown and owner/operators are sufficiently concerned to take action independent of the OEM (see following article). One utility reported clashing in 20 of 27 machines—all in peaking service. Those engines just run up to full load, operate for a few hours at most and then are shut down. Discussions among 7EA users focus on R1/S1 clashing; among 7F users the focus is R2/S2. In virtually all instances, reports indicated the clashing was concentrated at or near the 6 o’clock position.

What’s unnerving is that clashing remains unpredictable. One 7EA user told the group at an earlier meeting that clashing was found in five of six engines at his site. Four of the five affected units each had accumulated a nominal 1000 fired hours and 200 fired starts since COD. A year later, after only 20 more fired starts and 60 more fired hours, it was found on the sixth engine.

Shim migration has been discussed several times in the pages of the COMBINED CYCLE Journal, along with its solutions—such as vane pinning. Figs 7 and 8 show S2 and S3 shim migration. Shidler told the group that owner/operators should inspect the front ends of their compressors to develop a shim map for stages 1 through 4. There are 16 locations in those four stages where shims might have been installed. Locations having shims should be checked during each borescope inspection, he advised. Any shims protruding from the case by less than half an inch should be monitored every 25 starts until they reach one half inch. At that point, the shims should be removed or ground off as recommended in TIL 1562.

Stator vane platform stepping, which affects stages 5 through 8, was next on Shidler’s checklist (Fig 9), followed by rotor blade movement in stage 17 as described in TIL 1090-2R1. Movement can be significant: Shidler showed photos of blades that had moved by as much as 0.25 in. Discussion of inspection points in the combustion section followed along with photos of cracks in liners (Fig 10) and transition pieces (Fig 11). Shidler wrapped up his presentation with photos of relatively common turbine section ills: coating loss on first-stage nozzles, first-stage bucket pin liberation, damage to the first-stage nozzle discourager seal, and bucket impact damage downstream of the first stage.

 

 

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