Rod Shidler and Mike Hoogsteden of Advanced Turbine Support LLC, Gainesville, Fla, met with the editors a couple of weeks ago and passed on the photos shown here of an S1 tip liberation at the 6 o’clock position that caused extensive damage to a 7FA flared compressor. This was the second such incident witnessed by the engine inspection experts at ATS.
The OEM, to the editors’ knowledge, has not issued a Technical Information Letter pertaining to S1 airfoils. The closest the manufacturer comes is with its TIL 1509-R3 which addresses R0, R1 and S0. This document recommends annual visual inspection for R0 root cracking, R0/R1 tip discoloration, rolled metal, and/or tip loss. It provides detailed recommendations if any of those deterioration mechanisms are identified. For S0 stator vanes, inspection personnel are urged to look for trailing-edge cracks and if found immediate replacement is recommended.
Team ATS is not in complete agreement with the recommendations in TIL 1509-R3. It believes that annual visual inspections are not sufficiently comprehensive and have the potential to miss small or tight indications that could result in blade liberations and a catastrophic compressor event. Proof: The company’s inspection experts have identified multiple rotor-blade cracks with visible dye penetrant that were not seen during unaided visual inspections.
In addition, ATS’s state-of-the-art eddy-current inspection tools have identified S0 trailing-edge stator-vane cracks in the initiation phase that measured smaller than 0.080 in. These indications were not identified visually, even with the use of fluorescent dye. Company personnel also have found S0 leading-edge and radial-tip cracks in both flared and unflared compressors, which are not addressed in TIL 1509-R3. To those indications, you can now add the S1 experience.
ATS has recommended for several years dye-penetrant inspection of R0/R1 blades and dye-pen or eddy-current inspections of S0 vanes annually or every 100 fired starts. Subsequent inspections based on the results of the initial investigations would run parallel to the OEM’s recommendations. More recently it has recommended eddy-current inspections of S1 because of the earlier liberation.
The second and most recent liberation occurred in a base-load unit that had logged more than 55,000 hours of operation. The plant is not located in the so-called “Ring of Fire” near the coastline where some engine parts generally are believed more susceptible to damage. ATS visually inspected the unit only by borescope in November 2011, at the customer’s request. Vibration signature changed in December but the unit did not trip and continued in service until the spring inspection in early March when the damage was discovered.
Shidler said ATS can eddy-current check all the vanes in S0 and S1 during a top-on inspection. The company’s technicians can also get to some vanes in rows S2 and S3. He added that inspecting S1 after an eddy-current inspection of S0 only adds about 25% to the invoice. To learn more, visit with ATS in Booth 24 during the CTOTF exhibition Monday evening, April 16.
For us non-experts, what is the S1 vane / blade? More generally, what is the vanes / blades identification system?
Vanes are the stationary blades installed in the compressor case. The first row of vanes is typically is called S1. The first row of blades installed in the compressor rotor would be R1. The rows of vanes and blades are numbered consecutively from the front of the machine to the back. The 7FA has 17 rows. Looking at the Sulzer ad on the right-hand side of the screen, the machinist at the left is partially hidden by the first row of rotating blades, which are the largest in the compressor.
An E-class compressor has 17 stages, numbered 1-17. In GE’s F-class machines, there is an additional stage at the front, called “stage 0″. So, R0 would be the first rotating blade and S0 the first stator vane. GE has actually been communicating with its customers about forward stage compressor distress since 2006, through User Group meetings (including multiple CTOTF and 7FA Users conferences) and webinar presentations with users. The principle experience on “flared” 7FA machines has been on S1 and S3, where the crack initiates typically from corrosion pitting or foreign object damage. GE has released several TILs and communications warning of the dangers of corrosion and emphasizing the need to inspect for corrosion and maintain unit cleanliness. GE’s “enhanced compressor” packages offer increased robustness to this type of damage, and a large portion of 7FA users have implemented these upgrades. Users can contact their GE representative for more information.