The 2013 Conference of the 7EA Users Group, conducted at the Hyatt Regency Monterey (Calif) Hotel and Spa, October 22-24, exceeded expectations in terms of participation, venue, and content—at least according to an informal poll by the editors. There were nearly 140 owner/operators in attendance, including more than two dozen international delegates, plus multiple representatives from most of the 107 companies exhibiting on the second evening of the event.
The venue was the right size for this group, with adequate space in the exhibit hall and meeting and dining rooms. Food was good and the location on the California coast obviously well-received based on the attendance. About half of the information was disseminated through panel discussions focusing on clashing of compressor rotating and stationary blades, rotor life extension, controls upgrades, performance enhancement, and exhaust systems. The panels featured up to eight experts each, keeping the discussion lively and the subject matter stimulating.
There also were stand-alone presentations on liquid fuel systems, generator issues, inspection of generator retaining rings, ignition-system troubleshooting, station battery maintenance, torque converters, and flame scanners. A few open discussion sessions and a special half-day program by the OEM rounded out the program.
This and one more CCJ ONsite will review the conference highlights; prepared presentations made available to the group are posted on its website at ge7ea.users-groups.com. Access is user-only. Coverage begins here with a summary of the meeting’s traditional opening session on fleet operating experience presented by Advanced Turbine Support LLC’s President Rod Shidler and Field Service Manager Mike Hoogsteden.
The borescope inspection experts focused their presentation on 7E-class compressor hot topics, specifically the following:
• First-stage rotor blade/stator vane clashing (refer to Technical Information Letter 1884, “7EA R1/S1 Inspection Recommendations”).
• Tip distress on rotating blades (refer to TIL 1854, “Compressor Rotor Stages 2 and 3 Tip Loss”).
• Shim mapping (refer to TIL 1562, “Heavy-Duty Gas Turbine Shim Migration and Loss.”)
Many readers will recognize these as topics covered previously in both the electronic CCJ ONsite and the print CCJ. They also have been discussed before at 7EA annual meetings, both by the OEM’s engineers and in user-only discussions. Why cover them again? Employee turnover rates at powerplants are high. Experienced O&M personnel are in great demand and there is signification migration from E- to F-class facilities, which opens up opportunities for operators and technicians with no E-class experience. What Shidler and Hoogsteden had to say was “new” to them. By show of hands, about 50% of the attendees at this year’s conference were first-timers.
Clashing, Hoogsteden said, is caused by the closure of the axial clearances between rotor blades and stator vanes. Some experts attribute this phenomenon to increased tip deflection caused by “a rotating-stall-driven S1 vane frequency response during starts and shutdowns.” The frequency response, together with a loss of damping attributed to ring-segment “lock-up,” produces higher-than-normal operating stresses. Lock-up typically occurs because corrosion and oxidation products build up between the mating surfaces of vanes and ring segments.
Several years ago, when clashing was first identified, damage typically was found on the trailing edges of rotor blades (Fig 1) and the leading edges of stator vanes (Fig 2) at or near the 6 o’clock position (vanes 39-45 using the numbering convention illustrated in Fig 3). Reason: Corrosion is most prevalent at the bottom of the casing because of moisture accumulation there.
More recently, Advanced Turbine Support’s inspectors have found clashing damage in the top right quadrant of the casing. One example: Vanes 13-15 in a unit with nearly 10,800 hours of service and almost 2100 starts. Another: Nine vanes between 1 and 14 in a unit with nearly 40,000 hours and 2000 starts. Inspectors also have identified clashing in Row 2 on more than 20 machines (Figs 4, 5). Row 1 damage has been found in more than 90 7EA compressors.
Hoogsteden told the group that TIL 1884 recommends an NDE inspection in a specified area on the suction side of any vane suffering clashing damage. However, he reported, Advanced Turbine Support has not found any cracks in the TIL’s area of concern. The speaker suggested an NDE inspection on the leading-edge tips of the stator vanes, where the physical damage occurs, may reveal additional indications.
The take-away from this portion of the presentation: Clashing damage appears to be increasing year over year; scheduled inspections can mitigate damage.
Blade-tip distress in rows 1-3 (Figs 6-9). 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 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” (Fig 10).
Recommendations from Advanced Turbine Support based on more than 400 in-situ 7EA R1/R2 inspections since 2009 that have identified three dozen cracked rotor blades and found more than a dozen tip liberations:
1. For R1 and R2 blades with signs of tip distress, conduct 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.
2. 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.
Protruding shims, at a minimum, disturb air-flow patterns in your compressor and adversely impact performance. If shim migration continues, the resulting flow disturbance can produce a strong once-per-revolution stimulus at the tops of downstream rotor blades (Fig 11). High-cycle fatigue damage in neighboring blades is possible under such conditions. In the extreme, a shim can release into the flow stream, virtually assuring collateral damage.
Most in the industry are aware of shim migration and have taken appropriate action, Hoogsteden said, but inspectors still find some protruding shims. If they are located in the first few stages of the machine, a properly trained and equipped borescope technician can completely remove the protruding shims, or grind them flush with the compressor case.
In engines with many protruding shims, the best approach might be to remove the upper casing and rotor, and pin the shims to adjacent vane segments (first four rows) or vanes. Pinning of individual vanes into segments of several vanes each is a good approach for inexpensively dealing with hook-fit wear. The pinning technique was developed by Rodger Anderson of DRS-Power Technology Inc about 10 years ago. Collectively, the blades he has pinned on more than 150 compressors have accumulated more than 4 million hours of operation to date without a failure.