GE engineers closer to mitigation strategies for clashing, other 7EA S1 issues

Attendees benefit most from user group conferences involving robust participation by owner/operators, OEMs, and third-party services providers because it provides them perspective on various aspects of a particular issue. Inevitable disagreements among the parties are conducive to meaningful discussion, occasionally loud. This process contributes to issue resolution, often stimulating equipment modifications and upgrades and improvements in plant O&M practices.

CTOTF’s™ GE E- & EA-class Roundtable is the poster child for such a meeting. Its day-long program at the user group’s spring 2015 conference in Fort Myers, Fla, developed by Pierre Boehler and Ed Wong, PE, of NRG Energy Inc, Dan Giel of Duke Energy, and Dave Hollandsworth of Gainesville (Fla) Regional Utilities, exceeded expectations as usual with diversity in presentation topics and discussion.

The OEM’s engineering team presented in the morning on rotor design and casing alignment, the Mark V control system, lube-oil systems, compressor clashing, and parts configuration management. In the afternoon, a user demonstrated how to remove the upper- and lower-half aft-compressor casing of a Frame 5, with the rotor in-place, to enable a hook-fit repair. Mike Hoogsteden, field service manager for Advanced Turbine Support LLC, followed with a presentation on advancements in 7EA first-stage stator vane inspections.

GE’s 7EA product manager opened the meeting with a fleet overview—particularly worthwhile for first-timers, which comprised about one-quarter of the audience. Frame 7B-EA gas turbines, now installed in 99 countries, total about 224 GW in nameplate generating capability. Experience includes more than 143-million operating hours and 1.2-million starts. The Frame 7 series of engines was introduced in 1971 with the 7A. The most popular machines in the 7B-EA fleet are the 7B and 7EA.

A few of the major design differences between the E- and F-class engines identified by the rotor expert:

7E is a three-bearing, hot-end-drive machine with a 17-stage compressor and three-stage turbine section. Turbine wheels are made of alloy steel. The 7E base design was created with a simple-cycle configuration in mind.

7F is a two-bearing, cold-end-drive machine with an 18-stage compressor and three-stage turbine section. A sophisticated super alloy is used to make turbine wheels. The cold-end drive provides a straight path for the exhaust stream into the HRSG to maximize efficiency. However, this design increases the torque requirement through the compressor rotor and forward end of the turbine rotor.

The rotor presentation was a good backgrounder for all those concerned about rotor life (everyone). Here are some highlights:

During the segment on rotor design basics, the group was reminded that the tight-tolerance, rabbet-fit feature of turbine wheels assures wheel centering for vibration stability and that the wheel/shaft bolt faces provide rotor backbone stiffness support and contribute to the transmission of torque.

Creep, caused by stress and temperature, is the primary damage mechanism when engines are in base-load service. Fatigue affects starts-based units. Low-cycle fatigue (LCF) life is based on stress, which is influenced by speed and thermal condition. Forced cooling on shutdown adversely impacts fatigue life, but this effect is minimal on E-class engines. However, a combination of fast starts and forced cooling can get you in trouble over time.

Ageing observations noted by the speaker:

1. Embrittlement in E-class steel wheels. This is the change in a material’s toughness/ductility over time. Different forms of embrittlement exist, but most are driven by the intergranular effects arising from time at temperature.

2. Material softening—that is, the observed decrease in hardness readings on aft compressor wheels and turbine wheels. Ultimate and yield strengths move in the direction of hardness.

Clashing of rotating blades and stationary vanes in the first two stages of the 7EA compressor has been a topic of ongoing CCJ coverage for the last six years. Gain access to all the articles published by the editors by using the search function towards the top of this page. GE’s presentation on the subject was of considerable interest to most users in the room. According to the speaker, the OEM is aware of more than 100 clashing events in the last five years; midspan cracking in the area of concernwas identified at only one site.

Users have long been puzzled by the seemingly sudden appearance of clashing damage in one Midwestern 7EA peaker—and later all the units at that site—on a particularly cold winter evening about nine years ago. With the OEM mum publicly on the subject until recently, the 7EA Users Group and a few third-party equipment and services suppliers worked at identifying the cause of the phenomenon for years with limited success.

The speaker noted that operating profiles are changing today and engines are getting older so having new damage mechanisms crop up “later in life” should not be surprising. This is particularly true among E-class units, most of which have been relegated to low-hours peaking service, allowing them to sit for extended periods. In some environments these conditions are conducive to severe corrosion.

He said the key problem in 7EAs is vane lockup in ring segments, which reduces blade damping. This make the blade more susceptible to various phenomena, including rotating stall—the local disruption of compressor air flow during startup or shutdown which can cause a resonant response detrimental to materials. Note that S1 problems associated with the 7EA typically have not been identified in the 7A-E fleet because the compressor designs are different. Specifically, the 7EA has a flared compressor and it has a different aero loading than the 7A-Es.

Other issues associated with 7EA S1 are the following:

Tip liberations have been experienced in more than 30 7EAs—about 3% of the fleet—over the last seven years. Most tip liberations have occurred in coastal engines with airfoils made from GTD-450 material.

Root liberations reported previously in 7FAs have occurred recently in two 7EAs with vanes made of 403 Cb steel, which was replaced in 1988 by GE with GTD-450.

Of considerable help to owner/operators were these recommendations made by the subject-matter expert:

With 403 Cb vanes, you can expect to see cracks before clashing occurs because of the loss of ductility over time. Inspect regularly to TIL 1884 for 7EA R2/R3 tip loss.

Replace original vane carriers with ones made of stainless steel, to prevent lockup of vanes by corrosion products.

Protect GTD-450 vanes from corrosion by coating with GECC1. Virtually all users attending the session thought GTD-450 could not be coated. The speaker’s response: New solution for changing times; yesterday’s solution is no longer applicable.

Consult with the OEM about appropriate changes to the IGV schedule.

GE’s engineering team is pursuing the root cause of the rotating stall in the 7EA, which compounds the likelihood of S1 issues. Plus, it is fine-tuning recommended operating procedures to minimize the possibility of stall. A beta test site already has been agreed to by a major utility with a large number of GT assets.

End notes: The joint task force established by CTOTF and GE reported on its progress investigating casing-alignment and failed liquid-fuel starts in the 7B-EA fleet. Regarding the first initiative, users were referred to TIL 1819R1 and GEK131700A for recommended inspections during outages to establish benchmarks for future evaluations and assess the need to perform casing alignments during major inspections.

The presentation on lube-oil systems, which focused on fleet performance and issues associated with certain fluids, generated significant discussion on PAG (polyalkylene glycol), a synthetic lubricant manufactured by Dow, as a replacement for traditional mineral oils.

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