Experiences shared by colleagues may be the most important reason for attending the upcoming 7F Users Group meeting

The 7F Users Group’s Annual Conference and Vendor Fair is less than three weeks away—May 7-11, at the Atlanta Marriott Marquis. If you’re still debating whether to attend, it’s time to get off the fence and register for what surely is the most important meeting in 2018 for owner/operators of GE 7F gas turbines.

You can expect about 250 colleagues from around the globe at the conference, making it the world’s largest user meeting dedicated to one frame. Attendance enables you to access O&M know-how from engineers and technicians with deep 7F experience. There’s no better place for plant personnel to learn about this workhorse engine.

Perhaps the best way to gauge the meeting’s value is by review highlights from a few of last year’s user presentations (below). Registered owner/operators can access all the presentations on the PowerUsers website.

COMPRESSOR

Clashing event and root causes

Damage was discovered on rotating and stationary components in Rows 2 and 3 of a 7F.03 flared compressor (COD 2002) undergoing its first major inspection in spring 2016. Clashing was suspected immediately given the wear and tear at the roots of rotating blades and at the tips of stationary vanes. One or more clashing events had occurred since the unit’s last borescope inspection in fall 2015.

Investigations conducted after the unexpected discovery of damage noted that a Buffalo lube-oil (LO) pump coupling had failed and tripped the unit during the time period associated with clashing. The plant owner and GE collaborated on a root-cause analysis, finding the following:

    • The initiating event was a sheared LO pump coupling.

    • Inlet guide vanes (IGVs) closed quickly (in less than one second), before the unit tripped. A review of other trips with normal LO pressure showed slower IGV closure rates.

    • Compressor stalled prior to the trip because of IGV fast closure, the stall event forcing stator vanes into the rotating blades.

Key takeaways:  

    • Proper alignment of LO pump and motor is essential to prevent premature coupling failure. Note that the spigot fit between the motor and motor stand is not sufficient to control alignment.

    • Maintain the coupling assembly in accordance with the manufacturer’s recommendations.

    • Consider a fail-safe coupling with interlocking hubs to maintain system pressure in the event of a coupling failure.

    • Users with Moog actuators were urged to review Technical Information Letter (TIL) 2013 which pertains to IGV closure speed. The group was told that a closure time of between about 3 and 5 seconds was recommended—not faster.

The speaker went on to discuss coupling failures associated with Buffalo pumps, focusing on the need for proper alignment of Falk® Steelflex and Falk Rexnord Wrapflex® couplings and proper lubrication of the former. The Wrapflex coupling is a greaseless design but still is failure-prone—six times on 38 installations in one fleet alone. Reportedly, it is susceptible to parallel misalignment (not angular misalignment). Conservative selection of Wrapflex couplings reportedly is critical to their reliable service; four times the typical design margin was suggested by one rep.

S17 stator wear

A user reviewed his company’s S17 experience after walking attendees through an introductory slide that showed how the OEM’s recommendations on S17 hardware had evolved between 2004 and 2016. TIL 1850, “F-Class Shrouded Stator 17 Inspection,” was incorporated into the discussion.

Six 7FA.03 machines commissioned in 2007 at one of the company’s plants each was equipped with the G001 welded-design stator, engineered to run three cycles, or at least 144,000 hours. The recommended maintenance intervals for these units were hot-gas-path (HGP) inspection at 24,000 hours, major inspection (MI) at 48,000 hours. S17 stator wear was observed at the first major in 2012 and the OEM recommended replacement with the G002 staked-design stator. At 88,000 hours, progressive wear was observed on the G002 design and the OEM recommended the G004 enhanced-design stator.

Twelve 7FA.03 machines were commissioned in 2009 at another of the company’s plants with the G001 welded-design stator. The recommended maintenance intervals for these units were HGP at 32,000 hours, major at 64,000. During the first HGP, S17 wear was observed and the OEM recommended the hardware remain in service until the major and then replaced with the G002 staked design. 

However, bushing wear progressed faster than expected and the inner shroud was found moving towards the rotor at 50,000 factored fired hours. A forced outage was taken to install available G002 hardware as a containment action. Plan going forward is to replace S17 with the G004 enhanced design at the major (64,000 factored fired hours).

The owner/operator’s engineers were said to have a low level of confidence regarding the ability of G001 and G002 hardware to operate at a high level of reliability until the first major. While engineers conceded that the wear rate may be slowed with the new bushing material used in the G004 solution, they believe those stators too will require replacement at every major. The speaker then reached out to attendees for their experiences and if they might have identified alternative solutions to address S17 concerns.

The speaker next discussed his company’s experience upgrading the standard compressors installed in the machines for the 12-unit plant with the Package 2+ mod (upgrade of the first four compressor stages and enhanced R0 rotor blade). He closed by describing an internal evaluation of the Bigfoot and Littlefoot options for mitigating S14-S16 rocking concerns on two units. Access the presentation on the Power Users website [link again] for more information.

SAFETY ROUNDTABLE

Probably no one in the user group community is better equipped to conduct a safety roundtable than Jeff Gillis, a member of the 7F Users Group steering committee, co-chairman of the Frame 6 Users Group steering committee, and global gas-turbine lead for ExxonMobil Research & Engineering Co. He is knowledgeable about safety regulations and behaviors here and in many other countries. Keep in mind that OSHA is not global and America does not have all the answers.

Gillis provided the opportunity for attendees to ask questions on a wide range of topics, putting up the following opening slide of “reminders” to trigger questions.

General: Life-saving rules, compartment entry.

Safety systems: Hazardous gas detection, fire suppression systems.

Maintenance: Fall protection and PPE, scaffolding and access, safety professionals and other personnel, inlet-filter-house fire prevention and escape, rescue considerations.

Following several questions and open discussion of user experiences, Gillis reminded participants about the value of the 7F User Forum on the Power Users website [link to www. powerusers.org] regarding safety concerns. He reviewed several safety threads to illustrate the forum’s ability to quickly connect users with questions to those with answers. Sample threads included these:

    • Mounting location of turbine-compartment fire detectors.

    • Cable system fall arrest.

    • 7FH2 hydrogen leakage.

    • Combustible-gas-monitor alarm response.

    • 7F has-gas protective systems.

Perhaps the most valuable part of the roundtable was Gillis’ presentation of safety TILs affecting 7FAs. He listed 30 documents that should be reviewed by plant management to determine their applicability in safety training, outage preparation, etc; plus, eight product safety bulletins. You’ll likely want a copy of this PowerPoint in your files.

COMBUSTION SECTION

FlameSheet™ project update

Owner/operators of 7F and 501F engines have been following industry experience with PSM’s promising FlameSheet™ combustion system [link to http://www.ccj-online.com/combustion-hardware-enhancements-improve-performance-reliability-of-7fas/] since retrofit of two 7FAs at Eastman Chemical Co in 2015. [link to http://www.ccj-online.com/combined-cycle-journal-issue-51/7f-users-group-some-new-faces-same-great-program/] An Eastman engineer provided the user group its annual progress report on the technology, which is delivering on its promise.

    • The hardware is performing well—as expected. The speaker added that his company is very pleased with the FlameSheet project as a whole. The first engine upgraded to FlameSheet in March 2015 had about 16,500 fired hours/463 factored starts at the time of the 2017 meeting; the second unit upgraded in October 2015 had about 13,100 fired hours and 86 factored starts.

    • The addition of a chromium carbide coating to combat wear of the main-injector seals for the first gas turbine retrofitted with the new combustion system was said to be meeting expectations after a year of service.

    • FlameSheet has permitted the transition to 2 × 1 operation around the clock, increasing site reliability.

Two-year results:

Goals 

Turndown to 50% of the full-load rating 

Less than 9 ppm of NOx

Less than 9 ppm of CO

Equal or better efficiency compared to existing

Less than 130% of full-load efficiency at 50% load 

Elimination of visible emissions on startup 

 


Results

Achieved 40%

Less than 5/7 ppm at 40%/100% load

Less than 9/about 1 ppm at 40%/100% load

Confirmed

Less than 127%

Confirmed

TURBINE

TBC foreign-substance contamination

Some of the things you hear at user-group meetings might never have crossed your mind. Consider the speaker involved in a gas turbine’s first hot-gas-path (HGP) inspection and preparing to change buckets, nozzles, and shroud blocks for all three turbine stages. The replacement parts all were owned by the engine’s owner/operator.

He opens the box of new first-stage buckets (TBC-coated GTD-111 DS material; no internal coating) and finds writing on multiple airfoils. So, you ask, who wrote on the buckets and with what? As if you’d ever find out. Nobody knows anything except that if a Sharpie were used it’s not a good thing.

The OEM provided a list of approved markers, including these:

    • Carters Marks-A-Lot (black or blue).

    • Dykem® Hi-Spot Blue.

    • Joseph Dixon lead-free yellow lumber crayons.

    • Wallace Pencil Co #800 black marker.

    • Marco S-1141 (black and white).

Sharpie is not an approved marker for writing on airfoils. Reason given: Nickel-base alloys are subject to attack when heated in the presence of sulfur, lead, and other metals. These materials can cause embrittlement and cracking at elevated temperatures.

Recommended courses of action for this user: Replace with new buckets or clean the marked buckets with alcohol, install, and monitor. The latter translates to borescope at 4000 hours and check for spallation.

OTHER USER PRESENTATIONS

There were several other user presentations at the 2017 meeting. At a typical conference you can expect to listen to a dozen or so, all vetted by the steering committee. Some offer lessons learned of great value. Sharing information across the fleet is what user groups are all about. You don’t want to pay again for experience a colleague has already paid for.

Presentations available on the Power Users website and not profiled here that you might want to access include the following:

    • TIL 1584-R1 concerns weld cracking of turbine-shell extraction pipe that can result in a catastrophic failure. The original TIL, released in 2007, pertained only to 7FA+e machines. It was revised in 2011 to include the 7F, 7FA, and 7FA+. Recommendation is to inspect pipe flanges annually; the speaker suggested going beyond the TIL to include visual checking of similar connections.

    • A fuel nozzle issue was described that engineers attribute to a combustion flashback event caused by rapid variations in the heating value of delivered gas. This should be of interest to any plant with access to shale-gas reserves.

    • Case history of exhaust issues and repairs is worth a read before planning your next outage.

    • Thrust bearing instability giving you fits? This short presentation provides guidance on what to look for and how to identify the issues affecting your bearing.

    • Electric actuators for stop/ratio valves, PM valves, and IGVs allowed this user to eliminate hydraulic valve actuators prone to varnishing, turbine trips from inconsistent valve tracking, and annual cleaning or replacement of servos.

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