CTOTF tackles user issues on a wide range of aero, frame machines

Program covers virtually all GT models

The defining characteristic of the Combustion Turbine Operations Task Force is that it addresses user issues on virtually all gas turbine (GT) models—aeroderivative and frame— produced by the major manufacturers. That’s why CTOTF (pronounced cee-toff) often is the meeting of choice for plant and asset managers responsible for a variety of engines, especially when time and/or budget constraints militate against attending more than one user group meeting per year.

Chairman John Lovelace (jlovelac@aspc.com), Vice Chairman David Jeffery (djeffery@reliant.com), and others on the leadership team (see sidebar) schedule meetings (they call them “turbine forums”) in both the spring and fall to accommodate schedule conflicts. It is the only GT user group that has two major conferences annually.

Wickey Elmo (wickelmo@ctotf. com, 704-753-5377), who serves both as the group and conference coordinator, counts members from more than 150 owner/operators in eight countries.

At the Spring Turbine Forum in Annapolis, April 3-7, where CTOTF sponsored the presentation of the COMBINED CYCLE Journal’s first annual Best Practices Awards (CCJ, 2Q/2005, p 57, available at www.psimedia.info/ccjarchives.htm), the technical program was robust and well-attended.

Dale Linaweaver, VP east region for host Constellation Generation Group, got the meeting off to a productive start for Chairman Lovelace. Three presentations highlighted the opening general session, which precedes the roundtable program. Rich Curtis, VP engineering for Eta Technologies LLC, Coventry, Ct (www. etatechnologies.com), who had the same position at Sermatech International until that company left the power business, addressed end-user concerns and considerations regarding procurement of new aftermarket capital parts. Eta specializes in engineering and metallurgical consulting and offers GT parts for sale.

Curtis noted the Catch-22 facing some owner/operators of advanced technology GTs: Buy expensive OEM capital spares and run the risk of pushing power production costs above the market price or buy non- OEM parts to hold down costs and possibly face the adverse impacts of increased technical—and legal— risk.

He focused on legal issues such as generation and proper care of intellectual property, public domain, types of patents and proper “designarounds,” and contractual indemnification. Having knowledge in these areas is key to mitigating the risks associated with buying non-OEM parts.

TOPS is tops. Rich Evans, plant facility manager, Wolf Hills Energy LLC, Bristol, Va, highlighted the opening session with a compelling presentation on a methodology proven effective for reducing the risk of accidents, injuries, equipment damage, and loss of plant revenue. TOPS—for Teams Operating Plants Safely—is a behavioral-based program designed to create and reinforce a culture of operational excellence that can be applied universally to any organization.

The TOPS program captures STAR (Stop, Think, Act, and Review) verification techniques as an observable behavior and also transposes RCA (root cause analysis) findings into a set of functional behaviors necessary to prevent recurrence. Evans and the Wolf Hills staff earned the top Best Practices Award for Safety— The Best of the Best—for the TOPS program and its accomplishments. To learn more, access the entries for the Best Practices Awards, CCJ, 1Q/2005, p 67, at www.psimedia. info/ccjarchives.htm.

The ins and outs of long-term service agreements, a topic of continuing interest to CTOTF members because of LTSA impacts on O&M budgets and operating strategies, complemented Curtis’ presentation earlier in the morning. Topic was covered by Chris Haaland and Kenn Cygan, Data Mobility Systems LLC, Falmouth, Maine (www.datamobility. com) presenting in tandem with Scott Polhemus of Morris Law Firm LLC, Atlanta (www.morrislaw. net). Data Mobility Systems offers engineering and legal consulting to help evaluate maintenance options. Its core product/service on the engineering side is software that assists individual plants and fleets in analyzing the need for GT parts and services.

Generic Roundtable: Water-cooled liquid-fuel check valves

Two important presentations were on the agenda for the Generic Roundtable the afternoon of Day One: The first, “Mature transformer management program,” by ABB’s Craig Stiegemeier; the second, “Actively cooled fuel controls,” by Schuyer McElrath of Jansen’s Aircraft Systems Control Inc, Tempe, Ariz (JASC, www.jasc-controls.com), a manufacturer of liquid-fuel check valves and three-way purge valves for GT fuel systems.

Liquid-fuel-system issues in dualfuel applications are on the minds of many GT users these days. Reason is that coking—or solidification of petroleum—within the liquid-fuel check valve and other components downstream to the burner tip often causes costly operating problems. Check valves are particularly vulnerable.

In the most severe cases, downtime attributed to coking has forced owner/operators to forego the operating flexibility promised by dualfuel systems and modify the unit for natural-gas firing only. To dig deeper, read “Improve GT operating flexibility, reliability with fuelsystem mods,” p OH-22, in the 2006 OUTAGE HANDBOOK supplement bound into this issue.

McElrath began by reviewing the problems in dual-fuel systems caused by coking, including these:

  • Adverse impact on fuel-transfer reliability.
  • Multiple start attempt s required when burning distillate.
  • Excessive exhaust-temperature spreads.
  • Loss of flame trips.
  • Liquid-fuel system pressurized by purge-air system.
  • Need to refurbish or replace check valves or three-way purge valves after operation on gas.
  • Sticking of fuel distributor valves, causing valves to open partially or not at all.

Next, he analyzed the coking problem. Dual-fuel units are designed to operate on either distillate or natural gas and to switch fuels under load. What happens when transferring to natural gas after burning diesel oil is that the temperature of the liquid fuel trapped within the check valves continues to increase—up to as much as 400F because of the close proximity of the valve to the hot turbine section.

Such high temperatures are conducive to coking. The buildup of deposits over time impedes normal valve operation and can lead to false starts and lean trips under load. Safety aside, perhaps the last thing any plant manager wants is unnecessary trips under load because the thermal transients produced quickly consume the operating life of expensive hotgas- path parts (the financial impact is quantified in “Optimizing O&M for combined-cycle plants,” 2006 OUTAGE HANDBOOK, p OH-33).

A typical coking scenario relative to transfer capability is as follows: With the onset of coking, check valves do not open at the same pressures, thereby creating exhaust temperature spreads. As coking becomes more significant, fuel flow is not distributed equally to the combustors and the temperature spread becomes more acute.

Finally, when the check valves become very seriously fouled, they will not close properly. This allows the fuel lines to drain, thereby adversely impacting the unit’s ability to start on distillate. It also creates severe temperature spreads when the turbine transfers from gas to liquid fuel on the fly, because some fuel lines have more distillate in them than others. (For more on exhaust temperature measurement, see “Have you lost capacity or are you over-firing your gas turbine?” p OH-49, 2006 OUTAGE HANDBOOK.)

McElrath, who has an extensive background in fuel-system troubleshooting and problem-solving—experience acquired while working for a major GT OEM—noted that oil shows no tendency to coke at temperatures below about 250F. So the challenge was to design what JASC calls an “actively cooled check valve” to keep oil temperature as far below 250F as possible.

Laboratory testing of a prototype valve two years ago at an ambient temperature of more than 500F proved that at water temperatures as high as 150F, as little as 0.25 gpm of cooling flow maintained valve internal temperature below 210F.

A field demonstration under demanding operating conditions was next. JASC’s actively cooled check valves were installed at Valero Energy Corp’s (formerly Premcor) Delaware City refinery, home to a 160- MW cogeneration facility equipped with two GE Energy 6FA GTs. The refiner knew that after only four days (96 hours) of operation on gas, their conventional liquid-fuel check valves experienced enough coking to adversely impact the ability to switch fuels and to cause large exhaust-temperature spreads (Figs 1, 2).

Late update. Just prior to press time, Valero Energy reported that from commissioning in 1998 until August 2004, only two of 50 fuel transfers had been completed successfully on the two 6FAs. With the installation of water-cooled liquidfuel check valves during August 2004 on one unit and during April 2005 on the other, fuel switching reliability improved dramatically.

The Delaware City GTs have now operated with water-cooled valves for more than 5500 hours without the performance degradation that typically accompanies coking and they have successfully transferred between gas and liquid fuel 33 times this year. All starts on liquid fuel have been accomplished without experiencing a failure to fire as a result of evacuation of fuel tubing by the purge-air system.

McElrath told the group that if the existing cooling water system for the turbine’s major systems—that is, lube oil and atomizing air—has a pressure of at least 40 psig and can supply up to 1 gpm per combustor, you can retrofit a typical GT with water-cooled check valves inexpensively within two days.

Upgrade offerings dominate GE Roundtable

Day Two of spring CTOTF meetings are devoted to frame machines manufactured by GE Energy (Siemens Power Generation Inc gets Day Two of the Fall Turbine Forums) and the roundtable always gets a big turnout. This year approximately 60 users attended the session, which was supported by a team of engineering, field-service, and customer-service personnel from the OEM.

About half the program focused on the status of TILs and engine upgrades and modifications to improve performance and reduce emissions. Concentration in the afternoon was on testing and inspection of generators and controls, followed by a highly participative open discussion among attendees covering a wide range of topics. Issues with the 7E/EA fleet attracted much discussion—perhaps because the focus of the morning’s upgrades and mods presentation was on those machines—but there also was considerable give-and-take regarding application of new parts and techniques to the older “legacy” turbines.

GE’s Mike Adams updated the group on the company’s customer service improvement initiative. TVA’s Bob Kirn, who chairs the GE Roundtable, noted that GE has modified its customer-service performance metrics to more closely reflect customer needs. This action is at least partially a result of feedback from the 2004 Spring Turbine Forum in Savannah, and from several subsequent presentations by CTOTF members at GE’s corporate offices.

While you can’t hope to cover in one day every topic on the minds of conference delegates, there was a perceived “hole” in the program regarding 7FA machines, according to some CTOTF members. The 7FA was the GT of choice for the majority of combined-cycle units installed in the last several years and many in attendance have responsibility for those machines. In fact, the plant managers who accepted Best Practices Awards alone represented more than a dozen 7FAs. For an update on 7FAs, refer to coverage of 7F Gas Turbine User’s Group annual meeting elsewhere in this issue.

When the conference organizers refer to GE Day, they mean just that. Formal frame presentations end around the 5 o’clock hour. In Annapolis, GE Multi-Vendor Services, the group that provides spare parts and services for non-GE engines, conducted a special two-hour evening session (with refreshments) to update the group on its offerings. GE has made significant progress in the last two years leveraging its core competencies to serve this aftermarket sector. CTOTF, of course, is the perfect GT user group for such an information exchange because its members represent just about every engine model made in the last 30 years that is capable of producing more than 20 MW—give or take a couple of megawatts.

GE upgrades and mods

Atlanta-based J R (Bob) Johnston is a walking encyclopedia on performance and reliability improvements for GE frames. The 25+-year GT veteran lectured and entertained questions on CTOTF’s GE Day for two hours and never missed a beat.

Johnston’s basic message was this: GE is continually updating/ upgrading/uprating its designs and these technology advancements generally are available for application on older machines in various model series.

By way of example, he concentrated on the 7EA. The 7Es, Johnston said, first shipped in 1977, but design improvements have been ongoing and the engine has a well defined market niche. You may hear most discussion of GE frames revolving around the 7F model series, but the OEM actually will ship more 7Es this year than 7Fs.

So what does all this mean to owner/operators? Simply put, there are many avenues to capacity and efficiency improvements, if the market you serve can support the investment. Depending on the vintage of your MS7001E, an uprate to a firing temperature of 2055F could increase output anywhere from about 5% to nearly 19% and reduce heat rate by up to 4.6% (Fig 3).

One of the first things you have to consider with regard to an uprate is the capability of the generator and electrical auxiliaries. GE is respected for sizing its generators conservatively so uprates generally are possible, but an engineering review is required. Analysis of the GT components and accessories also is necessary to determine their compatibility with the enhancement being considered. And don’t forget to review your duty cycle—annual service hours and starts. What upgrades make most sense on a cost/benefit basis for your business case?

The optimum time to uprate

or upgrade units generally is during a major overhaul when new parts or significant refurbishment of existing parts is already necessary. Most users get started by selecting one or two upgrades that offer fastest payback; then add other upgrade modules later, when appropriate.

The manager of one 7EA-based cogeneration plant in California recently told the editors of the COMBINED CYCLE Journal that he believed his facility could remain competitive, even in such a difficult electricity market, if equipment was continually upgraded to be at the leading edge of technology. This plant had already benefited from being the first to complete a 2055F upgrade. Previously it had converted to DLN (dry low-NOx) combustors, switched to state-of-the-art gas control valves, and completed several other enhancements.

Early this year the facility became the first to retrofit third-stage turbine buckets and nozzles, an aerodynamic enhancement described by Johnston as being particularly beneficial. The upgrade was made possible by internal collaboration between frame and aero designers, who promised a 1% increase in output and a 1% decrease in heat rate at ISO conditions. The plant confirmed that these expectations had been achieved.

Compressor upgrades. Johnston also talked about the benefits of compressor upgrades. Approximately 20% of the inlet air is lost to the thermal cycle because of losses associated with cooling hot-gas-path parts and with excessive clearances between rotating and stationary components. Thus there are two main avenues available to get better performance from the compressor:

  • Increase air flow.
  • Reduce air-flow losses.

In the late 1990s, GE invested heavily in reducing losses, and significant savings were achieved. For example, use of brush seals in place of labyrinth seals can boost output by up to 1%, lower heat rate by 0.5%.

More recently, the 7EA compressor has been redesigned—the first major redesign of this component ever. The greater air flow associated with this enhancement allows an increase in output of up to 11% and an efficiency increase of up to 2.5% compared to the early 1970s base design. Johnston said that the company’s plan was to introduce this upgrade to the 7EA marketplace next year.

Johnston said that the company’s track record in performance upgrades has been excellent. One metric he cites to support this claim: GE has done more than 500 firingtemperature uprates and met project targets more than 99% of the time.

IGV (inlet guide vane) performance improvements have also been very popular, with 700 to 800 units opting for this upgrade. Extending the interval between combustor inspections is another way to reduce O&M costs. The company’s Extendor™ system for the combustion system (wear-resistant coatings and materials, enhanced clearances, and mechanical design improvements) was implemented with this in mind.

How to get started.

Generally speaking, the further away you are, the better things look. To get closer to what performance improvement options might benefit your facility, access on the Web GER-3571H, “Performance and Reliability Improvements for Heavy-Duty Gas Turbines,” www. gepower.com/prod_serv/products/ tech_docs /en/ downloads/ger3571h. pdf. Although the publication is several years old, it is good for preliminary work. Next step would be to access more detailed and more current information through your regular GE channels.

FT8 Users Roundtable

Chairman Ray deBerge of Ameren Corp noted three items on the minds of users:

  • Commitment from OEMs regarding equipment support in the future.
  • Identifying more third-party resources for parts and services.
  • Improving plant O&M practices.

The FT8 Users Roundtable was well represented by a diverse group of owner/operators from across the country who provided a spirited giveand- take on issues related directly to O&M of the aeroderivative platform. In addition to questions submitted prior to the conference that members requested be addressed, many new questions/comments were fielded during the session. In sum, they offered users options both for addressing fleet-wide issues and for improving operations.

The closed session, which elicited feedback from participants on a wide variety of O&M subjects, continues to be a key ingredient in candid problem- solving and information-sharing among users. In addition to this workshop, Pratt & Whitney Power Systems hosted a companion special session on the FT8 that is reviewed below.

Canadian-based GasTOPS Ltd, Ottawa, Ont, was invited to address the session on effective use of operating data to enhance condition-based maintenance (CBM) in the FT8 fleet. Dave Muir, VP product development, said a prerequisite for CBM success is having operating data related to engine condition collected, archived, and properly structured. He then showed how to classify different types of condition data and use that information in a continuous improvement process to boost asset productivity, increase the agility and flexibility of operations, and expand profit margins.

Muir noted the similarities between a fleet of airborne engines and land-based machines for electric generation, and how advanced CBM techniques— such as computed life usage indices—now being applied by the airlines can be used beneficially by the GT-based power industry. The payoff for proper implementation of CBM allows owner/operators to quantify component life consumption and to identify problem areas before failures occur. Examples of cost reductions possible with CBM were illustrated.

Muir said that the software package, while extremely important, is only one component of the CBM process. For CBM to be successful, he continued, the program must be driven by continuous improvement, and strong commitment at all levels of an organization. (For more on the lifetime management of critical parts, read “Proactive management of GT parts life key to controlling maintenance cost” on p OH-73 of the 2006 OUTAGE HANDBOOK supplement contained in this issue.)

P&W FT8 Special Session

If there was a comeback award in GT services, in the OEM category it would certainly go to Pratt & Whitney Power Systems. A related company, Turbo Power & Marine Systems, was a force in GT-based generation for electric utilities through about the 1970s—back when independent power was only an idea. Then P&W drifted away from the utility market. Things like that happen in large corporations.

Recent success in new engine sales for electric generation, in particular the FT8, has given the service business a shot of adrenaline. Over the last couple of years the service group has become a regular at CTOTF meetings and this spring expanded its role from a half-day program to a full day—a format similar to the one GE Energy and Siemens Power Generation use.

The P&W delegation in Annapolis was headed by Steve Kopf, manager of customer service. Along with Kopf were Customer Service Representatives A J Boonstra, Allen Markey, and Jerry McCormick. Engineering expertise was provided by Maurice Gabbidon, a controls specialist, and Jim Kennedy, a lubrication specialist.

Kopf opened the meeting with a review of P&W’s customer scorecard, which measures “satisfaction.” He talked about customer concerns and P&W’s initiatives to help solve problems. One such initiative is providing customers detailed control logic—including site-specific logic—so plant staff has the tools to do more troubleshooting than was possible previously.

The larger FT8 fleet, Kopf continued, has justified adding equipment to the lease pool. Another benefit of increased sales: The training program for field reps has been improved and expanded, and a transactional scorecard introduced to provide immediate customer feedback. Yet another benefit: P&W has standardized some aspects of its service proposals to respond more quickly to RFPs (requests for proposal).

Spare parts are easier to obtain, and repair depots are more responsive and able to complete jobs faster than only a few years ago, Kopf said. Note that depots are located in Connecticut, Europe, and Japan. Outside the US, the facilities are operated by a licensee. Good news for owner/ operators: Warranty on repair work and parts are now one year; formerly it was 90 days.

In addition, better forecasting tools assure parts are ready when requested—including pre-owned/ refurbished parts if the customer prefers. Plus new facilities, repair technologies, and procedures are in place to maximize parts lifetimes. A new test cell for the GG8-1-3 recently was commissioned and dedicated to aftermarket work. Benefits to owner/ operators of all these improvements include lower O&M costs and higher unit availability.

Most of the day was spent reviewing publications, technical updates, and responses to questions submitted to the OEM prior to and during the meeting. Perhaps surprisingly, a significant part of the discussion focused on commercial, rather than technical, concerns. Most of the technical questions were on ancillary equipment. In wrapping up, Kopf acknowledged that more robust communication— via a user-accessible website—would benefit customers.

Legacy Roundtable does rotors

Texas GenCo’s Steve Hedge, who chairs the Legacy Roundtable, always has an interesting program. Perhaps that’s because most of the subjects discussed go well beyond the conventional “fleet issues” that dominate sessions dedicated to late-model engines. Any machine more than about 15 years old is fair game for his program and because long-term operation gives rise to special challenges, both plant managers responsible for legacy equipment and the nostalgic attend Hedge’s session.

In Annapolis, the subject of the session was rotors. PSEG Power’s Rich Rebori, chairman of the Aero Roundtable, presented on the overhaul of rotors on two 7EAs. He made suggestions on what to look for when inspecting rotors so that others might avoid some of the problems his units experienced. Rebori talked about water collecting in the cavity between adjacent disks (wheels, if you prefer) and causing unbalance. Other problems discussed were cracking of stationary blades, hookfit and compressor casing wear, turbine casing cracks, proper torquing of through bolts, etc. Listening to “plant people” like Rebori is why you come to user-group meetings. The ideas you extract save outage time and expense.

One of Rebori’s most important messages was to build trust and work closely with your repair vendor. You have to monitor closely all repair work, but it’s generally not possible to “live in the shop” for an extended period. Next best thing is to put technology to work for you. For example, in the workscope, make sure you have defined “hold points” for inspection.

If you can’t get to the shop, have the vendor take a series of digital photos and e-mail them to you. Then review inspection results by phone.

Charlie’n Dave. No Legacy Roundtable would be complete without hearing from Charlie Pond and Dave Lucier of Pond and Lucier LLC, Clifton Park, NY (www. pondlucier.com). The two 30+-year veterans have been servicing GTs and teaching service technicians how to repair engines since before the Mark I control system was introduced. Difficult to identify a problem these guys haven’t solved or analyzed in their careers.

Their presentation included discussion of the following:

  • Rotor design and construction.
  • Bore fan problems. If you never heard of them, don’t fret. Early MS7000s used to have a bore fan inside the rotor to help pump air to the buckets. The bore fan could become loose and cause vibration. The cure: Remove the fan; designers decided that they were unnecessary.
  • Rotor coatings.
  • Internal debris.
  • Broken through bolts.
  • Rotor internal alignment.
  • Foreign object damage.
  • Inlet fogging overspray.

Rotor design and construction. Rotors are built-up of individual wheels or disks. Each wheel undergoes thorough nondestructive examination (NDE) prior to machining and is balanced after machining. The wheels are then stacked (Fig 4). Centerline is maintained by a Rabbet fit on GE machines (Fig 5), Curvic coupling on Westinghouse engines (Fig 6), and Hirth coupling on Siemens GTs (Fig 7). Note that Siemens uses one through bolt, other OEMs multiple bolts to hold the rotor assembly together.

After assembly, the entire rotor is balanced without buckets (blades if you prefer, Fig 9), then the buckets are installed (Fig 10), the rotor is rebalanced (Fig 11), and then installed (Fig 12).

Compressor coatings are important, say Pond and Lucier, for preventing corrosion and to help keep the rotor clean. A properly coated compressor rotor is shown in Fig 13. What happens when coating systems are not maintained is illustrated by the rusty rotor in Fig 14 and by the heavy oxidation, corrosion, pitting, and deposits of rotor and stator blades in Fig 15.

The wheel faces are coated on recent models. On the MS5000s, faces were allowed to rust (Fig 16). Random vibration sometimes was experienced from the several pounds of oxidation products that would flake off and remain trapped between adjacent wheels. On some MS7000s, epoxy coatings would peel, ball-up, and cause unbalance.

Another cause of vibration—one characterized by a sudden increase in vibration level—is the failure of a through-bolt (Fig 17). Failure of the through-bolt in single-bolt rotor designs is conducive to rotor destruction. A mod to prevent through-bolt failures on older GE units is described in TIL-1257-3. This is an important reference for any manager responsible for the first-time destacking of an older rotor. An NDE technique for inspecting through-bolts in service was presented by Pond.

If you find blade damage when inspecting your compressors, Figs 18-20 may help identify the source. Ice caused the damage in Fig 18, debris (foreign object damage) in Fig 19, and erosion from over-spray during inlet fogging in Fig 20. ccj