Coal, Gas

Service to the (GT) Fleet

Issue 8 and Volume 109.

While the building boom in gas turbine-based plants has been over for some time, repairing and refurbishing many machines commissioned during the boom remains an ongoing, essential industry. Repair shops stay active for both the OEMs and the many independent firms offering GT services.

For the past two decades, the gas turbine (GT) has been the technology-of-choice for new power-generation projects in North America. During the last six years alone, new projects increased the total installed GT base in the United States by 290 percent, to 318 GW, according to data gathered by Energy Ventures Analysis Inc (Arlington, Va.) for the North American Electric Reliability Council. As has been widely reported in the trade journals, these capacity additions kept business bustling for the four dominant original equipment manufacturers (OEMs)-GE Energy, Siemens Westinghouse Power Corp., Alstom Power and Mitsubishi Power Systems Inc.

Today, business is still bustling for the turbine OEMs-but in their service shops rather than their new-order office. The building spree that raised the GT’s total percentage of U.S. generating capacity to about 30 percent, though now stalled out, fielded a large fleet of sophisticated turbomachinery that requires ongoing, essential maintenance. The largest and most advanced GTs, in particular, demand frequent inspection and refurbishment, because of their high turbine-inlet temperatures, massive rotors, complex cooling schemes and touchy dry low-NOx (DLN) combustors. To meet the growing service demand, the OEMs have opened new field offices and expanded their repair-shop capacities.


Figure 1. Typical purchase order for repair of gas-turbine hot-section components can easily exceed $1 million for a mature model frame unit, and may top $4 million for the more advanced F- or G-class models. Photo courtesy of GE Energy.
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Siemens Westinghouse Power Corp.’s service expansion can be traced back to at least 2003, when it established Turbo Services Network. The move combined the strengths of one of the leading independent service companies, TurboCare Gas Turbine Services, with several joint ventures previously established between the OEM and independent service company Chromalloy Gas Turbine Corp. Last December, Siemens Westinghouse further expanded its repair business by acquiring Jet Turbine Services Inc., a leading independent service provider to users of aeroderivative gas turbines. Speaking to the importance of such service expansions, Randy Zwirn, Siemens Westinghouse Power Corp.’s president and CEO, stated at the time of the acquisition, “With a goal to have more than 40 percent of our global revenues coming from service activities, continued investment in this sector is one of our key business strategies.” Siemens Westinghouse Power Corp. already had not one but two major heavy-duty “Frame” GT repair centers in North America-Hamilton, Canada, and Houston, Texas.

The expansion of GT services for GE Energy also can be traced back several years, to when it acquired independent service company Preco Turbine Services Inc. In that deal, GE snared more than 200,000 square feet of shop space and an impressive array of turbine repair equipment, nicely located near Houston’s Intercontinental Airport.

Blurring of OEM versus non-OEM

It will come as no surprise that the turbine OEMs expanded their repair capabilities out of more than an altruistic desire to service the fleet (Figure 1). The typical purchase order for repair of GT hot-section components-combustion hardware, as well as rotating and stationary turbine parts-can easily exceed $1 million for a mature frame GT model , and may top $4 million for the more advanced F- or G-class models.

Those hefty purchase orders no doubt caught the eye of OEM executives. They also spurred many independent, non-OEM companies to leap into the GT service business. Today, more than three dozen non-OEM companies offer field-service expertise, repair facilities or newly manufactured aftermarket parts for heavy-duty frame GTs in the North American market. That’s according to a recent study conducted by IEM Energy Consultants Inc. (Alexandria, Minn.) for the Combustion Turbine and Combined Cycle Users Organization (CTC2).

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Inclusion in the CTC2 listing is a little fuzzier than it used to be, because the distinction between OEM and non-OEM is blurring. By acquiring TurboCare, Siemens Westinghouse essentially became a non-OEM supplier of GE parts. Meanwhile, GE Energy’s acquisition of Preco enabled it to enter the aftermarket business for Siemens Westinghouse machines. GE also owns Turbine Blading Ltd., an independent parts supplier based in the United Kingdom. In addition, Pratt & Whitney, a leading OEM of aeroderivative GTs, has become one of the leading suppliers of aftermarket parts for frame machines through its business unit Pratt & Whitney Power Systems (Marietta, Ga.).

Aftermarket parts

The aftermarket parts and refurbishment services offered by the independent shops are rarely endorsed by the OEM, but the independents are popular with many end users because of their competitive prices and customer responsiveness. To date, most of the aftermarket parts available from non-OEMs are for mature GT models-the E-class or older. “But that’s more because of commercial limitations than a lack of expertise or a shop constraint,” reports Jeff Fassett, president of IEM Energy Consultants and the principal author of the CTC2 study. Owners of most F-class turbines, Fassett explains, entered into some type of service agreement with the OEM-a long-term service agreement (LTSA), a contractual service agreement (CSA), or a third-party O&M contract-when the units were purchased. In these agreements, the owner essentially pre-pays for repair parts, services and engineering oversight by the OEM, thus excluding independent shops from even bidding on a repair job.

Significantly, these agreements also can specify that any component found to be unrepairable becomes the property of the OEM, thus further suppressing the independents by keeping F-class parts out of the hands of folks who might want to reverse engineer them. “The non-OEMs face substantial development costs if they want to manufacture F-class parts,” Fassett says.

But more and more non-OEMs are paying those development costs, and aftermarket parts for the F-class are beginning to show up in the power plants. “It’s only a matter of time before the repair business for F-class models reaches the same maturity level as for older GTs,” asserts John Yelincic, principal engineer for Progress Energy’s CT Repair Engineering Department. Yelincic says that as the LTSAs and CSAs near their expiration, many utilities, including his, are evaluating whether to extend the agreements or switch to what is being called the “self-perform” strategy. “The long-term, life-cycle costs of F-class units are dramatically higher than the OEMs or the owners originally expected,” Yelincic says. “So every owner is going to have to come up with a long-term strategy for capital component service interval and replacement.”

Manufacturing versus repair

While the supply of aftermarket parts may be somewhat limited, the supply of non-OEM shops capable of refurbishing existing parts or selling new parts is not-even for F-class components. In the CTC2 study, IEM surveyed a total of 39 non-OEM companies that are active in the North American market. The 21 companies that responded and are still actively selling parts or offering repair services are displayed in the accompanying table.

Indicative of the dynamic nature of the GT repairs business, two of the responding companies-Sermatech Power Solutions and Triumph Turbine Services-have shuttered their shops since the study was completed just last fall. Two others-Power Spares and TBRS-have merged since then under the common owner Allied Power Group. The industry’s dynamic nature was apparent years before that. When GE acquired Preco, the deal essentially removed one company from the non-OEM ranks. But at least two new GT repair companies rose from former Preco personnel: TriStar Turbine Technologies and Leading Edge, both of Houston, Texas.

The CTC2 study looked specifically at 19 of the most popular heavy-duty Frame GT models, and identified each non-OEM’s capabilities in 28 manufacturing categories and 45 repair categories. The categories included such detailed breakouts as:

• Combustor liners

• Combustor transition pieces

• Blades, vanes and shrouds in the various rows of turbine hot section

• Compressor inlet-guide vanes

• Compressor unstacking

• Numerous field and shop services, such as craft labor, combustor tuning and in-situ generator balancing

The CTC2 study results were compiled in a searchable MS Access database, which was provided to the organization’s participating members. Other industry professionals can purchase the study results. For more information on the study, visit www.ctc2.org, or contact the organization’s managing director, Doug Vandergriff at [email protected] or 704-553-3162.

Replacement combustors wanted

In the 1980s, the turbine OEMs began to integrate DLN combustion technology into their product lines to eliminate the need for steam or water injection-the traditional methods of NOx control. As emissions limits ratcheted downward, the DLN combustors grew more sophisticated in design to keep pace with the regulations. For example, Mitsubishi’s first DLN combustor was introduced in 1984 on a 50-Hz M701D gas turbine. By 1992, 25-ppm NOx levels were being commercially achieved in the OEM’s F-class turbine operating at full load, with a turbine inlet temperature of 2,460 F. Today the Mitsubishi DLN combustors achieve 15 ppm NOx.


Figure 2. Dry low-NOx combustors require frequent repair or replacement. Scoring is evident on this ‘hula skirt’ section of a combustor liner. One leaf (under the ‘7’ mark) has completely separated and traveled downstream. Damage is typical of that caused by vibration from combustor dynamics.
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GE has several generations of the dry combustor in service, including the DLN-1, -2, -2.6, and -2.6+. The DLN-1 is used exclusively in the lower firing-temperature units such as the B- and EA-class. The DLN-2.0 was used in the 2350F versions of the F- and FA- units to achieve 25 ppm NOx. The DLN-2.6 is the current offering for the F-class gas turbines, firing at 2,400 F to 2,420 F. It physically differs from the DLN-2.0 due to the addition of a sixth fuel nozzle in the combustion can (hence the “2.6” moniker). Impressively, the DLN-2.6 can achieve single-digit NOx emissions. The DLN-2.6+ is specifically for the MS7241 or the 7FA+E engine. GE also offers variations of these systems, some of which are available only to CSA customers.

Unfortunately for GT users, this impressive downward trend in emissions created an upward trend in the combustor replacement market, particularly with liners and transition pieces. These components are subjected to the highest temperatures in the machine, and therefore require more frequent inspection and replacement. The primary hazards for a DLN combustor in baseload operation are oxidation and corrosion. In cycling operation, DLN combustors are particularly susceptible to thermal mechanical fatigue (TMF) cracking, caused by the thermal expansion differences between thin and thick sections or between sections made of different materials. Whether in baseload or cycling duty, combustor life can be shortened further by fuel contaminants and combustion dynamic pressure oscillations, or “humming” (Figure 2).

When it’s time for a combustor replacement, the GT OEMs offer new and vastly improved models, compared to the DLNs supplied a decade ago. The newer models employ precipitation-strengthened materials, improved cooling schemes, and better thermal barrier coatings that effectively battle the onslaught of oxidation and corrosion (see Power Engineering, May 2005, p 42). These developments are helping return combustor life expectancy back to the 16,000 hours or 24,000 hours that once were typical with diffusion-style combustors. For example, GE’s EI24K model is guaranteed for 24,000 equivalent hours, at least in baseload service. The fact that some estimates peg the percentage of F-class machines operating in peaking service as high as 60 percent suggests that TMF cracking of DLN combustors may continue to be a problem for many GT owners.

Aftermarket combustor

Only one of the non-OEM companies offers a redesigned and newly manufactured DLN combustor for the most popular heavy-duty GTs. Power Systems Manufacturing LLC (PSM, Boca Raton, Fla.) entered the GT repair business in 1999, when it performed a B to E uprate on six GE 7B turbines. At the customer’s request, the project included redesigning the OEM’s original combustion hardware and manufacturing the new combustor hardware for all six units. The customer was so pleased with the result that it awarded PSM a contract to manufacture three more sets, and the repair company suddenly found itself in the manufacturing business.

When plant owner Calpine Corp., which boasts a fleet of more than 100 F-class GTs, acquired PSM it was able to greatly enhance its repair capabilities and manufacturing portfolio. Today, PSM offers a DLN combustion system called the Low Emissions Combustor (LEC) that achieves less than 5 ppm NOx and single-digit CO emissions for GE E-class machines. Sometime this year, the company plans to offer this combustion system for Siemens Westinghouse Power Corp.’s 501D5 machine, and eventually to offer it for GE’s F-class.

Key to the LEC design, according to PSM, is its more efficient combustion liner cooling, which uses effusion principles. Standard louver-style cooling has been found to create cold spots, a principle reason for higher CO emissions. These cold spots produce a cold layer of air causing less CO to be “burned off.” The trick is to put less air into the engine to make it more fuel-efficient by optimizing the air/fuel mixture. In essence, the more cooling air required, the less airflow directed into the reaction zone, resulting in higher flame temperatures and NOx levels, and a poorer degree of air/fuel mixing and distribution.

The LEC’s effusion cooling is composed of thousands of tiny holes around the combustion liner, which replace the OEM’s louvers. Using this approach, convective heat transfer effectively cools the liner with far less air without creating local cold spots, the manufacturer says. This approach also enhances durability compared to competing dry low-NOx systems, according to PSM, by reducing the thermal gradients within combustion system parts.

The LEC development has been a success for PSM, and it plans to further that good fortune by reducing the combustor’s guaranteed emissions level from 5 ppm to 2 ppm in the near future. But the development of an aftermarket combustor for another non-OEM was not so fortunate. A few years ago, Sermatech was one of the larger independent repair companies. Spurred by its success in the airfoil coating business, and recognizing the demand for replacement DLN combustors, the company attempted to develop its own aftermarket combustor. But the Sermatech combustor, installed on a GE 6B at a northeast refinery, broke apart shortly after startup. The failure stopped any further development of the aftermarket combustor, and the resulting litigation may have contributed to Sermatech’s financial trouble and eventual exit from the GT repair business.

Aftermarket hot sections

Looking downstream from the combustor at the turbine hot section, there are numerous non-OEMs offering aftermarket components, some even for the F-class models of both GE and Siemens Westinghouse. Wood Group APM (Advanced Parts Manufacture), for one, has dedicated component design centers in both the United States and Europe, from which it manufactures turbine blades and vanes, as well as other hot-section components. The company has been manufacturing turbine components for Middle Eastern utilities for 14 years, and in recent years has penetrated the North American market from its state-of-the-art facility in East Windsor, Conn. Wood Group APM claims to have delivered the world’s largest order for aftermarket turbine components for GE Frame 7 machines, in a 2003 deal with a Saudi Arabian operator that included the supply of more than 1,500 individual castings.

Another leading non-OEM in the manufacture of hot-section components is Pratt & Whitney Power Systems. Leveraging its parent company’s years of experience with aircraft engines-which are subjected to even tougher thermal conditions than F- or G-class Frame machines-Pratt & Whitney Power Systems has carved out a significant chunk of market share in hot-section components for GE models. For instance, Progress Energy purchased multiple new first-stage bucket sets for its vast fleet of GE 7EA units from this non-OEM. According to Yelincic, that purchasing decision was based not only on the competitive price, but also on the technical improvements and the service life guarantee that Pratt & Whitney Power Systems offers. Yelincic also is impressed with the non-OEM’s work with Siemens Westinghouse parts, specifically its cooling design for first-stage vanes on the D5A model.

Repair and refurbish

Refurbishment of GT parts, in both the combustor and the hot section, is vital to a GT’s economical service life, because even the most advanced repair costs a fraction of component replacement-whether at the OEM’s or the aftermarket price. Sulzer Hickham Industries Inc. (LaPorte, Texas) may be the top-ranking independent shop when it comes to combustion-section repair of F-class GTs. Progress Energy’s Yelincic states, “Hickham has shown that they have replicated the OEM’s coating technology for the GE F machine’s combustion section.” There’s no doubt that Sulzer-Hickham is one of the largest and most experienced non-OEMs. The company traces its roots to the Dutch entity Elbar, an independent GT repair company that was launched way back in 1973. In 1997, Elbar was acquired by Hickham Industry’s corporate parent, Sulzer Industries (Zurich, Switzerland), forming the Sulzer Hickham company that today dominates the independent repair business.


Figure 3. This first-stage nozzle from a GE 7FA+e hot section must be replaced, if it cannot be repaired. Sophisticated heat treatment and welding techniques, among other tasks, would be needed to restore the alloy structure and material properties.
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For hot-section components, refurbishment requires: (1) heat treatment and welding techniques to restore alloy structure and material properties; (2) a stripping process to remove old coatings and clean internal cooling passages; and (3) the selection and application of new protective coatings. Satisfactory completion of these three steps requires extensive technical expertise, tooling and experience (Figure 3). This is where many GT repair shops falter.

According to Lloyd Cooke of Luburdi Turbine Services (Dundas, Ontario), there are more than 50 different coatings commercially available for hot-section blades and vanes. Each has its strengths and weaknesses, making the coating-selection process a difficult one. Cooke points out that the user is actually in a better position than the GT designer to select the optimum hot-section coating, with the assistance of a skilled metallurgist. That’s because the optimum coating can be tailored to each component’s specific service environment and design limitations, which the user can determine by performing metallurgical analysis on his components after they’ve operated through their first service interval.

Hot-section coatings can be classified as diffusion aluminide, overlay MCrAlY, or thermal barrier coating (TBC). Diffusion aluminides are lightweight, thin coatings-typically 0.002 in. in total thickness-which makes them ideal for aeroderivative GTs. They provide excellent oxidation resistance, but their moderate corrosion resistance limits their service in Frame GT hot sections to second- or third-stage components where temperatures are lower.

The MCrAlY overlay coatings (the “M” stands for metal, which typically is nickel, cobalt or a combination of the two) are applied four or five times thicker than diffusion aluminides. Because they are highly resistant to both oxidation and corrosion, MCrAlY overlays are the most widely used coatings on hot-section airfoils of heavy-duty Frame GTs.

On top of the MCrAlY coating, a TBC may be applied to those hot-section components that are internally cooled. The TBC’s purpose is to assist the internal cooling in reducing the metal temperature. This is necessary in many of today’s high-firing GTs because the combustion-gas temperatures actually exceed the melting temperature of the components.

Reparable vs. irreparable

When it comes to the refurbishment of existing GT parts, end users have even greater choices, compared to the choices in the aftermarket parts business. Pratt & Whitney Power Systems is one of the leaders in this sector, thanks to its experience with aircraft engines where exotic coatings have been applied for decades. The company also benefits from the state-of-the-art repair facility it recently opened in San Antonio, Texas.

Liburdi Turbine Services is another clear leader among the independent shops doing hot-section refurbishment. The company has adapted its advanced aero-component repair techniques for application to the F-class components of both GE and Siemens Westinghouse. In recent years, Liburdi has repaired 7FA combustors, as well as first-, second-, and third-stage airfoils for 7FA and V84.3A machines, using nearly 100 percent repaired parts for each set. The company attributes such success to its rejuvenation heat treatments, reprofiling of shroud design, and proprietary technologies for automated welding of single-crystal and directionally solidified alloys. In June 2005, Liburdi was awarded a 10-year contract with Alberta-based TransAlta for hot-section repairs to the power producer’s entire fleet of 7EA machines.

Don’t forget the cold end

Traditionally, coatings have been applied only to hot-section components to protect against oxidation, corrosion and erosion. But coatings increasingly are being applied to compressor blades and vanes-the “cold section”- in the GT repair industry. Coatings are the best way to restore a surface finish that’s been damaged by fouling or erosion (Figure 4). Their low coefficient of friction also keeps the compressor cleaner and makes routine water-washes more effective. Prior to applying a new coating, airfoil components from both the compressor and hot-gas-path sections are grit-blasted, typically using aluminum oxide. An unwanted effect is a rough surface finish, which can substantially degrade engine performance if the surface is not properly restored before the airfoils are put back in service. Many users fail to account for this roughness in their overhaul specifications, specialists say, and as a result miss out on potential performance gains.


Figure 4. Advanced coatings, once limited to turbine hot sections to protect against oxidation and corrosion, can be applied to cold-section compressor airfoils to reduce friction losses. Because the compressor typically consumes more than one-third of a gas turbine’s power, the coating process can substantially improve gas turbine efficiency. Photo courtesy of Sermatech Power Solutions.
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One GT manufacturer estimates that an increase of 200 micro-inches over nominal surface roughness-defined as approximately 60 micro-inches-in just one stage of compressor vanes can cause a 1.4 percent loss in engine output and a 0.9 percent loss in efficiency.

The direct cost of coating compressor airfoils adds little to a GT overhaul price, but many users balk at the extra downtime that compressor coating might add to the outage. In response, one coating specialist developed a faster application method, applying the coatings to fully stacked compressor rotors, rather than to individual airfoil components. p