Fuel savings, durability demands are behind enhanced powerplant repairs
Technology being developed today for engine repairs or new production has a dual focus: lowering specific fuel consumption and increasing durability. OEMs spend considerable sums toward those ends.
“We are investing about $40 million, yearly in new repair development for engines that are currently in service, and we see that same level of investment continuing for the foreseeable future,” says Bill Moeller, director of aftermarket sales for Pratt & Whitney. “Over the past decade, we have spent in excess of $300 million on repair technology that will reduce fuel burn and extend material life.”
William J. Alibrandi, aero gas turbine analyst for Forecast International, a defense and aerospace consulting and production forecasting firm in Newtown, Conn., explains, “For at least the past six years, fuel has become the largest single cost of operating an airline, so anything that can be done to lower specific fuel consumption definitely impacts the bottom line. Any significant change in fuel cost savings is always engine-driven.”
Alibrandi cites the in-development. The OEM's approach is to increase the engine's thermal efficiency by increasing the compression ratios, resulting in higher exhaust gas temperatures. “That requires new materials in the form of ceramic matrix composites, which are installed in the hot section in the last stage of the high-pressure compressor. They provide increased heat resistance and save fuel,” he says.
“The engine OEMs are focusing on new technologies that increase engine temperatures to improve thermal efficiency, reduce overall engine weight, reduce fuel burn, maximize time on wing and improve maintenance costs,” says Richard Brown, London-based principal of international consulting firm ICF SH&E. “To achieve these objectives, the OEMs and their suppliers are investing in advanced materials and techniques including additive manufacturing, organic matrix composites, ceramic matrix composites, powdered metal and titanium-aluminide.”
At the same time, many of the high-tech repairs have focused on parts where the greatest savings can be made. “[They] tend to be on high-cost, high-value new parts such as HPT/LPT airfoils, LLPs, combustors and outer air seals,” Brown says. “Consequently, there are now very complex repairs available for engine blades and vanes and LLPs.”
In fact, for the Pratt & Whitney, a new Stage 3 vane bolt hole repair for the 94/100-in. models was introduced this year. According to the OEM, it restores the holes to OEM-acceptable limits, increases service life and extends time on-wing. The repair uses a proprietary process, which the company declined to discuss.
In that regard, Brown adds that the MROs have targeted engine models with high-volume shop visits—and parts—that offer the greatest savings potential. Those engines include the-80, PW4000, and International Aero Engines families. The largest independent MROs have invested heavily in high-tech designated engineering representative (DER) repairs that involve specialized coatings and state-of-the-art brazing and welding.
At Munich-headquartered MTU, many of the new technology insertions are based on new welding, coating and brazing methods, according to Bernd Kriegl, MTU Aero Engine's technical program manager for civil MRO. During 2012, the company spent €160 million (now $216 million) on research and development, of which 10% was for engine component repair processes. While no estimates have been released, Kreigl says the company's R&D investment, and the percentage devoted to engine component repair, should be about the same for 2013. A large number of the company's high-tech repairs are offered under the MTU Plus trademark.
“Some of the key developments have focused on the blades and vanes in the low- and high-pressure compressor sections, including erosion-protection coatings for the airfoils in those parts of the engine,” Kriegl says. “The objective is to maintain a high performance level for the compressors, and increase the life of the blades under harsh, abrasive operating conditions such as desert environments of the Middle East.”
The coatings, Kriegl explains, have led to fuel savings and lower emissions, coupled with greater on-wing time, since entering field trials in 2010. “Their performance throughout the field trials was very favorable, and we are now making them available for customer engine repairs,” he says.
Two years ago, notes Kriegl, MTU also introduced a new tip protection coating for the high-pressure turbine airfoils, which also increases fuel savings. “The tip protects the airfoils from hot-gas corrosion and degradation by enabling the blade to maintain the proper amount of clearance. This is a very unique process using induction-brazing, which has resulted in improved fuel burn, greater component durability, lower emissions and less frequent repairs.”
It also has cut scrap rates, Kriegl adds. “Without the coatings, the scrap rate of the airfoils was about 20%, but with the coatings, it's zero. When the blades or airfoils are overhauled, it's simply a matter of recoating them. They do not need to be scrapped.”
Going forward, he says, research and development will concentrate heavily on engine repair cost reduction by improving and increasing automation used in the processes. “R&D will also be done to reduce the impact of doing repairs on component base material,” Kriegl notes. “And with today's component designs, you also have to implement more advanced welding or brazing processes. That's presenting new challenges.”
For example, he points out that more new-generation engines incorporate integrated blades and discs—often referred to as “blisks,” or an integrated blade and rotor (IBR). The blades, Kriegl explains, do not detach from the disk. “When dealing with blisks or IBRs, there is a much more complex process of repair. MTU is developing proprietary processes and welding technologies to do this kind of work.”
The advanced repairs that Kriegl describes have been designed in-house, and are applicable to turbine engines now in service. MTU Aero Engines—under OEM licenses—specializes in a broad range of Pratt & Whitney and GE products, as well as the V2500, which is estimated to account for one-third of the 1,000 projected engine shop visits to the global MTU network of four shops—Berlin and Hanover in Germany, Vancouver, and Zhuhai, China—for 2013. Some additional growth is expected for 2014, notes Kriegl, although no official estimates are available now.
“Advanced blades, with higher corrosion resistance, can be applied to older engines and can replace or substitute new brazing technologies for older brazing processes,” he says. “In fact, we have developed a special high- temperature brazing process for hot-section components.”
To prepare for the firstshop visits, is expending considerable effort on repairs. The GEnx-2B engine went into service in 2011 on the -8 and the -1B, entered service on the last year. Those engines, he says, will start making their initial shop visits in 2015, with turbine blade recoating and repair, as well as static and rotating seals servicing. In tandem, GE, which spends $45-50 million annually on developing unique repairs—more than 1,300 this year (compared with 600 four years ago)—is focusing heavily on compressor blisk repairs, says Bill Dwyer, chief marketing officer for GE Aviation's service business.
“There is a lot of incentive to develop advanced repairs on compressor blisks, which are located on the forward end of the high-pressure compressor,” Dwyer notes. “The technology to repair an entire blisk unit is based on a more advanced welding process introduced by GE over the past year specifically for that purpose. This, in fact, was developed in anticipation of the first shop visits of the GEnx engine family.” Blisks are located in the GEnx compressor Stages 1, 2 and 5.
The incorporation of blisks has proliferated in turbine engine design due to the aerodynamic and weight-saving benefits they provide, says Dwyer.
But GE also is applying its new, improved welding technologies to older engines, with the introduction during the past 18 months of a 3C airfoil restoration process specifically for the CFM56-5B and -7B.
“It was a more sophisticated repair that was designed for application to the complex airfoil shape of the high- pressure compressor blades, and the complex design geometry of the high-pressure compressors in those engines. The benefit is that more parts can be repaired and fewer need to be scrapped,” Dwyer explains.
He further reports that GE has introduced advanced wear-resistant coatings for the high-pressure compressor blades on the CFM56-5B and -7B, providing a longer on-wing time, and scrap-rate reduction. “The technology uses a proprietary material composition, which has very robust wear resistance,” Dwyer says. “While it is available to all operators of these engines, it is being specifically marketed to airlines operating in very harsh environments, such as the Middle East and China.” The company is studying the application of the coatings to other engines.
The use of powdered-metal applications to hot-section rotating seals and other rotating parts is another recently introduced repair technology at GE. As Dwyer explains, it gives the component the capability to operate at very high temperatures. “Powdered-metal technology has excellent resistance to 'creep', the term which describes the deformation, or loss of shape—and ultimately failure—of the metal part at high temperatures,” he says.
Another key area being targeted to reduce maintenance costs involves what Dwyer calls “lean burn combustion.”
“Lean burn combustion results when temperature is uniform throughout the hot section—specifically in the combustor—rather than having hot spots, which will lead to peak stresses, cracks and wear,” he explains. “Much of the turbine maintenance is driven by the peak temperature versus the average temperature to which the part is subjected. If your combustor burns uniformly, it will last longer on wing. The GEnx uses this, as will the Leap, and it's proving to have a very big maintenance cost benefit.”
GE Aviation had 4,000 shop visits globally in 2013, and that is expected to increase in the low single-digit percentage range in the coming year, says Dwyer. The worldwide GE maintenance network totals 92 facilities, of which five are GE-owned and 19 are joint ventures. The GE-owned facilities handled about one-third of those shop visits. Of the 92, half work on the CFM family.
“At GE, our protocol is to develop repair processes on one engine, and apply them to others, where possible. The technology that is developed to repair a new engine will find its way to current-generation engines for upgrades during the overhaul process,” he says.
ICF SH&E's Brown agrees that as the OEMs develop new engines and manufacturing techniques, this protocol may benefit the repair programs they can develop for existing engines, and save on maintenance costs.
“Ultimately, the technology will reduce the number of parts in the engine, so there are potentially less parts to repair. But, it does raise some concern for non-OEM, independent MROs,” Brown adds. “As the latest technology is incorporated will these parts be repairable, or will they be so technologically complex and costly that only the OEM could repair them, or new parts have to be purchased from the OEM?”