Turbine Component Repair and Recoat

Turbine component repair and recoat New advancements make repair worth considering By Greg Napert June 1999 With increasing costs of engine overhauls over the years, many engine operators are continuously evaluating their options with...

Faster turn times
Obviously, the more rapidly a set of cold section airfoils can be repaired, the sooner the engine can be installed back on-wing, lowering the spares inventory requirements. Quite simply, days saved mean dollars saved.

Five years ago, the industry standard turnaround time for refurbishing a set of such airfoils was 3 to 5 weeks. Today, ATI has reduced that time to 7 to 10 days under normal conditions. As a result, the need for exchange services or maintenance of a large pool of inventory has diminished accordingly.

The main reasons for this improvement have been our commitment to reduce turntime and the resulting developments in automation. Today's high yields in compressor airfoil restoration simply would not be achievable with manual processing. Further, it is the orchestrating of these technologies that has led to today's shorter turn times. The craft and skills are there. However, they are embedded in artificial intelligence software rather than the individual craftsman controlling the various processes.

One lesson learned in developing the superfinishing processes for compressor blades is the importance of a smooth surface finish on airfoils throughout the engine. The relationship between airfoil shape, surface finish and performance is direct. Therefore, a good case could be made for using those three conditions rather than tip wear or damage as the trigger point for airfoil refurbishment. Once the airfoil surface finish is degraded, the leading edge blunt and the chord reduced by particulate matter, the compressor efficiency drops, and overall engine performance deteriorates. While the blade is still structurally sound and functional, it will waste valuable fuel dollars and reduce engine time on-wing. Often the saving in fuel will more than justify the cost of initiating airfoil refurbishment earlier.

The other principal lesson is that today you can and should expect airfoil refurbishment to produce blades that are not just structurally and geometrically sound, but perform literally like new parts. When this level of performance is achieved, you get airfoils that extend the cost-effective life of the engine plus fuel savings associated with having near-design shaped airfoils installed.

Fan blade refurbishment
All of the compressor blade maintenance strategies discussed above apply in much increased measure to the fan blades. In a high-bypass turbofan engine, the fan alone is responsible for consuming nearly one-half of the total engine fuel burn. Therefore, the impact of advanced fan maintenance scenarios can be significantly greater than for any other cold section component. Presently, ATI is in the process of implementing the RD305 process and a superpolish (down to 15AA micro-inches) surface finish for fan blades. This is in addition to the industry standard fan repairs that consist of patching, welding, blending and creep forming. To prove that the advanced refurbishment techniques of RD305 and superpolish for fan blades can have a significant impact on overall engine performance, a back-to-back test was conducted on the fan blades of a revenue service engine from a major airline.

The fan and engine were first overhauled per normal airline practice and cell-tested to establish that the engine performance met flight acceptance standards. This test, which the engine passed, also established a performance baseline for the overhauled engine. At this point, the RD305 process and superpolish had not been applied. After the cell test, the fan was removed from the engine and sent to ATI where the RD305 and superpolish process was done. Upon completion of this advance rework, the fan was mated to the waiting baseline engine, and the engine was retested in the same cell. Results indicated a drop of 0.7 percent in TSFC relative to the baseline engine after the enhanced fan overhaul. In addition, a comparison of the engine exhaust gas temperature over the range of thrust levels indicated a 3.5 C drop in EGT on average. The airline estimated that this performance improvement would translate to a fuel burn savings of $15,000 per year per engine and additional time on-wing of 1000 hours.

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