For other hot section components such as vanes and vane rings, the determination of whether or not to overhaul should really be left up to the overhaul and recoating facilities. There have been many repairs developed over the years that allow the overhauler to replace large sections of the vane and to weld and braze areas that, to the operator, would not appear to be salvageable. It's a good idea to call first to make sure that the overhaul facility does indeed repair that particular component.
"There are certain components that we don't overhaul anymore," says Palatine. "One of those components is the Cobalt vane ring used in some of the PT6 engine models. This is simply a business decision. We stick with what we're good at."
Related Article: Compressor Airfoilsand Fan BladesCompressor Airfoils and Fan Blades
New repairs and reduced turn-times
A major source of compressor efficiency loss in gas turbine operation is due to imperfections in airfoil shape. These imperfections exist for new airfoils and, as an engine deteriorates in service, the airfoil shapes become further distorted due to erosion. These distortions, along with degraded airfoil surface finish and increased tip clearance, penalize aerodynamic performance. This penalty is evidenced by decreased engine capability, increased fuel consumption and higher exhaust gas temperature.
This is why it's especially timely to take an overview of recent progress in the art and science of airfoil refurbishment for the cold sections of jet engines. Over the past five years progress, though evolutionary, has been substantial and mostly in the compressor section. The net benefit to commercial aircraft operators is fourfold:
• Higher repair yields of compressor airfoils, leading to lower refurbishment costs.
• Performance of restored airfoils that closely matches that of new airfoils
• Lower engine life-cycle cost and longer service life
• Much shorter turn times for airfoil refurbishment.
Today at our four global airfoil repair centers, the average incoming rejection rate for a set of cold-section airfoils is literally half of what it was in 1993. Usually, depending on engine type, 80 to 90 percent of the airfoils can be refurbished, up from 60 to 70 percent. In addition, with the proper treatment, refurbished parts give away nothing to new airfoils in operating performance. Turnaround times have been halved as well, especially at our new Singapore facility that opened in February 1998. The results have not only reduced airfoil replacement costs for operators of commercial aircraft, but also safely reduced their parts inventory levels.
The benefits related to airfoil refurbishment stem basically from five key advances. These include repair of newer-generation, 3-D design airfoils with end-bends; improved surface finish on refurbished airfoils; total automation of the welding process; chord, as well as tip restoration; and quicker turnaround. Let's look at each development separately.
Restoring "end-bend" airfoils
Over the past two years, ATI has pioneered the restoration of end-bend airfoils to like-new condition on a high volume basis. Examples include airfoils from the GE CF6-80C2 (Phase 2 Aero Design), and Rolls Royce RB211-535E4 engines. So-called 3-D Aero or end-bend airfoils are found in a number of new engines, and certainly represent the wave of the future in compressor airfoil design. Because of the complexity of their geometry, however, they were regarded for years as extremely difficult to refurbish. For one thing, their trailing and leading edges are not coplanar. It's the airfoil "twisted-closed" geometry near the endwall region (tip) that makes the machining difficult, as you must maintain the profile to recover the airfoil to near-design shape.
From a repair standpoint, the main problem lies in the machining that follows the welding process, not the welding itself. The finished airfoil geometry, and therefore the required toolpath, is simply beyond the capabilities of manual or 3-axis machining.
ATI's solution was to employ specifically designed and constructed 5 and 6-axis CNC machining centers, which represent substantial capital investment in both equipment and software development. But given the contours to be reproduced, there was no real alternative. Today, our protocol for refurbishing end-bend compressor airfoils involves a number of major steps.
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