Inspection results: Casing, seal damage concern FT8 users

A summary of recent borescope-inspection findings by Mike Hoogsteden, field service manager, Advanced Turbine Support LLC (ATS), Gainesville, Fla, pinpointed areas of the FT8 of greatest concern to owner/operators of the expanding fleet. The presentation on the popular aero engine, manufactured by Pratt & Whitney Power Systems, was made during a special roundtable for FT8 users at the CTOTF Spring Turbine Forum.

Hoogsteden walked attendees through the engine from the compressor bellmouth through the power turbine (PT). Rubs, tip discoloration, and minor deposits and impact damage characterized typical findings in the low-pressure (LP) compressor. Biggest surprise: compressor case damage. This had not been seen by ATS inspectors until recently and has been identified in four engines thus far (Fig 1).

The service manager’s photos of tip rubs and dings in the high-pressure (HP) compressor didn’t reveal anything every user in the room hadn’t seen previously. An inoperable engine-heater check valve seemed to generate the most interest. Hoogsteden said ATS finds inoperable check valves on 20% of all engines inspected. These valves are critical for keeping engines above dewpoint temperature when they are not in operation.

Wear and tear identified with the combustion section included relatively minor liner coating loss and cracking (Fig 2), plus a damaged J-seal on one engine. In the HP turbine, coating loss in spots is relatively common on the suction side of rotating blades. Erosion at the base of vane segments is found occasionally (Fig 3). Interestingly, erosion occurs gradually and little collateral damage has been experienced in engines inspected by ATS. Erosion can approach the air cooling holes in some instances.

Small areas of coating loss also can be found on the leading edges of LP turbine S1 vanes (Fig 4) and blades. Measurements taken by inspectors in the exhaust-case area can help ATS determine if the gas generator has moved and, if so, by how much. Thinking is that shifting of the GG case could be a factor contributing to R1 air seal damage found in some power turbines (PT).

Additionally, is believed that a crack in the PT’s R1 nozzle retainer ring may be conducive to a condition known as “lean back,” which also can contribute to R1 air seal damage (Figs 5 and 6). Next, trailing edges of S1 vanes are checked for platform wear (Fig 7) and R2 seals are inspected to be sure they’re in good condition. PT rows 3 and 4 are checked for rotor-blade tip shroud rub (Fig 8).

       
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