7FA exhaust system inspection: What took a bite out of these R3 turbine buckets?

1. Impact damage on trailing edge of R3 bucketPresident Rod Shidler and Field Service Manager Mike Hoogsteden of Advanced Turbine Support LLC had no sooner finished presenting on the results of recent inspections of Siemens 501FD2s and GE aeros at CTOTF’s™ 38th annual Spring Turbine Users Conference when one of the company’s technicians forwarded photos of significant damage to the trailing edges of 41 third-stage buckets on a GE7FA (Fig 1). It looks as if something took a bite out of the buckets.

What apparently had happened was that a repair weld holding a section of flex-seal ring pipe in place cracked allowing the pipe section to liberate and damage the buckets, located only an inch or two away from the flex seal. Two of the lessons learned: (1) Be sure this part of the engine is on your inspection check list. (2) Repair welds in the exhaust section have an element of risk given the high temperature (nominal 1000F) and very turbulent nature of the gas stream—especially so when the work is done on engines subject to daily thermal cycles.

To understand exactly what happened, please read on. A necessary first step is a review of the arrangem2. Exhaust system for a GE 7FAent and general design of the components involved. For this information, the editors reached out to David Clarida of Integrity Power Solutions LLC. The exhaust system expert, licensed by the OEM to make repairs downstream of the turbine section, had just participated in CTOTF’s GE E-class and Legacy Roundtable on the same subject. The exhaust systems on most GE E- and F-class units are nearly identical.

Clarida began by pointing out that the same components often are referred to by different names in industry discussions so it’s important to look at the diagrams as you read further. Fig 2 shows the arrangement of 7FA components from the R3 bucket row to a point about 6 ft beyond the turbine exhaust flange. Note, in particular, the locations of the flex seal, flex-seal ring pipe, and exhaust-frame outer diffuser, and the proximity of the flex-seal ring pipe to the shroud blocks and the rotating R3 buckets.

The flex seal essentially is formed by a couple of layers of thin-gauge metal sheet that slide into a slit in the flex-seal ring pipe on one side and a slit in the exhaust-frame casing on the other side (Fig 3). Its purpose: Provide a barrier between the hot exhaust gas and cooling air for the bearing housing while allowing the exhaust-frame casing and outer diffuser to expand and contract independently of each other.

By their nature, function, and environment, flex seals are subject to wear and tear conducive to failure. When the barrier between the exhaust and cooling air is breeched, one of two things is likely to happen: Air from the exhaust-frame blowers escapes into the exhaust stream, thereby starving the bearing housing of cooling, or if the backpressure is high enough, exhaust gas would flow into the cooling circuit and possibly overheat the bearing housing. Thus regular inspection by a trained professional is important.

3. Top half of exhaust-frame casing.png

Clarida said that the flex-seal ring pipe is in two sections—one for the upper half of the unit, one for the lower half. They meet at the horizontal joint. The flex seal is divided into several sections. When a flex-seal segment fails, one possible solution (not recommended) is to cut out a section of ring pipe in the affected area, replace the damaged seal segment, and reweld the section of ring pipe in place. The alternative is to remove the upper half of the casing and replace the entire ring pipe and flex seal in the affected half of the unit.

This obviously is the more expensive and time-consuming option, but Clarida said it is the only way to ensure against the weld cracking and ring-pipe segment liberation shown in the photos provided by Advanced Turbine Support (Fig 4-6). It is very difficult to make quality weld repairs of the type required, he continued, because of the tight spacing between the shroud blocks and the ring pipe. The circumferential welds at the ends of the pipe segment being replaced are particularly challenging.

4-6. Flex-seal ring pipe



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