Wear in a gas turbine is not evenly distributed. A small number of components in the combustion path absorb most of the thermal punishment, and they are the ones that decide how long a machine runs between outages.
The combustion path, in order
Air leaves the compressor, meets fuel in the combustion system, and the resulting gas passes through the turbine section. Every part along that path sees temperatures above what the base metal could survive unaided, which is why coatings and cooling air do so much of the work.
Combustion liners
The liner contains the flame. It runs hot, it cycles hard on every start, and it is usually among the first parts to show distress. Cracking around cooling holes and coating loss are the common findings.
Transition pieces
The transition piece carries gas from the combustion can to the first stage nozzle, and it changes shape while doing it. That geometry, plus the thermal gradient across it, makes it one of the most replaced items on the machine.
First stage nozzles and buckets
These meet the gas at its hottest. Nozzles are static and tend to show cracking and oxidation. Buckets rotate, so they carry centrifugal load on top of the heat, and creep becomes a factor as well as surface damage.
Shrouds and ring segments
Shroud blocks form the surface the bucket tips run against. Loss of coating or distortion here widens tip clearance, and wider clearance means lost efficiency long before it means a failure.
What actually drives the replacement interval
Two things, mostly. Fired hours accumulate thermal exposure. Starts accumulate thermal cycling, and cycling is harder on these parts than steady running. A peaking machine and a baseload machine of the same frame will not wear the same way even at similar hours.
Running a unit beyond its designed life does not change what fails. It changes how often.
Planning around it
The parts above have long lead times and they are needed together. An outage held up waiting for one transition piece costs far more than the piece. Frames covered and part families are listed in our parts catalogue.
