- Creep and creep fatigue
- Thermal fatigue & thermo-mechanical fatigue (TMF)
- Corrosion and oxidation
Creep and creep fatigue
Creep is time-dependent deformation of the component material under application of load at high temperatures. This mechanism is particularly relevant to the rotating blades in the first and subsequent stages, the stationary blades (nozzles) at the first stage, and the turbine discs.
The main concern here is:
- excessive creep deformation which will erode the axial working clearances
- creep rupture and the resulting loss of the rotating blades and
- creep cracking at the disc.
Creep-fatigue is a combination of creep and fatigue in which, for the case of thermal transients, residual secondary stresses can give rise to enhanced creep degradation in association with fatigue damage.
Thermal fatigue & thermo-mechanical fatigue (TMF)
Thermal fatigue and TMF are caused by intense differential thermal gradients across the component introduced during start up and shut down. It affects all components in the hot gas path including combustor, transition piece, turbine nozzles, turbine blades and turbine discs. The stress arising from the differential thermal gradients can be sufficient to cause localised yielding particularly at stress raisers associated with sharp radius intersections. Thus during each cycle of start/stop, a significant stress range is produced resulting in damage at that locality.
For each component material, there is a finite number of cycles at a particular strain before it can develop cracking. Once cracks initiate, they can propagate to failure by either further thermal cycling or by a creep cracking or creep-fatigue cracking mechanism.
The resistance of the hot gas path components to thermal fatigue is controlled by the type of material, cooling arrangement, corrosion resistant coating, thermal barrier coating and method of turbine start up and shut down operation. By taking these factors into account, it is possible to estimate the thermal fatigue life and hence the intervals for inspections and part replacements.
Fatigue can be the result of low cycle high strain similar to thermal fatigue or high cycle at a relatively lower strain.
Low cycle high strain fatigue is associated with centrifugal forces and is related to start-up. It is particularly relevant to the moving blades in the latter stages of the turbine. High cycle fatigue is generally caused by the vibration of the component. The mechanisms for the vibration include flow excitation, combustor pulsation and cyclic bending due to self weight. Interaction of any of these mechanisms with the natural frequency of the component and resonance efforts during starts and stops will accelerate the process of high cycle fatigue.
Corrosion and oxidation
High temperature corrosion and oxidation of the gas turbine material is a significant life limiting factor. The combustion gases, particularly from heavy fuels, contain several aggressive corrosion elements such as sodium, potassium, lead and vanadium which cause corrosion of the hot gas path components. For this reason, appropriate coatings are used to safeguard against oxidation/corrosion. Additionally, treatment of the heavy fuel by washing, filtering and addition of inhibiting additives is necessary.
Erosion by particles of dust present in the air is a common occurrence on compressor blading. The size and quantity of such particles depend on the location and meteorological factors such as fog, rain or snow. In coastal regions, for example, the air contains salt, some in crystalline form.
Erosion and fouling can cause a significant reduction in performance as a result of solids, oxides, silicates and other compounds which are normally not adequately removed by fuel treatment. These elements need to be removed by fuel filtration and the use of a duplex filtration system permits turbine maintenance without turbine shut down.