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5.3 The determination of life from new and operated components
It’s already old, our car. Will the exhaust, this expensive component , soon fail or will it keep up till the next vehicle inspection sticker? The synchronous belt and the water hoses look all like new. Why also the costly, according to the maintenence manual due exchange action?
Or, let’s look at the during a repair replaced parts which submits the shop to us. They look so fresh. Where they really at the end? Who did not himself pose similar questions? In all those cases a sure answer about the condition of the components, respectively their remaining service life is missing.
For an engine whose costs are multiple higher than for a car, such questions about the reuse are all the more urgent. Thereto some answers which indeed can not solve the problems up to now, but demonstrate efforts and approaches for a satisfactory solution.
Overhaul and repair costs can be noticeably sunk, if one succeeds, on the spot, to sufficiently acquire the condition of the components so as to make a statement on the expected amount of remaining life. Thus, the operation is made safer, the overhaul intervals are minimized, the cost of spare parts is saved, the storing of spare parts is decreased, repair is limited and the confidence between manufacturer and workshop, or rather operator is improved. Basically a requirement for the successful, i.e., practically applicable estimation of the residual lifetime is, that the damage/deterioration propagates sufficient slow ( "Ill. 5.3-1"). An example is the creep void (pores) formation ( "Ill. 3.3-13"). This becomes especially problematic when an accelerating damage occures, e.g., a crack propagation in a rotor disk ( "Ill. 5.3-1").
The question of the hot part condition and especially the high pressure turbine blades takes precedence because of their higher load and limited life. Towards judging the condition of the turbine blades are next to the measurable alterations through wear, creep deformations etc., the following (potential) failures relevant:
- Crack initiation outward ( "Ill. 3.3-10" and "Ill. 3.3-12") and inward ( "Ill. 3.3-14" and "Ill. 3.3-15").
- Deformations ( "Ill. 3.3-8").
- Microstructural changes.
- Creep failures ( "Ill. 3.3-13").
- Oxidation and hot gas corrosion ( "Ill. 3.3-9" and "Ill. 3.3-10").
- Blockage of the cooling air configuration ( "Ill. 3.3-10" and "Ill. 3.3-12").
- Damages of the protection layers ( "Ill. 3.2.3-1" and 3.2.3-7).
Today a multiple of new as well as (decidedly improved) conventional processes in laboratory standard is present. This does not, however, suffice for use under the typical overhaul conditions of a gas turbine on site. Such processes are:
- Thermography, has already proven in series application for statements about structure and bomding of overlay coatings (e.g., thermal barrier coatings).
- Three dimensional measurement of complex geometries with laser triangulation. Here, e.g., life determining creep deformations on the blades or impairments of the blade cooling (e.g., blockages, inner oxidation) are detectable.
- Micro focus X-ray examination is, meanwhile standard procedure of the new part quality assurance. For the use on site of inner cracks in the blades, however, transportable instruments are demanded, facilitating an immediate picture evaluation.
- Air flow measurements can also give important hints as to the cooling configuration and, through that, the thermal blade stress. Reliable use on site would demand at least some development work.
- Especially important would be to develop reliable methods that make a statement on the condition of coatings, especially of oxidation protection coatings and thermal barrier coatings. How far, here, adapted eddy-current and ultra sonic procedure have a chance to be examined. Certainly, the development of such examination technologies is an interdisciplinary task, in which the producer, the operator and technical institutes are to be included.