Table of Contents
4.1.2 Overhaul/'revision' (main inspeection)
The overhaul ( "Ill. 4.1-1") takes place in the widest possible open condition of the engine and comprises maintenance and repair work. If a necessary exchange of components is determined, repair can be required. It’s, consequently, an important measure towards protection from damages. A planned, naturally organized static condition of the engine is required, so the shut down occurs devoid of inappropriate repercussions on the normal work procedure and no production loss develops.
The time and expenditure during the main inspection depends on the construction of the engine and the affected parts. A time of at least some days has to be provided. If expensive parts are to be exchanged, because of the expired calculated life or due to unexpected damages, immediate availability is of interest, apart from the cost of the spare part. The storing of such parts is expensive and is, therefore, reduced, which can provoke unpleasant bottlenecks. Good maintenance planning is important. For this purpose, the affected parts have to be fully identified beforehand. It is often only possible to decide about the further usage of components, e.g., turbine blades, when these are accessible for examination. The latter can take days until, e.g., it can be decided whether the part can remain in the engine initially, to be rejected during the next overhaul, or if an exchange should follow, in order to repair these parts again ( "Ill. 4.1-2"). Repair delay, (e.g., after coating) may in most cases be forbidden because of shortage of time. A crash action can require a week, in a convenient situation.
A special set of problems for the exchange of used parts should be suggested. As private owner of a car we know the problem of building in into an old motor new tight pistons. This can lead that, from the now increased performance, an already worn crank shaft bearing or connecting rod bearing will be overloaded. In a short time a catastrophic failure is to anticipate.
Similar in an other sense is also possible with a gas turbine. That is the case when after an overhaul a higher power output loads other components higher. The operator should, if necessary, assure himself at the OEM if there are concerns in this direction.
An inspection which reveals the condition of the components on site, respectively in assembled state, is desirable. Mostly however, at present, the practical application of the current procedures such as the penetrant crack inspection, ultrasonic, X ray or eddy current inspection is insufficient. It would be very positive, if efforts in this connection were strengthened, with the idea of lowering inspection and maintenance costs (see Chapter 5.3.).
New labyrinth seals with lower lekage can influence the (axial) loads on the main bearings ( "Ill. 2.5-2"). After a module exchange ( "Ill. 4.2-3"), e.g., exists the raised danger that a labyrinth fails catastrophic during rub in at the remaining aged/deteriorated coating ( "Ill. 4.2-3").
Here should be pointed to a problem, shown in "Ill. 4.1-2". According to experience, statistically there is for every main inspection itself a certain risk to produce failures. Equivalent to the bath tub curve ( "Ill. 4.1-9") there is due to considerably works after a new start and during the first operation hours an increased failure risk. This rises especially when the casing must be opened and a partial disassembly took place. Reasons are assembling mistakes, inserting foreigen objects or not suitable media.
