Table of Contents
4.3.1 Repair friendliness of the gas turbine in the "shop"
If even the experts in the shop must surrender, it becommes desperate for the car owner. In former times this happened seldom. But today it is possible to observe totally helpless shop personnell at the electronic displays or watch as they cheesed off check lines and connectors. Also, the exchange of single, cheap component parts, apparently is over. A failing clock at the multifunction car cashboard, easily requires distinctly costs which approximate a nice, fully automatic washing machine. Dented bumpers with integrated headlamps depredate whole weekly wages. Electronic breakdowns, directly lead to the towing service… that’s the end of the ride: with a little bad luck the car will stay for days in the repair shop. Alternatives as reasonably priced parts from other, quite acceptable sources get more and more rare.
How much more important is it for a gas turbine, that repairs can be accomplished fast and economically priced. Extreme costly parts like the blading of the high pressure turbine should be repairable to an as much as possible, acceptable price. So for our gas turbine an economically operation with acceptable follow-up costs is facilitated.
The repair frendliness of an engine naturally must be seen in connection with transport possibility of the engin as a whole or in modules to an extern rapair. Primarily this is true for engine derivates with its low weight, compared with the low weight of the ‘hevy frames’. To guarantee the further operation of the plant an at least preliminary replacement by an exchange engine is assumed. So we expect from the responsible shop, that it can provide such an engine. A repair in the shop is subject to other demands as the repair of the same engine on-site.
At the begin it must be cleared if a repair pays off, or new parts must be supplied ( "Ill. 4.3.1-1"). Important criteria are:
- how often certain repairs can be repeated on a part?
- whether parts will be almost unavoidably damaged during disassembly. Is an expensive rework necessary?
Especially affected are parts such as bolts or disks. Stuck bolts can get overloaded during loosening and so not controllable damaged. In discs fit bores, centering faces and blade fixtings are highly LCF loaded. They take even a small damage amiss.
An other important question is, whether gas turbines contain technologies that make repairs only possible with great effort or if this is even entirely excluded. It is thikable that materials are in use which change their properties unacceptable, especially during a repair. This includes, e.g., the removal of coatings, when this implies a prohibited weakening (e.g., etching attack, thinning of the walls) of the component, or single crystal blades that perhaps necessitate heat treatment at high temperatures, absolutely problematic in relation to the danger of recrystallization. The high process temperature during brazing can critical change the micro and/or macro structure. Such problems can make an process not applicable.
Under this aspect, especially the use of „new technologies“ ( "Ill. 5-1") must be checked.
"Illustration 4.3.1-1": What is more convenient, the exchange of a part or a repair? This is a question, posed above all by an operator. The answer is not only dependent on the direct expenses involved, but also on expenses that emerge because of availability and logistics. There are, however, some typical components worth repair, such as turbine blades and labyrinths, if the damages are not too far gone ( "Ill. 4.1-2"). In the area of aviation, one speaks of a 60% decision line (Lit.4.3-1).Repair is only worth it when the costs lie below 60% of the expenses to be spent out on a new part. The pre requisite for a repair is then, that the part has no life and technical operation limits when compared to the new part.
Repair costs are also influenced by the development and the proof of being seviceable after repairs have been carried out. The development costs are worth it for the hot parts only for repairs supported by an adequate, added value. Otherwise, repair measures are not offered on the market.