Industrial Gas Turbines

4.2.1 Repair friendliness of the gas turbine locally

We all know of cars, where the exchange of a spark plug demands tremendous finger deftness and the replacement of a V-belt or a water hose is only with the demounting of the motor a answerable task. Those problems increase for modern cars. It gets especially awkward if we are struck by a breakdown. Even helpful specialists for field work from the breakdown service are today helpless confronted with the comulative might of electronics and encapsulation. Only the manufacturer and his contractual partners seem to can help in such cases. There the comes the feeling to be at their mercy. Such a situation beclouds the joy about his machine for every car fan.

Of similar importance is the repair friendliness of a gas turbine for operation security. Repair friendliness is in the end a significant cost factor.

The repair friendliness of a gas turbine on site depends especially on the design principles used. It’s, therefore, already assessable during acquisition and is an important criterion of choice (Chapter 1.1). In gas turbines with axial divided casings a half shell can be taken away, making the accessibility to blades of the compressor and turbine effective. A further example is the effort for the exchange of individual blades in compressor or turbine.

Gas turbines (frequently of such an older type) with multiple combustor cans at the periphery, permit an exchange of components usually more simply than modern gas turbines with annular combustors ( "Ill. 2.1-6.2"). Far more advanced would be an operator of a gas turbine with one tangential combustor (in engines of small performance), or operators of big engines with slanting or radial combustors easy to reach from the outside ( "Ill. 2.1-6.1" and "Ill. 3.2.1-2").

Gas turbines that stem from modern flight engines can have a modular design ( "Ill. 4.2-4"). This makes a complete exchange of an assembly group such as a compressor, combustor area and turbine possible. The advantage is an optimum overhaul of the module in the shop and a quicker and comparatively simpler exchange of the entire module.

Such an assembly has, however, entirely specific problems that show up at the connections in the module. Thus, one can be forced, e.g., to combine new labyrinth fins with appropriately large diameters and aged abradables ( "Ill. 4.2-3") with bad cutting in behavior. A dangerous situation can arise during rub of the labyrinths (see "Ill. 3.1.2.4-6" and "Example 4.2-4"). A further problem can arise at cramped or a bad accessible assembly at the joint ( "Ill. 4.2-5"). This is especially true, if the module is relatively heavy and big ( "Example 4.2-6"). There exists the danger during the assembly that unnoticed damages at components like roller bearings or labyrinths occur ( "Example 4.2-4"). To avoid this the OEM prescribes suited assembling aids. Those have to be used.

Example 4.2-1: The operator of a bigger gas turbine detected signs of hot gas corrosion on the high pressure turbine, well before the determined life expectancy had been reached. The manufacturer suggested an exchange of the entire blade system for new parts. An independent technical consultant removed a typical part and asserted, within the day, that the damage had progressed so much that a meaningful repair was no longer possible, however, the blades allowed operation of 20,000 hours more by the same amount of damage. In order to make this proposal with its inherent risks acceptable for the operator, borescope inspections were recommended with fixed failure limits of the outer findings at regular time intervals. The subsequent examination of the removed part explained the failure mechanism at hand and suggestions were made, regarding the insertion of new parts, which the manufacturer heeded. The operator followed this suggestion under the forces of availability of the engine and cutting costs. The correctness of this decision was verified through a disturbance free proceeding operation until the recommended time limit.The OEM considered the suggestions for the later build in new parts and so made a longer component lifetime possible.

From this example can much be learned: When there are cost intensive decisions the recommendation of an indepandent consultant of his trust is important for the operator. The operator then has a leeway in decision-making.

Example 4.2-2: ( "Ill. 4.2-2"): One of the manufacturer’s recommended re-fitting actions on the scavenge oil pump of a certain type of engine was carried out by the operator. There was a driven gear of the pump with the bearing flange to be removed. The accessibility of the pump in the lower area of the engine did not make it possible to observe the exact way the engine was built. This had to be assessed through the intuition of the fitter. When putting on the cover, a tooth of the very narrow driven gear abutted on the side face of the opposing wheel and snapped in only when the flange screws were tightened. Thus, the relatively thin power shaft of the pump was bent plastically. After a long operation of the engine, the damaged pump shaft broke as a consequence of fatigue crack with serious secondary damages. After the course of the damage was detected through a systematic failure analysis, the examination of other engines of the same type revealed many parallel cases, however, without the breakage of the pump shaft. In this manner great dangers and failure costs could be avoided.

Example 4.2-4: ( "Ill. 4.2-3"): In a gas turbine with modular design, the turbine module is to be removed and exchanged. The now, narrower labyrinth clearances in the joint area of the module leads after a few operation hours during run up of the engine to gap bridging over with hefty rub. A rub of the old liners led to extreme heating up of the labyrinth. The consequence was a catastrophic failure of the labyrinth and the adjacent assembly.

Example 4.2-5: During assembly of a turbine rotor, it occurred to the attentive fitter that the bolts, in contrast to previous occasions, reached just the minimum required torque after unusually many turns. He reported the incident. A check proved that the employed bolts belonged to a wrong delivery and evinced a much too low strength. Thus, extensive time consuming, expensive re assembling was avoided, to say nothing of the technical danger which may have been involved.

Example 4.2-6: ( "Ill. 4.2-4"): The exchange of a compressor module demanded that the shaft with the inner bearing ring was pulled out of the roller bearing. The assembly process of the new module was not sufficiently observable and a later visual control of the bearing was not possible ( "Ill. 4.2-5"). A damage of the bearing ring during assembly remained therefore unobserved. Only a few hours after restarting the engine, there was a bigger failure in the entire bearing area. The subsequent, extensive failure analysis clearly showed that the operator had not induced the damage (as was first assumed) through faulty service. A small damage of the bearing raceway, that, because of its characteristic features could only originate through assembly, which was responsible for the failure (see Chapter 4.3.3).

"Illustration 4.2-2": This failure is an example of a typical assembling problem. It was traced back not at least at unfavorable „human factors“. The description correlates with "Example 4.2-2".

"Illustration 4.2-3": ( "Example 4.2-4"): A typical potential problem can arise when there is a change from old to new module, if, through that, labyrinth components must necessarily be combined. In the new engine, the good cutting in behavior of the abradable ensures a non problematic insertion of the sealing fins (in the picture above).

After long service there is no bridging over of the abraded sealing clearance. A new sealing ring with a corresponding long sealing fins can, in contrast, when running against an old brittle abradable, stimulate a self propelling failure mechanism ( "Ill. 3.1.2.4-6") with extensive secondary damages.

"Illustration 4.2-4": Engines in modular design (example GE LM 5000) have advantages during assembly and overhaul. Attention should be paid to the concept dependent specialties of the engine types. The joints, respectively, points of separation, are frequently in the bearing area (Lit.5.5-1). Roller bearings offer themselves formally as separation points. The shaft is pulled away with the inner ring of the roller bearing. The zones “A” and “B” can have to do with such a point of separation (not necessarily in the engine type represented over here). In the joint areas of the modules, especially in the bearing area ( "Ill. 4.2-5") and on labyrinths ( "Ill. 4.2-3"), intensive attention is demanded during assembly. By stronger contact with the sealing surfaces, labyrinth fins can be damaged during axial assembly movement. Particularly when a new module is joined with one already longer in operation, distortions on the old parts can create problems. Another problematic in this connection is shown in "Ill. 4.2-3". It concerns the operation behavior of a combination of old and new modules.

"Illustration 4.2-5": ( "Example 4.2-6"): In the context of assembly work during inspection and overhaul, the situation arises that modules in the area of the bearing are joined. Here the joints (arrow in Fig. above) may sometimes be insufficiently visible. If it has to do with the difficulty to move masses, there is the possibility that the bearing, e.g. through the edge of the rolling body is damaged (arrow Fig. below). Such a damage can lead to fatigue of the raceway (spalling) and to failure of the bearing.

Thereby it is possible that the bearing fails so fast that this can not be captured or a fatigue failure ( "Ill. 3.5-10" and "Ill. 3.5-11") can no more be identified during the formation of chips. In such cases it must be reckoned with extensive secondary damages.

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4.2.1 Repair friendliness of the gas turbine locally