"Illustration 4.1-1": In Lit. 4.1.1 a process, according to F. Wotschofsky is typified in the illustration above. It presents intervals between inspections and main inspections (‘revisions’). They act in accordance with operation hours and/or startshut down cycles. With the possibilities of the continuous engine monitoring (chapter5.1), the „on condition“ principle lends itself ( "Ill. 4.1-0").

Maintenance work (even operation documentation and inspection work) does not impair the availability of the engine and can be undertaken by the responsible personnel on the periphery in the running or stand by condition of the engine. The repairs are carried out according to written instructions (e.g., lubrication point plan with component specific details regarding lubricants) Pressure measurements in the compressor, according to manufacturer’s requirements, enable one to recognize if a cleaning, respectively, washing process is to be undertaken ( "Ill. 4.2-1.1" and "Ill. 4.2-1.2") or not.

Inspection: without the disassembling of parts, visual and borescope inspections and the determination of data are implemented, in order to draw conclusions on the engine condition. The results are to be documented.

Overhaul (‘revision’) requires a disassembling of the engine, allowing one to carry out all inspections and monitoring and, if required, to introduce measures in order to guarantee a safe operation until the next inspection. One should especially notice signs of component typical damages such as oxidation, deformation, cracks and wear

Here also and when planning the intervals for inspections,it is imperative to refer to the manufacturer’s suggestions whenever there is doubt. Depending on the inspection findings (e.g., boroscope) one should, however, in discussion with the manufacturer, adapt to the necessary requirements. In industrial gas turbines such an interval should correspond to 20,000 equivalent operation hours (“eh”), and should follow, at the latest, after eight years. Periods of high stress, such as quick shut downs, starts (e.g., weighting 10) and quick load changes, weigh and count more than base load hours (a base load hour equals 1 eh) according to a prior possible key ( "Ill. 2.2-5").

"Illustration 4.1-2": The failure rate over time in gas turbines, typical for technical equipments follows the bath tub curve ( "Ill. 4.1-9"). This is also valid for individual overhaul cycles ( "Ill. 4.1-1"), where one has to reckon with an increase in failure, if even small, initially, after the actual overhaul. Without the latter, however, the failure rate would rise to prohibitive heights. With the overhaul cycles there is, indeed, an increase, but it is moderate and lies within limits of normal component fatigue.

One can understand failure rate (diagram 2, lit.4.1-1) as an addition of three individual effects:

The long term failure (“A”) of components excluded from the overhaul failures in connection with the overhaul itself (“B”) and the wear failures within an overhaul interval (“C”). It is expected that the sum of these failures is clearly less than if there had been no overhaul at all.

It is no secret that entirely different views (e.g. for flight engines) are also propagated. It’s, therefore, suggested, e.g., that the maintenance and inspection is extended and perfected (on condition, "Ill. 4.1-1"), so that an overhaul with total disassembly is, as far as possible, redundant. Perhaps the extensive expertise of the maintenance personnel, as well as the far reaching inspection work of the operator, is a necessary preliminary for such a process.

The overhaul interval ( "Ill. 4.1-2"): This is usually determined by the manufacturer and it is an important part of the decision for acquisition. The interval should be lengthened or shortened, depending on the findings of the inspection, after discussion with partners like the OEM or insurances.

One criterion is the period of time from when repair of important and costly (e.g., turbine blades) components is still meaningful.

Here are considerations, as in diagram 1 (Lit.2.3- 1). In the area “I” there are the lowest repair costs. If the condition deteriorates corresponding to the degree of impairment, the curve belonging to it enters zone II at “A”, the area of middle impairment. If a repair is carried out at this point of time, the utility length is extended by half the life if the repair takes place only when reaching zone III at point “B”, the damage is already so huge that the costs of an optimum life extension against the acquisition of a new part is not feasible any more.

The curve „C“ is true for in time repaired parts, i.e. in the region of „I“ to „A“. In the life time region up to an no more repairable condition lies the point „D“. This time is clearly longer than this of the not repaired part. The point of time of repair is also markedly influenced by effects at the damage of components. Typical are fuel, special operation conditions and the environment (e.g., contamination of the intake air).

If the specific fuel consumption is a criterion of examination, the quality of the repair shops, respectively, the overhaul can be of decisive importance.

In diagram 3 a scatter of 3,5% is recognizable according to Lit.4.1-2. It increases additionally, beyond the operation time by a high initial value. The overhaul interval is thus further shortened.

"Illustration 4.1-3": The inspection ( "Ill. 4.1-1", see also Lit.1-7) is made on the not disassembled engine as dirty inspection, respectively inspection in the as run condition. In contrast, an overall examination of the parts during inspection ( "Ill. 4.1-2") or overhaul is carried out in the disassembled or cleaned condition (Lit.4.1-3) described as clean inspection. Now, typical procedures in the framework of inspection are exemplarily introduced:

An outer visual inspection of the engine for particularities ( "Ill. 4.1-10") takes place, first of all. A photographic documentation, is recommended.

Fluids will be drained and samples taken according to the manufacturer’s instruction. Oil analyses ( "Ill. 3.5-4") can, e.g., hint as to problems of wear or unusual operation conditions (e.g., overheating). Oil filters ( "Ill. 3.5-3") and magnetic chip detectors ( "Ill. 3.5-5") are pulled, the deposits investigated if necessary.

Clearances (e.g., blade actuators and return cables from control systems and monitors) and gaps are to be controlled and documented.

Fastenings such as pipe clamps and cable fasteners are, if accessible, to be checked for proper clamping. If necessary, actuating forces and break torques are to be determined.

In addition, every module undergoes a precise inspection. Included here are function examinations of the components and boroscope inspections ( "Ill. 4.1-5" and "Ill. 4.1-6").

"Illustration 4.1-4": It is to be about an example for a structured decision (decision tree). Even the deposits on the magnetic chip detectors ( "Ill. 3.5-5") provide informations as to the condition of the engine and /or the trend of possible problems ( "Ill. 3.5-7" and "Ill. 3.5-8"), as does the filter residues. A combination with oil filter examinations ( "Ill. 3.5-3") can clearly improve the surety of statement and is, therefore, often demanded from the manufacture. Magnetic chip detectors ( "Ill. 3.5-5") that show, electrically, non permitted deposits should be checked for damaged indicators.

Relative to the findings of the filter and the magnetic chip detector deposits, the represented decision tree is a help. Black areas show results indicating that a further operation without manufacturer’s recommendation is not advisable. Grey areas demand ensuring actions suggested by the producer. White areas allow unlimited use of the engine according to the findings.

"Illustration 4.1-5": Borescope inspections (Lit. 4.1-6) belong for some time now to standard technique. They make the optical inspection of the components and a photographic documentation of the findings possible. The pre requisite is an optimum distribution and arrangement of the borescope openings that probably subject the entire flow canal (Lit.1.1-6), especially all hot parts, to inspection. For this, it is necessary that the angle of vision is also considered.

A particular problem is the explanation of the findings ( "Ill. 4.1-7" 1nd "Ill. 4.1-8"), especially with regard to new technologies such as thermal barrier coatings. Hot parts with the usual diffusion coatings can be a puzzle too, if there are cracks. If, e.g., one has to decide whether the failure involves a crack in the coating (coating crack) or a deeper fracture in the base material, a lineal type of coke deposit in the direction of flow of a blade, i.e., in preferred crack direction, can be mistaken for a crack. Elaborate repair measures in such cases turn out to be unnecessary.

In a dangerous case a crack propagating from the cooling channels ( "Ill. 4.1-7") can be concerned ( "Ill. 3.3-14" and "Ill. 3.3-15"). Then it is to reckon with a short term failure of the blade, that can no more be captured.

Precisely as important as the borescope openings and borescope instruments themselves is an experienced and trained personnel (Lit. 4.1-3), knowing where to search, in order to recognize and evaluate the fault. The evaluation in problem cases will be naturally, preferably made with the manufacturer. One should keep in mind, that optical findings are made available through picture transmissions for the manufacturer or expert, so that a distant diagnosis is possible and the evaluation specialist does not have to be locally present in every case.

"Illustration 4.1-6": Borescopy is available in different versions. (Lit.4.1-6). Rigid, as shown above, or flexible. It’s important for a successful borescope inspection to know where to look for what. Towards this purpose, the figures "Ill. 4.1-7", "Ill. 4.1-8", "Ill. 3.3-9" and "Ill. 3.3-10" can be helpful. Often, the explanation of the findings need consultations and specific examinations; especially the evaluation of the risks in connection with the further expected progress of failure (e.g., a crack growth or a corrosion attack) necessitates the expertise of the manufacturer

"Illustration 4.1-7": Some typical pictures of high pressure turbine blades, as the examiner views in the borescope, are represented as examples (see also Lit.4.1-4 and Lit. 4.1-5).

“A”: Local oxidation damage, also in connection with hot gas corrosion, in the area of a component specific hot spot, where the film cooling air is insufficiently effective. The protective diffusion coating is here already consumed, the base material becomes visible.

“B”: Typical thermal fatigue crack with delayed crack growth ( "Ill. 3.3-9") on one guide vane in the transition to the outer shroud.

“C”: OOD (Impact through internal foreign objects) on a rotor blade (secondary failure see “D” in "Ill. 3.3-10"). Typical for the turbine is the area on the suction side of the leading edge region. Foreign objects in the high pressure turbine are, e.g., coke particles from the combustor (carbon impact) or released ceramic particles from the thermal barriers ( "Ill. 3.2.3-4").

“D”: Burnt leading edge (“E”) in the tip region of a turbine rotor blade without shroud. Cause of the over temperature can be an inner blockage of the cooling air hole (e.g., a closed dust removal opening) or a narrowing of the cooling air hole, as a consequence of a deformation (OOD, “C”).

“E”: Heavy oxidation (burning) and thermal fatigue cracks on the leading edge of a turbine blade. This typical appearance at local over temperatures is also called the orange peel effect.

“F”: Turbine rotor blades from which foreign material, e.g., labyrinth or abradable abrasions from the compressor ( "Ill. 3.1.2.4-4"), emerges out of the film cooling holes and melt.

“G“: Black line on direction of the flow can originate from coked oil and is in this case not dangerous. But if it is a matter of an internal crack, originated from the cooling structure of the blade ( "Ill. 4.1-5") a fracture of the blade must be expected.

"Illustration 4.1-8": The picture shows some typical illustrations of compressor blades as viewed by the examiner in the borescope:

“A”: Radial fatigue crack in guide vanes and rotor blades without shroud, through high frequency vibrations of a higher order (lyramode). For this failure mode especially susceptible are blades and vanes of modern compressors with the typical thin profiles and a wide chord.

“B”: Break outs of soft abradables in the casings opposite the compressor rotor blades ( "Ill. 3.1.2.4-4"). Probably vibratory fatigue, through high frequency casing vibrations in connection with the blade passing frequency.

“C”: Foreign object damage (FOD) in the leading edge of a compressor blade.

“D”: Cracks through vibratory fatigue. “D 1” in a vane, fixed at both sides. Cause are torsional vibrations in the blade airfoil parallel to the edges with a so orientated node line. „D 2:“ Fatigue crack through a bending vibration of the blade, issuing from the edge.

"Illustration 4.1-9": Technical facilities follow a typical law in their temporal failure behavior, the so-called bath tub curve (Lit.0-4). Contrary to expectation, damages and failures of new parts occur abundantly during operation. With reference again to one’s private experience with a car or a computer, numerous examples can be found in which one experiences oneself as an external quality control of the supplier. In gas turbines, foreign object damages are not seldom the reason for this effect, because of forgotten fixing devices, tools and auxiliaries. Also assembling faults can play a role. An inceasing possibility that this phenomenon repeats during overhauls does not wonder. ( "Ill. 4.1-2"). In the region of the safe design lifetime the failure incidence/probability is almost constant. Here the curve runs horizontal. It’s determined by the coincidal influences from the outside, coupled with service and maintenance. Primarily, towards the end of life of the component (e.g., wear and corrosion) the conditional failure probability, along with the failure frequency, increases remarkably.

"Illustration 4.1-10": The service personnel and the operator can detect early signs of problems through attention and, if necessary, in conjunction with the manufacturer, introduce timely, suitable solutions. In the picture nearby, typical outer features of a gas turbine with relevant problems are depicted.

Wire mesh covered elastic bellows (expansion joint, A) can become chafed through after longer running times, on the grounds of the function specific component movements of the inner side. This leads to wire breaks and a splicing of the bellow, prior to a complete failure of the pipe connection.

Flange connections (B) from pressurized cases, such as the rear compressor, or from combustor and turbine can show non permitted leakages. Typical are locally failed bolt connections or through cracks. Evidences are local discolorations of the casing wall.

At pipes in the area of clamps (C), there is the increased danger of fretting and crack initiation when the clamps sit too loosely. To compound that, the vibration dampening effect of the clamp on the pipe is mitigated through a loose fit.

Cracks of restrained and vibrating pipes (D) occur constantly. That is why, one has to emphasize a stress free assembly of the pipes!

Chafing on contact faces of cables (E) can be the start of a functional failure, as such:

Oil, fuel, hydraulic fluids, especially in the area of bolting (F).

On flange clamps (G) the riveted lugs are vulnerable to cracking. Fractures of the fasteners in connection with corrosion or embrittlement are not to be excluded ( "Ill. 4.2.3.1-3").

Bulging (H) on pressurized casing walls.Local discolorations or increased oxidation can signify problems in the interior of the components (e.g., casings of the combustor and the turbine).

Welding areas (I) and deformation hindered regions, such as casing edges (K), are especially sensitive in the face of thermal fatigue and pressure cycles. Local conspicuous changes such as discoloration can be a hint at leaking hot gas through a crack.

This can be a streak in connection with local changed oxidation conditions (I). Also unusual accumulations (type and location) are an identifying feature.

"Illustration 4.1-11": (Lit. 4.1-7): The practitioner has means that support him in his senses. There are two examples:

Screwdriver (sketch left): Usuable during turning of the rotor by hand. Especially suitable is a design with end to end blade. This can lead the structure-borne sound of a vibrating wall (e.g., pipe line, casing) direct to the ear. Thereby the interesting sound will be stronger and clearer at the expense of background noise (airborne noise).

A coin for the estimation of vibrations only is usable in special cases when there is a horizontal plane like may be on a test rig. Careful the coin will be positioned vertical on a vibrating plane and its behavior evaluated (tilting, movements). This observation naturally is only predictable in comparison with the normal behavior. Naturally it must be safeguarded that the coin will not be left as a foreigen object.