Table of Contents
4. Maintenance, inspection, overhaul and repair
Naturally a thoroughly, approved and competent maintenance is the premise for a trouble free operation pleasure. For our car, after the rather short acquisition pase, these are the most important wishes that accompany us during the using. Easy maintenance work we can make our own, following the recommendations of the manufacturer. Inspections, overhaul and repairs however require the manufacturer and/or authorized shops with specialists.
Car owners exprerience, that after a shop visit the old problem further arises or a new is added. According to the motto: The clutch is functioning again but now a disconnected electric contact paralyzes the ventilator. The accumulation of failures after repairs and overhauls shows statistically the typical bath tub curve which also is true for gas turbines ( "Ill. 4.1-9").
Maintenance errors as cause for failures are not to neglect for gas turbines (Bild 0-4). They followup like all technical produkts a so called “bath tub curve”. The operator/owner contributes in his own interest his share that such failures are rare. Now some hints (without the claim of completeness):
The manufacturer/OEM gives us all the necessary clues (hopefully) in the maintenance manual. Crucial is, that it is clear to the operator that he also has to contribute towards the maintenance of an expensive investment. He should, thus, arrange for a technical training and literature for the operators. The technical education is necessary for correct interpretation of instructions and specifications; it is also the requirement for posing the right questions and appropriately assessing answers.
What influences the effort of repair, overhaul and maintenance?
At first, the question regarding the deciding influences on the (component) life of a stationary gas turbine interests us along with the effort required for maintenance, overhaul and repair. Experience gives the following amazing answers regarding influences and their importance:
- type of fuel,
- start frequency,
- amount of cycles,
- surrounding,
- maintenance.
The influence of fuels is of great importance for the failurefree and cost -efficient operation of a gas turbine. With reference to a low NOx emission the fuel gets an even higher significance if already relative small aberrations in the composition promote a instable combustion ( "Ill. 3.2.2-5").The lower the heat radiation during the combustion of fuel, the less the thermal load of the hot parts, especially the combustor. Decisive for the life of the hot parts is their heating up through radiation, because they are only protected against convection by film cooling. The heating up occurs essentially through the heat radiation of the flame. Flames from fuels, that emit soot particles during combustion have an especially high heat radiation. On account of natural gas, as the most widely used fuel with the lowest radiation energy (blue or invisible flame), one can expect the longest life from the affected components. A further problem of many fuels is impurities, bringing forth corrosion in fuel systems and/or hot gas corrosion (sulfidation, "Ill. 3.4-2" and "Ill. 3.4-3") on hot parts in the hot gas passage. Dirt in form of particles is dangerous because of the erosive effect it has on the fuel system (pumps, probes, valves, nozzles). Even in this sense, natural gas is advantageous.
The number and frequency of starts present life determining thermal load cycles for the hot parts. Thus, the type of gas turbine operation if, e.g., base or peak load must be produced, is of immense importance. Engines in pipe line operation and in process engineering are, according to experience, relatively little stressed in relation to the start amounts (cycles), as they show a comparatively high number of running hours per cycle (>10²h).
The load cycles (mini cycles), as a consequence of different power demands during service, are mostly of slight influence. By frequent and quick load alterations, however, they can be the cause of a noticeable part of the component load and will be considered during life assessment in the form of reference cycles (calculation in corresponding starts).
A damaging atmospheric influence of the engine components is described in Chapter 3 ( "Ill. 3.1.2.2-1"). The quality of the intake air is of particular significance. This is finally dependent on the choice and maintenance of the filter unit ( "Ill. 3.7.1-3"). One is challenged, here, as operator.
We have now reached the question of maintenance as being the most important factor capable of influence. An experienced, well trained and motivated team, together with the necessary ambience makes good maintenance possible. Requisites that the operator can influence are:
Making accessible the required space. This should be clean, have a moderat temperature and well lit.
An important field for the maintenance are medias like sealants, cleaning agents, labeling and lubrications/grease. In Chapter 4.2.2. details are explained. Take care for such, possibly required consumable material, like cleaning agencies, folia covers.If necessary everything has to comply with specifications of the OEM. In case of doubt there must be asked for.
The same is also true for tools, aids like hoists, spacers and special tools/assembling tools. Even such profane things like stop-blocks/wedges must be available.
Not to forget are documentation means like a suitable camera and inspection requirements as there are borescope, binocular and lamps.
If an additional service is suddenly necessary, e.g., oil analysis or microscopic examinations (chips, "Ill. 3.5-3" and "Ill. 3.5-5", damaged surfaces), it should be appropriately planned. This means to secure in time an appointment in a suitable laboratory.
In order, not to be surprised by breakdowns and repairs, prior assessments as to the condition of the components and their life expectancy ( "Ill. 3.5-1") are helpful. Appropriate inspections, e.g., borescope inspection ( "Ill. 4.1-6"), are necessary and are usually prescribed by the manufacturer/OEM. Various inspections, corresponding to the condition of the engine, combine to make the statements more precise and certain.
