Industrial Gas Turbines

During shut down as well as start, one has to deal with a transition operation, yet the load of the component is clearly different ( "Ill. 2.2-4"). Primarily there is thermal fatigue of the hot parts ( "Ill. 3.3-16"). After shut down, the air from the compressor quickly cools the turbine discs in the outer zone, via the relatively big blade surfaces. Through this, there are tensile stresses that balance the compression stress in the disc area close to the hub.The disc rim here is also temporarily, highly loaded (see "Ill. 3.3-17"). Integral turbine discs in small gas turbines show plastic elongation in areas that are blocked during heating up at the start. The consequence of starts and stops is a typical, thermal fatigue stress that not rarely leads to life determining, rim cracks ( "Ill. 3.3-17"). At least at the beginning, a stable crack propagation is present. That is why, such cracks are tolerated within the limits of experience.

Thin cross sections of the blades are especially quickly cooled and experience changing tensile loads of thermal fatigue load ( "Ill. 3.3-9"). Coating cracks can show up on blades with protective diffusion coatings, especially in the area of low component temperatures. These coatings become brittle by temperatures under 700°C ( "Ill. 3.3-7" and "Ill. 3.3-9"). Coating cracks are again origin for hot gas corrosion or fatigue crack propagation. Thus, it is important that the used protective coatings adapt to the particularities of the engine. It is, therefore, also understandable that by engine shut down, one has to pay attention to the manufacturer’s (OEM) instructions or suggested procedure.

After switching off, a strong convection occurs, especially in the innermost part of the engine; that means, through rising air, the lower casings and rotor zones cool quicker than the ones above. This leads, partially, to correspondingly large temperature gradients, with differences in thermal expansion, and, therefore, to elastic deformations. Clearances in the labyrinths and on tips of blades are not seldom bridged over and the rotor ceases ( "Ill. 2.2-2"). A “freeing“ of the rotor can, depending on the type of engine, take as long as an hour. Even when the rotor is free a rotor bow can be present, which brings about unacceptable unbalances, with corresponding vibrations at the start. For this reason, it is necessary to heed the required time intervals between starts. These by the OEM specified intervals are to be considered already at the time of purchasing the engine ( "Ill. 2.2-2").

An other potential problem is the coking in the oil system by the so called ‘heat soaking’ ( "Ill. 3.5-8"). It develops through the heat flow (conduction, radiation) to the hot parts during stand still. Emergency shut downs can increase the tensile stress in hot parts so much, through extremely quick cooling, that, in these zones, the life of a component is reduced 10 times more than during one start. Even during standstill, a gas turbine can be exposed to absolutely damaging influences. Corrosion during standstill affects especially clearly. If condensation occurs because of moisture and low temperatures and there are compressor corrosive deposits, e.g., out of the marine atmosphere, a noticeable burden of corrosion is present. On the grounds of these requirements, it is obvious that such gas turbines, in particular, are affected that have relatively little running time to show within a long span of time. Those areas of the compressor are susceptible in which low and high alloyed tempered steel are used. Typical are corrosion pittings on materials of type „13%-Cr-Steel. Such steels are called „rust free“, but are only so in a polished condition. This is not valid for typical operation- influenced surface erosion. Affected parts are usually protected with inorganic paints, containing aluminum, which, apart from some erosion resistance, also offers a cathodic, corrosion protection by glass bead peening Since, however, not all surfaces of this kind let themselves be protected, (interference fits, flanges, blade roots, and precision holes are not suited here for this), it remains the task of the operator to avoid conditions that bring about corrosion during standstill, as far as possibe. Further corrosion susceptible components are those made out of magnesium and aluminum alloys in contact with other metals, (e.g., steel threaded inserts ), or abradables made out of aluminum-filled, polyester resin in the front part of the compressor.

In the hot part, where, as a rule, only really corrosion resistant alloys of Ni base or austenitic Fe base materials are used, aqueous corrosion appears against expectations. There is mostly a connection with silver plated screws or nuts ( "Ill. 3.4-4"). The silver of the layers is loosened and deposits onto the neighboring components. During operation at elevated temperature, a corrosion pitting attack (sulfidation) begins. For this, obviously a special atmosphere and /or still unidentified, special deposits are necessary.

This corrosion could have been stimulated through unsuitable grease used during assembly (Chapter 4.2.2-4). In this connection, products containing MoS2 come under suspicion. The sulfur content of such auxiliary materials can be the source of sulfidation. When one is aware, already at the time of procurement (Chapter 1), that significant and unavoidable, noticeable, corrosion conditions are present, it becomes an important criterion for the expert’s choice of engine. He will make sure that the materials of the concerned parts and the technologies used are corrosion resistant.

The operator can contribute in the same way, so that corrosion is no problem, in that he regularly, (possibly from the outside), inspects the compressor for signs of corrosion, such as, e.g., discoloration through rust, peeling or blister like lift- offs of abradables. For this a borescope inspection is convenient.

It is possible to prevent corrosion by taking measures to avoid it. First of all we have to pay attention that no avoidable corrosive media are ingested ( "Ill. 3.1.2.2-1"). An engine temperature at stand still, that prevents the building of condensation water, is one possibility. It is also conceivable, through regular and meaningful „washing of the compressor“ (Chapter 4..2), to ensure that no such corrosive causing deposits accumulate.