Damages and failures by air pollutions are caused by fine dispersed gaseous, liquid or solid media that are carried by the air stream. It must be said here that even filter systems don’t guarantee absolute clean inlet air ( "Ill. 3.1.2.2-7").The limits of filter systems make it necessary to give further consideration to this subject:

Because filters are changed by the retained dust the selection by efficiency of the new condition is not recommendable. It’s better to look at the average effectiveness (Lit 3.1.2.3-1).

Under the term foreign object damages, we understand all macroscopic damages, such as deformations, geometric alterations, crack formation and fractures, which originate through solid or liquid substances, particles or larger objects.

Through roughening and geometrical alteration of the compressor blades, (profile) the compressor efficiency and the flow rate can fall remarkably. Parallel to this, there is the fuel consumption and output performance ( "Ill. 3.1.2.2-6"). These are important parameters for a diagnosis (Chapter 5.1). However, we observe that a similar effect can also be induced by a leakage of a bleed valve. Surface alterations of an aero dynamically effective surface (airfoil) originate through an accumulation of residue and /or through corrosion (Lit. 3.1.2.2-5). Frequently, only certain stages are affected, these are then chiefly responsible for the drop in performance.

Compressor fouling originates through the intake of “sticky“, dusty contaminations in the compressor. It is not necessary that the particles themselves are sticky, frequently an appropriate medium, such as oil, acts as a bonding agent. Such oil can be leakage oil from the front bearing chamber, from a leaking oil cooler, from an oilfilter or an oil tank breather. The deposit rate on gas turbines with inlet filters depend greatly on the quality of the inlet air. If the deposits are corrosive, (salt deposits), the effect can be stronger ( "Ill. 3.1.2.2-7"). The deposits are typically rough and scale like, and are found close to the blade leading edge.

A special form of deposits is the so called “insect roughness“. This happens through the sticking of ingested insects on the blade surface. Thus, e.g., a swarm of insects was drawn in by an engine of small performance by which the centrifugal compressor deteriorated seriously that the engine could not operated anymore without a cleaning process.

The filtering of inlet air is urgently recommended, if considerable air impurities are present (Lit. 3.1.2.2-6) . The latter can be attributed to ambient conditions ( "Ill. 3.1.2.2-1") as well as to the air inlet region (e.g., rust flakes, paint remnants etc.). Air impurities can also be caused by the gas turbine, as a consequence of recirculation of the exhaust gas. Technical literature advises that, when planning the unit (e.g., inlet area, exhaust duct), this should be heeded.

Of significance is, e.g., the arrangement of the neighboring buildings ( "Ill. 1.1-2" and "Ill. 3.1.2.2-1") relative to the favored wind direction. It is bewildering, when, through a recirculation of the exhaust gases in the compressor, deicing is undertaken. In such cases it’s no wonder if strong pollution and corrosion of the compressor blades is observable. Besides the raising of the compressor inlet temperature, an increase in the hot part temperature and/or a down rating of the engine takes place.

The filter itself can also be a source of air pollution. Particles from of fiber mats or filter tiles can damage the hot parts particularly ( "Ill. 3.7.1-2" and "Ill. 3.7.1-3").

It is to be noticed that corrosion inducing materials, are mostly bound with fine dust ( <5.10-3 mm) and demand an appropriate choice of the filter. If not, the filter can have the unexpected effect of stimulating a relative enrichment of pollutants.

The compressor itself can intensify pollutants in that it builds large amounts of deposits on the blades entering the subsequent area of the engine during washing. ( "Example 3.1.2.2-2").

Often, a mechanical, abrasive compressor cleaning is carried out. Fine shells (rice, apricots etc.) are brought into the compressor, implicit is the danger of erosion of soft abradables in the casings ( "Ill. 3.1.2.4-4") or the damage of paints.

It is easy to understand that the use of gas turbines for combustion, respectively, elimination of pollutants (Lit.3.1.2.2-7 and Lit. 3.1.2.2-8), brought into the inlet air is especially problematic and needs a prior intensive examination as to its suitability. Rather it is about a pollution dilution in the exhaust gas. Even if the emission limit is not reached the absolute amount of pollution is not really reduced. In contrast the suspicion exists that the fine distribution of the pollution and chemical processes during heating, the damage potential for the environment will rise. A foregoing intense ability examination of the engine and the purification effect is essential. From such foreign substances compressor and hot parts can be concerned. The "Example 3.1.2.2-1" can here serve as warning. From the OEM a practice relevant operator specific certificate about the endurance of the components under exposure of the inevitable impurities are required. If pollutants, like solvents, show ingredients such as halogen (bromine, fluorine and chlorine) are difficult to detect, long term failures in the hot parts must be expected (see Chapter 3.3. and 3.4). Free halogen can act itself corrosively in the compressor and, e.g., induce corrosion fatigue (Cr-steels) or stress corrosion cracking (Ti-alloys).

In connection with such an observation, it is important to know that many impurities in themselves do not signify a prohibited high failure risk. Whereas they would, in particular situations requiring the elimination of a combination of normal air impurities and media. The component and technologically specific long time compatibility should be examined.

• Painted components like casings and blades of the foreward compressor region made from Mg- and Al-alloys ( "Ill. 3.6.1-5").
• Components made out of Ti -alloys and martensitic steels (blades and discs).
• Casings with abradables (e.g., Al-polyester, "Ill. 3.1.2.3-1").
• Silver plated components like bolts and nuts are endangered by sulfur like in polluted gases or not suitable lubricants.

Particles in the cooling air flow can have an unpleasant effect on the life of the cooled, hot parts. This includes turbine guide vanes and rotor blades with inner cooling, but also combustion chambers with fine holes and slots for the cooling air film or wall structures with multiple cooling layers. The failure mechanism is the following:

Dust particles reach the cooling air flow, whereby, not seldom , a gas turbine’s sensitivity is born already during the time of design. Important is, e.g., the place of extraction of the cooling air and its guidance. Dust begins to block, initially, in especially narrow cross sections,( e.g., dust removal holes in turbine rotor blades, Fig. 3.3.2-4), Tixotrop molten dust, e.g., iron oxide particles, promote this procedure. As a result, the cooling air flow is lessened, leading to a strong life reducing increase of the local component temperature (see Chapter 3.6.2).

Solids ( "Ill. 3.1.2.2-1"):

Dust particles from outside the engine: paint flakes, rust and corrosion products, industry dusts, fertilizer and sprays, cleaning materials, (rice shells, apricot seeds etc.) fire extinguishing powder.

Dust from the engine itself: abradable particles (abradables for blade and labyrinth fins), material from sealing compounds, perhaps from the rotor blades in the compressor.

Liquids:

Wash solutions, pollution enriched condensation, moisture with sea salt (marine atmosphere), oil leakage.

Gases and steams:

Industrial atmospheres, combustion gases, oil fog, fuel mist, steam from cleaning and solvents, fire extinguishing gases, water.

Under the term FOD in the farthest sense are understood in the following all macroscopic causal failures (primary failures) and damages like deformations, geometric changes, cracks and fractures that are initiated by solid or liquid substances, particles and bodies. The origin of foreign objects can lie in front of the engine in the intake area (FOD = foreign object damage), or in the engine itself (DOD = domestic object damage, OOD= Own Object Damage). The last category includes, e.g.,loosened components or failed engine components which lead to notch damages, inciting blade fracture, linked with extensive secondary failures.

The gas turbine does not only take in air impurities, it can absorb massive foreign objects in distinct low pressure zones in the intake area of the compressor (‘ground vortex’, "Ill. 3.1.2.2-3").

Foreign objects in the gas turbine signify a noticeable danger for the engine itself and, in individual cases, because of secondary failures, e.g., rotor fracture or fire, also for the surrounding. Besides, these failures are not seldom connected with huge costs and a long break down of the engine. Amid all this the primary foreign object damage seems harmless and narrowly limited, e.g., a stone impact at blade edge. The subsequent failure, not seldom around many 100 operation hours later, then occurs as a result of a fatigue fracture ( "Ill. 3.1.2.2-4") of the component with extreme, secondary damage. If signs of a foreign object intake deformation exist in the compressor inlet as material deposits or notches, one must definitely inspect the rear area of the compressor (borescope, "Ill. 4.1-8"). If needed the casing must be opened, in order to be sure that no unrecognizable damages are present. Incidentally, there should be regular control for signs of FOD.

The avoidance of foreign object damages is an important task and must be observed during production, assembly, maintenance and service. Requirement to make this specifically possible, it is important to identify the foreign object, its origin and its causes ( "Ill. 3.1.2.2-5").

In general, it can be said that the sensitivity of a gas turbine for foreign objects and their resulting damages in small engines are clearly bigger than in big engines. This is due to the component cross sections and stiffness as well as the component distances and rotational speeds.

Already during the manufacture of the components, dangers emerge that are favorable to foreign object creation. Thus, insufficiently adhering abradables can break out and damage the blades. Since this foreign objects arise from the engine itself, one speaks of a DOD. With the use of screw and rivet joints in the gas flow, ( balance weights, damping rivets, rivets on static labyrinth carriers, locking plugs etc. ) there is, at unknown divergences, e.g., too low torque, missing securing, the potential danger of foreign object damage.

The assembly of the gas turbine in the shop or on site is a foreign body favoring procedure. It begins with forgotten instruments. Remedy: personal oriented labels, registration of the tools, as well as examination of completeness after the task on the basis of a check list. Often, left over screws and small parts are forgotten in the engine. According to experience, engines with lateral air intakes are endangered, as they do not allow visual control of the front compressor area.

Typical failures are notches at the blades through cut off safety wire ends (Fig.3.1.2.2-3). A normal, vertical assembly in the shop is particularly vulnerable in this regard. According to experience, safety wire ends and small parts, that fall in a being assembled engine, cannot be removed any more through upturn and shaking of the engine, because they are to be found in gaps and cavities, e.g., between compressor discs. The strip of at least one module is often necessary. Certainly, one needs to have a lot of insight to be able to report such a case in time. If the engine is operated, the foreign object will be forced into the gas flow, whereby the potential danger of big, secondary damages through later fatigue fractures ( "Ill. 3.1.2.2-4") exists. When one does not have to directly work on it, one should take care that the compressor inlet is always securely covered. This is compulsory during vertical assembly.

The long term result of unnoticed foreign object damages is especially malicious in compressor and turbines. As already described, small foreign objects can originate in the compressor through impacts, but also through typical corrosion pitting. Local notches, producing risky stress concentrations, dangerously increase the unavoidable, normally present, low vibration load clearly under the fatigue limit. It can take thousands of operating hours before a sufficiently high number of vibration cycles accumulate, as these appear only briefly in a transient operation phase ( "Ill. 3.1.2.2-4"). So, the time between the occurrence of failure and the establishing of the secondary failure is very long, needing failure analysis to prove these connections.

Not only hard parts are dangerous objects. Even a cleaning rag can destroy a compressor totally. Especially endangered are gas turbines of lower performance (102 up to approx. 103 KW), with axial compressor stages. Engines of smallest performance range find even paper handkerchiefs a problem. Engines, in comparison with centrifugal compressors, are clearly more robust.

The intake of foreign objects during operation is also possible with preliminary filters, e.g., when they themselves get destroyed. But all objects in front of the engine, capable of being sucked into it, can get into the compressor.

Not to be underestimated is the destructive effect of larger ice impact ( see chapter 3.6.6.3), that can be formed through the icing of the engine. Thus, an engine, e.g., in the 4000 KW range can undergo total damage of the compressor through a hand formed snow ball with an average diameter of 10 cm, infiltrating the running engine. The massive ice deposits, appearing on iced surfaces are to be considered correspondingly more dangerous.

Foreign objects are the cause of extensive failures and related high costs and it is of special concern to the operator that they be avoided. Precautionary measures and remedies on the grounds of concrete cases of failure are important. A requirement for a well aimed and successful solution of the problem is to establish the origin of the foreign object: this again presupposes its identification, which is invariably not always simple. Extensive compressor failures with blade fractures demand expertise, in order to recognize and prove the actual, primary releasing impact ( "Ill. 3.7.1-5"). If there are merely impacts and deformations present in the front area of the compressor, there is the possibility of finding parts of the foreign objects in the combustion chamber area (strainer effect of the combustor) and to identify them.

Relatively frequently, screws, nuts and washers emerge as foreign objects. If the length and type of thread can be ascertained from the impact diameter, the necessary identification for avoidance is often successful ( "Ill. 3.1.2.2-5"). So one can already establish if it is a metrical or an inch-thread and if the object is from the engine or surroundings. Frequently, in addition to the impression, residue of the metallic foreign object remains stuck onto the rotor blades and is relatively easy to identify.

Usually, in cases where many similar foreign objects are to be considered, one would compare all geometrically and analytically similar objects in the engine area. According to experience, this procedure has a high chance of success. Next to the impression, the micro analytical comparison of the contact surfaces with the uninfluenced neighboring component surfaces is a further important aid ( "Ill. 3.1.2.2-5"). As a metallic foreign object in the micro range welds together with the metallic surface opposite, a non- metallic foreign object frequently leaves particles on the opposite surface. One can then draw conclusions as to the composition of the foreign objects through comparative analysis.

3.1.2.2-1 M.K.Pulimood, „Field Experience With Gas Turbine Inlet Air Filtration“, ASME Paper 81-GT-193 (1981).

3.1.2.2-2 M.C.Manna, H.v.E. Doering, J.R. Patterson, „Experience and Application of Gas Turbine Inlet Air Filters“, ASME Paper 75-GT-105 (1975).

3.1.2.2-3 T.M.Higgins, R.J.Freuler, Ohio State University, „Experimental Determination of Bulk Swirl Attenuation Between Two Axial Stations in the LM2500 Inlet Bellmouth“, AIAA Paper 93-2203 (1993).

3.1.2.2-4 J.P.Stadler, P.v. Oosten, „Compressor Washing Maintains Plant Performance and Reduces Cost of Energy Production“, ASME Paper 94-GT-436 (1994).

3.1.2.2-5 N. Czech, „Korrosion und Beschichtungen“, Kapitel aus C. Lechner, J.S. Seume „Stationäre Gasturbinen“, Springer Verlag, ISBN 3-540-42831-3, Page 746 up to 748.

3.1.2.2-6 A.Rossmann, „Die Sicherheit von Turbo-Flugtriebwerken“, Band 1, ISBN 3-00-005842-7 , 2000, Axel Rossmann Turboconsult, Bachweg 4, 85757 Karlsfeld.

3.1.2.2-7 U.Benthien, „Schadstoffbelastete Abluft aus chemischen Produktionsanlagen als Verbrennungsluft für Gasturbinen“,ASUE 13, Page 61-65.

3.1.2.2-8 K.L.Sauer, „Gasturbine als thermische Verbrennungsanlage für lösungsmittelhaltige Prozessabluft“, ASUE 13, Page 66-72.

3.1.2.2-9 M.K.Pulimood, „Field Experience With Gas Turbine Inlet Air Filtration“, ASME Paper 81-GT-193 (1981).

3.1.2.2-10 A.Rossmann, „Die Sicherheit von Turbo-Flugtriebwerken“, Band 1, ISBN 3-00-005842- 7 , 2003, Axel Rossmann Turboconsult, Bachweg 4, 85757 Karlsfeld.