Table of Contents
3.1.2.4 Sealing problems and establishment of clearance of rub systems
Degradation of the seals in a gas turbine does this not lead to a comparable damage like the failure of the cylinder head gasket in our car with a breakdown shortly after a drop of power. The effects in a gas turbine are rather subtile. It comes also here to a efficiency loss but without at once observable failure. Thereby the fuel consumption of the gas turbine grows slowly. It will be clearly higher to get the same power output like new. This is also necessary to reach the required performance. It becomes additional noticeable in a higher gas temperature, hence with higher hot part temperatures. We keep in mind that a temperature rise of about 15 °C can bisect the lifetime of our expensive high pressure turbine rotor blades. That means besides the higher fuel it is also to reckon with higher repair and overhaul costs untill a sudden damage occurres. This shows the similarity with the at the beginning mentioned cylinder head gasket.
As shown in "Ill. 3.1.1-2", an especially minimization of leakage loss is important for the desired operation of the compressor. Hence this problem will be addressed here, although seals are present in the entire engine and have important tasks to fulfill everywhere.
Tip clearance
Tip clearance is the gap between the blade tips of the rotor and casings, respectively, between the guide vane (stator vane) tips and the spacers of the rotor ( "Ill. 3.1.2.4-1"). The clearances change according to the operation condition ( "Ill. 3.1.2.4-2" and "Ill. 3.1.2.4-3"). As presented in chapter 3.1.1., these clearances are of immense significance for the degree of efficiency and the operation behavior of the compressor, because of their influence on the leakage losses and the flow in the outer blade area. Due to the increasing pressure niveau and the dependent decrease of the blade size, the clearance influence in the rear compressor area is especially big.
Basically, a constant minimum clearance is desirable over the entire operation region. This is unattainable in practice. Through suitable measures one can successfully influence the clearances in such a way that the guaranteed operation data is maintained. If wanting to understand the design specialities of the engine and the resulting operation behavior, one must know the parameter of the clearance formation. Thus, the procedures at the rotor and at the casings will be described.
Expansion behavior of the casing
( "Ill. 3.1.2.4-3") Casings are usually, especially from derivates, relatively thin walled structures, behaving clearly less inertly to in comparison to thick walls of a ‘heavy frame engine’ during heating up through air flow. Additionally, like a pressure boiler, they experience a noticeable elastic expansion through the compressor pressure that clearly increases over 40 bar end pressure in modern engines. This expansion occurs in synchronization with the build up of pressure and with the centrifugal force expansion of the rotor. In order to delay the quicker heat expansion opposite (in contrast to ) the rotor, different designs are made:
- The applying of masses onto the casing.
- Isolation of the casing on the inner side.
- Double walled structures, whereby the inner (the air flow side) behaves differently compared to the colder, pressure absorbing outer wall.
In rotors, the expansion is concentrically symmetric, this is not so with casings, depending on the design. In casings divided lengthwise, certain deviations from the roundness, on the grounds of stiffness differences (horizontal flange) at the periphery, are to be expected.
Expansion behavior of the rotor
( "Ill. 3.1.2.4-3") The rotor expands during the operation centric symetric. During standstill however a nonuniform heating leads to a distortion, the so called rotor bow ( "Ill. 2.2-1"). When running up the engine, the centrifugal force rises with the square of the speed. Simultaneously, the rotor warms up, on grounds of compression temperature of the airflow. In modern engines, the temperature increases from room temperature at the inlet to approx. 600 °C at the compressor end. The centrifugal force leads, synchronous with the speed rise, to an elastic expansion up of the rotor components. The heat expansions follow clearly more slowly in contrast on account of the heating, depending on the rotor masses to be heated. In order to fit the time dependent course of the rotor heat expansion better to that of the casing, the rotor interior is often supplied with air (ventilation) from the compressor. It takes some minutes for derivates and small engines, clearly longer for heavy built engines, until the rotor is fully warm and a steady state temperature distribution has been adjusted, through which an expansion alteration does not follow anymore. During deceleration and /or shut down of the engine, the elastic expansion of the engine lessens with the speed. On grounds of the cooling off inertia of the rotor, the shrinking takes place more gradually. In static, as a consequence of the heat convection in the compressor, such big differences between the warm upper rotor part and the colder lower area can arise that the rotor bends and the clearance to the casing is bridged over. The rotor ceases for some time (see "Ill. 2.2-2"). The rotor can , however, through unbalances, vibrations and deflection, displace against the casing, influencing the clearance symmetrically or asymmetrically.
When rub of the rotor blades occur, these will be heated up in the tip area and experience a short term, additional heat expansion. This explains the phenomenon that during a measurement control just the shorter blade is discovered to be especially rubbed. Thus, also under normal conditions, the machining work is taken over by one blade; so shortened, it does not participate in the next rubbing.
Clearance condition through rub systems - Rub system rotor blade/casing
In older gas turbines, the assembly clearance is held sufficiently wide to hinder the different deformations between rotor and casing from provoking rub. For modern engines with continuously improving degrees of efficiency, increasing aero dynamic load of the blades and increasing pressure niveau and pressure ratio, ( at the individual stage as well as on the overall compressor), large clearances are not usable any more. Here the assembly clearance is consciously so narrowly chosen that rub of the tips during the operation of the engine is inevitable. The contacted casing walls show cutting in capable coatings, in which the blade tips are worked in by metal cutting. In this way, at least for one operation point, preferably for the one which is the most used, a small clearance is guaranteed. A problem is presented by the chip and/ or dust formation. Depending on the coating material, the chips, respectively, the dusts can act erosively at the following stages, in gas flow direction and/or narrow or block cooling air holes in the hot parts ( "Ill. 3.3-12").
The soft abradables are, necessarily, a compromise. On the one hand, during rub of the blade tips, they should not damage thermally or dynamically, i.e., their structure must be good and practicable . On the other hand, typical operation loads, like erosion, blade passing frequency and high frequency mechanical vibrations of the casing wall must be withstood over a long period of time. These different demands (good cutting properties, i.e., low hardness, but possibly high mechanical strength) lead, in the usual spray coatings, (e.g., nickel-graphite- plasma spray coating) to different failure pictures. Less sensitive are coatings made out of aluminum that are especially used in older engines. Here, the rub procedure is foreseen only as an exception.
Abradables are influenced by the ambient atmosphere. Free graphite in nickel-graphite-coatings begins to oxidize noticeably already at temperatures under 400 °C. This aging changes the abradable. It becomes more brittle, less erosion resistant and displays a deteriorated cutting in behavior. Coatings on the basis of Al filled polymer (Al/polyester) are used in the front area of the compressor and are known for their corrosion sensitivity during static under condensation of marine atmosphere ( "Ill. 3.1.2.3-1"). Similar problems can also occur with metallic Al spray coatings. The damages are, according to experience, relatively small.
Coatings out of elastomeric materials are only used in the front compressor area. Typical is filled silicon rubber, similar in consistency to india rubber. They can be attacked by different media like fuel, oil, unsuitable detergents and solvents. Damage through loss of strength, swelling or delamination occurs. Normally, the manufacturer has tested the coating resistance against all the normal media. Despite that, one should check with the manufacturer that no objections against possible operation/ operator specific impurities exist.
Rub system stator vane/rotor
The relative schort vanes in the rear of the compressor are often stiff enough to build the tip gap also without shrouds against a smooth spacer (distance ring between two rotor stages, "Ill. 3.1.2.4-1"). There are vanes with shrouds in two different design principles. Rings which connect all blades or segments with several vanes. Those seal faces serve as contact surfaces for the labyrinth tips. Here emerge typical labyrinth problems ( "Ill. 3.1.2.4-6" and "Ill. 3.1.2.4-7.1").
The clearance between the guide vane tips and the rotor should not be neglected and must also be minimized, although they are not of the same importance for the efficiency of the compressor as the clearance in the system rotor blade/casing ( "Ill. 3.1.2.4-1"). As the liners on the periphery surfaces of the rotor or the rotor spacer are deposited and, thus, feature high centrifugal forces and expansions (elastic expansion of the rotor through centrifugal force during acceleration as well as thermal expansions) they are usually implemented as hard, abrasive acting linings. Frequently, there are plasma spray coatings made of Al2 O3 or ZrO2 . In this case, the guide vane tips are ground.
The coatings of the spacers are spalling endangered. Should this happen, the blades can be damaged through the hard fracture bits. If the coating covers corrosion sensitive materials (steels), during static, condensation can penetrate through (manufacture dependent porous) the spray coating to the base material and act corrosively, weakening the bondage.
Structured casings
Often the casings over the blade tips are provided with a special geometrical structure (casing treatment). This has a rub function only in some cases, where one would like to avoid the load of the blade tips through cutting work. This structure should preferably be aero dynamic effective. In a simple case, it may be just a circumferential step groove ( "Ill. 3.1.2.4-1"). There are, however, also arrangements with a multiplicity of slanting grooves. Engines that are of Russian design frequently have a surrounding hollow chamber over the compressor stage, open to rotor blade tips through slits (“ Ivanow arrangement”). All these designs have the task of desensitizing the compressor against the inlet disturbances and of mitigating the negative effect of the big tip clearances. Thus, the surge behavior of the compressor can be noticeably improved. Not until now the designers succeeded to improve both effects, the surge margin and the efficiency.
Labyrinths
Labyrinths are, up until now, the most frequently used method to seal off cylindrical surfaces with relative movements against each other. Coupled with this, there is a certain leakage flow to be reckoned with. Depending on the principle, it should be constantly maintained or minimized during the entire operation. It is also unavoidable that during rub, abrasion on the fins and/or the coating emerges, leading to more air leakage in continued service, as a consequence of clearance enlargement.
Labyrinths are used in many areas in the entire turbine ( "Ill. 3.1.2.4-5"). In the compressor area, they work as sealing elements against the stator vanes and as one of the most important seals of the engines, the compressor exit seal. This closes the air flow at the compressor exit against the rotor region and must control an especially big pressure difference at high temperatures.
Labyrinths have a multiplicity of tasks in the gas turbine:
- Avoidance of not prohibited leakages of air, air/oil mixtures( "Ill. 3.1.2.4-5") and hot gases ( "Ill. 3.4-5").
- Guarantee of permitted and necessary axial bearing loads ( "Ill. 2.5-1").
- Guarantee of the cooling air delivery to hot parts ( "Ill. 3.3-11").
- Apportioning of cooling air as aimed leakage (e.g. for disc cooling, "Ill. 3.3-11").
Labyrinths are loaded in many ways through operation conditions. The design dependent, highest and most dangerous load is the rub of the labyrinth fins on the clearance forming opposite surfaces ( "Ill. 3.1.2.4-6"). This occurs, mostly, with relative speed, from 101 - 10² m/sec and corresponding short term local heating up ( to melting point). Hence, it is of great importance that the rub surfaces have a certain cutting in capability and that the labyrinth fins are concerted with them in form , material and, perhaps, tip coating. The problem lies exactly here, should such coatings lose their cutting in capability through aging after long running . This problem is to be observed in engines built like modules (see Chapter 4.2.1.). Where there are unfavorable rub conditions, (not necessarily high adjustment speed), an accelerating failure mechanism that leads to a total loss of the labyrinth can occur. On labyrinth fins with hard coating, (e.g., plasma spray coatings out of Al2 O3 or WC), there is less danger of prohibitive rub or a damage through crack and/ or strength drop, as opposed to uncoated fins, but rather the risk that the coating slivers ( "Ill. 3.1.2.4-7.1") and damages other parts. Often in just a position with production problems.
Brush seals
( "Ill. 3.1.2.4-8") In modern gas turbines, brush seals emerge more and more into use. Basically, this is related ( "Ill. 3.1.2.4-8") to the usual sealing principle of the swing door. The arrangement is annular, with radial, inwardly directed bristles made out of corrosion resistant wires. These seals have important advantages over labyrinth seals.
- In the case of rub, there is no catastrophic failure, because of the elastic, radial deflections and the brush structure.
- Because of the elastic deflection of the bristles, there is almost no rub; the sealing effect is hardly influenced for the further operation time.
- The brush seal necessitates an axial , smaller assembly space than a labyrinth seal of similar effect (1 brush corresponds to approximately 1 labyrinth sealing with 3 to 5 fins).
Practice (Lit. 3.1.2.4-2) shows that the brush seals have typical problems, especially during long term operation:
- In the end stadium, the leakage losses are often higher than those of a labyrinth seal.
- Damage and break out of the brush hair through air vibrations.
- Wear of the backing plate when the bristles rub during vibration excited by leakage air or/and whirl before the seal.
- Damage (bending up) of the brush while turning back the rotor in static, (e.g., during assembly or inspection).
- Damage of the contact surface sealing surface (break outs, roughening).
If brush seals are used in an engine, the operator should demand sufficient proof of long life in his conditions of operation.
"Illustration 3.1.2.4-1": Clearance in the compressor between rotor blade and casing (G1), (G2) and between rotor and guide vane tips (R1), (R2) are of special importance for the efficiency ( "Ill. 3.1.1-3") and the operation behavior ( "Ill. 3.1.1-2") of the compressor. There is a multiplicity of designs and their combinations related to this. The picture shows typical (G1) rotor blades opposite a deepened annular groove in the casing (structured wall ). Such a geometry is also gladly used in the turbine. Axial split casings lend themselves to this style. The tip clearance is kept so large that no rub occurs. A sufficiently big, axial gap is planned, in order to make axial rotor movements during load changes (e.g. shut down or full load, "Ill. 2.5-1") possible. The advantage of this option, as opposed to the unstructured casing wall, (G2) is that there are less clearance losses. This advantage is always more noticeable with larger clearances (diagram right).
Casings with this configuration allow rub and cutting in of the blade tips if the abradable is correspondingly soft (Ni/ graphite spray coating, Al spray coating). The guide vane tips can be equipped with a shroud containing an inlet coating, stiffening the tips opposite the labyrinth fins of the rotor (R1). Shorter guide vanes in the rear compressor do not normally have a shroud. Opposite them one finds a rotor, mostly in the form of a spacer, with an abrasive, hard, invariably ceramic spray coating made of Al2 O3 or ZrO2 and abrades with blade contact. Combinations that incite wearing off of the blade tips by contact can influence the maintenance costs remarkably, if the blades , aiming at the acceptance criteria of the engine, are too short and then of no use at least without costly repair (build up, weld and rework).
"Illustration 3.1.2.4-2": Even after shut down, the clearances change through axial and radial relative movements of casings and rotor. In Lit. 3.1.2.4-1, there are details to be seen that stem from the clearance measurements of a flight engine. Relative movements of many millimeters appear. Only after many hours does the position of the blade tips relative to the casing not alter any more. The more massive the rotor and casings, the slower their temperature behavior and the longer the time intervals, until there is no further relative movement. Differences between heavy engines and engine derivates lie at hand. This explains why rotors sometimes get stuck some time after shut down of the engine and release themselves only after a long time.
This situations force an engine specific time window in which can not be started without the risk of extensive damages (Chapter 2.2).
"Illustration 3.1.2.4-3": To keep the clearances between rotating and static components during all operation conditions is a difficult task. Heat expansions, as well as expansions under centrifugal forces and pressure alterations are very different for the neighboring components. Too large clearances deteriorate the efficiency as a result of leakage; too heavy rub bodes the risk of a dangerous failure of the components (crack formation, overheating, wear).
In the illustration, the radial expansion of a rotor blade tip is shown opposite the casing expansion. Coupled with that, because of typical design features, so called heavy frame engines behave clearly differently compared to engine derivates ( "Ill. 2.1-7"). This explains the diverse start and shut down times of both design principles.
A thick walled casing heats up slowly and, due to the inner pressure, expands less pronounced than a thin wall casing. The heat expansion of the rotors is normally slower than of the casings because of the relatively massive disc cross sections. The typically thin casing walls for derivates may favor this effect. The radial expansion of the rotor, on the grounds of the centrifugal forces, is immediately effective during acceleration, since it depends on the disc load. This expansion may be larger in derivates than in engines of a heavier design with lower disk load.
After the start, the assembly clearance is diminished through the prompt centrifugal force expansion of the rotor. The centrifugal force increases with the acceleration of speed and, with it, the expansion -quadratic with the speed. At point „ B“ contact with rub is possible. Usually, this process occurs during run in of the new engine, leading to abrasion of the contact surfaces and does not emerge noticeably during later normal operation. When the rotor decelerates, the clearance widens correspondingly quickly. If, after a long deceleration phase it is speedily accelerated again to full load, the casing has cooled off a little, the rotor expands quickly and it comes to point “V “again, to a potential rub situation.
Generally, one can conclude: thin casing walls and light rotors, like those typical for engine derivates, are less problematic during instant starts and load changes than heavy type engines. Therefore: Use derivates during peak load.
"Illustration 3.1.2.4-4": (Lit 3.1.2.4-3, Lit 3.1.2.4-4): illustrated are typical failure attributes of soft rub in coatings like thermal sprayed Ni-graphite (upper detail) and with metal (typical Al) powder filled resin (lower detail). Such coatings are preferred in compressor casings.
Abrasion and wear („A“): Soft abradables (abradable coatings) suffer due to their function during rub a remarkable abrasion. The surface of this wear face is at the beginnig relatively smooth, after some operation time ist is normally roughened by erosion and hardly to identify as rub surface.
Erosion („B“): With the air stream of the gas turbine sucked in or generated particles (e.g. abrasion by the rub in) soft abradables can heavy erode in some hundred operation hours. This erosion surface is normally rough with a structure, oriented in flow direction. A periodic division meets the spacing of the vanes. Aged (oxidized) abradables can erode accelerated, even by the air stream.
Material smearings at the rub surfaces („C“): They are unnormal by rotor blade tips at soft abradables. If they occur shows this a tribological system with poor rub in characteristic. Is the abradable rub to the base material or an bond coat, this can lead to pronounced smearing with signs of high friction temperatures (discoloration, bulges).
Impact damage („D“): Sucked in foreigen objects (FOD) or in the engine originate pieces (OOD, e.g. chipped of hard coatings, fracture pieces of blades or probes) can cut bigger notches in the abradable. Those features can help on the identification of the primary failure (location of the development of a fracture).
Coating breakout („E“): Several operation conditioned reasons lead to coating breakouts: Vibration fatigue (acoustic fatigue) due to the „blade passing frequency“, vibrating casings, thermal expansion and strains by mechanical loads. Typical production faults are high internal stresses or a poor adhasion ( "Ill. 3.1.2.4-7.2"). The position of the fracture face, relative to the base material, to the bond coat or the abradable itself can provide a first hint to the cause of the separation.
Phenomenon of a separation („F“): Does the abradable shows the formation of cracksoutside edge parallel to the bond coat than the danger of a larger breakout exists. This cracks can be due to the production and/or the the opreation. A microscopic investigation of the opened crack surfaces gives important hints at the cause of the failure (e.g. „bead problem“, "Ill. 3.1.2.4-7.2").
Aging of the abradable („G“): Do microstructural components of a multiphase coating oxidise or react (upper detail, schematic pictures: G1 = graphite , G2 = nickel particles, G3 = pores), the inner strength of the coating composite can decrease. That promotes erosion and/or a poor rubbing behavior. Thereby blade tips get overheated and/or vibrations excited.
Corrosion („H“): Some coatings respectively certain coating components are prone for corrosion. Typical example is Al-powder (H2) filled polyester resin (H3). Into fissures and racks (H1) penetrating sea atmosphere leads to corrosion of the Al-particles. The results are blistering and/or crack formation with the breakout of the coating ( "Ill. 3.1.2.3-1").
"Illustration 3.1.2.4-5": The gas turbine not least „lives“ from pressure differences in different component areas. This is important over the whole operation and significant for the function of the air system of the engine. Operation behavior and efficiency as well as the durability of crucial components are affected by the function of the labyrinths ( "Ill. 3.1.1-2"). We find labyrinths in almost all areas of the engine. This includes, besides the compressor especially the cooled hot parts and the main bearings. The picture shows labyrinths with its typical functions and arrangements at the compressor entrance of a derivat engine.
Sealing of the bearing chambers against oil leakage. Mostly in connection with sealing air and a convenient pressure niveau in the sump room.
Interstage labyrinths to seal against a back flow of the compressor air.
Labyrinths that influence the ventilation of the rotor respectively its discs. In the hotter part this affects amongst others the temperature respectively the life of the discs ( "Ill. 3.3-7" and "Ill. 3.3-11").
A further task of the labyrinths is to assure the design corresponding pressure niveau in the disc areas to meet tolerable axial bearing loads ( "Ill. 2.5-1"). Thus those labyrinths are important to reach the destined bearing operation time.
"Illustration 3.1.2.4-6": An important, if rare, failure mechanism leads to the catastrophic failure of labyrinth seals. The failure begins mostly with a light (not necessarily heavy) rub of one or more fins. When no cutting follows, through material removal or cutting in, fin material is smeared onto the area of rub. The material build up increases in a fracture of seconds. Hence, the process is increased alternately, with a heating up of the rub area, bending up the weakened rotor part and further aggravating the damage. Frequently, axial tearing and then circumferential fracture of a ring section of the labyrinth carrier occurs. The affected fins determine the ring width. Linked to this, structures can be formed on the rubbed surface that are similar to the thermal spray coatings. These kinds of damages are particularly probable, if the abrasion behavior of the abrading partners is bad. For example, if the abradable coating undergoes prohibitive deterioration, through aging during operation. Also not hard coated or too smooth coatings promote this type of failure.
"Illustration 3.1.2.4-7.1": Characteristic, load-specific damage symptoms in labyrinth fins: The depth of the wear and the circumferential length of the wear zone depend on the radial infeed motion. Infeed is caused by local distortion, shaft deflection, imbalances, or vibrations. Wear can occur along the entire circumference or only in a limited area. Typical symptoms in a rubbing zone are burring (“B“) and tarnishing or increased oxidation. These characteristics also indicate damaged sections of material (e.g., decreased hardness/strength; “D“). If the grain boundaries in these zones soften or melt, even small tensile stress can cause hot cracks. These cracks occur individually or in crack fields (see also “H“). They advance (“C“) due to sufficiently high LCF- (thermal fatigue) or HCF-loads (labyrinth vibrations). If material deposits remain on the fin (smeared “E“), they can increase the rubbing process to dangerous levels ( "Ill. 3.1.2.4-6").
Erosion particles in the shape of spalled rub coatings or removed material put erosive stress on the fins (“F“). The symptoms of this are similar to “peppering“ (the impact points of the individual particles are still recognizable).
Spalling or local delamination of hard-material armor (e.g. Al2 O3 ) is a common type of damage (“G“) and should be considered in connection with manufacturing flaws and/or handling mistakes (transport, mounting).
Not even armoring provides absolute protection against hot cracks (“H“). This is especially true if the armor is not sufficiently cuttable (which may be indicated by the coating roughness) which causes too much heat to be created in the rubbing zone (also see “C“).
The labyrinth fin materials that, for example, are exposed to the hot gas flow due to the leakage air in the turbine area, must be sufficiently resistant to oxidation. In oxidation-sensitive, highly heat-resistant materials or unsuitable welded deposits, oxidation damages have been observed that are similar to the orange-peel effect on the intake edges of overheated turbine blades (“I“). A typical symptom are many small cracks due to thermal fatigue caused by start-up/shutdown cycles.
"Illustration 3.1.2.4-7.2": The „bead problem“. This is a matter of a frequent production caused problem dealing with debonding respectively breake outs of thermal sprayed coatings. The effect is similar to the mechanism that accounts for a poor adherence of adhesive strips on a dusty surface. Lay ricocheted small solidified molten drops from the thermal spray process on the to be coated surface, the bonding strength suffers enormous. When failures of thermal sprayed coatings occur first of all is to look (scanning electronic microscope = SEM) at the debonding surface if such agglomerations of „splash beads“ exist.
"Illustration 3.1.2.4-8": (Lit. 3.1.2.4-3): We know brush seals from seal swivel doors. In brush seals for cylindrical geometries, the brush is bent to a ring, whereby the bristles point inwards in a radial or inclined direction. The bristles are made of thin, highly strong corrosion resistant wires. These are held in different ways, depending on the manufacture, in a supporting ring. On the side with the lower pressure there is a backing plate for the support of the brush against axial deflection through forces due to pressure differences. The brush is usually fixed onto the static portion. The rotating part comprises a cylindrical slide and sealing face that frequently reveals a special coating. With a brush sealing, one can attain almost the same sealing effect as with a good five tooth labyrinth seal. This enables advantages in the assembly region.
The well designed brush seal has the big advantage of not indicating a catastrophic failure behavior ( "Ill. 3.1.2.4-6") when there is an unfavorable rub, in comparison to labyrinth seals. When there is a deflection of the rotor, the brush yields elastically and thus avoids too much wear.
Brushes show a typical operation behavior. Elastically deflected bristles position themselves back again, mostly when there is a low pressure difference (hyteresis), if the friction within and against the backing plate is sufficiently low. Operation temperatures above 500 °C invoke oxidation problems if special measures are not taken. Strong flow turbulences in front of the brush can stimulate vibration and wear of the bristles. So it comes to the damage of the bristle with typical failure symptoms.
Literature of chapter 3.1.2.4
3.1.2.4-1 P.A.E. Stewart, K.A. Brasnatt, „The Contribution of Dynamic X-Ray to Gas Turbine Air Sealing Technology“, AGARD-CP-237 (1981).
3.1.2.4-2 S. Ingistov, „Compressor Discharge Brush Seal for Gas Turbine Model 7EA“, Proceedings of ASME Turbo Expo 2001, June 4-7, 2001, New Orleans, Louisiana, page 1-8.
3.1.2.4-3 A.Rossmann, „Die Sicherheit von Turbo-Flugtriebwerken“, Band 1, ISBN 3-00-005842-7, 2000, Axel Rossmann Turboconsult, Bachweg 4, 85757 Karlsfeld.
3.1.2.4-4 A.Rossmann, „Die Sicherheit von Turbo-Flugtriebwerken“, Band 2, ISBN 3-00-008429-0, 2000, Axel Rossmann Turboconsult, Bachweg 4, 85757 Karlsfeld.