Aeroengine Safety

Figure "Brush seals damage relevant features": Damage related influence in brush seals:

Manufacture: Brush types in which the bristles are welded to the frame have a higher risk of the bristles coming free. The bristles may not have been covered by the weld, been melted, or weakened so that they come off during operation.
Uneven density in the brush package or poor angles of individual bristles or whole bundles can be caused by the manufacturing process and can change the wear behavior of the brush.
If the bristles are stuck together by auxiliary materials, such as waxes that coke up when they melt down, it can cause an unallowable increase in the brush stiffness. Machining an exact internal diameter of the brush package is extremely important for the seal effectiveness. Melting procedures, such as laser cutting or electrical discharge machining, can change the brush angles and/or leave hard-to-remove residual particles.

Construction: All geometric sizes influence the functioning of a brush seal. This is especially true for the clearance during installation and operation, bristle height (technical terms see Fig. "Brush seal specific terms"), fence height, thickness and height of the backing plate, and the diameter of the bristles. The bristle diameters have a strong influence on the stiffness of the brush and its operating behavior.
The radial clearance determines the seal effectiveness. The amount of leakage air also influences the tendency of the brush to overheat during rubbing, as well as the blow down effect (pressure closure, Fig. "Preventing bending of brush seal bristles").
It is dangerous if the clearance between the backing plate and the rotor is insufficient, since the rotor can overheat and be seriously damaged during rubbing. If the clearance is too large (too much bristle height), it decreases the tolerable pressure ratios and presents the danger of bristles bending into the gap due to blow out, where they may be broken off or cause dangerous rubbing if the gap becomes smaller (Fig. "Brush seal partner surface damage").
An excessively long brush promotes damages during installation and handling of the brush. It will also wear down very rapidly, so that any benefits it may have on the seal effectiveness will disappear after a short initial period.
If the backing plate is too weak, the axial forces resulting from the pressure differences may plastically deform it (Fig. "Brush seal bristle deformation causes").
If the axial relative movements of the rotor occur against the rubbed-in surface of the rotor, it creates a poorer tribo-system.
Special attention must be given to the installability and removability of brush seals. If necessary, specially adapted tools must be made available. The brush must not be damaged during installation, and the prescribed radial clearances must be met around the entire circumference.

Materials: An important factor for the rubbing behavior is the selection of suitable materials for the tribo-partners, i.e. bristles and rub surface. For example, the wear behavior, tendency to deposit (smear), oxidation resistance, and strength at operating temperatures of the bristles must be considered. The use of brush seals at high temperatures demands especially careful selection of materials. Criteria include brush strength (e.g. to ensure spring-back over the entire operating period) and oxidation resistance.

Figure "Brush seals rules preventing damages": In principle, brush seals have the potential for a considerable safety advantage, compared with labyrinth seals. In labyrinth seals, catastrophic, self-increasing rubbing (Fig. "Labyrinth failure by rubbing") can cause the rotating labyrinth components to separate. The bristles of properly designed brush seals (see Fig. "Brush seals damage relevant features" and Chapter 7.3.3) are worn off during rubbing and/or bristle material is smeared onto the rotor, without self-increasing rubbing.
Characteristics of a poorly configured brush:

• too stiff
• unsuitable material combination - brush/rub surface
  
• insufficient leakage air flow
• overly high pressure ratios (see Fig. "Brush seal limits")
• insufficient fence height
• front plate is too long (top diagram)

These can cause dangerous self-increasing rubbing to occur in brush seals, as well. This causes the bristle tips to melt. Some of the fused material and separated bristles in the brush frame are carried away (detail). The blow-out effect presses softened bristles into the gap between the backing plate and the rotor. The same happens to bristles that have been melted down or broken off.
Evidently, brush configurations in which the front plate and backing plate are almost the same length are especially dangerous. If considerable clearance loss occurs, the brush is pressed into the brush frame. If the volume of the brush frame is not sufficient to accept the bristles, then the stiffness of the brush increases sharply while the amount of leakage air is minimal (bottom diagram). Therefore, the thin bristles are unable to dissipate the developing heat from their tips quickly enough. The heating-up of the relatively radially tall brush and the thin-walled rotor causes further tightening of the clearance gap and intensifies rubbing even more.

Figure "Brush seal bristles explaining damages": Determining causal influences on brush damage is not easy. It is especially important to know and make use of all relevant details. For example, bent bristles (top diagram) pose questions as to the cause, and therefore the time at which the damage occurred. It is also important to distinguish manufacturing flaws from overloads caused by operating factors.
In practice, the geometry of bristle tips (given angle of the contact surface to the bristle tip) can yield important clues (bottom diagrams).

“A”: The normal wear surface is parallel to the rub surface of the rotor and corresponds to an ellipse that matches the shape and location of the brush angle. These characteristics can also be used to determine a normal brush angle and normal functioning.

“B”: If the leakage air flow deflects bristles radially inward toward the rotor (blow down), they will be more worn and slower springing back than other bristles. Because these bristles were at a steeper angle during rubbing, the ellipse is less elongated and does not lie parallel to the rub surface of the rotor.

“C”: If bristles were bent so much in the new part (e.g. due to a manufacturing defect before the inner diameter was machined) that no rubbing occurred during operation, the shape and location of the tip surface will be different from a rubbed surface. If the bristle tip was machined along with the inner diameter, then its cross-section corresponds to that in “A”. In both cases, however, the surface shows no typical traces of rubbing, but rather characteristic signs of the process used to machine the inner diameter of the brush (e.g. laser cutting, wire-cut EDM, grinding).

“D”: “Compressed” bristles (e.g. during installation or due to bristle flutter) have a typical S-shape. Their wear surface is round due to their steep angle of attack. However, unlike “B”, it is parallel to the rub surface of the rotor. If several neighboring bristle rows have a similar bend, it indicates a manufacturing flaw.

“E”: Bristles bent during mounting (reverse rotation of the rotor against the bristles) or during production, but that are long enough to rub during operation, unlike “C”. These bristles have a rounder rubbing surface due to their steep angle (similar to “D”).

“F”: If bristles have normal rubbing surfaces (“A”), but are so bent that they do not make contact with the rub surface, then they were most likely bent due to bristle flutter. They were whirled by vibrations during operation and permanently deflected.

Figure "Brush seal bristle deformation causes": Permanent bending of bristles is a typical damage in brush seals (top diagram). There are various causes of this (compare Fig. "Brush seal bristles explaining damages").

Blow out: The fence height is in accordance with guidelines, and the seal is overloaded (Fig. "Brush seal limits"). The pressure differences at the seal are so large that the brush is plastically bent around the edge of the backing plate into the gap. The bristles are also bent in axial direction in this case, in the direction of the pressure gradient.

If the back rows of bristles (near the backing plate) are twisted and bent, it indicates overloading due to “bristle flutter”(Ref. 7.3.1-13). This danger is especially high in low hysteresis seals. In these seals, the backing plate is machined-out on the inside in order to minimize the friction on the bristles (Fig. "Designing brush seals"). The bristles are extremely deflected by the turbulent flow and twist into one another. In extreme cases, this results in noticeably uneven wear on the brush around the circumference (Fig. "Brush seal problems by turbulence").

Heavily bent and twisted bristles on the pressure side (front rows) indicate a very turbulent and/or uneven flow. These damages occur due to rotating screw heads or nozzles through which air enters (e.g. cooling air inflow in turbine rotors, TOBI).

The blow down effect causes bristles to be slightly bent against the direction of rotation (Fig. "Designing brush seals"). A strong leakage air flow presses the bristles radially inward against the rotor. The intense wear is noticeable on the bristles when they spring back.

Bristles that are sharply bent against the direction of rotation indicate reverse rotation of the rotor during the mounting process or engine shut-down while the clearance gap was bridged.

If several neighboring brush rows are evenly bent or have an improper angle, it is indicative of a flaw during manufacture.

If the brush seal configuration is not stable enough for a high pressure difference, the backing plate may be plastically bent out. The brush packet is bent along with it. Bristles become separated and the backing plate experiences heavy fretting. The bottom diagram shows this type of damage after several thousand hours of operation at temperatures of over 500 °C.

Figure "Brush seal leakage flow": Manufacturing tolerances allow variations in the spacing of the bristles. This creates gaps which interact with the leakage flow. This can cause movement or deflection of individual bristles in the flow. Knowledge of these processes is important for understanding the seal behavior and damages during operation. The leakage flow variations in the middle diagrams (Ref. 7.3.2-1) cause movements and “fluffing up” of the whole brush in the direction of the pressure gradient (Ref. 7.3.2-3). This causes the bristles to bend and twist (Figs. "Brush seal bristles explaining damages" and "Brush seal bristle deformation causes"). If the openings prevent free flow-through, then the individual flows create a self-sealing cross-flow (Ref. 7.3.2-1). The flow at the backing plate, especially, has a powerful influence on the seal mechanism. Bristle movements near the clearance gap are especially important. Here, small movements of the bristles can cause large changes in the amount and stability of the leakage flow. These changes are related to flow-induced strain, prestressing of the bristles, manufacturing problems, backing plate effects, and flow- and boundary layer fluctuations. The pressure ratios through the brush evidently depend primarily on the pressure in front of the brush. The flow through the brush incites self-increasing vibrations in the bristles (bristle flutter; bottom diagrams). Vibrating bristles alter the rubbing conditions and (fretting-) wear occurs between the individual bristles and the bristles and the backing plate (Fig. "Brush seal damaging turbulence"). This results in new, oxidation sensitive surfaces, which experience accelerated material removal due to oxidation. Additionally, local weakening of the cross-sections promotes fracturing of the bristles (Fig. "Brush seal damaging turbulence").

Figure "Brush seal damaging turbulence": Bristles can be damaged in various ways:
Bristle vibrations can be incited by the leakage flow (Fig. "Brush seal leakage flow"). Fretting takes place when the bristles rub against one another or against the backing plate. This local weakening (top left diagram) promotes fatigue fractures. The broken bristle tips can cause erosion damage in the brush or be carried into other parts of the engine by the leakage flow (top right diagram), either as whole fragments or ground dust. In the other parts of the engine, they can cause erosion, bearing damage, or other damages. This type of damage is especially pronounced in brittle bristles (ceramic bristles, e.g. made from SiC).
Increased operating temperatures cause oxidation of the bristles. If the oxide layers spall, they have an erosive effect, and cracks in a brittle oxide layer decrease the dynamic strength of the bristles considerably.
The glide capacity of the bristles is determined by wear, heat development, and friction forces, which can cause burring. In extreme cases, the tips of neighboring bristles can fuse together.
Whirl in the flow in front of the brush can create typical damage symptoms (bottom diagram). These include local tangling of the bristles and increased wear at the tips. This is due to the flow bending the bristles against the rotor, where they are worn down. After they spring back, the typical, seemingly implausible damage symptoms remain. Intensive bristle vibrations can cause so much friction wear in the back plate that it is no longer able to resist the forces from the pressure difference and fractures. In tandem brushes, the first brush in the air flow is at the most risk from this type of damage.
Dangerous whirls in the inflow air are caused (see Fig. "Brush seal problems by turbulence" and bottom right detail) by rotating screw heads or discrete air jets (e.g. from nozzles).

Figure "Brush seal problems by turbulence" (Refs. 7.3.2-1, 7.3.2-2 and 7.3.2-3): Experience has shown that highly turbulent and/or uneven inflow on a brush seal causes tangling and uneven material removal from the brushes around the circumference (Fig. "Brush seal damaging turbulence"). Typical causes of whirl are discrete air flows through openings or nozzles (top diagram). These configurations are found in the air infeed to the turbine rotor for cooling high-pressure turbine blades (TOBI).
Rotating screw heads/nuts in the ring chamber in front of the brush (bottom diagram) also potentially have a detrimental effect on the life span of a brush seal.

Figure "Brush seal bristle tips damage symptoms" (Ref. 7.3.1-2): Increased operating temperatures affect the tribo-behavior of the brush seals. The bottom diagrams depict observations of samples from a wear test. In this test, the bristles were subjected to 15 x10⁶ deflections of about 1 mm at a temperature of 650°C. The glide surface on the rotor was made from uncoated IN 718.
Micrsoscopic inspection of the various bristle materials showed that they had different behaviors, although in no case were dynamic cracks found in the bristles.
Haynes 25 bristles lost material due to spalling of the base material from the “infeed side” (relative to the glide movement). This behavior indicates embrittlement of the bristles during operation. The bristle surface is noticeably oxidized and there is buildup of wear products on the “back side” (also see top diagram). The thick oxide layer of the Haynes 25 material spalls if the bristles are bent sharply.
Inconel 718 exhibited less pronounced damage. However, the glide surfaces of the bristles had considerable foreign-material buildup.
NiCrAlY alloy bristles were the least damaged. The wear at the tips seems to have had primarily a polishing effect. There is no spalling on the bristle surface. Evidently the creation of a protective surface layer prevents material from breaking out.
Of course, these test results are not universally applicable. However, they show important trends. The nickel-alloy bristles can behave very differently opposite the same rub surface. Materials with good oxidation resistance tend to have good operating behavior at high temperatures, even with regard to abrasive wear.
In the end, however, experience from actual operation will be the determining factor for the suitability of the brushes in specific applications.

Figure "Brush seal track damages": Typical damage symptoms of rub surfaces on rotors, which make conclusions as to the specfic damage mechanism possible. The diagram shows a rub surface with a ceramic thermal spray coating (e.g. Al₂ O₃₎).

“A”: Particle erosion causes roughening of the rub surface. Erosive particles can be created by brush wear/anti-oxidation-coating spalling (see Fig. "Brush seal bristle tips damage symptoms"), leakage air contamination (e.g. particles from labyrinth rub coatings or armorings), and from spalling of the ceramic rub coating itself.

“B1”: Primarily axially-oriented crack network outside of the rubbing track (bottom diagram). These cracks are caused by strain differences (heat strain, different E-modules) between the base material and the coating.

“B2”: Large surface spalling down to the base material or bond layer. These outbreaks are promoted by the LCF loads during start-up/shut-down cycles.

“C”: Small spalling in the rub track down to the base material or bond layer. These outbreaks most likely align themselves with thermal fatigue cracks. The cyclical fatigue-induced spalling process is accelerated by rubbing.

“D”: Delaminated areas (outbreaks within the coating) can occur in connection with contraction strain in deposited material and/or temporary high strain in the coating. These types of stress occur during local heating-up due to heavy rubbing.

“E”: Micro cracks and grain spalling (Ref. 7.3.1-2) in the rubbing track (bottom detail). Zones near the surface are damaged if they are run over very often. This damage is usually fatigue spalling, similar to fatigue pittings in roller bearings (Ref. 7.3.3-3).

“F”: Running groove due to coating or rotor wear (middle diagrams). The surface of the groove can appear to be polished. This type of damage occurs when, for example, the bristles are made of a considerably more wear-resistant, i.e. harder material (e.g. ceramic bristles made from SiC) than the rub surface of the rotor.

“G”: Rub tracks in which an overly long backing plate comes into direct contact with the rub track, or in which the clearance gap is bridged by bent bristles (blow out, Fig. "Brush seal partner surface damage"), have deep damage due to overheating. It can also be assumed that the base material has been damaged (e.g. strength loss due to solution annealing).

“H”: Metallic deposits from the brush. If the deposits are thick enough, contractive stress causes crack initiation in the deposited material. This contractive stress can also cause the rub coating to delaminate (see “D”).

Figure "Brush seal partner surface damage": Contact between the backing plate and the rub surface of the rotor is one of the potentially most serious types of brush seal damage. This process is self-increasing, due to the heating-up and heat strain of the rotor. Underneath the rubbing track, the (usually) thin supporting ring cross-section is softened due to the high temperatures. Even insulating ceramic layers do not provide certain protection against overheating (top detail). The results can be spontaneous failures or permanent damage (top diagram).
If the bristle length is too great (i.e. the brush is overloaded by the pressure difference; Fig. "Brush seal limits"), blow-out can bend bristles into the gap between the rotor and backing plate, and cause hard contact between the backing plate and the rotor. Even seemingly minor axial deflection of the rotor can then cause dangerous rubbing. The rubbing process is especially intense if the front plate is drawn too far inward and the entire brush packet overheats and fuses, at least at the tips (Fig. "Brush seals rules preventing damages").

Figure "Brush seal inciting vibrations": Thin rotor rings, which support the rub surface of a single brush, can be incited to dangerous high-frequency vibrations. It is not clear to what extent the brush is causally related to the vibration incitement. In extreme cases, this can cause dynamic overloads, crack initiation, and fractures.
Incitement of vibrations is considerably less likely in tandem brushes than in labyrinth seals (Fig. "Brush seal distinguishes from labyrinth seal").

Figure "Brush seal causing damages": Brush seal damage, just as damage to other machine parts, is often followed by consequential damages.

If an increased leakage rate in the brush seal causes a large change in the pressure ratios around the rotor, it can overload the fixed bearing (see Fig. "Bearing loads influenced by gas-seals").

If brush material enters into a bearing chamber and gets onto a roller bearing track, it can cause premature bearing track fatigue.

If wear products from the brush (Fig. "Brush seal damaging turbulence") are caught in a housing with a powerful circumferential flow, they will travel around with the flow and can, over long operating times, erode through millimeter-thick wall cross-sections (similar to labyrinths; Fig. "Designing surrounding of brush seals").

If brush seal failure reduces the cooling air flow, or brush wear products block cooling air bores, it will cause overly high temperatures in hot parts. This decreases the life spans of these parts considerably (Ref. 7.3.1-10).