Aeroengine Safety

Fig. "Face seals for bearing casings" (Lit. 23.4.2.2-12, Lit. 23.4.2.2-13 and Lit. 23.4.2.2-14): Face seals consist of five typical elements (face seal, sketch above right). These can be designed in different ways. Every element has its specific problems.

„1“ Sliding ring: This part of the „split ring type” (sketch below right) can be, depending from the design, rotating or static. At the`face type' the sliding ring is static (sketch below left). Its material is usually relative `soft'. Typical are graphite/carbon mixtures. These are compressed and/or mixed for forming with binder. From this production, e.g., the binder and the porosity depends the sensivity against oil. Reactions with aeroengine typical, hot synthetic oil must not occur. The oil type (Fig. "Compatibility of oils with materials") is also in an other aspect crucial for the operation behaviour of the seal. Is the oil not sufficient temperature stable, a coking in the sealing gap can deteriorate the sliding ring (Fig. "Deteriorating effects by oil coking" and Fig. "Failure modes of axial face seals").
Frequently the carbon sliding ring is kept in a bearing structure. This guarantees the mechanical robustness. The type of joining, force fitted or bonded, influences the danger of distortion of the sealing face and with this the risk of a leak (Fig. "Failure modes of axial face seals").

„2“ Rotating sealing face (shaft): It contains of a harder material than the sliding ring. Concerned are hardened steels, hard metal/carbides like tungsten carbide. The selection also depends from the corrosion influences. The `natural' wear of the sealing face produces fresh reactive metal surfaces and so promotes corrosion. This is also true, if the wear is rather polishing or occurs as fretting at vibrating contact/guiding faces. Thereby attention must be payed at possibly touching corrosion cell forming metals like copper, brass, nickel and cromium. So the danger exists, that the Co/Ni-matrix of a TCcoating will be selectively etched out.
The material of the bond of coatings or brazed slide shoes must bear the expected overheating (hot run). Thereby the thermal stresses must not lead to crack development and/or separating/flaking. Also force-fitted and/or form-locking fixed ceramic inserts must take such conditions. Such a material is pure silicon carbide (SiC), sintered or reaction sintered. Important for the function is the transfer of a thin carbon sliding film from the graphite slide shoe at the rotating sealing face. With this property, the chance for an emergency operation (dry-running) during a short time drop out of the oil film in the sealing gap is given.

„3” Contact pressure system/spring: The necessary pressure force takes a mechanical spring and/or the inner pressure. The contact pressure force must not be too high during operation. Only so, a sufficient lubricant film can develop. However, it also may not be too low and with this endanger the sealing effect. Deposits at the centering face and guidances of the slide ring or setting of the springs can lower the contact pressure force.
The material of the contact pressure spring must withstand corrosive influences. To this also count decomposition products and contaminations of the oil (chapter 22.3). Especially dangerous are halogens (chlorine) for springs from stainless steel (Cr-Ni steels). These can produce notches (corrosion pittings) and/or cracks, caused by stress corrosion.

„4“ The seal/guidance of the slide ring serves to prevent a leak through bypassing the slidering. Depending from the operation conditions, O-rings or piston rings are used. O rings are markedly temperature limited. Therefore at main bearings piston rings are used. These must guarantee over long operation periods sufficient sliding properties. Jamming/blocking or stick-slip-effects must not occur. This requires an acceptable wear behaviour (fretting) of the tribological system.
Does the sealing face not close, contamination particles with leakage oil can intrude in the sliding face and produce unacceptable wear. At elastomers in face seals all material specific failure modes like swelling and cracking can appear (Fig. "Influences of O-ring materials").

„5” Prevention of the rotation of the sliding ring: Static sliding rings must be sure hindered to rotate. This takes place by form-locking. Usually are locking lugs or bolts (anti rotation pins). At supporting faces, respectively guidance faces, no unacceptable fretting wear must occur. Are the locks markedly dynamically loaded (bending), during material selection attention must be payed besides at strength and wear behaviour also at corrosion.

In the area of the aeroengine main bearingss (sketch above left) for tightening primarily two types of axial face seals against oil exit can be found.

Axial face seal (mechanical seal, face type, sketch above left):
In bearing chambers mainly `built' axial face seals (sketch below left) are used. For such, which can be taken-apart, a reassembly with inspected elements is possible. This requires an intense inspection for failures and, if necessary repairs.

Radial face seals (split ring type, sketch below right):
These are only used for low pressure differences in the region of main bearings. Its slide ring is segmented. It is pressed at the sliding face of the shaft by a circumferential spilral spring. Typical is the use for large seal diameters like main shafts and propeller shafts. Lubrication and cooling takes place through a special oil supply.
Applications can be found in the compressor region of elder aeroengine types and at propeller shafts. Thereby, different to labyrinths, the enduring sealing effect during stand still is important.

Cartridge seals serve as sealing ov shafts in gears and accessory devices. Its diameter is relatively small. Similar to a radial face seal, all static elements („1“, „3”, „5“) are in a metal casing (artrdge). An exchange takes place in one piece. Normally the reuse is not planned.

Fig. "Failure modes of axial face seals" (Lit. 23.4.2.2-2): This picture shall make aware the many damaging effects at an axial face seal and give so a support for the treatment.
Occurs a failure or problem at an axial face seal, the failure mode and symptoms can give the expert important hints at causative influences. For a statement, as sure as possible, some elements of the findings have to be combined. Wear patterns at the contact faces of the axial adjustment can be connected with track/contact patterns (Fig. "Failure causes oat axial face seals").
A survey about the consequences of typical influences follows:

Corrosion and other chemical reactions: It must be considered, that wear processes produce fresh reactive metal surfaces, which are unexpected susceptible to corrosion.

Oil contaminations (Ill. 22.3.3.2-1) and aging products (Ill. 22.3.1-2) can attack seal elements (Fig. "Problems due to change of the oil type"). In all cases, a specific sensitivity of the material is needed. This counts, e.g., for stress corrosion cracking (SCC) or hydrogen embrittlement of springs made from high strength steels. The formation of a corrosion cell develops during contact of different metals in an lubrication oil, acting as electrolyte. This can be traced back to not suitable additives, aging or contaminations. Hard thermal spray coatings and sinter layers at the rotating sealing face („2”) can be damaged by corrosion. From tungsten carbid (TC) in a Co-Ni-matrix, the matrix gets dissolved. So break out of the hard TC particles occurs. As result a high wear rate is to be expected.

Elastomers of the static sealing („4“, e.g. O-rings) of the face sealring can be damaged in different ways (Fig. "Problems due to change of the oil type" and Fig. "O-ring failures and its causes"). Is the axial move of the sealring hampered, a lekage occurs.

Wear can arise in different types.

• Vibration wear (fretting, volume 2, chapter 6.2) develops on contact surfaces of the axial adjustment. This can be alternately reinforced with corrosion.
• Sliding /abrasion wear at the sealing faces is caused by dry run after a breakdown of the lubrication film.

A further problem, are abrasive particles/oil contaminations or

• an unsuitable tribological system in the lubrication gap. * Erosion at the surfaces which are impinged by an intensive oil stream with hard contaminations.
  

Cavitation (volume 1, Ill. 5.3.1-11.2) depends from flow conditions in the oil. It is influenced by pressure, temperature and velocity. Also characteristics of the sealing faces like planarity, have a crucial effect. So it comes to the formation of gas bubbles and vapor bubbles in the lubrication gap with break outs and abrasion at the sliding faces.

New parts production and repair: Primarily the sealing faces of the seal ring and the shaft are affected by aberrations:

• Axial run-out /distortion,
• corrugation,
• topografy/roughness.
• No sufficient bonding strength in a slidecoating.
• Discrepancy of the slide ring material
  (carbon/graphite): Porosity, weak bonding.
• The cartidge type promotes the possibility ofthe use of unlicensed products (SUPs, chapter 20.2.1).

Assembling and handling problems: An appropriate storage must already guarantee the defect free condition of the seal. That is especially applied for the sensititve carbon sealring. Typical assembly relevant problems are

• alingnment of the sliding faces.
• Damage of the slide ring edges.
• Hampering of the axial adjustment.
• Unsuitable axial contact pressure.
• Contaminations (dust, abrasive particles).

Operational mechanical loads: Occur failures at axial face seals, they are probably secondary. Thereby it must be checked, if operation conditions existed, which explain unnormal high mechanical/physical loads:

• Severe vibrations (volume 2, Ill. 7.2.3-16 and volume 4, Ill. 16.6.3.3-9; unbalances, air oscillations or gas fluctuations in the combustion chamber `buzz' volume 3, Ill. 11.2.4-11).
• Large axial and radial movements of the shafts (bearing play, compressor surge, Ill. 3.1.1-6, thermal expansion), labyrinth failures.
• High pressures/pressure differences (volume 2, Ill. 7.2.1-2).
• Oilfire (volume 2, Ill. 9.2-2).
• Pressure shocks (compressor surge, volume 3 Ill. 11.2.1.2-1.

Fig. "Failure causes oat axial face seals" (Lit. 23.4.2.2-5, Lit. 23.4.2.2-10 and Lit. 23.4.2.2-11): The sliding face of an axial face seal can give important hints at problems and failure causes. The pictures show typical schematic features for the tribological system carbon slide ring - steel race. The associated causes and relevant influences are described more detailed in Fig. "Failure modes of axial face seals".
Typical symptoms of the contact pattern are (see also Ill. 4.2.3.2-8):

Rotating sliding face

• intensity, existence, absence(„d”),
• width („c“,”b“),
• radial displacement („f”,“g”),
• the race track exists only partly („f“),
• discontinuity and it's distribution („e”).

Failures of the race track at the rotating sliding face:

• Penetrative wear (k, Fig. "Damages at rotating sliding faces"),
• bigger out breaks of a hard facing („i“, Ill.23.4.2.2-4),
• smaller break outs (pits) at a hard facing(„h”),
• crack formation („i“ and Fig. "Damages at rotating sliding faces").

Static sliding surface (carbon ring)

• break outs at the edges („m”),
• pittings on the race track („n“),
• blistering on the race track („n”, Fig. "Seal carbon blistering").

Fig. "Damages at rotating sliding faces" (Lit. 23.4.2.2-5): In this picture it is attempted to display schematic details of typical failure modes with typical features of sliding ring tracks from axial face seals.

„A“ Nonuniform sliding track, possible causes:

• Distortion of the sliding ring by too highjoining/holding forces.
• Distortion due to production induced internalstresses.
• Insufficient/uneven supporting contactsurface of the sliding ring rear side.
• Misalignment of a segmented seal (Fig. "Face seals for bearing casings"). * Assembly mistake.
• Quality deficiency.

„B” Scratched sliding track, „C“ Erosion, possible causes:

• Contamination during assembly.
• Lifting of the sliding surface by
  Vibrations,
  short term dry contact (flashing),distortion by temperature influence,too high pressure in the sealing gap.
• Lubrication starvation on the sliding track.

„D” Coke formation, possible causes:

• Too high operation loads (temperature,pressure, circumferential speed),
• Contaminated, aged/deteriorated oil,
• insuffizient heat dissipation,
• Evaporation of the oil in the sealing gap,
• insufficient monitoring of the operationdata.

„E“ Thermal crack formation, possible causes:

• Too high operation loads (temperature,pressure, circumferential speed),
• Lubrication shortage at the sliding track, dryrun.
• Insufficient monitoring of the operation data.

„F” Break outs at the edges of the sliding race, possible causes:

• Lifting of the sliding surface.
• Operation in the evaporation region,
• vibrations,
• cavitation (Fig. "Failure modes of axial face seals"),
• production/quality,
• insufficient installation conditions,
• too high operation loads (pressure,temperature, circumferential speed),
• Lubrication shortage on the sliding surface.„G“ Break outs in the race track, blistering. Possible causes:
• Insufficient material/tribological system,
• Too high operation loads (pressure,temperature, circumferential speed),
• short term dry run (flashing).Comment: blistering at carbon seal rings will be dealt with in Fig. "Seal carbon blistering". It occurs during contact with oil.

Fig. "Seal carbon blistering" (Lit. 23.4.2.2-5 and Lit. 23.4.2.2-9): Carbon blistering at carbon sealing rings for the sealing of oil, apparently is the most important cause of leakages.
The failure mode are bright areas. Sometimes they show small radial cracks and/or pits at a broken up blister (detail below right). The blistering in the race track disturbs the evenness, needed for the sealing effect of the sliding face.

Three features are distinguished:
Type I: Bright spot.
Type II: Bright spot with origins of radial micro cracks.
Type III: Pitting shaped break out with radial micro cracks.

Formation mechanism of the blister (middle frame): This failure forms even at a few damaging cycles. This is a question of a self-energizing cyclic damage mechanism. In the initial phase (type I) a polishing effect creates the bright surface. The hydrodynamic oil film penetrates the pores near the surface in the carbon slidering („1” and „2“). During a fast temperature rise, due to high oil temperatures or friction heat, the oil expands in the pores („3”). Is the oil not able to escape fast enough, a zone below the sliding face bursts open (upper detail). Thereby the affected area (`cover') will be slightly lifted. Even about ten damaging temperature cycles can trigger blistering. This shows, that the temperature cycles are crucial. In the region of a blister the contact pressure and friction heat increases. So thermal stresses and mechanical bending stresses in the `blister cover' develop. Further loadcycles produce cracks (Type II) until the friction forces lift the blister cover (Typ III).

The blistering will be intensified

• markedly by the oil pressure to be sealed, which governs the pressure in the pores,
• markedly by the oil viscosity. It influences the pressure release in the pores,
• lesser pronounced by the sliding velocity;

This behavior is material dependend for all three parameters (cycles, oil pressure and oil viscosity). For example the damaging blister formation on the sliding ring is at a ceramic counter face generally weaker than at a steel face.

Fig. "Accident by a worn axial face ring" (Lit. 23.4.2.2-15): After the landing, the aeroengine was checked from the operator. It appeared, that the smoke in the cabin came from the left aeroengine. After this the aeroengine was transferred for an investigation to the OEM. The following failure sequence could be reconstructed:
The bearing chambers „2“ und „3” have labyrinth seals. These act function determined with a sealing gap. Obviously the leaking air led to a pressure rise in the casing of the accessory gear. This lowered the pressure difference to the sealing air at the bearing chambers (volume 2, Ill. 9.2-1). So the sealing effect of the labyrinth seals deteriorated further. The high pressure in the accessory gear let oil exit through a worn carbon axial face seal in the bearing chamber „1“. This got into the intake area of the low pressure compressor. The high operation temperature of the components evaporated and decomposed (smoke) the oil. The bleed air for the cabin is taken behind the high pressure compressor. It was corresponding contaminated by the oil.

To the contamination with the oil, obviously also the deterioration of a static seal between two casings has contributed. It let also oil into the cabin air. Also interesting is, that an improved sealing packing of fluorocarbon (PTFE) was concerned. It was only short time ago introduced, instructed by the OEM.
To solve the problem of the pressure rise in the casing of the accessory drive, from the OEM a vent pipe from the gear into the exhaust gas casing was drafted. This prevented a too high pressure in the gear casing.

Comment: From the cited literature the cause of the damage of the carbon face seals is not definitely clear. Besides the high sealing pressure, also seems the high temperature of the leaking air stream into the gear casing as cause absolutely plausible.
The remedy with a vent pipe shows, that this problem is not a single case. Probably it already exists for a long time. At this points the actual cause, i.e. the labyrinth seals at the bearing chambers as a design feature.
The introduction of the static sealing packing of PTFE seems to be a typical case of disimprovement (volume 1, Ill. 3.3). Here the question after the sufficient near operation-conditions testing arises. Above all a decomposition of PTFE may itself ad to an alarming air contamination (fluor).