9.2 Oil Fires

Figure "Main bearing chambers air systems" (Ref. 9.2-5): The main task of air systems in bearing chambers of the hot-part regions is to protect the bearing sump from the hot surroundings. Conditions surrounding an especially flammable main bearing chamber are the temperature and pressure at the compressor exit. The bearing chamber`s seals should ensure that the oil temperature stays at safe levels (Fig. "Oil fire at a supersonic aircraft bearing chamber"). The following example of a typical main bearing chamber with labyrinth seals emphasizes this:
The inner chamber, which contains the main bearing, is filled with an oil/air mixture (temp. about 180°C, pres. about 0.14 bar).

The bearing chamber is enclosed by the first mantel and is loaded with air from the high-pressure compressor (temp. about 200°C, pres. about 5.5 bar), which serves as blocking and cooling air for the bearing. The high pressure levels create a leak flow to the vent and prevent hot gases encroaching into the bearing. This also prevents foreign objects from entering the bearing from outside. Only a relatively cool leak flow from the blocking air reaches the bearing across the labyrinth seals.
The second mantel<U></U>is created by the vent, which blows out the leaked air from the labyrinths and prevents hot gases from reaching the bearing.
Around the bearing chamber, conditions are the same as those of the high-pressure compressor exit (temp. about 650°C , pres. about 25 bar).

It is worth noting that bearing chambers with a combination of labyrinth and washer bushing seals also exist. With these, very tight washer bushings are used around the bearing to seal off the oil/air mixture in the inner chamber.

Figure "Causes of oil fires in bearing chambers" (Ref. 9.2-5): The frequency of oil fires in the bearing chamber region and the oil sump system in the rear compressor and hot-part areas corresponds to the frequency of fires in an engine. This frequency rests in no small way on the multitude of potential causes (top diagram):

• Problems in the bearing chamber area
• Oil mist being released in and between concentric shafts.
• Leakages in the inner oil system and ignition of the leaked oil through hot gases (Example "Oil fire by hot air incursion").
• Leakages in the outer oil system.
• Perforation of oil lines and ignition of an oil fire by a titanium fire.
• If bearing chamber seals are poor, pressure waves in oil mists can create conditions suitable for ignition (diesel effect).

Oil fires occur most frequently in main bearing chamber regions. The factors that promote oil fires and their causes are shown in the bottom diagram in Ill. 9.2.-2:

Hot gas incursion into the bearing casing after crack initiation (Fig. "Oil fires by oil leaks in bearing chambers"). Bearing casings are subjected to high dynamic stresses from bearing thrusts, rotor vibrations, and cyclical heat strain. If fatigue cracks form, there is a danger that hot air (hot gas) could flow into the bearing chamber due to the usual pressure gradients which run towards the bearing (Fig. "Main bearing chambers air systems").
The same is true for return oil- and vent lines (Example "Oil fire by hot air incursion"). There are frequent reports of cracks and fractures of these lines (Fig. "Oil fire by cracks in oil lines and vent lines"). These low-pressure lines typically run through hot gas regions with high pressure levels (e.g. compressor exit conditions), which promotes the incursion of hot gases (Fig. "Main bearing chambers air systems").
Dangerous hot gas incursions into the bearing chamber at the shaft seals can be expected if the seals become very worn by friction.
Pressure waves can have an igniting effect on this type of leakiness (e.g. during a compressor stall).
A damage sequence can result in pressure gradients that promote the escape of oil from a leaky bearing chamber. This oil can then be ignited by the hot periphery of the bearing chamber.

Oil leakages in shaft seals: a dangerous oil leakage can be expected only if the seal fails. However, there are special cases where a change in temperature gradients (Fig. "Oil fire between shafts (inter shaft)") leads to overpressure in the bearing chamber and the escape of a highly flammable oil mist.

Sparks and heating-up during rubbing in seals (labyrinths) with an unfavorable tribo-system (see chapter 7) can ignite flammable oil mixtures.

Overheating of a bearing can cause a flammable air/oil vapour mixture to form and ignite. Since it follows bearing failure and the axial and radial shaft movements it causes, this type of fire can escape through the countersunk labyrinths and heat the rotor to dangerous levels (Fig. "Oil fire overstressing rotating engine parts").

If the temperature levels in the bearing chamber are too high (Figs. "Oil fire risk by bearing chamber temperature", Oil fire ignition dependent on operation data" and "Oil fire at a supersonic aircraft bearing chamber"), the danger of an oil fire is especially high. These temperature levels are reached in special missions (Ill 9.2-3). This also involves a “design weakness”, if this operating condition was not given enough attention during the engine development stage.

Figure "Operation data influencing oil fire risk" (Ref. 9.2-4): The danger of an oil fire in the bearing chamber and return oil regions rises with the engine temperature levels and the number and type of components connected to it (e.g. tanks). The situations in which this danger is present can easily be understood by looking at the aircraft`s flight envelope. When tactical aircraft (bottom right corner) fly at high speed and low altitude, they require high engine output and create high impact pressures (Fig. "Oil fire at a supersonic aircraft bearing chamber"). The top right corner shows the situations in which supersonic commercial aircraft are susceptible to oil fires. High speeds are critical, since they can heat up the fuel in the wing tanks to the point that the lubricating oil in the oil cooler (Fig. "Oil fire danger by aerodynamic fuel tank heating") is no longer sufficiently cooled (Ills. 9.2-5 and 9.2-6).

Figure "Oil fire danger by aerodynamic fuel tank heating" (Ref. 9.2-4): In this supersonic commercial aircraft, the engine oil is cooled by the fuel, which is the coldest liquid medium available for the task. The fuel tanks are located in the wings. It has become clear that during longer flights (about 2 hours) at speeds around Mach 2.0 the wing temperature at the leading edge and at the engine intake rises from an initial 92°C to 127°C. Also, before the fuel enters the engine`s fuel system, its temperature increases from 45° C to 115°C. Consequentially, the oil temperature also rises from 180°C to 230°C. The oil comes into contact with engine surfaces of up to 310°C, which the oil is intended to cool. According to document sources, from roughly 350°C upwards the oil being used is at risk of igniting.

Figure "Oil fire risk by bearing chamber temperature" (Ref. 9.2-5): Ignition of a sustained fire of a oil vapour/air mixture is only possible under certain pressure and temperature conditions (left diagram). If the oil vapour is very concentrated, it may be too rich for ignition, if it is not very concentrated, it may be too lean a mixture for burning.
The flow rate of the oil/air mixture can have a strong influence on the intensity and spread of an ignited fire (right diagram). The intensity of a fire is measured by the temperature increase after ignition. The results are registered in an oil fire testing rig with a bearing chamber. Evidently, there are limits of bleed air flow rates, under which no ignition (F_n) resp. self-sustaining fire (F_e) can occur.
The highest burn speed is reached if there is a stoichiometric relationship between the oil vapour and oxygen.
The flammability, however, is not determined by the flow rate of air and liquid oil, if one ignores their influence on temperature and thus on vapour pressure.

Figure "Oil fire ignition dependent on operation data" (Ref. 9.2-5): The left diagram shows the influence which oil flow in a bearing chamber testing rig has on the temperature of an oil/air mixture and the ignition at different oil inflow temperatures. The higher the oil flow from the injection nozzle is, the lower the temperature of the oil/air mixture in the bearing chamber drops, which in turn reduces the risk of a fire. The oil tested was type II Ester, the bleed air temperature was about 530 °C, and the ignition source was an electric spark.
As predicted, no ignition took place when the mixture was below ignition temperature (T_L). Even when the temperature was considerably higher than the ignition temperature (arrow), it was not always possible to ignite a fire. There is an upper flammability limit (T_U), above which ignition does not occur, because the necessary pressure and temperature conditions in the oil vapour are no longer present (Fig. "Oil fire risk by bearing chamber temperature", left diagram, intercept point of the upper concentration limit with the oil pressure/temperature curve).
The right diagram shows the influence which the oil inflow temperature (the oil stream from the injection nozzle) has on the temperature of the oil/air mixture and thus on flammability. Trial conditions: bleed air temperature of about 540°C, fires only occurred when the oil inflow temperature rose above 146 °C. This shows the importance of maintaining sufficiently low oil temperature.

Figure "Detecting and verifying an oil fire": Detection and verification of oil fires is required in order to take targeted countermeasures. There are signs that indicate oil fires while burning, and characteristic traces that can be used to identify oil fires afterward.

During an oil fire:

• CO₂ increase in the vent and/or return oil systems.
• Temperature increase (bearing chamber, oil)
• Impaired oil flow (filter blockage, coke build-up, and/or damaged oil lines)
• Imbalances, consequential damages (not necessarily specific to oil fires)
• Drop in pressure (leak in the pressure oil feed)

After an oil fire:
Without dissection (maintenance):

• Changes in the oil (chemical/oxidation, soot content)
• Soot deposits in the filter
• Unusual oil consumption
• Unusual soot traces in the engine exhaust area

With dissection:

• Traces of overheating on the bearing chamber (tarnishing, oxidation, deformation, crack initiation).
• Overheated parts, especially rotor components (plastic deformation, loss of hardness).
• Characteristic soot deposits and soot traces on parts outside of the oil circulation area; e.g. around labyrinth seals on shafts and discs. -Soot and coke deposits inside the bearing chamber. Coke is formed by localized, longer duration, and recurring oil fires.
  

Figure "Oil fire at a supersonic aircraft bearing chamber" (Ref. 9.2-4): During the development and adaptation of a military engine variant (top diagram) for a supersonic commercial aircraft, problems arose with the temperature levels of the lubricating oil and the indicated bearing chamber. The high supersonic speeds combined with the impact pressure caused the temperature levels of the engine to increase and air friction heated up the wing tanks. As a result, the fuel-cooled heat exchanger was unable to cool the lubricating oil (Figs. "Operation data influencing oil fire risk" and "Oil fire danger by aerodynamic fuel tank heating"). The problem was solved through appropriate measures.

Figure "Oil fire between shafts (inter shaft)": Intermediate-shaft oil fires in the intermediate-pressure turbine region of a tactical aircraft with multiple concentric shafts occurred during the development stage (top diagram, Fig. "Oil fire overstressing rotating engine parts"). During high-speed flights at low altitude (Fig. "Operation data influencing oil fire risk"), the pressure gradients around the main bearing chamber changed in such a way that oil vapour flowed from the bearing chamber into space between the shafts and ignited. The shafts were about 550°C. In a few minutes, this had softened the shaft and caused plastic torsion (bottom diagram).

Figure "Oil fire by cracks in oil lines and vent lines": This shaft-power engine from a large military helicopter (bottom diagram) had several oil fires occur in a ripped vent line (top detail). It was discovered that the line was subjected to high cyclical heat strain that its fasteners were not able to cope with. Corresponding prestressing occurred, promoting vibration-induced fatigue fractures in combination with stiffness and form notching (welding).

Figure "Oil fires by oil leaks in bearing chambers": Example "Oil leaks in bearing chambers" describes the background and probable causes of oil fires in the main bearing region of the turbine (schematic diagram bottom left, structural design see detail) of a large turbofan engine type. Fatigue cracks and fractures in the walls of the bearing chamber and lines seem to be the primary cause of oil leakages and/or hot gas incursions (return oil, vent).

Figure "Oil fires in low pressure turbines" (Ref. 9.2-10): This diagram shows an oil fire in the low-pressure turbine of a large turbofan engine. Evidently the low-pressure turbine is susceptible to oil fires in some engine types (see also examples 9.2-1 and 9.2-2, Fig. "Oil fires by oil leaks in bearing chambers"). There is also the danger of disks bursting and/or expanding and releasing the entire blading.

Figure "Oil fire overstressing rotating engine parts": A typical example of an oil fire in a low-pressure turbine (top diagram, Fig. "Oil fire between shafts (inter shaft)") after the failure of a labyrinth seal in the chamber of the main bearing. This type of oil fire in low-pressure turbines is especially dangerous to the integrity of the rotor (see also Fig. "Oil fires in low pressure turbines" and Example "Ruptured oil pressure line"). Within minutes, the oil fire heated up the shaft/disk flange to the point of softening. This resulted in noticeable plastic deformations as shown in the bottom diagram.