Aeroengine Safety

Fig. "Problems by water in lubrication oil" (Lit. 22.3.3.2.1-1): Water can affect the operation behavior of components in the oil system with changes of the lubrication oil (Fig. "Problems by water in lubrication oil" and Fig. "Water content in oil and lifetime of parts").

„A“ Shortening of the lifetime by rust formation: During stand still, water can trigger corrosion in anti friction bearings (chapter 23.1.2). Tereby water can act directly as condensate or in the oil. Additionally the corrosion attack is intensified by the acids, produced by the aging of the oil. Corrosion can be promoted if saliferous dust particles are in the oil (Fig. "Deterioration effect of particles in oil"). They can get into the oil by air exchange through labyrinth gaps. Barometer variations and/or temperature changes produce a pumping effect (Ill. 22.2.2-0). During operation, dust can get into the oil system by sealing air (bearing chambers). With water an aggressive electrolyte develops, which attacks not only rusting/oxidising steels but also „stainless” materials like noferrous metals („E“). The rust particles, transported by the oil stream act abrasive and produce fresh, reactive metal surfaces which easier rust. So a self accelerating process arises, which reduces the life of the part markedly (Fig. "Water content in oil and lifetime of parts").
A special type of corrosion at surfaces, especially races of anti friction bearings, is called „water etching”. This effect is caused from the formation of hydrogen sulphide (H₂ S). With the water sulphurous acid develops by decomposition of the oil.
„A type of drop impact“ („H”, volume 1, Ill. 5.3.1-11.1) arises, if free water (fine water droplets) impinge on a hot metal surface. This applies an erosion process with pitting like attack (erosion pittings).

„B“ Abrasive wear, when the separating oil film breaks through can occur, when the bearing capacity of the oil dropped by changes, caused from water (Fig. "Water content in oil and lifetime of parts"). This apply:

• Accelerated oxidation of the oil.
• Decomposition of additives like oxidationinhibitors and desulphurzation additives.
• Settling out of additives.
• Destabilisation above 80 °C of wear protectingadditives.
• Hinderance of the adherence of polar additi-ves on a metal surface.
  

Between the races of the rings and the rolling elements of bearings and the tooth flanges of gears an oil film must prevent the direct contact and so the wear. Thereby the behaviour of oils is used, whose viscosity rises with the hydraulic pressure (in the gap). Water has not this property. Its viscosity can even slightly drop under pressure.
Does this cause the touch of the metallic contact surfaces, the danger of seizing/galling wear and fatigue fractures arises (fatigue pittings, Fig. "Fatigue pittings at bearings", Fig. "Development of fatigue puittings" and chapter 23.2.2).

„C” Wear/abrasion by deposits from „hard water“: Did water absorb chalk (e.g., from dust), this can separate again and act abrasive.

„D” Hydrogen embrittlement (volume 1, chapter 5.4.4): To penetrate into metals, hydrogen must be in atomic condition. For this, water molecules must be dissociated. Obviously this is possible in microscopically small surface cracks under the extremely high pressure between the pitch surfaces. So the crack propagation is accelerated twice. Thereby the hydrogen (volume 1, Ill. 5.4.4.1-2) and the forcing effect of the high hydraulic pressure gets effective. This model seems questionable under the aspect, that hydrogen embrittlement gets effective under static load. Therefore also so called corrosion fatigue (volume 1, Ill. 5.4.3.1-2) under the influence of corrosive media, developed in the water (acids), can not ruled out. During crack growth fracture pictures can develop, which are similar to hydrogen embrittlement (Lit. 22.3.3.2.1-3).

„F“ Cavitation (volume 1, Ill. 5.3.1-11.2): Water can form vapour bubbles in the oil stream, depending from (high) temperature and (low) pressure. Does the pressure rise again at an other location, the bubbles implode. Thereby the liquid (condensed water, oil) impinges in a microscopic area on the metal surface. This multiple repeating process has a similar erosive effect like the drop impingment (volume 1, Ill. 5.3.1-11.1). Typical are pit like surface damages. Water also promotes the absorption of air from the oil. This contributes to the formation of bubbles and so intensifies the cavitation (gaseous cavitation).

Vapour cushions can form in hot oil. Similar to „fading of car brakes” they can degrade the function of the force transfer in a hydraulic system. In the oil system vapour cushions and air cushions act disturbing. For example when they hinder the intake of a pump (vapour lock). The function of control systems can be also dangerously affected. Thereby the springy effect in connection with vibrations plays a role.

„I“ Oxidation of babbitt: Slide coatings in oil lubricated friction bearings preferably consist of low melting alloys (babitt). They are used in planetary gears of turboprop engines to drive the propeller. Obviously alloying components can oxidize in connection with water in the oil.

Fig. "Water content in oil and lifetime of parts" (Lit. 22.3.3.2.1-1, Lit. 22.3.3.2.1-4, Lit. 22.3.3-5 and Lit. 22.3.3-6): Even in smallest amouts, water in the lubrication oil can markedly influence the lifetime of machine components (Fig. "Lifetime influenced by water in oil"). Thereby different damaging mechanisms act, influenced by water (e.g., corrosion, unsufficient bearing capacity, cavitation) or indirect (rust, aging of oil/oil oxidation, Fig. "Problems by water in lubrication oil"). Untortunately in the cited/available literature, there are no component specific hints.
The diagram shows the influence of water in the lubrication oil on the lifetime of anti friction bearings. Even with a water content of only 200 ppm, which lies at the limit of visibility (Fig. "Evaluating water content in oil") already a drop in lifetime of about 50% can be expected.

Fig. "How water can get in oil" (Lit. 22.3.3.2.1-1 and Lit. 22.3.3.2.1-6): There are many possibilities how water can get into the oil of aeroengines. Basically there are two possibilities for the absorption of water. By the absorption of air humidity from the oil, which is alwas to some extent hygroscopic. Thereby up to about 100 ppm (0,01%), water can be dissolved in the oil. The second possibility is condensation. This happens, when warm humid air contacts a cold surface (beer glass effekt). Can the air no more hold the air humidity during cooling down, it condenses at the cold wall.
In the following situations, water gets into the lubrication oil:

Storing of the oil: Primarily water from the air humidity can be absorbed from the oil also by condensate. Thereby a mechanism may arise, which also is known for fuel (Ill. 22.2.2-0). Sufficient air humidity can be expected in only oartly filled tanks and containers. Are caps not tight, the possibility of an air exchange with the outside exists. The pumping effect develops with changes of temperature and/or atmospheric pressure (hight, weather).
Also with oil changes or refilling, free water can get into the oil. This can origin from water accumulations in the oil-storage containers.

Stand still of the aeroengine: Similar half filled oil tanks, humidity can also get into the oil system of a shutdown aeroengine. Changes of the atmospheric pressure promote the exchange through vents and labyrinth gaps. Cools the aeroengine down, the air in the oil system contracts and air is sucked. If the temperature in the oil system is sufficient low, condensate forms. During heating the air in the oil system expands and is blown off. This process can happen as well in a mounted aeroengine (Ill. 5.4.1.2-11) during lasting stand still periods (military mission) as also in an unsufficient strored dismounted aeroengine. A further possibilty is the oxidation of markedly amounts of oil by „heat soaking” during shut down (Fig. "Problems of bearing chambers near hot parts", Ill. 22.3.2-6 and Fig. "Oil coaking endangers main bearings").

Compressor wash (chapter 19.2.3): Does this take place during turning with the starter, the rotation speed is so low, that no sufficient sealing pressure and pressure in the bearing chamber is build up. So spray water and water vapour can get into the oil system. To minimize this risk, the instructions in the maintenance manuals must be met.

During the aeroengine operation: At certain oxidation processes and corrosion processes water can develop in the oil. This can be estimated especially durig local temperatures with coke formation. In the extreme case of a limited oilfire which quenches (e.g., in a bearing chamber), water can form as a combustion product.

Ill. 22.3.2.1-4 (Lit. 22.3.3.2.1-1): Besides investigathion methods in the laboratory, there are comparatively simple possibilities, if there is sufficient experience.
The sketches above show the effect of free water at the appearance of an oil sample in a glass at room temperature. The changes are caused by tiniest droplets. Even if there are no such indications, deteriorating amounts of dissolved water can not be ruled out (diagram).
A simple test for free water, which demands only few devices, is the so called crackle test. Thereby a drop of the oil to investigate is heated in a metallic dish on a 135°C hot, plane metal plate. From the observed behaviour (sketch and description below) approximately the water content can be suggested (detection limit about 0,1 % water, equivalent 1000 ppm).

Ill. 22.3.3.2-5 (Lit. 22.3.3.2.1-1): The best remedy for water in lubrication oil is, like in many cases, to avoid the causes. To this, the summary gives a survey. Indirect hints are already given in Ill. 22.3.3.2.1-3. It deals with with the possibilities, how water gets into the oil of aeroengines. Basically can be assumed, that even smallest, also in the water dissolved amounts of water, can act deteriorating (Fig. "Problems by water in lubrication oil"). Therefore also seemingly negligible amounts must be prevented. That is especially true for air humidity. So basically the contact with larger amounts of air on the oil is alarming.