Table of Contents
3.5.1 Typical problems, their causes and remedies in oil systems and bearings
The oil coking and the oil fire.
Strong heat radiation upon oil systems can heat these up so much that a noticeable cross section reduction through coked oil occurs ( "Ill. 3.5-8", Lit.3.1.2.3-1). Endangered zones are, e.g., pipes in the combustion chamber region, bearing chambers of turbines and hollow shafts. Next to the danger of blockage, through growing coked oil or gathered coked oil particles, is the possibility of contamination of the oil circulation through coking products and, consequently, an influencing of the pumps, valves and bearings.
With increasing oil temperatures, the oil fire risk rises (Lit. 3.5-6). If the oil mist in the bearing chambers, scavenge pipes, intermediate shaft spaces or vents reach temperatures of self ignition, an oil fire can ignite and stabilize ( "Ill. 3.5-6"). Experience shows that next to the heating up through heat radiation from outside, a dangerous heating through high bearing temperatures or a secondary failure with hot gas ingestion (e.g., fracture through a scavenge oil pipe) can lead to an oil fire. The operation of the engine is endangered in different ways through an oil fire.
- Overheating of individual components (e.g., bearing or bearing chambers, shafts and discs).
- Formation of oil coke inducing bearing, seal and pump failures.
- Usage of the oil reserve (several liters in minutes), resulting in the danger of dry running with catastrophic secondary damages.
- Failure of seals, especially piston ring seals, can lead to the exit of huge amounts of oil that ignite outside the bearing chamber and heat up rotor parts extremely.
A sign for an at least temporary oil fire is inexplicably higher oil consumption and combustion products in the scavenge oil (discoloration through suspended substances). Proof on the operating engine is possible through a suitably positioned lambda probe, which registers an unusual CO2 formation. Such measurements can be necessary, if the suspicion of an oil fire exists. For the purpose of aimed remedies, the time of the oil fire and the place of origin must be determined.
Foreign objects in oil
As mentioned already, higher oil temperatures and, therefore, low viscosity oils lead to a greater sensitivity of the rolling element bearings, as opposed to foreign particles ( "Ill. 3.5-3" and "Ill. 3.5-5"). A rolling element bearing damage, especially a main bearing failure, can result in catastrophic secondary damages and is, therefore, definitely to be avoided. Due to this, there is a constant trend to finer oil filters. Oil filters for 0,010- millimeter particles are not rare. Usually, the foreign particles are transported into the bearing by the oil flow itself. This is only then possible, if the foreign particles reach into the fresh oil flow after the filter. A typical possibility are contaminations that already find themselves in suitable regions of new parts or overhauled components. This includes machining swarf, casting residues and blasting agents ( "Ill. 3.5-3").
Through the sealing system, impurities like sand and dust get into a bearing chamber. A further possibility exists during low rotor speeds (start, shut down, ‘washing’ of the compressor) if particles with the inlet air reach regions of the engine where the pressure of the sealing air is not yet sufficient. Those particles penetrate the sealing air and are blown into the bearing chamber.
In the engine itself, break outs can occur through fatigue damages or abrasion on rolling and sliding surfaces ( "Ill. 3.5-5") that re- enter the oil circulation ( "Ill. 3.5-1").
It is worth taking measures to monitor, so that the failure is recognized in time and secondary damages are avoided. Furthermore, there are many possibilities one could implement in combination.
Oil monitoring, magnetic chip detector and filter control
A regular oil analysis in intervals ( "Ill. 3.5-4") , mostly specified by the manufacturer (OEM), is effective. If not, an interval can be fixed that takes into account specific, individual conditions. A control of the oil is especially important and compulsory after the pass off test and, perhaps, flushing procedures. The oil analysis can show chemical changes (aging) in oils, giving a hint as to the necessity of a change of oil. An examination of suspended matter shows special operation influences like oil fires. Metallic abrasion gives clues as to worn components. It is naturally necessary to know the components. If one follows the analyses regarding the life of the engine, one can recognize and prevent failures in time, according to the trend.
The deposits on magnetic chip detectors (MCD) ( "Ill. 3.5-5") should be regularly gathered, examined and archived depending on the manufacture details ( "Ill. 3.5-7"). One can only expect magnetic particles. They can be microscopically (light and electron optical) inspected. The form and structure of the chips give hints as to the type of failure mechanism and its origin; the analysis permits conclusions regarding the component in question. The expert thus recognizes typical spalling, when there is fatigue on gears or rolling bearings. Swarfs from machining can be identified by their characteristic surface structure and, among other things, their spiral form. They are an indication of insufficient cleaning after the machining processes.
Filters ( "Ill. 3.5-3") should be inspected for residues after every change, because filters precipitate even non magnetic particles. Such a control is not replaceable through magnetic plug control. Typical filter deposits contain sealing abrasion, dust and sand. Aluminum oxide (= alumina) hints at core residues or abrasive blasting material from a cleaning process. Frequently, one finds large amounts of soft, colored plastic materials in filters, subsequent to the initial runs. This is related to surplus sealing compounds of the flange (e.g. from gearbox casings), which were washed into the oil circulation. Such residue is, according to experience, no reason for anxiety, in the context of the mechanical integrity of the components in the oil circulation. However, if they appear in substantial amounts, the filter or oil nozzles can get blocked (chapter.4.2.2). One should take care, already during the time of assembly, to ensure that not too much of the sealing compound is used, since it would then pour into the interior, while screwing the mating surfaces. Here also, “too much does not necessarily help too much” is valid. Non magnetic metal particles, like Ti abrasions or chips of Al-, Mg-, and nickel alloys are also detectable through filter monitoring and can serve as an important clue regarding erosion damage. A special skill is the art of recognizing unusual trends and their significance in connection with measures that may be required.
Early recognition of bearing damages through vibration monitoring
(see chapter.4.1.3) In conjunction with this, one is aware of the possibility of continual vibration monitoring and the evaluation of measurements as a means of early recognition of damages. In many gas turbines there are vibration pick ups in appropriate positions which emit evaluable signals ( "Example 2.5-2"). Normally amplitudes (vibration intensity), the distribution to different locations of the sensors, the frequency and the temporal development are analyzed. Especially we have to look after deviations to the normal condition ( "Ill. 5.1-1"). So there is a chance to isolate the affected components. If the manufacturer (OEM) refrains from offering such a service, one can turn to special firms which do so. Sufficient experience is vital in this area.
One should consider that modern gas turbines derived from flight engines have elastically suspended and damped bearings, allowing vibrations to penetrate only after the break through of the oil squeeze film, unusual for sensor recognition. In such cases, the damages can have progressed relatively far before there is the opportunity of recognition.
Evaluation of damages
If damages have already emerged, the greatest chance of successful evaluation is at the incipient stages ( "Ill. 3.5-10"). Thus, totally annealed bearings are hardly suitable for trustworthy research into causes, whereas a bearing with beginning fatigue fractures enables the expert to draw more certain conclusions relevant to the cause ( "Ill. 3.5-11"). E.g., it is possible to determine whether the bearing overload is due to the effect of foreign objects or an assembly fault. Foreign objects are often identifiable through the analysis of particles that have been rolled into the running path.
"Illustration 3.5-1": Typical design and components of the oil system of a gas turbine with the example of a bearing chamber in the turbine area. Each bearing chamber requires at least one feed oil supply, the drainage of the scavenge oil and a bearing chamber ventilation. The scavenge oil gets into the rear oil filter ( "Ill. 3.5-3") which is usually equipped with a magnetic chip detector ( "Ill. 3.5-5") . Here, the majority of particles from the engine are caught. From here the oil flows into the rear oil pump, through the oil cooler, oil tank, pressure oil pump and through the pressure oil filter with the magnetic plugs, back into the bearing chamber. The fineness of the pressure oil filter is of particular significance for the life of the bearing (Fig, 3.5.1-4). It complies with the thickness of the oil film between the rolling element and the bearing race ( "Ill. 3.5-2"). Air and oil mist from the bearing chamber are transported into the breather via the ventilator, the separated oil goes into the oil tank and is available again for circulation.
"Illustration 3.5-2": The trend to performance concentration and improved efficiency inclines to higher (rolling) bearing loads, rotating speeds and bearing temperatures. The oil temperatures rise corresponding to this trend. Rolling bearings should work in the contact zone between rolling elements and raceway surfaces under so called EHD conditions (elastic hydrodynamic load). The latter means that between the surfaces a bearing oil film builds up avoiding indentations of small particles and wear. If this lubrication film is breached (Lit. 3.5-2), the danger of secondary failures, mostly by rolling fatigue at the bearing races, exists. Typical causes for such a situation are:
- Unusually high bearing loads (mixed friction, Fig. above middle),
- Impurities (Fig. above right) in the oil, then the danger of fatigue failures ( "Ill. 3.5-10") occurring as the wear increases.
- Contamination particles. The thicker the lubrication film, the more insensitive the bearing. This connection is shown in the so called Macpherson curve. Deducible is that from fresh oil filters intercepting particles under 0,01 mm, one can expect an exponential life increase of the rolling bearing as opposed to coarser filters. The lubrication film thickness is especially dependent on the bearing forces and the oil viscosity. A higher oil temperature leads to a thinner fluidity of the oil and, thus, to a thinner oil film. This can be more easily broken through, damagingly, by smaller foreign matters. For this reason, the demands for fineness of the filter always tends to increase. If there is a particularly sensitive rolling element bearing of a type of engine for surface fatigue at the raceways (spalling), the introduction of finer filters should be considered and discussed with the manufacturer (OEM).
"Illustration 3.5-3": (Lit. 3.5-6): The deposits in the oil filters, especially from the scavenge oil filter, can give important information as to the condition of the engine and related problems. Those of us who have performed an oil change with filter exchange on cars are initiated in this regard, the resemblance is obvious. In the oil filter system represented, “1” is the actual filter cartridge with the filter lamellas. “2” marks the (cover) lid of the filter casing for the purpose of exchanging the cartridge.“3” indicates a stop valve that limits the exit of oil when the filter cartridge is removed. The magnification reveals a small bit of filter surface at the side of the inflow, with typical precipitated particles:
- “A”: non magnetic metal chip (if it was magnetic it would probably hang onto the magnetic chip detector).
- “B”: soot or coke particles
- “C”: surplus sealing compound
- “D“: mineral particle (peening/blasting material from cleaning processes during new production and overhaul)
- “E“: non metallic particle (labyrinth hard facing)
- “F”: non magnetic metal burr, e.g., from labyrinth fins or from a worn out rolling bearing cage.
"Illustration 3.5-4": An oil analysis can serve towards the identification and early recognition of problems of engines, if abrasion particles in the oil circulation are brought in that are so fine (<0,001mm = ca. 1/50 of a hair!), they pass through the oil filter and float as suspended substances in the oil. Typical concentrations are parts per millions of weight (ppm). Characteristic metals found as fine abrasions in the oil circulation are:
- Fe from gears and bearings ( "Ill. 3.6.1-7").
- Ni from Ni alloys of labyrinth fins or Ni-graphite abradables ( "Ill. 3.1.2.4-4").
- Cr from chromed sealing surfaces or chromed bearing seats.
- Ti from fretting between steel and Ti alloys e.g., bearing seats of Ti shafts or Ti casings.
- Cu or brass of bearing cages.
Crucial for the use of this monitoring possibility is that the experimental values and limits are specified by the manufacturer. Both of these typical analysis procedures are atom absorption and optic emissions spectrometry. In both cases a small oil sample is necessary. For a successful evaluation of this process, the time of sampling, respectively, the knowledge of the operation hours involved is a pre requisite. One needs to be aware that through oil refill (“A”) and oil exchange (“B”) the curve progression is influenced. A sudden ascent of the curve (“1”, “2”, “3”) is attentively to be observed. If the trend does not normalize, one has to consider impending failure. Filter ( "Ill. 3.5-3") and magnet chip detector deposits ( "Ill. 3.5-5" and 3.5-7) are to be evaluated in order to safely identify the problem.
"Illustration 3.5-5": (Lit. 3.5-6): If a ferromagnetic particle succeeds in entering the oil circulation, it can be held by a magnetic chip detector. Ferromagnetism can also show normally non magnetic, metallic materials: if they are, e.g., plastically deformed (e.g., Cr Ni steels of type 18/8) or concern oxides from Ni alloys.
There is a multiplicity of magnetic particles that are characteristic in kind and origin. Here some typical magnet plug deposits are set down: Fatigue fractures “A” on a ball bearing. Such fractures (spalling) can occur on all dynamically highly stressed contact surfaces like gear flanks ( "Ill. 3.7.2-5.1") and bearing raceways ( "Ill. 3.5-10"). These particles are found only after long operation and are an alarming sign ( "Ill. 4.1-4"). If they are visible, magnet plug monitoring within short periods of time is required to prevent a greater failure.
Swarfs, as shown in “B”, can originate during cutting processes, e.g., while cutting threads, during rub of rotating edges on an opposite surface, as well as during the start of engaging gears or the cutting in of labyrinth fins
Swarfs from the new machining of parts “C” have a typical geometry and surface structure. They show cleaning problems at the manufacturer’s or repair and emerge, if at all, within the first hours of operation. Small magnetic balls “D” or irregular spherical parts are often peening material from shot peening.
"Illustration 3.5-6": (Lit. 3.5-5): Increasing operation temperatures of modern engines stimulate the increase of the bearing and oil temperatures. And therewith, the probability of oil fires in the bearing chambers and the oil circulation (Lit.3.5-1). An inflammability diagram for a typical lubrication oil (ester oil acc. Mil-L- 23699) can impart information regarding the connections. Oil fires can be externally ignited in the gray marked fields under special conditions. The oil vapor/air mixture in the stoichiometric region lies at „S“. In the black field (from 430° C) self ignition is possible. These high temperatures can emerge in the bearing chamber, e.g., during a hot gas ingestion, as a consequence of seal failure or in especially hot regions of the bearing chamber wall. An ignition of the oil can follow, e.g., through a strong rubbing labyrinth or through a bearing failure. The conditions for oil fires are specified, if at all, only in certain conditions of operation. Outside these conditions, the fire can extinguish and remain unnoticed. If an oil fire breaks out, it signifies a potential overheating risk of affected parts.
Above the ignition area, the oil/air mixture is too rich (high portion of oil); below it is too weak to be ignited. As a measure for the oil portion in the mixture, the lubrication oil vapor concentration is drawn left at the ordinate; on the right is the vapor pressure.
"Illustration 3.5-7": The collected abrasion on the magnetic detector consists of ferromagnetic particles of diverse origins and formation mechanisms ( "Ill. 3.5-5"). Differ the agglomerations from the assesment scale of the OEM in type and amount from the acceptable limits its favorable to clear the further actions with a decision tree ( "Ill. 4.1-4"). Frequently after a review or an overhaul the amount of abrasion is the highest, declines then significantly.
A rerise is corresponding to the manufacturer instructions to monitor attentively. For this a documentation like in "Ill. 3.5-4" is helpful. Imprints of accetions secured with a special adhesive film. If not available also a adhesive film from the household can suffice. The imprints will be collected on a sheet of paper with the necessary comments. These include the datum and the cause of the sampling, the operation hours and the identification of the magnetic detector if there are more. So the findings are available to possible later investigations. The paper with the collected abrasion films will be added to the documentation of the engine. With this it is possible to check the point of time at which a bigger failure developed. So the chance for conclusions at the sequence of the failure exists.
"Illustration 3.5-8": (Lit.3.5-6): It is a persistent coking problem in aero engines that are also used as derivates. Obviously the cleaning of the tubes by a ‘burning out process’ had a detrimental influence. This lead to a decline of the adhesion of the coke layers. Now they were able to get loose "Ill. 3.5-8" during operation and to hinder the oil flow. This caused the OEM to shift to an alkaline rinsing (flushing method). Because evers method has its specific advantages, the operator used as precaution both methods one after an other.
This revealed that the manner of the oil change is crucial for the chipping just existing coke deposits in the fresh oil line. Responsible was the high additve concentration in the fresh oil. This supports an ablation of the coke deposites. An additional intensification of the coke formation as result of mixed old and fresh oil could not be ruled out.
A flow measurement was the only precautionary test fore blocked oil injectors. The required equipment was supplied by the OEM. The tests are repeated in intervals of a month.
A conversion of the procedure of the oil change by draining and fill-up (drain and flush) only to feed oil (top-up method) took place.
Additional the affected oil lines were desgined for easy removal. This facilitated the maintenance in critical areas.
From investigations by the OEM additional safety by the monitoring of the oil pressure could be expected.