Table of Contents
19.2.3 Cleaning/washing of compressor and turbine.
Deposits on the compressor blading can change the blade profile as well as increase the roughness and can trigger corrosion (fouling). Normally ingested mineralic or biological dust (e.g., flower pollen) is concerned. This combines on the blade surface with oil vapour from one of the front bearing chambers or as well ingested sticky materials (humid sea salt, unburned fuel). It is understandable that there is an influence from the arrangement and design (sealing) of the main bearing chamber and with this design characteristics of the aeroengine type. The proneness for contaminations is heavy influenced by the operation. Low flight altitudes over ground/land (dust) and/or sea (humid salty air), like it must be expected for helicopters and military aircrafts, promote the contamination. Compressors of aeroengines from airliners, which frequentlyn stand at the start for a longer time in a waiting line and ingest the exhaust gases of the previous airliners (unburned fuel, soot), can also show increasing deposits. So the pronness for contamination gets type and application depending (e.g., short-haul, APU) very different. The motivation for a washing/cleaning of the compressors and turbines can be operator specific. In the civil aviation a minimisation of the fuel consumption stands in the foreground. For aircrafts respectively aeroengine types which tend to compressor surges the operation behaviour respectively the surge margin can have priority (Ill. 19.2.3-13; volume 3, Ill. 18.104.22.168-7 and Ill. 22.214.171.124-8). In this connenction we come up in the literature especially on shaft engines and turboprop engines (Fig. "Importance of cleaning 1"). Aeroengines of elder versions frequently use in the compressor corrosion sensitive steels (Fig. "Unclean washing water"). The notch effect of developing corrosion pits can trigger fatigue failures (Fig. "Importance of cleaning 2"). At aeroengines which are prone for hot gas corrosion/sulfidation (Fig. "Potential dangers of hot washing", and Fig. "Negligence can lead to failures") the washing of the hot parts/turbine will be increased.
The following changes of the blading (profile, roughness) are able to deteriorate the operation behaviour of the compressor:
- Lowering of the surge margin (volume 3, Ill. 126.96.36.199-7)
- Decreasing of the efficiency/performance and with this of the aeroengine. This leads to increasing fuelconsumption (Fig. "Different methods of cleaning") and increasing hot parts temperatures with high follow-up costs/repair costs.
- Stall at single blades (rotating stall, volume 3 Ill. 188.8.131.52-1) with the danger of a fatigue failure.
- Corrosion/roughening of blade materials like steels and light metal alloys ( Al, Mg) of elder aeroenginetypes.
Also in the turbine can develop harmful deposites. These are primarily deposites of dust (chapter 21.2.2 and volume 1, chapter 5.3.2) with potential dangerous effects:
- Erosion or chipping off at coatings like diffusion coatings and thermal barrier coatings.
- Overheating of hot parts due to inner and outer blockage of the cooling air.
- In an extreme case during heavy access of dust (sand storm, volcanic ash) markedly reduction of the cross sections in the gas channel. The consequence is a drop in performance and/or surging of the compressor.
As remedies against contaminations/deposites serve specific adapted cleaning processes (Fig. "Different methods of cleaning"). Those can act in different ways. At elder aeroengine types which don't show yet potential endangered technologies like soft rub in coatings in the compressor casings more abrasive acting media which burn in the hot gas are used (Fig. "Abrasive cleaning of compressors"). Must glass like fused coatings be removed on the blading of a turbine harder particles are necessary (chapter 19.2.4). Serious disadvantages (Fig. "Abrasive cleaning of compressors") of abrasive cleaning procedures normally are allowed only for elder aeroengine types, if they are explicite type specific approved by the OEM.
Today normally cleaning/washing agents/detergents are sprays into the compressor. We speak about washing of the compressor. There will be distinguished between „Cold Wash“ (Fig. "Hot and cold wash" and Fig. "Advantages of cold washing") and „Hot Wash” (Fig. "Hot and cold wash" and Fig. "Potential dangers of hot washing") of the running aeroengine. During the washing processes basically exists the danger of deteriorating contaminations like salts which are transported from the compressor into the hot parts, especially on the turbine blading. Those deposites can trigger during the later operation hot gas corrosion.
Not only compressors are washed. In special cases also hot parts are washed (Fig. "Cleaning military aeroengines" and Fig. "Negligence can lead to failures"). Here we deal with the removal of water soluble contamionations like salts (e.g., sea salt, fire extinguishants, Ill, 19.2.2-1). Those deposites can be responsible for hot gas corrosion, especially sulfidation (volume 1, chapter 5.4.5) which remarkedlyshorten the lifetime.
Modern aeroengine types incorporate a multitude of new technologies which can react very sensitive at cleaning processes respectively washing processes (Fig. "Potential dangers of cold washing" and Fig. "Advantages and disadvantages of hot washing"). Therefore it is important to keep in each case with the valid instructions for the aeroengine type and the application spectrum. Often instructions/specifications give a leeway in decision making concerning frequency respectively intervals of the washing process (Fig. "Importance of cleaning 2" and Fig. "Negligence can lead to failures"). Problematic are recommended (not specified!) washing intervals which comply with inprecise terms like light or heavy conditions.
It is remarkable that a washing process can damage/deteriorate hot parts, especially turbine blading. This is the case if washed off contaminations respectively corrosion products in the compressor settle on the hot parts (Fig. "Potential dangers of cold washing"). Thinkable is a damage of the hot parts like aggravated sulfidation, deteriorated diffusion coatings (e.g., Al diffusion coating) or liquid metal embrittlement (LME, Ill. 4 Bild184.108.40.206-11). Dangerous metal combinations (cadmium, braze components like silver and copper) can derive from corrosion products of other components.
With growing environmental awarness the disposal of dangerous contaminated washing fluid during cold wash came to the fore. Especially with cadmium contaminated washing fluid (Fig. "Disposal of washing medium"), as it can be expected for elder aeroengine types with Ni-Cd corrosion protection, may not get into the environment and must be (costly) disposed. Obviously this does less count for an environmental impact by evaporated washing fluid in the aeroengine axhaust gas during hot wash, which seams questionable.
Figure "Different methods of cleaning" (Lit. 19.2.3-1, Lit. 19.2.3-3 up to Lit. 19.2.3-5): The sketch above left gives an impressive imagination about the relation between the washing of the compressors of an aeroengine and the amout of fuel which can be saved.
To benefit the specific advantages of the cleaning/washing processes it needs a targeted, at the particular application adapted cleaning media. The chart above right can assist the understanding.
The selection of the cleaning process is only a first, but important step. Also the process parameters must be optimised and freezed in mandatory specifications/instructions. The particle size (lower frame) is of especially importance for the effectiveness of the cleaning procedure. This is as well true for the droplets of a washing fluid as for abrasive acting hard particles for dry cleaning (Fig. "Abrasive cleaning of compressors"). Then optimal particle size will be a compromise. The cleaning is determined by effects like deflection/impact angle, impact energy, abrasion and transport of the contaminations. Thereby particle characteristics like size, density and speed play a role. An especial challenge is the development of optimal injectors/sprays for the insertion of the washing agent. The droplets must be sufficient small to avoid the danger of erosion. However the flow conditions of modern compressors guarantee a satisfactory wetting of the whole to be cleaned blading. This is especially true for big fan engines, at which the spray cone must enter through the fan into the booster and the compressor of the core. Undesirably is an attack by erosion (coating, base material) and the roughening of the component surface. So it must be found an acceptable compromise. This must be determined respectively verificated with practical tests at the aeroengine. For the particular aeroengine type respectively the operation/service the harmlessness of the cleaning process must be demonstrated. Aeroengine specific features can be materials in the gasflow. Typical examples are rub in coatings in the casings or a coating like a paint on the blading in old aeroengine types. Also design features like an especial good smoothness of the blading surfaces or thin edges/small radii, necessary for the operation behaviour of the compressor, dictate the selection of the cleaning process parameters. The proof of suitability of the cleaning process will perhaps carried out together with the OEM. In the end the cleaning process neads the approval by the OEM and/or the responsible authority.
Figure "Abrasive cleaning of compressors" (Lit. 19.2.3-1 and Lit. 19.2.3-8): The dry cleaning of a compressor takes place with abrasive acting particles (upper sketch). They must be adjusted at the deposited contaminations. A dusty cleaning media normally is brought with the help of hopper during idle into the intake of the aeroengine. This is a so called „hot cleaning“ (Fig. "Dont underestimate idle rotation speed"). The cleaning effect is limited to surfaces against which the flow has a sufficient large angle.
At aeroengines of old versions with corrosion susceptible materials protection coatings like paint must not be damaged/deteriorated by erosion. Fresh developed metallic surfaces whose increased reactivity promotes again corrosion Alarming are blade materials like chromium steels (e.g., with 13 % Cr) with an anorganic coating or painted aluminium alloys. Also corrosion sensitive casings made of magnesium alloys depend on a undamaged protective coating. Modern aeroengines are often equipped in the casing adverse to the tips of the rotor blades with a relatively soft rub in coating. Such a coating is very susceptible for erosion.
Does the cleanig lead to an increased roughness possibly an accelerated growth of deposits will be promoted.
The cleaning medium must not clog cross sections which are necessary for functions. This can be valves/openings for bleed air or cooling air channels/bores in hot parts. Sliding guides like at bleed valves or variable compressor guide vanes may not become sluggish or even stuck. A low idle rotor speed can affect tje function of seals which need sealing air (e.g., in main bearing chambers), cleaning particles can get into the oil system. React such contaminations with the oil this acts additionally damaging.
Because of the definitely alarming potential disadvantages a dry cleaning as possible will be carried out only in special cases, tolerated by the OEM (e.g., military).
Concluding it should be hinted at the abrasive cleaning of turbines in the cold condition („cold cleaning”/cold wash, Fig. "The Art of risk minimizing"). In those cases e.g., during a military mission under desert conditions, the blades will be blasted through the borescope opening during stand still. So it is possible to open blocked cooling air holes (Ill 19.2.4-1 and Ill. 19.2.4-2).
Figure "Hot and cold wash" (Lit. 19.2.3-2, Lit. 19.2.3-8, Lit. 19.2.3-18, Lit. 19.2.3-19 and Lit. 19.2.3-20): Because of the disadvantages of abrasive dry cleaning processes today the wet cleaning/washing has prevailed. Normally water, solvents or cleaning agents are used. Preferred is water because of its availability. Of it relatively high amounts are inserted into the compressor. This happens by a hand guided nozzle (sketch above left and below, Fig. "Dont underestimate idle rotation speed") or by a integrated injection system into the aeroengine (sketch above right and middle). In the cold condition, during rotation by the starter, we speak about „cold wash“ (Fig. "Advantages of cold washing"). A washing process during idle is called „hot wash” (Fig. "Advantages and disadvantages of hot washing" and Fig. "Dont underestimate idle rotation speed").
The application depends on a suitable injection device. Thereby the duration of the washing process plays an importens role. It dictates the costs. Therefore it's essential when the aircraft is available again. From experience this period of time is governed by the following factors:
- Availability of a suitable washing facility/device.
- Positioning of the airplane above a wash basin.
- Time consuming preparation of the washingprocess.
- Expected results seem not worthwile.
- Priority conflicts.Every washing agent has specific advantages and disadvantages.
Solvents can be particular effective for organic deposits like oil, pollen or insect roughness. Their disadvantages are dangers during handling. For anorganic contaminations like salt deposits solvents are not suitable. Successive even aggravated corrosion at the hot parts can occur (Fig. "Potential dangers of cold washing", Fig. "Typical problem zones" and Fig. "Negligence can lead to failures" ). Are elastomers like filled silicone rubber rub in coatings exposed to such washing fluids the danger of deterioration (swelling, embrittling, Fig. "Plastics are sensitive towards fuel" and chapter 23.4-1) exists. Solvent, draining during the washing process or rinsing process can deteriorate elastomer attachments/washers on clamps and tires or paints outside of the aeroengine. A further problem is the gathering and disposing of these contaminated solvent which counts to the most dangerous wastes (Fig. "Disposal of washing medium").
Chemical cleaning agents (detergents) are especial surface active media. They have a stronger cleaning effect as solvents for organic and anorganic matters. It is important that they are biological degradable. Thereby facilities for wastwater treatment must not be damaged.
„Surfactants“ contain water attracting (hygrophilic) and repellent (hygrophobic) components. The hygrophobic components dissolve oils and greases and keep those in dilution. Die active components act dynamic: For the cleaning effect a flow (e.g., air flow) at the contaminated surface is necessary. This removes the loosened contamination particles. For this reason the effectivity must be proven under near-service conditions.
An optimisation of the process parameters takes place with the concentration of the agent, its temperature and the effective kinetic energy of the fluid/droplets. The concentration is defined in specifications. The impact energy is determined by design and given rotation speed. As an influenceable parameter remains the temperature of the washing fluid.
This can only act where it comes in contact with the contamination. Precondition is the right droplet size and droplet velocity (Fig. "Different methods of cleaning"). They should guarantee a washing time as short as possible. Even at the beginning sufficient large droplets can steam off in the compressor so that they get much to small.
- Too small droplets can not overcome the boundary layer of the flow and will be ineffective.
- Too large droplets are centrifugalized to the casing.
Figure "Advantages of cold washing" (Lit 19.2.3-8): During cold wash (crank-soak, off-line washing), the compressor rotor is driven by the starter.
This process has specific advantages:
The relatively low rotor speed, compared with the aeroengine operation enables the cleaning medium a longer interaction. This improves the washing effect.
For the preparation of the washing fluid tap water with sufficient quality can be used. So in this case there is no demineralised water necessary whose availability not always is guaranteed.
The washing fluid does not evaporate in the hotter region. So it acts also in the turbine region.
Because during the washing process, especially at helicopters, the aeroengine may not run, for the washing process no pilot is necessary. The rotating of the compressor with the starter can be carried out by an approved mechanic.
Typical disadvantages of the cold wash are:
For the reason that the washing fluid stays sufficient long effective and does not evaporate, the aeroengine must cooled down enough. For this, depending from the engine type, 15-30 minutes are needed. This increases the time period in which the aircraft is not commercial available.
Starters are highly loaded high-speed devices. They are normally not optimised for longer running times. This is for example true for the lubrication supply and the power transmission. The number of the start cycles can additionally shorten the lifetime of the starter.
In several aspects it is alarming if contaminated washing agent as a fluid or fine dispresed gets into the environment. To the health hazards especially belongs cadmium. It is used as corrosion protection in old aeroengine types. Therefore the contamionated washing fluid must be collected and disposed (Fig. "Disposal of washing medium"). This can be an expensive process.
Must the washing fluid, like in the most cases be rinsed out of the aeroengine the danger of corrosion exists if humidity remains for a longer period of time. Typical examples of endangered components of the forward compressor region are polyester rub in coatings filled with aluminium powder (volume 1, chapter 220.127.116.11) or casings made of magnesium alloys. Generally porous rub in coatings are to consider, which are frequently used in the compresssor of modern aeroengines (volume 2, Ill. 7.1.1-6). They can be soaked with liquids/fluids. So the effectivity of the rinsing is put into question. Even after the drive out of the humidity, dumped contaminations can remain. Thereby unexpected effects like the debonding of coatings are possible. To avoid this, the aeroengine must be dryed-out with a following run.
The experience shows, that during a non optimalwashing process respectivekly rinsing process contaminations from the compressor can be washed to the hot parts, especially on the turbine blading. Remain there sticking contaminations and dry, the danger of hot gas corrosion exists (Fig. "Potential dangers of cold washing"). Presumably this is also related to a brazing like sticking of the turbine blade firtrees in the disk slots. Gets contaminated washing water into the cooling system of the turbine blading the effectiveness of the following rinsing process is at least doubtful. Does increasing oxidation respectively hot gas corrosion inside the cooling channels during operation develop acts this heat insulating. Additionally the heat transfer into the cooling air flow will be hindered by a constriction of the cross section. Then it must be reckoned with a markedly local increase of the temperature niveau. This can shorten the life of the hot parts considerably (volume 3, Ill. 12.5-4). A potential danger exists if contaminations like dust get into segmentation cracks of ceramic thermal barrier coatings, necessary for the function (volume1, Ill. 18.104.22.168-5 and volume 3, Ill. 22.214.171.124-4). Such a blockage of the thermal expansion triggers a spalling of the coating. Also thinkable is a reaction with the lower positioned undercoating/adhesion coating or the base material (volume 3, Ill. 126.96.36.199-5). An accelerating oxidation is connected with a shortening of the life by chipping of the coating.
A relatively low rotorspeed of the compressor can not build up enough pressure of the compressed air during the washing process. With this the function of the sealing air in the seals, especially in the main bearing chambers, is no more ensured (example 19.3.2-1). So contaminated washing fluid and/or rinsing water gets into the oil when it enters the area of the main bearing chambers. There is a certain operation time required to evaporate the water out of the oil. Extensive problems at the components in the oil system, especially the bearings are the result.
Illustratoin 19.2.3-5 (example 19.2.3-1, Lit. 19.2.3-25): The investigation of the aeroengine from the crashed helicopter (upper sketch) showed as cause a broken blade of the 1st gas generator stage (Skizze unten). The fracture was triggered by vibration fatigue. This started from a hot gas corrosion pitting (sulfidation). The blading of the turbine wheel showed pittings and also at many further blades fatigue cracks.
Befor the accident no performance test was carried out. Also a washing of the compressor could not be seen in the documents. The whole operation time of about 1400 hours of the crashed helicopter occurred under „non-corrosive conditions”. This „non-corrosive environment“ is defined as „in dry slightly populated inland”. This does not require mandatory the washing of the compressor. Before the assembly the turbine wheel operated about 200 hours in an other aeroengine under unnkown environmental conditions.
The OEM refers in a commercial service letter to the problem of corrosion-sulfidation. For this the conserned aeroengine type is already known.
During operation in corrosive and dirty atmosphere the washing of the compressor is recommended (Fig. "Negligence can lead to failures"). If during test runs the aeroengine performance accordant to the specification is too low, washing of the compressor is prescribed. There was the fear, that many a washing water itself could act corrosive.
For this reason operators in unproblematic atmosphere use compressor washing seldom. A washing only takes place if the intake area of the compressor is visible contaminated or the performance dropped too much. Thereby it is not clear, if the washing process also cleans the turbine wheel sufficiently.
By contaminations of the compressor polluted washing fluid can settle on the turbine blading. This is especially to expect if no sufficient rinsing process followed.
So contaminations concentrate at the hot parts and can trigger heavy sulfidation during operation.
Example 19.2.3-1 (Fig. "Potential dangers of hot washing", Lit. 19.2.3-25):
The helicopter used for forest operation was hovering about 50 meters above a clearing. Suddenly the cabin filled with black smoke. The pilot veered off and tried to reach a street.Then he noticed from the speed of the mainrotor a drop of the aeroengine preformance. With autorotation an emergency landing was tried. The helicopter had a hard touchdown beside the street, the nose down forward. Thereby the landing skid crippled and the helicopter laid itself aside. The engine run further till the pilot shut it down. He and both passengers were injured, the helicopter heavily damaged.
Subsequent an investigation of the aeroengine took place.
At first a run on a test bed should take place. However this test was cancelled because of heavy vibrations.
After the disassembly of the aeroengine a blade failure of the gas generator turbine wheel 1st stage arose. Perhaps this stood in causative connection with the corrosion due to a washing process. Obviously the smoke in the cabin was caused by the unbalance related failure of the labyrinth seals from the bearing chambres.
Figure "Potential dangers of cold washing" (Lit 19.2.3-27): In the last years several cases emerged, at which on a single engined fighter aircraft type it came to a sudden challenge of the oil flow in the aeroengine. This situation was frequently detected below 100 operation hours after the maintenance during cleaning of the filters. There was a dark read sticky deposit which reduces about 80% of the filter cross section. The depositions contained magnesium and phosphor. The used oil correlated the specification (Def.Stan 91-100, see chapter 22.2). An investigation of the cause showed:
Magnesium casings in the oil circuit were equipped with the following typical corrosion protection:
- Chromat layer on the base material,
- Epoxid coating ,
- Finishing coat/paint.
However the oil wetted surfaces without finshing coat showed at corners and edges of the flanges corrosion pittings (volume 1, chapter 188.8.131.52). So the phosphor containing additives in the oil could, under certain conditions attack these unsufficient protected areas. Corrosion tests at merely chromate protected or not at all protected magnesium surfaces showed no corrosion attack in clean washing fluid or clean oil. In contrast, in washing fluid contaminated with oil heavy corrosion occurred. The reaction products correlated with the deposits in the filters.
Obviously the increased oilflow during high aeroengine load (e.g., start) lead to the removal of the corrosion products and accelerated deposition in the filters.
Figure "Disposal of washing medium" (Lit. 19.2.3-23, Lit. 19.2.3-26 and Lit. 19.2.3-28): During washing of an aeroengine (compressor and/or turbine) not only the contaminations will be removed. Washing agent and rinsing also dissolve possible corrosion products and during slight erosion also small amounts of the base material. So it can come to many different contaminations, also problematic for the environment (hazardous waste) in the used washing fluid. Thereby naturally the operation respectively the mission play an important role. Aeroengines of helicopters can therefore considerably differ from those of airliners. Also fighters let expect specific contaminations. A mission in sea atmosphere remarkedly differs from missions over land (e.g., in a desert). Ingested contaminations can be expected from sea salt (Na, Cl) and dust (e.g., sulfur in gypsum dust). For fighters it must be thought about sucked in shooting residues and exhaust gases from rockets (volume 3, chapter 184.108.40.206).
Organic compounds primarily may derive from the aeroengines own oil circuit and from lubrications/greases in the compressor region (e.g., mounting of the variable compressor guide vanes). At airplanes which frequently or over a longer period of time stand in the exhaust gases of other airplanes (e.g., in a waiting line before the start) or for militäriy aeroengines during test runs also organic deposites like soot or fuel residues are no rarity.
Residues of the in the gas duct used aeroengine specific materials can be found in the washing fluid. Normally these will be corrosion procucts from the operation. The washing fluid is in the position to absorb those in form of easy water solvable salts or fine dispersed oxides. To those counts the environmental toxic pollutant cadmium respectively its compounds. It derives from elder aeroengine types of a corrosion protection coating (nickel/cadmium). This is used for casings, rotor parts and bladings made of unsufficient corrosion restistant steels e.g., of the type 13% Cr-steel and low alloy steels/“boiler steels”, Fig. "Unclean washing water"). This layer offers a cathodic protection which functiones by the faster dissolving as the base material. Those toxic reaction products are entrained by the washing water. Therefore there is a search for an alternative corrosion protection.
Further aeroengine own contaminations are paint particles, residues of chromium and nickel, lead, barium and copper. Nickel residues can be expected in aeroengines with the already mentioned nickel-cadmium-corrosion protection. Nickel like chromium can develop as oxide of the hot parts or as abrasion of nickel-graphite rub in coatings. Chromium contamionations can be traced back to corrosion of chromium steels (Fig. "Unclean washing water"). Those blading materials are typical for elder compressor designs. Lead has its source quite in the water supply itself. For copper and zinc brazing material (copper, brass), is under suspicion. This is used in compressor guide vanes of elder aeroengine types built by brazing. Is cadmium replaced by zinc as corrosion protection it must be reckoned with zinc contaminations, because of its also cathodic protection effect.
An own problem from experience have residues of silver. Silver can be especially found on bolts and nuts. Water soluble silver salts can from experience develop by the attack of aggressive humidity (e.g., with sulfur oxides from an industrial atmosphere). They should be hardly problematic for the environment. However if they are displaced on hot parts, they can later trigger during operation pittings (sulfidation, volume 3, Ill. 12.4-14). So this can dangerous lower the cyclic life of components like turbine rotor disks (volume 3, chapter 12.4). To avoid that, those problematic materials are displaced in the aeroengine and/or get into the environment as well in civil use as in military use there are facilities for collection (see picture) and aftertreatment. They are subject of official instructions and are not a matter of pleasure for the operator.
Figure "Unclean washing water" (Lit. 19.2.3-24): If there is unsufficient corrosion protection, washing can itself trigger corrosion in the compressor. This occurred in this case of a fighter airplane in the forward stages of the high pressure compressor. At several aeroengines guide vanes made from a corrosion sensitive steel with an aluminium diffusion coating showed already after about 400 operation hours considerably corrosion. It was located in small pustules at susceptible surfaces of the outer shrouds and the rub strip in 6 o'clock position (detail). Those pustules at the guide vanes are located at defects in the corrosion protection coating. There developed corrosion products (oxides) with aluminium, chromium and iron. At the corrosion medium pointed contents of sulfur, chlorine, calcium and potassium. Der compressor is washed with on-site tap water which contains the mentioned elements. The suspicion is not far, that after the compressor washing no sufficient drying took place. Obviously the dirty washing water remained at the „bottom“ of the compressor casing and could act corrosive over a longer period of time. Especially in military use over relative long stand still times this assumption suggests itself.
Corrosion may also develop at fretting zones of the contact surfaces whose corrosion protection was markedly degraded. If the casing of this aeroengine type, different as the steel rings of the guide vanes (outer shrouds) are made of a corrosion-resistant titanium alloy (can not be seen from the available documentation) above this it must be also reckoned with the formation of a corrosion cell.
Illustratoin 19.2.3-9 (19.2.3-8): Hot wash normally is carried out during idle. This means the combusion chamber is operating, hot gas powers the turbine(s) and so the compressor(s). This saves waiting time compared with cold wash if the washing process is carried out before or after the air traffic/flight (Fig. "Advantages of cold washing"). Also a starting sequence is omitted and with this the additional load of the starter. The often mentioned advantage of the hot wash that no contaminated washing medium accumulates which must be depolluted and so no leaking fluid which contaminates the environment as well as depollution cost are avoided must be seen rather crucial.
If the washing fluid carries contamionations which are problematic for health, they will at the latest evaporate in the hot section and/or be chemically modified. They exit with the hot exhaust gas the aeroengine and so get fine dispersed into the atmosphere. Then at least the question must be asked after the consequences for the maintenance personnel. Therefore for aeroengines where critical emissions must be expected (e.g., cadmium, Fig. "Disposal of washing medium") the acceptance of the process must be checked by responsible positions/authorities.
A further aspect is the possibility of a recirculation of the concerned exhaust gases. There rises the question if with this the risk of a new short-term contamination of the compressor during the washing process exists?
It is sure, that a sufficient sealing air supply of the bearing seals (sufficient high compressor rotor speed) is beneficial. So an alarming contamination of the oil (Fig. "Potential dangers of cold washing"), other than for cold wash, can be ruled out.
During hot wash different washing media are used (see also Fig. "Hot and cold wash"):
As washing fluid since long time a mixture of water and fuel (kerosene) is used. Emulsifier can be added. This washing fluid is used today more and more seldom, because aromatic hydrocarbons are banned for environment protection reasons in some countries. There also the danger exists that the emulsion again separates into water and kerosene. A washing process after this demixing can dangreous overheat the aeroengine and in an extreme case catastrophical damage it. Spontaneous hot part failures and/or overspeed of the rotors are the consequences (Fig. "Dangers of interchangeability" and volume1, chapter 5.5). Therefore many operators ban this washing method.
Gets kerosene in contact with elastomers like rub in coatings in casings and labyrinths (Fig. "Typical problem zones") these can swell or embrittle.
Normally cleaning fluids which base on solvents consist of 50-70% solvent agent and water plus a rest emulsifier. The mixture will be preparated before the washing process. Here there are environmental concerns as well for health reasons (aromatics) as because of odour nuisance. The handling is not hazard-free at least because of the flammability of the solvent agent.
It is extremely alarming if the solvent agent can damage elastomeres (rubber) and plastic coatings. This is filled silicone rubber of rub in coatings in casings or labyrinths of the forward compressor region (Fig. "Typical problem zones"). Gets washing fluid from outside on the aeroengine washers in pipeline clamps can be endangered. Also must be checked, if rub in coatings on the basis of plastics like polyester filled with aluminium powder can be damaged. A corrosion effect at some metals can also not ruled out.
Besides the potential damage of components (e.g., brazing) there must also considered a contamination of the environment by metal compounds. To avoid this, a complete and effective rinsing is needed.
Some solvents can embrittle metals and/or plastics like actylic glass (front shields). Therefore an aeroengine specific testing and approval of the solvent containing washing fluids is essential. To this belongs an affirmation of the OEM and the responsible authorities.
Aqueous-based cleaners enjoy itself an increasing popularity. These are solutions of cleaning media in water, sometimes by addition of inhibitors (against corrosion). The mixing ratio for example represents 4 parts of water at 1 part cleaner and is produced before the washing process. A low content of solvent avoids inflammability. Processing temperatures below the freezing point require the addition of antifreeze. Corrosion inhibitors are especially recommended for military aeroengines because here it must be reckoned with longer stand still periods in which corrosion can be active. With water based cleaners the following approach of military operators can be avoided. They spray water displacing liquids into the aeroengine if longer stand still times are to expect. Before a start of the engine those media must be completely washed out.
Figure "Typical problem zones" (Lit. 19.2.3-1 and Lit. 19.2.3-8): It must be prefixed, that for the case of application only sufficient tested, defined, approved and obeyed processes guarantee to avoid problems and failures. That this is obviously not always the case show the illustrations of this chapter. In the following those potential problems and risks are shown in a summary without the claim of completeness:
Compressor blading: In modern compressors the minimisation of the tip clearance has a high significance. To guarantee this, there are relatively soft, often porous rub in coatings in the casing. Those can be unacceptable eroded even by seemingly hardly erosive acting particles and droplets of the cleaning medium. Also the tips of the rotor blades of modern compressors with typical thin profiles and extremely thin edges can be changed by abrasion and so the operation behaviour of the compressor can suffer. Layer corrosion with blisters and ablation can occur at corrosion susceptible rub in coatings like resins filled with aluminium powder (Volume 1, Ill, 220.127.116.11-1). Rub in coatings like filled elastomeres (silicone rubber) can get inoperable by swelling or embrittling.
Corrosion susceptible casings, blading materials and disk materials like martensitic steels, aluminium alloys and magnesium alloys in elder aeroengine types can corrode during stand still periods if humidity remains (e.g., at military aeroengines).
Will be lubrications (oils, greases, infiltrated anti- friction agents) at variable compressor guide vanes washed out, the function will be impacted by stiffness or wear caused increased clearance. At such changes the operation behaviour of the compressor reacts sensitive.
The lifetime of cooled hot parts, especially turbine blades, reacts extrem sensitive on seemingly little increase of the material temperature. Reduction of cross sections from ducts for cooling air must therefore taken very seriously. A carryover of contaminations from the compressor can clog the bore holes from the outside and inside. Are contaminations like dust and/or low melting metals washed between the contact surfaces of the blade root (fir-tree) the high operation temperatures can fix the blades like braze. Is, for example, the vibration damping decreased this will promote fatigue cracks. Also a drop of the strength because of diffusion or hot gas corrosion is dangerous in the highly loaded root region. Are from contaminated washing medium metals, metal compunds, sulfur compounds (e.g., dusts) and chlorine compounds (sea atmosphere) deposited on the surfaces of hot parts it must be reckoned with an accelerated hot gas corrosion/sulfidation. This occurs inside of cooling air bore holes and on the outer surface of the blade/vane.
Also seemingly corrosion proof components like turbine disks and compressor disks made of forged nickel alloys like Waspalloy are endangered by pittinglike corrosion attack. This for example is the case if aggressive contaminations remains in washing media react during stand still. Such a situation arises, if silver from bolts and nuts will be dissolved and deposited during operation on an other area (volume 3, Ill. 12.4-14). At operation temperature these deposits dry and trigger sulfidation.
Figure "Significance of aircraft specific washing procedure" (Lit. 19.2.3-19): The selection of the cleaning process respectively washing process and its parameters must consider the particular features of the aeroengine and the expected contaminations (Fig. "Cleaning military aeroengines"). So the washing medium must not act damaging/deteriorating. Effects like erosion in the compressor (Fig. "Typical problem zones") or an attack of plastics/resin and paints must be considered. The spray device is also determined by design features like a particle separator in front of the aeroengine and the contaminations which must be removed (Fig. "Different methods of cleaning"). Attention must be payed that the contamintions are not washed into the turbine and can act there damaging during the operation (Fig. "Potential dangers of hot washing").
Figure "Cleaning military aeroengines" (Lit. 19.2.3-6): Especially in military mission of helicopters in desert environment it can come inspite an upstream dust deflector/particle separator to a inacceptable drop of the aeroengine performance. Besides the erosion of the compressor blading and/or the forming of deposits, the dust can soften in the combustion chamber and deposit as a coat on the turbine blading. Is the cooling of the blading in such a manner deteriotated it must be reckoned with the rise of the material temperatures and an exponential drop of the creep life. Therefore especially also the turbine blading must be cleaned. For this a washing injector is inserted through the bore hole of an ignition plug into the combustion chamber. Through this a soap containing washing fluid is sprayed on the hot parts while the gas generator is driven by the starter.
It can be supposed that glass-like molten coatings on the hot parts (volume 3, Ill. 18.104.22.168-14), especially the turbine blading can not be fully sufficient removed. In other extreme cases, for example at fighter engines with missions in desert areas, the turbine blades will be blasted with abrasive media through the borescope holes (Fig. "The Art of risk minimizing" and Ill. 19.2.4-2).
Figure "Surge events as a result of contamination" (Lit. 19.2.3-9): According to the literature, it seems that shaft aeroengines (helicopter, Fig. "Significance of aircraft specific washing procedure") respectively turboprop engines (Fig. "Importance of cleaning 1") are especially sensitive for compressor contaminations. An explanation is leaking oil from the forward shaft system (bearings, coupling) or gear. The leaking oil gets than with the intake airstream into the compressor.
Besides a drop of the performance obviously the decrease of the surge margin is a reason for concern. For the shown propulsion type the surge margin is sufficient again after compressor washing. Therefore time periods are recommended respectively claimed by the OEM for an inspection of the compressor for contaminations respectively washing processes.
Illustraton 19.2.3-14 (Lit. 19.2.3-21): This crash could be traced back on heavy contamination in the compressors of all four aeroengines. Two neighboring engines failed during touch-and-go and the airplane tilted in about 100 meters above ground. Eye wittnesses saw at a previous touchdown a cloud of dark smoke. Possibly this was the result of a surge in the compressor.
A later analysis of the cockpit voice recorder (CVR) permitted conclusions from the recorded noises at the behaviour of the aeroengines. Obviously after the short term touching the ground at two not identificated aeroengines the rotation speed dropped about 40 % .
The following investigation showed, that all surfaces and components in the compressor of the aeroengines were covered by a tar like coat. Besides a normal synthetic aeroengine oil for the adjustment of the propeller hydraulic fluid was used.
Especially the bleed valves (sketches in the middle and lower details) showed outside and inside the tar like coat. This hindered the function of the valves at the 5th compressor stage. At the contact surfaces of the valve pistons there were thick undamaged deposits. From these it could be suggested that the valves were partial open since some time. So the aeroengine performance was affected by an efficiecy loss and the malfunction of the bleed valves. Tests showed that the malfunction of the bleed valves caused about 20 % drop in surge margin and about 12 % drop of aeroengines performance.
Further investigations unfolded, that the washing of a compressor with a cleaning agent was too little effective to increase the performance. However this succeeded with walnut shells. After this again 100 % start power was reached and about half of the surge margin drop could be retrograded. The airflow rate was only about 2,4 % under a new aeroengine and the compressor efficiency within 1,3 % of the normal range.
Fig. "Importance of cleaning 2" (Lit. 19.2.3-10): The washing of a compresor serves in most cases the increase of the efficiency and the surge margin of contaminated compressors. The often corrosive acting contaminations (sea salt with condensate during stand still) are a danger for elder aeroengine types with martensitic steels. At these steels, frequently these are chromium steels, pitting corrosion develops (volume 1, Ill. 22.214.171.124-3). These corrosion pits have a high notch effect and therefore are origin of vibration fatigue cracks (detail). Especially concerned was the rotor disc which combined 2nd and 3rd stage (sketch).
The showed case is a very common aeroengine type (lower sketch) of which minimum 80 cases of cracked or broken blades emerged. This correlated with about one incident in one million flight hours. The corrosion protection was yielded to the operator. The failure rate of unprotected parts was about double as high than for parts with corrosion protection coating.
Already 20 years before this incident in a „commercial service letter” (not binding) the operators were remembered to clean the compressor daily with water during operation in a corrosive atmosphere. It was problematic, that the term „corrosive atmosphere“ was not sufficient defined. In the case on hand the operator decided for a washing interval of 100 operation hours.
It must be noted, that from experience not the operation time is the criterion for a corrosion load but the stand still times. During these contaminated condendsate as electrolyte can react (Fig. "Negligence can lead to failures"). Therefore the specification of a washing interval in operating hours is problematic. The stand still periods also relativise the term „corrosive atmosphere” in which the stand still times should be reasonable considered.
In the operation manual and maintenance manual meaures were specified for the case that already rust and/or pittings can be observed (not easy!). Preventative actions to avoid the damage are not indicated.
Unfortunately in the case on hand the washing processes were not satisfactory documented.
The supervision authority and the investigating authority required because of this accident:
Suitable information at the operator of this aeroengine type about the risk of corrosion of the compressor blading and the importance of regular washing processes.
Figure "Negligence can lead to failures" (Lit. 19.2.3-16): The airplane has a turboprop engine (sketch below). An investigation of the aeroengine after the failure occurred showed, that a blade of the gas generator 1st stage turbine wheel was broken in about half height (detail). The released fragment caused the fracture of two further blades following against the direction of rotation. The other blades were damaged. The fracture started from a deep sulfidation pitting. Similar craters were found in further seven blades of the same stage. The compressor showed also heavy corrosion like it is typical for operation in sea atmosphere.
The appropriate maintenance handbook claims for the operation in sea atmosphere a daily wash of the compressor and the turbine <U>after </U>the last flight. The operator undertook the washing of the compressor daily <U>before</U><U> </U>the first flight. The washing process of the compressor was carried out with an, in the aeroengine installed device. For the turbine the OEM offers a special device, by which the washing solution with the help of a pipe through the bore holes of the ignition plug is directly sprayed at the blades of the first turbine stage. This facility was not used by the operator. So it was possible, that dangerous saline deposits could form on the blades.
The aeroengine was supplied with an engine condition trend monitoring system (chapter 25.2.1). This facility releases the operator from a determined, regular hot parts inspection. Instead it will be preceeded, corresponding with the time indications of the monitoring system. The operator did not comply with these by the responsible aeronautical authority accepted and prescribed procedure. So it was hereon once more confirmed. Accordingly for an operation in sea atmosphere it is to apply:
- Daily washing of compressor and turbine after the last flight.
- Consideration of the engine condition trend monitoring system correspondent with the associated instructions.
- At the latest after 100 operation hours a borescope inspection of compressor and turbine.
Alternatively a hot part inspection after 750 operation hours is possible.Comment: A compressor is, different to a turbine, endangered by aqueous corrosion. This does not act during operation where the water evaporates. Water is also centrifuged by the rotor and/or blown off from the airstream. Corrosion in the compressor happens therefore during stand still periods (Fig. "Importance of cleaning 2"). For this reason it is from essential importance if corrosive media <U>before </U>a stand still time (after the last flight of a day) are removed or before the first flight the next day. In this case the corrosion can act over night.
For the turbine those considerations may not be so crucial. Here develops hot gas corrosion, especially sulfidation, during operation and this needs no watery electrolyte.
Figure "Dont underestimate idle rotation speed" (Lit. 19.2.3-17): Is a hot wash carried out, that is during running aeroengine (Fig. "Hot and cold wash"), despite idle running the danger to suck foreign objects exists. Therefore during compressor washing it must be looked after, that no loose equipment objects can get into the aeroengine intake.
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