Why Wash Compressors?
When gas turbine engines are run, they become fouled with airborne contaminants such as oil, soot, unburned fuel, soils and salt which encrust compressor components. Power loss follows as the compressor loses efficiency and degrades the performance of the engine, leading to higher operating temperatures, increased fuel consumption and shorter component life.
Some engines are more prone to fouling than others – low flying over land or sea, or operating in contaminated air, have led to helicopter engines and industrial engines traditionally needing cleaning – but nowadays it is recognised that all engines foul to some extent; for example, oil leakage and subsequent fouling can happen on any engine type. Contamination can also be caused by ingestion of exhaust gases from other aircraft during prolonged waits for takeoff clearance.
How Do You Clean an Engine?
There are three main methods:
Strip the engine and hand-clean it
This is effective but very costly, so it is seldom carried out for performance recovery only, but usually coincides with a shop visit for routine maintenance. Thus it is carried out so infrequently that a long deterioration of performance is inevitable between cleans.
The engine is run whilst crushed walnut shells, peach stones, coke or similar material is fed into the air inlet. During its passage through the compressor the abrasive material removes contaminants by “shot blasting.” Unfortunately, it can also erode blade protective coatings (which are becoming increasingly common), damage machined surfaces and undesirably enter bleed valves or other components, and for these reasons it is prohibited on many engines. This is really a last resort for performance recovery of a very badly fouled engine rather than a maintenance tool. There is also evidence that any performance recovery achieved can disappear very quickly after the clean
This is the best answer. Performance is regularly restored in a simple and quick operation which maintains power at optimum levels without abrasion or hand-cleaning.
Washing may be carried out with the engine running (here referred to as “hot washing,” also known as fired or on-line washing), or whilst the engine is being turned on the starter motor (“cold washing,” also known as crank-soak or off-line washing). Each method has advantages in certain operational conditions.
How Does Hot Washing Compare With Cold?
Cold Wash – Advantages:
This is the slower process, so the cleaner is in contact longer and has more chance to do the job.
Good quality tap water may be used for dilution of the cleaning fluid. The combustion and turbine areas are cold, so they get cleaned too. Highly qualified personnel may not be required (usually in helicopters a pilot must be in control when the engines are running, for safety reasons. However, maintenance personnel are permitted to turn the engine on the starter for a cold wash).
There is a time penalty as washing usually cannot be carried out until 15 – 30 minutes after engine shutdown because of engine temperature.
Component wear. Starter motors are subjected to extra cycles.
Dirty washing fluid is expelled from the exhaust onto the surrounding area.
Corrosion. Most washing fluids must be rinsed out with water, then the engine must be hot-run to dry out in an attempt to minimise corrosion.
Oil contamination. Some engines have air-blown sealing arrangements which only work at high engine speeds. Cold washing can allow the oil system to become contaminated with cleaning solution. The water thus admitted may be removed by hot-running the engine until the water is boiled off.
Disposal of effluent can be costly.
Hot Wash Advantages:
Speed of operation.
No engine shutdown needed.
No mess, as the washing fluid evaporates in the exhaust stream.
No effluent disposal necessary.
Less risk of corrosion, as all moisture is dried out.
Air-blown seals work properly, preventing ingress of fluid into oil.
Fewer starter cycles.
Very short dwell time, so a powerful cleaner is needed.
Selection of flow rate is critical. Too low a rate will not clean properly. Too high a rate may cause blade damage or flameout.
Fire risk. Flammable cleaners containing solvents can act as fuel, overspeeding the engine and causing massive damage.
Helicopter application usually requires a pilot to be present.
Attack of turbine materials at high temperatures is possible if washing fluids with poor formulation or manufacturing standards are used. Component failure may also be caused by these materials blocking turbine blade cooling holes. The presence of salt carried through from the compressor may add to this risk.
Demineralised water needed for dilution of cleaning fluid.
Hot washing is prohibited on some engine types.
Both methods have their advantages. We feel that cold washing is simpler and may give a better clean. Hot washing is useful where engine shutdown is undesirable or speed of operation is required.
In general, operators should follow the engine manufacturer’s instructions.
What Do You Wash With?
There are three main types of cleaning fluid.
Water and kerosene, with or without an emulsifier
This is the longest-established fluid. It cleans quite well and is cheap, especially since operators have kerosene available as fuel and sometimes forget the cost of this component.
It is not widely used now, partly because aromatic hydrocarbons (present in the fuel or the emulsifier) are banned in some countries, but mainly because the emulsion can split into water and kerosene. When this happens during a hot wash, the results can be catastrophic in overfuelling the engine and have led to the loss of aircraft in the past. The procedure is prohibited by many operators for this reason.
Usually these contain 50%-70% solvent such as White Spirit with the remainder being water with a detergent present to emulsify the solvent/water. They are diluted with water before use. Generally they are good cleaners, and are in wide use in the USA . However, the trend now is away from solvent-based cleaners. They tend to have an unpleasant smell and may be harmful, and handling is hazardous. Many contain aromatics and are banned in some countries. They are usually flammable and have restrictions on transport or storage. They can attack rubber seals causing swelling and possibly permanent damage to the rubber, and soften the paintwork of aircraft on which they splash. They can also be corrosive to some metals, and need many rinses to completely remove all traces of cleaner after a wash. Many specifications also prohibit the use of cleaners which cause embrittlement of high-strength steel or crazing of acrylic windows.
Although these products may be stored at +10ºF(-12ºC) without freezing, when used at low temperature an antifreeze must be added.
Aqueous-based cleaners (e.g. ZOK 27 and ZOK mx)
This is the most recent development in compressor cleaners and is steadily replacing older methods. Aqueous-based cleaners are solutions of detergent in water, sometimes with corrosion inhibitors. Usually diluted 1+4 with water before use, they have little solvent present and are non-flammable
They offer safe transportation and handling and ZOK 27 has corrosion- inhibition as a bonus. They tend to freeze around 0ºC, so for operation at low temperature an antifreeze must be added.
Is Inhibition Needed?
The more an engine runs, the less it needs corrosion inhibition. However, relatively low-time operators such as the military often spray water-displacement fluids into their engines after a wash to displace any water present and to protect against corrosion if the engine is not to be run for some days, or if it is going into long-term storage. This material must be completely washed out before the engine is run again.
These operations are not required if ZOK 27 has been used for the wash and not rinsed out, and if the engine is due to be used again within a week.
Offshore operators of industrial gas turbines have found that they can wash an engine with ZOK 27 before shutdown and then just leave it, without having to use dewatering fluids. If the engine is shut down for more than a week, the operator simply gives it another ZOK 27 wash – not to clean it again but to “freshen-up” the inhibition effect of the ZOK 27.