That’s certainly a leading question. Perhaps the answer is not that difficult in reality.
The compressor turbine of a gas turbine engine absorbs approximately 60% of the power generated by the hot section of the engine. Only 40% of the generated power is available to drive the aircraft. Therefore any power loss is significant to the performance. How does a dirty compressor affect performance? The blades of the compressor are highly tuned, carefully designed aerofoil sections that move at extremely high speed through the airflow. As contaminants build up on the blades, the aerofoil section changes and the efficiency of the compressor decreases. Even very small, changes in the aerofoil section have a dramatic impact on performance.
So where do the contaminants come from? Simple answer – from the air being drawn through the turbine. Oils and hydrocarbons are ingested by the engine, mostly whilst the aircraft is on the ground taxiing and holding. Often the hydrocarbons are coming from the engine’s own exhaust, or at least from the exhaust of the same aircraft, including the APU. These sticky hydrocarbon deposits then act as a glue for dust and other solid particles that are then ingested. It’s worth noting that the engine of an average wide bodied aircraft ingests around 100 tonnes of solid particulate every year!! This cycle of laying down the glue and sticking particles to it then repeats itself on every take-off and landing, slowly reshaping the aerofoil section of the blades and reducing performance and efficiency.
Compressor washing removes the contaminants from the blades and restores the original aerofoil section, thereby bringing the performance back to original specs. Some operators may say; “But I do a water wash every XXX hours on my engine”. Great, but let’s look at this. What do we want to wash off the blades? A mixture of dirt and hydrocarbons, right? And we are using water, often straight from a tap? Think about this for a minute! Do you wash the kitchen cookware in plain water? Do you wash the car with straight water? No, you use detergent to cut through the grease and/or oil. So why wouldn’t we use detergent to clean the hydrocarbons off the compressor blades?
The most common reason is that both OEM’s and engineers are concerned about corrosion. Many compressor washing detergents, if not completely removed from the engine, will cause corrosion damage and it is nearly impossible to guarantee that all traces of the detergent are indeed removed by the fresh water rinse. Thankfully, there are now detergent products that do not cause corrosion. ZOK 27, one product available in the market place, was developed for the British Ministry Of Defence and specifically prevents corrosion whilst cleaning! More than worth its weight in gold!
Studies have shown that it does not take much fouling to create a 1% drop in engine output performance. A 1% decrease will more than likely not even be noticed by aircrew and engineering, so therefore we don’t have an issue? This insignificant power loss is certainly not a safety issue either, so end of story then?
Hang on! Let’s take this issue to the accounting department! No joke here! Ask yourself: “How much do I spend a year on fuel?” Would you like to save 1% of that? The old familiar story is coming back… spend a little money and save a lot of money. This doesn’t even take into consideration cost savings from extending the life of hot section components.
So the bottom line to the question of compressor washing is not so much about engineering – it’s more about airline profitability and that makes good business sense!
Written by Solidus Industries Technical Director Charles Cheesman. This article was originally published in the November 2013 edition of the Aviation Trader publication Airwaves.