Compressor washes are a routine procedure for those who work on gas turbine engines. Some mechanics might think it is just another mundane task we must do. But why do we do compressor washes? Well, the answer is more than, “because it is written into our operations procedures.” Let’s take a look at compressor washes and how they affect engine performance and life cycles.
Taking a look back at what we learned in A&P school, we see that thermal efficiency is a prime factor in gas turbine performance. AC 65-12A tells us that thermal efficiency is the ratio of net work produced by the engine to the chemical energy supplied by the fuel. The three most important factors affecting thermal efficiency are turbine inlet temperature, compression ration, and the component efficiencies of the compressor and turbine. Other factors that affect thermal efficiency are compressor inlet temperature and burner efficiency.
Contamination to the compressor section affects the thermal efficiency, and therefore the performance of the engine. Not only does it affect performance, but damage to the blades caused by contamination can lead to engine failure.
So, what causes contamination? Well, it has to do with the atmospheric environment. The atmosphere, especially near the ground, is filled with contaminants. There are fine particles of dirt, oil, soot, and other foreign matter in the air. Because of the large volume of air introduced into engine compressors, a lot of this contamination is introduced into the engine. The centrifugal forces of the compressor throw this contamination outward so that it builds up to form a coating on the casing, vanes, and the compressor blades.
The accumulation of these contaminants reduces the aerodynamic efficiency of the blades, resulting in deteriorating engine performance. The efficiency of the blades is reduced in a way similar to the way ice buildup reduces the lift efficiency of a wing. This loss of efficiency can lead to unsatisfactory acceleration and high exhaust gas temperature (EGT). Contamination, especially in high salt operating environments, can also lead to corrosion of the engine components.
In order to maintain engine performance and reduce the corrosive effects on the engine, the debris that builds up in the compressor needs to be removed. We do this through routine compressor washes.
So, how does a compressor wash remove contaminants from an engine? AMT talked with Bruce Tassone, president of ECT inc. ECT manufactures R-MC compressor wash. “The chemicals in a compressor wash solution break down the organic bonds of the contaminants,” Tassone shares. “This then allows the air stream and/or the fresh water rinse to remove the contaminants out of the engine.”
The OEM specifies which chemicals can be used to wash the compressor. The approved list can vary from manufacturer to manufacturer. “Some OEMs have a specific list of approved washes,” says Tassone. “Others refer to a military specification like MIL-C–85704 or set specific chemical parameters.”
Tassone stresses the importance of using proper chemicals like R-MC. “There are different parameters you have to meet with a compressor wash,” he shares. “Using unapproved engine washes could cause damage to the engine or airframe such as corrosion, acrylic crazing, hydrogen embrittlement, stress corrosion cracking, and other defects.” In order to avoid damaging the engine, be sure to use only authorized chemicals. “You should ask your supplier or overhauler for certification that they meet the engine and airframe OEM specifications,” stresses Tassone.
Premix or concentrated?
Compressor wash can come in either premixed or concentrated forms. How do you know what type is right for you? “Some customers buy premixed if they are concerned with either the labor associated with mixing and/or the quality of water they can secure,” Tassone tells AMT. “If they have space concerns, such as inventory storage, or they want to be a little more cost-effective in terms of the transportation, and they have the capacity to blend the chemical in regards to labor pool and water quality, they may tend to go with concentrate.”
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