Under wet conditions, a salt-laden air filter can release salts into the clean air downstream side of the filter, posing a real danger for sulphidation corrosion damage to the hot section parts. This is especially worrying when considering today's OEM specifications for the latest technology engines - maximum allowable limits for alkali metal contaminants are stringent, whether the source be the fuel or air.rnWith evolving technology, specifications for trace metals have varied over the years with a very clear trend in the downward direction. For aeroderivative engines with standard coatings, one major manufacturer specifies a maximum limit of 0.1 ppmw sodium, potassium and lithium, from all sources. For units with special marinised blade coatings this upper limit is increased to 0.2 ppmw. Another supplier of smaller engines sets a maximum limit of 0.6 ppmw sodium and potassium in the fuel, which is also quoted as being equivalent to 0.02 ppmw sodium chloride in the air.
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