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Many oil analysis labs use the crackle test to screen for water contamination in lubricating oils and hydraulic fluids. If the crackle test is positive, the common Karl Fischer procedure is typically used to quantify the water content as an exception test. However, for certain “hermetically sealed” centrifugal-type refrigeration compressor applications (York, Trane, Carrier, etc.) the Karl Fischer method is the best routine test for water.
In hermetically sealed motor setups, the motor is cooled by a refrigerant vapor that flows directly through the motor body. CFC refrigerants are often exposed to water and high temperatures, which promote the production of hydrochloric and hydrofluoric acids. While motor winding deterioration is one effect of this condition, the oil analyst is more concerned with its effect on the lubricant. There are unavoidable refrigerant leaks through the impeller bearing labyrinth seals, and as the refrigerant enters the oil, a part of any acids and excess water present are transported with it.
For chiller systems that use napthenic mineral oils (low carbon-forming potential, low flocculent point, very lightly additized), oxidation and corrosive wear are easily catalyzed by small quantities of water and acids. This can be a double-barrelled attack when water enters the oil from the refrigerant side and the compressor environment simultaneously, or refrigerant-carried acids combine with the acids formed by oil degradation. Various combinations of driers and filters fight this water intrusion and acid formation, but it’s an ongoing challenge to manage the condition.
Since many OEMs recommend the use of this highly effective but contaminant-sensitive oil for their hermetically sealed centrifugal compressors, very low water guidelines (75 to 125 ppm in used oils) are often encountered. This level of moisture is undetectable using the crackle test, making Karl Fischer testing mandatory. It is important to note that the residual refrigerant always present in samples from this type of system has a very low boiling point, and will mask the water response when using the crackle test because the water and the refrigerant both scintillate on the hot plate.