Year of publication: 2007
During their use, all oils age. During the oxidation process, oil molecules react with oxygen, among other things. Acidic reaction products and oil-insoluble components are formed. Elevated oil temperatures of over 60 °C, impurities and intensive air intake accelerate the aging process. To delay it as long as possible, additives with phenolic or aminic antioxidants or with salicylates are added to the lubricants. They can buffer and neutralize oxygenated compounds. However, if the additives are completely loaded and used up, the aging process of the oil accelerates almost unchecked.
In large circulation systems, such as turbines, paper machines or rolling mills, oils must be able to remain in service for several years. But gear oils of wind turbines and larger hydraulic oil fillings should also not be changed for far more than 20,000 bh. Therefore, their oxidation stability in particular plays an important role. The degree of oil oxidation or aging that has already occurred is usually determined by FT infrared spectroscopy. However, it does not allow reliable conclusions to be drawn about the oxidation stability still present and thus the remaining serviceability of the oil.
The RPVOT test (RPVOT= Rotating Pressure Vessel Oxidation Stability Test) provides a remedy. It tests the resistance of the fresh oil or the oil in service under extremely oxidizing conditions. In this way, a statement on possible oil change intervals can be made within a very short time.
For the test, 50 g of the oil in use and 5 g of distilled water are weighed into a reaction vessel. A previously polished copper coil is added to the oil-water mixture as a catalyst. The reaction vessel is tightly screwed into a pressure-resistant stainless steel container. The internal pressure of the pressure vessel can be permanently recorded on a manometer. The apparatus is now filled with pure oxygen to a pressure of 620 kPa.