"How can you tell if an oil has exceeded the appropriate temperature range?"
Lubricants are formulated with base oils and additives to function under the conditions expected for the intended application in terms of speed, load, temperature, oil change interval, etc. These formulas may be designed for compressors, turbines, engines, gearboxes, etc. When their properties or performance are diminished as a result of contamination, temperature changes or additive depletion, the lubricants must be modified.
Operating temperature is a significant factor in determining a lubricant’s lifespan. The higher the operating temperature, the shorter the lubricant’s service life. This is because the lubricant oxidizes, or reacts with oxygen in the air, changing its chemistry and lubricating characteristics. At higher operating temperatures, the oxidation process speeds up.
A well-known principle that pertains to lubricants is called the Arrhenius Rate Rule. It states that for every temperature increase of 18 degrees F (10 degrees C), the lubricant life is cut in half. This rule is applicable to both high- and low-quality lubricants. Even though the comparative lifespans of high- and low-quality lubricants may be different, the principle of their life being shortened with temperature is similar. Therefore, although synthetic lubricants generally have a longer life and greater resistance to temperature than mineral oils, the law of reduced life with increased temperature still applies.
While there typically is no exact maximum temperature for a lubricant to operate, very high temperatures can result in a fire hazard or require the oil to be changed too frequently.
Oxidation also produces acid compounds in the lubricant. As long as the oxidation process continues during lubricant use, the acidity of the lubricant will increase to the point where it would be recommended to change it. An acid number test can be performed to establish if the oil is still in the proper condition for operation.
Fourier transform infrared (FTIR) spectroscopy can be used to measure various compounds in the oil, including the oxidized portion. In the field, oxidation can be detected by oil darkening and its characteristic odor. However, these parameters are somewhat subjective, so it may not be easy to decide if the oil has oxidized enough to change it.
Other effects of oil degradation due to high temperatures include damage to the additives in the lubricant formula, as well as the formation of sludge, varnish and lacquers in the equipment. These factors can be identified by an internal inspection of the machinery and additional laboratory tests.