Machinery Lubrication magazine published by Noria Corporation
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of the system oil. The only symptom was a darkening of the oil. When a sample of the oil was analyzed, it was determined that the acid number and viscosity had not changed, eliminating the likelihood of oil oxidation and suggesting that the change in color was the result of thermal degradation. Inspection with an infrared camera located the overheating valve in very short order. The valve was replaced, and significant varnish was noted at the point where the heat was generated. Oil analysis showed that the oil was perfectly suitable for continued service, but since there was no discernible change in system operation, the valve failure may very well have gone unnoticed until it became a system outage had there been no color change in the oil. While oxidation, the chemical union of oil and oxygen, is a common reason that hydraulic oil stability is reduced, the amount of color change is not a good indi- cation of the level of oxidation. Antioxidants will react as they do their job, frequently producing colors ranging from brilliant yellow to inky black. There are a number of factors that include formulation, oper- ating conditions and contaminants, any of which can cause a considerable color change without significant oil degrada- tion. Although the color change can be alarming, the oil can still retain good anti- oxidant potential, since a series of these reactions can occur before becoming truly exhausted. Again, the only way to be certain of the oxidation level is by oil anal- ysis. Look for an increase in the oil's viscosity and acid number as an indication of oxidation. The presence of metal catalyst parti- cles, heat, oxygen and water all contribute to oil oxidation. As the acid level increases, corrosion of components will become more likely. Viscosity will increase as soluble contaminants mix with the oil. This will leave sludge, varnish and tar deposits as a thin, insoluble film throughout the system's internal surfaces. The degrada - tion process accelerates with continued exposure to these elements. HYDRAULICS 24 | May - June 2016 | www.machinerylubrication.com