In common air compressors, oil injected screw air compressors (hereinafter referred to as air compressors) have very high requirements for the quality of lubricating oil. However, many users have inadequate monitoring and detection methods for lubricating oil in use. Therefore, visual observation of the appearance has become a simple and indispensable visual inspection and discrimination method for oil quality. When the oil color changes, many customers only rely on experience and intuition to make a hasty judgment and give a conclusion that the oil should be changed or continued to be used, but they often lack scientific theoretical understanding of the essence of color change. In this paper, the cause of color change of Air Compressor Oil is discussed. Color of new oil of air compressor The new oil color of air compressor oil is mostly light yellow or nearly transparent and colorless. The color and transparency of Base Oil produced in different regions are more or less different. Therefore, the color of oil products of the same model produced with base oil from different regions may also be different. Therefore, there is no color requirement for most lubricating oil products. Many people tend to think that the closer the oil is to colorless transparency, the higher the degree of refinement. The fact is that almost all lubricating oils can achieve colorless and transparent appearance as long as additives are properly used. Therefore, whether it is colorless and transparent cannot be the basis for judging the degree of oil refining. As long as the indicators meet the requirements, the initial color of the lubricating oil has no effect on the use effect. Color change during use With the installation and use of air compressor oil, the light yellow or nearly colorless and transparent oil gradually becomes darker and starts to show different color changes: yellow, dark red, blue, purple, dark brown, or even black, as shown in the figure.
It is generally believed that common color changes such as yellow and dark brown tend to be normal, while uncommon color changes such as dark red, blue and purple are considered abnormal. Obviously, there is no necessary scientific basis for this. To judge whether the color change of oil is normal, we should first understand the cause of color change. Reasons for common color changes Although oxidation and the mixing of various impurities will inevitably lead to certain color changes, the main reason for color change is the role of antioxidants. Antioxidants commonly used in lubricating oil include amine and phenol. Different types of antioxidants will have different color changes when acting. The color changes of most oils are caused by the color changes of antioxidants. The color change of air compressor oil can be regarded as normal in most cases when it is not accompanied by other obvious fault characteristics, while the sediment in the oil and the oil turbidity should be paid more attention. In the absence of necessary detection means, if the discoloration is not accompanied by precipitation, and the oil is clear without turbidity, it can be preliminarily judged that the use effect is good. Of course, the most accurate indicator is the oil detection. Cause of special green color change There is also a special discoloration of oil, that is, it turns green. After tracking and studying several cases, it is found that when the oil turns green, the acid value is detected to increase and increase greatly, and it is detected that the acid value reaches more than 20mgKOH/g for many times. You should know that even the relatively loose oil change standard will not allow the acid value to reach more than 2mgKOH/g, obviously, this 10 times acid value represents the serious deterioration of the oil.
Through a large number of oil testing, it is found that the additive content in the oil is extremely low, far exceeding the normal loss. At the same time, when detecting the mixed solid impurities, it was found that although the particle size of the impurities was very small, they had a microporous structure in the microcosm. Due to their unique pore structure and the van der Waals' attraction between molecules, they had strong adsorption capacity. Although the impurity particles mixed in the oil do not have the specific surface area of active carbon as high as 500~1500m2/g, they still have strong adsorption capacity. It is these impurities that absorb a large number of additive components, especially metal passivators and antioxidants, which make the oil react with the copper parts in contact (commonly known as copper biting). A large number of copper ions enter the oil, which acts as a catalyst and aggravates the deterioration of the oil. At the same time, the lack of antioxidants makes the oil oxidized rapidly, resulting in a sharp increase in acid value. At this time, a large number of copper ions are dissolved in water, and the oil appears green. From the above analysis, it can be seen that the greening of air compressor oil is often related to pollution, especially in the presence of fly ash, cement powder, etc. Another view is that reliable grounding is required during the installation of air compressor equipment. If the grounding connection resistance is too large or virtual, the equipment is at a high potential, and slow discharge to the ground may cause ionization of oil products, which may also cause green oil products, but the process is very light and difficult to detect.
