Can Lube Oil Varnish be Detected? 

Detecting something is the first step towards formulating a solution to minimize its effects or eliminate it from a system. In the case of varnish for lubricated assets, a few technologies are currently being used to detect its presence.

As seen at the beginning of this article, varnish can exist with various characteristics depending on the degradation mechanism which aided in its formation. For this article, the degradation mechanism of oxidation will form the main focus as it is the most prevalent pathway to lube oil varnish formation.

During oxidation, the first chemical change which can be observed in the lubricant is the depletion of the antioxidant additives. This is where the knowledge of phenols and amines is critical.

As per Livingstone et al. (2015), these antioxidants can form synergistic mixtures in mixed antioxidant systems. When the free radicals react with the phenols, they become depleted but can regenerate amines. Thereby, the phenols are sacrificial.

Thus, when performing the RULER analysis, one can find that the concentration of the phenols will typically deplete quicker than the amines. This provides the analyst with a good overview of the amount of oxidation that has taken place in the lubricant.

The RULER analysis is one of the oil analysis methods which can provide early detection of the occurrence of oxidation.

It has been shown that the physical changes, such as polymerization, will only begin after this chemical change of the depletion of antioxidants. It is at this point that the actual deposits will begin formation.

Unfortunately, oil analysis tests such as viscosity and acid number only show significant changes after the deposits have been formed. At this time, it may be too late to implement technologies to mitigate varnish formation.

The Membrane Patch Calorimetry (MPC) oil analysis test (ASTM D7843) can offer analysts insight into the estimated amount of insoluble varnish currently within the system. These results have three main ranges which identify the severity of the varnish, namely, 0-20 (Normal), 20-30 (Abnormal), and >30 (Critical). Oil analysis tests can effectively provide the operators with some awareness of the current condition of the lubricant and its tendency to form varnish.

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