OIL VARNISHING PART II: MITIGATION
As detailed in our October 2024 Technical Tip, oil varnishing is primarily caused by the oxidation of the lubricant which, at high temperatures in the presence of dissolved oxygen, causes the breakdown of the hydrocarbons in the base oil. This breakdown results in a degradation of the lubricating properties and accelerates the wear of components. As the lubricant becomes saturated with polar oxidation by-products, they precipitate and deposit on metal surfaces in a thin, relatively hard, ‘oil-insoluble’ layer made up mostly of organic residues with a varnish-like appearance.
The presence of varnish deposits is an indicator of degradation of the lubricant, which necessitates an evaluation of the degree of degradation and often requires the replacement of the lubricant. There are a number of methods to reduce and mitigate varnish problems. While the easiest/most obvious solution to address varnishing is to replace the saturated oil in the system with new oil and allow the new unsaturated oil to dissolve the varnish off the surfaces. However, this is not a viable long-term solution as the contamination is still present and even if the varnish does dissolve in the new oil, it will increase the level of the oxidation products in the new oil, which will redeposit onto the surface once the saturation point is reached.
There are four main approaches recommended to reduce and mitigate varnish-related problems, namely i) regular oil monitoring (maintenance strategies), ii) utilization of lubricants with a low varnish forming potential, iii) the use of appropriate flushing and cleaning procedures during oil changes, and iv) physical, electrostatic and/or chemical filtering.
Although often neglected, it is important to replace lubricants at appropriate intervals due to the deterioration of the oil (which increases as wear increases). When replacing the oil, flushing the system with oil (with or without additives) aids in removing varnish deposits, especially when the temperature of the flushing oil is increased to the maximum operating temperatures. Flushing with additives after the removal of the used oil and cleaning of the system using chemicals that dissolve the varnish can be used, but care needs to be taken to mitigate the potential risks associated with the compatibility of the chemicals with metals and seals in the system. As varnish deposition is also related to the solvency of the lubricants, adding solvency-enhancing additives to the oil will increase the ability to dissolve varnish and bring the deposits back into the solution. Solvency enhancers are effective as a preventative method, but not recommended for removal of varnish as they are not as effective at removing varnish from surfaces as chemical cleaners and don’t address oxidation.
Filtering of saturated oil can be used to remove particulates which can promote varnish formation as well as the varnish precursors. Cellulosic and depth filters which collect particles in a thicker graded medium (as opposed to membrane/surface filters which collect the particles on the surface of the filter element) are useful as they are less prone to become blocked. However, filtration only removes the degradation products in the suspension, and not those that have been deposited, or are in solution, and is therefore recommended that Depth Filters are only used with other cleaning methods.
Electrostatic oil cleaning systems remove the polar oxidation products and varnish precursors by using an electrostatic charge. These systems employ a high-voltage current-free electrostatic field between electrodes and as the polar varnish particles pass through the field, they are attracted to either one of the electrodes. As with depth filters, they remove varnishing products in suspension but do reinstate the solvency of the oil to some degree which in turn assists in the dissolution of the deposited varnish.
Charged agglomeration cleaning is similar to electrostatic cleaning, but the charge on the particles is not limited to the confines of filtration. Agglomeration occurs when the oil returns to the lubrication system. The process involves dividing the fluid into two streams, with positive and negatively charged particles in separate streams. When the streams re-join, the charged particles recombine to form neutral and larger particles that can be removed using conventional methods.
Contaminants can also be removed using resin-based chemical absorption processes, in which the oil is passed over a resin bed and contaminants are absorbed by the ion exchange within the resin bed. The process does not involve filtration but removes the varnish by chemically bonding the varnish to the surface of the adsorption medium.
Even with the various filtration technologies available, there is not one technology that provides a solution to all the problems. Often a combination of methods needs to be employed for optimal benefit (e.g. depth filters and a resin-based chemical absorption process)
The message is simple – oil varnishing is an indication of oil degradation and should not be ignored. Regular maintenance and inspection, replacement of oil at recommended service intervals and the addition of an appropriate filtering system will reduce the levels of oxidation by-products in the oil and hence reduce the chance of varnish deposition. However, every effort needs to be made to understand and address/limit the underlying cause of the oxidation.
This is the last Tech Tip for the year – we trust you have found the tips informative and they have been useful in helping improve integrity/prevent failure.