Varnish has become a popular term in the industry in recent years although it has been around since the first usage of lubricants.
Part of its popularity in recent times is aftermarket additive and filter manufacturers have realised they can remove the material, so it’s measurement has become highly promoted. Additionally the classical ultracentrifuge methods of measuring varnish have now been overtaken by Membrane Patch Colourimetry (MPC). The manufacture of the MPC devices has also been active in promoting the testing too.
Although the intentions of the manufactures of additives, lab instruments and filters is to sell more product off the back of testing varnish levels, the end result of awareness is a good thing to equipment owners by the fact problems that would normally remain silent within the instrument until it fails are now being highlighted earlier.
So let’s start with what is varnish. When a lubricant becomes oxidised through presence of air and heat it will form acidic sludges and deposits that precipitate out of the lubricant. These deposits can block pathways within the system causing blockages or seizures of valves. In larger systems the problem can be exponential, in that varnish formation is highest at localised hotspots, but usually remains soluble at high temperatures. As the oil cools, varnish can deposit on cooler surfaces, insulating the cooler and preventing heat release. This overall rising temperature promotes more oxidation and varnish formation. Additionally the acidic nature of oxidation products actually helps produce more varnish, so it is best to catch a varnish problem early before it gets out of control.
The trend in reducing varnish formation has been to use higher quality base stocks that are naturally more resistant to oxidation and hence varnish formation. However, these oils that are more resistant than less refined base stocks to oxidation are worse at solubilising the varnish when it does occur because they have less double bonds and dipoles within the molecules to be good solvents. This means although the oils are more resistant when they do form varnish they will struggle to cope with its presence and precipitate quickly.
Knowing this, it may seem like varnish formation is just a fact of life and not something that is avoidable or solvable. However, that is simply not true, but does require work and planning to reduce the risk and solve varnish problems. Let’s start with how to stop it forming.
Prevention: One of compressors major failure modes tends to be oil oxidation as compressing air naturally heats it up. It is no surprise then that many compressor manufacturers use polyglycol based lubricants to reduce thermal stress effects and this may be a viable option if you have had particular varnish issues in the past in non compressor applications. However this can sometimes be costly, they are not suitable for every application and have their own disadvantages too.
With regards to elimination there are 3 main solutions:
Oil change – this removes the varnish in suspension, but what happens when you add fresh oil is the deposited varnish goes back up into suspension. So it tends require flushing between the oil changes to remove the stubborn deposits, which can be a costly option and some really stubborn deposits may even require physically scraping off.
Solvents – depending who you speak to these can vary dramatically in price, especially if branded as varnish removal product and tend to be repackaged off the shelf solvents or different base stocks such as Naphthenic or aromatics rather than paraffinic to promote dissolving of the varnish. These can be added prior to a planned oil change and then allowed to dissolve the varnish. This has the advantage over just changing the oil in that it helps dissolve the deposited material before removal to give a cleaner system at the end. You need to be careful in selecting the products though as aftermarket additives may not be endorsed by your oil supplier or equipment manufacturer and so may invalidate warranties it the machine takes a turn for the worse, so it’s always best to check with them first before using such products.
Filtration – this works on Le Chatiller’s principle, I.e. removing an end product (varnish in the oil) to promote more deposited varnish to go back into suspension. This is especially useful on oils that are highly refined and so can’t hold onto the varnish very well. However, the filtration size to achieve good varnish removal results can sometimes be so small that additives are taken out of the oil as 1) more highly refined oils tend to be worse at solubilising in general, including additives, and 2) the pore size of the filter can be smaller than the size of the additives. Before anyone complains, as oil companies, and filter companies in my experience can be quite sensitive to suggestions filtration is removing additives, as they believe the general public may think it sounds like their product is doing a bad job, let me set the record straight . For me, this is simply not the case, in fact I am impressed daily by how the technology in both lubricant formulations and filters is leading to cleaner and more healthier machinery. So please don’t think that additive removal by filtration is either the oil company or filter manufacturer producing a poor quality product, as they are not. The best oils in the world with the best filters will still leave additive in the filters no matter what the size, it’s just a fact and it is not normally an issue and is taken into account when designing drain intervals and additive concentrations. However, as with each of these solutions, it is still good practice to regularly sample the oil for not only varnish potential, but full elemental and physical properties when doing any varnish potential filtration or using aftermarket additives to dissolve it because you are making changes to the system and no changes should be done blind.
So don’t let varnish be a silent killer to your machinery , test your oil to see if you are at risk and test it again when performing, and regularly after completing any varnish removal actions to keep your equipment safe and running reliably for longer.