Combined Cycle Journal recently published an article on turbine lube oil varnish written by Peter Dufresne Jr. and his team at EPT. We’ve divided the article into four blogs to make it easier to digest -- click here to find parts one, three and four.
The following factors play a major role in determining the solubility of varnish precursors in lubricants.
The polarity of a molecule refers to the distribution of positive and negative charges within it. In some molecules, these charges are well separated (like the poles of a bar magnet); such molecules are said to be polar. In others, there is little or no separation of charge; these molecules are said to be non-polar.
Molecular polarity is not simply black and white. The polarity scale incorporates shades of gray. Because polarity depends on the specific structure of every molecule, it is possible for one polar molecule to be more, or less, polar than another. The corollary for non-polar molecules is also true. The most basic axiom of solvency is that “like dissolves like.”
This accounts for the fact that polar alcohol will dissolve fully in polar water while polar water will not dissolve in non-polar mineral oil. Although the varnish precursors produced by oxidative degradation of mineral oil base stocks are polar, they are much less so than water. Consequently, these somewhat polar degradation products have some finite solubility in a lubricant’s non-polar mineral-oil matrix. Degradation products that are more polar will be correspondingly less soluble.
A lubricant has a finite capacity to dissolve other molecules (additives, contaminants, varnish precursors, etc). As the oil degrades and oxidation products accumulate, the solvency of the fluid decreases accordingly. Beyond a certain point (known as the saturation point), the fluid can no longer dissolve additional varnish precursors formed by continuing oxidation and varnish will begin to precipitate from solution in the solid form.
Oil temperature directly affects the solubilities of all the species dissolved within it. As temperature decreases, so does the solubility of varnish and its precursors. In the video to the right of this paragraph; the fluid on the left does not contain soluble varnish contamination, the fluid on the right does.
As the fluid cools the solubility of the varnish contaminants decreases causing them to become insoluble and fall out of solution. This same process is responsible for the precipitation of varnish deposits in cooler regions of a turbine’s lubricant circulation system.
Because metals are more polar than the lubricant’s base stock, the precipitated polar varnishes prefer to adhere to the metal and form potentially damaging deposits. When the level of varnish precursors in a lubricant is at (or near) the fluid’s saturation point, varnishing in cooler regions is very likely to occur.
Check out the next part, where we delve into the varnish
cycle and testing for varnish!
Dufresne Jr., Peter "Lubricant Varnishing and Mitigation Strategies." Combined Cycle Journal. Q4 2013: Pages 34-40.