Nearly 10 years ago, GE reported that approximately one in three large industrial gas turbines showed signs of oil varnishing. Because this condition affects the availability and reliability of GTs, the OEM recommended the use of varnish removal systems. Despite the recommendation, varnish-related turbine outages remain a significant issue for the industry.
The primary reason varnish is an ongoing problem: The mechanism by which varnishing occurs is poorly understood by many turbine owner/operators. Thus, strategies aimed at correcting or mitigating varnish-related problems often are misdirected, resulting in less than ideal outcomes. The goal of this article, based on a presentation by the principal author at the 2013 Conference of the Frame 6 Users Group, is to improve understanding of varnish by discussing its specific cause and how various mitigation alternatives work to minimize operational impacts.
Lubricant varnish generally is defined as a thin, hard, lustrous, oil-insoluble deposit composed primarily of organic residue. It is most readily defined by color intensity and is not easily removed by wiping.
While this definition provides an adequate description of varnish at the end of its life cycle (see image), it must be expanded as follows to account for the remainder of the varnish cycle: Varnish begins its life as a soluble degradation product before converting to an insoluble particulate form. The process responsible for the deposition of particulate varnish is reversible.
This expanded definition reveals that varnish is a shape-shifter; it can be insoluble (conventionally recognized particulate form) or dissolved (soluble) in the fluid. An understanding of lubricant solvency is the key to understanding the mechanism by which varnish deposits are formed and, more importantly, the mechanism by which they can be removed.
Under normal operating conditions, turbine lubricants are subjected to oxidation, which produces polar molecules (varnish precursors) from non-polar ones (lubricant mineral-oil base stocks). These polar species represent the starting point of the varnish life cycle. As a result, lubricants in service are a complex combination of base stocks, additives, and contaminants.
A lubricant’s solvency is defined as its ability to dissolve these distinct components. Everything in the oil has a finite solubility, which is affected by numerous variables (molecular polarity, contaminant levels, temperature, etc). This solubility determines if a particular molecule is soluble in the fluid or if it will precipitate from the fluid to form a potentially damaging deposit.
When the solubility of a molecule is low, the lubricant cannot dissolve it and will actively release it, producing deposits. However, when the solubility of a molecule is high, the lubricant will have a high capacity to dissolve it, avoiding the formation of varnish deposits.
Check out Part 2, where we'll discuss factors affecting lubricant solvency.
Dufresne Jr., Peter "Lubricant Varnishing and Mitigation Strategies." Combined Cycle Journal. Q4 2013: Pages 34-40.