Sludge and varnish in turbines, such as those used for power generation, have been observed more frequently in recent years due to increased use of API Group II and Group III base stocks in modern turbine oils. Contrary to previous published research, a study conducted at BASF points to aminic antioxidants as a key part of the solution to this costly problem.
Group II and Group III oils show better thermal stability than Group I base stocks. On the other hand, they exhibit poorer solvency for high molecular weight byproducts of oil degradation, which trigger sludge and varnish formation. There is not one type of varnish. For example, it could look yellowish, brownish or black and be soft or rather hard. More than 80 different varnish chemistries have been identified over the years.
Varnish-related failures can lead to unscheduled downtime with costs of up to $1 million for a single unit. Varnish in the turbine bearing can result in a much smaller clearance, increasing the oil film temperature and pressure. The loading capacity of the bearing is also decreased, which reduces the safety and reliability of bearing operation. Since varnish typically is a sticky residue, it may cause valve malfunction, blocked flow lines and premature filter blockage. Other effects of varnishing include decreased efficiency in the heat exchanger and seal malfunction, which could result in leaks or increased friction of the dynamic seals.
The process of varnish formation begins with polymerization of soluble oxidation products, forming oligomers. As the oil reaches the saturation point—when it is unable to hold any more of these oligomers in solution—so-called soft contaminants will drop out of solution and form insoluble particles of a median size around 80 nanometers. Since these particles are polar, they clump together in the environment of a non-polar mineral oil.