Formulators of turbine oils are gradually moving from API Group I and II base oils to Group III in response to original equipment manufacturers requirements to provide more cost-effective and efficient operation. According to George Dodos of Greek lubricant and grease company Eldons, formulating with Group III base oils provides both benefits and challenges.

Speaking at the ICIS ELGI Industrial Lubricants Conference in Vienna in November, Dodos noted that Group III formulations provide significant advantages in oxidation stability and film strength, resulting in longer service life. However, their lower solubility increases the potential for sludge formation. As a result, he said, formulators are seeking additive packages that are more compatible with Group III base oils to increase turbine oil service life.

The challenge for additive formulations is that a variety of Group III base oils are commercially available, and their properties differ significantly from those of Group I and II oils. For example, Group IIIs generally have higher volatility, with a typical Noack rating (of the evaporation of lubricants in high temperatures) of more than 13 percent. This can increase oil consumption, and the oil can thicken if it is not replenished, Dodos explained.

Also, due to the higher saturation levels of Group III oils, solubility is significantly lower than for Group I, Dodos said, adding, This can hinder additive performance.

Finally, since Group IIIs are less polar, i.e., they contain only fully saturated non-polar hydrocarbon molecules, they do not keep particulates in suspension and sludge formation can be more severe. Thus, polarity needs to be adjusted in formulations using base oils with high saturation levels, he said.

Common causes of turbine lubrication failures are insufficient oil flow, insufficient film strength and thickness, high oil temperature, excessive foaming due to inadequate air release and water in the system. Dodos
observed that in a typical turbine, more than 100 metric tons of steel, rotating at speeds of up to 3,600 rpm, is supported by plain bearings on a cushion of oil thinner than a human hair.

To prevent mechanical failure and filter plugging, the oil must resist oxidation and sludge formation and prevent bearing wear. In addition, oil cleanliness is critical. Moving from a Group I or II base oil to Group III with a premium additive package helps minimize oil-related headaches for power plant operators. Starting from a higher performance position ensures slower and more controlled oil degradation and longer oil interchange intervals, Dodos said. This provides value to the customer and cost savings by reducing maintenance and unscheduled downtime.

Turbine oils typically have a low additive treat rate, therefore careful balancing is required to provide good oxidation stability, corrosion control, anti-wear properties, emulsification and foam control.

Corrosion usually is not a problem because modern additive packages contain enough passivators (types of corrosion inhibitors) to provide good copper corrosion control. In addition, extreme pressure additive treat-rates for turbine oils, ranging from 0.03 to 0.5 percent, provide satisfactory load carrying capacity.

Power generation turbines are critical to modern society. With suitable replenishment, turbine oils can last between five and 15 years in service, Dodos said. To achieve this life, most power generation companies run condition monitoring programs either in-house or outsourced. These programs need to be reviewed to compensate for the different performance characteristics that synthetic and semisynthetic oils provide, he added and advised that end users adjust their preventive maintenance programs to account for the new oil performance characteristics.