Typically to achieve these greater efficiencies, higher firing temperatures have been introduced by designers, which have in turn pushed the boundaries of tribology and material performance.
Additionally, changes in power market structures have rewarded those power operators who can be responsive to fluctuations in power demand. This has required power plants to become more flexible, offering cyclic 'two shifting' or 'peaking' operation which can be particularly attractive to operators.
Just as cycling of power plant introduces stresses to the turbines, such operation also stresses the lubricants. As a result, turbine manufacturers have demanded specifications for turbine lubricants that have been both elevated to new levels and tightened to more exacting standards.
Lubricants manufacturers have therefore developed 'high thermal stability' turbine oils involving a combination of changes to both the base oil technology and the additive formulations. Over the last 10 to 15 years the trend has been to move from Group I to Group II base oils, partly as a result of the reduced availability of Group I base oils and partly to make use of Group II's inherently better thermal and oxidative stability.
However this move to Group II has resulted in the side effects of increased insoluble formation and system cleanliness issues. In the worst cases undesirable sludge and varnish deposits are experienced.