Varnish build-up in heavy duty gas turbines is a leading cause of costly unplanned shutdowns and resulting lost power generation capacity. The culprit is the conventional petroleum-based turbine oil, which breaks down to form varnish and sludge that cause servo valves to stick. Use of filtration to remove solid degradation by-products addresses a symptom but not the root cause of varnish formation: the petroleum-based turbine oils themselves. For the best protection against varnish-related shutdowns, a switch from petroleum-based turbine oil to non-varnishing polyalkylene glycol (PAG)-based synthetic turbine fluid was undertaken by four power plants in North America. PAG-based synthetic turbine fluid chemistry, its tribological characteristics and successful trials in GE 7FA heavy duty gas turbines (General Electric Company, Fairfield, CT, USA) are discussed in this paper. PAG-based synthetic turbine fluid has outperformed the hydrocarbon turbine oil in reduced sludge and varnish formation due to its polar nature and solvating power in plant trials as well as in a modified ASTM D 2893 test.
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