Varnish has had a significant impact on reliable gas turbine operation. According to a recent ExxonMobil Lubricants & Specialties survey of 192 US gas turbine power plants with a combined total of 626 gas turbines, approximately 40% reported current or historical varnish issues within six years of oil service life.
Industry sources estimate the cost of a utility trip or no-start to be $100,000 per event. The penalty costs associated with trips or no-starts are expected to increase dramatically in the future as these same turbines are dispatched for back up to renewable energy generation. In refineries, the economic impact of a turbine trip or no-starts can be measured in millions of dollars per event.
The impact of varnish on the operation of gas turbines with in common hydraulic and bearing reservoirs is often perceived as unavoidable. However, the selection of an oil specifically formulated to minimise the impact of varnish, in conjunction with improved maintenance strategies, may beneficially extend gas turbine/oil service life.
This article details the specific performance characteristics that may be considered when selecting a gas turbine lubricant, as well as certain lubricant condition monitoring tests for predicting varnish formation. The use of such oils and predictive methods may assist maintenance personnel in optimising productivity and decreasing unscheduled downtime.
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