In recent years, the power-generation industry has seen an increase in varnish-related problems in combustion turbines. This increase is attributed to higher operating temperatures, smaller fluid reservoirs, more peaking and cyclic service, highly refined base stocks that have lower solvency for varnish precursors, and a more widespread use of finer filtration that causes more electrostatic charging of the turbine oil. The solvency of varnish in turbine oil is temperature dependent, with the transition point being in the range of 130 to 135 degrees F. The temperature frequently falls below this threshold in the hydraulic control section of turbines, resulting in the formation of deposits on control valves. The most problematic aspect of varnish contamination of a turbine lubrication and control system is that the material plates out on servo-valve surfaces, leading to valve sticking, and plugs the last-chance filters (LCFs) that are part of the servo-valve assembly.
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