A film of polar oxidation and thermal breakdown products that drop out of solution when oil cools, especially in turbine, compressor and hydraulic systems running hot on Group II base oils. It forms on cold spots such as coolers, filters, servo valve spools and bearing shells, causing sticking valves, plugged filters and hotter bearings. Varnish potential is tracked with MPC, and the fixes are adsorbent or electrostatic filtration, stronger antioxidants and cooler running.
Inlet guide vane servo is sticking again, pull the spool and I bet it's coated in varnish.
Varnish often dissolves back into the oil when the system is hot, so samples from a running unit can look clean while the cold spots are caked.
Also heard as
- lacquer
- oil deposits
Related terms
MPC
Membrane patch colorimetry, a test that measures varnish-forming material in turbine and hydraulic oils.
RPVOT
Rotating pressure vessel oxidation test, which rates how much oxidation life is left in an oil.
Antioxidant
Additive that slows the chemical aging of oil by intercepting the radicals and peroxides that drive oxidation.
Turbine oil
Highly refined, lightly additized circulating oil built to last for years in steam, gas and hydro turbines.