This method is designed to evaluate the coking propensity of synthetic ester-based aviation lubricants under two phase air-oil mist conditions as found in certain parts of a gas turbine engine, for instance, bearing chamber vent lines. Based on the results from round robin data in 2008-2009 from four laboratories, this method is currently intended to provide a comparison between lubricants as a research tool; it is not currently a satisfactory pass/fail test. At this juncture a reference oil may improve reproducibility (precision between laboratories); a formal precision statement will be given when there is satisfactory data and an agreed on, suitable reference oil if applicable.
The Vapor Phase Coker apparatus bubbles a stream of dry air through a heated oil sump. The oil mist and vapor generated by the airflow travels through a furnace tube, where it is heated, and then travels through a stainless steel tube, where the deposition occurs. In general, vapor phase coking is a form of deposition which can be formed by a gas turbine lubricant when it is exposed to high temperatures in a non-oil wetted engine area. After engine shutdown, the airflow through the bearing vent tubes and seals ceases. This mist, vapor mixture is carried in the airflow condenses in the engine and is subjected to high soak back temperatures. The neopentyl polyol ester molecules typically experience thermal and oxidative degradation which form carbonaceous deposits. Over a period of time coke build up can cause back pressure in bearing compartments and lead to seal malfunction. Vent lines need to be clear for the pressure balance within vented bearing compartments to be maintained, allowing proper sealing of oil within these compartments.