ARP5921 Evaluation of Coking Propensity of Aviation Lubricants in an Air-Oil Mist Environment using the Vapor Phase Coker

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.