Varnish Problems of Modern Gas Turbines

摘要

Although the fluctuation of oil prices is damaging both personal lives and industries, we still depend on petroleum for energy which promotes the greenhouse effect. In order to increase thermal efficiency of fuels, the number of the combined cycle power generation systems is increasing rapidly and the inlet gas temperature of gas turbines has become higher and higher. It is reported that the combustion temperature of state-of-the-art gas turbines reaches 1500℃ and that such combined cycle power generation systems achieve almost 60% thermal efficiency. As turbines are exposed to such high temperatures, it is essential to protect turbine bearings by delivering a large volume of oil for cooling. Currently varnish due to oil oxidation is a hot topic in gas turbines. All commercially available turbine oils have oxidation inhibitors and satisfy oxidation stability test of turbine oils like ASTM D-943 (TOST) and D-2272 (RPVOT) standards but varnish problems are increasing. The author has reviewed past studies on turbine oils, which were published in Journals of ASLE and STLE, Institute of Petroleum, and Industrial and Engineering Chemistry. He is of the opinion that there must be some other factors causing the acceleration of oil oxidation which ASTM standards did not expect and other researchers did not discuss. He has investigated factors that may cause local hot spots in gas turbine oil systems and found several proofs that spark discharges of static electricity might play an important role in varnish formation in turbine oils. This paper will review the history of development of turbines and the past studies on oil oxidation stability, and discuss the possible root causes which initiate oil oxidation and suggest ways to (or how to) extend the life of turbine oils.