Turbine oil exists for more or less 97% of base oil and 3% of additives such as demulsifiers, corrosion inhibitors, oxidation inhibitors and defoamants. The different types of turbine oil are made for a specific application, a gas-turbine requires a different oil type than a steam turbine or a combined cycle turbine. Each have their typical ingredients and blends to meet their special needs and to counter specific problems.
A steam turbine has the highest risk of water ingress into the lubricating oil, due to the risk of leakage of steam through the labyrinth seals. Water is the worst enemy for lubricating oil and the lubricated parts. Water diminishes the lubricity of oil, generates corrosion, cavitations and micro-pitting as the result of hydrogen embrittlement.
Above: Left is clean oil, second from left is oil with excellent water separating ability. Second from right is new oil, on the right is the same oil with contaminations, emulsion and in an advanced state of oxidation.
Wet or dry steam ingress, albeit demineralised water, has the same destructive effect on the oil and the lubricated parts of the turbine, in particular the journal bearings. Journal bearing requires full-film lubricant to carry the load by the hydrodynamic lift. During the start-up phase the oil film is not sufficient to create the required oil film and results into a boundary lubrication which means metal to metal contact. Therefore, nearly all turbine oil has EP (extreme pressure) additives supporting the load during the start-up phase. This additive is sacrificial and can easily be wash-out by water. Typical dynamic oil film thickness for journal bearings lubrications varies between 1 to 75 µm.
Above: Hydrodynamic lubrication on journal bearing. Source: Noria-USA.
Water droplets or moisture in the oil-film under the extreme pressure of the load will evaporate during the implosions by cavitations. These cavitations create micro-pitting that eventually will damage the thread of the bearing.
Another important condition of oil to be watch out for is air-in-oil. Air entrainment reduces the lubricity and the air-release property of the oil. When the oil air-release property is reduced, more air will be entrained in the fluid. This will result in an adiabatic decompression of the air bubbles also known as micro-dieseling. The function of defoamants in the oil is to enhance the air-release property of the oil and by doing so prevention of the micro-dieseling. This defoamants additives is also easily washed-out by water.
Conclusion; water is the major nemesis of turbine oil and must be arrested efficiently in an early stage. Prolonged (high)water content in oil will accelerate wear of components and promote oil oxidations which in turn creates more wear.
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