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Energética India | September / October 2015

EXXON MOBIL Turbine Oil Condition Monitoring Modern steam and gas turbines subject the turbine lubricant to ever greater demands. Higher temperatures are encountered in bearings, smaller reservoirs reduce residence times, and issues with varnish deposits have become critical concerns. Since the oil is the lifeblood of reliable turbine operation, a sound oil condition monitoring is needed to ensure long trouble free operations. Turbine Oil Degradation There are four primary reasons that turbine oils degrade in service. First is oxidation. All oils oxidize in service when exposed to oxygen in the atmosphere. And oxidation is not limited to the reservoir as air is dissolved in the oil. With the increasing temperatures found in turbines, increasing flow rates and shorter reservoir residence times, the oxygen and oil have more opportunities to interact. Second is thermal degradation. The oil can be exposed to temperatures in a turbine that cause base oil and additive molecules to chemically change. The result of this reaction is the formation of materials that are not readily soluble in the oil. The materials then deposit within the oil system causing deposits, and in some cases, equipment failures. Third is contamination. Turbine oils are subject to a variety of contaminants such as water (especially in steam turbines), dust and other ingress materials, wash down chemicals, and internally derived contamination, such as wear metals. While none of these are a direct result of oil degradation, they often contribute to other degradation issues. Wear metals, such as copper iron and lead, catalyze the oxidation reaction. Water (especially chemically treated water) can have very adverse effects on the ability to dissipate foam and separate from water. Excess foaming can lead to sluggish response from hydraulic control systems, cavitations in pumps and bearings, and safety issues if the foam over fills the reservoir and spills on the floor. The fourth is additive depletion. Some additive depletion is normal and expected. Anti-oxidant additives are consumed as they perform their function. Demulsifiers help the oil shed water, but if exposed to large amounts of water contamination, the demulsifiers can be removed. Antifoam additives can be removed from ultra fine filtration or can agglomerate when the oil is not circulated for extended periods of time. All of these factors should be consistently monitored throughout the life of the turbine oil. The following describes the tests most commonly used for used turbine oil monitoring. They can be broken into three categories: Physical and Chemical Properties, Contamination Measurements and Performance Properties. Physical and Chemical Properties Viscosity Viscosity is the most important property of any lubricant. Viscosity is defined as the resistance to flow of oil at a given temperature and is measured via the ATSM D445 protocol. As it relates to turbine oils, significant changes to viscosity usually indicate that the oil has become contaminated with oil. In very severe cases, viscosity will increase as a result of excessive oxidation. Thermal cracking (from excessive heat) of the base oil can cause the viscosity to decrease. Results of this test are reported as centistokes at 40°C. The typical range of re- POWER SECTOR 4 energética INDIA · SEP | OCT15


Energética India | September / October 2015
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