Assessing oxidation condition and lubricant refreshment in turbine oils
March 2023Maintenance, Test & Measurement, Calibration
In turbines, lubrication undergoes a hydrodynamic regimen in which wear arises only after very poor lubricant condition. Conversely, the main root cause of turbine failure is the formation of deposits. Deposits produce several detrimental effects on these systems, such as sticking valves, orifice obstruction, and inefficient heat exchange.
In the case of gas turbines, where the lubricant suffers mainly from thermal stress, deposits are usually associated with the formation of varnish. Varnish is commonly associated with oxidation processes. It is composed of sacrificed antioxidants and oxidation products that coalesce to form sticky soft matter. The cost of varnish is very high, both in downtime and in equipment replacement. For this reason, monitoring of the oxidation condition of turbine oils is of utter importance.
To protect the base oil from oxidation, turbine lubricants are supplemented with about 1% of antioxidants. These are sacrificed to protect the base oil from free radicals and thermal and oxidative stress.
It is usually accepted that turbine oils can be used until their remaining active antioxidants are 25% of the original formulation. In many cases, however, depending on the oil, varnish issues arise before this point. Both laboratory tests and turbine oil condition monitoring, show that varnish may start to build up even when remaining antioxidants are as high as 60%.
In order to keep the operation reliable, the oxidation condition of a turbine must be kept between safe boundaries. This implies keeping a high dose of antioxidants, and the varnish potential low, to predict a high oxidation resistance.
Oil refreshment is a viable option to keep turbines free of varnish. By integrating the outcomes of lubricant analytics, such as RULER (Remaining Useful Life Evaluation Routine), RPVOT (Rotating Pressure Vessel Oxidation Test), and MPC (Membrane Patch Colorimetry), it is possible to estimate the refreshment required to keep the turbine under lean operation.
The lubricant’s oxidation condition can be effectively monitored through RULER, MPC, and RPVOT tests. Besides the nominal ASTM (American Society for Testing and Materials) value for these tests, significant information can be gathered from digging into these tests and integrating their outcomes.
One major application for this integration is the estimation of the lubricant refreshment for lean operation. Through laboratory tests, this can be accurately estimated, planning ahead of the upcoming maintenance intervention. This method is shown, together with case studies.
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