Oxidation condition and lubricant refreshment in turbine oils
WearCheck’s Technical Bulletin 81 features a comprehensive article by Andres B Lantos of WearCheck Argentina that details how lubricant refreshment life for turbines can be accurately estimated to achieve lean operation. MechChem Africa presents a summary. T urbines are critical pieces of equip- ment for power plants and industries, with varnish formation being the first root cause of downtime and
It is usually accepted that turbine oils can be used until their remaining active antioxi- dants are down to 25% of the original formula- tion. In many cases, however, depending on the oil, varnish issues arise well before this point. Both laboratory tests and turbine oil condition monitoring show that varnish may start to build up even when remaining antioxidants in the lubricant are as high as 60%. To keep tur- bine operations between safe boundaries, suf- ficiently high levels of antioxidants should be maintained so that varnish potential remains low and to proactively ensure high oxidation resistance. Complementary tests: MPC, RPVOT and RULER Membrane Patch Colorimetry (MPC) is a method for determining varnish formation in mineral turbine oils. MPC (ASTM D7843) is the standardised procedure used for turbines, in which the lubricant is heated at 60 °C for 24 hours to mimic the turbine operation tempera- ture and to redissolve varnish. After heating, the oil has to stand for 72 hours in the dark for varnish to re-precipitate. After filtering through a 0.45 μm pore membrane, the colour intensity (ΔE) of the patch is measured. The higher the ΔE or MPC indication, the more varnish has been retained by the membrane patch. Consensus for tur- bine oil is that an MPC>30 is condemning, MPC>20 is alarming and an MPC<15 is within the safety zone. RPVOT: The Rotating Pressurised Vessel Oxidation Test (ASTM D2272) is an oxida- tion simulator. Briefly, a sample of lubricant is pressurised at 190 psi under an oxygen atmo- sphere at 150 °C and rotated in the presence
of a copper catalyst and water vapour. The time curve of the vessel pressure is re- corded. During the test, the lubricant tends to oxidise due to the high oxygen potential. In the first stages of the test, antioxidants are sacrificed to protect the base oil and there - fore oxygen pressure remains steady. Once antioxidants are fully depleted, however, the bulk of the base oil oxidises and oxygen pres- sure drops. To accommodate different oil types, we have learned that the RPVOT test should not be stopped after a 25.4 psi pressure drop, but instead be continued until the pressure drops by at least 90 psi. RULER: Remaining Useful Life Evaluation Routine (ASTM D6971) is a voltammetric method for dosing antioxidants. Briefly, an oil sample (aliquot) is diluted in a vial, which extracts the antioxidants and decants the base oil. The sample is then probed under a potentiostat, with a linear increasing voltage applied. Each antioxidant, depending on its nature, is oxidised at a specific potential and an am - perometric peak arises. In oils with added aromatic amines and phenols, two peaks can be observed. The area under the curve for each peak is proportional to the antioxidant concentration. The area of an in-service lubricant com- pared to that of its original formulation dic- tates each remaining antioxidant percentage in the in-service oil. An educated reading of the amperogram provides additional information on the health of the in-service oil. As the oil degrades, it is seen how the antioxidant peaks become shifted from their original potential. Given that the method oxidises the antioxi-
reliability loss. The lubricant’s oxidation con- dition can be effectively monitored in many ways, such as RULER (Remaining Useful Life Evaluation Routine), MPC (Membrane Patch Colorimetry) and RPVOT (Rotating Pressure Vessel Oxidation Test) tests. Besides the nominal ASTM value for these tests, significant information can be gathered from integrating their outcomes which is particularly useful for estimating the optimum lubricant refreshment period for lean opera- tions. Through lab tests, this can be accurately estimated so that maintenance interventions can be planned well in advance of related reli- ability issues. In turbines, the main root cause of failure is the formation of deposits, which produce several detrimental effects, 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 varnish, which is a common product of oxidation processes. Varnish is composed of sacrificed antioxidants and oxidation products that coalesce to form sticky soft matter. The costs can be very high, both in downtime and in equipment replacement. For this reason, monitoring of the oxidation condition of tur- bine oils is of vital importance. To prevent the base oil from oxidation and varnish formation, turbine lubricant additives include about 1% antioxidants, which are sacrificed to protect the base oil from free radicals and thermal and oxidative stress.
Figure 1 . MPC is a standardised method in which precipitation time is critical. The MPC test measures the colour intensity (ΔE) of a patch after varnish has been precipitated through it.
6 ¦ MechChem Africa • January-February 2023
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