MechChem Africa January-February 2023

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 aredown to25%of theoriginal formula tion. Inmanycases, however, dependingon the oil, varnish issues arise well before this point. Both laboratory tests and turbineoil condition monitoring show that varnish may start to build up evenwhen remaining antioxidants in the lubricant are as high as 60%. To keep tur bineoperations between safeboundaries, suf ficiently high levels of antioxidants should be maintained so that varnish potential remains low and to proactively ensure high oxidation resistance. Membrane Patch Colorimetry (MPC) is a method for determining varnish formation in mineral turbine oils. MPC (ASTM D7843) is the standardisedprocedureused for turbines, inwhich the lubricant is heated at 60 °C for 24 hours tomimic the turbineoperation tempera ture and to redissolve varnish. After heating, the oil has to stand for 72hours 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 orMPC indication, themorevarnishhas been retained by the membrane patch. Consensus for tur bine oil is that an MPC>30 is condemning, MPC>20 is alarming and anMPC<15 iswithin 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 Complementary tests: MPC, RPVOT and RULER

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-serviceoil. Aneducatedreadingof the amperogramprovides 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 themethodoxidises 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 ASTMvalue for these tests, significant information can be gathered from integrating their outcomes which is particularlyuseful for estimating theoptimum lubricant refreshment period for lean opera tions. Through lab tests, this canbe accurately estimated so thatmaintenance interventions can be plannedwell 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, depositsareusuallyassociated with varnish, which is a common product of oxidation processes. Varnish is composed of sacrificedantioxidantsandoxidationproducts 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 fromoxidation 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