cant at turbine operating temperature for 24 hours to redissolve varnish, fol- lowed by 72 hours of standing for varnish re-precipitation. The colour intensity of the patch formed on a membrane filter is measured, and a higher MPC value in- dicates higher varnish retention. MPC values above 30 are concerning, above 20 are alarming, and below 15 are considered safe. RPVOT (Rotating Pressure Vessel
Oxidation Test) (ASTM D2272) is an oxi- dation simulator. This test simulates oxi- dation by pressurising lubricant samples in the presence of oxygen and a catalyst. The induction period, the time until the pressure drops by 25.4 psi, indicates anti- oxidant depletion. Different base oils and antioxidant formulations yield distinct RPVOT curves, offering valuable insights into the oxidation behaviour. RULER
(Remaining Useful Life
Evaluation Routine) (ASTM D6971) is a voltammetric method for quantify-
ing antioxidants. By analysing the oxi- dised products of antioxidants, this test provides an accurate assessment of re- maining active antioxidants. For highly degraded samples, extrapolation is nec- essary to obtain reliable results. RULER is an excellent methodology for
monitoring the remaining active antiox- idants. Given that the method oxidises the antioxidant, the result is dependable of the real remaining antioxidant poten- tial. However, when the remaining anti- oxidants are very low and the peaks are very shallow it is possible to make errors in the antioxidant quantification, typi- cally in excess. Excess quantification in degraded sam- ples is dangerous, because should antiox-
idants completely deplete, the lubricant will fall into massive oxidation in a very short period, causing huge damage. To avoid falling into this analytical pit, it is possible to better estimate the remaining antioxidant % by extrapolation. When preparing mixtures of new and used oil, we can define the Refreshment % such that Refreshment = 0% implies used oil; and Refreshment = 100% im- plies full lubricant replacement. Consider the following case study of
a Gas Turbine with a mineral ISO VG 32 lubricant supplemented with an R&O package and 43,000 operation hours. The RULER for this sample resulted in 28,3% remaining antioxidant, very close to the condemning limit.
Figure 2: RPVOT curves for different base oil formulations. Figure 3. RULER can be quantified by extrapolation in degraded samples. SEPTEMBER 2023 | LUBEZINE MAGAZINE 31
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