Antioxidants (AO) are critical components in turbine oil formulations and largely determine the life and performance of a lubricant. Even though these sacrificial warriors make up about 1% of a turbine oil formulation, they protect the base oil from oxidation and rapid deterioration. An oil void of antioxidants fails rapidly and dramatically in a thermally stressful environment such as a turbine. Antioxidants play the same role in our bodies by protecting cells from the damaging effects of free radicals which are produced when our bodies break down food. That’s why we read so much press about the value of antioxidant rich foods such as blueberries and other plant-based foods. (Although red wine and dark chocolate are my favorite sources of antioxidants.)
Turbine oils have reached the end of their life when their antioxidants have depleted to 25% of new levels. At this point, oil changes are scheduled. Alternatively, some users will remove a percentage of the oil from their sump replacing it with new oil in order to replenish some of their antioxidants. This “bleed and feed” practice however is an expensive way of replenishing the antioxidant.
It is common for plants to inquire about simply replenishing their oil’s antioxidant system while the fluid is still in-service. This is a lower cost option and is a similar practice to many other bulk fluid systems. Many power plants have familiarity with maintaining their cooling water chemistry in wet-recirculating or closed-loop cooling systems – turbine oil formulations can’t be more complex they reason. Similarly, swimming pools aren’t dumped when the water becomes dirty or algae emerge. The chemistry is simply tweaked and the fluid is filtered.
Replenishing antioxidants is a feasible and a cost-effective alternative to extend the life of your turbine oil. This is not suitable for all in-service turbine oils and requires a custom antioxidant formulation for each fluid. In order to mitigate risks, up-front testing must be done to qualify the oil. There are two main criteria that must be met in order for an in-service turbine oil to be qualified: compatibility and performance.
Compatibility is determined by running a battery of tests on the in-service oil, blending it with fresh antioxidants and then retesting the oil. If there are any adverse effects with the addition of the antioxidants, the oil is not qualified. ASTM D7155 is a turbine oil compatibility guideline that provides a good test protocol for performing these tests.
Performance of the replenished antioxidant system is another key factor in qualifying turbine oil. The antioxidant package should have synergism with the existing formulation. The replenished antioxidant system should also have a similar rate of depletion as new antioxidant formulation. This is a function of both selecting the correct antioxidants for the formulation and ensuring that the in-service oil is free of reactive degradation products. (i.e. low MPC values.) Laboratory stress tests can do an excellent job of measuring the effectiveness of the replenished antioxidants and estimating the long-term performance of the treated fluid. (Read this post on a suggested stress test: http://www.turbomachinerymag.com/blog/content/when-selecting-turbine-oils-look-beyond-spec-sheet)
Clearly a level of expertise is required before attempting to reformulate in-service turbine oils. This should not be attempted by those without a thorough understanding of the original oil’s formulation and oil blending expertise. However, this option has the potential of significantly reducing lubricant costs. It is also a more environmentally sustainable practice by doubling (or more) the life of your in-service lubes. With proper testing, the risks of antioxidant replenishment can be also mitigated. In fact, if done correctly, the risks of replacing depleted antioxidants in your in-service oil is lower than changing your oil out and performing a system flush.