The Rotating Pressure Vessel Oxidation Test (RPVOT) is a time-tested method for determining the oxidation stability of an oil. It is the most widely used indicator of a fluid’s oxidative resistance. Since it measures the existing resistance to oil oxidation rather than the oxidation that has already occurred, it is a predictive test.
RPVOT is most often utilized to estimate the fluid’s remaining useful life and can also be used to compare batches of the same oil formulation in different machines. One of the many reasons that RPVOT is so valuable is that it can head off oxidation before it has a chance to do some real damage.
And oxidation does create damage. In fact, it is one of the primary causes of lubricant degradation. As the oil oxidizes it forms acids and other insoluble products that will eventually form sludge and/or varnish—coating and compromising everything in its path including bearings and hydraulic components to valves.
RPVOT is usually performed annually—more frequently as oil ages. It’s best for large oil volumes with low makeup fluid requirements that are infused with antioxidant additive packages. This includes steam turbines and any machinery that operates in severe conditions.
RPVOT is conducted by placing a sample of oil along with water and a copper catalyst in a pressure vessel. It is then pressurized with pure oxygen. Antioxidants present in the oil will act to resist oxidation, but once they have been consumed the oil will begin to react with the oxygen and the pressure in the vessel will drop. The time it takes to reach the specified drop in pressure is recorded and trended against historical results as well as new oil specifications. Results are reported in minutes.
Legacy Group I fluids typically had RPVOT values of about 600 minutes. Group II and III fluids with comprehensive additive packages can have values in excess of 3000 minutes. Fluids are often alarmed when the RPVOT result is 25% of the new oil specification.
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