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been established between RPVOT and ASTM D 943 methods neither between actual field service. Mitsubishi specification for heavy-duty turbine oils requires RPVOT (ASTM D 2272) retention value in addition to the ASTM D-943 test.

ASTM D 7873: Determination of oxidation stability and insolubles formation of inhibited turbine oils at 120 °C without the inclusion of water (Dry TOST Method)

D 4310: Determination of the Sludging and Corrosion Tendencies of Inhibited Mineral Oils This method is a modified alternate to the ASTM D 943 test method, and is used to determine the tendencies of inhibited mineral oils, especially turbine oils, to form sludge during oxidation.

The test conditions described under ASTM D 943 are used. After 1000 hours, the test is stopped. The oil and water layers are separated and filtered. The weight of insoluble material is determined gravimetrically by filtration of the oxidation test tube content through a 5 micron pore size filter. The amount of copper in the oil, water, and sludge phases can be determined according to any suitable methods.

This method is used primarily for specification purposes. Formation of oil insolubles or metal corrosion products during this test may indicate that oil will form insolubles or corrode metals, or both, during field service. However, correlation with field service has not been established.

D 2272: Oxidation Stability of Steam Turbine Oils by Rotating Pressure Vessel Oxidation Test (RPVOT)

The RPVOT is a rapid method of comparing the oxidation life of lubricants in similar formulations, in the presence of water and a copper catalyst. This method can be used to evaluate the oxidation characteristics of turbine oils, hydraulic oils, and transformer oils. The test apparatus consists of a pressurised vessel axially rotating at 100 rpm, at an angle of 30° from the horizontal, in a bath maintained at 150°C. Fifty grams of test oil, 5g of distilled water, and a freshly polished copper coil are placed into a glass liner, and inserted into the vessel. The vessel is initially pressurized with oxygen to 600 kPa at room temperature.

At 150°C, the pressure in the vessel increases to approximately 1400 kPa. When oxidation occurs, the pressure drops, and the usual failure point is taken at 175 kPa from the maximum pressure obtained at 150°C. The results are reported as the induction time which is the number of minutes to reach 175 kPa loss.

The RPVOT is favoured as a quality control test because it is rapid. The RPVOT result is useful in controlling the continuity of this property for batch-to-batch acceptance of production lots, having the same composition. The RPVOT is also useful in determining the remaining oxidation life of in-service systems by charting the original RPVOT value versus subsequent samples of that system. It should be noted that the D 2272 test method is dependent on additive chemistry. No correlation has

A total of six to eight tubes containing 360 ml of sample without water are heated at 120 °C with oxygen in the presence of an iron-copper catalyst. Each tube is removed over time and the sample is analysed by Test Method D2272 and the insolubles are measured until the RPVOT residual ratio reaches below 25 %. The criterion is to maintain less than 100 mg/kg of sludge at a RPVOT value corresponding to 25 percent of the new oil. The 100 mg/kg limit was determined by Mitsubishi Heavy Industries (MHI) based on field experience with their turbines and hydraulic control systems.

IP-280: Determination of Oxidation Stability of Inhibited Mineral Turbine Oils

This method is commonly used for European specifications relating to turbine oils and other hydraulic fluids. This method is technically identical to the CIGRE method ’Turbine Oil Oxidation Stability Test.’ The test apparatus consists of a suitable sized test tube containing 30g of test oil, plus copper naphthenate and iron naphthenate as soluble catalysts. The sample test tube is placed in a heated bath, 120°C for 164 h. During the test period, oxygen is bubbled through the oil sample at a rate of 1.0 l/h. Both the test temperature and the oxygen flow rate must be carefully maintained throughout the test period. The volatile acids, soluble acids, and the sludge are used to calculate the “Total Oxidation Products” (TOP).

Because of the relatively short test time, this method is sometimes used as a replacement for the longer running ASTM D 943. However, no correlation between this test and the D 943 exists. Oils showing good results in D-943 test could fail in IP 280 test. Both the tests are antioxidant specific. Hindered phenols in adequate amount would show good result in D-943 test at 95°C but would sublime at 120°C in the IP 280 test and show poor result. The IP-280 test, therefore, require high-temperature antioxidants. It is, however, possible to design turbine oil by using complex mixtures of antioxidants, which will show up good results in both ASTM and IP tests.

To be continued in LUBE 141.

References [1] J. DENIS, J. BRIANT, J.-C. HIPEAUX., 1997, Physico-chimie des lubrifiants, analyses et essais. Editions Technip, PARIS

[2] Gerald J. Cochrac, Syed Q. A. Rizvi, 2003, Fuels and lubricants handbook – oxidation of lubricants and fuels, chapter 30, ASTM International, West Conshohocken, PA.

[3] S.P. Srivastava, 2014, Developments in Lubricant technology, TJ1077.S74, Edition Wiley, India

[4] ASTM test methods

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