Determine oxidation stability of steam turbine oils by rotating pressure vessel
9. Procedure
9.1 Charging - Weigh the glass sample container with a freshly cleaned catalyst coil. Weigh 50 more or less 0.5 g of oil sample into the container; also add 5 mL of reagent water. Add another 5 mL of reagent water to the vessel body and slide the sample container into the vessel body (see Note 2). Cover the glass container with a 57.2–mm (2 1/4 in.) PTFE disk and place a hold-down spring on top of the PTFE disk. Apply a thin coating of silicone stopcock grease to the O-ring vessel seal located in the gasket groove of the vessel cap to provide lubrication, and insert the cap into the vessel body.
NOTE 2 - The water between the vessel wall and the sample container aids heat transfer.
9.1.1 Tighten the closure ring by hand. Cover the threads of the gauge-nipple with a thin coating of stopcock grease (PTFE pipe tape is a suitable alternative to the use of stopcock grease) and screw the gauge into the top center of the vessel stem. Attach the oxygen line with an inline pressure gauge to the inlet valve on the vessel stem. Slowly turn on the oxygen supply valve until the pressure has reached 620 kPa (90 psi, 6.2 bar). Turn off the oxygen supply valve. Slowly release pressure by loosening the fitting or by using an inline bleeder valve. Repeat purging process two more times; purge step should take approximately 3 min. Adjust the regulating valve on the oxygen supply tank to 620 more or less 1.4 kPa (90 psi, 6.2 bar) at a room temperature of 25°C (77°F). For each 2.0°C (3.6°F) above or below this temperature, 5 kPa (0.7 psi, 0.05 bar) shall be added or subtracted to attain the required initial pressure. Fill the vessel to this required pressure and close the inlet valve securely by hand. Open the pressure valve one more time and watch the pressure gauge to make certain it is not decreasing. If not, then close the valve. If desired, test the vessel for leaks by immersing in water (see Note 3).
NOTE 3 - If the vessel was immersed in water to check for leaks, dry the outside of the wet vessel by any convenient means such as airblast or a towel. Such drying is advisable to prevent subsequent introduction of free water into the hot oil bath which would cause sputtering. For safety purposes, a face shield is recommended during the charging process.
9.2 Oxidation - Bring the heating bath to the test temperature while the stirrer is in operation. Switch off stirrer, insert the vessel into the carriages, and note the time. Restart the stirrer. If an auxiliary heater is used, keep it on for the first 5 min of the run and then turn it off (see Note 4). The bath temperature shall stabilize at the test temperature within 15 min after the vessel is inserted. Maintain the test temperature within 60.1°C (see Note 5).
NOTE 4 - The time for the bath to reach the operating temperature after insertion of the vessel may differ for different apparatus assemblies and should be observed for each unit. The objective is to find a set of conditions that does not permit a drop of more than 2°C after insertion of the vessel and allows the vessel pressure to reach a plateau within 30 min as shown in Curve A of Fig. 3.
NOTE 5 - Maintaining the correct temperature within the specified limits of more or less 0.1°C during the entire test run is an important factor assuring both repeatability and reproducibility of test results.
9.3 Keep the vessel completely submerged and maintain continuous and uniform rotation throughout the test. A standard rotational speed of 100 6 5 rpm is required; any appreciable variations in this speed could cause erratic results.
9.4 The test is complete after the pressure drops more than 175 kPa (25.4 psi, 1.75 bar) below the maximum pressure (see Note 6). The 175 kPa pressure drop usually, but not always, coincides with an induction-type period of rapid pressure drop. When it does not, the operator may question whether he has produced a valid experiment (see Note 7).
NOTE 6 - While termination of the test at a 175 kPa (25.4 psi, 1.75 bar) pressure drop is the standard procedure, some operators may elect to stop the test at lesser pressure drops or to observe the condition of the oil after a predetermined test period of perhaps 100 min; that is, well within the normal induction period of new inhibited oils.
NOTE 7 - A typical experiment is shown in Fig. 3 as Curve A. The maximum pressure is expected to be reached within 30 min, a pressure plateau is established, and an induction-type pressure drop is observed. Curve B, in which there is a gradual decrease in pressure before the induction break is recorded, is more difficult to evaluate. The gradual decrease in pressure could be due to a vessel leak, although some synthetic fluids will generate this type of curve. If a leak is suspected, repeat the test in a different vessel. If the same type of curve is derived when the test is repeated, the experiment is likely valid.
9.5 After termination of the test, the vessel shall be removed from the oil bath and cooled to room temperature. The vessel can be briefly dipped into and swirled around in a bath of light mineral oil to wash off the adhering bath oil. The vessel is rinsed off with hot water, then immersed into cold water to quickly bring it to room temperature. Alternately, the vessel can be cooled to room temperature in air. The excess oxygen pressure is bled off and the vessel opened.
