Determine oxidation stability of steam turbine oils by rotating pressure vessel
17. Procedure
17.1 Setup - Weigh the glass sample beaker 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 into the beaker. Place the sample beaker and spring clip into the magnetic cup. Make sure the anti-friction ring on the magnetic cup is not discolored. If so, it must be replaced according the operators manual. The spring clip should hold the cup so that it does not spin freely. Cover the glass container with the PTFE beaker cover. Add another 5 mL of reagent water to the vessel body and slide the magnetic cup with sample container into the pressure chamber. Place a new O-ring onto the chamber lid and place onto the pressure chamber.
NOTE 11 - The water between the vessel wall and the sample container aids heat transfer.
17.1.1 Tighten the chamber lid by screwing on the three knurled nuts in stages so the lid is securely in place and evenly gapped between the lid and the pressure chamber flange all around. Press the oxygen inlet valve until the pressure has reached at least 620 kPa (90 psi, 6.2 bar). Turn off the oxygen supply valve. Slowly release pressure by loosening the oxygen vent 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.
NOTE 12 - If desired, test the vessel for leaks by pressurizing an empty chamber, heat to 150°C and monitor overnight for a pressure drop. If the vessel drops more than 2 psi from the maximum with a dry chamber, contact the manufacturer for further instructions.
17.2 Oxidation - At this point, make sure the recording device is prepared and started. Place the PTFE lid cover over the pressure chamber lid. Turn on the motor switch to begin the rotation of the sample and then turn on the heat switch on the front of the console. Make sure the temperature controller is set for 150°C and maintains temperature stability within more or less 0.1°C after stabilization.
NOTE 13 - Leaving off the PTFE lid cover will invalidate the test.
NOTE 14 - 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.
17.3 The test is complete after the pressure drops more than 175 kPa (25.4 psi, 1.75 bar) below the maximum pressure (see Note 7). 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 15 - While termination of the test at a 175 kPa (25 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 16 - 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.
17.4 After termination of the test, remove the PTFE lid cover to allow the unit to cool more rapidly. If the auto-shut-off feature is activated, the metal block bath instrument will automatically turn off the heat and magnetic drive motor. For manual operation, turn off the heat and drive motor switches.
17.5 When the safe-opening indicator light is green, first open the oxygen vent valve to release the pressure. Then loosen the knurled lid nuts and screw the lid removal tool into the lid removal port to remove the relatively hot chamber lid.
17.6 Using the removal tool reach in to the chamber and remove the magnetic cup containing the glass sample beaker. Sometimes the beaker cover or the beaker comes out without the cup. If this is the case, then use the tongs to go back to get the magnetic cup.
17.7 Clean the apparatus according to Section 16.
