ASTM D7150 Standard Test Method for the Determination of Gassing Characteristics of Insulating Liquids Under Thermal Stress at Low Temperature
7. Method A
7.1 Filter 225 mL of insulating liquid through a 1 or 1.2-µm filter. Discard the first 25 mL. Collect the remainder in a flask that has been cleaned, rinsed with distilled water and dried for 4 h at 100 ± 5°C. Flasks that have been prepared beforehand are acceptable as long as all openings have been covered with aluminum foil.
7.2 Sparge 100 mL of the filtered insulating liquid with dry air for 30 ± 3 min. The air is sparged through the liquid at an approximate flow rate of 200 mL per minute. A typical setup is shown in Fig. 1.
7.3 Immediately fill two (2) glass syringes (either 30- or 50-mL) with the sparged oil (25 mL in the 30-mL syringe; 40 mL in the 50-mL syringe). Make sure the syringe is filled with enough oil to conduct a dissolved gas-in-oil analysis. The barrel and plunger of each glass syringe must be well-fitted to avoid air ingress at the junction of the plunger and barrel seal as the test specimen is being heated during the test procedure.
7.4 Immediately remove any trapped bubbles from the syringe as described in Practice D3613 and seal the syringe with a female-luer-to-closed-end adapter. Do not use plastic stopcocks as they tend to leak and melt during the heating phase of the procedure.
7.5 Sparge another 100 mL of the filtered insulating liquid with dry nitrogen for 30 ± 3 min. The nitrogen is sparged through the liquid at an approximate flow rate of 200 mL per minute.
7.6 Repeat the procedures listed in 7.3 and 7.4 for the nitrogen-sparged insulating liquid.
7.7 Place the syringes with the test specimens laying down in the oven with the leur fitting slightly above horizontal. Heat all the test specimens at 120 ± 2°C for 164 h ± 15 min. Remove the test specimens from the oven, allow to cool for at least 1 h.
7.8 After the test specimens have been allowed to cool (Note 1), remove the female-luer-to-closed-end adapter and replace with a normal stopcocks. Expel a small amount of oil (2-5 mL) from the stopcock to remove any air that may have entered into the syringe when changing out the female-luer-to-closed-end adapter.
NOTE 1 - Periodically check for air ingress during the cooling of the syringe. In those cases where it is visible, it is advisable to perform the test again.
7.9 Perform dissolved gas-in-oil analysis on all four test specimens according to Test Method D3612. Whichever Test Method D3612 method is employed (A, B, or C), the following minimum detection limits must be met:
7.10 The test results are to conform to the following:
7.11 If the test results do not conform to the provisions provided in 7.10, the results are to be discarded and the test repeated.
7.12 Acetylene should not be detected. If acetylene is detected, then the results should be considered suspect and the process reviewed.
7.13 Report the results as detailed in Section 9.
8. Method B
8.1 Prepare a stainless steel column such as the one shown in Fig. 2. Alternatively, construct a glass column (a vacuum filtering apparatus of 47 mm has worked well) using partial vacuum to pull the oil through as opposed to being pressurized. Make sure all the parameters listed in 8.2 through 8.6 are adhered to.
8.2 Fill the column with attapulgite clay filtering medium with a mesh size of 30/60 so that the final ratio is 1 g of clay to 30 to 35 mL of insulating liquid. The clay can be preheated if desired (recommended range is 80 to 100°C).
8.3 Place the appropriate volume of insulating liquid in the sample reservoir to achieve the correct clay to liquid ratio. No less than 275 mL of liquid is to be used as the clay will absorb some of the liquid and not all of it is recoverable.
8.4 Heat the clay and insulating liquid with an external heating tape so that the exiting temperature of the insulating liquid is between 60 and 70°C.
8.5 The rate at which the insulating liquid is passed through the column is 3 to 5 mL per minute.
8.6 Collect the eluent from the clay-filled column in a flask that meets the requirements of 7.1.
8.7 Continue with the testing procedure as detailed in 7.2 through 7.11.
8.8 Report the results as detailed in Section 9.