ASTM D96 Water and Sediment in Crude Oil by Centrifuge Method (Field Procedure)
8. Procedure
8.1 Fill each of two centrifuge tubes to exactly the 50-mL (100-part) mark with a sample taken directly from the sampling device (for example, a thief bottle, beaker, or LACT sample container) or the container in which the sample was collected. Then fill each tube with solvent to exactly the 100-mL (200-part) mark. Read the top of the meniscus at both the 50- and 100-mL (100- and 200-part) marks. If experience indicates that a demulsifier is required and one has not already been added to the solvent, add to each tube quantity of demulsifier-solvent stock solution that has previously been determined to be satisfactory for the crude oil under test. Stopper each tube tightly and invert the tubes a minimum of 10 times to ensure that the oil and solvent are uniformly mixed.
NOTE 7 - Caution: In general, the vapor pressures of hydrocarbons at 60°C (140°F) are approximately double those at 40°C (104°F). Consequently, tubes should always be inverted at a position below eye level.
8.1.1 Where the crude oil is very viscous and mixing of the solvent with the oil is difficult, the solvent may be added to the centrifuge tube prior to the oil to facilitate mixing. In this case, extreme care must be taken to fill the centrifuge tube to exactly the 50-mL (100-part) mark with solvent and then to exactly the 100-mL (200-part) mark with the sample.
8.2 Loosen the stoppers to prevent pressure buildup during heating and immerse the tubes to the 100-mL (200-part) mark in a preheater. Heat the contents to 60°C +/- 3°C (140°F +/- 5°F).
8.3 Secure the stoppers and again invert the tubes 10 times to ensure uniform mixing of the oil and solvent.
8.4 Place the tubes in the trunnion cups on opposite sides of the centrifuge to establish a balanced condition. Retighten the stoppers and spin for at least 5 minutes at a minimum relative centrifugal force of 500.
8.5 Immediately after the centrifuge comes to rest, verify the temperature. Do not disturb the oil-water interface with the thermometer. The test is invalid if the final temperature after centrifugation is below 52°C (125°F).
NOTE 8 - If the final temperature is found to be below 52°C (125°F), adjust the centrifuge heater to increase the final test temperature and reinitiate the procedure, beginning with 8.2.
8.5.1 Read and record the combined volume of sediment and water at the bottom of each tube as indicated in Table 4 and Fig. 3 (Table 5 and Fig. 4 for 200-part tubes). Reheat both tubes to 60°C +/- 3°C (140°F +/- 5°F), return the tubes without agitation to the centrifuge, and spin for another 5 min at the same rate. Repeat this operation until two consecutive consistent readings are obtained on each tube.
8.6 For the test to be considered valid, a clear interface must be observed between the oil layer and the separated water. No identifiable layering (that is, an emulsion) should be present immediately above the oil-water interface. In such cases, one or more of the following remedies may be effective:
8.6.1 Shake the mixture between whirlings in the centrifuge just enough to disperse the emulsion.
8.6.2 Use a different or an increased amount of demulsifier. (The demulsifier should not, however, contribute to the volume of sediment and water.)
8.6.3 Use a different or an increased amount of solvent. After a satisfactory procedure for a particular type of oil has been worked out, it will ordinarily be suitable for all samples of the same crude oil.
