ASTM D4007 standard test method for water and sediment in crude oil by the centrifuge method (laboratory procedure)
5. Apparatus
5.1 Centrifuge:
5.1.1 A centrifuge capable of spinning two or more filled cone-shaped, 203-mm (8-in.) centrifuge tubes at a speed that can be controlled to give a relative centrifugal force (rcf) of a minimum of 600 at the tip of the tubes shall be used.
5.1.2 The revolving head, trunnion rings, and trunnion cups, including the cushions, shall be soundly constructed to withstand the maximum centrifugal force capable of being delivered by the power source. The trunnion cups and cushions shall firmly support the tubes when the centrifuge is in motion. The centrifuge shall be enclosed by a metal shield or case strong enough to eliminate danger if any breakage occurs.
5.1.3 The centrifuge shall be heated and should be controlled thermostatically to avoid unsafe conditions. It should be capable of maintaining the sample temperature during the entire run at 60 +/- 3°C (140 +/- 5°F).
5.1.4 Electric powered and heated centrifuges must meet all safety requirements for use in hazardous areas.
5.1.5 Calculate the speed of the rotating head in revolutions per minute (r/min) as follows:
where:
rcf = relative centrifugal force and
d = diameter of swing measured between tips of opposite tubes when in rotating position, mm, or
where:
rcf = relative centrifugal force and
d = diameter of swing measured between tips of opposite tubes when in rotating position, in.
5.2 Centrifuge Tubes - Each centrifuge tube shall be a 203-mm (8-in.) cone-shaped tube, conforming to dimensions given in Fig. 1 and made of thoroughly annealed glass. The graduations, numbered as shown in Fig. 1, shall be clear and distinct, and the mouth shall be constricted in shape for closure with a cork. Scale error tolerances and the smallest graduations between various calibration marks are given in Table 1 and apply to calibrations made with air-free water at 20°C (68°F), when reading the bottom of the shaded meniscus.
5.3 Bath - The bath shall be either a solid metal block bath or a liquid bath of sufficient depth for immersing the centrifuge tube in the vertical position to the 100-mL mark. Means shall be provided for maintaining the temperature at 60 +/- 3°C (140 +/- 5°F).
NOTE 2 - By contractual agreement 49 +/- 3°C (120 +/- 5°F) may be used.
6. Solvent
6.1 Toluene (Warning - See Note 3.) conforming to Specification D362 or to the IP Specification for Toluole.
NOTE 3 - Warning: Flammable.
6.1.1 Typical characteristics for this material are
6.1.2 The solvent shall be water-saturated at 60 +/- 3°C (140 +/- 5°F) (see Note 2) but shall be free of suspended water. See Annex A1 for the solvent-water saturation procedure.
6.2 Demulsifier - A demulsifier should be used to promote the separation of water from the sample and to prevent its clinging to the walls of the centrifuge tube. The recommended stock solution is 25 % demulsifier to 75 % toluene. For some crude oils a different ratio of demulsifier to toluene may be required. Demulsifiers used in the concentration and quantity recommended will not add to the water and sediment volume determined. The solution must be stored in a dark bottle that is tightly closed.
7. Sampling
7.1 Sampling is defined as all steps required to obtain an aliquot of the contents of any pipe, tank, or other system and to place the sample into the laboratory test container.
7.2 Only representative samples obtained as specified in the API MPMS, Chapter 8 (or Practice D4057 and Practice D4177), shall be used for this test method.