10 4 e4 fm Manual of Petroleum Measurement Standards Chapter 10 4 Determination of Water and/or Sediment in Crude Oil by the Centrifuge Method (Field Procedure) FOURTH EDITION, OCTOBER 2013 ERRATA, MA[.]
General
Kerosine, Stoddard solvent, toluene, and xylene are effective solvents for field testing, as they help reduce viscosity and facilitate the separation of water and sediment in centrifuge tubes These solvents also aid in dissolving paraffin and asphaltenes, preventing them from contaminating test results It's important to note that different types of crude oil may necessitate the use of specific solvents for accurate results, as kerosine and Stoddard solvent, for instance, do not dissolve asphaltenes.
Testing should be performed to determine the best solvent to use on a particular crude type.
Annex A provides essential details about the physical properties of these solvents, along with important fire, safety, and health considerations for their usage It is required that kerosine complies with the ASTM D3699 specifications.
For accurate information on the physical characteristics, safety and health risks, and necessary precautions related to specific materials and conditions, it is essential to consult the employer, the manufacturer or supplier of the material, or refer to the Material Safety Data Sheet (MSDS).
Use of Solvents
Toluene and xylene must be saturated with water at the specified test temperature and maintained until use, as outlined in Annex B It is essential that the water-saturated solvent is free from suspended water at the time of application For water and sediment determinations in asphaltenic crude oils, toluene or xylene is the recommended choice.
Stoddard solvent and kerosine do not have to be saturated with water, since the solubility of water in these solvents is not significant at the test temperature.
Demulsifiers are essential for separating water from crude oil samples, unless testing shows they are unnecessary They should be mixed according to the manufacturer's guidelines without increasing the volume of sediment and water measured Always use demulsifiers in a stock solution or premixed with the testing solvent, and document the demulsifier-to-solvent ratio for reproducibility It is important to test different demulsifiers to identify the most effective one for each crude type For additional details on demulsifiers, refer to Annex A and B.
Centrifuge
A centrifuge must be able to spin multiple centrifuge tubes at a controllable speed, achieving a minimum relative centrifugal force of 500 g at the tube tips The required rotation speed to generate this force of 500 g for different swing diameters can be found in Table 1 or calculated using the provided equation.
The rotation speed (rpm) is measured in revolutions per minute, while the relative centrifugal force (rcf) is expressed in multiples of gravity The diameter of swing (d), which can be in millimeters or inches, is determined by measuring between the tips of opposite tubes when they are fully extended during rotation For a diameter measured in millimeters, the value of α is 1335 rpm, and for inches, it is 265 rpm.
Table 1—Rotation Speeds Necessary to Produce a Relative Centrifugal Force of 500 g for
Centrifuges of Various Diameters of Swing
Diameter of swing “ d ” a Rotation Speed
610 24 1210 a Measured between the tips of opposite tubes when the tubes are in their fully extended rotating position. rpm α rcf
The design of the centrifuge must include a robust revolving head, trunnion rings, and trunnion cups, along with durable cushions, to endure the maximum centrifugal force generated by the power source These components are essential for securely supporting the tubes during operation Additionally, the centrifuge should be encased in a sturdy metal shield to effectively contain any debris that may result from a tube failure or malfunction.
The centrifuge machine must be heated and maintain the sample temperature within 8 °C (15 °F) of the test temperature For instance, after centrifuging, the minimum acceptable temperature is 52 °C (125 °F) when the test temperature is 60 °C (140 °F), and 63 °C (145 °F) at a test temperature of 71 °C (160 °F).
Centrifuge Tubes
Centrifuge tubes must be cone-shaped and measure either 8 inches (203 mm) or 6 inches (167 mm) in length, adhering to the specifications outlined in Annex C These tubes should be constructed from thoroughly annealed glass Prior to field use, the accuracy of the tube graduation marks must be verified volumetrically or certified gravimetrically in accordance with ASTM E542, utilizing equipment that is traceable to the National Institute of Standards and Technology (NIST) or other internationally recognized standards.
The 6-inch tubes are preferred for field applications due to their smaller size and lighter weight, while the 8-inch tubes offer better readability with more spaced graduations Both types are available in 100-mL and 200-part sizes, but the methods for calculating total sediment and water differ For 100-mL tubes, the volumes are summed, whereas for 200-part tubes, the volumes are averaged Refer to Figure 1 for a 100-mL tube example and Table 2 for reading instructions, and see Figure 2 for a 200-part tube example along with Table 3 for its reading guide.
Preheater
The preheater must be a metal block or a liquid bath deep enough to allow the centrifuge tube to be immersed vertically up to the 100-mL (200-part) mark, and it should be capable of heating the sample to a temperature of 71 °C ±3 °C (160 °F ±5 °F).
Sample Thermometer
A sample thermometer shall have graduations at intervals of 1 °C (2 °F) or less and shall be accurate to ±0.5 °C (±1 °F) A bi-metal, dial-type thermometer is normally used.
Sampling involves the necessary procedures to acquire a representative sample from pipelines, tanks, or other systems This process ensures that the sample is carefully placed in a suitable analytical device, like a centrifuge tube, to preserve its compositional integrity before testing.
Samples obtained and handled as specified in API MPMS Ch 8.1 (ASTM D4057), Ch 8.2 (ASTM D4177), and Ch 8.3 (ASTM D5854) shall be used for this method.
9 Procedure—Determination of Water and Sediment
General
A centrifuge test is considered valid when two consecutive test results from identical sample tubes fall within the allowable tolerances specified in Section 10.
Figure 1—Reading a 100-mL Centrifuge Tube
Table 2—Procedure for Reading a 100-mL Cone-Shaped Tube
2.0 mL 1.9 mL 1.8 mL 1.7 mL 1.6 mL 1.5 mL 1.4 mL 1.3 mL 1.2 mL 1.1 mL 1.0 mL 95 mL 9 mL 85 mL 8 mL 75 mL 7 mL 65 mL 6 mL 55 mL 5 mL 45 mL 4 mL 35 mL 3 mL 25 mL
Figure 2—Reading a 200-Part Centrifuge Tube
Table 3—Procedure for Reading a 200-Part Cone-Shaped Tube
For accurate results, it is essential to use exactly 50 mL (100 parts) of the sample in the centrifuge tube The volume of the solvent solution added as a diluent is less critical, as it should not affect the sediment and water content of the sample.
Caution should be used when using solvents that require water saturation Water results will be affected if the volume of solvent is not properly water saturated.
NOTE Maintaining stoppered tubes throughout the entire process may help maintain the representative nature of the sample.
Procedure
The following steps shall be performed for determination of water and sediment See Annex D for an outline of the procedure.
1) Fill each of two centrifuge tubes with exactly 50-mL (100-parts) of the sample.
To ensure proper dissolution of the demulsifier-stock solution, add the necessary amount to 50 mL of solvent After mixing, fill each tube with the demulsifier-solvent solution up to the 100 mL mark.
When dealing with viscous crude oil that poses challenges for solvent mixing, it is advisable to add the solvent to the centrifuge tube before introducing the oil It is crucial to carefully fill the centrifuge tube to the 50-mL (100-part) mark with solvent, followed by adding the sample up to the 100-mL (200-part) mark to ensure proper mixing.
3) Stopper each tube tightly and invert the tubes a minimum of ten times to ensure that the oil and solvent-solution are uniformly mixed.
When handling hydrocarbons, it's important to note that their vapor pressures at 60 °C (140 °F) are roughly double those at 40 °C (104 °F) To ensure safety, always position tubes inverted below eye level to prevent contact in the event of a stopper blowout.
4) 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 a test temperature of 60 °C ±3 °C (140 °F ±5 °F).
NOTE Temperatures of 71 °C ±3 °C (160 °F ±5 °F) or more may be necessary depending on the properties of the sample Experience will provide the temperature(s) that will produce the most accurate results.
5) Secure the stoppers and again invert the tubes ten times to ensure uniform mixing of the oil and solvent-solution.
6) Place the tubes in the trunnion cups on opposite sides of the centrifuge to establish a balanced condition Spin for at least 5 minutes at a minimum relative centrifugal force of 500 g.
NOTE Experience may indicate that the initial spin time needs to be longer than 5 minutes, depending on the properties of the sample being tested.
7) Immediately after the centrifuge comes to rest use a sample thermometer to verify the temperature of the sample
Avoid disturbing the oil-water interface when using the thermometer For the test to be valid, the sample temperature must be within 8 °C (15 °F) of the test temperature If this condition is satisfied, proceed to step 8; otherwise, restart the procedure from step 4.
To accurately measure the volume of water and sediment, hold the tube vertically and record the readings as shown in Table 2 and Figure 1 for 100-mL tubes, or refer to Table 3 and Figure 2 for 200-part tubes.
Reheat both tubes to the initial test temperature and place them back in the centrifuge without agitation Spin the tubes for a minimum of 5 minutes at a relative centrifugal force of at least 500 g Repeat this step until two consecutive consistent readings are achieved, as outlined in Section 10.
For a test to be valid, a distinct interface must be observed between the oil layer and the separated water/sediment, without any identifiable layering such as asphaltenes, congealed paraffin, or emulsions above this interface To address these issues, one can increase the sample temperature to 71 °C ±3 °C (160 °F ±5 °F), add drops of a demulsifier-stock solution that does not increase the volume of water and sediment, or retest with new samples using the centrifuge method or other approved API/ASTM test methods.
10 Calculation and Reporting—Water and Sediment
General
A test consists of two identical tubes containing the same sample The readings from both tubes are compared, and if the difference exceeds one subdivision on the centrifuge tube, the test is deemed invalid and must be repeated.
For 100-mL tubes, measure and document the volumes of water and sediment in each tube Combine these readings to express the total as a percentage of water and sediment, as illustrated in Figure 1 and Table 2 Present the findings according to the format provided in Table 4.
For 200-part tubes, the average percentage of water and sediment is calculated to three decimal places based on the values obtained from the two tubes (refer to Figure 2 and Table 3) Accurate readings of the percentage from a 200-part tube are only possible when it contains 100 parts (50 mL) of oil The results should be reported as indicated in Table 4.
Table 4—Expression of Results for 100-mL Tubes and 200-Part Tubes
100-mL Tubes Volume of water and/or sediment
200-Part Tubes Volume of water and/or sediment
Read and Recorded Total water and/or sediment
If the combined volume of water and sediment in the 100-mL tubes is below 0.025% (equivalent to 0.0125 mL), it should be recorded as 0.000% (0.000 mL).
NOTE 2 200-Part Tubes—If the total water and/or sediment average percentage for the two tubes is less than 0.025 %, the water and
11 Procedure—Determination of Sediment Only
General
If other approved API/ASTM methods are used for water determination then the following test method for sediment may be used for the detection or quantification of sediment.
A valid centrifuge test requires two tubes of the same sample, with successive test results falling within the allowable tolerances specified in Section 12.
Solvents normally requiring water saturation do not have to be water-saturated for this procedure since water will be added to the test sample.
For accurate results, it is essential to use exactly 50 mL (100 parts) of the sample in the centrifuge tube The volume of the solvent solution added as a diluent is less critical, as it should not affect the sediment content.
Procedure
The following steps shall be performed for determination of sediment only See Annex E for an outline of the procedure.
1) Fill each of two centrifuge tubes with exactly 50-mL (100-parts) of the sample.
To ensure proper dissolution of the demulsifier-stock solution, add the necessary amount to 50 mL of solvent After mixing, fill each tube with the resulting demulsifier-solvent solution up to the 100 mL mark.
In cases where crude oil is highly viscous, making solvent mixing challenging, it is advisable to add the solvent to the centrifuge tube before the oil It is crucial to carefully fill the centrifuge tube to the 50-mL (100-part) mark with solvent, followed by adding the sample up to the 100-mL (200-part) mark.
3) Add approximately 3 to 4 drops of demineralized or distilled water to the centrifuge tube to allow for a clear and distinct reading.
4) Stopper each tube tightly and invert the tubes a minimum of ten times to ensure that the oil and solvent-solution are uniformly mixed.
When handling hydrocarbons, it's important to note that their vapor pressures at 60 °C (140 °F) are roughly double those at 40 °C (104 °F) To ensure safety, always position tubes below eye level and invert them to prevent contact in the event of a stopper blowout.
5) 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 a test temperature of 71 °C ±3 °C (160 °F ±5 °F).
6) Secure the stoppers and again invert the tubes ten times to ensure uniform mixing of the oil and solvent-solution.
7) Place the tubes in the trunnion cups on opposite sides of the centrifuge to establish a balanced condition Spin for at least 5 minutes at a minimum relative centrifugal force of 500 g.
NOTE Experience may indicate that the initial spin time needs to be longer than 5 minutes depending on the properties of the sample being tested.
8) Immediately after the centrifuge comes to rest use a sample thermometer to verify the temperature of the sample.
Avoid disturbing the oil-water interface when using the thermometer For the test to be valid, ensure the sample temperature is within 8 °C (15 °F) of the test temperature If this condition is satisfied, proceed to step 9; otherwise, restart the procedure from step 5.
To accurately measure sediment volume, hold the tube vertically and record the sediment level at the bottom of each tube, referring to Table 2 and Figure 1 for 100-mL tubes or Table 3 and Figure 2 for 200-part tubes.
Reheat both tubes to the initial test temperature and place them back in the centrifuge without agitation Spin the tubes for a minimum of 5 minutes at a relative centrifugal force of at least 500 g Repeat this process until two consecutive consistent readings are achieved, as outlined in Section 12.
For a test to be valid, a distinct interface must be visible between the oil layer and the separated water/sediment, without any identifiable layering such as asphaltenes, congealed paraffin, or emulsions above this interface To address issues with layering, one can increase the sample temperature, add drops of a demulsifier-stock solution that does not increase sediment volume, or retest with new samples using the centrifuge method or other approved API/ASTM test methods.
12 Calculation and Reporting—Sediment Only
General
A test involves two identical tubes containing the same sample For quantitative analysis, the readings from both tubes are compared; if the difference exceeds one subdivision on the centrifuge tube, the test is deemed invalid and must be repeated In cases where the test is solely for detection and sediment is found, the sample can be retested using an API/ASTM approved method to quantify the sediment.
To determine the percentage of sediment in 100-mL tubes, measure and document the volume of sediment in each tube Combine these measurements and express the total as a percentage of sediment, as illustrated in Figure 1 and Table 2 Present the findings according to the format provided in Table 4.
For 200-part tubes, the sediment percentage is calculated as the average of the values obtained from two tubes, rounded to three decimal places (refer to Figure 2 and Table 3) This percentage can only be accurately determined if the tube contains 100 parts (50 mL) of oil The results should be reported as indicated in Table 4.
The test method outlined in this article has not been established for precision, as it is a field method However, experience indicates that the determination of water and sediment using this method can achieve precision comparable to other accepted testing methods.
Due to the absence of a recognized reference material for assessing bias in the centrifuge method used to measure water and sediment in crude oil, no claims regarding bias can be made.
A.1 Physical Characteristics and Fire Considerations
Information regarding particular materials and conditions should be obtained from the employer, the manufacturer or supplier of that material, or the Material Safety Data Sheet.
The typical characteristics of kerosine are a distillation range of 205 °C to 300 °C (401 °F to 572 °F), a minimum flash point of 38 °C (100 °F), and a maximum freezing point of –30 °C (–22 °F).
Kerosene is highly flammable and must be stored away from heat sources, sparks, and open flames Always keep kerosene containers closed when not in use In case of a fire, use foam, dry chemical, or carbon dioxide to extinguish it For spills, absorb the liquid with clay, diatomaceous earth, or similar materials.
The typical characteristics of Stoddard solvent are a distillation range of 149 °C to 208 °C (300 °F to 407 °F), a minimum flash point of 38 °C (100 °F), and an aromatics plus olefins content of less than 20 percent by volume.
Stoddard solvent is flammable and must be stored away from heat sources, sparks, and open flames Always keep containers closed when not in use In case of a fire, use foam, dry chemical, or carbon dioxide to extinguish it For spills, absorb the liquid with clay, diatomaceous earth, or similar materials.
The typical characteristics of toluene are a minimum flash point of 4 °C (40 °F), an ignition temperature of 480 °C
Toluene has a boiling point of 111 °C (231 °F) and a boiling range from 2.0 °C (3.6 °F) to a recorded boiling point of 110.6 °C (231.1 °F) It exhibits an upper flammability limit of 7.1 percent and a lower flammability limit of 1.2 percent by volume With a molecular weight of 92, toluene has an American Public Health Association (APHA) color rating of ten, as per ASTM D1209, and leaves a residue of only 0.001% after evaporation Additionally, it meets the American Chemical Society (ACS) test criteria for substances darkened by sulfuric acid (H₂SO₄).
Toluene is a highly flammable substance that must be stored away from heat sources, sparks, and open flames It is essential to keep toluene containers closed when not in use In the event of a fire, it should be extinguished using water spray, foam, dry chemical, or carbon dioxide For spills, absorb the substance with clay, diatomaceous earth, or a similar material.
The typical characteristics of xylene are a minimum flash point of 27 °C (81 °F), an ignition temperature of 527 °C
Xylene has a boiling point of 139 °C (282 °F) and a boiling range of 137 °C to 144 °C (279 °F to 291 °F) It exhibits an upper flammability limit of 7.0 percent by volume and a lower flammability limit of 1.1 percent by volume With a molecular weight of 106 and an APHA color not exceeding ten (according to ASTM D1209), xylene also has a residue of 0.002 percent after evaporation Additionally, it successfully passes the ACS test for substances darkened by sulfuric acid (H₂SO₄).
Xylene is a highly flammable substance that must be stored away from heat sources, sparks, and open flames It is essential to keep xylene containers closed when not in use In the event of a fire, it should be extinguished using water spray, foam, dry chemical, or carbon dioxide For spills, absorb the liquid with materials such as clay or diatomaceous earth.
To reduce potential health risks from chemical exposure, it is essential to minimize contact with skin and eyes, as well as inhalation of vapors Chemicals should be kept away from the mouth to prevent harmful or fatal ingestion Always ensure that chemical containers are closed when not in use, and maintain well-ventilated and clean work areas Promptly clean up spills following relevant safety and environmental regulations, and adhere to established exposure limits while using appropriate protective clothing and equipment.
NOTE For permissible exposure limits (PELs), consult the most recent edition of the Occupational Safety and Health Standards,
The 29 Code of Federal Regulations Subpart Z addresses "Toxic and Hazardous Substances" under Section 19101000 and subsequent sections For comprehensive guidelines, refer to the latest edition of the ACGIH booklet on Threshold Limit Values and Biological Exposure, as detailed in Section 2.
Exposure to kerosene can lead to various health effects through skin and eye contact, inhalation of vapors, ingestion, or aspiration It may cause local irritations such as dermatitis, eye stinging, and respiratory discomfort Acute exposure can result in serious systemic effects, impacting the central nervous, respiratory, and gastrointestinal systems Symptoms may include vomiting, diarrhea, and in severe cases, drowsiness, central nervous system depression, and potentially fatal outcomes.
There may also be long-term (chronic) health effects of varying severity from exposure to kerosine.
For safety and health risk information regarding specific materials and conditions, it is essential to consult the employer, the manufacturer or supplier of the material, or refer to the Material Safety Data Sheet (MSDS).