ASTM D6468-22

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of the standard as published by ASTM is to be considered the official document.
Designation: D6468 − 08 (Reapproved 2019) D6468 − 22
Standard Test Method for
High Temperature Stability of Middle Distillate Fuels
This standard is issued under the fixed designation D6468; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope Scope*
1.1 This test method covers relative stability of middle distillate fuels under high temperature aging conditions with limited air
exposure. This test method is suitable for all No. 1 and No. 2 grades in Specifications D396, D975, D2880, and D3699. It is also
suitable for similar fuels meeting other specifications.
1.2 This test method is not suitable for fuels whose flash point, as determined by Test Methods D56, D93, or D3828, is less than
38 °C. This test method is not suitable for fuels containing residual oil.
1.3 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.3.1 Exception—The maximum vacuum includes inch-pound units in 6.5 and 11.2.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.5 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D56 Test Method for Flash Point by Tag Closed Cup Tester
D93 Test Methods for Flash Point by Pensky-Martens Closed Cup Tester
D396 Specification for Fuel Oils
D975 Specification for Diesel Fuel
D1500 Test Method for ASTM Color of Petroleum Products (ASTM Color Scale)
D2274 Test Method for Oxidation Stability of Distillate Fuel Oil (Accelerated Method)
D2880 Specification for Gas Turbine Fuel Oils
D3699 Specification for Kerosine
D3828 Test Methods for Flash Point by Small Scale Closed Cup Tester
D4057 Practice for Manual Sampling of Petroleum and Petroleum Products
D4175 Terminology Relating to Petroleum Products, Liquid Fuels, and Lubricants
This test method is under the jurisdiction of ASTM Committee D02 on Petroleum Products, Liquid Fuels, and Lubricants and is the direct responsibility of Subcommittee
D02.14 on Stability, Cleanliness and Compatibility of Liquid Fuels.
Current edition approved May 1, 2019May 1, 2022. Published July 2019May 2022. Originally approved in 1999. Last previous edition approved in 20132019 as
D6468 – 08 (2013).(2019). DOI: 10.1520/D6468-08R19.10.1520/D6468-22.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6468 − 22
D4625 Test Method for Middle Distillate Fuel Storage Stability at 43 °C (110 °F)
D5452 Test Method for Particulate Contamination in Aviation Fuels by Laboratory Filtration
3. Terminology
3.1 Definitions:
3.1.1 For definitions of terms used in this test method, refer to Terminology D4175.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 adherent insolubles—insolubles, n—material that is produced in the course of stressing distillate fuel under the conditions
of this test, and that adheres to the glassware after fuel has been flushed from the system.
3.2.2 filterable insolubles—insolubles, n—material that is produced in the course of stressing distillate fuel under the conditions
of this test, and that is capable of being removed from the fuel by filtration.
3.2.3 inherent stability—stability, n—the resistance to change when exposed to air, but in the absence of other environmental
factors such as water, reactive metal surfaces, and dirt.
3.2.4 storage stability—stability, n—the resistance of fuel to formation of degradation products when stored at ambient
temperatures.
3.2.5 thermal stability—stability, n—the resistance of fuel to formation of degradation products when thermally stressed.
4. Summary of Test Method
4.1 Two 50 mL volumes of filtered middle distillate fuel are aged for 90 min or 180 min at 150 °C in open tubes with air exposure.
After aging and cooling, the fuel samples are filtered and the average amount of filterable insolubles is estimated by measuring
the light reflectance of the filter pads. The 100 % and 0 % extremes of the reflectance rating range are defined by an unused filter
pad and a commercial black standard, respectively.
5. Significance and Use
5.1 This test method provides an indication of thermal oxidative stability of distillate fuels when heated to high temperatures that
simulate those that may occur in some types of recirculating engine or burner fuel delivery systems. Results have not been
substantially correlated to engine or burner operation. The test method can be useful for investigation of operational problems
related to fuel thermal stability.
5.2 When the test method is used to monitor manufacture or storage of fuels, changes in filter rating values can indicate a relative
change in inherent stability. Storage stability predictions are more reliable when correlated to longer-term storage tests, for
example, Test Method D4625, or other lower temperature, long-term tests. When fuel samples are freshly produced, aging for
180 min, instead of the traditional 90 min interval, tends to give a result correlating more satisfactorily with the above methods
(see Appendix X2).
5.3 The test method uses a filter paper with a nominal porosity of 11 μm, which will not capture all of the sediment formed during
aging but allows differentiation over a broad range. Reflectance ratings are also affected by the color of filterable insolubles, which
may not correlate to the mass of the material filtered from the aged fuel sample. Therefore, no quantitative relationship exists
between the pad rating and the gravimetric mass of filterable insolubles.
6. Apparatus
6.1 Aging Tubes, 25× 200 mm, heavy wall test tubes made of borosilicate glass.
6.2 Heating Bath, with liquid heating medium, thermostatically controlled to maintain the sample in the aging tube within 1.5 °C
Henry, C. P., “The du Pont F21 149 °C (300 °F) Accelerated Stability Test,” Distillate Fuel Stability and Cleanliness, ASTM STP 751, Stavinoha, L. L. and Henry, C.
P., Eds., ASTM International, 1981, pp. 22–33.
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of 150 °C. It must be large enough to hold aging tubes immersed in the heating liquid to a depth above the level of samples in
the tubes. The bath and its location shall be such to enable shielding of the samples from direct light during aging. The volume
of bath and its heat recovery rate shall be such that the temperature of the medium does not drop more than 5 °C when the
maximum number of aging tubes are inserted, and recovery to 150 °C shall not require more than 15 min. (Warning—The flash
point of the liquid heating medium must be at least 180 °C. Bath vapors and oil sample vapors shall be properly vented. Exposed
hot surfaces on the apparatus and hot heating medium can cause severe burns.)
6.3 Bath Thermometer, either glass or digital measuring temperature measuring device, whose accuracy in the 140 °C to 160 °C
range is certified or traceable to a certified thermometer.
6.4 Membrane Filter Holder, to fit 47 mm membrane filters, fitted to a heavy-walled 500 mL or 1 L vacuum flask.
NOTE 1—Several types of membrane filter holders are available. To reduce electrostatic hazards, an all metal filter holder equipped with grounding cables
is recommended. Such an apparatus and correct grounding practices are described in Test Method D5452. A fritted glass filter holder is less preferred
because of a tendency to become partially clogged during use so that filter pads that do not have uniform deposits are obtained. Glass filter holders that
use a 75 μm (200 mesh) screen to support the filter are available; however, since the screen can be an unbonded electrostatic charge collector, these are
not recommended for use with flammable liquids.
6.5 Vacuum Source, that limits the maximum vacuum to 27 kPa (200 mmHg) below atmospheric pressure. The vacuum should rise
to 27 kPa within 10 s to 15 s after the sample is added to the filtration funnel.
NOTE 2—Use of reduced vacuum improves retention of particulate on the relatively porous filter media.
6.6 Reflection meter, Photovolt Model 577 Digital Reflection Meter, complete with search unit Y with a green filter and polished
black glass standard.
NOTE 3—Other reflection meters or search units, or both, can be used, but they are likely to provide only similar (not identical) results. For example,
Photovolt Model 577 digital reflection meter equipped with search unit W usually gives somewhat lower percent reflectance values. Correlation of these
values is discussed in Appendix X1.
7. Reagents and Materials
7.1 Purity of Reagents—Reagent grade chemicals shall be used in all tests. Unless otherwise indicated, it is intended that all
reagents conform to the specifications of the Committee on Analytical Reagents of the American Chemical Society where such
specifications are available. Other grades may be used, provided it is first ascertained that the reagent is of sufficiently high purity
to permit its use without lessening the accuracy of the determination.
7.2 Acetone, (Warning—Extremely flammable.)
7.3 Adherent Insolubles Solvent (Trisolvent or TAM), a mixture of equal parts by volume of reagent grade toluene
(Warning—Flammable. Vapor harmful), acetone (Warning—see 7.2), and methanol (Warning—Flammable. Vapor harmful.
May be fatal or cause blindness if swallowed or inhaled. Cannot be made nonpoisonous).
7.4 Hydrocarbon Solvent, 2,2,4-trimethylpentane (iso-octane), 99.75 % purity minimum (Warning—see 7.2).
NOTE 4—Heptane is a satisfactory alternative hydrocarbon solvent. However, small differences may be seen due to slightly different solubility
The sole source of supply of the apparatus known to the committee at this time is a suitable filter holder available from Millipore Corporation, 80 Ashby Rd., Bedford,
MA 01730; Catalog No. XX20 047 20. If you are aware of alternative suppliers, please provide this information to ASTM International Headquarters. Your comments will
receive careful consideration at a meeting of the responsible technical committee, which you may attend.
The sole source of supply of the apparatus known to the committee at this time is available from UMM Electronics Inc., Photovolt Instruments, 6911 Hillsdale Court,
Indianapolis, IN 46250-2062. If you are aware of alternative suppliers, please provide this information to ASTM International Headquarters. Your comments will receive
careful consideration at a meeting of the responsible technical committee, which you may attend.
Reagent Chemicals, American Chemical Society Specifications,ACS Reagent Chemicals, Specifications and Procedures for Reagents and Standard-Grade Reference
Materials, American Chemical Society, Washington, DC. For suggestions on the testing of reagents not listed by the American Chemical Society, see Analar Standards for
Laboratory Chemicals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeia and National Formulary, U.S. Pharmacopeial Convention, Inc. (USPC),
Rockville, MD.
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characteristics. Iso-octane is specified to be in agreement with the hydrocarbon solvent used in other middle distillate stability test methods such as Test
Methods D2274 and D4625.
7.5 Filter Paper (Filter Pad), Whatman No. 1, 47 mm diameter, or equivalent.
NOTE 5—Filter papers of 42.5 mm or 55 mm diameter are technically satisfactory. Filters with a diameter of 47 mm permit a small unused margin for
identifying the sample and fit all filtration apparatuses.
8. Sampling
8.1 When samples of a fuel batch are obtained to determine stability, obtain samples in accordance with Practice D4057. Use only
epoxy-lined cans or borosilicate glass bottles. Shield clear glass bottles from sunlight to prevent photochemical reactions.
8.2 When samples are from a fuel or component rundown line, exercise care to ensure that the sampling line and valving are
thoroughly flushed with current mainstream sample.
8.3 Because stability of some fuels, as determined in this test method, changes over time, the sampling date shall be recorded;
record time and date if sample is from a fuel or component rundown line. Samples should be stored at temperatures below 5 °C.
If storage for more than a few days is expected, oxygen should be removed from the fuel by subsurface purging with a stream of
nitrogen; for example, by bubbling nitrogen for 1 min ⁄L of sample.
8.3.1 If samples are taken out of cold storage, warm them to ambient temperature and thoroughly mix prior to aliquot sampling.
NOTE 6—If multiple analyses are to be performed on a sample, it is not a good practice to warm the whole sample repeatedly for this purpose. One way
of doing this would be to pour an aliquot of the cold sample into a graduated cylinder, cap, allow to warm, and then dispense into the aging tube for
analysis. If the cloud point of the fuel is above 5 °C, warm to a temperature 5 °C higher than the cloud point before dispensing.
9. Preparation of Apparatus
9.1 Cleaning Aging Tubes—Clean new tubes using adherent insolubles solvent, then with a mildly alkaline or neutral laboratory
detergent, followed by copious rinsing with deionized or distilled water to remove all traces of detergent. Then rinse with acetone
and air dry. Rinse used tubes with trisolvent, dry, then clean as above for new tubes. Visually inspect tubes before use, and reclean
or reject if there is the slightest trace of contamination.
9.1.1 Because of the small sample size and the high surface to volume ratio in this test method, carefully avoid carryover from
past tests or from cleaning agents. There are especially strong effects from traces of copper, strong acids, and strong bases.
NOTE 7—Clean test tubes carefully to avoid carryover from past tests or from cleaning agents. As a result of the small sample size evaluated and the high
surface to volume ratio inherent to the test method, the results obtained can be
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