ASTM D2440-99(2004)
(Test Method)Standard Test Method for Oxidation Stability of Mineral Insulating Oil
Standard Test Method for Oxidation Stability of Mineral Insulating Oil
SIGNIFICANCE AND USE
The oxidation stability test of mineral transformer oils is a method for assessing the amount of sludge and acid products formed in a transformer oil when the oil is tested under prescribed conditions. Good oxidation stability is necessary in order to maximize the service life of the oil by minimizing the formation of sludge and acid. Oils that meet the requirements specified for this test in Specification D 3487 tend to minimize electrical conduction, ensure acceptable heat transfer, and preserve system life. There is no proven correlation between performance in this test and performance in service, since the test does not model the whole insulation system (oil, paper, enamel, wire). However, the test can be used as a control test for evaluating oxidation inhibitors and to check the consistency of oxidation stability of production oils.
SCOPE
1.1 This test method determines the resistance of mineral transformer oils to oxidation under prescribed accelerated aging conditions. Oxidation stability is measured by the propensity of oils to form sludge and acid products during oxidation. This test method is applicable to new oils, both uninhibited and inhibited, but is not well defined for used or reclaimed oils. Note 1
A shorter duration oxidation test for evaluation of inhibited oils is available in Test Method D 2112.Note 2
For those interested in the measurement of volatile acidity, reference is made to the work of IEC Subcommittee 10A.
1.2 The values stated in metric units are to be regarded as the standard.
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 and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: D2440 − 99(Reapproved 2004)
Standard Test Method for
Oxidation Stability of Mineral Insulating Oil
This standard is issued under the fixed designation D2440; 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.
This standard has been approved for use by agencies of the Department of Defense.
1. Scope D3487 Specification for Mineral Insulating Oil Used in
Electrical Apparatus
1.1 This test method determines the resistance of mineral
2.2 IEC Publication:
transformer oils to oxidation under prescribed accelerated
CEI/IFC 61125: 1992 Unused Hydrocarbon–Based Insulat-
aging conditions. Oxidation stability is measured by the
ing Liquids—Test Methods for Evaluating the Oxidation
propensity of oils to form sludge and acid products during
Stability
oxidation. This test method is applicable to new oils, both
uninhibited and inhibited, but is not well defined for used or
3. Summary of Test Method
reclaimed oils.
3.1 Atest specimen of mineral transformer oil is oxidized at
NOTE 1—A shorter duration oxidation test for evaluation of inhibited
a bath temperature of 110°C, in the presence of a copper
oils is available in Test Method D2112.
catalyst coil, by bubbling oxygen through duplicate test speci-
NOTE 2—For those interested in the measurement of volatile acidity,
reference is made to the work of IEC Subcommittee 10A. mens for 72 and 164 h, respectively. The oil is evaluated at the
end of each aging period by measuring the amount of sludge
1.2 The values stated in metric units are to be regarded as
and acid formed. The test specimen is diluted with n-heptane
the standard.
and the solution filtered to remove the sludge. The sludge is
1.3 This standard does not purport to address all of the
dried and weighed. The sludge-free solution is titrated at room
safety concerns, if any, associated with its use. It is the
temperature with standard alcoholic base to the end point
responsibility of the user of this standard to establish appro-
indicated by the color change (green-brown) of the added
priate safety and health practices and determine the applica-
p-naphthol-benzein solution.
bility of regulatory limitations prior to use.
4. Significance and Use
2. Referenced Documents
4.1 Theoxidationstabilitytestofmineraltransformeroilsis
2.1 ASTM Standards:
a method for assessing the amount of sludge and acid products
B1 Specification for Hard-Drawn Copper Wire
formed in a transformer oil when the oil is tested under
D664 Test Method for Acid Number of Petroleum Products
prescribed conditions. Good oxidation stability is necessary in
by Potentiometric Titration
order to maximize the service life of the oil by minimizing the
D974 Test Method for Acid and Base Number by Color-
formation of sludge and acid. Oils that meet the requirements
Indicator Titration
specified for this test in Specification D3487 tend to minimize
D2112 Test Method for Oxidation Stability of Inhibited
electrical conduction, ensure acceptable heat transfer, and
Mineral Insulating Oil by Pressure Vessel
preserve system life. There is no proven correlation between
D2272 Test Method for Oxidation Stability of Steam Tur-
performance in this test and performance in service, since the
bine Oils by Rotating Pressure Vessel
test does not model the whole insulation system (oil, paper,
enamel, wire). However, the test can be used as a control test
This test method is under the jurisdiction of ASTM Committee D27 on forevaluatingoxidationinhibitorsandtochecktheconsistency
Electrical Insulating Liquids and Gasesand is the direct responsibility of Subcom-
of oxidation stability of production oils.
mittee D27.06 on Chemical Test.
Current edition approved Oct. 1, 2004. Published November 2004. Originally
5. Apparatus
approved in 1965 as D2440 – 65 T. Last previous edition approved in 1999 as
D2440 – 99. DOI: 10.1520/D2440-99R04.
5.1 Aging (Oxidation) Bath—An oil bath, wax bath, or
ASTM Research Report No. RR:D27-1001, available from ASTM
aluminum block heater (see IEC Standard 61125) of a suitable
Headquarters, 100 Barr Harbor Drive, West Conshohocken, PA 19428.
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 International Electrotechnical Commission. Available from American National
the ASTM website. Standards Institute, 25 W. 43rd St., 4th Floor, New York, NY 10036.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D2440 − 99 (Reapproved 2004)
type capable of controlling the temperature at 110 6 0.5°C Appendix. Cellulose ester type membrane filters of 5-µm
with a temperature gradient of less than 1°C in the body of the porosity have been used successfully also.
liquid. Use any nontoxic liquid having low volatility at 110°C
6. Reagents and Materials
and containing no volatile additives and having a flash point
above the test temperature. Mineral oils, waxes and silicone
6.1 Purity of Reagents—Use reagent grade chemicals in all
oils have been used satisfactorily. Circulation of the oil or wax
tests.Unlessotherwiseindicated,allreagentsaretoconformto
heating medium by means of a pump or stirrer is recom-
the specifications of the Committee on Analytical Reagents of
mended. See also 5.4, relating to the depth of fluid in aging
the American Chemical Society, where such specifications are
bath and position of oil receptacles.
available.
5.2 Drying Tower—A drying tower 25 to 30 cm in height,
6.2 Chloroform, cp, or acetone, cp.
fitted with a ground-glass stopper and side tubes for condition-
6.3 n-Heptane,fordilutionoftheoxidizedoil,precipitation,
ing of the oxygen supply.
and washing of the sludge shall conform to the following
5.3 Oil Receptacle and Head—An oil receptacle consisting
requirements:
of a heat-resistant glass test tube 25 mm in outside diameter,
Relative Density at 20°C 0.6836 to 0.6839
1.25 mm in wall thickness, 210 mm in overall length including Refractive index at 20°C 1.3876 to 1.3879
Solidification temperature, min, °C −90.72
a standard taper 24/40 outer joint, with a Dreschsel-type head
Distillation 50 % shall distill between 98.38 and
consisting of a standard taper 24/40 inner joint with side outlet
98.48°C. Temperature rise between 20 and
tube 5.0 mm in outside diameter and an oxygen delivery tube 80 % recovery shall be 0.20°C maximum
5.0 mm in outside diameter and 3 mm, minimum, in inside
6.4 p-Naphtholbenzein Indicator Solution—The specifica-
diameter which extends to within 2.5 6 0.5 mm of the bottom
tions for p-naphtholbenzein are prescribed in Test Method
of the oil receptacle and has its end ground at an angle of 30°
D974. Prepare a solution containing 10 g p-naphtholbenzein
to the axis of the tube. The design is shown in Fig. 1.
per litre of titration solvent (see 6.7).
5.4 Position the oil receptacle in the aging bath in accor-
6.5 Oxygen—Obtainedfromliquidoxygen,minimumpurity
dance with the drawing shown in Fig. 2.
99.4 %.
NOTE 3—The oil receptacle and position in the aging bath are
6.6 Potassium Hydroxide Solution, Standard Alcoholic (0.1
essentially the same as specified in IEC Standard 61125, “Test Method for
N)—Prepare alcoholic potassium hydroxide solution and stan-
Oxidation Stability of Inhibited Mineral Insulating Oils,” Glassware
dardize as described in Test Method D974. Commercially
dimensionshavebeenalteredslightlytoconformtosizesreadilyavailable
preparedAlcoholic Potassium Hydroxide Solution is available.
in the United States.
6.7 Titration Solvent—Mix 3 parts by volume of toluene
5.5 Filtering Crucibles—A glass filter crucible having a
with 2 parts by volume of isopropyl alcohol.
maximum diameter of the pores between 5 and 15 µm as
determined in accordance with the method described in the
6.8 Silicon Carbide Abrasive Cloth, 100-grit with cloth
backing.
6.9 Wire Catalyst—AWG No. 18 (0.0403-in. (1.01-mm)
diameter) 99.9 % purity conforming to Specification B1. Soft-
drawn copper wire of equivalent grade may also be used.
6.10 Hydrochloric Acid, 10 Volume %.
7. Hazards
7.1 Consult Material Safety Data Sheets for all materials
used in this test method.
8. Preparation of Oxygen Supply
8.1 Oxidize the oil by contact with oxygen of 99.4 %
minimum purity conditioned as follows:
8.1.1 Use metal or glass tubing to deliver the oxygen to the
oxygendeliverytube.Eliminaterubberconnectionsifpossible.
If used, the rubber should be at room temperature. Limit the
length of rubber tubing exposed to the oxygen flow to 13 mm.
Do not use rubber connections in contact with the liquid of the
heating bath or under conditions where the temperature of the
“Reagent Chemicals,American Chemical Society Specifications,”Am. Chemi-
cal Soc., Washington, DC. For suggestions on the testing of reagents not listed by
theAmerican Chemical Society, see “Reagent Chemicals and Standards,” by Joseph
Rosin, D. Van Nostrand Co., Inc., New York, NY, and the “United States
FIG. 1 Oil Receptacle and Head Pharmacopeia.”
D2440 − 99 (Reapproved 2004)
FIG. 2 Position of Oil Receptacle in Aging Bath
rubber will be above room temperature. Where it is necessary following reagents have been found suitable: chromic acid,
to use rubber connections, only sulfur-free rubber is permis- aqua regia, and ammonium persulfate. Wash each receptacle
sible. FOA/USPNalgene tubing has been found satisfactory in free of acid, using tap water, and finally rinse with distilled
this application.
water. Dry in an oven at 105 to 110°C for at least 3 h, cool to
8.1.2 Dry the oxygen by forcing it through a solid desiccant
roomtemperatureinadesiccator,andkeepthereceptacleinthe
of high moisture-absorbing capacity. Arrange the desiccant in
desiccator until ready for use.
the drying tower to a depth of 205 to 254 mm. Change the
desiccant when the indicator begins to change colors from
10. Preparation of Copper Catalyst
moisture absorption. If an indicator is not used, change the
10.1 Immediately before use, polish the copper wire with
desiccant at least weekly.
silicon carbide abrasive cloth and wipe free of abrasive with a
8.1.3 After passing oxygen through the drying tower, admit
clean dry cloth.
it directly to the receptacle containing the oil to be tested. Do
not preheat the oxygen.
10.2 Wind a 300-mm length of the polished wire into a
8.1.4 Determine the rate of oxygen supply with an elec-
helical coil approximately 16 mm in outside diameter and 50
tronic flow meter, calibrated rotameter, or soap bubble buret,
mm in height. Clean the coil thoroughly with chloroform or
and adjust the flow rate to deliver the conditioned oxygen at a
acetone, air dry, and insert immediately into the oil receptacle.
rate of 1.0 6 0.1 L/h to each tube of oil being tested.
Handle the clean copper coil only with clean tongs to avoid
contamination.
9. Preparation of Oil Receptacle
9.1 Wash each oil receptacle thoroughly, first with acetone 10.3 Commercially available, prepackaged, preformed coils
and then with soap and water, and rinse in acid solution. The thatmeettherequirementsdescribedinthistestmethodmaybe
usedasanalternativemethodofcatalystpreparation.Cleanthe
6 coil in a 10 % HCl solution for 30 s, rinse three times with tap
Anhydrous magnesium perchlorate (Anhydrone or Dehydrite) is a suitable
desiccant for this purpose. water and with distilled water, chloroform or acetone and air
D2440 − 99 (Reapproved 2004)
dry immediately before use. Preformed coils were used in a meyerflaskfittedwithagroundglassstopper.Separate300mL
1997 Round Robin Study and found suitable for this applica- of n-Heptane into three equal aliquots to be used for the
tion.
sequential washing of the oxygen delivery tube copper catalyst
coil and the test receptacle to recover the adhering oil. Into the
11. Preparation of Glass Filtering Crucible
same flask, thoroughly rinse the catalyst coil, delivery tube,
11.1 Clean the glass filtering crucible and dry in an air oven
and test receptacle to recover adhering oil, using a total of 300
at 105 to 110°C until it has reached constant mass. Cool and
mL of n-Heptane.
store in a desiccator and, when needed, weigh to the nearest
15.2 Weighing Sludge—Allow the mixture to stand in the
0.0001 g.
dark for 24 h, at a temperature of 20 6 2°C, before filtering
12. Conditioning of Test Specimen throughaglassfilterorpolymermembrane,previouslydriedto
constant weight. To prevent sludge from passing through the
12.1 Filter the oil test specimen by gravity at ambient
filter, use a small pressure drop at the start of filtering. Pass
pressureoratreducedpressure,throughacid-freefilterpaperto
cloudy filtrates through a second time. Remove all traces of oil
remove traces of sediment, fiber, and excess water. Alterna-
fromthesludgebyrepeatedwashingwithn-Heptane.Washthe
tively, filter the test specimen under vacuum through a 0.45 µm
500-mL Erlenmeyer flask with n-heptane to remove adherent
mixed ester or cellulose filter. Discard the first 25 mLof the oil
oil, and empty each rinse into the filter. Use a total volume of
filtrate.Carefullyprotectthefiltrateduringthefiltrationagainst
150 mLof n-heptane, divided into three aliquots for sequential
dust and other contaminations.
washes, for washing the sludge. Dry the filter containing the
13. Procedure
sludge at 110°C to constant mass and weigh to the nearest
0.0001 g. Transfer sludge adhering to the catalyst, test recep-
13.1 Prepare two copper catalysts as described in Section
tacle, oxygen delivery tube, and the 500-mL Erlenmeyer flask
10.Insertoneineachoftwoclean,dryoilreceptaclesprepared
by dissolving the sludge in small quantities of chloroform or
as described in Section 9.
acetone (a total of 30 mL) into a tared porcelain vessel. Dry at
13.2 Transfer 25 6 0.01 g of the conditioned oil test
110°C after the evaporation of the chloroform or acetone to
specimen into each of the two prepared oil receptacles by
constant mass. Add the weight of the residue to that of the
means of a clean dry pipet. Immediately place the head on the
sludge obtained by precipitation with n-heptane. Express the
oil receptacle to protect the oil from contamination during the
total sludge as a percentage of the initial weight of the oil as
interval between placing the oil in the receptacle and the actual
follows:
start of the oxidation.
Sludge,% 5 weight of sludge/weight of oil sample 3100
~ !
13.3 Adjust the heati
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