2010-01-trafotech2010_trf_oil_specifications_for_indian_trf_industry.pdf

Trafotech-2010, January, 2010

TRANSFORMER OIL SPECIFICATIONS FOR INDIAN TRANSFORMER INDUSTRY P. Ramachandran ABB Ltd, Vadodara IS:335 was under taken in 1963 to bring the test methods in line with the practices in vogue. Second revision was in 1972, mainly to include oxidation stability test as given in IEC Pub 296(1969) ‘Specification for unused mineral insulating oils for transformers and switchgear’. Third revision in 1983 included ageing test based on ASTMD: 1934-1968 ‘Standard method of test for oxidation ageing of electrical insulating petroleum oils by open beaker method’, issued by the American Society for Testing and Materials. IS 335: 1993 (Fourth Revision) lined up to meet the requirements as per IEC Pub 296 (1982) and BS148:1984. In this revision, a new test method was added for detection of oxidation inhibitors. For inhibited transformer oil, a new Indian Standard was brought out as IS 12463:1988 “Specification for inhibited mineral insulating oils”. In between, IS: 12812: 1989 was issued for “Transformer Oil Feed Stock (TOFS) and Transformer Oil Base Stock (TOBS) Specification for the requirements of TOBS to be maintained by refineries”.

1. BACKGROUND Mineral lubricating oil was used in early transformers as an insulating medium in Europe (1890 Brown Boveri) and in America (1892- GE used Pennsylvanian Paraffinic type lubricating oil). Its efficacy as a heat carrier and cooling agent was recognized much later and early standard specifications for transformer oil came out in 1899. Indian transformer industry was dependent on imported oil (e.g. Shell Diala Grades) till 1969 when indigenous manufacture of transformer oil commenced at Mumbai and later at Chennai. Indigenous oil manufacturers were using the TOBS (Transformer Oil Base Stock) produced by the HPCL Refinery, Bombay and Madras Refineries, Chennai. These refineries were producing solvent-neutral oil with very high level of unsaturated compounds and sulfur / nitrogenous impurities. Initially the quality of locally produced oil, by acid treatment was poor, especially with regard to oil resistivity. Immediate impact of the indigenous oil on transformers was a sudden drop in insulation resistance during storage/service period. Industry could over come this problem and Indian Standard for oil added special requirements for oil resistivity, even though international standards were not specifying this property. After the economic liberalization, during 1990’s imported base oil made by hydro treating or hydro cracking the naphthenic and paraffinic crudes, was available for industry to deliver oils meeting any international specifications.

IEC standard on transformer oil was issued as IEC Pub 296 in 1969 and later revised in 1982 and in 2003 (Third Edition) as IEC 60296:2003 ‘Fluids for Electro-technical Applications-Unused Mineral Insulating Oils for transformers and switchgear’ to reflect the progress in industry and experience gained so far. Relevant American standard is ASTM D3487-08 Standard Specification for Mineral Insulating Oil Used in Electrical Apparatus, issued in 2008, revising the earlier 2000 and 1988 editions. Revision in 2008 was the deletion of upper limit of aniline point, so that iso-paraffinic oils can also meet with the ASTM standard.

2. EVOLUTION OF STANDARDS Even before the formulation of an Indian standard for transformers, Indian standard on Transformer Oil came out in1953 as IS: 335. This was based on BS 148: 1951 ‘Insulating Oil for Transformer and Switchgear’ issued by the British Standards Institution. First revision to

3. PROGRESS IN MANUFACTURING PROCESS OF TRANSFORMER OIL Until the middle of 1990’s, transformer oil was manufactured in India by using only local TOBS,

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hydrogen. By controlled hydrotreating the aromatics are saturated directly to naphthenic compounds. The advantages of such oils are the higher oxidation stability due to availability of optimum aromatics which act as natural inhibitors, lower pour points and lower gassing tendencies but there can be some disadvantage with respect to corrosive sulphur if the hydrotreating is not done to retain the right levels of aromatics. Naphthenic oil producers basically use this refining technique to date. Severe hydrotreating is becoming essential to meet the recently introduced stringent tests for corrosive sulphur and total sulphur content. (<0.15 % W/W)

made mainly from local and imported Paraffinic crudes by solvent extraction process, the technology available since 1930. With the refining technology available at that time in the country, very low pour point was not achievable and hence IS 335 stipulated a minimum pour point of -6oC against the prevailing IEC stipulation of -30oC for class I and -45oC for class II oils. In this process of making TOBS, base oil is treated with an extraction solvent (furfural / phenol) to selectively remove aromatics. After this process, de-waxing solvent (methyl– ethylketone/ Toluene / Propane) is mixed with oil and chilled to a low temperature to precipitate out waxes. This step is necessary for Paraffinic base oils which has higher wax content and is not necessary for Naphthenic base oils which contain very little wax content. Another process used for getting still lower pour point is catalytic de-waxing or hydro- dewaxing. Waxy oil is mixed with hydrogen gas at elevated temperatures and pressure and allowed to flow over catalyst bed. Wax is converted in to lighter hydrocarbons by this process.

3.2. Hydro-cracking and severe Hydrocracking – It is a severe form of hydro-treating, available from 1970, where base oil flows over a high activity catalyst bed at temperatures above 350oC and pressures above 70 bar. Almost all of the sulphur and nitrogen compounds in oil are removed and many aromatics are saturated with hydrogen. All unsaturates are converted in to smaller, saturated paraffinic molecules. The advantages of such oils are their higher electrical properties and highly non corrosive nature but the disadvantages can be due to their higher pour points, relatively lower oxidation stability (unless inhibited) and higher gassing tendencies.

Transformer Oil was made from TOBS using the Acid and Clay treatment process. In this process concentrated sulphuric acid is mixed with base oil to react with the polar and unsaturated compounds in it to form sludge that is then removed. Later, oil is neutralized with alkalies, extracted with solvent iso-propyl alcohol and then water washed and filtered through clay (Fullers earth), thereby removing aromatic and highly polar compounds to achieve the required electrical properties. Further filtering is done under vacuum to bring down the moisture level. The disadvantage with this process, apart from economic considerations is the environmental concerns about sludge and acids resulting from the process. Certain alternate transformer oil refining methods were tried abroad and became popular over the above conventional processes. These can be summarized as below:

3.3. Hydro-isomerisation – This process came in to use from 1990, for improving the properties of hydro-cracked paraffinic crude base oils. This is a process after hydro - cracking and similar to catalytic dewaxing. During this process, wax is selectively converted (isomerised) in to branched chain iso-paraffinic molecules. Paraffinic molecules are isomerised in to iso-paraffinic molecules, resulting in water-white, corrosive sulphur and aromatic free iso-paraffinic transformer oil, with pour point comparable to naphthenic oils, thus resulting in very high quality base oil. The advantages of such oils are their higher electrical properties and extremely non corrosive nature with lower pour points but the disadvantages can be due to relatively lower oxidation stability (unless inhibited) and higher gassing tendencies.

3.1. Hydro-treating – This was first used in 1960’s. Base oil is treated with hydrogen at a pressure above 35 bar and temperature above 315oC in the presence of a catalyst. Impurities are removed by the reaction of harmful sulphur and nitrogen hetero cyclic polar compounds with

The manufacturing of inhibited oils and high grade non-inhibited oils is quite different. Oxidation stability in non–inhibited oils is achieved by the presence of natural oxidation

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stabilizers available in the oil. In case of noninhibited oils, when refining is carried out, various compounds of nitrogen and sulphur are left in oil and these contribute to oxidation stability. The non-corrosive sulphur compounds left in oil for natural oxidation stability can some times turn in to corrosive compounds. These corrosive compounds can subsequently cause conductive coatings on insulating paper provided as inter turn conductor insulation, which can lead to inter turn fault from insulation breakdown. It is believed that certain compounds like DBDS (Dibenzyl Disulphide-antioxidant and antiwear additive) added to the oil for improving oxidation stability can also contribute to this corrosive sulphur property by cracking at higher operating temperatures forming reactive sulphur compounds. This corrosive property can be suppressed by adding copper passivators in oil. Passivating compounds used are benzotriazole based ones, in small concentrations e.g.: Irgamet 39 (0.01-0.03%) or benzotriazole amine (BTA0.003 – 0.01 %).

gas absorbing oils, but may be advantageous in special applications like EHV instrument transformers and bushings with thick paper insulation and also certain special EHV, HVDC transformers. Another option followed by some manufacturers is mixing iso-paraffinic oils with naphthenic oils to get optimum characteristics. It is recommended that oil manufacturers along with transformer manufacturers and utilities should monitor the field service experience of these iso-paraffinic oils that are now in service for more than a decade. The hydro treated, fully inhibited Naphthenic oils were used in all UHV (800 KV) transformers, commissioned so far in the country. Based on service experience, iso- paraffinic oils can be tried for such applications too. 4. NEW IEC STANDARD ON TRANSFORMER OIL The major changes in the latest IEC 60296 Specifications are the following: Instead of class I & II grades in earlier version, there is only one class of oil for transformer application, with maximum viscosity of 12 mm²/s at 40°C. A new parameter LCSET (Lowest Cold Start Energizing Temperature) has been introduced. It is the permissible lowest ambient temperature, at which the transformer can be energized safely from cold condition. As per new standard, this is -30°C, 5 degrees lower than the minimum service ambient temperature for power transformers stipulated in IEC 60076-2. LCSETs of 0°C, -20°C and -40°C are also specified in the standard as alternate options.

From 1997 onwards, Indian transformer oil manufacturers are increasingly using imported hydro-cracked, hydro-isomerised paraffinic oils as base oils. Such highly refined oils will not have any polar compounds, resulting in high resistivity but relatively poor oxidation stability. Synthetic antioxidants like 2,6 di-tert-butyl-p-cresol or 2,6 di-tert-butyl-p-phenol are added (0.08-0.4% by weight) to oil to give good oxidation stability. Normally oil ageing in inhibited oil will be slower than in uninhibited oil. Since some inhibitor depletion will definitely be there during service, it is required to monitor inhibitor content periodically and to replenish the inhibitor when it reaches half of its original level. But in normal situations, the added oxidation inhibitors will be sufficient to last long periods during the life of transformer. Another characteristic of such isoparaffinic oil is complete lack of aromatic content as all aromatics are converted in to stable molecules. This results in oil with positive gassing tendency, which can be a disadvantage in certain cases. So when negative gassing tendency (i.e. gas adsorbing property) is required or to reduce the positive gassing values, then aromatic additives like alkylbenzenes, benzyle toluenes or 1,2,3,4 Tetrahydronaphthalene (Tetralin) (3-8% by weight) are added to oil. Most of the transformer manufacturers do not demand

Three optional standard grades are introduced depending on the quantity of oxidation inhibitors present in oil. U – Uninhibited Oil-No detectable Antioxidant additive T – Trace inhibited Oil-Max 0.08% Antioxidant additive I – Inhibited Oil-0.08 to 0.4 % Antioxidant additive In addition, special grade inhibited oil with higher oxidation stability and low sulphur content characteristics is introduced in standard for use in transformers with higher operating temperatures or designed for extended service life.

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Acidity limit is reduced to < 0.01 mg KOH/g of oil with measuring method as per IEC 62021-1 (Earlier requirement was <0.03 mg).

1) Standard Grade (ST) –Trace inhibited - For General use in Power and Distribution Transformers 2) High Grade (HI) – Inhibited - For Use in Generator, Industrial, HVDC transformers 3) High Grade (HIG) – Inhibited, with negative gassing tendency - For EHV Instrument transformers, bushings and other special applications.

Total sulphur content of 0.15% is specified for high grade oil. For all standard grade oils, a corrosive sulphur test using silver strips is introduced (DIN 51353) instead of using copper strips. IEC 60296 is now under revision again, with the intention of introducing more stringent requirements for corrosive sulphur. IEC 62535 Ed 1.0 Test method for detection of potentially corrosive sulphur in used and unused insulating oil was issued in 2008, for detecting corrosive sulphur, together with the DIN silver strip test.

Even though in our country, un-inhibited oil was the standard in line with the practice in UK (except Indian Railways – Track side feeder transformers are specified with fully inhibited oil as per IS: 12463 for more than two decades). But in Europe inhibited oil is widely specified and ASTM standard specifies only trace inhibited or inhibited oils.

5. OIL STANDARD FOR INDIAN INDUSTRY Consequent to globalization, transformer industry has become truly an international one and the national standards are disappearing in favour of International Standards i.e. those of IEC. American Standards do rule over certain regions, but may line up with IEC in due course of time. Under such a scenario, it may no longer be necessary or desirable to have a separate Indian Standard for transformer oil. It will be prudent to adopt latest IEC standard with certain grades selected as preferred ones to reduce variety and to meet the requirements of industry. Like in many other countries (e.g. UK), in India too, base transformer oil is imported and only finishing is done locally to meet the plethora of local specifications with minor variations demanded by users. It will be appropriate to review these variations in specifications which defeat the whole purpose of standardization, and to suggest appropriate IEC grades to replace these specifications. It has become urgent as IEC was revised nearly 6 years back and we have still not fully incorporated these advances in technology in our oil specifications. (Table 1 Comparison of Transformer Oil Specifications).

The following additional requirements may be added to existing IEC standard reflecting progress during past 5 years. -Oil shall pass corrosive sulphur test as per IEC 62535 Ed1.0 (2008-10) Test method for detection of potentially corrosive sulphur in used and unused insulating oil. -Composition of oil in terms of percentage of aromatic content, hydrocarbon composition and proportions, used additives and inhibitors are to be furnished as a fingerprint for the reference of users. 6. SUPERFLUOUS REQUIREMENTS Oil specifications currently followed by users and manufacturers call for several additional requirements and tests over and above IEC standard. The following sections explain why these are not essential in the changed environment of imported oils made through improved manufacturing processes. 6.1. PONA Analysis / maximum aromatic content Some typical PONA requirements as per individual specifications, followed in India are a. Paraffinic 48-52 % Oliffinic Nil Naphthenic 38-42 % Aromatic 8 % Max

Based on the IEC standard, the following grades can be standardized for meeting the entire requirements of utilities in the country. Mineral Insulating Oil as per IEC 60296-2003 with maximum pour point of -400C (LCSET= 300C)

b.

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Paraffinic

-

46 % (Max)

Trafotech-2010, January, 2010

Naphthenic Aromatic c.

-

Aromatic as per ASTM D 2140 as per IEC 60590

7% (Max) 13% (Max)

d.

Aromatic Content

e.

Oil of Naphthenic type (made from Naphthenic Crude)

f.

Paraffins Aromatics Naphthenic

-

-

route. Oil is no longer manufactured by this method and hence there is no more relevance for S K value and this test has been removed from IS:335-1993 in April, 2006 vide its Amendment No 2.

46 % (Min) 8 % (Max)

6.3. Oxidation Stability Test Major differences between national standards in the limiting values for oxidation stability test are listed in Table 2-Comparison of Oxidation Stability Tests for oil.

4~8 %

1) IS:335 requirements for oxidation stability test are as given below: (a)Oxidation stability (testing as per Annexure C to IS:335 at 100oC for 164 hours with oxygen). Neutralisation value after oxidation (Max)-0.4 mg KOH/g Total sludge after oxidation (Max)-0.1% by Weight. (b)Ageing characteristics after accelerated ageing (open beaker method with copper catalyst) -Testing as per IS 12177: 1987 Method A Specific resistance at 27°C - Min 2.5 X 10¹² ohm cm Specific resistance at 90°C - Min 0.2 X 10¹² ohm cm Dielectric Dissipation Factor at 90°C - Max 0.2 Total acidity - Max 0.05 KOH/g Total sludge - Max 0.05 percent by weight 2) IEC 60296 requirements for oxidation stability are more stringent as mentioned below:

50 % Max 4-12 % Balance

Aromatic content in new transformer oils made through hydrogenation or hydro-cracking manufacturing route is very low so that there is no need for specifying a maximum limit for it. Good quality oils are made from both naphthenic and paraffinic crudes. Oil made from iso-praffinic base oils can match the characteristics such as oxidation stability, heat transfer and biodegradability as compared to common naphthenic mineral oils. With the advent of severe hydrocracking technology, oil should not be chosen on grounds of classification in to “naphthenic” or “paraffinic”. What matters is whether it fulfills the functional requirements of oil. There are more and more different ways of achieving this, making the former classifications less and less relevant.

Oxidation stability testing as per IEC 61125 Method C-at 120oC with Air, representing real service life situation Neutralization value after oxidation (Max)-1.2 mg KOH/g Total sludge after oxidation (Max)-0.8 % by Weight

6.2. S K Value – 4 to 8%: Is it necessary to specify this? S K Value is an index of degree of refining of oil measured by the reaction of oil sample with a volume of concentrated sulphuric acid. S K value measurement method was covered in Annexure D to IS:335 ‘Method of Determination of SK Value’. This was originally introduced in Germany and measurement was as per DIN 51533:1955 ‘Behavior of Insulating Oils in the Presence of Concentrated Sulphuric Acid’. The acronym S K came from German word Konzentriorter Schwefelsäure-sk-Zahl (Concentrated sulphuric acid). S K value was lower for oil made by high level of refining. In Europe this test is no longer used and new DIN standard on oil does not specify this requirement. This test is significant only in case of oil manufactured by acid refining

As per latest IEC norms, transformer oil conforming to IS: 335, is a trace inhibited oil because as per IS, oxidation inhibitor up to 0.05 % is to be considered as absence of DBPC while as per IEC 60296 clause 3.6, such an oil is treated as trace inhibited. Oxidation stability requirements of oil in such a case shall be as for T grade as given in Table 1 i.e. oxidation stability test shall be for 332 hours instead of 164 hours.

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distribution and power transformers), Inhibited High Grade( for generator and HV DC transformers) and Inhibited High Grade with negative gassing tendency((for instrument transformers and bushings), all with a pour point of - 40oC.

Test methods for detection and quantitative determination of the inhibitor are different as per IEC 60666 Ed1.0 (1979-01) ’Detection and determination of specified anti-oxidant additives in insulating oils’. For detecting small amounts of inhibitor (i.e. for U grade) thin layer chromatography (HPLC) is recommended (clause 5.2 of IEC 60666) and infra-red spectrophotometry method is specified for determination of higher quantity of inhibitor (clause 5.1 of IEC 60666).

It is hoped that such a measure will bring uniformity in specifications, resulting in savings to manufacturers and users. 8. REFERENCES [1] J R Nanda, Insulating Oils for Transformers and switch gear, Chary Publications, Bombay, 1971. [2] Lim Eng Seng, “Mineral Insulating Oil Manufacture and safe keeping”, Insulec 93, Session 1 Paper5, pages 41-49, IEEMA 1993. [3] Andrzej Sierota and Juris Rungis, “Electrical Insulating oils Part-1: Characterisation and pre treatment of New Transformer oils”, IEEE Electrical Insulation Magazine Vol11 No1, 1995 Pages 8- 20. [4] Paul W H Taylor, “The manufacture of transformer oils for the Power Industry” 64th Doble International Conference, Boston, MA 1997 Sec5-2. [5] T O Rouse, “Mineral Insulating Oil in Transformers”, IEEE Electrical Insulation Magazine, Vol. 14 No3, 1998 Page 6-14. [6] Kjell Sundkvist, “How to make a good transformer oil”, Nynas naphthenics, 2.1999. [7] B.Pahlavanpour, M.Eklund and KSundkvist, “International Specification for supply of unused oil, Revised IEC 60296”, 71st Doble International Conference, Boston, MA 2004 Sec IM-10. [8] H. Carl Manger and R. Reynolds, “Insulating Oil”- 101 Doble International Conference, 2005. [9] Jimmy M Rasco, “Petroleum Refining production of Naphthenic Transformer oil Control of Corrosive Sulphur”, 74th Doble International Conference, Boston, MA, 2007. [10] Steve Krawiec, “Production of Corrosive Sulphur Free Transformer Fluids”, Weidmann 7Th Annual Technical Conference, 2008.

6.4. Resistivity (Specific Resistance) As per IS:335, resistivity requirements are clearly stipulated and limiting values after ageing are also specified at 27oC and 90oC. IEC and other national standards are no longer specifying resistivity requirements, as new oils, made through modern manufacturing processes, inherently have very high resistivity due to the removal of all polar compounds by high degree of refining. IEC 60422:2005 on Oil Maintenance (clause 6.6) states: “There is generally a relation ship between DDF (Dielectric dissipation factor) and resistivity, with resistivity decreasing as DDF increases. It is normally not necessary to conduct both tests on the same oil and generally DDF is found to be the more common test. The measurement of resistivity is also considered to be of value for monitoring oils in service as it has been shown to be reasonably proportional to oxidation acids and to be affected by undesirable contaminants such as metal salts and water”. Hence resistivity values may be sometimes significant during maintenance of oil, but not much relevant for new oil as power factor test covers this requirement. 7. CONCLUSIONS It is necessary to standardize the specifications of Transformer oil used in the country, in line with the latest IEC Standard on transformer oil. Utilities and manufacturers may come to an understanding regarding the preferred grades to be selected as per latest IEC standard, considering also the availability in the country. Superfluous properties and test requirements may be avoided. Three grades of oil are proposed-Trace inhibited Standard Grade(for

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[11]

[12] [13] [14]

IEC 296:1982 Specification for unused mineral insulating oils for transformers and switchgearIEC Geneva, Switzerland, 2003. IS: 12463 -1988, Specification for inhibited Mineral insulating oil, Bureau of Indian Standards New Delhi, 1988. IS 335:1993, New Insulating oilsSpecification (Fourth Revision) Bureau of Indian Standards New Delhi, 1993. IEC 60296 :2003 (Ed 3.0) Fluids for Electrotechnical Applications –Unused Mineral Insulating Oils for Transformers and switchgear, IEC, Geneva, Switzerland 2003.

[15]

[16]

[17]

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IEC 60422:2005(Ed3.0) Mineral Insulating oils in electrical equipment –Supervision and Maintenance guidance, IEC Geneva, Switzerland, 2005. IEC 62535: 2008(Ed1.0) Test Method for detection of potentially corrosive sulphur in used or unused insulating oil, IEC Geneva, Switzerland, 2008. ASTM D 3487-08 Standard Specifications for Mineral Insulating Oil Used in Electrical Apparatus.

Trafotech-2010, January, 2010

TABLE 1 COMPARISON OF TRANSFORMER OIL SPECIFICATIONS

1

Grades

2

Kinematic Viscosity (Max)

IS:335-1993 New Insulating Oils – Specification IS:335 Grade (Uninhibited )

IEC 296:1982 Unused Mineral Insulating Oils

IEC 60296:2003 Unused Mineral Insulating Oils

Class I, Class II, Class III Uninhibited and inhibited grades in each class

U-Uninhibited T-Trace inhibited I- Inhibited (0.08-0.4%) HI – High grade inhibited (0.08-0.4%)

27 cst at 27°C

I 16.5(40°C)

II 11(40°C)

III 3.5(40°C)

1800(30°C) 130

150(40°C) 95

1800mm²/s at -30°C

-45

-60

-40

3

Flash Point (Min) oC

140

800(15°C) 140

4

Pour Point (Max) oC Acidity (Max) mg KOH/g IFT (Min) mN/m

-6

-30

5 6 7 8

Dissipation factor at 90oC (Max) PAC (Polycyclic Aromatics (max) % content)

12mm²/s at 40°C

135

0.03

0.03

0.01

40

40

-

0.002

0.005

0.005

-

-

3

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TABLE 2 COMPARISON OF OXIDATION STABILITY TESTS FOR OIL Standard Oil Grade

IEC 60296 : 2003

Grade

Standard U,T,I

Test Method

IEC 61125 Method C (1992) (Earlier IEC 813-1985)

Limiting Values Total acidity mg KOH/g, Max Sludge % Max Duration of Test hrs

Temp of oil during test

IEC 296 -1982

High Grade I

U

I

IEC 61125 Method A (Earlier IEC 74 1963)

IEC 61125 Method B (Earlier IEC 4741974))

IS 335 : 1993 U Annexure C to IS:335

IS:121771987Method A (Same as ASTMD 1934:1968.Op en Beaker with copper catalyst

I (IS :124631988) IS:124221988 (IEC4741974,Same as IEC 61125 Method B)

1.2

0.3

0.4

0.4

0.05

0.4

0.8

0.05

0.1

0.1

0.05

0.1

U (uninhibited oil ) 164 T(Trace inhibited oil) - 332 I ( Inhibited oil ) -500

164

Min 120 hrs Induction Period

164

96

Min 120 hrs and limit of Min 195 minutes by Rotating Bomb Test Method

120°C in Air

100°C in Oxygen

120°C in Oxygen

100°C in Oxygen

115o C in Air

120°C in Oxygen

Note: Oil Grade

U T I

= = =

Uninhibited Oil Trace Inhibited Oil Inhibited Oil

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