GB/T 8018-2015 Determination of oxidation stability of gasoline Induction period method

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1. Method number / full name / English name / replacement relationship

• Standard number: GB/T 8018-2015
• Chinese name: Determination of oxidation stability of gasoline Induction period method
• English name: Determination of oxidation stability of gasoline—Induction period method
• Replacement relationship: Replaces GB/T 8018-1987 "Determination of oxidation stability of gasoline (Induction period method)"
• Adoption relationship: Modified adoption using redrafting method of ASTM D525-12a "Determination of oxidation stability of gasoline (Induction period method)"
• Publication/implementation: Published on 2015-12-31, implemented on 2016-06-01
• Custodian: National Technical Committee for Standardization of Petroleum Products and Lubricants, Subcommittee on Petroleum Fuels and Lubricants (SAC/TC 280/SC1)
• Drafting organization: Sinopec Research Institute of Petroleum Processing; Participating drafting organization: Dalian Petroleum Instrument Co., Ltd.
• Main drafters: Liu Qian, Zheng Yu, Shen Zheng, Wang Enjie

2. Scope of application and non-applicability

• This standard specifies a method for determining the oxidation stability of gasoline under accelerated oxidation conditions.
• Applicable to automotive gasoline, blending component oils for automotive ethanol gasoline, automotive ethanol gasoline, and other products.
• Note: To determine the oxidation stability of gasoline by the method for determining potential gum, reference may be made to SH/T 0585.
• Non-applicability: No explicit "non-applicability" clause is provided in the available source. Appendix A.5 Warning 1 states: This standard is not intended to determine the stability of gasoline components, and even less to determine samples with high content of low-boiling, unsaturated components, because such samples may create an explosive atmosphere within the apparatus.

3. Method principle

• The sample is oxidized in an oxygen bomb; the oxygen bomb is first filled with oxygen at 15 ℃–25 ℃ to 690 kPa–705 kPa, and then heated under conditions of 98 ℃~102 ℃.
• Read the pressure at specified time intervals, or continuously record the pressure, until the break point is reached.
• The time required for the sample to reach the break point is the measured induction period at the test temperature. From this measured induction period, the induction period at 100 ℃ can be calculated.
• Term definitions:
  • Break point (3.1): A point on the pressure-time curve; before this point the pressure drop over 15 min reaches 14 kPa, and after this point the pressure drop over 15 min is not less than 14 kPa; this point is the break point.
  • Induction period (3.2): The time elapsed from placing the oxygen bomb into the 100 ℃ bath to the break point, expressed in minutes (min).
• Method application (Chapter 5): The induction period can indicate the tendency of automotive gasoline to form gum during storage. However, under different storage conditions and for different gasolines, the correlation between the induction period and gum formation during storage may differ significantly.

4. Reagents and materials (purity / preparation)

• 7.1 Toluene: analytical reagent, or purity ≥99%. Warning: Toluene is toxic and is a volatile hydrocarbon; inhaling its vapor or contacting its liquid with the skin will cause absorption into the body. Avoid skin contact with toluene and use it under good ventilation conditions.
• 7.2 Acetone: chemical reagent.
• 7.3 Gum solvent: a mixture of equal volumes of toluene and acetone.
• 7.4 Oxygen: pure oxygen, or purity ≥99.6%.
• 9.1 Cleaning detergent/strong oxidant: The strong oxidant may be chromic acid, a solution of ammonium persulfate dissolved in concentrated sulfuric acid (concentration about 8 g/L), or concentrated sulfuric acid alone. Warning: Chromic acid cleaning solution, as a strong oxidant, is a recognized carcinogen; when using it, do not splash it into the eyes, onto the skin, or onto clothing; contact with organic matter will cause fire; it should be used under good ventilation conditions and kept sealed during storage.

5. Instruments and equipment

• 6.1 Oxygen bomb, glass sample bottle and lid, accessories, pressure gauge, and oxidation bath: requirements are given in Appendix A.
• 6.2 Thermometer: temperature range 95 ℃~103 ℃; specifications shall comply with the requirements of GB/T 514-2005 No. GB-60 or ASTM E1 22C, IP 24C. Note: Other temperature-sensing devices with suitable temperature ranges (such as thermocouples, platinum resistance thermometers) may also be used as substitutes if they can provide equivalent or better accuracy and precision.
• 6.3 Graduated cylinder: 50 mL.
• Appendix A (normative appendix) Apparatus:
  • A.1 Oxygen bomb: shall be made of stainless steel; internal dimensions shall basically conform to Figure A.1; oxygen bombs and related accessories produced by different manufacturers shall all meet the requirements of Figure A.1; Figure A.1 gives specific dimensional ranges but they need not be very precise; oxygen bomb apparatus conforming to GB/T 8018-1987 is applicable, but a pressure relief valve device shall be added; for safety, the oxygen bomb wall thickness shall be at least 5 mm. Warning: Oxygen bomb accessories produced by different manufacturers may not match.
  • A.1.2 To facilitate cleaning and prevent corrosion, the inner surfaces of the oxygen bomb and the top of the oxygen bomb lid shall have a high finish, such as 0.20 μm~0.40 μm.
  • A.1.3 Other structural details (sealing method, gasket material, external dimensions, etc.) may be selected arbitrarily as long as the provisions of A.1.4 and A.1.5 are observed.
  • A.1.4 The oxygen bomb shall be capable of withstanding a working pressure of 1240 kPa at 100 ℃, and its ultimate strength shall be at least equal to that of an oxygen bomb made of an alloy of 18% (mass fraction) chromium and 8% (mass fraction) nickel. Alloy steel Y12Cr18Ni9 (S30317) or 06Cr19Ni10 (S30408) meeting the specification requirements of GB/T 20878-2007 is a suitable oxygen bomb material.
  • A.1.5 When the oxygen bomb is filled with oxygen at 15 ℃–25 ℃ to 690 kPa–705 kPa and immersed in a 100 ℃ liquid bath, the closed part of the oxygen bomb shall be able to form a seal without leakage. If the threads where the locking ring and the bomb body fit together move relative to each other when tightened, the locking ring should preferably be made of a different alloy steel from the bomb body.
  • A.2 Gasket: Any suitable gasket material that can pass the specified test may be used. Test conditions: Place a piece of gasket in an oxygen bomb without sample and seal the lid with the same gasket, fill with oxygen to 690 kPa~705 kPa, and immerse in a 100 ℃ liquid bath; if the liquid bath temperature is constant at 100 ℃±1 ℃ and the pressure within 24 h drops from the maximum value by no more than 14 kPa, the gasket is considered to meet the requirements.
  • A.3 Glass sample bottle and lid: shall conform to the dimensions shown in Figure A.2. The function of the lid is to prevent substances refluxing through the bomb stem from entering the sample, but it shall not prevent oxygen from freely contacting the sample. Figure A.2 is an example of a lid that meets the requirements; the dimensions shown may be used for reference.
  • A.4 Bomb stem: The bomb stem with filler rod shall be made of the same material as the oxygen bomb lid, and the dimensional requirements are as shown in Figure A.1. To facilitate cleaning and prevent corrosion, the filler rod and the interior of the oxygen bomb shall have a high finish, such as 0.20 μm~0.40 μm. A circular metal plate with a diameter of 89 mm shall be installed on the bomb stem (position as shown in Figure A.1), which can close the liquid bath when the oxygen bomb is placed in the liquid bath.
  • A.5 Pressure relief valve: The bomb stem shall be equipped with a stainless steel pressure relief valve; when the pressure is greater than 1530 kPa (±10%), this pressure relief valve can relieve pressure to ensure safety. This device shall be mechanically designed to ensure completely correct installation. Warning 1: This standard is not intended to determine the stability of gasoline components, and even less to determine samples with high content of low-boiling, unsaturated components, because such samples may create an explosive atmosphere within the apparatus; however, due to the unpredictability of certain unknown samples, the oxygen bomb apparatus shall be equipped with a safety pressure relief valve to protect operator safety. Warning 2: Precautions shall be taken in advance so that when the pressure relief valve ruptures, the discharged gas and flame are not directed toward operators, other personnel, or flammable materials.
  • A.6 Connections: As shown in Figure A.1, connect a pressure gauge and an airtight needle valve to the bomb stem. The connector attached to the needle valve can quickly release air and can also be used for rapid filling and discharging of oxygen.
  • A.7 Needle valve: A control valve that can be flexibly opened and closed after fine machining, assembled from a finely machined conical needle facing an orifice. Note: This needle valve is used for cleaning, oxygen pressurization, and discharge.
  • A.8 Pressure gauge: An indicating or recording pressure gauge with a reading of at least 1380 kPa may be used. Pressure sensors and digital readout devices may also be used provided that measurement accuracy is not affected. When measured along an arc scale, any 345 kPa interval between graduations from 690 kPa~1380 kPa shall be at least 25 mm. The graduation interval shall be no greater than 35 kPa, and the accuracy shall be 1% of the total scale range or less than 1%. Other suitable, equivalent metric gauges may also be used. The pressure gauge may be connected directly to the oxygen bomb, or connected to the oxygen bomb by flexible metal tubing or thermoplastic tubing (polyethylene nylon tubing) that meets the above pressure requirements. The total volume of the flexible tubing, fittings, and bomb stem equipped with the filler rod shall not exceed 30 mL. Warning: When ordering this apparatus, the manufacturer shall be required to guarantee that the pressure gauge and needle valve are suitable for use under oxygen conditions.
  • A.9 Oxidation bath: The liquid bath capacity for one oxygen bomb shall not be less than 18 L. If multiple oxygen bombs are placed, then for each additional oxygen bomb, 8 L of capacity shall be added, and the dimensions of the liquid bath shall maintain a bath liquid depth of not less than 290 mm. All liquid baths shall be equipped with a non-self-resetting thermostat to ensure that heating is cut off when the liquid level in the bath falls below a safe height, thereby preventing the liquid bath from boiling dry. Users of old liquid baths without this control device are strongly urged to improve the equipment to ensure operational safety. The top of the liquid bath shall have an opening of suitable diameter to accommodate the oxygen bomb and to mate with a cover plate fixed to the bomb handle. If a thermometer is used to monitor the liquid bath temperature, a position for inserting the thermometer shall be provided at the liquid bath cover, the thermometer shall be well secured, and its 97 ℃ scale line shall be above the liquid bath cover. If other temperature-sensing devices are used, corresponding positions shall also be prepared on the liquid bath for appropriate temperature control. After the oxygen bomb is placed in, the top of the oxygen bomb cover shall be immersed at least 50 mm below the surface of the bath liquid. When the oxygen bomb is not in the liquid bath, an auxiliary cover shall be used to cover the opening. The liquid bath shall be provided with a condenser and a heat source to maintain vigorous boiling of the bath liquid. If the liquid in the bath is not water, suitable mechanical stirring shall be used to maintain the uniformity of the liquid bath and to keep it at 100 ℃±2 ℃. Note: It is known that electrically heated baths are also currently used for heating. Parameters such as heating capacity, heating rate, and heat transfer characteristics of an electrically heated bath may differ from those of a liquid bath. Only when the sample heating rate and sample temperature are equivalent to those of a liquid bath can an electrically heated bath be used instead of a liquid bath. For the sample temperature calibration method for electrically heated baths, see Appendix B.
  • A.10 Thermometer: The required temperature range is 95 ℃~103 ℃, and the specifications shall comply with No. GB-60 in GB/T 514-2005 or with the requirements of 22C, IP 24C in ASTM E1. Other temperature-sensing devices with suitable temperature ranges, such as thermocouples and platinum resistance thermometers, may also be used in place of the thermometer required by 6.2 if they can provide equivalent or better accuracy and precision.
• Appendix B (Informative appendix) Solid bath temperature calibration:
  • B.2.1 Temperature probe: Calibrated, including: (1) a sheathed thermocouple (for example, a K type thermocouple (NiCr—Ni), complying with the requirements of GB/T 16839.1-1997), with a sheath length of at least 400 mm and a diameter of 3 mm; (2) a conical tip, with a length of at least 18 mm and a diameter of 1.5 mm (see Figure B.1).
  • B.2.2 Temperature probe connector: Fixes the thermocouple at the appropriate position and height on the oxygen bomb.
  • B.2.3 Temperature display: A measuring instrument or data logger used to display or store the oil sample temperature.
  • B.2.4 Centering cover: A PTFE cover with a hole of diameter 3 mm in the center, which can fix the thermocouple in the exact center of the sample bottle.

6. Recommended operating conditions

• Temperature before oxygen charging: 15 ℃~25 ℃.
• Oxygen charging pressure: 690 kPa~705 kPa.
• Oxidation bath temperature: 98 ℃~102 ℃ (during the test, observe the temperature at regular intervals, read to 0.1 ℃, calculate the average temperature and round to 0.1 ℃, as the test temperature).
• Pressure reading interval: Continuously record the pressure inside the oxygen bomb; if an indicating pressure gauge is used, record a pressure reading every 15 min or less.
• If leakage increases within 30 min of the start of the test (judged by a stable pressure drop exceeding 14 kPa within 15 min), the test is invalid.
• Cooling: Remove the oxygen bomb from the liquid bath and cool it in air or in water at a temperature not higher than 35 ℃ to near room temperature within 30 min.
• Pressure release rate: Shall not exceed 345 kPa/min.
• Liquid bath atmospheric pressure compensation (Note): If the atmospheric pressure in the test area is chronically lower than 101.3 kPa, it is permissible to add a higher-boiling-point liquid (such as ethylene glycol) to the water to maintain the liquid bath temperature at 98 ℃~102 ℃; if the liquid in the liquid bath is not water, it shall be ensured that this liquid is compatible with the sealing ring in the oxygen bomb.

7. Sampling and sample preparation

• Chapter 8 Sampling: Sampling shall be carried out in accordance with the requirements of GB/T 4756. When sampling manganese-containing gasoline, protect from light.
• Sample amount: 50 mL±1 mL (see 10.1).
• Sample preparation: No other preparation requirements are provided in the available source.

8. Analytical procedure

• 10.1 Bring the temperature of the oxygen bomb and gasoline sample to 15 ℃–25 ℃, place the glass sample bottle into the bomb, and add 50 mL±1 mL of sample. Alternatively, first pour 50 mL±1 mL of sample into the glass sample bottle, then place the glass sample bottle into the bomb. Cover the sample bottle, close the oxygen bomb tightly, and slowly introduce oxygen until the pressure inside the bomb reaches 690 kPa–705 kPa. Allow the gas in the oxygen bomb to slowly escape to flush out the original air in the bomb (release the pressure inside the oxygen bomb slowly and uniformly, with a release time of not less than 2 min). Then slowly introduce oxygen again until the pressure inside the bomb reaches 690 kPa~705 kPa, and observe for leakage. A rapid pressure drop that may be observed initially due to the dissolution of oxygen in the sample (generally not greater than 40 kPa) may be disregarded. If the pressure drop does not exceed 7 kPa within the subsequent 10 min, it is assumed that there is no leakage, and the test may be carried out without re-pressurizing.
• 10.2 Place the oxygen bomb containing the sample into a vigorously boiling water bath or another liquid bath with mechanical stirring, taking care to avoid shaking, and record the time of immersion into the liquid bath as the start time of the test. Maintain the liquid bath temperature at 98 ℃~102 ℃. During the test, observe the temperature at regular intervals, read to 0.1 ℃, calculate its average temperature, and round to 0.1 ℃ as the test temperature. Continuously record the pressure inside the oxygen bomb; if an indicating pressure gauge is used, record a pressure reading every 15 min or less. If leakage increases within 30 min of the start of the test (judged by a stable pressure drop exceeding 14 kPa within 15 min), the test is invalid. Continue the test until the break point is reached, that is, the pressure drop in the 15 min before this point reaches 14 kPa, and the pressure drop in the 15 min after this point is not less than 14 kPa. In addition, if the sample has specific product specification requirements, when the test exceeds the corresponding observation time but the break point has still not appeared, the test may be stopped as needed, and the report shall be given in accordance with the requirements of Chapter 12.
• 10.3 Record the number of minutes from placing the oxygen bomb in the liquid bath until reaching the break point, and take this as the measured induction period at the test temperature.
• 10.4 Remove the oxygen bomb from the liquid bath and cool it in air or in water at a temperature not higher than 35 ℃ to near room temperature within 30 min. Then slowly release the pressure inside the oxygen bomb through the line connected to the oxygen bomb; the release rate shall not exceed 345 kPa/min. Clean the oxygen bomb and sample bottle to prepare for the next test.
• Preparation work (Chapter 9):
  • 9.1 Wash the gum from the sample bottle with gum solvent, then rinse thoroughly with water, and soak the sample bottle and cover in cleaning agent. The criterion for judging whether the type of cleaning agent and cleaning conditions are satisfactory is comparison with the following method: soak with a strong oxidizing agent for at least 12 h (for example, chromic acid, a solution of ammonium persulfate dissolved in concentrated sulfuric acid (concentration about 8 g/L), or concentrated sulfuric acid alone), then wash separately with tap water, distilled water, and acetone. Comparison may be made by visual appearance and by the mass loss of the glassware upon heating under the test conditions. The surface of the glassware shall have no stains or particles, and the mass loss of the glassware shall not exceed ±0.5 mg. Cleaning with cleaning agent can avoid the potential hazards and inconvenience associated with corrosive chromic acid solution; the above cleaning process can serve as an optional method in addition to the strong oxidizing agent cleaning method.
  • 9.2 Remove the sample bottle and cover from the cleaning solution with stainless steel tweezers, and thereafter handle them only with tweezers. First rinse thoroughly with tap water, then with distilled water, and finally dry in an oven at 100 ℃~150 ℃ for at least 1 h.
  • 9.3 Pour out the gasoline from the oxygen bomb, and wipe the interior of the oxygen bomb and the gasket clean first with a clean cloth moistened with gum solvent, then with a clean dry cloth. Remove the filler rod from the bomb handle, and carefully clean off the gum or gasoline in the bomb handle, filler rod, and needle valve with gum solvent. Before each test, the oxygen bomb, valves, and all connecting lines shall be thoroughly dried. Warning: Volatile peroxides generated in previous tests may accumulate in the apparatus, creating a potentially explosive environment. Therefore, during the cleaning process after each test, special care shall be taken to ensure that no such peroxides remain in the filler rod, bomb handle, and needle valve.
• Appendix B Solid bath temperature calibration procedure:
  • B.3.1 Remove the filler rod from the bomb handle.
  • B.3.2 Screw the temperature probe connector onto the female thread at the top of the oxygen bomb (this location is generally used for connecting a pressure detector).
  • B.3.3 Fill the sample bottle with 50 mL of low-viscosity oil sample.
  • B.3.4 Replace the original glass sample cover with the centering cover.
  • B.3.5 Insert the thermocouple into the bomb handle, secure it with the temperature probe connector, so that the tip is located about 15 mm above the bottom of the sample bottle and positioned at the center of the oil sample.
  • B.3.6 Place the oxygen bomb into a preheated solid bath, where the solid bath is at the specified test temperature and the temperature is stable.
  • B.3.7 Keep the apparatus at constant temperature for 1 h, then record the internal oil sample temperature.
  • B.3.8 If there is a difference between the measured oil sample temperature and the solid bath temperature, adjust the solid bath temperature so that the oil sample temperature reaches the specified test value.
  • B.3.9 Repeat steps B.3.1~B.3.8 to confirm that the oil sample temperature reaches the specified test value.
  • Note: In the above temperature calibration procedure, oxygen charging is not required, and oxygen charging operation is unreasonable.

9. Qualitative and Quantitative Determination and Calculation Formulas

• 11.1 The number of minutes required from placing the oxygen bomb in the bath until reaching the turning point is the measured induction period at the test temperature.
• 11.2 Calculation method for the induction period of the sample at 100 ℃:
  • When the test temperature is higher than 100 ℃, the induction period t (min) of the sample at 100 ℃ is calculated according to formula (1):
    • t = t₁ / [1 + 0.101(tₐ − 100)] ……(1)
  • When the test temperature is lower than 100 ℃, the induction period t (min) of the sample at 100 ℃ is calculated according to formula (2):
    • t = t₁ / [1 + 0.101(100 − t_b)] ……(2)
  • Where:
    • t — the induction period of the sample at 100 ℃, in minutes (min);
    • t₁ — the measured induction period at the test temperature, in minutes (min);
    • tₐ — when the test temperature is higher than 100 ℃, tₐ is used to represent the test temperature, in degrees Celsius (℃);
    • t_b — when the test temperature is lower than 100 ℃, t_b is used to represent the test temperature, in degrees Celsius (℃);
    • 0.101 — constant.
  • Requires confirmation against the source standard (source-text extraction): the original formula is typeset in fractional form, source-text extraction appears as "L1+0.101(ta 100)]" and "i+0.101(100-10" and other noise; the above formula has been organized according to the standard conventional form, the specific symbols and bracket positions require confirmation against the source standard (source-text extraction).

10. Precision

• 13.1 Precision: judge the reliability of test results according to the following provisions (confidence level of 95%).
  • Note 1: The gasoline samples used for precision data determination are hydrocarbon-based gasoline containing no oxides.
  • Note 2: The following induction period precision values are derived from test results with a boiling water bath as the heat source. Therefore, when other heat sources are used, these precision values may not necessarily apply.
• 13.1.1 Repeatability, r: the difference between two consecutive test results obtained by the same operator, in the same laboratory, using the same instrument, on the same sample shall not exceed 5% of their arithmetic mean.
• 13.1.2 Reproducibility, R: the difference between two single, independent test results obtained by different operators, in different laboratories, using different instruments, according to the same method, on the same sample shall not exceed 10% of their arithmetic mean.
• 13.2 Bias: the bias has not been determined in this standard.
• 12.3 Mentioned: for the case of slowly oxidizing fuels, precision and bias have not been determined.

11. Report

• 12.1 According to the result calculated in 11.2, rounded to an integer, report as the induction period of the sample at 100 ℃.
• 12.2 If the sample has specific product specification requirements, when the test exceeds the corresponding observation time but the turning point still does not appear, the test may be stopped as needed, and report as the induction period greater than the product specification value of the sample.
• 12.3 If the sample undergoes slow oxidation without the turning point described in 10.2, the sample shall be reported as a slowly oxidizing fuel, and the total test time and the total pressure drop during the test shall be reported (total pressure drop refers to the difference between the maximum pressure in the oxygen bomb during the test and the pressure at the end of the test). For this case, precision and bias have not been determined.

12. Referenced Documents

• GB/T 514—2005 Technical conditions for glass liquid thermometers for petroleum product testing
• GB/T 4756 Manual sampling method for petroleum liquids
• GB/T 20878—2007 Stainless steel and heat-resistant steel — Grades and chemical composition
• ASTM E1 ASTM Specifications for glass liquid thermometers (Specification for ASTM liquid-in-glass thermometers)
• IP Compilation of standards for analytical test methods for petroleum and related products, Appendix A IP Standard thermometer specifications (IP Standard methods for analysis and testing of petroleum and related products—Appendix A: Specifications - IP standard thermometers)
• References:
  • [1] GB/T 16839.1-1997 Thermocouples — Part 1: Reference tables
  • [2] SH/T 0585 Determination of oxidation stability of aviation fuels (potential residue method)

13. Source and verification

This page is an AI translation of the available Chinese method card. Human verification is pending. It is not an official translation of the standard. Check the source standard and product certificate before use.

14. Quick Reference for Questions and Answers