by Raj Shah and Nathan Aragon
The need to develop processes with safety at the forefront leads to the analysis of a property of pure liquids and liquid mixtures known as flash point. The flash point of a liquid is defined as the minimum temperature, corrected to a barometric pressure of 101.3 kPa, at which the vapors of the liquid will briefly ignite given a nearby ignition source. The term “flashing” of a liquid has been defined as when a flame appears and spreads itself across the vapor formed by the liquid1. At the flash point temperature, the flammable vapor above the liquid reaches a concentration high enough to overcome the lower flammability limit2. There have been a few different test methods developed to determine the flash point of combustible liquids, most of which are standardized by ASTM International. In this article, we will further explain the significance of flash point testing and elaborate on these different test methods.
What is the significance of determining flash point?
The flash point is a crucial property to measure when dealing with combustible liquids. During the storage or transportation process of combustible fluids, the flash point needs to be determined so that personnel can take proper precautions when handling volatile substances in order to avoid accidental ignition. It is important to note that at the flash point temperature, a flame induced by the ignition source will burn only for a brief time and will typically cease to burn when the ignition resource is removed. When a liquid is heated further, another temperature called the fire point is reached at which the vapor of the liquid will continue to burn for more than five seconds.
Beyond the issue of safety and flammability, flash point can also be used along with other properties, such as viscosity and specific gravity, to assess the quality of a used liquid oil and how the oil was refined. Fuel dilution is another consideration that can be revealed by flash point testing. In the small-scale closed cup test, a small sample of fuel is supposedly diluted and is exposed to an ignition source. If the sample flashes, the fuel fails the test, which is usually indicative of some degree of dilution. This test is typically done by heating the sample to a temperature that corresponds with 1-2% dilution, and this temperature is usually much lower than the flash point of a new baseline oil.
Flash point can be used to determine other possible characteristics of used oil, such as the occurrence of base oil cracking, contamination of oil, use of a wrong oil, and removal of certain components of oil by evaporation. In most circumstances, base oil cracking lowers the flash point of an oil, while contamination can either raise or lower flash point depending on the substances that were introduced into the oil. Regarding different types of base oil, synthetic lubricants will generally have higher flash points than mineral oil-based lubricants. Table 1 describes factors that can cause flash point to change and illustrates the scope of utility that flash point can have in analytical testing1.
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Decreases flash point
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Increases flash point
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Changes in oil chemistry
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-Thermal cracking
-Radiation (cracking by gamma rays)
-Microdieseling
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-Polymerization
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Additions to the oil
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-Diesel fuel
-Gasoline
-Natural gas
-Solvents
-Wrong makeup oil
-Water (instrument interference)
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-Water
-Coal dust
-Glycol/antifreeze
-Wrong makeup oil
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Subtractions from the oil
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-Thermal evaporation (boiling off of light ends)
-Vacuum dehydration
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Table 1. The effect of various changes in oil on its flash point.
Flash point testing methods
Regarding testing for the flash point temperature, there are two primary methods—open cup and closed cup. In an open cup test, the liquid sample is placed in an uncovered container that is open to the atmosphere. An ignition source is placed just above the opening of the cup to raise the liquid’s temperature towards its flash point. The flash point can vary depending on the distance between the liquid and the ignition source. The closed cup method is almost identical to the open cup test, except that the liquid sample is contained in a closed vessel which is not open to the atmosphere. The ignition of vapors occurs within the closed vessel at a much closer proximity to the liquid sample. Closed cup test methods are generally preferred because they tend to yield lower flash points due to the proximity of the ignition source to the liquid. The practice of labeling combustible liquids is common practice in the industry in order to err on the side of caution. Additionally, the closed cup method would be affected less by ambient disturbances that could be present in the testing lab3.
Each of the test methods for flash point determination differ primarily based on the type of material being tested and the temperature range of samples. The Pensky-Martens test, standardized as ASTM D93, is a closed cup test that can be applied to oils within the range of 40°C to 360°C. Method A of this test is specifically for new lubricants and distillate fuels. Method B of this test is used for used lubricants, residual fuels, mixtures of petroleum-based liquids with solids, or petroleum-based liquids that may form some film on the surface during testing. At least 75 mL of sample must be used for this test. A number of alternative fuels have been introduced in the industry, but these new fuels can still be specified by the typical tests. The flash point of B100 biodiesel, for example, can still be determined as per ASTM D93. The D93 standard was also used to measure the flash point of a biofuel blend of 70% Cardanol and 30% kerosene in an experiment to test its performance in diesel engines4, 5, 6.
The Automatic Pensky-Martens Closed Cup Flash Point Tester manufactured by Koehler Instrument Company, as shown in Figure 1, is an example of an instrument that conforms to Procedures A, B, and C of ASTM D93, among other standards. The operating temperature ranges from ambient to up to 400°C. The stirring rate can range between 0 to 300 rpm and the cycle time is five minutes. There is a dual fan cooling system that cools the cup directly and the environment around the test cup. An additional safety consideration is an inert gas fire suppression system that comes standard with the tester. An integrated processor PC, running on Windows OS, is displayed on an 8.4” touch screen, with the option to utilize a multitude of user programs. The tester is fully automated by a mechanical lift system for the cover and motor, allowing for one-touch testing. The flash point temperature is detected by both a thermocouple and ionization ring and the tester comes in either an electric or gas ignition version7
The Tag test, standardized as ASTM D56, is another closed cup test method. This test is meant for fluids with low viscosity. The liquids need to have a viscosity lower than 5.5 mm2/s at 40°C or a viscosity lower than 9.5 mm2/s at 25°C. In addition, the flash point of the liquid must fall below 93°C as well. The Tag test uses a sample size of 50 mL, as opposed to the Pensky-Martens8, 9.
The Cleveland test (ASTM D92) is an open cup test that can determine both flash point and fire point. It is meant for petroleum products with flash points between 79°C and 400°C but excludes all fuel oils. The Cleveland test requires around 70 mL of sample10.
The Abel test is a closed cup test which is standardized by the International Organization for Standardization. It is designed for combustible liquids with flash point between -30°C and 75°C. However, the standard specifies that precision is only valid for flash points between -8.5°C and 75°C11.
ASTM D3828 is a small-scale, closed cup test that only requires the use of 2-4 mL of sample, with a test duration of only 1-2 minutes. This test covers liquids at a very wide range of temperatures, ranging between -30°C and 300°C, and allows for the test of different petroleum products and biodiesel fuels. Method A of this test involves determining whether a sample will flash at a specified temperature, as described in the previous section, while Method B is a typical test to determine the flash point of a sample12.
The original Pensky-Martens and Tag tests were developed over 100 years ago and since then, there has been much consideration regarding safer tests, since igniting 70 mL of sample can be quite hazardous depending on the type of oil. In 1999, ASTM International developed ASTM D6450 which was the first continuously closed cup method and then in 2004, ASTM D7094 was developed, which was a modified continuously closed cup method. This test method uses a sample size of 2 mL and fresh air is introduced into the sample cup after each ignition and the volume of air increases as the sample temperature is increased. After much testing was done, which displayed essentially equal results between this method and the Pensky-Martens method, ASTM D7094 was officially accepted in 2013 as an alternative to the traditional tests for specific types of fuels, such as fuel oils, diesel fuels, gas turbine fuels, and kerosene13.
Conclusion
The flash point of a combustible liquid is a very important characteristic that has been measured for over 100 years. It is especially useful in evaluating the lower flammability limits of liquids and in developing industry-wide standards for their storage and transportation. Numerous test methods have been developed that depend on specific characteristics of liquids and each method experiences varying degrees of use in current industry. As such, a basic understanding of the different applications for flash point and the various test methods is imperative, especially in the petroleum industry and in the field of tribology.
About the authors: Nathan Aragon is a student of Chemical Engineering at SUNY, Stony Brook University. Raj Shah is a Director at Koehler Instrument Company in New York, where he has worked for the last 25 years. He is an elected Fellow by his peers at IChemE, CMI, STLE, AIC, NLGI, INSTMC, The Energy Institute and The Royal Society of Chemistry.
References
1. Fitch, J. “How to test flash point.” Machinery Lubrication. https://www.machinerylubrication.com/Read/19/flash-point-test.
2. Vidal, M., Rogers, W. J., Holste, J. C., and Mannan, M. S. “A Review of Estimation Methods for Flash Points and Flammability Limits.” Process Safety Progress, vol. 23, March 2004. DOI: 10.1002/prs.10004.
3. “Open and Closed Cup Flash Point – What is the Difference?” Petro Industry News, June 26, 2014. https://www.petro-online.com/news/analytical-instrumentation/11/breaking-news/open-and-closed-cup-flash-point-ndash-what-is-the-difference/30654
4. ASTM Standard D93, “Standard Test Methods for Flash Point by Pensky-Martens Closed Cup Tester,” ASTM International, West Conshohocken, PA, 2003.
5. Alleman, T. L. and McCormick, R. L. “Biodiesel Handling and Use Guide.” United States Department of Energy, November 2016.
6. Ravindra, A. M. and Vardhan, H. “Performance Testing of Diesel Engine using Cardanol-Kerosene oil blend.” MATEC Web of Conferences 144, 04005 (2018).
7. “Automatic Pensky-Martens Closed Cup Flash Point Tester,” Koehler Instrument Company, Inc. https://koehlerinstrument.com/products/automatic-pensky-martens-closed-cup-flash-point-tester/
8. ASTM Standard D56-16a, “Standard Test Method for Flash Point by Tag Closed Cup Tester,” ASTM International, West Conshohocken, PA, 2016.
9. Mladin, D. “Flash Point Testing Methods ASTM D56 and ASTM D93.” Dell Tech, April 2020. https://delltech.com/blog/flash-point-testing-methods-astm-d56-and-astm-d93/#:~:text=Chemical%20Retail%20Labels%3A%20Consumer%20Chemicals%20and%20Containers%20Regulations,viscosity%20higher%20than%205.8%20mm2%2Fs%20at%2037.8%20C
10. ASTM Standard D92-05a, “Standard Test Method for Flash and Fire Points by Cleveland Open Cup Tester,” ASTM International, West Conshohocken, PA, 2005.
11. ISO 13736:2021, “Determination of Flash Point – Abel closed-cup method,” International Organization for Standardization, Geneva, Switzerland, 2021.
12. ASTM Standard D3828-16a, “Standard Test Methods for Flash Point by Small Scale Closed Cup Tester,” ASTM International, West Conshohocken, PA, 2016.
13. Wagner, C. and Schwarzmann, A. “ASTM D7094 - Modified Continuously Closed Cup Flash Point Standard Accepted as a Safe Alternative Method in Various Fuel Specs.” Petro Industry News, February/March, 2014.
The authors would like to thank Rey Montemayor, a world-renowned expert in flash point testing and the author of ASTM publication, “Practice of flashpoint determination: a laboratory resource” for reviewing and helping us edit this article.