Differentiating Flash Point and Autoignition Temperature: Flammability Testing

Posted by AnnMarie Fauske on Tue, Aug 02, 2016 @ 09 53 AM

To help assess your risk exposure by characterizing the flammability potential of your combustible gas, vapor or solids, the following is a list of the most common tests that are performed for flammability hazard characterization and the standards. (Specialized testing can be also performed to more closely match your process conditions and thereby better gauge your risk.)

TestsFLASH AND SPONTANEOUS IGNITION TEMPERATURE OF PLASTICS

• Flash Point Testing (open and closed cup)
• Autoignition Temperature (AIT)
• Flash and Spontaneous Ignition Temperature of Plastics
• Sustained Burning/Combustibility (Fire Point)
• Flammability Limits (LFL, UFL)
• Temperature Limits of Flammability (LTFL)
• Limiting Oxygen Concentration (LOC)
• Minimum Ignition Energy (MIE)
• Explosion Severity Pmax , KSt

Consulting 
• Desktop and/or on-site assessments per NFPA or OSHA for handing, processing or storing of flammable materials
• Flammable hazard calculations
- Estimating flammable properties (LFL, UFL, AIT, etc.) or explosion severity
- Modeling flammable gas distribution and hazards during an upset scenario
• Process Hazard Analysis (PHA) assistance

But, what is the difference in Flash Point and AIT Testing? 

The Flash Point is the minimum temperature at which vapors given off by a liquid pool form an ignitable mixture with air.  This test is used to assess the relative flash-fire hazards of handling and processing liquids. Results from this test, in conjunction with vapor pressure or boiling point tests, will help characterize a liquid as either being flammable or combustible based on the criteria of organizations such as the NFPA, EPA, OSHA, or UN.  Characterization of liquids using the flash point test will provide information on the proper packaging and shipping group for transportation purposes, in addition to storage and handling requirements.

Depending on material properties, testing can be performed to determine the flash point using one of these listed standards:

ASTM D1310 "Standard Test Method for Flash Point and Fire Point of Liquids by Tag Open-Cup Apparatus"

ASTM D3278 “Standard Test Methods for Flash Point of Liquids by Small Scale Closed-Cup Apparatus”

ASTM D3828 “Standard Test Methods for Flash Point by Small Scale Closed Cup Tester”

ASTM D56 “Standard Test Method for Flash Point by Tag Closed Cup Tester”

ASTM D92 “Standard Test Method for Flash and Fire Points by Cleveland Open Cup Tester”

ASTM D93 “Standard Test Methods for Flash Point by Pensky-Martens Closed Cup Tester”

ASTM D1929 "Standard test Method for Determining Ignition Temperature of Plastics"** (Note: This standard could potentially be listed under both Flash Point and AIT because we also find the “flash ignition temperature” where a flame is present above the sample as the ignition source and we also find the “spontaneous ignition temperature” which can be translated into the AIT of the sample - where no ignition source is present. See photo). 

- NFPA 30, Flammable and Combustible Liquids Code, National Fire Protection Association, Quincy, MA, 2012 

Autoignition Temperature (AIT) is a flammable property defined as the lowest temperature environment at which a gas or vapor will spontaneously ignite without a distinct/localized ignition source. It is helpful to know the autoignition temperature if chemicals are being handled or processed under elevated temperature and/or pressure conditions.  This flammability property is dependent on numerous factors including pressure, temperature, oxidizing atmosphere, vessel volume, and fuel/air concentration among others.  Therefore, it is important to characterize the autoignition hazard at as close to your process conditions as possible. 

Relevant standards a testing and consulting lab conforms to:

ASTM E659 “Standard Test Method for Autoignition Temperature of Liquid Chemicals”

ASTM D1929 "Standard test Method for Determining Ignition Temperature of Plastics"**(see above)

A certain concentration of vapor in the air is necessary to sustain combustion, and that concentration is different for each flammable liquid. The flash point of a flammable liquid is the lowest temperature at which there will be enough flammable vapor to ignite when an ignition source is applied. Unlike flash points, the autoignition temperature does not use an ignition source. In other words, the autoignition temperature is the lowest temperature at which a volatile material will be vaporized into a gas which ignites without the help of any external flame or ignition source. As a result, the autoignition temperature is higher than the flash point.

Per Petro Industry News article, "What is the Difference between Flash Point and Ignition Temperature?" from August 2014: "Open cup flash point testing occurs when the substance is placed into a vessel which is open to the outside atmosphere. Its temperature is then gradually raised, and an ignition source is passed over the top of it at intervals. Once the substance “flashes” or becomes ignited, it has reached its flash point.

Closed cup flash point testing is conducted inside a sealed vessel and the ignition source is introduced into the vessel. As a result, the substance is not exposed to the elements outside of the vessel, which can have an interfering effect on the results of the test. This, in turn, also leads to lower flash points, because the heat is trapped inside. Because it is lower, the flash point is also safer for widespread use, and as such is more generally accepted."

Autoignition temperature tests are measured by placing the substance in a half-litre vessel and inside a temperature-controlled oven. As mentioned the current standard procedures for such tests are outlined in ASTM E659.

Full scale labs will have additional equipment such as FAI's custom apparatus developed by Director of Flammability Testing and Consulting Services Paul Osterberg.  This 5L vessel is designed to perform high pressure AITs on a much larger scale versus the standard 550 ml vessel.  For more information regarding your specific testing needs, please contact:  Amy Paul, Mechanical Engineer, Fauske & Associates, LLC paul@fauske.com,  630-887-5292. www.fauske.com

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Tags: combustible liquid, combustible vapor, flash point, combustible gas, ait, autoignition temperature, flammable hazard, astm E659

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