The unacceptable frequency of reported fires and explosions suggests that many organizations do not have proper preventative measures and mitigative safeguards in place to reduce the number of fire-related incidents. Prior to scaling up a chemical process or working with a new chemical, it is critical to fully characterize the flammable properties of the chemical to get a strong understanding of the flammability potential and to set up the appropriate safeguards.
In any discussion of flammable properties it is important to first understand the three general elements that are required for a fire or an explosion to occur: a fuel, an oxidizer, and an ignition source (Figure 1). Through removal of one of these elements, a fire/explosion will not occur (in most cases1). Eliminating the ignition source is often not a practical method of prevention due to flammable vapors typically having very low minimum ignition energies (meaning they are very easy to ignite) as well as the likely existence of numerous different potential ignition sources (known and unknown). Therefore, moderating the fuel and oxidizer concentrations to avoid a flammable concentration of gases/vapors, otherwise known as the flammable region, is necessary for reducing the risk of a fire/explosion.
In the chemical industry, processing and handling of chemicals could result in the formation of a flammable or explosive atmosphere. For liquid chemicals, this may occur at temperatures other than at ambient conditions. Figure 2 shows the relationship between the flammable properties of a material and how they are related to temperature.
As temperature increases, the vapor pressure of a material exponentially increases, and there becomes a point where the concentration of the vapor is sufficient to create a flammable atmosphere in air. This temperature is commonly known as the flash point (FP). In theory, the lower flammability limit (LFL) should intersect the vapor pressure curve at the flash point temperature. As a result, this temperature is also referred to as the lower temperature limit of flammability (LTL). However, in reality, these two temperatures (FP and LTL), may not always be the same. Knowledge of the disparity between these two points will help better assess the flammability hazards of a specific chemical as well as help implement the proper safety precautions during handling.
To understand the variation between the lower temperature limit of flammability and flash point, tests were performed to compare the results. The lower temperature limit of flammability tests were conducted using ASTM E1232 “Standard Test Method for Temperature Limit of Flammability of Chemicals” modified to be conducted in a 5.3-L stainless steel spherical vessel using a fuse wire ignition source for safety and environmental purposes. The criterion for a positive ignition was a 7% pressure rise above the starting pressure. The flash point tests were performed using ASTM D3278 “Standard Test Methods for Flash Point of Liquids by Small Scale Closed-Cup Apparatus”. These tests were performed on four different chemicals and the results are summarized in Table 1.
The deviation between the values determined by these two tests is a result of differences in the test apparatus and methodology used in each of these experiments. It is important to understand that flammable properties are influenced by numerous factors. Below are a few factors that may provide an explanation for the differences between the two test results:
- Vessel Size and Geometry – As the size of a vessel increases, the heat losses to the vessel wall become negligible. Through minimizing heat losses to the vessel wall, more heat is transferred to the combustion reaction, promoting flame propagation. This results in a widening of the flammable region and potentially allowing for combustion to occur at lower temperatures. Furthermore, a study performed by Takahashi, Urano, Takuhashi, and Kondo (2003) determined that flammability properties should be determined using either a spherical vessel or a cylindrical vessel with a diameter of at least 30 cm and a height of at least 60 cm to minimize the effect of flame quenching which may artificially result in a narrower flammable region.
- Ignition Source Location – A lower ignition source elevation in a vessel has been shown to widen the flammable region as compared to a central ignition source location (Van den Schoor, Norman & Verplaetsen, 2006). With a lower placed ignition source, a larger percentage of the combustible mixture participates in the upward moving combustion reaction with minimal heat losses to the wall, thereby, causing more heat being transferred to the combustion reaction resulting in a wider flammable region.
- Homogeneity of Mixture – Slight changes in the vapor concentration could result in a mixture becoming flammable or not flammable. In the LTL tests, the vapor mixture is stirred to provide a homogenous mixture of the fuel in air, unlike the flash point tests where the vapor space is not stirred and thus allows concentration gradients to form. Furthermore, the LTL tests provide more uniform heating of the vessel as well as a longer mixing time to allow the vapor and the liquid to reach equilibrium. All of these factors will impact the concentration of the fuel in the vapor space and may influence the flammability results.
- Flame Propagation – Generally, the flammable region is wider for upward flame propagation compared to downward flame propagation due to flame buoyancy (EU-Project SAFEKINEX, 2006). Tests performed in the 5.3L vessel measure upward flame propagation as compared to the flash point tester which measures downward flame propagation. This wider range means that the LTL will generally occur at a lower temperature than the FP
These example results demonstrate that it is imperative to fully characterize the flammability hazards of chemicals. Determination of the flash point by itself may not always be sufficient in providing data that is used to implement proper safety measures to avoid flammable temperatures when assessing the hazards of flammable liquids. As shown from the LTL and FP tests, there can be potentially large deviations between the two values. Therefore, the use of a safety margin with the flash point value may not always be adequate. The safest approach would be to conduct an LTL test to assess the temperature at which there is sufficient vapor for flame propagation.
If you are interested in receiving a quote for flash point or lower temperature limit of flammability testing, or would
like to learn more, please do not hesitate to contact us at flammability@fauske.com.
References
- Crowl, D.A. (2003). Understanding Explosions. New York: American Institute of Chemical Engineers.
- EU-Project SAFEKINEX (2003-2006). Report on the experimental factors influencing explosion indices determination.
Programme “Energy, Environment and Sustainable Development”, Contract No: EVG1-CT-2002-00072. - Takahashi, Urano, Tokuhashi, Kondo (2003). Effect of vessel size and shape on experimental flammability limits of gases,
Journal of Hazardous Materials. - Van den Schoor, F., Norman, F., & Verplaetsen, F. (2006). Influence of the ignition source location on the determination of the explosion pressure at elevated initial pressure. Journal of Loss Prevention in the Process Industries.
