Explosion Dynamics Laboratory

Flammability and Explosion Limits

Flammability, explosion, and detonation limits are distinct.

Flammability limits refer to the range of compositions, for fixed temperature and pressure, within which an explosive reaction is possible when an external ignition source is introduced. This can happen even when the mixture is cold. Flammability limits are given in terms of fuel concentration (by volume) at a specified pressure and temperature. For example, the lean flammability limit for Jet A (aviation kerosene) in air at sea level is a concentration (by volume or partial pressure) of about 0.7%. The rich flammability limit is about 4.8% by volume or partial pressure. Flammability limits are not absolute, but depend on the type and strength of the ignition source. Studies on flammability limits of hydrocarbon fuels have shown that the stronger the source of the ignition stimulus, the leaner the mixture that can be ignited. Flammability limits also depend on the type of atmosphere (for example, limits are much wider in oxygen than in air), the pressure, and the temperature of atmosphere.

Explosion limits usually refer to the range of pressure and temperature for which an explosive reaction at a fixed composition mixture is possible. The composition has to be within the flammable range. The reaction is usually initiated by autocatalytic (sometimes called self-heating) reaction at those conditions, without any external ignition source. In practical terms, this means that the mixture needs to be sufficiently hot. Explosion limits are given in terms of a minimum autoignition temperature (AIT) for ignition of fuel injected into hot air. The minimum AIT is strong function of the fuel type (atomic composition and molecular structure), pressure, and fuel concentration. For common hydrocarbon fuels, the minimum AIT ranges between 600C (1350 F) for methane (CH4) to 200C (472F) for dodecane (C12H26). A minimum AIT of 190C (450F) is used for the purposes of hazard analysis for aviation kerosene. Note that the minimum AIT is much higher than the flash point and much lower than typical hot surface ignition temperatures, which can be as high as 900C (2000F) for common hydrocarbon fuels (Smyth, K. C.; Bryner, N. P. Combustion Science and Technology, Vol. 126, 225-253, 1997).

Detonation limits are the range of composition within which detonations have been observed in laboratory and field experiments. Detonation limits are a strong function of mixture composition, initial pressure and temperature but usually considered to be narrower than the flammabilty limits. In addition, detonation limits are much more strongly dependent on the ignition source, confinement, and the physical size of the experiment than flammability limits. The ability to initiate and propagate a detonation requires a set of critical conditions to be satisfied and despite extensive research into the subject, the limits are empirical in nature.

Data on the flammability and explosion limits of liquids and gases are given in Appendix A to Kuchta (1985). Since I often get questions about the flammability of various substances, I have scanned these pages in and made them available as a pdf files. The citation for Kuchta and other sources of flammability data are listed following the definitions. Data on detonations is available in the Detonation Data Base

Definitions of properties listed in Appendix A:

Mol wt Molar weight, modern term is molar mass. This the mass of the fuel molecule expressed in terms of g/mol or kg/kmol.

Specific gravity Density of fuel vapor relative to that of air.

Boiling Point (BP) The temperature at which the vapor pressure of a liquid is equal to one standard atmosphere (101.325 kPa). This is the temperature for the onset of boiling and formation of vapor bubbles on nucleation sites on the container surface.

Stoichiometric Concentration (Cst) The ratio of moles of fuel to moles of fuel-air mixture required to get complete oxidation of the fuel to carbon dioxide (CO2) and water (H2O) with no excess oxygen in the products. The values given in Kuchta are as a percentage of fuel in the mixture of fuel and air. Note that these values are identical for volume, molar, and partial pressure basis for gaseous fuels.

Heat of formation (Delta Hf) Heat of reaction for formation of fuel from elements at the standard state. Expressed in Appendix A in terms of kcal per mole of fuel.

Heat of Combustion (Delta Hc) Heat of reaction for stoichiometric combustion of fuel with oxygen of air. Expressed in Appendix A in terms of kcal per mole of fuel.

Flash point This is the minimum temperature at which the vapor above a liquid fuel will first support a combustion transient or "flash". The flash point is measured by a standardized test (ASTM D56) using a small quantity (50 cc) of liquid that is slowly heated (about 1 deg C/minute) until a flash is observed when an open flame is dipped down into a covered vapor space. The legal description of flammable is used for all liquids with a flash point less than 100 deg F (37.8 C), and the term combustible is used for liquids with a flash point in excess of 100 deg F (37.8 C).

Autoignition temperature (Minimum AIT) The temperature that a fixed volume of fuel-oxidizer mixture must be heated to before an explosion will take place without an external ignition source, i.e., spark or flame. The values of the minimum AIT used in conventional hazard classifications (e.g., Appendix A of Kuchta) have been measured in a standardized test (ASTM E659) which involves injecting a fuel into a heated flask filled with hot air. Explosion does not take place immediately when fuel is injected but occurs after a delay of between 5 and 600 s.

Flammability Limits (L25 or U25) A fuel-air mixture is flammable when combustion can be started by an ignition source. The main fact is the proportions or composition of the fuel-air mixture. A mixture that has less than a critical amount of fuel, known as the Lean or Lower Flammability Limit (LFL), or greater than a critical amount of fuel, known as the rich or Upper Flammability Limit (UFL), will not be flammable. The values in Kuchta are given in terms of molar percentages of fuel in the fuel-air mixture at 1 atm and 25 C.

References and links to flammability data

• Kuchta, J. M. (1985). Investigation of fire and explosion accidents in the chemical, mining, and fuel-related industries - a manual. Bulletin 680, U.S. Bureau of Mines. This is a summary with a lot of general information as the title suggests. No longer current in some areas but a useful introduction. (PDF)
• Zabetakis, M. G. (1965). Flammability characteristics of combustible gases and vapors. Bulletin 627, Bureau of Mines. Source of primary data with references. An update of Bu 503. (PDF)
• Coward, H. F. and G. W. Jones (1952). Limits of flammability of gases and vapors. Bulletin 503, Bureau of Mines. Primary data with references to literature back to the 1800s. (PDF)
  
  A secondary source of data are chemical engineering handbooks such as the CRC or Yaws' compilation.
  
  
• C. L. Yaws (1999). Chemical Properties Handbook. McGraw-Hill. One and one-half reams of tabular data, mostly thermophysical properties, no primary references or pedigree, no error bars or uncertainty ranges. Chapters 25 and 26 are regarding combustion. This is a mix of estimated and measured values, use cautiously. He refers to flammability limits as explosive limits.
  
  Professional organizations are also a good source of information.
  
  
• The NFPA has a number of publications such as the Flammable and Combustible Liquids Code Handbook that contain data on specific substances.
• The SFPE has a number of publications containing data and discussions of flammability and explosion. These include the SFPE Handbook of Fire Protection Engineering, and the Ignition Handbook - a CD-Rom database is available and contains information on 473 pure chemical compounds and over 500 commercial or natural products.