The material in this Safety Moment has been extracted from the ebook Safety in Design.
For many process and energy facilities fires and explosions represent the greatest potential for a catastrophic event. Hence it is vital that those responsible for the design and operation of these facilities have a proper understanding of the terms ‘Flammable Limit’ and ‘Flash Point’. Yet, all too frequently, these terms are used rather loosely and inaccurately. For example, many publications refer to a ‘Fire Triangle’ such as that shown below.
Triangles such as this say that there are three requirements for a fire: Fuel, Air (oxygen) and Heat. This is incorrect. The three requirements are: Fuel, Air (oxygen) and Temperature. (Heat is an extensive property, temperature is an intensive property.) This may sound like nit-picking, but it is not. The temperature of a fuel / oxygen mixture has to be within a certain range for that mixture to burn.
Another limitation of the fire triangle is that it does not take into account the fact that the fuel and flammable material have to be within the flammable range, otherwise they will not burn.
A simple fire triangle may be useful for basic education into the principles of fires, explosions and fire-fighting, but it is insufficient for those charged with designing and operating process and energy facilities.
Not all fuel vapor / oxygen mixtures will burn — the concentrations have to lie within the flammable range, which has upper and lower limits for the concentrations of the fuel in the vapor space. The flammability limits vary according to many factors, particularly the pressure and temperature of the mixture and the presence of inert components such as steam, carbon dioxide or nitrogen.
The flammable range for a fuel is defined by the Lower Flammable Limit (LFL) and the Upper Flammable Limit (UFL). These terms are also referred to as the Upper and Lower Explosive Limits. Below the Lower Flammable Limit (LFL) there is insufficient flammable material for a fire to occur — the mixture is ‘too lean’. It is the lowest concentration of a flammable vapor in air capable of producing a fire in the presence of an ignition source. For most flammable hydrocarbons the LFL is in the range 2 to 5%.
Above the Upper Flammable Limit the mixture is said to be ‘too rich’ — there is too high a concentration of flammable vapor for a fire to occur. For simple alkanes such as methane and ethane the UFL is generally in the 10 to 15% range. Some chemicals, such as hydrogen, ethylene oxide and acetylene, have much higher values for UFL.
Values for flammable limit ranges for many flammable materials are provided by NFPA 704 — Standard System for the Identification of the Hazards of Materials for Emergency Response NFPA 2017).
The flash point of a flammable material is defined as the temperature at which a vapor that is inside its flammable range that can be ignited. An ignition source such as a flame or spark is needed to make the material actually burn. It is important to recognize that an ignition source is required ― the flash point is not the same as the auto-ignition temperature. If a material’s temperature is below its flash point then it will not burn, even if a source of ignition exists.
(The word ‘flash’ is used in process plants in another context — if a liquid at high temperature has its pressure suddenly reduced then it will ‘flash’ or boil. This has nothing to do with fire or with it igniting.)
The sketch illustrates the concepts of ignition temperatures and flashpoints and flammable limits.
The left line in sketch is the flashpoint line. If the mixture has a concentration and temperature that puts it to the left of the line then it will not burn, even if a source of ignition is present. Similarly, mixtures above and below the LFL/UFL values will not burn, regardless of the mixture’s temperature.
Moreover, even when if the material is in the flashpoint range, and a source of ignition is present, the ignition source must contain sufficient energy to ignite the fuel. This minimum energy value varies with type of gas and concentration; for hydrocarbon vapors it is low, for high flash point liquids, such as diesel and fuel oil, it is much higher — usually in the form of an existing fire. This is why low energy flashes (such as might be created by a mobile phone or a digital camera) may not ignite a flammable mixture.
If a flammable mixture is heated to a high enough temperature it will spontaneously ignite — an ignition source such as a flame or spark is not needed. Spontaneous ignition occurs at the auto-ignition temperature (AIT), which is also shown in the sketch.
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Copyright © Ian Sutton. 2018. All Rights Reserved.