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Fire point is the temperature to which the liquid is heated that produces enough vapor for ignition to occur after the vapor flash occurs. The fire point temperature is one to three degrees higher than the flash point temperature. Fire point is so close to flash point temperature that it really isn't much of a concern to emergency responders. If a liquid is at its flash point in a spill or leak, then it is very likely to also be at its fire point, and prepare for a fire!
Photo by Robert Burke
This highway tanker is placarded for flammable liquids.
Another term associated with combustion, that is sometimes misunderstood, is ignition temperature, also known as auto-ignition temperature. The ignition temperature is the minimum temperature to which a material must be heated to cause auto-ignition without the need for an ignition source to be present. In other words, a flammable liquid is heated from an outside heat source and auto-ignites when its ignition temperature is reached.
For an ignition source to ignite a material at its flash point temperature, the ignition source must be at the ignition temperature of the liquid. For example, if you placed gasoline in a container and used a lit cigarette to ignite the gasoline, you would be wasting your time. The temperature of a lit cigarette is around 400F and the average ignition temperature of gasoline is around 800F; therefore, the cigarette is not hot enough to ignite the gasoline. On the other hand, if you placed diesel fuel in a container and used a lit cigarette to ignite the diesel fuel, you would have a fire on your hands! The ignition temperature of diesel fuel is about the same as the temperature of the lit cigarette.
Boiling point and flash point have a parallel relationship. When the boiling point of a compound is low, then the flash point is also low. Compounds that have high boiling points also have high flash points. If a flammable liquid that has a low boiling point, and a correspondingly low flash point, the quantity of vapor produced by the compound will be high. If the liquid is spilled on the ground, the vapor will travel farther away from the spill. This is referred to as the vapor content of the spill.
Inside a closed container, the vapor will increase the pressure inside the container. This is the vapor pressure within the container of liquid. If the boiling point and flash point are high, then the vapor content and vapor pressure will be low. Another term associated with combustion is the heat output when the vapor from a liquid burns. Large compounds have high heat output and small compounds have low heat output.
There is a relationship between boiling point, flash point, ignition temperature, vapor content, vapor pressure and heat output. Compounds that have high boiling points tend to have high flash points. Those that have low boiling points tend to have low flash points. The numeric values are different, but the ratio holds true.
Additional terms associated with boiling and flash point include vapor content, vapor pressure, heat output and ignition temperature. Compounds that have high boiling points and high flash points tend to have high heat output, low vapor pressure, low vapor content and low ignition temperatures. The illustration below shows the relationship of combustion characteristics.
If a flammable liquid is at its flash point temperature and the ignition source is at or above the ignition temperature of the liquid, there is still one more physical characteristic that must be correct for combustion to occur. This characteristic is referred to as the flammable range or explosive limits.
The flammable range is expressed on a scale from 0% to 100%. Between 0% and 100%, a flammable liquid vapor will encounter the proper mixture of air and fuel to complete the combustion process. Each vapor has an upper explosive limit (UEL) and lower explosive limit (LEL). Above the UEL there is enough fuel for combustion to occur, but not enough oxygen; it is too rich to burn. Below the LEL the mixture is too lean to burn; there is enough oxygen, but not enough fuel.
Large hydrocarbon compounds such as animal/vegetable oils each have a characteristic double bond somewhere within the structure. This double bond can be attacked by the oxygen in the air and the bond can be broken. When bonds break, it is an exothermic, or heat producing reaction. If the heat is confined, spontaneous ignition can occur.