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A flammable liquid must be at its flash-point temperature before combustion can occur if an ignition source is present.
Flammables can exist as solids, liquids or gases. Some solid materials undergo sublimation - they can go directly from a solid to a vapor or gas without becoming a liquid. These materials can have flash points and other characteristics of combustion. We will, however, limit this discussion to flammable liquids and gases. There are also flammable liquids that are pyrophoric, which means they will spontaneously combust when exposed to air.
For responders, flammable gases are the most difficult to deal with because you don't always know where the gas is going. Flammable liquids are easier to deal with than gases because you can see where the liquid is located in a spill. There are also procedures for stopping the flow of the liquid and keeping it from places you don't want it to go. However, some liquids are volatile, which means they readily produce vapor and can become fire hazards if all of the conditions for combustion are just right. The concept associated with vapor location is vapor density.
Vapor density is to air like specific gravity is to water. Air is given a value of one. Vapors that have a vapor density greater than one will be heavier than air. Those that have a vapor density less than one will be lighter than air. Knowing the vapor density in a spill will help responders find the location of the vapor. Liquids that will burn - or, in reality, liquids that produce vapors that will burn - can be divided into two general groups, flammable and combustible. The temperature that divides flammable and combustible liquids is usually agreed to be 100 degrees Fahrenheit. Those liquids with flash points below 100F are considered flammable and those above 100F are referred to as combustible.
The National Fire Protection Association (NFPA) further subdivides flammable and combustible liquids into subclasses for the purpose of safe storage. When responding to spills of liquids that may burn, all such liquids should be considered to be flammable until more information is gathered about the material.
Under certain conditions, such as increased ambient temperatures that heat road surfaces, combustible liquids can behave much the same as flammable liquids. Other factors such as ignition temperature and temperatures of ignition sources can also play a part and fool responders. Emergency responders should become familiar with the physical characteristics of flammable and combustible liquids and know where to get the necessary information from reference books and other sources.
One very basic characteristic of a flammable liquid or liquefied gas is its boiling point. The boiling point of a liquid is the temperature at which the vapor pressure of the liquid overcomes atmospheric pressure. Atmospheric pressure at sea level is 14.5 psi; it decreases as altitude increases. For example, in New York or Philadelphia, which are near sea level, water boils at 212F. In Denver, where the altitude is one mile above sea level, water boils at about 203F.
As atmospheric pressure is overcome, the vapors from a liquid start to move farther away from a spill; if inside a container, the pressure in the container starts to increase. Several physical characteristics determine whether a flammable liquid or family of liquids have high or low boiling points.
First, liquids with high carbon and hydrogen contents tend to have higher boiling points. They are heavier, which is referred to as the "size" or weight of a compound. For example, propane is a three-carbon alkane hydrocarbon with eight hydrogens, has a boiling point of around -40F, and is a gas at normal temperatures and pressures.