Flammable limits are the range of gas concentrations that can cause a fire. The lower limit is the minimum amount of gas needed to start a fire, while the upper limit is the richest concentration at which a fire can occur. Flammability limits vary depending on the gas. Proper ventilation is essential to prevent fires in confined spaces. Flammability limits are also important in designing combustion systems, such as car engines.
A flammable limit is a range of concentrations of flammable gas that could cause a fire if an ignition source is provided. The lower flammability limit indicates the minimum amount of gas that should be present to start a fire. At the other end of the scale, the upper end represents the richest possible concentration at which a fire could occur. Material Safety Data Sheets provide information on flammability limits along with other important characteristics so that people can adequately protect themselves.
Many gases are flammable under the right conditions. Below the lower limit, not enough gas is present to create combustion. Above the upper limit, too much gas is present and not enough oxygen to sustain a flame. The flammability limits for gases can be quite variable, depending on their characteristics. Ammonia, for example, can create a fire when it makes up between 15 and 28% of a gas mixture.
To determine the flammable limit, researchers use ideal conditions and it is important to be aware that real conditions can create fires outside the established range. Testers typically take measurements at approximately room temperature and one atmosphere of pressure. The gas is accurately released into the environment as instruments measure the concentration of the mixture. An ignition source is provided, to find the point at which the mixture ignites, illustrating the lower flammability limit. Gas mixtures can be diluted to find the upper limit.
It can be very easy to reach lower flammability limits in confined spaces, which is a significant safety concern. People using products such as kerosene heaters, solvents and flammable glues in small rooms could create a fire hazard if they do not use adequate ventilation. If conditions are right, an explosion could occur, increasing the risk of property damage and injury. Proper ventilation with open windows, fans, hoods and similar tools can be essential to protect safety in environments where flammable gases are present, even in very small quantities.
The flammability limit is not only of interest for safety reasons. They are also important in the design of systems that rely on combustion to function, such as automobile engines. Gas mixtures must be calibrated carefully to find a blend between lean and rich. The engine can be precisely engineered to run on a specific fuel blend, and mistakes can create problems. For example, if an oxygen intake is clogged, a car engine may not be able to draw in enough air to burn cleanly.
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