What Is a Flammability Limit?

Mary McMahon

A flammability limit is a range of concentrations of flammable gas that could result in fire if an ignition source is provided. The lower flammability limit indicates the smallest amount of gas that would need to be present to start a fire. On 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 about flammability limits along with other important characteristics so people can adequately protect themselves.

If an oxygen intake is clogged, a car engine might not be able to pull in enough air to burn cleanly.
If an oxygen intake is clogged, a car engine might not be able to pull in enough air to burn cleanly.

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 there is 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 represents between 15 and 28% of a gas mixture.

To determine flammability limit, researchers utilize ideal conditions, and it is important to be aware that real conditions may create fire outside the established range. Testers typically perform measurements at approximately room temperature, and one atmosphere of pressure. Gas is carefully released into the environment while 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 issue. People using products like kerosene heaters, solvents, and flammable glues in small rooms could create a fire hazard if they do not use adequate ventilation. If the conditions are just right, an explosion could occur, increasing the risk of property damage and injuries. Proper ventilation with open windows, fans, hoods, and similar tools can be critical to protect safety in environments where flammable gases are present, even in very small amounts.

The flammability limit is not just of interest for safety reasons. They are also important in the design of systems that rely on combustion to operate, such as car engines. Gas mixtures need to be carefully calibrated to find a mixture between lean and rich. The engine may be precisely designed to function with a specific fuel mixture, and errors can create problems. For example, if an oxygen intake is clogged, a car engine might not be able to pull in enough air to burn cleanly.

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