The relative danger of a flammable substance is determined by the conditions under which it can sustain combustion. The flammable range is a quantitative measure that directly correlates to how easily a material poses a fire or explosion risk in an open atmosphere. Understanding this range provides the most accurate assessment of a substance’s overall hazard level by defining the specific concentrations where ignition is possible.
Defining the Flammability Limits
The flammable range describes the specific concentration of a gas or vapor in the air that can ignite and sustain a flame when an ignition source is present. This range is defined by two boundaries, expressed as a percentage of the fuel vapor by total volume, and is valid only when sufficient oxygen is present (typically the approximately 20.9% found in normal air).
The lower boundary is the Lower Flammability Limit (LFL), the minimum concentration of fuel required for combustion. Below the LFL, the mixture is considered “too lean,” meaning there is insufficient fuel vapor relative to the air to propagate a flame.
The upper boundary is the Upper Flammability Limit (UFL), the maximum concentration of fuel that can still support a fire. Above the UFL, the mixture is “too rich,” containing too much fuel vapor and insufficient oxygen to complete the combustion reaction.
How Range Width Determines Relative Danger
The numerical difference between the UFL and the LFL defines the width of the flammable range, which is the primary indicator of a substance’s relative danger. A substance with a wide range can support combustion across a greater variety of atmospheric concentrations, making it inherently more hazardous. The wider the range, the more likely an accidental release will result in a combustible air-vapor mixture.
Consider the contrast between methane and hydrogen. Methane, the main component of natural gas, has a relatively narrow flammable range, typically from 5% LFL to 14% UFL by volume in air.
Hydrogen, by comparison, has an extremely wide range, extending from about 4% LFL up to 77% UFL. This 73 percentage point range means that nearly any hydrogen-air mixture encountered in a leak will be combustible, making it significantly more dangerous to handle and store.
Ignition Factors That Influence Practical Risk
While the flammable range defines if a substance can burn, other factors determine how easily it reaches that range in a practical setting. The Flash Point is the minimum temperature at which a liquid gives off enough vapor to form an ignitable mixture near its surface. A low flash point indicates that a substance will readily produce vapors that quickly enter the flammable range, even at room temperature.
Vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases, and is another measure of a liquid’s volatility. Substances with a high vapor pressure evaporate more easily and present a greater risk, as they rapidly establish a combustible atmosphere. A substance that has both a wide flammable range and a low flash point—such as gasoline—presents the highest practical risk because it is volatile and easily ignitable across many concentrations.
Using Flammability Data for Workplace Safety
Industrial safety management relies heavily on LFL and UFL data to implement engineering controls that prevent accidental ignition.
Ventilation and Monitoring
The most common control strategy is continuous ventilation, designed to keep the concentration of flammable vapors well below the Lower Flammability Limit. Safety standards often mandate that vapor concentrations must not exceed 25% of the LFL to maintain a large margin of safety. Continuous monitoring systems provide real-time tracking of vapor concentrations. These systems allow facilities to safely work closer to the LFL, sometimes up to 50% of the limit, while ensuring automated corrective actions are triggered before the atmosphere becomes hazardous.
Inerting
Inerting involves introducing an inert gas, such as nitrogen or carbon dioxide, into a vessel or space. This process reduces the available oxygen concentration below the Limiting Oxygen Concentration (LOC) required for combustion, effectively narrowing the flammable range until it disappears entirely.