Industrial fires and explosions happen more frequently than most people think. They cause downtime, property damage, injury and sometimes death. These fires and explosions result from a dangerous mixture of flammable vapors with air and a source of ignition.
- Solvents, chemicals and other sources of flammable vapors and gases are present in many manufacturing, production and converting processes and act as the fuel in process fires or explosions.
- In most industrial processes, enough oxygen is present to support combustion. But even in inert or purged processes, a sudden accidental introduction of air can mix with process vapors to produce a flammable mixture.
- There are many potential sources of ignition which can trigger a fire or explosion: electrical sparks, friction, static discharge, hot surfaces, air streams, and direct-fired burners in ovens and thermal oxidizers. Since we can never be guaranteed that an environment will remain completely free of air or of a source of ignition, the most reliable means of preventing fire or explosion is to measure and limit the amount of flammable vapors to a safe level.
For each flammable substance there is a level of concentration in air, usually expressed as a percent by volume, that is known as its Lower Flammable Limit, LFL, or Lower Explosive Limit, LEL. Below the LFL, the mixture of fuel and air is too lean to support combustion. For example, a mixture of 1.1 percent Hexane in air is equal to 100% of it’s LFL - just rich enough to burn.
- As the amount of fuel continues to increase, the mixture will eventually become too rich to burn—there will be too much fuel and not enough air. This concentration is known as the Upper Flammable Limit, UFL, or the Upper Explosive Limit, UEL.
- Between the LFL and the UFL lies the flammable range where, given a source of ignition, the mixture will readily ignite. While it may be theoretically possible to operate safely at concentrations up to 100% of the LFL, authorities world-wide have established safety regulations which require operation well below this point.
- Authorities have experimentally defined the limit to the solvent concentration in air, the LFL, below which the solvent and air mixture can not propagate a flame.
Almost all safety authorities require a 4:1 margin of safety below the LFL, based on worst-case conditions. This means that enough dilution air must be used to always maintain a concentration of less than 25% of the LFL, according to the National Fire Protection Association standard NFPA 86.
- However, a process oven or oxidizer is allowed to operate with only a 2:1 safety margin (up to 50% of the LFL), when continuous flammability analyzers are used. The requirements stipulate real-time, fast-response, continuous analyzers connected in such a manner as to trigger corrective action at predetermined alarm points.
- So unprotected processes, which may normally run at only 10 or 12 percent of the LFL to avoid reaching 25 percent in case of accidental upset, can operate at much higher vapor concentrations when LFL analyzers are used. The resulting cost savings due to reduced ventilation or increased throughput can be considerable.
Because Lower Flammable Limits are determined empirically, there are variations in the values published by different authorities at different times. These variations may be due to small inaccuracies in test procedures such as sample preparation, vessel size, air flow, temperature, observation methods, and monitoring instruments - nevertheless, they do exist. Measurement results are usually rounded-off to the nearest 0.1 percent by volume.
You may be surprised to learn that a comparison of the LFL value for fifty common solvents, published by six different authorities, shows that one-third of the solvents compared disagree by a standard deviation of more than ten percent. Given these differences, the LFL of a substance is certainly not an absolute value and ample allowances should be made to ensure safe operation.
Most published LFL values are calculated at room temperature. But as a given mixture is heated, it's flammability increases and thus the concentration required to achieve 100% of it’s LFL is less. This source of increased danger is often overlooked.