NFPA Flammable Limits

 

B.1 General

Some paints, varnishes, lacquers, and other coating materials contain volatile flammable solvents. In addition, such solvents are often added as “thinners.” When exposed to the atmosphere, these solvents give off vapors that mix with the surrounding air and, if the concentration reaches as much as approximately 1 percent solvent in air, these vapors can be ignited and an explosion can occur. Spray applications using only liquids that have relatively high flash points, although less likely to produce ignitable atmospheres than those using low flash point liquids, can, nevertheless, result in mists that are capable of propagating a flame in a manner similar to combustible solids in dust explosions.

Theoretical considerations can assist in hazard evaluation in some instances. For example, 3.8 L (1 gal) of the average solvent will occupy approximately 0.7 m3 (23 ft3) when evaporated into vapor at average room temperature. Therefore, if 3.8 L (1 gal) of liquid solvent is completely evaporated and thoroughly mixed with the surrounding air of an enclosure, the enclosure has to have a volume of more than 70 m3 (2500 ft3) to avoid an ignitable mixture, assuming the lower limit of the flammable range of the solvent is 1 percent in air. This is a conservative number; almost all of the solvents used in spray finishing have a lower flammable limit greater than 1 percent. In using such theoretical considerations, caution should be exercised to prevent erroneous conclusions. When liquids are sprayed, the area in the direct path of the spray will exceed the lower flammable limit. Vapors from most solvents are heavier than air, and small quantities of vapor can form an ignitable mixture in low, unventilated spaces in the vicinity of or even remote from the point of evaporation before they mix with the full volume of available air by natural diffusion and the mixture becomes too “lean” to burn. When liquid is sprayed, the rate of evaporation is greatly increased so that the lower flammable limit is quickly reached. For these reasons, a safety factor of 4 to 1 has been traditionally used and the ventilation requirement rounded off to 75 m3 per L (10,000 3ft/gal) evaporated at the maximum flow rate of the spray apparatus.

Adequate mechanical ventilation throughout all areas where ignitable vapors or mists might be present is essential to prevent the formation of flammable mixtures. The volume of air movement necessary will obviously vary with the arrangement of spraying operations, the amount of spray material used in a given length of time, and the rate of evaporation of theparticular solvent.