Definition of the Concentration Gradient Above Flammable Liquids

Areas where the potential for spill of flammable fluids is finite are defined as hazardous locations and are subject to code restrictions regarding location and protection from potential ignition sources. These locations require a specific separation distance between the potential spill plain and the ignition sources. This distance is universally accepted to be 18 in. for most U.S. codes. We found that acceptable separation distances have increased over the years, intuitively in the name of safety, although we found no data to support this. We surveyed the literature and performed experiments to find a quantitative basis for the increasing distances. The literature revealed a case for evaporating fluids with boundary conditions for the volatile vapor diffusing through a stagnant gas film. In containers with open tops, the calculated vapor-concentration gradients are monotonic and directly proportional to the vapor pressure of the fluid. We confirmed the validity of these calculated values by performing experiments to determine the elevations at which flammable mixtures formed above the surfaces of volatile, high-vapor-density liquids.