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Fractional Vaporization of Ignitable Liquids - Flash Point and Ignitability Issues
By: Patrick Kennedy and Andrew T. Armstrong |
ABSTRACT
Explosions or flash fires have occurred under circumstances in which suspected liquid fuel does not appear to explain the fuel source because of its high reported flash point. These instances have posed a conundrum to the fire investigation profession. In some cases, science can explain the fuel source by the application of the principal of fractional distillation.Commercial ignitable liquid products that are mixtures of various ignitable liquid components or ignitable and non-ignitable liquid components, with varying vapor pressures, can undergo fractional vaporization. In this process normal evaporation can separate the various ignitable or non-ignitable components, with the lighter end fraction (high vapor pressure) compounds evaporating first.
When applied to flammable and combustible liquids, this process is sometimes referred to in the fire and explosion investigation profession as "weathering." In these situations, the flash point of the remaining (not yet vaporized) liquid will be higher than the measured flash point of the "non-weathered" original liquid. It is possible for such mixtures to evolve concentrated vapors and be ignited even when the parent liquid is at a temperature below its reported flash point.
When applied to mixtures of ignitable and non-ignitable components in which the non-flammable component(s) evolve first, the process is referred to as "outgassing." When the earlier evolving volatile compounds are generally considered "non-combustible," such as halogenated hydrocarbons like methylene chloride, the flash point of the original liquid can actually initially increase and then decrease as vaporization continues.
In both situations, fractional vaporization can be extremely dangerous in that the perceived ignitability of the original liquid is masked or underreported in material safety data sheets, labels, warnings, and product use instructions.
In general, the process is observed more frequently in products that are designed to be used in coatings where the distribution or spreading over large surface areas is expected. Evaporation is an intended part of their application. These dangerous situations have been observed in products such as paints, stains, other surface coating materials, cleaning products, and strippers/removers.
VAPORIZATION (EVAPORATION) AND VAPOR PRESSURE
Liquids change to vapors (gases) at temperatures below their boiling points. This occurs at any temperature higher than absolute zero, -273° C (-459° F.) (0° on the Kelvin and Rankin scales). Vaporization of a liquid at a temperature below its boiling point is called evaporation, which occurs at any temperature when the surface of a liquid is exposed in an unconfined space. When, however, the surface is exposed in a confined space and the vaporization of the liquid is in excess of that needed to saturate the space with vapor, equilibrium is quickly reached between the number of molecules of the substance escaping from the surface and those returning to it. A change in temperature upsets this equilibrium. A rise in temperature, for example, increases the activity of the molecules at the surface of the liquid and increases the rate at which they fly off. When the new temperature is maintained for a short time, a new equilibrium concentration in the vapor is established.The pressure exerted by the vapor evolving from the surface of the liquid is called its vapor pressure. Vapor pressures differ for different substances at any given temperature, but each substance has a specific vapor pressure for each given temperature. At its boiling point, the vapor pressure of a liquid is equal to atmospheric pressure. For example, the vapor pressure of water, measured in terms of the height of mercury in a barometer, is 4.58 mm at 0°C (32° F.) and 760 mm at 100°C (212° F.), its boiling point at sea level.
The Vapor Pressure - Temperature Relationship
Patrick Kennedy is the Principal Expert Fire and Explosion Analyst for John A. Kennedy and Associates, the world's oldest established fire investigation firm. With over forty-five years of professional experience, Patrick Kennedy is also the senior-most active fire analyst, with more years of experience that any other active fire investigation professional.
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