THE VARIABLES OF FLASHOVER
Flashover can be described as the instantaneous ignition of a large area of combustible materials. This “area” may be a single room, a large indoor area, or even a narrow canyon. The important aspect of flashover is that the space must be contained enough to accumulate heat through any combination of convection, conduction, and radiation for the combustible materials within to reach their respective flash points (in the presence of an ignition source) or ignition temperatures. Fire within a space convects heat and combustible gases to the upper spaces that in turn radiate heat to nonburning contents within the space. This cycle repeats until the point of flashover when all the combustibles will ignite simultaneously.
Upper space gas temperature and the rate of heat release within the space (by either the burning objects themselves or radiated from the convection column gases) are believed to be the two principal variables determining flashover. First, temperatures of 930° to 1,100° F must be achieved within the confined upper space before flashover can be expected; second, the minimum rate of heat release is 20 kilow atts of pow er per square meter.
A number of factors affect the occurrence of the above criteria:
- The size and number of openings within a room or confined space. It follows logically that if flashover is dependent on the production of heat and its radiance to nonburning combustibles, then the less ventilation, or escape, of these heating products, the faster flashover will occur. As we know if the confined space is well ventilated, the heated gases will escape to the exterior and flashover will not occur.
- Rate and amount of heat release. If the fuel decomposes at a high rate of oxidation or if it produces high rates of heat release in relation to its mass, the production of heated gases and radiated heat to the nonburning combustibles is higher and flashover is more likely to occur and to occur more quickly. Conversely, if the rate and amount of heat production are insufficient, then flashover will not occur.
- The insulation qualities of the container itself. This factor expresses that the more heat that can be passed out of the confined space by conduction or radiation, the less likely that flashover will occur. The idea here is that good conductors such as pipes and metallic structural members will remove heat from the area of initial fire involvement and that poor insulation will allow heat to escape through walls, floors, and ceilings. The degree to which this escape occurs also will be a factor of when and if flashover occurs.
Of these three variables, the rate and amount of heat generated will be the most important single factor. As many wildland firefighters will attest to, flashover has occurred in narrow canyons. This can happen when the heat and combustion rate of hightonnage fuel loads heat nonburning combustibles to their ignition temperatures, overcoming the lack of a totally confined space that’s normally required to retain heat and create a flashover condition.
All of the above factors work in combination. Any of the negatives to flashover production such as poor insulators, well-ventilated spaces, or LOW heat rates may be overcome by closed spaces, high heat rates, or good insulators. These variables operate in a system of balances wTierein flashover occurs when the balance tips in its favor.