CAUSED BY SPONTANEOUS COMBUSTION
When most of us think of fires, we think of arson, or accidental causes such as overheated grease from a stove, or mechanical failures of a device. There is a third category of fire causes, which is more infrequent, but has resulted in some major property losses. This third category is naturally occurring fires.
The two prime causes of natural fires are lightning strikes, and spontaneous heating. This article will deal with area of spontaneous heating.
Spontaneous heating is the process whereby a substance increases in temperature without drawing heat from its surroundings. Spontaneous heating of a substance to its ignition temperature results in spontaneous ignition or spontaneous combustion.
Three conditions which have much to do with whether or not an oxidation reaction will cause dangerous heating are 1) rate of heat generation, 2) air supply, and 3) insulation properties of the immediate surroundings.
When exposed to the atmosphere, organic substances capable of combination with oxygen will oxidize at some critical temperature with the evolution of heat. The rate of oxidation at normal temperatures is usually so slow that the released heat is transferred to surroundings areas as rapidly as it is formed, with no increase in temperature of the material. This is not true of all combustible materials, since certain reactions generate heat more rapidly than it can be dissipated, with a resulting temperature increase in the material.
In order for spontaneous ignition to occur, there must be sufficient oxygen available for the reaction to proceed, but not so much draft that the heat is carried away as quickly as it is generated. For example, a linseed oil-soaked rag may heat spontaneously in the bottom of a container, but would not do so if hung on a clothes line where air movement would remove the heat as quickly as it was formed. Because of the many possible combinations of air supply and insulation, it is impossible to predict with certainty whether or not a material will heat spontaneously.
The moisture content of agricultural products has a definite influence on their spontaneous heating. These crops initially are oxidized by bacteria. These bacteria usually perish at temperatures of 160 - 175°F. Wet, or improperly cured hay is very likely to heat in barns.
Experience has indicated that heating in hay mows may result in ignition within a period of two to six weeks after storage. Many agricultural products are susceptible to spontaneous heating. These include corn meal feed, linseed, rice, bran, and pecan meal.
If the spontaneous heating of a hay mow is detected early enough, the hay can be moved out prior to ignition. In many cases, the heating is not detected until fire breaks out. As we all know, barn fires are very difficult to extinguish, and normally result in total destruction of the building.
Another common area of spontaneous ignition is within silage in silos. These fires are very difficult to fight, and have resulted in many serious injuries and fatalities to fire fighters.
Vegetable oils have a high tendency to heat spontaneously, while oils such as hydrocarbons, do not have any heating tendency.
Some of the materials which have a high tendency to heat are linseed oil which is commonly used in furniture refinishing, fish meal, fish oil, alfalfa meal, cod liver oil, corn meal feeds, oiled fabrics, oiled rags, peanuts, varnished fabrics, and charcoal.
Large coal piles must be carefully prepared to prevent heating of the material. Large industrial users such as hydro generating plants and similar facilities, monitor the moisture content, and compaction, of their coal piles very closely to ensure that spontaneous heating and ignition does not occur.
One of the more common causes of spontaneous ignition in residential areas is the heating of linseed oil-soaked rags. Fabrics impregnated with this oil are extremely dangerous. They should be stored in closed containers, preferably metal, and in small quantities. These containers should be stored outdoors whenever possible.
At Walters Consulting Corporation, we have had the opportunity to investigate many fires caused by spontaneous heating. IF you have a situation where you suspect spontaneous heating may have occurred, it is important to retain an expert who can assess the variables involved, and possibly set up a recreation test to substantiate the theory.
One such large fire caused by spontaneous heating occurred in a subdivision in Markham, where a pile of agricultural products heated, ignited, and eventually spread to 105 homes under construction. This clearly demonstrates how a small oxidation reaction can lead to a major inferno, with serious financial repercussions for insurers.
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Walters Forensic Engineering | 277 Wellington
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