Spontaneous Ignition Causes And How to Prevent Them
Industrial Fire Safety
Frequently a building fire is blamed on spontaneous ignition or on faulty electric wiring when no other cause can be pinpointed. In this article we will consider spontaneous ignition, what causes it, and how best to prevent it.
In general, there are certain flammable liquids which have a tendency to oxidize and generate a small amount of heat when spread thinly over a large surface. These fall into two categories, namely vegetable oils—such as linseed oil, turpentine, olive oil and cottonseed oil—and fish oils—such as cod liver, menhaden and whale oil. Also subject to spontaneous heating and ignition are charcoal and wood. Bituminous coal and agricultural products will not be considered here.
The oils mentioned are relatively safe as liquids in proper containers. But when absorbed in combustible fibrous materials such as cotton waste, cloth, dust mops, or work clothes, they may be very hazardous. Due to their greater use, linseed oil, turpentine, varnish, and colors-in-oil used by painters are probably responsible for most fires of spontaneous origin.
Watch those rags: So it behooves all of us to watch our paint-coated wiping rags, drop cloths and splattered work clothes when putting them aside for the night. If thrown, wadded up, in a comer or in a closet or on a work bench, air containing enough oxygen for combustion but not enough for conducting away the heat may reach the oil absorbed in the material, and slow oxidation will take place. Due to being more or less confined in the material, little heat will escape, and the temperature of the oil-soaked material will continue to rise, possibly until flaming ignition occurs. However, if the cloths are spread for better exposure to the moving air, the air will cool the material by carrying away most of the heat.
Spontaneous ignition is very erroneous and cannot be predicted with any degree of accuracy. It frequently occurs accidentally; yet it is difficult to reproduce it in the laboratory with any uniformity. The temperature in most cases rises slowly to about 200°F, but after that it may take one of three courses. In many instances, soon after reaching the 200° mark, the material begins to cool off, as indicated by Curve A in Figure 1.
In other instances, heating continues to above 500°, as indicated by Curve B. Smoke and gases are liberated, and some internal charring occurs. Then the material begins to cool off.
But in the third course, as indicated by Curve C, after reaching the 500° mark, the temperature continues to rise, but more slowly. Smoke and gases are given off, then the material begins to glow, and finally flames appear with a slight puff. Flaming combustion then causes the temperature to rise suddenly.
Linseed oil test: Laboratory tests indicate that a considerable amount of liquid must be absorbed to cause spontaneous ignition in a relatively short time. In one test, 3’A ounces of colored cotton waste was saturated with boiled linseed oil and squeezed until the oil content was reduced to 4 ounces. It was then placed in a pasteboard box 4 by 4 by 3 inches with small air vents in its sides. Flaming combustion occurred in about two hours.
To demonstrate that it is the oil rather than the combustible fibrous material which causes the fire, another test was conducted. Two and a half ounces of very fine steel wool was saturated with 3 3/4 ounces of boiled linseed oil and placed in a similar box. Again, as the oil became oxidized, its temperature rose until in about 90 minutes there was flaming combustion. Yes, very fine steel wool will bum.
Raw linseed is just as hazardous as boiled linseed oil, but it requires a longer time to ignite spontaneously. Three and a half ounces of raw linseed oil was absorbed in 1 3/4 ounces of cotton in a closed but ventilated container. Autoignition occurred in about 15 hours, but it was just as hazardous as if it had occurred in less time.
Use covered metal can: When quitting a paint job at the end of a day’s work, all paint-spotted or oil-soaked wiping rags, drop cloths and spattered work clothing should be placed in a metal can with a self-closing cover. If such is not available, spread the cloths on a concrete floor or hang them from a clothesline so that there will be adequate air circulation to carry away any heat generated by oxidation of the oil. Don’t roll or bundle them up and throw them in a comer where air circulation is restricted and the material will consequently become heated.
In the home, don’t place an oily mop in a small closet where air circulation may be further restricted by other mops and brooms placed against it.
Charcoal, especially when damp, may ignite spontaneously, so watch where you place the partially filled bag of charcoal remaining after the barbecue. Don’t, in the interest of economy, extinguish the charcoal fire with water and then try to salvage the wet charcoal for the next picnic.
One source of spontaneous ignition which might hardly be expected is dry wood, without the presence of any oils. The NFPA “Fire Protection Handbook” gives as the self-ignition temperature of wood the following data. Sticks of yellow pine 1 1/4 by 4 inches ignited when exposed to heated air at:
Pyrophoric carbon: Dry wood may absorb from 30 to 90 times its own volume of air. If the room temperature is constantly high, this may help the entrained air, which contains oxygen, to change the surface of the wood into a thin layer of pyrophoric carbon. This process may require months to years of exposure to the heat, whether continually or intermittently, for it is the total time of exposure which is important. The higher the temperature the shorter the time needed to produce pyrophoric carbon. And this pyrophoric carbon may ignite spontaneously into flaming combustion when least expected.
Investigations have revealed cases in which phrophoric carbon has been produced and spontaneously ignited when wood structural members have been touching, or have been in close proximity to, steam pipes having a steam temperature of 212 °F in a poorly ventilated location.
Spontaneous ignition has caused many fires, both in industry and in the home, but before we give “spontaneous ignition” as the cause of a fire, let us try to determine exactly what materials caused the ignition. Then we will be better able to detect hazards when making a fire inspection.