What is self-ignition?
Self-ignition is a process in which a substance catches fire without an ignition source. Some oils and varnishes are able to engage in these reactions, but solid biological matter, e.g. straw, grain, wood chips and fish flour are also known to present a self-ignition hazard. This article is limited to the former group of substances, which are commonly used inside buildings and therefore represent a significant fire hazard to critical structures and equipment.
Occurrence and loss potential
Where self-ignition is the source of a fire the resulting damage is similar to that of fires developing from other fire sources; ranging from negligible to the complete destruction of an area. The following examples are from our files and show examples of damage caused by fire resulting from self-ignition.
- At one company cleaning was undertaken before the weekend break. The filter unit of a spray paint booth was removed and discarded in a roughly 1x1 m metal tray without a lid. The tray was left in the building when the staff went home. On Monday morning the first shift found massive soot and smoke damage in an extensive area, but fortunately the fire had been confined to the tray. It took one week to clean up after this small fire.
- Another company was preparing to move into their new 7,500 square meter office building. What remained were the final touches, including oil treatment of their floor boards. Improper handling of the oil-soaked rags allowed for ignition to take place, resulting in soot damage to a vast area of the building as well as to technical equipment. A building fire did not develop, probably because of the absence of significant fire load in the empty offices. After a 10 week delay and a 2 million EUR loss, the company could finally move into the new offices.
- Probably the most publicised fire in Denmark caused by self-ignition is the fire in 2003 at the North Sea Oceanarium. Floors in the canteen area had been treated with oil, but the rags had been carelessly discarded. A fire started and caused massive damage to the entire structure and contents. The total cost of this loss was in the range 5 to 10 million EUR.
The mechanism of self-ignition
Self-ignition is a combustion process in which oxygen in the air reacts with an organic compound under the generation of combustion products and heat. At normal ambient temperature the combustion proceeds slowly, but under certain circumstances the temperature may increase sufficiently to cause ignition. In terms of heat, ignition may take place when released heat from the combustion process locally exceeds the heat loss over an extended period of time.
The rate of ventilation determines the amount of oxygen that will be available for the combustion process and thereby the rate of heat release of the reaction. Certain oils undergo a curing process which is auto-oxidative, i.e. the compound does not need access to oxygen to undergo the combustion reaction. Although the rate of combustion for such auto-oxidative compounds may well be increased by the easy access to oxygen, ventilation is not a prerequisite for the combustion process to take place.
While ventilation provides oxygen for the combustion reaction, it also removes heat from the combustion zone through convection. This may cool the reaction sufficiently to keep it at a sub-critical level in terms of self-ignition.
Just as ventilation will cool the combustion process, some heat is also lost through transmission. This heat loss is reduced when the area of combustion is well insulated, e.g. when oil-soaked rags are discarded into a waste bin.
Heat loss is also reduced if the ambient temperature is elevated. In a test by SENTEF NBL on the self-ignition potential of oil used to treat floors, it was found that a sub-critical temperature development at 24 centigrade was changed to a critical one by raising the ambient temperature to 44 centigrade. The same study showed that rags soaked in this oil and left in ordinary waste bins were very unlikely to self-ignite at 10-15 centigrade.
Where are you exposed to this fire hazard?
The risks associated with rags soaked in linseed oil are well-known, and indeed this oil is extremely prone to self-ignition. It should be borne in mind, however, that many other oils with similar chemical properties are in use domestically and in industry where they represent a serious, and perhaps unrecognised, fire hazard. Oil-treated floors are in fashion at the time being which increases the hazard at an industrial level.
In industrial surface coating applications, filter units from spray booths and piles of semi-cured film or over-spraying are also known to present a self-ignition hazard.
Loss control measures
The first step in controlling the hazard of self-ignition is to understand where the risk occurs in the particular environment: Make a list of applications and areas then check to see if adequate control is exercised for each application.
If a contractor is hired to undertake service involving materials that represent a self-ignition hazard, check that he is familiar with the risk and knows how to safely handle the hazardous wastes. Also ask the contractor to remove these wastes from your facility. Any oil-soaked rags, used filter units and piles of semi-cured film from your spray booths should be considered hazardous unless you have reliable evidence that they are not.
As described above the most important critical factors in self-ignition are the following:
Self-ignition may occur when the balance of these factors allows for more rapid heat release from the combustion process than the heat loss rate. Fortunately it is possible to control the balance by affecting these factors.
Incineration under safe conditions is an efficient way of controlling the spontaneous fire hazard of contaminated application. However, before the materials can be incinerated, the conditions under which they are stored must minimise the risk of self-ignition starting a fire.
Clearly, discarded hazardous material should be stored in such a place that a fire would not cause a significant loss. Outside and away from buildings is the best place to keep such waste. Where this is not possible, the following measures will help to reduce the chances of a loss occurring:
Ventilation
Ventilation is the most important factor in controlling the self-ignition hazard. Keeping these hazardous wastes in airtight metal containers designed for fire hazardous wastes or immersed in water are safe manners of keeping ventilation at a sufficiently low level until the materials can be properly discarded.
Insulation
Separating wastes that represent a self-ignition hazard from ordinary waste is an important loss control measure. Mixing wastes in a bin might shift the heat balance toward a lower level of heat loss because of the possible insulating effect of other wastes. It is essential to keep track of the hazardous waste and keep them separated from ordinary waste streams to ensure that they are properly handled at all times.
Ambient temperatures
Higher ambient temperatures emphasises the need for good loss control. Unsafe practices may not reveal themselves until an elevation of the ambient temperature has reduced the heat loss to a level allowing for a local net build-up of heat in the combustion zone, elevating the temperature beyond the ignition point. When wastes that may potentially ignite, are discarded where the ambient temperature is high, particular vigilance should be exerted in securing compliance with the other loss control measures.
Halldor Machholm
