The Fire Triangle and the Exceptions

Usually, fires can only start where there is a chain reaction to three elements – fuel, oxygen(air) and an ignition source – all coming together at the same time. This is the “Fire Triangle” and is responsible for most fires. By removing one element, the fire should be extinguished, or not be initiated. A simple example would be a fire in a metal bin – if the lid is then closed the fire uses up all the air and self-extinguishes. This principle is used in vehicle workshops where drums of flammable solvents are used with a tray for cleaning located on top – the lid is fitted with a lead fusible link that is anchored to keep open the lid, so that if there is a fire in the tray, the lead melts and the open lid can then automatically close and cut off the air supply to extinguish the fire.

There are some fires, however, that are not caused in this way, and some, such as lithium-ion battery fires that are exacerbated by creating their own oxygen during the combustion process, thus maintaining the “Fire Triangle” for as long as there is fuel from the burning battery, irrespective of enclosing a burning battery.

Other fires can be caused by a material merely coming into contact with air or even water. Such materials include pyrophoric gases such as silane or phosphine (if there is a trace of diphosphane) and can only be controlled by keeping them within an inert environment such as in nitrogen. There are also pyrophoric liquids such as butyllithium which is supplied in hexane for the same reason.

Combustible/flammable materials also have auto-ignition temperatures, this being the temperature at which the material catches fire without an ignition source being present. For instance, petrol auto-ignites at 400⁰C, and wood can auto-ignite at 300⁰C. These are elevated temperatures – spontaneous combustion, on the other hand, starts at ambient temperatures.

Materials that spontaneously ignite also do not rely on the “Fire Triangle” as the process is where there is oxidation of the material where heat cannot escape – the heat builds up until the material catches fire with no external ignition source. An oily rag is an example of this and if the rag is roughly folded up it is much more likely to self-combust than the same rag that is laid flat, as in the latter case the heat can escape and the required temperature for self-combustion is not reached.

Activities That Can Result in Spontaneous Combustion

• Hay Storage

There are many rural activities in use at UMAL Member properties, whether it is for research farms, stables or animal husbandry. Many of these establishments have a hay store. Hay can spontaneously ignite at 55⁰C, and moisture content is the key component. It is important to keep the moisture content below 20% to help prevent self-combustion, with 16% – 18% moisture recommended for the larger round/rectangular bales. As the process starts with mould, mildew and bacterial build-up, a musty smell or a slight caramel odour can result, this being a sign that the hay store is heating up, with action needed to exclude air from the heart of the hay store or reduce moisture. It is believed most fires occur within 6 weeks of baling.

Manure has also been known to be subject to spontaneous combustion.

• Refectories

Many refectory kitchens have cooked fried fish, and this creates “batter scraps”. If these are stored in a tightly packed container they can start to heat until they self-combust – if the container is open, they can self-combust within one day. Storage in a metal lidded metal bin for the purpose is recommended, and ideally emptied daily. The addition of some water during storage also helps prevent self-combustion. Similarly, some animal or vegetable oils, such as linseed oil, soybean oil, corn oil, cottonseed oil, cooking oil, fish oil, peanut oil, powdered eggs, lard oil and margarine, can also spontaneously combust when in contact with rags, cardboard, paper or other combustible material in the correct conditions – tightly packed with little/no heat dissipation possible.

• Hair and Beauty Training Salons

There is the use of some essential oils in Salons which impregnate towels. These towels are then washed and often stored on open shelves. The combination of washing at low temperatures (under 60⁰C) and high drying temperatures, together with packed storage on shelving can precipitate the self-heating process as the chemicals in the towels have not properly broken down and been washed away. It is thought some animal and vegetable oils can cause the same issue. Spontaneous combustion can then occur, and large fires have resulted in both Colleges and commercial salons. The key is to wash at 60⁰C to eliminate the chemicals that produce the exothermic reaction.

• Artwork

There is a wide range of material that could self-combust that can be found in art studios. Vegetable oils, turpentine, linseed oil and pine tar derivatives used in paint manufacture can oxidize rapidly in the right conditions – this can include impregnated rags in tightly packed storage in an open area, as small as a student locker. The issue can also occur with spray booth filters, paint overspray residues. The key is to store oily cloths in tightly sealed metal lidded metal bins for the purpose and keep clean cloths in separate bins. Paint booth filters should also be changed on a frequent basis and never compacted into a waste storage area such as a skip.

Oil paint on a canvas dries through oxidation and causes no issues as the heat generated when drying (curing) is easily dissipated. Dry, solid oil paint also causes no issues, nor do cleaning cloths impregnated with acrylic paints. Some paint coatings, usually alkyd enamels, naturally cure through auto-oxidation and there is no issue if in an open area, where the heat generated can easily dissipate. Spontaneous combustion can occur if compacted in a space where the heat cannot dissipate, so such storage of waste materials having this coating must be avoided.

• Printing/Vehicle Workshops

There are some oily rags that are used in traditional printing, such as letterpress and litho printing, these to clean down the printing equipment. Separate bins must be used for the clean rags and for the impregnated rags, the latter always stored in metal lidded mental bins. Similar risk mitigation measures are required for vehicle workshops where oily rags can self-ignite.

 

Conclusion

Risk mitigation for spontaneous combustion is as easy as creating the conditions to promote this risk. In most cases:

• Separate oily impregnated combustible materials (rags, paper, card, etc) from clean materials
• Store soiled materials in tightly closed metal lidded metal bins
• Empty soiled material bins regularly
• If washing impregnated materials, always do so at elevated temperatures (60⁰) and dry at lower temperatures
• If storing organic material, such as hay, keep the moisture content as low as possible – less than 20%
• Food waste such as ‘batter scraps’ should also be stored in metal lidded metal bins