The phenomena supplying heat energy may be grouped into four fundamental categories as to their origin (Sax 1979):
1. heat energy generated during chemical reactions (heat of oxidation, heat of combustion, heat of solution, spontaneous heating, heat of decomposition, etc.)
2. electrical heat energy (resistance heating, induction heating, heat from arcing, electric sparks, electrostatical discharges, heat generated by lightning stroke, etc.)
3. mechanical heat energy (frictional heat, friction sparks)
4. heat generated by nuclear decomposition.
The following discussion addresses the most frequently encountered sources of ignition.
Open flames
Open flames may be the simplest and most frequently used ignition source. A large number of tools in general use and various types of technological equipment operate with open flames, or enable the formation of open flames. Burners, matches, furnaces, heating equipment, flames of welding torches, broken gas and oil pipes, etc. may practically be considered potential ignition sources. Because with an open flame the primary ignition source itself represents an existing self-sustaining combustion, the ignition mechanism means in essence the spreading of burning to another system. Provided that the ignition source with open flame possesses sufficient energy for initiating ignition, burning will start.
Spontaneous ignition
The chemical reactions generating heat spontaneously imply the risk of ignition and burning as “internal ignition sources”. The materials inclined to spontaneous heating and spontaneous ignition may, however, become secondary ignition sources and give rise to ignition of the combustible materials in the surroundings. Although some gases (e.g., hydrogen phosphide, boron hydride, silicon hydride) and liquids (e.g., metal carbonyls, organometallic compositions) are inclined to spontaneous ignition, most spontaneous ignitions occur as surface reactions of solid materials. Spontaneous ignition, like all ignitions, depends on the chemical structure of the material, but its occurrence is determined by the grade of dispersity. The large specific surface enables the local accumulation of reaction heat and contributes to the increase of temperature of material above spontaneous ignition temperature.
Spontaneous ignition of liquids is also promoted if they come into contact with air on solid materials of large specific surface area. Fats and especially unsaturated oils containing double bonds, when absorbed by fibrous materials and their products, and when impregnated into textiles of plant or animal origin, are inclined to spontaneous ignition under normal atmospheric conditions. Spontaneous ignition of glass-wool and mineral-wool products produced from non-combustible fibres or inorganic materials covering large specific surfaces and contaminated by oil have caused very severe fire accidents.
Spontaneous ignition has been observed mainly with dusts of solid materials. For metals with good heat conductivity, local heat accumulation needed for ignition necessitates very fine crushing of metal. As the particle size decreases, the likelihood of spontaneous ignition increases, and with some metal dusts (for example, iron) pyrophorosity ensues. When storing and handling coal dust, soot of fine distribution, dusts of lacquers and synthetic resins, as well as during the technological operations carried out with them, special attention should be given to the preventive measures against fire to reduce the hazard of spontaneous ignition. Materials inclined to spontaneous decomposition show special ability to ignite spontaneously. Hydrazine, when set on any material with a large surface area, bursts into flames immediately. The peroxides, which are widely used by the plastics industry, easily decompose spontaneously, and as a consequence of decomposition, they become dangerous ignition sources, occasionally initiating explosive burning.
The violent exothermic reaction that occurs when certain chemicals come into contact with each other may be considered a special case of spontaneous ignition. Examples of such cases are contact of concentrated sulphuric acid with all the organic combustible materials, chlorates with sulphur or ammonium salts or acids, the organic halogen compounds with alkali metals, etc. The feature of these materials to be “unable to bear each other” (incompatible materials) requires special attention particularly when storing and co-storing them and elaborating the regulations of fire-fighting. It is worth mentioning that such hazardously high spontaneous heating may, in some cases, be due to the wrong technological conditions (insufficient ventilation, low cooling capacity, discrepancies of maintenance and cleaning, overheating of reaction, etc.), or promoted by them. Certain agricultural products, such as fibrous feedstuffs, oily seeds, germinating cereals, final products of the processing industry (dried beetroot slices, fertilizers, etc.), show an inclination for spontaneous ignition. The spontaneous heating of these materials has a special feature: the dangerous temperature conditions of the systems are exacerbated by some exothermic biological processes that cannot be controlled easily.
Electric ignition sources
Power machines, instruments and heating devices operated by electric energy, as well as the equipment for power transformation and lighting, typically do not present any fire hazard to their surroundings, provided that they have been installed in compliance with the relevant regulations of safety and requirements of standards and that the associated technological instructions have been observed during their operation. Regular maintenance and periodic supervision considerably diminish the probability of fires and explosions. The most frequent causes of fires in electric devices and wiring are overloading, short circuits, electric sparks and high contact resistances. Overloading exists when the wiring and electrical appliances are exposed to higher current than that for which they are designed. The overcurrent passing through the wiring, devices and equipment might lead to such an overheating that the overheated components of the electrical system become damaged or broken, grow old or carbonize, resulting in cord and cable coatings melting down, metal parts glowing and the combustible structural units coming to ignition and, depending on the conditions, also spreading fire to the environment. The most frequent cause of overloading is that the number of consumers connected is higher than permitted or their capacity exceeds the value stipulated.
The working safety of electrical systems is most frequently endangered by short circuits. They are always the consequences of any damage and occur when the parts of the electrical wiring or the equipment at the same potential level or various potential levels, insulated from each other and the earth, come into contact with each other or with the earth. This contact may arise directly as metal-metal contact or indirectly, through electric arc. In cases of short circuits, when some units of the electric system come in contact with each other, the resistance will be considerably lower, and as a consequence, the intensity of current will be extremely high, perhaps by several orders of magnitude higher. The heat energy released during overcurrents with large short circuits might result in a fire in the device affected by the short circuit, with the materials and equipment in the surrounding area coming to ignition and with the fire spreading to the building.
Electric sparks are heat energy sources of a small nature, but as shown by experience, act frequently as ignition sources. Under normal working conditions, most electrical appliances do not release sparks, but the operation of certain devices is normally accompanied by sparks. Sparking introduces a hazard foremost at places where, in the zone of their generation, explosive concentrations of gas, vapour or dust might arise. Consequently, equipment normally releasing sparks during operation is permitted to be set up only at places where the sparks cannot give rise to fire. On its own, the energy content of sparks is insufficient for the ignition of the materials in the environment or to initiate an explosion.
If an electrical system has no perfect metallic contact between the structural units through which the current flows, high contact resistance will occur at this spot. This phenomenon is in most cases due to the faulty construction of joints or to unworkmanlike installations. The disengagement of joints during operation and natural wear may also be cause for high contact resistance. A large portion of the current flowing through places with increased resistance will transform to heat energy. If this energy cannot be dissipated sufficiently (and the reason cannot be eliminated), the extremely large increase of temperature might lead to a fire condition that endangers the surrounding. If the devices work on the basis of the induction concept (engines, dynamos, transformers, relays, etc.) and are not properly calculated, eddy currents may arise during operation. Due to the eddy currents, the structural units (coils and their iron cores) might warm up, which might lead to the ignition of insulating materials and the burning of the equipment. Eddy currents might arise—with these harmful consequences—also in the metal structural units around high-voltage equipment.
Electrostatic sparks
Electrostatic charging is a process in the course of which any material, originally with electric neutrality (and independent of any electric circuit) becomes charged positively or negatively. This may occur in one of three ways:
1. charging with separation, such that charges of subtractive polarity accumulate on two bodies simultaneously
2. charging with passing, such that the charges passing away leave charges of opposed polarity signs behind
3. charging by taking up, such that the body receives charges from outside.
These three ways of charging may arise from various physical processes, including separation after contact, splitting, cutting, pulverizing, moving, rubbing, flowing of powders and fluids in pipe, hitting, change of pressure, change of state, photoionization, heat ionization, electrostatical distribution or high-voltage discharge.
Electrostatic charging may occur both on conducting bodies and insulating bodies as a result of any of the processes mentioned above, but in most cases the mechanical processes are responsible for the accumulation of the unwanted charges.
From the large number of the harmful effects and risks due to electrostatic charging and the spark discharge resulting from it, two risks can be mentioned in particular: endangering of electronic equipment (for example, computer for process control) and the hazard of fire and explosion. Electronic equipment is endangered first of all if the discharge energy from the charging is sufficiently high to cause destruction of the input of any semi-conductive part. The development of electronic units in the last decade has been followed by the rapid increase of this risk. The development of fire or explosion risk necessitates the coincidence in space and time of two conditions: the presence of any combustible medium and the discharge with ability for ignition. This hazard occurs mainly in the chemical industry. It may be estimated on the basis of the so-called spark sensitivity of hazardous materials (minimum ignition energy) and depends on the extent of charging. It is an essential task to reduce these risks, namely, the large variety of consequences that extend from technological troubles to catastrophes with fatal accidents. There are two means of protecting against the consequences of electrostatic charging:
1. preventing the initiation of the charging process (it is evident, but usually very difficult to realize)
2. restricting the accumulation of charges to prevent the occurrence of dangerous discharges (or any other risk).
Lightning is an atmospherical electric phenomenon in nature and may be considered an ignition source. The static charging produced in the clouds is equalized towards the earth (lightning stroke) and is accompanied by a high-energy discharge. The combustible materials at the place of lightning stroke and its surroundings might ignite and burn off. At some strokes of lightning, very strong impulses are generated, and the energy is equalized in several steps. In other cases, long-lasting currents start to flow, sometimes reaching the order of magnitude of 10 A.
Mechanical heat energy
Technical practice is steadily coupled with friction. During mechanical operation, frictional heat is developed, and if heat loss is restricted to such an extent that heat accumulates in the system, its temperature may increase to a value that is dangerous for the environment, and fire may occur. Friction sparks normally occur at metal technological operations because of heavy friction (grinding, chipping, cutting, hitting) or because of metal objects or tools dropping or falling on to a hard floor or during grinding operations because of metal contaminations within the material under grinding impact. The temperature of the spark generated is normally higher than the ignition temperature of the conventional combustible materials (such as for sparks from steel, 1,400-1,500 °C; sparks from copper-nickel alloys, 300-400 °C); however, the ignition ability depends on the whole heat content and the lowest ignition energy of the material and substance to be ignited, respectively. It has been proven in practice that friction sparks mean real fire risk in air spaces where combustible gases, vapours and dusts are present in dangerous concentrations. Thus, under these circumstances the use of materials that easily produce sparks, as well as processes with mechanical sparking, should be avoided. In these cases, safety is provided by tools that do not spark, i.e., made from wood, leather or plastic materials, or by using tools of copper and bronze alloys that produce sparks of low energy.
Hot surfaces
In practice, the surfaces of equipment and devices may warm up to a dangerous extent either normally or due to malfunction. Ovens, furnaces, drying devices, waste-gas outlets, vapour pipes, etc. often cause fires in explosive air spaces. Furthermore, their hot surfaces may ignite combustible materials coming close to them or by coming in contact. For prevention, safe distances should be observed, and regular supervision and maintenance will reduce the probability of the occurrence of dangerous overheating.