Sources of Ignition

A dust explosion will occur only if a dust is dispersed in air or oxygen within the explosive range and if, at the same time, a suitable source of ignition is present. Preventive measures are aimed at avoiding the presence of either, or preferably both, these factors in the system. In many cases, however, because of the heterogeneous nature of dust clouds and the difficulty in foreseeing or excluding every possible source of ignition, explosions cannot be positively prevented by these means alone. Although every effort should be made to avoid flammable concentrations of dust and to eliminate all sources of ignition, it is usually necessary to supplement such measures by taking precautions designed to reduce the spread and the effects of an explosion. Sensitivity to ignition can be measured in the laboratory and a range of different tests have been developed over many years to characterise the ease in which of a particular material can be ignited. Ignition sources for dusts and gases are described in detail in EN 1127-1 Explosive Atmospheres - Explosion Prevention and Protection - Part 1: Basic concepts and Methodology. The most important ignition sources are described below and commercially available dust explosion tests allow any material to be assessed for potential explosion risk.

Flames and hot surfaces

Open flames or hot surfaces are by far the most common sources of ignition in factory processes. The flame or hot surface may be produced inadvertently. For example there could be a welding or cutting operation on or near to a vessel or plant containing flammable dust. The flame or hot surface may be there by design, as in the case of drying and heating plant associated with the processes and the factory building. The lowest surface temperature that will ignite a dust cloud is known as the minimum ignition temperature. In the case of welding and similar operations no such work should be carried out until steps have been taken to remove the dust from the area. No dust should be readmitted to the area until all sources of ignition have been removed and the surfaces cooled. Indirect methods of heating processes and buildings are preferable to those involving open flames. If direct heating is the only practicable solution to this problem then it will be necessary to ensure that (a) the combustion air for the heating process is drawn from a dust-free zone away from sites where people are normally employed (b) flammable dust is not allowed to accumulate on the heater unit.

Spontaneous ignition

The storage of many dusts is liable to promote spontaneous heating. If the heat is not dissipated the ignition temperature of the dust may be reached. The immediate result of spontaneous ignition is fire, but explosions may follow if the dust is subsequently dispersed in a cloud. Materials particularly prone to spontaneous heating are organic substances such as sewage sludge, corn meal, fertilizers, fish meals and scrap rubber, especially when they are contaminated with unsaturated oils. More recently, the move to renewable energy sources and the use of biomass in large quantities - especially wood pellets - has given rise to a significant number of self-heating fires. Metal powders are also susceptible to spontaneous heating, particularly when produced in the absence of oxygen or when in a very finely divided state. When processing has caused a rise in temperature of such material it should be allowed to cool before being stored or packed. Means of measuring the temperature of the stored product should be provided in silos and other plant used for the storage of materials subject to spontaneous heating. If appreciable temperature rises are noted, the material should be re-circulated to dissipate the heat. With certain materials it is desirable to avoid extremely low or high moisture contents in storage though some carbonaceous materials should be stored and kept quite dry - see also ‘static electricity’ below. Where metal powders are manufactured in an inert gas this should contain a small proportion of oxygen to allow a protective film of oxide to form on the surface of the particles. Otherwise it will usually be necessary to store such powders in sealed containers having an inert atmosphere.

Friction sparks

Many explosions have been caused as the result of foreign objects entering a grinding mill or disintegrator with the feedstock. The entry of ferrous tramp metal should be prevented by the installation of a magnetic separator at a suitable point in the feed to the mill. Where the feedstock is a paste or is so coarse that the field strength of a magnetic separator is insufficient to remove the extraneous material, a metal detector may be employed which will stop the plant so that the tramp metal may be removed by hand. If electromagnetic separators are used a device should be included to divert the feed from the mill should there be failure of the current actuating the magnet. Foreign bodies, both magnetic and non-magnetic, may be removed from the feedstock by the use of a pneumatic separator, provided that there is an appreciable difference in density between the materials. The machinery itself may cause ignition. Friction in bearings may generate sufficient heat to ignite dust, and it is important that bearings should be properly lubricated and maintained. Ideally, bearings should be external to the dust cloud as part of the machinery design.

Electrical plant

Flammable dusts may be ignited by sparks generated by electrical equipment, for example, during the operation of switchgear, when fuses blow or cables or equipment are damaged. Ignition may also occur if the surface temperatures of equipment are excessive, particularly if accumulations of dust are allowed to form on such surfaces. Where practicable all electrical equipment should be situated in positions where it is not exposed to concentrations of dust. Switchgear can often be located in a control room separated from the main workroom, and lighting fittings can usually be installed outside the hazardous area and separated from it by transparent dust-tight windows. If it is essential for certain electrical fittings to be situated in hazardous locations they should be certified for use in the designated hazardous area. The maximum surface temperature of the equipment should not be capable of igniting the combustible material, and it must not be possible for dust to collect to such an extent that proper cooling is prevented, thus causing a dangerous rise in temperature. Note that a knowledge of the layer ignition temperature (LIT) of the dust is required in order to designate the temperature class of the electrical equipment. Explosions have occurred in silos and bins as a result of the use of portable lead lamps. The provision of a permanent lighting system, appropriate for the area, is recommended. Facilities should be provided to maintain the fittings from outside the enclosure; alternatively battery hand- lamps approved for the purpose may be used.

Static electricity

Precautions should be taken to minimize the possibility of the formation of static charges on plant. All conducting materials should be efficiently earthed by the use of bonding strips, and routine tests for electrical resistance should be made at frequent intervals. Where static charges are likely to be developed on plant made of non-conducting materials such as belts and rubber sleeves, these components can often be replaced by anti-static rubber or similar conducting materials. Dust in bulk may retain charges for a very long time if its electrical resistivity is high (e.g. over10 9 ohm.m) - see powder resistivity.