Vigilance is essential to prevent all possible mine explosions. Maurice Jones reports
The explosion (or fire depending on which reports you believe) at Soma coal mine in Turkey, in which 302 miners lost their lives, comes as a reminder to mining engineers, if any were needed, of the continued hazards in mining, particularly of coal.
Turkey, it is reported, now has the unenviable distinction of the worst safety record in coal mining in terms of deaths per ton of coal mined. Even if the validity of this means of measurement can be questioned, it does demonstrate both the huge production, and safety improvements in China where largely under-reported explosions and fires have claimed many lives. A China government policy to close smaller, largely uncontrolled collieries in favour of more efficient, and more easily monitored, large mines may be paying off in safety terms.
In recent years mining explosions with large loss of life have also occurred in Australia, India, Japan, New Zealand, Rhodesia/Zimbabwe, Russia, South Africa, Ukraine, and the US, so no one can afford to be complacent.
Those disasters that have been investigated independently have shown that the primary causes are dominated by failures of owners and management, sometimes with outside inspectors, to implement best practice in explosion prevention. The often simple technology, as well as known good practice, have been available for many years, and just require implementation and maintenance. There have been developments in such technology, chiefly in gas monitoring, communications and for the preservation of life once an incident has occurred, but not offering the degree of safety improvement that good management could achieve.
Basically an explosion requires the three participants of fuel, oxygen and ignition to occur, and an enclosed space to promote the explosion. The fuel and oxygen must also be present in the correct proportions to form an explosive mixture. Prevention can be achieved by removal of any one of these from the source. Removal of oxygen is largely impractical except in extreme circumstances when an area has to be sealed off.
Dealing with atmospheric explosions rather than the less likely possibilities of blasting explosives misfires, liquid fuel explosions or battery hydrogen ignition, the fuel in mining usually consists of mainly methane (not exclusively in coal mines) or coal dust. Methane, or ‘firedamp’ mixture, can be removed by adequate ventilation, not forgetting that the low density of methane can cause the formation of roof layering in some circumstances. Although this will introduce oxygen the ventilation must be sufficient to lower the proportion of methane to below its lower explosive limit.
Although the danger of coal dust has long been recognised in propagating an explosion probably starting with methane, the possibility for explosions involving only coal dust in atmospheric suspension took a lot longer to be recognised. Even so UK text books in the 1950s referred to this danger. The increase of mechanisation has led to the creation of more fine coal dust making this fuel an increasing hazard through both explosions and inhalation to miners’ lungs.
Coal dust can be dealt with successfully by removal by extraction ventilation and air scrubbing, suppression with water mists at source, and/or clearing up accumulations by manual means. Water barriers have also been tested, but are less effective unless a spray on the flame can be assured to dissipate the cooling effect.
Although active barriers, triggered by flame or heat detectors to apply water or special chemical fire extinguishers have been tested for effectiveness and practicality, they have not shown to be substantially more effective than passive stone-dust barriers in mining.
Sources of ignition resulting in explosions can vary from open flames, through high temperature surfaces to sparks from mechanical action and electrical arcing. Equipment capable of creating a hot enough arc flash must be housed in strong flameproof enclosures to prevent the ignition of any methane in the enclosure from spreading out to the general atmosphere, and to retain any explosion.
Another common and troublesome source of ignition can be the spontaneous combustion of volatile coals through passage through the mass, usually fractured in some way. Improved ventilation to remove firedamp could actually increase danger without attention to adequate sealing. Side effects can include the generation of poisonous carbon monoxide and even hydrogen from any water applied to such a fire.
Video: This video is based on one produced by the US MSHA following the Framington coal mine explosion in West Virginia in 1968