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Sunday, 13 March 2011 16:34

Fires and Explosions in Mines

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Fires and explosions pose a constant threat to the safety of miners and to the productive capacity of mines. Mine fires and explosions traditionally have ranked among the most devastating industrial disasters.

At the end of the nineteenth century, fires and explosions in mines resulted in loss of life and property damage on a scale unmatched in other industrial sectors. However, clear progress has been achieved in controlling these hazards, as evidenced by the decline in mine fires and explosions reported in recent decades.

This article describes the basic fire and explosion hazards of underground mining and the safeguards needed to minimize them. Fire protection information on surface mines can be found elsewhere in this Encyclopaedia and in standards such as those promulgated by organizations such as the National Fire Protection Association in the United States (e.g., NFPA 1996a).

Permanent Service Areas

By their nature, permanent service areas involve certain hazardous activities, and thus special precautions should be taken. Underground maintenance shops and related facilities are a special hazard in an underground mine.

Mobile equipment in maintenance shops is regularly found to be a frequent source of fires. Fires on diesel-powered mining equipment typically arise from leaking high-pressure hydraulic lines which can spray a heated mist of highly combustible liquid onto an ignition source, such as a hot exhaust manifold or turbocharger (Bickel 1987). Fires on this type of equipment can grow quickly.

Much of the mobile equipment used in underground mines contains not only fuel sources (e.g., diesel fuel and hydraulics) but they also contain ignition sources (e.g., diesel engines and electrical equipment). Thus, this equipment presents an appreciable risk for fires. In addition to this equipment, maintenance shops generally contain a variety of other tools, materials and equipment (e.g., degreasing equipment) that are a hazard in any mechanical shop environment.

Welding and cutting operations are a leading cause of fires in mines. This activity can be expected to occur regularly in a maintenance area. Special precautions need to be taken to ensure that these activities do not create a possible ignition source for a fire or explosion. Fire and explosion protection information relating to safe welding practices can be found elsewhere in this Encyclopaedia and in other documents (e.g., NFPA 1994a).

Consideration should be given to making the entire shop area a completely enclosed structure of fire resistant construction. This is particularly important for shops intended for use longer than 6 months. If such an arrangement is not possible, then the area should be protected throughout by an automatic fire suppression system. This is especially important for coal mines, where it is critical to minimize any potential fire source.

Another important consideration for all shop areas is that they be vented directly to the air return, thus limiting the spread of products of combustion from any fire. Requirements for these type of facilities are clearly outlined in documents such as NFPA 122, Standard for Fire Prevention and Control in Underground Metal and Nonmetal Mines, and NFPA 123, Standard for Fire Prevention and Control in Underground Bituminous Coal Mines (NFPA 1995a, 1995b).

Fuel Bays and Fuel Storage Areas

The storage, handling and use of flammable and combustible liquids pose a special fire hazard for all sectors of the mining industry.

In many underground mines, mobile equipment is typically diesel-powered, and a large percentage of the fires involve the fuel used by these machines. In coal mines, these fire hazards are compounded by the presence of coal, coal dust and methane.

The storage of flammable and combustible liquids is an especially important concern because these materials ignite more easily and propagate fire more rapidly than ordinary combustibles. Both flammable and combustible liquids are often stored underground in most non-coal mines in limited quantities. In some mines, the main storage facility for diesel fuel, lubricating oil and grease, and hydraulic fluid is underground. The potential seriousness of a fire in an underground flammable and combustible liquid storage area requires extreme care in the design of the storage areas, plus the implementation and strict enforcement of safe operating procedures.

All aspects of using flammable and combustible liquids present challenging fire protection concerns, including the transfer to underground, storage, dispensing and ultimate use in equipment. The hazards and protection methods for flammable and combustible liquids in underground mines can be found elsewhere in this Encyclopaedia and in NFPA standards (e.g., NFPA 1995a, 1995b, 1996b).

Fire Prevention

Safety for fires and explosions in underground mines is based on the general principles of preventing fire and explosion. Normally, this involves using common-sense fire safety techniques, such as preventing smoking, as well as providing built-in fire protection measures to prevent fires from growing, such as portable extinguishers or early fire detection systems.

Fire and explosion prevention practices in mines generally fall into three categories: limiting ignition sources, limiting fuel sources and limiting fuel and ignition source contact.

Limiting ignition sources is perhaps the most basic way of preventing a fire or explosion. Ignition sources that are not essential to the mining process should be banned altogether. For example, smoking and any open fires, especially in underground coal mines, should be prohibited. All automated and mechanized equipment that may be subject to unwanted buildup of heat, such as conveyors, should have slippage and sequence switches and thermal cutouts on electric motors. Explosives present an obvious hazard, but they could also be an ignition source for suspended dust of hazardous gas and should be used in strict conformance with special blasting regulations.

Eliminating electrical ignition sources is essential for preventing explosions. Electrical equipment operating where methane, sulphide dust or other fire hazards may be present should be designed, constructed, tested and installed so that its operation will not cause a mine fire or explosion. Explosion proof enclosures, such as plugs, receptacles and circuit interrupting devices, should be used in hazardous areas. The use of intrinsically safe electrical equipment is described in further detail elsewhere in this Encyclopaedia and in documents such as NFPA 70, National Electrical Code (NFPA 1996c).

Limiting fuel sources starts with good housekeeping to prevent unsafe accumulations of trash, oily rags, coal dust and other combustible materials.

When available, less hazardous substitutes should be used for certain combustible materials such as hydraulic fluids, conveyor belting, hydraulic hoses and ventilation tubing (Bureau of Mines 1978). The highly toxic products of combustion that may result from the burning of certain materials often necessitates less hazardous materials. As an example, polyurethane foam had previously been widely used in underground mines for ventilation seals, but more recently has been banned in many countries.

For underground coal mine explosions, coal dust and methane are typically the primary fuels involved. Methane may also be present in non-coal mines and is most commonly handled by dilution with ventilation air and exhaustion from the mine (Timmons, Vinson and Kissell 1979). For coal dust, every attempt is made to minimize the generation of dust in the mining processes, but the tiny amount needed for a coal dust explosion is almost unavoidable. A layer of dust on the floor that is only 0.012 mm thick will cause an explosion if suspended in air. Thus, rock dusting using an inert material such as pulverized limestone, dolomite or gypsum (rock dust) will help to prevent coal dust explosions.

Limiting fuel and ignition source contact depends upon preventing contact between the ignition source and the fuel source. For example, when welding and cutting operations cannot be performed in fire-safe enclosures, it is important that areas be wet down and nearby combustibles covered with fire resistant materials or relocated. Fire extinguishers should be readily available and a fire watch posted for as long as necessary to guard against smouldering fires.

Areas with a high loading of combustible materials, such as timber storage areas, explosives magazines, flammable and combustible liquid storage areas and shops, should be designed to minimize possible ignition sources. Mobile equipment should have hydraulic fluid, fuel and lubricant lines re-routed away from hot surfaces, electrical equipment and other possible ignition sources. Spray shields should be installed to deflect sprays of combustible liquid from broken fluid lines away from potential ignition sources.

Fire and explosion prevention requirements for mines are clearly outlined in NFPA documents (e.g., NFPA 1992a, 1995a, 1995b).

Fire Detection and Warning Systems

The elapsed time between the onset of a fire and its detection is critical since fires may grow rapidly in size and intensity. The most rapid and reliable indication of fire is through advanced fire detection and warning systems using sensitive heat, flame, smoke and gas analysers (Griffin 1979).

The detection of gas or smoke is the most cost-effective approach to providing fire detection coverage over a large area or throughout the entire mine (Morrow and Litton 1992). Thermal fire detection systems are commonly installed for unattended equipment, such as over conveyor belts. Faster-acting fire detection devices are considered appropriate for certain high-hazard areas, such as flammable and combustible liquids storage areas, refuelling areas and shops. Optical flame detectors that sense either ultraviolet or infrared radiation emitted by a fire are often used in these areas.

All miners should be warned once a fire has been detected. Telephones and messengers are sometimes used, but miners are often remote from telephones and they are often widely scattered. In coal mines, the most common means of fire warning are shutdown of electric power and subsequent notification by telephone and messengers. This is not an option for non-coal mines, where so little equipment is powered electrically. Stench warning is a common method of emergency communication in non-coal underground mines (Pomroy and Muldoon 1983). Special wireless radio frequency communication systems have also been used successfully in both coal and non-coal mines (Bureau of Mines 1988).

The primary concern during an underground fire is the safety of underground personnel. Early fire detection and warning permit the initiation of an emergency plan in the mine. Such a plan assures that the necessary activities, such as evacuation and fire-fighting will occur. To assure smooth implementation of the emergency plan, miners should be provided with comprehensive training and periodic retraining in emergency procedures. Fire drills, complete with the activation of the mine warning system, should be performed frequently to reinforce the training and to identify weaknesses in the emergency plan.

Further information on fire detection and warning systems can be found elsewhere in this Encyclopaedia and in NFPA documents (e.g., NFPA 1995a, 1995b, 1996d).

Fire Suppression

The most common types of fire suppression equipment used in underground mines are portable hand extinguishers, water hoselines, sprinkler systems, rock dust (applied manually or from a rock dusting machine) and foam generators. The most common type of portable hand extinguishers are typically those using multi-purpose dry chemicals.

Fire suppression systems, either manual or automatic, are becoming more common for mobile equipment, combustible liquids storage areas, conveyor belt drives and electrical installations (Grannes, Ackerson and Green 1990). Automatic fire suppression is especially important for unattended, automated or remote control equipment where personnel are not present to detect a fire, to activate a fire suppression system or to initiate fire-fighting operations.

Explosion suppression is a variation of fire suppression. Some European coal mines use this technology in the form of passive or triggered barriers on a limited basis. Passive barriers consist of rows of large tubs containing water or rock dust that are suspended from the roof of a mine entry. In an explosion, the pressure front that precedes the arrival of the flame front triggers the dumping of the contents of the tubs. The dispersed suppressants quench the flame as it passes through the entry protected by the barrier system. Triggered barriers utilize an electrically or pneumatically operated actuation device that is triggered by the heat, flame or pressure of the explosion to release suppressant agents that are stored in pressurized containers (Hertzberg 1982).

Fires that grow to an advanced stage should be fought only by highly trained and specially equipped fire-fighting teams. Where large areas of coal or timber are burning in an underground mine and fire-fighting is complicated by extensive roof falls, ventilation uncertainties and accumulations of explosive gas, special action should be taken. The only practical alternatives may be inerting with nitrogen, carbon dioxide, the combustion products of an inert gas generator, or by flooding with water or sealing part or all of the mine (Ramaswatny and Katiyar 1988).

Further information on fire suppression can be found elsewhere in this Encyclopaedia and in various NFPA documents (e.g., NFPA 1994b, 1994c, 1994d, 1995a, 1995b, 1996e, 1996f, 1996g).

Fire Containment

Fire containment is a fundamental control mechanism for any type of industrial facility. Means for confining or limiting an underground mine fire can help ensure a safer mine evacuation and lessen the hazards of fire fighting.

For underground coal mines, oil and grease should be stored in closed, fire-resistant containers, and the storage areas should be of fire-resistant construction. Transformer stations, battery charging stations, air compressors, substations, shops and other installations should be housed in fire-resistant areas or in fireproof structures. Unattended electrical equipment should be mounted on non-combustible surfaces and separated from coal and other combustibles or protected by a fire-suppression system.

Materials for building bulkheads and seals, including wood, cloth, saws, nails, hammers, plaster or cement and rock dust, should be readily available to each working section. In underground non-coal mines, oil, grease and diesel fuel should be stored in tightly sealed containers in fire-resistive areas at safe distances from explosives magazines, electrical installations and shaft stations. Ventilation-control barriers and fire doors are required in certain areas to prevent the spread of fire, smoke and toxic gas (Ng and Lazzara 1990).

Reagent Storage (Mills)

Operations that are used to process the ore produced in a mining operations may result in certain hazardous conditions. Among the concerns are certain types of dust explosions and fires involving conveyor operations.

The heat generated by friction between a conveyor belt and a drive roller or idler is a concern and can be addressed by the use of sequence and slippage switches. These switches can be effectively used along with thermal cutouts on electric motors.

Possible explosions can be prevented by eliminating electrical ignition sources. Electrical equipment operating where methane, sulphide dust or other hazardous environments may be present should be designed, constructed, tested and installed such that its operation will not cause a fire or explosion.

Exothermic oxidation reactions can occur in both coal and metal sulphide ores (Smith and Thompson 1991). When the heat generated by these reactions is not dissipated, the temperature of the rock mass or pile increases. If temperatures become high enough, rapid combustion of coal, sulphide minerals and other combustibles can result (Ninteman 1978). Although spontaneous ignition fires occur relatively infrequently, they are generally quite disruptive to operations and difficult to extinguish.

The processing of coal presents special concerns because by its nature it is a fuel source. Fire and explosion protection information relating to the safe handling of coal can be found elsewhere in this Encyclopaedia and in NFPA documents (e.g., NFPA 1992b, 1994e, 1996h).



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