Dust Explosion Prevention

A dust cloud of flammable concentration can be produced in industry either by design or by chance. For example, lean-phase pneumatic transfer systems handling flammable dust have an almost continual dust cloud present. In processes such as the filling and emptying of dust containers a dust cloud of flammable concentration is produced transiently. However, in certain circumstances, e.g. the collapse of a 'bridge' of dust in a bin or the bursting of a bag at the bagging-off point, a flammable dust cloud can be produced quite accidentally. In view of this, it is not sufficient to design plant such as dust-conveying systems to operate above or below the limits of flammability. In the selection of plant, consideration should be given to such matters as dust tightness, avoiding the formation of dust clouds and to the rapid removal of dust clouds if their formation cannot be prevented. Knowledge of the material’s ignition properties is essential and generic data is freely available from companies that carry out dust explosion testing.

Choice of Plant

Wherever practicable, plant which does not produce dust clouds should be used. Conveying and elevating machinery provides useful illustrations of this principle. Conveyors of the drag-link type transfer dust in a solid mass with the virtual elimination of dust clouds, although the return leg of the conveyor may provide a space in which flammable concentrations of dust will occasionally form. This can be minimised by reducing the cross section of the return leg to a narrow channel in which the plates or links fold up. In bucket elevators, on the other hand, because of spillage from the buckets, the interior of the casing tends to be very dusty, and serious explosions have frequently occurred within them. Where practicable the use of bucket elevators to convey flammable dusts should be avoided. In many plants dust clouds cannot be completely avoided, and safety must be obtained by restricting the spread and effects of the explosions.

Milling Processes

Numerous dust explosions have started in size reduction equipment (mills, grinders, granulators and classifiers), yet some simple precautions can greatly reduce the chance of an ignition occurring. In particular: • Remove ferrous metal from the feed with magnets. • Remove non-ferrous metal and stones from the feed with pneumatic separation, ensuring the separator is regularly emptied and maintained. • Control feed input rate. • Install overload monitoring on the the drive motor and maintain and lubricate bearings regularly. • Check and maintain the alignment of mechanical components regularly. • Periodically check for excessive heating of mechanical components. Infra-red cameras may be useful to quickly identify hot spots in complex manufacturing processes. • Ensure the correct number of V-belt drive belts are fitted and that they are maintained at the correct tension. • All components should be checked for earth connection, including frames and guards. Earth resistance should be 10 ohms or less. • Feed stock should not be kept within the mill room. • Any mechanical or electrical equipment within a designated hazardous area must be suitable for the zone (Ex rated for zone 20, 21 or 22).

Wet-type dust collectors

Many serious explosions have occurred in cyclones and bag filter units, which are the commonest forms of dry dust collectors. Where the use of such collectors is essential they should be situated in a safe place, outside occupied workrooms. Where the dust is an unwanted by-product - for example where articles are ground, sanded or polished on abrasive wheels or similar devices - a well-designed wet scrubber avoids the explosive concentrations of dust inherent in a dry-type collector and offers the safest method of collecting such dust. The collector should be situated as close as possible to the point of origin of the dust in order to avoid explosive concentrations in long ducting systems. Certain materials such as aluminium and magnesium will react with water in a wet collector to form hydrogen. The risk of a hydrogen explosion is negligible during normal operation, provided the water supply and exhaust ventilation are properly maintained. To avoid the formation of flammable concentrations of hydrogen at times when the scrubber is not in use the sludge should be removed from the collector at frequent intervals. Wet collectors must never be allowed to run dry.

Inert gas protection

The flammability of a dust cloud is reduced by decreasing the oxygen content of the medium in which it is dispersed. As the oxygen content is reduced, the minimum hot- surface temperature necessary to ignite a dust cloud is progressively raised; and when a dust cloud is exposed to a source of ignition at a given temperature there is a critical percentage of oxygen below which the dust will not explode. This limit depends on the inerting properties of the other components of the atmosphere. Carbon dioxide and nitrogen are effective inerting agents for carbonaceous materials, but some metal powder may ignite and burn in carbon dioxide and even nitrogen under certain circumstances. Explosions may be prevented by inert gas protection in many plants and processes where the dust is confined within an enclosure by the replacement of the normal atmosphere by an inert gas. In such a system, oxygen is excluded, or the oxygen content of the atmosphere in the plant is reduced to a level at which combustion cannot occur. The use of inert gas ensures that if a dust cloud does form it will not be a flammable dust cloud. In addition the inert gas may extinguish sources of ignition. Careful consideration must be given to the source, chemical composition, and reliability of supply of the inert gas in relation to the flammable material and the plant. The safe oxygen content varies with the size, shape, and duration of the igniting source, the flammable material and the inerting gas. Carbon dioxide is more effective than nitrogen as an inhibitor of ignition in dust clouds of most carbonaceous materials. Certain metal dusts will ignite and propagate flame in an atmosphere of carbon dioxide. Nitrogen is more suitable in such cases, but at elevated temperatures it may be necessary to use argon or a similar gas. Inert gas may be obtained from inert gas generators by burning oil or gas in a strictly controlled supply of air, from adjacent plants emitting waste gases, or, for small units, directly from compressed gas cylinders. Steam is commonly used in drying operations involving sewage sludge. In all cases where inert gas is used as a precaution against the dust explosion hazard it is essential that the gas actually in circulation in the plant is continually monitored for its oxygen content. If the oxygen content reaches an unsafe level the instrumentation should be such that the plant is automatically closed down and warning devices operated. Where metals are ground in an inert gas from which oxygen is entirely excluded a very reactive dust is produced, which may ignite spontaneously on exposure to air. In certain cases this danger may be avoided by the inclusion of a small proportion of oxygen in the inert gas, which reduces the surface activity of the particles. Plant operators should not be exposed to concentrations of an inert gas, owing to its lack of oxygen. Some inert gases may also contain toxic components. Entry to confined spaces with depleted atmospheric oxygen must be rigorously controlled.

Limiting Spread and Effects of

an Explosion

Where explosive concentrations of dust inside the plant cannot be prevented and sources of ignition eliminated, precautions should be taken to ensure that: 1 Any explosion is minimized by limiting the quantity of material involved 2 Any explosion is prevented from spreading within the plant by the use of individual plant units or by the sub-division of continuous plant with chokes 3 The flame and hot gases produced by any explosion are contained within the plant or alternatively relieved to a safe place in the open air.

Explosion Chokes

By far the most common method of explosion isolation is the power-driven rotary valve which will prevent the passage of a blast wave through the valve. Rotary valves used in this capacity should be designed to have high-strength casings and close tolerance blade clearance. The use of explosion isolation tested and certified valves to an appropriate standard (e.g. EN 15089) is recommended. Note that even certified valves may allow particles of burning dust to pass from one section of plant to another. This risk may be reduced by arranging for the power supply to the driving motor to be cut off in the event of an explosion. In dust extraction systems, explosion isolation flaps are often fitted at entry to the dust collection unit. In normal operation, the flap is held open by the airflow into the collector: under explosion conditions, the pressure generated by the deflagration closes the valve against its seat and prevents back propagation down the inlet ductwork. Again, the use of certified valves is recommended. Alternatively, active explosion isolation systems may be installed comprising explosion detection, control system and slam-shut valves or flame extinguishing suppressors. The design and installation of such systems should be left to specialist suppliers, but compliance with the appropriate standard (e.g. EN 15089) should be specified in the purchase contract.