Dust explosions have very extensive damage possibilities and they usually occur in industrial plants leading to personal injuries or fatality, facility/plant destruction and loss of production. Industries are therefore required to equip themselves with the necessary tools and means for the immediate implementations in case the calamity occurs. (Rolf, 2009, pp. 1). This paper describes in details the causes of dust combustion and the necessary steps to take to avoid its occurrences in the industries.
Dust Explosion Prevention and Mitigation
1. Employee awareness creation
- Give the relevant information concerning the hazardous properties of the materials.
- Create awareness and employee training on safety and preventive measures on dust explosions and how to use and apply the industry codes and standards in dust explosion preventions.
- Train employees on the dust cloud explosion safety initiatives and measures to avoid the occurrence of an explosion.
- Introduce and store equipments and facilities that can be used to curb the hazards when it strikes to protect the lives of people and property.
2. Specific points on dust control
Dispersion and escape of dust in the air at the workplace should be prevented and taken into account to avoid explosion. The amount of excess gas produced should be put in check. Equipments and enclosures should be put into place at the scene where gases are handles so as to cub the spillage and drift of the unwanted gasses in a safer way or even avoided in totality. The dust collecting equipment need be built in such a way that the dust collected does not leak in the air. Dust of a larger volume need to be handled with a lot of care by storing and transporting them in containers that do not leak or even drift. The use of wet machinery is one of the most reliable means to use because they have the capability of converting dust into slur. Cleaner processes need be kept separately from the dusty processes so as to allow the safety measures to be set in a specific area.
Dust does accumulate over a period of time in plants and equipments. This is where housekeeping comes in. housekeeping is mandatory and it is advisable to be conducted constantly. This can be conducted by means of:
- By regularly inspecting and cleaning the surfaces such as the floors, roofs, rafters and other equipments.
- Avoiding the piling up of oily rags and other rubbish.
- Avoid cleaning methods which aid in the spread of dust clouds, such as compressing and bowing of air and sweeping with a lot of energy.
- Compressed air among the dust should totally be avoided.
- More proficient dust removal methods should be assigned to all the dust generating machines and other processes. (Department of labor, 1985, pp.13)
4. Elimination of ignition sources
All the explosion sources should be removed from the danger zones. This can be achieved by:
- Avoiding the use of lights or flames and elimination smoking habits.
- Avoiding the use of welding tools and gas operating materials.
- Maintenance at the industry should be conducted at and after all the dust producing machines have been shut down and all the necessary and mandatory precautions have been put into place.
- A work permit should be introduced into the system before such kind of activities have been cleared to go ahead in the industry and this needs to be introduced be completed before the commencing of any type of work in the dangerous area zones.
- A suitable checklist should be introduced to ensure that all the precautions observed at the hazardous areas are safe i.e. the working temperatures should be optimal to enable a safe working environment for the workers involved and avoid the possibility of explosion.
- Electrical and other associated equipments should be bonded and earthed to avoid the increase of electrical and static charges.
- Non ferrous blades should be used in fans which enables the passage of dust.
- Sparking free tools should be used.
- Non magnetic separators should be used in the dust-producing crush mills to stop the intake of ferrous materials.
- Friction sparks should be eliminated in entirely and high speed belts and shafts avoided.
- Hot surfaces should be eliminated to avoid the ignition of dust deposits and layers. (Department of labor, 1985 pp. 16), (Vahid Evadat, 2009, pp. 37), (Rolf, 2009, pp. 2)
5. Fire as a source of ignition
- Settled dust on the ground should not be disturbed while fire fighting exercise is being carried out as this can cause the formation of dust cloud which then triggers and spread fire.
- In another concern, the use of dry powder fire extinguishers should be limited because they do add to the formation of dust clouds. (Rolf, 2009, pp. 2).
6. Spontaneous heating
- The dust which does undergo under unprompted burning should not be stored in large portions as this may cause fire.
- Materials which have the natural heating capability without even the aid of external sources should be carefully handle and stored, i.e. fertilizers, fishmeal, rubber and cornmeal. (Rolf, 2009, pp. 2).
7. Relief of explosion pressure
- The ignition of combustible cloud in enclosed working stations like in a plant or machinery should be avoided as this will cause a pressure built-up and eventually lead to the damage of the plant and premises. (Rolf, 2009, pp. 2).
- The prior release of this excess pressure should be restricted early enough so as to avoid the damage of the plant.
- Explosion reliefs like explosion doors, vents and bursting panels need to be taken into account while installing and providing explosion protection mechanisms. These reliefs have to be ensured that they are properly and effectively functional because the safety factors between the bursting pressure and the vent operation is very small. Explosion products should be ensured that they are discharged; this will ensure that the explosion pressure is maintained at a lower level than the design strength to ensure the vessel protection from explosion defects and effects.
- Explosive reliefs'siting should be handled with utmost care and explosion products safely vented in an area with good ventilation. (Department of Labor, 1985, pp. 18), (Rolf, 2009, pp. 2).
9. Explosion relief vents
- The explosion reliefs should be located very closely to potential sources of ignition and be strong enough to resist the tear and wear which they are prone to in the everyday activities.
- For vents to be active enough, they should be left open though the practice has its limitations because it does not support the product contamination, weather and atmosphere contamination from dust particles.
Examples of closures commonly used:
Burst panels and discs
These are panels made out of different range of materials capable of handling variable amount of pressure at which the materials are able to burst and allow the resistance of pressure from within the plant.
Weather proof and fire resisting materials should be put into place; these includes, brown wrapping paper, polythene sheeting, waterproof papers, cellophane, rubber and metal foil. The provision of cutters and knives at the centre of the panel enables the acceleration of the pressure relief.
- Light hinged doors are equally effective as vents in terms of relieving the pressure brought about by the explosions carrying less rate of pressure. These kinds of doors needs to be lightly locked to tone down the inertia force which holds back the operation. The weight of these kinds of doors should not exceed 10 kg/m2.
- Explosive doors should be securely and strongly fastened to the plant by strong chains to avoid the dangerous missiles from vent closures while in operation.
- Other explosive doors are made of light but much rid covers held in place by friction, spring clips, magnets and thin metal tongues. (Department of Labor, 1985, pp. 18-22),
- Locating the plant in an open air is the key to safe venting. This can be achieved by locating the plant under light and waterproofed construction to aid in the safely emission of burning dust, flames and hot gases.
- The explosion relief vents on the plant located in the chamber should be linked to the one outside by means of strong piping so as to withstand the maximum subjected to it. The pipes used for this purpose has to be short, straight strong and bigger in diameter.
- Relief vents ducts' recommendable maximum length for optimum explosion should have a distance of 6meters long and its cross section must be twice the vent area. It should also be capable of resisting twice the required pressure in the vent area. (Department of labor, 1985, pp. 19-22)
10. Explosion detection and suppression
Suppression method system is usually used to curb explosion in the circumstances where venting method is not applicable. In this kind of arrangement, the explosion detector picks up the initial explosion by detecting the rise in pressure. The sequence is then relayed to the main control point causing gas suppressant to flow killing big explosions intending to flow. This communicates to the plant which enables it to be automatically shut down by the control unit.
Suppressants do contain liquid vapours like halogenated hydrocarbons which are equally toxic and a lot of caution should be put into practice while using them. The plant should be able to withstand the pressure increase of approximately 20 kPa in order to use the suppression system. Explosion isolation should be ensured by preventing dust cloud explosion by means of controlling its spread all over the place. (Department of Labor, 1985, pp. 23), (Rolf, 2009, pp. 2).
this is the detection of pressure waves or flame fronts and the use of appropriate suppressant to put off the fire. The limitation of chemical barriers is that they cannot stop the explosion of pressure from spreading. In this case, the downstream method will be applied as a substitute.
this consists of rotary valves which adequate amounts of blades forming a barrier and screw feeders modified to consistently contain material plugs and rapid shutoff regulators.
With this kind of method, gas explosion prevention is done by replacing the normal atmosphere inert gas by inert gas in the plant and processes whereby dust particles are confined within an enclosure. It works by reducing the oxygen content in the atmosphere of the plant to a level which cannot support combustion. The inert gas neutralizes the dust cloud and prevents it from exploding and also puts off all other possible ignition sources.
Dilute ventilation to enable the provision of fresh air circulation all over the surrounding atmosphere and to control dust concentration. (Department of Labor, 1985, pp. 24), (Rolf, 2009, pp. 2).
12. Friction and impact sparks prevention
- Avoid and control the coalition and striking of metallic materials such as rusty steel surfaces and aluminums as this causes frictional sparks.
- Avoid striking of metallic surfaces, rocks, grit and other surfaces containing flint with equally hard objects as they have a potential of causing ignition.
- Avoid the habit of carelessly dropping objects, and metal equipments and other working tools.
- Overheating caused by misalignment of loosely placed objects should be prevented by the regular equipments maintenance and plant inspections.
- Foreign materials which may trigger ignition in the system should be prevented.
- The use of electromagnets screens and pneumatic separators should not be looked down upon while milling and grinding feed locations.
13. Electronic equipments and instruments
Suitable and correct electronics equipments specification should be ensured to accommodate the intended surroundings of application. This can be done by defining the hazardous location where possible flammable atmosphere might take place.
14. Electrostatics hazards management
The precautions include:
Insulation materials' use restriction:
Nonconductive materials should not be used in the environment where the process is likely to create high surface charges until satisfied that the material is less than 4 kV. Flexible connectors, bags, plastic pipes, coatings, containers and liners are the basic examples of non-conductive objects.
Bonding and grounding:
Conductive materials such as fiberboard drums metal equipments, low-resisting powders, conductive/static dissipative plastic liners and containers and people should be electrically grounded to avoid spark discharges.
Charge reduction by ionization:
The level of electrostatic charges from the particles of powder entering a container is capable of dropping as a result of localized ionization discharges from grounded, sharp, wire and conducting probes.
Charge reduction by humidification:
Relatively high moisture content can lessen the resistance of insulating objects and selective powders and equally increase rate of charge decay to ground. The success of this can only be achieved given that the relative humidity is in excess of 65% and is maintained at a favourable temperature. (Vahid Evadat, 2009, pp. 38), (Rolf, 2009, pp. 2).
15. Fire protection equipment and facilities
Building plans should be shown to the fire fighting service for their own opinions before building and constructing new factories that will produce, handle or even process combustible dust. The installation of water sprinkler system and an alarm connected to the fire fighting brigade control rooms should be entrenched in the plan. Industries should seek the consultations of the best fire extinguishing equipments depending on the hazards type. Precautions should be taken not to directly jet water on the burning dust as this could propel dust and activate the explosion. (Department of labor, 1985, pp. 25)
Laws, directives and technical solutions entrenched in the company's rules and regulations usually just help in avoiding the most common minor dangers. Personal responsibility for every single employee is the most vital thing to consider while handling and storing Hazardous materials and substances in our day to day industries. Any ignorance of the above precautionary measures will definitely lead to a disaster.