The industries in which flammable dusts occur most frequently and in the greatest quantities can be classified into three main groups:1The milling industries where these materials are converted into powders, flours, meals or dusts2The industries that use such powders, flours, meals or dusts3The industries in which metal castings, or articles of wood, cork, plastics, or other materials are smoothed or polished on abrasive wheels, polishing mops or bands, the dust being produced as an unwanted by-product.Precautions must be taken in connection with processes, such as grinding, atomising, conveying, collecting, drying, screening, grading, blending, weighing and packing, in these industries.
Almost all combustible materials can form explosive dust clouds in certain circumstances. Unless, therefore, there is positive knowledge to the contrary it should be assumed that any organic or carbonaceous material may give rise to a dangerous dust. This category includes many naturally occurring products of animal and vegetable life such as fish meal, grain, seeds, coal, cork, malt, starch, wood, sugar and resin. It also includes products of chemical synthesis such as synthetic resins and plastics, dyes and intermediates, fine chemicals and pharmaceuticals. In addition to these organic materials many other easily oxidisable substances form explosive dust clouds. Of these, the metal powders (particularly magnesium and aluminium), have caused the most serious explosions. Of the non-metals sulphur has produced the largest number of explosions.
Make a list of all the materials used in the factory which may form a dust cloud, either as part of the process or by some other means (e.g. accidental discharge). Through Material Safety Data Sheets, dust testing or published explosibility data, identify those materials which are known to be explosible.Conduct explosibility classification tests (Group A/Group B) on the remaining materials to prove non-flammability.Consider possible ignition sources. These might include:(a) heat energy, e.g. heating installations, internal combustion engines, open fire and flame, hot surfaces, smoking, hot work (including welding spatter), laser or other intense radiation sources;(b) electrical energy, e.g. electrical lighting devices, electromagnetic radiation, short circuit, electrical arc,earth fault, conductor fault, lightning strike, discharges of static electricity, loose contact, excessive temperature rise due to overload, induction heating, resistive heating, connection to inappropriate electrical supply;(c) mechanical energy, e.g. friction (binding, rubbing, overheating), ultrasonic, impact, grinding, compression (including adiabatic compression and shock waves);(d) chemical energy, e.g. self-heating, impact and heat sensitive materials such as pyrophoric substances and thermite reaction sparks, runaway exothermic reaction.In particular, consider activities, such as maintenance, where there is the potential for a highlevel of risk.
Consider the outcome of an explosion. Will it spread to other parts of the factory (for example, by propagating as a secondary explosion) or will it be confined to the vessel of origin. Is the explosion likely to give rise to a fire?
Step 3 : Evaluate the risks and decide on precautions
Generally speaking, the outcome of dust explosions are severe; fatalities, serious injuries, extensive building and plant damage, long-term loss of production. In many ways, this simplifies the risk assessment as measures must be taken to prevent the explosion occurring or to mitigate its effects.Explosion prevention(a) Substitute a non-flammable material for a flammable one.(b) Prevent the formation of an explosive atmosphere. This may be by oxygen reduction (inerting) or by reducing the formation of dust cloud below the minimum explosible concentration. Dust suppressed materials, for example, use oil additives to stop dust clouds forming. Screw elevators, unlike belt and bucket elevators, produce no internal dust cloud. In some instances, it may be prudent to stop using a particular piece of plant or operation altogether until the safety issue is resolved.Risk reduction(a) Reduce the quantity of dust cloud to a minimum.(b) Eliminate as many potential ignition sources as possible. For example, use a robust permit-to-work system to prevent ignition through hotwork procedures, including the issue of written instructions for the carrying out of the work.(c) Provide suitable personal protective equipment.(d) Provide adequate training for plant personnel, including the findings of the risk assessment.(e) Maintain work processes in an efficient state, in efficient working order and in good repair.Explosion protection(a) Install adequate explosion relief, discharging to a safe area. Alternatively, use flameless explosion vents.(b) Install explosion suppression.(c) Install explosion containment.(d) Install explosion isolation equipment.
Step 4 : Record the findings and implement them
Record the findings of the risk assessment and share them with the appropriate factory staff.You should be able to shown that:(a) A proper check was made.(b) You considered who might be affected.(c) You have dealt with all the significant hazards, taking into account the number of people who could be involved.(d) The precautions are reasonable, and the remaining risk is low; and you have involved staff or their representatives in the process.
Step 5 : Review the risk assessment and update if necessary
Few plants stay the same. Changes made to the process plant will need to be documented and assessed in relation to its effect on the risk assessment. This applies whether the change is physical, e.g. new equipment installed or change of materials handled; or operational, e.g. certain processes used less frequently.