By Craig Carlyle, director at Maintenance Transformations
A silent menace lurks within industry that is mostly forgotten but can strike with deadly results. The explosive properties of many dusts are not widely recognised, but in the right (or wrong) conditions, the dusts created by many materials will explode if mixed with air in a cloud formation, with a source of ignition present. Dust explosions can occur in grain silos and elevators, flour and icing sugar mills, malthouses, thermal coal dryers, and milk spray-dryers. There have been many dust explosions overseas and within New Zealand. One, in a Masterton plastics factory in 1965, killed four people.
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 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. Metal powders (particularly magnesium and aluminium) have caused the most serious explosions. Of the non-metals sulphur has produced the largest number of explosions.
The industries in which flammable dusts occur most frequently and in the greatest quantities can be classified into three main groups:
1. Milling industries where these materials are converted into powders, flours, meals or dusts.
2. Industries using powders such as flours, meals or dusts.
3. 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.
WorkSafe guidance on dust is broken down into processes and materials but the archived Department of Labour ‘Dust Explosions in Factories’ (1985) document provides a compelling overview of the risks and controls. Mitigation methods are detailed as well as hazard ratings of common dangerous materials. Zero rated materials are tabled, providing a reference point for assurance.
So, what can you do if you are unsure of the situation in your plant? After recognising the dust situation, either in the environment, or contained in vessels, research may provide existing guidance for your material. If no information exists, a Dust Hazard Analysis (DHA) can be conducted, but there is some cost to that. The DHA should identify safe operating ranges, check safeguards are in place to mitigate the hazards and should recommend additional safeguards where warranted.
In some cases, if there is legitimate doubt, it may be more cost effective to carry out the mitigation solutions regardless.
Craig Carlyle is director at Maintenance Transformations. His expertise lies in the practical application of maintenance and health and safety management systems in the workplace. He is also a life member of the Maintenance Engineering Society of NZ.
The information and opinions within this column are not necessarily the views or opinions of Xpress Engineer NZ, NZ Engineering News or the parent company, Hayley Media.