Factors Affecting Fire And Explosion Risks
The following physical and chemical characteristics of materials can influence the level of risk of a fire or explosion occurs.
Form and physical state
The form or physical state of chemicals, substances, or other materials can significantly influence the level of risk of a fire or explosion. The physical state of a material is generally considered as either solid, liquid, or gas. However, materials can be further categorized as aerosolized droplets, vapors, fumes, mists, powders, dust, or fibers.
Bulk materials in solid, liquid, and gas forms behave differently and present different risks. Liquids spread readily compared to solids and have a greater risk of coming into contact with an ignition source if spilled. Gases present a greater risk as concentrations in air are generally higher than for liquids (and their vapors) and can spread more rapidly. Depending on the vapor density, some gases can flow across surfaces similarly to liquids rather than dissipate quickly. For example, vapors with a density greater than air can move along the floor and spread to adjacent work areas where ignition sources may be present, creating a significant risk in those areas.
Temperature and pressure
Changes in temperature and pressure can affect the properties of a chemical.
The explosive range of a chemical (for instance, its lower and upper explosive limits) can change with temperature. At higher temperatures, the lower explosive limit is usually lower, meaning that the substance is more likely to ignite at lower concentrations in the air. Heating solid or liquid combustible substances can also increase the vapor pressure (for instance, the concentration of vapors emitted) of the substance making it more likely to ignite.
Handling chemicals under pressure increase the risk in several ways. Any loss of containment will occur more rapidly than under normal atmospheric pressure so that more hazardous chemicals are released. Increasing pressure generally increases the temperature of the material, and some chemicals also become unstable at higher temperatures and pressures, causing an uncontrolled decomposition or reaction.
The effects of an explosion can be exacerbated where the fuel and air mixture is contained, for example, in a tank, duct, or pipework, as well as in larger structures like silos, rooms, or buildings. Explosions can be more violent than unconfined, and flying debris (such as from the container or building) can cause serious injuries or death.
Fire risks involving chemical oxidizers
Chemical oxidizers can react violently and unexpectedly with many chemicals such as organic material (for example, wood, paper, cellulose products), hydrocarbon solvents (for example, mineral turpentine, petrol, diesel), and other organic (carbon-based) chemicals (for example,, ethanol, mineral oils).
You should assess any situation where an oxidizer could come into contact with these materials. This includes containers and other equipment used to handle or transfer the chemicals. Oxidizers should be handled in compatible containers and with compatible equipment to avoid a dangerous reaction.
It is important to note that since oxidizers provide oxygen through the chemical reaction, rather than air being the oxygen source, a risk of fire or explosion can still exist even if these materials are handled under an inert atmosphere like nitrogen.
Fire risks from other chemical reactions
Fires and explosions can occur as a result of chemical reactions. Many chemical reactions are exothermic – that is, they give off heat during the reaction. This heat can act as an ignition source igniting any fuels present; pressure can build up in enclosed systems (for example, containers, flasks, and pressure vessels), causing the container to rupture or even explode.
You should assess any situation where incompatible chemicals could interact and cause a dangerous or uncontrolled violent reaction.
Dust explosion risks
Dust explosions present a significant risk in some workplaces. However, they are often overlooked. Dust explosions usually occur where combustible dust (or fibers, for example, from paper, grain, finely divided organic compounds, and metals) have accumulated and are then disturbed and released into the air, coming into contact with an ignition source. Common ways dust can be disturbed include from the wind when opening doors or windows, during cleaning or sweeping up of waste, or using compressed air to blow out material accumulated in crevices, gaps, or machinery.
Dust may also be generated by transferring materials, such as filling the hold of a ship or a silo with a grain (liberating grain dust).
When the dust cloud comes into contact with an ignition source such as a flame, hot surface, or spark, ignition can occur, causing an explosion. Dust-air mixtures can be classified as hazardous atmospheres similar to other flammable materials like vapors from flammable liquids and gases.
Dust clouds can be generated by pressure from an explosion in another area, causing damage and propagation much greater than the original explosion.
Effect of particle size on dust explosion risk
The particles’ size in dust can significantly impact the explosion risk. Smaller particles have a more excellent surface-to-mass ratio and present a greater risk; for example, a block of metal, such as a metal ingot, may be practically inert but could be highly reactive when in the form of filings or shavings, dust, or powder. Similarly, the risk from an aerosol (fine droplets in the air) form of flammable liquid is much greater than the bulk liquid. Processes that generate fine particles, like grinding and milling flour and nanomaterials, can present significant risks. Special control measures may be needed for handling such materials.
The classification of hazardous dust atmospheres is complex and depends on many factors, including the rate of dust dispersion, sedimentation characteristics, and particle size. Further information is contained in the following Australian Standards:
- AS/NZS 4745: Code of practice for handling combustible dust
- AS/NZS 60079.10.2: Explosive atmospheres – Classification of areas – Combustible dust atmospheres
Common examples of the types of industries and processes that potentially present a fire, explosion, or implosion risk are listed in Appendix I.
Some activities, work systems, structures, and equipment not directly involved with the use, storage, and handling of hazardous chemicals in the workplace may create a hazard you need to be aware of when undertaking your risk assessment. These include:
- Hazardous chemicals on adjacent or nearby premises that could be ignited by activities at your workplace and other substances and materials that are not hazardous chemicals but could add to the overall fire load, such as wooden pallets, paper, combustible liquids, or other combustible materials.
- Activities and installations on adjacent premises include the plant operation, equipment, and vehicles, deliveries of hazardous chemicals, personnel movements in routine and emergency situations, visitor access, and the trial of site emergency procedures.
- The proximity of sensitive facilities may be put at risk by hazardous chemicals and during an emergency, such as schools, hospitals, child and aged care facilities, theatres, shopping centers, and residences. These may require special consideration when planning for emergencies.
- The presence of incompatible materials, either other chemicals or the materials that plant, equipment, storage, and handling systems are made of, which could react with the chemicals being stored or handled.
- Foreseeable plant, equipment, and storage systems failures, as well as natural disasters or extreme weather events such as temperature extremes, wind, lightning, or rainfall, including the potential for flooding.
- Other failures could occur and events that may give rise to new hazards or more significant risks. Any examination should be systematic and consider the possibility of human error in the system’s operation.
RISKS FROM CORROSIVE SUBSTANCES
Hazardous chemicals that are corrosive to metals can cause damage to plants and equipment, such as containers, pipes, fixtures, and fittings. Corrosion can lead to leaks or complete failure and loss of containment of the chemical, resulting in severe damage to property, exposure of workers to hazardous chemicals, and potential injury and death.
Compressed and liquefied gases are used as fuel, a source of oxygen, or shielding gases in certain types of welding. The hazards associated with compressed and liquefied gases include fire, explosion, toxicity, asphyxiation, oxidation, and uncontrolled pressure release. Gas leakage is one of the most significant hazards.
Cylinders contain large volumes of gas under high pressure, and precautions must be taken when storing, handling, and using cylinders.
Asphyxia is a condition that occurs when there is a lack of oxygen. This can occur either through:
- consumption of oxygen in the air (burning of fuel or oxidation process such as microbial activity or rusting)
- an accumulation of gases displacing oxygen in the air.
- Inhalation of the chemical affects the body’s ability to use oxygen (for example, hydrogen cyanide can asphyxiate a person by binding to hemoglobin in the blood following inhalation).
All gases, including fuel gases (for example, hydrogen, acetylene, and liquid petroleum gas) and inert gases (for example, argon, helium, and nitrogen), are an asphyxiation hazard in high concentrations.
Too little oxygen in the air we breathe can cause fatigue and, in extreme cases, death. Using compressed and liquefied gases can result in dangerously low levels of oxygen. For example, gases that are heavier than air can accumulate in low-lying areas such as pits, wells, and cellars, and gases that are lighter than air can accumulate in high areas such as roof spaces and lofts. Working in an enclosed or confined space with inadequate ventilation, where hazardous vapors can accumulate, is a potential asphyxiation hazard.
You should identify possible causes of asphyxiation in your workplace. Asphyxiation can occur in welding and allied processes from gas slowly leaking in a work area.
Compressed air can be hazardous and should be handled carefully by workers. For example, sudden gas release can cause hearing damage or even rupture an eardrum. Compressed air can also profoundly penetrate the skin resulting in an air bubble in the bloodstream known as an embolism. Even a small quantity of air or other gas in the blood can be fatal.
Ensuring workers are trained to properly handle compressed air can eliminate many associated risks. Training and work procedures should emphasize the safe use of air tools and safeguard against the deliberate misuse of compressed air. Also, maintaining air receivers properly prevents the potential for an explosive rupture.