Dangers of fire and explosion in asphalt mix plants
Fires and explosions have occurred at asphalt mix plants in Canada over recent years, including in B.C.
What is the potential risk?
A fire or explosion can occur when a source of ignition, such as a propane torch, is used in areas where combustible or flammable products are present. Combustible materials are materials that will burn via fire. Flammable liquids have a flashpoint below 37.8°C (100°F).
Sources of ignition in asphalt mix plants include:
- Propane ignition torches (e.g., Tiger Torch), used to thaw plugs of congealed bitumen in pipes or pumps
- Electrical equipment (e.g., monitors, control circuitry, and pumps)
Is bitumen a flammable or combustible material?
Bitumen — also known as liquid asphalt, or asphalt cement (AC) — is the heavy residue by-product of oil refining. It is considered a combustible material. However, bitumen contains traces of light hydrocarbons, which are flammable. The very slow breakdown of bitumen also continues to generate low levels of light hydrocarbons. Off-gassing of the light hydrocarbons occurs when asphalt is handled at the elevated temperatures (approximately 150°C or 300°F) used in asphalt plants. This results in flammable environments in the headspace of AC tanks that make it critical to control ignition sources in and around the tanks.
Lighter hydrocarbons can also flow into piping associated with an AC tank, extending the fire hazard. Further, a “breather” vent on a bitumen tank may extend the fire hazard to the plume zone around that discharge opening.
What other flammable or combustible materials may be present?
Diesel fuel has been used as a cleaning solvent at asphalt mix plants. Diesel becomes flammable in hot (above 52°C or 126°F) environments. Diesel can vaporize in an AC tank and be ignited. Sparking in steel conveyors, as well as hot work (including welding, or using torches or grinders) contribute to the risk of ignition.
How can the risk be reduced in the workplace?
As an employer, it’s your responsibility to regularly inspect your workplace and ensure your safe work procedures and practices control risk. The following measures can help reduce the risks posed by using torches in flammable and combustible work areas. They generally follow the hierarchy of controls.
Manage flammable hazards:
- Establish fire hazard zones in and around AC tanks and ancillary equipment.
- Eliminate the use of diesel fuel for cleaning:
- Hang bitumen (liquid asphalt) reception hoses immediately after use to effectively drain residues before they congeal.
- Manually clean conveyors before fresh asphalt mix cools.
- Substitute less flammable cleaners for diesel.
Manage ignition sources:
- Eliminate or effectively control the use of torches in fire hazard zones.
- Develop safe work procedures, including the use of an LEL (Lower Explosive Limit) monitor, to minimize the risk of ignition while a torch is in use.
- Ensure that electrical equipment in use is rated for the type of hazardous location where it is being used. Refer to the Canadian Electrical Code for area classification and rating information.
Eliminate or reduce the need to heat pipes, valves, and pumps with a propane torch:
- Insulate all piping, valves, and pumps that handle bitumen.
- Use an engineered heating system (e.g., oil jacket, heat trace, AC piping) to reduce the likelihood of bitumen plugs.
- Remove bitumen plugs in supply lines by use of a high-pressure water jet system. All tanks and supply lines must be isolated (e.g., disconnected) from the plant system and at ambient temperature prior to initiating the work. Mixing water with hot bitumen must be prevented to avoid a potential explosion from rapid vapour expansion.
Use alternative methods to spot-heat pipes, valves, and pumps:
- Use steam, or electrically powered heating, blankets, or tracing tape to heat pipe that is plugged by solidified bitumen. Those methods can also be used to prevent plugging.
Where can I find more information?
- Workers Compensation Act
- Section 21(2)(e), on employers’ duty to provide information to workers
- Occupational Health and Safety Regulation
- Part 5, Chemical Agents and Biological Agents — Section 5.27, Ignition sources
- Bitumen Safety Code (Model Code of Safe Practice Part 11: Safety, health and environmental aspects of design, construction, operation, inspection and maintenance of bitumen manufacture, blending, storage, distribution, product handling and use, and sampling), Energy Institute
Learn more about managing risk in your workplace.