Acetone is a common industrial solvent used to clean metal surfaces prior to welding. It effectively removes dirt, grease and staining from metals which allows for a better, and sometimes stronger, weld.
Although Acetone is a highly flammable chemical it is widely used in welding workshops, mostly without injury or safety incident. For this reason its fire and explosion risks are often ignored or under-considered. Unfortunately, when acetone fires and explosions do occur in the workplace, the consequences are usually devastating. Serious injuries, burns, toxic smoke inhalation and even death are the common outcomes.
Extreme Flammability
Acetone is highly flammable liquid. The flashpoint of Acetone is -17.8° Celsius, meaning it can easily ignite at room temperature. Liquid acetone is often mixed with water for industrial use to reduce its ignition risk, however it remains highly flammable even when highly diluted.
In workshops, cloths and rags are used to wipe away Acetone from surfaces being prepared for welding. If these materials are not safely disposed of, they create ready fuel for a fire to ignite from a spark or flame within the workplace.
Explosive Vapour
Acetone has a high vapour pressure which means the liquid evaporates quickly. This vapour forms an explosive mixture when it combines with air. Open flames, sparks, static discharge and heat must be kept away from acetone vapour at all times to avoid a violent explosion. This can be difficult in workshops with limited space, where surface preparation and welding are done in close proximity. Even when separation is possible, it is not always effective due to vapour flashback.
Vapour Flashback
Acetone vapour is heavier than air, tending to drop and collect at ground level. This makes it difficult to remove from workshops via overhead ventilation systems. Instead the vapour tends to pool in areas away from the immediate area it is used, reaching explosive concentrations within workshops. Sometimes vapour pools collect at significant distances away from where surface cleaning is being done.
Flashback occurs when a trail of flammable vapour is ignited by a spark or flame, then travels back to the source, such as an open acetone vessel or leak, causing a serious fire or explosion. Vapour flashback makes safely managing ignition risks by physically separating surface preparation and welding functions very challenging.
Explosive Limits
The flammable or explosive range of acetone vapour is 2.6% to 12.8%. This means that it only takes a small amount of vapourised acetone to create an extreme fire and explosion risk.
When acetone vapour is concentrated above the upper explosive limits, reducing the concentration through ventilation or extraction fans will take it down through the danger zone until the concentration is blow the lower explosive limit. What this means is that relatively small concentrations of acetone vapour in the air create the biggest risk for explosions. This can be counter-intuitive for workers who may feel safer when acetone use is light or infrequent.
Hazardous Combustion Products
In the event of a fire, acetone combustion generates a range of dangerous and toxic substances including carbon monoxide, carbon dioxide, flammable formaldehyde and corrosive acetic acid.
Water does not effectively extinguish an acetone fire, so workplaces that use this chemical must have working carbon dioxide, alcohol foam or dry chemical extinguishers to rapidly put out the flames and reduce heat injuries and worker exposure to hazardous combustion products.
Managing Acetone Flammable Risks
Strict safety procedures must be followed for the use, transport and storage of acetone to avoid fires and explosions. Welding workshops should remove all ignition sources, including spark-producing mechanisms from zones where acetone is used, or areas connected to surface preparation stations. Spills must be cleaned up safely and promptly. Used cloths and rags must be removed and safely disposed of immediately. Closed containers of acetone must be stored in a cool and stable environment to avoid violent eruption.
These basic safety processes may be difficult for welding workshops to implement and maintain consistently due to the nature of the work, space restrictions and the equipment used. The best way to avoid an acetone fire or explosion is to remove the chemical entirely from the workplace.
Conclusion
Acetone cannot be used without risk of fire or explosion in any workplace which has sources of ignition. Despite the chemical’s widespread application as a surface preparation solvent prior to welding, it remains a serious hazard for workers. Acetone’s flammable and explosive properties are very difficult to adequately and consistently control.
The prevalence of the chemical in welding workshops can create familiarity and a false perception of safety. Fires and explosions from acetone will happen because of the chemical’s extreme flammability and ready evaporation. And when these accidents occur they can be deadly. The only way to fully protect workers from acetone fire and explosion risk is to use a suitable non-flammable and low evaporation chemical substitute for surface preparation.
