The advent of fiber laser cutters and flexible manufacturing cells is creating growth opportunities for metal fabrication shops. However, the new processes and heavier demands on growing shops may also increase the potential for a combustion event.
Because metal shops are inherently high-risk environments for combustion, risk mitigation must be a top priority. To manage these hazards, there are three points every shop owner should know:
Know Your Regulations and Standards
Guidance for managing combustible dust is provided by the National Fire Prevention Association (NFPA). These standards are frequently adopted by cities, states, and municipalities, and OSHA has been known to identify them as industry best practices during inspections. A good place to start is to review these three key standards:
- NFPA 652, Standard on the Fundamentals of Combustible Dust
- NFPA 654, Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids
- NFPA 484, Standard for Combustible Metals
Under NFPA Standard 652 (2019 edition), all facilities that produce or handle combustible dust are required to complete a dust hazard analysis (DHA) which includes a risk assessment and mitigation plan.
Know Your Process and Airstream
The first step in a dust hazard analysis is to identify all dusts, fumes, and gasses produced in your facility and their combustion risk. Laboratory testing may be necessary to determine this information. Be sure to review all areas of your operation and all dusts present, including dust and fume mixtures. For many thermally generated processes, the dust that is produced may change over time. Or, metals that are stable in one process can be volatile when oxidized or mingled with other metals. Collecting a representative sample of dust will ensure you have accurate test data to use for making mitigation strategy decisions.
Because metal fires can be very challenging to extinguish, NFPA 484 provides specific guidance, such as:
- Restricting which dusts and fumes can be collected together
- Dedicating work cells, ducting, and dust collectors to separate processes
- Applying fire-extinguishing agents – such as water, carbon dioxide, or argon – that are appropriate for the particular metal in your process
Know Your Mitigation Options
A fire and explosion risk management strategy should consider both preventive and protective measures. Prevention methods help reduce the likelihood of an event, while protective methods are intended to limit the damage if one occurs.
You can integrate many of the following mitigation options into your new or existing dust and fume collection system. The combination you choose may depend on factors such as your risk tolerance level, downtime concerns, and resources.
Housekeeping — NFPA 484 requires that metal dust be removed from the dust collector at least daily and possibly more often, depending on the process. Also, schedule regular housekeeping of facility surfaces to reduce dust accumulation. Be careful to clean out-of-sight areas, because an initial explosion can suspend hidden dust leading to a larger secondary event.
Collector Location — NFPA recommends locating your dust and fume collector outside your facility, and for some dust types this is a requirement. An outdoor location, however, gives you additional mitigation strategies. For example, a fire in an outdoor collector, properly isolated, could be allowed to burn itself out.
Dedicated Equipment — Separate volatile and reactive metal dusts from each other. If you work with multiple types of metals, consider dedicating dust collection equipment to each type. Point-of-use dust and fume collectors can help accommodate this strategy. Ensure you label your dedicated dust collectors and train employees on appropriate use.
Spark Control — Prevent sparks from reaching the fuel source (dust) in the collector. Because air velocity is high through a dust collection system (up to 4,500 fpm), very long lengths of duct may be necessary for active spark detection systems to be effective. A good alternative may be a passive spark arrestor – a device that creates turbulence to accelerate the rate at which sparks extinguish in the duct before they can reach the collector.
Explosion Venting — Specialized panels installed on a dust collector and designed to break open during an explosion, releasing explosive materials into a dedicated safe area or “exclusion zone.” Note that equipment, controls, or areas where people could congregate should not be located within this zone.
Explosion Suppression — This equipment senses rising pressure in the dust collector and sets off a chemical suppressant to halt a combustion event. Although more expensive, explosion suppression devices are very effective at stopping initial as well as secondary events.
Isolation — Isolation devices are used to limit the impact of deflagration to the dust collector. There are many types of isolation devices and strategies, many of which will depend on your dust types and facility layout. Be aware that deflagrations involving metals can grow faster than some equipment can react, so consult with the device manufacturer about the appropriate selection for your application.
Fire Extinguishing System — Choose a fire extinguishing system that is compatible with your metals and processes. Water-based systems may be lower in cost but can require significant clean-up after an event. Carbon dioxide or argon-based systems may allow for faster recovery, but potentially at a higher cost.
Summary
The owners of manufacturing facilities processing combustible metals need to understand their risks and develop an appropriate mitigation strategy. The DHA process helps metal shop owners address these risks early which may help reduce the likelihood and consequences of a combustion event. By knowing what your combustible dust is, where it occurs, and how to mitigate the risks it presents, you can develop a mitigation strategy that supports new growth opportunities.
