Combustible dust is a very dangerous and very legitimate threat that can create fire hazards or even cause an explosion. Manufacturers need to be aware of and address the intrinsic hazards when machining, grinding, drilling, or polishing materials like magnesium, titanium, aluminum, stainless steel, carbon fiber, and iron oxides.
According to NFPA 484 Annex // A.220.127.116.11, combustible metal dust is “a particulate metal that presents a fire or explosion hazard when suspended in air or the process specific oxidizing medium over a range of concentrations, regardless of particle size or shape.”
A system that addresses the process requirements, materials, and the volumes processed must be put in place. When constructing equipment that will be in contact with combustible dust or hazardous materials, it must be done in compliance with HazLoc Class II disciplines and applicable N.F.P.A., A.N.S.I., and A.S.M.E. regulations. Customized systems are normally particular to the application and can be sized correctly for the conveyance, collection, and control of the ratios, materials, and expected volumes of debris that will be recovered.
A general comprehension of the problems and solutions as they relate to developments that are currently occurring in systems configurations and industry best practices demands an explanation for why such devastating events can occur. Since we are working to protect workers and make plants safer, we will also be reviewing measures that can be taken to avoid such events.
Direct: Fires related to combustible dust happen whenever combustible materials are exposed to an ignition source. This can happen during machining, or after machining when materials have ignition sensitivity levels that have the ability to cause a deflagration.
Transfer: Combustible dust can also become a problem during the transfer for collection. It can affect duct work elbows and other restricted joints and can create a spark moving the ignition source towards a collection point as a result of high speed impact. The ignition source can also be created by non-grounded mechanisms building up an electrostatic charge, exposure to electrical motors, or other equipment that could possibly emit sparks.
Collected: A further removal of the ignition to additional materials in the process stream can occur as a result of a spark or ember being transferred to a collection location and maintaining its ignition energy.
Accumulated Residuals: You can also transmit an initial deflagration to secondary areas when layers of dust accumulate over time. If the dust particles are suspended as a dust cloud, then you would create a worst case scenario with an explosion easily being created as the deflagration will gain significant and rapid expansion due to the higher volume of combustibles in the area.
Suspension: Explosions from dust happen when combustible materials are suspended in an air/fuel concentration that is consistent with rapid ignition transmission. If even a primary explosion is caused by the initial deflagration, then the shockwaves associated will extricate any and all dust that has collected on walls, overhead beams, duct work, machinery, or collection vents. Once these materials have been extricated, they will go airborne and contribute to a much larger secondary explosion due to the presence of the initial flame front ignition source created by the first deflagration. The devastation caused by this second explosion has resulted in several injuries, deaths, and major destruction of property all across the United States.
Comingled Materials: Safety Data Sheets (SDS) hardly ever refer to the intrinsic hazards of finite particles of dust that are created during the machining process. They don’t mention the minimum ignition energy (MIE) and minimum ignition temperature (MIT) thresholds and barely mention the problem of reactivity to other materials. In industries like aerospace, usually there are many materials in a waster stream. For instance, drilling and assembling aircraft structures can create titanium, carbon fiber, stainless steel, and aluminum in many different combinations. Comingled materials such as these will cause a secondary handling problem that SDS specifications do not mention. Using a single collection system to recover different materials from different operations should be analyzed with sample testing. The sample testing will determine if a volatile combination of materials with lower MIE than each by themselves is present.
Housekeeping: Housekeeping will add to the overall cost of manufacturing operating expenditures, but it will heighten worker and plant safety. You will need special equipment to remove all combustible dusts from an area. Sometimes, entire areas need to be shut down in order for access to be possible. Since this is the case, a lot of cleaning duties are not performed as often as they should be. This only increases the risks associated with combustible dust that has accumulated. If you want to minimize risks, then efforts must be made to provide safety awareness materials, safe handling protocols, and training. For instance, “blowing off” debris from recessed areas using plant compressed air should be avoided. The resulting dust in suspension can easily create an explosion with the right conditions.
Testing: One of the first steps when dealing with any debris created within the production waste stream is to include combustible dust testing of the materials for explosive severity and ignition sensitivity as they are generated in the work environment. It is imperative to send a sample or samples to an independent lab so they can be tested under N.F.P.A. Code 68. It is mandatory to refer to this data when designing explosion protection and fire prevention equipment. It’s also mandatory when designing process systems with enough capacity to safely support subsequent collection, conveyance, and containment of materials being recovered.
Take a Proactive Approach; Collect-Control-Contain
To optimize collection efficiencies and transfer capabilities of heavy combustible dusts, modern systems have implemented both high volume air flow and high vacuum. The capability to control and collect the transfer of debris and dust is directly related to the size, volume, weight, specific gravity, and surface area of the material being addressed. Vacuuming in itself does not give the means to act on any materials unless there is significant air volume available to create the motive force behind transfer. Generally speaking, dust collectors depend on high volumes of air flow and, as a result, impart minimal vacuum on materials to maintain their velocities in collection ducts.
Collect Dust As It Is Created
Most hazardous dust migration has been eliminated using special high volume-high vacuum/dust recovery systems to collect debris simultaneuous to generation, further containing the recovered materials and minimizing the burden of housekeeping. In the aerospace industry, complete recovery of drill chips and dust at the work piece is currently being used on many different projects as well as in automotive applications. Recovery of debris has been achieved within assembly and machining operations which have the same process.
Control Dust While It Is Being Transferred
Recovering combustible particulate solids must assure that air/material ratios never approach critical Minimal Explosive Concentrations (MEC) which could support ignition resulting in a deflagration. This could travel either up or down stream of the event. MEC ratios can vary based on materials and process requirements, however, the speed at which materials are transferred and the separation of these materials by excessive air volumes creates a means to isolate one particle from the next. High volume rate transfer also maintains the materials in suspension. This minimizes their contact with duct work, and thus, reduces the build-up of fines, clogs, static, and sparks.
Contain Dust for Safe Disposal
Several terms apply to the initial separation of the combustible dusts from the recovering air stream such as Centrifugal, Vortex, or Gate type systems. Initial contact between recovered dusts and the receiving receptacle will act to slow the materials in the air stream to the point that heavy materials drop out of the air stream before making contact with any filtering media. General dust collectors employ filter “bag-house” configurations that have high pressure air jets to back flush the filters occassionally to maintain collection abilities as the finer materials in these types of systems have a tendency to migrate into the filter media. Rotary valves or gates at the bottom of the recovery receptacles allow recovered materials to be collected for disposal.
Items which may have been considered as “OPTIONAL” in the past should now be considered as MINIMAL requirements in systems conveying and collecting combustible dusts in compliance to HazLoc Class II. Terminology may differ between industries and suppliers, but the intent to provide safety in the workplace remains the same.
Industry Best Practices Include:
- Grounding – and incorporating non-spark producing elements, materials, motors, switches.
- Spark / Heat detection – includes multiple high temperature rise sensors.
- Explosion detection – detect first pressure wave of an initial explosion ( see isolation below ).
- Explosion venting – pressure relief and / or rupture disk, vents explosion toward a safe area.
- Flame / Fire / Deflagration Suppression – appropriate to the material(s) being encountered.
- Flameless venting – prevents flame travel beyond location of occurrence.
- Isolation-Explosion gates – prevents flame travel “up-stream” or to other process areas.
- Minimized debris contained for safe daily removal.
- Plant personnel education, awareness and training.