Combustible dust explosions or fires resulting from combustible dusts can be devastating events. The frequency of these events in North America and the destruction, injuries and loss of life that they have caused is alarming. A common trend with the occurrence of these events is when hazard mitigation strategies have been neglected or ignored and when general housekeeping practices are lacking.
This is the negative side of things, but fortunately combustible dust fires and explosions are preventable. Our fire safety standards in Canada recognize combustible dust hazards and outline requirements to control the threat.
Fire protection and life safety professionals need to be able to recognize a potential combustible dust hazard, ask the appropriate preliminary questions and (if required) come to a determination as to how to advise the building owner / operator on methods to mitigate the hazard by properly applying the applicable Fire Code requirements.
The subject of combustible dusts and their associated processes / hazard mitigation strategies can become quite in-depth and will change on a case-by-case scenario as processes can be unique and material properties vary widely. As such, this blog focuses on the basics of approaching a combustible dust hazard and is not meant to be all-inclusive. This blog focuses on the combustible dust requirements in Division B, Section 5.3 of the National Fire Code of Canada – 2015 (NFC). Depending on your province / region, different Codes / standards may apply, and it is the responsibility of the Code user to determine the applicable requirements.
What Are We Dealing With?
Is the dust combustible?
The National Fire Code (NFC) defines “combustible dust” as: “dusts and particles that are ignitable and liable to produce an explosion”.
NFC, Division B, Sentence 5.3.1.1.(1) indicates: “This shall apply to buildings or parts of buildings where combustible dusts are produced in quantities or concentrations that create an explosion or fire hazard.”
The above NFC Sentence serves as a great starting point as it invokes the Code user to ask two very important questions when encountering a dusty environment or dust producing process:
- Is the dust in question combustible?
- Is it produced in a quantity or concentration that will create an explosion or fire hazard?
In some circumstances it may be fairly easy to come to a conclusion, for example in a woodworking facility. However, there may be other circumstances where dust material is being produced, or is proposed to be produced, and its combustibility properties are unknown. This is where the owner / operator of the building will need to employ one of two methods to determine if a combustible dust hazard exists:
- Historical or published data representative of the materials / process conditions
- Analysis of representative samples of the material being used to determine its properties (typically a laboratory analysis)
Although not referenced in the NFC, an excellent resource as to how to determine if you are dealing with a combustible dust and how to perform a Dust Hazard Analysis (DHA) can be found in NFPA 652 “Standard on the Fundamentals of Combustible Dust”. Additionally, NFPA 654 “Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids” is another great resource for information on how to approach combustible dust safety measures, including, but not limited to, Dust Hazard Analysis, risk assessments and facility and process design.
If it is determined through appropriate analysis that the dust is non-combustible or is not being produced in quantities or concentrations to create an explosion or fire hazard, then the application of NFC Section 5.3 is not applicable.
The dust is combustible and there is a confirmed hazard: now what?
Once it is confirmed that there is a combustible dust hazard in the building, the dust hazard will be required to be addressed using the applicable provisions in Section 5.3 of the NFC.
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How Can We Reduce the Risk?
The following is a brief synopsis of the various Articles in Section 5.3 of the NFC that stipulate methods to control a combustible dust hazard. If assessing a combustible dust scenario in accordance with the NFC, you will need to refer to the specific Code requirements in Section 5.3 for the complete details.
Dust Removal
One of the main causes of combustible dust explosions / flash fires is due to building owners / operators ignoring good housekeeping practices and allowing combustible dusts to accumulate on surfaces in the facility. The fire protection and life safety professional will need to take note of the facility’s dust removal and housekeeping practices.
A significant hazard with poor combustible dust housekeeping practices is that, if an initial combustible dust explosion occurs at a source, this initial explosion creates a pressure wave and can cause the accumulated combustible dust on the surfaces in the facility to be thrown up into the air. This can then cause a secondary explosion and it is this secondary explosion that can be extremely violent due to the large dispersion and ignition of the combustible dust that has not been properly disposed of.
NFC Division B, Article 5.3.1.2. outlines that building and machinery surfaces are required to be kept clean of combustible dusts and discusses methods to safely remove the dust from the facility.
Dust Collecting Systems / Dust Collectors
If combustible dusts are being produced in a building, the NFC identifies that the accumulation as well as the suspension of dusts in air need to be kept at a safe concentration.
This can be achieved through the installation of a dust collection system / dust collectors. The NFC identifies that dust collection systems be designed in conformance with good engineering practice, be non-combustible in its construction and not create sparks upon physical contact in the fan assembly.
Article 5.3.1.4. provides two options for the location of dust collectors. One option is for dust collectors having a capacity greater than 2.36 m3 / s to be located outside the building and have “explosion venting” to the outdoors.
The NFC provides a second option for locating a dust collector indoors, provided one of the following conditions are met:
- The dust collector is provided with explosion venting to the outdoors as specifically described in the Article
- The dust collector is equipped with an explosion prevention system, or
- The dust collector is located in a room constructed as fire separation with a fire-resistance rating not less than 1 hour
Additionally, there are circumstance where a building owner / operator may wish to exhaust air from the dust collector back into the building (for example, saving on heating costs). If this is done, Article 5.3.1.4. indicates that the dust collecting system be designed so that the returned air will not create an explosion hazard inside the building. Exhaust fans and any ancillary equipment will also be required to automatically shut down in the event of a fire or explosion within the dust collector.
Bonding and Grounding
Controlling the build-up of static electricity on equipment and machinery in atmospheres containing combustible dusts is extremely important to prevent static electricity from becoming an ignition source.
Article 5.3.1.4. of the NFC discusses grounding and bonding equipment and machinery in a combustible dust atmosphere. As such, the building owner / operator will need to be asked about the methods they have used to satisfy this requirement, including any sign-off documentation related to this (e.g.; electrical engineers sign-off or any authority having jurisdiction sign-off with respect to electrical safety).
Explosion Venting / Explosion Prevention Systems
As noted above in this blog, Section 5.3 of the NFC outlines options to mitigate the effects of a dust explosion through two methods: explosion venting and explosion prevention systems.
Buildings, and processes within buildings, can obviously be unique. Depending on the situation, the NFC gives the option of using one of the methods noted above. The desired approach will depend on many factors, but most often it is the practicality of one versus the other.
NFC Article 5.3.1.6. outlines the requirement for a building with a combustible dust atmosphere to have explosion venting to the outdoors. Simply put, explosion venting allows for pressure relief during an explosion event to prevent critical structural and mechanical damage. If this option is used, the NFC points to NFPA 68 “Explosion Protection by Deflagration Venting” as a guide for good engineering practice.
NFC Article 5.3.1.7. discusses the requirement for an explosion prevention system if explosion venting is not an option or is impractical. There are various types of explosion prevention systems that use different methods of detecting and controlling an explosion event such as spark detection and suppression, pressure sensors and chemical isolation, oxygen reduction, etc. The NFC indicates that an explosion prevention system shall be designed in accordance with good engineering practice such as outlined in NFPA 69 “Explosion Prevention Systems”.
Electrical Interlocks
It would not make a lot of sense if building operators / employees were capable of using dust producing equipment (for example, a table saw in woodworking shop) without the dust collection system operating at the same time. This would defeat the purpose of having a dust collection system. As such, NFC Article 5.3.1.8. identifies that equipment required to have a dust collecting system be “interlocked” so that the equipment cannot operate unless the dust collecting system is operating.
In other words, the table saw used as an example above should not be able to be turned on unless the dust collector is turned on (one is electrically interlocked with the other).
Separators
Spark producing foreign material mixing with combustible dust is an explosion waiting to happen. NFC Article 5.3.1.9. identifies that “separators” are required to stop the entrance of foreign material in any equipment located in an atmosphere of combustible dust. An example of this could be a magnetic separator strategically located upstream within the process to trap any tramp metal before it enters the process.
Ignition Sources
The NFC Article 5.3.1.10. seeks to control ignition sources in general. It very clearly indicates that unless controlled in a manner that will not create a fire or explosion hazard, sources of ignition are not permitted. Sources of ignition include, but are not limited to, open flames, smoking, cutting and welding, hot surfaces, frictional heat, static, electrical and mechanical sparks, spontaneous ignition, heat-producing chemical reactions, and radiant heat.
Woodworking Operations / Grain Handling and Storage Facilities
NFC Subsection 5.3.2. and 5.3.3. discuss specific requirements for woodworking operations and grain handling and storage facilities, respectively. While the intent of this blog is not to delve into specific operations, it should be noted that the general dust producing process requirements previously discussed are applicable in addition to these process specific requirements.
Building / Electrical Permits
The installation of a dust collection system and the associated building and life safety systems will likely require the submission of a building permit application, drawings, manufacturers’ equipment specifications, etc. Anyone dealing with the installation or upgrade of these systems will need to inquire with their local Authority Having Jurisdiction (AHJ) as to the requirement for permits, including physical final testing and sign-off documentation required by the AHJ.
It is highly recommended that the AHJ be consulted very early in the process to iron out the details and understand their perspective.
And Finally…
If not treated with respect and not dealt with in a manner that complies with fire safety regulations, combustible dust processes in buildings can pose a significant threat to not only life safety but property protection and business continuity. By applying and implementing the appropriate codes and standards specific to the combustible dust producing process, building owners / operators can reduce their risk of an explosion and the subsequent harm that it can cause.
The subject of combustible dust safety can become quite interesting as the variety of processes, equipment and safety approaches regarding the subject are plentiful. Hopefully, this blog post will help people to be more aware of this fire safety hazard and will serve as a good starting point to address the hazard when it is encountered.
