For decades, wood shops and wood manufacturing facilities have been using high-pressure compressed air to blow down dust from elevated surfaces during cleanup. As this practice seems effective as dust is cleaned from a surface, the reality is that the combustible dust has been relocated into harder to reach and more dangerous areas. This is known as the combustible dust relocation program, where combustible dust which is readily reachable with a vacuum hose or brush is suspended into the plant air by compressed air and redistributed and settles in unaccessible surfaces.
NFPA (National Fire Protection Association) 664, Standard for the Prevention of Fires and Explosions in Wood Processing and Woodworking Facilities, restricts the pressure for localized compressed air usage to 30psi and this is only after the surfaces are cleaned as much as practical with vacuum and brushes with minimum dust suspension. However in real life cleanup procedures, mill plant compressed air exceeding 100psi is more likely used. As NFPA is the fire code of Canada, violations to the fire code can cause serious injury and in some cases criminal prosecution.
The main concerns on high pressure blow down for cleanup include:
- The high-pressure compressed air breaks down the dust particles into smaller particles creating a higher risk of explosion and increased levels of respirable suspended particulates.
- Dormant dust is not explosive until it has been disturbed, i.e. by the compressed air. The easiest way to create a dust explosion is to use compressed air to suspend combustible dust into a flame/spark. This is exactly what is happening every time compressed air is used to suspend and distribute the fine dust.
- NFPA (National Fire Protection Association) is recognized by the Canadian fire code and limits the accumulation of combustible dust on flat surfaces to 1/8”. This is recognized as the minimum dust depth required for the suspended dust to create a flash fire/explosion. So consider every time someone using compressed air for cleanup, it only takes a spark to create the sequence of events that can led to a catastrophic explosion.
For explosion risk, inspectors reference primary and secondary explosions. For wood shops and wood processing facilities, dormant dust is not a risk until disturbed and suspended. Once a pocket of dust becomes suspended (this could be as small as a handful of dust), The expanding fireball gains enough energy during expansion to dislodge larger quantities of dust, in some cases catastrophic. This is called the secondary explosion and can and has been deadly in wood processing facilities. The question is why would someone knowingly use compressed air to start the sequence of a catastrophic and deadly dust explosion.
The answer is human nature takes the easiest route. When using compressed air, the combustible dust appears to be cleaned as it “disappears.” This could not be future from the truth. All the compressed air has done is relocate dangerous combustible dust to another area. The combustible dust did not leave the building.
Perhaps education on the science of dust explosions would help. The math is easy, if the suspended dust reaches a concentration of 40 grams/cubic metre, it has reached what is called the minimum explosion concentration (MEC). As it is difficult for the lay-person to identify the air-borne concentration, safety professionals and NFPA have determined that 1/8” of dust depth when suspended can reach the MEC. In reality, the 1/8th inch MEC suspended dust concentration is thick enough that you could not see a 25w bulb six feet away or you could not see beams and columns on the opposite side of the facility. All are good examples, however, when doing cleanup activities and to error on the side of safety, it should be considered that any suspended dust cloud can be explosive and certainly exceeds OSHA’s STEL or TWA for respirable suspended particulate.
How do we solve this problem?
The best way to solve this problem is to install a dust extraction hood at every location where combustible dust is liberated under normal operating conditions. This is also a requirement of NFPA. For this solution, dust is captured at the source and does not leak and deposit to unsafe levels. A properly designed and installed dust extraction system can provide dust capture at the source where no dust levels exceed 1/8” between cleanup. It is expected that some dust will leak into the plant air space even with a properly designed and operating system as the complexity of some capture areas, such as a CNC machine, cause multidirectional dust patterns with sometimes complex moving hoods. It should also be noted that some hoods, for example at head pully of a conveyor, capture nearly 100 per cent of the dust.
In the absence of a properly working dust extraction system, manual cleanup will be required. The best method is by vacuum. The vacuumed dust is captured in an explosion protected enclosure and is physically removed the plant, which 100 per cent lowers the MEC and explosion risk. The main issue with vacuums is the accessibility and cost of larger vacuum trucks. Although perfect for the job, the cost can be prohibitive.
The second best manual solution is to install a vacuum system just for combustible dust. For this system, the vacuum receiver is located outside due to explosion risks and metal piping is distributed throughout the building with connections to which vacuum hoses can be attached. These systems can also be cost prohibitive.
The third solution for manual cleanup is the use of explosion proof portable vacuums, which are less costly than vacuum systems, however, they are heavy and difficult to move around the plant.
The most cost-effective temporary manual cleanup system is the use of portable vacuums (shop vacs). The use of portable vacuums can be safe, providing acceptable operating conditions for the hazardous area classification exist. Hazardous area classification follow three main guidelines.
- Zone 20 would be an enclosure where a combustible dust concentration is normal. For example, inside a dust collector. A shop vac could not be used inside these type of enclosures.
- Zone 21 is where combustible dust layers are above 1/8” and can be easily suspended, shop vacs could not be used.
- Zone 22 is where dust layers are normal under 1/8” and no suspension would result in an explosive suspended mixture.
Shop vacs cannot be used in zone 20 or 21. Shop vacs could be used to vacuum zone 22 areas providing the shop vac motor is in an unclassified area and only the hose is brought into the zone 22 area. In the case where a portable shop vac is the chosen solution it is important to recognize that over 50 per cent of ignition sources in wood manufacturing occur from a hot bearing. If the bearings are covered with dust, this is a perfect condition for a flash fire which can be the catalyst for a catastrophic secondary explosion. So if a shop vac vacuum program results in no dust is on the bearings, the risk of a primary/secondary explosion is significantly reduced.
How do we change human nature?
Once an operator has used compressed air for cleanup, it is unlikely they would be amicable to reverting to a push broom and shovel, even when they have been educated that the air hose is a major hazard. Imagine if a cleanup crew did not have any experience or even knew that an air hose was available for cleanup and they were trained to only use brushes, brooms and shovels, the cleanup would be done many times safer than blowdown.
This exact scenario is unfolding in western Canada, where facilities are successfully eliminating, due to the high risk, compressed air blowdowns and replacing it with manual cleanup with vacuums, brooms and shovels. The new hires have never seen blowdown and hence are quite happy doing the cleanup as trained without compressed air. The problem continues to be the false convenience of operator blowdown versus the harder, more effective manual cleanup, hence the new hires and new methodologies. In some cases, manual cleanup programs have been modified where manual cleanup is faster than blowdown. Considering that high-pressure blowdown does nothing to reduce the potential dust concentrations in the plant, we should consider that the new manual cleanup programs are significantly safer and many times more effective removing the dust from the building versus blowdown.
The methodology and examples described in this article are for illustration purposes only. As with any modified safety program, it is advisable to seek the experience of combustible dust specialists to assist and advise what works best for your facility.
John E. Bachynski, P.Eng, is the president of EPM Consulting Ltd.
This article is part of Dust Safety Week 2022. To read more articles on dust safety, click here.
