In Prof. Paul Amyotte's "An Introduction to Dust Explosions: Understanding the Myths and Realities of Dust Explosions For a Safer Workplace," Amyotte offers a section on Practical Guidance.
"These observations help to explain the advice given by experienced industrial practictioners on the matter of acceptable combustible dust layer thicknesses. Their comments, although anecdotal, have a firm foundation in the physics and chemistry of dust explosions. Scientific underpinning by the aforementioned difficulties in physically dispersing and chemically reacting excessively thick dust deposits is intrinsic to the following expressions:
There's too much layered dust if you can see your initials written in the dust
There's too much layered dust if you can see your footprints in the dust (Anonymous, 1996. Personal communication, with permission)
There's too much dust if you can't tell the color of the surface beneath the layer (Freeman, R., 201). Personal communication, with permission)
I tell my plant manager to write their name on their business card. It's time to clean up when they can't read their name because of layered dust. (Anonymous, 2012. Personal communication, with permission)"
So, what is a combustible dust? You might be wondering this before you worry about how to ship it off to be tested. Per the Canadian Centre for Occupational Health and Safety:
What is a combustible dust?
Essentially, a combustible dust is any fine material that has the ability to catch fire and explode when mixed with air. Combustible dusts can be from:
most solid organic materials (such as sugar, flour, grain, wood, etc.)
many metals, and
some nonmetallic inorganic materials
Some of these materials are not "normally" combustible, but they can burn or explode if the particles are the right size and in the right concentration.
Therefore any activity that creates dust should be investigated to see if there is a risk of that dust being combustible. Dust can collect on surfaces such as rafters, roofs, suspended ceilings, ducts, crevices, dust collectors, and other equipment. When the dust is disturbed and under certain circumstances, there is the potential for a serious explosion to occur. The build-up of even a very small amount of dust can cause serious damage.
The technical definitions for combustible dust vary. In Canada, one example is Alberta’s Occupational Health and Safety Code which defines combustible dust as "a dust that can create an explosive atmosphere when it is suspended in air in ignitable concentrations".
The Occupational Safety and Health Administration (OSHA) in the United States defines combustible dust as "a solid material composed of distinct particles or pieces, regardless of size, shape, or chemical composition, which presents a fire or deflagration hazard when suspended in air or some other oxidizing medium over a range of concentrations.
What workplaces are at risk for a dust explosion?
Dust explosions have occurred in many different types of workplaces and industries, including:
Chemical manufacturing (e.g., rubber, plastics, pharmaceuticals)
Metal processing (e.g., zinc, magnesium, aluminum, iron)
Recycling facilities (e.g., paper, plastics, metals)
Coal-fired power plants
Dusts are created when materials are transported, handled, processed, polished, ground and shaped. Dusts are also created by abrasive blasting, cutting, crushing, mixing, sifting or screening dry materials. The buildup of dried residue from the processing of wet materials can also generate dusts. Essentially, any workplace that generates dust is potentially at risk."
So, have something that might be hazardous in your facility? You need a simple test to find out if its explosible. That's a "Go/No-Go Test". Collect a dust sample and find out if and what it takes to ignite. Air sampling is not necessary to determine whether or not a dust is combustible.
Dust testing is performed on the sample as it is received (“as received”) from your facility. It may be screened to less than 420 μm (40 mesh) – OSHA’s and NFPA’s demarcation of a “dust” – to facilitate dispersion into a dust cloud. Particle size may vary widely depending on the sample.
It's easier than you think:
A Go/No-Go Screening Test, based on ASTM E1226, “Standard Test Method for Explosibility of Dust Clouds”, is an abbreviated set explosion severity testing at two or more dust concentrations to determine if the sample is explosible. This test is generally performed with samples tested ‘as received’ or sieved with >100 grams (~¼ lb) of sample less than 420μm required.
A Combustible Dust Screening Test is based on VDI2263 and UN 4.1 combustion testing. This test is to determine if a dust in a pile supports self-sustaining flame propagation. [>30 grams (~1oz) of sample less than 420μm required; >300 grams (~2/3 lb) of sample less than 420μm required if testing metal dusts]
It's that easy. If you would like to know more about all available tests or have further questions, please contact Jeff Griffin, firstname.lastname@example.org, 630-887-5278, www.fauske.com.