We’ve been writing a lot recently on the basics of combustible dust. Makes sense, after all, what safety professional knows exactly how much of their particular dust, in their particular facility under a certain set of circumstances is ok? And, if you don’t yet have a dust collection system, how do you know what you need? Even if you have a collector, do you have the proper Process Hazards Analysis (PHA) to understand your dust’s potential for explosibility?
Plant and facility safety professional customers will often call and say they think they need a dust test but do not know what the next step is. How do I collect a sample? What is a Go/No-Go test?
While we offer a list of testing services to determine the deflagration hazards of dust samples per ASTM International, OSHA (Occupational Health & Safety Administration, NFPA (National Fire Protection Agency) and UN (United Nations), knowing the basic tests can go a long way for tackling your safety needs.
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 practitioners 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
• 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.
OSHA in the U.S. 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:
• Grain elevators
• Food production
• Chemical manufacturing (e.g., rubber, plastics, pharmaceuticals)
• Woodworking facilities
• 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: (see image 1)
** Please note: If you suspect you may have an electrostatically charged dust, collect the sample by using a plastic coated shovel or scoop.
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 420 µm 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 420 µm required if testing metal dusts]
The chart (see image 2) discusses the outcomes for your tested dust. If your test sample is a "Yes, it explodes" then further tests can be run to determine how quickly and how severe the explosion will be (KSt/Pmax Test), followed by testing what concentration of dust in the air will cause a risk of explosion (MEC Test). Next, another test can determine if a spark will cause an explosion (MIE) test.
But, what if your Go/No-Go test result is a "no"? We next look at what temperature it will take make your dust ignite. To find the Minimum Autoignition Temperature (MIT) of a dust cloud in the air, the MIT tests the minimum temperature that would cause your dust cloud to ignite. Next, is the Layer Ignition Test (LIT), which determines the hot-surface ignition temperature of a dust layer. Finally, a VDI 2263 burning behavior test is conducted to determine if a dust will burn and if it does, how quickly it will spread. It is followed up by a UN 4.1 Burn Rate test for additional confirmation.
All of these tests start with the Go/No-Go Test. A comprehensive Process Hazards Analysis (PHA) can apply your test results to real world scenarios at your facility. Better to know what you are dealing with so you can plan safely.
Here are some other tests run for dust explosibility screening:
• Go/No-Go Screening + Combustible Dust Screening Package - Both tests run in tandem as a screening package
• Sample Characterization Test - includes determining the sample moisture content and particle size distribution [>30 grams of samples less than 420 ?m required]
• “Hard-to-ignite” Explosibility Test - Tested as above but with a 400 J ignition source. [>100 grams (~¼ lb) of sample less than 420 µm required]
Unless otherwise instructed, dust testing is performed on the sample as it is received (“as received”) from your facility as mentioned earlier. 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.
Furthermore, please note that per ASTM recommendations (and some NPFA requirements); samples should be tested at a particle size less than 75 µm and less than 5% moisture. Please note that testing materials in a method not complying with the ASTM/EU recommendations may produce explosion severity and explosion sensitivity data that is not considered conservative enough for explosion mitigation design.
AnnMarie Fauske, MBA, is marketing manager, Fauske & Associates LLC. Ashok Dastidar, PHD, MBA, is vice president, dust & flammability testing and consulting services, Fauske & Associates LLC. For more information on your dust testing questions or needs, contact Jeff Griffin at [email protected] or 630-887-5278.
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