In order to "screen" for the possibility of dust cloud explosibility in your facility, we perform a Go/No-Go Explosibility Screening Test. Based on ASTM E1226, "Standard Test Method for Explosibility of Dust Clouds", this test is an abbreviation of the explosion severity test method looking at a limited number of dust concentrations to determine if the sample is explosible. This test is generally performed with samples tested "as received" or sieved with a 40 mesh (420μm) screen and using one 5-kJ chemical igniter as the ignition source. [>100 grams (~¼ lb) of sample less than 420μm required]
In a previous post: "How To Collect and Ship Combustible Dust Samples For Testing", we discussed the simple steps for getting your samples to a lab for testing. The chart provided in this article discusses the outcomes for your dust tested. 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"? Well, 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:
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.
In our next article: "Combustible Dust Basics, Part II: What Testing Do I Need?" we will discuss more detail on the why and the how of available tests.