Combustible Dust- Minimum Explosive Concentration (MEC)

A common question that I receive is; if I have been operating with this material for years why have I not had any incidents? The underlying cause for a combustible dust incident is achieving the minimum explosive concentration (MEC) in a dust cloud in the presence of a credible ignition source.  Companies can operate for days or decades before seeing this situation. The particles involved have a specific flammable range where the air fuel mixture has to be between the minimum and maximum in order to ignite the cloud. Most people have experienced this with gasoline powered small engines on lawn mowers. The motor will not start because there is not enough fuel (too lean) or too much fuel (too rich).

Understanding this characteristic is very important to prevention of incidents. Ensuring the process does not intentionally create dust clouds with material transfer or clean-up activities will reduce the risks substantially. By simply maintaining dust accumulations below the mass required to create the minimum explosible concentration you can ensure that secondary explosions will not occur in your main work areas. The dust quantity can be easily calculated using the volume of the space (m3) and the MEC (grams/m3). Dust layer depth can be calculated using the bulk density of the material.

As a general guideline for materials, organics such as wood fall into a 1/8 of an inch layer depth maximum, whereas metals being denser have 1/32 of an inch layer depth maximum accumulation.   Understanding MEC will also assist you in designing process equipment to operate outside of the explosive range of the material to prevent primary explosions. If the process equipment operates inside the explosive range, then appropriate explosion protection control devices should be installed.

This article is not intended to be a definitive explanation on combustibility parameters. It is just a primer to allow people to become more aware of the components of an effective Combustible Dust Management Program, and to understand which questions to ask when seeking assistance with setting up a program.