Understanding Dust Explosions (and Determining Combustibility)

Dust Explosion Terminology

In order to understand dust explosion risks, it's important to familiarize yourself with the terminology used. Below is a list of several key terms* used by many manufacturers and experts:

• Minimum Exposible Concentration (MEC): The minimum concentration of combustible dust suspended in air, measured in mass per unit volume, that will support a deflagration.
• Minimum Ignition Energy (MIE): The lowest capacitive spark energy capable of igniting the most ignition-sensitive concentration of a flammable vapor–air mixture or a combustible dust–air mixture as determined by a standard test procedure.
• Kst: Rate of pressure rise for suspended combustible dusts. An index is used to categorize dusts with certain Kst values.

• Pmax: Maximum pressure created by a deflagration of suspended combustible dusts.
• Limiting Oxygen Concentration (LOC): The amount (%) of oxygen required by a suspended combustible dust to support a deflagration.

Each of these product characteristics helps manufacturers like us to understand how best to design systems to prevent a deflagration or explosion.

Determining if Your Material is Combustible

Upon learning and understanding these terms, it’s important to know whether your material is classified as a combustible dust. There are a few ways to determine this. You can:

• Confer with the manufacturer/supplier
• Check online resources (NFPA Standards)
• Send a sample of your product to a testing center. Most testing centers will be able to provide you with all of the information above. As part of ASTM E1226, these test centers will perform a “Go/No-go” screening test to determine whether or not your product would be classified as a combustible dust.

If determined that your product is not a combustible dust, the owner/operator of the process shall maintain documentation demonstrating the dusts are not combustible or explosible. (NFPA 652). If determined that your product is a combustible dust, the next step would be to perform a Dust Hazards Analysis (DHA). A DHA should only be performed by a qualified person.

Dust Hazards Analysis (DHA)

Per NFPA 652, the qualified person who is leading or performing the DHA should be familiar with the hazards of combustible dusts. Typically, a team performs a DHA. For some processes, this team might be as little as two persons, or for larger, more complex processes it can require more. This team is comprised of a variety of persons (such as facility operators, engineers, owners, equipment manufacturers and consultants) whose background and expertise can include the following:

1. Familiarity with the process
2. Operations and maintenance
3. Process equipment
4. Safety systems
5. History of operation
6. The properties of the material
7. Emergency procedures

A couple of other things to be aware of relating to a DHA are:

• For existing processes, with the exception of the food industry, DHAs should have been completed by September 7, 2020.
• A DHA should be reviewed and updated every 5 years.
• For all new processes handling combustible dusts, a DHA should be performed.

Addressing Dust Explosion Risks and Hazards

There are two primary methods to address explosion risks and hazards identified during the DHA: a prescriptive approach and a performance-based approach. In this post, we will be focusing specifically on the prescriptive approach.

Per NFPA 652, explosion protection systems shall incorporate one or more of the following methods of protection:

1. Oxidant concentration reduction (in accordance with NFPA 69)
2. Deflagration venting (in accordance with NFPA 68)
3. Deflagration venting through listed flame-arresting devices (in accordance with NFPA 68)
4. Deflagration pressure containment (in accordance with NFPA 69)
5. Deflagration suppression system (in accordance with NFPA 69)
6. Dilution with a non-combustible dust to render the mixture noncombustible

Here at De Dietrich Process Systems, we utilize the oxidant concentration method. This may be more commonly referred to as "inerting" or "inertion."

When looking at the fire triangle, we are essentially removing the oxygen side of the triangle in order to not create a combustible atmosphere. For those unfamiliar with the fire triangle, a fire requires 3 things: fuel, an ignition source, and oxygen.