Combustible Dust Series: Part 1 – How Explosive Is Combustible Dust?

As we learned in the introduction of this combustible dust series, combustible dust can cause catastrophic flash fires and explosions, resulting in loss of life and serious injuries. In many of these tragic incidents, workers and managers were unaware of the potential for dust explosions and their serious consequences. A 2006 Combustible Dust Hazard Study conducted by the U.S. Chemical Safety and Hazard Investigation Board (CSB) researched 140 known combustible dust materials. They found that 41% of these materials' Material Safety Data Sheets (MSDSs) did not warn users about potential explosion hazards. This inadequacy underscored the need to recognize the potential explosiveness of various forms of dust materials. Tyndale’s Vice President of Corporate Strategy and Technical, Scott Margolin, explains the importance of understanding the key measure of the explosivity of combustible dust – its Kst value.

What is a Kst Value?

The mathematical abbreviation “Kst” stands for the German words for “Constant” (Konstante) and “dust” (staub). Technically speaking, Kst is the maximum rate of pressure rise generated when the dust is ignited in a confined testing enclosure. As Scott points out, Kst values help determine the combustible dust hazard that may be present in your facility by quantifying the potential explosiveness of various types of materials. OSHA’s Hazard Communication Guidance for Combustible Dusts lists some of the materials from which flash fires or explosions could occur if processed in powdered form:

• Metal dust, such as aluminum and magnesium
• Wood dust
• Plastic or rubber dust
• Biosolids
• Coal dust
• Organic dust, such as flour, sugar, paper, soap, and dried blood
• Certain textiles

The list above is not all-inclusive, and not all these materials are equally explosive. The Kst value measures the relative explosion severity of a dust when compared to other dusts. A Kst value is a single number and, as Scott explains, the higher the number, the more explosive the dust is. The chart below shows dust explosion classes based on Kst ranges and typical combustible dust materials that fall within those classes.

* OSHA CPL 03-00-008 - Combustible Dust National Emphasis Program.

** NFPA 68, Standard on Explosion Prevention by Deflagration Venting.

It must be noted that the actual Kst value is specific to the sample tested. The severity of a combustible dust explosion is also dependent on other variables, such as the size and shape of the dust particles, moisture content, ambient humidity, available oxygen, and the concentration of dust in the air.

How Meaningful is a Low Kst Value?

As the chart above indicates, Kst values are divided into four classes, St 0 through St 3. Classes St 1, St 2, and St 3 – with Kst values above zero – indicate that the substance is flammable. An St 3 class signifies high explosivity, suggesting this class of combustible dust has the potential to “take your roof off and blow your walls down,” as Scott describes. However, even St 1 class dust materials can ignite and cause a flash fire throughout your facility.

In fact, most catastrophic multiple-fatality combustible dust events in the United States have occurred from low Kst value dusts. One tragic example is the 2008 sugar dust explosion and subsequent fire that occurred at a sugar refinery near Savannah, Georgia, which caused 14 deaths and left multiple workers with severe burns. Sugar dust falls into the St 1 category with a “weak explosion” characteristic. However, in this case, the initial explosion set off a chain reaction of subsequent explosions as the accumulated dust throughout the facility became airborne from the initial explosion and also ignited.

Knowing the Kst value of the combustible dust in your facility is essential to help determine the risk of a devastating incident. And you must recognize that any Kst value above zero poses a flash fire hazard calling for the use of flame resistant clothing to protect against injuries. Additional steps are required to identify and protect against this potentially deadly hazard. Stay tuned for Part 2 of this series to learn about implementing a dust hazard analysis to identify the specific combustible dust hazards present in your facility.