Imperial Sugar Shares “The Science of Dust Explosions”

When Kevin Jeffries, Imperial Sugar Company’s corporate safety systems manager, recently spoke about “the science of dust explosions” during Customer Safety Day, he began with a fundamental truth not everyone knows: Under the right conditions, most finely ground organic materials, when dispersed in air, can ignite and lead to an explosion.

“I wanted to bring everyone’s level of awareness up,” Jeffries says. “Since our customers are often handling not just sugar, but flour, cornstarch and other material, my goal was to help them understand the potential hazards and how certain factors relate to their specific operation.”

The potential hazard can be best understood by examining what Jeffries calls the “pentagon of combustible dust.”

Kevin Jeffries, Imperial Sugar Company’s corporate safety systems manager, speaks about “the science of dust explosions” beginning with a fundamental truth not everyone knows: Under the right conditions, most finely ground organic materials, when dispersed in air, can ignite and lead to an explosion.

“A source of ignition, fuel (the dust), oxygen, confinement and dispersion – you’ve got to have all five of these things before you can have a dust explosion,” he says.

While a dust fire occurs when combustible dust is exposed to heat in the presence of air, a dust explosion requires the simultaneous presence of two additional elements – dust suspension and confinement. Suspended dust burns more rapidly and confinement allows for pressure buildup.

Removal of either the suspension or the confinement elements prevents an explosion, although a fire may still occur.

“An explosion happens quickly, think milliseconds … the event often occurs inside equipment – silos, granulators, bucket elevators, enclosed conveyor belts or powder mills,” he added. “Dust accumulations on rafters, beams and building steel can also be a concern.’’

A combustible dust is, technically, “any finely divided solid material that is 420 microns or smaller in diameter,” according to the National Fire Protection Association (NFPA). How dense that dust is will control how combustible it is. Engineers measure the Minimum Explosible Concentration (MEC) inside process equipment – such as conveyors and silos – to see whether the amount of dust dispersed in air is concentrated enough to spread an explosion.

For example, explains Jeffries, with an MEC concentration of coal dust in the air, a 25-watt light bulb six feet away from you would not be visible.

To gauge the likelihood of explosion and the potential intensity of an explosion, engineers also can measure Minimum Ignition Energy (how much energy is required to ignite a dust cloud), Minimum Ignition Temperature (the temperature that will ignite a dust cloud), Kst (the maximum rate of pressure rise) and Pmax (the maximum explosion pressure).

Engineers use some of these calculations, such as Kst and Pmax, to design venting or isolation systems that prevent explosions.

“No one wants an explosion to happen,” he says. “But the best guidelines and standards tell us to design pressure-release or isolation systems, and to design walls that absorb and deflect energy generated by a blast.”

Jeffries emphasizes that no one wants an explosion.

As guests on Customer Safety Day – almost all of whom had engineering or other technical backgrounds – toured the refinery with Jeffries, many had questions about dust-collection systems and other state-of-the-art techniques now in place at the Port Wentworth refinery. Even among engineers, Jeffries says, many people “just don’t fully understand the qualities of combustible dust.”

The consensus standards published by the NFPA aren’t easily found and aren’t widely known within the industry, though they have been around for decades. Adding to the knowledge gap is the fact that different states have different fire codes, and inspectors may have different levels of experience. That’s one reason OSHA recently re-started its National Emphasis Program on combustible dust to allow for more education, outreach and inspection.

Jeffries says helping Imperial Sugar’s customers be more aware of prevention tactics was a primary goal of his “science of dust explosions” presentation. The company’s management and engineering team has gained extensive knowledge from the process-safety experts at Chilworth Technology Group, which was brought in as consultants for the rebuilding of the Port Wentworth refinery.

“If we can give that information to a customer in an eight-hour segment, so he can make a determination for his own facility, then we’ve done our job. We don’t want to be one of those companies that hordes information. We want to share what we know with others.”