Kudos! to the U.S. Chemical Safety Board (CSB) for providing an update on their investigation of the devastating explosion at the Imperial Sugar/Dixie Crystals refinery near Port Wentworth, Georgia. As I’ve noted in previous posts, because the CSB makes it part of their business to provide regular update for the public—even if they don’t have much at all to report—their effort increases the likelihood that worker and environmental safety and health issues will be covered by the press. In turn, it means that these critical public health topics stay in the public’s and policymakers’ consciousness.
At today’s press briefing, we learned that there may have been a history of dust explosions at the refinery, with at least one occuring about three weeks ago. The Associated Press reports that company officials and the CSB confirmed that the previous (much smaller) explosion was associated with a build-up of dust in the refinery’s rooftop dust-collection system. The AP story says “a metal fragment caused a spark when it got sucked into a dust collector and ignited the dust inside it.” A spokesman for Imperial Sugar said that no one was injured in the earlier incident and it caused minimal damage.
Today, the CSB’s Stephen Selk, P.E., offered a primer on dust explosions.
His lesson on combustible dust goes like this:
“It is necessary for five elements to be in place for a dust explosion to occur.
“First is the presence of a combustible dust itself. That can be almost any organic material – grain flour, plastic, corn starch, pharmaceuticals, and even powdered metals such as aluminum. And as was the case here in Savannah sugar particles are a combustible dust. An important parameter is the particle size. Finer particles are more likely to be both ignitable and dispersible. Additional parameters are particle shape and the molecular composition of the substance itself.
“A second needed element is a source of oxygen. Because air contains appreciable amounts of oxygen, air is all that is necessary to support an explosion.
“Thirdly, the dust needs to be dispersed into the air.
“Finally, some energy source is required to ignite the mixture. That may be something with as little energy as static electricity or a stronger energy source such as an open flame or an electrical fault.
“A final element is confinement. And because buildings have walls, ceilings, floors and roofs, they create confinement. However, another form of confinement may be process equipment and even ducting. It can be ironic that ducting used for dust extraction and other equipment such as dust collectors can themselves be conducive for the initiation of dust explosions.
“An important attribute of dust explosions is that they may propagate. In such instances some primary event occurs that kicks up larger amounts of dust that may have accumulated and disperses the dust into the air. When this happens the stage can be set for catastrophe. A very large flammable dust cloud ignites with devastating consequences. In other instances an initial explosion may simply propagate as the blast wave ahead of a rapidly advancing flame front – the fireball – which disperses more dust and ignites as the fireball expands.
“When a dust explosion occurs in a building, walls may blow out, floors may heave, and ceilings may collapse. This can all occur in a few seconds. It is therefore not unusual for local fire protection and electrical systems to be almost instantly crippled. Occupants may at first find themselves burned, or blown about, or struck, or among rubble. At worst they may experience all of that. At first they may find themselves in darkness or the obscurity of smoke. But fires initiated by the thermal energy of the explosion may follow and grow. The scene is set for tragedy.”
Engineeer Selk goes on to describe what they’ve observed and learned to-date at the scene of the Imperial Sugar disaster:
“The tops of two silos [where sugar was stored] are missing or largely missing [their tops] suggesting that at some time in the sequence of events, explosions occurred within them.”
Referring to a photograph taken by a CSB investigators from an elevated platform, he notes
“to the left side of the picture there are the remains of a burned out building. It is my understanding that this interior building dates back to a very early time in the refinery’s evolution. It was constructed many decades ago and while it had brick walls and a steel truss much of the construction was timber. …The building is almost completely consumed by fire.”
It’s refreshing to see a government agency—the CSB—keeping us apprised and providing mini-tutorials on basic workplace hazards like combustible dust. I cringed, however, when I read part of the AP’s story which said, the CSB
“has just begun looking into the refinery blast after criminal investigators determined Friday the explosion was accidental — caused by clouds of tiny sugar dust particles that, when airborne in confined spaces, can ignite like gunpowder.”
I’m sure the reporter meant that criminal experts had concluded that the powerful blast was not due to a bomb or other nefarious evil plot. His use of the term “accidental” suggests, however, the combustible dust explosion was unexpected or happened by chance.
As professional engineer Selk of the CSB explained in his primer, a combustible dust explosion needs five elements to occur. If a company is manufacturing a product and using a production method that fosters these elements, the owners and managers better have a plan in place to prevent these factors from converging. If they don’t, we shouldn’t accept claims that the event was unexpected or it simply happened by chance.