Figure 1. Locations of workers inside the mill at the time of the incident. Circles represent workers who sustained fatal injuries, and the pentagons represent injured workers. Red dotted outlines - buildings collapsed or severely damaged. Yellow outlines - buildings sustained severe damage but accessible to investigators.
A recent incident investigation report of CSB (Ref. 1) revealed a very remarkable and sad fact: 3 years ago, all 19 employees on a nightshift of a corn milling plant (Fig. 1) were either killed or injured in a single incident of dust explosions. It would be more disastrous if this incident occurred during a dayshift because 60 people would be working on a normal dayshift.
The CSB investigation shows that the incident seemingly began with a fire in a mill. The initial fire soon triggered 4 secondary dust explosions in the plant, which resulted in extensive structural collapses. Strikingly, some employees witnessed the initial fire and all the explosions, and miraculously escaped some structural collapses during the explosions.
Milling processes have inherent dust fire and explosion risk. However, it is worth thinking about why no one in the plant was spared in this incident.
While all relevant factors or causes have to be evaluated systematically in order to answer this question, one important factor in this case is plant additions and changes, which constantly happen in many facilities. As shown in Figure 2, the milling plant began in 1991 with a single milling unit (A Mill) constructed at the site. At the time of the incident, 4 other mill buildings (B, C, D, and F milling) had been added. In addition, there had been additions of a train and truck bulk-loading facility, front office, and a multi-purpose building that housed C Mill, packaging equipment, offices, a laboratory, a maintenance shop, and mill control room.
Figure 2. Pre-incident overhead view of the corn mill facility.
Adding a milling unit to an existing plant could introduce ignition and flame propagation hazards. The location of the added mill can also change the equation of dust explosion risk.
It is also noteworthy that the milling processes are complex. Many product streams pass through various pieces of equipment in different mills and on different floors multiple times before bagging and shipping. Apparently, an explosion that happens in one equipment can propagate to other connected equipment.
What Can Be Done to Reduce the Risk?
Suffice it to say that whenever a facility makes a change the hazard and risk of combustible dusts or other flammable/combustible materials should be assessed systematically, and adequate safety measures be implemented based on the hazard and risk analysis.
Unlike building a new plant, there are often more constraints on changing an existing plant or adding a process unit to an existing facility, and on the prevention and mitigative measures. Therefore, when making changes, the potential effects of changes to the fire and explosion hazards/risks and life safety should be carefully examined. Moreover, an existing plant may not satisfy some requirements of today’s codes and standards on fire and explosion prevention and protection. Often, assessing the performance of fire and explosion safety measures for an existing plant based on risk analysis by considering all relevant factors can help developing effective and adequate solutions to minimize the risk of consequences like those in the subject incident.
Ref. 1 Factual Investigative Update, Didion Milling, the US Chemical Safety and Investigation Board, Washington, DC.
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