A sugar dust explosion in an enclosed conveyor caused by many years of neglect
‘I am amazed at the extent of destruction. I understood the hazard, understood the risk […] but never imagined the propagation that occurred at Port Wentworth’ – this is how a member of the management staff at Imperial Sugar Refinery commented on the disaster during a hearing. Years of neglect at the Imperial Sugar Refinery in Port Wentworth led to a series of explosions that killed 14 people and seriously injured another 36. The plant was completely destroyed.
Management knew of the danger but ignored it
Let us go more than 100 years back, when the story began. The Imperial Sugar Refinery complex was established in 1917 and quickly became one of the largest sugar refining and packaging facilities in the United States. At the time, hazards related to sugar dust explosions were not yet known. The sugar industry became aware of the problem only in 1925, when William Gibbs published a scientific paper entitled ‘The Dust Hazard in Industry’. From then on it was clear that sugar dust explosions may have fatal consequences. However, the management of Imperial Sugar Refinery did not recognize this fact until 1958, when a communique was issued on the threat of a sugar dust explosion and the need to prevent it through proper process design and the implementation of good practices that would minimize the accumulation of dust in the production area. Although the management of the plant were aware of the danger, they failed to act accordingly and implement actual changes in the production facilities operated by the corporation.
A photo with the location of the explosion circled. The photo shows a building with three protruding silos. The packing house, where the secondary explosions began, is to the right of the silos.
Footage from nearby CCTV cameras shows the enormity of the explosion. Frames from the recording that captured the primary explosion are shown at the top, whereas frames from the recording of the secondary explosion are shown on the bottom.
One could say that they remained blind to the issue for the next 50 years, as no changes were implemented in the area of explosion safety and intermittent fire incidents in their plants were downplayed. Then, on 7 February 2008, at approximately 7:15 PM, everyone gained a sudden clarity of vision – but by then it had been too late. They were blinded by the glare of an explosion that completely destroyed a large portion of the Port Wentworth plant, taking a deadly toll.
Video – catastrophic effects of a sugar dust explosion
WHAT CAUSED THE EXPLOSION?
For many years, the conveyor belt used to collect sugar from three silos passed through a concrete tunnel. The tunnel had a large volume and was ventilated, which kept dust concentration below the lower explosive limit.
A year before the tragic event, management of Imperial Sugar Refinery decided to tightly encase the said conveyor belt inside metal panels. However, the enclosure was not equipped with a dust collection system, which, as it later turned out, was a serious error. From that point on, the concentration of dust in the air could easily exceed the lower explosive limit.
A sketch of the installation transporting material to and from the silos. The conveyor belt where the first explosion occurred is marked in yellow.
Archival photo of the conveyor prior to its enclosure (top) and steel plates forming the tunnel enclosure, torn out by the force of the explosion, marked with arrows.
This was made all the easier by the fact that sugar clumps often became lodged between the sugar chute and the conveyor belt, causing product to spill onto the conveyor frame and floor. It also caused dust clouds to form. Investigators concluded that the same situation must have occurred on the day of the disaster.
On February 7, 2008, at approximately 7:15 p.m., the sugar dust encountered an ignition source – most likely an overheated belt feeder bearing that led to the explosion. The explosion tore apart the tunnel enclosure. Accumulated settled dust Archival photo of the conveyor prior to its enclosure (top) and steel plates forming the tunnel enclosure, torn out by the force of the explosion, marked with arrows. The sugar was lifted and ignited by an accelerating fireball. The resulting dust clouds provided fuel for a chain reaction of secondary explosions that travelled throughout the building. The concrete ceilings collapsed, releasing large amounts of sugar and sugar dust straight into the flames, dramatically increasing the force of the explosion.
How could the explosion have been avoided?
‘We can implement various solutions to reduce the risk of explosion related to conveyor belts constructed inside concrete tunnels under silos, as was the case at the Imperial Sugar Refinery. One such solution is the previously mentioned dust extraction system. A line system to monitor temperatures of bearings is another solution that’s frequently recommended. The simplest thing we can do is ensure regular cleaning, e.g. using a central vacuum cleaning system,’ says Mariusz Balicki, an experienced explosion safety specialist at WOLFF GROUP.
Balicki also mentions that using conveyor enclosures to protect the transported product from contamination is a fairly common practice. However, this is usually limited to installing canopies. This solution is safer from the perspective of dust explosion risk, as well as much cheaper. We don’t know if there was a rationale in favour of building a closed tunnel in this case, or if it was a case of overzealousness.
Three inch (about 7.62 cm) thick ceiling slabs were torn from their steel supports by the pressure of the explosion.
The enclosure around the conveyor referred to above was installed in order to reduce the risk of the product becoming contaminated during transport. However, the impact of the new tunnel on the risk of explosion was not analysed in this case, and this risk was considerable.
European law explicitly requires the explosion risk assessment to be updated if changes are made to an installation which may affect the level of explosion safety. Such an assessment would have undoubtedly revealed the associated risk. What the management would have done with this knowledge, however, is a different matter. Would they have followed the recommendations in the documents, or would they have disregarded them?
Why are secondary explosions more dangerous than primary explosions?
When an explosion occurs, the blast wave propagates in all directions simultaneously and seeks the quickest possible outlet. It therefore travelled not only through the horizontal tunnel, but upwards as well, through the space between silos No.1 and 2. It then encountered the most vulnerable element of the entire structure – a vertical brick wall. After destroying that wall, the blast wave broke through into the staircase that lay beyond it, into every floor of the building. As the pressure wave made its way into the packing house, it blew the deposited sugar dust up into the air. The flame wave following right behind the pressure wave therefore encountered another explosive atmosphere, the explosion of which was the direct cause of death of most of the victims, according to CSB’s report.
‘The explanation of this fact is related to the energy of the ignition source of the primary and secondary explosions. Usually, only a small amount of ignition energy is sufficient to trigger a primary explosion – this can often be a single spark, an electric arc or heat from an overheated machine part. In the case of secondary explosions, the explosive atmosphere is ignited by the wave of flames and pressure generated by the primary explosion. This means that the energy of the secondary explosion is much higher than the in the case of the primary explosion from the very start, and worse yet, it will increase more rapidly. Regular cleaning, which I mentioned above, could have prevented this tragedy,’ states Mariusz Balicki.
One of the photos documenting the deposited dust that was not cleaned. Of note is the fact even a thin layer of dust can lead to tragedy, let alone the depicted amount.
Prevent secondary explosions! Video explains the secondary explosion process. This type of explosion is more powerful and more destructive than the primary explosion.
The CSB’s report revealed a number of irregularities and negligence related to the manufacturing process and cleanliness at the Port Wentworth site. In consequence, large amounts of sugar dust were deposited throughout the production area. Some of the most egregious oversights present on the site included:
• devices for transporting sugar and maize starch had not been designed or maintained so as to minimise the release of sugar and sugar dust into rooms
• the dust extraction system in packaging buildings was not sufficiently effective, and in addition an explosion had occurred in one of the filters three weeks prior to the explosion. The decompression panels on this filter did work properly on that occasion, but on the day of the explosion the filter was still out of use, further reducing the effectiveness of dust removal
• apart from the dust extraction system, the room ventilation system was faulty as well. Furthermore, it turned out that pipelines forming part of this installation had been improperly made, causing them to become clogged with sugar. All of this caused the fans to run well below the required performance curve.
‘The settling of dust around the work area is a negative phenomenon, but often cannot be fully eliminated from the manufacturing process. That’s why it’s so important to not only ensure the extraction of dust, but also perform regular clearing operations using vacuum solutions, such as the use of vacuum cleaners. Unfortunately, the CSB’s report revealed that dust deposited around the Imperial Sugar Refinery site was cleaned up extremely infrequently, and even when such operations were performed, they were done using compressed air. As a result, deposited dust would be blown up into the air during every cleanup, and would first mix with the air, each time creating an explosive atmosphere, and then settle back down, often in locations where access for cleaning was even more difficult. Had a dust cloud encountered an ignition source during such cleanup, the explosion would have occurred not in 2008, but much earlier’ – continues Mariusz Balicki.
Why did the management keep downplaying the threat for so long?
Because ‘we haven’t had any explosions for 30 years…’. This is a quote which we have heard very frequently at WOLFF GROUP from representatives of various manufacturing companies over the 25 years of the group’s existence. Unfortunately, the management of Imperial Sugar Refinery had the same mindset.
In this particular case, the facility experienced multiple sugar fires over the years, caused by overheated bearings or electrical equipment. However, none of these incidents carried consequences that were serious enough. Because of this, the company’s board of directors, management and employees in all Imperial Sugar Refinery plants downplayed the threat posed by sugar dust. Only a small number of facilities were blast-proofed, and evacuation plans were inadequate. Besides, the whole area of the plant was inadequately prepared for evacuation. When the explosion cut off the power supply to the interior lighting, it was then very difficult to evacuate in the dark maze of damaged corridors and stairs. This was exacerbated by the fact that the plant’s employees had not completed any evacuation drills beforehand.
An interesting fact is that a risk assessment performed at the Port Wentworth facility by the Zurich Services Corporation insurance company in May 2007 also failed to identify any risk of dust explosion. That’s why no actions were taken to improve the plant’s safety during a security review in October 2007.
The photo shows the torn down brick wall, through which the explosion reached the packing house, raised the deposited dust up into the air and started propagation of the secondary explosion.
