Dust explosion identified as cause of major sugar plant blast

21st February 2013

An explosion at Imperial Sugar’s plant in Port Wentworth, Georgia, US on 7th February, left 12 workers dead and a dozen more in critical condition in hospital with burns. 

The blast occurred at about 7pm when there were about 100 workers on site. It was felt in neighbouring towns. 

On 17th February, Chemical Safety Board (CSB) investigations manager Stephen Selk gave an update into the organisation's investigations to date. He started by highlighting concerns that the CSB has had about dust explosions for more than four years. 

“In 2003 the Board investigated three catastrophic dust explosions. One at West Pharmaceutical Services in Kinston, North Carolina, where plastic powder that had accumulated above a suspended ceiling exploded, killed six and gravely injured many others. At CTA Acoustics in Corbin, Kentucky, phenolic resin – another plastic powder – exploded killing seven and again injuring many others. And at the Hayes-Lemmerz automobile wheel plant in Indiana, aluminium powder exploded killing another worker. That plant has since closed. Both the other plants had to be demolished and rebuilt.” 

After investigating these three explosions in just one year the CSB undertook a larger study of the extent of the industrial dust explosion problem. The Board identified 281 fires and explosions over a 25-year period that took 119 lives and caused 718 injuries. Some 24 per cent of these incidents were in the food industry. 

“Pursuant to its findings the Board made several recommendations – including recommendations to the Occupational Safety and Health Administration (OSHA) – which OSHA has so far partly acted on. But the tragic event that occurred here in Savannah demonstrates that the problem of dust explosions in industry has yet to be solved. It is a problem that requires further attention,” he said. 

After describing how CSB investigators arrived 18 hours after the blast and how they were currently still conducting interviews and examining damage, Selk gave a primer on dust explosion theory. 

“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 aluminium. 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. Fourthly, 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.”

Events at Savannah

Following his primer, Selk turned to the circumstances of the Imperial Sugar Company refinery explosion in Savannah. To begin with he showed a photograph in which three large sugar storage silos were visible. 

“The granular table sugar produced in the refinery passed through and was stored in these silos. The tops of two of the silos are missing or largely missing suggesting that at some time in the sequence of events, explosions occurred within them. In the foreground there is a building in which there has obviously been an explosion. The brick walls are largely blown out. Sugar was packaged in this building. Additionally, granular sugar was pulverised into powdered sugar. There were other operations in the building as well. This building will figure prominently in the ensuing investigation. 

To the left or southwest is an area where cartons of the packaged sugar were palletised and then transferred to the storage warehouse. This area displays fire damage as opposed to explosion damage. There has been a fire which has caused the steel structure of the building to soften from heat and to collapse.” 

Selk then turned to a second photograph, one taken by CSB investigators from elevated platform suspended by a large crane. 

“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. A maintenance shop and the refinery laboratory were in this building. The building is almost completely consumed by fire. To the east adjacent to the silo, known as the number one silo, there was a bucket elevator that lifted the sugar from the refinery and carried it to the silos. Explosion and fire damage appear in this area and further eastward.” 

He went on to describe how CSB investigators will further analyse the widespread and extensive damage to the plant, as well as attempting to reconstruct the events that lead up to the explosions and fires. 

“It is also apparent in the photographs that many structures within the refinery have been compromised. It will be necessary to perform a strategic disassembly of these areas while, simultaneously allowing for investigative access. The Imperial Sugar Company has engaged structural engineers to assist them and we will also do so if necessary,” Selk concluded. 

While the CSB does not issue citations or fines itself, the organisation is charged with making safety recommendations to a variety of bodies including regulatory agencies such as the Environmental Protection Agency (EPA) and OSHA.l


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