In 2006, the U.S. Chemical Safety Board published the findings of a major study outlining the scale and devastating consequences of combustible dust cloud explosions that have occurred between 1980 and 2005 in US chemical processing operations.
In that period 281 explosions were caused by ignitable combustible dust atmospheres, resulting in 199 fatalities and the injury of 718 workers. In the UK the Health and Safety Executive recorded 303 dust explosions over a nine-year period and German records demonstrate 426 similar incidents over a 20-year period.
During one 10-year period a single insurer listed a total of 450 incidents across their client base that were attributed to dust fires and explosions. The total cost of damages amounted to $580 million, with the average gross loss for dust explosions costing $1.9 million and dust fires costing $1.2 million.
Since their report was published the CSB has repeatedly requested that OSHA take more action to regulate the safety of operations processing combustible and flammable powders. The 2008 sugar refinery explosion at the Port Wentworth plant of Imperial Sugar should be a warning to a broad range of industries just how risky and relevant dust explosions are. Approximately 70% of all chemical processing industry operations handle powders in a combustible form at some point in their manufacturing process.
CSB study 1980 to 2005: sectors with recorded incidents of combustible dust fires and explosions.
Several contributing factors need to be present to support the ignition of a combustible dust cloud in a static hazardous area, comprising:
- A dispersed dust cloud-oxygen mixture that is above its Minimum Explosion Concentration (MEC).
- Physical containment of the dust cloud that will lead to rapid pressure build-up causing deflagrations out of process equipment and into open workspaces.
- A heat source with enough energy to ignite the combustible atmosphere.
The locations of primary deflagrations normally occur within process equipment such as dust collectors and blending machines. Secondary explosions result from a containment breach, with the primary deflagration propagating through conveying systems or through mechanical breaches in the processing machinery. Secondary explosions cause the bulk of devastating damage to workers, buildings and equipment by unsettling and igniting layers of dust that have collected on surfaces. A 1.6 mm layer of dust that gets dispersed from primary explosions is all that is required to initiate secondary deflagrations.
Three separate studies with collective data totalling 1100 dust explosions gathered in the US, UK and Germany highlight process equipment that have proven to be known sources of primary dust explosions. The main processes that suffer from explosions are dust collection, powder grinding and pulverising, powder conveying operations, silo and container filling and powder mixing and blending.
German study: recorded sources of ignition in combustible dust explosion incidents.
The German data, which totalled 426 incidents, provides a percentage breakdown of known primary sources of ignition. Electrostatic discharges make up 10% of known primary ignition sources. The “unaccounted” category accounts for incidents where no physical evidence (electrical or mechanical causes) has been detected. The prime suspect in the “unaccounted” category is very often electrostatic discharges, but as no witnesses can provide evidence of seeing or hearing a spark, ignition sources of this type go unreported and unaccounted for.
Even though the majority of combustible dusts have higher MIE’s than flammable vapours the amount of energy available from electrostatic discharges within contained environments will ignite the vast majority of combustible dusts. This is because the rate of electrostatic charge generation and accumulation in powder processing operations is extremely high.