FIRE & ELECTRICAL SAFETY
LETTING THE DUST SETTLE
Kevin Spiess, EMEA Sales Manager for Explosion Protection at BS&B Safety Systems, discusses how to minimise the risk of dust explosions to ensure staff and building safety.
In health and safety terms, we generally think of dust as a health hazard that causes skin irritations and respiratory illnesses. However, from a safety angle, dust can be a destructive force with the potential to cause serious damage and physical harm. Just a few burning embers caused by a spark entering a dust collector, bin or silo is enough to trigger a dust explosion which can cause a rapid rise in explosive pressure. This may trigger a violent explosion in milliseconds, placing personnel and property at risk. This explosion risk is highest in any industry which handles, stores and processes organic materials, generating large quantities of fine organic dusts.
Dust that has settled on floors or walls can be thrown into the air by a dust explosion, thus providing fuel for secondary explosions, which can cause more damage than the first.
The effects of accidental fires or explosions can be devastating in terms of lives lost, injuries, damage to property and the environment, and to business continuity.
UNDERSTANDING
DUST DANGERS In the UK and Europe, there is a requirement to identify any potentially explosive substances in the workplace. In the UK this requirement is governed by the Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR). DSEAR classifies dangerous substances as those used or present at work that could, without adequate controls, cause harm as a result of a fire or explosion.
In Europe, ATEX is the common name given to two European directives that
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specify the controls employers should use to prevent explosion risks, which are divided into danger ‘zones’.
Both DSEAR and ATEX recognise dusts as explosive risks.
Different types of dust have different particle sizes, properties, ignition temperatures, and ignition sources. Dusts are given explosion severity classifications; St1 to St3. ‘Not specified’, means the material is non- explosive and St3 is the most explosive type of material.
Most people think that dust is, for the most part, the same. However, the range and qualities of dangerous dusts is expansive across the process industries and include organic materials that most of us would not even consider as an explosive risk. Nevertheless, for the purposes of manufacturing and processing, a closer analysis of dust characteristics is required to specify operational conditions and protection.
According to the HSE’s guidelines Safe Handling of Combustible Dusts, wheat has an average particle size of 80 µm; wood flour is 65µm, tissue paper is 54µm.
Dust testing is designed to identify two key performance characteristics, which in turn influence explosion protection equipment design and their application as well:
• The first measures maximum pressure of a dust explosion (Pmax in bar)
• The second identifies the speed of the rise in explosive pressure (KSt in m/sec)
Testing under laboratory conditions will tell you what St class your dust
is, based on its KSt value. It will also reveal the dust’s performance under specific equipment operations and processes; is it being milled, ground, poured or dried? What is its Minimum Ignition Temperature (MIT) while being subjected to these processes – in other words, what temperature will the dust withstand before it becomes an ignition risk? All aspects of the dust’s behaviour in relation to its process environment can be determined to inform the correct precautionary measures against a potential explosion.
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