Of the many powders and particulates that could help give rise to an explosion in the process industries, dust is perhaps the most lethal.
According to Mike Bradley, director at the Wolfson Centre for Bulk Solids Handling Technology, there are significant and often-overlooked issues concerning dust in the workplace.
“Most people in the process industries do not realise how hazardous dust can be,” Bradley says.
Fortunately, Bradley says there are a number of measures process plant operators can employ to help control levels of dust at their facilities.
“First and foremost, plant operators have to make sure their facilities are kept clean,” Bradley says.
“Most people in the process industries do not realise how hazardous dust can be"
“Good housekeeping is of vital importance. Housekeeping is what really protects life and limb.”
Bradley cites the catastrophic secondary dust explosion at the Imperial Sugar refinery in Georgia, US during 2008, in which 14 people lost their lives and a further 40 sustained injuries, as an example of the disastrous effects of bad housekeeping.
In that instance, Bradley is certain that had the facility been kept clean, nobody would have died that day.
Bradley says plant operators can also follow a simple ‘pyramid’ of best practice rules to help control the risks created by dust.
To avoid a secondary dust explosion, which unlike a primary explosion is not contained within a piece of machinery, plant operators must first attempt to reduce the dust at source.
“That could be as simple as handling materials with more care, which could involve having better-designed transfer points so that materials are not being moved at high velocities, which can create a lot of dust,” Bradley says.
“That’s the most basic thing you must always do first.”
However, better materials handling may not necessarily alleviate a plant’s dust problem.
Bradley says the next stage, and the second part of the ‘pyramid’, is containment.
“Containment essentially means putting a box around any process within your facility that has the propensity to create dust. You must also reduce the amount of airflow because if you have a draft, it will carry the dust through the workplace,” Bradley says.
“If you prevent the draft, the dust will not hang in the air and will therefore be much easier to remove as it will not be able to spread.”
A combination of controlling the dust at source and containing any dust, while reducing airflow, is often enough to solve a plant’s dust problem and keep dust to a safe level.
However, plant operators may also need to install fire and explosion suppression systems, such as misting and fogging technologies.
Suppression systems rely on an automated response to the detection of an explosion, usually triggered by a rapid change in pressure, and the system responds by purging the atmosphere with an inert substance, specifically chosen for its ability to prevent explosion propagation.
However, Bradley says there are instances, such as within the handling of biomass materials, where suppression systems cannot be used.
He also says suppression systems may not alleviate dust issues completely.
“The last port of call is dust extraction and extrusion. If you have tackled the dust at source, contained it, and then suppressed it, you should only have a small dust problem left,” Bradley says.
“I always say to people: ‘the last thing you need is a dust extractor’. I am not saying you don’t need dust extraction, rather, you must ensure you do everything else first and only come to dust extraction as a last resort, and if you still have a dust issue at your facility.”
Steve Bell, managing director of explosion protection firm StuvEx, says filtration systems also play a vital role in managing dust.
“Having isolation protection built-in is fundamental,” Bell says.
“I consider isolation filters as more important than the filters themselves, but you’d be surprised at the number of companies that do not have isolation built into their systems.”
Unfortunately, poor housekeeping and a lack of adequate equipment are not the only issues.
Legislation – namely the Dangerous Substances and Explosive Atmospheres Regulations (DSEAR) and the ATEX Directive – are also often poorly adhered to.
DSEAR and ATEX guidelines put the onus on employers to ensure the safety of those who work in the presence of hazardous, flammable and explosive substances.
“In the UK, DSEAR combine the Explosive Atmospheres Directive (1999/92/EC) and the ATEX (Explosive Equipment) Directive (94/9/EC) into a single health and safety regulation,” says Tom Westcott, compliance engineer at Lorien Engineering Solutions.
Jeremy Gadd, head of instrumentation, control and automation at power plant training simulation specialist GSE systems, says that many firms, particularly small and medium-sized enterprises (SMEs), do not appreciate that DSEAR applies to any company that handles explosive or flammable materials.
“I think there is a lack of understanding,” Gadd says.
“I think there are essentially two elements: one is the lack of appreciation that ATEX and DSEAR exist and, although many firms try and stay on top of the legislation, they are perhaps not well-staffed enough to ensure consistent compliance.”
Gadd also says there is an issue with mind-set, that can lead to “obvious mistakes” being made by engineers.
“The age-old line of ‘it’ll never happen to me’ is one I hear on a regular basis. Operators often back this up by suggesting they have run a facility for over 20 years without a single issue,” Gadd says.
“It’s a really dangerous position to get into. Plant operators must always remember that past performance is not reflective of future performance.”
A major part of mitigating the risks associated with damage caused by dust explosions in the workplace is identifying situations where explosive atmospheres may be present.
Dust zone classifications are defined in DSEAR by three scenarios. Zone 22 relates to explosive atmospheres that may occur under a fault condition – such as failure within pipework.
However, that atmosphere will only be present for a short period of time, says Tom Westcott, compliance engineer at Lorien Engineering Solutions.
Zone 21 relates to areas where an explosive environment is created, normally during the manufacture process, but which only lasts for a short period of time, and is deemed to occur only occasionally.
Meanwhile, Zone 20 relates to areas where an explosive environment is created, normally during the manufacturing process, but the environment is repeatedly sustained, constant and lasts for a long period of time, Westcott says.
“Once the manufacturer has identified and classified areas they must ensure that appropriate equipment and safety control measures have been put in place to ensure protection from explosion,” Westcott says.
Steve Bell, director of explosion protection firm StuvEx, says that to zone effectively, most plants will need to call in a specialist.
“If a plant is zoned incorrectly it can cost thousands of pounds to rectify as the equipment in that area will not comply with Health and Safety Executive (HSE) regulations,” Bell says.
“If the zoning is wrong, and the HSE inspects your facility and says you did it wrong, then you have to rip out all the equipment as it will not comply with necessary ATEX-approved regulations for that zone.”
Bell says StuvEx has a unique approach to effectively zoning a plant.
“We will not just go to a textbook and parrot what the book says. We will actually look at your installation and analyse the existing conditions and come up with zoning that is practical in relation to what [processes] you’ve actually got on site…not what it says in the textbook. That is a big difference,” he says.