Dust Explosion Safety
Dust Safety Science has released its annual Combustible Dust Incident report which captures and details combustible dust incidents from around the globe. The report shows that in total, 165 fires and 60 explosions caused 88 injuries and 10 fatalities during 2020. The statistics from the report show a decrease in incidents and injuries relating to combustible dust incidents compared with 2019. In 2019, 250 fires and 75 explosions resulted in 118 injuries and 8 fatalities. Before that, in 2018, 213 fires and 68 explosions caused 114 injuries and 23 fatalities.
The report lists some of the most severe incidents that occurred in 2020, including the December 3 explosion at Wessex Water’s treatment plant near Bristol which killed four people and injured one. In terms of the materials involved in combustible dust incidents, food and wood products made up over 75% of incidents and resulted in 57% of the injuries and 40% of the fatalities. Similarly, wood processing, wood products, agricultural activity and food production also make up a large portion of the overall fire and explosion incidents, a historical trend when it comes to combustible dust incidents. The report shows that storage silos had the highest percentage of combustible dust incidents in 2020 with 30 fires and 13 explosions reported. This differs from 2017 and 2018, which found collections systems to have the highest number of incidents.
ATEX Explosion Hazards have been providing solutions to these problems for nearly 50 years. To understand these issues we have written papers specific to most industries that have issues with explosion safety with dust as the primary hazard.
Dust Explosion Safety in Chemical & Pharmaceutical, Dairy, Food and beverage, Cement and Biomass.
To be a real explosion hazard the dust or fine powder needs a Minimum Explosion Concentration; MEC or Lower Explosion Level; LEL of typically 30 to 60 g/m3 enough fuel to sustain an explosion. You cannot see the hand on the end of your arm at 30 g/m3; it is not a light dust loading. The explosibility KST of most Powders is typically 50 to 200 bar m/s or less. That is 200 bar/s in a 1-m3 vessel reaching typically a Pmax 9-10 barg. European guidance define explosive dust as less than 500 micron and fines as <100 micron. Explosion tests are standardized at 63 micron. Metal dust or some pharmaceutical dusts can be higher up to 500 bar m/s.
Ignition energies may differ for similar substances with differing moisture content and particle sizes. They may be as low as the static discharge experienced when taking off a synthetic fibre jumper, or as high as that from a ‘fixed flame’ such as a gas fired boiler. In evaluating the self-heating properties for biomass pellets or dairy powder, there are limitations to basket tests when scaled up to large silos installations.
Standard EN 1127-1 distinguishes thirteen types of ignition source from hot surfaces to chemical reactions.
Strength of vessel. It is important to know the strength of the plant, so that one can design the protection system within that vessel containment. Take care of transition points and socks. The weakest part of most plant is the roof. In old plant, this could be lower than 0.1barg but in modern plant typically 0.3 to 0.7 barg. Fluidbed and cyclones can be very strong with filters usually 0.5 to even 1 barg.
Select the appropriate Zones/Equipment.
Simple things like a viewing lamp have caused fires and explosions in plant. Obviously, do not use any equipment, instrumentation or tools, which generate an ignition source.
Train your staff
Wherever there is the potential for harm at the workplace, this also implies that the work organization must meet certain requirements
Prevention. Outside the above, there are systems that can minimise the probability of a fire or subsequent explosion. Temperature sensors are useful to control process conditions but tend to be too slow to detect a fire. Spark detection has been tried but with limited success, in the milk industry it causes more trouble than it has is worth and most plant operators now favour ACOM or ATEX CO monitoring
Protection
Once the explosion starts, you need to mitigate the consequences. Venting is the preferred option but only if you can vent the flame outside, usually only feasible on the silo and bag filter. If you cannot find a safe vent area outside, the flame from large vessels can do a lot of damage in a congested site. With product upgrades flameless vents are now been accepted into the Food industry for small vessels venting inside the building.
Explosion Suppression is an alternative when venting is not feasible using Hot Water or Powder fast fire extinguishing usually in the Fluidbed and cyclones. Explosion Isolation is possible using rotary valves if ATEx approved, barriers using hot Water and powder between the driers, fluidbed, cyclones and bag filter are common.
Maintenance
Maintenance, should never be done while the plant is running especially blockages and hot work. Proper risk and method statements even for regular maintenance should be in place supervised & sign off by a competent person. Ensure the plant is returned back to the operator in a safe condition.
Changes
Modern companies are now in the business of multiple ingredients where the hazards and their properties are different with each product, having different safety implications. Some factories can be processing various products in one week, which product is their worst-case scenario? Is one product stickier, is it more susceptible to self-heating, leading to the possibility of smouldering nests. Review your safety assessments.
