The possibility of dust explosions during waste and recycling processes is par for the course in the daily risk management of these sectors. Although the Health and Safety Executive (HSE) has driven the safety agenda and accident rates have declined by 24% in the past five years, RIDDOR (Reporting of Injuries, Diseases and Dangerous Occurrences Regulations 2013) reports that injury rates remain four times higher than the national average.
Whether pelleting compacted sawdust, processing paper, metal or plastics, or even recycling print toner cartridges, there is an explosion risk in handling these substances as they generate combustible dusts. Every year, damaging dust fires start in dust extraction and waste conveying equipment. Just a few embers entering a dust collector, bin or silo is enough to trigger a dust explosion, which would typically lead to fire.
How does a dust explosion occur?
The originating causes of dust explosions may be varied; a spark, friction on badly maintained machinery, an electrical fault, or grinding and milling friction. However, the fuelling and propagation of a possibly fatal explosion is almost always caused by suspended dust in the atmosphere.
Explosions are caused by the trinity of ignition, fuel and oxygen. When these three elements come together in an enclosed area, with rising pressure and rapid increases in temperature, a deflagration can occur. This primary explosion can cause a pressure wave carrying with it a flame that disturbs accumulated dust.
Once the agitated dust is in suspension, this is where the extremely dangerous secondary explosion risk is created and can spread to other parts of processing equipment, risking its complete destruction. It travels through pipes, ductwork and silos until no part of the processing facility is safe from mobile explosion risk.
Protecting against dust explosions
The UK and European process industries are subject to guidance when it comes to protective measures. Explosion safety in the UK is governed by requirements according to DSEAR (Dangerous Substances and Explosive Atmospheres Regulations 2002).
Distilled to its basic elements, DSEAR requires employers to:
• Assess and identify the risk
• Eliminate the risk
• Provide controls to minimise and protect against the risk
• Provide additional controls to mitigate the consequences, for staff and equipment. For the purposes of manufacturing and processing, a closer analysis of dust characteristics is also required to specify operational conditions and protection.
For example, according to the HSE’s guidelines, Safe Handling of Combustible Dusts, wood flour has an average particle size of 65μm; tissue paper is 54μm. Based on these profiles, dust testing is designed to identify two key performance characteristics, which in turn influence explosion protection equipment design and their application:
• 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.
Theoretically, when one element of the incendiary trinity is interrupted or controlled, an explosion can be prevented. This invites different methods of protection to mitigate against fire risk.
Chemical suppression systems detect the start of an explosion (point of ignition) and deliver dry chemical extinguishing agents into a developing internal deflagration. It suppresses further flame propagation and protects interconnecting process equipment from any spreading explosion damage.
Spark detection devices detect hot particles, sparks and flame that might become the ignition source for a fire or explosion if allowed to travel on through pneumatic ducting and conveyors towards other materials-handling equipment.
The most advanced flameless vents intercept, extinguish and retain all burning materials, preventing them from hazardous release into the surrounding environment.
These are all vetted methods by which effective control of ignition, combustion and explosions can be safely controlled.
Why protect against dust explosions?
Economic loss due to halted production because of destruction of property and equipment is an undesirable and unprofitable position to be in. There are other advantages to adequately safeguarding your process facility against dust explosion risk; possible insurance benefits in terms of reduced premiums if an employer demonstrates due care and diligence by installing correct explosion controls.
Employers have a legal obligation to comply with safety regulations because endangering or losing the lives of workers because of a dust explosion is a loss too great to put a price on, and a cost to reputation that any business can ill-afford.