Dust explosions are an ongoing hazard in wood-processing plants and mills. Workplaces handling fine organic dusts in such operations are at risk of these materials burning and exploding.
Every year, premises where wood products are manufactured are severely damaged or destroyed by dust fires that usually start in dust extraction equipment. Just a few burning embers entering a dust collector are enough to trigger a dust explosion. Likewise, smouldering combustible material conveyed into a storage silo could become the nucleus of a fire.
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.
A fire or explosion results from ignition of a combustible material (dust, gas or vapour) when mixed with oxygen in the air. When this takes place inside a process or storage enclosure, the rapid rise in pressure could potentially cause an explosion in milliseconds, placing personnel and property at risk.
Most materials handling, processing and storage equipment is not designed to resist the pressure of an expanding flameball which proceeds below the speed of sound in air – known as a deflagration – as compared to a detonation, which exceeds the speed of sound in air.
Risk assessment
An explosion risk assessment will typically recommend that a series of protection measures be implemented. These range from investment in protection equipment to improvements in housekeeping to eliminate the build-up of deposits of combustible dust which may accumulate on beams in the factory. These deposits may be disturbed by a primary explosion in the process equipment and result in a more severe secondary explosion.
Codes and standards are now very clear in requiring isolation of vented equipment to prevent secondary explosions, which typically have much greater potential for damage and destruction.
BS&B Safety Systems has developed a bespoke spark detection and extinguishing system for the wood industry, designed to detect sparks, glowing embers and hot particles that might become the ignition source for a fire or explosion if allowed to travel on through pneumatic ductwork and conveyors towards other material handling equipment.
By preventing sparks, embers and hot particles from reaching dust-rich downstream process equipment such as dust collectors, bins and silos, both fire and explosion risks can be managed.
Using infra red detectors, the system recognises the radiated light emitted from sparks, glowing embers and hot particles travelling past the detection point and activates control circuits. The greatest sensitivity occurs when these sensors are employed in a dark area, such as closed ductwork, although optional daylight detectors are also available for use on open conveyor belts and for applications where light is likely to be visible. If hot particles are detected the system provides several options to manage the ignition risk:
? An electrical signal generated by the sensor activates control circuits typically used to operate an automatic water extinguishing curtain. Sparks can be extinguished without stopping production.
? An automated shutdown of the process can prevent the feed of combustible material.
? Alarm and control systems can be activated for other control devices such as diverter valves.
The detectors and control unit are ATEX (Directive 94/9/EC) certified for use in a dusty working environment. This means there are no expensive costs for additional wiring to run from detectors to remote mounted control units, and plant personnel have direct access to the status of the equipment. In the event of an alarm condition, the operator can see immediately which process is at risk. A web-based monitoring system, which allows plant managers to receive system alarms and faults to their smartphones, as well as monitoring the systems live on the internet, is also available.
In addition to this preventative solution, there are systems for protection, should an explosion occur. Our chemical isolation system isolates an explosion and prevents it from affecting interconnected processes. If unprotected, the ductwork and piping, as well as all the connected vessels and equipment, are at risk. This system is used in combination with explosion protection equipment on each vessel, bin, or dust collector and could include chemical suppression, explosion venting or indoor venting.
Legal obligation
Employers are legally obliged to protect their workers from being harmed by dangerous substances in the workplace, which includes risks related to wood dust. Employers must carry out risk assessments, and take steps to ensure they prevent or adequately control exposure.
ATEX is the name commonly given to Directive 94/9/EC. This provides the technical requirements and the relevant conformity assessment procedures to be applied before placing on the European market equipment and protective systems intended for use in potentially explosive atmospheres.
By investing in explosion protection, companies can protect themselves economically, follow appropriate health and safety standards and protect workers from potential risks.
