Wood chips and pellets belong to the important secondary fuels category in modern power plants, furnaces or heat generators. Yet it is still not common knowledge that this combustibility makes wood handling processes some of the most hazardous in industry from an explosion standpoint. Statistics about explosion incidents are evidence enough.
But what makes wood material and wood handling processes so special in terms of explosion hazards? There are three basic requirements of a dust explosion phenomenon:
• combustible dust
• air / oxygen
• effective ignitions sources
Nearly all wood handling installations have this “perfect mixture” for dust explosions to occur. Additionally, given mechanical moving parts as well as drying processes, ignition sources are easily generated.
PROTECTING AGAINST EXPLOSION HAZARDS
The explosion safety concept for such plants typically is made up of a combination of explosion prevention measures and explosion protection measures.
|Woodworking plants must perform a risk analysis to identify the hazards.|
Explosion prevention means taking measures to prevent the formation of explosive dust clouds as well as
avoiding ignition sources by dedusting, housekeeping, grounding, proper maintenance and/or spark extinguishers.
Even if all preventative measures are applied, this approach might lead to misapplication of spark extinguishers which:
• might not work if the particles are large;
• cannot suppress an explosion;
• are only addressing the ignition risk arising from small, hot particles; and
• do not prevent ignition sources from tramp metal or from hot surfaces.
That is why protective measures also have to be applied in most wood handling installations. They typically apply one of three approaches:
• explosion resistant design (making equipment so sturdy it will withstand explosion overpressure of up to 10 bar);
• explosion pressure venting (pressure and flame relief by applying a predetermined breaking point on the installation);
• explosion suppression and (a rapid fire extinguisher that stops the flame ); plus
• explosion isolation (prevent flame and/or pressure propagation to down or upstream units).
Due to minimal maintenance requirements and low investment costs, passive explosion protection approaches such as explosion pressure venting is the most commonly used in wood handling facilities. The fact that these burst panels can even be combined with flame-trapping mesh materials allows various applications to be protected by so called flameless vents.
As with any comprehensive safety concept, even a fully protected plant can only be secured when all relevant persons, situations and conditions are taken into account. This means that plant management in the wood handling industry has to be aware of the explosion risk in general, implement available explosion safety measures and educate plant personnel.
The awareness of the need for combustible dust explosion safety has to be raised so that catastrophic events are not likely to endanger health, lives and business objectives such as profitability, continuity and productivity.
Therefore, a risk analysis should be carried out to identify the hazards and to allow the implementation of appropriate safety measures.
Johannes Lottermann, Dr.-Ing (PhD-Engineering) is Senior Consultant Explosion Protection; Head of Projects and Expansion Department, REMBE GmbH. For more information call (704) 716-7022 or visit Rembe.us