It´s 'Only Wood' Understates Dust Explosion Hazards
By Dr.-Ing. Johannes Lottermann, REMBE GmbH | Posted: 12/10/2013 11:02AM
Rendering of flames at a dust collection system. Wood can burn. That's obvious. 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.
|Explosion Ratio %|
|Food (flour, starch, etc.)||26.7|
|Table 1: Ratio of Comdust Explosions by Industry
But what makes wood material and wood handling processes so special in terms of explosion hazards? This question can be answered quite simply, considering there are just 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.
Picture 1: Explosion Triangle [source: Non-binding standard for 1999/92/EC]
In Autumn 2012, this deadly combination led to one of the most severe wood dust explosions in history in South America, where 5 people lost their lives and the entire plant was shut down for more than 5 months.
How to protect against explosion hazards in wood handling installations?
The explosion safety concept for such plants typically is made up of a combination of explosion prevention measures (to reduce the likelihood of explosion) and explosion protection measures (to reduce the effects of an explosion to an acceptable level).
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.
We know that even if all preventative measures are applied (especially with regard to the latter), this approach might lead to misapplication of spark extinguishers which
• might not work if 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 hot surfaces.
click image to zoomPicture 2: Overview of explosion protection measures [www.ivss.org] That is why protective measures also have to be applied in most wood handling installations. They typically apply one of three approaches:
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