August 24, 2017

25/08/2017: From ember to explosion: how to prevent an inferno caused by dust

by Mark Shannon, UK & Ireland Sales Manager for Explosion Protection at BS&B Safety Systems

The grain and milling industries are well known for being under constant risk of dust explosions

If not stopped in time, devastating damage can occur, including harm to employees, which is sometimes fatal. Grains, feeds, meals and flours that are handled produce fine organic dusts that are waiting fuel to cause untold damage when ignited by the smallest spark.

What does it take to cause an explosion?
Dust is generally thought of as an innocuous product, a nuisance to be literally brushed away or worse ignored. However, fires and explosions in dusty environments can occur when suspended dust is ignited, and when mixed with oxygen in the air, fueled into a raging inferno.

When this takes place inside a grain silo, process or storage enclosure, the rapid rise in pressure could cause a violent explosion in milliseconds, placing staff and property at risk. It only takes a few embers entering a dust collector to trigger a dust explosion. Likewise, combustible material passing into a storage silo could become the start of a fire.

The risk of sparks generated from milling operations may spread from the mill to other equipment. Some materials handling, processing and storage equipment has not been designed to resist the pressure of an expanding flame ball. Whatever the organic dust, when it is ignited, the ensuing effects of fires or explosions can be devastating in terms of damage and injury.

Acceleration to secondary explosion

All dust explosions can have a cascade effect. For example, grain dust that has settled on floors or walls can be thrown into the air by a primary dust explosion, thus providing fuel for secondary explosions. Often, these secondary explosions cause more damage than the first.

In this way, a dust explosion can move from room to room or from silo to silo. This is a common phenomenon in grain dust explosions. One of the most dangerous areas is in the bucket elevator or conveying system linked to a silo. The grain is always in motion, so dust is constantly present.

Tried and tested protection measures
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 mill.

These may be disturbed by a primary explosion in the process equipment and result in a more severe secondary explosion. Theoretically, when one element of the trinity of ignition, fuel and oxygen is interrupted or controlled, an explosion can be prevented.

This invites different methods of protection to mitigate fire or explosion risk including better housekeeping to reduce or eliminate dust accumulation, use of venting systems, spark detection devices and explosion suppression systems, or a combination of all of them. For example, chemical suppression systems detect the start of an explosion (point of ignition) and cannons™ 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. Chemical suppression and isolation systems often work in tandem. These cannons can be installed in the boot, head and legs of a bucket elevator system, for example.

Otherwise, if ignition or an explosion does occur, a flame can propagate at high speed up or down the legs and spread to other parts of the connected system whilst building explosive pressure. 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 flame free™ 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.

Read the full article, HERE.

The Global Miller
This blog is maintained by The Global Miller staff and is supported by the magazine Milling and Grain
which is published by Perendale Publishers Limited.

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