Expert view
Expert view
Preventing dust explosions during flour production
Devastating dust explosions pose one of the most significant safety risks in flour production . The milling , handling and storage of grain and flour constitute explosion risks in spite of their inherently harmless nature . Given the right conditions , flour dust becomes a highly explosive fuel , capable of destroying a building and even taking lives .
By Mark Shannon , BS & B Safety Systems
The originating causes of dust explosions may be varied ; a spark , electrical fault , or grinding and mil - ling friction . Dust explosions occur when there is a source of ignition , fuel ( in this case , the flour dust ) and oxygen . When combined in an enclosed area , with rising pressure and ra - pid increases in temperature , de - fla gration can occur . The primary dust explosion causes a pressure wave that disturbs accumulated flour dust in the factory , giving rise to the risk of a secondary ex - plosion . In such a case , the risk of serious damage to property and personnel becomes very real .
POST IGNITION HAZARDS AND PROTECTING AGAINST THEM When agitated flour dust is in sus - pension , the extremely dangerous secondary explosion risk arises . If such an explosion occurs , it has the ability to spread to other parts of the factor and processing equip ment , thus risking a buil - ding ' s complete destruction .
GOOD HOUSEKEEPING An explosion risk can be increased by the build-up of combustible flour dust on factory surfaces and ceiling spaces . In the UK , for example , the Health and Safety Executive ( HSE ) recommends that in addition to a rigorous cleaning regime , there should be the total elimination of dust from high-le vel horizontal surfaces through , e . g ., the use of sloping surfaces in order to minimize dust accumu lation .
DUST TESTING As different dusts have different explosive properties , they are of - ten handled and stored in dif - ferent ways and locations . Dusts are given explosion severity clas - sifi cations , ranging from St1 to St3 . A “ not specified ” classification means that the material is nonexplosive , while St3 is the most explosive type of material . Flour is rated at St1 , as is sugar and paper , which are well known to carry the risk of explosion . Dust testing identifies two key per - formance characteristics of dust :
• The maximum pressure of a dust explosion ( Pmax in bar )
• The speed of the rise in explosive pressure ( KSt in m / sec ). Knowing the severity of these two parameters can influence what explosion protection equipment one may need to protect proces - sing equipment .
SPARK DETECTION Spark detection systems sense hot particles , sparks and flames that may serve as ignition sources for a fire or explosion . These systems can incorporate automated shutdown systems to interrupt the feed of combustible material in pro cessing equipment . These pro - ces ses can be monitored by an operative via a control panel and can be utilized to assess any further risks . By preventing sparks , embers and hot par ticles from reaching downstream processing equipment such as dust collectors , bins and silos , both fire and ex plo - sion risks can be managed .
CHEMICAL SUPPRESSION During the milling of grain and handling of flour , friction / heat be - comes a real ignition risk . Ad - vanced chemical suppression sys - tems are designed to detect the start of an explosion ( point of ignition ) and deliver dry , inert che mical extinguishing agents into a developing internal defla - gra tion . These suppression and
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Spring 2016 • BISCUIT WORLD