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Avoiding the big bang!

09 April 2017

Mark Shannon explains why food manufacturers need to be taking dust explosion risks seriously. 

A 2011 study carried out by HFL Risk Services, specialists in process safety management, identified that 2,000 dust explosions occurred in factories and refineries across Europe, with 50 of these being in the UK. In spite of the explosion risk management requirements set out in Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR) and ATEX, devastating dust explosions still occur. 

As far as the food industry is concerned, there have been notable dust explosions in the last decade. In 2006 a man was killed in a dust borne explosion at a milk powder production facility in Holland. The Visalia milk powder processing plant in California succumbed to an explosion that blasted a hole through the side of the facility in 2008. In 2012, an explosion caused nearly AU$300,000 worth of damage when a powdered milk facility was significantly damaged in an explosion.

Sugar dust too has shown its destructive potential in several incidents around the world; one of the most notable in recent times being the Imperial Sugar Plant explosion of 2008 in the US. Accumulations of sugar dust in the factory were ignited, probably by a very small spark. However the ensuing secondary explosions and fire claimed 14 lives and left nearly 40 people with serious burn injuries. 

In 2003 in the UK, the British Sugar refinery in Cantley (Norfolk) also suffered a series of devastating explosions. They propagated throughout the facility causing severe damage starting in the bucket elevators and through dust extraction systems. The most alarming fact about this incident was that the factory was not operational and hadn’t been for some time. The ignition of the sugar dust was caused by welding repairs on the exterior of a bucket elevator. The real shock was how fast and vigorous the explosion propagation was from the moment of ignition by the welder’s spark.

It begs the question, has awareness about combustible dusts grown? And is enough being done to prevent dust explosions?

Combustible dusts 
It is commonly known across varied processing industries that wood-based dusts are highly flammable. Biofuel, recycling and wood pelleting processes are documented causes for dust explosions. We expect these substances to be a fire or explosion risk.

However, these are closely followed by dust created during the food manufacturing process – familiar suspects like flour, sugar, powdered milk, powdered starches, tea and coffee. Indeed, the HFL Risk Services study identified that 24% of dust explosions occur in the food industry. However, the range of dangerous dusts is so expansive and encompasses organic materials that most people would not even consider to be an explosive risk. So, is there a need to improve industry’s damage limitation knowledge base?

In the US, OSHA has compiled a comprehensive list of combustible dusts that pose an explosion risk during processing. (www.osha.gov/Publications/combustibledustposter.pdf)  The list comprises a myriad of organic dusts that are generated in various industries, ranging from food manufacturing, pharmaceutical and petro-chemical industries, recycling and agriculture. Substances listed include lesser known food dust sources such as tomatoes, egg-white and onion, for example, are listed as potentially combustive materials when they are being processed in powdered form.

In 1989 a paper presented by B. Porter at ‘Dust Explosions: Assessment, Prevention and Protection,’ gives one of the most accurate, documented set of dust explosion statistics in the public domain. Apart from the expected suspects, like wood based products and metals, foodstuffs rank quite high on the risk list.

How an explosion occurs
The originating causes of dust explosions may be varied; a spark, friction on badly maintained machinery, an electrical fault, or grinding and milling friction. However, the fuelling and propagation of a possibly fatal explosion is almost always caused by suspended dust in the atmosphere.

Levelling explosions are caused by the trinity of ignition, fuel and oxygen. When these three elements come together in an enclosed area, with rising pressure and rapid increases in temperature, a deflagration can occur. This primary explosion can cause a pressure wave carrying with it a flame that disturbs accumulated dust. 

Once the agitated dust is in suspension, this is where the extremely dangerous secondary explosion risk is created and has the ability to spread to other parts of processing equipment, risking its complete destruction. It travels through pipes, ductwork and silos until no part of the processing facility is safe.

Protective measures
To eliminate the relationship between chance and disaster, the UK and European process industries are subject to guidance when it comes to protective measures. In the UK employers are subject to certain basic requirements according to DSEAR, which simply, requires employers to assess and identify the risk; eliminate the risk; provide controls to minimise and protect against the risk; and provide additional controls to mitigate the consequences, for staff and equipment.

ATEX specifies what controls employers should use to prevent explosion risks, divided into danger ‘zones’ as shown in Table 1.

In theory, when one element of the incendiary trinity is interrupted or controlled, an explosion can be prevented. This invites different methods of protection to mitigate against fire risk including laboratory dust testing, better housekeeping to reduce or eliminate dust accumulation, to venting systems, spark detection devices and explosion suppression systems.

For example, chemical suppression and isolation systems detect the start of an explosion (point of ignition) and 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.

Mechanical isolation uses rapid-response valves that isolate each stage of the process so should an ignition occur, it won’t spread throughout the process equipment, fuelled by dust along the way.

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 flameless vents intercept, extinguish and retain all burning materials, preventing them from hazardous release into the surrounding environment.

These are all approved methods by which effective control of ignition, combustion and explosions can be safely controlled.

From a business viewpoint, economic loss because of halted production due of destruction of property and equipment is an undesirable and unprofitable position to be in. 

There are other advantages to adequately safeguarding your process facility against dust explosion risk; such as possible insurance benefits in terms of reduced premiums if an employer demonstrates due care and diligence by installing correct explosion controls.

As an employer there is a legal obligation to comply with regulations for the purposes of safety. Given how dangerous food dust explosions can be, it is vital to remain vigilant. Endangering or losing the lives of workers as a result of a dust explosion is a loss too great to put a price on and a cost to reputation that is damaging to future prosperity.

Mark Shannon is European sales manager at BS&B Safety Systems.


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