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How to Extract Combustible Dusts at the Source

August 15, 2019

Closing the barn door when the horse has bolted is widely held to be pointless, with the only option being to chase after and catch the horse. The same is true of industrial dust suppression: we can devise a plan B but there’s no substitute for catching it before it escapes.

Rather than relying upon general dust extraction ducting, it is much more effective to go directly to the source of the processes that produce dust and fumes. A sanding or metalworking dust collector can often be fitted directly onto a handheld tool. For larger machinery such as bandsaws, woodworking dust collection systems might take the form of an extraction hood, or a downdraft system that creates a steady airflow toward an extraction duct.

These can be directly integrated with the equipment in use - so that both activate together. This solution can work well for many types of dust-producing machines and processes, including materials conveying.

Flexible extractor arms are another option for many processes, enabling the user to concentrate power only where and when it is needed, folding it out of the way when not in use.

Despite the many tools available, unfortunately, safe dust control is not as simple as just buying a tool attachment or an extractor hood. Dust is toxic, often inflammable, and potentially explosive. However you collect it, moving it from one place to another can solve a hazard in one place only to create another somewhere else [1].

That is why a great deal of government legislation focuses upon factory dust extraction systems. Before you start browsing for packaged solutions, you need a designer who can apply sophisticated calculations to determine your risks and ensure that you have a correctly designed and integrated end-to-end solution.

Statutory Requirements

General best practices to control toxic risks are outlined in COSHH guidelines [2] but a host of more specific legislation also exists, including the Health and Safety at Work Act 1974 (HASAWA), and the Registration, Evaluation, and Authorisation of Chemicals (REACH). A series of other directives (e.g. EH40 - the Work Place Exposure Limits) have established occupational exposure limits for a long list of specific substances.

Dangerous substances are found in almost every place of work. They include commonplace examples such as printer ink, solvents, paints, varnish and nail varnish, LPG and refrigerant gases, sawdust, dry foodstuffs, slowly oxidizing packaging and plastics, together with metallic dusts from milling, fettling, and drilling activities. Under the right circumstances, almost anything can pose a danger of one kind or another.

While assessing worker exposure levels to toxins is often difficult, assessing fire and explosion risks is even more demanding. Fire risk assessments and mitigation are mandated for all UK businesses. Failure to comply can risk an unlimited fine and up to two years in prison. In addition to fire safety regulations, combustible dusts usually need to be assessed under the Dangerous Substances and Explosive Atmosphere Regulations (DSEAR) [4].

Given the devastating consequences of industrial fires and explosions, it’s important that you tackle these obligations in a professional way, documenting it carefully to prove that you have been fully compliant and responsible.

Why Dusts Explode

Despite the way in which many of us were taught to think about these things at school, dusts and other chemicals cannot simply be divided into one of two categories: flammable and non-flammable. Whether or not a substance will combust depends upon a variety of factors that are considerably harder to predict, including their density, temperature, chemistry, air mixture, impurities, ignition point, and sources of ignition.

A professionally conducted assessment considers all the factors. A good place to begin is by investigating the precise circumstances under which dusts are being generated in the first place.

However, dust clouds become explosive when they reach a critical dust-to-air ratio. In fact, as a dust cloud gets thicker, beyond a certain point it becomes less likely to explode. The principle is the same as fuel injected into a car engine. If the mixture is too fuel-rich, the engine’s efficiency declines and acceleration suffers.

Unfortunately, increasing your dust generation doesn’t usually offer a very satisfactory solution. In your car, the excess fuel mixes with more air and backfires in the exhaust. Something similar could happen inside dust extraction ducting - with dangerous consequences.

Any dust needs to be assessed chemically (and sometimes structurally) to determine precisely where the dangerous concentration point lies. An explosivity rating is calculated based upon parameters called Pmax and the Kst value which are calculated in a laboratory. Pmax is the maximum explosion pressure of a dust cloud and is defined in BS EN 14034-1:2004. Kst is the maximum rate of explosion pressure rise and is defined in BS EN 14034-2:2006.

Dust particle size is another important factor. Usually, the finer the dust, the more likely it is to ignite or explode, but it’s not a steady sliding scale. The particle size at which a particular dust chemical becomes an explosion hazard is often quite precise.

Even when working with the same raw material, different industrial processes are inclined to produce dust particles of different sizes. As a result, substances such as aluminum dust may be completely harmless in one workshop, but dangerously explosive in another. Finishing processes are notoriously inclined to produce aluminum dust and other metals at the most dangerous diameters.

Similarly, even the shape of the particles can have a dramatic effect. Inspection under a microscope often makes the reason for this clear. Some particles are spherical while others are anything but. The further a particle departs from spherical, the greater surface area it has exposed to oxygen.

Detailed laboratory assessments can help to determine a variety of useful information. The size, shape, and chemistry of dust components will serve to identify where and how it is being produced, how hazardous it is likely to be, and what amount of air flow will be required to effectively capture and remove it.

Causes of Ignition

The cause of some of the world’s worst industrial fires and explosions will remain forever unknown. Investigators have a hard time piecing the evidence together - quite literally. In almost all cases, there will be more than one cause, and this is a leading reason as to why we still fail to foresee disasters.

Complacency is the other side of this coin. If we have worked with a material for much of our working lives, we start to assume that it is safe, and that the legislation is just a nuisance. But the factors that can transform it from something banal into something explosive are often subtle and complex. Sources of ignition can be even more invisible and come out of left field.

An often-neglected risk factor is static electricity. Under the right weather and turbulence conditions, moving air can become a huge generator of static. If you’ve had fun with a Van der Graaf generator then you know that static frequently reaches extraordinarily high voltages that like to discharge to earth in a blue bolt, easily hot enough to ignite an inflammable gas, or under the right conditions a fuel/air mixture.

Ignition can also result from an indirect static field induction. Aging electrical components such as condensers, coils, and capacitors, can induce a static discharge even when disconnected from the mains electricity supply, perhaps at night.

More commonly, a hot motor, bearing, or overheated electrical component provides the initial heat source. Dust that has not been properly extracted will find its way inside engines, motors, casings, and bearings, adding to frictional heat, making it harder for components to cool down and providing an initial “tinder” in a chain that leads to a more general ignition.

The ATEX Equipment Directive (ATEX 2014/34/EU) [5] and ATEX Workplace Directive (ATEX 1999/92/EC) [6] are intended to address these risk factors. If static is a possibility, steps may need to be taken to prevent electrostatic build-up and to introduce electrostatic discharge precautions. In some extreme cases, major pieces of plant machinery must be replaced if there is no other way to eliminate a potentially flammable dust or fume situation. Fortunately, there are usually cheaper alternatives.

Other Risk Factors

One has already been alluded to. Airborne dust does not exist in isolation. Wherever there is dust in the air, there is also dust settling on the ground and other hard surfaces, inside machines, above suspended ceilings, stuck to walls in nooks, crannies, and corners, or even inside factory dust extraction systems themselves. Dust that has settled is not safe dust; it is dust waiting for the chance to become airborne again.

The explosion at the Pepcon chemical plant in Henderson, NV in 1988 was largely caused by settled (and “non-flammable”) dust. A chain of explosions demolished the plant, causing two deaths, 372 injuries, and $100 million of damage. A small initial fire or explosion stirred up dust in adjacent areas, causing a series of secondary ignitions that, in turn, stirred up more dust, causing an ever-widening conflagration. The plant was believed to have a good cleanliness record; workers regularly swept dust up with brooms, probably helping to spread it into less reachable recesses.

Settled dusts should always be removed with vacuum cleaners, not brushes. Finer and higher risk dusts such as titanium or aluminum may need wet scrubbing.

Vacuum cleaners with appropriate attachments also have a much better reach and can be used to clean inside ductwork. However, well-designed extraction ductwork with an adequate throughput of air, should not accumulate any substantial quantity of static dust. In some cases, the ducts themselves are at fault.

To maintain evenly moving air, ducts need to have a particular width and profile, especially where they take turns, rise, or fall. When necessary, airflow can be accelerated with additional fans, or steadied with various types of gate and baffle. Again, this is an area that requires the skills of a professional ventilation engineer.

The material from which the vent is constructed also makes a significant difference. Surfaces that aren’t smooth can encourage dust to settle, especially at snagging points such as sharp corners or bolt heads. Filters are also sensitive points because at a filter, there is often an ample supply of oxygen and a reservoir of accumulating dust.

Most Dust Clouds Have a Silver Lining

The complexity of the threats that can be posed by dust mean that most enterprises need the help of a professional to arrive at a realistic assessment of their risk exposures. The silver lining resides in the fact that there are many possible solutions and not all of them are expensive. Some may even save you money.

If you already have an antiquated factory-wide ducted extraction system, you probably know that there is nothing more expensive than constantly running an industrial dust extraction system that is far more elaborate than it is effective. Poorly designed solutions are frequently energy inefficient.

Why pay to suck the air out of an entire plant if you can simply capture hazardous dusts and fumes at source by attaching vacuum hoses to your hand tools, or zone-off problem processes and concentrate effort where it is needed?

Dust is also waste. When poor storage is part of your problem, improving it can reduce overheads and raise product quality. Re-thinking product design to avoid wastage and the labor of substantial milling and finishing is never a bad thing either.

A professional survey may even find that you have no unmanaged risk factors. That’s something you can proudly present to the regulators and pull out the next time you are negotiating your plant and liability insurance policies.

Paul Riddick is co-founder and technical director at Vodex (Southampton, UK), a fume and dust extraction specialist. For more information, call +44 (0)1489 899 070 or visit www.vodex.co.uk.

More articles on combustible dust:

 

What You Need to Know About the New Edition of NFPA 652

How to Use Dust Explosion Test Data to Ensure Facility Safety


Explosion Containment: A Comparison

Dust Collection Systems: 10 Common Questions

[1] Typical example of a dust extractor causing a fire https://www.eadt.co.uk/news/margaretting-road-factory-catches-fire-near-hylands-park-1-5858631.

[2] Control of Substances Hazardous to Health Regulations 2002 (SI 2002 No. 2677) https://www.legislation.gov.uk/uksi/2002/2677/made, or https://www.hse.gov.uk/coshh/index.htm

[3] Fire risks in the workplace. https://www.gov.uk/workplace-fire-safety-your-responsibilities/fire-risk-assessments

[4] Dangerous Substances and Explosive Atmosphere Regulations (DSEAR) https://www.hse.gov.uk/fireandexplosion/dsear.htm and https://www.hse.gov.uk/pubns/indg370.pdf

[5] Equipment Directive ATEX 2014/34/EU https://osha.europa.eu/en/legislation/directive/directive-201434eu-equipment-and-protective-systems-intended-use-potentially-0)

[6] Workplace Directive ATEX 1999/92/EC https://osha.europa.eu/en/legislation/directives/21