The combustibility of dust is often over looked. In terms of extraction, we often focus on keeping the dust away from operators breathing spaces, to minimize their exposure and reduce the risks and damages from breathing solid dust particles into their respiratory system.
And this is an essential consideration when it comes to protecting people from airborne contaminants and particulates in the work place. That is why extraction systems exist – to stop people breathing in dust, fumes, vapours, gases, fibres and so on, that are hazardous to health. Check out our Dust Extraction page for more information on dry dust extraction.
But what if the dust is combustible?
Dusts like flour, or wood, aluminium or resin powder for example. Whilst these dusts do pose a risk to us from breathing them in in the form of a foreign body entering our respiratory system, they are not overly toxic. But they are potentially highly combustible.
What is combustible dust?
Combustible dust refers to a metallic or non-metallic airborne dust that is capable of igniting fast enough to cause an explosion in your work space. The determining factor on whether or not an industrial dust is combustible lies in its Kst value, which measures how explosive dust is when compared to other types of dust. Any reading higher than zero indicates the risk for explosion. Kst values are broken into ST Classes.
- ST Class 0 - Kst Value of 0
- ST Class 1 - Kst Value of 1 - 200
- ST Class 2 - Kst Value of 201 - 300
- ST Class 3 - Kst Value of 301+
Although a cloud of flammable dust in the air may explode violently, not all mixtures will do so. The concentration of dust and air must be within the upper and lower explosive limits for the dust involved.
You need the right mix of dust to air ratio with the dust particles being with in a designated size range.
The most violent explosions usually result from dust/ air mixtures that are fuel rich. This means that the oxygen available in the air cannot burn all the dust, creating partly burnt, glowing particles or embers that remain after the explosion. These embers in turn can reignite additional dust clouds – such as those shaken loose from the force of the initial explosion – and if enough air is available. These multiple explosions often result in catastrophic damage and loss of life.
A dust explosion involves the rapid combustion of dust particles that releases energy and usually generates gaseous reaction products.
The dust provides the fuel for the explosion. The dust particles are very small and so ignite easily and very quickly. The dust cloud is also full of oxygen from the air – another essential ingredient in a fire.
As the cloud burns it sucks more air in from the surrounding area, increasing the rate of deflagration – combustion through heat transfer; hot burning material hits the next layer of cold material and ignites it.
This can happen in milliseconds and the energy released from such a fast reaction causes a rapid increase in volume and energy to the extreme, generating high temperatures and release of gases – an Explosion.
A mass of solid combustible material as a heap or pile will burn relatively slowly owing to the limited surface area exposed to the oxygen of the air.
Example of a ST1 Class dust igniting
On the 7th of February in 2008 at the Imperial Sugar refinery in Port Wentworth, Georgia, United States, a dust explosion occurred in the main factory. It was believed to have originated beneath storage silos in an area where employees packed the sugar.
Tragically 14 people lost their lives and 38 were injured. 14 of the injured suffered life changing injuries.
The entire main factory was destroyed and secondary explosions occurred.
Within 24 hours of the event, the explosive substance was identified as sugar dust.
Sugar dust has a Kst value of 35 and is a ST Class 1 dust – the lowest class of combustible dust!
Extracting combustible dusts
Many dusts we find in the work place – metals, wood, flour, resin, paper and more have the potential to combust and explode. When extracting potentially combustible dusts, we have to allow for this. After all, an extraction system creates a dust cloud in the ductwork of the LEV system.
We then store this potential fuel in a pressurized container (the interior of the dust extraction unit and filters) and force more air through it. In specific circumstances this makes LEV’s a potential prime candidate for a dust explosion. So how do we stop dry dust extraction systems blowing up?
By this stage we should already know what dust is being extracted and how is it being produced, so first off, we need to look at ignition sources. How could the dust ignite? What are the ignition sources? could the extract fan ignite or over heat, are there external factors – grinding or cutting works nearby, hot works such as welding (common in metal fabrications) is the application producing a heat source such as a laser cutter or does the application create sparks or hot swarf.
Non-sparking fans, fans out of air stream, spark arrestors, properly maintained equipment and designated working areas, proper risk assessments and controlled working practices, inline filters, fire boxes and pre-separators help prevent hot materials from reaching the dry dust in the dust collector. All of these helps to control the risk of an explosion.
If we can control or remove ignition sources, we have taken a big step in saving lives and property.
Next, we can look at what happens if there is an explosion in the extract system. Measures include explosion relief panels (panels slightly weaker than the rest of the system that will blow out first before the pressure in the extractor becomes to great and direct the explosion away from people, usually to outside) and high-grade ducting to help contain any potential explosion.
We can also look at fire suppression – inbuilt extinguishers, fire boxes and sprinklers in the extraction unit or duct, to prevent fires reaching the critical point where it can explode.
Some metal dusts are very highly combustible in certain forms and pose significant risks. For these, dry dust extraction is not safe enough and wet extraction is needed.
Wet extractors come in the form mainly of wet extraction benches or wet collector systems.
These work by basically immersing the volatile dust directly into water. Either directly at the process/ source point (such as a wet bench) or terminate via a specially ducted system in to a wet collector.
Both wet extraction systems work by having a reservoir of water in and agitate the water using a “screw” or “weir” type device creating lots of moisture droplets in the air. As the dust is drawn into the wet chamber these airborne water droplets adhere to the dust particles making them heavy. The heavy, wet dust then falls out of the air stream and into the water reservoir below. The clean air then passes over the top of the water and out the exhaust.
Wet collectors and wet benches need to be cleaned regularly as the sodden dust will create a mud like sludge at the bottom of the reservoir and the water needs to be changed regularly to prevent stagnation and risks such as Legionnaires disease.
Wet collectors that have a completely encased wet chamber may need to have considerations to vent of hydrogen that can build up in the chamber. Wet benches that have open wet chambers such as wet benches allow hydrogen to escape naturally in to the air.
Vodex MDD Mini Wet Bench
Titanium is a combustible metal. As with all metal dust, the size of the dust particles determines the degree of hazard. The finer the dust, the higher the risk. Fine dust ignites easily and burns very hot. Fine titanium dust and chips can be ignited with a match. As larger chips are mostly mixed with fine dust you should assume a titanium dust will ignite easily.
Fine titanium dust does not ignite spontaneously, as for example zirconium and magnesium does, unless it is as fine as flour. Ignition temperatures range from 300° to 500° C.
To prevent explosions titanium dust must be extracted via wet dust collectors with titanium specification. This includes a design that prevents hydrogen gas accumulating inside the extractor and specific water level controls. It is the formation of hydrogen that increases explosions. By venting the hydrogen, we can control the risk.
If the Titanium does ignite the dust clouds are highly explosive and the ignited powder can burn even when wetted with water. Titanium fires are extremely difficult to extinguish and react violently with water at high temperature. Water fire extinguishers must never be used on a titanium fire.
Wet collection systems dramatically reduce the risk of the titanium dust igniting in the first place but will not completely remove the risk.
The melting point of magnesium is 650° C. However fine magnesium dust can be ignited below 400° C.
Many magnesium grades can contain other metals, e.g. aluminium, manganese and zinc and some of these have significant lower ignition temperature. Magnesium dust can also ignite spontaneously.
Magnesium dust from grinding and extracted via a wet dust collector will also produce hydrogen gas which must be allowed for if the wet chamber on the wet collector is completely encased. Make sure the dust collector is designed to let hydrogen gas escape and not to collect inside the collector. Magnesium dust collectors must be grounded and have an interlock between the fan motor and the water level control. Never mix magnesium dust with other metal dusts. Use dedicated dust collectors only.
Other highly combustible metals include:
Sodium, potassium, lithium, zirconium, hafnium, calcium, zinc, plutonium, thorium, uranium. All ignite readily in fine dust form but not in solid form.
Aluminium is not considered a fire risk unless in powder and dust form with approx. 20% of the dust in particle sizes below 44 microns. Aluminium alloys can contain up to 15% of alloying metals like chromium, copper, iron, magnesium, manganese, nickel, titanium and zinc.
Aluminium is very reactive and the greatest hazard is chemical reactions. Aluminium reacts with water or even moisture in the air to form hydrogen gas. It is therefore essential that dust collectors used for aluminium are designed to vent hydrogen to the atmosphere and not to allow it to collect inside the extractor.
Aluminium dust clouds can ignite and cause considerable damage, particularly if the dust cloud has formed in a confined space, e.g. inside a dust collector. Explosions have been reported with concentrations of about 40 grams of aluminium per cubic metre (0.04 ounces per cubic foot).
The finer the aluminium dust the higher the risk of fire and/or explosion. Aluminium should be extracted into a wet extractor with aluminium specification or into a dry extractor with an explosion relief and a blast barrier of the correct size and installed with the correct specification ductwork and is sited correctly. As far as medical risks are concerned you must make sure employees are not exposed to more than the maximum exposure limit set by COSHH. Aluminium dust mixed with bodily fluids presents a serious health risk.
Ensure you know the dangers of the dust you are creating, not just the respiratory dangers but also the combustible dangers.
In the cases of highly combustible materials it is not just the Control of Substances Hazardous to Health Regulations (COSHH) regulations that need to be considered. The Dangerous Substances and Explosive Atmosphere Regulations may also come into effect. It is imperative that proper risks assessments are carried out by competent, designated persons or professionals.
LEV systems should also be designed and specified by persons whom understand the potentially explosive risks of combustible dusts and how to properly control and mitigate the risks.
Vodex Extraction Services
Vodex Ltd has over 30yrs experience in handling dust extraction and we offer a whole range of extraction products for dust and combustible dusts. We have worked in a very wide range of applications. As always if you need any further information, have any questions or just want to chat about your application or requirements then please feel free to contact us. Its really easy to do.
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