Combustible Dust

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.

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!

Wet Extraction

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.