What are dust explosions that can happen at industrial processes and filtration plants?
Any combustible material will burn faster if the area exposed to the air is larger. A simple example of this is burning wood in a fireplace; a well-built pile of thin branches burns away a lot faster than large blocks of wood. The main difference is that in the former case, a much larger surface area is in contact with oxygenated air. With fine atomized dust, every particle of dust is in contact with the oxygenated air, and therefore combustion can occur very quickly. Under the right conditions, this can be so fast that it is referred to as an explosion.
1: In a grain silo during loading, the light chaff sometimes forms an explosive dust-air mixture
It is well known to most people that a fire requires three factors:
- Fuel;
- Oxygen source;
- Heat source.
For a dust explosion, a fourth factor is added, namely dispersion. A bag of fine wood dust will burn but not explode. The same wood dust finely dispersed through the air will burn so quickly that it becomes an explosion. Such explosions can take place with materials not normally considered explosive.
Examples of industries where dust explosions may occur are:
- Processing organic foods such as sugar, flour, spices, cocoa or coffee;
- Grain and flour, especially when dry grain is transported in silos, can stir up a lot of dust in an enclosed space;
- Wood industry, especially when fine sawdust is released from sawing and sanding, for example;
- Metal industry, especially if fine metal is released by metallization of zinc or aluminum, for example.
- Processing/recycling of plastics or fibers.
In any industry where non-inert dust is released, it is wise to consider whether there is a risk of a dust explosion.
How is it determined whether a dust explosion can occur?
The dispersion of dust is an important factor in determining whether or not an explosion occurs. The Lower Explosive Limit (LEL) and Upper Explosive Limit (UEL) are two limiting values between which concentration of dust in the air must be present to be explosive. In clean or slightly dusty air, there is not enough fuel in the air for the explosion to happen. With very dusty air or dust collected in a bag, the fuel is there, but there is not enough oxygen present for the explosion to occur.
Thus, for an explosion to occur, it is essential that the fuel and air be in a certain ratio. In most industrial processes with good extraction, dust generation is low enough that the dusty air remains below the LEL everywhere, and there is no risk of explosion. Good continuous dust extraction is very important here to keep the dust concentration low. In manufacturing processes where people are present, this is usually ensured because no one likes to work in a very dusty room and good extraction is already in use for this reason.
In areas where people are not present, this is not always guaranteed. For example at grain silos or other silos for bulk goods, these are usually filled at the top via an elevator where fine dust swirls up. This can cause a very dusty smell in the silo. If there is a spark, the silo can explode with far-reaching consequences.
Why dust explosions are a risk to compressed air cleaned filters
With self-cleaning dust filters, a disadvantage is that here the dust is concentrated, so there is always a lot of fuel. During cleaning, the collected dust is blown off the filter cartridge with a blast of compressed air, which then creates a fine dispersion of dust particles.
Because it is inevitable that an explosive situation will occur in the filter, it is critical that when extracting such substances, the filter be explosion proof. By using check valves, it is also possible to isolate any explosions to the filter, protecting the rest of the plant from the expanding explosion.
What ATEX standards apply to industrial dust filters?
Equipment approved according to the ATEX 114 directive may be used in different conditions depending on zoning. The zoning depends on the type of explosion risk (gas/vapor or dust), and the frequency of the explosion risk.
Machines can also have different zoning for different parts. This is especially true for dust filters, usually the dusty air is drawn in through an intake duct and on the outside of the filter there is very little or no dust in the air.
| Frequency risk of explosion | Gas explosion hazard | Dust explosion hazard |
| < 0.1% of operating time | Zone 2 | Zone 22 |
| > 0.1% and <10% of operating time | Zone 1 | Zone 21 |
| >10% of operating time | Zone 0 | Zone 20 |
What ATEX zonings are typical for industrial air filters?
In an extraction plant, different parts of this plant are in contact with different concentrations of dusty air. Indeed, the function of a filtration system is to separate the dust from the extracted air so that clean air and concentrated dust remain, which are discharged separately. Therefore, the different parts of the installation must usually meet different standards.
Due to pulse cleaning, there is very regularly a cloud of dust created in the dirty part of the filter. ATEX filters are therefore almost always designed for ATEX zone 20 on the “dirty side” at the filter cartridges. All components that are in direct contact with the inside of the filter must therefore be suitable for ATEX zone 20, such as, for example, full detectors or rotary valves.
When the filter is working properly, the fan will only draw in clean air. However, it can happen that a filter cartridge ruptures and dusty air still passes the filter cartridges. The chance of this happening is very small and <0.1% of the operating time, but cannot be ruled out. That is why the fan is internally designed in ATEX 22.
The outside of the filter is usually in an area where no large amounts of dust are present in the air. No additional ATEX measures therefore need to be taken here
How is the explosivity of industrial dust determined?
The explosiveness of substance is highly dependent on several properties. First, of course, there are the chemical properties; substances that oxidize more easily releasing a lot of energy will be more explosive than substances that are relatively inert. Especially organic substances and petrochemicals are often explosive, although various metals can also be highly explosive.
Secondly, the particle size of the dust is important, typically an explosion with fine dust will be faster than an explosion with coarser dust. Therefore, it is very important to know the particle size of the dust being released. Dust released from metallization or shoveling, for example, is finer than the dust released from grinding or sawing metals. In wood processing, the dust released from sawing is similar a coarser than the dust released from sanding.
An explosion ball in which the explosivity of substances is determined.
Numbers of explosiveness
For the explosivity of dust, there are two important key figures:
- Pmax: The maximum pressure increase that occurs when all the dust is burned.
The higher the pressure created, the more hot gases must be discharged to lower the pressure again. - Kst: The rate at which the pressure increases.
The faster the explosion occurs, the less time there is for the resulting high pressure to dissipate.
The determination of these key figures is done in an explosion sphere. Here the dust is finely distributed throughout the sphere with a blast of compressed air, and then the dust mixture is ignited with an igniter. The pressure build-up is measured where the total pressure created and the rate at which it is reached determine the values for Pmax and Kst.
Customer example: Stainless steel explosion-proof filter for chemical processes
For a customer working with various chemicals, we supplied a large stainless steel filter for a new plant where reactive chemicals are handled. These substances are both chemically reactive and explosive.
For this purpose, Naaykens’ has designed a filter made of chemically inert stainless steel that can withstand the explosions that might take place.
For this project, the dust production is also relatively high with an expected dust production of 500 kg/h. Therefore, dust disposal via screw conveyor has also been chosen so that the collected dust can be disposed of in bulk.
ATEX measures applied
| Order Reference | 84418-010 | 84418-020 |
| Filter type | PJ EA 80/25 – ATEX – STAINLESS STEEL | PJ EA 4/25 – ATEX – STAINLESS STEEL |
| Flow rate | 75,000 m3/h | 2,000 m3/h |
| Dust load | Max. 6.6 g/m3 – Max. 2000 kg/h | 1.5 g/m3 |
| Explosion values dust | Pmax = 4.76 barg and Kst = 37.5 bar.m/s | |
| Material | STAINLESS STEEL 304 | |
| Specifications | ATEX 20 inside, ATEX 22 outside. 4 Flameless pressure reliefs | ATEX 20 inside, ATEX 22 outside. 1 Flameless pressure relief |
For this project, several measures were taken to make the filters safe. The measures taken for ATEX filters can be roughly divided into two categories: explosion prevention and measures to make an explosion occur safely. Measures to prevent explosions are primarily aimed at preventing a source of ignition. After all, due to the function of the filter, the other three conditions cannot be prevented – the presence of oxygen, fuel and the dispersion of this fuel.
Prevention of the occurrence of ignition sources consists of:
- Preventing the generation of sparks due to static electricity.
All metal parts are connected to a grounding system to prevent the buildup of static electricity, and antistatic filter cartridges are applied; - Preventing the generation of sparks from applied equipment.
Only ATEX certified equipment such as full detectors and conveyor screws are used.
Photos of the large filter and fans, before transport to the factory for assembly. At the suction trays you can clearly see the reinforcements.
In the middle is a flange connection for the suction pipe, next to it a connection for the flameless pressure reliefs.
Safe operation of an explosion is guaranteed by:
- Drain the resulting pressure in a safe manner; in this case via flameless pressure reliefs;
- Explosion isolation by means of check valves;
- Sizing and reinforcing the filter so that it does not deform plastically due to the high pressures.
In the pictures above, the additional reinforcements on the hatches, inlet trays and funnels are clearly visible.
Flameless pressure relief
Because of where these filters will be located, a rupture disk is not a convenient option. The pipe for pressure relief would become so long that the pressure build-up in the filter could become very high.
Therefore, a flameless pressure relief system was chosen here. This flameless vent is sealed with a plate which is kept closed with a spring. During an explosion, the high pressure pushes this plate open after which the hot gases can flow past the plate.
The flameless pressure detonator has a so-called flame stopper, where hot gases can flow past and these are cooled, extinguishing the flames. Because explosions involve only a relatively small amount of gas, it is possible to have enough passive cooling so that the resulting gases are cold enough that they cannot cause a further fire outside the filter.
Flameless pressure reliefs on filters
Reference ATEX filter: Metallization with aluminum
For a client that metallizes/shops with aluminum, we delivered an ATEX filter to capture overspray. Aluminum dust is one of those substances that is not easily expected to be explosive. However, pure aluminum oxidizes fairly quickly, with the resulting oxidation forming a self-protective layer to prevent further oxidation of the underlying material.
This is easy to see for yourself by scraping an aluminum part with a steel knife, the dull oxide layer is then scraped off after which the shiny pure aluminum is visible. However, it does not take long for the dull oxide layer to recover.
The dust produced by metallization is also not yet fully oxidized and, with the right (or wrong) dispersion, therefore explosive. Using an explosivity test, this has also been demonstrated.
So for this client, we provided ATEX filters to prevent explosions from happening, and when they happen that they happen safely.
Since the location of the filter in a building is on the third floor, a convenient option here is to apply a simple rupture disk. This rupture disc leads to the outside, where in the event of an explosion it is immediately relieved to the outside. Since this occurs at a high altitude there is no risk of a hazardous situation, except perhaps a bird flying by at an unfortunate moment.
ATEX measures at a metallization filter
Technical data
| Order Reference | 85561 |
| Filter type | 2x PJ EA 20/25 – ATEX |
| Flow rate | 13,500 m3/h |
| Fabric Type | Aluminum Dust |
| Dust load | Max. 3 g/m3 |
| Explosion values dust | Pmax < 10 barg and Kst < 200 bar.m/s |
| Specifications | ATEX 20 dust side, ATEX 22 clean side. |
Explosion isolation dust filters with check valves
The explosion is also isolated in the filters by the use of check valves. These are clearly visible in the photos by their recognizable red color. These check valves are mounted at a distance from the filter for a good reason – during an explosion, a pressure wave forms in front of the flame front which propagates through the pipe. This pressure wave causes the shut-off valve to close before the flame front reaches it. The piping between the filters and the check valves is a critical factor here, ensuring that there is enough time for the pressure wave to close the valve.
So, now you have a better idea of what ATEX filters Naaykens’ has provided. Do you also need an extraction for explosive dust? Don’t take any unnecessary risks and let us advise you!