First, in this article, we take a look at how a dust explosion occurs. Then we look at the ATEX zonings used for environments with (dust) explosion risk and what protective measures can be applied. Finally, we provide insights on sensors that are useful in explosive atmospheres and manufacturing processes with a risk for a dust explosion.
How does a dust explosion occur?
An explosion is a phenomenon that depends on three elements, which are the conditions for an explosion to occur, these are gathered in the so-called danger triangle.
These three factors together form the basic condition under which a dust explosion can occur. The three elements of the explosion triangle consist of:
- oxygen, food source to grow a flame/explosion
- fuel, the combustible substance, for example, milk powder
- ignition source, the factors that can cause a spark and thereby ignite the fuel
Eliminating at least one of these factors prevents an explosion from occurring.
When a layer of dust is allowed to build up, static discharge or scalding in the dust layer can create a spark. When this spark is fed by oxygen in the atmosphere, the dust layer can ignite, leading to a dust explosion.
In practice, the main focus is on ruling out the source of ignition, as the fuel often cannot be replaced for an alternative and ruling out oxygen is often not an option either.
A fuel is not always a flammable liquid or gas!
Industrial accidents and large-scale (dust) explosions have shown that it is necessary to look beyond the obvious.
Dust explosions can occur in all kinds of industrial processes where unoxidized substances susceptible to oxidation are handled. Application examples include factories where wood dust, flour, milk, graphite, metal powders and so on are handled.
Examples of at-risk situations include:
- Milk powder in big bags or sugar in silos in the food processing industry
- Flour and flour in an industrial bakery or flour mill
- Cement powder in a silo at a concrete plant
- Rubber granules in a storage terminal
- Sawdust in a wood processing hall
- Animal feed in storage halls or silos at animal feeds
In these situations, an explosion hazard can occur due to the presence of dust particles and oxygen. When dust layers can build up sufficiently, friction (heating) in the dust layer can cause the core temperature to rise. When the temperature rises high enough there is a risk of self-ignition of the dust mixture, thus fusion is a potential ignition source.
ATEX zoning for dust: Zone 20,21 and 22
According to the ATEX 153 directive, explosive atmospheres in dust environments are classified based on the explosion hazards present and the degree to which the explosion hazards are or may be present.
The following zones are distinguished:
Zone 20: An area in which an explosive atmosphere is present regularly and for prolonged periods during normal operation. As a starting point, it can be assumed that the explosion hazard is present for at least 1,000 hours per year, or at least 10% of operating time.
Zone 21: An area where there is a high probability of the presence of an explosive atmosphere in normal operation. This assumes a duration of at least 10 to 1,000 hours per year, or 0.1% to 10% of operating time.
Zone 22: An area where there is a low probability of the presence of an explosive atmosphere in normal operation. This is assumed to be less than 0.1% of operating time, or less than 10 hours per year.
Atex Zone 22 is the most common Ex zone, an estimated 95% of ATEX applications have a Zone 22 designation. Of the remaining 5%, the vast majority receive a Zone 21 designation and Zone 20 is relatively infrequent.
What are possible measures?
- Apply only Ex-certified equipment that complies with the ATEX 114 directive and is sufficiently protected to be used in explosive atmospheres
- Take measures to eliminate ignition sources
- Ensure that dust cannot accumulate to form a dust layer in which scalding can occur
- Ensure that no (sudden) air swirls can occur that could blow up the combustible dust and expose the brewing layer and ignite and explode by mixing with oxygen
- Provide a thoroughly designed installation that avoids the build-up of static electricity as much as possible and eliminates static discharges
When the build-up of a dust layer cannot be prevented to eliminate the risk of scalding, apply a fire and scalding detection system to actively monitor the dust layer.
ATEX-compliant sensors
Sensor Partners has several options in the field of ATEX certified sensors and solutions. In addition, Sensor Partners works with several partners who have the necessary expertise to build and deliver Ex-certified solutions. The following is a sampling of our range of Ex-certified sensors.