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Good housekeeping : avoid secondary explosions

Making sure the process area remains dust free to avoid secondary explosions

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Section summary
1. What it is about
2. What must be done

1. What it is about

It is not uncommon, when visiting a plant processing powders, to realize that a fine layer of dust is covering the equipment, the surrounding of the process, sometimes the floor.

One must realize that it is a safety risk.

Indeed, dust layers outside the process equipment can have the following effect :

- Cover motors and increase their temperature : it can lead to an ignition of the powder, but if the motor has been designed according to the ATEX zone it is positionned, the temperature should be lower than the MIT and SIT of the powder.

- Create a dust cloud if the powder is put in suspension

This last risk is very often overlooked. Dust deposits may not be seen as harmful, but they can trigger a secondary explosion that can be very dangerous and may even destroy a building. Indeed, if an explosion happens in the process, the explosion shock wave and draft can put in suspension all the dust in the surroundings. If the dust cloud reaches a non ATEX equipment, like a very hot light bulb, a secondary explosion will be triggered.

This type of accident has been very frequent in large grain or sugar facilities. One can refer to the Imperial Sugar Accident in 2008.

2. What must be done

Housekeeping, means cleaning the process area, is MANDATORY for any process industries using powders.

The maintenance must be such that no dust leak is allowed from the process to the surroundings. The operators must be trained to understand the risk and take actions to stop dust emission sources and clean any spill of powder in the process area.

Good housekeeping must be part of the actions from the dust explosion risk analysis and the procedures put in place to do so must be described and respected. Conclusions of the risks analysis must be implemented by the factory.

In order to control dust spillage, the factory must be equipped with the adequate tools, which is typically vacuum cleaners. Those vacuum cleaners should be used to clean the floor, the equipment but also reach any place where dust can have accumulated (on top of pipe racks, cable trays for example). If the vacuum cleaner needs to be manipulated in an area were a dust explosion risk exist, it must be itself certified ATEX (or another norm relevant in the country). The key characteristics of such a vacuum cleaner are the following [Watkins]:
  • All the conductive parts of the vacuum cleaner must be grounded.
  • A particular attention must given to the hoses which should be conductive and grounded or static dissipative.
  • No dust should go through the drive, which must be on the clean side of the filter.
[Watkins] Combustible dust compliance via proper housekeeping, Watkins, PBE, January 2020