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|2. Explosion vents
|3. Flame quench
When studying how a dust explosion unfolds, one can see that the pressure is increasing slightly at the beginning and then very steeply, with an increase of pressure over time equal to Kst, in to finally reach a maximum pressure Pmax. If nothing is done, the explosion will destroy the process equipment and injure or kill people working around. One of the strategies to protect against the explosion is to vent the process equipment as soon as the pressure starts to increase. This way, the pressure inside the equipment stays within the design limit and the pressure shock and flames are vented in a safe area.
The venting, depending on the size of the equipment, can be done thanks to a bursting disc or an explosion door.
The most common type of explosion venting is to cover an opening in a hopper or silo with a pre-cut membrane, typically in steel. The membrane is designed to withstand the normal process conditions, in terms of vacuum or pressure, but to open if the pressure reaches a certain level, which would be the sign of an explosion. Such vents are grouped in the generic term "explosion vents" but are sometimes called explosion panels if they have a rectangular shape or bursting disc in case of circular shape.
The design of such venting panels must follow some norms in order to be set properly in terms of opening pressure and size (area). This type of design must be carried out by trained consultants.
The panel must be equipped with a sensor that will detect its opening, the automation system must then shutdown the process as it means that an explosion may have happened (bursting disc are fragile and may open without an explosion if not installed properly for example). The panel must be positioned in an area where it is safe to vent, typically where nobody is present during production. If not possible, in a building for example, a venting duct can be installed, but it should be considered in the design.
Note that for big silos, it may be possible to install explosion doors, or flaps. These are not based on a membrane but rather a hinged flap that will open, then re-close in case of an explosion.
It may be necessary to vent the explosion in a building. This can happen if the route for an exhaust duct is too complicated or if for hygienic reasons it is not recommended. Just venting the explosion inside a building would have catastrophic consequences as the pressure shock could damage the building, other equipment, or worse trigger a secondary explosion by lifting dust in the production area that would be ignited by the primary explosion flames.
In this case, one possibility is to install a flame quench, also called flame arrester. It is a large device that will cover the explosion panel and will stop the propagation of flames. It is made of a stainless steel grid and filters that allow to stop flame and also reduce the pressure shock.
The installation of the flame quench requires some precautions : it should not be in a passing area where people are supposed to stand, and should not be too close to the wall. Each manufacturer has some recommendations that it is necessary to follow.
This device must also be maintained, especially to make sure that the grid and filter do not get clogged over time. For this purpose, a cover supplied by the manufacturer can be installed.
Explosion vents needs must be defined thanks to a dust explosion risk analysis. Conclusions of the risks analysis must be implemented by the factory.