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|3. Explosion mitigation
A dust explosion risk analysis (often called ATEX risk assessment) aims at identifying the ATEX zones, where an explosion can occur, defines the risk associated with any potential source of ignition, and propose measure to avoid or mitigate the risk.
This page is detailing the solutions to avoid an explosion, or to mitigate it if it happens.
When an ATEX area has been identified, which means that any point of time, a dust cloud can be present in explosive concentration, the 1st attempt of the process designer or operator should be to avoid the risk of explosion. Most of the time the approach is based on making sure that there will be no ignition source in the area.
The sources of ignition, and possible measures to avoid them is detailed in the following table :
Table 1 : sources of ignitions and how to avoid them
|Source of ignition||Main actions to avoid the source of ignition
For details, please refer to the links
|Electrostatics||Sparks : ensure electrical continuity all along the
installation. No conductive part must stay isolated.
Cone discharge : limit the diameter of hopper / silos
Propagating brush discharge : for non conductive material, select material with low breakdown tension < 4 kV
|Mechanical sparks or hot spots||Mechanical sparks : for stainless steel equipment, make sure
the tip speed of rotating equipment is < 1 m/s
Hot spots : if parts are rubbing against each other, heat is generally contained if tip speed is < 1 m/s
|Electrical equipments||When selecting an electrical equipment for an ATEX area,
make sure the equipment is certified for the appropriate zone
20, 21 or 22
For motors, make sure the maximum temperature is less than MIT and SIT of the powder that may become in contact with it
||Works as welding and cutting must be clearly regulated and submitted to work permit in ATEX area|
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When the measures listed above cannot be ensured, it is still possible to avoid the ignition by inerting the process. The oxygen concentration is then not enough to allow a combustion. The process can be gassed with nitrogen or carbon dioxide for this purpose. This can be quite costly and requires some specific process measures and safety chains to make sure the process is actually inerted.
It also works only inside the process equipment, it is not valid in
the immediate surrounding of the process which may also be an ATEX
Making sure there is no deposit of dust outside the process is also a measure, for certain zones and situation, to make sure dust will not be put in suspension and create an hazardous atmosphere.
In certain cases, the explosion risks cannot be controlled by simply removing the source of ignition, or inerting is too complex and costly. In this case, the ATEX risk assessment should show that the risk has to be mitigated.
Of course, all good practices MUST STILL BE IMPLEMENTED TO PREVENT THE EXPLOSION, even if mitigation measures are in place.
The 1st possibility to mitigate the effect of an explosion is to have a system able to resist to the present increase in case of explosion and isolate the explosion inside a process area. It can typically be used for powder having a Pmax < 10 bar g and is often used in milling equipment.
The equipment must be calculated and certified for Pmax and
physical barriers like valves, must be used to isolate the
explosion. This solution is efficient but the equipment may be
damaged during the explosion and have to be replaced.
The 2nd solution is to allow a part of the installation to open at the very beginning of the explosion, and vent the explosion outside of the process equipment. Such venting is usually done thanks to a bursting disc or an explosion panel positioned on a hopper or silo.
The size and opening pressure of the venting equipment is very important to ensure a good protection in case of explosion and should be calculated by a reputable company, from the explosion characteristics of the dust. As for other explosion protection systems, it is necessary to isolate the explosion if it happens and keep it in the area of the vent. The explosion should not be allowed to propagate to several process sections.
There is a potential hazard where the explosion is vented, due to the pressure wave and to flames. It is necessary to vent the explosion in a safe area, without the presence of people. In some cases, it is possible to install a flame arrester if the venting is to be done inside a building.
The last strategy that can be put in place to mitigate an explosion is to suppress it. The suppression can be done thanks to chemicals injected in the process and that will stop the explosion prior it reaches hazardous pressures.
The installation of such system requires specific knowledge and a reputable company must be contracted to carry the design and the installation.
Such system should be coupled with an isolation system as well.
As mentionned above, it is necessary to isolate the explosion happening in one area of the process, to the other area, to avoid that an explosion on one side triggers another explosion somewhere else.
Active isolation systems : Quick acting valves
Passive isolation systems : Ventex valves, airlock rotary valves
The measures put in place to avoid ignition sources and the type of explosion mitigation chosen, must be described in the ATEX risk analysis and the measures to put in place explained. Conclusions of the risks analysis must be implemented by the factory.