The ability of dust explosions to propagate is widely recognized but not fully understood, and this can lead to some myths or misconceptions. We will touch on these myths here, however this article does not provide the complete technical information.
Dust explosions do not propagate against process flow
When a dust deflagration takes place in a vented vessel, the resulting reduced explosion pressure reaches and temporarily exceeds the static opening pressure of the explosion vent panel. Because the pressure differential from the fan (which is used to convey particulate solids into process pipes) will be exceeded almost immediately, this results in a flow reversal and flame propagation downstream the pipe.
Dust explosions do not propagate in small diameter pipes
The primary arguments for this, are that flame propagation is challenged due to heat loss to the pipe walls and the increasing pipe resistance against the expansion flow. A number of experiments observed consistent flame propagation, even when using a smaller pipe diameter. So, the possibility of dust explosion propagation through pipes of small diameters must be carefully considered in the design and operation of powder handling systems.
Dust explosions do not propagate in dust free pipes
Dust explosions do not need large amounts of fuel to propagate over long distances, if any. Experiments have shown that even a 1 mm layer can create a dust explosion hazard in a typical room. During a dust explosion, the volume of combustion gases increases by a factor 8, leading to significant flow expansion. In addition, unburned dust is ejected and able to burn at a further distance as a secondary combustible cloud. These two mechanisms favor dust explosion propagation, even without the presence of dust layers.
EXPLOSION ISOLATION TECHNOLOGY
To prevent explosion propagation, isolation should be used. There are a number of solutions explosion isolation can provide:
- To prevent flame propagation and therefore ignition of a dust cloud in a secondary enclosure
- To prevent pressure piling and flame jet ignition
- To prevent deflagration to detonation transition in pipes
Explosion isolation can be achieved in two ways: either through active or passive systems. Passive systems are activated by the explosion itself, while active systems require tripping by a sensor for activation. Essential is the placement of the device: if the isolation device is placed too far from the ignition, the deflagration can transit into a detonation prior to reaching the device and damage it. If it is place too close to the ignition, it may not be entirely closed and able to block the passage of flame when it arrives.
By reviewing past incidents and experiments we now know that dust explosion propagation in industrial-scale piping is a reality, not a myth, and must be carefully considered in the design and operation of powder handling systems. History has shown that the most devastating dust explosions occur when the initial fireball is allowed to propagate further, far away from the initial event, causing human losses and widespread damage. Explosion propagation can be affected by many parameters. Large scale testing forms an important step in the verification of the dynamics of the isolation barrier formation, its explosion resistance and sealing against hot combustion gases.