Silos for fresh wood chips are mostly located outside of buildings and are protected with explosion relief devices.
Especially in the warm season, the wet material carries the risk of spontaneous combustion, and hence ultimately the risk of an explosion.Explosion vents are the most economical solution, to prevent greater damages.
Thereby, it must be observed that sufficiently dimensioned safety areas are established, if the discharge is performed at the height of internal footpaths or roads.
Alternatively it is possible to utilize integrated deflectors, which reduce the danger area and accordingly increase the usable area of operation (Fig. 5).
In the drum dryers themselves as well as the interconnected conveyors and cyclones the process temperatures are inherently very high, so that glowing embers can develop. During the design phase of Explosion Safety it must, however, be considered that due to the elevated humidity a reduced oxygen content is developed in the air.
The latter means that the severity of a possible explosion is lower than with the usual oxygen content of the ambient air. In the fire-lock silos, which are downstream from the drying, any possible glowing embers should be secured in a protected area.
They serve as a “buffer” to avoid carrying glowing embers into the further production process.
This buffer is missing in many legacy facilities – a high risk potential.
Just like the fresh wood chipping silos, fire protection silos are mostly outdoors and can be relieved with explosion vents.
While fresh wood chips are still relatively wet and the spontaneous combustion poses a great threat, there is a threat of combustion through introduced ignition sources for the dry chips in fire protection-silos and the following process steps. Both lead to explosion protection being necessary.
The downstream screens are, in contrast to the silos, usually inside of buildings. Especially when there is no fire-protection silo in the system, sparks or glowing embers can reach the screens.
Through the variety of screen decks within a screen, the utilization of explosion suppression is virtually impossible. In this case, several high rate discharge suppressors must be mounted, in order to ensure a reliable protection. The use of explosion vents or the flameless venting devices is useful here.
Inherently, every mill has metallic parts that approach and touch each other at high velocities, and therefore very probably creates ignition sources. And because these are also confined spaces with a high dust concentration which also contain oxygen, there is basically nothing to prevent an explosion.
Therefore, several mill manufacturers offer devices with an explosion pressure resistant design (up to 10 bar). Depending on the size of the mill this can be very costly for the operator, resulting in explosion protection concepts being considerably more economical. At the air inlet of the mills, the Q-Rohr LF protects against the entry of foreign parts in the process.
Usually the explosion risk leading to protective approaches of particleboard factories ends with surface and core layer of silos. The coating with glue greatly reduces the probability of the build-up of an explosive atmosphere, which is why explosion prevention is rather use here instead of explosion protection.
Last but not least, filter systems play an important role in many places throughout the production of wood-based materials. Dust collection takes place during almost every step of the process. As an example, we will observe the dedusting of the sanding line: Foreign bodies in chipboards can unfortunately not always be excluded.
If the sander then passes over a metallic foreign object sparks will be created, which could reach the filter through the suction force. If the filter is situated within a building, a flameless venting is recommended. Outside of buildings explosion vents are sufficient. Because filter systems are usually installed on the ground level, an integrated deflector on the explosion vent can be useful to channel flames and pressure upwards, into non-critical areas without passenger and vehicle traffic.
In every production facility, even beyond wood processing industry, individual plant components are connected through pipes.
Therefore, an individual observation of the system elements is not sufficient here: The aim of the explosion isolation is to prevent the propagation of flames and pressure, thus protecting adjacent plant components. These are various safety devices available for this.
Figure 6, for example, depicts a non-return valve.
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