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|4. Explosion vents
There is quite a large variety of grinding and milling systems
to reduce the size of bulk solids. However, more or less all of
them are using high speed rotating equipment which can be, in
certain circumstances, the source of ignition of a explosion of
powder. This has always to be confirmed by a
dust explosion risk assessment, but in general, as long as
the powder produced can explode, an ATEX risk exist with many
milling systems and must be managed through specific measures. 2
types of mitigation measures are usually used : a
system resistant to explosion, and explosion vents. In
specific cases, inerting can also be used.
It is critical to 1st PREVENT explosion. To do so, the following measures are advised :
- Maintenance of the equipment following the manufacturer recommendation
- Equip the milling system with instrumentation able to detect a malfunction like vibration sensor or temperature sensors on bearings
- Pressurize bearing seals with compressed air
- Install foreign body prevention equipment at the product infeed like sieve, magnet or metal detectors
It is also critical to be able to detect if the system is jammed, clogged with product, as product trapped in the milling chamber can increase in temperature and ignite.
Manufacturer of pin mills, universal mills, cone mills... often propose this option : the mill is explosion resistant up to 10 bar g. If it is in line with the Pmax of the powder being processed, such strategy can be implemented.
It is however fundamental that the rest of the installation in direct communication with the milling system be also resistant to explosion. To ensure this, the mill is usually installed between 2 isolating elements like airlock rotary valves explosion resistant. If an explosion happens, the explosion will be blocked by the valve upstream and downstream of the mill. In case an air pipe is connected to the mill, a Ventex valve should be installed.
It must be possible to detect the explosion thanks to a pressure sensor or a spark detector in order to stop the whole installation, and especially the valves in and out of the milling system.
The milling systems are usually discharging the powder to a hopper, or to a pneumatic conveying system and then a hopper. It is possible to install an explosion vent on this hopper to release the explosion before it reaches hazardous levels.
The explosion will happen on the mill and will go in all direction, upstream and downstream, it is thus necessary to install isolation equipment to direct the explosion towards the vent and block its propagation to other parts of the installation. It can be through quick acting valves, airlock rotary valves or ventex valve.
Another possibility to manage the risk of explosion is the prevent it through inerting. The oxygene level is kept below the low explosivity limit so that, in case of malfunction leading to a source of ignition, the explosion cannot happen due to a lack of oxygen, the comburant.
Such strategy requires a closed loop for the gas in the installation, in order to be able to keep the level of oxygen low while reducing the consumption of nitrogen. The system must be equipped with analysers to control the system by injecting nitrogen when necessary, or, even more importantly, shutting down the system the oxygen levels are too high.
A milling system must be the object of a dust explosion risk analysis to identify the sources of ignitions and define the right prevention and mitigation measures. Conclusions of the risks analysis must be implemented by the factory.