Except for self-igniting materials such as biologically active solids, silos do not normally contain their own sources of ignition. This means that they lack any significant components that might cause explosions. Nevertheless, there is the hazard that a source of ignition might enter the silo from upstream sections of the plant. Outdoor silos therefore need to be protected with bursting discs, while indoor silos must have flameless venting or explosion suppression systems. Depending on the material that is conveyed by the system, it is also possible to take preventative measures. One prevention system, for instance, is to combine spark detectors with spark extinguishers or quench valves, as this may prevent sources of ignition from entering the silo. However, the use of explosion protection measures should always be discussed with and assessed by the relevant experts.
The same is valid for screens, destoners, etc. These are devices that do not normally have their own sources of ignition, so that explosion protection is usually only required in a very small number of cases, despite the high probability of an explosive mix within a screen.
However, it is important to maintain a comprehensive perspective, as they can often be extremely hazardous, especially in combination with upstream dryers (e.g. spray dryers in the dairy industry, drum dryers in the wood industry and flush dryers in the starch industry). The tumbling and rotating movements of a screen may allow it to ‘open’ to hot embers that were created in a dryer and which have often survived mechanical conveyors where they did not previously ignite. The latest stage at which such embers may be broken down and then cause an explosion is in the screen. This is precisely the scenario that caused one of the biggest explosions in a particle board factory in South America in 2012, when six people lost their lives.
In such cases it is important to provide explosion protection for screens, although this poses special challenges to safety engineers, especially within buildings. The pendular movements usually require dedicated solutions where any vibration-prone elements must be decoupled. Moreover, such solutions need to be based on flameless venting principles (Figure. 2). Other protection methods, such as explosion suppression systems, are often unsuited, as the extinguishing powder cannot be distributed homogeneously within the screen on account of the built-in screen deck.
Figure 2. Protecting a screen with a Q-Box.
It is in the nature of a mill to have metallic parts that collide with one another at high speed, so that ignition sources are highly probable. Again, this is a closed container with a high concentration of dust that may include oxygen, making an explosion extremely likely. Many mill manufacturers therefore offer their machinery in a design that is resistant to explosion pressure (up to 10 bar). Depending on the size of the mill, this may be very expensive for the operator.
Alternatives would be flameless venting solution. In either case a decoupler should be attached both above and below the mill. One particularly smart solution for the air intake of the mill is to install a Q-Rohr LF. This is a modified version of the Q-Rohr, well-known in flameless venting, which does not contain the explosion vent that is normally included. As a result, it is possible to aspirate air under normal operation, and the air can then be fed through the stainless steel mesh filter without any trouble. If an explosion occurs, this filter removes the resulting heat and protects the environment from the flames and the explosion pressure.
This is part two of a three-part article written for World Cement’s February issue and abridged for the website. Subscribers can read the full issue by signing in, and can also catch up on-the-go via our new app for Apple and Android. Non-subscribers can access a preview of the December 2015 issue here.
Read the article online at: https://www.worldcement.com/special-reports/05022016/explosion-protection-part-two/