Dust explosion protection - how to protect selected devices
Although it is very important to make explosion protection design for every installation because there may be (and usually are) small details which can change the way of protection, there are some standards in protecting typical, frequently used devices. Here is their short list with description of correct dust explosion protection. This list should not be understood as unchangeable and definite. Local conditions must always be taken in mind!
Dust transporting devices
Generally, we can devide dust transporting devices into vertical (elevators, bucket elevators) and horizontal
(screw transporters, chain conveyors, redlers, vibration or fluid transporters, etc.). Horizontal transporters are being protected
against explosions only rarely (usualy when explosion spreading can occur beacuse of short length or specific construction of the transporter)
because the way of operation of these transporters is not whirling the material. Vertical transporters, on the other hand, are protected
almost in all cases becuase there is whirling material in the bottom and on the head of the elevator and there is high risk
of explosion source presence.
If the elevator is placed out of the building, it can be protected by
explosion venting protective system (always consider
the risk of explosion propagation!), if the transporter is placed indoor, they are being protected by
explosion suppression system with one action
element on the head, on on the bottom and if the elevator has significant length (over 20 m), there are also one or more action element
on the legs as a explosion barrier. This action element is
usually activated together with the action element on the head or on the bottom, depending on from which side the explosion spreads.
Devices for millin/crushing the material
With the exception of small opened milling machines used sometimes for producing flour (small volume and big opened parts of the body),
all crushers, mills and similar devices in agriculture, food and fodder industry and for preparing the coal and other similar fuels
for burning are always protected by
explsion suppression systems.
Action element is placed always under the net, to the hopper where the fine material is present.
Filters, cyclones and other dedusting devices
The way of protection of filters and cyclones and other dedusting devices depends on their position and connection to other technology.
If the filters stands alone outdoor (or very next to the external wall) and there can be safety zone set, the
explosion venting can be used (caution!
recoil forces must be considered too!). In other cases (or if the material from the filter cannot get outside - poisons),
the hopper of the filter is protected by one or more action elements of
explosion suppression system.
Inlet pipeline is usually protected by backflow
explosion valve, if
pressure and flame must be stopped (for example if the filter is used also as a industrial vacuum cleaner) or by
explosion chemical barrier (if the pipeline is not
round, has bigger diameter than 500 mm or because of other reasons, such as the sticky material like wet sugar). The bottom of the
hopper is usually protected by rotary valve or double valve (both must be in Ex design with ATEX certificate!).
Silos
Steel, stainless steel or aluminium silos for storing bulk materials, standing alone out of the buildings are almost always
protected by explosion venting system using
membranes (best placement is in the top lid, so the static pressure of the material cannot press the membranes and also for better
settings of the safety zone.
Inlet pipelines (DN 100 and higher) are protected by
explosion valve, placed usually
directly on the beggining of the pipeline (as close to the inlet socket as possible). Inlet pipelines DN 80 of the minimal length of 5 m
are not protected because it was proved experimentally that such small diameters are not transmitting the dust explosion (caution!
We are speaking only about dust explosion! Gas and vapour explosion are being transmitted of pipeline of any diameter).
Pneumatic transport system
Speaking about the pneumatic transport system, there must be considered if the risk of explosion exists in both ways or only
in one way. According to this decision,
explosion chemical barrier (for both ways) or
mechanical barrier (for
explosion spreading opposite to the normal stream) are applied.
The end device of transportation systems are cyclones, rarely filters. These devices are protected the same way as other dedusting
device (see above).
Sorting of the material
Net, vibrating, rotating or other sorting and separating devices are protected only rarely because there is usually only small risk of explosion (the ignition source usually cannot get through the net causing ignition in the part of the machine with fine dust). On the other hand, always consider the risk of ignition source caused by hot surfaces or static energy. The interior of these machines is also usually divided into small volumes, which can be unsufficient for effective explosion (100 l is being used as a minimal volume considered as dangerous).