Filters: an abundance of dust

Filters are used to separate dust from the air stream. Important applications are exhaust systems and central vacuum cleaning systems. But filters are also applied on pneumatically filled silos and at the end of e.g. a ring or flash dryer. In all cases the air passes through the filter elements and the (fine) dust remains behind on the filter element. To prevent clogging of the filter after a certain time, the filter elements must be regularly cleaned. This is usually done by means of a short pulse with pressurised air. Filters: frequent explosions In most cases the average dust concentration is (far) below the lower explosion limit. Yet explosions in filters do frequently occur. This is caused by the fact that, when the filter elements are cleaned, there is always a cloud of very fine dust that develops around the filter elements. Filters: the risks Ignition sources can be imported from the outside. E.g. from dust extraction on machines. Even if the extracted spark is not strong enough to ignite a dust cloud, it can, if settling in a dust layer, be the origin of a smouldering fire on a filter element and eventually lead to an explosion. Another frequent cause of explosions is static electricity. Powders become, by the pneumatic transport, often heavily charged. If the filter elements used are not antistatic and badly earthed, this can lead to dangerous discharges. Inevitably in filters there are often in dead corners. If the cleaning is insufficient, and the product handled is vulnerable to it, this might lead to auto-ignition. Filters: how to protect them Ignition source detection If there is a real risk that sparks could be imported with the product, ignition source detection, combined with an extinguishing system or possibly a shutting valve, is to be recommended. Dependant on the products, ignition sensitivity, antistatic filter elements are chosen or necessary. Thorough earthing of the metal filter cages is always necessary. If the filter casing is coated on the inside it is important that this coating's breakdown voltage is not too high, because otherwise dangerous propagating brush discharges are possible. Constructive protection Both pressure resistant design (particularly for small filters), and explosion venting and explosion suppression are appropriate constructive protections. In such a case it has to be taken into account that a filter is no “empty” casing, but that it is filled with filter elements. Frequent mistakes A frequently made mistake is to place the filter elements in front of the vent opening. In case of an explosion an enormous air flow towards the outside develops. The (relatively slack) filter elements are pushed into the opening and clog it. Bursting discs should be positioned preferably below the filter elements. If necessary, some filter elements have to be cut down to size, at the bursting disc. Another possibility is to eliminate some filter elements and to construct a kind of cage structure. This way free passage towards the bursting disc is always guaranteed. When explosion suppression is applied, sometimes only a few bottles are placed on the cone, below the filter elements. This way it is however impossible to inject the powder far enough between the filter elements. And it is exactly between the filter elements that an explosion often starts and consequently can develop nearly undisturbedly. For an effective suppression of the filter either injection points have to be installed in several places between the filter elements or corridors must be created that can be filled with extinguishing powder. Filters: what about in- and outlets? The product inlet: as a consequence of the explosion pressure the explosion is “blown back” into the inlet: isolation is certainly necessary. The following systems are preferable: a purpose-built check valve, an explosion relief stack or a chemical barrier. The air outlet: if explosion suppression is applied, in most cases isolation of the air outlet is not necessary, because the explosion will be extinguished before it has been able to burn itself through the filter elements. In the case of explosion venting the fire will certainly get through the filter elements. If the clean air is blown out at a safe place, no isolation is necessary. If this is not the case, isolation is necessary. In most such cases a Ventex valve is applied. The product outlet: here an additional protection is not necessary if the outlet is equipped with an explosion resistant rotary valve or, if can be guaranteed that there will always be that much product in the outlet that propagation becomes impossible.

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