How should Atex be applied to LEV filter units?

 

 

Employers must undertake a Risk Assessment and classify areas where hazardous explosive atmospheres may occur into zones.

 

Only equipment appropriate to the designated zone should be used.

 

The LEV Filter (Extraction) Unit

 

Most filters will have an explosive dust cloud inside at some point during normal operation.

The inside may be areas in which an explosive atmosphere caused by air/dust mixtures are present continuously, for long periods or frequently, or areas in which such an atmosphere is likely to occur, depending on the operating conditions.

 

Many filters are located in the open air, or in a room in a building which does not need to be classified as hazardous.

 

With the exception of 5(a) and 7 the description overleaf of different cases assumes that filters themselves will not be a source of dust release that would give rise to a potentially explosive atmosphere in the surrounding area.

 

This description also considers that many apparatuses with filters inside are fitted with explosion protection devices, such as vent panels, doors or suppression equipment.

 

Most filters and silo bins will have an explosive dust cloud inside at some point during normal operation.

 

The inside may be areas in which an explosive atmosphere caused by air/dust mixtures are present continuously, for long periods or frequently, or areas in which such an atmosphere is likely to occur, depending on the operating conditions.

 

Seven different sets of circumstances were considered, advice is given overleaf for each of these cases: 

  1. The filter or the silo bin has no moving parts or electrical equipment on the inside, and is located  in a non hazardous area.

  2. The filter has moving parts inside that can be considered as mechanical equipment, such as a bag shaking mechanism, or a screw feeder to remove collected dust. The whole filter is located in a non-hazardous area.

  3. The complete filter or the silo bin has electrical equipment inside. In filters those electrical equipment may be a pressure switch, or level switch odn the container that collects the dust, in silos level indicators are widely used.

  4. The silo bin or the complete apparatus with the filter is fitted by the manufacturer with explosion vent panels or doors, supplied by another manufacturer.

  5. The silo bin or the complete apparatus with the filter is fitted with explosion vent panels or doors produced and integrated into the filter or silo by the filter/silo manufacturer themselves.

  6. A - normally small - apparatus with only a filter sock, plastic collection bag and fan, but no metal enclosure.

  7. The silo or an apparatus with a filter is intended to be installed in an area, in which air/dust mixtures are unlikely to occur or, if they do occur, are likely to do so only infrequently and for a short period only.

 

1- The filter or the silo bin has no moving parts or electrical equipment on the inside, and is located in a non hazardous area.

 

Conclusion:

These filters or silos are not in scope of the Directive 94/9/EC.

Electrostatic hazards may exist from insulating surfaces inside the filter, from the filter elements or from cone discharges in silos. This risk depends for example on the properties of the dust being collected, and other operating conditions. But any electrostatic risks are not considered as giving the filter or silos its own potential source of ignition, so these filters or silos do not fulfill the definition of equipment in Article 1(3)a.

 

Remark: This filters or silos even do not fulfill the other criteria of the definition.

 

The electrostatic risks can be covered by other directives, for example the Machinery Directive when the filter is part of a machine. In this case the manufacturer of the machine is responsible to avoid this risk according to the regulations of the Machinery Directive. In all cases these risks must be controlled by the user under Directive 1999/92. The electrostatic risks are covered in

EN 13463-1.

 

2 - The filter has moving parts inside that can be considered as mechanical equipment, such as a bag shaking mechanism, or a screw feeder to remove collected dust. The whole filter is located in a non-hazardous area.

 

Conclusion:

 

The manufacturer must assess whether the moving parts create its own potential source of ignition. If the moving parts do not create any potential source of ignition, perhaps because they have low power, or move very slowly, the situation is the same as case 1, and the filter is not in scope of the directive.

 

Remark: Low power in this sense is not given, when for example the power source is strong and only the power inside the equipment is reduced by protection methods in order to avoid an ignition risk. There is a similar situation in case of the electrical type of protection the "intrinsic safety".

If the mechanical equipment on the inside does create an ignition risk, this equipment (as part of the complete apparatus) must comply with the Directive 94/9/EC (see chapter 4.1.2.3 of the Guidelines).

 

If inside the filter an explosive atmosphere caused by air/dust mixtures is present continuously, for long periods or frequently, according to Annex I for the equipment inside conformity with category 1 should be reached. But this will in respect of the state of the art not always be possible. In these cases according to

  •  Annex II A technological knowledge must be taken into account, AND

  •  Annex II 1.0.1 the principles of integrated explosion safety must be applied.

That means when it is not possible to prevent the ignition source sufficiently - according to the "state of the art" - to reach category 1, category 2 can be sufficient when the manufacturer takes additional measures "to halt it immediately and/or to limit the range of explosion flames and explosion pressures to a sufficient level of safety" (see Annex II 1.0.1 indent 3). It is in the responsibility of the manufacturer to take this decision.

 

The explosion vent can be seen as an example of integrated explosion safety as described under Annex II 1.0.1. In this case, and if the complete apparatus (filter with explosion vent panel or doors) is produced and integrated by the same manufacturer, not only the mechanical but all equipment inside falls under the scope of Directive 94/9/EC. Consequently the manufacturer takes the following measures:

  • Preventing sufficiently the ignition source inside (according to the "state of the art");

  • Selecting an appropriate protective system in order to limit the range of explosion flames and pressure;

  • Designing the filter in such a way that it can withstand an internal explosion without rupturing (design for the reduced explosion pressure in conjunction with explosion pressure relief or explosion suppression).

 

3 -The complete filter or the silo bin has electrical equipment inside. In filters those electrical equipment may be a pressure switch, or level switch on the container that collects the dust, in silos level indicators are widely used.

 

Conclusion:

 

This electrical equipment is equipment in the sense of Article 1.1 of the Directive 94/9/EC and therefore must comply with this directive.

 

4 - The silo bin or the complete apparatus with the filter is fitted by the manufacturer with explosion vent panels or doors, supplied by another manufacturer.

 

Conclusion:

 

These panels or doors are 'protective systems' in the sense of the Directive 94/9/EC and the manufacturer of these systems has to apply the directive when placing this as an autonomous system on the market. That means the procedure set out in Article 8.2 has to be applied and they must be CE and Ex marked. Selecting the correct panel or door (for example: size, quality, function) depends on the application and has to be done by the manufacturer of the apparatus