Figure 1: A filter box equipped with a fire extinguisher

Filter boxes are frequently used for cleaning air flows in industrial applications. Precautions must be taken to prevent explosions if there are combustible powders, fibres, or particles present in the air. Filter boxes are often part of a collection system intended to dilute released powder into harmless concentrations and collect it in a single, central location. The link to explosion protection is clear.

There is a lot of uncertainty regarding filter boxes and explosion protection. Frequently asked questions on the subject include:

  • Do explosion-proof filter boxes exist?
  • What must be done by the employer, and what can be expected from the manufacturer?
  • How should we deal with an “old” filter box?

This article answers some of these questions.

EXPLOSIVE ATMOSPHERES

To prevent explosions, the employer must first determine where explosive atmospheres could present, both in and around the filter box. These areas are subdivided using a zone classification to illustrate the correlation between the presence of a combustible mixture and the filter box’s operating time, or the duration of work on the filter box (such as opening an inspection hatch).

Filter boxes often use the following zone classifications:

  1. The internal area of the filter box on the dust side is classified as zone 20
  2. The internal area of the filter box on the clean side is classified as zone 22, or a non-hazardous area (NGG)
  3. The area around the filter box is classified as zone 22, or a non-hazardous area (NGG)
Zone 20 Explosive atmosphere >10% present in relation to operating time or activity.

Zone 21 Explosive atmosphere >0.1% <10% present in relation to operating time or activity.

Zone 22 Explosive atmosphere <0.1% present in relation to operating time or activity.

NGG Likelihood of explosive atmosphere negligible due to the organisational and/or technical precautions taken. This area does not require measures against ignition sources.

IGNITION SOURCES

In the zones, measures are taken against ignition sources and to limit the consequences of an explosion. The Working Conditions Act therefore requires all equipment and safety systems in use to be explosion proof. That means that they must not be able to ignite the explosive atmosphere and must be constructed according to the ATEX 114 product guideline. The physical and chemical properties of the product to be captured are essential to choosing the right equipment.

In practice, however, the choice is often based on invalid product characteristics or process conditions. The following table illustrates the relationship between the ambient temperature and the minimum ignition energy of skim milk powder. As the temperature increases, the ignition energy decreases. If this is not taken into account when selecting equipment and safety systems, there is a good chance that the wrong choices will be made, resulting in a false sense of securit

Process temperature 25 °C 45 °C 80 °C
Minimum ignition energy 30 mJ 10 mJ 5 mJ

EXPLOSION-PROOF FILTER BOXES

Explosion-proof filter boxes offer employers peace of mind. The manufacturer can use an EU statement of conformity to prove that the filter box meets the relevant product guidelines. All guidelines for safe use are included in the relevant user manual.
However, explosion-proof filter boxes do not officially exist. This is because the ignition sources associated with a filter box are not inherently tied to the operation of the filter box, but instead depend largely on the correct use thereof.

For that reason, filter boxes must be equipped with potential equalisation and an anti-static filter medium to limit the buildup of static electricity. User instructions must also be created. The users of the filter box must be made aware of the instructions, and a supervisor must be appointed to monitor proper compliance. Instructions can be safeguarded using a work permit system.

Incorrect use is a frequent occurrence in practice. People sometimes forget to connect the potential equalisation or to check it regularly, allowing static electricity to build up and create sparks (figure 2). If the filter medium becomes clogged, the resistance in the pipeline can also increase, reducing the air speed in the pipeline and allowing the powder to accumulate.

Figure 2: A spark caused by lack of potential equalisation.
Figure 3: Accumulation of powder in pipeline.

APPLICATION OF EXPLOSION-PROOF EQUIPMENT AND SAFETY SYSTEMS

In some cases, equipment is used in filter boxes which are inherently associated with ignition sources. This includes things like level metres or rotary valves. For that reason, such devices must be explosion proof and comply with the ATEX 114. The same applies to safety systems intended to stop an incipient explosion or limit the effects of an explosion. These could include things like an explosion relief panel or a ‑suppression system.

The manufacturer will often add a comprehensive type plate to the filter box to inform the user that the box uses explosion-proof equipment and safety systems. This type plate only applies to the products used inside, not to the filter box itself. This is permitted as long as the original type plate on the products are not removed, and the products are used within the scope of the original manufacturer’s intended use. In this context, the filter box is explosion proof as an “assembly”.

FILTER BOXES OLDER THAN THE CURRENT ATEX GUIDELINE

Figure 4: The relationship between zone classification and ignition source.

Prior to 2003, there were almost no explosion-proof devices for dusty environments. The employer must therefore conduct a risk assessment to demonstrate the safety of the filter box. What the risk assessment basically comes down to is that, as the likelihood of the presence of an explosive atmosphere increases, measures against ignition sources must be increased accordingly. The consequences of an explosion must also be taken into account. Such a risk assessment is usually recorded in an explosion protection document or in the filter box’s technical documentation.

Temporary additional measurements can be taken if the risk assessment indicates an unacceptable safety level. For instance, an explosion relief panel can be used to limit the effects of an explosion. Calculations and strength tests must be conducted to demonstrate that the filter box is suitable for this. However, in many situations, it is easier to write off an outdated filter box and replace it with a properly equipped model. As employer, you can ensure a safe and healthy workplace for your employees by operating within the scope of the filter box’s intended use and continuing to perform the required inspections and maintenance on the filter box.

Written by Frank de Jager, Senior Consultant at D&F Consulting B.V. Frank is an ATEX expert at D&F’s Business Unit Process Safety. www.denf.nl