Acceptable Explosion Protection for Dust Filters

Introduction

Given the number of technical questions raised recently by our customers, filter manufacturers and explosion experts on how to protect industrial filters, it appears there is some confusion on how to deal with acceptable explosion protection for dust filters.

This document aims to present some guidelines and highlight important considerations concerning protection strategies for industrial dust filters. 

The German VDI 2263 suggests explosion protection is only required when the Minimum Ignition Energy (MIE) of the dust is below 1 mJ. For MIE values in excess of 10 mJ, preventive measures are sufficient. In between 1 and 10 mJ “expert advice should be sought”. Yet, according to other sources, explosion protection of filters is always required. 

In this document, we will investigate, starting from the ATEX directives, under which conditions it is possible to rely on explosion prevention only for protecting dust filters.

Explosive atmospheres

Although dust filters can be used in applications where there are high dust concentrations, many filters are used in extraction systems where the average dust concentration is below the Lower Explosion Limit (LEL).  Therefore it is tempting to state that in such filters an explosive atmosphere is unlikely to arise. 

However, when the air (with low dust load) enters the filter, part of the dust fines will settle down on the filter elements.  In order to prevent clogging of the filter elements, from time to time an air pulse strikes the filter elements, which will release the fines.  As a consequence, with each cleaning pulse, a dense cloud of very fine dust is typically created around the cleaned filter elements.  And since in common filters such pulses are created frequently, an explosive dust cloud is to be expected frequently, at least in a part of the filter housing. According to the zone definitions as per the ATEX directive 1999/92/EC (ATEX 153), the consequence of this pulse cleaning is that a zone 20 situation will be present.

Of course there are exceptions to this general rule.  In some situations the dust concentration in the extracted air is extremely low so that manual cleaning once a day will suffice to keep the filter operating properly. In other situations, no cleaning at all over long periods of time can be acceptable such as in a secondary filter in the outlet of a common filter.  

Acceptable risk according to ATEX

According to ATEX 153 equipment in a zone 20 should be certified as category 1D, with reference to ATEX directive 2014/34/EU (ATEX 114). According to ATEX 114, the requirements for cat 1D equipment are that ignition sources may not even arise in case two independent fault situations or a “rare” fault situation occur.

The two faults or rare fault scenarios should be the starting point for performing the process risk analysis in order to determine if preventive measures intended to avoid ignition sources, will be sufficient to reach an acceptable safety level.

In other words, for a zone 20 environment inside a filter, relying on preventive measure solely, is only then sufficient if ignition sources are not to be expected even in case of two independent faults or a rare fault occurrences.

Example - if a (conductive) filter element is not earthed, it may become charged and create spark discharges towards the filter housing.  In order to prevent such discharges, the filter element should be earthed.  Even then, such discharges still need to be considered: suppose one element was forgotten, or a filter element becomes detached and falls down. 

This is certainly not to be considered as a normal situation, but can hardly be excluded in fault situations. A spark discharge should at least be considered as a rare fault situation. But even such a situation is not acceptable in a zone 20!

Hazardous ignition sources

The spark energy of an isolated filter element is limited.  It depends on the type and size of the filer element, but it is very unlikely that such a spark discharge will ever exceed 10 mJ.  The VDI limit of 10 mJ therefore does make sense.

A similar approach is possible for mechanical sparks: if fast moving machinery is extracted towards a filter, it is very hard to prove that even in rare fault situations no spark will ever arrive in the filter.  But incidental mechanical sparks will only ignite rather “sensitive” dusts (MIE lower than 10 mJ and Minimum Ignition Temperature MIT lower than 400°C). However this needs to be evaluated with care. For dusts having a very low MIT, sparks could be able to ignite even if the MIE is greater than 10 mJ.  Therefore, apart from the MIE also the MIT is an important variable when verifying whether protection is required.

“A single spark, even if it is not capable of igniting a dust cloud, may settle down on a filter element and start a fire.”

An event that is often overlooked is that a single spark, even if it is not capable of igniting a dust cloud directly, may settle down on a filter element and start a smouldering fire. The surface temperature of such a smouldering fire is far above the MIT of almost any dust cloud. A smouldering fire is a guarantee for a dust explosion as soon as an explosive mixture arises (i.e. with the next pulse cleaning). Therefore, before it can be concluded that explosion protection of a specific filter can be excluded, proof is required that such an event of a smouldering fire due to sparks (or due to auto-ignition of deposits) can be excluded, even as a rare fault condition.  Please remember that even a dust with a Burning Number BZ of 1 or 2 (meaning it will not support a smouldering fire in a dust layer) might very well support a smouldering fire when the dust layer involved is on a filter element with continuous air flow!

Prevention

If sparks are to be expected in an extraction line (like on machinery) spark detection and extinguishing might help to prevent sparks from arriving in the filter. While this certainly helps, it is not immune to failure: the spark detector may become blinded due to deposits, the water pressure can drop, the water valve can be accidentally closed, etc. Therefore it is usually impossible to exclude sparks as rare fault conditions.

Conclusion

In considering and applying ATEX requirements, explosion protection of filters will be required in nearly all situations.

Except for those special cases, where form a specific risk assessment it can be concluded that either explosive dust clouds are unlikely or all potential ignition sources can be excluded with almost 100 % certainty, protection will not be required.

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