Dust comes under the ATEX microscope

ATEX is not just about potentially explosive gaseous environments, dust is equally dangerous, explains Steve Meadows, Product Manager for process valve products at Asco Joucomatic.

The impending ATEX directive, which comes into force in July 2003, has concentrated the minds of plant engineers in all sectors of industry on the dangers of potentially explosive atmospheres.

Although this is of necessity, it is timely as research shows that there has been a substantial growth in hazardous areas in all sectors of manufacturing and processing industry over last few years; estimates revealing that 50% of all industrial applications now contain a hazardous area of one type or another.

As well as being timely, ATEX is welcome for the focus that it provides on industrial dust as a potential source of explosions.

Almost all types of industrial dust can be considered to be potentially explosive, so it comes as no surprise that the procedure for technical evaluation of safety measures used to avoid the risks of dust explosions is both complex and extensive.

In order to describe the explosion risk posed by dust a number of factors need to be described.

These include particle size, explosion limits, the maximum explosion pressure, the destructive power of the combustion, moisture content, and the minimum ignition energy required.

Once the dust has been characterised, an examination then needs to be made of the industrial processes concerned.

This takes into account possible ignition sources, explosive volumes, operating temperatures and an assessment of the possibility of a dust explosion under given conditions.

Helpfully for engineers involved in safety evaluations of dust-laden atmospheres, ATEX simplifies explosion protection with a three-zone concept.

Zone 20, the most critical of the three, is an area in which an explosive atmosphere in the form of a cloud of combustible dust in air is present continuously, or for long periods, or frequently.

Typically, these conditions would be encountered on the inside of containers or pipelines and enclosed conveying equipment.

Zone 21 is a place in which an explosive atmosphere in the form of a cloud of combustible dust in air is likely to occur in normal operation occasionally for example when discharging and filling equipment.

Zone 22 is a place in which an explosive atmosphere in the form of a cloud of combustible dust in air is not likely to occur in normal operation but - if it does occur - will persist for a short period only.

Areas in which dust escapes and forms deposits are included in this category.

Whatever the zone, one of the biggest risks when it comes to preventing dust explosions, is posed by enclosures.

The ATEX directive defines the type of protection provided by enclosures, based on limiting the maximum surface temperature of the enclosure and using dust-tight and dust-protected enclosures to prevent dust entry.

The legislation covers two degrees of protection: dust-tight, for use of equipment in Zone 20, 21 and even 22 in the case of the presence of conductive dust; and dust-protected, for use of equipment in Zone 22 areas in the presence of nonconductive dust.

The scope of the ATEX directive on enclosures is comprehensive, extending down to electrical actuators used on individual valve types.

This is important due to the increasing use of solenoid valves in the dust collector systems that reduce industrial air pollution.

One of the major manufacturers of these valves is Asco Joucomatic.

The company's unique Power Pulse range optimises the performance of pulse jet cleaning systems, one of the most effective techniques for removing dust from the atmosphere in cement plants, foundries, waste incinerators, power stations, food processing, plants and many, many more.

Asco Joucomatic has been planning for ATEX for some time with the result that the company offers the widest range of solenoid coils and enclosures complying with the new directive for use in gaseous and dust laden environments.

The enclosures meet the needs of all industry types, being available in metals such as aluminium, cast iron and stainless steel and also as convenient epoxy encapsulations.

The range of ATEX certified categories covered by Asco Joucomatic is exhaustive and includes: d - flameproof (Zone 21); e - increased safety (Zone 21); m - encapsulation (Zone 21); em- encapsulation with increased safety (Zone 21); ia - intrinsically safe for use in continuously explosive atmospheres (Zone 20); ib - intrinsically safe for atmospheres where combustible dust is likely to occur during normal operation, occasionally (Zone 21); and n - non-sparking (Zone 22).

In addition to meeting the needs of industry at large, Asco Joucomatic's ATEX certified range is particularly relevant to OEMs.

The range is employed in Asco Joucomatic's Power Pulse tank system, which provides a neater package for dust collector installations The tank system is a fully integrated package that effectively overcomes the problem of CE certification for OEM's who traditionally build their own tank systems.

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