Dust environments require special assessment

Steven Hill of Schmersal discusses the application of the ATEX regulation for dust environments and warns that compliance needs to be checked carefully.

Applications for explosion-proof switchgear are usually associated with gas environments in the chemical and petrochem industries, but in applications such as the wood and light-metal processing industries for example, the danger of dust ignitions also has to be considered.

The ATEX Directive introduced on 1st July 2003 applies to all current products fitted in potentially explosive environments.

It refers to devices, safety systems and components with their own ignition point as well as to facilities for the secure operation of devices and systems, and includes a dedicated regulation for explosion protection against dust.

The directive defines new safety levels that will mean major changes for manufacturers of the components, machines and plant used in explosive environments.

For example, in equipment group II (for potentially explosive atmospheres except mining) there are now three categories (1G, 2G, 3G) for areas with explosive gases, mists and vapours appropriate gas zone 0, 1 and 2.

Although this is an important new development, what is even more important is that for the first time the ATEX Directive uniformly governs explosion protection against dust.

This is an issue that will concern many safety engineers because all flammable substances are capable of reacting explosively when they are finely spread, like a dust cloud, in the atmosphere.

The directive is therefore valid for wood and plastics as well as for many food ingredients (coffee and flour for instance), and even for combustible metal dusts such as aluminium and magnesium, which are employed more and more in the automotive industry for weight-saving.

In these environments one spark is enough to cause serious damage.

Experts estimate that there are far more dust explosions than gas explosions, which confirms that ATEX is right to regulate this potential hazard.

Potentially explosive dust zones are defined in the directive 1999/92/EC, and in the European Standard EN50281-1-2.

As with gas environments, there are three zones that correspond with three equipment categories: equipment category 1D (zone 20) covers explosive atmospheres caused by a cloud of flammable dust in the air, which is present continuously or for long periods.

These conditions generally occur only inside containers, pipe conduits, devices etc.

Equipment category 2D (zone 21) covers areas in which explosive atmospheres, caused by a cloud of flammable dust in the air, are likely to occur.

Typically, this applies to areas close to where materials are introduced or drawn off, or where dust deposits occur, or where in normal operation an explosive concentration of flammable air/dust mixture can be generated.

Equipment category 3D (zone 22) covers areas where, in normal use, explosive atmospheres caused by a cloud of flammable dust are unlikely to occur or, if they do occur, are likely to do so for a short period only.

The new regulations mean that manufacturers of machines used in explosion endangered areas, must identify the zone and choose components that are ATEX compliant for that zone.

As for manufactures of products for zones 20 and 21, a "notified body" must certify both the products and the production facility.

The importance of certification was highlighted recently when Schmersal was asked to provide an ATEX certified zone 22 version of one of our safety position switches by a manufacturer of rotary valves and ancillary equipment for the bulk materials handling industry.

Rota Val of Chippenham in Wiltshire had been asked to provide a fully tested and certified ATEX version of its flagship Hypergienic valve to a leading pharmaceutical customer.

The specific application consists of two units - a top valve that meters material into a cone mill at a particular rate, and a bottom valve that takes material away.

Although the valves are independent of each other they operate simultaneously as a single unit.

Because of the operating environments and the type of materials processed, both valves are designed as explosion/flame containment devices capable of stopping the propagation of an explosion-through flame if it occurs in the line above or below either valve.

As the valves are also designed for CIP installations, access to the fully washable internal surfaces is provided via two removable end covers that slide out on rails.

To ensure that operators and maintenance staff cannot withdraw the end covers while the valves are running, Rota Val engineers specified safety switches from a well-known manufacturer of safety equipment, to indicate if the end covers were being removed.

Because of the potential for explosion in the operating environment, engineers specified switches with ATEX certification to zone 22 (dust).

However, as Rota Val's Contracts Engineer Paul Bailey explains, when the safety position switches were delivered and Rota Val requested formal certification, the safety component manufacturer advised that they were ATEX compliant only for gas.

"We then had to search for a supplier that could provide a dust certificated switch", he said.

"After extensive research, the only company we found that was able to meet the specification was Schmersal".

The unit Schmersal supplied was a stainless steel EEx 13 safety position switch that is ATEX certified for both gas zones 1 and 2 and dust zones 21 and 22.

On the Hypergienic valve it acts as a positive break position switch indicating that the valves are in operation and enabling the safety system to automatically stop the process if the covers are removed.

Although Schmersal was able to solve Rota Val's problem, the company's experience does raise concerns that some safety component manufacturers may not fully understand the specific and separate requirements for gas and dust environments.

It is therefore essential that safety specifiers ensure that components specified for potentially explosive atmospheres actually comply with the relevant EC Directive 94/9/EC (ATEX).

EEx Certification ensures that the appropriate certificate of conformity has been issued for every device, based on the European CENELEC standards EN50014.

Fortunately, a broad range of explosion-proof switchgear is available for gas and dust explosion endangered areas including electromechanical position switches, noncontact magnetic sensors, safety guard interlocks (Schmersal's was the first of its kind to comply with the ATEX regulations), safety switches, pull-wire and foot switches and emergency stop devices.

All are available with different contact configurations and all are certified by a notified body for use in gas zones 1 and 2 and dust zones 21 and 22.

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