EXPLOSION HAZARD – ONE SPARK IS ENOUGH

In many industrial sectors, powdered and dust-like materials are processed or formed during production. Whether classified as useful or waste dust, most dust substances present a fire hazard and, in certain conditions, even an explosion risk. Most industrial dusts are combustible—just a thin layer of dust in an enclosed space can ignite and trigger an explosion when disturbed. Unfortunately, many people exposed to this risk are unaware of the potential for a dust explosion, which is why it’s crucial to understand this topic in greater detail.

Small Ignition, Big Impact

Using this case study, on February 6th, 1979, a fire at the Rolandmühle industrial mill in Bremen led to a catastrophic event. A series of explosions, set off by swirling flour dust, resulted in a chain reaction. The disaster claimed 14 lives, injured 17 others, and caused over 50 million euros in material damage.

Flour explosions are not uncommon. In fact, any combustible substance can explode when finely ground—examples include carbon, flour, cocoa, coffee, starch, cellulose, wood chips, or shavings. Even inorganic materials like magnesium and aluminium can be explosive or at least combustible in this form.

It doesn’t take much to spark an explosion. Only three elements are needed: combustible dust, enough oxygen, and an ignition source. The right mixture of dust and air can be explosive. Approximately 80% of all industrial dusts are combustible. Air is everywhere, and an explosion can be triggered by unnoticed embers or sparks from welding. Even a dust layer as thin as one millimetre in an enclosed room can cause an explosion if disturbed and ignited, leading to potentially fatal consequences for both the company and its employees.

Despite the risks, many people in this sector are not sufficiently aware of the hazard. This is partly because, in many companies, even supervisors may not realize they are working with combustible dust. Industries most affected include food and wood processing, paper and plastics production, chemical and pharmaceutical manufacturing, agriculture, metal processing, and waste management. On average, one dust explosion occurs daily in Germany, with one in four of these caused by food or animal feed dust.

Preventing Explosions with Dust Explosion Protection Measures

To prevent explosions, at least one of the factors that lead to ignition must be eliminated or minimized. This can be achieved in various ways, such as reducing the concentration of combustible material through effective extraction. Alternatively, the hazard can be completely removed by eliminating the presence of combustible substances altogether.

Removing ignition sources is crucial to preventing the ignition of hazardous explosive atmospheres. This starts with identifying potential sources of ignition. Open flames, embers, hot surfaces, mechanical or electrical sparks, and electrostatic discharge are all potential ignition sources. Even less obvious risks, such as those from repair work or carelessly discarded cigarette butts, must be considered. Once identified, the appropriate protective measures are determined, and suitable equipment is selected.

In some situations, it may not be possible to reliably implement these preventive measures because dust clouds cannot be entirely avoided, or certain ignition sources cannot be completely eliminated. In such cases, measures that limit the hazardous effects of any ignited explosions to a minimal level must be implemented. This "constructive explosion protection" includes using explosion-proof structures, explosion pressure relief, explosion suppression, and explosion barriers.

Strict Equipment Requirements

There is no "one size fits all" solution for dust explosion protection. Each system is uniquely designed to meet the specific needs of its operators and production processes. Analysing potential hazards can be simplified by classifying dust hazardous areas into zones, based on the frequency and duration of hazardous explosive atmospheres. The higher the frequency and longer the duration of these conditions, the stricter the requirements for the devices used in those areas. The definitions of these zones can be found in the following table.

Equipment Classification for Hazardous Areas

Equipment used in hazardous areas is categorized according to the hazardous explosive zone they are designed for. Devices in Category 1 must ensure a high level of safety, meaning they must prevent ignition even if two rare faults occur simultaneously. These devices are suitable for use in Zone 20. Category 2 equipment is designed to maintain the required safety level even under frequent faults expected during normal operation, making them appropriate for Zone 21. Devices that generate no ignition sources fall into Category 3 and can be used in Zone 22. This method of classifying dust hazardous areas into zones has been highly effective in explosion protection, similar to the approach used for gas atmospheres.

Types of Protection – Preventing Explosions with Effective Measures

To ensure no ignition sources arise, technical measures must be implemented according to the safety parameters of an assumed explosive mixture. Current standards outline four types of dust protection below:

1. Type "t" Protection – Enclosure Protection: This is considered the most important by many experts. It works by limiting the maximum surface temperature of the enclosure and preventing dust ingress using dust-tight and dust-protected enclosures.

2. Type "p" Protection – Pressurized Enclosure: This allows "non-Ex" approved devices to be used in hazardous areas, such as protecting cabinets in dust hazardous zones. The surrounding atmosphere is kept out of the enclosure by maintaining a higher pressure inside with an ignition protection gas.

3. Type "i" Protection – Intrinsic Safety: This involves intrinsically safe equipment containing circuits with very low energy levels. In these circuits, neither sparks nor thermal effects can ignite a dust-air mixture.

4. Type "m" Protection – Encapsulation: In this method, parts that could ignite an explosive atmosphere due to sparks or heat are embedded in a compound, preventing ignition of the explosive atmosphere.

Selecting Equipment for Dust Hazardous Areas

Once the requirements for equipment in dust hazardous areas have been established, careful selection of the equipment is essential. Only machines and components specifically designed for dust explosion protection and carrying the appropriate certifications should be used. When operating in an explosive atmosphere, the equipment’s maximum temperature must not exceed the ignition temperature of either the dust cloud or dust layer, with certain safety factors taken into account.

Device Groups – Proper Assignment and Selection

The standard categorizes devices into groups to determine their suitability for different explosive environments, such as gas or dust. Group III is designated for devices intended for areas where explosive dust atmospheres are expected. Since different types of dust have varying requirements, further classification is provided:

• IIIA: Combustible lint

• IIIB: Non-conductive dusts

• IIIC: Conductive dusts

Devices are thus classified according to the type of dust they are designed to handle, ensuring they meet the specific requirements of each dust class.

Ultimately, the quality of the product determines its effectiveness in providing protection. Experts in explosion protection, like R. STAHL, offer a fully certified range of products, along with comprehensive consultation and training on the subject.

Visit R. STAHL West and Central Africa for more information.