Warning: Explosion hazard – one spark is enough
In many industrial sectors, powdered and dust-like products are processed or formed during the production process. Regardless of the classification as useful dust or waste dust, the vast majority of all dust substances can result in a fire hazard and, under certain circumstances, even an explosion hazard. Most dusts that are found in industry are combustible – even just a small layer of dust in an enclosed room is enough to trigger an explosion after being stirred up and ignited. Many people affected by this risk are not aware of the hazards of a potential dust explosion, which is a good reason to get to grips with this topic in more detail.
Small ignition – huge effect
On 6th February 1979, a fire at the Rolandmühle industrial mill in Bremen caused a disaster. In a chain reaction, a number of individual explosions caused more and more flour dust to swirl around, causing yet more explosions. The devastating result killed 14 people, leaving a further 17 injured and causing material damage amounting to over 50 million euros.
It is not uncommon for flour to explode. As a rule, any combustible substance can explode if it is finely ground – take carbon, flour, cocoa, coffee, starch, cellulose, wood chips or wooden shavings, for instance. Even anorganic materials and elements such as magnesium and aluminium are explosive or at least combustible in this form.
It doesn't take much to spark an explosion. Only three elements are required – combustible dust, sufficient oxygen and an effective source of ignition. Only the right mixture of dust and air is explosive. 80% of all dusts used in industry are combustible. Air is present everywhere and an explosion can be triggered by unnoticed embers or sparks like those generated during welding work. Even a dust layer just one millimetre thick in an enclosed room can trigger an explosion, if the dust has been disturbed and ignited. This can have fatal consequences for the company and employees.
Despite this, many people within this sector are not aware enough of this hazard. But why? Well, in many companies, even the supervisors themselves are not aware that they are working with combustible dust. The branches of industry that are most affected are foodstuff and wood processing, paper and plastics production, chemistry and pharmaceuticals, agricultural operations, metal processing, waste management, etc. On average, one dust explosion occurs every day in Germany – one in four of these explosions is caused by foodstuff dust or animal feed dust.
Eliminate explosions with dust explosion protection measures
To prevent an explosion, at least one of the factors that lead to ignition must be eliminated or minimised. This can be achieved in a number of ways, for instance by keeping the concentration of the combustible content low using extraction. Alternatively, the hazard can be eliminated completely, for instance by releasing combustible substances.
Eliminating ignition sources prevents hazardous explosive atmospheres from being ignited. This can be achieved by initially analysing potential sources of ignition. Open fires, flames or embers can ignite an explosive atmosphere; hot surfaces, mechanical or electrical sparks, and electrostatic discharge are also potential sources of ignition. Less obvious sources of ignition such as those caused by repair work or carelessly throwing away a cigarette butt must also not be forgotten. The required scope of protective measures is then determined and suitable equipment is selected.
In some cases, the processes described above cannot be implemented reliably because it is impossible to avoid generating dust clouds or to eliminate certain sources of ignition. In these cases, measures that limit the hazardous effect of any ignited explosions to an insignificant extent must be implemented. "Constructive explosion protection" includes the use of explosion-proof structures, explosion pressure relief, explosion suppression and explosion barriers.
Strict requirements for equipment
However, there is no "one size fits all" solution for dust explosion protection. No two systems are the same – they are uniquely designed to meet the specific needs of their operators and production. Analysing the potential hazards can be made simpler by classifying dust hazardous areas in zones. In this process, they are assessed according to the frequency and duration of the occurrence of hazardous explosive atmospheres. The higher the frequency and the longer the duration of the explosion, the stricter the requirements are for the devices used in the area. The definitions of the zones can be found in the following table.
|Zone 20||An area in which a hazardous explosive atmosphere in the form of a cloud containing combustible dust in the air is present continuously, for long periods or frequently.|
|Zone 21||An area in which a hazardous explosive atmosphere in the form of a cloud containing combustible dust in the air is likely to occur under normal operating conditions.|
|Zone 22||An area in which a hazardous explosive atmosphere in the form of a cloud containing combustible dust in the air is normally not present, or is only present for a short period of time, under normal operating conditions.|
Dividing dust hazardous areas into zones is a method that has been used to great success in explosion protection for gas atmospheres.
Equipment that is to be used in a hazardous area is classified using device categories according to the hazardous explosive zone. Devices in category 1 must guarantee a high degree of safety; this means that even if two errors occur on the device, no ignition can occur even if the faults occur rarely. These devices can be used in Zone 20. Equipment in category 2 must be designed so that it guarantees the required degree of safety in the event of frequently occurring faults that are expected to occur in normal operation. This equipment can be used in Zone 21. Devices which generate no sources of ignition can be used in Zone 22. These devices belong to category 3.Dividing dust hazardous areas into zones is a method that has been used to great success in explosion protection for gas atmospheres.
Types of protection – prevent explosions with effective protection
Technical measures must be implemented in order to ensure that no sources of ignition can arise in accordance with the safety-related key figures of an assumed explosive mixture. According to current standards, four types of protection for dust exist. These can be found in the table below. The "t" type of protection – protection by enclosure – is seen as the most important version by many experts. It is based on the principle of limiting the maximum surface temperature of the enclosure and restricting dust ingress through the use of dust-tight and dust-protected enclosures. The "p" type of protection – pressurised enclosure – enables "non-Ex"-approved devices to be used in hazardous areas and could, for instance, be used to protect cabinets in dust hazardous areas. The surrounding atmosphere is prevented from entering the enclosure of electrical equipment because an ignition protection gas is stored inside the enclosure at a pressure higher than that of the surrounding atmosphere. Intrinsically safe equipment with the "i" designation only contains intrinsically safe circuits. This refers to electrical circuits with very low energy levels, in which no sparks or thermal effects can cause a dust-air mixture to ignite. In the case of the "m" type of protection – encapsulation – parts that could ignite an explosive atmosphere as a result of sparks or heating are embedded in a compound to prevent the explosive atmosphere from being ignited.
|Abbreviation||Principle||Type of protection||IEC/EN|
|t||Protection by enclosure||IEC/EN 60079-31|
|p||Pressurised enclosure||IEC/EN 60079-2|
|i||Intrinsic safety||IEC/EN 60079-11|
Types of protection for dust according to current standards.
Once the requirements for equipment to be used in dust hazardous areas have been determined, the equipment can be selected. This process requires careful attention. Only machines and components that have been developed for dust explosion protection and bear the corresponding equipment designations may be used. If equipment is to be used in an explosive atmosphere, its maximum temperature must not reach the ignition temperature of either the dust cloud or dust layer. Within this context, certain safety factors must be considered.
Device groups – correct assignment and selection
The standard uses groups to establish the suitability of devices for the different explosive areas (e.g. gas or dust). Group III represents devices intended for use in areas where explosive dust atmospheres are expected. However, different types of dust pose different requirements for devices. Further classification is therefore provided:
- IIIA: Combustible lint
- IIIB: Non-conductive dusts
- IIIC: Conductive dusts
The devices can therefore be assigned to different dust classes and, accordingly, their individual requirements.
Ultimately, however, the quality of the product determines its actual protective function. Explosion protection experts with years of expertise, such as R. STAHL, offer a fully certified product range, as well as comprehensive consultation and training on this topic.