The danger of an explosion always exists when the conditions according to the ATEX fundamentals and the hazard pentagon are fulfilled. This includes, for example, the presence of combustible dust, atmospheric oxygen, and an effective ignition source. These prerequisites result in starting points for protecting the plant and personnel from the effects of an explosion. We will show you which ones in this article.
Classification of protective measures
To avoid an explosion, it is sufficient if one of the above conditions is not met. A distinction is made between preventive and constructive protective measures:
Preventive protective measures
These protective measures serve to prevent explosions. If they are implemented, an explosion cannot occur because at least one of the conditions of the hazard pentagon is not met.
First of all, this includes primary explosion protection: the presence of a flammable substance is avoided. If no flammable material is present, it cannot explode. If flammable material is present, then only in non-hazardous concentrations.
For dust, the following explosion indicators should be considered in this context:
- Mixing ratio (lower as well as upper explosion limit)
- Limiting oxygen concentration SGK
- Particle or grain size
- Smoldering point of the dust
Preventive protection measures also include secondary explosion protection. These protective measures ensure that no explosion can occur in the presence of a flammable substance by avoiding ignition sources or eliminating the oxidizer. For example, the plant is grounded to prevent static charges, or there is insufficient oxygen due to inerting of the process gas.
Constructive protective measures
The design protective measures are intended to reduce the effects of explosions should they occur in the plant.
The design protection measures include tertiary explosion protection. The system is designed to withstand the pressure in the event of an explosion. The selection of constructive explosion protection is about containing the effects after an explosion at a plant. Important factors here are the maximum overpressure generated by the dust when it explodes (Pmax) and the rate of pressure rise (severity), i.e. the STS value.
Options for tertiary explosion protection include the use of decoupling systems to shield plant areas from each other. In the event of an explosion, neighboring systems are separated from each other – either by active systems that use sensors to detect the explosion, or passive systems that react to the explosion due to their design.
Another possibility is the selection of materials or the design of the equipment itself so that it can withstand the effects of an explosion accordingly. A distinction is made between the pressure-resistant and pressure-shock-resistant designs:
- The flameproof construction is designed for the maximum explosion overpressure – permanent deformations are not permissible here.
- The pressure shock resistant construction is designed for a reduced maximum explosion overpressure and works in conjunction with a pressure relief system (the pressure created is relieved either externally or internally so that a set level is not exceeded) or an explosion suppression system (the explosion is detected by sensors and the pressure that builds up is already eliminated during the creation phase). Permanent deformations are permissible (within certain limits) in the pressure-shock-resistant design. After an explosion, the equipment can remain in operation if this has been confirmed by a test beforehand.
Protective measures of the manufacturer
When designing a plant, knowledge of the hazard potential of the mixture of substances must be available in advance. This results in the measures that must be taken at the plant itself.
Using a dust collection system as an example, these would be the following:
2. preventive protective measures: Measures for the absence of ignition sources
- all components – especially the electrical ones – comply with the Ex category of the entire plant. This results during the planning of the plant
- all components are force grounded (zone 1+0) to prevent spark discharges
- the materials used are dissipative and the size of the insulating surfaces is limited to prevent tuft discharges
- Coatings must not insulate, perforate if necessary as well as pay attention to the thickness of the coatings in order to prevent sliding stem tuft discharges
2. constructive protective measures:
- Installation of decoupling systems such as raw gas dampers or securing of filter cells to prevent lift-off in the event of an explosion.
- pressure shock resistant design with internal pressure relief due to the presence of a relief chamber
Protective measures of the operator
The operator of systems with explosive mixtures or in explosive atmospheres must also ensure that there is no danger to people or the environment during operation. For example, he must provide the manufacturer with the necessary information on the process and the substances used, which the manufacturer needs in order to select the correct measures to be taken at the plant. In the case of a dust collection system, it must ensure the following:
- The process temperature must always be below the minimum ignition temperature of the substance used to prevent hot surfaces.
- Exothermic reactions between the materials of the plant and the substance and between the substances themselves must be avoided.
- Sucking in metallic foreign bodies must be avoided to prevent mechanical sparks.
Conclusion
Despite the existing risk of an explosion, measures can be taken to minimize this risk. A distinction is made between those measures that prevent the hazard from occurring in the first place (preventive protective measures) and those that prevent it from spreading after an explosion has occurred (constructive protective measures). It is essential that the properties of the substance and the process are known and that all parties involved are aware of them.
