ATEX explosion protection: Protective measures for the prevention of explosions

The danger of an explosion always exists when the conditions of the ATEX fundamentals and the hazard pentagon are fulfilled. Included here, for example, are the presence of combustible dust, atmospheric oxygen, and an effective ignition source. These conditions result in approaches for the protection of personnel and equipment from the effects of an explosion. Details will be shown in the following article.

Classification of protective measures

To avoid an explosion, it is sufficient if one of the mentioned conditions is not fulfilled. A differentiation is made between preventive and constructive protection measures:

Preventive protection measures

Such protection measures help to prevent explosions. If they are implemented, an explosion cannot occur because at least one of the conditions of the hazard pentagon is not fulfilled. 

Initially, this involves primary explosion protection: The presence of a combustible substance is avoided. No flammable material - no explosion. If flammable material is present, then only in non-hazardous concentration.
For dust, the following explosion parameters should be considered in this context:

• Mixing ratio (lower as well as upper explosion limit)
• Limiting oxygen concentration LOC
• Particle and grain size
• Smouldering point of the dust

Preventive protection measures also include secondary explosion protection. These protection measures ensure that no explosion can occur in the presence of a flammable substance by avoiding ignition sources or eliminating the oxidizing agent. For example, the system is connected to ground to avoid static charging, or there is not enough oxygen due to inertization of the process gas.

Constructive protection measures

Constructive protection measures are intended to reduce the effects of explosions, should they occur inside the system. 

Tertiary explosion protection is a constructive protection measure. The system is designed to withstand the pressure in the event of an explosion. When selecting constructive explosion protection, the objective is to contain the effects following an explosion in a system. Important here are the maximum overpressure that the dust generates when it explodes (P_max) and the speed of pressure increase (intensity), i.e. the K_ST value.

Options for tertiary explosion protection include the use of separation systems to shield off system parts from each other. In the event of an explosion, neighboring systems are separated from each other - either through active systems that detect the explosion using sensors, or passive systems that react to the explosion based on their design. 

Another possibility is the selection of materials or the dimensioning of the system itself, so that it can adequately withstand the effects of an explosion. A differentiation is made between pressure-resistant and pressure-blast-safe design:

• Pressure-resistant design is dimensioned for maximum explosion overpressure, where permanent deformations are not admissible.
• Pressure-blast-safe design is dimensioned for reduced maximum explosion overpressure and operates based on pressure relief (the pressure created is relieved either externally or internally so that a set level is not exceeded) or on an explosion suppression system (the explosion is detected by sensors and the building-up pressure is eliminated already as it develops). Permanent deformations are admissible (to a certain extent) with the pressure-blast-safe design. After an explosion, the system can remain in operation if this has been confirmed by a prior test.

Protection measures by the manufacturer

When dimensioning a system, knowledge of the hazard potential of the mixture of substances must be available beforehand. This results in measures that must be taken on the system itself.

Looking at a dedusting system, these would be the following:

1. Preventive protection measures: Measures for removal of ignition sources
   • All components - especially electrical ones - comply with the Ex category of the complete system. This results during the planning of the system.
   • All components are positively connected to ground (zone 1+0) to prevent the discharge of sparks.
   • Materials used are dischargeable and the size of the insulating surfaces is limited to prevent corona discharges.
   • Coatings may not insulate; perforate them, if required, and pay attention to the coating thickness in order to prevent propagating brush discharges.
2. Constructive protection measures:
   • Installation of separation systems such as raw gas valves or securing of filter cells to prevent them from being lifted off in case of an explosion.
   • Pressure-blast-safe design with internal pressure relief through provision of a relief area.

Protection measures by the operator

The operators of systems with explosive mixtures or in explosive atmospheres must also ensure that there is no danger to humans or the environment during operation. For example, by providing the manufacturer with the necessary information on the process and the substances being used for correct dimensioning and component selection of the system. For dedusting systems, the following must be ensured:

• The process temperature must always be below the minimum ignition temperature of the substance used, in order to prevent hot surfaces.
• Exothermic reactions between the materials of the system and the substance as well as between the substances themselves must be avoided.
• The drawing-in of metal foreign particles must be avoided to prevent mechanical sparks.

Summary

Despite the existing danger of an explosion, measures can be taken to minimize this risk. A differentiation is made between those measures that prevent the hazard at the outset (preventive protection measures) and those that prevent spreading out after an explosion has occurred (constructive protection measures). It is generally essential that the properties of the substance and the process are known and that all parties involved are aware of them.