Area Classification
Primary explosion protection
Primary explosion protection aims to replace combustible agents or atmospheric oxygen with something else or reduce their quantity to such an extent that there is no danger of forming an explosive mixture.
Increasing air circulation flushing air through ventilation can be achieved by constructive measures, for example, open-plan gas stations where there is a minimal potentially explosive atmosphere.
Replacement of atmospheric oxygen is not possible in rooms where people work. For this reason, the measures available for such places are limited:
• Avoiding or limiting the use of combustible agents capable of forming an explosive atmosphere
• Preventing or limiting the release of combustible agents and thus the formation of the explosive mixture in and around the valve, e.g., by:
- Limiting their concentration
- Using covers filled with an inert substance
- Natural or artificial ventilation
- Monitoring the concentration using a gas detection system which gives an alarm/shuts down the system
Secondary explosion protection
Suppose, despite primary explosion protection measures, a dangerous, potentially explosive atmosphere is likely to form (to the extent that actions are required to protect workers from combustible factors). In that case, the ignition of this hazardous, potentially explosive atmosphere must be effectively prevented. All possible ignition sources are evaluated, and appropriate protective measures are applied.
Effective ignition sources on equipment and installations can, for example, be prevented using protection appropriate to the level of protection required. The classification of potentially explosive areas into zones (frequency and duration of occurrence of a dangerous explosive atmosphere and local environmental conditions) serves as the basis for determining the level of protection of equipment. In addition, it is necessary to know the essential explosion characteristics for flammable materials (temperature classes, dust ignition temperatures, explosion sub-groups, etc.) and the local environmental conditions.
Explosion characteristics help the control operator accurately determine the risk in a given area, allow the equipment manufacturer to select the right solution for the work equipment, and finally oblige the installation engineer to select and assign suitable devices. Ultimately, this data is contained in the labeling of the device.
Tertiary explosion protection
If the primary and secondary explosion protection measures are insufficient, additional protection measures must be taken. Their purpose is to limit the explosion’s impact and/or reduce it to a non-dangerous level. The most common actions to limit the hazardous 8 www.sigma-industries.kz AREA CLASSIFICATION effects of an explosion are as follows:
• Blast-resistant Design: containers, apparatuses, pipelines are built in such a way as to be resistant to pressure shocks to withstand the explosion inside.
• Blast Relief: rupture discs or blast flaps are installed that open in a safe direction in the event of an explosion and ensure that the plant is not subjected to a load that exceeds its blast resistance.
• Explosion Suppression and Explosion Propagation Prevention: explosion suppression systems prevent reaching maximum explosion pressure by quickly injecting extinguishing agents into containers and installation. Explosive decoupling limits possible explosions to individual parts of the installation.
Relevance and benefits of zone classification in the workplace
The practice of dividing potentially explosive atmospheres into zones has developed. This classification considers the various hazards from explosive atmospheres and allows explosion protection measures to be taken that reflect safety engineering and economic efficiency. For the European Community, zone definitions are uniformly presented in Directive 2014/34/EU, and it should be applied with a technical understanding of the particular situation.
Potentially explosive atmospheres are classified into zones depending on the frequency and duration of the explosive atmosphere. Many details and influencing factors must be considered to classify the zones in a particular case.
IEC 60079-10-1 assumes a roughly similar classification for gases and vapors, also applying to future facilities built to US NEC 505. IEC 60079-10-2 provides support for classifying areas with dust.
Potentially hazardous areas are classified according to the frequency and duration of the explosive atmosphere. This classification defines the scope of the measures to be taken under Section A of Annex II of Directive 2014/34/EU in conjunction with Annex I of Directive 2014/34/EU.
In workplaces, potentially explosive atmospheres are usually classified as zones 1 and 2, 21 and 22. Zones 0 and 20 are limited to tiny inaccessible areas in workplaces or are typically limited to the interior of technological facilities.
Notes:
Under atmospheric conditions, an explosive atmosphere is a mixture of air and inflammable substances in gases, vapors, mists, or dust. After ignition, combustion spreads to the entire unburned mixture.
A dangerous explosive atmosphere is an explosive atmosphere that causes damage in an explosion and requires introducing measures to protect workers from combustible factors.
