Safety and area classification
Safety principles
Installations in which flammable materials are handled or stored should be designed, operated and maintained so that any releases of flammable material, and consequently the extent of hazardous areas, are kept to a minimum, whether in normal operation or otherwise, with regard to frequency, duration and quantity.
In the case of maintenance activities other than those of normal operation, the extent of the zone may be affected but it is expected that this would be dealt with by a permit-to-work system.
In emergency situations, reliance should be placed on the isolation of unsuitable electrical equipment, shut-down of the process, isolation of process vessels, containment of spillages and, if possible, the provision of additional emergency ventilation.
In a situation in which there may be an explosive gas atmosphere, the following steps should be taken:
- eliminate the likelihood of an explosive gas atmosphere occurring around the source of ignition, or
- eliminate the source of ignition.
Where this is not possible, protective measures, process equipment, systems and procedures should be selected and prepared so the likelihood of the coincidence of a) and b) is so small as to be acceptable. Such measures may be used singly if they are recognized as being highly reliable, or in combination to achieve an equivalent level of safety.
Area classification procedure
The area classification should be carried out by those who have knowledge of the properties of flammable materials, the process and the equipment, in consultation, as appropriate, with safety, electrical and other engineering personnel.
The following subclauses give guidance on the procedure for classifying areas in which there may be an explosive atmosphere and on the extent of zones 0, 1 and 2.
Sources of release
The basic elements for establishing the hazardous zone types are the identification of the source of release and the determination of the grade of release.
Since an explosive gas atmosphere can exist only if a flammable gas or vapour is present with air, it is necessary to decide if any of these flammable materials can exist in the area concerned. Generally speaking, such gases and vapours (and flammable liquids and solids which may give rise to them) are contained within process equipment which may or may not be totally enclosed. It is necessary to identify where a flammable atmosphere can exist inside a process plant, or where a release of flammable materials can create a flammable atmosphere outside a process plant.
Each item of process equipment (for example tank, pump, pipeline, vessel, etc.) should be considered as a potential source of release of flammable material.
If the item cannot contain flammable material, it will clearly not give rise to a hazardous area around it. The same will apply if the item contains a flammable material but cannot release it into the atmosphere (for example an all-welded pipeline is not considered to be a source of release).
If it is established that the item may release flammable material into the atmosphere, it is necessary, first of all, to determine the grade of release in accordance with the definitions, by establishing the likely frequency and duration of the release. It should be recognized that the opening-up of parts of enclosed process systems (for example during filter changing or batch filling) should also be considered as sources of release when developing the area classification. By means of this procedure, each release will be graded either "continuous", "primary" or "secondary".
Having established the grade of the release, it is necessary to determine the release rate and other factors which may influence the type and extent of the zone.
Type of zone
The likelihood of the presence of an explosive gas atmosphere and hence the type of zone depends mainly on the grade of release and the ventilation.
NOTE A continuous grade of release normally leads to a zone 0, a primary grade to zone 1 and a secondary grade to zone 2.
Extent of zone
The extent of the zone is mainly affected by the following chemical and physical parameters, some of which are intrinsic properties of the flammable material; others are specific to the process. For simplicity, the effect of each parameter listed below assumes that the other parameters remain unchanged.