Wednesday, November 10, 2010

Hazardous Area Classification

The technology of application of electrical equipment in explosive atmospheres is very old, dating almost from the original application of electricity to apparatus other than lighting. From its origins it has been developed in most industrialized countries. The determination of the likelihood and the areas contaminated or likely to be contaminated by explosive atmospheres produced by fuels such as gas, vapour, mist, dust or a combination of these. This is still the least researched of the areas of this technology, principally because there are so many variations, in particular circumstances occurring in practical locations.

It is essential to identify the risk of the presence of an explosive atmosphere in a given area. This is determined by the likelihood of release, the nature of the release and the ventilation in the area of release. Clearly, with the exception of releases inside vessels, the persistence of explosive atmospheres is determined to a significant extent by such things as the level of ventilation at the location of the release and the characteristics of dispersion of the flammable material. It is necessary to define the risk of the presence of an explosive atmosphere in some way which is related to the source of release and these other parameters.

There are two main categories here.
1. Gases/vapours/mists and
2. Dusts.

Each category is further sub-divided into different zones which are detailed below.

1. Gases/Vapours/Mists

Zone 0

Zone 0 is an area (Zone) which includes the interiors of closed or ventilated vessels containing flammable liquids or vapours and air. Clearly this only applies to the vapour spaces of closed or ventilated tanks containing flammable liquids but as these tanks usually have a varying quantity of liquid within them, the Zone 0 extends to the lowest liquid level possible. A Zone 0 will also occur in the vapour space of an open-topped tank containing a flammable liquid. All other places where flammable gas or vapours normally released will also be Zone 0.

Zone 1

Zone 1 is any area containing vessels, pumps, compressors, pipes fittings and similar items of equipment which may be considered to leak in normal operation (e.g., with some degree of regularity) and any area containing such things as relief valves, vents and similar devices which are designed to release flammable gases, vapours and liquids in normal operation will be classified as Zone 1. Areas containing sample points which are not specially designed to prevent release in normal operation are in Zone 1. In the case of sample points the Zone 1 can, however, be limited to the immediate location of the sample point particularly if sampling is manual. Finally, areas where releases are so rare as not to be assumed to occur in normal operation will also be identified as Zone 1 if ventilation is restricted. Such areas include both indoor areas and other areas where, as a result of any form of containment or airflow restriction, ventilation is considered to be restricted.

Zone 2

An area containing vessels, pumps, compressors and similar equipment which are so well maintained that leaks can be assumed only to occur very rarely (in abnormal operation) and relief valves which only operate very rarely (abnormally) will be classified as Zone 2. The vents associated with such things as bursting discs will also normally give rise to Zone 2 in
areas in which they occur. In all these cases the Zone 2 classification is only acceptable provided that the areas in question are freely ventilated, that is, the equivalent of a normal outdoor situation, so that the released vapour is rapidly dispersed. Less well-ventilated zones in these Zones 2, such as pits and trenches, will be defined as Zone 1. As a result of the danger to personnel carrying out sampling, sampling points are normally designed to ensure that any area into which personnelaccess for sampling purposes are Zone 2.These definitions are given in BS 5345 part 2 and similar ones with the same intended meaning. The current reference to places where explosive atmosphere can occur specifies them as zones but the term is no different in meaning to more historic references to areas.

The nomenclature used to describe the various grades of hazardous area are shown in Figure.
 Many countries have national area classification systems, either historic or current, which are related to the now internationally recognized system described above and as shown in this table
Relationships between national area classification systems. 
IEC/CENELEC/ British/India
Zone 0
Special area in Ex order
Division 1
Division 1
Zone 1
Normal area in Ex order
Division 1
Division 1
Zone 2
Hazardous area not within Ex order
Division 2
Division 2
2. Dusts

In respect of dusts the situation is more fluid. The degree of formality is less than that for gases and vapours and has only been effectively addressed in the UK and internationally in the last 15-20 years. Within the UK, hazardous areas caused by dusts are currently defined in a different way to those caused by gases, vapours and mists.

Zone Z

Zone Z is a zone in which a combustible dust is, or may be, present as a cloud during normal processing, handling or cleaning operations in sufficient quantity result in an explosible concentration of combustible dust in mixture with air (explosive atmosphere).

Zone Y

Areas not classified as Zone Z in which accumulations or layers of combustible dust may be present under abnormal conditions and give rise to ignitable mixtures of dust and air (explosive atmosphere) are designated as Zone Y. These definitions, which are similar to those appearing in BS 6467, Part 2 and other documents, are on the face of it very different to those for gases, not least because only two Zones exist as a result of the exclusion from the scope of BS 6467, Part 2 of the interior of dust handling equipment. Taking this fact into account, however, the differences are not as great as would initially seem the case and, allowing for the differences in performance of released dust from that of released gas, vapour or mist, there appears to be an easily identified relationship between Zone 1 and Zone Z and likewise between Zone 2 and Zone Y.

These definitions in respect of dusts will be replaced in the near future by three new definitions in IEC 1241-34 which is an international document and thus will bring the definitions for dust hazardous areas up to the same status as those for gases, vapours and mists.

Zone 20

Zone 20 is a Zone in which combustible dust, as a cloud, is present continuously or frequently, during normal operation, in sufficient quantity to be capable of producing an explosible concentration of combustible dust in mixture with air (explosive atmosphere), and/or where layers of dust of uncontrollable and excessive thickness can be formed. An example of this Zone is the inside of processing equipment.

Zone 21

Zone 21 is a Zone not classified as Zone 20 in which combustible dust, as a cloud, is likely to occur during normal operation in sufficient quantities to be capable of producing an explosible concentration of combustible dust in mixture with air (explosive atmosphere). Examples of these areas are given as those immediately surrounding powder filling or discharging sites.

Zone 22

Zone 22 is a Zone not classified as Zone 21 in which a combustible dust, as a cloud, can occur infrequently, and persist only for a short period, or in which accumulations or layers of combustible dust can give rise to an explosive concentration of combustible dust in mixture with air (explosive atmosphere). Examples of this are given typically as areas of mills where dust released from leaks can settle and give rise to dust layers which can be agitated into a cloud by physical shocks or air turbulence.

These newer definitions show a much clearer relationship to the zonal definitions for gases, vapours and mists while still drawing attention to the differences which exist. Typical of these are the ability of dust to settle and persist even in well-ventilated locations and its ability to burn as a layer in addition to its dangers as a cloud. The general approach has historically been to attempt, in Zones 21 and 22 (Zones Z and Y) to limit dust layers to less than 5mm thickness and electrical equipment is designed assuming a 5 mm thickness of dust (See BS 6467, Part 2 and IEC 1241-3). The latter of these two documents also identifies 1 mm as the thickness of the layer of dust above which an explosive atmosphere is possible.

1 comment:

  1. Sajith, that was a very good explanation. I want to follow your blog. I wanted to ask help also if you could give me a pdf sample of the hazardous area documentation diagram. I want to use it as reference on how to plot the area. It would really be a great honour if you could be my mentor. send it to my mail.