Classification of Hazardous Areas

The requirements for Electrical apparatus for explosive gas atmospheres have been detailed in the IEC 60079 series.

This article, an extract from IEC 60079-10, will be the first of a series of articles discussing the requirements for explosive gas atmosphere electrical installations. To understand better the requirements for hazardous areas installations, IEC 60079-10 provides the classification of explosive atmospheres.

The objective of IEC 60079-10, Classification of hazardous areas, is to set out the essential criteria against which the risk of ignition can be assessed, and to give guidance on the design and control parameters which can be used in order to reduce this risk.

In areas where dangerous quantities and concentrations of flammable gas or vapour may arise, protective measures are to be applied in order to reduce the risk of explosions.

Area classification objectives
Area classification is a method of analysing and classifying the environment where explosive gas atmospheres may occur so as to facilitate the proper selection and installation of apparatus to be used safely in that environment, taking into account gas groups and temperature classes.

In most practical situations where flammable materials are used, it is difficult to ensure that an explosive gas atmosphere will never occur. It may also be difficult to ensure that apparatus will never give rise to a source of ignition. Therefore, in situations where an explosive gas atmosphere has a high likelihood of occurring, reliance is placed on using apparatus which has a low likelihood of creating a source of ignition. Conversely, where the likelihood of an explosive gas atmosphere occurring is reduced, apparatus constructed to a less rigorous standard may be used.

It is rarely possible by a simple examination of a plant or plant design to decide which parts of the plant can be equated to the three zonal definitions (zones 0, 1 and 2). A more detailed approach is therefore necessary and this involves the analysis of the basic possibility of an explosive gas atmosphere occurring.

The first step is to assess the likelihood of this, in accordance with the definitions of zone 0, zone 1 and zone 2. Once the likely frequency and duration of release (and hence the grade of release), the release rate, concentration, velocity, ventilation and other factors which affect the type and/or extent of the zone have been determined, there is then a firm basis on which to determine the likely presence of an explosive gas atmosphere in the surrounding areas.

This approach therefore requires detailed consideration to be given to each item of process equipment which contains a flammable material, and which could therefore be a source of release.

In particular, zone 0 or zone 1 areas should be minimized in number and extent by design or suitable operating procedures. In other words, plants and installations should be mainly zone 2 or non-hazardous. Where release of flammable material is unavoidable, process equipment items should be limited to those which give secondary grade releases or, failing this (that is where primary or continuous grade releases are unavoidable), the releases should be of very limited quantity and rate.

In carrying out area classification, these principles should receive prime consideration. Where necessary, the design, operation and location of process equipment should ensure that, even when it is operating abnormally, the amount of flammable material released into the atmosphere is minimized, so as to reduce the extent of the hazardous area.

Once a plant has been classified and all necessary records made, it is important that no modification to equipment or operating procedures is made without discussion with those responsible for the area classification. Unauthorized action may invalidate the area classification. It is necessary to ensure that all equipment affecting the area classification which has been subjected to maintenance is carefully checked during and after re-assembly to ensure that the integrity of the original design, as it affects safety, has been maintained before it is returned to service.

Scope of IEC 60079
The classification of hazardous areas where flammable gas or vapour risks may arise is a primary concern in the proper selection and installation of apparatus for use in such hazardous areas (see notes 1 and 4). IEC 60079-10 is intended to be applied where there may be a risk of ignition due to the presence of flammable gas or vapour, mixed with air under normal atmospheric conditions (see note 2).

Exclusions to IEC 60079

1. mines susceptible to firedamp;
2. the processing and manufacture of explosives;
3. areas where a risk may arise due to the presence of ignitable dusts or fibres;
4. catastrophic failures which are beyond the concept of abnormality dealt with in this standard (see note 3);
5. rooms used for medical purposes;
6. areas where the presence of flammable mist may give rise to an unpredictable risk and which require special consideration (see note 5). This standard does not take into account the effects of consequential damage.

Definitions and explanations of terms are given together with the main principles and procedures relating to hazardous area classification.

For detailed recommendations regarding the extent of the hazardous areas in specific industries or applications, reference may be made to the codes relating to those industries or applications.

Notes

1. For the purpose of this standard, an area is a three-dimensional region or space.
2. Atmospheric conditions include variations above and below reference levels of 101.3 kPa (1 013 mbar) and 20 °C (293 K), provided that the variations have a negligible effect on the explosion properties of the flammable materials.
3. Catastrophic failure in this context is applied, for example, to the rupture of a process vessel or pipeline, and such events that are not predictable.
4. In any process plant, irrespective of size, there may be numerous sources of ignition apart from those associated with electrical apparatus. Appropriate precautions will be necessary to ensure safety in this context. This standard may be used with judgement for other ignition sources.
5. Mists may form or be present at the same time as flammable vapours. This may affect the way flammable material disperses and the extent of any hazardous areas. The strict application of area classification for gases and vapours may not be appropriate because the flammability characteristics of mists are not always predictable. Whilst it can be difficult to decide upon the type and extent of zones, the criteria applicable to gases and vapours will, in most cases, give a safe result. However, special consideration should always be given to the danger of ignition of flammable mists.

Definitions and terms
explosive gas atmosphere
a mixture with air, under atmospheric conditions, of a flammable material in the form of gas or vapour in which, after ignition, combustion spreads throughout the unconsumed mixture.

NOTE Although a mixture which has a concentration above the upper explosive limit (UEL) is not an explosive gas atmosphere, it can readily become so and, in certain cases for area classification purposes, it is advisable to consider it as an explosive gas atmosphere.

hazardous area
an area in which an explosive gas atmosphere is present, or may be expected to be present, in quantities such as to require special precautions for the construction, installation and use of apparatus

non-hazardous area
an area in which an explosive gas atmosphere is not expected to be present in quantities such as to require special precautions for the construction, installation and use of apparatus

Zones
Hazardous areas are classified into zones based upon the frequency of the occurrence and duration of an explosive gas atmosphere, as follows:

zone 0
an area in which an explosive gas atmosphere is present continuously or for long periods

zone 1
an area in which an explosive gas atmosphere is likely to occur in normal operation

zone 2
an area in which an explosive gas atmosphere is not likely to occur in normal operation and, if it does occur, is likely to do so only infrequently and will exist for a short period only

NOTE Indications of the frequency of the occurrence and duration may be taken from codes relating to specific industries or applications.

source of release
a point or location from which a flammable gas, vapour, or liquid may be released into the atmosphere such that an explosive gas atmosphere could be formed [IEV 426-03-06, modified]

Grades of release
There are three basic grades of release, as listed below in order of decreasing likelihood of the explosive gas atmosphere being present:

1. continuous grade;
   a release which is continuous or is expected to occur for long periods
2. primary grade;
   a release which can be expected to occur periodically or occasionally during normal operation
3. secondary grade.
   a release which is not expected to occur in normal operation and if it does occur, is likely to do so only infrequently and for short periods

A source of release may give rise to any one of these grades of release, or to a combination of more than one.

release rate
the quantity of flammable gas or vapour emitted per unit time from the source of release

normal operation
the situation when the equipment is operating within its design parameters

NOTES

1. Minor releases of flammable material may be part of normal operation. For example, releases from seals which rely on wetting by the fluid which is being pumped are considered to be minor releases.
2. Failures (such as the breakdown of pump seals, flange gaskets or spillages caused by accidents) which involve urgent repair or shut-down are not considered to be part of normal operation.

ventilation
movement of air and its replacement with fresh air due to the effects of wind, temperature gradients, or artificial means (for example fans or extractors)

Explosive limits

lower explosive limit (LEL)
the concentration of flammable gas or vapour in air, below which the gas atmosphere is not explosive

upper explosive limit (UEL)
the concentration of flammable gas or vapour in air, above which the gas atmosphere is not explosive

NOTE For the purpose of this standard, the terms "explosive" and "flammable" should be considered synonymous.

relative density of a gas or a vapour
the density of a gas or a vapour relative to the density of air at the same pressure and at the same temperature (air is equal to 1,0)

flammable material
a material which is flammable of itself, or is capable of producing a flammable gas, vapour or mist

flammable liquid
a liquid capable of producing a flammable vapour under any foreseeable operating conditions

flammable gas or vapour
gas or vapour which, when mixed with air in certain proportions, will form an explosive gas atmosphere

flammable mist
droplets of flammable liquid, dispersed in air so as to form an explosive atmosphere

flashpoint
the lowest liquid temperature at which, under certain standardized conditions, a liquid gives off vapours in a quantity such as to be capable of forming an ignitable vapour/air mixture

boiling point
the temperature of a liquid boiling at an ambient pressure of 101,3 kPa (1 013 mbar)

NOTE For liquid mixtures, the initial boiling point should be used. Initial boiling point is used for liquid mixtures to indicate the lowest value of the boiling point for the range of liquids present, as determined in a standard laboratory distillation without fractionation.

vapour pressure
the pressure exerted when a solid or liquid is in equilibrium with its own vapour. It is a function of the substance and of the temperature

ignition temperature of an explosive gas atmosphere
the lowest temperature of a heated surface at which, under specified conditions, the ignition of a flammable substance in the form of a gas or vapour mixture with air will occur

NOTE IEC 60079-4 and IEC 60079-4A standardize a method for the determination of this temperature.

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:

1. eliminate the likelihood of an explosive gas atmosphere occurring around the source of ignition, or
2. 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.

Release rate of gas or vapour

The greater the release rate the larger the extent of the zone. The release rate depends itself on other parameters, namely:

1. Geometry of the source of release
   This is related to the physical characteristics of the source of release, for example an open surface, leaking flange, etc.
2. Release velocity
   For a given source of release, the release rate increases with the release velocity. In the case of a product contained within process equipment, the release velocity is related to the process pressure and the geometry of the source of release. The size of a cloud of flammable gas or vapour is determined by the rate of flammable vapour release and the rate of dispersion. Gas and vapour flowing from a leak at high velocity will develop a cone-shaped jet which will entrain air and be self-diluting. The extent of the explosive atmosphere will be almost independent of wind velocity. If the release is at low velocity or if its velocity is destroyed by impingement on a solid object, it will be carried by the wind and its dilution and extent will depend on wind velocity.
3. Concentration
   The release rate increases with the concentration of flammable vapour or gas in the released mixture.
4. Volatility of a flammable liquid
   
   1. Flashpoints of flammable liquids are not precise physical quantities, particularly where mixtures are involved.
   2. Some liquids (for example certain halogenated hydrocarbons) do not possess a flashpoint although they are capable of producing an explosive gas atmosphere. In these cases, the equilibrium liquid temperature which corresponds to the saturated concentration at the lower explosive limit should be compared with the relevant maximum liquid temperature.
   
   This is related principally to the vapour pressure, and the heat of vaporization. If the vapour pressure is not known, the boiling point and flashpoint can be used as a guide. An explosive atmosphere cannot exist if the flashpoint is above the relevant maximum temperature of the flammable liquid. The lower the flashpoint, the greater may be the extent of the zone. If a flammable material is released in a way that forms a mist (for example by spraying) an explosive atmosphere may be formed below the flashpoint of the material for example.
5. Liquid temperature
   The vapour pressure increases with temperature, thus increasing the release rate due to evaporation.
   
   NOTE The temperature of the liquid after it has been released may be increased, for example, by a hot surface or by a high ambient temperature.

Lower explosive limit (LEL)
For a given release volume, the lower the LEL the greater will be the extent of the zone.

Ventilation
With increased ventilation, the extent of the zone will be reduced. Obstacles which impede the ventilation may increase the extent of the zone. On the other hand, some obstacles, for example dykes, walls or ceilings, may limit the extent.

Relative density of the gas or vapour when it is released

If the gas or vapour is significantly lighter than air, it will tend to move upwards. If significantly heavier, it will tend to accumulate at ground level. The horizontal extent of the zone at ground level will increase with increasing relative density and the vertical extent above the source will increase with decreasing relative density.

NOTES

1. For practical applications, a gas or vapour which has a relative density below 0,8 is regarded as being lighter than air. If the relative density is above 1,2 it is regarded as being heavier than air. Between these values, both of these possibilities should be considered.
2. Experience has shown that ammonia is hard to ignite and a gas release will dissipate rapidly in the open air, so any explosive gas atmosphere will be of negligible extent.

Other parameters to be considered

1. Climatic conditions.
2. Topography