Hazardous Area Classification and Control of Ignition Sources

This Technical Measures Document refers to the classification of plant into hazardous areas, and the systematic identification and control of ignition sources

The relevant Level 2 Criteria are 5.2.1.3(29)c, 5.2.1.11(63)f, 5.2.1.13 and 5.2.4.2(93)a.

Design of plant, pipework and general plant layout is considered in Technical Measures Documents on Plant Layout, Design Codes - Plant, Design Codes - Pipework, Plant Modification / Change Procedures, Maintenance Procedures.

The Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR) provide for the first time a specific legal requirement to carry out a hazardous area study, and document the conclusions, in the form of zones.

General Principles

Hazardous Area Classification for Flammable Gases and Vapours

Area classification may be carried out by direct analogy with typical installations described in established codes, or by more quantitative methods that require a more detailed knowledge of the plant. The starting point is to identify sources of release of flammable gas or vapour. These may arise from constant activities; from time to time in normal operation; or as the result of some unplanned event. In addition, inside process equipment may be a hazardous area, if both gas/vapour and air are present, though there is no actual release.

Catastrophic failures, such as vessel or line rupture are not considered by an area classification study. A hazard identification process such as a Preliminary Hazard Analysis (PHA) or a Hazard and Operability Study (HAZOP) should consider these abnormal events.

The most commonly used standard in the UK for determining area extent and classification is BS EN 60079 part 101, which has broad applicability. The current version makes clear the direct link between the amounts of flammable vapour that may be released, the ventilation at that location, and the zone number. It contains a simplistic calculation relating the size of zone to a rate of release of gas or vapour, but it is not helpful for liquid releases, where the rate of vaporisation controls the size of the hazardous area.

Other sources of advice, which describe more sophisticated approaches, are the Institute of Petroleum Model Code of Practice (Area Classification Code for Petroleum Installations, 2002), and the Institution of Gas Engineers Safety Recommendations SR25, (2001). The IP code is for use by refinery and petrochemical type operations. The IGE code addresses specifically transmission, distribution and storage facilities for natural gas, rather than gas utilisation plant, but some of the information will be relevant to larger scale users.

Zoning

Hazardous areas are defined in DSEAR as "any place in which an explosive atmosphere may occur in quantities such as to require special precautions to protect the safety of workers". In this context, 'special precautions' is best taken as relating to the construction, installation and use of apparatus, as given in BS EN 60079 -101.

Area classification is a method of analysing and classifying the environment where explosive gas atmospheres may occur. The main purpose is to facilitate the proper selection and installation of apparatus to be used safely in that environment, taking into account the properties of the flammable materials that will be present. DSEAR specifically extends the original scope of this analysis, to take into account non-electrical sources of ignition, and mobile equipment that creates an ignition risk.

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

Various sources have tried to place time limits on to these zones, but none have been officially adopted. The most common values used are:

Where people wish to quantify the zone definitions, these values are the most appropriate, but for the majority of situations a purely qualitative approach is adequate.

When the hazardous areas of a plant have been classified, the remainder will be defined as non-hazardous, sometimes referred to as 'safe areas'.

The zone definitions take no account of the consequences of a release. If this aspect is important, it may be addressed by upgrading the specification of equipment or controls over activities allowed within the zone. The alternative of specifying the extent of zones more conservatively is not generally recommended, as it leads to more difficulties with equipment selection, and illogicalities in respect of control over health effects from vapours assumed to be present. Where occupiers choose to define extensive areas as Zone 1, the practical consequences could usefully be discussed during site inspection.

As an example:

A proposal was made to zone an aircraft hanger as Zone 1, although the use of fuels handled above their flash point would be a rare event. It proved difficult to obtain a floor-cleaning machine certified for Zone 1 areas, though the floor needed sweeping regularly. The option of writing out an exception to normal instructions to allow a non Ex-protected machine to be used regularly is not recommended. Instead, a more realistic assessment of the zones is needed, and special instructions issued for the rare event of using more volatile fuels.

A hazardous area extent and classification study involves due consideration and documentation of the following:

These factors enable appropriate selection of zone type and zone extent, and also of equipment. The IP code gives a methodology for estimating release rates from small diameter holes with pressurised sources, and shows how both the buoyancy and momentum of the release influence the extent of a zone. It tabulates values for an LPG mixture, gasoline, natural gas, and refinery hydrogen for pressures up to 100barg. Similarly the IGE code gives a methodology for natural gas, relating the leak rate to the hole-size and the operating pressure. The tables of dispersion distances to the zone boundary address in the main quite large diameter deliberate vents. There is in practice little overlap between the codes.

The results of this work should be documented in Hazardous Area Classification data sheets, supported by appropriate reference drawings showing the extent of the zones around (including above and below where appropriate) the plant item.

Selection of Equipment

DSEAR sets out the link between zones, and the equipment that may be installed in that zone. This applies to new or newly modified installations. The equipment categories are defined by the ATEX equipment directive, set out in UK law as the Equipment and Protective Systems for Use in Potentially Explosive Atmospheres Regulations 1996. Standards set out different protection concepts, with further subdivisions for some types of equipment according to gas group and temperature classification. Most of the electrical standards have been developed over many years and are now set at international level, while standards for non-electrical equipment are only just becoming available from CEN.

The DSEAR ACOP describes the provisions concerning existing equipment.

There are different technical means (protection concepts) of building equipment to the different categories. These, the standard current in mid 2003, and the letter giving the type of protection are listed below.

Zone 0 Zone 1 Zone 2
Category 1 Category 2 Category 3
'ia' intrinsically safe
EN 50020, 2002		 'd' - Flameproof enclosure
EN 50018 2000		 Electrical
Type 'n' - EN 50021 1999
Non electrical
EN 13463-1, 2001
Ex s - Special protection if specifically certified for Zone 0 'p' - Pressurised
EN 50016 2002		  
  'q' - Powder filling
EN 50017, 1998		  
  'o' - Oil immersion
EN 50015, 1998		  
  'e' - Increased safety
EN 50019, 2000		  
  'ib' - Intrinsic safety
EN 50020, 2002		  
  'm' - Encapsulation
EN 50028, 1987		  
  's' - Special protection  

Correct selection of electrical equipment for hazardous areas requires the following information:

Temperature Classification Maximum Surface Temperature, °C Ignition Temperature of gas or vapour, °C
T1 450 >450
T2 300 >300
T3 200 >200
T4 135 >135
T5 100 >100
T6 85 >85

If several different flammable materials may be present within a particular area, the material that gives the highest classification dictates the overall area classification. The IP code considers specifically the issue of hydrogen containing process streams as commonly found on refinery plants. Consideration should be shown for flammable material that may be generated due to interaction between chemical species.

Ignition Sources - Identification and Control

Ignition sources may be:

Sources of ignition should be effectively controlled in all hazardous areas by a combination of design measures, and systems of work:

Direct Fired Heaters, Hot Oil Systems and Processes Operating Above Auto-Ignition Temperatures

A range of petrochemical and refinery processes use direct fired heaters, e.g. steam crackers for ethylene production. Clearly, if the fuel supply to the heater or the pipework carrying the process fluid leaks close to the furnace, any leak must be expected to find a source of ignition, either directly at the flames, or by a surface heated by a flame. In these circumstances, hazardous area classification, and appropriate selection of ATEX equipment is not suitable as a basis of safety for preventing fire and explosion risks.

Instead, safety should be achieved by a combination of a high standard of integrity of fuel and process pipelines, together with a means of rapid detection and isolation of any pipes that do fail. The consequences of the failure of a pipe carrying process materials within the furnace should be considered in any HAZOP study.

Other processes (such as hot oil heating circuits) may handle products above their auto-ignition temperature. Any such processes should be specifically identified in a safety case. Again, area classification is not a suitable means of controlling the ignition risks, and the same considerations apply, as with fired heaters.

Lightning Protection

Protection against lightning involves installation of a surge protection device between each non-earth bonded core of the cable and the local structure. Further guidance can be found in BS 6651:19991 - (Code of practice for protection of structures against lightning). Ignitions caused by lightning cannot be eliminated entirely, particularly with floating roof tanks, where vapour is usually present around the rim seal. In these circumstances, measures to mitigate the consequences of a fire should be provided.

Vehicles

Most normal vehicles contain a wide range of ignition sources. These will include electrical circuits; the inlet and exhaust of any internal combustion engine; electrostatic build up; overheating brakes, and other moving parts. Site rules should be clear where normal road vehicles may be taken, and areas where they must be excluded.

Standard EN 17551 sets out the requirements for diesel powdered ATEX category 2 or 3 lift trucks. Electric powered vehicles can also be built using a combination of this standard and the normal electrical standards. No specification is available for vehicles with spark ignition engines, and it is unlikely that such an engine could be built economically. Vehicles certified to ATEX requirements are however expensive, and for many applications an unprotected type has to be extensively rebuilt. Consequently, many employers are likely to try and justify not zoning storage compounds, where lift trucks handle flammable liquids or gases in containers. In some stores, perhaps with limited use of a vehicle, this may be acceptable. Discussions have been held with the British Chemical Distributors and Traders Association, with the objective of clarifying when storage areas should be classified as zone 2. The conclusions from this exercise will be made available in due course. Discussions are also ongoing, about vehicles with gas detection systems, designed to shut the engine and isolate other sources of ignition in the event of a gas release. At present these are sold without any claim for ATEX compliance, but with the suggestion they may be useful in cases of remote risk.

For the purposes of COMAH, an assessment is needed of the risk that an ignition within a storage compound will produce a major accident, either directly or because a fire or explosion spreads to involve other materials. If this is possible, it is more appropriate to provide controls to prevent the spread, rather than simply apply more conservative zoning, and more restrictive rules on the equipment used in the store.

Where specialist vehicles (e.g. cranes) are needed during maintenance operations, proper controls and plant isolation may allow the normal zones to be suspended. Typically these will involve written instructions, as specified in DSEAR schedule 1, or a formal permit to work system.

Many sites will have operations of filling and emptying road tankers with flammable materials. Controls will be needed to prevent or minimise the release of gas or vapour but controls over ignition sources are also needed. Hazardous areas may be considered to exist during the transfer operation, but should not be present once the transfer is complete. Safe systems of work are needed to ensure safety where such 'transient' zones exist.

Factors for Assessor of a Safety Case to Consider

Factors that could be considered during an on site inspection

Dust Explosions

The COMAH Regulations do not apply to any material if the only risk created is that of a dust explosion. However, many toxic materials are handled in fine powder form, and a serious dust explosion could cause a major accident. A dust explosion involving a non-toxic dust like polyethylene would not result in a major accident as defined in the regulations, unless it also led to loss of containment of a COMAH substance. A dust explosion could then be an initiator of a major accident. Measures to prevent major accidents should address all potential initiators.

DSEAR requires that hazardous area classification for flammable dusts should be undertaken in the same manner as that for flammable gases and vapours. Zoning as described above may be applied, replacing 'gas atmosphere' with 'dust/air mixtures'. The zone numbers used are 20, 21 and 22, corresponding to 0,1 and 2 used for gases/vapours

The only relevant standard to help people zone their plant is BS EN 50281 part 3, 20021, which is an adaptation of the IEC equivalent.

Where toxic dusts are processed, releases into the general atmosphere should be prevented, and the extent of any zone 21 or 22 outside the containment system should be minimal or non-existent. The inside of different parts of the plant may need to be zoned as 20, 21 or 22, depending on the conditions at particular locations.

Classification of dusts relating to autoignition and minimum ignition current is undertaken similarly to gases/vapours, but involves additional complications.

The explosibility of dusts is dependent upon a number of factors:

Where toxic dusts are handled, in most cases occupiers will need to carry out testing of the product for its explosion properties. Companies able to undertaken such testing are listed in the IChemE's book on the prevention of dust explosions. There is no legally defined test for an explosible dust. However, for many years we have used a small-scale screening test, the vertical tube test, described in HSG 103. The issues about representative samples of dust, and other factors that might cause the results to vary are also discussed in this guidance. In general, dusts with a particle size greater than 500 µm are unlikely to cause an explosion. For most chemical products it is preferable to test dust taken from the process, but if the particle size distribution varies, it is common to test material that passes a 63-micron sieve, and take this as the worst case.

Ignition due to a hot surface is possible, but the temperature needed to ignite a dust layer depends on layer thickness and contact time. For COMAH sites with toxic dusts, the most likely hazard would arise in drying processes, if substantial quantities were held for extended periods hot enough to start self heating or smouldering combustion.

Status of Guidance

Existing codes of practice provide information with respect to good practice for hazardous area classification. The standards detailing selection of appropriate electrical apparatus have been updated to take into consideration ventilation effects.

European equipment standards may become 'harmonised' when a reference to them is published in the Official Journal of the European Community. A list of ATEX harmonised standards can be checked on the EU web site:

Equipment built to such a harmonised standard may assume automatic conformity with those essential safety requirements of relevant directives that are covered by the standard. The EPS regulations describe the conformity assessment procedures that apply to different types of equipment.

Reference Documents

Electrical Equipment

Non-electrical equipment

Electrostatic ignition risks

The most recent general source of advice was drafted by a European standards working group, but was published in the UK as BS PD R044-001 and not as a full standard. It contains much useful advice about limiting pumping speeds, electrostatic risks from clothing, and many detailed operations. The two parts of the older BS 5958: 19911 Code of Practice for the control of undesirable static electricity remain current, because they contain some useful information not duplicated by the PD. The two parts are:

Part 1: 1991 General considerations;

Part 2: 1991 Recommendations for particular industrial situations

Further Reading Material

Cox, A.W., Lees, F.P. and Ang, M.L., 'Classification of Hazardous Locations', 1993.

This was a study led by a consortium of the chemical, electrical and mechanical engineering institutes, and showed how the subject spanned the traditional divides. It was important in the development of ideas, but provides no new methodology for users.

A Guide to Safety in Aerosol Manufacture, BAMA, Third Edition, 1999.

Dust Explosion Prevention and Protection: A Practical Guide, IChemE, ISBN 0852954107

A practitioner's handbook - Electrical installation and maintenance in potentially explosive atmospheres, Publication No. 186, The Engineering Equipment and Materials Users Association.

References

1. For further information on the relevant British Standards, please access the British Standards web site and use the search facility. Back

Page last updated: 22nd September 2004

2022-02-10