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April 2, 2009

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Blast resort

The Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR) formalise the requirement to undertake an assessment of the explosion risk due to vapours, gases, or dusts. With reference to an oil-storage terminal, Alex Hills discusses the factors such an assessment should consider.

A risk assessment of an explosion due to vapours, gases, or dusts – as set out by the DSEAR 20021 – is likely to include a Hazardous Area Classification (HAC) to ensure that risks are effectively controlled within certain boundaries. However, as with any risk assessment, the first stage is to identify the hazards, i.e. the locations where a flammable or explosive atmosphere may exist. This exercise requires a review of activities undertaken and substances used.

Create an inventory

It is often helpful to produce an inventory of dangerous substances and their properties. This will include flammable solvents, gases, and pyrophoric materials (i.e. substances that can ignite spontaneously).
It is especially important to consider three properties:

  • The flash point of a flammable liquid – the lowest temperature at which it can form an ignitable mixture in air;
  • The auto-ignition temperature of a substance – the lowest temperature at which it will spontaneously ignite in a normal atmosphere without an external source of ignition, such as a flame or spark; and
  • The lower explosive limit (LEL) – usually expressed in volume per cent, this is the lower end of the concentration range of a flammable solvent at a given temperature and pressure for which air/vapour mixtures can ignite. The flammability range includes an upper and lower flammability limit. Outside this range, the mixture will not ignite, unless it is heated.

A highly-flammable liquid has a flash point of less than 21°C, which means its vapour will ignite at room temperature. A flammable liquid has a flash point closer to 55°C, in which case its vapour should not ignite at room temperature. A flash-point temperature of 32°C, which is cited in the (now largely repealed) Highly Flammable Liquids and Liquefied Petroleum Gases Regulations 1972, may be a more realistic measure of whether a liquid presents a risk of a flammable vapour at room temperature; at this temperature a flammable vapour is unlikely to form even on hot days, which may not be the case for flash points just above 21°C.

Other important properties include the vapour pressure of the material and the relative density to air. Heating increases the risk of a vapour igniting, and a further risk arises when fluids are pumped under pressure. Even a mist from a non-flammable fuel oil can form an explosive mixture.

Having established what is used, it is then necessary to know how it is used, or stored, e.g. at what quantities, pressures, temperatures, and flow rates.


Where risks of a flammable atmosphere developing cannot be eliminated they need to be carefully controlled within zones. This involves the undertaking of a Hazardous Area Classification (HAC) study, or zoning exercise.

There is no single correct way to undertake a HAC study. In practice, it is a mixture of calculations, as outlined in guidance documents; reference to guidance documents from organisations, such as the HSE, British Standards Institution, the Institute of Petroleum, and trade associations; and professional judgement, based on an understanding of the materials and process conditions present.

The convention for classifying zones for gases and vapours can be summarised as follows:

  • Zone 0: An area in which the explosive gas/air mixture is continuously present, or present for long periods;
  • Zone 1: An area in which the explosive gas/air mixture is likely to occur in normal operation;
  • Zone 2: An area in which the explosive gas/air mixture is unlikely to occur in normal operation and, if it occurs, will exist only for a short time.

The HAC exercise needs to determine both the zone classification and the extent of the zone. One factor used to determine the zone classification is the type of release, categorised in BS EN 600079 part 102 as follows:

  • Continuous Grade Release – a release that is continuous, or is expected to occur. Such releases usually result in a Zone 0.
  • Primary Grade Release – a release that can be expected to occur in normal operation. Typical examples include seals of pumps, compressors, or valves if a release of flammable material is expected during normal operation. Other examples include vent valves and drainage points, or sample points, which could be expected to contain flammable materials. Such releases usually result in a Zone 1. 
  • Secondary Grade Release – a release that is not expected to occur in normal operation and, if it does, will only do so infrequently and for short periods. Typical examples include seals of pumps, compressors, or valves where release of flammable material is not expected during normal operation. Other examples include relief valves, vents, flanges, pipe fittings, and drainage points. Such releases usually result in a Zone 2.

Having determined the zone classification, it is necessary to determine the extent of the zone. This is the distance (with a factor of safety built in) at which any release is likely to be at a concentration below the LEL (see table 2 above).

Controlling the risk

Where hazardous areas are identified, elimination of the risk should be the first consideration. Options include the substitution of a higher-risk substance with a lower-risk alternative, or, in the example of a storage tank, applying a nitrogen blanket to the top of the vessel.

Where flammable atmospheres cannot be eliminated, there needs to be a strict regime of control to ensure that the risk of ignition is minimised. This includes the selection of equipment in accordance with legal requirements.3 The correct equipment must be installed for each zone and the type of gas or vapour that may be present. Where the substance has a flash point above 32°C, the risk is from a leakage at pressure forming a spray mist. This can be eliminated, and the need for zoning minimised, by the application of anti-spray tape, or shrouding around the joints.

Static electricity is another ignition source, which must be controlled. Effective earthing and bonding arrangements need to be put in place, and maintained, in order to minimise this risk.

Just one element

Remember that undertaking the zoning exercise is just a part of the risk assessment and of fulfilling the requirements of DSEAR. The zoning should be made clear to all relevant persons, preferably via the use of diagrams. Control procedures, including emergency mitigation measures, must be developed, implemented and maintained. It is also important not to neglect procedural and behavioural controls, which must be implemented along with technical control measures.

References and further reading
1   HSE (2003): Approved Code of Practice and Guidance to the Dangerous Substances and Explosive Atmospheres Regulations 2002, L138, HSE Books
2   BSI (2003): Electrical apparatus for explosive atmospheres. Part 10: Classification of hazardous areas for gases and vapours, BS EN 60079-10:2003
3   The Equipment and Protective Systems Intended for Use in Potentially Explosive Atmospheres Regulations 1996, SI 1996, no. 192; see
4   HSE (1998): The Storage of flammable liquids in containers, HSG 51, HSE Books
5   HSE (1996): Safe use and handling of flammable liquids, HSG 140, HSE Books
 HSE (1998): The storage of flammable liquids in tanks, HSG 176, HSE Books
 Institute of Petroleum (2005): Area Classification Code for installations handling flammable fluids, Model Code of Safe Practice Part 15, third Edition
 Association for Petroleum and Explosives Administration (2005): Design, construction, modification, maintenance and decommissioning of filling stations, second edition
 HSE (1999): Safe use of petrol in garages, INDG 331, HSE Books, available at
 HSE (1996): Dispensing petrol – Assessing and controlling the risk of fire and explosion at sites where petrol is stored and dispensed as a fuel, HSG 146, HSE Books
 West Yorkshire Fire & Rescue Service (2007): Petrol filling stations – Guidance on managing the risks of fire and explosion, revised

Alex Hills is principal health and safety consultant at Connaught Compliance Training Services. Brian Mallon works in the licensing department of a council in Northern Ireland. See page 4 for more details.


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In this episode of the Safety & Health Podcast, we hear from Matt Birtles, Principal Ergonomics Consultant at HSE’s Science and Research Centre, about the different approaches to managing the risks associated with Musculoskeletal disorders.

Matt, an ergonomics and human factors expert, shares his thoughts on why MSDs are important, the various prevalent rates across the UK, what you can do within your own organisation and the Risk Management process surrounding MSD’s.

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