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Explosive Atmospheres

Published: 10th May 2016

Working in explosive atmospheres can be highly hazardous, and nowhere are the dangers more prevalent than in the oil and gas industry.

According to the definition by the UK’s Health and Safety Executive (HSE), an explosive atmosphere is a mixture of air and dangerous substances such as dust, gas, mist, or vapour, with the potential to catch fire or explode.

In most cases explosive atmospheres do not result in explosions, but when they do happen the flames travel quickly. It is important to know that vapours, mists, gases and dusts can all form explosive atmospheres when in contact with air. For the purposes of standardisation, normal atmospheric conditions are defined from -20°C to 40°C and 0.8 to 1.1 bar respectively. Several catastrophic incidents have been recorded as a result of explosive atmospheres. In fact, the oil, gas and chemical industries often generate many fire and explosion accidents.

The disastrous Buncefield incident was a major conflagration caused by explosions on 11 December, 2005, at the oil storage terminal in Hertfordshire, UK. This terminal was the fifth largest oil-products storage depot, with a capacity of 60,000,000 gallons of fuel. Further explosions followed that eventually overcame approximately 20 large storage tanks.

From all accounts, the incident came as a result of an unconstrained vapour cloud explosion of unusually high strength, also known as a fuel-air explosion. Because of the inversion layers, the explosions were heard up to 125 miles (200 km) away; there were reports that effects were noticeable in neighbouring countries. An investigation report revealed that Tank 912 at Buncefield oil storage depot was overfilled with petrol. The tank had a normal level gauge that employees used to monitor the flow level manually, and independent high-level switch to shut off inflow when the level filled above a certain set point. Prior to the incident, the manual gauge became stuck and the independent shut off was inoperative. Eventually, when the tank was completely filled, the petrol overflowed through the vents at the top and formed vapour clouds, which were later ignited and detonated. The fires from the explosion lasted for five days. Hundreds of homes in the area were displaced, with 2,000 people needing alternative homes. In fact, emergency services asked residents of the smoke-affected areas to close their windows and doors and stay inside. The concerns for public safety resulted in 227 schools and other public buildings closing throughout the incident. The Court ordered costs against the defendants totalling £4,081,000.

Work in explosive atmospheres should demand that certain health and safety measures are put in place to mitigate the potential hazards of operating in these conditions. Oil and gas industries implement these measures to ensure that nobody gets hurt. Some of the important measures are described as follows.

Assessing the risk

In general terms, a hazard is anything that has potential to cause harm, but when considering dangerous substances and preparations a more restricted meaning is specified in the Dangerous Substances and Explosive Atmospheres Regulation (DSEAR). The term ‘hazard’ in DSEAR is confined to the properties of a substance that can potentially lead to fire, explosion or other similar effects that could affect personal safety. Before working in explosive areas, therefore, it is compulsory to conduct a risk assessment to identify hazardous areas and apply appropriate mitigations to protect personnel and assets from injury and damage. Identifying the hazardous or non-hazardous areas should be carried out in a systematic manner. The risk assessment is used to determine if hazardous areas exist and assign zones to those areas.

At a minimum the following criteria should be considered:

• Hazardous properties of dangerous substances

• Number of dangerous substances involved

• Work processes and various interactions

• The temperatures and pressures at which the substances are being handled

• The containment strategy and controls provided to prevent gases, liquids, vapours or dusts escaping to the workplace atmosphere

The overall risk assessment principle must be applied by identifying the hazard, identifying who could be harmed, evaluating the risks and deciding on control measures, recording findings, reviewing the assessment and updating as and when necessary.

The properties that need to be known are boiling points and flash points of flammable substances that could be lighter or heavier than air. Flammable substances are grouped into flammable gas, flammable liquids/vapours and flammable solids. It is important to note that there are specific lower and upper flammability limits for every flammable gas or vapour.

Where the concentration is below the specific value for low flammability limits or above a specific value for upper flammability limit, ignition will occur. Where the flammable gas or vapour cloud is unconstrained and ignited, all the material may be consumed in one explosion. Where the flammable gas or vapour cloud is not ignited, convection and diffusion will eventually disperse the flammable cloud, and the immediate danger will pass out, meaning the particular fuel source will be lost. Table 1 describes examples of various flammable substance and their characteristics.

For dust, information on particle size and density will be required, as it must be shown that a particular dust can give rise to an explosive atmosphere. Very often, relevant hazard information is usually contained on the safety data sheet provided with the product. However, the size of potential releases, temperature, pressure, ventilation and extent of hazardous areas will be examined. The assessment needs to identify areas within a workplace that are connected to places with potentially explosive atmospheres. This will provide information on areas to exclude from the source of hazard where the explosive atmosphere could spread, for example in ducts. Such areas would be included in the classification for areas in which explosive atmospheres could occur. The approach to assessing this risk is described in BS EN 60079/10.

Fires and explosions

The major difference between fires and explosions is the rate of energy release. Fires release energy slowly, while explosions release energy that travels very rapidly, typically in microseconds. Fires can also result from explosions and explosions can result from fires. However, in order for a fire to start or be sustained there must be fuel, oxygen and an ignition source. If one of the three components is eliminated, then there will not be a fire (or explosion). In many cases an explosive atmosphere can make any ignition cause a fire rather than an explosion.

Both fire and explosion have the potential to cause danger to workers, and in many situations the precautions required to prevent an ignition are the same. Yearly losses due to fires and explosions are quite substantial. The property losses for explosions in the United States, for instance, are estimated at over 200 million. Additional losses that were recorded due to business interruptions are estimated to exceed 250 million annually.

In order to prevent accidents due to fires and explosions, employers must be familiar with:

• Properties of materials in use

• The nature of fire/explosion that could occur

• Arrangements to reduce fire and explosion hazards

The overall kind of precautions to take would depend on the possible consequence of a fire or explosion if it occurred. Many factors could potentially influence risks of fire involving dangerous substances occurring. In particular, what all employers should consider would be whether a particular fire could lead to an explosion, if it does happen how fast the fire can spread, what other possible materials might be rapidly involved, what dangers could come as a result of the smoke and toxic gases given off, and whether personnel in the vicinity would be able to escape or not.

The fire triangle

The existence of fire requires heat, oxygen and fuel. These key ingredients are known as the fire triangle. If any side of the triangle is missing, fire cannot exist, and any pre-existing fire will extinguish.


As mentioned previously, for fire to occur fuel must be present within certain concentrations. Typical cases where fuel occurs are when there is excessive dust, or when a leakage occurs during fuelling or transfer operations. Employers often cannot always eliminate these sources, but can help to prevent the build up of vapours by ensuring adequate ventilation, locating things outside and using grating on floors.


Oxygen is a key ingredient for fire. Field operations often use ‘inerting’ blankets of nitrogen and helium over flammable materials to reduce the oxygen content to a safe level.


Heat is another common ignition source. Although we can eliminate ignition sources where practicable, it is almost unavoidable that an ignition source will be available where there is a large release of flammable material that cannot be diluted quickly.

Controlling fire and explosion

Since the removal of all potential hazards will not always be possible in the oil and gas industry, removal of one element from the fire triangle can provide protection and prevent both fire and disastrous explosions.

Since flammable substance and oxidisers cannot be frequently eliminated, inhibiting ignition of a potentially explosive atmosphere can eliminate danger at the source. An acceptable level might be determined by selecting protective measures and installing means to ensure the likelihood for explosion is not increased by the presence of electrical apparatus. The most failsafe method of preventing an explosion would be to locate electrical equipment outside of hazardous zones whenever possible. In situations where this is not practicable, installation techniques and enclosures should be available that meet the requirements for locating electrical equipment in such areas.

Zoning hazards

Part of the important safety measures on oil and gas platform to ensure safe working atmosphere would be to classify the hazardous areas into zones. Hazardous areas are where flammable liquids, gases, vapours or combustible dusts exist in sufficient quantities to generate explosion or fire. In hazardous locations, specially designed equipment and installation methods must be used to protect personnel against the explosive and flammable potential of these substances. The area should be classified into zones based on the frequency and persistence of the potentially explosive atmosphere. This will help to determine the controls that are required to manage the source of ignition that could be present in the area. This is applied to the fixed equipment that holds the risk of generating explosion and is extended to the control of mobile equipment and other sources of ignition in the area, such as matches and lighters, as well as the risks from electrostatic discharges.

International standards such as BS EN 60079/10 provide clarification of the basic principles for area classification for gas and vapours; however, dust has an equivalent standard that was published in 2002: BS EN 61241/3. These standards provide guidelines for complying with the requirements of DSEAR. Industry specific codes have also been published by various organisations and, provided they are applied appropriately, they are valuable in encouraging a consistent interpretation of the requirements.

It is important to note that area classification studies usually take the form of drawings identifying the hazardous areas and zones. Whenever such drawings and documents have been produced, they should be included in the risk assessment record as required by DSEAR standards. These documents should be referred to when new equipment is to be introduced into a zoned area. The hazardous areas are classified in terms of zones on the basis of the frequency and duration of an explosive atmosphere.

Zone classifications for gases, vapours and mists are 0, 1, and 2 respectively:

• Zone 0 - Area with concentrations of flammable gases or vapours that are ignitable and are present continuously or for long periods of time

• Zone 1- Area with concentrations of flammable gases or vapours that are ignitable and are expected to occur in normal operation occasionally

• Zone 2 - Area with concentrations of flammable gases or vapours that are ignitable and are not expected to exist under normal operating conditions

For dusts the zone classifications are:

• Zone 20 - Area with explosive atmosphere in the form of combustible dust in air that are present continuously, or for long periods

• Zone 21 - Area with explosive atmosphere in the form of combustible dust in air that may occur in normal operation occasionally

• Zone 22 - Area with explosive atmosphere in form of a cloud of combustible dust in air that may not occur in normal operation but, if it actually does occur, will continue for a short period of time only

Where the zone system is practiced, it is the responsibility of the user or designer to make a choice and apply the appropriate protection for each zone. Directive 94/9/EC, however, requires supplementary markings to specify precisely which categories and zones. For all protection methods, this rule applies to the parts to which the potentially explosive atmosphere has unhindered access – they mustn’t reach unacceptable temperatures. The temperatures must fall within the class that applies to the particular potentially explosive atmosphere.

Equipment in hazardous areas

Equipment is usually a source of ignition in oil and gas facilities. While working in an explosive atmosphere, therefore, special precautions should be taken to prevent equipment from being a source of ignition. In a situation where an explosive atmosphere has a high likelihood of occurring, efforts should be placed on using equipment with a low probability of creating a source of ignition. Where the likelihood of an explosive atmosphere occurring is reduced, equipment constructed to a less rigorous standard may be used, within reason. Equipment is usually categorised into (1, 2 or 3) depending on which zone is intended to be used. A number of ways of constructing equipment to prevent ignition risks have been published as harmonised European Standards. Zone 0 and Zone 20 use category 1 equipment, Zone 1 and Zone 21 will use category 2 equipment, and Zone 2 and Zone 22 use category 3 equipment. A standardised marking scheme must be applied to identify equipment that is suitable for a specific location. Equipment built to the requirements of EPS will be inscribed with the explosion protection symbol “Ex” in a hexagon, the equipment category number is (1, 2, or 3), the letter G and/or D, depending on whether or not it is intended for use in gas or dust atmospheres. Many will also include temperature rating marked as “T”. These indicate limitations to safe use. Employers and those installing equipment should consider the marking and provided documentation with “Ex” equipment inscribed when it is being installed. Depending on the country certifications, slight variations to the marking are shown in Table 2.

Selection of equipment

DSEAR established requirements in July 2003 that new equipment and protective systems used in hazardous areas must be selected based on the requirements identified in the DTI’s Equipment and Protective Systems for Use in Potentially Explosive Atmospheres Regulations 1996 (as amended), unless the risk assessment finds otherwise. For much electrical equipment, employers only notice comparative changes from recent situations, except in the details of the marking on equipment. The regulations also apply to mechanical equipment that is a potential ignition source; however, a harmonised European standard for category 3 mechanical equipment is now available as BS EN 13463 Part 1. Importantly, it would be expected that equipment used in zoned areas must comply with DTI regulations and that equipment of a lower or higher category than usually required would only be present in some certain circumstances; for example, where equipment of the required category is not obtainable, a lower category can be used in combination with other protective measures to comply with these regulations. Where employers intend to use the flexibility provided in DSEAR, their decision must be fully justified in their risk assessment.

PPE for explosive atmospheres

The type of protective clothing to be in use when operating in an explosive atmosphere is earmarked in the PPE directive 89/686/EEC in clause 2.6. This directive clarifies that clothing is to be of anti-static material and must be certified to EN 1149-5:2006 standard. It is of importance that the garment is built to minimise risk of unintended sparks and other factors from the work environment. The PPE must be manufactured in such a way that it cannot be a source of electric, electrostatic items or impact induced spark with potential to causing an explosive mixture to ignite. It must be of a surface resistance of less than or equal to 2.5x109Ω, on at least one surface, tested according to EN1149-1. This requirement should be emphasised to employers to achieve a safe vision.


It is the responsibility of employers to recognise the hazardous potential of dangerous substances and/or explosive atmospheres identified through risk assessment. They must identify measures to reduce to the lowest possible level reasonably practicable and ensure safety controls are implemented.

Published: 10th May 2016 in Health and Safety Middle East

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Mohammed Abdul Karim
Mohammed Abdul Karim (GradIOSH, MIIRSM, MAIChE, IRCA Certified Safety Auditor, and MCSSE) is a safety expert with 15 years of experience in safety, health and environmental management and consulting to internationally recognised multinational oil and gas companies.