The effects of accidental fires or explosions can be devastating in terms of lives lost, injuries, damage to property and the environment, and to business continuity. Working with flammable liquids, dusts, gases and solids is hazardous because of the risk of fire and explosion.
About dangerous substances
Dangerous substances are any substances used or present at work that could, if not properly controlled, cause harm to people as a result of a fire and explosion. Explosive atmospheres can be caused by flammable gases, mists or vapours or by combustible dusts.
ATEX and explosive atmospheres Explosive atmospheres
in the workplace can be caused by flammable gases, mists or vapours or by combustible dusts. Explosions can cause loss of life and serious injuries as well as significant damage.
These pages will tell you more about explosive atmospheres and ATEX:
- What is an explosive atmosphere?
- Where can explosive atmospheres be found?
- What is ATEX?
- Explosive atmospheres in the workplace
- Equipment and protective systems intended for use in explosive atmospheres
- BIS information on equipment and protective systems intended for use in explosive atmopsheres
- ATEX and DSEAR Frequently asked questions
Explosive atmospheres can be caused by flammable gases, mists or vapours or by combustible dusts. If there is enough of the substance, mixed with air, then all it needs is a source of ignition to cause an explosion. Explosions can cause loss of life and serious injuries as well as significant damage. Preventing releases of dangerous substances, which can create explosive atmospheres, and preventing sources of ignition are two widely used ways of reducing the risk. Using the correct equipment can help greatly in this. The Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR) place duties on employers to eliminate or control the risks from explosive atmospheres in the workplace. A summary of those requirements can be found below.
What is an explosive atmosphere?
In DSEAR, an explosive atmosphere is defined as a mixture of dangerous substances with air, under atmospheric conditions, in the form of gases, vapours, mist or dust in which, after ignition has occurred, combustion spreads to the entire unburned mixture. Atmospheric conditions are commonly referred to as ambient temperatures and pressures. That is to say temperatures of –20°C to 40°C and pressures of 0.8 to 1.1 bar.
Where can explosive atmospheres be found?
Many workplaces may contain, or have activities that produce, explosive or potentially explosive atmospheres. Examples include places where work activities create or release flammable gases or vapours, such as vehicle paint spraying, or in workplaces handling fine organic dusts such as grain flour or wood.
D. What is ATEX?
ATEX is the name commonly given to the two European Directives for controlling explosive atmospheres:
- Directive 99/92/EC (also known as ‘ATEX 137’ or the ‘ATEX Workplace Directive’) on minimum requirements for improving the health and safety protection of workers potentially at risk from explosive atmospheres. The text of the Directive and the supporting EU produced guidelines are available on the EU-website. For more information on how the requirements of the Directive have been put into effect in Great Britain see the information in the section Explosive atmospheres in the workplace below.
- Directive 94/9/EC (also known as ‘ATEX 95’ or ‘the ATEX Equipment Directive’) on the approximation of the laws of Members States concerning equipment and protective systems intended for use in potentially explosive atmospheres. The text of the Directive and EU produced supporting guidelines are available on the EU website. For more information on how the requirements of the Directive have been put into effect in Great Britain see the section on Equipment and protective systems intended for use in explosive atmospheres.
E. Explosive atmospheres in the workplace
In Great Britain the requirements of Directive 99/92/EC were put into effect through regulations 7 and 11 of the Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR). The requirements in DSEAR apply to most workplaces where a potentially explosive atmosphere may occur. Some industry sectors and work activities are exempted because there is other legislation that fulfils the requirements. These exemptions are listed in regulation 3 of DSEAR.
What does DSEAR require?
DSEAR requires employers to eliminate or control the risks from dangerous substances. In addition to the general requirements, the Regulations place the following specific duties on employers with workplaces where explosive atmospheres may occur.
Classification of areas where explosive atmospheres may occur
Employers must classify areas where hazardous explosive atmospheres may occur into zones. The classification given to a particular zone, and its size and location, depends on the likelihood of an explosive atmosphere occurring and its persistence if it does. Schedule 2 of DSEAR contains descriptions of the various classifications of zones for gases and vapours and for dusts. Further information and guidance on the classification and zoning of areas where potentially explosive atmospheres may occur and the selection of equipment for use in those areas:
Explosive atmospheres – Classification of hazardous areas (zoning) and selection of equipment
Selection of equipment and protective systems
Areas classified into zones must be protected from sources of ignition. Equipment and protective systems intended to be used in zoned areas should be selected to meet the requirements of the Equipment and Protective Systems Intended for Use in Potentially Explosive Atmospheres Regulations 1996. Equipment already in use before July 2003 can continue to be used indefinitely provided a risk assessment shows it is safe to do so.
Identifying areas where explosive atmospheres may occur
Where necessary, the entry points to areas classified into zones must be marked with a specified ‘EX’ sign.
Providing anti-static clothing
Employers must provide workers who work in zoned areas with appropriate clothing that does not create the risk of an electrostatic discharge igniting the explosive atmosphere, eg anti-static footwear. The clothing provided depends on the level of risk identified in the risk assessment.
Confirming (verifying) overall explosion safety
Before a workplace containing zoned areas comes into operation for the first time, the employer must ensure that the overall explosion safety measures are confirmed (verified) as being safe. This must be done by a person or organisation competent to consider the particular risks in the workplace, and the adequacy of the explosion control and other measures put in place.
Equipment and protective systems intended for use in explosive atmospheres
The aim of Directive 94/9/EC is to allow the free trade of ‘ATEX’ equipment and protective systems within the EU by removing the need for separate testing and documentation for each Member State.
In Great Britain, the requirements of the Directive were put into effect through BIS Equipment and Protective Systems Intended for Use in Potentially Explosive Atmospheres Regulations 1996 (SI 1996/192).
The Regulations apply to all equipment intended for use in explosive atmospheres, whether electrical or mechanical, and also to protective systems.
Manufacturers/suppliers (or importers, if the manufacturers are outside the EU) must ensure that their products meet essential health and safety requirements and undergo appropriate conformity procedures. This usually involves testing and certification by a ‘third-party’ certification body (known as a Notified Body) but manufacturers/suppliers can ‘self-certify’ equipment intended to be used in less hazardous explosive atmospheres. Once certified, the equipment is marked by the ‘EX’ symbol to identify it as such. Certification ensures that the equipment or protective system is fit for its intended purpose and that adequate information is supplied with it to ensure that it can be used safely.
Additional requirements where explosive atmospheres can occur
The correct implementation of control measures aims to prevent the formation of potentially explosive atmospheres, or limit their extent. However, due to the way dangerous substances are stored, handled and used, you cannot fully avoid the risk of potentially explosive atmospheres occurring.
The areas where hazardous explosive atmospheres may occur should be identified and classified into zones, based on their likelihood and persistence. This is known as Hazardous Area Classification.
The zone classifications are:
Zone 0 (Zone 20)
That part of a hazardous area in which an explosive atmosphere is continuously present, or present for long periods, or frequently.
Zone 1 (Zone 21)
That part of a hazardous area in which an explosive atmosphere is likely to occur occasionally in normal operation.
Zone 2 (Zone 22)
That part of a hazardous area in which an explosive atmosphere is not likely to occur in normal operation but, if it does occur, will persist for a short period only.
Zones 0, 1 and 2 are used for explosive atmospheres formed of flammable gases, vapours or mists.
Zones 20, 21 and 22 are used for explosive atmospheres formed of combustible dusts.
For the hazardous areas identified, you should ensure that:
■ all potential ignition sources, including sparks, hot surfaces, smoking materials, naked flames, unsuitable equipment etc are excluded;
■ only equipment and protective systems, including portable equipment that meets the requirements of the Equipment and Protective Systems Intended for Use in Potentially Explosive Atmospheres Regulations 1996,7 should be used and installed. Such equipment will be CE marked and carry the Ɛx symbol in a hexagon – see below. Equipment and protective systems in use before July 2003 can continue to be used provided the risk assessment shows it is safe to do so;
■ before bringing them into operation for the first time, as part of the commissioning procedure, a person competent to do so should verify that the equipment and protective systems provided are suitable and sufficient to make sure the fire and explosion risks are properly controlled;
■ people who provide, maintain or verify electrical installations and equipment in, or associated with, the hazardous areas are competent to undertake the task (eg CompEx trained);
■ where necessary, a warning sign is posted at the entry points of places that have been classified as hazardous areas to warn those entering those areas that special precautions are required;
Example of warning sign with supplementary text
■ employees working in these areas are provided with appropriate clothing that does not create a risk of an electrostatic discharge capable of igniting the explosive atmosphere.
The main hazards from the use of flammable liquids are fire and explosion. For a fire or explosion to occur it requires three things to come together:
■ flammable liquid;
■ source of ignition; and
Always present in the air Additional sources from oxidising substances
Hot surfaces Electrical equipment Static electricity Smoking/naked flames
Common causes of incidents
Fires or explosions can occur when vapours or liquids are released from a controlled environment to areas where there may be an ignition source, or, alternatively, when an ignition source is introduced into a controlled environment. Common causes of such incidents include:
■ lack of training and awareness of the properties of flammable liquids;
■ inadequate design of equipment;
■ failure or malfunction of equipment;
■ inadequate installation or maintenance;
■ overheating flammable liquids, for example above the flashpoint, boiling point, auto-ignition temperature, or decomposition temperature;
■ procedural errors or omissions;
■ inadequate control of potential ignition sources, including electrostatic discharge;
■ dismantling or disposal of plant and equipment that contains or has contained flammable liquid;
■ hot work on or close to flammable liquid containers;
■ exposure to heat from a nearby fire;
■ incorrect use of solvents for cleaning operations;
■ misuse of flammable liquids; for example, to burn waste material or ’brighten’ fires.
Incidents involving flammable liquids commonly arise during transfer operations, including:
■ movement from storage and within premises;
■ decanting and dispensing;
■ process activities;
■ emptying plant and equipment, including vehicle fuel tanks prior to maintenance;
■ dealing with spillages.
Combustion of liquids occurs when flammable vapours released from the surface of the liquid ignite. The flashpoint is the lowest temperature at which a flammable liquid gives off vapours in sufficient concentration to form an ignitable mixture with air near the surface of the liquid. Generally, a flammable liquid with a flashpoint below ambient temperature will always give off sufficient vapour to form an ignitable mixture. Ignitable mixtures are less likely to form from liquids with a flashpoint greater than the ambient temperature unless they are heated, mixed with low flashpoint materials or released under pressure as a mist or spray. However, flammable liquids below their flashpoint can also be readily ignited if present as a thin film over a surface, even one that is non-combustible, or when spilt onto absorbent material such as clothing or packaging material.
The amount of flammable vapour given off from a liquid, and therefore the extent of the ignitable flammable vapour mixture with air, depends on a variety of factors, including the temperature of the liquid, its volatility, the surface area of the liquid exposed, how long it is exposed for, and air movement in the locality.
The extent of the ignitable mixture is referred to as the explosive atmosphere, ie the mixture of flammable vapour with the air, which has the potential to catch fire or explode. An explosive atmosphere does not always result in an explosion but, if it does catch fire, the flames will travel quickly through it. If this happens in a confined space (eg within plant or equipment, or a compartment, room or building), the rapid propagation of the flames through the explosive atmosphere may result in an increase in pressure and subsequent damaging explosion.
The explosion limits define the concentrations (normally by volume) of vapour/air mixtures that will propagate a flame. The lower explosion limit (LEL) is the minimum concentration of vapour in air below which propagation of a flame will not occur in the presence of an ignition source. This may also be referred to as the lower flammable limit or lower explosive limit. The upper explosion limit (UEL) is the maximum concentration of vapour in air above which the propagation of a flame will not occur. This may also be referred to as the upper flammable limit or the upper explosion limit.
The explosion limits vary for different flammable liquids, with typically many in the range of circa 1% (LEL) to perhaps between 10 and 20% (UEL). While flames will not propagate through flammable vapour mixtures with air above their UEL, they are still readily ignitable at their interface with air; in which event, a fire will result.
Some mixtures (preparations) that contain a flammable liquid (for example, water/solvent mixtures or emulsions) may only release flammable vapours slowly. However, the vapours may still achieve sufficient concentration to form an ignitable mixture with air and for the mixture to be assigned a flashpoint, even though the rate of vapour release may be insufficient to sustain combustion.
Other physical properties, such as viscosity, auto-ignition temperature (AIT) and conductivity, also indicate the potential extent of an explosive atmosphere in relation to possible ignition sources.
The viscosity of the substance/mixture (preparation) determines how far any spillages will spread and therefore the size of any exposed surface of flammable liquid. Solvents generally have a low viscosity and when spilt, spread quickly allowing a rapid build-up of vapours from the surface of the liquid. Some liquid formulations, such as paints and resins, may have a high viscosity; if they are spilt they spread and produce vapours more slowly than would the individual solvent constituents.
The ignition energy required to ignite flammable liquid vapours is relatively small. The typical minimum ignition energy (MIE) is in the range of 0.1 to 10 mJ, which can be readily produced in much plant and equipment in the form of heat, electrical, mechanical or chemical energy.
In addition to the potential ignition sources presented by external plant and equipment, you should consider the properties of the flammable liquid itself to ensure it is not handled in a manner that might result in an incident. For example, to ensure it is not heated above its auto-ignition temperature; or where it has low (electrical) conductivity (ie less than 50 picoSiemens (pS)) it is handled in a manner to guard against incendive electrostatic discharge that is potentially capable of causing ignition of the flammable vapours.
The physical environment in which flammable liquids are handled can also determine how the hazards may develop. Whether the released vapour is able to build up to form an explosive atmosphere will depend on whether it can freely disperse and if indoors the degree of ventilation of the enclosure, taking account of any obstructions and plant congestion that might interfere with this.
You should also consider the proximity of pits, gulleys and drains to where flammable liquids are stored and handled to ensure the ingress and accumulation of any releases of flammable liquids and/or vapours is prevented. Flammable liquid vapours are heavier than air and will, unless dispersed, tend to accumulate at lower areas. If ignition occurs in such locations, the flame is likely to travel or ‘flash’ back to the source of the vapours, ie the flammable liquid.