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. 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.
Explosive atmospheres occur when flammable gases, mist, vapours or dust are mixed with air. This creates a risk of explosion. The amount of a substance needed to create an explosive atmosphere depends on the substance in question. The area where this possibility exists is defined as a potentially explosive atmosphere. These atmospheres can be found throughout all industries, from chemical, pharmaceutical and food, to power generation and wood processing. The areas may also be known as hazardous areas or hazardous locations. The number of substances that are flammable when mixed with air is very large. This means there are many industrial sectors that can have a potentially explosive atmosphere somewhere in their process. Some of these are not so obvious. For example, sawmills by default are not a potentially explosive atmosphere, but if the sawdust is allowed to gather in large amounts, the area in question will become hazardous.
Risk assessment of explosive atmospheres
Dangerous Substances and Explosive Atmospheres Regulations 2002(DSEAR) define ‘hazardous’ as a place where an explosive atmosphere may occur in quantities that require special precautions to protect the health and safety of workers. A place where an explosive atmosphere is not expected to occur in quantities that require such special precautions is deemed to be non-hazardous. Identifying hazardous or non-hazardous areas should be carried out in a systematic way. Risk assessment should be used to determine if hazardous areas exist and to then assign zones to those areas.
The assessment should consider such matters as:
- The hazardous properties of the dangerous substances involved
- The amount of dangerous substances involved
- The work processes like cleaning, repair or maintenance activities that will be carried out
- The temperatures and pressures at which the dangerous substances will be handled
- The containment system and controls provided to prevent liquids, gases, vapours or dusts escaping into the general atmosphere of the workplace
- Any explosive atmosphere formed within an enclosed plant or storage vessel
- Any measures provided to ensure that any explosive atmosphere does not persist for an extended time, e.g. ventilation
Along with the above factors, the following points should be considered in detail.
Hazardous nature of the substance
The properties of a dangerous substance that need to be known include the boiling point and flash point of any flammable liquid, and whether any flammable gas or vapour that may be involved is lighter or heavier than air. For dusts, information on particle size and density will be needed, once it has been shown that a particular dust can form an explosive atmosphere. Often, relevant information is contained on a safety data sheet provided with the product.
Size of potential releases
Some potential sources of release may be so small that there is no need to specify a hazardous area. This will be the case if the consequence of an ignition following a release is unlikely to cause danger to people in the vicinity. However, in the wrong circumstances ignition of quite small quantities of flammable gas/vapour mixed with air can cause danger to anyone in the immediate vicinity. Where this is the case, as in a relatively confined location, from which rapid escape would be difficult, area classification may be needed even where quite small quantities of dangerous substance are present. The size of any potential explosive atmosphere is, in part, related to the amount of dangerous substances present. Industry specific codes have been published by a variety of organisations to provide guidance on the quantities of various dangerous substances that should be stored.
Temperatures and pressures
Information relating to the processes that involve the dangerous substances should also be taken into account, including the temperatures and pressures used in the process, as this will influence the nature and extent of any release, and the extent of any subsequent hazardous areas. Some substances do not form explosive atmospheres unless they are heated, and some liquids if released under pressure will form a fine mist that can explode even if there is insufficient vapour.
Ventilation, either natural or mechanically produced (e.g. by fans), can both dilute sources of release and remove dangerous substances from an enclosed area. As a result there is a close link between the ventilation at any given location, and the classification and extent of a zone around a potential source of release. Well-designed ventilation may prevent the need for any zoned areas, or reduce it so it has a negligible extent.
Extent of hazardous areas
The assessment needs to identify areas within a workplace that are connected to places where an explosive atmosphere may occur. This will provide information on any areas away from the source of the hazard to which an explosive atmosphere may spread, for example through ducts.
When considering the potential for explosive atmospheres, it is important to consider all dangerous substances that may be present in the workplace, including waste products, residues, materials used for cleaning or maintenance, and any used only as a fuel. Also some combinations of dangerous substances may react together, forming an ignition source or in combination may form an explosive atmosphere, where singly this does not occur.
Some repeated activities such as refuelling cars, or loading and unloading tankers intended for use on the public roads, involve the introduction of potential sources of ignition into an area where a spill is possible, and which would meet the description of a hazardous area. In these circumstances, safety can be achieved by isolating power sources (e.g. turning off engines) while a transfer is taking place, and making suitable checks before and after a transfer, before moving a vehicle into or out of a hazardous area.
Activities such as maintenance may incur risks not covered by the normal area classification of the area where the activity is taking place, for instance the introduction of sources of ignition into a hazardous area. Sometimes the dangerous substance can be removed before the maintenance work activity starts. Sometimes, special control measures can be taken to prevent the release of any dangerous substance during the work. In such cases the additional risks associated with the activity should be assessed before work starts.
Hazardous zones and ATEX directives
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. 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.
Within industries, all potentially explosive atmospheres are required to have area classifications called zones. It is the employer’s responsibility to ensure that the classification of their site is performed before suitable products can be selected and installed at the location. Globally, a zone system is used to classify potentially explosive areas. The Worker Protection Directive 1999/EC and the international standards IEC 60079-10-x, EN 60079- 10-x define these zones. In all cases, zone classification is the responsibility of the owner of the site where the potentially explosive atmosphere exists.
There are 6 zones:
- Zone 0 – A place in which an explosive atmosphere consisting of a mixture with air of dangerous substances in the form of gas, vapour or mist is present continuously or for long periods or frequently.
- Zone 1 – A place in which an explosive atmosphere consisting of a mixture with air of dangerous substances in the form of gas, vapour or mist is likely to occur in normal operation occasionally
- Zone 2 – A place in which an explosive atmosphere consisting of a mixture with air of dangerous substances in the form of gas, vapour or mist is not likely to occur in normal operation but, if it does occur, will persist for a short period only
- Zone 20 – A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is present continuously, or for long periods or frequently
- Zone 21 – A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is likely to occur in normal operation occasionally
- Zone 22 – A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is not likely to occur in normal operation but, if it does occur, will persist for a short period only
Equipment categories are used in ATEX. The category indicates which safety level of product must be used in each zone. In Zone 0/20, category 1 devices must be used; in Zone 1/21, category 2 devices; and in zone 2/22, category 3 devices. Classification into categories is of particular importance, because all the inspection, maintenance and repair duties of the end user will depend on the category of the product/equipment and not on the zone where it is installed.
Explosive atmospheres and safety
Generally, a practice of strictly no smoking will be followed in all industries. In the explosive atmosphere this has to be followed as a mandatory rule. In these areas “No Smoking” signs should be clearly displayed on the entrance doors and in prominent positions within the laboratory/workshop or store. Persons who operate equipment in areas using dangerous substances should be trained in the use of that equipment, the associated dangers with dangerous substances and flammable liquids.
Individuals should be made aware of:
- The properties of dangerous substances and flammable liquids
- Use of correct containers and processes
- Not allowing hot work close to or within areas using dangerous substances
- Correct design of equipment, installation and maintenance
- Control of ignition sources and heating materials above their auto-ignition point
- The safe disposal and dismantling of equipment containing flammable liquids
- Correct storage and limited quantities kept in the area
- Correct decanting, dispensing or movement of dangerous substances, correct method of dealing with spillages
In addition, appropriate protective clothing including eye and hand protection must be worn by all persons operating/working with dangerous and flammable substances. The provision of eye wash facilities and medical first aid should be made in the immediate area. It will also be necessary to consider the provision of emergency decontamination/deluge showers. The latter items are normally considered at the design stage of the premises. Where these are not provided and new processes are being considered, these provisions should form part of the risk assessment process.
There are various types of approved storage cabinet for dangerous substances and flammable liquids. In general terms they should provide a minimum fire resistance of half an hour, offer a secure latched door, lipped shelving to prevent spillages and be clearly identified. The identification should indicate the type/class of Dangerous Substances stored i.e. “Highly Flammable Substances” or “Corrosive”. It is dangerous to mix storage i.e. oxidising agents, halogenated solvents (non-flammable) with flammable solvents. A regular assessment of the contents of the storage cabinets is needed. Inappropriate storage should be removed and re-located to a suitable store. Hazardous substances that produce a source of flammable vapour must not be stored in refrigerators, freezer cabinets or other closed containers with internal sources of ignition, i.e. electrical contacts. Spark-proofed equipment, specifically designed and built for this purpose should be used. These units must be clearly indicated “Spark-Proofed” on the outside of the door.
In areas/workplaces where flammable sources are in use, non-proofed refrigerant appliances should be clearly indicated “Explosion Hazard – Not Suitable for Use with Highly Flammable Substances”. External storerooms used to store flammable liquids should be constructed with a concrete impervious “bund” floor (152mm), brick walls (114mm), and concrete cast roof (102mm). Cross flow ventilation is required using high and low level vents fitted with internal flame gauzes. A minimum one hour fire resisting, self-closing fire door (outward opening) is also required. All electrical fittings should be flameproof to comply with Category 1 equipment. Shelving should be constructed of slatted non-ferrous metal or wooden shelves. To assist in preventing spills the shelves should be provided with lipped edges. Signs indicating “No Smoking” and “Highly Flammable” should be displayed in a prominent position on the outside of the access door. Fire extinguishers (normally dry powder) and a sand bucket should be provided.
The following general points should also be considered while storing flammable or explosive substances in external stores:
- An emergency switch to isolate the electrical supply to the building should be provided on the outside of the store clearly indicated “electrical isolation switch”.
- All substances kept within the storeroom must be accurately and clearly labelled. Where emptied chemical containers are used to store waste substances, all inappropriate labels to the present contents must be removed to avoid any confusion.
- The new regulations emphasise the need to risk assess the compatibility of the contents of the store. Explosives, oxidising agents, radioactive substances and cylinders should not be stored with general flammable substances.
- The cross flow vents should be cleared of all obstructions to ensure a clear airway. Regular checks should be made to ensure storage or plant growth around these units is cleared.
- Unnecessary combustible storage, cardboard boxes, wood shavings and packaging materials should be removed and disposed of in an approved manner.
- Vehicle parking or storage items should not be allowed within five metres of the storeroom door. The area should be clearly indicated with cross hatched yellow lines.
Disposal of flammable and dangerous substances
Dangerous/flammable substances must be disposed of safely and in a manner that minimises environmental risks. In this respect, and to demonstrate your organisation’s responsibility, waste products will generally need to be stored and handled according to the same standards as the products from which they were derived. They will also be subject to the same legislation, unless their properties have been significantly altered by the processing. This will require the provision of suitable containers that are correctly labelled appropriate to the contents. Waste product should not be put into drains or water courses. There is the potential to create a local explosive mixture within the drainage system or damage biological purification systems at the water treatment works. Waste materials collected from different processes should not be mixed before disposal unless the various components are known to be compatible. Consideration will need to be given to the eventual disposal techniques at the early stages of the risk assessment. Methods of decanting waste into the containers should be assessed and the correct ancillary equipment provided. Any system of decanting the products should be assessed to account for the dangers of static electricity and suitable provision of earth leads made. When not in use, containers of waste product should be securely closed to prevent leakage and should be returned to storage areas. Waste product must not be decanted into containers within a storage area; the container should be taken to the designated work area.
Explosive atmospheres are unavoidable in process industries like petroleum refining and explosives manufacturing, but by adopting active monitoring methods like executing risk assessments, involving in regular inspection, doing regular maintenance along with implementing and using equipment like flameproofing and dust ignition proofing in the workplace as per the legal guidelines, safe disposal of flammable waste will reduce the accidents in explosive atmospheres while working.