Potentially explosive atmospheres

When assessing the risk of explosion, the employer must assess all work and production processes. Each process and equipment operating alternative must be assessed separately. In the first phase of assessing the explosion risk, the employer should identify all flammable substances that are a normal part of the production process. In normal operations, machinery and equipment is used for their intended purpose and within their capacity.

The following should also be taken into account when assessing the risk of explosion:

• normal operating conditions, such as maintenance, sampling, leakages of seals, connections and valves, breaking of packages;
• commissioning and decommissioning, including start-up, shutdown and stoppage;
• malfunctions and foreseeable fault conditions, such as faulty air conditioning;
• misuse which may reasonably be foreseen, such as overfill of a tank.

Next the employer must assess where and for how long the potentially explosive atmosphere is present. Factors to note in the assessment include ventilation conditions, spread of gases, liquids and dusts and spaces that are connected to spaces with potentially explosive atmospheres. The source of emission may be

• continuous, meaning that a potentially explosive atmosphere is present continuously or for long periods or frequently;
• primary, meaning that a potentially explosive atmosphere is likely to occur in normal operation occasionally;
• secondary, meaning that an explosive atmosphere is not likely to occur in normal operation but, if it does occur, will persist for a short period only.

The employer must categorise hazardous places into zones. The employer must carry out the zoning in accordance with standards and examples published in guides.

Potentially explosive atmospheres are categorised into zones 0, 1, 2, 20, 21 and 22 according to the explosive substance and duration of the potentially explosive atmosphere. Safety requirement for equipment present, installed or temporarily used in the space are determined based on the zone.

Zone	Definition
0	A place in which an explosive atmosphere consisting of a mixture with air of flammable substances in the form of gas, vapour or mist is present continuously or for long periods or frequently.
1	A place in which an explosive atmosphere consisting of a mixture with air of flammable substances in the form of gas, vapour or mist is likely to occur in normal operation occasionally
2	A place in which an explosive atmosphere consisting of a mixture with air of flammable 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.
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.
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.
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.

Examples of zones

Zone 0, 20

• inside containers, pipes or equipment (e.g. tanks, silos, cyclones)

Zone 1, 21

• Vicinity of zones 0 or 20, vicinity of feed openings, area around fragile vessels or pipes (made of glass), area around inadequately sealed glands of pumps and valves, sampling areas, filling and emptying points.

Zone 2, 22

• Vicinity of zones 0, 1, 20 or 21, coupled bolted joints, filter emptying points, places where dust deposits may occur.

No zone

• area around pipes, storage of unopened metal containers.

The employer must mark places with a potentially explosive atmosphere with and EX marking. If the entire space is not part of a zone, the limits of the zone should be marked with a black and yellow line, for example. Potentially explosive atmospheres must also include other necessary safety-related markings, such as signs forbidding open flame, smoking or use of a mobile phone.

Measures for explosion protection implemented by the employer include all such arrangements and measures that are used to

• prevent the formation of hazardous explosive atmospheres,
• avoid the ignition of hazardous explosive atmospheres or
• mitigate harmful effects of explosions.

Measures for explosion protection may be technical or organisational. Technical measures for explosion protection are related to factors such as the process, production conditions, used substances, equipment and controlling and regulating systems. Organisational measures for explosion protection are part of the site’s general safety management. They include measures such as training and orientation of employees, identification and assessment of dangerous situations, work and operating instructions for performing different duties, work permit practices and maintenance and monitoring of equipment. Organisational and technical measure complement each other in the prevention and mitigation of explosions.

Preventing the formation of explosive atmospheres

The formation of explosive atmospheres can be prevented by ensuring that the concentrations of flammable substances remain below the explosive limits typical for each substance. Examples include the tightness and regular inspection of equipment, maintenance, adequate ventilation, cleaning, effective local removal, inerting and use of gas alarms.

Preventing the ignition of explosive atmospheres

If the formation of a potentially explosive atmosphere cannot be prevented, it must be prevented from igniting. The employer can influence this though means, such as

• equipment selection: both mechanical and electrical equipment;
• ensuring proper earthing and condition of electric wires and casing;
• choice of materials: tools, flooring, shoes, clothing;
• isolation: hot surfaces;
• cooling: exothermic reactions;
• siting of operations;
• instructions (e.g. prohibitions on open flames and mobile phone use).

The most common ignition sources include flames, hot surfaces, hot work, other mechanically generated sparks, static electricity, smoking, electrical apparatus and self-ignition.

Mitigation of the effects of explosions

If the risk of explosion cannot be completely eliminated, the employer must prepare by mitigating the effects of a possible explosion. Measures that the employer can implement to mitigate the effects of an explosion include

• explosion relief: lightened walls, explosion hatches;
• explosion suppression: rapid injection of extinguishing agents into equipment;
• explosion-resistant design: (explosion pressure resistant control rooms and supporting structures);
• prevention of explosion propagation: rapid-action mechanical isolation, flame extinction in narrow gaps, safety gaps.

An explosion often results in a fire and the employer must ensure that the exits lead to a safe direction and the preparedness to put out fires and first aid skills of personnel are sufficient.

Select equipment used in explosive atmospheres

The employer is responsible for selecting suitable equipment for potentially explosive atmospheres. Equipment may also be selected by installation planners and builders based on source information, such as zoning of hazardous places and characteristics of explosive substances.

The employer must select equipment and protection systems in accordance with the categories defined in ATEX equipment regulations, unless otherwise specified in the explosion protection document. ATEX equipment categories are based on the explosive substance and their likelihood of occurrence.

In addition to the equipment category, other factors, such as the temperature class, method of protection and explosion group, must also be taken into account when selecting equipment.

Further information on ATEX equipment is available here.

Zone	Equipment category (G = gas, D = dust)
0	1G
1	1G or 2G
2	1G, 2G or 3G
20	1D
21	1D or 2D
22	1D, 2D or 3D

 

The employer must draw up an explosion protection document for workplaces where an explosion could potentially occur. The explosion protection document must be drawn up before the premises are taken into use. The explosion protection document is intended to provide an overview of the results of the risk assessment and the protective measures. The company must update the explosion protection document if work spaces, equipment or arrangements go through significant changes.

The explosion protection document should include:

• name of the persons in charge of the spaces where explosive atmospheres might occur and the number of employees working in the space;
• potentially explosive substances used;
• conditions in which there is a risk of explosion;
• results of risk assessment and assessment methods;
• floor plan that indicates all exits;
• zoning of hazardous places (zoning images);
• list of equipment (electrical and mechanical equipment);
• verification of explosion safety by a competent person;
• persons responsible for the implementation of explosion protection measures and drawing up and updating the document;
• description of technical and organisational explosion protection measures.

Supervision

The supervision of explosive atmospheres has been divided between several supervisory authorities.

• Occupational health and safety authorities supervise the safety of employees who work in conditions with hazards caused by potentially explosive atmospheres, pursuant to the Occupational Safety and Health Act.
• Tukes supervises plants with large-scale chemical storage and handling operations that require a permit based on the Act on the Safe Handling and Storage of Dangerous Chemicals and Explosives.
• Rescue authorities supervise small-scale chemical handling and storage.