Important note: We have been allowed to publish this excerpt from the book ”Opbevaring af laboratoriekemikalier” (Storage of Laboratory Chemicals), which details the considerations that should be made when storing potentially explosive liquids/materials. The handbook is written (in Danish) by experienced experts in the field; Jørgen Stage Johansen, Hanne Troen and Lene Hjerrild and published by Praxis- Nyt teknisk forlag.

Prevention principles

The rules on zone classification are designed to prevent explosions by, in order of priority,

• Avoiding explosive atmospheres.
• Limiting the spread of explosive atmospheres.
• Avoiding explosion by protecting against sources of ignition.
• Limiting the harmful effects of an explosion.

ATEX WPA

All companies that work with flammable materials must take ATEX into account. The likelihood of an explosive atmosphere occurring must be mapped out. Companies must consider whether the presence of ignition sources and the extent of the expected consequences need to be described.

Zone classification

If it is not possible to avoid the formation of an explosive atmosphere, the areas must be classified in zones according to danger, and all sources of ignition in the area must be removed or explosion-proofed in line with the relevant zone. If the warehouse contains areas that are to be zone classified, signs must be posted with “risk of explosion” and “smoking and use of open flames prohibited”. This can be done, for example, by displaying signs on the outside of the cabinet and on the door to the room. The following is a proposal of how explosive atmospheres in chemical storage can be avoided or limited, and how to ensure the rules in ATEX are followed.

In practice

If the flash point is above 30°C (10 °C above working temperature), there is usually no risk of an “explosive atmosphere”. This is also the case if the explosive atmosphere is less than 10 litres (mixture of air and steam/gas/mist/dust). Measures to avoid an explosive atmosphere include replacing flammable liquids with non-flammable liquids or with liquids that have a higher flash point. For example, petrol (flash point approx. -20°C) can be replaced with kerosene (flash point above 30°C). The spread of explosive atmospheres can be limited by using smaller quantities or by changing work processes so that the time during which the liquid can evaporate is reduced. Finally, it is possible to remove the oxygen from the air in tanks by, for example, introducing inert gas, e.g. nitrogen. Protection against ignition sources can be achieved by removing them and all devices that could generate sparks, or by spark-proofing equipment in the relevant area. Sparks can be generated by electrical equipment, mechanical equipment, static electricity, electromagnetic waves and lightning. Limiting the harmful effects of an explosion can, for example, be achieved by installing extra screens to protect the person working in fume cupboards where there is a risk of explosion.

Storage of flammable liquids and gases - Avoid explosive atmospheres

When storing flammable liquids, pressurised cylinders or aerosol cans, ensure that explosive atmospheres do not form. This is done by ensuring that the packaging is unbroken or sealed just as effectively as unbroken packaging. For pressurised cylinders, for example, valves must be closed. Remember that pressurised cylinders should be stored separately. The chemical cabinet or storage room must also be ventilated to ensure air exchange at least once an hour. Warehousing that meets the above must not be classified as an explosion-hazard area.

Limit the explosive atmospheres

If packaging containing flammable liquids cannot be completely sealed, it must be stored in cabinets with a ventilation system that ensures that the concentration of flammable vapours does not exceed 25% of the lower explosive limit. If, at the same time, ignition is prevented in the event of ventilation failure, the warehouse does not need to be classified as an explosion-hazard area.

This can be done, for example, by:

• an automatically connected emergency generator ensuring continued ventilation in the event of power failure, or
• not switching on sources in cabinets/warehouses, e.g. lighting and electrical installations, or
• automatic cut-out of electricity if vapours exceed 25% of the lower explosive limit – but not EX-proof ventilation.

If these measures or equivalent measures do not exist, the warehouse must be classified as zone 2.

Avoid spark generation

In cabinets that cannot be ventilated internally, e.g. refrigerators and freezers, flammable liquids must be stored in sealed packaging. Electrical installations must comply with the protection principles corresponding to the zone in which the cabinet is placed. If explosion-hazardous gases and vapours only occur in the event of faults or accidents, the electrical installations in the cabinets must meet the requirements for zone 2. If this is a frequent occurrence, they must meet the requirements for zone 1 or 0, but efforts should be made to achieve conditions with the lowest possible classification. When the refrigerator is opened, the zone is both the storage room and the area around the cabinet. Therefore, both the electrical and mechanical construction must comply with the protection principles for the relevant zone. However, there should always be a special suction on refrigerators so that harmful vapours are not inhaled when opening (see section Refrigerators and freezers). The same precautions must be taken for freezers if highly evaporating liquids are stored there. Refrigerators and freezers in which flammable liquids with a flash point below 10 °C above storage temperature are stored must be classified as Zone 2 as a minimum.

Example of zone classification in refrigerators and freezers

* Mixtures with water will have a higher flash point, ** If the power supply or thermostat fails, please be aware that the temperature may rise. It must then be possible to move the contents to an explosion-proof area. This should be taken into account in the laboratory instruction.

Decanting flammable liquids

In chemical stores, flammable liquids are often stored in large packages of e.g. 25 litres. When the chemicals are to be used, the desired quantities are decanted into smaller packaging. In these cases, a ventilated drain facility must be established, e.g. in a fume cupboard. Do not decant in rooms with large flammable storage (>800 OE).

Avoid explosive atmospheres

Explosive atmospheres can be avoided by purchasing the flammable liquids in the packaging sizes that are going to be used in the laboratory or by replacing the flammable liquids with non-flammable liquids or with liquids with a flash point of above 30 °C.

Limit the explosive atmospheres

When decanting liquids, the explosive atmosphere can be limited if the decanting is done with extraction in a fume cupboard. If small quantities of less than 1 litre (duration less than 0.5 min per pour) are decanted into an effective fume cupboard, this area does not need to be classified as an explosion hazard area.

Avoid spark generation

When transferring volumes of up to 5 litres into an effective fume cupboard, the fume cupboard must not be classified as an explosion hazard area if it is secured against spillage from the fume cupboard (e.g. by a backsplash) and if it is secured in the event of ventilation failure. Fuses in the event of ventilation failure (usually power failure) may consist of ventilation being connected to an automatic UPS or the process may be stopped or omitted. Alternatively, electrical equipment inside the fume cupboard must be secured, and if there is no backsplash on the fume cupboard, electrical equipment and installations in the area in front of the fume cupboard (0.5 m) must also be spark-proofed. The fume cupboard and the area in front of it must be classified as zone 2. The transfer of flammable liquids from large packaging, e.g. alcohol from drums to 1-litre bottles, takes place in many areas. Such decanting can create significant quantities of explosive atmospheres if ventilation is weak. Decanting can normally not take place in a standard fume cupboard because there is no space for a drum. When draining to smaller containers indoors but outside the fume cupboard, ventilation must ensure that the spread of the explosive atmospheres is kept close to the drain point. According to an example from the Danish Emergency Management Agency, the area horizontally around the drum 1 m from the opening and vertically from the floor up to 1 m above the opening of the tap must be classified as zone 1. An area of 2 m outside zone 1 must be classified as zone 2.

Accidents

When assessing the risk of explosive atmospheres, possible accidents must be taken into account. Accidents must, as far as possible, be prevented by proper organisation and maintenance of the warehouse and by having safe means of transport available. Trained personnel and laboratory instructions are also essential to avoid accidents. If the consequences of a possible accident can result in significant personal injury, this must be prevented, even if this is rare. It may therefore be necessary to store flammable liquids so that they are spark-proof (zone 2), even though accidents such as loss of a broken container may occur very rarely. The local emergency services should also have the prerequisites to handle any accident situations in the form of accident instructions, collection media, extinguishing equipment and courses in elementary firefighting and first aid.