An explosion is a very rapid expansion and release of energy. Often, explosions are the result of rapid combustion, which is a particular concern in explosive atmospheres. Combustion is a high-temperature chemical reaction involving the oxidation of a fuel. It requires three things: fuel, oxidant and heat. An explosive atmosphere contains both fuel and oxidant. Preventing explosions from occurring therefore means ensuring that a heat source sufficient for ignition is not added to this mix. In most explosive atmospheres, a fuel is added to air, which provides the source of oxygen. However, it is also possible for other chemicals to act as an oxidant.
Explosive environments in industry include offshore platforms, petrochemical plants, paint spraying, refuelling, and the generation of fine organic dusts such as wood or grain flour. Mines (such as pictured above) are another area where explosive atmospheres are often encountered. This is especially hazardous in coal mines, where gases such as methane, traditionally referred to as ‘firedamp’, are naturally released from exposed coal seams.
A broad range of standards, legislation and certifications cover working practices and equipment operating within explosive atmospheres. International Electrotechnical Commission (IEC) 60079 is a very large series of international standards related to explosive atmosphere safety, running up to IEC 60079-35. IEC 60079-0 covers general requirements for equipment operating in explosive environments, including test procedures, for example.
ATEX is the name commonly used to refer to two European Directives related to the control of explosive atmospheres: Directive 99/92/EC, also known as ‘ATEX 137’ or the ‘ATEX Workplace Directive’; and Directive 94/9/EC, also known as ‘ATEX 95’ or ‘the ATEX Equipment Directive’. Equipment sold within the EU, and for the time being also within the UK, are certified to ATEX.
The Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR) is UK legislation placing duties on employers to eliminate or control the risks from explosive atmospheres in the workplace. It incorporates key elements of the European ATEX Workplace Directive and the international IEC 60079 standard. The UK implementation of the ATEX Equipment Directive is Equipment and Protective Systems Intended for Use in Potentially Explosive Atmospheres Regulations 1996 (EPS). UKCA Ex is the new post-Brexit certification that products sold in England, Scotland and Wales will require to replace ATEX certification. This will be mandatory from 1 January 2023. Other standards, such as HAZLOC, CCC Ex and EAC EX, operate outside the UK.
For an equipment manufacturer that intends to export globally, the number of different certifications required for different territories can present a significant non-tariff trade barrier. The actual requirements of the standards are often very similar. This means that it is typically relatively easy to design equipment that meets the requirements of multiple certifications. However, equipment must be stamped with the correct standards markings, and this apparently simple step can often cause significant issues. For example, injection-moulded enclosures may require modifications to mould tooling to imprint products with new standards marks.
The recent IEC System for Certification to Standards Relating to Equipment for Use in Explosive Atmospheres (IECEx System) is an attempt to create a single certification to international standards. Currently this remains largely aspirational.
HOW TO APPLY THE RULES
The first step in controlling the risk of explosion in a workplace is to categorise hazardous areas which have the potential for explosive atmospheres and therefore require special precautions (hazardous places), as well as identifying where special precautions are not required (non-hazardous places). Hazardous places should be further classified into zones according to the frequency and persistence of the explosive atmosphere, with specific criteria defined in the standards.
LOST IN TRANSLATION
ATEX uses zones based on the IEC standard. The US National Electrical Code (NEC) and the Canadian Electrical Code (CEC) classify areas in the same way, but use a different naming convention.
A systematic risk assessment must be carried out (see also guidance: www.is.gd/imefen). Any dangerous substance should be documented. For flammable liquids, this should include the boiling point and flash point, and whether any flammable gas or vapour that may be evolved is lighter or heavier than air. For dusts, it should include particle size and density.
The potential size of any release should be considered; it will only present a hazard if it could result in a dangerous explosion. Temperatures and pressures of processes must also be considered in terms of how this will affect the combustibility of dangerous substances. Ventilation should also be considered, if this will effectively remove fuel sources from the atmosphere, preventing them accumulating to the point where an explosive atmosphere could be formed. In that case, an area may not be considered hazardous.
In essence, the special precautions required in hazardous places are all about preventing ignition sources. This is achieved through the construction, installation and use of equipment. Equipment may be classified as 1, 2 or 3 depending on the hazard level of the zone where it is to be used. Potential ignition sources may be entirely removed by preventing electrical arcs and other sources of heat, or they may be separated from the explosive atmosphere with encapsulation or sealed enclosures.
Equipment is divided into two groups: group I for the mining industry and group II for all other uses. Each of these had different categories depending on the zone in which it is to be used. Group I M1 remains functional in presence of an explosive atmosphere; group I M2 de-energises in the event of an explosive atmosphere.
Group II (non-mining equipment) is more complicated:
The requirements for explosion- proof equipment for use in explosive atmospheres are complex. It is critical that the right level of protection is provided. Expert advice should therefore always be obtained before specifying equipment for use in such a potentially hazardous environment.Equipment category 1: For use in zone 0 (continually present gas) or zone 20 (continually present dust). This equipment must give very high levels of protection, providing protection even in the event of rare faults, including two independent faultsEquipment category 2: For use in zone 1 (likely gas) or zone 21 (likely dust), with a high level or protectionEquipment category 3: For use in zone 2, where explosive atmospheres are less likely and a normal level of protection is sufficient.