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From grain flour to vehicle paint spraying, when things go wrong in explosive atmospheres they have a tendency to go spectacularly wrong, extremely quickly.
When it comes to working in environments with the potential for explosions, the stakes have always been high. While following safety procedures is of course important in any industry, just one check missed in an explosive atmosphere can lead to anything from serious injury to death, and catastrophic failure – as the atmosphere’s name would suggest – from explosions. Fortunately, using the right equipment can make a vast difference.
Explosive atmospheres are defined in the Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR) 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. The aforementioned ‘atmospheric conditions’ are more generally referred to as ambient temperatures and pressures. These will be found in the ranges of –20°C to 40°C and 0.8 – 1.1 bar pressure, respectively.
An explosion is any uncontrolled combustion wave. Many of us will already be familiar with the Fire Triangle. Just in case you’re not, it’s the name for the three precursor elements that facilitate fires. In order to create an explosion the environment will need to have:
- Fuel – an explosive gas such as hydrogen
- An oxidiser – such as the oxygen in air
- A source of ignition energy – such as a hot surface or an electrical spark, even just from flicking on a light switch
In addition to this, two further facets are required: something to mix the fuel with the oxidiser (such as the turbulence created in a gas leak under pressure); and containment. It is, however, common industrial practice to use the term ‘explosion’ for both confined and unconfined combustion.
Activities that produce explosive or potentially explosive atmospheres take place in many workplaces; for example, places handling fine organic dusts such as grain flour or wood, and in environments that solvents, varnishes, flammable gases (such as liquid petroleum gas) and paints are used; think paint spraying, for example.
ATEX explained
The term ATEX gets thrown around a lot when there’s a risk of explosive atmospheres, but what does it mean? Well, ATEX is the name given to the two European Directives for controlling explosive atmospheres: Directive 99/92/ EC and Directive 94/9/EC.
Directive 99/92/EC
Also known as ‘ATEX 137’ or the ‘ATEX Workplace Directive’ this looks at the 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.
Directive 94/9/EC
Also known as ‘ATEX 95’ or ‘the ATEX Equipment Directive’, Directive 94/9/EC is focused on the approximation of the laws of Members States concerning equipment and protective systems intended for use in potentially explosive atmospheres.
Critical concentrations
It’s a position you’ll hope to never find yourself in, but when you reach the most easily ignited concentration (MEIC), the amount of energy required to cause ignition is minimal.
If the ignition experiment is conducted under conditions that allow one to assume that all the energy injected into the gas/vapour cloud is utilised in the combustion process, the critical energy at the MEIC is called minimum ignition energy (MIE). As the concentration is varied from the most easily ignited concentration the amount of energy required to cause ignition increases, approaching asymptotical at lower concentration, the lower explosive limit, LEL (often called lower flammable limit), and at higher concentration, the upper explosive limit, UEL (often called the upper flammable limit). It’s worth noting, these are not inherent properties of an explosive atmosphere; their values depend on the nature of the experiment by which they are determined – especially the size of the vessel and the energy available from the ignition source. No prudent person controlling concentration to reduce the risk of an explosion would operate much higher than 50% of the LEL except under very carefully controlled conditions, and in most situations the limit is set at 25% or lower.
A hazardous area is defined as an area in which explosive atmospheres, or may be expected to be, present in quantities such as to require special precaution for the construction and use of electrical equipment.
“when you reach the most easily ignited concentration, the amount of energy required to cause ignition is minimal”
As mentioned earlier, an explosive atmosphere consists of a mixture of flammable substances with air in the form of gas, vapour, and mist in such proportions that it can be exploded by excessive temperatures, arcs or sparks; however, the gases, vapours or mists in question will only explode when mixed with air between specific percentage mixtures in the range from LEL to UEL.
In process industries, many explosions take place due to a lack of proactive safety implementation. To avoid this, the following strategies can be adopted:
- Hazardous area classification and explosive zones identification
- Execution of risk assessment
- Special audits like electrical safety audit can be implemented
- Identification of appropriate control measure
Strategy 1: Hazardous area classification and explosive zones identification
Industrial processes involving flammable or combustible materials may produce explosive atmospheres. The concept of assessing and limiting the risk associated with the installation of electrical devices in areas where potentially explosive atmospheres may be present is referred to as “Area Classification”. Hazardous area classification assessment is a probability analysis and risk assessment evaluation of a manufacturing or process area processing a potentially flammable atmosphere that focuses exclusively on the minimisation or elimination of electrical energy as a potential source of ignition. To fully understand what is meant by the definition above, it is also important to understand what area classification is not.
Hazardous area classification is not intended to be a secondary line of defence against poor:
- Process design
- Elastomer compatibility causing excessive emissions from valve packing, pump seals, and/or pipe flanges
- Facility and equipment maintenance
Hazardous areas are divided into three distinct classes that are totally dependent on the type of material encountered in the process. Class I areas are locations in which flammable gases and/or vapours are, or may be, present in the air in quantities sufficient to produce an explosion or ignitable mixture. Class II areas are locations that are hazardous because of the presence of combustible dust. Combustible dust is defined as any solid material of 420 microns or less in diameter that presents a fire or explosion hazard when dispersed in air. And finally, Class III areas are locations that are hazardous because of the presence of easily ignitable fibres and filings.
Zone classification is defined by taking into account the different dangers presented by the following potentially explosive atmospheres:
- Zone 0 – ignitable concentrations of flammable gases or vapours are present continuously, for long periods of time
- Zone 1 – ignitable concentrations of flammable gases or vapours are likely to exist under normal operating conditions and may exist frequently because of repair, maintenance operations, or leakage
- Zone 2 – ignitable concentrations of flammable gases or vapours are not likely to occur in normal operation. It occurs for only a short period of time and becomes hazardous only in case of an accident or some unusual operating condition
By doing this assessment, the type of explosive substance (Class) and its likelihood (Zone) can determined.
Strategy 2: Execution of risk assessment
A risk assessment methodology must be developed prior to beginning the actual area classification assessment itself. This methodology sets the ground rules by which the assessment is conducted. The deliverables presented at the completion of the assessment methodology are as follows. Key members of the assessment team are identified along with their respective roles and responsibilities required to support the assessment process. All potential point source of emissions will be identified. Point sources are process equipment that continuously or intermittently releases flammable vapours into the atmosphere during routine modes of operation.
Out of Sight, Out of Mind?
Mitigating the heightened stresses faced by lone workers
You may already work in a high-risk industry, but if you’re a lone worker those risks shouldn’t automatically increase just because of your solo status.
Lone workers are defined by the UK’s HSE as “those who work by themselves without close or direct supervision.” They’re divided into two camps: those working alone in fixed establishments, such as in factories and warehouses, potentially outside of usual hours carrying out maintenance works; and those working away from their fixed base, such as engineers, and people involved in construction and plant installation.
Working alone has its highs and lows. On the one hand, you’ve no human error around you – except your own – to create hazards. On the flipside, however, when things do go wrong help may be much further away when it’s needed.
It’s vital that employers take steps to avoid or control risks. As well as the familiar hierarchy of control for workplace safety, this includes being aware that some tasks may be too difficult or dangerous to be carried out by an unaccompanied worker.
The HSE recommends that, among many other variables, employers should consider:
- Is there machinery involved that one person cannot operate alone safely?
- If the lone worker’s first language is not that of their residing country, as in the case of many migrant workers, are suitable arrangements in place to ensure clear communications, especially in an emergency?
- May the worker be more vulnerable than others, (e.g. if they are a trainee, disabled, young or pregnant)?
Strategy 3: Special audits like electrical safety audit can be implemented
In factories, around 8% of all fatalities are due to accidents caused by electricity. Data compiled by international organizations like Fire Protection Association (FPA), UK and the National Fire Protection Association (NFPA), USA indicate that nearly one fourth of all fires are caused by electrical appliances or installations. To overcome this, detailed Electrical Safety Audits (ESA) can be conducted. The plant must be visited to identify electrical hazards as per the scope of the audit. Clarification/ discussion is carried out with the plant officials during the field visit. A senior electrical engineer and preferably, safety officer should also be part of the external electrical safety audit team. All the relevant maintenance documentation, test records, electrical records, electrical inspector reports, OEM (Original Equipment Manufacturer) service manuals, History cards are subjected to detailed examination.
Strategy 4: Identify appropriate control measures
The following control measures can be implemented based on the Class and Zone assessment. Firstly, place or relocate the normal arc producing electrical devices to a non hazardous area. Next up is explosion confinement. This means that the device enclosure is designed and tested in a manner guaranteeing that if a flammable vapour enters the enclosure and is ignited by an electrical arc or hot surface within, then the resulting explosion will stay contained within the enclosure. Further to this, the electrical apparatus contained within the enclosure should still be operational. Another fitting control measure is energy limiting, or intrinsic safety. This prevents ignition by limiting the released energy resulting from wiring and component failures or faults. Hermetically sealed protection ensures that the arc or heat producing devices are sealed against the intrusion of the hazardous vapour.
Conclusion
In a process industry, explosive atmospheres cannot be avoided. If appropriate control assessment, control measures not implemented it may leads to devastating catastrophic accidents. It leads the employer to face the consequences like legal prosecutions, fines and other direct indirect cost. By attempting the above mentioned proactive arrangements, working in explosive atmospheres can be done more swiftly and safely.
