The ATEX equipment directives 94/9/EC and 2014/34/EU and the ATEX worker directive 1999/92/EC require an explosion risk assessment.
Hierarchy of Protection Measures
The Directives apply the hierarchy of protection measures common to risk reduction methods.
- firstly to eliminate or reduce the presence of an explosive atmosphere.
e.g. by substituting the dangerous product with a safer one or limiting its amount used or released in the process, etc. If successful this results in a ‘safe zone’ with no explosive mixture or a zone of negligible extent with a small enough amount of explosive mixture that it can ignite without hazard
- If a hazardous explosive atmosphere cannot be avoided, prevent its ignition.
e.g. sometimes ignition sources can be avoided or equipment can be protected to prevent it becoming a potential ignition source from occurring. The use of ATEX-rated equipment, where the manufacturer or his certification body declares that the equipment is adequately protected, is the most common method
- If the risk of explosion cannot be avoided, mitigate the detrimental effects of an explosion with sufficient measures to ensure health and safety.
Risk assessment steps
An explosion risk assessment consists of:
- a hazard identification of substances and their explosion properties,
- area classification to identify where an explosion hazard may occur and how often
- an ignition hazard assessment to identify sources of ignition and how often they may become active and the protective measures to reduce the occurrence of ignition hazards.
With general purpose equipment, the hazardous substances and zones are not known, and the manufacturer can only do the ignition hazard assessment and declare which zones and hazardous substances it is suitable for by marking and declaring a Category, gas group and temperature restrictions.
Hazardous substance classification
The purpose of the hazardous substance classification is to:
- identify the flammable substances present in the equipment or work area and their explosion properties
- Determine whether they are capable of causing an explosion
- List the properties that affect the ignition and development of an explosion.
For gases vapours and mists these properties are: Flash point, flammability limits, ignition temperature, ease of ignition by sparks (energy or group), vapour density, boiling point, maximum explosion pressure, maximum rate of rise of explosion pressure.
In addition for dusts: particle size, dust layer ignition temperature, conductivity, moisture content, St class (severity of explosion) are important.
Area Classification
List of Sources of Release
List parts of equipment where the hazardous substances may be released continuously or frequently (Continuous), occasionally (Primary) and rarely (Secondary) with release rates and ventilation characteristics at those points to help determine how the substance will disperse.
Zone estimation
The standards EN 60079-10 Parts 1&2 define zones based on the likelihood of presence of an explosive atmosphere are:
The standards give more guidance and examples of sources of release and a calculation method to estimate zone sizes from the amount of substance released and ventilation conditions. This calculation method is adequate where gases and vapours mix by diffusion but are overcautious where there is flow and turbulent mixing. More precise zones can be estimated with Computational Fluid Dynamics (CFD) where over dimensioning of zones presents a problem.
The ATEX equipment directive does not classify hazardous areas with Zone numbers but refers to their definitions (e.g. 'An area where an explosive atmosphere occurs frequently or continuously' for Zone 0)
The ATEX worker directive requires employers to estimate the scale of a possible explosion.
Ignition hazard assessment
Ignition hazard identification
EN 1127-1, “Basic concepts and methodology for Explosion prevention and protection in Explosive atmospheres” lists the types of ignition sources that can ignite an explosive atmosphere. They range from the common ones such as hot surfaces and electrical apparatus through to the more unusual ones such as ionizing radiation and ultrasonics. It explains their physical mechanisms and offers general advice on how to stop them becoming effective. This list is compared to the equipment or work area in order to identify and list all the potential ignition hazards present. EN 1127 and the non-electrical equipment standard, EN 13463-1 describe which mechanisms and phenomena are considered to be a potential ignition hazard.
Ignition risk assessment
Assessing the risk of ignition of an explosive atmosphere means making a detailed analysis of each of the ignition hazards identified:
- Evaluating its likelihood of becoming effective
- If the likelihood is not low enough for the zone it is in, applying protective measures
- Re-evaluating its likelihood of becoming effective with the protective measures in place.
- When the likelihood is acceptable, assigning a category from the table below to each ignition hazard.
- If the ignition hazard is safe with certain substances but not others due to the temperature or spark energy being limited but not avoided, apply restrictions in the form of a temperature class or gas group.
When each individual ignition hazard has been assessed and given a category and restrictions, an assembly can be given a category equal to that of the least protected hazard and a temperature class and gas group equal to that of the most restricted hazard. These are the category, temperature class and gas group that are marked on equipment and assemblies according to the ATEX equipment directive.
Evaluating the likelihood of an ignition source becoming effective requires a detailed knowledge of the environment, of the process performed and of the equipment features and function.
Electrical apparatus manufacturers do not generally perform a full ignition hazard assessment but instead follow protection methods set out in the harmonized standards with prescriptive design, test and marking requirements.
Non-electrical apparatus standards require manufacturers to complete a full ignition hazard assessment and have less prescriptive design and test requirements.
Documentation
The steps above must be documented clearly with supporting data and reasoning behind the assessment decisions. Types of document include:
risk assessment activities completed on templates from the standards
Compliance with design requirements of standards is usually demonstrated with checklists.
Tests are usually documented in test reports.
Supporting documentation is often in the form of datasheets and certificates for components.
The ATEX worker directive requires these documents to be kept in an Explosion Protection Document. The ATEX equipment directive requires them to be kept in a technical file. In the stricter categories this file must be sent to a Notified Body for storage or for approval.
Further advice
For further advice specific to your products, please contact us and we will be pleased to discuss your needs. Our consultancy services regarding the application of the 99/92/EC Directive and of the DSEAR Regulations include:
- training for Health and Safety Managers and engineers
- hazardous area classifications including CFD
- assessment of the explosion risk of workplaces
- issuing and updates of the Explosion Protection Document
- consultancy regarding the technical and organizational measures required where an explosion risk is present