Many areas may contain hazards, but the term “Hazardous Area” is specifically applied to locations where the likelihood of coincidentally creating both a flammable atmosphere and a source of ignition is high enough to justify implementing safeguards to prevent the harmful consequences.
The identification and classification of such hazardous areas is a significant aspect of the risk assessment for the plant, to meet formal safety requirements. Yet how is this done?
In AU & NZ, hazardous areas are classified based on the AS/NZS variant of the IEC 60079-10-1 standard.
Where the releases are considered to be part of the normal operation (drainage, sampling, atmospheric storage venting, etc.), the release is graded as either Continuous or Primary, depending on the duration. A Continuous release totals more than 1000 hours in a year (i.e. 8760 hours). Primary releases are more than 10 hours a year.
Where a gas release is not expected during normal operation, i.e. it only occurs as the result of a failure and won’t last long (more than 1 hour per year), this was graded as a Secondary release.
Based on the grades of release, three hazardous area Zones (0,1 & 2) are identified in decreasing order of duration of the flammable atmosphere created by the gas release. This allows the impact of dispersion and material flammability to be integrated.
Zone 0 is used for areas where the flammable gas is present for more than 1000 hours in a year (i.e. 8760 hours), effectively a Continuous release.
Where the flammable atmosphere is present for between 10 to 1000 hours per year it is classified as a Zone 1 and the release is called a Primary release.
For releases totalling less than 10 hours per year the release is designated as Secondary and generates a Zone 2 classification.
Where a flammable atmosphere would exist for 1 hour per year or less, the area would typically be classified as Non-Hazardous.
The default relationship between the release grade and Zone depends on the ventilation's adequacy to disperse/dilute the gas releases to the point where they are no longer flammable.
Adequate ventilation disperses a release of gas, reducing the duration of a potentially flammable atmosphere below the 10 hour limit for a Zone 1. Therefore, a small Zone 1 may be surrounded by a larger Zone 2 to account for the dilution of released gas further from the release point.
The airflow around a release point is high-enough to rapidly dilute the gas below the flammable concentration (LFL). The Zone may be designated as having a Negligible Extent (e.g. less than 100mm radius). For example, a Local Exhaust Ventilation (LEV) hood may turn a Zone 1 classification into a Zone 1 NE.
A small, infrequent release in a poorly ventilated area (e.g. a small leak in an enclosed plant without ventilation) may not disperse and fill the enclosure with a flammable atmosphere for more than 10 hours – making the whole space a Zone 1 area.
The extent of the hazardous area depends on the distance from the flammable gas release source to the edge of the gas cloud at which it is no longer flammable. The gas concentration within the cloud depends on the gas release rate and the airflow rate dispersing/diluting the gas into the surrounding atmosphere.
The current AS/NZS 60079.10.1(2009) HAC standard contains examples of typical equipment scenarios where the zones classes are based on conservative assessments agreed by a committee of ‘experts’. These zoning examples may need tuning to meet the needs of a specific application where the fluid properties, ventilation flow, release rate etc. differ from those specified for the examples. It does not include a methodology to quantify the extent of the flammable cloud and the associated Zones. The standard states that the ventilation methodology incorporated is not intended to directly determine the extent of the hazardous areas. However, it may provide guidance for tuning the impact of ventilation on the example Zones.
Estimation of the extent of hazardous areas has been an area of considerable development since the issue of the current AS/NZS standard. Guidance based on CFD modelling is now available in other, more recent international standards issued by IEC and Energy Institute - IEC 60079-10-1 (2020) and EI15 (2015). Future releases of the AS/NZS standard (due in 2022) will incorporate the IEC methodologies and local guidance based on such fundamental approaches.
These new models require an improved understanding of the chemical and physical properties of the materials, the process conditions of the releases, risk assessment and the subsequent dispersion modelling.
The impact of these changes on the required competencies has been recognised in the latest standard (AS/NZS 4761: 2018) but this may take time to filter down to the basic technical & engineering competencies, training courses features and Professional Development requirements for those performing hazardous area classification.