Process-based facilities can cause flammable atmospheres from time to time during normal operation. Fire risk is an ever-present concern. Hazardous area classification is a key aspect to ensure fit-for-purpose ignition source control.
Hazardous area classification (HAC) is standardized through several international standards and industry-specific guidelines. However, it can still be challenging to perform, requiring the need for expert judgment for specific applications.
The Purpose of Hazardous Area Classification
The aim of HAC is to avoid ignition of those flammable atmospheres that are expected to occur from time to time in normal operation either by design intent (e.g. venting, sampling, etc) or by operational or design failures that are expected to occur at some point (e.g. small leakages from flanges, corrosion holes, etc).
The approach of HAC is to reduce the likelihood of coincidence of a flammable atmosphere and an ignition source (e.g. hot surface, electrical spark formation, etc.). You do this by assigning ATmospheres EXplosible (ATEX) zones (type and extent/size) for different flammable release sources. For flammable gas/vapor releases, areas are classified as Zone 0, Zone 1, Zone 2, or as non-classified areas. The zone type depends on how often and for how long a flammable atmosphere is expected to be present.
Firstly, in zone 0 the flammable atmosphere is present continuously or for long periods;
Secondly, in zone 1 the flammable atmosphere will be present occasionally in normal operation;
Finally, in zone 2 the flammable atmosphere is not likely to be present in normal operation, and only for short periods.
The zone type, therefore, depends on three things; how often the release source occurs, the duration of the release, and the persistence of the flammable atmosphere formed by the release. The persistence is very dependent on the ventilation of the area (natural or forced).
The resulting ATEX zones are applied to select proper Ex-rated equipment to be applied for the specific zone to reduce the ignition risk. Furthermore, the zones also provide input for operational awareness for personnel in different contexts e.g. hot work control.
The Steps when Performing a Hazardous Area Classification
There are different methods for assigning hazardous areas. Broadly speaking, you need to take the following steps:
Identify flammable release sources for the studied area;
Determine the grade of the release of individual release sources (based on frequency and duration of releases);
Work out the release rates of release sources;
Determine the effectiveness of ventilation (forced and/or natural ventilation);
Work out the availability of the credited ventilation;
Determine the type of zone for the flammable release source;
Find out the extent of the zone for the flammable release source;
Prepare HAC drawing for the area.
Document the steps so the basis for the HAC drawing is clear. In principle, another ATEX engineer/consultant should be able to trace all assumptions and parameters applied in the above steps. In this way, they should be able to reproduce the HAC drawing. If the only thing documented is the HAC drawing, or if vital information or assumptions are missing, as is often the case, this often results in problems assessing if the HAC is correct. It can also make it more challenging to perform future updates.
Dealing With Other Risks
Area classification does not address large or catastrophic failures of equipment (e.g. pipework rupture) leading to large releases of flammable material. You shall manage these largest accidental releases by other methods. Methods such as following legislative requirements, international standards, best industry practices performing proper risk assessments, and having proper operating and maintenance processes and procedures in place, etc.
It is a common misunderstanding that you cannot have flammable atmospheres in non-classified areas. Furthermore, even if you properly apply ATEX equipment in classified areas, ignition of flammable atmospheres in such zones is still a risk. ATEX equipment that is faulty or incorrectly installed can cause ignition. Hence hazardous area classification and use of ATEX equipment will not completely eliminate the risk of fires and explosions. However, they are still key activities for reducing the risk for fires and explosions in process-based facilities.
Do you have any questions or comments regarding hazardous area classification or process safety? Don't hesitate to get in touch with us.