In this article:
Introduction to ATEX
The concept of ATEX originates from the French term Appareils destinés à être utilisés en ATmosphères EXplosibles (which means "Equipment intended for use in explosive atmospheres"). The terminology of “ATEX” is used in the EU, whereas other terminology is applied in e.g., the US.
ATEX assessments deal with assessments of explosive atmospheres. Explosive atmospheres can be defined as mixtures of flammable gas, vapor or combustible dust in such concentrations that ignition is possible. ATEX deals with minimum safety requirements for workplaces and equipment used in explosive atmospheres. These requirements are specified in two EU directives (one for the manufacturer and one for the user):
The ATEX 114 - the "equipment" Directive 2014/34/EU - Equipment and protective systems intended for use in potentially explosive atmospheres.
The ATEX 153 the "workplace" Directive 1999/92/EC - Minimum requirements for improving the safety and health protection of workers potentially at risk from explosive atmospheres.
So, when do the ATEX directives apply, and when might an ATEX assessment be necessary?
When working with flammable gases, liquids or combustible dusts, there is always a risk of explosive atmosphere. ATEX therefore generally refers to the hazard of explosive atmospheres occurring in the workplace due to the presence of flammable gases, liquids or combustible dust mixed in air. The risk of an explosive atmosphere should always be assessed according to the above ATEX directives.
Hazardous areas and ATEX zones
If an explosive atmosphere requires special precautions to protect the safety of workers it is referred to as a hazardous area. Hazardous areas are classified based on the frequency of occurrence and duration of an explosive atmosphere. The classification is further split between flammable gases and combustible dusts. For gases the following classification is made:
Zone 0: An area in which an explosive gas atmosphere is present continuously or for long periods;
Zone 1: An area in which an explosive gas atmosphere is likely to occur in normal operation;
Zone 2: An area in which an explosive gas atmosphere is not likely to occur in normal operation and, if it occurs, will only exist for a short time. For dusts, the following classification is made:
Zone 20: A place in which an explosive atmosphere, in the form of a cloud of combustible dust in air, is present continuously, or for long periods or frequently for short periods.
Zone 21: A place in which an explosive atmosphere, in the form of a cloud of combustible dust in air, is likely to occur occasionally in normal operation.
Zone 22: A place in which an explosive atmosphere, in the form of a cloud of combustible dust in air, is not likely to occur in normal operation but, if it does occur, will persist for a short period only.
The different zones further guide which type of equipment is allowed in a specific zone. For instance, electrical equipment such as switches or motors may cause sparking. Furthermore, static electricity can build up on persons, clothing or items which may discharge generating a spark. Sparks may further ignite a surrounding explosive atmosphere. Hence, if an explosive environment is unavoidable (i.e., the preferred risk management approach of removing the risk source is not possible), all equipment within a hazardous area must be suitably rated and protected from ignitions from sparks or other sources of heat. Furthermore, precautions such as anti-static clothing and shoes may be necessary to prevent static electrical discharges igniting the explosive atmosphere.
But how does one then go about classifying and knowing the extent of a hazardous zone? As part of this insight – let us limit ourselves to explosive gas atmospheres excluding combustible dusts.
Classification and calculation of the extent of zones
To aid in the classification and calculation of the extent of hazardous zones, the International Electrotechnical Commission (IEC) has provided an international standard – IEC 60079. Part 10-1 of the standard deals with the classification of areas for gas atmospheres.
The type of zone can be estimated with the help of Table D.1 provided in the standard.
The table requires inputs in the form of three parameters:
Grade of release (i.e., how often leaks/releases of flammable liquids or gases are expected to occur)
Continuous – i.e., expected to occur frequently or for long periods Examples of sources of continuous grade of release include flammable liquid surfaces which are open to the atmosphere, or flammable liquid containments with permanent vents to atmosphere.
Primary – i.e., expected to occur periodically or occasionally during normal operation. Examples of sources of primary grade of release include expected releases from seals of pumps, compressors or sampling during normal operation.
Secondary – i.e., not expected to occur during normal operation and, if it does occur, is likely to do so infrequently and for short periods. Examples of secondary grade of release may be unexpected leaks from flanges, seals of pumps, compressors or vents during normal operation.
Availability of ventilation (i.e., how often discontinuities in the ventilation are expected)
Good – i.e., ventilation is present virtually continuously.
Fair – i.e., ventilation is expected to be present during normal operation. Discontinuities are permitted provided they occur infrequently and for short periods.
Poor – ventilation does not meet the standard of fair or good, but discontinuities are not expected to occur for long periods.
Effectiveness of ventilation (i.e., how well the ventilation can dilute the release)
High dilution – concentration near the source of the release reduces quickly with virtually no persistence after the stop of the release.
Medium dilution – A stable zone boundary is given whilst the release is in progress. The explosive gas atmosphere does not persist unduly after the release is stopped.
Low dilution – There is a significant concentration whilst the release is ongoing and significant persistence of an explosive zone once the release has stopped.
The effectiveness of ventilation, i.e., the degree of dilution, will depend on the ventilation velocity and the release flow. IEC 60079-10-1:2020 provides guidance on how to assess the degree of dilution as given in Figure C.1. Note that specific exceptions and rules to apply for certain situations are further described in the standard.
Furthermore, guidance is provided on how to assess the ventilation velocity as well as the release characteristic (Qc).
For outdoor ventilation velocities, Table C.1 may be applied.
For indoor applications, the ventilation velocity may be provided by knowing the ventilation flow in a room and the room dimensions. It is further important to consider any background concentration of the flammable gas, as the dilution should be considered low if the background concentration exceeds 25% of the lower flammability limit (LFL) of the released gas.
The background concentration (vol/vol) is given by the following formula:
Where Xb is the background concentration, f is a measure of the degree of mixing of the ventilation (f=1 means that the background concentration is uniform and the ventilation outlet is far from the release meaning that the concentration at the ventilation outlet reflects the mean background concentration), Qg is the volumetric flow of the release, i.e.,
where Wg is described in equation (4) below and ρg 𝜌g is the gas density at ambient conditions. Further,
Where C is the air change frequency in the room (s^-1 ) and V0 is the volume under consideration (e.g., volume of the room).
Finally, the release characteristic, Qc, is calculated through the formulas given below. In this case, the release characteristic is given by equation (1) where equation (3) is the equation for mass release of a liquid, (4) is the equation for subsonic gas release (i.e., when the gas in the system has a pressure below the critical pressure):
where:
k - Safety factor attributed to LFL
LFL - Lower flammable limit (vol/vol)
M - Molar mas of gas or vapour (kg/kmol)
R - Universal gas constant (8314 J/kmol K)
ρa - Air density (kg/m3)
ρg - Density of the gas or vapour (kg/m3)
Ta - Absolute ambient temperature (K)
Wg: Mass release of flammable substance (kg/s), for mixtures only the total mass of flammable substance should be considered
CD - Discharge coefficient
S - Cross section of the opening (hole), through which the fluid is released (m2). Table B.1 in IEC-60079-10-1:2020 provides guidance on selection of hole sizes.
p - Pressure inside system (Pa)
Z - Compressibility factor (-)
γ - Polytropic index of adiabatic expansion of ratio of specific heats (-)
Knowing the grade of release, effectiveness of ventilation and availability of ventilation further makes it possible to assess the zone from Table D.1.
Once the type of zone is known, the extent of the zone can be calculated. The extent of the zone implies the estimated or calculated distance at which an explosive atmosphere exists before it is dispersed to a concertation in air below its lower flammability limit (LFL).
The IEC 60079-10-1:2020 provides a guide to determine the extent of the hazardous zone in the form of Figure D.1.This guide is only applicable for high dilution with a zero-background concentration. In case of low or medium dilution the estimated extent will be too small and another assessment technique should be applied. It shall be noted that consideration should be given to the fact that gases can be heavier or lighter than air, further guiding their spread after release. Heavier gases will flow into areas below ground level whilst lighter gas may be retained in high points. The estimation of the extent of the hazardous zone requires input in the form of release characteristic given by equation (1) above.
Conclusion
With the above, we hope that you are more familiar with ATEX and the background on how calculations are performed. Need more help? Do not hesitate to contact ORS Consulting. We perform multiple ATEX Assessments and Hazardous Area Calculations every year for a large variety of industries!
