Understanding ATEX and Explosive Areas

Explosive atmospheres in the workplace can be caused by flammable gases, mists or vapors or by combustible dusts. Explosions can cause loss of life and serious injuries as well as significant damage.

Overview

Explosive atmospheres can be caused by flammable gases, mists or vapors or by combustible dusts. If there is enough of the substance, mixed with air, then all it needs is a source of ignition to cause an explosion. Explosions can cause loss of life and serious injuries as well as significant damage.

There are two primary ways of reducing the risk of an explosion:

1. Preventing releases of dangerous substances, which can create explosive atmospheres, and
2. Preventing sources of ignition

Selection and use of the correct equipment can help greatly in preventing explosions.

What Is An Explosive Atmosphere?

An explosive atmosphere is defined as a mixture of dangerous substances with air, under atmospheric conditions, in the form of gases, vapors, mist or dust in which, after ignition has occurred, combustion spreads to the entire unburned mixture.

Atmospheric conditions are commonly referred to as ambient temperatures and pressures. That is to say temperatures of –20°C to 40°C and pressures of 0.8 to 1.1 bar, i.e. temperatures of -4°F to 104°F and pressures of 11.6 to 16 psi.

Where Are Explosive Atmospheres Found?

Many workplaces may contain, or have activities that produce, explosive or potentially explosive atmospheres. Examples include places where work activities create or release flammable gases or vapors, such as vehicle paint spraying, or in workplaces handling fine organic dusts such as grain flour or wood.

What is ATEX?

ATEX is the name commonly given to the two European Directives for controlling explosive atmospheres:

1. Directive 99/92/EC (also known as 'ATEX 137' or the 'ATEX Workplace Directive') focuses on minimum requirements for improving the health and safety protection of workers potentially at risk from explosive atmospheres.
2. Directive 94/9/EC (also known as 'ATEX 95' or 'the ATEX Equipment Directive') addresses the approximation of the laws of Members States concerning equipment and protective systems intended for use in potentially explosive atmospheres.

Understanding USA Equivalent Classifications To ATEX 94/9/EC Designations

How Should Explosive Atmosphere Zones Be Divided?

The ATEX directive distinguishes between two types of explosive atmospheres: (1) Gas, and (2) Dust.

Areas within these two kinds of explosive atmospheres are each divided into three sub-zones.

Although the zones characteristics are identical for both gas and dust, their numbering is different. Zone 0, 1 and 2 refer to gas. Zone 20, 21 and 22 refer to dust.

Zone 0 / 20: Constant Danger
Permanent presence of explosive gasses or combustive dust.

Zone 1 / 21: Potential Danger
Occasional presence of explosive gasses or combustible dust during normal duty.

Zone 2 / 22: Minor Danger
Presence of explosive gasses or combustible dust is not likely to occur or only for a shorter period of time.

Potentially Explosive Atmosphere

• Oil and Gas Drilling
• Petrochemical Refining and Processing
• Fuel Storage
• Chemical Manufacturing
• Car Manufacturing
• Water Treatment
• Power Generation
• Pharmaceutical
• Distilleries
• Food Manufacturers
• Aviation
• Military
• Painting

Fuel Properties

All flammable gasses, vapours and mists require to be mixed with oxygen to make them burn. There is about 20% to 21% of oxygen in the air we breathe. Mixtures of a flammable gas and certain percentages of air will burn if ignited.

Too much or too little oxygen, the mixture will not ignite. The upper and lower concentrations of gas in atmospheric air, by volume, are known as their flammability or explosive limit.

Lower Explosive Limit (LEL)

This is the concentration of flammable gas or vapor in the air below which the gas atmosphere is not explosive.

Upper Explosive Limit (UEL)

This is the concentration of flammable gas or vapor in the air above which the gas atmosphere is not explosive.

Examples of Explosive Limits 

Explosion Properties

At a critical concentration called the most easily ignited concentration (MEIC), the amount of energy required to cause ignition is minimal.

The critical energy at the MEIC is called minimum ignition energy (MIE).

The Effect of Temperature

Every material has a spontaneous ignition temperature (SIT), or auto ignition temperature (AIT) at which it will ignite spontaneously.

If the temperature of a mixture is raised, the amount of electrical energy required for ignition will decrease, reaching zero at the AIT.

Temperature Classification

• Equipment is identified with a temperature class
• Either identified by a ‘T’ rating or by a temperature in degrees C.
• The temperature class identifies the hottest temperature that the equipment can obtain.

An Explanation of ATEX, NEC and IEC Systems

• Environments that generate potentially explosive atmospheres classify the dangerous areas based on the likelihood and duration of the explosive atmospheres presence.
• This is referred to as a “Zone” (Europe and NEC 505) or “Division” (NEC 500)

Equipment Groups

Apparatus is divided into Equipment groups:

• Group I for mines susceptible to methane (firedamp).
• Group II for explosive gases for locations other than mines
• Group III for dusts

Hazardous Area Classification: Europe

Zone System

• Gases Mists & Vapors - Zone 0,1 or 2
• Combustible Dusts - Zone 20, 21 or 22

Class/Division Definitions

• Class I - Contains flammable gases or vapors in quantities large enough to produce an explosion.
• Class II - Is hazardous due to the presence of combustible dust in the air.
• Class III - Contains easily ignitable fibers or flyings in the air. However, the quantities of fibers and flyings suspended in the air are not likely to be large enough to cause an explosion.

• Division 1 - There is a high probability of an explosive atmosphere in normal operation. This can be for part of the time, up to all the time.
• Division 2 - There is a low probability of an explosive atmosphere being present during normal operation.

*Group designations further define the types of gases, and dusts (A, B, C, D) (E, F, G)

*Example of an American certification would be:

Class I, Division 1, Groups A, B and C;
Class II, Division 2, Groups F and G

Hazardous Area Classification: North America

Class & Division System

• Gases Mists & Vapors - Class I Division 1 or 2
• Combustible Dust - Class II Division 1 or 2
• Fiber & Flyings - Class III Division 1 & 2

Zone System

• Gases, Mists & Vapors Only – Zone 0, 1 or 2

Note: The majority of Areas Classified in North America are according to Class and Division

Ingress Protection

Two numbers indicate the level of ingress protection of an instrument or apparatus. The first is the level of protection against solid objects and the second against liquids.

Note: IP54 is the minimum protection allowed for Hazardous Area Equipment. The US practice is to use NEMA standards to describe ingress protection (i.e. NEMA 3 is similar to IP 54, NEMA 4 is similar to IP 55, NEMA 4x is similar to IP 56, NEMA 6 is similar to IP 67).