1. Formation of an Explosive Atmosphere

Flammable Substances and Their Classification

Correct classification of flammable substances is essential for
identifying explosive atmosphere risks
and selecting suitable Ex-proof equipment.

Flammable and Non-Flammable Substances

• Flammable substances:
  Materials such as coal, paper, gasoline, natural gas and hydrogen
  that can ignite under suitable conditions.
• Non-flammable substances:
  Materials such as concrete, sand, metal, water and nitrogen that
  do not normally burn under standard conditions.

When evaluating substances, not only their chemical properties but also
their physical form and environmental conditions
must be taken into consideration.

Classification of Equipment and Flammable Substances

Equipment and flammable substances used in explosive atmospheres are
classified according to the
groups defined in the EN IEC 60079 series of standards.
This classification helps determine the environment in which the equipment
may be used.

• Group I:
  Covers equipment intended for use in mines and mining applications.
• Group II:
  Covers non-mining applications where flammable gases or vapors may
  be present. Group II is divided into
  IIA, IIB and IIC subgroups.
• Group III:
  Covers non-mining applications where combustible dust may be present.
  Group III is divided into
  IIIA, IIIB and IIIC subgroups.

The criteria used to determine the A, B and C subgroups should be evaluated
together with the explanations provided in the
Ignition of an Explosive Atmosphere section.

Classification of IIIA, IIIB and IIIC Dust Groups

What Are Flammable Volatile Substances?

Flammable volatile substances are materials that can evaporate under
normal operating or ambient conditions and combine with air to
form a flammable or explosive atmosphere.

What Is Conductive Dust?

Conductive dusts are combustible dusts that may create additional risks
for electrical equipment due to their electrical conductivity.
Therefore, when selecting equipment for dusty explosive atmospheres,
the electrical properties of the dust must also be evaluated.

What Are Explosion Limits?

The concentration of flammable gases, vapors or dusts in air is one of
the key factors determining whether an explosion can occur.

• Explosions can only occur within the
  explosion limits
  specific to the relevant flammable substance.
• For combustible dusts, airborne concentration can vary significantly
  due to settling, accumulation and re-suspension.
• Explosion limits may vary under non-atmospheric pressure and
  temperature conditions.

Lower Explosive Limit (LEL) and Upper Explosive Limit (UEL)

The Lower Explosive Limit (LEL) is the lowest
concentration of a flammable gas or vapor in air at which an explosive
mixture can exist.

The Upper Explosive Limit (UEL) is the highest
concentration at which an explosion is still possible.

Hazardous Quantity in an Explosive Atmosphere

The quantity of an explosive atmosphere is an important criterion in an
explosion risk assessment.

• Regardless of room volume, approximately
  10 liters of explosive atmosphere
  may be considered a hazardous quantity.
• In small enclosed spaces, even an explosive atmosphere corresponding
  to one ten-thousandth of the room volume (0.01%)
  should be taken into consideration.
• Where an explosive atmosphere forms directly around people,
  even smaller quantities may be critical.

Explosive Atmosphere Example: Gasoline

Gasoline Evaporation and Explosion Risk

Due to its high volatility, even a small quantity of gasoline can
generate a significant amount of flammable vapor. When gasoline vapor
mixes with air in suitable proportions, an
explosive atmosphere may form.

• Explosion limits:
  approximately 0.6%–8.0% by volume.
• 1 liter of liquid gasoline
  can produce approximately 196 liters of gasoline vapor.
• At the LEL concentration, approximately
  32,700 liters of explosive atmosphere
  may be formed.
• At the UEL concentration, approximately
  2,450 liters of explosive atmosphere
  may be formed.
• Even approximately 0.3 ml of liquid gasoline
  can form around
  10 liters of explosive atmosphere
  under suitable conditions.

This example demonstrates why even very small quantities of flammable
liquids can create a significant
explosion and fire risk
in enclosed or inadequately ventilated areas.

2. Ignition of an Explosive Atmosphere

Potential Ignition Sources in Explosive Atmospheres

For a fire or explosion to occur in an explosive atmosphere, a source
of ignition with sufficient energy must be present in addition to the
flammable substance and oxygen.
Typical ignition sources found in industrial facilities include:

• Hot surfaces
• Flames and hot gases
• Mechanically generated sparks
• Electrical installations
• Electrical equalizing currents and cathodic protection systems
• Static electricity
• Lightning
• Electromagnetic waves (10⁴ – 3 × 10¹¹ Hz)
• Electromagnetic waves (3 × 10¹¹ – 3 × 10¹⁵ Hz)
• Ionizing radiation
• Ultrasonic sound
• Adiabatic compression and shock waves
• Exothermic reactions and spontaneous ignition of dust

Ignition Risk from Hot Surfaces

When Can Hot Surfaces Occur?

• In electrical equipment, such as
  lighting fixtures, and mechanical equipment,
  such as pumps
• During normal operation, for example in heat-transfer oil systems,
  heat tracing systems or steam pipelines
• During faults, for example in bearings or motor windings
• During maintenance work such as welding, drilling and grinding
• In cases of inadequate monitoring, such as insufficient lubrication
  or dry running

When Can Ignition Occur Due to a Hot Surface?

• When the surface temperature exceeds the
  ignition temperature
  of the gas, vapor or dust cloud
• When smoldering or heating occurs in accumulated combustible dust layers

Flammable gases and vapors are divided into different
temperature classes
according to their ignition temperatures.

What Is Ignition Temperature?

Ignition temperature is associated with the temperature
at which a flammable gas, vapor or other combustible substance can be
ignited by a hot surface without an external flame or spark.
This value is critical when selecting equipment for explosive atmospheres.

Temperature Classes for Gases

Flammable gases and vapors are divided into temperature classes according
to their ignition temperatures.

• Classification depends on the specific ignition temperature of the
  flammable substance.
• The T1, T2, T3, T4, T5 and T6
  temperature classes are used for gases and vapors.
• Combustible dusts are not classified according to the T1–T6 temperature classes.
  In dusty environments, actual surface temperatures and the ignition
  characteristics of the dust must be considered.

Examples of Temperature Classes

Temperature Classes and Ex Equipment Selection

Example: Solvent Storage Area

A flammable substance may, for example, be characterized by the
T4 temperature class.
For T4 equipment, the maximum surface temperature is associated with
a limit of 135 °C.

Therefore, the ignition temperature of the flammable substance and the
maximum surface temperature that the equipment can reach must be
evaluated together.

Example: Ex-Proof Lighting Fixture

Ex equipment is characterized by its permitted maximum surface temperature.
For example,
the maximum surface temperature of T4 equipment must not exceed 135 °C.

Characteristics of Combustible Dust

Small combustible dust particles can generally occur in industrial
environments in two basic forms:

• Airborne combustible dust cloud
• Combustible dust layer accumulated on surfaces

The form in which the dust is present, its particle size and other physical
characteristics may affect its
ignition behavior and ignition temperature.

Ignition Temperature of a Dust Layer

Ignition Temperature of a Dust Cloud

Surface Temperature and Equipment Selection for Combustible Dust

Example: Flour Packaging Facility

In areas where combustible dust is present, two different ignition
temperatures must be taken into consideration:

• Ignition temperature of the dust layer
  (for example T_{5 mm} = 300 °C)
• Ignition temperature of the dust cloud
  (for example T_CL = 460 °C)

Example: Ex-Proof Lighting Fixture for a Dust Hazardous Area

Equipment intended for hazardous areas containing combustible dust is
marked with its actual maximum surface temperature.
The required safety margins must be taken into account when selecting
suitable equipment.

Ignition Sources in Electrical Installations

Electrical installations can become ignition sources in explosive
atmospheres in two main ways:

• Formation of hot surfaces above the permitted temperature
• Formation of electrical sparks with sufficient energy to cause ignition

When evaluating the ignition risk associated with electrical sparks,
flammable gases and vapors are divided into
gas groups
according to the ignition energy required and their relevant characteristics.

Gas Groups and Ignition Energies

Ignition Energies and Ex Equipment

Example: Hydrogen-Cooled Power Plant Generator

Some power plant generators are cooled using hydrogen.
Hydrogen is classified within the
IIC gas group
and has a very low ignition energy.

Therefore, electrical equipment used in environments containing hydrogen
must be selected with consideration for the fact that even very
low-energy sparks may cause ignition.

Example: Intrinsically Safe Barrier

Intrinsically safe barriers
help prevent spark energy from reaching hazardous levels by limiting
voltage, current and power within the circuit.

In a properly designed system, the potential spark energy is maintained
below the level required to ignite flammable substances within the
IIC gas group.

Mechanically Generated Sparks

When Can Mechanical Sparks Occur?

• When foreign particles enter moving machinery; for example,
  a stone entering a mill or a screw entering a fan
• As a result of using hand tools during maintenance, repair or
  installation activities

The ignition risk may increase where oxidizable materials are present
or where an intense shower of sparks is generated.

Static Electricity and Explosion Risk

Static electricity can occur as a result of electrical
charges accumulating on a surface or material. In an explosive atmosphere,
the sudden discharge of these charges may become an ignition source.

When Can Static Electricity Occur?

• During normal operation of industrial systems; for example,
  films moving over rollers or liquids flowing through pipelines
• During cleaning activities such as wiping surfaces
• When people walk
• During contact, separation, transfer or friction between materials

To reduce risks associated with static electricity in explosive atmospheres,
appropriate
grounding, equipotential bonding and electrostatic charge control
methods should be implemented.

3. Effects of Explosions

Characteristic Values of Explosions

The behavior of explosive gas, vapor or dust mixtures can be determined
using tests performed under suitable experimental conditions.
These tests are important for understanding the severity and propagation
characteristics of an explosion.

Key Parameters Evaluated in Explosion Tests

• Propagation velocity
• Explosion pressure
• Rate of pressure rise
• Explosion propagation behavior through small openings

Explosion Propagation Through Openings

During an explosion, hot gases and flames may propagate to the surrounding
atmosphere through openings or joints in equipment.
This behavior is particularly important in the design of
flameproof Ex equipment.

Gap Width and Gas Groups

Ignition tests performed with different flammable gases and vapors
demonstrate that explosions can propagate through certain gap widths
specific to the substance involved.

This characteristic is used to classify flammable gases and vapors according
to their
Maximum Experimental Safe Gap (MESG)
values.

The MESG value is an important criterion for determining
IIA, IIB and IIC gas groups
and for selecting suitable Ex equipment.

What Is MESG?

MESG (Maximum Experimental Safe Gap)
refers to the limiting gap dimension under controlled test conditions
that prevents an explosion from propagating through an opening and
igniting an explosive mixture in the external atmosphere.

Because flammable gases have different MESG values, they are classified
into IIA, IIB and IIC
groups according to their explosion characteristics.

Gas Groups and MESG Examples

Flammable gases have different MESG values.
These differences are among the primary criteria used to determine whether
a gas belongs to the IIA, IIB or IIC gas group.

Classification of IIA, IIB and IIC Gas Groups

Gas group classification is an important factor when selecting equipment
for explosive gas atmospheres. Classification is based on the ignition
and explosion characteristics of the gases.

• IIA:
  Includes gases and vapors with relatively higher MESG values.
• IIB:
  Includes substances with lower MESG values than those in Group IIA.
• IIC:
  Includes gases and vapors with the lowest MESG values and therefore
  more demanding equipment requirements.

In general, equipment suitable for more demanding gas groups can also
provide suitability for explosive atmospheres with less demanding
gas-group requirements, subject to the complete equipment certification
and application conditions.

MESG and Ex Equipment Selection

MESG is used not only for classifying flammable substances but also for
selecting suitable Ex-proof equipment.
Equipment joints are designed to prevent an internal explosion from
propagating to the surrounding explosive atmosphere.

Example: Solvent Storage Area

A solvent or flammable gas may be classified within a particular gas
group according to its MESG value. For example, certain substances may
belong to the IIC gas group.

For substances in the IIC group, hot gases and flames may pass through
very narrow gaps.
Therefore, the design of equipment joints is particularly important.

Example: Flameproof Ex-Proof Lighting Fixture

Flameproof Ex equipment
is designed to contain an explosion that may occur inside the enclosure.

The joints between the enclosure body and cover are designed so that hot
gases and flames are sufficiently cooled as they travel through the
flamepath, helping to prevent ignition of the surrounding explosive atmosphere.

For example, a
flameproof lighting fixture suitable for the IIC gas group
must be designed with joint clearances and flamepath characteristics
that meet the requirements applicable to the IIC gas group.