Flammable material
Flammable material can be either gaseous, a
vapour from liquid or solid. For a general discussion
relevant to work places, their reactivity with atmospheric oxygen is considered.
Flammable gases
A flammable gas may be an element such as
hydrogen which can be made to react with oxygen
with very little additional energy. Flammable gases
are often compounds of carbon and hydrogen.
These flammable gases and vapours require only
small amounts of energy to react with atmospheric
oxygen.
A vapour is the proportion of a liquid - if talking
about the explosion protection of flammable liquids
- which has evaporated into the surrounding air as
the result of the vapour pressure above the surface
of the liquid, around a jet of that liquid or around
droplets of the liquid. Mist is a special type, which
because of its explosion behaviour, can be included
with the vapours, for the purposes of fulfilment of
safety considerations.
Flammable liquids (actually the vapour only)
Flammable liquids are often hydrocarbon compounds such as ether, acetone or petroleum spirit.
Even at room temperature, sufficient quantities
of these can change into the vapour phase so that
an explosive atmosphere forms near their surface.
Other liquids form such an atmosphere near their
surface only at increased temperatures. Under
atmospheric conditions this process is strongly
influenced by the temperature of the liquid.
For this reason the flash point, or rather the flash
point temperature, is an important factor when
dealing with flammable liquids. The flash point
relates to the lowest temperature at which a flammable liquid will, under certain test conditions,
form a sufficient quantity of vapour on its surface
to enable an effective ignition source to ignite the
vapour air mixture.
The flash point is important for the classification
of potentially explosive atmospheres. Flammable
liquids with a high flash point are less dangerous
than those with a flash point at room temperature
or below.When spraying a flammable liquid, a mist can form
consisting of very small droplets with a very large
overall surface area, as is well-known from spray
cans or from car paint spraying stations. Such a
mist can explode. In this case the flash point is of
lesser importance. For a fine mist - made from a
flammable liquid - the behaviour relevant to safety
can be roughly derived from the known behaviour of
the vapour.
Flammable solids (actually dust only)
Flammable solids in the form of dust or flyings can
react with atmospheric oxygen and produce disastrous explosions. Normally more energy is required
for activating the explosion in air than with gases
and vapours. However, once combustion starts, the
energy released by the reaction produces high temperatures and pressures. In addition to the chemical
properties of the solid itself, the fineness of the particles and the overall surface area, which increases
with increasing fineness, play an important role.
The properties be determined by processes which
take place immediately at the surface of the solid
particles. Igniting and extinguishing a paraffin wax
candle provides a demonstration of a series of processes undergone by a solid material within a short
period of time which cannot easily be presented in a
simplified form.
An experiment shows that when the wick of a
candle is lit, the paraffin wax melts and then vaporises and that this vapour feeds the flame. After
extinguishing the candle, the paraffin vapour can
still be smelled, the melted paraffin wax solidifies
and the paraffin vapours disperse. Now the paraffin
wax candle is once again a harmless object.
Dust reacts very differently, depending on whether
it is in a deposited layer or whether it is in a swirled
dust cloud. Dust layers are liable to begin smouldering on hot surfaces, while a dust cloud which has
been ignited locally or through contact with a hot
surface can explode immediately. Dust explosions
are often the consequence of smouldering dust
layers which become swirled up and already carry
the ignition initiation. When such a layer is stirred
up, for example by mechanical cleaning methods
during transportation or inappropriate extinguishing
attempts, this can lead to a dust explosion.
A gas or vapour/air explosion can also swirl up the
dust, which then often turns from the first, the gas
explosion, into the second, the dust explosion. In
deep coal mines methane/firedamp explosions
often have triggered off coal dust explosions whose
consequences were more serious than those of the
original firedamp explosion.
Oxygen
The quantity of oxygen available in the air can only
oxidise/burn a certain quantity of the flammable
material. The ratio can be determined theoretically,
it is called the stoichiometric mixture. When the
quantity of the flammable material and the available atmospheric oxygen are near to the optimum
(most ideal) ratio, the effect of the explosion - temperature and pressure increase - is most violent.
If the quantity of flammable material is too small,
combustion will only spread with difficulty or will
cease alltogether. The situation is similar when the
quantity of flammable material is too great for the
amount of oxygen available in the air.
All flammable materials have their explosive range,
which also depend on the available activation
energy. This is usually determined by igniting
the mixture with an electric spark. The explosive
range is bounded by the lower flammable (previous referred to as explosive) limit and the upper
flammable (previous referred to as explosive) limit.
This means that below and above these limits,
explosions will not happen. This fact can be utilised
by sufficiently diluting the flammable substances
with air or by preventing the ingress of air/oxygen
into parts of the equipment. The latter option is,
however, not or only with restrictions possible in
environments where people regularly work (inerting
means danger for suffocation) and must therefore
be reserved for technological equipment only.
Sources of ignition
With the use of technical equipment a large number
of ignition sources are possible. In the following
overview the numbers given behind the ignition
sources refer to the appropriate clauses of the basic
standard: EN 1127-1: 2019 “Explosive atmospheres
- Explosion prevention and protection- Part 1: Basic
concepts and methodology.”
Hot surfaces (5.1)
arise as a result of energy losses from systems,
equipment and components during normal operation. In the case of heaters they are desired. These
temperatures can usually be controlled.
In the event of a malfunction - for example with
overloading or seized bearings - the energy loss, and
therefore the temperature, increases unavoidably.
Technical equipment must always be assessed as to
whether it is stabilizing - for example whether it can
attain a final temperature, or whether non-permissible temperature increases are possible which need
to be prevented by taking appropriate measures.
Examples: coils, resistors or lamps, hot equipment
surfaces, brakes or overheating bearings
Flames and hot gases (including hot particles)
(5.2)
can occur inside combustion engines or analyser
equipment during normal operation and when a
malfunction has occurred. Protective measures are
in such case required which are able to permanently
prevent them from leaving the enclosure.
Examples: exhausts from internal combustion
engines or particles which are formed by the switching sparks of power switches eroding material from
the switch contacts
Mechanically generated sparks (5.3)
are produced for example by grinding and cutting
devices during normal operation and are therefore
not permitted in a potentially explosive atmosphere. Cracks in rotating parts, or parts sliding
over each other without sufficient lubrication or
similar situations can generate such sparks when
malfunctioning.
Specific requirements to the materials used to
produce enclosures serve to reduce the risks from
such ignition sources.
Examples: tools such as a rusty hammer and chisel
in contact with light alloys or the metal fork of a fork
lift truck
Electrical equipment and components (5.4)
must normally be regarded as a sufficient ignition
source. Only very low energy sparks with energies of
only a few micro Joules (= micro Watt seconds) may
be regarded as too weak to start an explosion. For
this reason, suitable measures must be adopted to
prevent these ignition sources.
Examples:switching sparks, sparks at collectors or
slip rings
Stray electric currents, cathodic corrosion
protection (5.5)
which then may result in a potential difference
between different earthing points. This is why a
highly conductive connection to all the electrically
conductive parts of the equipment must be provided
so that the potential difference is reduced to a
safe level. It is not relevant whether the conductive
equipment is electrical or non-electrical parts of
the installation, as the cause of the current may be
found outside of the equipment.
An equipotential bonding shall always be provided,
irrespective of whether or not such currents are
expected or whether its sources are known.
Examples: Electric railways and other earthed
voltage supplies for example for electric corrosion
protection of equipment
Static electricity (5.6)
Independently of whether or not there is an electrical voltage supply, electrical sparks can be caused
by static discharges. The stored energy can be
released in the form of sparks and function as an
ignition source. Because this ignition source can
arise quite independently of an electrical voltage
supply, it must also be considered with non-electrical devices and components. It is connected with
separation processes; therefore these cases must
be assessed where this ignition source needs to be
taken into account.
Friction during normal operation can be the cause
of electrostatic charging. For example, portable
devices cannot - due to their portability - either
be earthed or connected to potential equalization.
When interacting with the clothes of the user, static
charging can occur during normal operation. Static
electricity must be prevented from becoming an
ignition source by taking appropriate measures.
Examples: Transmission belts made from plastic
materials, enclosures of portable devices, synthetic
clothing material. Separation processes when rolling
out paper or plastic film, plastic transport tubing
systems
Lightning (5.7)
and the impact of lightning can result in the ignition
of an explosive atmosphere. Lightning always results
in the ignition of an explosive atmosphere, so there
is a need for lightning distraction. However, there is
also a possibility of ignition due to the high temperature reached by lightning distraction routes.
Large currents flowing from where the lightning
strikes can produce sparks in the vicinity of the
point of impact.
Radio frequency (RF)
electromagnetic waves from 104 kHz to 300 GHz are
not the only ignition sources where radiation energy
enters the explosive mixture, the following needs to
be listed:
Electro-magnetic radiation -
radio RF waves (5.8),
Electro-magnetic radiation - IR,
visible and UV light (5.9),
Ionising radiation -
röntgen and gamma (5.10),
Ultrasonic (5.11).
Systems, devices and components that use radiation
may be set up and operated in the Ex area if their
parameters are limited permanently and reliably and
this equipment is checked.
Examples: transmitting and receiving equipment,
mobile telephones, photoelectric barriers and
scanners
Adiabatic compression and shock waves (5.12)
inside tube-shaped structures operated at negative
pressure can also become a source of ignition.
Examples: transport tubes with narrow passage,
breakage of a long fluorescent tube in a hydrogen/
air atmosphere
Exothermic reactions (5.13)
are together with self-ignition of dusts the finally
defined possible type of ignition sources.
