FPASA BULLETIN SF 16
HAZARDOUS ATMOSPHERES AND ELECTRICAL EQUIPMENT
Hot surfaces or sparks associated with electrical equipment can ignite flammable
atmospheres causing explosions and fires. Process and storage areas are
classified according to the likelihood of an explosive atmosphere being present and
safe electrical equipment chosen accordingly.
Similarly certain dusts are
hazardous. Three materials classifications are used:
Zone 0, 1, 2 Zone 21
Zone 22
-
gases and vapours
dusts, fibres in suspension
fibre and dust accumulation
Zones in which flammable gases or vapours can be present in air in quantities
sufficient to become hazardous.
Zone
0
1
2
Definition of Location
Those in which flammable gases or
vapours are continuously present in
concentrations within the lower and
upper limits of flammability.
Where:
a) Hazardous
concentrations
of
flammable gases or vapours occur
intermittently or periodically under
normal operating conditions.
b) Hazardous
concentrations
of
flammable gases or vapours can
occur frequently because of repair or
maintenance operations or because
of leakage.
c) Breakdown or faulty operation of
equipment or process which might
release hazardous concentrations of
flammable gases or vapours, might
also cause simultaneous failure of
electrical equipment.
Where operations concerned with
flammable or explosive substances,
gases, vapours or liquids are so well
controlled that an explosive or ignitable
concentration is only likely to occur under
normal conditions.
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Some Examples
Usually confined spaces such as the vapour
space of closed process vessel or closed
storage tank.
Can occur in larger rooms such as rooms in
chemical plants.
All hazardous areas in mines.
Interiors of spray booths and areas in the
vicinity of spraying and painting operations
where volatile flammable solvents are used.
Open tanks or vats of volatile flammable
liquids.
Drying rooms for the evaporation of
flammable solvents.
Flammable liquids or liquefied flammable
gases are transferred from one container to
another.
A distillation unit on open premises where
relief valves are connected to a closed
system. Open air loading and unloading
areas for road and rail tankers where:
The use of flexible pipe is confined to the
connection to the vehicle.
A closed system is used.
Rapid drainage for escaping liquid is
provided.
Valves are well maintained.
Blank flanges are fitted over pipe ends
whenever pipes are not in use.
Zones which are hazardous because of the presence of combustible dusts or
fibres.
Zone
21
Definition of Location
Where:
a) Under normal operating conditions
dust or fibres are or are likely to be
in suspension in the air in
quantities sufficient to produce an
explosive
or
ignitable
and
b)
22
Metallic dusts could be present.
Where deposits or accumulations of:
a) Dusts or fibres can be great
enough to interfere with the safe
dissipation of heat from electrical
apparatus
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or
b)
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Some Examples
Rooms that contain machines (such as
grinders and pulverisers) that are not provided
with suitable dust extraction or exhaust
systems.
Open bins and hoppers.
Terminal points on open conveyors.
Spouts in grain, starch, sugar, malting, hay
and coal plants.
All working areas where metallic dusts and
powders are produced, processed, handled
and packed.
Working and storage areas of grain elevators
Some parts of rayon, cotton and other textiles
mills.
Combustible
fibre
manufacturing
and
processing plants.
Woodworking plants.
Cotton gins and cotton seed mills.
Dust or fibres in, or in the vicinity of
electrical apparatus might be
ignited by arcs, sparks or burning
material from such electrical
apparatus.
Protection methods
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Segregation of electrical equipment from a hazardous atmosphere, should
always be the first consideration for protection eg: a motor driving machinery
situated in a hazardous area could be in a safe area with the drive passing
through a gland in the separating wall.
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Limit temperature of the apparatus’ external surfaces to below the ignition
temperature of the gas, vapour, dust or fibre. SABS IEC 79-0 lists common
gases and vapours according to their ignition temperatures. Maximum surface
temperature attained by equipment is designated by classifications T.1 to T.6.
Temperature
Classification
T.1
T.2
T.3
T.4
T.5
T.6
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Maximum
Surface
Temperature
450°C
300°C
200°C
135°C
100°C
85°C
Examples
Acetone, ethane, hydrogen
Acetylene, butane, ethylene
Cyclohexane, hexane, naptha
Acetaldehyde, diethylether
Carbon disulphide
Dust layers: naphthalene, phenolic
resins, novolak
Prevent production of high energy sparks by using intrinsically safe equipment
and circuits.
Prevent propagation of an explosion out of the equipment to the external
atmosphere
–
flameproof
equipment.
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Prevent access of flammable atmospheres to the equipment – pressurised, oil
immersion and powder filled apparatus.
Types of protection
Electrical equipment should ideally be located in non-hazardous areas only, but
where this is not practical, varying degrees of safety are achieved by constructing
the apparatus in specific ways.
Type of protection
Flameproof enclosure
Explosion protected
apparatus.
Intrinsically safe
Pressurised apparatus
Increased safety
Encapsulated
Powder filled
Dust ignition proof
S
U
S/V
S/I
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-
Symbol
Exd
0
U
S
1
S
S
Zone
2
S
S
21
S/V
S
22
S/V
S
U
S
U
U
S/I
S
U
U
S/I
S
S
U
S/I
S
S
S
S/i
S
S
S
Exia
Exp
Exe
Exm
Exq
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Suitable
Unsuitable
Suitable if without vents
Suitable if provided with isolating interlock
Flameproof enclosure, Exd.
The oldest concept in flameproof enclosures was originally developed for use in
fiery mines but has now found many applications in other industries. An
explosion will not propagate through a gap smaller than a certain size for a
given path length (joint width). A particular gas or vapour has its own critical
gap size determined by experiment. Safe gap sizes range from 0,1 mm to 0,75
mm depending on the joint width and the particular vapour involved. The casing
of the apparatus must also be constructed to withstand internal explosion
pressures. The flameproof enclosure is not gas-tight and allows flammable
gases to enter the enclosure. Internal electrical equipment is not designed to
be non-sparking and internal explosions, therefore, are possible. The resulting
enclosure is usually heavily constructed and expensive.
Flameproof enclosures are rated in three groups. Group 1 is reserved for mining
and methane situations, while Group 2 is for other applications.
Groups for flameproof enclosures
Application
Mining
Industrial
Enclosure
rating
1
11A
11B
11C
Examples
Methane (fire damp)
Propane,
butane,
carbon monoxide.
Ethylene
oxide,
butadiene.
Hydrogen, acetylene
ammonia,
ethylene,
SABS 0108 lists group classifications of common gases and vapours.
Explosion protected apparatus
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Intrinsically safe apparatus, Exia or Exib.
A gas or vapour will be ignited by an electrical spark possessing a certain
amount of energy. The energy value corresponds to the minimum ignition
energy of the vapour. Hydrogen’s minimum ignition energy is only 0,019
millijoules while methane’s is 0,28 mJ. Whether in fault or working conditions,
intrinsic circuits and equipment will be incapable of producing a spark to ignite
the particular gas or vapour.
Exia equipment has been tested with two
simultaneous faults occurring and is the only equipment permissible in 0 areas.
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Increased safety, Exe
Equipment constructed to be non-sparking in ordinary use through strict
insulation quality design and by limiting surface temperatures via cut-outs. The
maximum surface temperature rating must be specified on the equipment.
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Pressurised apparatus, Exp
Clean air drawn from a safe area and pressurising the equipment, prevents the
ingress of gases and vapours by providing a positive pressure inside the
enclosure. The air flow will leak through any openings and interlocks will isolate
the equipment if the flow stops or the pressure decreases. Another method is
utilised in chromatographs for example. Gases and vapours enter the
apparatus but are diluted by a supply of fresh air or inert gas – this method is
known as pruging.
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Powder filled, Exq
Sparking or live equipment filled with inert powder, to suppress any sparks.
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Non-Sparking equipment, Exn or ExN
Only suitable for Zone 2, 21 or 22 areas. Similar to Exe but without such
stringent insulation quality and surface temperature specifications.
Selection, installation and maintenance
Ever certified equipment in a hazardous location may cause ignition if it has been
incorrectly selected, installed or maintained. Visual inspection alone will not ensure
correct operation of the apparatus. Corrosion will jeopardise the effectiveness of
any explosion protected apparatus if not controlled. Apparatus and glands of a
reputable corrosion resistant design will ensure integrity of the enclosure or
apparatus. If, for example, a steel Exe junction box with aluminium or brass glands
is installed in a chemical plant or on an oil rig, corrosion will occur. Exe apparatus
contains Exn (non-sparking) electrical components and any dust or moisture
entering the enclosure could cause sparking. Corrosion in the above instance will
usually occur at the glands and even a rubber seal will be ineffective protection
against the ingress of moisture.
A corrosion resistant apparatus may encase the entire metal gland in a noncorrosive body (glass filled, polyester) which will also protect the cable’s wire
armour from corrosion.
Evaluation considerations:
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division classification
apparatus or gas grouping
environmental conditions (corrosion, heat, dust etc)
installation and maintenance requirements
All equipment for use in hazardous areas must be certified and labelled
accordingly. Such certification would consist of a logo (eg: SABS mark), the rating
(Exd), gas group (11B), and temperature group (T.5).
For example, if an area of a plant is hazardous due to ammonia fumes during
normal working conditions, the selection of electrical equipment is determined by
the following questions:
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what division is the area? Zone ), 1, 2, 21 or 22.
apparatus group? Exia, Exib, Exd, Exe, or Exp (Exia and Exib would probably
be unsuitable due to high cost)
gas group? Group 11A
temperature classification? (ignition temperature of ammonia 651°C)
Flameproof apparatus suitable for this ammonia atmosphere should therefore be
marked as follows:
Exd.2A.T.1.
References:
FPAUK Compendium of fire safety data, Vols. 2 & 4
SABS 0108 – The Classification of hazardous locations and the selection of
electrical
apparatus for use in such locations.
Franz Lackinger, CCG Systems.
Published by
Fire Protection Association of Southern Africa
(Incorporated Association not for Gain)
Reg. No. 73/00022/08)
P O Box 15467
Impala Park
1472