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 - gases and vapours

Zone 21 - dusts, fibres in suspension

Zone 22 - fibre and dust accumulation

Zones in which flammable gases or vapours can be present in air in quantities sufficient to become hazardous.

Zone
/ Definition of Location / Some Examples
0 / Those in which flammable gases or vapours are continuously present in concentrations within the lower and upper limits of flammability. / ·  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.
1 / 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. / ·  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.
2 / 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. / ·  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 / Definition of Location / Some Examples
21 / 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)  Metallic dusts could be present. / ·  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.
22 / Where deposits or accumulations of:
a)  Dusts or fibres can be great enough to interfere with the safe dissipation of heat from electrical apparatus
or
b)  Dust or fibres in, or in the vicinity of electrical apparatus might be ignited by arcs, sparks or burning material from such electrical apparatus. / ·  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.

Protection methods

·  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.

·  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 / Maximum Surface
Temperature / Examples
T.1 / 450°C / Acetone, ethane, hydrogen
T.2 / 300°C / Acetylene, butane, ethylene
T.3 / 200°C / Cyclohexane, hexane, naptha
T.4 / 135°C / Acetaldehyde, diethylether
T.5 / 100°C / Carbon disulphide
T.6 / 85°C / 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.

·  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 / Symbol / Zone
0 / 1 / 2 / 21 / 22
Flameproof enclosure / Exd / U / S / S / S/V / S/V
Explosion protected apparatus.
Intrinsically safe / Exia / S / S / S / S / S
Pressurised apparatus
Increased safety / Exp
Exe / U / S/I / S/I / S/I / S/i
Encapsulated / Exm / S / S / S / S / S
Powder filled / Exq / U / U / S / S / S
Dust ignition proof / - / U / U / U / S / S

S - Suitable

U - Unsuitable

S/V - Suitable if without vents

S/I - 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 / Enclosure rating / Examples
Mining / 1 / Methane (fire damp)
Industrial / 11A
11B
11C / Propane, butane, ammonia, carbon monoxide.
Ethylene oxide, ethylene, butadiene.
Hydrogen, acetylene

SABS 0108 lists group classifications of common gases and vapours.

Explosion protected apparatus

·  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.

·  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.

·  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.

·  Powder filled, Exq

Sparking or live equipment filled with inert powder, to suppress any sparks.

·  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 non-corrosive body (glass filled, polyester) which will also protect the cable’s wire armour from corrosion.

Evaluation considerations:

-  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:

-  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