IEC 60079-10:2002 pdf download

IEC 60079-10:2002 pdf download.Electrical apparatus for explosive gas
atmospheres – Part 10: Classification of hazardous areas.
An explosive gas atmosphere cannot exist if the flashpoint is above the relevant maximum temperature of the flammable liquid. The lower the flashpoint, the greater may be the extent of the zone. If a flammable material is released in a way that forms a mist (for example, by spraying) an explosive atmosphere may be formed below the flashpoint of the material, for example.
NOTE I Flashpoints of flammable liquids are not precise physical quantities, particularly where mixtures are invoived
NOTE 2 Some liquids (for example, certain halogenated hydrocarbons) do not possess a flashpoinl although they are capable of producing an explosive gas atmosphere. In these cases, Ihe equilibrium liquid temperature which corresponds to the saturated concentration at the lower explosive limit should be compared with the relevant maximum liquid temperature
e) Liquid temperature
The vapour pressure increases with temperature, thus increasing the release rate due to evaporation.
NOTE The temperature of the liquid after it has been released may be incteased, for example by a hot surfac, or by a high ambient temperature
4.4.2 Lower explosive limit (tEl.)
For a given release volume, the lower the LEL the greater will be the extent of the zone.
Experience has shown that a release of ammonia, with an LEL of 15 % by volume, will dissipate rapidly In the open air, so an explosive gas atmosphere will normally be of negligible extent.
4.4.3 VentilatIon
With increased ventilation, the extent of the zone will normally be reduced. Obstacles which impede the ventilation may increase the extent of the zone. On the other hand, some obstacles, for example. dykes, walls or ceilings. may limit the extent. A compressor shelter with a large roof-ventilator and with the sides open sufficient, to allow free passage of air through all parts of the building is considered well ventilated and should be treated as an outdoor area (i.e. medium degree and good availability).
4.4.4 RelatIve density of the gas or vapour when It Is released
If the gas or vapour is significantly lighter than air, it will tend to move upwards. If significantly heavier, it will tend to accumulate at ground level. The horizontal extent of the zone at ground level will increase with increasing relative density and the vertical extent above the source will increase with decreasing relative density.
NOTE I For practical applications, a gas or vapour which has a relative density below o,a is regarded as being lighter than air If the relativ• density is above 1,2. it is regarded as being heavier than air Between these values, both of these possibilities Should be conSidered.
NOTE 2 With gases or vapours lighter than air. en escape at low velocaty will disperse fairly rapidly upwards. the presence of a roof will, however, inevitably increase th, area of spread under lt. If the escape is at high velocity in a free Jet ihe action of the jet, although entraining al which dilutes the gas or vapour, may increase the distance Over which the gas)air mixture remains above its lower flammable limit.
NOTE 3 With gases or vapours heavier than air. an escape at low velocrty will tend to flow downward and may travel long distances over the ground before it is safely dispersed by atmosphenc i*tfusiori. Special regard therefore needs 10 be paid to the topography of any Site under consideration and also to surrounding areas WI Order to determine where gases or vapours might collect in hollows or run down inclines to lower levels If the escape is at high velocity in a free Je the el-mixing action by enlraining air may well reduce the gasiaar mixture to below its lower flammable limit in a much shorter distance then in the case of a low.veloclty escape
NOTE 4 Car, n..d to be latin when classifying ereas conteining cryogenic flammable gases such cc liquefied natural gas. Vapours emitted can be heavier than air s.f low temperatures and become lighter than air on approaching ambient temperature.