Release rate of gas or vapour
The greater the release rate the larger the extent of the zone. The release rate depends itself on other parameters, namely:
- Geometry of the source of release
This is related to the physical characteristics of the source of release, for example an open surface, leaking flange, etc. - Release velocity
For a given source of release, the release rate increases with the release velocity. In the case of a product contained within process equipment, the release velocity is related to the process pressure and the geometry of the source of release. The size of a cloud of flammable gas or vapour is determined by the rate of flammable vapour release and the rate of dispersion. Gas and vapour flowing from a leak at high velocity will develop a cone-shaped jet which will entrain air and be self-diluting. The extent of the explosive atmosphere will be almost independent of wind velocity. If the release is at low velocity or if its velocity is destroyed by impingement on a solid object, it will be carried by the wind and its dilution and extent will depend on wind velocity. - Concentration
The release rate increases with the concentration of flammable vapour or gas in the released mixture. - Volatility of a flammable liquid
- Flashpoints of flammable liquids are not precise physical quantities, particularly where mixtures are involved.
- Some liquids (for example certain halogenated hydrocarbons) do not possess a flashpoint although they are capable of producing an explosive gas atmosphere. In these cases, the equilibrium liquid temperature which corresponds to the saturated concentration at the lower explosive limit should be compared with the relevant maximum liquid temperature.
This is related principally to the vapour pressure, and the heat of vaporization. If the vapour pressure is not known, the boiling point and flashpoint can be used as a guide. An explosive 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. - 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 increased, for example, by a hot surface or by a high ambient temperature.
Lower explosive limit (LEL)
For a given release volume, the lower the LEL the greater will be the extent of the zone.
Ventilation
With increased ventilation, the extent of the zone will 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.
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.
NOTES
- For practical applications, a gas or vapour which has a relative density below 0,8 is regarded as being lighter than air. If the relative density is above 1,2 it is regarded as being heavier than air. Between these values, both of these possibilities should be considered.
- Experience has shown that ammonia is hard to ignite and a gas release will dissipate rapidly in the open air, so any explosive gas atmosphere will be of negligible extent.
Other parameters to be considered
- Climatic conditions.
- Topography