Behaviour of gas/vapour plumes
When volatile or gaseous substances are spilled, the following processes determine the displacement and the size of the vapour/gas plume.
- Release rates
- Subsequent spreading and movement of any slick on the water surface
- Dispersal (diffusion) and movement of the subsequent gas/vapour plume
- Movement and dilution of the gas/vapour plume
Release rate
The rate of release depends on the mode of transportation and on the point of release.
- Liquid may be discharged from a tank below the surface level of the liquid in the tank
- Vapour may be discharged from a tank Above its liquid level
- Gas may be discharged from a tank Containing pressurised gas only
- Evaporation may be from a slick on the water surface. The spreading and movement of a slick on the water surface is described in section 4.2.1.
Dispersal (diffusion) of gas plumes formed
Gas plumes released into the atmosphere tend to disperse/dilute with increasing volume as they travel downwind. As the plume becomes larger, its pollutant concentration falls effectively to zero. The flash point, explosion limits and concentration of the pollutant gas determines whether it is explosive or not or whether it is toxic or not.
Movement of gas/vapour plume
The movement depends on:
- Density of the plume
- Wind speed & direction
When plume density is lower than that of air, the plume rises into the atmosphere where response other than avoidance and the issuance of warnings is impossible.
When plume density is higher than that of air, the plume remains close to the water surface and thus may be a danger to sea traffic and offshore platforms.
Plumes move in the wind direction at the wind speed.
