Pool evaporation

Pool evaporation

The formation of a pool results from the instantaneous or continuous release of liquid or rain out from pressurised liquefied gas release. When a liquid spills, it forms a pool on land or water. On land, the pool spread is influenced by surface roughens and it is often confined by a bund. On water, the pool is unconfined and spreads more easily (Bosch 2005).

Immediately after a liquid spill, the pool receives heat from the surroundings and begins to vaporise. Depending on the type of pool, a dominating heat transfer mechanism affects the evaporation rate of the pool.

Heat is transferred into the pool from the internal energy of the source (in the case of continuous release of the inflowing liquid), by convection (e.g., air, water), radiation (sun), and conduction (ground). Not only does the atmosphere influence the evaporation rate, but the surface on which the pool forms also plays a significant role. Different subsoil types have varying thermal conductivity and diffusivity coefficients. Surface roughness not only presents resistance for pool spread but also increases the surface for heat transfer from the soil. It is important to note that water provides not only conductive but, due to its motion, also convective heat transport.

Material storage or process conditions significantly influence how the pool behaves. In general, we distinguish between non-boiling pools and boiling pools.

Non-boiling pool

A non-boiling pool is associated with liquid stored and released at a temperature lower than its boiling point (typically ambient temperature), such as gasoline or acrolein. In case of release, the escaping material creates a pool. The material from the pool subsequently evaporates and creates a vapour cloud that can be flammable or toxic. Because the temperature of the pool is at a very similar temperature than the temperature of the subsoil it spreads on, evaporation is highly influenced by heat transfer through convection due to wind speed​ and solar heat radiation (and not by heat transfer from the released material and the subsoil as the temperature gradient is not very big).

Boiling pool

A boiling pool is formed when liquid is stored under pressure at a temperature higher than its boiling point, such as LNG or other substances stored at cryogenic conditions. In the event of a release of liquefied gas, the liquid phase flashes, creating a two-phase cloud. Heavier liquid droplets rain out, creating a pool that evaporates. Because the temperature of the pool is similar to the normal boiling point temperature of the material (the normal boiling point of methane is -161^oC), evaporation is highly influenced by heat transfer between the subsoil (which is at ambient temperature) and the pool (cryogenic temperature), and by the subsoil type.

Shape of the pool

A flammable pool is not necessarily static; it may spread or contract, with additional material potentially coming from a leak. The dimensions of the pool are usually dictated by surrounding constructions, such as bunds, preventing the pool from spreading. On a flat, unbounded surface, pool spread is affected by the roughness of the surface (Bosch, 2005).

Conclusion

Understanding pool evaporation is crucial for assessing the risks associated with liquid spills in industrial settings. The evaporation process is influenced by heat transfer mechanisms, the type of surface on which the pool forms, and the storage conditions of the liquid. By analysing these factors, we can better predict the behaviour of hazardous pools and implement effective safety measures.

References

Bosch, C. v. (2005). Methods for the calculation of physical effects 'Yellow book' CPR 14E. The Hague: Ministerie van Verkeer en Waterstaat.