What is the Rollover Effect of LNG?
LNG “rollover” refers to the rapid release of LNG vapours from a storage tank caused by stratification. The potential for rollover arises when two separate layers of different densities (due to different LNG compositions) exist in a tank. In the top layer, liquid warms up due to heat leakage into the tank, rises up to the surface, where it evaporates. Thus light gases are preferentially evaporated and the liquid in the upper layer becomes denser. This phenomenon is called “weathering”. In the bottom layer, the warmed liquid rises to the interface by free convection but does not evaporate due to the hydrostatic head exerted by the top layer. Thus the lower layer becomes warmer and less dense. As the density of two layers approach each other, the two layers mix rapidly, and the lower layer which has been superheated gives off large amount of vapour as it rises to the surface of the tank.
The main hazard arising out of a rollover accident is the rapid release of large amounts of vapour leading to potential over-pressurization of the tank. It is also possible that the tank relief system may not be able to handle the rapid boil-off rates, and as a result the storage tank will fail leading to the rapid release of large amounts of LNG to the atmosphere.
Also, it is important to emphasise the, at some point, the excess pressure can result in cracks or other structural failures in the tank.
To prevent stratification and rollover, operators unloading an LNG ship measure the density and temp of the cargo and, if necessary, adjust their loading or unloading procedures accordingly.
Source: https://lnkd.in/dcyQYB8p
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