Threat of gas blasts minimised
15 Jan 2000
There has been speculation that the infamous explosions at Flixborough and La Spieza were a consequence of rollover, a phenomenon associated with the storage of liquid natural gas (LNG).
Will Zimmermann is leading a CASE award project, funded by the EPSRC and the Health and Safety Executive, to develop a predictive tool to inform operators whether conditions are suitable for transfer of a new LNG cargo.
The problem can arise in the following way: if the new cargo contains a higher percentage of relatively heavy isobutane than that of the tank (the `heel') and it is fed into the tank's base, the lighter fraction will overlie the heavier.
The external environment heats the cool internal layers. The lower fluid rises at the tank walls, but if the density difference between cargo and heel is large enough, the lower liquid must recirculate, forming a stable stratification. Here, stable does not mean safe.
As more heat is absorbed by the system, the densities of the layers will eventually equalise and rapid mixing occurs, with a sudden burst of pressure - the rollover effect.
Reassuringly, Zimmermann added: `My gut feeling is that there are actually very few dangerous industrial situations.'
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