Basic Description of Rollover and a Stratification Event

The convective instability, reached by the stratification of two multi-component cryogenic liquid layers with different densities, was originally believed to cause the stratified layers to reverse position by rolling over. This is not correct 

What happens in practice is that the densities of the two layers approach each other, for different reasons, over a period of time. As the layers approach density equilibration, a phenomenon called double-diffusive convection, involving both thermal and compositional equilibration at the same time, takes over and cause the layers to spontaneously mix. 

This mixing, or so-called rollovef , is uncontrolled and may cause a rapid rise in evaporation rate, and tank pressure, associated with the uncontrolled release of the thermal overfill energy from the lower layer, and the heat of compositional mixing of the two layers, within the tank. 

If the safety and vent valves on the tank are under-sized, they will not be able to release the large volume of evaporated gas, the tank pressure will rise and may lead to rupture of the tank. 

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