Mechanisms Due to Density Differences

Since stratification is the necessary precursor of a rollover event, it is important to consider the various ways in which stratification can occur, can be anticipated, and can be prevented. 

We have already noted that density differences between LNG or LPG layers can occur from combinations of temperature differences and composition differences. Also, that wall boundary layer flow cannot penetrate a density difference greater than about 0.5 % between the layers, in which case the heated wall boundary layer flow is trapped in the bottom layer. 

Likewise at constant temperature, a 1 % density difference corresponds to a 5 % change in methane concentration in LNG as a binary mixture of methane and ethane, and a 15 % change in propane concentration in LPG. 

These figures suggest that auto-stratification can be expected to be a common occurrence, since the necessary density changes are so small. 

There are several examples of auto-stratification particular to cryogenic mixtures of LNG. 

---