Factors Creating Stratification Management of LNG Rollover

Abstract Stratification or the creation of two convectively stable layers of liquids with different densities is the beginning of events leading to possible roll-over. 

The main factors are the convective mechanism whereby heat inflows into the lower layer become locked into that layer causing its temperature to rise and density to fall with time also the preferential evaporation of methane from the upper layer of LNG (or propane from stratified LPG) will lead to an increase in density with time also the heat of mixing is large and may evaporate up to 10 % of the liquid mixtures. 

The chapter discusses how stratification can be prevented, and/or removed, by adequate mixing operations. However, auto-stratification after filling is possible and needs to be countered by mixing at a later time. 

The chapter concludes with some advisory points towards anticipating roll-over, if the mixing is insufficient, or too late, to remove stratification. 

There are several ways in which LNG, a multi-component mixture, can become stratified in a storage tank. 

---