Auto-stratification

If Stratification is detected or auto-stratification is suspected, then urgent action is needed to mix the contents of each tank Allowing the stratification to continue wiU allow more thermal overfill to bmld up in the lower layer, and increase the amount of vapour produced subsequently by a mixing operation or a rollover. 

The creation of density stratification, with small but significant density discontinuities (of the order of 0.5-2.0 %) between layers, by custody management errors, or by unexpected auto-stratification. 

However, there are also auto-stratification mechanisms, and if flie above mixing is inadequate, then a follow-up mixing procedure is needed urgently to get rid of any residual stratification in every tank. 

In the next 11 years to 1982, some 40 rollover events were recorded, including four incidents ascribed to nitrogen induced auto-stratification. Since then, development of good practice has reduced the number. The last recorded near miss in LNG shipping was in 2008 on a Moss-type 125,000 m LNG carrier, when two of the five spherical tanks experienced stratification that nearly resulted in rollovers (see [2], the contents of which have been circulated as a reminder to the whole LNG industry via SIGTTO). 

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. 

There are a number of scenarios whereby auto-stratification may occur, including the following. 

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. 

During storage of LNG with, say, a composition containing 3 % nitrogen, preferential evaporation of the more volatile nitrogen with MW=28, liquid density 808 kg/m from LNG with MW 16, liquid density 430 kg/m will result in a liquid density reduction towards 420 kg/m of 2.5 %, which is more than enough for auto-stratification. 

Another source of auto-stratification arises when the boil-off gas is recondensed and returned to the same tank. Generally, the boil-off gas contains a higher proportion of the lowest MW components and condenses to a liquid with a lower density than the... 

Auto-stratification in Both Single Component Liquids and Mixtnres... 

Auto-stratification is not confined to cryogenic liquid mixtures, and can occur in a number of other ways relevant to handling LNG. 

The superheated wall boundary flow feeds into this surface layer without evaporating, and a saturated upper layer builds up above a lower layer at the initial filling temperature. As the pressure rises, the density difference between the two layers remains small, so that the auto-stratification is very stable wifli no sudden spontaneous mixing and therefore no rollover (see Chap. 7). 

Pressure variations in the vapour return lines will also produce temperature and density variations in a similar manner, If the pressure rises by more than 100 mbar, then the triggering of auto-stratification may be expected. 

If a tank is topped up with thermally subcooled liquid—a dangerous practice—then auto-stratification will take place immediately (see Fig. 6.2). 

It may not be realised that the unwetted walls above the stratified LNG will be covered with differential surface-tension-driven Marangoni films of ethane rich liquid up to the saturation temperature of ethane, and propane rich liquid up to the saturation temperature of propane. Ethane and propane rich droplets will run down the walls against the film flow and collect in the bottom of the tank, as an example of auto-stratification of a high density layer at the bottom of the tank. 

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