1、 Storage issues

1. Liquid stratification

LNG is a multi-component mixture, because of the change of temperature and component, the difference of liquid density, the LNG in the tank may be stratified. Generally, when the temperature difference in the vertical direction of the liquid in the tank is greater than 0.2 and the density is greater than 0.5kg/m3, it is considered that the liquid in the tank is stratified.

2. Aging phenomenon

LNG is a kind of multi-component mixture. In the storage process, the evaporation capacity of each component is different, resulting in the change of composition and density of LNG. This process is called aging.

3. Rolling phenomenon

The rolling phenomenon refers to the phenomenon that two layers of LNG with different density are rapidly mixed up and down in the tank, and a large amount of gasified gas is produced in an instant. At this time, the vaporization amount of LNG in the tank is 10-50 times of the normal natural evaporation amount, which will cause the air pressure in the tank to rise rapidly and exceed the set safety pressure, and make the tank overpressure. If it is not discharged through the safety valve in time, it may cause mechanical damage to the storage tank, resulting in economic loss and environmental pollution.

The fundamental reason for the rolling phenomenon is that the liquid density of different layers in the tank is different. The existence of layered components has an important impact on the time and severity of evaporation and rolling.

In the long-term storage of LNG tanks, the lighter components (mainly N2 and CH4) first evaporate and roll spontaneously. There was LNG left in the LNG storage tank. After a period of time (several hours or even dozens of days) when the new LNG with different filling density and temperature was filled, it suddenly turned over.

The density of LNG in the upper part is small, while that in the bottom of the tank is large. When the LNG in the tank is stratified, with the introduction of external heat, the bottom LNG temperature increases and the density decreases. The LNG in the top layer becomes heavier due to the volatilization of bog. After mass transfer, the LNG in the lower part rises to the upper part, the pressure decreases and becomes supersaturated liquid. The accumulated energy is released rapidly and a large amount of bog is produced, that is to say, rolling phenomenon.

It is worth noting that LNG stratification is the premise of causing rollover.

2、 Methods of detecting and eliminating delamination

1. Temperature monitoring

2. Density monitoring

3. Bog monitoring

Once the tank is stratified, LNG at the bottom of the tank shall be pumped out first during export. After LNG stratification, the top entry device shall be used for circulation operation to promote the mixing of LNG in the tank and avoid overturning. But at the same time, it increases the amount of evaporation gas and the cost of dealing with these increased evaporation gas.