Due to the differences in natural gas composition, especially from different gas fields, the density of LNG will also vary. If LNG is stored in the same tank and pump with different densities, it is easy to cause liquid stratification. How to prevent it?
Liquids with higher density accumulate at the bottom of the pump in the tank, while liquids with lower density are at the top. The bottom liquid is also subjected to the gravity of the top liquid, resulting in a higher pressure than the top liquid and a corresponding increase in evaporation temperature. Compared to the evaporation temperature corresponding to this pressure, the temperature of the bottom liquid increases. Due to the increase in temperature, the density will decrease. When the density of the bottom liquid is lower than that of the top liquid, the layered equilibrium will be disrupted, forming a so-called "roll over" or "vortex". Mixing will cause a large amount of evaporation, resulting in a significant increase in evaporation rate. If the exhaust device cannot discharge a large amount of gas in time, the pump inside the tank will exceed the designed working pressure, which is very unfavorable for safe storage.
When filling a fixed LNG tank with a large capacity pump, in order to prevent liquid stratification in the tank pump, it is first required that the thermal properties of the filled liquid be close to the original liquid in the tank pump (composition, temperature, and density). In addition, it is usually required that the inlet system of the tank pump has both a top inlet pipe and a bottom inlet pipe. When there is a difference in thermal properties between the filled liquid and the original liquid in the pump inside the tank, the liquid is uniformly transported through different parts to reduce the possibility of liquid stratification. The specific method is:
a) Separate storage of LNG from different origins and sources can avoid LNG stratification caused by density differences.
b) Choosing the correct filling method based on the difference in density between the LNG to be stored and the existing LNG in the storage tank can effectively prevent stratification. The selection of filling method should generally follow the following principles:
i) When the density is similar, it is generally filled at the bottom.
When filling light LNG into heavy LNG storage tanks, it is advisable to fill the bottom.
When filling heavy LNG into light LNG storage tanks, it is advisable to top fill.
c) By using a mixing nozzle and porous tube for filling, the newly filled LNG can be fully mixed with the existing LNG, thereby avoiding stratification.
