The principles of aging, stratification and vortex of LNG during storage, and measures to prevent the occurrence of vortex in LNG

Stratification is a unique phenomenon during LNG storage. Implement separate storage or internal stirring charging methods to make the density and temperature difference zero. Preventing stratification is a means to avoid LNG tumbling, but it is not direct and convenient. Internal stirring will also cause an increase in evaporation. Research shows that to prevent delamination and tumbling, measures such as: 1) Reduce the remaining LNG thickness to avoid a large peak evaporation rate ； 2) Choose a suitable injection method to avoid a large amount of heavy liquid at the bottom ； 3) Monitor the degree difference and thickness of the injected LNG and remaining LNG, determine the tumbling time, and start the circulation pump to eliminate tumbling. LNG stratification is a prerequisite for tumbling. Therefore, how to detect LNG fraction during actual operation is very important. Generally, the following methods can be used to detect LNG stratification:: (1) Install a temperature measuring device every 1.5m vertically on the liquid receiving part of the storage tank for temperature monitoring. ； Generally, when the temperature difference is as high as about -14℃, rolling phenomenon may occur. At this time, in order to prevent rolling, the storage tank should be cooled. (2) Install a density measuring device every 1.5m vertically on the liquid receiving part of the tank for density monitoring. ； Generally, when the density reaches 5kg/m3, attention should be paid and a cyclic method should be used to eliminate the density difference. (3) Monitor the OG evaporation rate. After stratification, the BOG evaporation rate will decrease significantly, and changes in atmospheric pressure will affect the monitoring of LNG generation speed. Absolute pressure can be used to monitor or record changes in atmospheric pressure. Once LNG rollover occurs, it will be difficult for the large amount of gasified natural gas to be discharged through the safety valve in time. In order to prevent accidents, a release flare, recovery and compression system, etc. were added to prevent LNG evaporation and tumbling from causing overpressure in the storage tank. Generally, the above measures are adopted at the same time, and the variation range of the storage tank components is controlled as much as possible to prevent the tumbling phenomenon of LNG. Generally, the density difference of LNG poured into the storage tank should be controlled within 15kg/m3. It can be seen that density monitoring is crucial.

This post was last edited by tracily on 2012-9-7 08:23. I would like to ask someone upstairs why the BOG evaporation rate decreases when stratification occurs?

My understanding is that after stratification occurs, the gases that volatilized first have all volatilized, and the remaining gases are relatively difficult to volatilize, so the BOG evaporation rate decreases.

It's just stratification. If the density changes, it shouldn't affect the BOG evaporation, or it may not necessarily decrease the BOG evaporation, right?

In operations without feeding and discharging materials, stratification is mainly caused by the heat absorption of the tank causing different components in the medium in the tank to evaporate one after another. If it is serious, tumbling will occur, resulting in the generation of BOG. ​​However, after the tumbling is completed, the volatile components are discharged, so the remaining ones are relatively difficult to volatilize. At this time, the amount of BOG is relatively reduced.