I think reading BS EN 1160 makes my writing ideas grows. As a follow of of this post (LNG Basics: Understanding Its Typical Properties), I want to share you what I read and browsed about Rollover in LNG Storage Tank.
Rollover in one of physical phenomena explained in BS EN 1160, other than Rapid Phase Transition (RPT) and Boiling Liquid Expanding Vapor Explosion (BLEVE).
Rollover refer to process whereby large quantities of gas can be emitted from an LNG storage tank over a short period of time. How this could happen?
In figure below, I created mind map of rollover in LNG storage tank summarized from “Rollover in LNG Storage Tank” published by GIIGNL.
Nature of LNG
As you already know, LNG composition is typically methane (CH4), ethane (C2H6), propane (C3H8), butane (C4H8), a little bit heavy hydrocarbon, and nitrogen (N2). It is stored at -160oC and at about 0.14 barg for flat bottom tank. The tank is insulated to prevent heat leak. Although it is insulated, LNG is still heated up, so that about 0.15%-kg/day LNG is turned into vapor. Light components, which are methane and nitrogen, are vaporized. The process is called weathering. Because LNG temperature increases, its composition is changing over time. Changes in LNG temperature and composition causes LNG density to change too. LNG layers will be formed, with upper layer is “lighter LNG” and bottom layer is “heavier LNG”. This phenomenon is called stratification.
Prevention of Stratification
When density of incoming LNG is lower than heel, it is recommended to use bottom filling to prevent stratification and ensure complete mixing of LNG. I think it is very logical, because when lighter LNG is stored via bottom filling, it will rise to the top of surface thus ensure complete mixing.
How to do bottom filling?
There is a kind of bottom filling device to help distribute LNG in the tank. It consists of a tube attached to tank supports and goes down vertically from the top to the bottom of tank. At the bottom, there are some slots that directs LNG to several directions to promote mixing. It is positioned at the edge near tank wall.
If incoming LNG is heavier than LNG at heel, it is recommended to do top filling. However, this method will generate excessive vapor due to flashing. To prevent this, we can reduce LNG flow rate or use top-filling device. The later method is more preferable.
Top filling devices, such as sprays or spray plates are effectively reduce boil off gas rate. It enters into the top of tank through dome chamber. When incoming LNG in contact with plate, it produces droplets of LNG that fall down the tank into the heel.