LNG storage tanks are generally cryogenic vessels that work under high vacuum and deep low temperature conditions, so the air tightness of the weld is very high. According to the conditions of the components, a reasonable selection of welding forms is one of the main contents in the design of welded structures.

　　General principles of welded structure design for LNG storage tanks:

　　1. First consider the working medium and working conditions of the joint, such as temperature, vibration, fatigue and corrosion, to ensure sufficient strength and air tightness.

　　2. For large components, the preheating of welding and the heat treatment after welding should be reduced as much as possible.

　　3. Try to minimize the deformation and stress of the welded material. For example, when welding materials of different thicknesses, try to use equal-thickness joints. Minimize the processing workload after welding.

　　4. The welding method of LNG storage tank should use advanced semi-automatic welding or automatic welding as far as possible. LG tank welding must also adopt straight, round or other regular shapes as much as possible.

　　5. Welding with strict requirements on gloss (such as the flange sealing surface processed after welding is not allowed) should be designed to avoid locations that require welding. Due to the rare defects such as pores and gaps in welding, the entire workpiece will be discarded.

　　6. ​​The connection of the welded joint shall adopt a style with the highest possible static load and fatigue strength, instead of the style shown in Figure 2.2a.

　　7. LNG storage tank avoids excessive welding concentration, reduces stress concentration and joint deformation, and finally adopts the style shown in Figure 2.2a.

　　8. When welding two materials with different coefficients of expansion, in order to avoid leakage caused by different degrees of thermal expansion and contraction at low temperatures, most of the materials with a large coefficient of linear expansion are wrapped outside.

　　9. When applied to welding in a vacuum environment, the dead corner after welding should be the surface and base structure far away from the vacuum.

　　In order to ensure the vacuum performance, the welding should be located inside the vacuum as much as possible to avoid the "dead zone" that is not conducive to vacuum exhaust. It is strictly forbidden to have a welding structure on the inside and outside at the same time, but the outer layer can be welded intermittently for positioning and strengthening.

　　Several welding structures under the vacuum environment of LNG storage tanks:

　　In the cryogenic container of LNG storage tank, in order to meet the requirements of use and inspection, many sealing structures are commonly used, which are roughly divided into four types.

　　1. Low-temperature static sealing is related to vacuum, mostly low-temperature sealing under vacuum;

　　2, dynamic seal, the seal between several moving parts;

　　3. The window of the cryogenic container is sealed;

　　4. Wire sealing.

　　The above 4 types of seals are used only at low temperatures, and some are used under low temperature vacuum. Many physical properties of the material will change greatly at low temperature, such as thermal expansion and contraction, hardening and brittleness at low temperature, etc., resulting in a large reduction in the sealing specific pressure or complete disappearance.