# The Evolution and Development Trends of LNG Loading and Unloading Arms
## 2 Development of Marine LNG Loading and Unloading Arms
### Overall Structure of the Marine Loading Arm
#### 2.1.1 Rotary Joints and Sealing of the Marine Loading Arm  
LNG leakage is the main source of danger during the LNG transportation process. Once LNG leaks, it will immediately evaporate into gas and is extremely likely to explode once it encounters a fire source. Moreover, LNG is a cryogenic liquid at \(-162\)\({}^{\circ}\)C. Contact with the skin can cause severe frostbite, posing a great threat to the safety of surrounding workers. Whether in the early design stage or the later production and assembly process, preventing the leakage of cryogenic loading and unloading arms is of particular importance [18]. During the production and R&D process, there are many possible causes of leakage, including unqualified welding quality, unqualified flange assembly quality, unreasonable rotary joint design, sub-standard quality of standard parts such as valves, etc. Among them, the rotary joint is a common factor leading to the leakage of cryogenic loading and unloading arms. The cryogenic rotary joint is the \"heart\" of the LNG loading arm and a key component for the long-term and stable operation of the loading arm [19]. The cryogenic rotary joint consists of rotating parts and sealing parts, including sealing flanges, gaskets, inner races, outer races, balls, etc. [20]. Its typical structure is shown in Figure 4.  
The sealing of the rotary joint includes two parts: dynamic sealing and static sealing. The dynamic sealing side is the rotating connection surface, which is mainly used to prevent the medium in the process pipeline from leaking into the ball chamber. During the rotation of the rotary joint, the seal and the sealing surface will produce corresponding friction. The static sealing side is the static connection surface, which is used for the sealing of the process pipeline medium and the isolation sealing of the ball chamber from the outside atmosphere [21]. [PERSON] et al. [22] conducted a rigorous assessment of the main seal failure in rotary joints using the RCA modeling approach. They found that the residual LNG  
Figure 3: Three balancing types of marine loading and unloading arms. (**a**) Full-balanced type; (**b**) rotary-balance type; (**c**) double-balance type. underwent explosive evaporation after being purged with nitrogen, which subsequently pushed the seal out of its groove. In addition, the sealing device of the rotary joint in the world mostly adopts a double sealing ring structure. The main seal is mainly responsible for preventing the fluid from leaking between the rotating part and the fixed part of the rotary joint. The secondary seal is used as a backup or auxiliary seal for the main seal, which can play a sealing role when the main seal leaks and ensure the safe operation of the joint. For example, the double-sealed rotary joint of Niigata Company uses low-temperature-resistant stainless steel as the material. The main seal structure is a series arrangement of double polytetrafluoroethylene seals, and the secondary seal chooses a water seal to prevent the ball bearing from being damaged by water or dust from the outside world. The extremely low-temperature characteristic of LNG will cause the water vapor accumulated in the rotary joint to freeze, thereby damaging the internal raceway. In order to solve this problem, most of the existing rotary joints use the cavity formed between the bushing and the inner ring as the heat insulation cavity of the rotary joint [23]. Although this structural design solves the problem of raceway damage, not only does the structure of the rotary joint become more complex, but the sealing of the heat insulation cavity also becomes a difficult problem. Against this background, [PERSON] [24] and others adopted a double sealing ring structure and provided an annular heat insulation cavity inside the inner ring to reduce the traditional bushing structure; the case is depicted in Figure 5. The active sealing ring is pre-compressed to compensate for the reduced sealing effect of the sealing material due to low-temperature shrinkage and wear.