# The Evolution and Development Trends of LNG Loading and Unloading Arms
## 3 Development of Land-Used LNG Loading and Unloading Arms
### Overall Structure of the Land-Used Loading Arm
#### 3.1.1 Rotary Joints and Sealing of the Land-Used Loading Arm  
Common sizes of rotary joints for land-used LNG loading and unloading arms include DN50, DN80, DN100, etc., while those for marine loading and unloading arms are ND150, DN200, DN250, DN300, DN400, etc. There is a significant difference in size between the two, and there are many similarities and differences in terms of structure, sealing, and  
Figure 10: Schematic diagram of forces on the outer arm when it is horizontal. Figure 9: Structure diagram of spring cylinder. 1—pull rod; 2—spring-loaded positioning plates; 3—cylinder block; 4—spring; 5—spring cylinder lugs; 6—clapboard; 7—end caps. design requirements. The rotary joints of land-used loading and unloading arms are smaller in size, with a more compact internal structure, and it is easier to ensure the fitting accuracy between components. The rotary joints of marine loading and unloading arms are larger in size, capable of withstanding greater loads. At the same time, to avoid the impact of error accumulation during the manufacturing process on the sealing performance, higher machining accuracy is required [52]. In terms of sealing, both adopt a multi-layer sealing structure and use a nitrogen purging system to keep the raceway dry and prevent water vapor from freezing at low temperatures. However, marine LNG loading and unloading arms work in a bumpy, rocking, and humid marine environment for a long time, so they have higher requirements for the sealing performance, corrosion resistance, shock resistance, and structural strength of the rotary joints. The rotary joints of land-used and marine LNG loading and unloading arms have many similarities in basic structure, main functions, and technical requirements. However, due to the different working environments they face, their design characteristics also vary. Marine rotary joints focus more on shock resistance, stability, and complex sealing structures, while land-used rotary joints pay more attention to simplifying the structure and reducing costs while ensuring sealing performance. The sealing forms of domestic cryogenic rotary joints in China mainly include active sealing, secondary dynamic sealing, auxiliary sealing, main static sealing, secondary static sealing, and nitrogen sealing. Among them, the most important and also the most prone to problems is the main sealing. Regarding the problem of main sealing leakage, Shanghai Qiyao Power Technology Co., Ltd. [53] proposed a double-leak-prevention structural design. The first-layer sealing selects liquid sealing, adopting a structure of two sealing ring retaining rings sandwiching one sealing ring. The retaining rings on both sides of the sealing ring can effectively alleviate the end-face wear of the sealing ring. The second-layer sealing adopts a metal knife-edge sealing structure, which is to add a metal ring in the shape of a knife-edge on the flange and use a metal gasket to assist in sealing. When the liquid sealing between the flange and the rotating shaft fails, the metal sealing can still prevent the leakage of LNG [54]. In the past six years, there have been several LNG leakage accidents at the Jiangsu LNG tank truck station, all of which were caused by problems with the static sealing device of the rotary joint. Such cryogenic rotary joints all choose spring-energized sealing rings as the main static seal and PTFE gaskets as the secondary static seal. The spring-energized sealing ring can still maintain good sealing performance when worn or under pressure, but the maintenance cost is high in case of failure. The PTFE sealing ring has excellent chemical stability and low-temperature flexibility, but as a secondary static sealing ring, its compensation performance is insufficient and it is prone to the cold-flow phenomenon.