# 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  
The land-used LNG loading arm is a device used to connect LNG transport vehicles to the pipelines of the receiving terminal and transfer LNG. The loading arm consists of a riser, rotary joints, a liquid-phase arm, a gas-phase arm, pipeline supports [12], a balancing device, an electrostatic-conducting device, an emergency disconnection device, etc. It is usually fixed on the loading platform or storage tank, with a relatively fixed structural form and a stable working environment. There are two loading methods for land-used loading and unloading arms: top-loading and bottom-loading; the case is shown in Figure 7.  
The top-loading loading spout, also known as the upper-loading loading spout, is mainly used for the transfer of liquid or gas between the loading platform and the top of road or railway tank cars. It comes in two types, open-type and enclosed-type, and is connected to the tank car by inserting from the top. The bottom-loading loading spout, also known as the lower-loading loading spout, is mainly used for the transfer of liquid media between the loading skid and the tank car. It is connected to the outlet and the receiving port through a 90-degree angle flange connection, enabling quick docking and sealed loading and unloading, with a relatively high loading efficiency. Currently, bottom-loading is mostly used for LNG loading and unloading. Compared with the top-loading method, bottom-loading is more convenient to operate, eliminating the need for workers to work at heights on the trestle. The bottom-loading method can achieve static liquid loading, greatly reducing the volatilization of LNG and the generation of static electricity. Moreover, the skid-mounted system for bottom-loading has a higher degree of automation, which can significantly improve work efficiency. The case is depicted in Figure 8.  
To ensure smooth movement of the loading arm during operation, a balancer is usually used to balance the rotational torque of the outer arm. According to different balancing principles, there are seven types of balancing methods, including the spring cylinder type, air cylinder type, counterweight type, and counterweight-air cylinder type. Currently, the spring cylinder-type balancing method is widely adopted for land-based LNG loading and unloading arms. This method requires relatively little operating force. It utilizes the spring force generated by spring deformation to counteract the self-weight of the loading arm and external loads, thus achieving the balance of the loading arm. The structure diagram of the spring cylinder is shown in Figure 9.  
Figure 8: Structure diagram of land-based LNG loading arm. Figure 7: Land-based LNG loading arm [16]. (**a**) Top-loading loading arm; (**b**) bottom-loading loading arm. When the loading arm is in a horizontal position, the schematic diagram of forces on the outer arm is shown in Figure 10. At this time, the moment of gravity of the outer arm and the extension arm relative to the swivel joint is equal to the tension moment of the spring cylinder on the outer arm, that is, the following obtains:  
\[G\cdot AE\cdot\cos\theta=K\cdot I\cdot\frac{a\cdot b}{c}\cos\beta \tag{1}\]  
In order for the outer arm to remain in a horizontal state in any position, two conditions must be met: (1) \(\beta+\theta=90^{\circ}\); (2) in any position, the compression amount of the spring in the spring cylinder is equal to the distance between point B and point C. Therefore, when installing the spring cylinder, the pre-compression amount of the spring should be equal to the length of BC when the outer arm moves upward to the highest point. The sealing rings inside the rotary joint, due to factors such as wear and aging, can seriously affect the sealing performance of the rotary joint [51]. Therefore, when conducting the overall design of the loading arm and the design and selection of components, the convenience of replacing the sealing rings of the rotary joint should be taken into key consideration.