The pump inside the LNG tank is a submersible electric pump, which is an important equipment of the storage unit. It consists of two parts: a motor and a pump body. The motor is located above the pump body and is a three-phase asynchronous motor. There are rolling bearings on the upper and lower parts of the motor rotor. The main components of the pump body include inducers, impellers, diffusers, shells, etc. The motor shaft and the pump shaft are coaxial or connected by splines. A bottom valve is installed at the bottom of the pump well, and its main components include springs, bottom cover plates, sealing rings, etc. Under the condition of no external interference, due to the elastic force of the spring, the bottom cover plate is closed and the bottom valve is in a closed state. At this time, the function is similar to a one-way valve, allowing only the liquid inside the pump well to enter the outside of the pump well under appropriate pressure, while the liquid outside the pump well cannot enter the inside of the pump well. When the pump in the tank is installed normally, due to the gravity of the pump, the bottom cover plate is pressed downward, which will cause the bottom valve to be in a normally open state, and the pump can freely suck in liquid at this time. Compared with ordinary submersible pumps, LNG tank pumps have the following characteristics:
(1) Low temperature resistance. The fluid medium (LNG) transported by the pump inside the LNG tank is an ultra-low temperature liquid, usually around -160 ℃. Therefore, the pump itself is required to have good low-temperature resistance performance.
(2) No need for a separate explosion-proof motor. The motor and pump body are integrated and immersed in LNG. LNG enters the motor through the clearance between the balance mechanism and the bearing near the pump outlet, and then returns to the pump inlet from the top. The stator and rotor are filled with flowing LNG, which not only ensures safety but also plays a decisive role in cooling the motor.
(3) No need to move the sealing structure. The pump body and motor are completely immersed in LNG due to their unique structure, which determines that there is no need to set up corresponding dynamic sealing structures on the shaft, only the sealing of the bottom joint surface of the pump needs to be ensured.
(4) Coaxial design. The motor and impeller can be designed coaxially to eliminate eccentric vibration caused by couplings.
(5) LNG self-lubricating. The pump adopts a unique LNG self-lubricating system. At the pump outlet, the LNG pressure is high, and a small portion of LNG flows into the motor chamber through the balance drum clearance and bearing clearance. It then flows into the LNG return pipe from above the motor chamber through the bearing clearance, and re enters the pump inlet through the return pipe. This part of LNG provides lubrication for the upper and lower bearings and other relative moving parts of the motor, without the need for a specialized oil lubrication system.
(6) Unique axial force balancing mechanism. In the LNG tank pump, there is no thrust bearing, and the currently advanced axial force balancing mechanism is the EBARA patented technology impeller self balancing mechanism.
(7) Nitrogen sealing system. To prevent LNG evaporation gas from leaking along the cable and causing danger to the junction box, an independent nitrogen sealing system must be installed at the front end of the junction box.
(8) Positioning tool. For the convenience and reliability of installation, there is a specific positioning tool for the positioning roller.
(9) Lifting system. For the convenience of maintenance and inspection, specialized lifting systems are required, such as support cables and lifting cables.
