OIL AND GAS
LNG pumps: key considerations By Michelle Goldsmith, Contributing Editor, Pump Industry Magazine
T
o safely, efficiently and economically transport natural gas long distances, it is converted to liquified natural gas (LNG), which reduces it to around 1/600th of its original volume. Keeping LNG in its liquid form requires cryogenic conditions of at least -162ºC to be maintained throughout the transportation process. The pumps that transport LNG, such as those that move it into, around, and out of the storage tanks on LNG tankers, must therefore operate at cryogenic temperatures. Due to the significant safety and economic consequences should equipment fail or leak LNG, the pumps used must be highly reliable, durable and constructed of compatible materials.
Pumps for LNG service
For LNG applications, specialised submerged motordriven cryogenic pumps are used. Some of the benefits of submerged operation include that, as a cryogenic fluid, LNG is an excellent electrical insulator, and the motor is completely isolated from oxygen in the atmosphere, eliminating the risk of fire or explosion. This design also removes the need for rotating seals, which can be problematic and introduce a potential point of failure. Submerged LNG pumps are constructed as a single unit, with the pump mounted on the same shaft as the motor, to be submerged in the LNG. Some of the cryogenic LNG fluid moves through the gap between the pump’s rotor and stator to cool the motor. There are three main types used – fixed, vessel-mounted and retractable – with various adaptations available to suit the needs of a specific project and application. For applications where a LNG pump is used in a piping system, it is mounted in its own suction vessel. The suction vessel takes the loads of the piping system and other structural elements, reducing the forces affecting the pump itself. This design eliminates the need for alignment, as the pump bolts to the headplate of the suction vessel. Additionally, the vessel and liquid dampen the noise from the pump, and the pump can remain in the liquid, ready to operate whenever needed. A retractable in-tank pump design allows the pump to be removed from the storage tank for maintenance while the tank itself continues to operate. These pumps are arranged in vertical pump columns, with a foot valve at the bottom, which prevents LNG from entering the column when the pump is removed. The pump can be raised or lowered to open or close the foot valve without removing the headplate at the top of the column. The operators purge the column with nitrogen, and it can be safely removed at the top of the column. The only penetration from the pump to the outside atmosphere is the electrical leads to the motor.
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pump industry | Winter 2021 | Issue 36
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