Pumps for LNG service

For LNG applications, specialised submerged motor-driven 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.

Considerations for LNG pumps

Cryogenic LNG pumps incorporate unique features to meet the requirements of their specialised application. Some of the important requirements and considerations for pumping LNG include:

• Basic type – fixed, retractable, or vessel-mounted

• Construction materials

• Cooldown and warmup considerations

• Hydraulic considerations (e.g. operation at low NPSH)

• Handling considerations for pump size and weight

• Thrust equalising

• Bearing configuration

• Junction box assembly and electrical penetrations

• Vibration monitoring

• Adequate testing at the cryogenic temperatures and the power levels expected in service