Multistage pump and subsea pumping arrangement

A multistage pump configured for installation on a sea ground, comprising:

• a common housing configured to withstand subsea pressure exerted at least at a depth of 100 meters;
• a pump unit arranged in the common housing;
• a drive unit arranged in the common housing,
• the common housing comprising a pump inlet and a pump outlet, the pump unit comprising a plurality of impellers configured to convey a compressible fluid from the pump inlet to the pump outlet, the pump outlet being an only opening through which the conveyed compressible fluid is capable of exiting the common housing, and a pump shaft, on which each impeller of the plurality of impellers is mounted, each said impeller is a radial or semi-axial impeller, the drive unit comprising a drive shaft configured to drive the pump shaft, and an electric motor configured to rotate the drive shaft about an axial direction;
• a seal disposed between the pump unit and the drive unit to seal around the pump shaft;
• and
• a coupling disposed in a portion of the common housing with the drive unit and configured to couple the drive shaft to the pump shaft, such that the pump shaft extends from the portion of the common housing with the drive unit through the seal and into a portion of the common housing with the pump unit,
• the pump unit configured to convey the fluid being in a dense phase at the pump outlet, and at least two impellers of the plurality of impellers have a different specific speed,
• the plurality of impellers comprising a first stage impeller and a last stage impeller, and the last stage impeller having a lower specific speed than the first stage impeller.