The basic mechanism is simple. The operating consequences are not.
1. The rotor does not sit in the center
The rotor axis is offset from the casing. When the rotor turns, the seal liquid forms a ring around the casing while the available volume between the blades and liquid ring changes around the rotation.
Where the volume expands, gas enters. Where the volume contracts, the trapped gas is compressed and directed to discharge.
2. The seal liquid is part of the process
The liquid provides sealing between compression spaces and carries away heat. Its vapor pressure, temperature, cleanliness and compatibility with the process gas matter.
Condensable vapor may condense as it is cooled, changing the gas volume handled by the pump. That is one reason a dry-air nameplate comparison may not tell the complete process story.
3. The pump belongs to a system
Discharge separation, seal-liquid supply or recirculation, heat exchange, piping losses and instrumentation all influence what operators see. A pump can be mechanically sound and still miss the process need when the system conditions move away from the design basis.
Technical references
General industrial guidance only. Qualified personnel should review equipment design, process conditions and site risk before changes.
