Why do regenerative pumps need liquid to seal their close clearances?

Regenerative pumps, also known as turbine or peripheral pumps, feature an impeller with vanes that rotate within a ringlike channel in the pump casing. Because these pumps are designed with very close clearances between the impeller and the casing, they require the presence of liquid to effectively seal these gaps.

This liquid seal is essential when the pump is used to move gases or liquids containing vapors. Without sufficient liquid to fill these tight spaces, the pressure differential necessary for the pump to function would be lost, preventing the unit from operating correctly. Due to these precise internal tolerances, regenerative pumps are unsuitable for handling fluids that contain solid particles, as such debris would interfere with the close clearances.1