Clearance gap cavitation

The static pressure differential up- and downstream of clearance gaps – especially controlled gap seals such as the impeller clearance gap (see Clearance gap width) between the pump casing and the vane tips of open axial and mixed flow impellers (not fitted with outer shroud) and the clearances at the suction eye of closed radial and mixed flow impellers – may, in combination with the sharp edges at the clearance gap, result in extremely high local flow velocities. As a consequence, a correspondingly low static pressure develops in the clearance gap which may decrease as far as the vapour pressure of the fluid, even if the NPSHa (NPSH of the system) is sufficient. Clearance gap cavitation results from partial evaporation of the fluid in the clearance gap.

In the region of lower flow velocity downstream of the clearance gap, a portion of the vapour bubbles collapses (implodes), a phenomenon which leads to cavitation erosion, particularly at the pump casing in the impeller clearance area.

Another portion of the vapour bubbles is entrained by the fluid into the main flow where they may act as cavitation nuclei and contribute to incipient cavitation.