Brochure

1919
Primary flow
Achieving an optimal balance between leakage and friction is an essential
goal when designing an impeller seal. A small gap limits the leak flow but
increases the friction and risk of drag and noise. A small gap also increases
requirements to machining precision and assembling resulting in higher
production costs. To achieve optimal balance between leakage and friction,
the pump type and size must be taken into consideration.
1.2.5 Cavities and axial bearing
The volume of the cavities depends on the design of the impeller and the
pump housing, and they aect the flow around the impeller and the pump’s
ability to handle sand and air.
The impeller rotation creates two types of flows in the cavities: Primary
flows and secondary flows. Primary flows are vorticies rotating with the
impeller in the cavities above and below the impeller, see figure 1.14.
Secondary flows are substantially weaker than the primary flows.
Primary and secondary flows influence the pressure distribution on the
outside of the impeller hub and shroud aecting the axial thrust. The axial
thrust is the sum of all forces in the axial direction arising due to the pres-
sure condition in the pump. The main force contribution comes from the
rise in pressure caused by the impeller. The impeller eye is aected by the
inlet pressure while the outer surfaces of the hub and shroud are aected
by the outlet pressure, see figure 1.15. The end of the shaft is exposed to the
atmospheric pressure while the other end is aected by the system pres-
sure. The pressure is increasing from the center of the shaft and outwards.
Figure 1.14: Primary and secondary flows
in the cavities.
Cavity above impeller Cavity below impeller
Secondary flow