3.63 Cavitation effect pump performance
Cavitation effect pump performance
Cavitation effect pump performance by reducing hydraulic efficiency and mechanical wear. Cavitation, if allowed to continue for prolonged periods of time, can cause mechanical damage to the pump. The hydraulic performance is also reduced by the onset of cavitation. The performance is affected in different ways depending upon the type of centrifugal pump.
Figure 3.63a Cavitation, in radial pump (specific speed nq ≈ 20).
The full lines shown in figure 3.63a represent the pump suction head and efficiency for cavitation-free operation. The pump’s cavitation sensitivity in the form of NPSHr is also shown. The pump is a radial type with low specific speed (nq=20). The pump is intended for a system having a certain static suction head. When the volume flow increases, the NPSHr also increases while pmin decreases. Eventually the limits of cavitation-free operation are reached and the pump performance declines rapidly (dashed lines). The rapid decline in performance is associated with the narrow blade channels which are characteristic of pumps having low specific speed. The complete length of the front edges of the blades are at approximately the same diameter and are subjected to the same velocity. With the onset of cavitation it requires only a slight increase in volume flow to completely fill the blade channels with vapour. The pump’s delivery head can thus be reduced to zero.
The relationship for axial pumps is different. Here cavitation occurs on the suction side of the profiles at the tips of the blades where the relative velocity is greatest (Δp greatest). Despite cavitation at the tips of the blades there still remains a large usable amount of blade channel, figure 3.63b. The decline in performance is therefore a more gradual process in the case of axial pumps (propeller pumps).
Figure 3.63b Cavitation, in axial pump (specific speed nq ≈ 200).