Impeller erosion damage

Q  -  One of our 1850 KW slurry pumps is fitted with a mixed flow impeller. The
pump is handling an abrasive mixture of water and sand. At the point where the
blade is fixed to the outer shroud (opposite the point where the blade is fixed to
the hub of the impeller) the blades experienced severe (fast progressing) wear.
The blades are quite blunt at the tip so my guess is that because the flow cannot
follow the blade, a sort of spiral vortex forms at the tip of the blades that causes
the wear.

Has anyone ever had similar experience? Is there a way to modify the existing
impeller to reduce its wear?

A  -  This appears to be a classical example of erosion/corrosion damage. The
presence of sand particles is definitely the main factor and, in cases like this,
velocity is a major aggravating factor (that is why slurry pumps should be run at
low RPM.) This explains the severe erosion/corrosion damage at the outer tip
diameter of the impeller blades - this is the point of maximum peripheral velocity
at the inlet side of the impeller. If the pump is operating at off-peak capacity, it is
probable that suction recirculation may also be a contributing factor and the area
of the damage is typically where suction recirculation is more pronounced.
Compared to radial flow, mixed-flow impeller is more susceptible to suction
recirculation because of its larger eye diameter/maximum diameter ratio. The
blunt tip of the inlet blades does not help either because it causes a greater
amount of flow disturbance or entrance shock. The sharp corner in a
square-shaped blade tip is also a stress-concentration point. If possible slow
down the pump but this is probably not an acceptable option depending on the
service. You can consider using a more abrasion-resistant impeller material such
as CD4MCu, or other duplex stainless steel. There are a number of available
technologies - diffusion type coating and plasma spray coating, among them - to
coat the blade inlet tips with hard overlay material to make the blades more
erosion resistant. It also helps to grind a smooth radius at the blade tips and
remove any sharp edge (profile the blade tip similar to that of airplane wing.)

R: 1109-IMER
C: trouble-shooting
F: impeller erosion

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