The profile of the impeller vanes gives an indication of the average flow relative to best efficiency flow. Even wear across the leading edge of the vane generally means that the pump is operating close to the best efficiency point. (Figure 1)
Figure 1 - Even Vane wear
Operation well below best efficiency can cause recirculation in the eye of the impeller. This is evidenced by vortices which remove material from the front of the vane preferentially. These can sometimes wear through the front shroud entirely. (Figure 2 and 3)
Figure 2 - Low %BEP operation
Figure 3 - Front shroud holed due to low flow
Operation past BEP can cause high inlet velocities and increase the impact energy of solid particles as they hit the impeller surface. However, large particles can also possess high energy due to their mass. Therefore, impact wear on the back shroud can either be due to operation past BEP or to coarse solids. (Figure 4)
Figure 4 - Impeller wear due to coarse solids
The two main mechanisms for wear of hard iron parts are impact and sliding abrasion. Sliding bed is defined by a smooth, polished surface (figure 5). Sometimes the surface develops a distinctive pattern like sand dunes (Wilson, 2006) It has been suggested that the wavelength of these dunes correlates with the particle size of the slurry (Walker, 2016)
As the particle size increases the coarse particles push through this bed and cause damage by impact. This is a much more damaging mechanism, and impact wear will result in much faster material loss (figure 6). The surface will not be as smooth and may develop a frosted appearance. (Wilson, 2006)
Figure 5 - Fine particle wear
Figure 6 - Frosting by particle impact
Parts which are exposed to corrosive liquids and abrasive solids can suffer from extreme rates of material loss. The most obvious feature of a badly corroded wear part is the colour, with the Martensite matrix turning a reddish brown colour as it is oxidized (Figure 7).
This also weakens the matrix that supports the hard carbides. These are then easily knocked out by the abrasive particles causing rapid wear.
Fortunately, it is often possible to improve the wear life of these parts if the chemistry of the melt is altered by adding more alloying elements such as Chromium or Molybdenum.
Figure 7 - Wear and corrosion
At Metso we believe that every detail matters, the parts removed from your pump tell a story about the operating conditions. With a little experience, it is possible to diagnose the factors that cause failure. This gives you an advantage should you wish to trial new materials or geometry. The key is to keep good records and to discuss your concerns with the OEM supplier of your pumping equipment.
Walker, C. I. (2016). Slurry pump selection based on particle size. Hydrotransport 20.
Wilson, A. S. (2006). Slurry Transport using centrifugal pumps. New York: Springer.
Director of Sales, Pumps Business Area