Page 25 - Chemical Technology

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Chemical Technology • January 2013
solids handling
hardness, generated predominately erosion of the
leading edges of the impeller blade.
• The higher the hardness of the blade material, the
lower the wear rate of the blade.
• The wear rate of the leading edge was not a function
of pitch angle.
• Sheet erosion of the blades exhibits a maximum ero-
sion rate between 20° and 45°.
Zheng6 and others studied the erosion-corrosion syn-
ergistic effect in an acidic slurry. The slurry was 10% by
weight H2SO4 and 15% by weight –60 mesh (<0,251 mm)
corundum sand. Their apparatus was a rotating disk with
four specimen holders on its edge. They determined the
rate of erosion by making electrochemical measurements
during rotation. All of their studies were done under
turbulent flow conditions. Erosion rate velocity exponents
ranged from 1,9 to 4,0. A model was proposed and used
which divided the overall erosion rate into an erosion rate
via corrosion, an erosion rate via erosion, and an erosion
rate due to synergism. The synergism rate was very large
and varied between 32 and 99% of the total. The percent
contributed by synergism diminished as the alloy became
more statically corrosion-resistant.
Amelyushkin and Agafonov7 studied the erosion of
cogeneration steam turbine blades caused by water drop-
lets. If kinetic energy is high enough, even water droplets
can cause erosion. They found that the toroidal and near
root vortices were very intense and caused enhanced
wear of the rotor blades. Also they found that they were
able to eliminate erosion by making the water droplets
small enough. It is expected that these effects are
related to grain size. In ductile erosion, plastic deforma-
tion may occur first, before metal is removed. If erosion is
due to inter-granular grain fracture, then if particles are
significantly smaller than the metal’s grain size, erosion
should be minimized. As ductile alloy grain sizes are in
the order of 20 μm, particles smaller than this should
have little erosive effect.
Khalid and Sapuan8 studied wear for a centrifugal
pump impeller in a slurry application. Weight and diameter
losses were very nearly linear with time over 480 hours of
operation. Blade height and depth loss did exhibit some
non-linearity but were modelled as linear. Typical of rotat-
ing devices in slurries, more material was lost near the
periphery of the impeller than in the centre because linear
velocities increase with radial distance from the shaft.
Lopez 9 and others studied the effect of corrosion-
erosion at relatively high velocities on 304 and 420 stain-
less steel. Velocities ranged from 4,5 to 8,5 m/s. Such
velocities are not common in mixing equipment except in
high shear devices. They used a rotating disc device with
erosion samples attached to the periphery of the disc.
The aqueous liquid for the slurry was composed of 70%
by weight H2SO4 and 3,5% NaCl. The slurry solids were
30% by weight SiO2 particles with a mean diameter about
0,25 mm. They found that high velocity impacts were
beneficial. The combined action of erosive and corrosive
mechanisms did not lead to a significant increase in
mass loss if compared to corrosion tests. They suggested
that pores, small cracks and fresh pits can be covered
by the prows and lips that are formed as a consequence
of the wedge action of round particles which produces a
smoother, uniformly corroded surface. Thus even though
the transport mechanisms which remove corrosion
products are greater due to higher velocities, the surface
area exposed is smaller.
Fasano and Corpstein10 studied slurry wear through
multilayer paint modelling. The paint layers are in the
order of 0,0381 mm thick. The three layers of paint used
had an overall thickness of approximately 0,114 mm.
This is only about 7% of the blade thickness and did not
change the fluid hydraulics over the blade. Erosion was
studied using 8,3 inch diameter scaled down axial flow
mixing impellers in a sand water slurry.
These studies pointed out the strong effects that blade-
shedding vortices could have on erosive wear. Impellers, such
as the four bladed-pitched impeller that generated stronger
vortices, suffered the highest degree of localized erosion.
The effects of these vortices can completely wear through an
impeller blade, leaving holes where the vortices contacted
the surface. High efficiency impeller blades created signifi-
cantly smaller vortices and as a consequence exhibited much
lower localized wear. Vortex erosion can be severe and occurs
on the backside (low pressure side) of the impeller blade.
Comparisons of the backside wear pattern for a Chemineer
HE-3 impeller and a standard generic 45° pitched four-blad-
ed impeller are shown in Figures 1 and 2. The impellers were
each 211 mm diameter and operated at 870 rpm in a 10%
by weight sand slurry in water. The weight mean particle size
of the sand was 360 μm. These backside erosion patterns
were made after only 30 minutes of operation and it is obvi-
ous that the erosion on the backside of the 45° four-bladed
pitched impeller was much more severe than the erosion for
the Chemineer HE-3 high efficiency impeller.
Wu 11 and others were also successful in using
this technique to study five different style radial flow
impellers and a low attack angle (~15°) (6-bladed)
pitched impeller. As expected, the hydraulically more
efficient six bladed-pitched impeller experienced the
least erosion.
Figure 2: Backside of HE-3 high eff. impeller
Figure 1: Backside of pitched blade impeller