Page 27 - Chemical Technology

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Chemical Technology • January 2013
impeller blades are almost always double wrapped. The
most popular elastomeric coverings are: natural rubber,
neoprene, butyl, chlorobutyl, and hypalon. Improperly ap-
plied linings on high-efficiency impellers that significantly
change the profile of the blade can cause increased
erosion problems. Typically linings are double layered on
the leading, trailing and outside edges of impeller blades.
These linings must be adequately feathered such that
the transition from the double layer to the single layer is
smooth to avoid generation of additional vortices.
Both thermoplastic and thermoset polymers do not
have the ability to restore back to the particle most
of the kinetic energy and are generally not as good in
mixing slurry service. Hercules 1900 UHMWPE, touted
as being a very abrasion-resistant polymer, was tested
by the authors against Polymeric Protective Coatings'
elastomers 2001-B (natural rubber Durometer A 30-40)
and 1054-B (chlorobutyl rubber Durometer A 35-45).
The testing procedure was identical to that specified in
the Hercules 1900 UHMWPE bulletin. A 50% sand-water
slurry was used with a sand weight mean particle size of
53 μm. The specimen tip speed was 2,22 m/s. Weight
loss was determined at various time intervals over an 8
hour period. The weight loss versus time was found to
be linear with R2 values for all three falling between 0,93
and 0,94. The rates of erosion were:
Material Rate of Weight Loss, g/hr
Hercules 1900 UHMWPE 0.0975
Polymeric Protective Coatings
2001-B (natural rubber) 0.0104
Polymeric Protective Coatings
1054-B (chlorobutyl) 0.0124
As can be observed, the rate of weight loss for the
thermoplastic polymer is 7 to 10 times greater than that
for the elastomers tested.
Dickey and Fasano have provided a general reference
on materials' selection considerations.16
Impeller selection
There are many different impeller styles available to the
designer. Selecting the correct impeller can often make
a difference in impeller life of two or three times. Erosion
of impeller blades can depend heavily on the flow regime,
with flow regime being determined by the impeller Reyn-
olds number.
N NDRe
ρ
μ
2
where ρ = density
N = impeller rotational speed
D = impeller diameter
μ = viscosity
NRe = Reynolds Number, dimensionless
Impellers in turbulent flow create shedding vortices
that attach themselves to the back of impeller blades.
There are a number of techniques that can be used to
visualize these vortices. In Figure 4 and Figure 5 telltales
Figure 5: HE-3 impeller vortex Figure 4: Pitched blade impeller vortex
solids handling
attached to the blade are used to visualize these vorti-
ces.17 They are shown for both a relatively inefficient 45°
pitched four-blade impeller and the Chemineer HE-3 high
efficiency impeller.
In these figures, the impellers are rotating clockwise
when viewed from above. Thus the blades are moving
into the page, and the view is of the backside of the
blades. These vortices cause very localized wear emanat-
ing from the back-side or low pressure side of impeller
blades. These impeller vortices begin to diminish at Reyn-
olds numbers below 10 000, become very weak below a
Reynolds number of 500, and have completely disap-
peared below a Reynolds number of 10. As most slurry
particles are heavier than the fluid, the centrifugal effect
caused by the vortex will cause particles caught in the
vortex to migrate to the OD of the vortex. Thus the con-
centration of solids at the periphery of the vortex is much
higher and the rate of solid particle to surface impacts
is much greater, increasing locally the rate of erosion. Of
course, particles must be small enough to be captured
in these vortices before this effect would be observed.
On an industrial scale, however, the greatest majority of
slurry applications would have particles sufficiently small
to be captured by these vortices.
There are a number of relatively efficient wide blade
impellers used in solids suspension service. Fasano and
Reeder18 compared the erosion rate between a Chem-
ineer Maxflo W impeller, Figure 6, and a standard 45°
four-bladed pitched impeller (refer to Figure 4).
For the same level of solid suspension, these impel-
lers utilize the same impeller diameter at the same
speed. Therefore velocities at the impeller are the same.
The rate of erosion however for the pitched-blade impel-
ler was, on a percentage basis, 59% greater than the
erosion rate of the Maxflo W impeller.
Radial flow impellers are not very efficient in suspend-
ing solids. However, radial flow impellers are efficient in
dispersing gasses. In applications where solids are pres-