Chemical Engineering March 2017 - 72

0·06
0·04
0·03
0·04
0·02
Impact angle
0·02
0·01
30
60
Impact angle, deg
FIGURE 4. Impact angle is an important variable for erosive wear of various surface materials
30
For:
mild steel bends
53-mm bore
140-mm radius in horizontal plane
20
Conveying
70 m sand at a solids loading ratio of 2
90 0
Surface material
Particle
10
10
20
Conveying air velocity, m/s
FIGURE 5. Erosive wear is also influence heavily by particle velocity in pipeline bends
100
80
60
40
20
40
80
30
Material before
conveying
Material after
conveying
120
160
Particle size, m
FIGURE 6. Pneumatic conveying can shift the cumulative particle size distribution of a material, especially
for friable solids
there will be a small pressure drop,
but the major part of the loss in pressure,
as a result of the flow though
the bend, will be in the acceleration
of the particles back to their terminal
velocity following the bend. The two
pressure gages included in Figure 2,
at the bend inlet and outlet, will give
a false reading for the actual pressure
drop, which results from the
flow of material through the bend.
In pneumatic conveying situations,
most of the pressure drop that can
be attributed to the bend occurs
72
after the bend, in terms of the need
to re-accelerate the particles back to
their terminal velocity.
Erosive wear
The erosive wear of bends in pneumatic-conveying-system
pipelines is
well illustrated by the work of Mason
and Smith [2]. They carried out tests
on 25- and 50-mm (1- and 2-in.)
square-section 90-deg bends with a
flow of alumina particles from vertical
to horizontal. The bends were
made of Perspex (polymethylmeth200
240
acrylate)
and were constructed with
substantial backing pieces so that
the change in flow pattern and wear
over a period of time could be visually
observed. The results from one
of their tests are given in Figure 3.
With a new bend, the particles tend
to travel straight on from the preceding
straight pipeline until they impact
against the bend wall. After impact,
they tend to be swept around the
outside surface of the bend. They
are then gradually entrained in the
air in the following straight length of
pipeline. In Figure 3, the flow pattern
is shown after substantial wear has
occurred. This shows quite clearly
the gradual wearing process of a
bend and the effect of impact angle
on the material in the process. Erosion
first occurred at a bend angle
of about 20 deg, which became the
primary wear point, as one would
expect. After a certain depth of wear
pocket had been established, however,
the particles were deflected
sufficiently to promote wear on the
inside surface of the bend, and then
to promote a secondary wear point
at a bend angle of about 75 deg.
A small tertiary wear point was
subsequently created at a bend
angle of about 85 deg. If such a
highly reinforced bend were to be
used in industry, in preference to
replacing worn bends, the deflection
from the latter wear points
would probably cause erosion of the
straight section of pipeline downstream
from the bend. Because this
pattern of particle deflection in worn
bends is now well recognized, some
companies manufacture steel bends
with thicker walls. They are also
slightly thicker on the inside surface
to allow for the fact that particles can
be deflected to the inside surface, as
illustrated in Figure 3.
Impact angle and surface material.
A curve presented by Tilly [3],
and shown in Figure 4, illustrates the
variation of erosive wear with impact
angle for two different surface materials,
and is typical of the early work
carried out to investigate the influence
of these variables. Both materials
showed very significant differences
in both erosion rate and the effect of
impact angle. These materials do, in
fact, exhibit characteristic types of
behavior that are now well recognized.
The aluminum alloy is typical of
ductile materials: it suffers maximum
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
MARCH 2017
Percentage mass over size
Specific erosion, g/metric ton
Erosion, aluminum, cm3/kg
Erosion, glass, cm3/kg

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Chemical Engineering March 2017

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