Chemical Engineering March 2017 - 73

erosion at an impact angle of about
20 deg, and offers good erosion resistance
to normal impact. Glass is
typical of brittle materials: it suffers
severe erosion under normal impact,
but offers good erosion resistance to
low-angle, glancing impacts.
These tests were carried out with
sand particles sieved to particle sizes
of between 60 and 125 μm. The particles
were impacted at a velocity of
about 100 m/s, since they were undertaken
specifically for the investigation
of the potential wear of aircraft
engines. That brittle and ductile materials
respond to erosion in very different
ways can be clearly seen from
Figure 4, and it is obvious that different
mechanisms of material removal
must be involved. Note that the two
vertical axes relate to the two different
materials that were eroded, and
that they have very different scales.
Theories proposed. From early
thoughts on the matter, it was suggested
that
for
ductile
materials
(annealed low-carbon steel, copper,
aluminum and so on), removal
of material occurs predominantly by
plastic deformation. No cracks propagate
ahead of the cutting particle
and the volume removed is due entirely
to the cutting action of the particle,
rather like the cutting edge of
a machine tool. For brittle materials
(glass, basalt, ceramics, cast iron,
concrete and so on), it was thought
that material removal occurs in large
part due to the propagation of fracture
surfaces into the material.
These erosion processes, however,
have subsequently proved to
be less straightforward. Photographs
taken of impact craters, produced
as a result of single-particle impact
studies, have shown clear evidence
that melting of the pipe material can
take place. The melting only occurs
over a small part of the impact crater,
but it must be considered to contribute
to the erosive wear process.
Influence of velocity. The author
has undertaken research into the
erosion of pipe bends in pneumatic
conveying systems at velocities appropriate
for dilute-phase suspension
flow. Tests were carried out over
a range of conveying air velocities
from 15 to 35 m/s (3,000 to 7,000
ft/min). Steel bends of 53 mm (2 in.)
bore having a ratio of bend diameter,
D, to pipe bore, d, of about 5:1
were eroded by 70-μm sand. Over
40
Particle
size limits
30
Material after
conveying
20
Material after
conveying
10
40
80
120
160
Particle size, m
FIGURE 7. Pneumatic conveying can affect the fractional size distribution of a friable material
100
80
60
40
20
8
12
16
20
Particle velocity, m/s
FIGURE 8. These data show how particle velocity can influence the degradation of 5-mm aluminium
oxide particles
the ranges tested, the velocity exponent
was found to be consistent at
2.65. A graph showing the influence
of conveying air velocity on the specific
erosion of the bends is given in
Figure 5 [1].
The solids-loading ratio is the dimensionless
ratio of the mass flowrate
of the bulk particulate material
conveyed, to the mass flowrate
of air used to convey the material.
For dilute-phase, suspension-flow
conveying, solids-loading ratios are
typically up to about 15. For lowvelocity,
dense-phase conveying,
solids-loading ratios generally need
to be greater than about 40.
The erosion is in terms of the mass
of metal eroded from a bend per ton
of sand conveyed through the bend.
With a velocity exponent of 2.65, this
means that the wear rate will increase
by a factor of six with a doubling of
the air velocity. This explains why the
curve rises so steeply in Figure 5. If
a positive-pressure conveying system
operates at a pressure of 1 barg
(15 lb/in.2), a doubling of the velocity
will be achieved in a single-bore
pipeline discharging to atmospheric
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MARCH 2017
pressure. With a vacuum conveying
system, a doubling in velocity will be
achieved with a system exhausting
at 0.5 bar absolute (7.5 lb/in.2). In
such a system, a bend at the end of
the pipeline will wear approximately
six times as fast as a bend at the
start of the pipeline.
If an abrasive material is to be
conveyed, therefore, it would always
be recommended that the pipeline
be stepped to a larger bore partially
along its length in order to limit the
maximum value of velocity achieved,
in order to minimize the erosive wear
of bends toward the end of the pipeline.
It is essential, of course, that
the step to the larger-bore pipeline is
correctly positioned along the pipeline,
for if the velocity falls below the
minimum value of conveying air velocity
at the step, the pipeline is likely
to block at this point.
Particle degradation
In some bulk-solids-handling processes,
intentional breakdown of the
material is required (crushing, grinding
and comminution). In many handling
and storage situations, how73
Particle
size: 5 mm
Impact angle: 90 deg
Target material: steel
200
240
24
28
Percentage of unbroken particles
Percentage mass within particle size ranges

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Chemical Engineering March 2017

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