Chemical Engineering May 2012 - 73

bend due to impact and wall friction.
This reduction can be 5-50% depending
upon the solid. For a 50% reduction,
solids velocity entering the bend
must be at least 50% higher than the
saltation velocity. In general, length
of this horizontal line is at least 20 ft,
but use a longer line where possible.
* After the first bend, locate the second
bend to provide a sufficiently
long horizontal line that will allow
the solids to reaccelerate to the
same velocity as was at the inlet of
the first bend
* Do not install bends back to back but
at a sufficient distance from each other
to allow the solids to re-accelerate
* For long conveying lines, increase
the pipe diameter at a suitable point
in the pipeline route to prevent very
high velocities. Select this point at
which the solids velocity does not
fall below the minimum conveying
velocity. Figure 4 shows a pipeline
with two increases in pipe diameter
* Conveying gas can leak from pipe
couplings, diverter valves, rotary
valves, and other fittings. Use gas
tight equipment or increase the supply
air flow for these losses
* Avoid using upward sloping pipelines
because of loss of solids velocity
due to wall friction.
Minimize solids breakage. High conveying
velocity results in breakage of
fragile solids particles. For fragile solids,
this breakage can be proportional
to the velocity to the third or fourth
power. If this breakage is unacceptable,
use a low velocity, dense-phase system
or keep the velocity as low as possible.
Minimize pipeline erosion. When
handling abrasive solids, it is important
to use special material of construction
for the pipeline to prevent
pipeline erosion. Bends in the pipeline
are eroded faster than straight
pipe. To minimize this problem, use
special bends with wear-resistant
surfaces, or bends that have replaceable
outer backs. Do not use soft materials
such as aluminum. Determine
the abrasiveness of the solids before
selecting a suitable material for the
pipeline. Reduce conveying velocity
as much as possible, because wear increases
to velocity to the 2.5 power or
more. If the velocity is doubled, wear
increases six times. See Figure 5 for a
relationship of this wear between different
materials.
■
Edited by Rebekkah Marshall
Author
Amrit Agarwal is a consulting
engineer with Pneumatic
Conveying Consultants (7
Carriage Road, Charleston,
WV, 25314; Phone: 304-5531350,
Email: polypcc@aol.
com). He started his consulting
work after retiring from
Dow Chemical Co. in 2002 as
a senior research specialist.
He has more than 47 years of
design and operating experience
in bulk solids handling and pneumatic conveying.
He holds an M.S. in mechanical engineering
from the University of Wisconsin-Madison,
and an M.B.A. from the West Virginia College of
Graduate Studies in Charleston.
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