Chemical Engineering May 2012 - 69

Solids Processing
Rules of Thumb For Pneumatic
Conveying Pipelines
Proper design, configuration and layout
of the conveying pipeline are critical
for successful and troublefree operation
Amrit Agarwal
Pneumatic Conveying Consultants
Q
uite often, during the design of
conveying systems, not enough
attention is paid to the design
of the conveying pipeline and
its components. This disregard often
leads to some major conveying system
problems such as low conveying rates,
conveying line plugging, excessive wear
and tear in the conveying line, high conveying
system pressure drop, product
breakage, fines and streamer generation
and product cross-contamination.
Guidelines presented below help mitigate
or avoid such problems.
General considerations
Materials of construction. For the
pipe itself, use carbon steel if contamination
is not an issue. Otherwise, use
aluminum or stainless steel. Use stainless
steel for food and pharmaceutical
applications. For special applications
such as abrasive solids, high temperatures,
and so on, it is necessary to find
a suitable material for the conveying
line and its components.
Pressure rating. Pressure rating of
the conveying pipeline should be suitable
for the maximum conveying pressure
of the conveying system. For most
applications with a Roots type blower,
a pressure rating of 30 psig is satisfactory.
This rating corresponds to
the rating of a Schedule 10 pipe. Use
thicker pipes, such as Schedule 20, 30
or 40 for higher pressure applications.
Temperature rating. The temperature
rating of the conveying pipeline
should be suitable for the minimum
and maximum temperatures experienced
by the conveying line. These temperatures
depend upon the ambient
temperature, conveying air temperature
and solids temperature.
Pipeline joints. Pipeline segments
must not be welded to each
other because of the need for dismantling
of the pipeline. These
segments should instead be joined
to allow easy dismantling of the
pipeline. Flanges can be used, but
they are expensive and not easy to
unbolt. They should be used where
the joint must be 100% leak proof.
Use of easy-to-open pipeline couplings
is common. Locate the joints for
easy access to allow dismantling of the
pipeline when necessary.
The inside diameter of the couplings
or the flanges must be equal to the inside
diameter of the pipe. Both ends
of the adjacent pipe segments must be
truly aligned with each other so that
there is no internal protrusion and
there is no gap between the two ends.
Pipeline internal surface. The inside
surface should be clean and free
of oil and rust. A smooth interior can
be used, except when handling plastics
that can generate so-called streamers
(a product of plastic degradation). In
that case, inside surface is roughened
by special tools.
For instance, dilute-phase conveying
lines in pelleted plastics service
are generally roughened on the inside
surface to prevent streamer formation.
(Other common names for streamers
are angel hair or snake skins).
Streamers are formed when a plastic
particle at a high conveying velocity
strikes the smooth pipe wall at a low
angle of incidence. The energy of impact
is enough to melt the particle sur1.2
1.0
0.8
0.6
0.4
0.2
D/2
d
Rough
pipes
f = 0.0075
Smooth pipes
f = 0.0045
Bend radius vs bend pressure drop
10
20
Ratio of D/d
Heat loss for 90 deg bends,
where f = friction coefficient,
k = number of velocity heads lost
FIGURE 1. Studies show that long radius
bends (larger D/d) have a lower pressure drop
(lower k)
face at the point of contact and leave a
thin crayon-like film mark on the pipe
wall. This thin film continues to grow
in length due to subsequent impacts,
cools quickly, and peels off from the
pipe wall in the shape of a streamer.
The length of this streamer may be a
fraction of an inch or two to three feet
in length, or even longer. Long thin
streamers tend to float and swirl on
the air current in storage vessels until
they ball up to form something that
resembles a bird-nest.
In any case, the net result of streamers
is plugged slide gates, feeders,
screeners, mechanical conveyers, and
hoppers. The best solution to solve this
problem is to avoid making streamers,
which is done by roughening the
inside of the pipeline. Roughening
(or scoring) is done by a special shotpeening
method, either using inhouse
specifications or vendors who have
this expertise. Vendors have their own
proprietary techniques for applying
the scoring. A well scored pipe should
last at least one year before it needs to
be re-scored.
Aluminum elbows should be anCHEMICAL
ENGINEERING WWW.CHE.COM MAY 2012 51
30
Value of k
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