Chemical Engineering March 2014 - 53

Solids Processing
Diverter valves
Dilute-PhaSe Pneumatic conveying:
Instrumentation and
Conveying Velocity
Follow these guidelines
to design a well-instrumented
and controlled system, and to
optimize its conveying velocity
Amrit Agarwal
Consulting Engineer
D
ilute-phase pneumatic conveying
systems must be
operated in a certain sequence
and have sufficient
instrumentation and operating controls
to assure reliable operation
and prevent problems. This article
discusses two subjects that are important
for successful dilute-phase
conveying. Discussed below are design
guidelines for instrumentation
and controls that can prevent
operating problems, such as pipeline
plugging, downtime, equipment
failure, high power consumption,
product contamination and more.
The article also provides a simple
methodology for finding out if the
presently used conveying velocity
is too low or two high and for
making the required changes in
this velocity.
The required instrumentation
depends on the degree of automation
that is necessary, and whether
the system is to be controlled locally
or remotely. When manual
control of the conveying system
is used, problems can arise, especially
if the operators do not have a
thorough understanding of the design
and of the required operating
method of the conveying system, or
if they do not pay close attention to
day-to-day operation of the system.
For conveying systems - where
even a single error can result in a
large financial loss - a well-instrumented
and automated control system
is highly recommended.
Air
inlet
Blower
Run or position light
FIGURE 1. This figure is a schematic flow diagram of the
conveying system with run and position lights to show the
operating condition of each component of the system
Process logic description
Feeding solids into a conveying line
that does not have an airflow with
sufficiently high conveying velocity
will result in plugging of the line.
To prevent this, solids must be fed
into the conveying line only after
the required airflow has been fully
established. This requirement is
met by allowing the solids feeder
to start only after the blower has
been running for at least five
minutes. To do this, the rotaryvalve
motor should be interlocked
with the blower motor so that the
blower motor has run for five minutes
before the rotary-valve motor
can start.
When the conveying system is
running, the rotary-valve motor
must stop immediately in the event
that the blower motor stops for any
reason. If the rotary valve is not
stopped, solids feed will continue
and will plug the pipeline below
the feeder. To remove this plug, the
pipeline will need to be opened. This
required control option is implemented
by interlocking the rotaryvalve
motor with the blower motor
so that the rotary-valve motor stops
when the blower motor stops.
Should the conveying system need
to be stopped, certain steps must be
followed: The first step is to stop the
solids feed, after which the blower
is allowed to run until the conveying
line is empty and the blower
discharge pressure has come down
to the empty-line pressure drop. Do
54 ChemiCal engineering www.Che.Com marCh 2014
not stop the blower and the solids
feed at the same time.
When a conveying cycle has
been completed and the solids flow
into the conveying line has been
stopped, the blower motor must
continue to run for at least a few
more minutes to ensure that all of
the solids that are still inside the
conveying line have been conveyed
to the destination bin. If these solids
are allowed to remain in the
conveying line, they may plug the
line when the system is restarted.
These solids may also cause contamination
if a different solid is
conveyed in the next cycle.
Solids feed must stop immediately
if the normal operating pressure of
the blower increases by 10% and
continues to rise. This is because
the pressure increase is most likely
due to the conveying line starting to
plug. If the ongoing feed stream is
not stopped, the pressure will keep
increasing, making the plugging
situation worse.
After stopping the feed, the
blower is allowed to run for about
five minutes in an effort to flush the
plug. If the plug does not flush out
and the blower pressure remains
high, the blower motor should be
stopped. The plug is then removed
by tapping the pipeline to find the
plug location and opening up the
plugged section of the pipeline.
Solids feed must also be stopped
if the receiving bin or silo becomes
full, as indicated by its high-level
Feed
bin
Rotary
valve
High
level
Low
level
Receiving bins with bin
vent filters
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Chemical Engineering March 2014

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