Increasing Conveying Rates in
Pneumatic Systems
Guest article by A. Tim Agarwal, Pneumatic Conveying Consultants
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Introduction
Low conveying rates are a common
problem in pneumatic conveying systems, particularly in systems that
handle a variety of products through the same system. In many cases,
a few minor changes in the conveying system can increase these rates
appreciably. But before making these changes it is necessary to
understand the reasons for the low conveying rates. Low conveying
rates are generally caused by the following conditions:
- Rotary valve speed is too high, or
its fill efficiency is too low, resulting in a "Feed
Limit".
- System pressure drop is near the
maximum discharge pressure rating of the conveying blower,
resulting in a "Pressure Limit".
- Solids start to salt out when the
conveying rates are increased or a "Velocity Limit".
- One or more new materials are
being conveyed but the system was not designed for these
materials or "System Design Limit".
Diagnostics and Solutions
Feed Limitations - Rotary Valve
Speed and Leakage
 In
vacuum-type conveying systems, one can check the speed of the rotary
valve and find out if it is flood-fed or starve-fed. The speed of
the rotary valve can be determined by counting the number of
revolutions per minute of the drive shaft. A normal speed range for
a typical rotary valve is 15 to 25 RPM. At speeds higher than this
rotor pockets don't have enough time to empty out completely and the
capacity of the rotary valve decreases instead of increasing (See
Figure 1).
Maximum speed of a rotary valve can
also depend upon the size and density of the solids. For example,
the maximum rotational speed will be higher for plastic pellets than
for powdery materials. As most of the rotary valves will have chain
and sprocket drive, one can change sprockets and increase the speed
of the rotary valve beyond which its capacity starts to reduce. An
easy way to check the capacity is to monitor the blower discharge
pressure.
For pressure-type conveying systems,
the rotary valve "pocket-fill" efficiency depends greatly
upon how the return leakage and displacement gases are vented out
from the rotary valve. If they vent out directly from the rotary
valve inlet, fill efficiency will reduce whether the rotary valve is
flood fed or starve-fed. When conveying fine powders this fill
efficiency will fall dramatically. Therefore, it is vital to have a
correctly designed venting system for the rotary valve. Good venting
systems have gas flow channels separate from the solids flow
channels, and for powdered materials have body vents in the rotary
valve to an external vent line.
Pressure Limitations - Blower
Performance and Pressure Drops
Most of the conveying systems have
pressure gauge at the discharge of the conveying blower. Very few
conveying systems have pressure indicators or recorders in the
control room or on a central control panel. Pressure gauges located
outside, near the blower, are often unreliable because of poor
maintenance or weathering. For diagnostics, it is necessary to find
out the correct pressure reading of the blower. If the existing
pressure gage does not seem to be in a good condition, it should be
replaced by a new gage.
Get a good reading of the blower
discharge pressure including any changes or pulsations in the
pressure. From the nameplate that should be mounted on the blower's
body, find out the design pressure of the blower. Also, check the
set point of the blower safety valve.
Get the performance curve for the
blower. It should be easily available from the blower vendor if it
is not available at the plant site.
Compare the blower discharge pressure
with the maximum pressure rating of the blower. Allow 10% margin to
prevent pressure fluctuations from too frequent lifting of the
blower's safety valve. As most of the lobe-type blowers have a
pressure rating of 15 psig, the maximum operating pressure should
not exceed 13.5 psig.
If the blower's discharge pressure
has reached 13.5 psig, it is not possible to increase the conveying
rates any more. This condition is called "Pressure Limit".
It means that the maximum pressure that is available from the
conveying blower is the reason for the low conveying rate.
The solutions for this rate limit are
either to increase the available conveying pressure or to reduce the
conveying system pressure drop. One solution for pressure-type
systems, using a suction blower at the discharge of the conveying
system can increase the available conveying pressure.
For reducing the conveying system's
pressure drop, the first thing to find out is the conveying
velocity. This can be calculated easily by using the well-known
Darcy Equation. Calculate both the velocity at the pick-up point and
at the end of the conveying line. Reduce these velocities if they
are much higher than necessary for the line diameter and materials
being conveyed. Since the pressure drop is a function of the square
of the velocity, reducing the velocity by 10% can reduce the
pressure drop by about 20%.
 To
reduce the velocity, the simplest change is to reduce the blower
speed. Change the sheaves on the blower to reduce the blower speed.
If the blower has a variable speed drive, blower speed can be
reduced without making any changes. Another option to reducing the
velocity is to blow off a measured flow of the conveying gas from
the blower outlet. Use a vent line after the blower and a flow
control valve in this line to do this. The effect of reducing the
velocity on the solids conveying capacity is shown in Figure 2.
Other options to reduce the pressure
drop are to reduce the number of bends in the conveying line,
especially if two or more bends are located very close to each
other. Solids velocity is always less than the gas velocity and this
difference becomes greater when the solids pass though a bend. The
result is that a higher pick up velocity or initial gas velocity is
needed to prevent solids from salting out after a bend. This higher
velocity then increases the line pressure drop. Another option is to
use a stepped pipeline, i.e., to increase the line diameter along
the route of the conveying line so that the velocity us reduced. In
general, try to reduce the overall length of the pipeline and the
number of bends.
Velocity Limitations -
Velocity limit means that at the
existing conveying rates, any increase in the conveying rate solids,
in the conveying line, will start to "salt out". Consider
the following changes to solve velocity limitations:
- Leakage: Make sure that
there is no gas leakage from the conveying line due to erosion
of bends or worn out pipeline or poor alignment or poor design
of pipeline joints or diverter valves.
- Blower Speed: Increase the
blower speed, provided the resulting pressure is within the
pressure limit of the blower. This will result in a higher gas
flow from the blower and a higher gas velocity. The higher gas
velocity will be able to increase the solids conveying rate.
- Bend usage: Relocate bends
in the conveying line if they are located too close to each
other. Do not use more than two bends next to each other. This
is because the solids velocity reduces when the solids pass
through a bend and need sufficient length of straight pipe after
the bend to reaccelerate to the original conveying velocity.
Make sure that there is sufficient straight pipe after each
bend. In general, provide at least 20 pipe diameters of straight
pipe after each bend.
- Pipeline changes: Reduce the
pipeline diameter at the beginning of the system. This will
increase the conveying pressure so make this change only if the
resulting pressure is within blower's pressure limits. If the
pipeline diameter is constant, higher velocity at the beginning
will result in higher velocity in the pipeline.
- Feed point piping: Make sure
that there is sufficient straight section of the pipeline after
the material feed point. This is important because material must
be accelerated from its zero velocity at the pick up point to
its minimum conveying velocity.
- Piping alignment: Check
alignment of the pipeline at the joints to make sure that the
pipeline's interior is concentric and the pipe is not
obstructing the flow of solids and thus reducing the solids
velocity.
- More blowers: Use a second
blower in parallel with the existing blower. This second blower
will increase the conveying velocity. As this will increase the
conveying pressure, consider this only if the blower is not
pressure-limited.
System Design Limitations - Get
Professional Assistance
 If the conveying rates are low
because some new materials are being conveyed in an existing system,
the overall design of the conveying system must be re-evaluated to
find the changes that are needed. Difficult conveying
conditions may occur because the conveying properties of the new
materials may be different from the previous materials.
For this evaluation, the best option
is to obtain actual operating data for the new materials by running
a few tests on them at low conveying rates. Then use this data to
find out the maximum conveying rate that the existing conveying
system can handle. Sometimes, a simple change such as a higher
blower speed may be enough to run the existing system at a higher
conveying rate.
Other options would be to optimize
the conveying line route by eliminating a few bends or by shortening
the pipeline. If these options are not enough, an increase in the
pipeline diameter for a portion of the pipeline may have to be
considered. In these cases, it may be a good idea to seek the help
from people who are knowledgeable in pneumatic
Contact our author:
Mr. A. Tim Agarwal
Consulting Engineer
Pneumatic Conveying Consultants
7 Carriage Road
Charleston, WV, 25314
Email: polypcc@aol.com
Web site: http://www.powderandbulk.com/pcc
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