Solving Closed-Loop Pneumatic
Conveying System Problems
Guest article by Tim Singer of Pneu Solutions
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Without a doubt, the closed-loop
system is the most problematic type of pneumatic conveying system in
existence. The majority of these systems are dilute-phase pressure,
but vacuum and dense phase systems are also run in closed loop as
well.
Closed-loop pneumatic conveying
systems are commonly used in conjunction with an inert gas as the
conveying medium. Nitrogen is the most commonly used inert gas. The
closed system provides a means to recycle and conserve the inert gas
transport medium. The inert gas serves several purposes. It can be
used to dry a material, to keep a hygroscopic material dry, or to
provide an atmosphere around the material in which to inhibit
potential explosions of the dust clouds produced by that
material. Figure 1 is a simplified diagram of a closed-loop
system.
Figure 1: Closed-Loop System
Diagram
Many conveyed materials are highly
explosive in specific concentrations. Three things must be present
to cause any type of explosion: an ignitable energy source, oxygen,
and a spark or flame. Pneumatic conveying provides an easy means to
transport explosive materials because it can economically eliminate
two of the items that are necessary to generate a dust explosion.
The air, or oxygen, in the system is displaced by the inert gas
medium and the equipment and piping are positively grounded to
prevent any possibility of a spark caused by the release of a static
electricity charge.
When a closed-loop pneumatic
conveying system is not functioning properly, it often exhibits wide
pressure and vacuum fluctuations. This situation can lead to upset
conditions that release potentially explosive concentrations of
material outside the system or let oxygen into the system. The
typical place for material to be released and oxygen to enter the
system is on top of the receiving silo through the pressure/vacuum
release safety valve. When this upset situation occurs, the safety
conditions of the system have been breeched. It is time to correct
the system immediately because all three factors necessary to fuel
an explosion may be present.
The key to proper design of a closed
pneumatic conveying system is in the details. Even if all the
necessary equipment is in place, an improperly set switch can throw
the system out of balance and into upset conditions. A thorough
understanding of how the closed system is designed to operate is the
key to safe operation. What are the most common problems plaguing
these systems? The following four items are most common key points
missed during the design stage that cause most of the problems.
Filter and Silo Design
The receiving filter and silo must be
designed to withstand a higher than normal internal pressure.
The normal internal design pressure
of a silo in an open-type pneumatic conveying system is usually
about 0.5 PSIG. This is enough to withstand the differential
pressure of a bin vent filter in an upset condition. In a
closed-loop system, the design pressure of the receiving filter and
silo will usually be a minimum of 3 PSIG. Within the closed system,
there is no longer a condition of atmospheric pressure after the
silo filter. There exists a return line between the silo and the
blower inlet. A positive internal pressure is always maintained in
this section of piping with the lowest gage pressure in this section
occurring at the blower inlet flange. This must be the situation to
avoid the possibility of a vacuum leak that may let in unwanted
oxygen from the atmosphere.
When the system is in operation with
a gas flow, there will be a differential pressure in the return line
associated with the piping and other equipment within this section.
The differential pressure is produced by: a blower inlet silencer
(0.3 PSI), blower inline protective filter (0.3 PSI), a bin vent or
receiver filter (0.2 PSI), and return line piping and elbows (0.3
PSI or more). The unknown factor is still the return line piping and
elbow losses. The equipment layout between the receiving silo and
blower must be determined prior to determining the final design
pressure ratings of the silo and filter.
Once the physical relationship
between the silo and blower are set and the pipe routing is
complete, the final pneumatic calculation can be performed to
determine total pipeline pressure losses. Starting with a slight
positive pressure at the blower, this quickly puts the normal
operating pressure inside a silo at 1.5 PSIG or higher. In order to
minimize the silo internal design pressure rating and minimize
project costs, it is advisable to minimize the pipe losses by
locating the blower package as close to the receiving silo as
possible.
Make-up Gas and Overpressure
System
There must be a gas make-up and a gas
overpressure system installed.
No closed system is truly perfect and
there is going to be times that the system will require rebalancing
of gas volume. The gas can exit the system through airlocks, slide
gate valves, and pipeline leaks.
A system can become "off
balance" during start-up as material enters the system and
displaces [compresses] the gas that was at rest in the system before
start-up. The material entering the system will offset some of the
volume of the compressed gas, but a net loss of gas volume [actual
cubic feet] is usually the result. A gas make-up valve needs to be
incorporated and sized accordingly.
Conversely, as the system is
shut-down and the gas decompresses, there may be too much gas in the
system and the need to safely expel some gas may be required.
A typical example of this is during
the unloading of a pressurized bulk truck similar to the one shown
in the photograph, Figure 2. These trucks hold approximately 1,000
cubic feet of material. When they are near empty of material and
filled with compressed gas at 15 PSIG, they can hold 2,000 cubic
feet of gas. Where does the additional gas go upon shutdown? The gas
volume will produce a net increase in pressure when the system is at
rest. The need for a gas overpressure valve will release the excess
gas volume and solve this problem.
Figure 2: Pressurized Bulk
Truck
Proper system design and equipment
selection can minimize the adverse effects of these conditions and
conserve the net gas loss.
Blower Design
A system requiring a positive
displacement blower producing approximately 1000 CFM or more of gas
will require a bypass line around the blower.
A positive displacement blower is the
typical gas flow device used for these systems. The blower will
reach full flow condition in a matter of a few seconds as the motor
reaches full speed. The blower must pull in as much gas as it pushes
out.
Is the return line capable of
allowing this to happen without drawing some amount of vacuum at the
blower inlet? With a surge tank in front of the blower, a small
blower may not have a problem. As the blower increases in volume and
size, some amount of vacuum at the inlet is not possible to avoid
without an aid. The addition of a bypass pipe and control valve from
the blower discharge side to the blower inlet side will allow the
blower to start and recirculate the gas under a no-load condition
and without disturbing the pressure conditions of the system. The
bypass line control valve can be slowly closed and the gas will
start its way through the main piping loop in a controlled
manner.
The gas pressure will remain nearly
constant at the inlet of the blower and the internal pressure at the
blower inlet can be set at just a few inches of water column.
Without the bypass line, the return line pressure must typically be
set at 1 to 2 PSIG in order to absorb the vacuum surge at start-up.
Remember the problem above? If the internal pressure is at 2 PSIG at
the blower inlet, than the pressure is 3 to 4 PSIG at the receiving
silo. This situation results in higher silo costs. Also, the
conveying pressure is higher than necessary and conveying capacity
may be reduced as a result.
Baghouse Design
A top-loading pulse-jet, baghouse
design is highly recommended for closed systems that convey
materials with very small particle size or having many fines.
 A
top-loading filter design will increase the initial capital cost of
the project, but will absolutely pay for itself down the road in
maintenance costs.
Anyone who has had responsibility for
the operation of a closed-loop system conveying a fine powder with
bottom-loaded filter bags has an experience to share of the
maintenance nightmares they experienced. The fine material has a
tendency to migrate through the area at the top of these filter bags
and into the clean side of the filter and return line. This
situation will eventually lead to a high differential pressure in
the blower protective filter and will add an unnecessary pressure
drop to the return line.
The result is increased operating
pressure in the receiving silo. The problem is easily avoided with a
properly designed top-loading filter unit. The sealing arrangement
is improved and filter bag replacement is simplified. The photo
[below right] shows a typical top-loaded filter bag where the bag
cage and venturi arrangement are loaded from the top of the filter
tube sheet.
Other filter arrangements may be
possible from a new design or retrofit perspective. Another
possibility is a pleated filter installation. Most baghouse
equipment suppliers can provide details of the different filter
arrangements along with the installation procedures.
Summary
The key to safe and proper operation
and maintenance of these systems lay in the knowledge of how the
components and controls function together to maintain a balanced
system. A pneumatic conveying system operating with material
spillage is a potential human and environmental hazard in need of
immediate attention. The addition of the above mentioned design
considerations to a pressure dilute, closed-loop pneumatic conveying
system will increase plant safety and provide a safe work
environment for you and your co-workers.
About our author:
Tim Singer is currently an
independent pneumatic conveying consultant and holds a BSME from the
University of Illinois in Chicago. Since 1987, he has held various
technical positions with leading suppliers of pneumatic conveying
systems. If you have any questions about this article, please
contact the author at:
Mr. Tim Singer
Pneu Solutions
16823 Cimarron Drive
Magnolia, TX 77355
Telephone: 281-252-8850
Web site: http://www.pneusolutions.com/
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