Dense Phase Conveying, Is It The
Right Choice?
Guest article by Tim Singer of Pneu Solutions
Printer friendly PDF
Pneumatic transportation of powders
and bulk solids through pipelines has numerous advantages when
compared to the alternative modes available today. Many materials
can be transported over one-half of a mile. The systems have a
relatively long life expectancy with low maintenance costs. Product
contamination is eliminated and fines emissions to the environment
are minimal.
One type of technology used to
pneumatically transport these materials is Dense Phase. It has been
around for quite a while, but was typically only used to transport
just a few materials like cement and fly ash. Over the last ten
years, there have been a rapidly growing number of pneumatic
conveying installations that are using the dense phase technology to
transport a very wide range of materials from dog food to plastic
pellets.
Dense phase installations have grown
rapidly in the market because of the cost savings for the producers
of the products and materials that we consume. The producers of
foods, plastics, pharmaceuticals, building materials, and dry
chemicals are all global suppliers. The material handling systems
that are necessary to move these products are designed to transport
millions of tons annually. By reducing product waste by only a
fraction of a percent, any money spent to make improvements to these
systems will quickly be recovered.
What are the applications for
dense phase?
Assuming the product is a candidate
for the dense phase conveying method (product candidates can be a
topic for discussion of their own), the number one reason to apply a
dense phase design is because the product being handled is highly
friable. Many products within the food industry fall into this
category. As an example, if a consumer opens a bag of cheese puffs
and finds that half of them are broken into little pieces, they will
quickly change product brands to someone with whole cheese puffs.
Degradation to the final product is a highly important priority to
any manufacturer of goods.
The next best application for dense
phase is on abrasive materials. The higher velocities associated
with dilute phase conveying can lead to rapid pipe wear. Many
materials are so abrasive that they can wear a hole in a pipe elbow
in just a few weeks. Many minerals and chemicals fall into this
category. The pipe wear can also result in a contamination problem
because the pipe material will become mixed with the product.
There is really only one other main
reason for selecting dense phase. This is typically found within the
plastics and petrochemical industry. Some of the softer plastics,
such as polypropylene and polyethylene, will smear onto the pipe
wall when the product slides along the outer wall of an elbow in a
dilute phase transport system. The plastics will actually melt from
the frictional contact with the pipe wall and will leave a long thin
layer of material. The layers are peeled off into strips and
re-entrained into the system. These strips are commonly referred to
as a "streamers" and they will quickly build up in awkward
places and stop product flow. The build-up of these streamers is
usually referred to as "bird-nests" because of the way
they look when they are finally removed from the process. Dense
phase transportation will eliminate the streamers that are commonly
associated with dilute phase conveying. There are several other
methods to greatly reduce the streamer generation and they may be a
more cost effective solution. These other methods would be a
separate topic for discussion.
How does dense phase work?
How does dense phase transportation
achieve the stated advantages over dilute phase conveying? During
the pneumatic transport process, the products are accelerated to a
design velocity, come in contact the pipe walls, change direction in
bends, and fall back to the original resting state at the
destination point. As one can probably imagine, some products hold
up better than others during this process. In all cases, though, the
percent of material breakage is very much proportional to the
average velocity of the product in the pipeline. The main principle
of a dense phase conveying system is to slow down the velocity of
the product in the pipe to a point that is below the speed at which
the product breaks or degrades. In a dense phase system, the
velocity range at the source can be as low as 3 to 5 miles per hour
for the majority of products. The product velocity at the
destination is always a function of the system differential
pressure. Because of the compressibility of the air or gas used as
the conveying medium, the gas will expand as it moves to the end of
the pipeline and the product velocity will increase accordingly. The
terminal velocities of a dense phase system can vary, but in most
cases they rarely exceed 20 miles per hour (mph). In a typical
dilute phase system, the starting velocities begin at about 40 mph
while the terminal velocities can reach 100 mph.
A natural phenomenon occurs within
the pipe as the velocities are reduced from dilute phase mode to
dense phase mode. In a dilute phase conveying system, the product is
transported by lift, or suspension, of the individual particles in
the gas stream. As the gas velocity is subsequently reduced, the
larger particles cannot sustain this lift and they begin to fall
from suspension to the bottom of the pipe. The technical term used
in the industry that describes the velocity at which particles fall
from suspension from the gas stream (in horizontal pipes) is
"saltation velocity". The best, single description for
identifying if a system is dense phase is whether the product
velocities in the pipe are designed to be operating below the
saltation velocity. Most systems will operate erratically when
velocities are just below the saltation point causing rapidly
changing conveying pressures. The pressure fluctuations result in
acceleration and deceleration of the product and will cause
excessive product damage. A system operating in this manner is
usually described as operating in an unstable dense phase mode. A
further reduction in gas velocity will create the natural
combination of wavelike-flow and plug-flow of product as it moves
throughout the pipes. The system pressures are much higher and still
fluctuate somewhat, but are controllable and consistent. This
condition is described as stable dense phase conveying.
What are the potential problems
with dense phase design?
Many operating problems occur as a
result of poor planning, inexperience, or little understanding of
how dense phase systems work. When a dense phase system is applied
properly, designed correctly, commissioned by a qualified start-up
engineer, and has the proper operation and maintenance procedures,
these systems will work very well for many years. Below are some of
the reasons that can create problems down the road.
The first step in the decision
process is to review the application. The following questions must
be answered before proceeding down the dense phase path. Is the
material a good candidate for dense phase? What problems lay ahead
because of the physical material characteristics? Does the decision
for selecting dense phase over dilute phase fall into one of the
three categories stated above: product degradation, pipeline
erosion, or streamer formation (with plastic pellets)? If the main
reason for selecting a dense phase system does not fall into one of
these categories, then an alternate solution for a dilute phase
equivalent design should be sought.
There are many possibilities in
designing a dense phase system. This should not be left to the
beginner or novice person. The single pressure vessel design,
stacked vessel design, side-by-side vessel design, high pressure
rotary feeders are just some of the ways to introduce material into
the pipeline. Each method has certain advantages over the next one.
The gas source can be from a screw compressor, piston compressor,
plant air, or other sources. The gas flow control equipment is
highly dependent on the source of the motive gas. This scenario
creates many combinations when selecting the proper gas control
system. One can begin to see that this is not an easy task to put
together the best system and only persons who are trained and
experienced with all the possible combinations will make the correct
choices.
Dense phase systems are more
complicated than dilute phase systems to control properly and it is
not an intuitive process. The reason being primarily because of the
narrow operating window in which the systems will perform with
stability. It is necessary to constantly change the gas amounts for
varying conditions in the system. Therefore, the heart of all dense
phase systems is the gas control device. It must be flexible and
accurate. In addition, the owner/operator of the system must have a
good in-depth knowledge of the dense phase process in order to
troubleshoot and correct problems if they occur. A lack of training
by the customer can have negative results on system performance.
Much information is needed for proper
design. Dense phase systems are not as flexible as dilute phase
systems when it comes to varying product transfer capacities,
varying products, or varying product grades that result in
dissimilar physical product characteristics. Even product
temperature variations need to be known at the design stage and
incorporated into the system design. The system designer can build
in some additional flexibility, but they need to know the customer's
process and process requirements for the transport system. It is
extremely important for the system designer to have an understanding
of the total process leading into and out of the conveying system.
Are dense phase systems meeting
customer's expectations?
How have these systems been
performing over these latter years? Many systems are working great.
There have been some common complaints about these systems, though,
such as: line plugging, not making design rate, excessive product
breakage, or destroying pipe supports. Why are there so many
problems associated with this type of technology? The majority of
the complaints associated with these systems stems from fact that
they are not operating in a stable dense phase mode. The gas flow
may be too high. It may be too low. How does one know which? How
does one get back to the correct gas flow? These questions can be
answered, but are mostly specific to the individual system
configuration.
Line plugging is the most common
problem associated with dense phase systems. The lines can plug
because of either too much or too little gas flow. If the product is
permeable, such as a pelleted material, a line plug is typically
associated with too little airflow. If the product is a fine powder,
however, the lines can plug from either too little or too much gas
flow. For this reason, it is crucial to know how much gas is being
supplied to the system at all times.
Excessive product breakage is the
next problem. As mentioned already, this is a direct function of
velocity of the product. This is typically a result of too much gas
flow into the system. Many times there are other physical design
modifications that can be made to the system that will help reduce
degradation.
Not reaching the design transfer rate
is a common problem. This is usually because the system has reached
a maximum convey pressure or differential pressure for some reason.
The number one cause is that the product being transferred is not
performing in the actual installation as it was expected to do
according to the manufacturer. Because of the nature of dense phase,
the pressure drop in the line is highly dependent on the physical
properties of the material (and the transfer rate). Friction between
the product and the pipe walls create a much higher pressure in the
line as compared to dilute phase. Therefore, small changes in
friction coefficients between the pipe surface and the product
(adhesive forces) can have large effects on the line pressure. The
manner in which the product interlocks with itself (cohesive forces)
as it moves through the pipe will, also, have a large affect on
system pressure drop. These two forces are not the only variables
for determining convey pressure, but they are the most predominant
ones.
The pipes shake enough to break the
supports. This problem has a two-part solution. The natural
phenomenon of dense phase is to make the waves and plugs. When these
plugs change direction in an elbow, it will produce two reaction
forces onto the pipe at that point. First, an impact force from the
plug hitting the elbow, because it wants to move in a straight line.
Second, a centripetal force is induced into the pipe and supports as
the mass of product changes direction. These two forces occur at
every elbow and each time a plug comes through it. The cycle time
between plugs is typically on the order of magnitude of one every 30
seconds. If the system were run continuously for 24 hours per day,
this is equivalent to over one million cycles annually. In general,
larger pipe diameters mean larger magnitude of reaction forces. Most
of the problems start to occur with convey pipe diameters as small
as 3 inches.
The first solution to the problem is
to give special attention to the design of the pipe support system.
The conveying system supplier should supply the expected magnitude
of these dynamic reaction forces to the engineers who are designing
the piping and structural steel systems. It is then the
responsibility of the piping designers to follow through with this
information to determine fatigues, moments, stresses, pipe anchoring
designs, and steel support configurations. The second part to the
solution of this problem is to establish periodic maintenance
inspections on these systems. They must be operating in a stable
dense phase mode. A system that is not operating with the correct
airflow can induce dramatically larger forces into the pipe
supports. If left unattended, this situation can accelerate the
destruction of a piping system.
About the 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:
Tim Singer
Pneu Solutions
16823 Cimarron Drive
Magnolia, TX 77355
Telephone: 281-252-8850
http://www.pneusolutions.com/
Email: Singertimothy@aol.com
Help others by posting your comments, suggestions and
experiences with bulk solids feeding or any other materials handling concerns you may have
on our On-Line Help Forum. For
past Ask Joe ! Articles, visit the Ask Joe! Archived Articles.
Guest articles for the Ask Joe! Column are always welcome,
for more information please contact Joe Marinelli directly at his email address:
joe@solidshandlingtech.com.
|
