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Does Your Silo Honk and Thump?
by David S. Dick, Solids Handling Technologies, Inc.
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 The problems of thumping and
vibration in silos are relatively common in some bulk solids
handling industries. There are many reasons for these problems, some
better understood than others. They vary from relatively high
frequency to very low frequency and from being a minor nuisance to a
serious structural concern.
High frequency vibrations will often
be a nuisance to neighbors and plant personnel and may require
operators to wear ear protectors. Low frequency vibrations and
sporadic thumping can be much more serious. Significant energy may
be released with each thump, which at least can be disconcerting and
at worst may cause catastrophic structural failure in the silo
itself and in adjacent buildings.
Dynamic loads in funnel flow
Dynamic
loads in funnel flow silos have created structural problems for as
long as products have been stored in silos. When a flow channel
empties out it is called a rathole. A rathole may be very stable and
require a flow aid to restore flow or it may collapse spontaneously.
The amount of product that falls depends on when and how the sides
of the rathole will give way.
 There are many examples of collapsing
ratholes creating a vacuum and sucking in the roof and top part of a
silo and/or very high pressure blowing off the hopper or causing
damage to the support structure and foundation. We have had
experience with loads large enough to tear a hole in a 0.5" (12.7
mm) thick
stainless steel plate and enough to move a 2000 ton silo by shearing
its foundation bolts. (See photo)
The solution to these problems will
usually involve either retrofitting the silo to convert it to mass
flow or expanded flow. Expanded flow is a short mass flow hopper
that expands the flow channel up to a diameter that will always be
unstable. This prevents the flow channel from emptying out and
therefore prevents dynamic loads while ensuring complete cleanout of
the silo. Sometimes the solution is to correct the design details of
the feeder to restore mass flow in a silo. Even a hopper steep
enough for mass flow will flow in funnel flow if the hopper and
feeder combination is not working properly.
Vibrations in mass flow
There are a number of mechanisms that
can cause vibrations in a mass flow silo. High frequency vibrations
are almost always due to a slip-stick phenomenon as the particles
slide on the silo walls. ‘Honking’, or ‘singing’ silos can create
noise with sound pressure levels in excess of 110dB.
 Depending
on the frequency, the sound has been described as honking like a
train horn or singing. (Click on the image to hear recordings of
actual silo sounds)
The phenomenon is seen in aluminum
and stainless steel silos. It only occurs at a certain level of
contact pressure between the particles and the silo wall. One method
of dealing with the problem is to convert the silo to funnel flow.
This may be done with an internal tube or tubes either in the center
of the silo or attached to the walls to force a first-in-last-out
flow sequence. With a free-flowing, non-degrading product this may
be acceptable. If it is necessary to maintain mass flow, a number of
other remedies are available. One is to reduce the loads.
Experience has shown that if the silo
is allowed to empty, the honking stops at some point when the level
of product reaches a certain point. Laboratory tests confirm that
the slip-stick phenomenon is a function of wall contact pressure and
it disappears when the pressure is reduced. It is possible to build
relatively simple internal structures in the silo to prevent the
solids contact pressure reaching levels where slip-stick and honking
would start. The internal structures shed the loads due to the
weight of the product into the silo walls higher up in the silo
rather than allowing the loads in the cylinder section to build up
to a level where honking would start.
Vibrations at an intermediate
frequency (or pulsing) are often due to a high-pressure region in
the flowing mass near a region of low pressure. Elastic potential
energy builds up in the highly stressed region which, when released
into the low-pressure region allows a mass of solid to move
suddenly. A large amount of solid is usually involved but since the
frequency is relatively high, the amount of movement is small.
Pulsing may also be due to slip-stick
but instead of occurring at the interface between the product and
the silo wall, it occurs as the product shears on itself. This will
often occur if the solid is relatively compressible and springy.
 Low
frequency vibrations in mass flow also may be due to a number of
mechanisms. Plate-like products such as wood chips, chopped plastic
bottles, chopped plastic film etc are not homogeneous and can
transfer stresses in unusual ways when stored in a silo. It is not
uncommon for fairly large voids to open up in the lower portion of
the silo when this type of product is discharged. The voids are not
necessarily like ratholes and have been described as ‘caverns’ in
the hopper. When the voids collapse, the material above falls and
the results are similar to a collapsing rathole in a funnel flow
silo. (See photo)
There is another low frequency
vibration mechanism that occurs in mass flow that has been more
difficult to pin down. This mechanism usually occurs with relatively
free-flowing, relatively incompressible products like plastic
pellets, dry sand, hard granules etc. These vibrations have a
relatively fixed period and are what is often referred to as
‘thumping’. The reason it has been difficult to pin down the actual
cause is that thumping is due to a number of properties of the bulk
solid.
When certain values of
compressibility, internal friction and angle of sliding friction on
the hopper wall combine, portions of the stored volume cannot move
unless other parts move first. This creates an intermittent movement
pattern in the bulk of the stored product and while it appears that
there is mass flow, the flow is in fact switching between funnel
flow where only some of the material is in motion and mass flow
where all of the silo contents are in motion.
Thumping can be a nuisance, a
discomfort or a dangerous condition causing structural damage. The
magnitude of the thumps is a function of the period. The longer the
period between thumps the higher the magnitude. Since the flow of
product at the outlet is constant, the longer the period between
thumps the more potential energy of the stored product will be
converted to kinetic energy when it starts to move. Structural
distress can range from dents in the silo wall to catastrophic
failure of the weakest element in the structure – the wall, the
hopper or the foundation. Thumping can also pose a risk to nearby,
and possibly connected, buildings.
It is possible to identify
combinations of compressibility, angle of internal friction of the
product, angle of sliding friction between the product and the
hopper wall and the angle of the hopper wall that are likely to
cause thumping. When the conditions have been identified the
critical combination may be designed out of the system. If an
existing silo is found to be thumping, making relatively simple
modifications to change one of the parameters so that the
combination falls outside the critical values will stop the thumps.
Comments, Suggestions and More!
If you would like to comment on the
article or discuss silo design and silo flow problems, please feel free
to click the link below and post your comments. Its a great way to
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comment to our discussion about it in our Help Forum, click this
link:
http://www.powderandbulk.com/cgi-bin/yabb/YaBB.pl?board=general;action=display;num=1139427790
About our Author
David Stuart Dick has a BS degree in
Civil Engineering from the University of Natal in South Africa.
He spent 12 years designing large mining structures and silos as
well as developing a solid’s flow testing laboratory in South
Africa.
In 1982 David joined Jenike &
Johanson Inc where he managed their West Coast office and was Vice
President and Director of R&D for 10 years. He returned to
England in 1997 and started a new consulting firm, working closely
with Solids Handling Technologies, Inc during that time. In 2005 he
relocated to the USA to add to, and participate in the growth of
Solids Handling Technologies.
For more information contact:
- David S. Dick
Solids Handling Technologies, Inc.
1631 Caille Ct
Fort Mill, SC 29708
Telephone: (704) 962-0925
Web site:
http://www.solidshandlingtech.com/
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on our On-Line Help Forum.
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