Hopper Retrofits Save Money
by Joseph Marinelli
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As our economy continues to slow down
companies are resorting to retrofitting their existing equipment as
opposed to buying new equipment. This is not an entirely bad
situation. Sometimes existing equipment that has been retrofitted
can work quite reliably. A scientific approach is required to
identify your problem areas, evaluate your material properties and
rectify your problems with existing solids handling equipment. The
following is a discussion of what is necessary to provide a properly
designed retrofit:
Identify your problem areas
This requires knowledge of solids
flow problems and their results. If you have read our column before,
you are aware of the types of solids flow problems that can occur;
namely, no flow (due to arching or ratholing), erratic flow,
flooding, segregation and limiting discharge rate. Sometimes it is
difficult to determine if your product is arching or ratholing, or
both. Even if you look into the container from the top, you may not
see a rathole, even though it is forming within the product.
Segregation problems are somewhat more difficult to solve and
typically require a site visit by an expert in the field.
Evaluate your material properties
If you read some of the articles
archived on this website, you will definitely notice a common thread
to most of them. In order to reliably identify a flow problem and
handle any bulk solid, you need to evaluate the flowability of your
bulk solid. This requires you to use a reputable testing laboratory
capable of determining your materials':
- Cohesive properties to predict
opening sizes to prevent bridging and ratholing, by determining
your material's cohesive strength as a function of consolidation
pressure.
- Wall friction properties to
determine wall slopes for mass flow, by measuring the friction
that develops between a bulk solid and any hopper wall material.
- Compressibility properties to
determine a material's bulk density as a function of
consolidation pressure, which is required in the analysis of
bridging, ratholing, and hopper angles.
- Limiting discharge rate which
predicts the limiting discharge rate of a solid by measuring its
permeability to air or gas.
Once you have identified your flow
problems and evaluated your material's flow properties, you can
develop possible solutions to those problems and retrofit your
existing equipment.
Modifications to existing
equipment
Modifications to your existing
equipment may be something as simple as installing a liner of
stainless steel or TIVAR 88 (which is an ultra high molecular weight
polyethylene supplied by Poly Hi Solidur). A liner that develops
less friction may allow your material to flow in mass flow and
eliminate ratholing.
Figure 1: Modified Hopper
Perhaps you can "cut off" a
portion of your funnel flow conical hopper and install a properly
designed mass flow hopper to provide an expanded flow pattern. The
mass flow pattern developed by the mass flow hopper will expand into
the funnel flow hopper and, if designed properly, will cause a
stable rathole to become unstable and allow material to flow
reliably. See the Figure 1, which shows an expanded flow approach.
Sometimes it is easier to replace the
entire funnel flow hopper with a properly designed mass flow hopper.
Remember to keep in mind, that the feeder design is as important as
the bin design. If the feeder is not designed properly, modifying
your hopper for mass flow may be a waste of time and money.
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.
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