When Things Go Wrong : How to Address a Solids Flow
Problem
by Joseph Marinelli, Solids Handling Technologies
 Introduction
As a consultant to the bulk solids handling industry for so many
years, I have been involved in numerous approaches to solids
handling problems. The process usually starts with a telephone
conversation concerning a particular problem. This typically
results in the submission of a proposal for material testing and
conceptual design recommendations.
Some of the problems and their
results that are frequently brought to my attention are as follows:
"I can’t get my material out
because it bridges and ratholes in my bin.” Material
to your process stops flowing as a result of a flow obstruction such
as a bridge or rathole. Your operator opens the gate or turns
on the feeder expecting material to flow freely from the bin.
All of sudden, material stops flowing and he is required to use a
flow aid device such as a sledgehammer to initiate flow. Maybe
you have designed your silo to store 100 tons of product and because
of a stable rathole forming, only 10 tons of your product is
actually live.
“My process is upset due to
erratic material flow.” Flow within your system is erratic
such that processes (driers, kilns, packaging lines, etc.) have to
turn off and on frequently to accommodate flow stoppages. Not
only is the handling equipment affected by this erratic operation,
but the upstream and downstream equipment is also affected by it.
“Sometimes my fine powder
floods from the bin all over the floor.” This is nasty
problem in that fine powders can flow uncontrolled like a liquid and
end up all over the floor. Perhaps you are filling packages
and your product floods the process, overfilling bags and
containers, or at the very least, creating a tremendous dust cloud.
 “The
quality of my final product is affected by the fine and course
particles of my material segregating.” Segregation can
affect the quality of the end product you produce or make. It can
also result in dissatisfied customers who are expecting material
that is within a certain particle size range or density and receive
off spec material. The coarse and fine particles have
separated during handling. (see photo to left)
"No matter how fast I turn my
feeder, I can’t get my material to flow at a higher rate.”
Fine materials can exhibit limiting discharge rate problems such
that no matter how fast you turn your feeder, you may not be able to
achieve the desired rate. The interaction between solids and
the air that surrounds the particles can actually hold them up, thus
limiting your discharge rate. Turning the feeder speed up has no
effect.
“Help! My silo has collapsed!”
(see photo above) Asymmetric flow patterns caused by ratholes,
preferential flow channels, etc. can cause silos to dent, buckle and
even collapse. Collapsing ratholes and bridges can also lead
to silo failure problems. Some things to remember when
discussing flow problems:
Material Properties
- What material(s) are you handling?
- What is their moisture content and
how is it measured?
- What is their particle size?
- Are they exposed to temperature
variations such as dried product entering a bin at 150°F and
cooling to room temperature?
- How long does the material remain
at rest in the silo? This is critical to determine the
effect storage at rest on the cohesive strength of the product.
- What is the bulk density?
- What discharge rate do you require
from the silo or bin?
Silo Geometry
- What is the current bin diameter
or capacity requirements for a new bin?
- What is the shape of the existing
hopper?
- What is the size of the opening?
- How is it filled?
- What is the silo or bin fabricated
from, lined or coated with?
- What type of feeder is used to
control discharge rate?
- What equipment is upstream and
downstream of the bin or silo?
This list is just the beginning of
the information one needs to collect to evaluate a flow problem or
design a new bin or silo. Other information such as drawings
or sketches of the existing equipment is always necessary.
Segregation problems typically require a site visit in order to
fully understand the cause of the segregation problem and provide
reliable solutions to those problems.
Contact our author
Joseph Marinelli
Solids Handling Technologies
1631 Caille Court
Fort Mill, South Carolina 29708
Telephone: 803-802-5527
Fax: 803-802-0193
E-mail
Joe@SolidsHandlingTech.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.
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