Silo Damaged by Plastic Powder - A
Case Study
Guest article by David Stuart-Dick of Powder Engineering Systems.One of the largest manufacturers of
Purified Terepthalic Acid (PTA) in the world has had some serious
structural problems with their silos. PTA is a plastic powder that
is processed into terelene and other polymers for manufacturing
fabrics. During production of PTA, the un-purified form, (TA) is
usually stored in large silos before being washed to become PTA.
Problem
At one of this manufacturers
production plants that had been operating for more than twenty years
a problem was noticed that appeared to be serious. One morning,
operators noticed that powder was leaking from the side of one of
two 17-m diameter, 5,000-ton TA silos. On closer examination it was
found that powder was leaking from a crack in the hopper. The hopper
sections of both silos were slightly deformed but the one that was
leaking was worse than the other.
One of the immediate concerns was to
get the silo emptied and to determine the cause of the deformations
and the crack so that a new hopper, with thicker walls if necessary,
could be commissioned. The existing hopper walls varied from
approximately 26-mm thick at the top to 6-mm thick at the outlet.
The crack was at the top of the 6-mm thick plate sections.
The plant operators investigated
various possible operational causes. The possibilities of a high gas
pressure in the silo or a sudden vacuum were considered. Previous
problems with pressure and vacuum relief valves made this a real
possibility.
Powder Engineering Systems was asked
to investigate the cause of the problem and to predict hopper loads
so that remedial work could proceed. With very high production rates
involved and the holiday season approaching, the results were
required quickly.
Silo Analysis
Our engineers determined that the
problem had its beginnings in the way the powder flowed in the silo.
Although the stainless steel walls of the hopper were steep enough
for mass flow, other factors were causing the powder to flow in
funnel flow. In funnel flow a flow channel forms within the powder
so that some powder remains stagnant - sometimes for long periods.
In the PTA silo, temperature gradients driving moisture into the
stagnant regions and other factors allowed the stagnant product to
form hard lumps. Evidently, falling lumps had deformed the hoppers
of both silos and a large lump had caused the one hopper section to
tear.
As a result of our analysis we were
able to confirm that replacing the damaged hopper section with one
based on the same structural design would be adequate. Rather than
trying to design for dynamic loads, which would be more difficult to
predict and would probably require excessive structural stiffness,
the silo should be made to flow in mass flow, as originally
designed. In mass flow the product loads are predictable and are
reasonable. In addition, avoiding lumps in the product would make
production of PTA easier.
Our report included an analysis of
initial and flow loads due to product in the silo. Subsequent
analysis by the client's metallurgists of the piece of plate that
had been torn confirmed that the tear was a result of a local
overstress.
Restoring Mass Flow
To restore mass flow, we recommended
that problems with a slide gate valve interface be corrected and
that the design of the screw feeder be corrected. The existing
feeder had been designed to increase in capacity along its length.
However, while some of the principles of making a correct mass flow
feeder are well known, there are pitfalls. Unfortunately, the
existing screw feeder probably had pinch points along its length.
Screw feeders under both silos in the plant had broken regularly but
they had always been replaced with screws of identical design.
The PTA silo has been corrected and
has been back in operation for a year. According to the
manufacturer: "We are happy that Powder Engineering Systems
could help to identify the root cause of the problems and
recommended course of action. The job was well done and I have not
heard of any problems since".
Thanks Dave for the case history.
Dave can be contacted at:
Mr. David Stuart-Dick
Powder Engineering Systems
Carrington Business Park
Manchester, M31 4YR
United Kingdom
Phone: +44 161 667 4523
Fax: +44 161 667 4524
Web site: http://www.powderengineering.com
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