Over
the last few years several inground floating
molasses storages were constructed in Queensland.
The largest structure has a capacity to hold
up to 10,000 tonnes of molasses.
Structures
are located at Bingira, Isis, South Johnstone & Kingsthorpe.
The molasses stored is used largely as stockfeed
during the mills none processing period. The
sale of molasses directly to farmers by the
mills is a profitable alternative to the sale
of molasses to the molasses pool for those
mills which are located close to the point
of sale. The largest part of the milling operations
in Northern NSW is utilised as stockfeed in
the west of that state.
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The
largest of the storages recently constructed
in Queensland involves an excavation of approximately
140m long, 80m wide and 7m deep. The excavation
and any associated filling was carefully controlled
in accordance with AS3798-1990. The design of
earthworks for such a large structure is preceded
by a preliminary foundation investigation. The
earthworks can be configured to reduce the effects
of particular site weakness`s. The finished surface
of the completed excavation/embankment is required
to comply with close tolerances and surface roughness
standards. Possible foundation movements need
to be reduced to a minimum.
Large
storage structures lined with various polymer
materials have been constructed in the United
States and Australia over the last 20 years.
In the United
States structures with areas of several hundred
thousand square meters have been built.
The
polymer material used is generally Polythene
or Polypropylene based, matched to the application.
The installation of the liner material is carried
out under supervision and quality assurance.
The material is delivered to site in prefabricated
panels and flat rolls and is cut and welded
on site to suit the excavation.
The
welding process is carried out with self-propelled,
self-aligning automatic welders which are fitted
with pre-heating devices. Welds are generally
pressure (Non-destructive) and strength (Destructive)
tested. The long term success of the liner
is dependant upon care taken during the installation
process.
In
the case of a molasses storage a floating roof
is fitted above the floor liner with molasses
being pumped between the two layers. As the
tank fills the floating roof rises. The roof
has a much larger surface area when the tank
is empty than when it is full. The roof is
required to fold as it rises. The folding must
occur in a way that stress points are not created.
The roof is designed to fold into the molasses
along lines that run radially. The roof is
constructed from a mix of different geosynthetic
materials to provide a combination of strength
and flexibility.
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The
case of Molasses fermentation needs to be addressed.
A degassing system is incorporated in the roof
structure to allow the collection and removal
of gas and prevent the entry of water. The quality
of the materials used in construction of the
roof is dependant on the design life of the structure
and significantly effects costs. The all in design
and construction cost of an inground structure
with a capacity of approximately 5000 tonnes,
excluding incoming lines and outlet structure,
is likely to be less than AUD$35/tonne. This
is less than 30% of the cost of a steel structure.
For a structure with a capacity of 25000 tonnes
this unit cost will significantly reduce. |
