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Description  |
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This invention relates to a means and method for the reduction of hoop
stresses in silos which contain grain, or other bulk solids.
BACKGROUND OF THE INVENTION
Many silos which were built before the dynamics of discharge were fully
understood, were designed for static loading, but it has been shown that
material pressures exerted on the cell walls increases by a factor of up
to about two and a half when the outloading valve is opened and material
begins to move.
The opening of the valve removes vertical support from the material
directly above it and the stress field changes from "peaked", with lines
of major stress vertical, or near vertical, to "arched" with lines of
major stress arching across the cell.
The "arched" stress field occupies a conical zone which diverges upwardly.
At the point where this conical zone intersects the cell walls, the large
lateral component of force causes a high hoop stress in the cell walls.
This stress exceeds the static stress by a factor of up to two and a half,
and is often large enough to cause overstressing and cracking of cell
walls.
The cost of replacement of a silo is prohibitive, and the main object of
this invention is to provide improvements whereby the hoop stress can be
substantially reduced.
Several methods are available to strengthen the cylindrical walls of an
upstanding silo. One widely used (but basically unsound) method, is the
repair of bulged areas, but even this is expensive. The second alternative
is the use of external strapping on the external surfaces of external
cells only of a group of silos, but this is many times more expensive than
the cost of local bulge repairs. Another possibility which has been
examined has been the use of a steel liner spirally wound within a silo to
lie against the inner surface of a concrete wall but this is even more
expensive than the external strapping. The other alternative (apart from
this invention) is the use of a concrete liner constructed for the full
height of the cell and within an old cell, but the cost of this is so
great that it is not viable.
The object of this invention is to provide improvements which are
economical and feasible.
STATE OF THE ART
This problem has already been the subject of various studies and the
following references are pertinent:
(a) Arnold, P. C., McLean, A. G. and Roberts, A. W. BULK SOLIDS: STORAGE,
FLOW AND HANDLING. Tunra Bulk Solids Handling Research Associates.
(b) Jenike, A. W. GRAVITY FLOW OF BULK SOLIDS, BULLETIN 108, Utah
Engineering Experiment Station, University of Utah.
(c) Riembert, M. & A. SILOS, THEORY AND PRACTICE, Trans Tech Publications,
1976.
(d) Warner, R. F. STRENGTHENING, STIFFENING AND REPAIR OF CONCRETE
STRUCTURES, IABSE SURVEYS S17/81.
(e) Riembert, A. U.S. Pat. No. 4,372,466.
The reader's attention is drawn to a central tube known as an "anti-dynamic
tube" proposed by Riembert, and this employs a tube containing a plurality
of apertures throughout its length, placed at the cell centre and
extending for full cell height, and supported by guy wires fixed to the
cell wall. In principle, the tube and portholes are intended to ensure
that the grain flows into the tube only close to the grain surface, thus
emptying the cell from the top downwards. No mass flow occurs, and no
switch pressures are generated against the cell walls. Although there is
available supporting literature, there appear to be some practical
problems. The small portholes are liable to blockage, thus causing
unsymmetrical flow which in turn generates large lateral forces on the
tube with the possibility of collapse. If flow into the tube is able to
occur at lower tube levels than close to the free grain surface, there
will be mass flow within the bin.
The Reimbert U.S. Pat. No. 4,372,466 also disclosed use of a central tube
(5) which had imperforate walls, and was separately valved from the rest
of the silos. Although this arrangement is capable of effective use, it is
also capable of incorrect use, and if for example, the second discharge
orifice (4) is opened before the first discharge orifice (3), the
arrangement is ineffectual, and high stresses can be imparted to the silo
walls.
Many silo cells have a height to diameter ratio of about three, and the
hoop stress is excessive only when the ratio exceeds about 1.5 (depending
upon the grain used and its moisture content), and this invention seeks to
resolve the excessive hoop stress by dividing a silo cell into a plurality
of notional cells one above the other.
BRIEF SUMMARY OF THE INVENTION
Briefly in this invention the hoop stress in a silo is reduced by
positioning an open ended tube in the lower part of a silo cell, the wall
of the open ended tube having apertures near the silo base allowing the
entry of granular material and a restrictor (or choke) below the
apertures, restricting the material flow, so that when the silo cell first
discharges the material, all of that discharge is through the upper end of
the tube, downwardly through the tube, and outwardly from the silo cell
through valve means beyond the lower end of the tube and restrictor. The
restrictor below the tube apertures ensures that, during that initial
discharge, the tube remains full and granular material does not flow
through those apertures from the silo cell.
More specifically the invention consists of means for reducing hoop stress
in a silo of the type having upstanding cylindrical walls comprising a
grain tube having an open upper end located centrally within the silo and
upstanding from its base and extending part way up a cell, apertures
through the wall of the tube lower end and symmetrical about the central
vertical axis of the tube, a restrictor beneath the apertures restricting
flow through the tube, valve means at the lower end of the tube below the
restrictor, and support means supporting the upper end of the tube from
said cylindrical silo walls.
The zone of granular material flow causing dynamic forces when grain begins
to move upon opening of the outloading valve, is approximately conical in
shape and will have a variation of included cone angle dependent on the
material type used, and the environmental conditions within the silo cell.
However the highest pressure which would otherwise be imparted to the silo
walls, will be avoided if this cone extends through the surface of the
grain, or to the walls near that surface, and not at the interface between
the grain and the inner surface of the silo cell wall at an effective
distance below the surface. Since the cone angle is likely to be small and
its point of intersection with the cell wall relatively elevated, it is
usually sufficient to have within a silo cell a tube with apertures at the
base only, and this greatly reduces the incidence of very high pressures
if the upper part of the silo is emptied first before any material flow or
emptying begins in the lower part. However, for very tall, narrow, silos,
a tube may require further apertures, for example at a height of
1.5.times.silo diameter above the base.
The lateral forces imposed on the tube can be considerable in the event of
asymmetrical flow of the material, and it is necessary to limit the
possibility of such flow by having the tube apertures symmetrical, and
most bulk of the tube body centrally placed within the silo cell, but
nevertheless to resist those forces it is desirable that support stays
should be of sufficient cross-sectional area and of sufficiently high
tensile material that such loads will be fully resisted with a minimum of
further damage to the walls of the silo.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the invention are described hereunder in some detail with
reference to and are illustrated in the accompanying drawings in which:
FIG. 1 illustrates a silo cell showing how grain will discharge in a
conical or funnel manner,
FIG. 2 shows diagrammatically the modification of a silo cell according to
this invention,
FIG. 3 is a section through a cell showing the details of construction and
the open ended tube therein,
FIG. 4 is a section on line 4--4 of FIG. 3 which indicates the manner in
which the support stays are utilised,
FIG. 5 shows the construction of the open ended tube,
FIG. 6 is a diagrammatic view of a silo wherein there is an additional side
discharge tube,
FIG. 7 is a diagrammatic view of a silo wherein a flat bottom silo has a
discharge valve near one edge,
FIG. 8 is a diagrammatic view of a silo wherein the height/diameter ratio
exceeds 3:1, and
FIG. 9 is a diagrammatic view of a silo wherein the silo has more than one
discharge valve in the bottom, in this case two discharge locations.
Referring first to FIG. 1 which is representative of prior art, a silo cell
10 contains grain 11 which discharges through a gate valve 12 shown
diagrammatically, and in so doing there is an interface 13 between the
flowing grain 14 and the stationary grain 15, and it is the existence of
this interface which causes hoop stresses in the walls of silo 10 which
are in the order of two and a half times greater than the static stresses
for which many silos have been designed.
FIG. 2 illustrates a first embodiment of this invention. As will be seen
from FIG. 2, an open ended tube 18 the walls of which are imperforate
except for apertures 19 at its lower end (there being four in all) and
these apertures 19 are placed above a restrictor or choke, in this
embodiment choke plate 20, being an annular plate or moveable diaphragm at
the lower end of the open ended tube 18, and located above the gate valve
12.
The existence of a choke plate 20 reduces the likelihood of grain flowing
through apertures 19 until the grain above the open ended tube 18 has
discharged.
During the discharge of the flowing grain 14 above the tube 18, an
interface 21 develops but this is so high in the cell that it is located
in a low pressure area and will not impart excessive hoop stress to the
cell walls. In some instances interface 21 will pass through the surface
of the grain and thereby impart no dynamic forces at all to the cell
walls. However once the flow has taken place and grain ceases to discharge
from above the tube 18 it will adopt an elevated conical surface (21a)
above the top of the tube and, further outward flow will then take place
through apertures 19 and by this time the pressure will be reduced so that
excessive hoop stresses will not be imparted to the silo cell walls.
As shown in FIG. 3, the open ended tube 18 comprises a lower portion 24, an
upper portion 25 and a plurality of upper extensions 26 which can be
arranged so that the effective length of the open ended tube 18 can be
adjusted for optimum working conditions. It will be seen that the tube 18
even with extensions does not extend as high as half way up the cell 10.
Near the upper end of the upper portion 25 of tube 18, there is provided a
band 29 which extends around it and this is held fast to the walls of cell
10 by support stays 30 which extend through the walls and are secured with
nuts 31 which abut bearer plates 32.
FIGS. 4 and 5 show some details of construction of silos which embody the
invention. FIG. 4 shows how the stays 30 are best secured to a silo which
is part of a group of silos. FIG. 5 shows details of construction of the
lower end of grain tube 18.
FIG. 6 shows how the invention is applied to a silo having a side discharge
tube 34, which projects from grain tube 18 through the wall of silo 10,
and also terminates in an additional gate valve 12. The final emptying of
the silo 10 must be effected through gate valve 12 in the base of the
silo.
In FIG. 7, silo 10 has a flat bottom 35, and the discharge is offset, gate
valve 12 being near a side wall of the silo. Grain tube 18 has a sloping
portion 36 near its lower end. There is a support structure 18a below the
vertical portion of the grain tube.
FIG. 8 shows the arrangement wherein the silo 10 is very tall, exceeding
three times its diameter. In this embodiment the silo has a grain tube 18
which extends for nearly two thirds of its height, and an intermediate set
of apertures 19a set just above an upper choke level plate 20a. In some
applications such a choke plate will not be required. The diameter of the
aperture of choke plate 20a exceeds that of choke plate 20, so that, after
the contents of the upper end of silo 10 have been discharged, a secondary
discharge will take place through apertures 19a, before a tertiary
discharge through apertures 19.
In FIG. 9, silo 10 has two discharge outlets, each with gate valve 12
offset from the silo centreline. The grain tube 18 is bifurcate below an
intermediate set of apertures 19a into two sloping portions of tube, 36a,
each with a lower set of apertures 19, with choke plates 20. There is a
support structure 18a below the vertical portion of the grain tube.
Tests have indicated that notwithstanding the simplicity and low cost of
this invention, hoop stress within the silo can be reduced very
considerably and this avoids the excessive costs of replacement or
expensive repairs referred to herein.
Operation is entirely automatic, and danger of unsymmetrical flow or other
malfunction is slight. The invention is easily applied to existing silos
with a minimum of rework.
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