Note: Descriptions are shown in the official language in which they were submitted.
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HYDROSTATIC PRESSURE RETAINMENT SYSTEM
This application is based upon a provisional application assigned serial
number 60/095,509,
1o filed in the United States Patent and Trademark Office on 08/06/98, by
Robert J. Setlock, Jr., and
titled "Hydrostatic Pressure Retainment System And High Strength Bonding
Method."
This invention relates generally to the field of pressure vessels, and
pertains more
15 specifically, to an apparatus and method for retaining pressurized fluid
within a three-dimensional
structural matrix, and bonding a component to the matrix.
The containment of gases under high pressure generally requires the use of
hollow vessels
2o having outer walls in the form of spheres, cylinders, ellipsoids, tori or
composites of these shapes.
These are the most efficient shapes to withstand the tensile stresses induced
in the walls by internal
pressure, but they are not the most efficient configurations possible. In
addition, these conventional
outer shell shapes seldom fit into the available space efficiently. There are
many examples in the
prior art where interior supports of various configurations have been
introduced inside of pressure
25 vessels to facilitate non-conventional shapes. However, all of the non-
conventional pressure vessels
in the prior art share the characteristic of excessive weight. They all weigh
more than equivalent
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conventional pressure vesseis due to inefficient use of the structural
material. This invention offers
the novel characteristic of weighing less than equivalent conventional
pressure vessels fabricated of
the same material.
Another shortcoming of conventional vessels is that in the event of structural
failure of the
walls due to stresses induced by pressure, the vessel will fail
catastrophically. The sudden release of
compressed gas and wall fragments results in an explosion, with serious
consequences to nearby
personnel and equipment.
In my prior Patent Specification No. W097/27105, USSN 08592004, it describes
the
possibility of having a high pressure storage tank for gases, in particular
having a reinforcement
1o matrix disposed in the tank body and attached to the inner walls thereof,
the matrix displacing no
more than 50% of the interior volume of the tank. However, the high pressure
storage tank suffers
from the disadvantages that it is difficult and expensive to manufacture and
is in general not of
sufficient strength to withstand the high pressures commonly associated with
such tanks.
The previously disclosed, but unpatented work of ERG Aerospace which specified
use of its
15 DUOCEL open cell aluminum foam as an interior support within an irregularly
shaped pressure
vessel also suffers from the disadvantages that it is difficult and expensive
to manufacture and is in
general not of sufficient strength to withstand the high pressures commonly
associated with such
tanks.
Patent Specification No. W098/33004 in the name of Mannesmann AG, describes a
container
2o for the storage of compressed gases in which an outer metallic wall
encloses a hollow chamber in
which is disposed an open-cell metal foam materially connected to the outer
wall. However, again,
such reinforced containers will again not generally be capable of withstanding
the pressures
commonly associated with high pressure storage tanks.
A homogeneous isotropic material such as steel has mechanical properties,
including tensile
z5 strength. that are equal at all points (homogeneous) and in every direction
(isotropic). Thus. steel can
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withstand tensile loads in all three Cartesian axes simultaneously. However,
materials are seldom
used in this manner. Usually, materials are loaded in one direction (axial
loading) or in two
directions (planar loading). Some portion of the material's capacity to
support loading is left unused,
thereby limiting structural efficiency. A filament or fiber in tension is
under an axial load. The
material's capacity to support loading in the remaining two directions is
unused. A conventional
shell type pressure vessel holding compressed gas is under a planar load. The
material at any given
point in the wall is stressed in two directions defined by a plane tangent to
the outer surface at that
point, as shown in FIGS. 1 and 3. The capacity of the material to support
loading in the direction
perpendicular to the given plane remains unused. Further, axial and planar
loading will cause
to angular distortion in the material, which is a primary trigger of
structural failure.
It would be advantageous to load materials equally in all three directions
(hydrostatic loading,
triaxial loading , or three-dimensional loading). Materials under hydrostatic
loading (or triaxial
loading, or three dimensional loading) are fully utilized, and exhibit no
unused capacity for further
loading. This ideal condition also presents no angular distortion or
deformation. The absence of
is angular distortion results in the additional benefit of greatly increased
yield strength for materials in
hydrostatic loading. Thus, less volume of material would be needed to
fabricate the vessel, resulting
in weight savings when compared to conventional outer shell pressure vessels.
Pressure vessels are
an ideal application for structural hydrostatic tensile loading, since the
fluid (liquid and/or gas)
pressure to be counteracted is by definition hydrostatic.
2o Such a structure will take the form of an internal matrix having cavities
or interstices for
containing the pressurized fluid. The matrix carries most of the pressure
induced loading. A lightly
stressed thin solid outer covering is attached to the matrix to retain the
fluid. The ability of each
portion of the matrix material to carry loads in one axis (the worst case), or
in all three axes (the best
case), depends upon the three-dimensional details (or morphology) of the
matrix.
3
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An example of a matrix structure which embodies the characteristics required
to be a best
case matrix is a body of material with a series of substantially spherical
voids which are
interconnected at their point of contact. These points of contact shall form
apertures, wherein the
size of the aperture between adjacent voids will generally not exceed more
than 10%, preferably 5%,
more preferably 2% and advantageously no more than 1 %, of the surface area of
the void. Such a
matrix structure results in a substantial amount of the material being in
hydrostatic (triaxial, or three-
dimensional) tension. The voids are substantially spherical in shape and
preferably of similar size.
The voids are substantially uniform in distribution throughout the matrix, and
preferably in a face
to centered cubic orientation. FIG. 6 is a graphic representation showing the
desired interior
morphology, with homogeneously distributed spherical voids with very small
passages 18,
connecting each void to the adjacent void at each point of contact. FIG. 7
shows the more preferred
face centered cubic orientation. Continuity is maintained in all three
dimensions. The structure of
this hydrostatically optimized morphology demonstrates substantially ideal
structural efficiency,
I S because it places a high proportion of the matrix material under
hydrostatic tensile loading when
used as an inner matrix in a pressure vessel.
FIG. 8 is a photograph of a conventional foam. The support is almost entirely
simple axial
members because of the irregular structure and absence of cell walls. The
morphology of such a
structure will not efficiently support hydrostatic tensile loading of the
matrix material when used as
2o an inner matrix in a pressure vessel.
Relative density is the volume percentage of parent material in the total
volume of the matrix.
In the situation of a best case matrix morphology, the strength of the matrix
will vary linearly with
the relative density. The density required is thus the ratio of the required
hydrostatic tensile strength
(maximum pressure to be stored) to the hydrostatic tensile strength of the
parent material (including
25 the design factor). A face centered cubic orientation provides the greatest
gas capacity obtainable for
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a uniform spherical void morphology, about 67°, o. This equates to a
relative density of about 33%.
Therefore, the matrix will have a relative density preferably from about 30%
to about 35%, or from
about 2% to about 30%, or about 35% to about 50%, depending upon the parent
material and packing
method.
Turning now to FIGS. 1 and 2, the volume of parent material required to
fabricate a
conventional shell type spherical pressure vessel 10 is compared to the volume
required to fabricate a
matrix type spherical pressure vessel 12. These are approximations for thin
walled spheres.
maximum stress: , = a~, = 2t shell thickness: t = 2a
surface area: S = 4~rr' material volume: sn,r = St = 4~r r2 2~
m pr'
to smn = 2 ~ (eq. 60)
Where: a~= maximum stress, parent material
p = maximum pressure stored
r = pressure vessel radius
t = shell thickness
S = surface area
Vfm~, = volume of shell material
For the matrix type spherical pressure vessel 12, the outer covering is not a
critical load
2o bearing structure, and thus will not be considered. The volume of material
required to form the
matrix is the product of sphere volume and matrix density.
Volume of sphere: Vfr,,, = 3 ~r r'
4 p
p n r3
Therefore: v~~~nrrW n r x _ 1.33 (eq, 62)
3 a a
Where: I soh = volume of sphere
V,~m,, = volume of matrix material
5
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Comparing equations 60 and 62 reveals the material efficiency advantage. An
optimum
matrix structure can theoretically yield a 33% improvement over a spherical
shell. Actual practice
indicates an improvement of over 20% is achievable.
s Turning now to FIGS. 3 and 4, the volume of parent material required to
fabricate a
conventional shell type cylindrical pressure vessel 14 is compared to the
volume required to fabricate
a matrix type cylindrical pressure vessel 16. These are approximations for
thin walled cylinders,
ignoring the end closures. Hoop stress , is twice the longitudinal stress ~ in
all outer shell
cylinders.
to maximum stress: "~ax = Q, = pr shell thickness: t = pr
t
surface area: S = 2~r rl material volume: fm~, = St = 2~c rl pr
Q
~ pr21
smt! = 2 (eq. 66)
Q
Where: l = length of cylinder
For the matrix type cylindrical pressure vessel 16, the outer covering is not
a critical load
t s bearing structure, and thus will not be considered. The volume of material
required to form the
matrix is the product of cylinder volume and matrix density.
Volume of cylinder: ~,., _ ~ r 2l
P n p r~ 1
Therefore: 1'mmrr = n ~ l x _
a a (eq.68)
Where: V~,., = volume of cylinder
6
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Comparing equations 66 and 68 reveals the material efficiency advantage. An
optimum
matrix structure can theoretically yield a SO% improvement over a cylindrical
shell. Actual practice
indicates an improvement of over 40% is achievable.
A relatively thin, light outer covering can either be attached to the matrix,
or formed
monolithically with the matrix as one piece. For purposes of analysis, the
thin outer covering can be
modeled as many small, interconnected, substantially circular shaped plates.
Circles of widely
varying radii will be present. The maximum radial size of these circles will
be determined by the
size of the matrix interstices. FIG. 5 shows a polygon having an inscribed
circle of radius = a, and
number of sides = n. The required outer covering thickness is a function of
the fluid pressure, the
allowable material stress, and the polygon radial size. The maximum stress in
polygon shaped plates
is on the outside edge of each plate, according to Roark's Formulas for Stress
and Strain. The
following data is from case number 20 of table 26 in Roark's.
n 3 4 5 _ 6 7 8 9 10
1.423 1.232 1.132 1.068 1.023 0.99 0.964 0.944 0.75
a'' , a''
I S maximum stress: - ~',_q thickness: t '
."ex -
t Amax
Where: Q~,~ = maximum allowable stress
~3z = factor from table
t = ~zqa (eq. 72) a = radius of polygon
Amax q = fluid pressure
t = membrane thickness
An example will illustrate the efficiency of a matrix type pressure vessel.
The circular
polygon, with number of sides = oo, and radius = matrix void radius, has a
,13, value = 0.75, and will
be used as the worst case condition. Assuming a hydrostatically optimized
matrix with a void size of
20 0.0625 inch radius, and made of 6061 T6 aluminum having a yield strength of
40k psi.; thickness
from equation 72 is tabulated below for q = 450 psi, 6k psi, and 15k psi.
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Parameter Low Pressure Med. Pressure High Pressure
Max. Working Pressure,1 SO 2,000 5,000
psi
Burst Pressure 450 6,000 15,000
(3 x Max.), psi
Required Outer 0.00597 0.021$0 0.03447
Covering Thickness,
inches
Solid surface components such as the outer covering, fill nozzle, etc., may be
formed
monolithically with the matrix, or may be attached later.
SUMMARY OF THE INVENTION
In accordance with the invention there is provided a hydrostatic pressure
retainment apparatus
for compressed fluid which is capable of withstanding high internal pressure.
The pressure
retainment apparatus comprises a matrix for carrying loading induced by
pressure of the compressed
to fluid. The matrix structure comprises a body of material with a series of
substantially spherical voids
which are interconnected at their point of contact. These points of contact
shall form apertures,
wherein the size of the aperture between adjacent voids will generally not
exceed more than 10%,
preferably 5%, mare preferably 2% and advantageously no more than 1 %, of the
surface area of the
void. Such a matrix structure can be described as one with nearly closed open
void morphology
15 resulting in a substantial amount of the material generally in hydrostatic
(triaxial, or three-
dimensional) tension. The voids are substantially spherical in shape and
preferably of similar size.
The voids are substantially uniform in distribution throughout the matrix, and
preferably in a face
centered cubic orientation.
Preferably the matrix is a metal, for example aluminum, steel, stainless steel
and the like.
zo
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An outer covering surrounds the matrix, for retaining the compressed fluid
within the matrix.
The outer covering has an inner surface attached to the matrix outer boundary
surface or region. The
outer covering is impermeable to the retained fluid, and is substantially
contiguously supported over
the matrix outer boundary surface.
Transfer means is provided for admitting the fluid into the matrix, and for
discharging the
fluid from the matrix.
A hydrostatic pressure retainment apparatus with a nearly closed void
structure is not
generally attainable by standard metal (or other) foam manufacturing
techniques. Apparatus of this
type can be constructed by using an investment casting technique using small
uniform spherical
balls. For example, a gas containing structure can be made by preparing a
shaped outer skin of a
metal similar to or compatible with the to be formed internal matrix,
generating the internal matrix
by coating beads of a volatile substance such as carbamide with powdered metal
and then adding
these beads to the container, finishing with a layer of powdered metal and
also making sure that there
15 is a manifold or tube access to the system. The unit is then heated at
about 200°C which enables the
metal slurry to stick together whilst the carbamide beads are volatilised and
escape through the
manifold. This 'green' container can then be subsequently sintered in a higher
temperature furnace
to provide the finished structure. Internally, because of the method of
construction, the cells will all
be spherical and inter-connections will be small and at the point of contact
with adjacent cells, hence
2o this will produce a porous morphology with only tiny inter-connections
between each cell, thereby
maximizing the strength of the overall matrix.
Addition of the organic spheres to the canister is best achieved by a "snow-
storm" packing
method to minimize irregularity and structure, however, a simple single sized
cellular structure will
only leave empty 67% of the volume as free space. This volume fraction can be
increased by using
2s spheres of smaller size which will effectively fit in the spaces between
the other spheres.
9
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Experimental work has demonstrated that this is optimized when trie ratio of
diameters is between
7:1 and 10:1 and the proportion of small spheres is 1$-20%. Again by using the
"snow-storm"
packing technique the structure can be made very homogeneous by using the
required size range and
proportion of spheres.
A further embodiment would be to use a powdered metal skin which would avoid
any
shrinkage problems.
The space between the spherical voids orientated in a face centered cubic
morphology can
also accommodate smaller voids where the radius ratio is ds",a" = d,,r~~(~-1 )
wherein ds",a" is the
diameter of the smaller void and d,a,xe is diameter of the larger void. This
type of procedure could be
to repeated with ever smaller spherical voids resulting in lower density
structures. Alternative
manufacturing techniques include, but are not limited to, metal foaming
methods as described in U.S.
Patent No. 5,151,246 in the name of Fraunhofer-Gesellschaft, the contents of
which are incorporated
herein by reference. Such methods can be tailored to achieve the necessary
structure of the
invention, rapid prototype technology and the like.
The invention will be more fully understood, while still further features and
advantages will
become apparent, in the following detailed description of preferred
embodiments thereof illustrated
in the accompanying drawing, in which:
2o FIG. 1 is an elevational view of a spherical shell type pressure vessel
showing the planar
stresses imposed upon the shell;
FIG. 2 is an elevational view of a matrix type spherical pressure vessel
showing the matrix
without the outer covering;
FIG. 3 is an elevational view of a cylindrical shell type pressure vessel
without the end
's closures. showing the planar stresses imposed upon the shell;
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FIG. 4 is an eievational view of a matrix type cylindrical pressure vessel
showing the matrix
without the outer covering;
FIG. 5 is a polygon having an inscribed circle of radius = a, and number of
sides = n;
FIG. 6 is a graphic representation of a hydrostatically optimized morphology,
showing
homogeneously distributed spherical voids with very small passages connecting
each void to the
adjacent void at each point of contact;
FIG. 7 is a graphic representation of a hydrostatically optimized morphology,
showing the
1 o preferred face centered cubic orientation of the spherical voids;
FIG. 8 is a photograph of a conventional foam;
FIG. 9 is a partial sectional, isometric view of a hydrostatic pressure
retainment apparatus
constructed in accordance with the invention;
FIG. 10 is an enlarged view of detail 12 of FIG. 10, showing an artery in
section; and
t5 FIG. 11 is a cross-sectional view through a matrix and a solid surface
component showing the
component material penetration into the matrix.
Referring now to FIGS. 6, 7, 9, and 10, a hydrostatic pressure retainment
apparatus for
2o storing a compressed fluid is shown at 20. The apparatus, which weighs less
than an equivalent
conventional pressure vessel fabricated of the same material, comprises a
three dimensional matrix
22, for canrying loads induced by pressure of the compressed fluid (not
shown). The matrix can be
fabricated in a variety of materials and structural configurations. Polymers,
metals and composites
can be utilized to form a hydrostatically optimized morphology matrix. FIG. 7
is a graphic
?5 representation of a hydrostatically optimized morphology with face centered
cubic orientation of the
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spherical voids, which is the preferred structure to embody the invention. l
he individual cells are
substantially spherical in shape and distributed in a substantially
homogeneous fashion throughout
the matrix structure. (This shows a nearly closed cell structure) Each cell 26
has a continuous wall
28 almost fully enclosing a space or interstice 30 for containing the
compressed fluid. The interstices
s 30 communicate with one another through relatively small openings or pores
32 in the cell wall 28.
The pores 32 ensure generally homogeneous distribution of the fluid throughout
the matrix 22. The
external dimensional limits of the matrix 22 define an outer boundary surface
34.
Substantially all portions of the matrix 22 are in substantially hydrostatic
tension when
carrying the pressure induced loads, by virtue of the three dimensional nature
of the hydrostatically
to optimized morphology 24. The matrix 22 has a relative density preferably
from about 30% to about
35%, or from about 2% to about 30%, or about 35% to about 50%, depending upon
the parent
material and packing method. However, the novel characteristic of weighing
less than conventional
pressure vessels is independent of the relative density. The characteristic of
lower weight depends
entirely upon the structural efficiency of the matrix morphology.
15 An outer covering 36 surrounds the matrix 22, for retaining the compressed
fluid within the
matrix 22, and is impermeable to the fluid. The outer covering 36 has an outer
surface 38 and an
opposite inner surface 40. The outer covering inner surface 40 is attached to
the matrix outer
boundary surface 34. The outer covering 36 is substantially contiguously
supported over the matrix
outer boundary surface 34. The outer boundary surface 34, and therefore the
outer covering 36, can
2o assume any imaginable shape. This is because most of the stress is carried
by the matrix and very
little by the outer covering 36, so that hoop stress is no longer a limiting
factor. Thus, the outer
boundary surface 34 configuration can be symmetrical, or can be of reduced
symmetry (irregular
shape).
The matrix 22 and the outer covering 36 of the hydrostatic pressure retainment
apparatus 20
25 have a total structural mass significantly less than the total structural
mass of an equivalent
12
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conventional shell type pressure vessel of identical total volume measured
over the outer surface 38
of the outer covering 36, made of identical parent material, and designed to
withstand identical fluid
pressure with an identical design factor.
Transfer means is provided for admitting the fluid into the matrix 22, and for
discharging the
fluid from the matrix 22. Specifically, the transfer means comprises at least
one nozzle 42 attached
to the outer covering 36. The nozzle 42 has an inner surface 44, and an
orifice 46 therethrough
communicating with the matrix interstices 30. An optional network of arteries
48 can be provided,
communicating with the nozzle orifice 46 and with the matrix interstices 30.
FIG. 9 illustrates the
artery system 48 in section. The arteries 48 comprise tubes 50 that become
ever smaller and more
1o numerous while progressing from the nozzle orifice 46 toward the matrix
interstices 30. The tubes
SO include a multiplicity of holes 52 to convey the fluid, as depicted in FIG.
10. The arteries 48
enhance the fluid flow rate throughout the system for more rapidly
distributing the fluid to the matrix
22 during admitting, and more rapidly collecting the fluid from the matrix 22
during discharging.
The arteries 48 and matrix 22 may be fabricated as one monolithic structure,
or they may be
15 fabricated separately. The arteries 48 may be fabricated of the same
material as the matrix 22 or of a
different material which is compatible with and attachable to the matrix 22
structure.
When a solid outer covering 36 is not present as a monolithically formed
portion of the
matrix, a novel attachment system, shown in FIG. 11, is employed to integrally
mount a solid outer
covering 36, or a solid surface component 54 to the outer boundary surface 34
of the matrix 22. The
20 outer covering 36 or component 54 is fabricated onto the solid phase matrix
22 while the component
inner surface, or the entire component 54, is in a liquid phase. The component
inner surface is
allowed to penetrate or extend a predetermined distance 56 into the matrix 22.
The component then
solidifies so as to anchor the component to the matrix 22. A strong integral
bond structure results.
This inexpensive attachment is compatible with irregular outer boundary
configurations. Application
25 methods include, but are not limited to, potting, dipping, spraying,
brushing, vacuum dipping and the
13
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like. The outer covering can also be thick enough to provide mechanical
protection from impacts,
piercing and the like. Further, labels can be introduced providing information
on identity, safety
instructions and the like. The covering may also have cosmetic properties and
in some systems
biocompatable material can be employed.
A hydrostatic pressure retainment method is disclosed for storing a compressed
fluid. The
method comprises the steps of: extending a matrix structure 22 in three
dimensions to an outer
boundary surface 34; surrounding the matrix 22 with an outer covering 36
impermeable to the fluid;
attaching an inner surface 40 of the outer covering 36 to the outer boundary
surface 34 of the matrix
22; supporting the outer covering 36 substantially contiguously over the
matrix outer boundary
to surface 34; admitting the fluid under pressure into the matrix 22;
retaining the compressed fluid in
interstices 30 within the matrix 22; retaining the compressed fluid within the
matrix 22 with the outer
covering 36; inducing substantially hydrostatic loading in the matrix 22
material by the pressure of
the compressed fluid; carrying the loading in substantially hydrostatic
tension in substantially all
portions of the matrix 22 material; and discharging the fluid from the matrix
22.
15 Further steps include: attaching a nozzle 42 to the outer covering 36; and
communicating an
orifice 46 through the nozzle 42 with the matrix interstices 30.
Still further steps include: communicating a network of arteries 48 with the
nozzle orifice 46
and with the matrix interstices 30; distributing the fluid through the
arteries 48 to the matrix 22
during admitting; and collecting the fluid from the matrix 22 through the
arteries 48 during
2o discharging.
Additional steps include: juxtaposing an inner surface of a component with the
outer
boundary surface 34 of the matrix 22 while inner surface material is in a
liquid phase; impregnating
the matrix interstices 30 to a predetermined depth with the inner surface
material; and changing the
inner surface material to a solid phase, thereby anchoring the component to
the matrix 22.
25 Another step comprises forming the outer boundary surface in an irregular
configuration.
14
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Yet further steps include forming the matrix 22; and forming the matrix by
using an
investment casting process.
Yet further steps include forming the matrix 22; and foaming the matrix from a
Fraunhofer
type metal foam which has been modified to open small apertures between
adjacent voids.
Yet further steps include forming the matrix 22; and forming the matrix by
using a rapid
prototyping process.
As seen from the foregoing description, the present invention satisfies the
need to provide a
system for retaining pressurized fluid that does not induce planar loading in
a relatively thick wall,
but that utilizes more efficient hydrostatic loading with less material to
significantly reduce weight;
1o that is not limited to the form of spheres, cylinders, ellipsoids, or tori,
but could assume a reduced
symmetry configuration to fit within any given envelope; that will not explode
in the event of
structural failure of the walls; and that includes a method for attaching
solid surface components
securely to a matrix of any surface configuration.
Although the invention has been described and illustrated in the preferred
embodiments,
15 those skilled in the art will make changes that will be seen to be
functional equivalents to the present
invention. For example, the hydrostatic pressure retainment apparatus
described above and depicted
in FIG. 9 is a rectangular parallelepiped. It will be appreciated that any
shape or configuration,
symmetric or unsymmetric can be utilized. It is therefore to be understood
that the above detailed
description of embodiments of the invention is provided by way of example
only. Various details of
20 design and construction may be modified without departing from the true
spirit and scope of the
invention as set forth in the appended claims.
