Note: Descriptions are shown in the official language in which they were submitted.
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~PPARATUS AND METHOD FOR
ARRESTING DUCT:[LE FRACTURE PROPAGATION
BACKGROUND OF THE INVEWTIO~ -
Objects of metal and other materials which are
subjected to high stresses tend to fail by developing one Gr
more propagating .duc,ile fractures, which are commonly
called free-running cracks. The range of objects which
exhibit such.propagating ductile fractures is too large for
all of the objects to be mentioned specifically in this
application, but it includes compressed gas cylinders,
blades of fans and windmills, vehicle axles, pipes and so
on. One of the areas which presently appears to be facing
the greatest potential problems due to propagating ductile
fractures is that of pipes, and especially the relatively
large diameter pipes which are used in pipelines.
Pipelines of a relatively large diameter that are
formed by pipes of steel or other meta~ are playing increasingly
important roles in the transportation of gases, such as
natural gas, CO2, and other volatile fluids. For example,
with the increase in domestic gas reserves and a corresponding
increase in the use of natural gas, there will be a corresponding
increase in the need for pipelines for transporting the gas
at gathering pipelines and through long-range transmission
lines. '.
Especially if flawed or damaged, and if they
contain fluid at relatively high pressures, these types of
pipes are susceptible to a propagating ductile fracture, or
free-running crack, that will travel at velocities of 400-
900 feet per second. The probability of this type of fracture
initiating is increased if the product in the pipeline is
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corrosive, either because of the gas itself or because of
moisture, which is often contained ln the gas. The probability
is also increased in the case of steel pipes carrying natural
gas containing large quantities of hydrogen sulfide, which
has been known to create hydrogen emb~ittlement. Furthermore,
any moisture in the gas and any C02 which may be present
will generate carbonic acid, which can also damage steel
pipes. In addition, the problem of failure is significantly
compounded in a cold climate. ~loreover, a propagating ductile
fracture or free-running crack can be caused by impact from
an external force such as a trenching machine accidentally
striking the pipeline, earthquakes, and the like.
The tendency for objects to fail by a propagating
ductile fracture is gener'ally due to the nature of the
material of which the object is made'and the manner in which
the material is worked or shaped to form the object. For
example, many pipes for pipelines are made from a metal
block which'is elongated more in one direction than in another
by being passed between pairs of rollers or similar means.
Such a method produces an elongated plate which is then
formed into a U configuration by bending it about its longitudinal
axis. The U-shaped plate is further bent into a cylindrical
configuration by bringing the sides of the U into abutment
and joining the sides along the length thereof, such as by
welding, to form a substantially straight longitudinal seam.
'In such a pipe, there is considerably greater strength in
the longitudinal direction than there is in the circumferential
or hoop direction, which defines an axis of inherent weakness,
and, as a result, the pipe is able to withstand greater
stress in the longitudinal direction than in the hoop direction.
Therefore, when such a pipe fails, it is the result of a
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hoop of indeterminate width being broken and the ends of the
broken hoop being separated. Then, adjacent hoops are
broken as the result of the failure of the first hoop, and a
line of separating hoop ends moves longitudinally along the
pipe to define a propagating ductile fracture.
Although there ha~e been numerous proposals to
limit ductile fracture propagation in objects and especially
in pipelines, including the use of heavy walled pipes,
cables, concrete abutments, valves and metal sleeves, they
have all bee`n less than completely satisfactory.
For example, U.S. Patent No. 4,195,669 to Ives et
al discloses arresting ductile fracture propagation by
providing an encircling mass of material around the pipe at
preselected intervals as a circumferential restraint.
However, the steel collar, the steel cable windings and the
reinforced concrete cast disclosed by Ives et al are all
quite heavy and difficult to handle and install. In addition,
the crevice between the encircling masses and the pipe, and
the crevices between adjacent windings in the case of steel
cables, are subject to the ingress of dirt and moisture and
the resultant crevice corrosion, which weakens the pipes.
Moreover, the corrosion attacks the encircling masses themselves,
especially in the case of cables.
V.S. Patent Nos. 4,148,12~ and 4,224,966 to Somerville
disclose a method of applying a band-type crack arrestor
over the outer diameter of a pipe and applying radial force
to the inner diameter of the pipe to engage the arrestor in
a tight fit. The.disclosed crack arrestors are preferably
bands or rings of the same metal as the pipeline. Therefore,
they suffer from the same g_eat weight and handling and
installation problems as the encircling masses of the Ives
- et al patent. In addition, the tight fit of the pipe with
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the edges of the crack arrestors tends ~o cause stress
concentrations in the pipe where the crack arrestor edges
engage it. Of course, stress concentrations can lead to
premature failure of the pipe. The Somerville patents also
disclose that the bands can be fiberglass, but state nothing
more about fiberglass bands. If the band is a typical
molded ring of randomly oriented glass fibers, the thickness
required to stop a crack would be such as to make the band
very bulky and unwieldy.
U.S. Patent No. 3,870,350 to Loncaric discloses a
pipe having zones of increased crack resistance due to
cylindrical steel members surrounding the pipe, to which
they are at least partially welded. The steel members are
heavy and, thus, difficult to handle and install, as well as
being subject to corrosion.
Besides the need for crack arrestors, corrosion
protection is desirable for pipes and other objects, even
where no crack arrestors are involved, but to provide corrosion
protection for a pipe in the region of a crack arrestor and
to apply it in a manner which is compatible with the presence
of crack arrestors presents additional problems.
SUMMARY OF TB INVENTION
Accordingly the present invention seeks
to provide an improved apparatus and method for arresting a
propagating ductile fracture in an object, such as a pipe.
The present invention further seeks to
provide an apparatus and method of the above type in which
the arrest of propogating ductile fractur~ is assured with a
minimum of added labor and materials, and at a reasonably low
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cost and which is lightweight, corrosion-resistant and capable
of protecting the object from impacts.
Toward the fulfillment of these and other objects,
a plurality of crack arrestors are placed around the object,
encompassing and engaging the object at spaced intervals,
each crack arrestor defined by a band of continuous,
unidirectional, lightweight, high-strength non-metallic fibers
encapsulated in a resinous material.
The invention in one aspect pertains to a crack
arrestor for arresting a propagating ductile fracture in a
pipe containing fluid under a pressure sufficient to subject
the pipe to the possibility of a ductile fracture propagating
at a high speed in a predetermined direction, wherein the
crack arrestor comprises means for stopping the propagating
ductile fracture, and the stopping means includes a band for
encompassing and constraining the pipe. The improvement
comprises the band including a plurality of substantially
unidirectional continuous, high tensile strength, non-
corrodible, lightweight non-metallic fibers encapsulated in
a resin matrix, the fibers defining with the resin matrix
a fluid impervious mass.
The invention also comprehends a method of arresting
a propagating ductile fracture in an object in which the
ductile fracture tends to propagate in a predetermined
direction comprising wrapping a band of continuous, high
tensile strength, non-metallic fibers in a resin matrix around
the article in a direction perpendicular to the direction,
and curing the matrix. An alternative is to wrap the band
first around a mandrel, cure the resin matrix to form a
hardened band, remove the band from the mandrel and secure
the hardened band in place on the object so that the fibers
are perpendicular to the predetermined direction.
Some non-metallic fibers have very high tensile
strengths, while being lightweight, corrosion-resistant
and very inexpensive. Glass fibers, for example, have
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a tensile strength which is approximately four times the
strength of steel. As a result, continuous glass fibers which
are aligned in one direction, such as in a roving of a
plurality of parallel flexible glass strands, have tremendous
strength in the direction parallel to their length. Therefore,
a plurality of continuous unidirectional glass fibers in a
plurality of parallel rovings wrapped around an object which
tends to develop propagating ductile fractures or free-
running cracks, in a direction perpendicular to the cracks,
equalizes the hoop strength of the object with its longitudinal
strength and greatly increases the stress that the object
can endure without failing. Moreover, glass fibers wrapped
around an object in a band in the circumferential or hoop
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direction can stop a failur~, in the form of a free-running
crac~, after it has begun.
~ ands of such continUous~ unidirectional slass
fibers are well suited for application to pipes as crack
arrestors, and can be wrapped ~t selected positions on ~ipe
sections in the shop or can be wrapped around pipe already
installed at a job site. The glass fi3ers can also be
applied where there is concern fox stress concentrations,
such as at pipe joints and at corners-or elbows. ~oreover,
such bands can be wrapped either by machine or by hand.
In the crack arrestors, the continuous unidirectional
glass fibers are encapsulated in a matrix of resinous material
which is later cured, such as asphalt enamel, coal tar
enamel, polyethylene, or epoxy. The glass fibers are lightweight
and not subject to corrosion and, once they are encased in
the cured resin matrix, the resultant crack arrestor is
immune to the corrosion and other environmental problems
which plague the crack arresting devices of the prior art.
In addition, the crack arrestor according to the present
~0 invention forms a bond with the object, providing corrosion
protection for the pipe, eliminating the ingress of dirt
and moisture, and preventing any crevice corrosion. ~oreover,
the crack arrestor is resilient and energy-absorbent, thereby
protecting the pipe from external impacts and avoiding the
induction of stress concentrations in the pipe.
The crack arrestors according to the present
invention can be wrapped on bare pipe; they can be wrapped
on pipe completely coated with a corrosion-resistant substance;
and they can be wrapped on pipe having a wrap laden with a
corrosion-resistant substance. The glass fibers of the
crack arrestors can be saturated with the resinous material
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by being passed through a resin bath as they are wrapped on
the pipe, or the fibers can be saturated with the Iesinous
materials by coating rollers ~r spray heads after they are
in place on the pipe. As another alternative, the fibers
can be previously impregnated with the resin, wrapped on the
pipe while dry, and then heated to allow the resin to flow.
The glass fibers can be drawn from a plurality of spools and
aligned in parallel as they are wound on the pipe, or they
can be pre-aligned and held together by cross filaments to
form mats which are wound around the pipe.
When the band of glass fibers in a resinous matrix
is wound around the pipe to the appropriate thickness, the
matrix is cured to form a hardened, fluid impervious, corrosion
resistant, energy absorbing, lightweight mass. The curing
is accomplished by such suitable means as the application of
heatr ultraviolet radiation, or catalyst. Where the pipe is
previously treated in a process which requires the application
of heat, as in the case of coating the pipe with fusion bond
epoxy for corrosion resistance, the glass fibers and the resinous
matrix can be applied immediately after 'he aforementioned
process, and the residual heat from the process can be
utilized to cure the resinous matrix, thereby eliminating
the need for any additional step to cure the matrix.
The present invention also contemplates forming
the crack arrestor separate from the pipe by wrapping a band
of glass fibers in a resin matrix on a mandrel, curing it,
removing it from the mandrel, and then slipping it over the
pipe when and where it is convenlent. The crack arrestors
of the present invention are ve~y lightweight, so that a
crack aXrestox many feet in diameter can easily be lifted
and installed by one man. No special means of securing the
- crack arrestor to the pipe is required. All that is needed
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is some means of preventing the crack arrestor from moving
along the pipe, such as filling the space between the crack
arrestor and the pipe with foam material, such as a foa~
plastic, which eliminates the possibility of crevice corrosion.
Due to the strength of the glass fibers and the
ability of the resin to absorb energy, the crack arrestors
protect f~om external impacts the portions of the pipe which
they cover and also prevent or reduce fragmentation of the
pipe in the area of the crack arrestor upon failure of the
pipe.
BRIEF DESCRIPTION OF THE DRAWINGS
The above brief description, as well as further
objects, features and advantages of the present invention
will be more fully appreciated by reference to the following
detailed description of a presently preferred but nonetheless
illustrative embodiment in accordance with the present
invention, when taken in conjunction with the accompanying
drawings wherein:
Fig. 1 is a perspective view of a crack arrestor
according to the present invention being wound around a pipe;
Fig. 2 is an elevation of the crack arrestor of
Fig 1 in place on a bare pipe;
Fig. 3 is an elevation of an alternate form of the
crack arrestor being wound around a coated pipe;
Pig. 4 s an elevation of the crack arrestor of Fig. 1
in place on a pipe wrapped for corrosion resistance;
Fig. 5 is an elevation of another embodiment of
- he crack arrestor according to the present invention;
Fig. 6 is an elevation of still another embodiment
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of the crack arrestor according to the present invention;
Fig. 7 is an elevation of yet another embodiment
of the crack arrestor according to the present invention;
Fig. 8 is a cross section of a furthe~ embodiment
of the crack arrestor according to the present invention;
Fig. 9 is a cross section of a still fuFther
embodiment of the crack arres~or according to the present
invention;
Fig. 10 is yet a further e~bodiment of the crack
arrestor according to the present inYention; and
Fig~ ll is a transverse cross section of a Pipe
having mounted thereon another, separately formed embodiment
of the crack arrestor according to the present invention.
DETAILED DES~ OF THE ~x~R~ EMBODIMENT
As can best be seen from Fig. l, the crack arrestor
according to the present invention, which is ge~erally
designated by the reference numeral 10, is wound around an
object, such as a bare pipe 12, which is mounted on a rotating
mandrel 14. Although a pipe is illustrated in Fig. l, it is
understood that the crack arrestor can similarly be wound
around a compressed gas cylinder, a windmill blade or other
object which might develop propagating ductile fractures, or
free-running cracks.
The crack arrestor comprises a band made up of a
plurality of rovings 16, each of which includes a large
number of continuous unidirectional, lightweight, high-
strength, inorganic, ~onconductive, nonmetallic fibers
such as glass fibers. Although glass fibers will be
referred to throughout the specification, similar fibers
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having a high tensile strength, such as Kevla~ may also be
used. The rovings 16 are drawn from a plurality of spools
18 contained in a creel 20 and are fed through gathering and
aligning devices 22 and 24 prior to being drawn through a
bath 26 containing a resin 27 such as epoxy, asph~lt enamel,
coal t2r enamel or polyethylene, in which they are saturated
with the resin. The ro~ings 16 cQntinue on from the bath 26
to a roving applicator 28 which is mo~nted for movement on
a bar 30 arranged parallel to the rotating mandrel 14, and
some device is usually pro~ided for removing excess resin.
The roVing applicator 28 can be moved back and forth along
the bar 30 between predetermined limits to wind on the pipe
12 a band having the desired width. Moreover, additional
layers of the continuous, resin-encapsulated fibers can be
wound onto the pipe 12 until a band or crack arrestor 10
having the appropriate thickness is achieved.
The apparatus shown for winding the crack arrestor
10 on the pipe 12 is merely exemplary, it being understood
that the glass fibers contained in the rovings 16 can be
wound on the pipe 12 without the resin, and the resin 27 can
be applied to the glass fiber windings on the pipe 12 by
coating rollers, spray nozzles or other suitable devices.
However, applying the resin 27 to the rovings 16 prior to
the winding of the rovings 16 on an object has the advantage
that the tackiness of the resin 27 causes the rovings 16 to
adhere to the object, there~y aiding in the starting of the
windings. In any of the methods described, sufficient resin
is applied so that the crack axrestor 10 comprises continuous,
unidirect~onal glass fiber~ encapsulated in a matrix of the
resin 27, and so that it foxms a continuous bond with the
object, leaving no space for the ingXess of dirt oX moisture
which is apt to cause corrosion.
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Beside~ the pipe 12 on the rotating mandrel 14
shown in Fig. 1, the crack arrestors 10 can be.applied to
stationary objects by other machines, and the crack ar~estors
can be applied to either rotating or.stationary objects by
hand. Some of the machinery is.suitable for applying the
crack arrestors l0 in the f~eld as well as in the shop. When the
crack arrestors 10 are in place, the r-esin 27 is cured by
the application of heat, ultxaviolet radiation, catalysts or
other suitable means to form a haxdençd, ~fluid LmperVioUs,
corrosion resistant, energy absorbing, lightweight mass.
The glass fibers can be previously impregnated with
the resin 27 and applied to the pipe 12 in a dry state. Once
the pre-impregnated glass fibers aXe in place, the application
of heat to the fibers causes the resin 27 to run, thereby
forming a single mass of glass fibers encapsulated in a resin
matrix. The further application of heat will cure the resin,
as will any of the other curing processes previously described.
Preparatory to treating a pipe according to the
method of the present invention, the pipe is preferably cleaned
by either sand or grit blasting or by mechanical scraping and
wire brushing to render the pipe surface free from oil,
grease, dust, moisture and non-adhering mill scale. As is
illustrated in Fig. 2, the crack arrestor 10 is shown in
place, bonded to the bare pipe 12. Although the pipe 12 in
Fig. 2 is bare, the crack arrestor 10 can also be applied to
coated pipe~such as pipe have a coating for corrosion resistance.
For example, a hot, relatively ViscoUs substance, such as coal tar
enamel, or an asphalt enamel can be applied to the primed
outer surface of the pipe, as disclosed in detail in Applicant's
30 copending application, Cdn.. Serial No. 409,~572, ~iled August 17,
1982. Alternatively, the pipe can be precoated with a
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fluidized epoxy powder which is sprayed onto the preheated
outer pipe surface, by an el'ectrostatic process, or the
like. Still further, a coating of extruded polyethylene can
be extruded onto the outer pipe surface in a conventio~al
manner.
~ s is-illustrated in Fig. 3, a crack arrestor 10'
is partially wound ar'ound a coated pipe 12', the continuous
glass fibers in the crack arrestor 10' being contained in
rovings 16' held together by cross filaments 32 to define a
mat 34. The mat 34 is saturated with resin 27 and wound
around the pipe 12' to form another embodiment of crack
arrestor 10' according to the present invention. Mats of
other configurations can also be employed.
Where a crack arrestor according to the present
invention is applied to a coated pipe, as in the case illustrated
in Fig. 3 in which the crack arrestor 10' is wound around
the coated pipe 12', the coating process involves the heating
of the pipe. In such a case, the wrapping of the crack
arrestor 12' can be done immediately after the coating so
that the residual heat from the coating process cures the
resin matrix of the crack arrestor, thereby eliminating the
need of a separate step for curing the resin matrix.
As is illustrated in Fig. 4, the crack arrestor 10
can be wound around pipe 12'' already having a wrapping,
such as a helical wrapping of paper, fabric or other material
treated with asphalt or other anti-corrosion substance to
provide corrosion protection fo~ the pipe 12''. The wrapping
can also be an adhesive coated t~pe of polyethylene or
polypropylene. The crack arresto~ 10 of Figs. 1 and 2 is shown in
place over the wrapping 36, but the'crack arrestor 10' of Fig. 3,
defined by the resin saturated mat 34, can also be used over
the wrapping 36, as can other forms of the crack arrestor
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according to the present invention hereinafter to be described.
The crack arrestors are ordinarily wound to a
width and thickness which will stop the propagation of a
crack, and are placed at predeter~ined inter~als along a
pipeline. For exa~ple, one crack ar~estor 10 can be provided
for each section of pipe tn the PiPeline. .It has been found
that, in most instances, a crack a~restor according to the
present invention, having a width less than the diameter of
the pipe and a thickness less than the thickness of the pipe
is sufficient, by itself, to.stop a propagating fracture.
Bands having a width equal to approximately two-thirds of
the diameter of the pipe and having a thickness equal to 60
of the thickness of the pipe have successfully stopped a
crack.
Crack arrestors can also ~e applied such that a
band of glass fibers in a resin matrix is insufficient to
stop the crack by itself, but a series of bands are provided
so that, acting together, they decelerate and stop the
propagation of the crack. For example, as is shown in Fig.
5, a continuous helical band 36 of glass fibers in a resin
matrix can be applied in a helix along the entire length of
the pipe 12, being wound right across joints 38, usually in
the form of welds, between pipe sections 12a, 12b and 12c.
The helix has a pitch such that there is a space between
adjacent revolutions of the helix. As an alternative, a
plurality of closely spaced mini-bands 40 can be applied
along the pipe 12 to decelerate and stop the propagation of
a crack, as is illustrated in.~i~. 6.
In another arrangement, alternate or periodic
30 sections ~f the pipe 12, such as pipe sections 12b and 12d,
can be wrapped with continuous glass fibers in a resin
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matrix ~o define a crack arrestor 42 extending the entire
length of the pipe section, as is shown in Fig. 7. Of
course, with this arrangement, one crack arrestor 42 is
sufficient to arrest a crack, and only half of the pipe
sections or ~ewer need any cxack axrest treatment.
ordinarily, the windings of a crack arrestor accord-
ing to the invention have rectangular cross-sectional configur-
ations, although configurations other than rectangular can
be used. As-is -shown in ~ig. 8, the lateral edges 45 of the
crack arrestor 44 are be~elled. As is shown in Fig. 9, the
cross section of the crack arxestor 46 h~s an arcuate section
such that the crack arrestor 46 is thickest at its center
and is tapered at its edges 47. The cross sections of the
crack arrestors 44 and 46 of Figs. 8 and 9, respectively, are
stippled to represent the cross sections of the glass fibers.
However, there are thousands of fibers in parallel, uni-
directional relationship, each fiber preferably having a
diameter less than .001 inch. The crack arrestors according
to the present invention are wound essentially without tension,
~ only sufficient tension being applied to permit the glass
fibers to be wound neatly and orderly. ~s a result, the crack
arrestors do not transmit significant stress to the pipes
they wrap, and thus do not tend to produce stress concentrations
in the pipes, especially along the edges of the crack arrestor.
However, both of the embodiments of Figs. 8 and 9 further reduce
the stress between the crack arrestor and the pipe 12 along
the edges of the crack arrestor, thereby eliminating any
possibility of the crack arrestor causing stress concentrations
in the pipe 12 at those places.
Since the crack arresto~s according to the p~esent
invention can be wrapped onto the pipe in the field, they
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can be placed directly over the joints 38 in the pipe 12,
between the pipe sections 12a, 12b and 12c, as shown by the
crack arrestors 10 in Fig. 10,` thereby providing crack
arrest capabilities and at thç same time protecting the
joints 38, which u~ually requi~e some separate form of
protection. Although ~ig. 10 shows the crack arrestor 10 of
Figs. 1 and 2 in place ove~ the ioints 38, the other embodiments
of the crack arrestor according to the present invention,
such as the crack arrestor 10' of ~ig. 3 employing the mat
34, could be placed over the joints 38.
Fig. 11 is a transverse cross section of the pipe
12 having mounted thereon a separable crack arrestor 48,
which is formed of the same materials as the other embodiments
of the crack arrestor described herein, but which is formed
separate from the object to be treated, 2S by being wound
onto a rotating mandrel, cured and removed. Since the glass
fibers and the resin are very lightweight, a crack arrestor
for a pipe many feet in diamete~ is light enough to be
lifted and put into position by one man. The crack arrestor
48 is slipped over the pipe 12 and secured to the pipe to
prevent it from sliding along the length of the pipe. One
suitable mechanism for holding the separable crack arrestor
48 in place on the pipe 12 is the provision of a foam filling
50, such as a plastic foam, in the space between the pipe 12 and
the crack arrestor 48. The foam filling 50 not only bonds
the crack arrestor 48 to the pipe 12, but also fills in the
space and prevents the ingress of dirt and moisture, thereby
preventing the possibility of crevice corrosion.
Although it is apparent from the foregoing that
the present invention pro~ides s~gnificant advantages in the
arrest of propagating ductile fractures or running cracks,
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it is understood that various changes and modifications may
be made without departing from the spirit and scope of the
present invention as recited in the appended claims and
their legal equavilents.
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