Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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FINNED STRAINER
FIELD OF THE INVENTION
The present invention pertains to the field of filters used to remove debris
from water
being sucked into a piping system. More particularly, the present invention
relates to the field of
filters used in nuclear power plants.
BACKGROUND
Nuclear plants have various safety systems to ensure that the nuclear fuel in
the reactor
core remains cooled in all credible accident scenarios. One such scenario is a
"loss of coolant
accident," in which an external pipe is postulated to break, allowing a large
amount of water to
escape from the reactor cooling system. This water may dislodge solid debris
from neighbouring
pipes or other reactor structures. The water, along with some of the dislodged
debris, will flow
to the lowest parts of the reactor building into a sump. Plants are equipped
with safety systems
that pump water from the sump back into various reactor cooling systems.
Strainers on the pump
intakes ensure that any debris large enough to clog equipment in these systems
is prevented from
entering. Depending on the type of debris, the first layer to deposit on the
strainer may form a
finer filter than the underlying screen, and catch many smaller particles.
Strainers must have enough screen area that the debris layer on the strainer
is not too
thick to cause unacceptably high restriction to flow. Strainers must also be
as small as possible
to fit into the available space. Therefore compactness, i.e., accommodating
the most screen area
in the smallest volume, is important.
Conventional strainers in many nuclear plants are simple box-type devices
mounted over
the pump intakes. Newer more advanced strainers often have an irregular
surface to increase the
surface area.
This background information is provided for the purpose of making known
information
believed by the applicant to be of possible relevance to the present
invention. No admission is
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necessarily intended, nor should be construed, that any of the preceding
information constitutes
prior art against the present invention.
SUMMARY OF THE INVENTION
In accordance with one aspect of the invention there is provided a strainer
for filtering
debris from a fluid comprising an elongated header defining an enclosed
flowpath, having an
outlet in fluid communication with a suction source and a plurality of inlet
apertures disposed
along the length of said flowpath, said flowpath exhibiting a pressure drop in
the direction of
fluid flow; a strainer element disposed in each said inlet aperture for
straining debris from fluid
entering said flowpath; and a flow controlling device for maintaining
substantially uniform fluid
flow through strainer elements located at different positions along said
flowpath.
In accordance with another aspect of the invention, the flow controlling
device comprises
an orifice for producing a pressure drop between an inlet aperture and the
flowpath located at a
position closer to said suction source that is greater than the pressure drop
between an inlet
aperture and said flowpath located at a position farther from said suction
source. The orifice can
be in the form of a nozzle for accelerating the fluid entering said flowpath
in a direction
substantially parallel thereto and can be formed in a baffle disposed in said
header, the baffle
defining a collection channel enclosing a plurality of apertures.
In accordance with another aspect of the present invention, the header has a
generally
planar side-wall and the inlet apertures are a series of substantially
parallel slots formed in the
side-wall in a direction transverse to said flowpath. The strainer elements
can be in the form of
flat-surface fins projecting outwardly from the apertures in the planar side-
wall.
In accordance with another aspect of the present invention, there is provided
a strainer for
filtering debris from a fluid comprising a header defining an enclosed volume
and having an
outlet in fluid communication with a suction source, said header having a
plurality of inlet
aperture slots formed therein, a fin-like strainer element projecting
outwardly from each aperture
slot for straining debris from said fluid, each said strainer element
comprising a perimeter frame
and a pair of fluid permeable screens fixed thereto in opposed spaced
relation, and at least one
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fluid flow channel therebetween in fluid communication with said enclosed
volume through a
marginal side edge of said frame and said aperture slot. The fluid permeable
screens can be
formed of perforated metal sheet or mesh. A corrugated metal spacer can be
disposed between
the fluid permeable screens for maintaining said fluid permeable screens in
spaced relation, and a
plurality of flow channels can be defined between said corrugated metal spacer
and said fluid
permeable screens. The perimeter frame can be impermeable to fluid except at
said one marginal
side edge.
In accordance with another aspect of the present invention, the fluid
permeable screens
are each formed of corrugated metal mesh having a plurality of parallel peaks
and valleys, said
screens being maintained in opposed spaced relation by contact at alternating
peaks and defining
a plurality of said flow channels therebetween.
BRIEF DESCRIPTION OF THE FIGURES
Figure 1 is an isometric view of a strainer module for connection to a pump
intake in
accordance with an embodiment of the present invention.
Figure 2 is an exploded view of the strainer module shown in Fig. 1;
Figure 3 is a cut-away isometric view of a strainer module mounted directly
onto a sump
in accordance with an embodiment of the present invention;
Figure 4 is a cut-away isometric view of a section of a flat-surface fin in
accordance with
an embodiment of the present invention;
Figure 5 is a cut-away isometric view of a section of a corrugated-surface fin
in
accordance with an embodiment of the present invention;
Figure 6 is an exploded view of a corrugated-surface fin in accordance with an
embodiment of the present invention;
Figure 7 is an isometric section view of flow equalization devices in
accordance with an
embodiment of the present invention; and
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Figure 8 depicts a corrugated-surface fin in accordance with an embodiment of
the
present invention.
DETAILED DESCRIPTION OF THE INVENTION
Referring to Figures 1 and 2, the strainer module of the present invention
comprises
elongated header 3 that defines an internal fluid flowpath that is in fluid
communication with a
suction source through pump intake pipe 2 which may be located in the floor or
wall through one
or more connection(s) 1. Header 3 has a generally planar sidewall with a
plurality of inlet
apertures 9 in the form of a series of substantially parallel elongated slots
disposed along the
length of the header to accommodate fins 4. The inlet apertures are oriented
in a direction
transverse to the fluid flowpath within header 3. Strainer elements in the
form of hollow flat-
surface fins 4 may be mounted on the sides (as shown in Figure 1), top, or
bottom of header 3
and project outwardly from inlet apertures 9. Fins 4 may have a uniform or
variable spacing and
are located by mounting frames 5 and braces 6. In a preferred embodiment, fins
are easily
removable, using a pins 10 and bolts 11, but they may also be permanently
attached to header 3.
Water enters the strainer through fluid permeable screens 7 on the surface of
the fins 4,
leaving debris on the screens. Water then flows through the fluid flow channel
in hollow core 8
of the fin 4 towards header 3. Various portions of header 3, particularly the
portion between fin
slots 9, may be made using fluid permeable material to increase the filtration
area. Header 3 may
have one or more baffle plates 12 to provide structural support for the sides
to resist high suction
pressures. Baffle plates 12 have large holes 13 to ensure flow velocity in the
fluid flow channel
in header 3 is the same above and below the baffle.
The ends of each header 3 have flanges 14 that allow adjoining modules to be
attached
together. Modules may be attached together, or they may be independently
mounted with seals
between modules. Mounting frames 5 may be provided under the module. Mounting
frames 5
have adjustable-height mounts 15 that allow the device to be installed on
floors that are not level.
Figure 3 shows an alternative embodiment of the present invention. This
embodiment is
useful for situations where there is a pre-existing sump 45 with a cover 46
(which could be pre-
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existing or installed specifically to support the strainer modules) and a pump
intake 47 inside the
sump 45. Strainer module 48 comprises fins 49 mounted in a frame 50 with
appropriate bracing.
For simplicity, only one module has been shown in Figure 3. If required, a
plurality of strainer
modules 48 can be mounted in a similar manner over sump 45.
Flow enters fins 49 in the same manner as described above in relation to the
embodiment
shown in Figures 1 and 2, but then flows directly into sump 51, and then into
the pump intake 52.
Pump intake 52 may be modified to reduce inlet losses. No separate collection
header is
required for this arrangement, because sump 51 itself performs this function.
Undesirable bypass
flow between the sump cover 46 and the module frame 50 can be prevented using
close
tolerances between mating parts, or using wire mesh gaskets or any other
suitable type of seal,
such as item 25 shown in Figure 5 on the edge of the fin. Suitable portions of
the module frame
50 or sump cover 46 may be made from perforated metal sheet to increase the
filtration area. To
make use of some of the volume of sump 51, the strainer module can be
recessed, either partially
or completely, into the sump below the level of the floor. In such a case, the
frame 50 of the
module would extend down from the floor level to the bottom of the module in
order to prevent
flow from bypassing the filtration elements of the strainer.
Air ingestion can be prevented by ensuring that there is a sufficient height
of water above
the strainer. In the alternative, a horizontal cover (not shown) can be added
over the fins. This
cover allows the fins to be closer to the water surface without ingesting air
or causing hollow-
core vortices.
Various types of bracing, such as those shown in Figures 1, 2 and 3, are used
to ensure
that the strainer is sufficiently rigid to resist the predicted seismic and
pressure loads. In
addition, external bracing, such as indicated by reference numeral 6 in Figure
2, may also be
placed between fins.
For all applications, it is desirable to optimize the design for the type and
quantity of
debris that the strainer is required to handle. Two basic factors need to be
considered: the
filtration area required, and the potential volume of debris that must be
accommodated within the
strainer. The number of fins is determined by the required filtration area,
and then fin spacing
can be varied to ensure that there is sufficient space between fins to
accommodate the potential
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debris volume. The strainer module is advantageously fabricated in a size that
is easily
manageable and can be moved into place without interference with surrounding
equipment. In
addition, a complete strainer assembly may contain as many strainer modules as
necessary.
Two types of fins that may be incorporated in the apparatus of the present
invention are
discussed below in relation to Figures 4, 5, 6 and 8.
Flat-Surface Fin
Referring now to Figure 4, in accordance with one aspect of the present
invention, flat-
surface fins, have a pair of planar fluid permeable screens 16 of perforated
metal sheet on each
side of the fin in opposed spaced relation. Screens 16 filter debris from the
water entering the
strainer. The two screens 16 are separated by a corrugated metal spacer 17.
Spacer 17 provides
stiffness and strength, and also forms flow channels between screens 16 to
header 3. The edges
of screens 16 are enclosed by perimeter frame 18. The flow channels between
screens 16 are
open to fluid communication with the enclosed flowpath in header 3 through the
marginal side
edge of frame 18 that fits into aperture 9 of header 3. Frame 18 also adds
significantly to the
structural strength of the fin.
If the application requires smaller filtration holes than are achievable using
standard
perforated metal mesh, a layer of fine wire mesh may be laminated onto the
surface of perforated
metal screens 16 of the fin.
The advantages of the fin construction shown in Figure 4 include simplicity of
manufacture and minimal internal volume.
Corrugated-Surface Fin
Referring now to Figures 5, 6 and 8, in accordance with another aspect of the
present
invention, corrugated-surface fins are constructed of two layers of perforated
metal screen 19
that have been corrugated to increase their exposed surface area. Debris
filtered out of water
entering the strainer is deposited on these corrugated surfaces.
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The corrugations provide a number of advantages. The large increase in
filtration surface
area over a flat screen is a very significant advantage for thin debris
layers, which often pose
more of a problem than more porous thick debris layers. The increased area
reduces the
restriction to flow entering the strainer by decreasing the water velocity
through the screen and
reducing the thickness of debris (because it is spread over a larger area).
The "peaks" of the
corrugations also reduce the pressure drop by tending to encourage a locally
non-uniform debris
bed. Even with layers of debris that are thicker than the height of the
corrugations, there can be a
significant benefit, because fine particulates often migrate through the
debris bed and concentrate
near the filtration surface, causing a thin relatively impermeable layer at
the surface. The
resistance of this thin layer to flow entering the strainer is reduced with
the larger screen area
achieved by the corrugations.
Another important feature of this design is that the corrugated screens can be
made strong
enough to be relied on as the sole structural element in the fin. Moreover,
the screen can also be
formed using relatively thin gauge material. This minimizes the amount of
material required to
make a fin, saving cost and making the fins easier to handle because of their
reduced weight.
The corrugated metal mesh screens have a plurality of parallel "peaks" and
"valleys" and
are positioned in opposed spaced relation such that alternating peaks in one
screen are in tip-to-
tip contact with alternating peaks in the opposed screen. This configuration
forms hollow
internal channels for fluid entering the strainer to flow towards the
collection header. These flow
channels are unobstructed and can be made large enough to offer minimal
restriction to flow.
The internal volume of the design is minimized, therefore maximizing the space
outside the
strainer to collect debris.
As shown in Figure 5, the perimeter frame about the fluid permeable screens
can
comprise flat bar 20 to seal the edges parallel to the corrugations and to
provide strong
attachment points 21 for bracing 6 and fin attachment hardware 10 and 11.
These edges can also
be sealed with perforated metal screen 22 to further increase the filtration
surface area.
The perimeter frame about the fluid permeable screens can also comprise
perforated
metal caps 23 to seal the ends of the corrugations. The advantage of this type
of end cap is that it
adds to the perforated screen area and does not restrict flow access to the
space between the fins.
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In the embodiment shown in Figures 6 and 8, end cap 40 is formed from a
channel, which is then
welded over the end of the fin. The advantage of this type of end cap is that
it is simple to
manufacture and adds significantly to the strength of the fin. It can be fully
or partially
perforated if extra screen area is needed.
The marginal side edge of the perimeter frame at the edge of the fin fitting
into the header
is adapted to a rectangular cross-section to fit the rectangular slots 9 in
the header. This can be
done using a toothed strip of perforated metal 24, with the edge sealed into
the collection header
with a flexible metal strips 25 as shown in Figure 5. Figures 6 and 8 also
shows a simpler design
of a cap 41 for the portion of the perimeter frame at the header end of the
fin. Cap 41 is formed
from a channel that is welded over the ends of the corrugated metal mesh
sheets. Cap 41 has
large openings through which flow 43 from the channels between the
corrugations communicates
with apertures 9 and the flowpath enclosed in header 3. The sides of the end
cap provide
surfaces to attach seals 42, which ensure a good fit of the fin into the
header.
Flow equalization
Reasonably uniform flow is desirable to prevent formation of hollow-core
vortexes and to
ensure that debris depositing onto the strainer is not packed too densely. If
the flow concentrates
at one spot, debris will quickly build up at this spot in a very dense mat,
raising the flow
resistance enough that flow will enter at an adjacent spot, causing a dense
bed to build up there
also. If unrestrained, this can progress throughout the whole strainer,
causing a much higher
pressure loss than if the debris had built up uniformly.
In a further embodiment of the present invention shown in Figure 7, the flow
entering at
various points along the length of the header is controlled using flow-
balancing devices that
boost the pressure inside the header. As fluid flows along the header,
frictional pressure drop and
acceleration pressure drop cause decreasing pressures closer to the suction
end of the header (i.e.,
in the direction of fluid flow). This would normally provide more driving
pressure for flow
entering the fins, causing somewhat non-uniform flow (more flow entering fins
closer to the
pump intake, or suction end). To ensure that water entering any fin is
subjected to the same
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driving pressure differential, calibrated flow-balancing devices are added,
which provide greater
restriction for fins closer to the suction end of the strainer than at the far
end.
In accordance with a further preferred embodiment of the present invention,
the flow-
balancing devices provide flow restriction in a partially reversible fashion.
Thus, the energy
required to accelerate the flow through the flow-balancing devices is
converted first to kinetic
energy as a jet of water in the header pointed in the direction of flow
towards the suction end.
The momentum of this jet is used to boost the pressure in the header in a
manner that partially
offsets the upstream friction and acceleration losses. This pressure boost
reduces the amount of
pressure imbalance along the length of the header. A substantially uniform
flow can be achieved
at the same time as providing a lower overall pressure loss.
Figure 7 shows the details of this embodiment in which all but one fin have
been
removed for clarity. Flows at various locations are illustrated with arrows.
Fluid flow 25 enters fin-like strainer element 32 through perforated screens,
passes
through the internal channels formed by the corrugations which are in fluid
communication with
header 35 through end cap 41 (see Figure 6 and 8) and slot 26. Flow from
neighbouring fins
similarly enters header 35 through slots 27 and 28. A relatively narrow
collection channel 38
inside header 35 is defined by vertical baffle 31 running inside outer wall 36
of the header 35.
Flow from the collection channel is accelerated through an orifice 33 forming
a jet 39 that joins
the flow 29 from upstream fins. Because the velocity of the jet 39 is
substantially parallel to the
velocity of the main flow 29, the pressure at downstream flow 30 is raised
over that if the water
were injected perpendicular to the main flow. Furthermore, because the
velocity of the jet 39 is
greater than the velocity of the main flow 29, momentum is added to the main
flow, which
boosts pressure at flow 30 over that that would exist if the flow velocity of
jet 39 were the same
as that of main flow at 29. In addition, the orifice 33 provides a smooth
contraction for the flow
so that there is minimal energy loss in creating the jet 39.
The pressure in the main header 35 drops as one moves closer to the pump
intake because
of friction and acceleration pressure drops. The differential pressure across
orifices closer to the
pump intake are therefore greater than across orifices farther away. In order
to balance the flows
entering the main header, the width of each orifice 33, 34 is selected so that
the pressures
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upstream of all orifices, e.g., through each of the collection channels 38, 37
are equal.
Substantially equal pressure can be achieved by providing that orifices closer
to the pump intake
have smaller flow area than orifices farther from the pump intake with the
result that
substantially uniform fluid flow is maintained through strainer elements
located at different
positions along the flowpath in header 3.
A flow control device in the form of an appropriately sized and shaped orifice
can be
provided for individual collection channels each of which encloses a plurality
of apertures (as
shown in Figure 7), or in the alternative, can be provided for each individual
inlet aperture.
Unless defined otherwise, all technical and scientific terms used herein have
the same
meaning as commonly understood by one of ordinary skill in the art to which
this invention
belongs.
The invention being thus described, it will be obvious that the same may be
varied in
many ways. Such variations are not to be regarded as a departure from the
spirit and scope of the
invention, and all such modifications as would be obvious to one skilled in
the art are intended to
be included within the scope of this application.
