Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to a filter underdrain
apparatus.
Filter underdrain apparatuses are by no means new.
Examples of such apparatuses are found in United States Patent
Nos. 2,873,857, which issued to ~.L. Schied on February 17,
1959; 3,189,181, which issued to J.S. Couse on June 15, 1965;
3,247,971, which issued to R.E~. Kastler on April 16, 1966;
3,313,420, which issued to A.A. Hirsch on April 11, 1967;
2,615,019, which issued to F.J. Early, Jr., on October 26,
1972; 3,762,559, which issued to M.G. Knoy et al on October 2,
1973; and 3,968,038, which issued to D.H. Nilsson on July 6,
1976.
In general, underdrain apparatuses or systems of the
type disclosed by the prior art possess serious flaws,
including non-uniform or uneven backwash distribution which
occurs because the momentum of the water passing through a
perforated header or channel is an important but overlooked
factor. Water at a high velocity across an orifice will not be
discharged through the orifice as readily as it will when
flowing at a lower velocity across another orifice. Other
problems include structural failures because the underdrain
system is not sufficiently strong or anchored strongly enough
to resist the large upward thrust generated during a backwash
operation.
Some underdrains are expensive to purchase and many
are difficult and expensive to install, require tedious
grouting procedures or cumbersome and expensive false bottom
structures. Channelling and jetting and spouting bed action in
.
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the filter media occurs in many strainer type designs. Many
underdrains have no ability to cope with trapped air which on
explosive release is very disruptive in a filter particularly
those with gravel bedding. Many underdrains lack the
flexibility to operate in the air scour assisted backwash mode
or air scour simultaneous with backwash. Some types of
underdrains, for example those of tile or porous tile, are
quite fragile and much breakage during installation results.
Inadequate corrosion resistance is a factor with some
underdrains. Many types of underdrains require gravel layering
as an inflexible requirement. Thus, in spite of the large
number of different apparatuses or systems presently available,
there is still much room for improvement in the filter
underdrain field.
The object of the present invention is to meet the
above need by providing a relatively simple filter underdrain
apparatus, which substantially reduces the likelihood of most
or any of the above mentioned problems being encountered.
While the term filter underdrain is used throughout
for brevity, the application of the invention is not restricted
by any means to filters only. There are various types of
water\waste and process equipment that are not filters at all
but where improved flow collection and backwash distribution
would be most desirable. Examples of such equipment, and this
list is by no means intended to ~e all encompassing are:
- upflow or down flow contact clarifiers or filters
- activated carbon contractors
- ion exchange units
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- iron removal units - greensand/catalyzed sand/birm
- catalyst bed contactors, e.g. desilicizers
- neutralizing media contac-tors
Thus rather than writing underdrains or flow
distributors for all of the various types of equipment to which
this invention is applicable (filters, carbon contactors,
process contactors, ion exchangers, catalyst beds, etc.) every
time, the term filter underdrains i5 used and understood to
encompass units other than true filters.
In some process equipment vessels (upflow mode
filters and contact clarifiers as examples) the underdrain
serves a somewhat different function than in downflow, i.e. it
serves to distribute incoming service flow as well as
backwash.
Backwash in filters is clearly defined as a periodic
reverse flow through the media to flush out trapped impurities.
The term is used in ion exchange and carbon contactors as well,
but means a somewhat different thing. In filters dirt is
flushed from the bed by backwash. In ion exchange and carbon
contactors, and the like, water is typically filtered in
advance so backwash serves to "fluff up the bed" to eliminate
packing and flow channelling so that contact is improved and
short circuiting averted in carbon contactor units. In ion
exchangers a "backwash" is required to wash any dirt from the
bed, but more to "fluff the bed" 50 that regenerant contact is
maximized, i.e. regenerant short circuiting avoided.
Accordingly, the present invention relates to a
filter underdrain apparatus comprising plate means of inverted
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V-shaped cross section, whereby a plurality of said plate means
can be assembled in juxtaposed relationship to define
alternatlng V-shaped troughs and inverted V-shaped arches;
perforate grid means for extending aGross a trough proximate
top thereof between ad~acent arches or across such arches for
supporting filter media; a first row of air vent holes
extending longitudinally of said plate means on the top or near
the top of said arches; and a row of water holes extending
longitudinally of said plate means beneath said first grid
means proximate the bottom of each side of each said arch.
The invention will be described in greater detail
with reference to the accompanying drawings, which illustrate
preferred embodlments of the invention, and wherein:
Figure l i8 a schematic, perspectlve, partly
sectioned view of a common t~pe of filter tank or basin
incorporating an apparatus in accordance with the present
invention;
Figure 2 is a plan view of a panel or plate used to
form a filter element for use in the apparatus of the pre~ent
invention;
Figure 3 i5 a schematic, cross-sectional view of a
portion of a filter element in accordance wlth the present
invention;
Figure 4 i5 a perspective view of one end of a
portion of the filter element of Figure 3;
Figure 5 i9 a schematic, side elevational view of a
~oint between two sections of the filter element;
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Figure 6 is a cross section taken generally along
line VI-VI of Figure 5;
Figure ~ is a side elevational view of one end of the
filter element of Figures 3 and 4;
Figure 8 is a cross section of a trough in the
filter elements of Figures 3 and 4;
Figures 9, 10, and 11 are perspective views of
sections of grids used in the apparatus of Figure l;
Figure 12 is a cross-sectional view of an alternate
form of a grid used in the apparatus of the present invention;
Figure 13 is a cross section of one side of a filter
element in accordance with the present invention;
Figures 14 and 15 are plan views of two additional
forms of panels or plates used to form filter elements in the
present invention;
Figure 16 is a perspective view of a section of
another embodiment of the filter apparatus of the present
invention;
Figure l~ is a cross section of a portion of the
filter apparatus of Figure 16;
Figure 18 is a perspective view from above of yet
another embodiment of the filter apparatus of the present
invention;
Figure 19 i5 a partly sectioned, side elevational
view of a portion of the apparatus of Figure 18;
Figure 20 is a side elevational view of one end of a
filter element incorporating a different form of air inlet;
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Figure 21 is a schematic, cross-3ectional view of a
portion of another embodiment of the filter element of the
present invention incorporating gravel or fine media
res~raining grlds;
Figures 22 and 23 are perspective view~ of two types
of grids;
Figure 24 is a perspective view of portlons of two
grids and a seal;
Figure 25 is a perspective view of another
embodiment of the grid using strainers as media retainers;
Figure 26 is a cross section taken generally along
line XXVI-XXVI of Fig. 25;
Figure 27 is a view similar to Fig. 1 showing an
inlet flume maldistribution corrector plate; and
Figures 28 and 29 which appear on the ~econd last
sheet of drawings, are sections of the plate of Fig. 2~,
showing a palr of anti-turbulence, flow director devices.
Wlth reference to Figure 1, the filter underdrain
apparatus of the present invention is shown with a bed 1 of
filt~r media of the type which includes a top layer 2 of
anthraclte coal followed by a layer 3 of sand, then several
layers of progressively coarser gravel, down to a base layer of
coarse gravel. The mode illustrated is water backwash only.
It should be clearly understood that the
configuration and type of filter media shown in Figure 1 are
for illustration only. The underdrain ha~ embodiments where
for example no support gravel layering is required, and is
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applicable to the wide variety of materials which are used as
filter media.
Commcnly, filters do incorporate a top layer of
anthracite over a layer of fine sand as shown in Fi.gure 1.
Many filters operate with no anthracite layer. Many filters
incorporate a layer of fine heavy material such as garnet or
ilmenite under the filter sand. On occasion filters operate
with other materials entirely for example coke, magnesium
oxide, activated carbon, etc.
The underdrain apparatus is not restricted by any
means to the layering shown for illustration in Figure 1.
The apparatus and the bed 1 are located in a
concrete, open top tank or basin 6, which is defined by a
bottom slab 8, side walls 9 and end walls 10. A partition 11
parallel to one side wall 9 defines an overflow trough or
gullet 13 for receiving backwash water from semicylindrical
metal/concrete/fiberglass troughs 14, which extend transversely
of the basin 6 above the bed 1.
A transversely extending trough or flume 15 is
provided in the bottom of the basin 6 at one end thereof for
receiving filtered and backwash water. Filtered water is
discharged from the flume 15 via a pipe 16, which is also used
to introduce backwash water into the basin ~. A shoulder or
ledge 17 is provided at the outer end of the flume 15 for
25 supporting one end of filter elements generally indicated at
18.
It should be again clearly understood that Figure 1
is for illustration only of one type of filter. Other
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configurations are common and the underdrain apparatus is
readily adaptable to these. Examples of varied configurations:
-transversely extending trough or flume or embedded
pipe across the centre width, with filter outflow and backwash
inlet at the side.
-trough or f lume or embedded pipe running the length
of the filter down the center line, or along one side, or
externally down one side. In these cases the troughs and
channels of the underdrain apparatus run transversely.
-circular filters with cross diameter inlet/outlet
f lume or trough or embedded pipe with the underdrain apparatus
running transversely to such flume or trough or embedded pipe.
Frequently, the side gullet 13 of Figure 1 is across
the end of the filter with backwash troughs 14 of E'igure 1 then
running down the length of the f ilter. The plethora of
drawings required to illustrate such variations is not
considered essential to this application.
The filter elements 18 include a first set of
elongated, rectangular plates 20 (Figure 2), which are bent to
an inverted V-shape to define arches 21 (Figures 1, 3 and 4).
Starting with a flat sheet of, for example 4" x 10" x 1/8"
thick 304 stainless steel, holes 22 for receiving anchor bolts
24, water inlet and drain holes 25, and air vent holes 26 are
punched through the sheet. The sheet is then bent along lines
2~ and 28 to form the sloping sides 30 of the arches 21 and the
bases 31 of the troughs therebetween. While two arches 21 are
formed in the same sheet, a narrower sheet can be used to form
a single arch. The bases at the outer edges of the sheet are
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defined by flanges 33, which overlap similar flanges on
adjacent sheets (Figures 3 and 4) to form a plurality of
parallel arches alternating with troughs. It is normally not
necessary to provide a seal between overlapping flanges 33.
However, when conditions warrant such a seal, the seal can be a
strip of rubber or an elastomeric material. The filter
elements are ins~alled on the bottom slab 8 of the basin 6
before the bed 1 of the filter media, and are secured to such
bottom wall 8 by the bolts 24.
An extremely important feature of the invention
relates to the sizing of the water inlet/outlet orifices in the
inverted V-shaped arches. Such orifices are shown as number 25
in various of the accompanying figures. ln the embodiments
shown, the orifices vary in diameter along the length of the
inverted V-shaped arch conduits to compensate for the velocity
and momentum changes in the backwash flow. Water flowing at
high velocity across an orifice will not be discharged through
such orifice as readily as when flowing at a lower velocity
across another orifice of the same diameter. What occurs
then, in a conduit having uniformly sized orifices or lateral
connections along the length is a maldistribution of the
backwash flow so that the far end of the conduit passes more
flow than the inlet end. Such maldistribution is common in
many types of underdrains and is very disruptive. The key to
the design of this filter underdrain apparatus is to vary the
size of the orifices 25 so that the coefficient of flow
discharge through each orifice is the same. The variation in
orifice diameter is calculated for each filter on a custom
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design basis, such calculation taking into account the
variables of flow velocity entering and along the conduit,
fluid viscosity, allowable pressure drops through the orifices
and along the conduit, orifice spacing center-line to center-
line and desired maximum maldistribution required. The properorifice size spacing and variation in diameter thus calculated
is then used in the fabrication of plates shown in Figure 2,
14, 15.
For the air scour mode the orifices 2S are
compensated for in the calculation for the water which passes
through the air distribution holes on backwash. For the
simultaneous backwash air scour mode of operation where the
conduit is divided by plate 6~ (Figs. 16 and 17) into an upper
air passage 71 and a lower water passage 70, orifices 25 are
calculated based on actual flows pertaining in the passage 70.
Side conduit orifices are made slightly larger than all the
others to compensate for the somewhat greater area of filter
media subtended.
Thus, the inverted V-shaped arched conduits or
passages are fabricated with varying orifice sizes to insure
essentially no maldistribution along the conduit length.
The size and distribution of the air and water holes
makes it expensive and difficult to produce the holes by
drilling, particularly by stacked plate drilling. The holes
are readily made by a computerized punch press. While 1/8"
stainless steel is considered to be standard, in many filters
the underdrain can be made of thinner guage stainless steel and
still be strong enough for the intended purpose.
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A variety of materials could be used to fabricate
this underdrain. For example: steel (painted or galvanized),
aluminum, fiberglass, various types of plastics and fiber
reinforced plastics, concrete, etc. The material preferred
for the great majority of installations is 304 or other grade
st~inless steel because of its corrosion resistance properties
and great strength. However, stainless steel is difficult to
drill, machine and weld so the underdain was designed so that
fabrication would be by punching and bending and assembly by
bolting, with the use of seal strips, thus eliminating any
requirement for drilling, tapping, machining, welding, etc.
Referring to Figures 5 and 6, the junctions between
the ends 35 of aligned sections 36 of filter elements are
sealed by flexible strips 38 of generally H-shaped cross
section. The ends of the arches 21 are closed by rectangular
end plates 39 with seals 88 (Figure 7) of generally C-shaped
cross section, of neoprene or other elastomer, between the
sections and the plates. Corner brackets 40 of L-shaped cross
sections are used to connect the sections 36 to the end plates
39 by bolting. Referring to Figures 1, 3, and 8 to 11, the top
end of each trough between adjacent arches 21 is closed by an
elongated grid 41 defined by a rectangular plate with a
plurality of openings 42 therein, and inclined side edges 44
for connecting the grid to the sloping sides 30 of the arches
21.
As shown in Figure 8, anchor bolts 24 extend through
the grid 41, nuts 45, washers 46 and the bases 31 or flanges 33
of the elements 18 into the floor 8 of the basin 6. When
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sections of grid 41 are to be joined end-to-end, a strip 38 of
H-shaped cross section is used (Figure 10). When the grid 41
is to be used with fine sand or another fine filter media,
screens 47 are provided over the openings 42 and a cover plate
48 is mounted on the grid to sandwich the screens 4~ in
position. Bolt holes 49 are provided near the side edges of the
grid 41 and of the cover plate 48 for securely connecting the
elements to each other. Openings 50 similar in size to the
grid openings 42 are provided in the cover plate 48. The
openings 50 are aligned with the openings 42.
The half troughs formed by the sides of the filter
elements 18 and the side 9 of the basin 6 or the partition 11,
are closed by a grid 51 (Figures 1 and 13). The grid 51 has
one inclined side edge 52 for engaging the side 30 of the
element 18. The other edge 53 is supported in the horizontal
position by a generally C-shaped channel member 55. An anchor
bolt 56 extends through the member 55, and through nuts 57,
washers 58 and the flange 33 of the element 18 into the bottom
slab 8 of the basin 6.
In cases where the filter media 1 i9 to be cleaned by
water backwash only, i.e. without a preliminary air scour, or
air addition simultaneous with water backwash, the grids 41
and 51 are mounted at approximately the middle of the sides 30
of each arch 21 or trough (Figures 8 and 13). As shown in
Figure 12, when air scour is to be used prior to or
simultaneous with backwashing of the filter media 1, a wider
grid 60 is mounted near the top of the arches and troughs so
that the air holes 61 are under the grid. Of course a wider
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half grid (not shown) similar to the grid 51 is provided at the
sides of the elements 18.
As shown in Figure 14, additional air distribution
openings 61 can be provided in a plate 62 used to form a filter
element. Such openings 61 are in parallel longitudinal rows
above the water inlet and outlet openings 25 on each side 30 of
the arches 21 formed using the plate 62. This type of
structure is preferred in the air scour mode.
Another embodiment of the invention (Figures 15 and
17) for use when the media îs subjected to simultaneous air and
water backflushing includes a sheet 64 which incorporates the
lower water openings 25, air outlet openings 26 and 61, and the
bolt holes 65 for receiving bolts 67 (Figures 16 and 17) which
are used to mount a horizontal partition 68 i beneath each arch
when the sheet 64 is bent to form two adjacent arches with a
trough therebetween. Neoprene or other elastomeric sealing
strips 69 are provided between the ends of the partition 68 and
the sides 30 of the arches. The partition 68 divides the
interior of the arch into a lower water passage 70 and an upper
air passage 71. Air is introduced into the passage 71 via an
inlet tube 72, a header 73 (Figure 16) in the flume 15 and a
pipe 75 extending through the wall of the basin 6 to a source
of air under pressure air (not shown). The tube 72 is
surrounded by a sleeve 76, which extends downwardly from the
partition 68 to receive the top end of the tube 72.
Figure 18 shows the type of air introduction system
used when air scour not simultaneous with backwash is used. In
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- 1313504
this case there are no division plates such as plate 68 of
Figure 1~ or downwardly extending sleeves such as sleeve 76 of
Figure 1~-~6.
~he lnlet tubes ~2, closed ended header 73, air inlet
~5 are identical in both Figures 16 and 18. Such lnlet tubes
72 are open at thelr bottom ends and extend downwardly into
the header ~3. Openings 83 of Figure 190 for controlled
metering of air are provided in the sides of each tube 72.
In the embodiment of Figure 19, optional shoulder 85
is shown to extend into the flume 80 from the inner side
thereof. In underdrain systems of the type disclosed herein,
it i5 desirable to be able to alter the flume size to limit
the flow velocity of backwash water in such flumes.
With reference to Figure 20, another form of air
inlet includes a top inlet tube 86, which is connected to the
top end of the arch 21 by an internally threaded connector 8~,
which is welded or bolted to the top of the arch near one end
39 thereof.
Figures 21 to 26 show embodiments of grids 90
intended to replace the grids 41, 60 and 51. The grids 90 of
Figs. 21 to 26 arre not mounted in the trough areas of the
apparatus but span the arches transversely. Larger grid
sections can be used with this embodimènt with co~t savings in
larger filters in fabrication and installation. The problem of
protecting the air vent holes 26 from fine media ingress by
individual air vent hole screening ~s eliminated, because the
vent holes are beneath the grids. The air vent holes 26 are
14
1 31 3504
punched slightly spaced from the arch 30 apex so the holes are
not blocked by the top grid.
Figure 21 shows the transverse spanning position of
the grids 90 in this embodiment of the invention. The grids 90
are held in position by extended anchor bolts 24. At the
sides, the grid ends are fastened to the side edge channel
member 55 by anchor bolts 56. The member 55 and the anchor
bolts 56 are lengthened to conform with the horizontal
spanning position of the grid. The grid ends are overlapped
over an arch apex and fastened at that point with self tapping
screws 91. The anchor bolts 24 support the grid 90. However,
self tapping screws at the apex of each or some of the arches
spanned is workable, to eliminate anchor bolt e~tensions.
As best shown in Fig. 22, a perforated grid 90 for
supporting graded gravel with fine media above such gravel is
provided in lengths and widths to suit. Slots 92 are punched
longitudinally with slots 93 (Fig. 24) of the overlapping
adjoining grid punched transversely for ease of matching and
installing self tapp~ng screws. Circular openings 94 similar
20 to the openings 42 (Fig. 9) are provided in the grid 90.
A laminated perforated structure similar to that of
Fig. 11 includes stainless steel mesh 96 sandwiched between
grids 90 and 97 to act as a retainer for fine media with no
necessity for gravel barrier layering.
There are numerous ways of constructing grids
including the use of flat profile wire (wedge wire) sheets.
Referring to E'ig. 24, the edges of adjacent grids 90
are interconnected by I-shaped seal strips 38 of neoprene or
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other elastomer. Sealing the grid 90 where it rests on side
channel members 55 or where ends overlap is not normally
necessary, al~hough such sealing could readily be accomplished
using strips of neoprene or another elastomer.
As best shown in Figs 25 and 26, strainers 9~ can be
used on the grid 90 to act as fine media retainers. each
strainer 98 includes a frusto-conical body 99 with slots 100
therein, and a threaded bottom end 102 for mounting the
strainer in the grid 90.
During normal use of the basic apparatus practicing
water backwash only (Figures 1 to 4), water is filtered
through the media 1, the aches 21 and the grids 41, flowing
into the flume 15 for discharge through the pipe 16.
Periodically, the flow of water is reversed to backwash the
media. Backwash water and impurities dislodged from the media
1 overflow into the troughs 14 for discharge via the gullet 13.
When backwashing operations involving a preliminary
air scour and water backflush or a combination air scour and
water backflush are desirable modes it is necessary to use the
apparatuses of Figures 14 to 20 for such procedures. During
air scouring, water is drained to beneath the trough level 14,
and air is introduced through the pipe 75, the header 73 and
the tubes 72 into the arches 21. The use of the header 73 and
metering tube ~2 ensures uniform distribution of the air (which
may be another gas; however, for the sake of simplicity the
term "air" is used throughout this case) to the individual arch
shaped conduits. By varying the number and size of the air
openings 26 for approximately 3"-4" of water head loss uniform
16
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distribution of air from conduits 21 to all parts of the media
1 is assured.
In the air scour mode, air is introduced to bubble
through the media 1 to cause vigorous agitation of the media to
S loosen material adhering thereto. Air scour is performed for
a variety of periods of time, typically three-five minutes.
The airflow is then stopped and, after a delay of a few minutes
to permit air release from the media l, backwash flow is
initiated as in an ordinary hydraulic backwash.
When operating with long arches 21, it may be
necessary to stagger the air openings 61, or to provide
additional holes. Custom designs of the air openings
compensates for filter floors which are not dead level, and
assists in proper air distribution in long conduits where wave
action results due to the velocity of the air over water
surface. Concerning the surface of the bottom wall 8 of the
basin 6, normal construction tolerance on concrete filter
floors is l/8" for ten feet of length. When operating in an
air scour mode, the floor level is much more critical than when
using a water backflush only. When normal construction
tolerance has not been achieved, a concrete floor or bottom
wall 8 can be made dead level using self-levelling grout. It
is preferable when using a preliminary air scour to provide a
dead level floor in the basin 6 by grouting, rather than
compensating by changing the openings 61.
During a simultaneous air scour/water backwashing
operation, air or another gas is bubbled through the media 1
while water flows therethrough. The water flow rate is too low
~ 1313504
to cause fluidization of the media 1, but is sufficiently high
to sweep out dirt loosened by the air agitation of the media.
In dual media or multi-media filters, it is necessary to
increase the backwash rate sufficient to cause fluidization, on
completion of backwashing, to restratify the media. When
effecting a simul~aneous air scour/water backwash operation in
long arches 21, there is a marked tendency to generate severe
wave action. The wave action can be so severe that some of the
water openings 25 pass air, and some of the air openings 26
and/or 61 pass water. Thus, the distribution of air and water
is very poor. If water velocities are kept low, i.e. less
than l fps, the tendency to wave action is minimal. The
preferred solution to the problem of wave action is to provide
the horizontal partition 68 (Figures 16 and 17), which divides
each arch 21 into a lower water passage 70 and an upper air
passage 71.
Further embodiments of the invention relating to
inlet flumes (see Figure 1, flume 15) will now be considered.
Figure 1 shows a front flume design, though other
configurations are common, for example:
-trough flume or pipe-in-trough or embedded pipe
running the length of the filter down the centre line or along
the inner side. The troughs and conduits of the underdrain
apparatus run across the width of the filter.
-trough flume or pipe-in-trough or embedded pipe
running across the center width of the filter with such trough
flume receiving backwash flow or discharging filtered water via
an embedded pipe through the side or via an embedded pipe
18
1 31 3504
through the end of the filter. The troughs and conduits of the
underdrain apparatus run the length of the filter.
-on occasion the conduits of the apparatus may be fed
via a side or end pipe or conduit through wall sleeves
connecting to the ends of the conduits.
-circular filters with across the diameter
inlet/outlet trough flume or pipe-in-trough or embedded pipe,
with the underdrain apparatus conduits running transversely to
such trough flume or pipe.
The underdrain apparatus is compatible with and
readily adaptable to any of these (and other) modes of backwash
introduction/filtered water outlet.
It is readily apparent, however, from previous
discussion of velocity/momentum considerations in the conduits
of the apparatus that maldistribution from the flume or pipes
introducing backwash water into the conduits of the apparatus
is important to consider.
On any of the sunken concrete flumes in any
configuration the provision, as part of the underdrain, of
stainless steel flume cover plates, with orifices sized using
the same hydraulic calculation method as for orifices 25 in the
conduits of the underdrain ensures an evenly distributed flow
of backwash water into each conduit section. Figure 2~ shows
such a cover plate 104 having such sized orifices 105 therein
to introduce backwash water into each conduit section.
Similar cover plates for any flume configuration are proposed.
A further embodiment would be the addition of a flow
director apparatus to eliminate turbulence from the orifices
19
1 31 3504
~.
105. As shown in Figure 28, one form of flow director
includes a 90 elbow 10~ where the introduction of backwash
water i8 at the bottom end of the conduits, or a tee 108 (Fig.
29) where ~uch introduction is mid way of a conduit.
On pipe-in-trough type of backwash-in/filtered water
out systems orlfices would be drilled in the top of the pipe,
such orlfices located under each conduit section. These
orifices are sized using the same hydraulic calculation method
as for orifices 25 in the conduits of the underdrain, to ensure
even flow dlstribution from the pipe into each conduit section.
Again flow director devices 10~ and 108 of Figs. 28 and 29 may
be desirable to eliminate turbulence.
On embedded pipe much the ~ame procedure would be
used. Varied orifices would be provided in the pipe with
welded-on riser pipes upward to the top of the concrete bottom
slab. These risers would all be the same diameter, l.e.
slightly larger than the largest orifice hole. Flow director
devices 107 and 108 may be desirable on the risers to elimlnate
turbulence.
Where the underdrain condults are fed through their
end via an external conduit or pipe with wall sleeves, the same
hydraulic treatment would be used to vary orifice sizes in such
pipe with the sleeves all the same size, or to calculate plate
orifice si~e variation for wall sleeves from a concrete flume.
In this case, the term "proximate" is intended to
mean at or near.
