Note: Descriptions are shown in the official language in which they were submitted.
Screening rakes
The invention relates to a process for removing solids from flowing
sewage using a screening belt which can be driven and placed in the
s flowing sewage and has one link chain at either side with screening
elements made of perforated plates extending between the two chains.
The invention further relates to an apparatus for carrying out the process.
Before sewage is clarified in a sewage treatment plant, solids which
exceed a pre-defined particle size must be removed mechanically.
1o Processes are known whereby the flowing sewage is fed through a
screening rake which retains the solids. These solids must be removed at
regular intervals or continuously to prevent the screening rake from
becoming clogged.
Different designs of screening rakes or filter rakes are already known.
15 The majority of these screening rakes consist of an endless, driven
screening belt revolving like a paternoster, the belt comprising individual
screening links, with the gaps between these links forming the screening
surface through which the sewage flows.
In addition to screening rakes whose screening elements are made up of
2o groups of adjacent screening links (WO 92/18 221), screening rakes are
also known of the type described in the introduction, e.g. from the DE-U
296 19 891, where the screening elements are each formed by a flat
perforated plate. These screening rakes with perforated plates have the
advantage of their relatively simple structure, making them relatively
2s cheap to manufacture. Also, if there is any damage to the screening
elements, it is relatively easy to replace a single perforated plate and a
replacement is easy to obtain because the perforated plates are
manufactured to a simple design and at low cost, and because the link
chains on either side remain in place when individual perforated plates are
3o changed.
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On these screening rakes with perforated plates, only the revolving
movement of the screening belt can be controlled. If there is a low level of
water to be handled, the screening belt is at a standstill for longer periods
of time because there is no or only a very slight difference in the water
35 level on both sides of the screening rake. Due to the low flow speed of
the water, the material that settles in the sewer is mainly coarse particles
and the material reaching the screening belt is almost exclusively fine
grain. The greater part of this fine material is flushed through the holes in
the perforated plates. This material contains both inorganic and organic
4o substances.
If there is a high level of water, for example after heavy rainfall, the sewer
is flushed out due to the high flow speed of the water. The surface of the
screening belt becomes covered over very quickly and its control system
switches to continuous operation, possibly also to a higher revolving
45 speed, as a result of the difference in water level. When the material in
the sewer has been flushed through, there is only a small amount of
material left in the water, but the water level remains unchanged for a
longer period. The screening belt remains in continuous operation for
safety reasons. As a result of the flow pattern, fine material is flushed
5o through the screening belt and thus, is not removed.
The aim of the invention, therefore, is to improve material discharge and
to design a screening rake of the type mentioned in the introduction that
can meet this requirement.
According to the invention, this task is achieved by the open area of the
55 screening elements in the screening belt being regulated or controlled
according to the flow patterns, whereby the flow patterns are recorded by
a sensor
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and the open area can
be controlled in relation to the difference in water level upstream and
s downstream of the screen, As a result, the screening effect can be
influenced such that an optimum removal rate is achieved under all
operating conditions occurring.
The screening rake according to the invention is characterised by each
screening element consisting of two perforated plates whose edges. over
lap and which are movable in relation to one another, and by one of the
two pertorated plates being designed as a movable pertorated plate and
engaging an adjusting drive at least at one point on the revolving path of
the screening belt.
Due to the relative adjustment of the two overlapping pertorated plates,
which together form one screening element, the open area of the hole in
the screening elements changes. As a result, there is a further means of
controlling the screening efficiency in addition to the drive control of the
screening rake and permitting optimum adjustment to the conditions pre-
veiling in each case_
2o By changing the gap 'width (size of the holes) in the screening elements,
more material is removed in the operating mode for low water level, where
. . . the difference in wafer level caused by .reducing the gap width causes
the
drive of the screening belt to be switched on again, The screening belt
removes more material at a smaller gap width,
2s If the water level is rising, the gap width is only enlarged to the extent
required for the prevailing operating status. As a result, optimum screen-
ing efficiency is still achieved, even at a high water level.
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The improved removal rate allows the operator to react more quickly to
unexpected weather conditions and further reduces the risk of subsequent
pumps and pipes becoming clogged. The operating periods for the
screening belt can be optimised such that the material deposited on the
rake acts as an additional fine screen and also retains fine particles. This
means that larger quantities of organic material can also be washed out
and fed to a clarifying process if a washing stage is used after screening.
Advantageous configurations of the invention principle are covered in sub-
claims.
1o The invention is described in more detail below in examples and with
reference to the drawings, where
Fig_ 1 shows part of a fop view of a screening belt in a screening rake for
removing solids from flowing sewage,
Fig. 2 shows a section through the line marked il-lI in Fig. 1,
Fig. 3 shows an enlarged part section through the screening belt in the
carrying run and return run, and
Fig. 4 shows a partial top view in the direction of the arrow marked IV in
Fig. 3.
The screening rack illustrated in Fig, 1 is used to remove solids from
2o flowing sewage. A revolving, driven screening belt 1 has a link chain 2 at
either side, with screening elements made of two-layer pertorated pls~tes
3a, 3b between the finks.
One of the two overlapping perforated plates 3a in each screening
element is attached, using screws for example, at both ends to a retaining
angle 4, which is connected to a fink 2a in the link chain 2. Each retaining
angle 4 has a stop plate 5, which rests on the underside of the perforated
plate 3 and extends almost over its entire width.
Reg. 4013 Ausl
CA 02282445 1999-08-30 r
A joint bridging profile 6 is inserted between each of the two longitudinal
edges 3c of adjoining perforated plates 3. In the example illustrated, this
bridging profile 6 is made of two profile strips 7, each of which are made
from a shaped metal strip. The two profile strips 7 are joined together by
screws 8 spaced at equal distance. As shown in Fig. 2, each screw 8 is
connected to the bottom profile strip 7 at one end, welded for example,
while the other end projects through a bore in the top profile strip 7 and is
covered by a secured cap nut 9.
A spacer sleeve 10 pushed over the screw 8 and between the two profile
strips 7 acts as a distance piece between the two profile strips 7.
Between the two longitudinal edges of the profile strips 7 mounted at
equal distances from one another there is one retaining channel 11 on
either side of the joint bridging profile 6, each channel holding one of the
two longitudinal edges 3c, while still allowing movement and swivel. Each
of the two retaining channels 11 of the joint bridging profile 6 is bordered
by two lips 12 at the channel inlet. The clear width of the channel inlet is
greater than the thickness of the perforated plates 3. The illustration also
shows that the retaining channel 11 broadens towards its floor. The inner
surfaces of the channel lips 12 which face the perforated plates 3 are
2o convex. This permits the two perforated plates 3 to pivot in relation to
one
another in the sector where the screening belt is deflected. In addition,
the two perforated plates 3a, 3b can be displaced in relation to one
another due to the flexible means of securing the longitudinal edges 3c in
the retaining channels 11.
As is shown particularly in Fig. 3, each screening element 3 comprises two
overlapping perforated plates 3a and 3b, where one of the plates 3b is
attached to the link chain 2 as the fixed plate.
5
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An angle 13 is mounted at one end of the movable plate 3a resting on the
s _ fixed plate 3b and a guide fork 14 is fitted to the arm 13a of this angle
that
points outwards. The guide fork 14 runs on a guide rail 15 such that the
movable pertorated piste 3a is retained in its pre-set position in relation to
the fixed pertorated plate 3b white the screening belt is revolving, for
example in a position where the holes 16 in the movable pertorated plate
3a exactly cover the holes 17 in the fried perforated plate 3b. At this
setting, illustrated in Fig_ 3, the screening elements 3 have the largest
possible gap width.
At a pre--determined point in the revolving path of the screening belt 1,
preferably in the return run, the guide fork 14 engages an adjusting
section 15a of the guide rail 15, which can be adjusted at right angles to
the running direction of the screening be(t_ As shown in Fig. 3, the adjust-
ing section 15a is connected to an adjusting drive 19 via a shifting rod 18,
for example an air-oil actuated cylinder, which is connected to a control
device 20 only shown here in a diagrammatic view. The control device 20
2o can be connected to a sensor 21, which measures the flow pattenl at any
given time, for example a device to meter the wafer level upstream and
downstream of the screening rake.
The two pertorated plates 3a and 3b are held together at both their longi-
tudinal edges by the joint bridging profile 8. In addition to or in place of
2s this joint bridging profile there may also be profile sections 23 inserted
in
longitudinal holes 22 in the two perforated plates 3a and 3b. The two
widened ends of these profile sections 23 extend over the edges of the
longitudinal holes 22 and hold the two perforated plates 3a, 3b together.
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The adjusting drive 19 changes the gap width of the screening elements
by displacing the perforated plates 3a, 3b in relation to one another and
thus, it influences the screening efficiency such that an optimum discharge
rate is achieved under all operating conditions that may occur.
s If the sensor 21 detects an increase in the difference in water level up-
stream and downstream of the screening rake, the screening belt 1 is
switched fast of all to continuous operation and possibly also to a higher
speed. Only when this measure is not sufficient at the narrow gap setting,
as a result of the large volume of water, is the gap width extended by
~o actuating the adjusting device 19_
When the material in the sewer has been flushed through and there is
only a little and very fine material carried in the water, but the water level
remains high for a longer period, the very high wafer level keeps the
screening best running in continuous mode for safety reasons. Only a
15 small amount of fine material is deposited. In order to prevent this
material flowing through the screening rake unhindered, as soon as the
water level starts to drop; the gap width is reduced in such a way that the
fine material can be removed more efficiently.
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