Abstract:
The water intake rotary screen comprises a mobile filter designed as an endless loop formed of a succession of flexible and articulated filter panels. It is associated with a suction system adapted to generate a local contraflow through the filter to take up successively elements retained thereby and direct them to the exterior.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates generally to the screens with filter elements that are usually fitted to water intakes, either river water intakes or seawater intakes, to block debris and particles carried by the water drawn in. 
   A water intake screen is normally on the downstream side of a grid with bars a few centimeters apart protecting it from large debris, either on its own or in combination with one or more other screens of the same type forming part of the same screening station. 
   Prior art filter elements, which have a mesh size of only a few millimeters, are mobile so that they can be periodically cleared of debris and particles larger than their mesh size, which progressively obstruct all of the mesh and therefore block the filter element. 
   The present invention relates more particularly to the rotary screens known as “chain filters” that are commonly encountered in water intakes in the United States and in Japan in particular. In these rotary screens, consecutive flexible articulated rectangular filter panels or elements are carried by two chains, one on each side, forming an endless loop of elongate cross section constituting said chain filter. 
   A filter element of this kind passes cyclically from an immersed position, in which it is progressively charged with debris and diverse particles, to an out of the water position, in which it is subjected to contraflow pressurized water jets to clear it of the particles and debris that have accumulated on its surface in the above manner, to enable it to resume its filtering function when it is next immersed. 
   At present, the particles and debris entrained by the washing water are usually collected in and drained off via a channel provided for this purpose. 
   It would seem that until now the fact that the debris and particles are of mineral and vegetable origin and contain lifeforms, in particular fish, has not been a matter for concern except in a few particularly sensitive locations. 
   This is no longer the case today, when the widespread concern to protect the natural environment is taken much more seriously, especially in the case of protecting aqueous lifeforms, when greater amounts of water are taken from rivers and the sea, in particular by nuclear power stations, and in view of the fact that the problem is aggravated by the current tendency to install for this purpose water intakes with large flowrates on ecologically sensitive estuaries or on the seashore. 
   Now it is clear that these lifeforms, and in particular fish trapped and entrained by the filter element of a screen of a water intake of this kind, are inevitably removed from the water, often for a long time, are exposed to the usually brutal effect of the washing water jets applied to the filter element while it is out of the water, and suffer the resulting violent splashing in the channel for recovering particles and debris temporarily blocking the filter element in question. 
   Various systems to be fitted to a water intake screening station to save lifeforms and in particular fish carried by the water drawn in have already been proposed. 
   However, at present, these are either special filters that in themselves merely save fish, with no general screening function, or conventional screening filters that still remove fish from the water. 
   One prior art system which avoids removing fish from the water and the resulting trauma is disclosed in French patent No. 7720258. It is used only in rigid drum filters and provides a recovery channel below the lowest water level. The recovery channel is provided with pressure reducing means that induce a reverse flow of water through the filter element that is sufficient to entrain lifeforms retained thereby. 
   However, this kind of recovery channel is designed only to be installed on rigid filter drums and not on chain filters whereof the filter comprises a succession of flexible articulated filter panels. 
   SUMMARY OF THE INVENTION 
   One object of the present invention is precisely to save fish at the same time as providing the required screening means for a chain filter. 
   The invention relates to a chain filter rotary screen for a water intake comprising a filter mobile as an endless loop and formed of a succession of flexible and articulated filter panels. 
   According to a general definition of the invention, the rotary screen is associated with a suction system adapted to generate a local contraflow through the filter to take up successively elements retained thereby and direct them to the exterior. 
   In practice, the suction system is fixed below the lowest water level of the water intake. 
   One embodiment of the screen further comprises a washing manifold and an evacuation unit on either side of the filter. 
   Each filter panel advantageously comprises openings individually delimited by parallel channels elongated perpendicularly to their plane and having a hydrodynamic profile, the thickness of the panel being perpendicular to the plane of the openings to constitute a honeycomb structure. 
   In another embodiment, each filter panel includes a horizontal median partition dividing the panel longitudinally into two compartments. 
   In practice, the suction system comprises a suction pump and a suction tube. 
   For example, the suction pump comprises a volumetric centrifugal pump comprising an enclosed impeller and a booster screw. 
   The suction tube preferably extends horizontally over the width of the filter panel and includes an opening defined by two branches. 
   According to another feature, the screen comprises a fixed mask above the suction system. 
   According to a further feature, the suction tube comprises shields forming retaining shoes above and below the opening of the suction tube and is associated with guides for the panels on either side of the suction tube. 
   The suction tube further comprises flexible lips at the entry of the opening of the suction tube to distribute the flow over the width of the suction tube whilst allowing the occasional passage of larger elements. 
   The suction tube is advantageously articulated about a pivot axis below the lower shield. 
   Apart from the essential advantage of obtaining the required protection of elements conveyed by the water drawn in, the suction system of the invention also has the two-fold advantage of being totally static, which guarantees ruggedness, reliability and a long service life, and of accommodating current water intake filter element screen technology, regardless of the nature of the filter elements, so that it can easily be added to screens of this kind that are already in service, if required. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages of the invention will become apparent in the light of the following detailed description and the drawings, in which: 
       FIG. 1  is a view in cross section of a screening station including a device of the invention for saving fish; 
       FIG. 2  is a plan view of the screening station in section taken along the line II-II in  FIG. 1 ; and 
       FIG. 3  shows to a larger scale the portion III of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The above figures show the application of the invention to a direct passage rotary chain filter  10 , filter elements  11  known as filter panels being disposed across the flow and carried by two chains, one on each side, constituting an elongate vertical endless loop, the upper and lower shafts being disposed longitudinally. 
   In the example shown, the chain filter  10  is inside a channel  12  between two vertical walls  13  and therefore delimits two areas, namely, on the upstream side of the channel, an area  14  in which the water to be screened arrives in the direction of the arrow F 1 , and a downstream area  15  from which the screened water exits in the direction of the arrow F 2  for subsequent use. The construction of a direct passage chain filter is well known in the art and is described here only in outline. 
   The consecutive panels form a loop supported by coaxial sprockets with the upper portions whereof the chains carrying the panels mesh. 
   The shaft carrying the upper sprockets rotates in two bearings, one on each side, and is driven in rotation in the direction of the arrow F 3  by a drive system that is known in the art. The bearings are carried by a frame that generally slides in vertical guides embedded in the walls. On each lateral edge of each panel a mobile sealing system known in the art prevents unfiltered water from bypassing the filter elements by passing between the fixed vertical uprights of the frame and the edges of the panels. 
   Similarly, at the junction of two panels at the horizontal edges where the panels are articulated, a sealing system known in the art prevents unfiltered water passing between the two panels. 
   In the lower part, where the panels turn on lower rails before rising, a curved transverse plate  16  provides a clearance between the upstream lips of the panels and said curved plate that is smaller than the mesh size of the filter element. This prevents the upstream to downstream passage of any debris larger than the mesh size. 
   In the embodiment shown, the water flows through the upstream face of the chain filter from the chamber  14  to the chamber  15  and then, cleared of debris and lifeforms, through the downstream face. 
   As shown in  FIG. 1 , in practice the filter  10  is only partially immersed in the water to be filtered. 
     FIG. 1  shows the lowest water level PB and the highest water level PH. 
   The part of the filter  10  that is out of the water is usually associated with a washing station  22  above the highest water level PH and on the upstream side of the portion of the chain filter  10  that is out of the water relative to the direction of movement of the filter element, i.e. the rotation direction, as indicated by way of example by the arrow F 3  in  FIG. 1 . 
   As known per se, the washing station  22  includes, inside the filter  10 , one or more washing manifolds  23  adapted to spray a substantially perpendicular water curtain  24  through the filter element  11  of the filter and, outside the filter  10 , in corresponding relationship with the manifold or manifolds  23 , a channel  25  for collecting debris detached from the filter by the water curtain  24 . 
   Each filter panel  11  passes in front of a suction tube  30  just below the level PB. The suction tube  30  extends horizontally the entire width of the filter panel  11 . It has a rectangular opening  50  whose height is of the same order of magnitude as the height of the compartment of the filter panel to be cleaned. This opening  50  extends the entire length of the suction tube and the panel. The edges of the opening  50  are fitted with flexible lips  32  to restrict the flow of water to the downstream side of the opening  50  in order to distribute the flow over the whole of the length of the suction tube without preventing the occasional passage of more bulky debris. 
   The general shape of the suction tube converges in the direction of the upstream side of the channel  14 . The suction tube comprises two branches  31  defining the opening  50  of the suction tube, namely an upper branch  31 S and a lower branch  31 I. The upper branch  31 S is supported by an intermediate arm  33  and the lower branch is supported by an intermediate arm  35 . The two intermediate arms  33  and  35  are connected together by a pivot  38 . 
   The suction tube  30  feeds a suction pump  39  which has an inlet  29 . The pump  39  is of the type combining a volumetric booster screw  34  with a single-channel centrifugal outlet. The booster screw is enclosed in an external cone. 
   Compared to other pumps able to provide the required flowrate and pressure of a fluid charged with debris, this type of pump, often called a Hydrostal pump, has the characteristic of not creating even the slightest pressure drop anywhere in its internal circuit. No fish or other aquatic creature can indeed survive a sudden pressure drop, even of short duration. Moreover, with its enclosed rotor, this pump prevents fish being injured by rigid walls moving at relatively high speed relative to the water. 
   The pump is fixed either to supports fastened to the frame of the filter or to a chassis fixed to the lateral walls  13 . It discharges the water, debris and aquatic lifeforms either into a pipe  40  or into a channel  42  for discharging them into a safe area. 
   The suction tube  30  is fitted with a shield  36 S above the suction opening  50  and a shield  36 I below it which cooperate with the lips  32  of the compartments of the panel to ensure that water passes only in contraflow through the filter element  11  being washed by suction. The upper shield  36 S is at the end of the upper branch  31 S and lower shield  36 I is at the end of the lower branch  31 I. 
   The suction tube  30  is articulated about a pivot axis  38  lower than the lower end of the shield  36 I. In this way, if a large body on the filter panel projects far enough to interfere with the shield of the suction tube on rising, the suction tube  30  is pushed upward, disengaging it from the filter  11  and thus allowing the large debris to pass before it returns to its service position. 
   A wall  45  above the shield  36 S of the suction tube is just above the highest water level PH. This wall (or fixed mask)  45  is fixed to the frame of the filter. It prevents the passage of unfiltered water if the water level is above the level of the upper shield  36 S of the suction tube. This is particularly advantageous in that it reduces the size and the mass of the suction tube  30  whilst assuring the same function. 
   The filter panels  11  comprise a frame whose height is equal to the pitch  41  of the chains that carry them (which is generally from 500 mm to 600 mm). The width of the filter panels  11  is slightly less than the distance between the walls  13  of the channel. 
   To be efficient, it is desirable for the countercurrent flowrate drawn in by the suction tube to be at least equal to that passing through the filter and depositing debris thereon. This flowrate conditions the size and power of the pump and the diameter of the evacuation pipes. It is therefore beneficial, for economic reasons, for this flowrate to be as low as possible. 
   The size of the filter panels being fixed, the area to be washed is reduced by installing a horizontal median separator  11 B that divides the panel into two compartments  11 A and  11 C. 
   Thus the panel consists of a longitudinal upper membrane  9 S, a median separator  11 B and a lower membrane  9 I. Two vertical uprights  7 I and  7 S situated on either side connect the membranes  9 I and  9 S to each other. The sections are chosen so that the upstream edges of all these elements lie in a common vertical plane, thereby providing a minimum clearance from the shields  36 I and  36 S of the suction tube  30 . 
   The filter element is fixed in the plane of the downstream edges of the frame of the panel. The height of the membranes  9 I and  9 S thus produces a box in which debris stopped by the filter element can be stored pending washing. In practice, the depth of these compartments is at least 10 cm. 
   Fish and other lifeforms stopped by the filter must be protected from injury while they remain on the filter element pending washing. 
   The filter element of French patent No. 8700429 is particularly beneficial in this situation because it has a totally plane upstream surface with components with rounded edges made from a composite material that is softer than a metal filter, for example. In this kind of filter, each filter panel comprises openings individually delimited by elongate parallel channels perpendicular to their plane and having a hydrodynamic profile, the thickness of the panel being perpendicular to the planes of the openings, resulting in a honeycomb structure. 
   Moreover, contraflow washing by means of the suction tube is improved because fibrous elements and jellyfish are unable to attach themselves to the frame. 
   Note that the contraflow through the filter elements created by the suction tube generates a head loss that tends to move the filter panel toward the shield of the suction tube and can cause jamming. 
   Jamming is prevented by providing on each side of the panel local retaining slideways  52  and  54  at the level of the suction tube  30 . These slideways or retaining shoes  52  and  54  therefore prevent movement in the upstream direction of the filter, which is relatively flexible compared to a rigid filter drum. 
   Thus the present invention is installed on a chain filter and includes a suction tube  30  adapted to remove elements stopped on the filter panels  11  by a contraflow created by means of a volumetric centrifugal pump  34 ,  39 . 
   The panels  11  are held so that they are not themselves drawn toward the suction tube  30 , which would interfere with their movement in translation. The pump discharges elements detached from the filter panels via a water pipe and/or channel to an area of the natural environment in which fish and other aquatic lifeforms can return to their usual environment without risk of being recycled to the water intake.