Abstract:
Screen apparatus comprising a screen ( 10 ) which allows fluid to pass through it whilst providing a barrier to non-fluid matter in the fluid. A jet outlet ( 22 ) is arranged to direct a jet of fluid across that side of the screen ( 10 ) on which such matter would otherwise tend to collect when the apparatus is in use.

Description:
TECHNICAL FIELD 
   The present invention relates to screen apparatus comprising a screen which allows fluid to pass through it whilst providing a barrier to non-fluid matter in the fluid. 
   BACKGROUND 
   Such a screen is used in sewage storm overflow apparatus. In one previously proposed such construction the screen comprises a multiplicity of bars which are parallel to one another and which are spaced from one another across the screen. One problem encountered with this prior construction is that non-fluid matter, for example debris, can be trapped against the bars as fluid flows between them, so that after a prolonged period the non-fluid matter can build up and inhibit the free-flow of fluid through the screen. 
   SUMMARY OF THE INVENTION 
   The present invention seeks to provide a remedy. 
   Accordingly the present invention is directed to screen apparatus having the construction as set out in the opening paragraph of the present specification, in which the apparatus further comprises a jet outlet which is arranged to direct a jet of fluid across that side of the screen on which such matter would otherwise tend to collect when the apparatus is in use. 
   The jet of fluid prevents the screen from clogging up, or at least increases the time it takes to be clogged up. This in turn reduces the load on the screen, being the total force exerted by fluid on the screen. 
   Such screen apparatus is particularly advantageous when it is constructed as sewage storm overflow apparatus. 
   In the event that non-fluid matter, such as for example a disposed hypodermic syringe, approaches the screen during storm conditions it will either be swept away by the jet of fluid before it contacts the screen, or, should it reach the screen, it will be rolled over by the jet of fluid across the screen until it is cleared therefrom. 
   A particularly inexpensive embodiment of the present invention uses water as the fluid of the jet. In a preferred construction, therefore, the jet outlet is connected to receive water from a source thereof. 
   Advantageously the source of water comprises a tank of water. This can be filled with water in a very convenient manner by being an open tank placed below the screen to catch the fluid flowing therethrough. Suitable passageways and a pump may be provided to transfer fluid from the tank to the jet outlet under pressure. 
   An air injector may be connected to inject air into the water so that the jet of fluid comprises a mix of water and air. This provides the advantage of increased turbulence to improve the scouring action of the jet of fluid to clear non-fluid matter from the screen, and helps to float the screenings. At the same time it improves the biological breakdown of the sewage in the event that the screen apparatus constitutes sewage storm overflow apparatus. This aids the cleaning process. 
   The screen apparatus may be installed alongside a channel along which sewage or other fluid flows, in such a manner that in overflow conditions the fluid will reach the screen. It is preferable that the screen extends in the direction of flow as defined by the channel. It is also preferable that the jet outlet is arranged to direct the jet of fluid in that direction, that is to say that the direction of this jet should be generally parallel to the direction of the flow as defined by the channel. 
   The channel may be defined at one side thereof by a weir. The screen may be located along the top of the weir. It may be recessed from the inside surface of the weir which faces the channel. This affords the advantage that the jet of fluid can be at least partly contained within the screen. It is then preferable for a deflector to be arranged in the screen at a position downstream of the jet outlet to deflect the fluid of the jet into the mainstream of the fluid flowing along the channel. The screen may be curved so that it has a generally semi-circular cross-section open on a diameter thereof. One readily obtainable material for the screen is sheet, for example, metal sheet, uniformly perforated, for example with 6 mm diameter holes. 
   The deflector may be an integral part of the screen, or it may be a separate component which is secured to the screen. Either way, it is desirable for the deflector to be impervious to fluid to reduce the likelihood that non-fluid matter will be pressed and held against it by the jet of fluid. 
   The deflector may be constructed so that it acts as a venturi, to improve the flow of the jet. 
   An example of screen apparatus embodying the present invention will now be described with reference to the accompanying drawings, in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a cross-sectional elevational view of the apparatus; 
       FIG. 2  shows a side-elevational view of the apparatus shown in  FIG. 1 ; 
       FIG. 3  shows a plan view of the apparatus shown in  FIG. 1 ; 
       FIGS. 4 to 7  show respective cross-sectional views of parts of modifications to the apparatus shown in  FIGS. 1 to 3 . 
   

     FIGS. 1 to 3  show sewage storm overflow apparatus comprising a screen  10  in the form of a metal sheet  12  perforated uniformly over its area with 6 mm diameter holes  14 . The ratio of open area created by these holes across the sheet to solid area is about 1:1. The screen allows fluid to pass through it whilst providing a barrier to non-fluid matter such as debris in the sewage. The sheet  12  is curved so that it has a generally semi-circular cross-section as shown in  FIG. 1 , the diameter across the semi-circle being entirely open. The screen could then be said to constitute a basket. At the same time, the sheet  12  as curved in this fashion is generally elongate with a lower edge secured to an upright wall  16  which constitutes a weir. The said diameter slants at an angle of about 30 degrees to the wall  16 . 
   The wall  16  has a cut-away  18  along its top edge, so that the top edge of the wall  16  is at a lower level in this cut-away region than it is elsewhere. It will be appreciated that the wall is built in this way, and that it is not to be inferred from the words “cut-away” that anything has been removed. The lower edge of the screen  10  is secured to the wall  16  along the upper edge and to one side thereof at the cut-away region  18 . The other side of the wall  16  defines one side of a channel  20  along which sewage waste flows. 
   A jet outlet  22  is secured to one end of the screen  10 . The jet outlet  22  is provided with a venturi  24  upstream thereof. A pump  26  is connected via a passageway  28  to receive water  30  from a tank  32  positioned below the screen  10 , and to pump that water through the venturi  24  and out through the jet outlet  22 . The jet outlet  22  is arranged to direct a jet of water through the interior of the screen  10 . 
   The venturi  24  is provided with an inlet  34  to draw air therethrough into the water passing through the venturi  24  so that air is mixed with the water in a mixing compartment  35  before it exits the jet outlet  22  to constitute the jet of fluid. 
   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The other end of the screen  10  is provided with a deflector  36  which, as is most evident from  FIG. 3 , is curved to deflect the jet of fluid at that end of the screen  10  into the main flow of the fluid waste in the channel  20 . 
   A float switch  38  is provided in the tank  32 , and is electrically connected by circuitry (not shown) to trigger operation of the pump  26 . 
   In normal conditions sewage waste flows through the channel  20  on its way to a treatment plant, for example. In the event of storm conditions, the level of the waste water rises until it starts to tip over the top edge of the wall  16  at the cut-away portion  18 . The water is freely able pass through the holes  14  and falls into the tank  32 . 
   If the storm conditions are sufficiently bad, the tank  32  will fill with water  30  and eventually trigger the float switch  38 . This operates the pump  26 , drawing water from the tank  32 , which water is then pumped through the venturi  24 , drawing air in through the inlet  34  and forming a jet of water/air mix, or wash water, to issue as a jet stream from the jet outlet  22 . The jet stream flows along the inside of the screen  10  and is deflected by the deflector plate  36  back into the main flow of the waste sewage in the channel  20 . 
   Any non-fluid matter in the sewage fluid which approaches the screen  10  is thrust along the screen  10  by the jet stream from the jet outlet  22  and is then deflected by the deflector plate  36  back into the main flow of the waste sewage in the channel  20 . It is thereby prevented from clogging the holes  14  of the screen  10 . In the event that non-fluid matter should reach the screen  10  it will be rolled therealong by the jet stream to the deflector plate  36  where it will be urged out into the main flow. 
   A number of variations to the construction in the  FIGS. 1 to 3  are shown respectively in  FIGS. 4 to 7 . Thus in  FIG. 4  the median bisector of the U defined by the semi-circular cross-section of the screen  10  is generally parallel to the wall  16 , with the base of the U lowermost. Furthermore, that side of the screen  10  which is further from the wall  16  is extended to a level significantly higher than the top of the wall  16  at the cut-out  18 . 
   In  FIG. 5 , the edge of the screen  10  further from the wall  16  is extended by a generally upright lip  40 . 
   In the modification shown in  FIG. 6 , the said bisector is generally horizontal, and a portion of the screen  10  close to the base of the U rests on a support plate  42 . 
   In the modification shown in  FIG. 7  the U of the screen&#39;s cross-section is entirely inverted with opposite ends of the U supported respectively on two weirs  16  spaced apart from one another with the channel  20  defined therebetween. 
   Numerous variations and modifications to the illustrated constructions may occur to the reader without taking the resulting construction outside the scope of the present invention. For example, the deflector plate  36 , could be made as an integral part of the screen  10  instead of as an insert therefor. It would, however, in that case be necessary for the holes  14  to be absent from the deflector plate region of the screen, or else those holes  14  should be filled in in that region. Unfiltered water could be used for the jet of fluid, and a standard pump could be used without air injected. The perforated screen cross-section could be continuously reduced from the pump end to the deflector plate end to improve its self-cleaning function.