GB2044624A - Skip raked bar screen - Google Patents

Abstract

A rake (28) co-acts with a screen grid (10) installed in a flow channel (12) to remove debris collected against the upstream side of the grid. The rake is fixedly attached to carriages (26) which are mounted on rollers (24) which run between rails (22). Power means (42) move the rails (22) towards and away from the grid (10) so as to bring the rake (28) into engagement with the grid and to retract it. During a cycle the rake (28) is retracted from the grid (10) and is then moved down the rails (22). The rake (28) is next brought into engagement with the grid (10). To ensure that the rake (28) can be forced through large amounts of debris which may have collected at the base of the grid (10) the rake (28) is held at a fixed angle with respect to the rails (22) due to the rollers (24) being contained within the rails (22). Excess current-sensing device monitors motor (38) and causes the rails to be moved, retracting the rake from the grid on meeting a solid obstruction. <IMAGE>

Description

SPECIFICATION Skip raked bar screen The present invention relates to a skip-raked bar screen.

Skip-raked bar screen are employed inter alia in water treatment plants to remove solid debris from incoming raw water and in sewage works to remove debris from a flow channel. Skip-raked bar screen are suitable for use in channels or screen chambers of a depth which requires the debris or screenings to be lifted more than about 1 1/2 metres to the discharging point. In one form the screen includes a number of spaced parallel bars, usually of steel rigidly fabricated into a grid. The grid is placed in and across the channel so that the bars extend vertically or are somewhat inclined to the vertical. Liquid flow can thus pass through the grid whilst solid debris can be held by the bars.To remove the debris a skip rake consisting of a scraper blade having tines which can project between the bars of the grid is mechanically arranged to travel up the grid collecting the debris held back by the bars and depositing it at a discharge point above the channel.

The skip rake has to be arranged to be lowered into the channel at a position clear of the grid and of any debris collected against it. Once lowered the skip rake has to be moved towards the grid to engage the bars to collect the debris on its next upward stroke.

Conventionally the skip rake has been pivotally mounted on a carrier which is fitted with guide rollers running in or on fixed tracks adjacent and parallel to a straight screen grid. The pivotal mounting allows the skip rake to be swung away from the grid and lowered into the channel. The skip rake can thus be allowed to re-engage the grid by pivoting towards the grid under its own weight; control of the skip rake engagement can be achieved by means of a lowering or positioning rope, or by means of a second set of guide tracks.

Flow in sewers consists of domestic effluents having a fairly high organic solid content, and treated and untreated industrial effluents; in addition, surface water drainage is normally fed into the same sewers.

Such known skip raked bar screens as described above can be satisfactory under normal conditions where the debris content of the channel does not exceed a certain level.

In storm conditions however, flows are much increased, and the surface water tends to entrain much solid debris; it is also the case that, at the entrance to sewage treatment works, normal sewage flows have a high organic solids content. Under such conditions a pivotally mounted skip rake designed to engage the grid under gravity can be prevented from full or proper engagement with the grid due to heavy solid content collected against the bars. Moreover under conditions of low flow little impedance to engagement with the grid is offered by the effluent and engagement can take place too violently leading to increased screen wear.

To try to overcome these problems it is known to control pivotal movement of the skip rake on the carrier by means of a pneumatic or other power cylinder attached to the carrier. The cylinder is supplied with power by a free trailing pipe or cable.

Such an arrangement however can involve the undesirable immersion of a power cylinder in the sewage flow and can present handling problems of the cable or pipe.

Rotary or semi-rotary power operated rakes are known in which the rakes extend across the channel and rotate about an axis transverse to the channel to co-act with a curved bar screen. Such power driven rakes are however only suitable for shallow channels with flows of up to about 2 metres in depth.

It is an object of the present invention to overcome these problems and to provide a skip raked bar screen suitable for use in medium depth channels under conditions of high and low flows.

According to the invention we provide a skip raked bar screen including a screen grid having a plurality of rigidly spaced parallel bars, a skip rake adapted, in use, to co-act with the grid for raking out debris therefrom and carrier means to carry the skip rake, the carrier means being adapted to move the skip rake along the grid, there being provision to cause relative movement between the grid and the carrier means towards and away from each other so as to bring the skip rake and the grid into engagement with each other and to retract the one from the other, the skip rake on being brought into engagement with the grid being held at a predetermined angle relative to the carrier means.

The bar screen can be a straight bar screen appropriate for installing in a medium depth channel, for example a channel of up to about 5 metres deep.

The carrier means conveniently includes an elongate member which is adapted to extend parallel to the grid when the skip rake is moved along the grid.

The elongate member may be a carriageway along which the skip rake can be moved by any suitable means. For ease of movement the skip rake can be fixedly attached to a carriage which is adapted to move along the carriageway. Preferably the carriageway includes at least one rail and the carriage means includes engagement means adapted to engage the rail(s) and to move along the rail(s). The engagement means conveniently engages each respective rail at at least two points in the longitudinal direction of the carriageway, so as to hold the skip rake at a fixed predetermined angle relative to the carriageway, on bringing the skip rake into engagement with the grid.In a preferred form of the invention the carriageway includes two rails each of which includes a pair of runners spaced from one another and the engagement means includes at least two longitudinally spaced rollers or sliding blocks arranged to run between each pair of runners. The rollers or sliding blocks, the blocks being preferably of a low friction material, are thus contained between a pair of runners so that pivotal movement of the skip rake relative to the carriageway on being brought into initial engagement with the grid can be prevented.

Preferably the grid is held stationary and the relative movement towards and away from the grid is due to movement of the carrier means. The move ment can for example be a pivotal movement or be a translational movement in a given direction. Where the carrier means include an elongate member such as a carriageway, the member can be conveniently pivotably mounted on a frame structure above the grid so that in use the member can swing about that pivot towards and away from the grid, occupying a position parallel to the grid for the skip rake to move therealong and rake out debris from between the bars of the grid. To provide translational movement an elongate member such as a carriageway can conveniently be mounted parallel to the grid in horizontally extending guides. Movement of the carriageway towards and away from the grid can be facilitated by providing roller guides.Movement of the grid towards and away from a fixed carriageway could, if desired, be achieved in a similar manner by mounting the grid either pivotally or in horizontal guides. Movement of the carriageway is however preferred as it can lead to a more economic construction. In both constructions (movable grid or movable carriageway) the pivotal mounting or the horizontal guides can be located above the channel clear of any flow to be screened.

To move the carriageway towards and away from the grid any suitable power means can be used.

Examples of such power means include one or more fluid power cylinders, one or more electric or electro-hydraulic actuators, or one or more pneumatic cylinders. The cylinders or actuators can be arranged to act upon the carriageway at a position above the channel clear of any flow. An appropriate cylinder or actuator can similarly be used to act on the grid, or a back plate extending above the grid and lying out of the channel, if the carriageway is held fixed and the grid is arranged to move.

In either arrangement the use of fluid power cylinders, pneumatic cylinders or electro-actuators permits the partial retraction of the carriagway (or grid) to permit the skip rake to pass any immovable obstruction. A control circuit in which the cylinder or actuator is incorporated can include one or more pressure sensitive valves or current sensing devices respectively to detect any resisted motion of the skip rake and on such detection to cause the power means to operate to retract the carriageway (or grid).

The skip rake may thus pass the obstruction; the pressure sensitive valves or current sensing devices detect the fall in resistance to motion and send a signal to cause the power means to replace the carriageway (or grid) in parallel relationship with the grid (or carriageway).

One embodiment of the present invention will now be described by way of example with reference to the accompanying drawings; wherein: Fig. 1 shows in sectional side elevation a screen installed in and over a sewage channel, as seen along line I-I of Fig. 2; Fig. 2 is a view taken on line Il-Il in Fig. 1; and Fig. 3 is a detail in elevation illustrating engagement of a skip rake with a screen grid.

Referring to Figs 1 and 2 the screen includes a screen grid 10 fixedly installed in and across the sewage channel 12 at an angle a of 110 to the flow of sewage which, in Fig. 1, flows along the channel 12 in the direction of arrow A. The grid 10 includes a number of spaced, straight, parallel bars extending from a base of the channel 12 to above the top level of sewage flows. A back plate 16 lying above and parallel to the grid 10 acts as a continuation of the grid 10 and leads to a hopper 18. A discharge chute 55 is provided on the back plate 16 to prevent debris accumulating on the side plates of the hopper 18. In place of the hopper a conveyor could be use.

A frame head structure 20 lies over and across the channel 12 and carries straight parallel carriage rails 22 which extend downwardly into the channel 12.

The rails 22 are pivotally mounted on a drive shaft 40 itself carried by the head structure 20 such that a track which they form can swing towards and away from the screen grid 10 on its upstream side. At a position of closest proximity to the grid 10 the track lies parallel to the screen grid 10. The rails 22 are formed of four angle bars arranged in two distinct pairs 22' and 22". The two bars making up each pair are closely spaced so as to provide a two sided runway for rollers 24. Two rollers 24 are provided between each pair of rails and are attached to respective carriages 26. Pivotal movement of the carriage with reference to the rails is thus prevented due to the containment of the sequentially mounted rollers between their respective two-sided runways.A skip rake 28 of width equal to the effective width of the grid 10 is fixedly and rigidly attached to the carriages 26 and projects forwardly from the track towards the grid 10. The skip rake 28 includes an open ended through 30 lying across the track and having at its lower longitudinal edge a row of forwardly projecting tines 32 whose number, dimensions and position are such that they can project between the bars of the grid 10.

Separate roller chains 34 are attached to each carriage 26 and extend between their respective pairs of rails 22', 22" to respective sprocket wheels 36 driven by a motor 38. A drive shaft 40 from the motor to the wheels 30 acts also as a pivot for the track. Ends of the chains remote from the carriages are attached to a rearward part 35 of the head structure 20. In place of the roller chains and sprocket wheels, a rope and hoist drum system can be used instead.

Attached to the head structure 20 at a position clear of the sewage flow is an electric actuator 42 which acts on the track to swing it towards and away from the grid 10. Pivot pins 44 and 46 allow for articulated movement of the electric actuator 42 with respect to the track and the head structure 20. Actuation of the electric actuator 42 and the motor 38 is controlled by limit switches (not shown) which detect when the carriages 26 have reached a lowermost and an uppermost position on the track.

Stiffening members 48 hold the track rigid and the rails 22' and 22" at a constant width. The members 48 project behind the track so as not to hinder movement of the skip rake 28 along the track. A wiper blade 50 of greater width than the skip rake 28 lies across and in front of the track at a position above the back plate 16. The blade 50 is supported at each end by an arm 52 which lies outside the track and is pivotally mounted on a pair of trunnions 54 projecting from outside edges of the tracks. The blade 50 is thus freely suspended.

In use the screen is raked out cyclically. At the start of a cycle the skip rake 28 is held at an uppermost position on the track with the chains 34 hanging freely between the motor 38 and the head structure 20 in the position shown by broken lines in Fig. 1.

When the skip rake 28 reaches that uppermost position the limit switch actuates the electric actuator 42 to return and to retract the track from the grid 10. The track is thus swung away from the grid 10 about the drive shaft 40 and the skip rake 28 can be freely lowered to a lowermost position. The tines 32 make no contact with the grid 10 whilst being lowered and are held at a sufficient distance from the grid to be clear, under normal circumstances, of any debris collected against the grid 10. The chains 34 are closely contained between their respective pair of rails 22 so that being driven by the motor they do not buckle but provide a downward thrust on the carriages. The carriages 26 and skip rakes 28 can thus be forced downwardly th rough any debris adhering to the rails 22.

On reaching the lowermost position the limit switch actuates the electric actuator 42 which swings the track towards the screen grid 10. The skip rake 28 being held rigidly by the carriage 26 has the power of the electric actuator behind it to force it through any debris collected at the bottom of the grid 10. The end of the cylinder stroke places the track parallel to the grid 10 with the tines 32 of the skip rake 28 inserted between the bars of the grid 10. Engagement of the tines 32 with the grid 10 is shown in enlarged section in Fig. 3. Motor 38 now operates to pull the carriages 26 up the track. The electric actuator 42 maintains its position to hold the track parallel to the grid 10 so that the tines 32 scrape debris off the grid into the trough 30.As the skip rake 28 moves past the top of the back plate 16 it contacts the wiper blade 50 which sweeps freely across the trough 30 and the tines 32 due to their continued upward movement pushing the collected debris off the skip rake 28 and into the hopper 18. The skip rake 28 has now reached its uppermost position and the limit switches are actuated to start the beginning of a new cycle. As the track is retracted and the skip rake 28 lowered the wiper blade 50 swings back to its rest position ready for the next upward movement of the skip rake.

Should the skip rake meet an obstruction on its upward travel which the tines are unable to scrape off the grid, the resistance to upward travel increases the load on the drive motor with a consequent increase in current through the motor. The provision of current sensing devices in the electrical circuit of the drive motor can detect an increase in current to the motor and cause the electric actuator to retract, carrying the track and skip rake with it to withdraw the tines from the grid and to permit them to pass the rigid obstructions. When the obstruction is passed the current will fall and a signal is sent to return the actuator to the end of its stroke and thus hold the track parallel to the grid again and the tines reinserted between the bars of the grid.

Current sensing devices are also provided in the electrical circuit of the actuator, and these will detect an excessive increase in current to the actuator if the tines of the skip rake meet a solid obstruction when moving into engagement with the grid. The sensing of this increase in current is arranged to cause the drive motor to start and hoist the skip until such time as the tines are clear of the solid obstruction and the actuator can continue its stroke without obstruction until the tines are fully engaged with the grid.

As alternatives to an electric actuator a fluid power cylinder, a pneumatic cylinder or an electrohydraulic actuator can be used instead. In place of current sensing devices in the electrical circuit of the actuator, pressure relief valves can be employed to detect an excessive increase in pressure caused by the skip rake contacting a solid obstruction in its grid engagement action and to actuate a signal causing the track to be temporarily retracted.

The present invention can thus provide a skip raked bar screen in which the skip rake is rigidly held when it initially engages the grid at the base of the channel. The force for moving the skip rake into engagement comes from the electric actuator moving the track. The skip rake can thus be forced through large amounts of debris collected at the base of the grid under storm flow conditions.

To prevent lateral movement of the track or other carriageway, guides can be inserted in the side wall of the channel. To ensure the correct positioning of the track in relation to the grid, abutment stops can be provided to locate the track at the conclusion of the stroke of the actuator or cylinder.

In order to provide additional advantages, the bar screen described above embodying the present invention can be improved by incorporating two stops or limit switches (not shown in the figures) in the outward stroke of the electric actuator 42. The first limit switch locates the carriageway 22 in the correct position for raking and the second outermost limit switch and associated control circuit allows the actuator to extend further towards the grid 10 when the skip rake 28 has been lifted to a position above the top of the back plate 16 and discharge chute 55.

The further extension of the actuator 42 is stopped before the carriageway 22 strikes the grid 10, but is sufficient to permit the skip rake 28 to overlie the reception hopper (or conveyor) 18 and so avoid the accumulation of debris on the discharge chute 55.

Claims (13)

1. A skip raked bar screen including: a screen grid having a plurality of rigidly spaced parallel bars; a skip rake adapted, in use, to co-act with the grid for raking out debris therefrom; and carrier means to carry the skip rake, the carrier means being adapted to move the skip rake along the grid, there being provision to cause relative movement between the grid and the carrier means towards and away from each other so as to bring the skip rake and the grid into engagement with each other and to retract the one from the other, the skip rake on being brought into engagement with the grid being held at a predetermined angle relative to the carrier means.

2. A bar screen according to claim 1 wherein the carrier means includes an elongate member which is adapted to extend parallel to the grid when the skip rake is moved along the grid.

3. A bar screen according to claim 2 wherein the elongate member is a carriageway and the carrier means includes carriage means which are adapted to move along the carriageway and to which the skip rake is fixedly attached.

4. A bar screen according to claim 3 wherein the carriageway includes at least one rail and the carriage means includes engagement means adapted to engage the rail(s) and to move along the rail(s).

5. A bar screen according to claim 4 wherein the engagement means engage each respective rail at at least two points in the longitudinal direction of the carriageway.

6. A bar screen according to claim 5 wherein the carriageway includes two rails each of which includes a pair of runners spaced from one another and the engagement means includes at least two longitudinally spaced rollers or sliding blocks arranged to run between each pair of runners.

7. A bar screen according to any one of the preceding claims wherein the relative movement between the grid and the carrier means is a pivotal movement.

8. A bar screen according to any one of claims 1 to 6 wherein the relative movement between the grid and the carrier means is a translational movement.

9. A bar screen according to claim 7 or claim 8 wherein the grid is held stationary and the relative movement is due to movement of the carrier means.

10. A bar screen according to any one of the preceding claims wherein the relative movement between the grid and the carrier means is provided by power means which includes sensing means to detect resisted motion of the skip rake and a subsequent fall in resistance to motion such that relative movement between the grid and the carrier means away from each other occurs on detection of said resistance and relative movement towards each other occurs on detection of said fall.

11. A bar screen according to anyone of the preceding claims being a straight bar screen.

12. A skip raked bar screen substantially as herein described with reference to Figs 1 to 3 of the accompanying drawings.

13. A skip raked bar screen according to any one of the preceding claims installed in a flow channel.