GB2438076A

GB2438076A - Rotary screen drum

Info

Publication number: GB2438076A
Application number: GB0708946A
Authority: GB
Inventors: William Edmondson
Original assignee: Individual
Current assignee: Individual
Priority date: 2006-05-12
Filing date: 2007-05-10
Publication date: 2007-11-14
Anticipated expiration: 2027-05-10
Also published as: GB2438076B; GB0609442D0; GB0708946D0

Abstract

A rotary screen drum, such as used in the dewatering of sludge, has an inlet 6 at a lower level than an outlet 8, such that if the apparatus becomes blocked, material flows of preference back into the inlet 6 rather than out of the outlet 8. In other terms, the screen drum is angled upwards from inlet 6 to outlet 8. During normal usage, when not blocked, the drum may be used in a horizontal orientation. The drum is driven rotationally by a motor, and an internal screw thread arrangement 23 may progress material along the length of the drum. The orientation of the drum may be adjusted with an actuator 19.

Description

time to time without the whole of the waste processing plant to which

it has been fitted having to be closed down while the blockage is cleared.

According to a first aspect of the present invention there is provided a rotary drum screen apparatus for processing particulate material comprising a drum screen comprising at least one cylindrical screen section defining an inlet opening at one end through which material to be dewatered is introduced into the screen and defining an outlet opening at the other end for the discharge of dewatered material; drive means for rotating the drum screen at a predetermined rotational speed about its longitudinal axis; a feed pipe for discharging material to be dewatered into the interior of the drum screen through the inlet opening; a casing that surrounds the drum screen and that is adapted to collect liquid filtered by the drum screen for further processing; and wherein, in use with the longitudinal axis of the drum screen horizontal or angled upwards in a direction towards the outlet opening, the lowest level of the inlet opening is at a lower level than the lowest level of the outlet opening so that when the screen is blocked unfiltered material preferentially flows out of the drum screen through the inlet opening rather than the outlet opening.

According to a second aspect of the present invention there is provided the use of a rotary drum screen apparatus according to the first aspect of the present invention wherein under normal operation particulate material to be dewatered is discharged through the feed pipe into the drum screen which is rotated by the motor at a predetermined circumferential speed, filtered liquid is collected at the bottom of the casing for further processing and dewatered material is discharged through the outlet opening, and characterised in that on detection of unfiltered material passing out of the drum screen via the inlet opening, the discharge of further material through the feed pipe into the drum screen is stopped and the unfiltered material passing out of the drum screen via the inlet opening is channelled by an overflow arrangement to a recirculation arrangement.

According to a third aspect of the present invention there is provided a material processing plant comprising a rotary drum screen apparatus according to the first aspect of the present invention.

According to a fourth aspect of the present invention there is provided a method of operating a material processing plant comprising a rotary drum screen apparatus according to the first or the second aspect of the present invention wherein under normal operation material to be dewatered is discharged through the feed pipe into the drum screen which is rotated by the drive means at a predetermined circumferential speed, filtered liquid is collected at the bottom of the casing for further processing and dewatered material is discharged through the outlet opening, and wherein on detection of unfiltered material passing out of the drum screen via the inlet opening, the discharge of further material through the feed pipe into the drum screen is stopped and the unfiltered material passing out of the drum screen via the inlet opening is channelled to a recirculation arrangement.

Preferred additional features of all the various aspects of the invention are described in the dependent claims.

The various aspects of the present invention will now be described by way of example with reference to the accompanying drawings, in which:-Fig. 1 is a schematic side elevation in partial cross-section of a rotary drum screen apparatus according to the present invention when operating normally; Fig. 2 is a an end elevation of the drum shown in Fig. 1; Fig. 3 is a transverse cross-section of the drum shown in Fig. 1; Fig. 4 is a view similar to that of Fig. 1 but showing the rotary drum screen apparatus when in an overflow situation; and Fig. 5 is a process and instrumentation diagram of a waste processing plant incorporating a rotary drum screen apparatus in accordance with the present invention.

One embodiment of a rotary screen filter apparatus is shown in Figs i to 4. The illustrated apparatus is designed primarily for use in a waste water processing plant and can be retrospectively fitted to an existing plant as will be described in more detail with reference to Fig. 5.

The apparatus comprises a casing i within which is located a rotary drum screen 2 comprising, in the illustrated embodiment, two cylindrical sections 3 and 4 and a conical discharge section 5. It should be appreciated, however, that the dependent on the size of drum screen 2 required for any particular application, the drum screen 2 may comprise only a single section or a plurality of sections in excess of that shown in Figs. 1 and4. However, in the illustrated embodiment, the first cylindrical section 3 defines an inlet opening 6 at one end through which material 7 to be dewatered is introduced into the screen 2 and the discharge section 5 defines an outlet opening 8 at the other end for the discharge of dewatered material 9. The screen 2 is driven about its longitudinal axis 10 by a motor ii and belt 12 located at its inlet end, as shown in Fig. 2. The motor ii is preferably a variable speed motor in order that the rotational speed of the screen 2 can be controlled to optimize the relative speed at which the material 7 hits the screen 2 and the dwell time of the material 7 within the screen 2. Dependent on the type of material 7 to be dewatered, there is an optimal circumferential speed for the screen 2 which causes liquids to be sheared away efficiently from solid material as the material 7 first contacts the screen surface. The motor 11 can therefore be set up to rotate the screen 2 at this speed dependent on the size of the screen and the nature of the material 7 being processed.

The first and second sections 3 and 4 of the screen 2 are preferably made of wedge wire, which has a triangular cross-section, arranged with the apex of the cross-sectional profile pointing radially outwards from the drum screen 2. This allows liquids to be separated from and to drain away from the sludge material 7 being dewatered more readily and therefore improves the efficiency of the dewatering process. However, in some cases it may be more appropriate to make the sections 3,4,5, of the screen 2 from perforated plate.

The conical discharge section 5 also assists in the retention of material 7 within the screen 2 to increase the time available for dewatering and decontamination of the material 7. The screen 2 is housed within the casing 1 that at its underside, beneath the sections 3 and 4 of the screen 2, is formed into a chute 13 to collect liquid 14 filtered from the material 7 by the screen 2 and to channel the liquid 14 for delivery to another part of a processing plant, as will be described in more detail with reference to Fig. 5. The casing i is also provided with two additional but smaller chutes 15 and i6 located at each side of the larger chute 13. The chute 15 is located beneath the outlet opening 8 and collects the dewatered material 9 as it is discharged from the screen 2. This chute 15 enables the dewatered material 9, usually called screenings, to be collected, for example in a skip 17 for further processing. In contrast, the chute i6 is located at the other end of the casing 1 beneath the inlet opening 6 of the screen 2 and collects unfiltered material 7 that overflows out of the screen 2 if a blockage occurs. This circumstance is described in more detail below. The chute 16 is intended to channel material flowing through it to a recirculation arrangement and is provided with an overflow detection sensor 18 that detects when an overflow situation starts and stops by detecting the flow of material 7 through the chute 16. The sensor 18 is preferably linked to controls means for other apparatus in the system in which the rotary filter screen apparatus is incorporated, as is described in more detail below.

Retention of material 7 within the screen 2 is also affected by the degree of tilt of the screen's longitudinal axis 10. This can be changed by a level adjustment means which comprises a hydraulic cylinder 19 that can rotate the discharge end of the casing i upwards via a supporting beam 20 about a pivot 21 at the other end of the casing i. However, regardless of the whether the longitudinal axis 10 is horizontal or tilted upwards towards the outlet opening 8, the lowest level of the inlet opening 6 is arranged at a lower level than the lowest level of the outlet opening 8. This ensures that if the screen 2 becomes blocked, unfiltered material 7 preferentially flows out of the drum screen 2 through the inlet opening 6 rather than building up within the screen 2 or flowing out through the outlet opening 8. In this regard, the size of the inlet opening 6 is controlled by the size of a baffle 22 which is provided at the free end of the first section 3 of the screen 2, The baffle 22 is in the form of an annulus with a central opening forming the inlet opening 6, the interior diameter of which determines the lowest level of the inlet opening 6. The baffle 22 is otherwise imperforate to prevent leakage of liquid therethrough other than via the inlet opening 6.

Flow of material 7 through the screen 2 is assisted by the provision of two helical vanes 23, 24 which are located around the interior periphery of the screen sections 3, 4 and 5. In the illustrated embodiment shown in Figs. i and 3, one of the vanes 23 is arranged along the full length of the screen 2 from the inlet opening 6 to the outlet opening 8. However, the second vane 24 is only provided in the first section 3 of the screen 2. The pitch of the helical vanes 23, 24 is the same but the second helical vane 24 is arranged to be substantially 1800 out of phase with the first helical vane 23. This arrangement speeds material 7 out of the first section 3 of the screen where the greatest degree of dewatering takes place as the material 7 enters the screen 2 but then slows the passage of the material 7 through the sections 4 and 5 of the screen 2 to allow time for further dewatering and decontamination to occur thereafter prior to discharge of the dewatered material 9 from the outlet opening 8.

However, in some applications it may be advantageous for the helical vane arrangement to be such that the passage of material through the drum screen 2 is the opposite of that described above, with material being held back within the first section of the drum screen 2, where the greatest degree of dewatering takes place, and then, relatively speaking, to be speeded up through the later section of the drum screen 2. In these circumstances the second vane 24 may be located in the second section 4 and/or the discharge section 5 of the drum screen 2 rather than in the first section 3.

In an alternative arrangement (not shown), a single helical vane could be provided through the screen 2 but having a variable pitch, such that its pitch in a first section 3 of the screen 2 adjacent the inlet opening that 8 is different from its pitch in the section 4 and/or section 5 of the screen 2. It may be appropriate, dependent on the application, for this difference in pitch to be either up to half or alternatively double that of the pitch in the first section 3 of the screen 2, dependent on the application.

The material 7 to be dewatered is fed into the screen 2 via a feed pipe that runs along the longitudinal axis 10 of the screen 2 through the inlet opening 6 formed in the baffle 22. Preferably, the feed pipe 25 discharges the material 7 centrally of the first section 3 of the screen 2 and is cut-away along its length to form an outlet orifice 26 that is directed downwardly and to one side of the screen 2. This ensures the material 7 is discharged into the screen 2 over a large surface area to facilitate and improve dewatering.

In order to assist in the prevention of blockages of the screen 2 and in the dewatering and decontamination of the material 7, the apparatus is provided with two batteries 27, 28 of spray nozzles. The first battery 27 of spray nozzles is located within the casing i outside and above the screen 2.

This battery sprays low volume, high velocity water onto the exterior of the first two sections 3 and 4 of the screen 2 to wash away any small particles that have passed through the screen 2 and to wash back into the body of the material 7 within the screen 2 any larger particles that may be sticking to the screen's interior surface. The second battery 28 of spray nozzles is located internally of the screen 2 and only within the second section 4. This second battery 28 is intended to wash and to decontaminate the material 9 prior to discharge. This is particularly important, for example, if the dewatered material 9 is to be used for landfill, for fertilizer or similar applications. Both of the batteries 27, 28 are fed from a source 29 of high pressure water via an isolation valve 30 and a pressure reducing valve 31 to individual solenoid valves 32 and 33 for each of the batteries 27, 28 respectively. In addition, a further pressure reducing valve 34 may be provided for the battery 28 to allow the pressure of the water to be reduced further to allow the water to thoroughly wash the material 9. In this way, the water flow to the batteries 27, 28 can be controlled individually.

In use, the screen 2 is tilted at an appropriate predetermined angle and rotated by the motor ii at a predetermined rotary speed according to its size and the nature of the material 7 to be dewatered. This material 7 is then fed into the screen 2 via the feed pipe 25. Under normal operating circumstances as shown in Fig. 1, the material 7 is transported through the screen 2 by the vanes 23, 24 and is dewatered primarily as it passes through the sections 3 and 4 of the screen 2. The liquid 14 egressing from the screen 2 is collected in the chute 13 and transported away, for example to an effluent outlet, and the dewatered material 9 left behind in the screen 2 is discharged via the outlet opening 8 into the chute 15 for collection, for example in the skip 17 for further processing. As explained above, the spray batteries 27, 28 assist in this process. However, if the screen 2 should become blocked, then material 7 builds up within the screen 2, as shown in Fig. 4. In this case, very little liquid can pass through the screen 2 into the chute 13 so that the material 7 retains a high liquid content. As material 7 is continuing to be delivered into the screen 2, eventually a sufficient build-up occurs that the material starts to flow out of the screen 2 through the inlet opening 6. As described above, the lowest level of the inlet opening 6 is arranged to be at a lower level than the lowest level of the outlet opening 8 so that the material 7 preferentially flows out of the screen 2 in this direction. When this happens, the overflowing material 7 is collected in the chute i6 and can be transported to recycling arrangement. At the same time, the overflow detection sensor i8 detects that an overflow situation has occurred. This enables other parts of the system of which the apparatus forms a part to be controlled accordingly, as will now be described with reference to Fig. 5. It should also be appreciated that the sensor i8 will also detect when an overflow situation is no longer occurring and normal operation of the apparatus can be resumed.

In a conventional waste processing plant such as a sewage treatment farm as shown in Fig. 5 in chain-dot lines, the raw sewage is first stored in a sludge reception tank 40 before being pumped via a pump arrangement 41 to a feed chamber 42 via pipework 43. From the feed chamber 42, the waste is piped to a screening arrangement such as a rotary bar screen arrangement 44 for dewatering. The liquid waste removed by the screening arrangement 44 is collected in chute 45 and transported to a distribution chamber 46 whence it is taken for further processing to consolidation tanks or other processing apparatus. The screenings produced by the arrangement 44 are discharged usually collected in a skip 47, again for further processing.

The dewatering achieved by the rotary bar screen arrangement 44 is, in many cases, relatively rudimentary and this results in a considerable quantity of solid matter escaping through the arrangement and ending up in the distribution chamber. This matter may include grit and similar material that can cause damage and clog apparatus intended to process liquid waste.

It is therefore proposed to improve the dewatering process by the inclusion of a rotary drum screen apparatus 50 as described above into a conventional processing plant as just described. One way that this can be achieved will now be described with reference to Fig. 5 where the new additions to the conventional system are shown in solid lines and the various parts of the apparatus 50 schematically depicted in Fig. have been given the same reference numerals as those shown in Figs. 1 to 4.

In the new system, instead of pumping the waste from the sludge reception tank 40 to the feed chamber 42, it is now pumped via a first actuated valve 51 to the feed pipe 25 of the rotary drum screen apparatus 50.

A second actuated valve 52 is also provided to close off the inlet into the feed chamber 42 to ensure that the material travels to the apparatus 50. Here, the waste material is more efficiently dewatered than previously and the liquid -10 -waste collected by the chute 13 is channelled into the feed chamber 42 where it can be led either to the screening arrangement 44 and thence to the distribution chamber 46 or be channelled directly to the latter dependent on whether the screening arrangement 44 is operational. For example, it may be undergoing routine maintenance or could be blocked. The dewatered material produced by the apparatus 50 is discharged into the chute 15 and piped either to a screenings skip 53 or to a screw compacter 54. To this end, the pipework attached to the chute 15 includes a diverter valve 55 that preferentially directs the screenings to the screw compacter 54 but can be operated to direct material directly to the skip 53 if the compacter 54 is out of action. The screw compacter 54 will extract further liquid from the screenings and this is collected in a sump 56 from where if may be pumped back into the distribution chamber 42 or, if the sump 56 is in danger of becoming too full, allowed to overflow into a drainage system via a weir and associated pipework 57. The compacted waste produced by the compacter 54 is discharged to the skip 53.

Under normal operation of the system, any further solid material that is screened out of the material pumped to it from the feed chamber 42 by the rotary bar screen arrangement 44 may also be piped to the compacter 54 via a screw conveyor 58. A diverter valve 59 is preferably included in the pipework so that if the conveyor 58 or the compacter 54 is out of action, the material can be diverted from the arrangement 44 directly to the skip 47.

It should be appreciated that although the skips 47 and 53 are shown as two separate skips in the drawing and described above as such, it may be possible in some processing plants for them to be one and the same receptacle.

In the case where the rotary drum screen 2 of the apparatus 50 becomes blocked or clogged and an overflow situation occurs, unfiltered material will start to flow out of the screen 2 via the chute 16 and the overflow detection sensor i8 will signal that an overflow situation has -11 -occurred. When this happens, the valve 51 is closed so that no further material is pumped to the apparatus 50 and instead the valve 52 is opened so that the material is pumped directly to the feed chamber 42. The overflowing material egressing from the chute 16 is also fed into the feed chamber 42 and the system starts to operate as a conventional system, the material in the chamber 42 being pumped to the rotary bar screen arrangement 44 for dewatering. However, the screen 2 of the apparatus 50 continues to be rotated and the spray batteries 27, 28 continue to operate to try to clear the blockage. When the sensor i8 detects that no further material is being discharged by the apparatus 50 through the chute i6, then the valve 51 is opened and the valve 52 is closed to recommence pumping of material back to the apparatus 50. A control system warning may also be flagged so that an operative can investigate if the apparatus 50 remains blocked.

However, the advantage of the system is that the plant will remain operational, albeit operating at a lower efficiency, during a blockage. This is a considerable advantage over current systems where a blockage in, for example, the rotary bar screen arrangement 44 results in operation of the whole plant being brought to a standstill.

Hence, the fitment of a rotary drum screen apparatus 50 of the present invention into an existing processing plant not only increases the efficiency of the plant because dewatering of material can be more efficiently carried out, for example separating grit and fine solid particles from liquid waste, but also enables blockages too be dealt with in a much more efficient manner with considerable savings in cost and manpower. Although the rotary drum screen apparatus has been particularly designed for use in waste water and sewage processing apparatus, it will be appreciated that it could be used in many applications where the processing of particulate material, such as dewatering or grading and washing, is required in, for example, waste water processing applications and applications within the food industry.

Claims

-12 -

CLAIMS

1. A rotary drum screen apparatus for processing particulate material comprising a drum screen comprising at least one cylindrical screen section defining an inlet opening at one end through which material to be dewatered is introduced into the screen and defining an outlet opening at the other end for the discharge of dewatered material; drive means for rotating the drum screen at a predetermined rotational speed about its longitudinal axis; a feed pipe for discharging material to be dewatered into the interior of the drum screen through the inlet opening; a casing that surrounds the drum screen and that is adapted to collect liquid filtered by the drum screen for further processing; and wherein, in use with the longitudinal axis of the drum screen horizontal or angled upwards in a direction towards the outlet opening, the lowest level of the inlet opening is at a lower level than the lowest level of the outlet opening so that when the screen is blocked unfiltered material preferentially flows out of the drum screen through the inlet opening rather than the outlet opening.

2. An apparatus as claimed in Claim 1, wherein an overflow arrangement collects any unfiltered material passing out of the drum screen via the inlet opening.

3. An apparatus as claimed in Claim 1 or Claim 2, wherein a detector is located adjacent the inlet opening to detect when unfiltered material passes out of the drum screen via the inlet opening.

4. An apparatus as claimed in any of Claims 1 to 3, wherein the drum screen comprises a conical discharge section that defines the outlet opening at its narrower end.

-13 - 5. An apparatus as claimed in any of Claims 1 to 4, wherein an annular baffle is provided at said one end of the drum screen that defines the inlet opening through which the feed pipe passes and which has an interior diameter that determines the level of inlet opening.

6. An apparatus as claimed in any of Claims i to 5, wherein the apparatus comprises a level adjustment means whereby the level of the longitudinal axis of the drum screen can be adjusted to alter the lowest level of the inlet opening relative to the outlet opening.

7. An apparatus as claimed in any of Claims 1 to 6, wherein the interior of the drum screen is provided with a helical vane to control movement of dewatered material along the length of the drum screen.

8. An apparatus as claimed in Claim 7, wherein at least part of the interior of the drum screen is provided with first and second helical vanes.

9. An apparatus as claimed in Claim 8, wherein the pitch of the first and second helical vanes is the same but the second helical vane is approximately 1800 out of phase with the first helical vane.

10. An apparatus as claimed in Claim 9, wherein the drum screen comprises a first section that defines the inlet opening and that is provided with the first and second helical vanes and a second section that defines the outlet opening and that is provided only with the first helical vane.

ii. An apparatus as claimed in Claim 7, wherein the drum screen is provided with a single helical vane which has a pitch in a first section of the drum screen adjacent the inlet opening that is different from its pitch in a second section of the drum screen adjacent its outlet opening.

-14 - 12. An apparatus as claimed in Claim ii, wherein the pitch of the helical vane arrangement is smaller in the first section of the drum screen than it is the second section in order than material is moved more quickly through the first section than it is through the second section.

13. An apparatus as claimed in Claim ii, wherein the pitch of the helical vane arrangement is greater in the first section of the drum screen than it is the second section in order than material is moved more slowly through the first section than it is through the second section.

14. An apparatus as claimed in any of Claims 1 to 13, wherein a first battery of spray nozzles is located within the casing and adapted to spray over the exterior of the drum screen.

15. An apparatus as claimed in any of Claims i to 14, wherein a second battery of spray nozzles is located within a part of the drum screen adjacent the discharge section.

i6. An apparatus as claimed in any of Claims i to 15, wherein the feed pipe has a cut-away outlet opening.

17. An apparatus as claimed in any of Claims i to 16, wherein the drive means comprises a variable speed motor.

18. An apparatus as claimed in any of Claims i to 17, wherein the drum screen is made from wedge wires arranged to run parallel to the longitudinal axis of the drum screen.

19. The use of a rotary drum screen apparatus as claimed in any of Claims 1 to 18, to dewater material in a material processing plant wherein under normal operation material to be dewatered is discharged through the feed pipe into the drum screen which is rotated by the -15 -drive means at a predetermined circumferential speed, filtered liquid is collected at the bottom of the casing for further processing and dewatered material is discharged through the outlet opening.

20. A material processing plant comprising a rotary drum screen apparatus as claimed in any of Claims ito i8.

21. A plant as claimed in Claim 20, comprising a first valve means controlling the flow of material to the feed pipe.

22. A plant as claimed in Claim 20 or Claim 21, comprising a second valve means controlling the flow of any material that has overflowed from the inlet of the drum screen.

23. A method of operating a material processing plant comprising a rotary drum screen apparatus as claimed in any of Claims 1 to i8 wherein under normal operation material to be dewatered is discharged through the feed pipe into the drum screen which is rotated by the drive means at a predetermined circumferential speed, filtered liquid is collected at the bottom of the casing for further processing and dewatered material is discharged through the outlet opening, and wherein on detection of unfiltered material passing out of the drum screen via the inlet opening, the discharge of further material through the feed pipe into the drum screen is stopped and the unfiltered material passing out of the drum screen via the inlet opening is channeled to a recirculation arrangement.

24. A method as claimed in Claim 23, wherein when unfiltered material is no longer detected passing out of the drum screen via the inlet opening, the delivery of material to the drum screen through the feed pipe is recommenced.

-16 - 25. A method as claimed in Claim 23, wherein the flow of material unfiltered material from the drum screen is controlled by valve means that is opened when the delivery of material to the drum screen through the feed pipe flow of first valve is closed and vice versa.

26. A rotary drum screen apparatus for dewatering material substantially as described herein with reference to Figs. 1 to 4.

27. A material processing plant comprising a rotary drum screen for dewatering material substantially as described herein with reference to Figs. ito 5.