Field of the invention

The invention relates to a liquid filter with several built-in filter elements that are automatically cleaned while the filter is in continuous filtering operation. The filter is intended for use for fine or coarse filtering e.g. of water from natural water sources or of process fluids. The filter is designed to be mounted in the main flow in a pipe system to protect occurring process equipment from damage or clogging.

Background

The problem with pressure filters of today is that the filter baskets cannot be cleaned over its entire surface. The cost of e.g. pumping energy during operation becomes unnecessarily high when the pressure drop cannot be restored to its original value.

Such is the case for e.g. a pressure filter according to Swedish patent 7802969-1. This filter type has only one filter basket and the backwash of the basket' s filtration surface occurs through the reverse of the direction of flow by means of a flow diverter which is placed within the filter basket. This method only allows for the rear region of the basket's filtration surface to be backwashed and the pressure drop therefore cannot be restored to its original value, unless the filter is dismounted. If the flow diverter is placed close to the filter basket inlet to maximize the backwash filtration surface, all or part of the flow will stop for a shorter or longer time.

The pressure drop also increases markedly during the cleaning phase, which may also require larger and more expensive pumping capacity. These factors result in a strongly varying liquid flow to the disadvantage of the continuous process the filter is to serve. Another major drawback is the geometric size limit of the filter for mechanic and structural strength reasons. When large liquid flows are required, e.g. in central cooling plants for an entire factory, many filters installed in parallel are required which adversely effects the installation costs for the piping systems, space needs and service possibilities. A similar filter type is shown in Swedish patent application 9000803- 8. For this filter the backwash of the basket filtration surface occurs according to the so-called Bernoulli-effect which means that the filter, instead of a flow diverter inside the basket such as in the example above, is equipped with a plunger which is pushed through part of the filter basket interior at cleaning, however not along its entire length. This filter type has in principle the same weaknesses as the above mentioned filter type according to Swedish patent 7802969-1.

Another filter type according to patent application DEI 02009012444 has a number of filter baskets mounted in parallel in common filter housing where pistons are pushed through the baskets during the cleaning phase and providing flow reversal of the liquid. The pistons work independently during the cleaning. Each filter basket has its own debris collector and each debris collector has its own flush outlet. Each flush outlet must have its own backwash valve. Since this filter type has such a complex design it is expansive to manufacture and install. The method for flushing the filter baskets is less effective compared to filters according to the present invention, since the pistons work independently. Each filter element backwash outlet is connected to a common outlet header which requires an extensive pipe system with many bends which increases the pressure drop and risk of clogging from mayor pollutants. If the flow is occluded in this manifold due to clogging, the cleaning effect is affected negatively.

Summary of the invention

The above and other problems are solved, or at least mitigated, by the present invention by providing many benefits as shown below compared to the previously described, commercially available filter types:

- The filter has only one common debris collector with a backwash valve and a flush outlet, regardless of the number of filter baskets. This simplifies and cheapens the manufacture, installation and service.

- The filter basket whole filtration surface is cleaned effectively by means of an automatic system with interlocking valves. Even stuck pollutants are flushed from the filter baskets.

- Compared to existing single basket filters, the flushing flow is small in comparison to the total flow through the filter since only one or a few of the filter baskets are cleaned at a time while all others are in normal filtering operation. This makes the variation in total filtered fluid volume low, which benefits the process the filter is serving, and that no additional pumping capacity for compensating the flow loss during the cleaning is required.

- The pressure drop is reset to the initial value after each cleaning due to the effective backwash and due to that 100 % of the basket filtration surface is backwashed.

- The filter has a far greater flow capacity range than the described, existing single basket filters which results in cheaper installation and operation for larger capacity needs.

- The construction is flexible in the way that the filter baskets can be exchanged in the existing filter housing when filtering needs change, such as from coarse to fine filtering.

Description of the figures

These and other aspects, features and advantages of which the invention is capable of will be apparent and elucidated from the following description of embodiments of the present invention, reference being made to the accompanying drawings, in which the invention is described with reference to the accompanying schematic drawings, Figs. 1-7, which shows a multi- basket filter with three filter baskets. In the images, the three filter baskets have been placed vertically above each other for the flow streams through the different filter baskets to be visualized. The actual location of the filter baskets in the filter housing are shown by sections in figs. IB, 2B, 3B and 11.

Fig. 1A shows the filter with its included components under normal filtering operation, exclusive valve devices;

Fig. IB shows a section through the filter housing with real placement of the filter basket;

Figs. 2-3 show filters with different valve devices when all the filter baskets are in normal filtering operation;

Fig. 2A shows the filter with valve device type A with flip valves at the inlets of the baskets and piston valves at the outlets of the baskets, when all filter baskets are in normal filtering operation;

Fig. 2B shows the drive device of the flip valves in a section through the filter in front of the inlet of the filter baskets; Fig. 3 A shows a multi-basket filter with valve device type B with flip valves at the inlet and outlet of the filter baskets, when all filter baskets are in normal filtering operation;

Fig. 3B shows the drive device of the flip valves in a section through the filter in front of the outlet of the filter baskets;

Figs. 4-7 show filters with the different valve devices, when one of the filter baskets (8) is during cleaning while the others are in normal filtering operation;

Fig. 4 shows the flushing phase for filter basket (8) at valve device type A;

Fig. 5 shows the backwash phase of the filter baskets (8) filtration surface at valve device type A;

Fig. 6 shows the flushing phase for the filter basket (8) at valve device type B;

Fig. 7 shows the backwash phase for the filter baskets (8) filtration surface at valve device type B;

Figs. 8 - 10, logic diagrams for control of the valves, showing the coordination of the valve movements and positions during the different phases of the multi-basket filter;

Fig. 8 shows logic diagrams for valve device type A with piston valves acting at the outlets of the filter baskets and flip valves acting at their inlets;

Fig. 9 shows logic diagrams for valve device type B with flip valves at the basket inlets and outlets;

Fig. 10 shows logic diagrams for a cleaning sequence with pre-flush step;

Fig. 11 A shows a one piece front partition wall; and

Fig. 1 IB shows the front partition wall with mounted flip valves and drive means.

In Figs. 1 - 7 the black dots symbolizes pollutants.

Reference numerals

Identical parts have the same reference numerals in the different images.

Main inlet 1

Main outlet 2 Backwash outlet 3

Backwash valve 4

Inlet section 5

Filter basket 6-8

Filtration surface of the baskets 9

Filter basket inlet 10

Filter basket outlet 11

Outlet main line 18

Inlet main line 19

Flushing pipe 20

Debris collector 21

Front partition wall 23

Rear partition wall 24

Filter housing 25

Front body 25a

Main body 25b

Rear body 25c

Valve shaft 26

Drive means 27

Flip valves at the inlet of the baskets 28-30

Flip valves at the outlet of the baskets 31-33

Piston valve disc / piston valves 34-36

Piston valve rods 37-39

Flange mount 40

Guide flange 41

Protruding flange 42

Guide hole 43

Gasket 44

Through-passage hole 45

Plate 46

Mount 47

Shaft cylinder 48

Multi-basket filter 100

Flip valve carrying unit 200

Inlet fluid pressure PI Outlet fluid pressure P2

Fluid pressure in the flushing pipe P3

The velocity through the filter basket (8) ul at open backwash valve (4)

The velocity in the flushing pipe (20) u2 at open backwash valve (4)

Detailed description of the invention

Fig. 1A discloses a Multi-basket filter 100 in normal filtering operation and its different cleaning steps, without valve members.

The Multi-basket filter 100 is a liquid filter with several built-in filter elements that are automatically cleaned while the filter is in continuous filtering operation. The filter is intended for use for fine or coarse filtering e.g. of water from natural water sources or of process fluids. The filter is designed to be mounted in the main flow in a pipe system to protect occurring process equipment from damage or clogging. In this context fine and coarse filtering is intended to mean filtering of particles, i.e. separation of particles from liquid, wherein said particle size is 50 microns to 5 mm.

The filter consists of a filter housing 25. The filter housing preferably has a cylindrical shape. Additionally the filter housing 25 is manufactured in e.g. a metallic material. The filter housing 25 can also, in whole or in part, be manufactured from other materials, such as reinforced plastic. The filter housing 25 has a main inlet 1 and a main outlet 2 for the liquid. Main inlet 1 is connected to the front body 25a via an inlet section 5 volume. The main outlet 2 is preferably centrally positioned on the filter housing 25 periphery. In the filter housing 25 is placed three filter baskets 6-8 in parallel. The number of filter baskets 6-8 may be different from three but more than one, such as 2, 4, 5 or 6. However, three parallel filter baskets 6-8 per multi-basket filter 100, is an appropriate number considering capacity loss during cleaning of the filter baskets 6-8 and product cost. The filter basket inlets 10 and outlets 11 are connected to the corresponding through-passage holes 45 in a front partition wall 23 and a rear partition wall 24 respectively.

The front partition wall 23 and/or rear partition wall 24 can be made as one piece flip valve carrying unit(s) 200, as shown in figure 11A and 1 IB. The flip valve carrying unit 200 is manufactured in e.g. a metal material, but can also, in whole or in part, be manufactured from other materials such as reinforced plastic. By manufacturing the flip valve carrying unit 200 as one piece, details such as valves and/or valve shafts 26 can be integrated and run within the flip valve carrying unit 200. Production, service and maintenance is simplified by having one removable section, since the flip valves 28-30 and drive means 27 are integrated into one module, as shown in figure 1 IB.

The flip valve carrying unit 200 comprises a plate 46. The plate 46 is manufactured in e.g. a metal material, but can also, in whole or in part, be manufactured from other materials such as reinforced plastic.

The plate has three through-passage holes 45 distributed around a normal axis of the plate 46, as seen in figure 11 A. The number of through- passage holes 45 may be different from three but more than one, such as 2, 4, 5 or 6. The normal axis of the plate around which the through-passage holes 45 are distributed may be positioned at the center of the plate 46 or may be positioned at a non-central position of the plate 46.

The distribution of the through-passage holes 45 around the normal axis of the plate 46 may be evenly or unevenly distributed.

For the evenly distributed through-passage holes 45, the angle between neighbouring through-passage holes 45 is 360 n (where n is the total number of through-passage holes 45), with regards to their positions around normal axis of the plate 46.

For the unevenly distributed through-passage holes 45, at least one angle between two neighbouring through-passage holes 45 is >360 n (where n is the total number of through-passage holes 45), with regards to their positions around the normal axis of the plate 46. In this case, at least one other angle between two neighbouring through-passage holes 45 is <360 n (where n is the total number of through-passage holes 45), with regards to their positions around the normal axis of the plate 46. The sum of the angles between neighbouring through-passage holes 45 is 360° with regards to their positions around the normal axis of the plate 46.

The positioning of the through-passage holes 45 may be used to improve the water flow characteristics from the filter baskets 6-8 to the main outlet 2. One such optimization is to ensure that the total filter area of the filter baskets 6-8 which has unobstructed line of sight to the main outlet 2 is enlarged. An example of such through-passage hole 45 positions is shown in figures 11A and 11B, where the consecutive angles between neighbouring through-passage holes 45 are 108°, 108° and 144°, with regards to a normal axis of the plate 46. This positioning ensures that a larger total filter area of the filter baskets 6-8 has an unobstructed line of sight to the main outlet 2, which ensures a less obstructed water flow to the main outlet 2, compared to a configuration where the angles between the neighbouring through-passage holes 45 are 120°, 120° and 120°, with regards to their positions around a normal axis of the plate 46.

The water flow characteristics may be improved by positioning the through-passage holes 45 so that a central axis of the main outlet 2 does not intersect with the filter surfaces 9 of the filter baskets 6-8 located in the half of the filter housing closest to the main outlet 2 in the transversal plane. Such a configuration is shown in figs. IB, 2B, 3B, 6B and 7B.

Other water flow optimizations, such as simulations of water flow or practical experimental trials can be used to optimize the positioning of the through-passage holes 45 to ensure an effective water flow from the filter baskets to the filter basket outlet 11.

For a flip valve carrying unit 200 with three through-passage holes 45, it is preferable that one angle between neighbouring through-passage holes 45 is at least 140°, with regards to their positions around the normal axis of the plate 46.

In the through-passage holes 45, flip vales 28-33 can be mounted.

The plate 46 has a circumferential periphery. Along the periphery there are three mounts 47 distributed around a normal axis of the plate 46, as seen in figure 11 A. The number of mounts 47 may be different from three but more than one, such as 2, 4, 5 or 6. The normal axis of the plate 46 around which the mounts 47 are distributed may be positioned at the center of the plate 46 or may be positioned at a non-central position of the plate 46.

The mounts 47 may be evenly or unevenly distributed around the circumference of the plate.

For the evenly distributed mounts 47, the angle between neighbouring mounts 47 is 360 n (where n is the total number of mounts 47), with regards to their positions around normal axis of the plate 46.

For the unevenly distributed mounts 47, at least one angle between two neighbouring mounts 47 is >360 n (where n is the total number of mounts 47), with regards to their positions around the normal axis of the plate 46. In this case, at least one other angle between two neighbouring mounts 47 is <360 n (where n is the total number of mounts 47), with regards to their positions around normal axis of the plate 46. The sum of the angles between neighbouring mounts 47 is 360° with regards to their positions around the normal axis of the plate 46.

It is often preferable to have the mounts 47 of a plate 46 distributed in a similar manner as the as the through-passage holes 45 of the plate 46. Figure 11 A and 1 IB shows such a configuration, where the mounts 47 and through- passage holes 45 are positioned around the same normal axis position on the plate 46, sharing the same angles between neighbouring mounts 47 as between the corresponding neighbouring through -passage holes 45.

Each mount 47 has a flange 40 at its outmost end, as seen in figure 11 A. Drive means 27 for the flip valves 28-33 can be mounted on the flange 40, which ensures a tight connection. Each mount 47 has a shaft cylinder 48 which runs from the outmost end of the mount 47 to a corresponding through- passage hole 45 in the plate 46. The shaft cylinders 48 are preferably positioned along the central plane of the plate 46. The shaft cylinders 48 are preferably aligned along a line running from the center of the outmost end of the mount 47 to the center of the corresponding through-passage hole 45. However, the shaft cylinders 48 may also be aligned along a line running from the outmost end of the mount 47 to a non-centered position of the

corresponding through-passage hole 45. The shaft cylinders 48 can be fitted with for instance insulating sleeves, bearings, plain bearings, slide bearings, roller bearings, o-rings and seals.

In each shaft cylinder 48, a valve shaft 26 can be mounted. A mounted valve shaft 26 can rotate about its length axis. The valve shafts 26 can connect drive means 27, mounted at the periphery of the plate mount 47, with the flip valves 28-33, mounted in the through-passage holes 45. The drive means 27 can be of for instance pneumatic, hydraulic or electrical type. The drive means 27 can control the movement of the flip valves 28-33. Insulating sleeves, bearings, plain bearings, slide bearings, roller bearings, o-rings and/or seals mounted in a shaft cylinder 48 ensures low friction movement of a mounted valve shaft 26 and prevents water from leaking out through the shaft cylinder 48. The flip valve carrying unit 200 has a guide flange 41 for each through-passage hole 45 in the front partition wall 23 and/or rear partition wall 24 corresponding to filter basket inlets 10 or outlets 11.

The guide flanges 41 are fitted inside the through-passage holes 45, covering the inside walls of the through-passage holes 45. The guide flanges 41 have holes corresponding to the shaft cylinders 48 for the drive shafts 26, to enable mounting of the flip valves 28-33 inside the guide flange 41.

The guide flange 41 comprises a washer- shaped part on each side of the through-passage hole 45, locking the guide flange into place, as seen in figures 11A and 1 IB. The washer-shaped part extends outwardly from the through-passage hole 45 along the surface of the plate 46, laterally from the central axis of the through-passage hole 45. The guide flange 41 is

manufactured in e.g. a non-corrosive metal material, but can also, in whole or in part, be manufactured from other materials such as rubber.

The guide flange 41 has a protruding flange 42 extending towards the main body 25b of the filter housing 25. The guide flange 41 protruding flange 42 is possibly of a conical shape with a diameter that decreases the further the protrusion extends from the flip valve carrying unit 200. A conical shape of the guide flange 41 protruding flange 42 helps to guide a filter basket 6-8 to a uniform tight fit around a guide flange 41. When the filter basket 6-8 is mounted, the guide flange 41 also works as a filter basket 6-8 support.

The front partition wall 23 is mounted between the flanges of a front body 25a and the flanges of a main body 25b of the filter housing 25. A rear partition wall 24 can be mounted between the flanges of the main body 25b and the flanges of a rear body 25c of the filter housing 25. The flip valve carrying unit 200 has one or more, preferably four, guide holes 43 for flange bolts. The guide holes 43 help positioning the flip valve carrying unit(s) 200 between the front body 25a and main body 25b, and/or possibly the main body 25b and rear body 25c. Other positioning systems, such as guide pins, may be used to help positioning the flip valve carrying unit(s) 200. In such configuration, a number of guide pins, preferably 4, are fixed to the main body 25b having

corresponding guide holes in the front body 25a and/or rear body 25c. The flip valve carrying unit 200 may include gaskets 44, such as flat gaskets or oval ring type gaskets, for a leak free connection, as seen in fig. 1 IB. In one configuration, the front partition wall 23 can be welded to the front body 25 a and/or the main body 25b of the filter housing 25. Such a configuration may not require guide holes 43 and gaskets 44.

In one configuration, the rear partition wall 24 can be mounted inside the filter housing 25, attached, for instance bolted, to a flange running along the inner wall at the end of the main body 25b. The filter baskets 6-8 can be held, for instance screwed, in place. The rear partition wall 24 may have such a filter support for each through-passage hole 45 in the wall corresponding to a filter basket outlet 11.

The front partition wall 23 and rear partition wall 24 prevent mixing of unfiltered and filtered liquid. The filtration surface 9 of the filter baskets 6-8 is equipped with a large number of mesh holes whose size is adapted to the required minimum size of separated particles, in accordance with the particle sizes indicated above. The total cross sectional area for the mesh holes, summarized for all the baskets 6-8, is normally considerably larger than the area for the liquids main inlet 1, which makes the total pressure drop over the filter baskets 6-8 being relatively low.

The filtration area 9 of the filter baskets 6-8, preferably of a cylindrical shape, may consist of metal sweep with punched holes, be constructed from wire rods (wire wedge) or wire meshes (wire cloth). The filter baskets 6-8 can also, in whole or in part, be made of materials other than metal, such as plastic. Rear partition wall 24 defines the space of the debris collector 21 which all the filter basket outlets 11 ends. The debris collector 21 has a backwash outlet 3 fitted with a backwash valve 4 and connected with a flushing pipe 20 for discharging debris contaminated liquid.

Unfiltered liquid from connecting inlet main line 19 is flowing in through the inlet 1 and passing into the interior of the filter baskets 6-8. The difference between the pressure at the main inlet 1 (PI) and the main outlet 2 (P2) drives the liquid through the many small holes in the filtration surfaces 9 and particles which are larger than the holes are separated and retained in the filter basket 6-8 or passed to the debris collector 21. Filtered liquid continues towards the main outlet 2 to the connecting outlet main line 18.

Filter baskets 6-8 and debris collector 21 are freed from collected, separated particles in two different steps. In a first step, a stream of fluid is lead from the total flow through the inner of for example the filter basket 8, whose outlet 11 is open while the other filter baskets 6, 7 outlets are closed. In the descriptions below this step in the cleaning process is termed as "flushing" which is described in Figs. 4 and 6. Particles collected within the filter basket 8 are collected in the debris collector 21 and flushed out through opening of the backwash valve 4. The cross sectional area of the flushing pipes 20 is significantly smaller than the cross sectional area of the main inlet 1 which limits the flow-through in the flushing pipe 20.

The flow velocity u2 in the flushing pipe 20 is high. The flow velocity ul through the filter basket 8 increases due to the pressure difference between P1-P3. Loose contaminants that have been accumulated on the inside of the filter basket 8 and in the debris collector 21 are flushed out through the flushing pipe 20.

At a second step, the outlet 11 of the filter basket 8 remains open while its inlet 10 is closed and the fluid stream is forced through the small holes in the filtration areas 9 in reverse direction, from the outside of the filter basket 8 to its inside, wherein fully or partially filled holes are reset by flushing the loosened particles out through the backwash valve 4.

Other filter basket 6, 7 outlets 11 are closed. In the descriptions hereinafter, this step in the cleaning process is designated as "backwash", which is described in Figs. 5 and 7.

While the cleaning of one or many filter baskets 6-8 is ongoing, the rest of the filter baskets 6-8 are in normal filtering operation. The cleaning is repeated for all the filter baskets 6-8 in succession after a given program in the controlling system. The liquid streams in the cleaning steps are operated by the pressure differential between the main inlet 1 (PI) and the flushing pipe 20 (P3).

The above described two cleaning steps can, depending on type of application and liquid, be reversed. There may also be cases where one of the above said two steps in the cleaning process is sufficient to achieve satisfactory operating conditions.

The opening and closing of the backwash valve 4 is controlled automatically in conjunction with other valves in the arrangement of valves controlling fluid flows at the various cleaning phases. The interworking control of all valve movements is pre-programmed as are the duration and frequency for the different cleaning steps. Alternatively, the frequency of the cleaning of the different filter baskets can be controlled by a pre- set maximum value for the measured pressure difference between unfiltered and filtered liquid, which increases with the increasing soiling of the baskets filtration surface. Logic diagrams according to Figs. 8 and 9 show how the valves cooperate.

If there is extensive dirt and debris collected in the filter baskets 6-8 and debris collector 21, above described flushing and back- washing steps can be preceded by a pre-flush step. A pre-flush step starts from normal filtering operation of all filter baskets, where the inlet valves 28-30 and outlet valves 31-36 are open for all filter baskets 6-8 and the backwash valve 4 is closed. In a first step of a pre-flush the backwash valve 4 opens, flushing water through all filter baskets 6-8 and dirt chamber 21. Thereafter, two outlet valves 31-36 are closed to initiate flushing of one filter basket 6-8. Next, the inlet valve 28- 30 of said flushed filter basket 6-8 is closed to initiate the backwash of the filter basket. Finally all inlet valves 28-30 and outlet valves 31-36 are opened while the backwash valve 4 is closed. This is repeated for the next consecutive filter basket 6-8 until all the filter baskets 6-8 have been cleaned. Logic diagrams according to Fig. 10 shows pre-flush step. The sudden rush of water through all filter baskets 6-8 and the debris chamber 21 during the pre-flush helps to remove a large part of the collected dirt and debris, making the succeeding flushing and back-washing steps more effective.

As disclosed in Figs. 2A, 2B, 4 and 5, the Multi-basket filter 100 comprises basket inlet valves 28-30 at the inlets 10 of the filter baskets 6-8, and basket outlet valves 34-36 at the outlets 11 of the filter baskets 6-8, respectively. The basket outlet valves 34-36 may be piston valves acting at the filter basket outlets 11. The basket inlet valves 28-30 may be flip valves, such as butterfly valves, acting at the filter basket inlets 10. Thus, the filter baskets 6-8 have flip valves 28-30 acting at their inlets 10 and piston valves 34-36 acting at their outlets 11.

The inlet valves 28-30, which in number and placement corresponds to the number of filter baskets 6-8 in the filter housing 25, such as inlet valves 28-30 in form of flip valve type, such as butterfly valves, can be driven by separate drive means 27, such as of pneumatic, hydraulic or electrical type. The inlet flip valves 28-30 and the drive means 27 are linked by valve shafts 26. The drive means 27 are positioned on the outside of the filter housing 25, such as illustrated in Fig. 2B. Other drive devices may also be used in addition to the drive means 27.

The piston valves 34-36, which in number and placement correspond to the number of filter baskets 6-8 in the filter housing 25, consist of piston valve discs attached to piston valve rods 37-39, running through the debris collector 21 outer boundary wall. The movement of the piston valves 34-36 is accomplished by the piston valve rods 37-39 being connected to a drive device acting on the outside of the debris collector 21, which can consist of pneumatic or hydraulic cylinders alternatively electric drive motors. The different positions of the piston valves 34-36 during the cleaning steps are determined by pre-set position sensors or other equipment.

The piston valves 34-36 are shifted between their start positions adjacent to the debris collector 21 outer boundary wall and the basket outlets 11. Their stroke is thus short, meaning shorter valve rods 37-39 with less need for space which reduces the installation cost. Also, there is less risk of vibrations, caused by liquid flow, being to harmful for the valve rods 37-39.

Fig. 2A shows the valve positions at normal filtering operation. The piston valves 34-36 are in starting position with piston valve discs adjacent to the debris collector 21 outer boundary wall and the filter basket outlets 11 are then fully open. The inlet valves 28-30 are in open position and the backwash valve 4 is closed.

Fig.4 shows the valve positions at the first step of the cleaning process, the flush phase of the filter basket 8. The piston valve discs of piston valves 34, 35 are shifted forward to the outlets 11 of the filter baskets 6, 7 whose outlets are shut off. The piston valve disc of piston valve 36 remains in its starting position and the filter basket 8 outlet 11 is open. The flush begins when the backwash valve 4 opens to the flushing pipe 20.

Fig. 5 shows the valve positions during the second step of the cleaning process, the backwash phase of the filter basket 8. Its inlet 10 is shut off through its flip valve 30 being closed while the inlets 10 of the filter baskets 6, 7 remain open. The outlets 11 of the filter baskets 6, 7 remain closed and the piston valve 36 remain in its starting position and the outlet 11 of the filter basket 8 remains open. The backwash is initiated when the backwash valve 4 is opened to the flushing pipe 20. In Figs. 3 A, 3B, 6 and 7 a Multi-basket filter 100 with flip valves 28- 30 positioned at the filter basket inlets 10 and outlets 11 is disclosed. The filter baskets 6-8 have flip valves 28-30 positioned at their inlets 10 and flip valves 31-33 at their outlets 11. The latter can be operated in the same manner as described for the Multi-basket filter 100 with flip valves 28-30, in accordance with Figs. 2A, 2B, 4 and 5. This Multi-basket filter 100 requires shorter installation dimensions than the two described above, thus becoming the cheapest for installation.

Fig. 3 shows normal filtering operation, Fig. 6 shows flushing of the filter basket 8 and Fig. 7 shows backwash of the filtration surface 9 of the filter basket 8. The positions for the filter basket inlet and outlet valves for the different steps are identical to those for the Multi-basket filter 100 with flip valves 28-30, in accordance with Figs. 2A, 2B, 4 and 5.

Although the present invention has been described above with reference to (a) specific embodiment(s), it is not intended to be limited to the specific form set forth herein. Rather, the invention is limited only by the accompanying claims and, other embodiments than the specific above are equally possible within the scope of these appended claims.

In the claims, the term "comprises/comprising" does not exclude the presence of other elements or steps. Furthermore, although individually listed, a plurality of means, elements or method steps may be implemented by e.g. a single unit or processor. Additionally, although individual features may be included in different claims, these may possibly advantageously be combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. In addition, singular references do not exclude a plurality. The terms "a", "an", "first", "second" etc do not preclude a plurality. Reference signs in the claims are provided merely as a clarifying example and shall not be construed as limiting the scope of the claims in any way.