WO2012002812A1

WO2012002812A1 - Shaft seal for a pump

Info

Publication number: WO2012002812A1
Application number: PCT/NL2011/050471
Authority: WO
Inventors: Timothy Alexander Van Opstal; Hasan Hüseyin BUGDAYCI; Floris Walther Pierre Smeets; Niels Den Hartog; Cornelis Stefanus Kraaij
Original assignee: Ihc Holland Ie B.V.
Priority date: 2010-06-29
Filing date: 2011-06-29
Publication date: 2012-01-05
Also published as: NL2004997C2

Abstract

The present invention relates to a pump (30), in particular a centrifugal pump, for pumping a mixture of liquid and solid particles, wherein the pump has a pump housing (31) which comprises; a pressure chamber (34) provided with an intake (32) for introducing and an outlet for delivering the mixture to be pumped; a rotating drive shaft (35); a shaft seal (50) comprising at least one sealing element (61) arranged around the drive shaft and bearing against the drive shaft and at least a flushing chamber (60) adjoining the drive shaft; a connection between the pressure chamber and the flushing chamber along which a portion of the mixture can be passed to the flushing chamber in order to flush the latter; and a filter module (51) arranged in the connection for at least partly removing solid particles from the mixture flowing through the latter.

Description

SHAFT SEAL FOR A PUMP

The invention relates to a seal for an element which is rotatable with respect to a stationary housing, in particular a rotatable shaft or a rotatable combination of a shaft and a bushing, wherein a mixture of solid particles and liquid is situated in the housing. The invention also relates to a pump for pumping a mixture of liquid and solid particles provided with such a seal. In particular, the invention relates to a centrifugal pump. The invention also relates to a method for pumping such a mixture.

A centrifugal pump can be used for various purposes. On the suction side of the pump housing of the pump, more specifically the pressure chamber thereof, the mixture is supplied via an intake which may be connected to a suction line. On the delivery side of the pressure chamber, the mixture can be discharged via a discharge connection which is connected to a pressure pipe. In the pressure chamber, a rotating impeller is provided which can be driven by means of a drive shaft which extends as far as inside the pressure chamber. The drive shaft is in turn made to rotate by means of a drive mechanism, such as for example an electric motor which is arranged in line with the pump housing. The mixture in the pressure chamber is displaced by rotation of the impeller. The impeller is in particular configured to pressurize the mixture. The mixture is discharged to the delivery side (outlet side) of the pressure chamber.

The pump may comprise one or more seals which are arranged along the shaft and are enclosed in one or more sealing chambers. The seals together form a sealing element and in principle abut the shaft. The seals prevent, or at least reduce, mixture leaking from the pressure chamber along the shaft in the direction of the drive mechanism. A further function of the seal is reducing pressure loss in the pressure chamber.

The seal may be formed by packing material which is arranged in rings or strands in the sealing chamber around the shaft. In practice, there is always some (slight) leakage with such seals around the packing, which leaked fluid can be used for lubrication and dissipation of frictional heat which occurs when the drive shaft rotates inside the pump housing. It is furthermore known to use a flushing seal in the case of certain applications of centrifugal pumps. One of the functions of a flushing seal is to provide (additional) cooling. Another function of the flushing seal is to prevent solid particles from accumulating in the seal and thereby damaging the seal due to, for example, the abrasive action of the particles on the seal. The flushing liquid therefore has to be sufficiently clean in order to prevent wear of the drive shaft or of the protective bushing of the drive shaft. An example of such a seal is described in Dutch laid-open application 7811 164, the contents of which are deemed to be incorporated in this application by way of reference.

Fig. 1 shows an example of a seal 1 which is known per se. The figure is a partially open longitudinal section of such a seal, used on a centrifugal pump. This seal, in the relevant field also referred to by the name Liquidyne™ seal, is composed of three modules, i.e. a dirt-stopping module 4, a pressure-reducing module 3 and a sealing module 2. The three modules are arranged around a rotatable shaft 9 or, as in the embodiment from Fig. 1 , around a rotatable bushing 15 which is provided around the shaft.

The dirt-stopping module 4 comprises a housing part 5 which is attached to a pump housing 7, for example the housing of a centrifugal pump, in a known manner using fastening means 6. In the pump housing, a medium 20, such as dredge slurry, is pumped. In the housing part 5 of the dirt-stopping module, a cylindrical lip seal 8 which is L-shaped in cross section is fitted. When the shaft 9 and the surrounding bushing 15 rotate, a horizontal leg 10 of said lip seal 8 is positioned against the outer side of the bushing 15. At the location of the lip seal 8, the bushing is smooth so that the lip seal 8 can produce a good sealing.

The central module, that is to say the pressure-reducing module 3, comprises a housing part 12 which is coupled to the housing part 5 in a known manner and which is provided with a flushing chamber 22 (indicated in a hatched manner by broken lines). The flushing chamber 22 can be filled with water or a similar medium for flushing the seal. The housing part 12 furthermore comprises a similar sealing lip 13 which is substantially L-shaped in cross section. In the illustrated embodiment, the sealing lip 13 rests against the bushing 15. In the external surface of the bushing 15, a spiral-shaped groove 16 is provided which, due to the helical shape, can displace the flushing water in the direction of the sealing module 2 when the shaft 9 rotates (depending on the direction of rotation of the shaft and the orientation of the spiral-shaped groove).

The last module, that is to say the sealing module 2, is composed of a housing part 17 which is provided with a similar sealing lip 18. The horizontal leg 19 of the sealing lip 18 presses against the outer side of the bushing 15. The housing parts 5, 12 and 17 are in principle sealed off from each other except at the locations where the respective lips 8, 13, 18 bear against the bushing 15.

The dirt-stopping module 4 is adapted to keep the soiled medium, such as for example process water or dredge slurry 20 which can be pumped via the pump of which the seal forms part, separate from the flushing water which can be supplied via a flushing water supply line 21 in the direction Pi to the aforementioned flushing chamber 22 of the pressure-reducing module 3. The flushing chamber 22, that is to say the space between the dirt-stopping seal and the pressure-reducing seal, fills with flushing water and in such a way that the pressure in this flushing chamber 22 substantially corresponds to the pump pressure prevailing inside the pump housing 7, more particularly in the passage space for the shaft 9. Due to the presence of flushing water, there are only small pressure differences, if any, thus preventing excessive loads on the seal.

The function of the pressure-reducing module 3 is to reduce the pressure of the flushing water to a lower level, for example atmospheric level. The central lip seal 13 runs on a bushing 1 by means of the abovementioned spiral-shaped groove 16. The peripheral velocity of the bushing makes it possible for the water of the flushing water supply 21 to flow to a flushing water discharge 25 of the sealing module 2 and can be discharged by means thereof (in direction P₂). A hydrodynamic layer is formed which lubricates said pressure-reducing module 3 in an effective manner. In addition to the lubricating function, the water flow also has a cooling function so as to keep the temperature of the shaft and the seal thereof low.

Furthermore, the flushing water can flush the seal. The function of the sealing module 2 is to seal the flushing water which has flowed through the pressure- reducing module 3. The text above describes how the flushing water from the supply line 21 is moved in the direction of the sealing module 2 via the groove in the bushing 15. Depending on the pressure in the flushing chamber 22, it is also possible for a small portion of the flushing water to move in the other direction, that is to say, in the direction of the dirt-stopping module 4. This portion of the flushing water ensures that the lip seal 8 can be flushed and that any dirt contained in the pump water 20 cannot reach the flushing chamber 22.

Said seal is in particular (but not exclusively) used for pumping mixtures of liquid (for example (sea) water) and solid particles, in particular abrasive particles, such as cooling water, mud or slurry. The flushing water for the flushed seal is obtained via an external flushing water supply so that clean flushing water can be supplied via the supply 21. However, the costs for such a separate flushing water supply can be quite high. Furthermore, in certain situations, there may not be any clean water available or it may be difficult to establish a supply of clean water.

It is an object of the present invention to provide an improved flushing seal in which at least one of the abovementioned disadvantages has been eliminated or at least been reduced.

According to a first aspect of the invention, this object can be achieved with a seal of the kind mentioned in the preamble, comprising:

- a first module comprising a first housing part which is or can be connected to the housing and is provided with a filter device which bears against the rotatable element;

- a second module adjoining the first module and comprising a second housing part provided with a sealing element which bears against the rotatable element and with a flushing chamber via which said sealing element can be flushed; wherein the filter device of the first module comprises a filter element which is configured and arranged in order to block a first mixture portion essentially containing solid particles and to allow a second mixture portion essentially containing liquid to flow through into the flushing chamber of the second module.

The mixture portion which has been allowed to flow through can be used to flush the sealing element of the second module and no external supply of flushing medium is therefore necessary. This can simplify the construction of the seal or the pump in which the seal is fitted. The filter device prevents or at least counteracts (an excessive amount of) solid particles from ending up in the flushing chamber.

Depending on, inter alia, the particle size, this can reduce wear or even damage to the rotatable element or other elements coupled thereto, such as a drive motor, or to the seal itself.

As an external supply of flushing medium is no longer required, the flushing chamber of the second module can be closed off, that is to say that there is no connection to the environment for the supply of flushing medium.

According to an embodiment of the invention, the filter device comprises a third module adjoining the second module and comprising a third housing part provided with a further sealing element bearing against the rotatable element. A flushing water discharge may be connected to the third module for the discharge of flushing water which has arrived in the third module.

According to an embodiment of the invention, the filter device comprises a first perforated plate, a second perforated plate and filter material arranged between the first and second perforated plate. The function of the perforated plates is, inter alia, to keep the filter material in position and to prevent filter material from being carried along by the mixture stream. In a further embodiment, the first and second perforated plates extend substantially at right angles to the rotatable element.

According to an embodiment of the invention, the filter element comprises an amount of filter material which, optionally retained between perforated plates, bears directly or indirectly against the rotatable element. When the filter material comes into contact with the rotatable element and in particular when the filter material is pushed against the rotatable element under some prestress, the risk of mixture containing abrassive solid particles seeping through unfiltered is reduced.

The filter material is preferably water permeable so that sufficient water can pass through the filter device in order to serve as flushing water in the next module.

In a further embodiment, the filter device is configured in such a manner that the flow of the first mixture portion to the second module is at least as large as the flow of flushing water to the third module, so that no undesirable (large) pressure differences are produced. The filter device can furthermore be configured as a labyrinth. Medium (usually water) and relatively small solid particles can pass through the labyrinth, while relatively large solid particles are substantially trapped in the labyrinth.

In a further advantageous embodiment, the filter device is configured to cause at least a part of the solid particles of the first mixture portion to accumulate upstream, in the module itself and/or in said housing. By causing solid particles to accumulate in the device itself, but also (in some cases in particular) in an area in front of the device, the accumulated solid particles themselves can act as a filter for other solid particles, for example the relatively large (abrasive) particles.

In a further embodiment, the filter device is configured in such a manner that the density of the filter material inside the filter device varies locally. It is conceivable for the density of the filter material to increase upstream, specifically to increase continuously. It is also conceivable that the density of the filter material decreases upstream, specifically decreases continuously. Variations in the density of the filter material vary the water permeability, specifically increase the water permeability, and offer structural advantages when using, for example, fibre material as filter material.

Examples of suitable filter material are wire or fibre material (wire mesh or fibre mesh), such as MPPM polypropylene fibres, ceramic fibres, steel wool, stainless steel wool, aluminium wool, glass wool, mineral wool and/or other polymer fibres.

In an embodiment of the invention, the filter device is configured to block relatively large particles (which have abrasive properties) and to allow relatively small particles (which cause very little wear, if any) to pass. It has been found that a satisfactory separation of small and large particles can be achieved if the first mixture portion substantially contains solid particles having a mean particle size of at least 100 μηι and if the second mixture portion contains liquid and substantially solid particles having a mean particle size of less than 100 μπι.

The sealing element in the second module and/or in the third module may comprise a lip-shaped seal, in particular an elastic sleeve. An example thereof is a lip seal which is, for example, substantially L-shaped in cross section and in which, in use, the horizontal part (the leg) of the L bears against the rotatable element.

However, other seals are also possible. Furthermore, the drivable element, more particularly the outer surface thereof, is preferably provided with a spiral-shaped groove in order to move flushing water in the desired direction when the rotatable element rotates.

In certain embodiments of the invention, the entire first module and/or only the filter device are configured to be replaceable, so that maintenance on the seal can be carried out quickly and efficiently.

The seal can be used in numerous different ways. A non-limiting example of such a use is a pump for pumping a mixture of liquid and solid particles. In these embodiments, said housing is formed by the pressure chamber of a pump, wherein the pressure chamber is provided with an intake for introducing and an outlet for delivering the mixture to be pumped.

It is possible to have a mixture stream of pumped liquid flow from the pressure chamber to the flushing chamber of the flushing seal even though the pumped mixture may be 'polluted', that is to say can contain a large number of solid particles, for example abrasive particles. However, due to the presence of the filter device, the number of abrasive particles entering the flushing chamber is greatly reduced or brought down to zero.

According to another aspect of the invention, a method is provided for pumping a mixture of liquid and solid particles in a pump, the method comprising:

- supplying the mixture via an intake in a pressure chamber and discharging the mixture via an outlet;

- supplying a portion of the mixture from the pressure chamber to a filter device;

- substantially blocking the first mixture portion and substantially allowing the second mixture portion to pass through by means of the filter device;

- flushing the sealing element of the second module using the second mixture portion which was passed through.

Further advantages, features and details of the present invention will be explained by means of the following description of some embodiments thereof. In the description, reference is made to the attached figures, in which:

Fig. 1 shows a schematic, partially cut-away longitudinal section through a known flushed seal; Fig. 2 shows a schematic cross section through an embodiment of a centrifugal pump provided with an embodiment of the seal according to the invention;

Fig. 3 shows a detail in cross section and in perspective of the embodiment of the seal from Fig. 2; and

Fig. 4 shows a schematic representation of the operational principle of a filter according to a particular embodiment of the invention.

The embodiment of a centrifugal pump 30 according to the invention illustrated in Fig. 2 has a pump housing which is designated overall by reference numeral 31 and a rotatable impeller 33. The pump housing 31 has an intake 32, an outlet and a pressure chamber 34 from which the medium which has been pressurized by the impeller 33 can be discharged. The impeller 33 is attached to the end of the shaft which is denoted overall by reference numeral 35, which is rotatably supported in the known manner with respect to the pump housing 31.

A bushing 40 is shown which surrounds the shaft 35 and serves as protection. The bushing 40 can be seen as forming a part of the shaft 35. The bushing rotates together with the shaft 35 and impeller 33 because the shaft is driven by a drive motor (not shown). The shaft is provided with a flange 42 which keeps the shaft in position with respect to the pump housing 31.

A substantially annular seal 50 is illustrated diagrammatically in Fig. 2 and, in more detail, in Fig. 3. The seal runs around and adjoins the abovementioned shaft 35, specifically the bushing 40, and is composed of three sealing modules arranged one behind the other.

The seal comprises three modules 51-53 which can be releasably attached to the housing 31. The first module 51 which is situated on the side of the pump chamber comprises a housing part 54 which can be attached to the pump housing 31 by means of screws 55. Between the housing part 54 and the pump housing 31, a packing flange 59 is provided in order to produce a good connection to the pump which is free of leaks. The housing part 54 furthermore contains a filter device 56, the operation of which will be explained in more detail below.

The second module 52 is arranged next to the first filter module 51. The second module 52 comprises a housing part 57 (in the illustrated embodiment three housing parts 57', 57" and 57"'). Housing part 57' can be attached to the housing part 54 by means of the abovementioned screws 55. Housing parts 57" and 57'" can be screwed onto the first housing part 54 by means of further screws 71. Packing flanges 58, 75 are arranged between housing parts 57' and 54 and between housing parts 57" and 57', respectively, in order to ensure a mutual connection which is free from leaks. In the housing part 57 (that is to say housing parts 57',57" and 57"'), a flushing chamber 60 is provided where flushing medium (for example flushing water) can collect which can be used to flush the seal. Furthermore, a sleeve or sealing lip 61, which is L-shaped in cross section and is also referred to as a lip seal, is attached in the housing part 57. This sealing lip comprises an upright lip part 62 which is attached to the housing part 57 and a horizontal lip part 63 which bears against the outer side of the rotatable bushing 40 of the shaft 35. The sealing lip 61 is made from elastic material and is prestressed against the bushing 40 by means of a spring 64. The bushing 40 is provided with a groove (not shown here) identical to the spiral-shaped groove 16 of the known seal 1 from Fig. 1. The groove serves to transport the flushing medium inside the second module 52, usually away from the pump and in the direction of the drive mechanism.

The third module 53 is arranged next to the second module 52. The third module 53 comprises a housing part 80 which is releasably attached to the housing part 57 (more specifically housing part 57"') of the second module 52 by means of screws (not shown) or similar coupling means. Between the housing parts 57 and 80 a packing 82 is again provided in order to ensure a liquid-tight connection between both housing parts.

The abovementioned flushing chamber 60 extends into the housing part 80 of the third module 53. In the housing part 80, a sealing lip 73 may be provided, for example in the shape of a sleeve or lip which bears against the outer side of the bushing 40 and is substantially L-shaped in cross section. In the illustrated embodiment, the sealing lip 73 is substantially identical to the sealing lip 61. Other seals are of course also possible, for example a gland packing or a mechanical seal. Furthermore, a discharge line (not shown) is connected to the flushing chamber 60 via which the flushing medium in the third module 53 can be discharged to the outside. It is conceivable for the flushing medium to be returned to the suction side of the pump by means of the discharge line. The flushing chamber 60 in the second module 52 is closed off. In this context, closed off means that no supply line 21 for supplying flushing medium is connected, as is illustrated in Fig. I. According to this embodiment of the invention, the flushing medium therefore does not come from an external flushing medium supply; the flushing medium is diverted from medium to be pumped in the pressure chamber 34 of the pump housing 31. In order to prevent abrasive particles from medium to be pumped from the pump housing from ending up in the flushing chamber 60, the filter device 56 is provided. In the illustrated embodiment, the filter device 56 comprises a first perforated wall 90 which extends at right angles to the longitudinal direction of the shaft 35 and the surrounding bushing 40. At some distance from the first wall, there is a second wall 91 which is also provided with perforations. The two walls are prestressed against the bushing 40 in a manner which is not shown, so that there is little tolerance, if any, between the underside of the walls and the outer surface of the bushing 40. An amount of filter material 92 is enclosed under prestress between the two plates 90, 91, in which case, due to the resilient properties of filter material also prestressed against the bushing 40. This filter material can be embodied in various ways. The main objective of the filter material is to make it possible for medium to flow from the pump chamber to the flushing chamber 60 and simultaneously prevent abrasive particles in the medium in the pump chamber from ending up in the flushing chamber 60. The medium which has reached the flushing chamber 60 can be used as flushing medium for the flushed sealing lip 61 of the second module 52. Furthermore, it is preferable if the filter material is flexible so that it can move concomitantly in case of any swinging movements of the shaft 35. The flexible material can then compensate for the swinging movements and ensure that the material remains pressed against the shaft or the bushing as much as possible during the swinging movements.

The filter material preferably forms a labyrinth for the solid particles in the mixture which are larger than a predetermined size. Relatively small particles are allowed to pass through as these particles can cause little or no damage further down the seal 50. Research has shown that particles (for example sand particles in dredge slurry) smaller than 100 μιη cause hardly any damage to a seal. It is therefore desirable that the filter material filters the abrasive particles larger than approximately 100 μιη out of the flushing water.

In a particularly advantageous embodiment, the filter device 56 is configured such that, as a result of the pressure build-up between the perforated plates, a large proportion of the solid particles, and in particular those particles which are larger than a predetermined size, accumulates in front of the filter device 56 instead of in the filter device 56 itself. This effect is diagrammatically illustrated in Fig. 4. On the right-hand side and the left-hand side of the seal, the pump housing and the drive motor (both not shown) are respectively provided. The solid particles (v), for example sand of a certain minimum grain size in dredge slurry, mainly accumulate in an area 95 upstream from the first perforated wall 90 of the filter device 56, while only a small proportion ends up in the filter device itself. The solid particles collected in area 95 in this case form a filter for further solid particles in the mixture. This has the advantage that, on the one hand, the filter action is improved and, on the other hand, that the filter does not become clogged up so quickly. This is advantageous for the life of the filter, so that this has to be replaced less often.

More generally, the purpose of the filter device 56 of the seal is to filter the abrasive particles out of the pumped medium. This filtered water can then be used to flush the other modules of the seal. In order to prevent a pressure difference from occurring across the filter device 56, it is important that the filter material is able to allow more water through than the second and third module of the seal need. At the same time, the abrasive particles have to be blocked. In order to achieve this, a porous material is preferably used. This material is water-permeable and acts as a labyrinth for the abrasive particles. There are several materials available which are suitable for use as filter material in the filter device 56 according to the invention. At least three material groups can be distinguished. First, the space between the perforated plates 90, 91 can be filled with fibres, such as steel wool, stainless steel wool, aluminium wool, bronze wool, glass wool, mineral wool, ceramic fibres and/or MPPM polypropylene fibres. A second group relates to synthetic filter materials, such as PPC 6.9 and/or other polymer fibres. A third group relates to granular material, for example lava stone, ceramic foam pieces, and the like. In the illustrated embodiments, the seal 50 is of modular construction and the respective modules can be replaced individually. In some embodiments, it is for example possible to replace the filter material 92, optionally together with the perforated plates 90, 91, with new filter material. In other embodiments, the entire housing part 54 including the filter device 56 can be replaced.

This component can then be fastened to the pump housing by means of fastening bolts. Depending on the pump construction, the assembly can be placed on the suction side or on the motor side. By way of example, the attached pump cross section in Fig. 2 shows a situation in which the seal 50 is placed on the suction side. This makes it necessary to remove the suction cover and the impeller. Then, the assembled component can be pushed over the shaft and fitted to the pump housing. Thereafter, the wearing bushing can be pushed over the shaft and positioned.

In some embodiments, the third module 53 comprises a lip seal 73 in order to prevent flushing water which has flushed the second module 52 from ending up in the environment. In other embodiments (not shown), the third module comprises a shaft bushing or wearing bushing for the same purpose. In yet other embodiments, the third module is omitted and the flushing water flows into the environment. This is, for example, possible in the case of an underwater pump or in a situation where it is permissible to dump flushing water.

The centrifugal pump can be used for different applications, such as a pump for cooling water, for mud and/or for slurry. The pumped liquid may be polluted. According to the invention, the liquid may 'leak' to the flushing chamber and be used as flushing liquid for the seal, after filtration.

The present invention is not limited to the above-described preferred embodiments described here. The rights sought are defined by the following claims, which allow for numerous adaptations, additions and modifications.

Claims

1. Seal (50) for an element (35, 40) which is rotatable with respect to a

stationary housing (31), in particular a rotatable shaft (35) or a rotatable combination of a shaft and a bushing (40), wherein a mixture of solid particles and liquid is situated in the housing, the seal comprising:

- a first module (51) comprising a first housing part (54) which is or can be connected to the housing and is provided with a filter device (56) which bears against the rotatable element;

- a second module (52) adjoining the first module and comprising a second housing part (57) provided with a sealing element (61) which bears against the rotatable element and with a flushing chamber (60) via which said sealing element can be flushed;

wherein the filter device of the first module comprises a filter element (65) which is configured and arranged in order to block a first mixture portion essentially containing solid particles and to allow a second mixture portion essentially containing liquid to flow through into the flushing chamber of the second module.

2. Seal according to Claim 1, comprising;

- a third module (53) adjoining the second module and comprising a third housing part provided with a further sealing element bearing against the rotatable element.

3. Seal according to Claim 1 or 2, wherein the filter element comprises porous filter material (92) bearing against the rotatable element.

4. Seal according to one of Claims 1 - 3, wherein the filter device comprises a first perforated plate (90), a second perforated plate (91) and filter material (92) arranged between the first and second perforated plate.

5. Seal according to Claim 4, wherein the first and second perforated plates extend substantially at right angles to the rotatabie element.

6. Seal according to one of the preceding claims, wherein the filter device is arranged in the first housing part so as to press against the rotatabie element under prestress.

7. Seal according to one of the preceding claims, wherein the filter material is water permeable and forms a labyrinth for the solid particles in the first mixture portion.

8. Seal according to one of the preceding claims, wherein the filter device is configured in such a manner that the flow of the first mixture portion to the second module is at least as large as the flow of flushing water to the third module.

9. Seal according to one of the preceding claims, wherein the filter device is configured to cause at least a part of the solid particles of the first mixture portion to accumulate upstream in said housing.

10. Seal according to one of the preceding claims, wherein the density of the filter material inside the filter device varies locally.

11. Seal according to one of the preceding claims, wherein the filter material comprises fibre material, such as MPPM polypropylene fibres, steel wool, stainless steel wool, aluminium wool, glass wool, mineral wool and/or polymer fibres.

12. Seal according to one of the preceding claims, wherein the flushing chamber is closed off.

13. Seal according to one of the preceding claims, wherein the first mixture portion contains solid particles having a mean particle size of at least 100 μηα and the second mixture portion contains liquid and solid particles having a mean particle size of less than 100 μπι.

14. Seal according to one of the preceding claims, wherein the sealing element in the second module and/or in the third module comprises a lip-shaped seal, in particular an elastic sleeve.

15. Seal according to one of the preceding claims, wherein a flushing water discharge can be or is connected to the third module.

16. Seal according to one of the preceding claims, wherein the entire first module and/or only the filter device are configured to be replaceable.

17. Seal according to one of the preceding claims, wherein the housing (31) is formed by the pressure chamber (34) of a pump (30), wherein the pressure chamber is provided with an intake (32) for introducing and an outlet for delivering the mixture to be pumped.

18. Pump (30), in particular a centrifugal pump, for pumping a mixture of liquid and solid particles, wherein said housing (31) is formed by the pressure chamber (34) of the pump and wherein a seal (50) according to one of the preceding claims is attached to or in the pressure chamber.

19. Pump according to Claim 18, comprising a drive motor fitted in a pump

housing of the pump for driving the drive shaft, wherein the shaft seal is arranged between the pressure chamber and the drive motor.

20. Pump according to Claim 18 or 19, comprising an impeller (33) which is rotatably mounted in the pressure chamber and connected to the drive shaft.

21. Filter device (56) of a seal (50) as defined in one of Claims 1-17.

22. Method for pumping a mixture of liquid and solid particles in a pump (30) as defined in one of the preceding Claims 17-19, the method comprising:

- supplying the mixture via an intake in a pressure chamber and

discharging the mixture via an outlet;

- flushing the sealing element (61) of the second module (52) using the second mixture portion which was passed through.

23. Method for a pump comprising the step of;

- fitting a seal (50) according to a preceding claim, or

- replacing a filter element (65) of the filter device (56) of the first module (51) of a seal (50) according to a preceding claim.