EP4000739A1

EP4000739A1 - Centrifugal separator comprising a disc stack

Info

Publication number: EP4000739A1
Application number: EP20207214.6A
Authority: EP
Inventors: Leonard Borgström; Ola CURTIUS
Original assignee: Alfa Laval Corporate AB
Current assignee: Alfa Laval Corporate AB
Priority date: 2020-11-12
Filing date: 2020-11-12
Publication date: 2022-05-25
Anticipated expiration: 2040-11-12
Also published as: CN116438010B; JP7575599B2; WO2022100954A1; CN116438010A; KR20230097193A; EP4000739B1; JP2023549387A

Abstract

The present invention provides a centrifugal separator configured to separate a heavy phase and a light phase from a liquid feed mixture. The separator is comprising a frame (2), a drive member (3) and a rotating part (4). The drive member (3) is configured to rotate the rotating part (4) in relation to the frame (2) around an axis of rotation (X), and the rotating part (4) comprises a centrifuge rotor (5) enclosing a separation chamber (9). Further, the separation chamber (9) comprises a stack (10) of separation discs (10a) arranged coaxially around the axis of rotation (X) and axially under a top disc (11) and the centrifugal separator (1) further comprises an inlet (14) for receiving the liquid feed mixture into the centrifuge rotor (5), a first outlet (6) for the heavy phase and a second outlet (7) for the light phase. The second outlet (7) is arranged in an outlet chamber (8) that is integrated in said top disc (11), said outlet chamber (8) being arranged such that its lowermost axial position (P1) is positioned axially below the uppermost axial position (P3) of the stack (10) of separation discs (10a).

Description

Field of the Invention

The present invention relates to the field of centrifugal separators, and more particularly to a centrifugal separator for cleaning oily water onboard a ship.

Background of the Invention

Centrifugal separators are generally used for separation of liquids and/or for separation of solids from a liquid. During operation, liquid mixture to be separated is introduced into a rotating bowl and heavy particles or denser liquid, usually water, accumulates at the periphery of the rotating bowl whereas less dense liquid accumulates closer to the central axis of rotation. This allows for collection of the separated fractions, e.g. by means of different outlets arranged at the periphery and close to the axis of rotation, respectively.
A centrifugal separator may be used for treating bilge water onboard ships. Bilge water must be treated to reduce the oil content to levels that meet international regulations for release into the environment. However, cleaning of bilge water poses distinct challenges. Not only does the composition and flow of bilge water constantly change, making continuous and efficient treatment difficult, but treatment onboard also presents another set of constraints. Treatment methods must meet individual ship requirements and demands for e.g. compactness and the ability to withstand rough weather conditions.
Even though centrifugal separation has been used for decades onboard vessels due to its superior efficiency in cleaning liquids, there is still a need in the art for a centrifugal separator for cleaning bilge water that is more compact and further facilitates operation during rough weather at sea.

Summary of the Invention

It is an object of the invention to at least partly overcome one or more limitations of the prior art. In particular, it is an object to provide a centrifugal separator that is compact. Further, it is an object to provide a centrifugal separator having increased capabilities of operating during rough weather at sea.
As a first aspect of the invention, there is provided a centrifugal separator configured to separate a heavy phase and a light phase from a liquid feed mixture; comprising
a frame, a drive member and a rotating part,
wherein the drive member is configured to rotate the rotating part in relation to the frame around an axis of rotation (X), and
wherein the rotating part comprises a centrifuge rotor enclosing a separation chamber;
wherein the separation chamber comprises a stack of separation discs arranged coaxially around the axis of rotation (X) and axially under a top disc;
wherein the centrifugal separator further comprises an inlet for receiving the liquid feed mixture into the centrifuge rotor, a first outlet for the heavy phase, a second outlet for the light phase,
wherein the second outlet is arranged in an outlet chamber that is integrated in the top disc, the outlet chamber being arranged such that its lowermost axial position is positioned axially below the uppermost axial position of the stack of separation discs.
The centrifugal separator is for separation of a liquid feed mixture. The liquid feed mixture may be an aqueous liquid or an oily liquid. As an example, the centrifugal separator may be for separating impurities, such as oil and or particles, from an aqueous liquid. The liquid feed is thus separated into at least two phases; a light phase and a heavy phase. The heavy phase has a density that is higher than the density of the light phase. The light phase and/or the heavy phase may be liquid phases. As an alternative, the heavy phase may be a solid phase. The centrifugal separator may be arranged to separate the liquid feed mixture into a liquid light phase, a liquid heavy phase and a solids phase, i.e. a sludge phase.
The frame of the centrifugal separator is a non-rotating part, and the rotating part may be supported by the frame by at least one bearing device, which may comprise a ball bearing. The rotating part of the separator may be arranged to be rotated around vertical axis of rotation, i.e. the axis of rotation (X) may extend vertically. The rotating part comprises a centrifuge rotor. The centrifuge rotor is usually supported by a spindle, i.e. a rotating shaft, and may thus be mounted to rotate with the spindle. Consequently, the rotating part may comprise a spindle that is rotatable around the axis of rotation (X). The centrifugal separator may be arranged such that the centrifuge rotor is supported by the spindle at one of its ends, such at the bottom end or the top end of the spindle.
The drive member for rotating the rotating part of the separator may comprise an electrical motor having a rotor and a stator. The rotor may be fixedly connected to the rotating part, such as to a spindle. Advantageously, the rotor of the electrical motor may be provided on or fixed to the spindle of the rotating part. Alternatively, the drive member may be provided beside the spindle and rotate the rotating part by a suitable transmission, such as a belt or a gear transmission.
The centrifuge rotor encloses by rotor walls a separation chamber in which the separation of the fluid mixture takes place. The separation chamber further comprises a stack of separation discs. The separation discs may e.g. be of metal. Further, the separation discs may be frustoconical separation discs, i.e. having separation surfaces forming frustoconical portions of the separation discs. The angle of inclination of the separation surface may be within the range of 30-50 degrees, preferably about 40 degrees, to the radial direction.
The separation discs are arranged coaxially around the axis of rotation (X) at a distance from each other such that to form passages between each two adjacent separation discs. The separation discs in the disc package may be arranged such that the liquid mixture to be separated flows radially inwards in the passages between each two adjacent separation discs of the stack.
The stack of separation discs is further arranged axially under a top disc.
As used herein, the term "axially" denotes a direction which is parallel to the axis of rotation (X). Accordingly, relative terms such as "above", "upper", "top", "below", "lower", and "bottom" refer to relative positions along the axis of rotation (X). Correspondingly, the term "radially" denotes a direction extending radially from the axis of rotation (X). A "radially inner position" thus refers to a position closer to the axis of rotation (X) compared to "a radially outer position".
The top disc may however have an outer radius that is larger than the separation discs of the disc stack in order to guide separated heavy phase out of the separation chamber axially above the top disc. A top disc may further have a larger thickness as compared to the separation discs of the disc stack.
Consequently, in embodiments of the first aspect of the invention, the top disc has a larger thickness than the separation discs of the stack of separation discs.
In embodiments, the top disc is integrated with the wall of the centrifuge rotor.
Thus, the top disc may be formed by e.g. the inner wall of the upper part of the centrifuge rotor.
The centrifugal separator may be arranged such that a separated heavy liquid phase move towards the periphery of the separation chamber and then is guided along the upper side of the top disc to the first outlet.
The centrifugal separator also comprises an inlet for liquid mixture to be separated (the liquid feed mixture). This inlet may comprise an inlet pipe extending into the centrifuge rotor. Further, this inlet pipe may be arranged at the axis of rotation (X).
The first outlet for the heavy phase may be a liquid outlet or a sludge outlet. The centrifuge rotor may for example comprise at its outer periphery a set of radially sludge outlets in the form of intermittently openable outlets. These may be for discharge of higher density component such as sludge or other solids in the liquid feed mixture. The centrifuge rotor may also comprise at its outer periphery open nozzles through which certain flow of sludge and/or heavy phase is discharged continuously.
The second outlet for a light phase is a liquid outlet. This is arranged at a smaller radius than the first outlet.
As an example, the centrifugal separator may comprise a first liquid outlet for a heavy phase, a second liquid outlet for a light phase and sludge outlets for separated sludge.
The centrifugal separator may be arranged such that a separated heavy liquid phase move towards the periphery of the separation chamber and is then guided along the upper side of the top disc to the first outlet.
Further, the second outlet is arranged in an outlet chamber that is integrated in the top disc such that the outlet chamber is arranged with its lowermost axial position is positioned axially below the uppermost axial position of the stack of separation discs. The second outlet chamber may thus be arranged as a chamber integrated in the top disc.
Thus, according to the centrifugal separator according to the first aspect, the second outlet chamber is formed within the top disc such that it extends axially down into the stack of separation discs, such as down into a central position of the disc stack. The central portion may thus be formed within the inner radius of the separation discs of the disc stack.
As an example, the whole second outlet chamber may be arranged axially within the stack of separation discs. As an alternative, a portion, such as a lower portion, of the second outlet chamber may be arranged axially within the stack of separation discs.
The first aspect of the invention is based on the insight that forming the second outlet chamber integrated within the top disc such that it extends axially down into the stack of separation discs, the height of the rotating part, and thus the whole centrifugal separator, may be decreased with maintained separation capacity. This provides for a rotating portion of the separator having a lesser height in relation to its width, which may be beneficial during rough weather at sea.
In embodiments of the first aspect, the second outlet comprises a stationary paring device for discharging the light phase.
Thus, during operation, the liquid light phase may form a liquid body rotating with the rotating part in the second outlet chamber, and a stationary paring device arranged in the second outlet chamber may be arranged for paring the liquid light phase out of the centrifugal separator via the second outlet. The stationary paring device may be a paring disc, such as an annular paring disc.
As an example, the paring device may be arranged axially at or below the uppermost axial position of the stack of separation discs. Thus, the second chamber may extend axially down into the stack of separation discs such that also a paring device within the second outlet chamber is arranged axially at or below the uppermost axial position of the stack of separation discs.
As an example, the stationary paring device may be a paring disc. Such a disc may thus extend around the axis of rotation (X). However, the stationary paring device may be any other type of outlet centripetal pump.
In embodiments, the separation discs comprise a central through hole with an inner radius R, and the second outlet chamber may thus be integrated in the top disc such that it extends down into the opening formed by the central through holes of the separation discs.
In embodiments of the first aspect of the invention, the stack of separation discs comprises a first set of separation discs having a first inner radius R1 and a second set of separation discs having a second inner radius R2, wherein R2>R1, and wherein the lowermost axial position of the outlet chamber is positioned radially within the second set of separation discs.
Thus, the second set of separation set may enclose the second outlet chamber. In other words, the whole, or a lower portion of, the second outlet chamber may be at the same axial position as the second set but radially arranged the inner space formed radially within R2.
The second set may be arranged as the axially uppermost set of separation discs in the stack.
Thus, in order to get enough space for a second outlet chamber, e.g. for fitting a desired paring device within the second outlet chamber, the inner radius of the uppermost separation discs may need to be enlarged.
As an example, the second set comprises at least 10 separation discs, such as at least 20 separation discs.
Moreover, R2 may be at least 10% larger, such as at least 15% larger, such as at least 20 % larger, than R1.
The first and second set of separation discs may have the same outer radius.
Furthermore, as discussed above, the inlet may comprise an inlet pipe extending into the centrifuge rotor around the axis of rotation (X). The second outlet chamber may be arranged around such an inlet pipe. The inlet pipe may thus also extend in the opening formed by the central through hole of the set of separation discs.
In embodiments of the first aspect of the invention, the stack of separation discs are arranged axially between a distributor and the top disc, and the distributor is arranged as a separate piece from the top disc and the outlet chamber.
Thus, the stack of separation discs may be supported by the distributor and compressed axially between the top disc and the distributor. The distributor is arranged for guiding the liquid feed mixture from the inlet to the separation chamber. Therefore, the distributor may comprise or form a set of channels for guiding the liquid feed mixture. These channels may be arranged for guiding the feed radially outwards. The inlet may comprise an inlet pipe extending through the distributor.
The distributor may be arranged as a separate piece from the top disc and the outlet chamber for the liquid light phase, meaning that the outlet chamber for the liquid light phase may not be integrated into the distributor, but only in the top disc.
In embodiments of the first aspect of the invention, the top disc further defines the radial level of the inlet to the second outlet chamber from the separation chamber.
The radial position of the interface between a formed heavy liquid phase and light liquid phase in a centrifugal separator may be controlled by the radial position of weirs, which define overflow borders from the separation chamber. An overflow border thus forms an inlet to an outlet chamber, from which a separated phase may be discharged. A weir on the light phase side is usually called a level ring while a weir on the heavy phase side is called a gravity disc. The top disc may thus be arranged such that the level ring is integrated in the top disc. Thus, in embodiments, the centrifugal separator does not comprise a separate weir or level ring for defining the radial overflow border to the outlet chamber for the liquid light phase, but this function is instead integrated in the dimensions of the top disc.
An alternative way of controlling the interface position is to use back pressure control on one or both phases. When the level ring goes submerged (not controlling the interface) there is still a positive effect as the outer wall of the paring chamber shields off turbulence caused by the paring disc, and in that way the flow in the disc stack is not disturbed.
In embodiments of the first aspect of the invention, the centrifugal separator is for separating marine oil pollution from bilge water onboard a ship.
The bilge water may thus form a separated liquid heavy phase, whereas oil pollution may be discharged from the centrifugal separator as a liquid light phase.
The separator may be arranged such that the bilge water is safe for discharge overboard after separation.
As a second aspect of the invention, there is provided a method of separating impurities from an aqueous feed mixture comprising the steps of

a) providing a centrifugal separator according to the first aspect above and rotating the rotating part of the separator;
b) introducing the aqueous feed mixture into the separation chamber; and
c) discharging a purified aqueous phase and separated impurities as two different phases from the separator.

This aspect may generally present the same or corresponding advantages as the former aspect. Effects and features of this second aspect are largely analogous to those described above in connection with the first aspect. Embodiments mentioned in relation to the first aspect are largely compatible with the second aspect.
The impurities may be an oily phase. The impurities may be discharged via the second liquid outlet, whereas the purified aqueous phase may be discharged via the first outlet.
In embodiments of the second aspect, the aqueous feed mixture is bilge water onboard a ship.
Bilge water collects in a ship's bilge wells, which are located in the lowermost part of the vessel just above the hull. Bilge water may contain fluids from machinery spaces, internal drainage systems, sludge tanks and various other sources, and may thus comprise of a mixture of water with detergents and other chemicals, fuel oil, lubricating oil, hydraulic oil, cat fines, oil additives, soot and dirt. Thus, the separated impurities may comprise e.g. fuel oil, lubricating oil, hydraulic oil, cat fines, oil additives, soot and/or dirt.

Brief description of the Drawings

The above, as well as additional objects, features and advantages of the present inventive concept, will be better understood through the following illustrative and non-limiting detailed description, with reference to the appended drawings. In the drawings like reference numerals will be used for like elements unless stated otherwise.

Figure 1 shows a schematic drawing of a centrifugal separator according to an embodiment of the present invention.
Figure 2 shows a schematic drawing of a cross-section of a centrifuge rotor.
Figure 3 further shows details of the top disc, the outlet for the light phase and the separation discs.
Figure 4 shows an embodiment of a separation disc.

Detailed Description

The centrifugal separator according to the present disclosure will be further illustrated by the following description with reference to the accompanying drawings.
Fig. 1 and Fig. 2 show a cross-section of an embodiment of a centrifugal separator 1 configured to separate a heavy phase and a light phase from a liquid feed mixture. The centrifugal separator 1 has a rotating part 4, comprising the centrifuge rotor 5 and drive spindle 4a.
The centrifugal separator 1 is further provided with a drive motor 3. This motor 3 may for example comprise a stationary element and a rotatable element, which rotatable element surrounds and is connected to the spindle 4a such that it transmits driving torque to the spindle 4a and hence to the centrifuge rotor 5 during operation. The drive motor 3 may be an electric motor. Alternatively, the drive motor 3 may be connected to the spindle 4a by transmission means. The transmission means may be in the form of a worm gear which comprises a pinion and an element connected to the spindle 4a in order to receive driving torque. The transmission means may alternatively take the form of a propeller shaft, drive belts or the like, and the drive motor may alternatively be connected directly to the spindle 4a.
The centrifuge rotor 5, shown in more detail in Fig. 2, is supported by the spindle 4a, which is rotatably arranged in a frame 2 around the vertical axis of rotation (X) in a bottom bearing 22 and a top bearing 21. The stationary frame 2 surrounds centrifuge rotor 5
The centrifuge rotor 5 forms within itself a separation chamber 9 in which a stack 10 of separation discs 10a is arranged coaxially around the axis of rotation (X) and axially under a top disc 11, and is thus arranged to rotate together with the centrifuge rotor 5. The separation discs 10a provide for an efficient separation of the liquid mixture into at least a light phase and a heavy phase. The light phase may be a liquid light phase. Thus, in the separation chamber 9 centrifugal separation of e.g. a liquid feed mixture to takes place during operation.
The separation chamber 9 is in this embodiment confined between the upper part 28 of the centrifuge rotor 5, the distributor 13 and an axially movable operating slide 18.
The stack 10 is supported at its axially lowermost portion by distributor 13. The distributor 13 comprises an annular conical base portion arranged to conduct liquid from the center of the rotor to a predetermined radial level in the separation chamber 9 of the rotor, and a central neck portion extending upwards from the base portion.
The centrifugal separator 1 further comprises an inlet 14 in the form of a central inlet chamber formed within or under the distributor 13 into which a stationary inlet pipe 14a extends for supply of the liquid feed mixture to be separated. The inlet 14 communicates with the separation chamber 9 via passages 20 formed in the distributor 13.
The top disc 11 and an upper inner wall part of the centrifuge rotor 5 delimits at least one channel 25 extending from at least one radially outer portion of the separation chamber 9 towards a central portion of the centrifuge rotor 5. The first outlet 6 is arranged in a first outlet chamber 15, which is in fluid communication with the at least one channel 25 for discharge of a separated liquid heavy phase. A stationary paring device 16 in the form of a paring disc is arranged in the outlet chamber 15 for the separated heavy phase for discharging the heavy phase into outlet pipe 6a.
The radially inner portion of the disc stack 10 communicates with a second outlet 7 for a separated light phase of the liquid feed mixture. The second outlet 7 is arranged in a second outlet chamber 8 that is integrated in the top disc 11 provided at the upper axial end of the disc stack 10. The second outlet chamber 8 for the separated liquid light phase comprises a stationary paring device 12 in the form of a paring disc for discharge of the light phase into outlet pipe 7a. The paring disc 12 is supported in the second outlet chamber 8 by the stationary inlet pipe 14a.
The centrifuge rotor 5 is further provided with outlets 17 from the radially outer periphery of the separation chamber 9. These outlets 17 are evenly distributed around the rotor axis (X) and are arranged for intermittent discharge of a sludge component of the liquid feed mixture. The sludge component comprises denser particles forming a sludge phase. The opening of the outlets 17 is controlled by means of an operating slide 18 actuated by operating water in channel 19, as known in the art. In its position shown in the drawing, the operating slide 18 abuts sealingly at its periphery against the upper part 28 of the centrifuge rotor 5, thereby closing the separation chamber 9 from connection with outlets 17, which are and extending through the centrifuge rotor 5.
However, the centrifugal separator 1 could also be a solid wall bowl centrifuge, i.e. a centrifuge lacking outlets at the periphery of the centrifuge rotor 5.
During operation of the separator as shown in Fig. 1 and 2, the centrifuge rotor 5 is brought into rotation by the drive motor 3. Via the inlet pipe 14a, liquid feed mixture to be separated is brought into the separation space 9. Depending on the density, different phases in the liquid feed mixture is separated between the separation discs 10a of the stack 10. Heavier component, such as a water phase and a sludge phase, move radially outwards between the separation discs 10a, whereas the phase of lowest density, such as an oil phase, moves radially inwards between the separation discs 10a and is forced through second outlet 7 arranged in the second liquid outlet chamber 8. The liquid of higher density is instead forced out through the liquid outlet 6 for the heavy phase that is arranged at a radial distance that is larger than the radial level of the outlet 7 for the light phase. Thus, during separation, an interphase between the liquid of lower density and the liquid of higher density is formed in the separation chamber 9, such as radially within the stack of separation discs. Solids, or sludge, accumulate at the periphery of the separation chamber 9, in the sludge space, and is emptied intermittently from within the centrifuge rotor by the sludge outlets 17 being opened, whereupon sludge and a certain amount of fluid is discharged from the separation chamber 17 by means of centrifugal force. However, the discharge of sludge may also take place continuously, in which case the sludge outlets 17 take the form of open nozzles and a certain flow of sludge and/or heavy phase is discharged continuously by means of centrifugal force.
Fig. 3 shows a close-up view of the top disc 11 and the upper part of the stack 10 of separation discs 10a. As seen in Fig. 3, the second outlet 7 for the light phase is arranged in an outlet chamber 8 that is integrated in the top disc 11. In this embodiment, the second outlet chamber is thus arranged as a chamber within the top disc 11, i.e. outlet chamber 8 and top disc11 form a single piece. Further, as visualized, the outlet chamber 8 for the light phase is arranged such that its lowermost axial position P1 is positioned axially below the uppermost axial position P3 of the stack 10 of separation discs 10a. P3 is in this embodiment the axial position of the central through hole of the separation disc 10a that is arranged as the upper disc in the disc stack 10. Also the stationary paring disc 12 may arranged axially below the uppermost axial position P3 of the stack 10 of separation discs 10a. However, the stationary paring disc 12 may be arranged approximately at the uppermost axial position P3 of the stack 10 of separation discs 10a.
Thus, outlet chamber 8 is integrated in the top disc 11 between an axial upper position P2 and an axial lower position P1. In embodiments, also the axial upper position P2 is positioned below the uppermost axial position P3 of the stack 10 of separation discs 10a.
In order to make room for the integrated outlet chamber 8 in the top disc 11, the stack 10 of separation discs 10a comprises a first, lower set 10b of separation discs 10a having a first inner radius R1 and a second, upper set 10c of separation discs 10a having a second inner radius R2. As illustrated in Fig. 3, R2 is larger than R1 so that the lowermost axial position P1 of the outlet chamber 8 is positioned radially within the second, upper set 10c of separation discs 10a.
In this example, the outer radius R_outer of the first 10b and second 10c set of separation discs are the same.
The second set 10c may for example comprise at least 10 separation discs 10a, such as between 10 and 30 separation discs 10a. Further, the second set 10c is arranged as the axially uppermost set of separation discs 10a in the stack 10. In Fig. 3, only a few of the separation discs are illustrated. In total, the disc stack may comprise e.g. 80-180 separation discs.
A typical distance between the separation discs 10a, generated by spacing members 21, may be below 0.75 mm, such as below 0.6 mm, such as about 0.5 mm. In embodiments, the distance between the separation discs are 0.4-0.75 mm, such as 0.4-0.6 mm, such as about 0.4- 0.5 mm.
The separation discs 10a in the stack 10, i.e. in both the first 10b and second 10c set of discs, may have the same thickness. However, the top disc 11 may have a larger thickness than the separation discs 10a of the stack 10. Also, the top disc 11 may have a radius R_top that is larger than the radius R_outer of the stack 10 of separation discs 10a.
Moreover, as illustrated in Fig. 3, the top disc 11 further defines the radial level r1 of the inlet to the second outlet chamber 8 from the separation chamber 9. Thus, the level ring is integrated in the top disc, i.e. the top disc defines the outer radial level of the passages 27, i.e. an overflow outlet, for transport of the separated liquid light phase to the outlet chamber 8.
In other embodiments, the top disc 11 is integrated in the wall of the centrifuge rotor 5. As an example, the top disc may be formed in the inner wall of the upper part 28 of the centrifuge rotor 5. Then, the channels 25 extending from at least one radially outer portion of the separation chamber 9 towards a central portion of the centrifuge rotor 5 could be formed as channels within the upper portion 28 of the centrifuge rotor 5.
Fig. 4 shows an example of a separation disc 10a that may be used in the stack 10 of separation discs. The separation disc 10a frustoconical separation portion 22 with an inner and an outer separation surface, around a central through hole 26 that defines the inner radius R1.
The separation portion 22 is provided with a plurality of distance members in the form of straight elongated caulks 21 providing distances to form passages between each two adjacent separation discs in the stack 10 formed by a stack of separation discs 10a. The caulks 21 in Fig. 4 form in an angle with the radius of the disc 10a, but the caulks 21 could also be straight radial caulks, i.e. caulks that do not form an angle with the radius of the disc 10a. The caulks are fastened to the outer surface of the frustoconical separation portion of the disc 10a and distributed around the circumference of the disc. The caulks 21 may also or as an alternative be provided on the inner surface of the disc. The caulks may also be formed as an integral part of the disc 10a. The separation disc 10 a is further provided with a plurality of cut-outs in the form of slits 24 at the radially outer portion 23 of the separation disc 10a, which slits 24are open towards the outer radius of the separation disc 10a. The number of slits 24 correspond to the number of caulks 21 and the slits are distributed around the circumference of the disc 10a in-between the caulks 21.
In addition, or as a complement, the separation disc 10 a could be provided with a number of through holes in the separation portion 22. The cut-outs and such through holes may then be aligned in the stack 10 of separation discs 10a so as to form axial distribution channels for the liquid feed mixture through the stack.
The invention is not limited to the embodiment disclosed but may be varied and modified within the scope of the claims set out below. The invention is not limited to the orientation of the axis of rotation (X) disclosed in the figures. The term "centrifugal separator" also comprises centrifugal separators with a substantially horizontally oriented axis of rotation. In the above the inventive concept has mainly been described with reference to a limited number of examples. However, as is readily appreciated by a person skilled in the art, other examples than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended claims.

Claims

A centrifugal separator (1) configured to separate a heavy phase and a light phase from a liquid feed mixture; comprising
a frame (2), a drive member (3) and a rotating part (4),
wherein the drive member (3) is configured to rotate the rotating part (4) in relation to the frame (2) around an axis of rotation (X), and
wherein the rotating part (4) comprises a centrifuge rotor (5) enclosing a separation chamber (9);
wherein the separation chamber (9) comprises a stack (10) of separation discs (10a) arranged coaxially around the axis of rotation (X) and axially under a top disc (11);
wherein the centrifugal separator (1) further comprises an inlet (14) for receiving the liquid feed mixture into the centrifuge rotor (5), a first outlet (6) for the heavy phase, a second outlet (7) for the light phase,
wherein the second outlet (7) is arranged in an outlet chamber (8) that is integrated in said top disc (11), said outlet chamber (8) being arranged such that its lowermost axial position (P1) is positioned axially below the uppermost axial position (P3) of the stack (10) of separation discs (10a).
A centrifugal separator (1) according to claim 1, wherein the second outlet (7) comprises a stationary paring device (12) for discharging the light phase.
A centrifugal separator (1) according to claim 2, wherein said stationary paring device (12) is arranged axially at or below the uppermost axial position (P3) of the stack (10) of separation discs (10a).
A centrifugal separator (1) according to claim 2 or 3, wherein said stationary paring device (12) is a paring disc.
A centrifugal separator (1) according to any previous claim, wherein the stack (10) of separation discs (10a) comprises a first set (10b) of separation discs (10a) having a first inner radius R1 and a second set (10c) of separation discs (10a) having a second inner radius R2, wherein R2>R1, and wherein the lowermost axial position (P1) of the outlet chamber (8) is positioned radially within said second set (10c) of separation discs (10a).
A centrifugal separator (1) according to claim 5, wherein the second set (10c) comprises at least 10 separation discs (10a).
A centrifugal separator (1) according to claim 5 or 6, wherein the second set (10c) is arranged as the axially uppermost set of separation discs (10a) in the stack (10).
A centrifugal separator (1) according to any previous claim, wherein the top disc (11) has a larger thickness than the separation discs (10a) of said stack (10) of separation discs.
A centrifugal separator (1) according to any previous claim, wherein the stack (10) of separation discs are arranged axially between a distributor (13) and said top disc (11), and wherein the distributor (13) is arranged as a separate piece from said top disc (11) and said outlet chamber (8) .
A centrifugal separator (1) according to any previous claim, wherein the top disc (11) further defines the radial level (r1) of the inlet to said second outlet chamber (8) from the separation chamber (9).
A centrifugal separator (1) according to any previous claim, wherein the top disc (11) is integrated with the wall of the centrifuge rotor (5).
A centrifugal separator (1) according to any previous claim, wherein the centrifugal separator (1) is for separating marine oil pollution from bilge water onboard a ship.
A method of separating impurities from an aqueous feed mixture comprising the steps of
a) providing a centrifugal separator (1) according to any one of claims 1-12, and rotating said rotating part (4) of said separator (1);

b) introducing the aqueous feed mixture into the separation chamber (9); and

c) discharging a purified aqueous phase and separated impurities as two different phases from said separator (1).
A method according to claim 13, wherein an aqueous feed mixture is bilge water onboard a ship.