Patent Publication Number: US-11660614-B2

Title: Heavy phase liquid discharge element for a centrifugal separator, centrifugal separator and method for separating two liquid phases

Description:
TECHNICAL FIELD 
     The present disclosure relates to a heavy phase liquid discharge element, a centrifugal separator configured to separate a first liquid phase, a second liquid phase and a solid phase from a slurry, wherein the liquid phases have different densities and a method of separating a first liquid phase and a second liquid phase from a slurry by means of centrifugal forces in a centrifugal separator, as defined in the appended claims. 
     BACKGROUND ART 
     In the processing industry where different slurries are handled, there may be a need to separate solids from liquids at some point during a manufacturing process. For this purpose, a decanter centrifuge may be used. Such decanter centrifuge utilizes centrifugal forces, whereby liquids can be separated from solids. The liquids may comprise one or two phases, i.e. the liquids have different densities. When the slurry is subjected to the centrifugal forces, the denser solid particles are pressed outwards against a rotating bowl wall, while the less dense liquid phase forms a concentric inner layer. Different dam plates, also referred to as weir edges, are used to vary the depth of the liquid, so called pond. The sediment formed by the solids is continuously removed by means of a screw conveyor arranged with the bowl of the decanter centrifuge. The screw conveyor is usually arranged to rotate at a different speed than the bowl, whereby the solids can be gradually removed from the bowl. Thus, the centrifugal forces compact the solids and expel the surplus liquid. The clarified liquid phase or phases overflow the dam plates situated at an end opposite to the solids removal end of the bowl. Baffles within the centrifuge casing direct the separated liquid phases into correct flow paths and prevent risk of cross-contamination. 
     Reference is made to  FIG.  1   , which shows a prior art centrifugal separator or decanter centrifuge schematically. For example WO2008138345 discloses a centrifugal separator of this type. The centrifugal separator comprises a rotating body  1  comprising a bowl  2  and a screw conveyor  3  which are mounted on a shaft  4  such that they in use can be brought to rotate around a horizontal axis  5  of rotation. The axis  5  of rotation extends in a longitudinal direction of the bowl  2 . Further, the rotating body  1  has a radial direction  5   a  extending perpendicular to the longitudinal direction. For the sake of simplicity directions “up” and “down” are used herein as referring to a radial direction towards the axis  5  of rotation and away from the axis  5  of rotation, respectively. The bowl  2  comprises a base plate  6  provided at one longitudinal end of the bowl  2 , which base plate  6  has an internal side  7  and an external side  8 . The base plate  6  is provided with a number of liquid phase outlet passages  9  having external openings in the external side  8  of the base plate. Furthermore the bowl  2  is at an end opposite to the base plate  6  provided with solid phase discharge openings  10 . The screw conveyor  3  comprises inlet openings  11  for feeding a feed slurry to the rotating body  1 . The slurry comprises a light, liquid phase  12  and a heavy, solid phase  13 . During rotation of the rotating body  1 , separation of the liquid phase  12  and solid phase  13  are obtained. The liquid phase  12  is located radially closer to the rotation axis than the heavier solid phase  13 , and the liquid phase is discharged through the outlet passages  9  in the base plate  6 , while the screw conveyor  3  transports the solid phase  13  towards the solid phase discharge openings  10  through which the solid phase  13  is eventually discharged. Each liquid phase outlet passage  9  may be partly covered by a weir or dam plate  14 , as shown in  FIG.  1   . The weir plate  14  determines a level  15  of the liquid in the bowl. 
     Furthermore, centrifugal separators adapted for separation of two liquid phases are known for example from WO2009127212. Reference is made to  FIG.  2   a   , which shows an example of a prior art centrifugal separator or decanter centrifuge schematically, which is adapted to separating two liquid phases, but the solid phase separation works in a similar way as in  FIG.  1   . The centrifugal separator comprises a rotating body  1 ′ comprising a bowl  2 ′ and a screw conveyor  3 ′ which are mounted on a shaft  4 ′ such that they in use can be brought to rotate around a horizontal axis  5 ′ of rotation. The axis  5 ′ of rotation extends in a longitudinal direction of the bowl  2 ′. Further, the rotating body  1 ′ has a radial direction  5   a ′ extending perpendicular to the longitudinal direction. The bowl  2 ′ comprises a base plate  6 ′ provided at one longitudinal end of the bowl  2 ′, which base plate  6 ′ has an internal side  7 ′ and an external side  8 ′. The base plate  6 ′ is provided with a number of heavy liquid phase outlet passages  19 ′ and a number of light liquid phase outlet passages  19 ″. Furthermore the bowl is at an end opposite to the base plate provided with solid phase discharge openings (not shown) in a similar manner as in the variant shown in  FIG.  1   . As in  FIG.  1   , the screw conveyor  3 ′ comprises inlet openings (not shown) for feeding a feed slurry to the rotating body  1 ′. The slurry comprises a solid phase (not shown), light liquid phase  21 ′ and a heavy liquid phase  22 ′. During rotation of the rotating body  1 ′, separation of the liquid phases  21 ′ and  22 ′ and the solids are obtained. The light liquid phase  21 ′ is located radially closer to the rotation axis  5 ′ than the heavier liquid phase  22 ′. The light liquid phase  21 ′ is discharged through the outlet passages  19 ″ in the base plate  6  to an outlet chamber  20 ″, the heavy liquid phase  22 ′ is discharged through outlet passages  19 ′ to an outlet chamber  20 ′, while the screw conveyor  3 ′ transports the solid phase towards the solid phase discharge openings at the opposite end of the separator as described in connection with  FIG.  1   . Each liquid phase outlet passage  19 ′ and  19 ″ is partly covered by a respective heavy phase weir and dam plate  14 ′ and a light phase weir plate  14 ″. The respective weir plates  14 ′ and  14 ″ determine a respective heavy phase level  15 ′ and a light phase level  15 ″ in the bowl, whereby it is possible to discharge respective liquid phases. 
     Liquid discharge elements have been incorporated in base plates of centrifugal separators, which include outlet housings, also called “power tubes”. WO 2012/062337 shows an example of such centrifugal separator, in which an outlet housing is arranged in fluid connection with an outlet passage which extends through the base plate. The outlet housing receives liquid from the bowl of the rotating body via the outlet passage and has an outlet opening discharging liquid from the outlet housing. The outlet opening comprises a weir edge defining in normal use a level of a surface of the liquid in the bowl. The outlet housing may be rotatable around an adjustment axis and the outlet opening is placed in a side wall of the housing, offset from the adjustment axis. In this document, two different types of channel members or liquid discharge elements are arranged for the respective two different liquid phases. The liquid channel members are in turn connected to a respective type of outlet housing, which are arranged to discharge liquid phases to a respective liquid compartment. In the arrangement, when adjusting the angular position of the outlet housings, care is taken that an outlet opening in the housing faces rearwards relative to a direction of rotation in order to discharge the liquid phase in an opposite direction relative to the direction of rotation. Thereby, energy can be recovered from the discharged liquid. 
     Thus, it is previously known how to separate liquids from solids and two liquid phases from each other by means of centrifugal separators. However, especially in connection with separation of two liquid phases, it has been noted that outlet passages for heavy phase liquids may suffer from a drawback of rendering pressure losses during discharge. Therefore, there is still a need to further improve the centrifugal separators. 
     SUMMARY OF THE INVENTION 
     The pressure losses mentioned above may affect the separation process of two liquids in different ways. It has been noted for example that the pressure losses may lead to losses of the light phase during the separation. This may be due to the fact that heavy phase cannot be discharged at the same rate as the light phase, whereby a position of an interface, i.e. a level between the two liquid phases, becomes unstable. Thus, the level settings in the outlet arrangement may not correspond to the actual interface level position, which is unstable. 
     It is thus an objective of the present invention to provide an outlet passage with reduced pressure loss for the heavy phase in centrifugal separators. It is especially an objective to reduce pressure losses in outlet arrangements including channel members or liquid discharge elements which are incorporated in base plates to provide liquid outlet passages connected to outlet housings. 
     It is a further objective to provide more stable interface position even in case of large flow variations. 
     The objectives above are attained by a heavy phase liquid discharge element, a centrifugal separator and a method for separating a first liquid phase and second liquid phase as defined in the appended claims. Accordingly, the present invention relates to a heavy phase liquid discharge element for a centrifugal separator, which is configured to separate two liquid phases having different densities. The heavy phase liquid discharge element has a longitudinal extension, a transversal extension perpendicular to the longitudinal extension, a first inlet side and an opposite second outlet side, both extending in the longitudinal direction and in the transversal direction, a first longitudinal portion comprising a first transversally extending edge, a second longitudinal portion comprising a second transversally extending edge, and two longitudinally extending side edges, in between which a longitudinally extending center line extends The heavy phase liquid discharge element comprises at least one inlet opening on the first side of the heavy phase liquid discharge element. The at least one inlet opening being is adapted to face an interior of the centrifugal separator. Further, the heavy phase liquid discharge element comprises at least two separate outlet channels defining an outlet on the second side of the heavy phase liquid discharge element. At least a portion of each of the outlet channels overlaps with the at least one inlet opening, thereby forming a liquid pathway between the at least one inlet opening and the outlet defined by the at least two outlet channels through which the liquid can pass. Additionally, each of the at least two outlet channels has an extension in the longitudinal direction of the heavy phase liquid discharge element, which is longer than the extension of the at least one inlet opening in the longitudinal direction. 
     By providing at least two outlet channels, the tangential dimension of the outlet channel is reduced by introducing at least two separate outlet channels. It has been surprisingly noted that in this way pressure losses can be limited substantially, since the vortices in the radial movement will be reduced. This is a huge advantage, since the separation process in the centrifugal separator thus becomes less sensitive to flow rate variations and the interface between the light and heavy liquid phases becomes more stable. 
     The at least two outlet channels may be arranged in parallel along the longitudinal extension of the heavy phase liquid discharge element. The at least two outlet channels may be positioned symmetrically and mirror-imaged in respect to the center line. In this way the flow of the liquid may be equal in the channels. 
     The at least two outlet channels may extend in the first and second longitudinal portions (I; II). The number of the outlet channels may be from 2 to 6. Thus, the liquid may be pressed in the channels radially inwards, while the pressure losses may be further reduced. 
     The two outlet channels may have respective channel end portions which taper symmetrically and in a mirror-imaged way towards the center line and the second transversal edge in the second longitudinal portion and wherein the tapering end portions may have a rounded shape. In this way, the channels may better adapt to a shape of an outlet housing. 
     The at least one inlet opening may be comprised in the first longitudinal portion. In this way, it is possible to place the intake of the liquid close to the bowl wall, when mounted in a separator. 
     The amount of the inlet openings may correspond to the amount of the outlet channels. In this way, the pressure losses may be further reduced. 
     The at least one inlet opening may comprise a first transversally extending inlet edge on the first inlet side towards the first transversal edge of the liquid discharge element. Each of the outlet channels comprises a first transversally extending outlet edge on the second outlet side towards the first transversal edge of the liquid discharge element. A longitudinal distance between the first transversal inlet edge and the first transversal edge of the liquid discharge element is smaller than a longitudinal distance between the first transversally extending outlet edge and the first transversal edge of the liquid discharge element. In this way a peripheral wall for the inlet opening can be provided. Additionally, an extension of the first transversal inlet edge in a plane of a thickness dimension may be perpendicular to the central line and to a peripheral wall. The perpendicular extension and/or the peripheral wall may in mounted position reduce suck up of particles from the area close to the bowl wall. 
     The present disclosure also relates to a centrifugal separator configured to separate a first liquid phase, a second liquid phase and a solid phase from a slurry, wherein the liquid phases have different densities providing the same advantages as described above. The centrifugal separator comprises a rotating body comprising a bowl, which comprises a base plate at an end of the bowl. The base plate has an inner surface and an opposite outer surface and the inner surface faces an interior of the bowl. The base plate comprises one or more first liquid phase outlet passages and one or more second liquid phase outlet passages. The first and second liquid phase outlet passages are configured to discharge liquid from the rotating body. The second liquid phase outlet passages are associated with the heavy phase liquid discharge element as defined above. 
     The one or more first liquid phase outlet passages may be configured to discharge the first liquid phase, which is lighter than the second liquid phase. Thus, different outlets can be used for different liquid phases. 
     The one or more first liquid phase outlet passages may comprise a light phase liquid discharge element comprising an opening passage in fluid connection with the first outlet passage comprised in the base plate. Thus, by having liquid discharge elements for both light and heavy phase, rotational symmetry may be obtained. 
     The light phase liquid discharge elements and the heavy phase liquid discharge elements may be arranged in association with the inner surface of the base and at different angular positions relative to the axis of rotation. The amount of the light phase liquid discharge element and the heavy phase liquid discharge element may vary from 2 to 16. The amount may be equal. Alternatively, the amount of the heavy phase liquid discharge elements may be larger or smaller than the amount of the light phase liquid discharge element. Thus, in this way it is possible to adapt the separator to the slurry to be separated. 
     The light phase liquid discharge element and the heavy phase liquid discharge element may be associated with a respective outlet housing. Each of the outlet housings may be rotatably adjustable around an adjustment axis, and each of the outlet housings may comprise a respective outlet opening comprising a respective weir edge. The outlet housings may enable energy recovery from the liquid. 
     Furthermore, the present invention relates to a method of separating a first liquid phase and a second liquid phase from a slurry by means of centrifugal forces in a centrifugal separator. The liquid phases have different densities and the method comprising steps of
         bringing the slurry to a rotational movement in a cylindrical bowl and thereby separating the slurry into two liquid phases,   separating the liquid phases from each other by
           bringing the first light liquid phase in fluid contact with at least one first outlet passage comprised in a base plate of the centrifugal separator, the first outlet passage being connected to a weir plate adapted for keeping at least part of the second, heavy phase inside the rotating bowl, wherein the at least one outlet passage provides a liquid pathway to the light phase to be discharged from the bowl   bringing the second heavy phase in contact with at least one second outlet passage comprised in a base plate of the centrifugal separator comprising a heavy phase liquid discharge element being adapted for keeping the first light phase inside the rotating bowl and for providing a liquid pathway to the heavy phase to be discharged from the bowl,
 
wherein the method is characterized by discharging the heavy phase by using at least two separate liquid outlet channels connected to a respective at least one second outlet passage.
   
               

     Further features and advantages of the present invention are disclosed in the detailed description below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    shows schematically a partially cut view of an example prior art centrifugal separator; 
         FIG.  2    shows schematically a cut view of an end portion of an example prior art centrifugal separator; 
         FIG.  3   a    shows a perspective view of a prior art liquid discharge element from a second surface comprising an outlet channel; 
         FIG.  3   b    shows the liquid discharge element of  FIG.  3   a    from a first surface comprising an inlet opening; 
         FIG.  4   a    shows a perspective view of a liquid discharge element according to the present disclosure from a second surface comprising two outlet channels; 
         FIG.  4   b    shows the liquid discharge element of  FIG.  4   a    from a first surface comprising two inlet openings; 
         FIG.  5   a    shows a view from a first surface of a liquid discharge element comprising two inlets, according to the present disclosure; 
         FIG.  5   b    shows a cut side view along the line X-X of the liquid discharge element shown in  FIG.  5     a    
         FIG.  5   c    shows a view of the liquid discharge element of  FIGS.  5   a  and  5   b    from a second surface of the liquid discharge element comprising two outlet channels; 
         FIG.  6    shows an enlarged view of  FIG.  5     c;    
         FIG.  7    shows an enlarged view of  FIG.  5     a;    
         FIG.  8   a    shows schematically a partially cut view of an example centrifugal separator according to the present disclosure; 
         FIG.  8   b    shows an enlarged view of a portion of  FIG.  8   a    corresponding to  FIG.  5     b;    
         FIG.  9    shows a view of a centrifuge base plate from an inner surface, the base plate comprising the liquid discharge element of the present disclosure; 
         FIGS.  10  and  11   , respectively, shows schematically a partially cut view of an example centrifugal separator comprising an outlet housing according to the present disclosure; 
         FIG.  12    shows comparative test results relating to oil losses. 
     
    
    
     DETAILED DESCRIPTION 
     Thus, according to the present disclosure the pressure losses in an outlet passage for a heavy phase liquid can be reduced by using a heavy phase liquid discharge element as described more in detail herein. The heavy phase liquid discharge element is especially usable for a centrifugal separator configured to separate two liquid phases having different densities. 
     An example of the heavy phase liquid discharge element  200 ′ according to a prior art solution is shown in  FIGS.  3   a  and  3   b    in a perspective view. An example embodiment of the heavy phase liquid discharge element  200  according to the present invention is shown in  FIGS.  4   a  and  4   b    in a similar perspective view as the prior art heavy phase liquid discharge element of  FIGS.  3   a  and  3   b   . The heavy phase liquid discharge element  200 ′  200  is herein below also referred to as “the element  200 ′,  200 ”. 
       FIGS.  3   a  and  4   a    view a second, outlet, side  220 ′,  220  of the elements  200 ′ and  200  that is adapted to face an external side of the centrifugal separator. The details of the element  200  are described more in detail below, but as can be seen, the liquid charge element  200  according to the present disclosure comprises at least two separate outlet channels  271 ;  272  defining at least one outlet opening on the second side  220  of the heavy phase liquid discharge element  200 . The prior art liquid charge element  200 ′ comprises only one outlet channel  270 ′. Additionally, a view of a first, inlet, side  210 ′ and  210  of the respective elements  200 ′ and  200  is illustrated in  FIGS.  3   b  and  4   b   . As illustrated, the liquid discharge element  200  according to the present disclosure comprises two separate inlets  211 ;  212  on the first side  210  of the heavy phase liquid discharge element  200 . The prior art liquid charge element  200 ′ comprises one inlet opening  211 ′. Further the prior art liquid charge element  200 ′ comprises holes  213 ′ for attachment means, such as a screw. It can also be seen that in both the prior art liquid charge element  200 ′, and in the present liquid charge element  200 , in between the outlet side  220 ′,  220  and the inlet side  210 ′,  210 , around the periphery of the respective element  200 ′,  200 , a track  215 ′,  215  is arranged. In the track, a sealing means  216 ′,  216 , for example an elastic O-ring, is arranged to prevent liquid leakage. 
     The shape and structure of the heavy phase liquid discharge element  200 , referred to as “the element  200 ” below, is shown in more detail in  FIGS.  5   a ,  5   b  and  5   c   .  FIG.  5   a    shows that the element  200 , which in the illustrated example is a plate having a shape resembling a triangle with rounded corners, has a longitudinal extension L and a transversal extension T, which is perpendicular to the longitudinal extension. Any other outer shape could be utilized for the heavy phase liquid discharge element  200 , referred to as “the element  200 ”, e.g. rectangular, elliptical or circular. By having the slightly triangular shape, it is possible to utilize only three mounting screws. The rounded corners have an advantage of facilitating the placement of the sealing means in between the inlet and outlet sides, while preventing wear and tear of the sealing means against sharp edges. 
     The maximal longitudinal extension, i.e. the length, and the transversal extension, i.e. the width, of the element  200  can vary depending on the application. The maximal longitudinal extension corresponds extension in a radial direction, when the element is mounted on the base. The maximal longitudinal and transversal extensions can be adapted to the diameter of the bowl and the base thereof. For example, a ratio longitudinal extension of the element to the bowl diameter may be from 1:10 to 1:2.5, such as 1:3, but is not limited thereto. A ratio transversal extension of the element  200  to the longitudinal extension of the element 1:3 to 1:1.1, such as 1:1.5, but is not limited thereto. However, the longitudinal extension is suitably longer than the transversal extension so that outlet channels may be provided with sufficient length in relation to the width of the channels, whereby the pressure losses of the heavy phase can be minimized. 
     The element  200  comprises a first longitudinal portion (I) comprising a first transversally extending edge TE 1 , which is illustrated as an upper edge in  FIG.  5   a - 5   c   . The element  200  also comprises a second longitudinal portion (II) comprising a second transversally extending edge TE 2 . The first longitudinal portion (I) transitions to the second longitudinal portion (II), and vice versa, at a point corresponding to half of the maximal length of the element  200  along the longitudinal extension. For example, if the maximal length of the element  200  is 130 mm from the transversally extending edge to edge, the first longitudinal portion (I) transitions to the second longitudinal portion at a transversal line drawn through a position corresponding to 65 mm from edge to edge. 
     The element  200  further comprises a center line (CL), which extends centrally in between two longitudinally extending side edges SE 1  and SE 2 . The center line (CL) extends longitudinally through a point corresponding to half of the maximal width of the element  200 . Thus, the centre line (CL) may divide the element  200  into two symmetrical, but mirror-imaged, portions. The centre line may be in a mounted position be arranged in the direction of the radius of the base plate. 
     The element further comprises a first inlet side  210 , or inlet side surface, and an opposite, second outlet side  220 , or an outlet side surface, both extending in the longitudinal direction and in the transversal direction. At least one inlet opening  211  is arranged on the first side  210  of the heavy phase liquid discharge element. The at least one inlet opening is adapted to face an interior of the centrifugal separator, when installed in the centrifugal separator, and as described more in detail below. In the illustrated example of  FIG.  4   b   , there are two inlet openings depicted with numerals  211  and  212 , respectively. According to a variant, the amount of the inlet openings may correspond to the amount of outlet channels, whereby the interface will be more stable even in case of large flow variations. Thus, in case of two outlet channels, there may be two inlet openings, etc. 
     According to the present invention, the element  200  comprises at least two separate outlet channels  271 ;  272  defining an outlet on the second side  220  of the element  200 . The outlet channels  271  and  272  are arranged in parallel along the longitudinal extension of the element  200 . Generally, the amount of the outlet channels may be more than two, for example 2-6 or 2 to 4, and can be adapted to the application in question. Further the liquid charge element  200  comprises holes  213  for attachment means, such as a screw. 
     The maximal width of each channel, i.e. extension in the transversal direction of the element  200 , may vary, but may be generally less than about ⅓ of the maximal transversal extension of the element  200 , for example up to about 30%, 25% or 20% or 15% of the maximal transversal extension of the element  200 . The lower limit for the width depends on the liquid in question, but should be adapted so that the channel width is not too narrow and thereby does not negatively affect the flow through the element  200 . Each of the channels may thus have a maximum width of for example less than about 35 mm, for example from 10 to 30 mm, but is not limited thereto. 
     The at least two channels may be arranged in a parallel manner on the second, outlet side  220  of the element  200 . However, the length of the individual channels may vary so that the channels can adapt to an outer shape of the element. At the same time the flow in the separation process should not be negatively affected by the length of the channels. Generally it is advantageous that the at least two outlet channels  271 ,  272  are positioned symmetrically and mirror-imaged in respect to the center line CL. However, at least a portion of each of the outlet channels  271  and  272  overlaps with the at least one inlet opening  211 ,  212 . Thereby, a liquid pathway between the at least one inlet opening and the at least one outlet defined by the at least two outlet channels through which the liquid can pass, is formed. Furthermore, each of the at least two outlet channels  271 ,  272  has an extension in the longitudinal direction, i.e. the length, which is longer than the extension of the at least one inlet opening in the longitudinal direction. Suitably, the at least two outlet channels  271 ;  272  extend in the first (I) and second (II) longitudinal portions. The at least one inlet opening  211 ,  212  may be comprised in the first longitudinal portion (I). Thereby, the outlet channels may be substantially longer, such as 3-5 times longer than the inlet openings. Thus, the heavy phase liquid can be effectively pressed in a radial direction during the discharge of the liquid. 
     The purpose of the outlet channel/channels is to press the heavy phase liquid, which enters a liquid passage at a radial position near an inner wall of a bowl of a centrifugal separator radially inward towards a rotating axis of the centrifugal separator. Coriolis forces will create turbulence and vortices in the radial movement, which is one of the reasons for the generation of pressure losses. By reducing the tangential dimension of the outlet channel by introducing at least two separate outlet channels, it has been surprisingly noted that the pressure losses can be limited substantially, since the vortices in the radial movement will be reduced. This is a huge advantage, since the separation process in the centrifugal separator thus becomes less sensitive to flow rate variations and the interface between the light and heavy liquid phases becomes more stable. Therefore, e.g. light phase liquid (e.g. an oil) losses can be decreased. 
     Reference is now made to  FIG.  6    and  FIG.  7   .  FIG.  6    shows the element  200  in an enlarged view from the second, outlet, side  220 .  FIG.  7    shows the element  200  in an enlarged view from the first, inlet, side  210 . The  FIG.  6    shows that the two outlet channels  271 ;  272  may have respective channel end portions CE 1  and CE 2  which taper symmetrically and in a mirror-imaged way towards the center line CL and the second transversal edge TE 2  in the second longitudinal portion (II) of the element. Each of the outlet channels  271  and  272  also comprises a first transversally extending outlet edge TOE 1  and a second transversally extending outlet edge TOE 2 , which may provide a point of longest extension in the longitudinal direction towards the second transversal edge of the element. The tapering end portions CE 1  and CE 2  have a rounded shape, approximately resembling a quarter of an ellipse or a circle. In case of several channels, the described shape of channel end portions CE 1  and CE 2  could be provided for the channels locating closest to the side edges SE 1  and SE 2 . The shape can then better adapt to a circular peripheral shape of an outlet housing, also referred to as a power tube, which may be in close proximity or connected to the element  200 , as explained more in detail below. 
       FIG.  6    further shows that each of the outlet channels  271 ,  272  comprises a first transversally extending outlet edge TOE 1  on the second outlet side  220  and towards the first transversal edge TE 1  of the liquid discharge element  200 . The first transversally extending outlet edge TOE 1  has a longitudinally extending distance dig to the first transversal edge TE 1  of the heavy phase liquid discharge element  200 . Each of the channels also comprises a second transversally extending outlet edge TOE 2 , which is opposite to the first transversally extending outlet edge TOE 1 . 
       FIG.  7    shows in a corresponding manner that each of the at least one inlet openings  211 ,  212  comprises a first transversally extending inlet edge TIE 1  on the first inlet side  210  and towards the first transversal edge TE 1  of the liquid discharge element  200 . Each of the inlet openings also comprises a second transversally extending inlet edge TIE 2 , opposite to the first transversally extending inlet edge TIE 1 . The first transversally extending inlet edge TIE 1  has a longitudinally extending distance di 1  to the first transversal edge TE 1  of the heavy phase liquid discharge element  200 . 
     As can be seen, the longitudinal distance di 1  between the first transversal inlet edge TIE 1  and the first transversal edge TE 1  of the liquid discharge element  200  is smaller than the longitudinal distance di 2  between the first transversally extending outlet edge TOE 1  and the first transversal edge TE 1  of the liquid discharge element  200 . In this way, the inlet opening edges can be arranged closer to the first edge of the element  200  than the outlet channel edges. Thus, as displayed in  FIGS.  5   b  and  5   c   , a peripheral wall portion  280  can be provided in connection with the inlet openings  211 ,  212 . The peripheral wall  280  assists in pressing the liquid downwards along the extension of the channels towards the second transversal edge TE 2 . In this way, the total length of the liquid pathway can be maximized, and thus the pressure losses can be further decreased. Also, as shown best by  FIG.  5   b   , an extension of the first transversal inlet edge TIE 1  in a plane of a thickness dimension (d) of the element  200  is perpendicular to the central line, and also perpendicular to the peripheral wall  280 . Thereby suck up of particulate material, which may be drawn with the liquid when pressing it radially inwards from the location near the bowl wall through the liquid pathway between the inlet openings and the two outlet channels, can be decreased. Additionally, the stability of the interface position can be further improved. 
     As shown by the  FIGS.  5   a ,  5   c   ,  6  and  7 , the side edges SE 1  and SE 2  may taper symmetrically, and mirror-imaged from the first longitudinal portion towards the center line (CL) and the second transversal edge (TE 2 ). The tapering angle in respect of the extension of the center line (CL) may vary, but could be from 5-15 degrees and/or could correspond to the circumferential angle depending on the distance to a center of a base plate, in which the element  200  is mounted.  FIG.  7    further shows that the second longitudinal portion (II) of the liquid discharge element  200  may comprise a second end portion E 2 , which is semi-circular or has a shape of a circular segment. Thus, a shape resembling a triangle with rounded corners may be provided. Such shape enables attachment of the element to the bowl by means of three attachment means, such as screws. 
     The present invention also relates to a centrifugal separator or decanter centrifuge configured to separate a first liquid phase, a second liquid phase and a solid phase from a slurry. 
     Reference is now made to  FIGS.  8   a  and  8   b   .  FIG.  8   a    shows schematically a portion of a centrifugal separator including a base plate and  FIG.  8   b    shows an enlargement of the cut view of the heavy phase liquid discharge element described above, and also shown in  FIG.  5     b.    
     The centrifugal separator comprises a rotating body  101  comprising a bowl  102  and a screw conveyor  103  which are mounted on a shaft  104  such that they in use can be brought to rotate around a horizontal axis  105  of rotation. The axis  105  of rotation extends in a longitudinal direction of the bowl  102 . Further, the rotating body  101  has a radial direction  105   a  extending perpendicular to the longitudinal direction of the bowl  102 . The bowl  102  comprises a base plate  106  provided at one end of the bowl  102 . The base plate  106  has an internal side  107  and an external side  108 . The base plate  106  is provided with one or more second, heavy liquid phase, outlet passages  145  and one or more first, light liquid phase, outlet passages  115 . According to the present disclosure, the first and second liquid phase outlet passage are configured to discharge liquid from the rotating body, wherein the second liquid phase outlet passages  145  are associated with the heavy phase liquid discharge element  200  as described above. By “associated with” is meant that the parts are joined together in a working relationship, and may thus be for example directly or indirectly connected together. 
     Furthermore the bowl  102  is at an end opposite to the base plate  106  provided with solid phase discharge openings (not shown) in a similar manner as described in connection with the prior art separator shown in  FIG.  1   . Additionally, the screw conveyor  103  shown in  FIG.  8   a    may comprise inlet openings (not shown) for feeding a feed slurry to the rotating body  101 . The slurry comprises a solid phase (not shown), light liquid phase  21  and a heavy liquid phase  22 , with a liquid interface  15 ′ there between. By light liquid phase is meant a liquid phase having a smaller density than the density of the heavy liquid phase. The light phase liquid level is depicted with reference sign  15 ″. Analogously, by heavy liquid phase is meant a liquid phase having a higher density than the density of the light liquid phase. The heavy phase liquid level corresponds to the liquid interface  15 ′ in the shown example. The light liquid phase may be for example an oil or an organic solvent and the heavy liquid phase may be water, but the liquids are not limited thereto. 
     During rotation of the rotating body  101 , separation of the liquid phases  21  and  22  and the solids are obtained. The light liquid phase  21  is located radially closer to the rotation axis  105  than the heavier liquid phase  22  in the radial direction  105   a . The light liquid phase  21  is discharged through the one or more first liquid phase outlet passages  115  in the base plate  106  to an outlet chamber  121 . The heavy liquid phase  22  is discharged through the second outlet passages  145  to an outlet chamber  122 , while the screw conveyor  103  transports the solid phase towards the solid phase discharge openings at the opposite end of the separator as described in connection with  FIG.  1   . Each first liquid phase outlet passage  115  may be partly covered by a respective weir or dam plate  114 , or a light phase liquid discharge element  300  (see  FIG.  9   ) comprising an opening passage  315 , which may define or be a part of a weir edge in fluid connection with the first outlet passage  115 , and being comprised in the base plate  106 . Each of the second, heavy, liquid phase outlets  145  is associated with the heavy phase liquid discharge element  200  as described above, which may define an intake level for the heavy phase liquid. In this way it is possible to discharge the respective liquid phases. 
     Reference is now made to  FIG.  9   , which schematically shows an example of a base plate  106  in a centrifugal separator, viewed from the internal side  107 . It can be seen that the base plate  106  is associated with three light phase liquid discharge elements  300 , each comprising an opening passage  315  in fluid connection with a first outlet passage (not shown) associated with the base plate. Additionally, the base plate  106  is associated with three heavy phase liquid discharge elements  200 , each comprising an opening passage two inlet openings  211 ,  212  in fluid connection with a second outlet passage (not shown) associated with the base plate. Further, the light phase liquid discharge elements  300  and the heavy phase liquid discharge elements  200  are arranged at different angular positions relative to the axis of rotation, and thus at a distance from each other. The center line (CL) of each of the liquid discharge elements is arranged in the radial direction of the base plate  106 . The base plate may comprise pockets or similar means in which the liquid discharge elements  200 ,  300  can be fitted and secured. In the shown example, every other liquid discharge element is a heavy phase liquid discharge element  200 , and every other is a light phase liquid discharge element  300 . However, the liquid discharge elements can be arranged in any other way, and the amount of the liquid discharge elements for the heavy phase and light phase, respectively, do not need to be the same. Therefore, the liquid discharge elements preferably have the same outer shape, so that the amount of the respective heavy phase/light phase liquid discharge element can be easily varied. By varying the amount of the respective liquid discharge elements and the arrangement thereof, the liquid removal without pressure losses, respectively, can be adapted to the slurry to be separated. This means that the amount of the heavy phase liquid discharge elements  200  may be larger, if for example water-content is higher than oil content in an oily slurry. Generally, the amount of the light phase liquid discharge elements  300  and the heavy phase liquid discharge elements  200  may vary for example from 2 to 16, and the amount may be equal. Alternatively, the amount of the light phase liquid discharge elements  300  and the heavy phase liquid discharge elements  200  may vary from 2 to 16, but the amount of the heavy phase liquid discharge elements  200  is larger than the amount of the light phase liquid discharge element  300 . In this way, the heavy phase may be removed with less pressure losses from the bowl. Alternatively, the amount of the light phase liquid discharge elements  300  is larger than the amount of the heavy phase liquid discharge elements  200 . In this way, the light phase may be removed more efficiently from the bowl. 
     Reference is now further made to  FIG.  10    and  FIG.  11   , which show a further variant of a centrifugal separator base plate  106  with the heavy phase and light phase liquid discharge elements  200  and  300  as described above. The function of the centrifugal separator is the same as described in connection with  FIG.  8   a    and the base plate  106  may have the same features as described in connection with  FIG.  9   , and reference is made thereto. However, the embodiment shown in  FIGS.  10  and  11   , includes another type of outlet arrangement for the outlet passages  115  and  145  than described above. The light phase liquid discharge element  300  displayed in  FIG.  11    is associated with an outlet housing  1115 , which is also referred to as a “power tube”, and the heavy phase liquid discharge element  200  is associated with a respective outlet housing  1145 . The heavy phase  22  is discharged through the outlet housing  1145  to a respective outlet compartment  1122 . The light phase  21  is discharged through the outlet housing  1115  to a respective outlet compartment  1121 . A liquid interface  15 ′ is shown in between the light and heavy liquid phases. Outlet housings of this type are previously described in WO 2012/062337. However, it has been noted that the heavy phase (second) liquid discharge element  200  of the present disclosure is also usable in connection with such outlet housing arrangements. Each of the outlet housings  1115  and  1145  comprises a respective outlet opening  1118 ,  1148 , through which the respective liquid is discharged. The first outlet opening comprises first weir edge  1129 , and the second outlet opening  1148  comprises a second weir edge  1159 . The outlet housings  1115  and  1145  can be rotatably adjustable around an adjustment axis, whereby the weir edges can be brought to a desired level in a simple manner. Also, discharge of the liquid phase can be made in an opposite direction relative to the direction of rotation whereby energy can be recovered from the discharged liquid. Thus, more accurate separation can be provided and unnecessary losses of the desirable liquid phase can be decreased. 
     The present invention also relates to a method of separating a first liquid phase and a second liquid phase from a slurry by means of centrifugal forces in a centrifugal separator. As described above, the liquid phases have different densities. The method comprises the steps of:
         bringing the slurry to a rotational movement in a cylindrical bowl and thereby separating the slurry into two liquid phases,   separating the liquid phases from each other by
           bringing the first light liquid phase in fluid contact with at least one first outlet passage comprised in a base plate of the centrifugal separator, the first outlet passage being connected to a weir plate adapted for keeping at least part of the second, heavy phase inside the rotating bowl, wherein the at least one outlet passage provides a liquid pathway to the light phase to be discharged from the bowl   bringing the second heavy liquid phase in contact with at least one second outlet passage comprised in a base plate of the centrifugal separator, the second outlet passage being associated with a heavy phase liquid discharge element adapted for keeping at least part of the first light liquid phase inside the rotating bowl, wherein heavy phase liquid discharge element provides a liquid pathway to the heavy phase to be discharged from the bowl,   
           wherein the method is characterized by discharging the heavy phase by using at least two separate liquid outlet channels in the heavy phase liquid discharge element through which the heavy phase liquid is arranged to flow.       

     By having the two outlet channels in the liquid discharge element, it is possible to decrease pressure losses during the separation process. In this way, it is possible to minimize the losses of a desirable liquid phase and obtain a stable separation process with a stable liquid interface. 
       FIG.  12    shows results from an experiment in which the heavy phase liquid discharge element of the present invention (“New Separation Plate design”) was compared with a prior art liquid discharge element (“Conventional Separation Plate Design”), similar to “the second channel member  167 ” disclosed by WO2012062337. In the test, a decanter centrifuge with a diameter of 500 mm was operated at 2800 rpm bowl speed on a 3-phase process, where the goal was to minimize the content of light phase (oil) in the discharged heavy phase liquid. This oil loss will depend on the choice of weir radius for the liquids and in particular the difference between the weir levels. On the right side of the graph in  FIG.  12    the solid line represents the optimal performance at a feed flow rate of 35 m 3 /h based on the test results marked by triangles. If an oil loss level of 0.75% is taken as the limit, the difference in discharge levels must be between 20 and 22 mm, which is a quite narrow range. As indicated with the interrupted line the optimal operating window for the discharge level will change to the interval 22 to 24 mm for a flow rate of 40 m 3 /h indicating that the pressure loss in the heavy phase discharge line increases significantly at increased flow rate. In order to get acceptable performance at 40 m 3 /h the discharge level setting would need to be adjusted. The test result for the present invention are shown in the left part of the graph, where the solid line based on the test results marked by circles indicates the optimal performance for this design. It is noted that there is a wider operational window covering differences in discharge levels from 11 to 16 mm. Comparing to the original design the change in level difference is equivalent to approximately 1 bar of reduced pressure loss in the discharge line for the heavy phase liquid. The reduced dependency of pressure loss is also noted by the significantly reduced change of operational window when the flow is changed from 35 to 40 m 3 /h. The new window of 12 to 17 mm difference in discharge level will normally not require a change of level settings for the higher flow rate. This reduces the dependency of flow rate significantly and results in a much more stable interface position even for large flow variations. 
     The foregoing description of the embodiments has been provided for illustration of the present invention. The embodiments are not intended to limit the scope of the invention defined in the appended claims and features from the embodiments may be combined with one another.