Patent Publication Number: US-9849467-B2

Title: Disc package for a centrifugal separator

Description:
BACKGROUND 
     The invention relates to a disc package for a centrifugal separator comprising a plurality of separation discs, each provided with a separation surface with is inclined with respect to the radial direction and wherein the separation discs are arranged coaxially around an axis of rotation at a distance from each other such that to form passages between each two adjacent separation discs. The invention also relates to a centrifugal separator comprising such a disc package. 
     From the early days of development of centrifugal separators it is known to provide each separation disc in a disc package with a brim extending radially outside the frustoconical portion of the disc, in order to improve the mechanical stability of the discs, see e.g. SE 22981. 
     A disc stack having a single separation disc provided with a brim extending radially outside the rest of the separation discs of a disc stack is also previously known, see SE 227107. This is used to divide the disc stack into a first section where the cleaning of the light phase is optimised (purifier mode of operation) and a second section where the cleaning of the heavy phase is optimised (concentrator mode of operation). 
     SUMMARY 
     It is an object of the invention to improve the separation efficiency of the centrifugal separator. In particular it is an object to minimise the risk of separated particles being recirculated into the separating passages in-between the separation discs of the disc package. None of the documents cited above provides a satisfactory solution to this. 
     Thus the invention relates to a disc package for a centrifugal separator, comprising a first set of separation discs and a second set of separation discs. Each separation disc of the first and second sets is provided with a separation portion having a separation surface with is inclined with respect to the radial direction. The separation surfaces may be a frustoconical portion 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 of the first and second sets are arranged coaxially around an axis of rotation at a distance from each other such that to form passages between each two adjacent separation discs. The separation discs are preferably arranged such that the base portions of the inclined separation portions of the separation discs in the disc package are facing in the same direction. The separation discs in the disc package may be arranged such that the fluid to be separated flows radially inwards in the passages between each two adjacent separation discs of any of the two sets. 
     The separation discs of the first set have outer diameters A or below, the separation discs of the second set have outer diameters B or above, and the diameter B is larger than diameter A. At least two separation discs of the first set are arranged between every two separation disc of the second set. 
     As an effect of this the risk of formation of currents in the separation space in the region immediately outside the disc package is minimised. Such currents having an axial flow component may have the undesired effect to bring along particles already separated from a liquid mixture of components. The risk of separated particles being recirculated into the separating passages of the disc package is thus minimised and the separation efficiency of the centrifugal separator increased. The disc package is also easy to fabricate and assemble. 
     The outer diameters of the separation discs of the first set may vary as long as they have an outer diameter of A or below. Alternatively the separation discs of the first set have an outer diameter A. Similarly, the outer diameters of the separation discs of the second set may vary as long as they have an outer diameter of B or above. Alternatively every separation disc of the second set may have an outer diameter B. 
     The diameter B may be 3-15%, preferably 5-12% larger than diameter A, or the diameter B may be 14-50 mm, preferably 20-30 mm larger than diameter A. Thus the risk of separated particles being recirculated into the separating passages of the disc package is minimised while maintaining an open space for separation radially outside the disc package. 
     The separation discs of the first and second set may extend from a common inner radial position. The radial extent and inclination of the inclined separation portion may be similar over the separation discs of the two sets and over the disc package as a whole. 
     The separation discs of the second set may have a brim portion formed radially outside the diameter A, which brim portion has an inclination to the radial direction which is different from the inclination at the separation portion. The radial extent of the brim portion may be 1.5-7.5%, preferably 2.5-6% of diameter A, or the radial extent of the brim portion may be 7-25 mm, preferably 10-15 mm. The radial extent of the inclined separation surface may be similar for the separation discs in the first and second set. 
     The angle of the brim portion to the radial direction may be less than 45 degrees, preferably less than 30 degrees, more preferably less than 15 degrees, most preferably zero degrees. If the angle is close to zero or zero, i.e. the brim portion is in a plane perpendicular to the axis of rotation, the brim portion acts to define a flow zone radially outside the separation discs of the first set without acting as a separation surface. Thus the inclined separation surface of each separation disc of the second set may extend to the diameter A. The angle of the brim portion may be the same or may vary over the separation discs of the second set in the disc package. 
     The brim portion may be ring shaped and the surface of the brim portion may be plain and formed as a continuous sheet of material circumventing the separation surface, thereby being provided essentially without any apertures or protrusions. Thus the amount of turbulence caused by the brim portion is minimised. 
     There may be 2-10, preferably 3-5 separation discs of the first set arranged between every two separation disc of the second set. This configuration provides a particularly advantageous flow formation in the flow zone radially outside the separation discs. The total number of separation discs of the second set to the total number of separation discs in the disc package may be below 30%, preferably below 25%, and above 5%, preferably above 10%. More preferably the total number of separation discs of the second set to the total number of separation discs in the disc package is within the range of 12-20%. 
     A lower portion of the disc package, at the end of the package in the direction of the base of the separation surface of the separation discs (i.e. in the axially diverging direction of the separation surface of the separation discs), may be provided only with separation discs of the first set. The lower portion of the disc package may be at least 20%, preferably at least 25%, more preferably at least 30% of the disc package. In a disc package having 82 discs this lower portion may thus contain 18-26 discs. Similarly, an upper portion of the disc package, at the end of the package in the direction of the top of the separation surface of the separation discs (i.e. in the axially converging direction of the separation surface of the separation discs), may be provided only with separation discs of the first set. The upper portion of the disc package may be at least 15%, preferably at least 20%, more preferably at least 25% of the disc package. In a disc package having 82 discs this upper portion may thus contain 12-21 discs. Thus the risk of separated particles being recirculated into the separating passages of the disc package is minimised while maintaining a desired flow in the portions of the axial ends of the disc package. 
     The inclined separation surface of each separation disc of the first set may extend to the outer diameter of the separation disc. Thus the separation discs of the first set may be provided essentially without any brim portion, maximising the separation surface. 
     Each separation disc may be provided with cut-outs arranged such that to distribute the flow of fluid to be separated through and over the disc stack. The separation discs of the first set may be provided with cut-outs in the form of notches which are cut-outs that are open towards the outer radius of the separation disc. This has the effect that the risk of clogging in the area of the cut outs is minimised. The separation discs of the second set may be provided with cut-outs in the form of apertures that are closed towards the outer radius of the separation disc. This has the effect to improve the mechanical properties of the separation discs of larger diameter, to be able to cope with the centrifugal forces. The combination of cut-outs in the form of notches on the separation discs of the first set and cut-outs in the form of apertures on separation discs of the second set further minimises the risk of clogging in the area of the cut-outs on the separation discs of the second set. 
     The invention further relates to a centrifugal separator for separating a liquid mixture of components, comprising a rotor enclosing a separation space wherein a disc package as described is arranged. The rotor may comprise outlets from the radially outer portion of the separation space for intermittent discharge of a sludge phase. 
     The interior wall of the rotor may be provided with a wall portion, which may be conical, and the separation discs of the second set may be arranged in the disc stack such that there is a passage between the outer brim of each disc and the rotor wall portion of at least 1 mm, preferably at least 1.5 mm. 
    
    
     
       DRAWINGS 
         FIG. 1  shows an embodiment of a separation disc of the kind comprised by the first set in the disc package. 
         FIG. 2  shows an embodiment of a separation disc of the kind comprised by the second set in the disc package. 
         FIG. 3  shows a cross-section of a portion of a centrifugal separator having a disc package comprising a first and a second set of separation discs. 
     
    
    
     DETAILED DESCRIPTION 
     In  FIG. 1  a separation disc  1  of the kind comprised by the first set in the disc package is shown, having a frustoconical separation portion  2  with an inner and an outer separation surface. The separation portion is provided with a plurality of distance members in the form of elongated caulks  3  providing distances to form passages between each two adjacent separation discs in a package formed by a stack of separation discs. The caulks are fastened to the outer surface of the frustoconical separation portion of the disc and distributed around the circumference of the disc. The outer diameter A of the separation disc is 165 mm, and the inclined separation surface extend all the way out to this outer diameter. Thus the radially outer portion  4  of the separation disc is part of the inclined separation surface. The disc is provided with a plurality of cut-outs in the form of notches  5  at this radially outer portion  4  of the separation disc having a diameter A, which notches are open towards the outer radius of the separation disc. The number of notches  5  correspond to the number of caulks and the notches are distributed around the circumference of the disc in-between the caulks. 
     In  FIG. 2  a separation disc  6  of the kind comprised by the second set in the disc package is shown, having a frustoconical separation portion  2 ′ with an inner and an outer separation surface. The separation portion is provided with a plurality of distance members in the form of elongated caulks  3 ′ providing distances to form passages between each two adjacent separation discs in a package formed by a stack of separation discs. The caulks are fastened to the outer surface of the frustoconical separation portion of the disc and distributed around the circumference of the disc. The separation surface extends to the diameter A and radially outside the separation surface the disc is provided with a flat brim  7  (i.e. having an angle of zero degrees to the radial direction) extending to the outer diameter of the separation disc B. The diameter B is 185 mm and thus the radial extension of the brim is L=(B−A)/2, i.e. 10 mm. The diameter B is thus 12% larger than the diameter A. Each disc  6  has a generally circular outer edge at B as seen in,  FIG. 2 . The disc is provided with a plurality of cut-outs in the form of apertures  8  at the radially outer part of the separation portion, which cut-outs are closed towards the outer radius of the separation disc by means of the brim. The number of apertures  8  correspond to the number of caulks and the apertures are distributed around the circumference of the disc at positions corresponding to the notches of the separation disc  1  in the first set. 
       FIG. 3  shows a portion of a centrifugal separator  9  for separation of a liquid mixture of components, the separator having a rotor  10  supported by a spindle  11  (partly shown) which is rotatably arranged in a frame around an axis of rotation (x). The rotor forms within itself a separation chamber  12  wherein a disc package  13  is arranged. The rotor further comprises an inlet chamber  14  formed within a distributor  15  into which a stationary inlet pipe  16  extends for supply of a liquid mixture of components to be separated. The inlet chamber communicates with the separation chamber via passages  17  formed in the rotor. The radially inner portion of the disc package communicates with an outlet  18  for a lighter liquid component of the mixture. The outlet  18  is delimited by a top disc  19  provided at the upper axial end of the disc package  13 . The top disc  19  and the upper wall part of the rotor  10  delimits a passage for a denser liquid component of the mixture, the passage extending from the radially outer part of the separation space  12  to an outlet  20  for the a heavier component of the liquid mixture. The rotor is further provided with outlets  21  from the radially outer periphery of the separation space  12  for intermittent discharge of a sludge component of the liquid mixture comprising denser particles forming a sludge phase. The opening of the outlets  21  is controlled by means of an operating slide  22  actuated by operating water, as known in the art. 
     The disc package  13  comprises a first and a second set of separation discs, the first set comprising separation discs  1  of the kind shown in  FIG. 1 , and the second set comprising separation discs  6  of the kind shown in  FIG. 2 , provided with brims  7 . The separation discs are arranged coaxially around the axis of rotation (x) at a distance from each other by means of the caulks  3 ,  3 ′, such that to form passages between each two adjacent separation discs. There are three separation discs of the first set arranged between every two separation disc of the second set. The passages extend from the radially outer portions of the separation discs to the radially inner portions of the separation discs. In the figure the distance between each separation disc is exaggerated and the disc package is schematically shown to have 28 discs. A typical disc package comprises 82-120 discs and a typical distance between the separation discs, generated by the caulks, is 0.4-0.75 mm. Thus the typical distance H between separation discs of the second set in the disc package is 1.6-4.5 mm, preferably 2-4 mm. 
     The cut-outs in the form of notches on the separation discs  1  in the first set and the cut-outs in the form of apertures on the separation discs  6  in the second set are aligned in the disc package to form distribution channels  23  for the liquid mixture. 
     A lower portion of the disc package, i.e. at the end of the package in the direction of the base of the separation surface of the separation discs and indicated as D, is provided only with separation discs of the first set. This portion is 25% of the height of the disc package (7 of the 28 separation discs shown), and in a typical disc package this portion is 21-26 of 82 separation discs. The clearance C between the radially outer end of the brim of the first (lower) separation disc of the second set and the interior wall of the rotor is at least 1.5 mm to provide a passage for sludge toward the outlets  21 . Similarly, an upper portion of the disc package, i.e. at the end of the package in the direction of the top of the separation surface of the separation discs and indicated as E, is provided only with separation discs of the first set. The clearance F between the radially outer end of the brim of the last (topmost) separation disc of the second set and the interior wall of the rotor is at least 1.5 mm. 
     An embodiment of a disc package includes, from bottom to top, a lower portion of 21 separation discs of the first set, followed by 12 separation discs of the second set each having three separation discs of the first set in-between (i.e. all in all 11*4+1=45 separation discs), and an upper portion of 16 separation discs of the first set on top. Thus the total number of separation discs of the second set to the total number of separation discs in such a disc package shown is 12/82, which is below 20% but above 10%.