Abstract:
The invention relates to a centrifugal pump ( 1 ) comprising a pump housing ( 2 ) which is provided with an axial inlet ( 6 ), an outlet ( 5 ) and a rotor ( 7 ) which is attached in the pump housing ( 2 ) such that it can rotate about an axial rotation axis A. The rotor ( 7 ) is provided with a central boss ( 9 ), a shaft shield ( 11 ) fastened to the boss ( 9 ), a suction shield ( 12 ) attached so as to be axially set apart from the shaft shield ( 11 ), which suction shield ( 12 ) has an axial supply ( 14 ) aligned with the axial inlet ( 6 ) of the pump housing ( 2 ), and a plurality of rotor blades ( 15 ) which are fastened between the shields ( 11, 12 ). The radial inner ends ( 18 ) of the rotor blades ( 15 ) are connected to the suction shield ( 12 ) by a substantial perpendicular connection.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates to a centrifugal pump, in particular for the pumping of a mixture of substances possibly including soil, comprising: 
         [0002]    a pump housing which is provided with an axial inlet and an outlet attached tangentially to a circumferential wall of the pump housing, 
         [0003]    a rotor which is attached in the pump housing such that it can rotate about an axial rotation axis A, which rotor is provided with a central boss, a shaft shield fastened to the boss, a suction shield attached so as to be axially set apart from the shaft shield, which suction shield has an axial supply aligned with the axial inlet of the pump housing, and a plurality of double bent rotor blades which are fastened between the shields and each extend substantially transversely to the rotation axis A between a radial outer end and a radial inner end. The invention further relates to a double bent rotor blade for use in such a centrifugal pump and a vessel, comprising such a centrifugal pump. 
       BACKGROUND 
       [0004]    Centrifugal pumps are known. Two types of pumps can de distinguished: pumps with single bent rotor blades, such as shown in EP2236836A2 and JP 8 284 885, and pumps with double bent rotor blades. Examples thereof are known from European patent application EP 1 903 216 A1 and EP1906029. This document relates to problems and solutions thereof associated with double bent rotor blades. 
         [0005]    An example of such a centrifugal pump with double bent rotor blades is depicted in  FIGS. 1 and 2 . 
         [0006]      FIGS. 1 and 2  schematically depict a centrifugal pump  1 , comprising a pump housing  2  shaped like a volute (spiral casing). The pump housing  2  has a circumferential wall  3  and a spout-shaped outlet  5  attached tangentially to the circumferential wall  3  of the pump housing  2 . The junction between the inner surface of the tangential outlet  5  and the inner surface of the circumferential wall  3  of the pump housing  2  defines what is known as a cutwater  4 . The pump housing  2  also has an axial inlet  6 . 
         [0007]    A rotor  7  is attached in the pump housing  2  such that it may rotate about an axial rotation axis A. The rotor  7  has a central boss  9  which may be fastened to a drive shaft (not shown). A shaft shield  11  extends from the central boss  9 . The shaft shield  11  forms a first wall for delimiting the flow within the rotor  7 . Axially set apart from the shaft shield  11 , the rotor has a suction shield  12  which defines a second wall for delimiting the flow within the rotor  7 . The suction shield  12  has an axial supply  14  which is aligned with the axial inlet of the pump housing  2 . 
         [0008]    A plurality (three in  FIGS. 1 and 2 ) of rotor blades  15  are fastened between the shields  11 ,  12 . In this illustrative embodiment, the rotor  7  comprises three rotor blades  15 . The rotor blades  15  each extend substantially radial to the rotation axis A. Each rotor blade  15  comprises a radial inner end  18  (leading edge) and a radial outer end  17  (trailing edge). The radial outer ends  17  and radial inner ends  18  run from the shaft shield  11  to the suction shield. Between the radial outer ends  17  of the rotor  7  and the inner surface of the circumferential wall  3  of the pump housing  2  there is a circumferential channel  19 . The circumferential channel  19  has a passage surface area which increases somewhat in the circumferential direction from the cutwater  4  toward the outlet  5 . 
         [0009]    The characteristic dimensions of the centrifugal pump  1  are indicated in  FIGS. 1 and 2 . These characteristic dimensions largely determine the characteristics of the pump. The rotor  7  has an outermost diameter Dw which is defined by the radial outer edges of the shields  11 ,  12 . The rotor  7  has a width Bw extending between the mutually facing surfaces of the shaft shield  11  and the suction shield  12 . The axial supply  14  of the rotor  7  defines a suction diameter Dz. An inlet pipe can be connected to the axial inlet  6  of the pump housing  2 . The centrifugal pump  1  also has what is known as a spherical passage Bol which is defined by the diameter of the largest sphere able to pass between the rotor blades (indicated in  FIG. 1 ). 
         [0010]    The rotor blades  15  are double bent rotor blades, which means that the rotor blade is curved in a first direction from the radial inner end  18  (leading edge) to the radial outer end  17  (trailing edge) and is curved in a second direction perpendicular to the first direction. Also indicated in  FIG. 1  is a throat diameter Dk which is defined by the narrowest passage of the spout-shaped outlet  5  of the pump housing  2 . Said narrowest passage is located in proximity to the cutwater  4 . The spout-shaped outlet  5  also has a press diameter Dp located at the tip thereof. The distance, extending parallel to the centre line B of the outlet  5 , between the crossing  4  and the level of the rotation axis A is indicated by the parameter T. The thickness of the circumferential channel  19  at the location of the cutwater  4  is represented in  FIG. 1  by V. 
         [0011]    During operation, the rotor rotates about the rotation axis A. Between the rotor blades  15 , the mass to be pumped is forced radially outward into the pump housing  2  under the influence of centrifugal forces. Said mass is then entrained in the circumferential direction of the pump housing  2  toward the tangential outlet spout  5  of the pump housing  2 . The pumped mass which, after leaving the rotor  7 , is entrained in the circumferential direction of the pump housing  2  flows largely out of the tangential outlet of the pump housing  2 . A small amount of the entrained mass recirculates, i.e. 
         [0012]    flows along the cutwater back into the pump housing  2 . 
         [0013]      FIG. 3  shows a perspective view of an example of the rotor  7  described above with reference to  FIGS. 1 and 2 . 
         [0014]    Said centrifugal pump  1  can be used in dredging operations. If the centrifugal pump  1  is located on board a dredging ship, such as a cutter suction dredger or hopper suction dredger the centrifugal pump  1  has to fetch a loose mixture of substances, possibly including soil, stones and/or pebbles, from the sea floor. The main characteristics of a centrifugal pump used in dredging operations are  1 ) suction capacity,  2 ) durability and  3 ) spherical passage Bol (see  FIG. 1 ). In use, the mixture of substances, possibly including stones and/or pebbles, flows through the centrifugal pump  1 . In order to prevent blockage, said stones and/or pebbles have to be able to pass through the centrifugal pump  1  (Bol). A wide centrifugal pump  1  having few blades is suitable for this purpose. However, by widening the centrifugal pump  1  and reducing the number of rotor blades  15 , the suction characteristics and durability of the centrifugal pump  1  are adversely affected. 
         [0015]    DE 101 49 648 A1 describes an example of a pump with double bent rotor blades. This pump is of a different design than the pumps described above, i.e. the leading edge does not run from the shaft shield to the suction shield, but originates at a central shaft and does not end at the suction shield. 
         [0016]    An object of the invention is to provide an improved centrifugal pump, which combines a relatively high suction capacity, with an improved durability and spherical passage. 
       SUMMARY OF THE INVENTION 
       [0017]    According to the invention, this object is achieved in a centrifugal pump, in particular for the pumping of a mixture of substances possibly including soil, comprising: 
         [0018]    a pump housing which is provided with an axial inlet and an outlet attached tangentially to a circumferential wall of the pump housing, 
         [0019]    a rotor which is attached in the pump housing such that it can rotate about an axial rotation axis, which rotor is provided with a central boss, a shaft shield fastened to the boss, a suction shield attached so as to be axially set apart from the shaft shield, which suction shield has an axial supply aligned with the axial inlet of the pump housing, and a plurality of double bent rotor blades which are fastened between the shields and each extend substantially transversely to the rotation axis A between a radial outer end and a radial inner end, wherein the radial inner ends form a substantially right angle with respect to the suction shield. 
         [0020]    The radial inner ends end at the suction shield with a substantial perpendicular connection. By providing a substantial perpendicular connection between the radial inner ends  18  to the suction shield  12 , less material is needed thereby increasing the inflow area of the centrifugal pump  1 . 
         [0021]    According to a further aspect there is provided a double bent rotor blade for use in a centrifugal pump, in particular for the pumping of a mixture of substances possibly including soil, the rotor blade comprising a radial inner end and a radial outer end, the rotor blade being arranged to be mounted on a mounting position between a shaft shield and a suction shield in a centrifugal pump, the radial inner end being formed such that in the mounting position it forms a substantially right angle with respect to the suction shield. 
         [0022]    According to a further aspect there is provided a vessel, comprising a centrifugal pump as provided above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The invention will now be described in greater detail with reference to an illustrative embodiment represented in the drawings, in which: 
           [0024]      FIG. 1  is a front view in cross section of a centrifugal pump according to the prior art, 
           [0025]      FIG. 2  is a side view in cross section along the line II-II in  FIG. 1 . 
           [0026]      FIG. 3  is a perspective view of the rotor shown in  FIGS. 1 and 2 , 
           [0027]      FIGS. 4   a  and  4   b  schematically show an embodiment of a centrifugal pump, 
           [0028]      FIG. 4   c  schematically shows an example from the prior art, 
           [0029]      FIGS. 5   a  and  5   b  schematically show a further embodiment, 
           [0030]      FIGS. 6   a - b,    7   a - b  and  8  schematically show further embodiments of a centrifugal pump, 
           [0031]      FIGS. 9   a  and  9   b  schematically show a cross section of part of a centrifugal pump according to further embodiments. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0032]    As explained above with reference to  FIGS. 1-3 , the rotor  7  is provided in the pump housing  2  in which it is rotatable suspended. In use, mass enters the pump housing  2  which is transported to the outlet  5  by the rotor  7 . In the Figures, the outlet  5  is depicted as a spout-shaped outlet  5 , but it will be understood that the outlet may also be provided with another suitable shape, such as a straight outlet  5 . 
         [0033]    The embodiments depicted in the Figures show a rotor comprising three rotor blades  15 . The rotor blades  15  according to all embodiments are double bent rotor blades, which means that the rotor blade is curved in a first direction from the radial inner end  18  (leading edge) to the radial outer end  17  (trailing edge) and is curved in a second direction perpendicular to the first direction. 
         [0034]    The radial outer ends  17  and radial inner ends  18  run from the shaft shield  11  to the suction shield  12 . 
         [0035]    It will be understood that any suitable number of rotor blades  15  may be provided, such as for instance four or five rotor blades  15 . Each rotor blade  15  comprises a radial inner edge, also known as the leading edge and a radial outer edge, also known as the trailing edge. The trailing edge may be straight, possibly slanted. 
         [0036]      FIGS. 4   a - 4   b  schematically depicts a centrifugal pump  1  as described above with reference to  FIGS. 1-3 , wherein the radial inner ends  18  of the rotor blades  15  are connected to the suction shield  12  by a substantial perpendicular connection. The radial inner ends  18  form a substantially right angle with respect to the suction shield  12 . In other words, the radial inner ends  18  are connected to the suction shield  12  in a substantial perpendicular way. 
         [0037]    As shown in  FIG. 4   a,  the end part of the radial inner end  18  that is connected to the suction shield  12  is directed in a radial direction away from the rotation axis A. 
         [0038]    The suction shield  12  comprises a bent near an inner edge of the suction shield  12  defining a nozzle projecting in a direction towards the supply the mass to be pumped. The nozzle forms the axial supply  14 . The nozzle comprises an inner wall to which the radial inner end  18  is connected. The radial inner end  18  is provided with a bent to provide a perpendicular connection between the radial inner end  18  and the inner edge of the suction shield  12 . 
         [0039]    In the technical field of centrifugal pumps for dredging, angles in the range of 85°-95°, or at least angle in the range of 88°-92°, will considered to be substantial perpendicular. 
         [0040]    By providing a substantial perpendicular connection between the radial inner ends  18  to the suction shield  12 , less material is needed thereby increasing the inflow area of the centrifugal pump  1 . 
         [0041]      FIG. 4   b  shows in more detail the connection between the radial inner end  18  and the suction shield  12 . As can be seen in  FIG. 4   b,  due to the casting process, casting curvatures  41  are formed in the corners. 
         [0042]      FIG. 4   c  shows a connection according to the prior art, wherein the angle between the radial inner end  18  to the suction shield was sharp, e.g. 60°. As can be seen by comparing  FIGS. 4   b  and  4   c,  the casting curvatures  41  are relatively small when providing a substantial perpendicular connection. This results in a larger inflow area and an increased suction capacity of the centrifugal pump  1 . Also, in case the same amount of mass is being pumped by a centrifugal pump  1  according to this embodiment, the flow velocity of the mass in the pump can be smaller, thereby reducing the wear and enhancing the decisive vacuum and the net positive suction head required. 
         [0043]    Accordingly, according to an embodiment there is provided a double bent rotor blade  15  for use in a centrifugal pump  1 , in particular for the pumping of a mixture of substances possibly including soil, the rotor blade  15  comprising a radial inner end  18  and a radial outer end  17 , the rotor blade  15  being arranged to be mounted on a mounting position between a shaft shield  11  and a suction shield  12  in a centrifugal pump, the radial inner end  18  being formed such that in the mounting position it forms a substantially right angle with respect to the suction shield  12 . 
         [0044]    A further embodiment is depicted in  FIGS. 5   a  and  5   b.  According to this embodiment there is provided a centrifugal pump  1 , wherein the radial inner ends  18  of the rotor blades  15  extending between the shaft shield  11  and the suction shield  12  have a substantially S-curved shape comprising a first part  181  near the suction shield  12  being convex towards the axial supply  14  and a second part near the shaft shield  11  being concave towards the axial supply  14 . Accordingly, also provided is a double bent rotor blade  15 , wherein the radial inner end  18  has a substantially S-curved shape, which comprises a first part  181  which in the mounting position is near the suction shield  12 , the first part  181  being convex with respect to an axial supply  14  of the centrifugal pump and a second part which after mounting is near the shaft shield  11 , the second part  182  being concave with respect to the axial supply  14 . 
         [0045]    Instead of providing substantial straight radial inner ends, the radial inner ends  18  are substantially S-shaped, wherein the first part  181  is curved in a first direction and the second part  182  is curved in a second direction, opposite to the first direction. 
         [0046]    The radial inner ends  18  may comprise further parts that are substantially uncurved, an example of which is schematically depicted in  FIG. 5   a.  According to  FIG. 5   a  a straight part is provided in between the first part  181  and the second part  182 . 
         [0047]      FIG. 5   b  depicts an embodiment in which the curved first part  181  and curved second part  182  are directly connected to each other. 
         [0048]    As can be seen, the S-shaped radial inner ends  18  allow for an easy substantial perpendicular connection of the radial inner ends  18  to the suction shield  12 . 
         [0049]    The shape of the rotor blades  15  in a direction running from the radial inner ends  18  to the radial outer ends  17  determines the energy transfer from the rotor blades  15  to the mass being pumped. Different parts of the rotor blade  15  may be provided with different curvatures which transfer different amounts of energy to the mass being pumped. 
         [0050]    According to an embodiment there is provided a centrifugal pump  1 , wherein the rotor blades  15  comprise a strip  185  along the radial inner ends  18  that is shaped such that in a direction perpendicular to the respective radial inner end  18  the radius R of the rotor blade  15  is a function of an angle φ with respect to the rotation axis A: R(φ)=C 1 .+C 2 , wherein C 1  and C 2  are constants. Accordingly there is provided a rotor blade that is shaped as such. This embodiment is schematically depicted in  FIG. 6   a.  An example R(φ) is schematically shown in  FIG. 6   b.    
         [0051]    Angle φ is indicated in  FIG. 6   a  and is defined in a plane substantially perpendicular with respect to the rotation axis A (perpendicular to the plane of drawing in  FIG. 6   a ). Radius R and angle φ together thus form polar coordinates, with respect to the rotation axis A. For strip  185  applies that the angle a at which the strip  185  intersects (imaginary) concentric circles positioned concentrically around the rotational axis A in a plane perpendicular to the rotational axis A is constant. This is shown in  FIG. 6   a.    
         [0052]    The strip may cover up to 10% of the total length of the rotor blades when measured from the radial inner end  18  to the radial outer end  17 . The remaining portion of the rotor blade, thus between the strip  185  and the radial outer end  17  may be curved. The exact shape of this curved part may be designed to achieve an optimal energy transfer from the rotor blades  15  to the mass that is being pumped, as will be explained in more detail below. 
         [0053]    The strip  185  may be given an orientation that is substantial parallel to the flow direction of the mass being pumped. This has the advantage that the radial inner ends  18  of the rotor blades and the strip  185  transfer no or relatively little energy to the mass being pumped, thereby reducing the wear of the radial inner ends  18 . Furthermore, possible wear of the radial inner ends  18  has only little effect on the characteristics of the centrifugal pump  1 . 
         [0054]    The direction of movement of the mass near the radial inner ends  18  may depend on the characteristics of the centrifugal pump  1  and on the operational parameters (revolutions per minute, type of mass to be pumped, etc.). Therefore, the direction of the strip  185  may be determined by the direction of movement of the mass when the centrifugal pump is operated at the best efficiency point (BEP), which is the flow at which the efficiency of the pump is highest. This parameter is known to the skilled person for a specific centrifugal pump. 
         [0055]    As mentioned above, the shape of the rotor blades  15  in between the radial inner end  18  and the radial outer end  17  may be designed to optimize the energy transfer from the rotor blades  15  to the mass being pumped. According to an advantageous embodiment depicted in  FIG. 7   a,  wherein at least part of the rotor blade adjacent the suction shield  12  comprises 
         [0056]    a first part  187  in which the radius (R 12 ) of the rotor blade  15  with respect to the axial rotation axis A increases at a first rate as a function of the angle φp, 
         [0057]    a second part  188  in which the radius (R 12 ) of the rotor blade  15  with respect to the axial rotation axis A increases at a second rate as a function of the angle φ, and 
         [0058]    a third part  189  in which the radius (R 12 ) of the rotor blade  15  with respect to the axial rotation axis A increases at a third rate as a function of the angle φ, wherein the second rate is greater than the first and third rate. 
         [0059]    Accordingly, also provided is a double bent rotor blade formed as such. 
         [0060]    R 12  refers to the radius near the suction shield  12 . 
         [0061]    Radius R 12  is a function of φ. Angle φ is indicated in  FIG. 7   a  and is defined in a plane substantially perpendicular with respect to the axial rotation axis A (perpendicular to the plane of drawing in  FIG. 7   a ). Radius R and angle φ together thus form polar coordinates, with respect to the axial rotation axis A. 
         [0062]    The derivate DR 12 /dφ&gt;0, while d 2 R 12 /d φ 2 &gt;0 at the transition between the first part  187  to the second part  188  and d 2 R 12 /d φ 2 &lt;0 at the transition between the second part  188  to the third part  189 .  FIG. 7   b  schematically depicts a graph of R 12  as a function of φ. 
         [0063]    The second rate for instance may at least locally be 1.5 times as high as the first and third rate. 
         [0064]    As can be seen in  FIG. 7   a,  the second part  188  faces the radial inner end  18  of the next rotor blade  15 , thereby providing an increased spherical passage Bol. 
         [0065]    The radius R increases continually as a function of φ, both indicated in  FIG. 7   a.  the increase is relatively low in the first part  187 . In use, the direction of the first part  187  of the rotor blade  15  is parallel to the direction of the flow. The first part  187  therefore transfers no or relatively little energy to the flow. 
         [0066]    In the second part  188 , the radius R increases relatively strong as a function of φ to provide a relatively large spherical passage Bol. 
         [0067]    It has to be noted that this shape is provided along the edge of the rotor blades  15  adjacent to the suction shield  12 . As the rotor blade  15  is double bent the part of the rotor blades  15  adjacent the shaft shield  11  may have a radius R 11  which increases as a function of φ, the amount of increase decreasing as a function of φ. In other words: dR 11 /dφ&gt;0, while d 2 R 11 /dφ 2 &lt;0. R 11  refers to the radius near the shaft shield  11   a.    
         [0068]    This embodiment may be combined with the embodiment of the non-curved strip  185  along the radial inner end  18 . According to such an embodiment, the rotor blades  15  comprise (in a direction from radial inner end to radial outer end) a non-curved strip  185 , a concave first part  188  and a convex second part  189 . 
         [0069]    The embodiments described with reference to  FIGS. 6 and 7   a - b  may also be combined with the other embodiments as described, such as the embodiments described with reference to  FIGS. 5   a - 5   b.    
         [0070]    According to a further embodiment the rotor blades  15  comprise a thickened strip  186  along the radial inner ends  18 , the thickened strips  186  being substantially thickened in a direction perpendicular to the surface of the rotor blades  15 . 
         [0071]      FIG. 8  schematically depicts a centrifugal pump with three rotor blades  15  each comprising a thickened strip  186  that extends along at least part of the radial inner ends  18 . The thickening may extend to both sides of the rotor blades  15 , i.e. on an inner side of the rotor blade  15  facing the rotation axis A and on an outer side of the rotor blade  15  facing away from the rotation axis A, as is shown in  FIG. 8 . 
         [0072]    Providing a thickening on the inside and/or outside of the rotor blade  15  has the advantage that the shape of the rotor blades  15  better match the flow lines of the mass being pumped. 
         [0073]    Flow separation mainly occurs on the outside of the rotor blade  15  and will most likely occur near the inner radial ends  18 . This has a negative effect on the suction capacity of the centrifugal pump. It also may result in cavitation and subsequent wear of the centrifugal pump. By providing a thickening on the outside of the rotor blade  15 , flow separation is reduced. Thus, according to an embodiment, the thickening may be provided on the outside of the rotor blade  15  (i.e. the side of the rotor blade  15  facing away from the axial rotation axis A), thereby preventing or at least reducing flow separation. 
         [0074]    By providing a thickening on the inside and/or on the outside of the rotor blade  15 , the rotor blade  15  is strengthened and comprises more material, so allowing the rotor blade  15  to withstand wear for a longer operation time.  FIGS. 9   a  and  9   b  show further embodiments of the centrifugal pump, wherein the suction shield  12  comprises an inner edge  121  defining the axial supply  14  of the rotor  7  having a suction diameter Dz (similar to  FIG. 2 ) and wherein the radial inner ends  18  of the rotor blades  15  connect to the suction shield  12  on a location of the suction shield  12  away from the inner edge  121  having a diameter that is larger than the suction diameter Dz. The inner edge  121  is formed as a nozzle arranged to receive the mass to be pumped. In between the inner edge  121  and the remainder of the suction shield  12  is a bend to which the radial inner ends  18  are connected. 
         [0075]    By providing rotor blades  15  that are pulled inward the flow area of the axial supply  14  is somewhat increased as it no longer comprises rotor blades. As a result, the local flow velocity will be reduced, reducing (the risk of) cavitation and the suction characteristics will be improved. Also, by providing rotor blades  15  which are connected to the suction shield  12  at a location away from the inner edge  121 , it is relatively easy to a right angle between the radial inner ends  18  and the suction shield  12 . 
         [0076]      FIG. 9   a  shows an embodiment with a similar suction shield  12  as depicted in the embodiments described above and in  FIGS. 1-3 .  FIG. 9   b  shows a suction shield  12  which is curved differently. In use, a medium enters the rotor  7 , the rotor  7  providing kinetic energy to the medium, which is later transferred into static pressure. The embodiments provided above provide improved suction characteristics, which mainly relate to the suction characteristics. 
         [0077]    The centrifugal pump according to the embodiments has improved characteristics, especially with regard to the efficiency, the suction characteristics and wear. The centrifugal pump may be used in all kind of situations, including situations with a relatively high or low hydrostatic inlet pressure. 
         [0078]    The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.