Patent Publication Number: US-2019170154-A1

Title: Assembly for reducing size of suspended solids

Description:
TECHNICAL FIELD 
     The present invention generally relates to an assembly for reducing the size of suspended solids upstream of a pump impeller. 
     BACKGROUND 
     Pumps are often required to transfer a fluid that contains suspended solids. Such suspended solids may include scale that can build up on pipes or within process vessels, where they may dislodge and become suspended in a fluid stream in a pump suction. The presence of such suspended solids in the suction line of a pump may be problematic in that they may become clogged in a pump impeller or volute and may reduce the net positive suction head and reduce the efficiency of pumping operations. In particular, oversized suspended solids may cause blockages and may damage parts of a pump such as the pump impeller. 
     In some applications, a strainer or a filter may be used to remove or reduce large suspended solids in a fluid stream upstream of a pump. Such strainers or filters may become blocked, leading to a drop-off in pump performance, such that the pump and associated piping and processes may need to be shut down and isolated so that the strainer or filter may be removed and cleaned. For fluid streams carrying a heavy burden of suspended solids, the strainers or filters may require frequent cleaning leading to severe disruption of the pump operation. 
     Accordingly, it would be desirable for a system to remove or reduce large suspended solids upstream of a pump in a manner that minimizes interference with the operation of the pump. 
     The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as, an acknowledgement or admission or any form of suggestion that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates. 
     BRIEF SUMMARY 
     The present invention seeks to provide an invention with improved features and properties. 
     In a first aspect the present invention provides an assembly for reducing the size of suspended solids in a pump intake including a rotatable element and a screen configured to locate in a position between a pump and the rotatable element. 
     In an embodiment, the rotatable element may be rotated in a forward direction about a rotation axis, the rotatable element including two arm members extending generally radially from a central hub of the rotating element in generally opposing radial directions, wherein each arm member includes opposed upstream and downstream sides, each arm member further including a leading side facing the forward direction and an opposed trailing side facing a rearward direction, wherein the leading side is configured with a curvature along a radial direction of the leading side such that the leading side curves inwardly towards the trailing side. 
     In an embodiment, the trailing side is configured with a curvature along a radial direction of the trailing side such that the trailing side curves outwardly from the leading side. 
     In a second aspect the present invention provides the rotatable element is rotatable in a forward direction about a rotation axis X-X, the rotatable element including two or more arm members extending generally radially from the rotation axis, each arm member including a leading side facing in the forward direction, the leading side having a peripheral edge distal from the rotation axis; and, a trailing side facing in a rearward direction, the trailing side having a peripheral edge distal from the rotation axis, wherein the leading side is configured with a curvature relative to the radial extension of the arm member such that the leading side is generally concave relative to a radial line Y-Y extending from the rotation axis X-X through the peripheral edge of the trailing side. 
     In an embodiment the trailing side is configured with a curvature relative to the radial extension of the arm member such that the trailing side is generally convex relative to a radial line Z-Z extending from the rotation axis X-X through the peripheral edge of the leading side. 
     In an embodiment, the curvature of the leading side is continuous from a region proximal to the central hub to a region distal from the central hub. 
     In an embodiment, the leading side includes a chamfered surface proximal to the downstream side and a forward surface proximal to the upstream side, wherein the chamfered surface is configured to incline towards the trailing side. 
     In an embodiment, the chamfered surface is configured to incline towards the trailing side at an angle of between about 40° and about 60°. 
     In an embodiment, the forward surface is substantially normal to the forward direction. 
     In an embodiment, the trailing side includes a rounded surface proximal to the upstream side and a rearward surface proximal to the downstream side. 
     In an embodiment the rearward surface is substantially normal to the forward direction. 
     In an embodiment the rounded surface is configured with a curvature smoothly transitioning between the rearward surface and the upstream surface such that the rounded surface encompasses an edge of the trailing side proximal to the upstream side. 
     In an embodiment the rounded surface is configured with a curvature that is generally shaped as a circular arc. 
     In an embodiment the rounded surface comprises approximately one third of the thickness of the trailing side as measured between the upstream side and the downstream side. 
     In an embodiment the screen is configured with apertures and wherein the apertures are size to resist the passage therethrough of suspended solids above a maximum size. 
     In an embodiment the screen is configured with a shroud attached to a perimeter of the screen, and wherein the shroud is adapted to seat within an inner dimeter of a pump intake. 
     In an embodiment the screen comprises a plurality of ribs interconnected to form a network thereby defining a plurality of apertures, wherein the network includes a plurality of junctions formed by at least two ribs meeting at an obtuse angle. 
     In an embodiment, the network comprises interconnected axial arrays of ribs, the network extending between an internal support and an external support, wherein the plurality of ribs are arranged into: a first axial array of ribs connected to and arranged around an external perimeter of the internal support; and, a final axial array of ribs connected to and arranged around an internal perimeter of the external support. 
     In an embodiment, the arrangement of the plurality of ribs further includes one or more further axial arrays or ribs arranged successively at an increasing radial distance from the first axial array, 
     In an embodiment, the internal support is configured in the shape of a ring defining a central aperture therethrough, wherein the central aperture is configured to receive a rotating shaft. 
     In an embodiment, the internal support is configured in the shape of a star defining a central aperture therethrough, wherein the central aperture is configured to receive a rotating shaft. 
     In an embodiment, the profile of the ribs have a region of maximum thickness between an upstream periphery and a downstream periphery of the ribs. 
     In an embodiment, the thickness of the ribs taper from the region of maximum thickness to the upstream periphery. 
     In an embodiment, the thickness of the ribs taper from the region of maximum thickness to the downstream periphery. 
     In an embodiment, the region of maximum thickness occurs at approximately 15% to 25% of the distance between the upstream periphery and the downstream periphery. 
     In an embodiment, the region of maximum thickness occurs at approximately 20% of the distance between the upstream periphery and the downstream periphery. 
     According to a third aspect, the present invention provides an apparatus for reducing the size of suspended solids upstream of a pump impeller wherein the apparatus may be rotated in a forward direction about a rotation axis, the apparatus including two arm members extending generally radially from a central hub of the apparatus in generally opposite radial directions, wherein each arm member includes opposed upstream and downstream sides, each arm member further including a leading side facing the forward direction and an opposed trailing side facing a rearward direction, wherein the leading side is configured with a curvature along a radial direction of the leading side such that the leading side curves inwardly towards the trailing side. 
     In an embodiment the trailing side is configured with a curvature along a radial direction of the trailing side such that the trailing side curves outwardly from the leading side. 
     According to a fourth aspect the present invention provides an apparatus for reducing the size of suspended solids upstream of a pump impeller which can be rotated in a forward direction about a rotation axis X-X, the apparatus including two or more arm members extending generally radially from the rotation axis, each arm member including a leading side facing in the forward direction, the leading side having a peripheral edge distal from the rotation axis; and, a trailing side facing in a rearward direction, the trailing side having a peripheral edge distal from the rotation axis, wherein the leading side is configured with a curvature relative to the radial extension of the arm member such that the leading side is generally concave relative to a radial line Y-Y extending from the rotation axis X-X through the peripheral edge of the trailing side. 
     In an embodiment the trailing side is configured with a curvature relative to the radial extension of the arm member such that the trailing side is generally convex relative to a radial line Z-Z extending from the rotation axis X-X through the peripheral edge of the leading side. 
     In an embodiment the curvature of the leading side is continuous from a region proximal to the central hub to a region distal from the central hub. 
     In an embodiment the leading side includes a chamfered surface proximal to the downstream side and a forward surface proximal to the upstream side, wherein the chamfered surface is configured to incline towards the trailing side. 
     In an embodiment the chamfered surface is configured to incline towards the trailing side at an angle of between about 40° and about 60°. 
     In an embodiment the forward surface is substantially normal to the forward direction. 
     In an embodiment the trailing side includes a rounded surface proximal to the upstream side and a rearward surface proximal to the downstream side. 
     In an embodiment the rearward surface is substantially normal to the forward direction. 
     In an embodiment the rounded surface is configured with a curvature smoothly transitioning between the rearward surface and the upstream surface such that the rounded surface encompasses an edge of the trailing side proximal to the upstream side. 
     In an embodiment the rounded surface is configured with a curvature that is generally shaped as a circular arc. 
     In an embodiment the rounded surface comprises approximately one third of the thickness of the trailing side as measured between the upstream side and the downstream side. 
     According to a fifth aspect the present invention provides an assembly for reducing the size of suspended incident on a pump impeller, including the apparatus according to any one of the preceding aspects; and, a screen located between the apparatus and the pump impeller, wherein the screen is configured with apertures and wherein the apertures are size to resist the passage therethrough of suspended solids above a maximum size. 
     In an embodiment the screen is configured with a shroud attached to a perimeter of the screen, and wherein the shroud is adapted to seat within an inner dimeter of a pump intake. 
     In a sixth aspect the present invention provides a screen for a pump inlet comprising a plurality of ribs interconnected to form a network thereby defining a plurality of apertures, wherein the network includes a plurality of junctions formed by at least two ribs meeting at an obtuse angle. 
     In an embodiment the network comprises interconnected axial arrays of ribs, the network extending between an internal support and an external support, wherein the plurality of ribs are arranged into: a first axial array of ribs connected to and arranged around an external perimeter of the internal support; and, a final axial array of ribs connected to and arranged around an internal perimeter of the external support. 
     In an embodiment the arrangement of the plurality of ribs further includes one or more further axial arrays or ribs arranged successively at an increasing radial distance from the first axial array. 
     In an embodiment the internal support is configured in the shape of a ring defining a central aperture therethrough, wherein the central aperture is configured to receive a rotating shaft. 
     In an embodiment the internal support is configured in the shape of a star defining a central aperture therethrough, wherein the central aperture is configured to receive a rotating shaft. 
     In an embodiment the profile of the ribs have a region of maximum thickness between an upstream periphery and a downstream periphery of the ribs. 
     In an embodiment the thickness of the ribs taper from the region of maximum thickness to the upstream periphery. 
     In an embodiment the thickness of the ribs taper from the region of maximum thickness to the downstream periphery. 
     In an embodiment the region of maximum thickness occurs at approximately 15% to 25% of the distance between the upstream periphery and the downstream periphery. 
     In an embodiment the region of maximum thickness occurs at approximately 20% of the distance between the upstream periphery and the downstream periphery. 
     In a seventh aspect the present invention provides a kit comprising the rotatable element of an above aspect and the screen according to an above aspect. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
       Example embodiments should become apparent from the following description, which is given by way of example only, of at least one preferred but non-limiting embodiment, described in connection with the accompanying figures. 
         FIG. 1  illustrates a cutaway view of a pump fitted with an embodiment of an assembly of the present invention; 
         FIG. 2  illustrates a top view of an embodiment of a rotatable element; 
         FIG. 3  illustrates a perspective view of the rotatable element of  FIG. 2 ; 
         FIG. 4  illustrates an alternative perspective view of the rotatable element of  FIG. 1 ; 
         FIG. 5  illustrates a side view of the rotatable element of  FIG. 1 ; 
         FIG. 6  illustrates a screen as known to the prior art; 
         FIG. 7  illustrates a front view of an embodiment of a screen according to the present invention; 
         FIG. 8 a    illustrates the embodiment of  FIG. 7  with highlighted areas and a section line. 
         FIG. 8 b    illustrates a view of a highlighted area of  FIG. 8   a;    
         FIG. 8 c    illustrates a view of a highlighted area of  FIG. 8   a;    
         FIG. 9  illustrates a cut-away view along the section line of  FIG. 7 ; 
         FIG. 10  illustrates a front perspective view of the screen of  FIG. 7 ; 
         FIG. 11  illustrates a rear perspective view of the screen of  FIG. 7 ; 
         FIG. 12  illustrates a side view of the screen of  FIG. 7 ; 
         FIG. 13  illustrates a front view of an embodiment of a screen according to the present invention with a section line; 
         FIG. 14  illustrates a cut-away view along the section line of  FIG. 13 ; 
         FIG. 15  illustrates a front perspective view of the screen of  FIG. 13 ; 
         FIG. 16  illustrates a rear perspective view of the screen of  FIG. 13 ; 
         FIG. 17  illustrates a side view of the screen of  FIG. 13 ; 
         FIG. 18  illustrates a view of an embodiment of a network of a screen according to the present invention; 
         FIG. 19  illustrates a schematic view of the open area of the screen of  FIG. 1 ; 
         FIG. 20  illustrates a schematic view of the open area of the screen of  FIG. 13 ; 
         FIG. 21  illustrates a view of a cross-sectioned junction of a screen of the prior art; 
         FIG. 22  illustrates a T-shaped junction of a screen of the prior art; 
         FIG. 23  illustrates a view of a junction according to an embodiment of the present invention; 
         FIG. 24  illustrates a schematic cut-away view of a screen according to the present invention in profile; 
         FIG. 25  illustrates a schematic cut-away view of a screen and a rotatable element according to the present invention in profile; 
         FIG. 26  illustrates a cross sectional view of an embodiment of a rib according to the present invention; 
         FIG. 27  illustrates a front view of an embodiment of an assembly according to the present invention with a section line; 
         FIG. 28  illustrates a cut-away view along the section line of  FIG. 27 ; 
         FIG. 29  illustrates a front perspective view of the assembly of  FIG. 27 ; 
         FIG. 30  illustrates a rear perspective view of the assembly of  FIG. 27 ; 
         FIG. 31  illustrates a rear view of the assembly of  FIG. 27 ; 
         FIG. 32  illustrates a side view of the assembly of  FIG. 27 . 
     
    
    
     PREFERRED EMBODIMENTS 
     The following modes, given by way of example only, are described in order to provide a more precise understanding of the subject matter of a preferred embodiment or embodiments. 
     In the Figures, incorporated to illustrate features of an example embodiment, like reference numerals are used to identify like parts throughout the Figures. 
     Referring to the  FIG. 1 , shown is an assembly  22  for reducing the size of suspended solids  41  upstream of a pump impeller  3 . The assembly  22  may be locatable in a pump  2  intake. The assembly  22  may include a rotatable element  1  configured to locate in an upstream position relative to a screen  7 . By this arrangement, the rotatable element  1  may rotate in order to break up suspended solids  41  such as scale into smaller pieces before flowing to the pump impeller  3 . Also by this arrangement, the screen  7  may act to prevent the throughflow of suspended solids  41  above a maximum size. Further, the rotatable element  1  may be positioned in close proximity to the screen  7  such that suspended solids above a maximum size caught on the surface of the screen  7  may be ground down into a smaller size by attrition due to contact with the rotating rotatable element  1 . 
       FIG. 1  shows a section view of a pump  2  fitted with the assembly  22 . The assembly  22  is located in the pump  2  intake upstream of the pump impeller  3 . The rotatable element  1  may be mounted to a shaft  4  that is appended to a main shaft  5  driving the pump impeller  3 . By this arrangement the rotating element  1  may be rotated at the same speed as the main shaft  5  and pump impeller  3 . The rotatable element  1  may be located upstream of a screen  7  configured to prevent the through flow of suspended solids  41  above a maximum size. Otherwise stated, a screen  7  may be located between the rotatable element  1  and the pump impeller  3 . The location of the rotatable element  1  may be offset from the location of the screen  7  by a relatively small distance such that suspended solids  41  above a maximum size may only build-up on the surface of the screen by a relatively small amount before they contact with the rotatable element  1 . By this arrangement, the rotatable element  1  may grind down suspended solids  41  built-up on the surface of the screen  7 , thereby minimizing blockages of the screen  7  by oversized solids  41  that were not sufficiently reduced in size when flowing past the rotatable element  1 . 
     Referring to  FIGS. 2 to 5 , shown is a rotatable element  1  for reducing the size of suspended solids  41  upstream of a pump impeller  3 . The rotatable element  1  may be configured to be rotated in a forward direction about a rotation axis. The rotation axis may be shared by the pump impeller  3 . The rotatable element  1  may include two arm members  10  extending generally radially from the axis of rotation. By this arrangement, the rotating arm members  10  may come in contact with solids suspended in a fluid flowing toward the pump impeller  3 , and may thereby break the suspended solids  41  into a smaller size before the solids reach the pump impeller  3 . 
     Referring to  FIG. 2 , shown is an embodiment of a rotatable element  1  for reducing the size of suspended solids upstream of a pump impeller  3 . The rotatable element  1  includes two arm members  10  that extend from a central hub  11  in a generally radial direction. The central hub  11  may be in engagement to a rotatable shaft  4 , thereby defining an axis of rotation. The direction by which the arm members  10  travel about the rotation axis may be termed the forward direction. The arms members  10  may be generally elongate in the radial direction and may join with the central hub  11  at opposing radial directions. In some embodiments, the rotatable element  1  may include three or more arm members  10 , with each arm member  10  extending in a generally radial direction from the central hub  11  and wherein each arm member  10  is joined to the central hub  11  at equal or approximately equal intervals about the central hub  11 . 
     The arm members  10  may have opposed leading sides  14  and trailing sides  15 . The leading side  14  is the side of the arm member  10  facing in the forward direction of rotation and the trailing side  15  is side of the arm member  10  facing in a rearward direction that is opposite to the forward direction. The leading side  14  and the trailing side  15  may be offset from each other by a distance such that the arm member  10  is configured with a thickness in the direction of rotation. The thickness between the leading side  14  and the trailing side  15  may taper away from the central hub  11 . 
     The arm members  10  may have opposed upstream sides  12  and downstream sides  13 . The upstream side  12  is that facing the upstream direction of a pump suction intake when the rotatable element  1  is in use. The downstream  13  side is that nearest to the pump impeller  3  when the rotatable element  1  is in use, according to the usual conventions of describing the direction of fluid flowing in a pipe. 
     Although the arm members  10  extend generally radially from the central hub  11  and hence the axis of rotation, the arms are configured with a curvature such that they are swept towards the forward direction.  FIG. 1  shows a top view of the rotatable element  1  from an upstream position, looking towards a downstream position where a pump impeller  3  may be located in normal use of the rotatable element  1 ; in this view, the arm member  10  may be considered to take the general form of a cubic curve, with an inflection in the region of the central hub  11 /rotation axis, and with each of the arm members  10  curving toward the forward direction of rotation. As depicted in  FIG. 1 , both the leading side  14  and the trailing side  15  of the arm members  10  curve forwardly, such that both the leading side  14  and the trailing side  15  curve forwardly in the forward direction of rotation. 
     Otherwise stated, the leading side  14  may be configured with a curvature along the radial extension of the arm member  10  such that the leading side  14  curves inwardly towards the trailing side  15 . The trailing side  15  may also be configured with a curvature along the radial extension of the arm members  10  such that the trailing side  15  curves outwardly from the leading side  14 . The forward sweep/curvature of the arm members  10  is such that portion of the arm members  10  distal from the central hub  11  may be forwardly positioned compared to the portion of the arm members  10  proximal to the central hub  11 , relative to the forward direction. 
     As shown in  FIG. 2 , the curvature of the leading side  14  may be configured such that the leading side  14  is generally concave relative to a radial line Y-Y extending from the rotation axis X-X through a peripheral edge  21  of the trailing side  15 . The curvature of the trailing side  15  may be configured such that the trailing side  15  is generally convex relative to a radial line Z-Z extending from the rotation axis X-X through a peripheral edge  20  of the leading side  14 . 
     By this arrangement, portions of the leading side  14  distal from the central hub  11  may be in advance of portions of the leading side  14  proximal to the central hub  11  relative to the forward direction, such that solids flowing past the rotatable element  1 , and solids coming into contact with the leading side  14  of the rotating arm members  10 , may be encouraged toward the rotation axis. In comparison, if the arm members  10  did not display such curvature of the leading side  14 , such as substantially straight arm members or backwardly curving arm memebers, the centrifugal force generated by the rotation of the arm members  10  may encourage solids  41  to agitate away from the rotation axis. Accordingly, the forward curvature of the arm members  10  may act to counteract the centrifugal force generated by the rotating arm members  10  such that solids  41  may be encouraged to agitate toward the rotation axis rather than being encouraged to agitate distally from the rotation axis. 
     In some embodiments, such as that shown in  FIG. 1 , the forward curvature of the leading side  14  along the radial direction may be continuous, such that leading side  14  curves smoothly and continuously from the central hub  11  to the peripheral edge  20  of the leading side  14  and such that the peripheral edge  20  of the leading side  14  is in advance of the region of the leading side  14  proximal to the central hub  11  relative to the forward direction of rotation. The curvature of the trailing side  15  may also be continuous from the junction region of the arm member  10  and central hub  11  to the peripheral edge  21  of the trailing side  15 . 
     Referring now to  FIGS. 3 and 4 , shown are perspective views of the rotatable element  1  including showing the leading side  14  and the trailing side  15 . The leading side  14  may be configured with a forward surface  17  and a chamfered surface  16 . The forward surface  17  and chamfered surface  16  may both substantially extend along the entire span of the leading side  14  and the forward surface  17  and chamfered surface  16  may be adjacent to each other and may further be generally parallel to each other. The forward surface  17  may be proximal to the upstream side  12  and the chamfered surface  16  may be proximal to the downstream side  13 . Transitions/edges between the upstream side  12  and the forward surface  17 , the forward surface  17  and the chamfered surface  16 , and the chamfered surface  16  and the down stream side  13 , as well as other transitions/edges between surfaces/sides, may be smooth and rounded to some degree as depicted, or may alternatively be sharp. 
     The forward surface  17  is so called as it is in advance of the chamfered surface  16  relative to the forward direction. The forward surface  17  may be normal to the forward direction or substantially normal to the forward direction. The forward surface  17  may also be at right angles or approximately at right angles to the upstream  12  and/or downstream sides  13 . The forward surface  17  may be orientated parallel or substantially parallel to the axis of rotation. By this arrangement, the forward surface  17  may pose a blunt surface relative to the flow of suspended solids towards a pump impeller  3 , which may facilitate effective impact between the forward surface  17  and the suspended solids  41  flowing past for reducing the size of the suspended solids  41 . 
     The chamfered surface  16  is so called as it is configured in the form of an angled or chamfered edge between the forward surface  17  and the downstream side  13 . Otherwise stated, the chamfered surface  16  is configured to recede towards the downstream side  13 , such that the chamfered surface  16  is inclined away from the forward surface  17  towards the downstream side  13 . The angle at which the chamfered surface  16  recedes from the forward surface  17  may be between about 40° to about 60°. The angle at which the chamfered surface  16  recedes from the forward surface  17  may be about 45° to about 55° or about 50°. Otherwise stated, the chamfered surface  16  may be configured with an angle with respect to the axis of rotation of between about 40° to about 60°, between about 45° to about 55°, or about 50°. 
     The upstream side  12  and the downstream side  13  may be offset from each other such that the arm members  10  have a thickness in the direction of the rotation axis. Otherwise stated, the arm members  10  have a thickness in the superficial direction of fluid flow towards the pump impeller  3  when the rotatable element  1  is in use. Accordingly, the leading side  14  and trailing side  15  also have a thickness in this direction. In the embodiments of the Figures, the thickness of the forward surface  17  measured between the upstream side  12  and the downstream side  13  is about one third of the thickness of the leading side  14 . Similarly, the thickness of the chamfered surface  16  may be about two thirds of the distance between the upstream side  12  and the downstream side  13 . By this arrangement, the leading side  14  may have a relatively stream-lined profile in comparison with the trailing side  15 , with a prominent forward surface  17  and a receding chamfered surface  16 . This streamlined arrangement may improve the breaking action of the forward surface  17  on suspended solids during rotation of the arm members  10 . Furthermore, the receding angle of the chamfered surface  16  may enhance the flow of fluid containing suspended solids  41  towards the impeller  3  such that flow is induced towards the pump impeller  3  thereby potentially lowering the required net positive suction head of the pump  2 . Although the depicted embodiment has a forward surface  17  thickness of about one third of the leading side  14  and a chamfered surface  16  thickness of about two thirds of the leading side  14 , other thicknesses of the forward surface  17  and the chamfered surface  16  are possible. For example, the forward surface  17  may be configured with a thickness of up to one half of the thickness of the leading side  14  or greater than one half of the thickness of the leading side  14 , with the remainder of the thickness of the leading side  14  being substantially occupied by the chamfered surface  16 . 
     The trailing side  15  may be configured with a rearward surface  18  and a rounded surface  19 . The rearward surface  18  and the rounded surface  19  may both substantially extend along the entire span of the trailing side  15  and the rearward surface  18  and the rounded surface  19  may be adjacent to each other and may further be generally parallel to each other along the radial direction of the arm member  10 . The rounded surface  19  may be proximal to the upstream side  12  and the rearward surface  18  may be proximal to the downstream side  13 . 
     The rearward surface  18  is so called as it is generally positioned behind the rounded surface  19  relative to the forward direction of rotation. The rearward surface  18  may be normal to the forward direction of rotation or substantially normal to the forward direction of rotation. The rearward surface  18  may also be at right angles or approximately at right angles to the upstream and/or downstream sides  13 . The rearward surface  18  may be orientated parallel or substantially parallel to the axis of rotation. The rearward surface  18  may be in parallel or substantially parallel orientation with respect to the forward surface  17 . 
     The rounded surface  19  is so called as it is configured with a curvature that transitions from an edge of the rearward surface  18  to an edge of the upstream side  12 . Otherwise stated an edge of the trailing side  15  adjacent to the upstream side  12  may be rounded to form the rounded surface  19 . In the embodiment of the figures, the thickness of the rounded surface  19  measured between the upstream surface and the downstream surface may be about one third of the thickness of the trailing side  15 . Similarly, the thickness of the rearward surface  18  may be about two thirds of the distance of between the upstream side  12  and the downstream side  13 . The rounded surface  19  may be configured with a curvature in the form of a circular arc. In some embodiments, the radius of curvature of the rounded surface  19  may be about 5 mm, though other embodiments are equally permissible and may depend on the size of the rotating element  1 . The curvature of the rounded surface  19  may induce a low pressure zone as the arms of the rotatable element  1  are rotated, such that a pressure differential is induced to encourage flow of fluid towards the pump impeller  3 . By this arrangement, the rounded surface  19  may enhance the flow of fluid towards a pump intake and lower the required net positive suction head of the pump  2 . 
     Although the embodiment of the Figures has a rounded surface  19  thickness of about one third of the overall thickness of the trailing side  15  as measured between the upstream surface  12  and downstream surface  13 , other relative thicknesses of the rounded surface  19  are possible. For example, the rounded surface  19  may be configured with a thickness of up to about one half of the thickness of the trailing side  15 , or greater than one half of the thickness of the trailing side  15 , with the remainder of the thickness of the trailing side  15  being substantially occupied by the rearward surface  18 . 
     The central hub  11  may be configured with an internal thread configured for attachment with a corresponding thread on a shaft  4 . The internal thread may expend through the central hub  11  such that the central hub  11  is in a socket style configuration. By this arrangement, when the rotatable element  1  is threaded onto the shaft  4 , a portion of the thread of the shaft  4  may emerge from the central hub  11 , thereby allowing a locking nut  6  to be threaded onto the shaft  4  to maintain the rotatable element  1  in place on the shaft  4  at the rotatable element  1  rotates. In some embodiments, the locking  6  nut may be integral with the rotatable element  1 . 
     Referring now to  FIG. 5 , shown is a side view of the rotatable element  1 . The view show an end side of the arm members  10  distal from the rotation axis showing the profile of the leading side  14  including the forward surface  17  and the chamfered surface  16 , as well as the trailing side  15  including the rearward surface  18  and the rounded surface  19 . This view also shows the profile of the upstream and downstream sides  13 . The profile of the end side is similar in relative proportions to a cross section of the arm member  10  taken along the forward direction of rotation, noting that the distance/thickness between the leading surface and the trailing surface decreases distally in the radial direction of the arm member  10  in the depicted embodiment, as is visible in  FIG. 1 . 
     The rotatable element  1  herein described may cause the reduction of size of suspended solids upstream of a pump impeller  3  with several surprising benefits. The forward curvature of the arm members  10  may increase the strength and wear performance compared to arm members  10  that radially extend without curvature. Furthermore, the forwardly curved blades may encourage agitation of suspended solids towards the center of a screen  7  located downstream of the rotatable element  1  which may result in lower torque requirements for the arm members  10  and reduced power consumption. The relatively streamlined forward surface  17  may improve the breaking action on suspended solids during rotation, whilst the chamfered surface  16  and the rounded surface  19  may enhance fluid flow past the rotatable element  1  and may reduce the required net positive suction head of the pump  2 . 
     Described herein with reference to the  FIGS. 6 to 32 , shown are embodiments of a screen  7  adapted for placement in a pump  2  inlet upstream of a pump impeller  3 . The screen may be configured as part of an assembly  22  for reducing the size of suspended solids upstream of a pump impeller  3 , which may include the rotating element  1  The screen  7  is configured to reduce the through passage of solids above a certain size, thereby reducing the incidence of solids above a certain size flowing into a pump. The screen  7  is formed of a plurality of ribs  23 , which are in the form of relatively short and substantially straight members. The ribs  23  are interconnected to form a network  27  with spaces between the ribs  23  of the network  27  thereby defining a plurality of apertures  34  through the screen  7 . The sizes of the apertures  34  are adapted to prevent or reduce the through passage of solids  41  above a certain size which may otherwise flow to the pump impeller  3 . Each rib  23  is connected with at least one other rib  23  in the network  27  and the region of connection between any two or more ribs  23  is termed a junction  26 . The angle formed between at least two of the ribs  23  meeting at any junction  26  may be an obtuse angle. The screen  7  may include an axial shroud  8  extending from a perimeter of the screen  7 . The shroud  8  may extend from a perimeter of the screen in an upstream direction. The shroud  8  may be configured to reside within an internal diameter of a pump intake such that the screen  7  will be positioned substantially normal to the superficial direction of flow in the pump intake. 
     Referring to  FIG. 6 , shown is an example embodiment of a prior art screen  7  where the angle formed between any two ribs  23  meeting at a junction is a right angle or substantially a right angle. These junctions  26  may be termed “T” junctions  26  or “cross” junctions  26  to reflect the shape of these junctions  26  owing to the 90° angles, or substantially 90° angles formed between the ribs  23 . 
     Referring now to  FIG. 7 , shown is an embodiment of a screen  7  according to the present invention. The network  27  comprised of the interconnected plurality of ribs  23  extends from an internal support  28  to an external support  29 . The external support  29  may be configured as a ring or a sheaf, such as a shroud  8 , adapted to fit coaxially with the inner diameter of the pump inlet. The internal support  28  may be configured as a ring with a central aperture  33  therethrough. 
     As shown in  FIG. 7 , the network  27  of interconnecting ribs  23  may be organized in arrays arranged axially/circumferentially around the internal support  28 . A first axial/circumferential array  30  may comprise a plurality of ribs  23  depending radially outward from the central aperture  33 . A final axial/circumferential array  32  may comprise a plurality of ribs  23  depending radially inward from the external support  29 . A further axial/circumferential array termed the second axial/circumferential array  31  may arranged between the first axial/circumferential array  30  and the final axial/circumferential array  32 . The ribs  23  comprising the second array may connect at one end with ribs  23  from the first axial array  30  and at their other end with ribs  23  from the final axial/circumferential array  32 , thereby interconnecting the axial/circumferential arrays. In other embodiments, additional further arrays may locate between the first axial/circumferential array  30  and the final axial/circumferential array  32 , with each further axial/circumferential array being staged at a radial distance from the preceding axial/circumferential array and connected to the preceding axial/circumferential array and the succeeding axial array. 
     Otherwise stated, the first axial array  30  includes a plurality of ribs  23 , each having a first end  24  and a second end  25 . The first end  24  of each rib  23  of the first axial array  30  joins with the internal support  28 , such that each rib  23  of the first axial array  30  depends outwardly from the internal support  28 . The second end  25  of each rib  23  of the first axial array  30  joins with the first end  24  of at least one rib  23  of the second axial array  31  at a junction  26 . In the embodiment of  FIG. 8 a   , the second end  25  of each rib  23  of the first axial array  30  joins with the first ends  24  of two ribs  23  of the second axial array  31 , thus forming a Y shaped junction  26 , wherein the angle formed between the rib  23  of the first axial array  30  and either rib  23  of the second axial array  31  is obtuse. 
     The final axial array  32  includes a plurality of ribs  23 , each having a first end  24  and a second end  25 . The second end  25  of each rib  23  of the final axial array  32  joins with the external support  29 , such that each rib  23  of the final axial array  32  depends inwardly from the external support  29 . The first end  24  of each rib  23  of the final axial array  32  joins with the second end  25  of at least one rib  23  of the second axial array  31 . In the embodiment of  FIG. 8 a   , the first end  24  of each rib  23  of the final axial array  3232  joins with the second ends  4  of two ribs  23  of the second axial array  31 , thus forming a Y shaped junction  26 , wherein the angle formed between the rib  23  of the final axial array  32  and either rib  23  of the second axial array  31  is obtuse. 
     Referring now to  FIG. 8 b   , shown is an exploded view of a junction  26  between a rib  23  of the first axial array  30  connected to two ribs  23  of the second axial array  31 . The junction  26  is formed at a second end  25  of a rib  23  of the first axial array  30  and a first end  24  of two ribs  23  of the second axial array  31 . In this junction  26 , the three ribs  23  connect in a Y shape, with an obtuse angle being formed between any two ribs  23  in the junction  26 . In particular, the angle (α) formed between the rib  23  of the first array and either of the ribs  23  of the second array  31  is obtuse, rather than at 90° as is typical for a prior art screen  7  such as that of  FIG. 1 . 
     Referring now to  FIG. 8 c   , shown is an exploded view of a junction  26  between two ribs  23  of the second axial array  31  and a rib  23  of the final axial array  32 . The junction  26  is formed at the second ends  4  of the ribs  23  of the second axial array  31  and a first end  24  of the rib  23  of the final axial array  32 . In this junction  26 , the three ribs  23  also connect in a Y shape, with an obtuse angle (α) being formed between either of the ribs  23  of the second array and the rib  23  of the final array. 
     Referring to  FIG. 9 , shown is a cut-away view along section A-A of  FIG. 8 a    demonstrating an example cross section of the ribs  23  as well as the axial shroud  8 , which may taper in thickness towards the screen  7 . Shown in  FIGS. 10 and 11  are perspective views of the screen  7  from an upstream position and a down stream position respectively. Shown in  FIG. 12  is a side view of the screen  7  showing that the network  27  of ribs  23  are contained within the internal diameter of the circumferential shroud  8 . 
     With reference to  FIG. 13 , shown is an alternative embodiment wherein the internal support  28  is configured in the shape of a star with a central aperture  33  similarly in the shape of the star/regular polygon. Specifically, the depicted embodiment is in the shape of a symmetrical eight point  35  star. As depicted, the star showed central aperture may have rounded points  35 . 
     The first axial array  30  of  FIG. 13  includes a plurality of ribs  23 , each having a first end  24  and a second end  25 . The first end  24  of each rib  23  of the first axial array  30  joins with the internal support  28 , such that each rib  23  of the first axial array  30  depends outwardly from the internal support  28 . In the depicted embodiment, two ribs  23  of the first axial array  30  join with the internal support  28  at the region of each point  35  of the star shaped internal support  28 . The second end  25  of each rib  23  of the first axial array  30  joins with the first end  24  of at least one rib  23  of the second axial array  31  at a junction  26 . In the embodiment of  FIG. 13 , the second end  25  of each rib  23  of the first axial array  30  joins with the first ends  24  of two ribs  23  of the second axial array  31 , thus forming a Y shaped junction  26 , wherein the angle formed between the rib  23  of the first axial array  30  and either rib  23  of the second axial array  31  is obtuse. 
     The final axial array  32  includes a plurality of ribs  23 , each having a first end  24  and a second end  25 . The second end  25  of each rib  23  of the final axial array  32  join with the external support  29 , such that each rib  23  of the final axial array  32  depends inwardly from the external support  29 . Specifically, each rib  23  of the final axial array  32  depends inwardly from the external support  29  in a generally radial direction. The first end  24  of each rib  23  of the final axial array  32  joins with the second end  25  of at least one rib  23  of the second axial array  31 . In the embodiment of  FIG. 13 , the first end  24  of each rib  23  of the final axial array  32  joins with the second ends  25  of two ribs  23  of the second axial array  31 , thus forming a Y shaped junction  26 , wherein the angle formed between the rib  23  of the final axial array  32  and either rib  23  of the second axial array  31  is obtuse. 
     The central aperture  33  through the internal support  28  of the embodiments of  FIG. 7  may define a passage for receiving a rotating shaft  4 . The rotating shaft  4  may attach with a rotatable element adapted to rotate in advance of the screen  7  to reduce the size of solids in the pump inlet as hereinbefore described. In the alternative embodiment of  FIG. 13 , the internal support  28  is configured in the shape of a star with a central aperture  33  defining a passage for receiving a rotating shaft  4 . By this arrangement, a gap  36  may exist between the cylindrical shaft  4 and the space of the central aperture  33  formed by the points  35  of the star shaped internal support  28 . The gap  36  between the rotating shaft  4  and the space of the central aperture  33  formed by the points  35  of the internal support  28  may act as apertures  34  preventing the through passage of solids  41  over a certain size, thus adding to the open area of the screen  7 . The star shaped internal support  28  allows for an increased flow area over the internal support  28  of the prior art screen  7  of  FIG. 6  while still controlling the size of particles flowing into the pump. The area within the central aperture  33  of  FIG. 13  may be 1714 mm 2  compared with an area of 844 mm 2  within the central aperture  33  of the embodiment of  FIG. 6 , which represents a 94% improvement. 
     The network  27  arrangement of  FIG. 13  leads to a different pattern of apertures  34  to the network  27  pattern of  FIG. 7 . The apertures  34  arranged adjacently around the internal support  28  of  FIG. 13  are in the form of alternating hexagons and irregular diamonds. If further axial arrays were provided between the second axial array  31  and the final axial array  32 , this pattern of apertures  34  may be repeated at a radial displaced intervals from the apertures  34  adjacent to the internal support  28 . 
     Referring to  FIG. 14  shown is a cut-away view along section A-A of  FIG. 13  demonstrating an example cross section of the ribs as well as the axial shroud  8 . Shown in  FIGS. 15 and 16  are perspective views of the embodiment of  FIG. 18  from an upstream and down stream position respectively, whereas  FIG. 17  shows a side view. 
     Referring now to  FIG. 18 , shown is a network  27  similar to that of  FIG. 13  but disembodied from the external support  29 .  FIG. 18  provides an indication of the position of the rotating shaft  4  through the central aperture  33  of the internal support  28 , showing the gaps  36  between the rotating shaft  4  and the indents of the central aperture  33 . 
     The embodiments of  FIGS. 13 and 18  may provide a screen  7  with a greater percentage of open area compared to the screen  7  of embodiments depicted in  FIG. 7 . Similarly, the screen  7  of  FIGS. 13 and 18 , as well as the screen  7  of  FIG. 7  may provide a greater percentage of open area compared with the prior art screen  7  of  FIG. 6 . The provision of a screen  7  with a higher open area may reduce the pressure loss of a fluid flowing through the screen  7 , thus reducing the NPSH required by a pump with the screen  7  installed in the pump inlet.  FIGS. 19 and 20  compare the open area of the screens  1  of  FIGS. 7 and 13  respectively, with the shaded portions representing the open areas e.g. the apertures  34 . The increase in open area may be achieved while still providing the same or similar sized apertures  34 , thus increasing flow through the screen  7  whilst still protecting the pump  2  from over-sized solids  41 . Providing this increase to the open area may allow thicker ribs  23  to be used which may increase the strength and robustness of the screen  7 . The increase in open area may improve the NPSH of the pump. Furthermore, the larger apertures  34  toward the radial periphery of the screen  7  and the associated increase in flow therethrough may serve to improve the flushing of solids  41  through the screen which may reduce the buildup of solids  41  on the screen  7 . 
     The embodiments of screens  7  herein described include a plurality of junctions  26  between ribs  23 , where at least two of the ribs  23  involved in each of the junction  26  meet at an obtuse angle. In the depicted embodiments, any rib  23  joining with a rib  23  belonging to an axial array further displaced from the internal support  28  in the radial direction will meet at an obtuse angle. For example, any rib  23  of the first axial array  30  joining with a rib  23  from the second axial array  31  will meet at an obtuse angle. Similarly, any rib  23  of the second axial array  31  joining with a rib  23  of the final axial array  32  will meet at an obtuse angle. By this arrangement, the structural quality of the junctions  26  are improved compared to the prior art screen  7  of  FIG. 6 , wherein the junctions  26  involve ribs  23  meeting at 90°. When the screens  7  are cast from liquid metal, the T-shaped or cross-shaped junctions  26  of the prior art screen  7  result in a relatively high mass of metal at that junction  26  that takes longer than the surrounding areas to cool down, which in turn results in increased porosity which may reduce the strength and wear life of the screen  7 . In contrast, the junctions  26  of the screens  7  according to the present invention result in a lower mass, as the “Y” shape of the junction  26  allows for a smaller junction  26  region between ribs  23  as may be noted by the smaller radii of the junction  26  in  FIG. 23  compared with  FIGS. 21 and 22 , where D 3 &lt;D 2 &lt;D 1 . This in turn produces a junction  26  that takes less time to cool down and cast compared to the prior art junctions  26 , leading to a screen  7  that may display increased strength and wear life. 
       FIGS. 21 and 22  show a T and cross shaped prior art junction  26  respectively, whereas  FIG. 23  shows a Y shaped junction  26  consistent with the present invention where at least two of the ribs  23  involved meet at an obtuse angle.  FIGS. 21 and 22  display the greater diameter and hence mass at the junction  26 . In contrast, the Y shaped junction  26  of  FIG. 23  shows a reduced diameter and hence mass at the junctions  26 , allowing the part to cool at an even rate during the casting process. 
     Referring now to  FIG. 24 , shown is a schematic cutaway view of a solid  41  entering an aperture, showing two ribs  23  in profile. The ribs  23  include an upstream periphery  38  in the upstream direction of flow, and a downstream periphery  39  in the down stream direction of flow towards a pump. The ribs  23  may include a region of maximum thickness  40  between the upstream periphery  38  and the down stream periphery. The thickness of the rib may taper from the region of maximum thickness  40  towards the upstream periphery  38 . The thickness of the rib may also taper from the region of maximum thickness  40  towards the downstream periphery  39 . By this arrangement, the ribs  23  may define a streamlined profile that reduced the drag coefficient for liquid flowing through the screen  7 . The upstream periphery  38  may be configured with a substantially blunt or flat profile orientated substantially normal to the direction of fluid flow in the pump intake. Similarly, the downstream periphery  39  may be configured with a substantially blunt or flat profile orientated substantially normal to the direction of fluid flow in the pump intake. 
     The region of maximum thickness  40  may occur nearer to the upstream periphery  38  than the downstream periphery  39 , at about 15% to about 25% of the distance between the upstream periphery  38  and the downstream periphery  39 . The region of maximum thickness  40  may occur at about 20% of the distance between the upstream periphery  38  and the downstream periphery  39 . 
     Referring to  FIG. 25 , shown is a schematic cutaway view similar to that of  FIG. 25 , but also showing a rotatable element  1  in cross section moving in the direction of the arrow. As depicted in  FIG. 25 , the chamfered surface  16  may impart a shearing action on solids  41  caught on the screen  7 , and may thereby improve the breaking-up of such solids  41 . The action of the trailing side  15  including the rearward surface  18  moving across the screen  7  may also act to apply break-up a suspended solid  41  caught on the screen  7 . The chamfered surface  16  of the rotatable member  1  may also impart a substantially streamlined aerofoil shape to the rotatable member, which may also induce fluid flow toward the impeller and thus may lower the NPSHr of the pump  2 . 
       FIG. 26  shows a cross section view of a rib  23 , demonstrating the transition from the blunt surface of the upstream periphery  38 , with an increasing thickness towards the region of maximum thickness  40 , and decreasing in thickness towards the blunt profile of the downstream periphery  39 . 
     Referring now to  FIGS. 27 to 32 , shown are example embodiments of an assembly  22  according to the present invention, including the rotatable member  1  of  FIGS. 2 to 5  with the screen of  FIGS. 7 to 12 .  FIG. 26  shows a front view of the assembly from an upstream position showing the orientation of the rotatable member  1 .  FIG. 27  shows a cross section view of the assembly  22  taken along A-A of  FIG. 26 , demonstrating the position of the rotatable element  1  relative to the screen  7 . The embodiment of  FIG. 27  shows the rotatable element  1  as a monolithic design, inclusive of the locking nut  6 . Such a monolithic design may facilitate the easy mounting and dismounting of the rotatable element  1  to the shaft  4 . In other embodiments, the locking nut  6  may be separate to the rotatable element  1 . 
       FIGS. 28 and 29  provide perspective views of the assembly from an upstream and downstream position respectively.  FIG. 30  provides a back view of the assembly from a downstream position whereas  FIG. 31  provides a side view of the assembly. 
     Many modifications will be apparent to those skilled in the art without departing from the scope of the present invention.