[0001] This invention relates to an impeller in an end suction slurry pump and, more particularly, to the construction of primary vanes in the impeller. [0002] In an end suction centrifugal slurry pump slurry enters the pump in an axial direction. The direction of slurry flow is then gradually changed by 90°, by impeller action, to flow generally perpendicularly to the axial direction in order to impart to the slurry kinetic energy which is necessary to generate a designed head and slurry flow rate.

[0003] The profile of the impeller and, more particularly, the profile of each primary vane included in the impeller, have a direct influence on the efficiency of the aforementioned pumping action.

[0004] Typically the profile of a primary vane can be described as having a "backward sloping form as a radial angle changes along a camber line of the vane". In one construction technique a radial vane profile is extruded in an axial plane to produce vanes with parallel sides, also referred to as "straight vanes". These vanes, although of a relatively simple shape which facilitates consistent, volume production, do not deliver optimal operating efficiency, nor do they possess an extended operating life. PRIOR ART

[0005] Figure 1 of the accompanying drawings is a side view of an impeller 10 which includes a front shroud 12, a rear shroud 14, and a plurality of primary vanes 16 located between the shrouds. In use the impeller 10 is rotatable about a central axis 20. [0006] Figure 2 is a perspective view of the rear shroud 14 (with the front shroud 12 omitted for clarity of illustration) and shows the primary vanes 16 upstanding on an inner face 18 of the rear shroud 14.

[0007] Figure 3 is a plan view of the components shown in Figure 2. Each primary vane 16 has an arcuate profile with sloping sides 24 and 26 respectively. In use, the impeller 10 rotates in a direction 28 and the vanes 16 slope "backwardly" with respect to the rotational direction 28.

[0008] Although the sides 24 and 26 are curved in plan, the sides 24 and 26, in a direction which is parallel to the central axis of rotation 20, are parallel to the axis 20. It is this feature which facilitates high volume production of the primary vanes 16 although, as indicated, this benefit comes at the cost of a lower efficiency of operation and a shortened operating lifetime.

[0009] As the slurry which is pumped changes flow direction from an axial path to a radial, path the velocity of the slurry flow towards the front shroud 12 differs from the velocity of the slurry flow towards the rear shroud 14. This means that to increase efficiency of operation and to extend the operating lifetime of the impeller, the profile of each primary vane should change angle not only in a radial sense but also in axial plane. This calls for a fully profiled vane which changes in curvature in all three planes over the vane's external surface.

[0010] In use, the abrasive nature of the slurry which is being pumped, can rapidly abrade the primary vanes. To combat this the impeller is cast in a wear-resistant iron alloy of elevated hardness chosen for a specific application. This type of casting is produced using a sand mould which is prepared with a pattern which, as far as is possible, replicates the desired vane shape. In the process a container is filled with sand that is compacted around the pattern which, in turn, comprises a number of interconnected solid profiles which are positioned inside the container. Thereafter the profiles of the pattern are extracted from the sand leaving a series of interconnected cavities which are then filled with molten iron. Once the iron solidifies the required moulded product is produced.

[001 1] The pattern profiles must be shaped so that it is possible to extract the profiles from the compacted sand. The desired vane pattern has a complex shape, with opposing angles at "inlet" and "outlet" sides, and the removal of the profiles without disturbing the compacted sand is not possible. As vane castings are produced in large numbers it is necessary, for each casting, to repeat the mould making process.

[0012] To address this aspect in practice the pattern for a primary vane can be divided into smaller segments which can be individually extracted from the compacted sand mould. This approach does, however, lead to some undesirable outcomes for in practice the segments cannot always be accurately matched to one another. In a slurry application, these mismatches result in areas of localised wear. [0013] Figure 4 shows from one side an impeller 30 which has primary vanes 32. Each primary vane 32 has a complex shape of the kind described i.e. each primary vane 32 changes in shape in three dimensions. Figure 5 shows the impeller 30 of Figure 4 but with a front shroud 34 omitted and with a rear shroud 36 shown in dotted outline. Parts of the primary vanes 32 are shown in solid lines in Figure 4 and in dotted outline in Figure 5.

[0014] Figure 6 has three curves designated 32A, 32B and 32C respectively which are superimposed on one another. The curve 32A represents the outline of a primary vane 32 on a section A-A indicated in Figure 4. The curve 32B depicts the cross sectional profile of the primary vane 32 at a section line B-B in Figure 4. The third curve 32C represents the profile of the primary vane 32 at plane which is between the sections A-A and B-B. At places the profile 32C overlaps (crosses) the profile 32A and, similarly, the profile 32B crosses the profile 32A, and the profile 32C, at various locations. It is apparent that in order to extract a vane 32 of such complexity from a compacted sand mould, it is necessary to break the mould first.

[0015] An object of the present invention is to address the aforementioned situation. SUMMARY OF THE I NVENTION

[0016] The invention provides an impeller wherein the cross sectional outline of each primary vane on a shroud surface, taken on a plane which is parallel to the shroud surface, lies on a closed path, and wherein no part of that closed path crosses any similar closed path which is between the closed path and the shroud surface. [0017] Similarly, a cross sectional profile of the vane surface, taken on a plane which is at a right angle to the shroud surface, traces a path which moves "inwardly" into the body of the vane, with increasing distance from the shroud surface.

BRIEF DESCRIPTION OF THE DRAWINGS [0018] The invention is further described by way of example with reference to the accompanying drawings in which:

[0019] Figure 7 is a perspective view of an impeller but depicting in dotted outline only one primary vane according to the invention,

Figure 8 is a plan view of a rear shroud of the impeller and of the primary vane, of Figure 7, and

Figure 9 depicts cross sectional profiles of the primary vane shown in in Figures 7 and 8. DESCRIPTION OF PREFERRED EMBODIMENT

[0020] Figure 7 of the accompanying drawings illustrates an impeller 40 which includes a front shroud 42, a rear shroud 44 and a number of primary vanes 46 positioned between opposing faces of the front shroud 42 and of rear shroud 44. Figure 7 shows, in dotted outline, only one primary vane 46 - the other primary vanes are similar in shape and size although, as is known in the art and as is exemplified for example in Figure 2, the primary vanes are circumferentially spaced from one another around a central rotational axis 48 of the impeller 40. The front shroud 42 has an inlet 50 through which slurry, to be pumped, can flow axially into a volume 52 which is between opposing surfaces 42A and 44A of the first shroud 42 and the rear shroud 44 and within which the primary vanes 46 are located. [0021] Each primary vane 46 extends from the axis 48 radially and "backwardly" relative to a rotational direction 54 of the impeller. This is similar to what has been described in connection with Figures 1 , 2 and 3.

[0022] Figure 8 shows the rear shroud 44 and a single primary vane 46. The primary vane 46 has a body 58, of complex shape, which has a first end 58A which abuts the inner face 42A of the front shroud 42, a second end 58B which abuts the inner face 44A of the rear shroud 44 and an outer surface 60. The first end 58A is parallel to the second end 58B and is larger in cross sectional area than the second end 58B.

[0023] Figure 9 shows three curves 62A, 62B and 62C respectively superimposed on one another. The curve 62A is produced by taking a section through the body 58 on a plane which corresponds to the plane A-A shown in Figure 4. The curve 62A thus depicts the cross sectional shape of the body at the first end 58A. The section is thus on a plane which is perpendicular to the central axis 50 and represents a closed path along which the section intersects the outer surface 60 of the body 58. [0024] The profile 62B represents a second closed path at which a section through the body 58 on a plane which corresponds to the plane B-B of Figure 4 intersects the outer surface 60 of the body i.e. the cross sectional shape of the body 58 at the second end 58B.

[0025] The intermediate profile 62C is a closed path produced by taking a cross section through the body 58 in any plane which is parallel to the plane A-A or the plane B-B and which is between the first end 58A and the second end 58B. [0026] The closed path 62C lies on or lies within the closed path 62A. Similarly the closed path 62B lies on or lies within the closed path 62C. This means that no portion of the path 62C goes outside the boundary of the closed path 62A, and no portion of the path 62B goes outside the boundary of the closed path 62C. The cross sectional areas of the sections decrease, moving from the section 62A to 62B. Thus each cross sectional profile proceeding from the first end 58A to the second end 58B at any plane which is perpendicular to the axis 48 is enclosed in any preceding profile and similarly encloses any succeeding profile. As no succeeding profile crosses a boundary of a preceding profile the ability to extract a vane body 58 from a compacted sand mould is not compromised. Thus the vanes 46 can be made with complex three-dimensional shapes and can be made in sand moulds without resorting to the segmented type approach. Discontinuities associated with mismatches between smaller vane segments are therefore eliminated. Production rates of the primary vanes are also increased.