VANE CORE ASSEMBLY FOR MAKING CENTRIFUGAL ELASTOMER ELASTOMERIC IMPELLERS

Abstract: An elastomeric-covered shrouded impeller, having a pair of disk-like shrouds (16, 17) enclosing vanes and vane pas­ sages, which has an improved throat opening (10) having sweeping internal sidewalls is disclosed. A three-piece mold core (A,B,C) is utilized to form the vane passage and the sweeping curved throat inlet of the impeller.

Description: The invention relates to a vane core assembly used in the molding of a shrouded impeller, of the kind described in the pre-characterizing part of the annexed claim 1. Shrouded impellers in which the vanes and vane passages are enclosed between a pair of opposed shrouds or disks are relatively commonplace in centrifugal pumps. Shrouded impellers are available in both metal and elastomer-covered metal constructions. Metal impellers are typically utilized in non-abrasive, non-corrosive environments. Elastomer-covered impellers, because of expense and difficulty in making same, are typically utilized only where abrasive or corrosive resistance is required, for example, in slurry pumps handling abrasive or gritty solids in a liquid media or in dealing with corrosive liquids such as acids and the like. An elastomeric-covered impeller is formed about a metal insert. The technique involves placing the metal insert within a mold and providing core elements which provide for the voids within the impeller after molding. Elastomeric material is forced generally under pressure into the molds so that those spaces which exist between the metal insert and the core elements are filled with rubber thereby forming the elastomeric-covered shrouded impeller. A typical rubber-covered impeller is shown in Fig. 1 in an elevational view showing the peripheral edge of the impeller with vane passage openings shown at the periphery. The formation of the throat opening and vane passages in the molding process is relatively straight-forward in this type of construction. An elevational view of the vanes of a shrouded impeller along section lines 2-2 of Fig. 1 is illustrated in Fig. 2. The spacing between adjacent vanes is closer near the center of the impeller than around the outer edges of the vanes. In the orientation of the impeller illustrated in Fig 2, the rotation of the impeller is counter-clockwise. In the arrangement illustrate in Fig. 3, the vane core, which is a portion of the mold which forms the vane passage, has a uniform width "w" between the inner walls of the front and rear shroud. For the purposes of description herein of shrouded impellers, the front shroud is the shroud containing the inlet opening in the throat of the impeller. Thus, the vane core may be easily extracted by a force perpendicular to the central axis of the impeller. A slight variation to the arrangement illustrated in Fig. 3 is that illustrated in Fig. 4 which is another prior art arrangement. The illustration of Fig. 4 shows some curvature of the inner walls of the front and rear shrouds. This wall curvature is to provide a flow channel from the inlet throat of the impeller into the vane passage which provides a gradual change of direction to accomplish the 90° change of direction from axial inlet flow to radial outlet flow. the vane passage of the impeller of Fig. 4 is formed by a pair of vane core members, A and B, whereby the width W A and width W B of each core member is smaller than the width "b" of the peripheral vane passage width. Extraction of these core members is perpendicular to the central axis of the impeller and is in the order of core A being first removed and then core B being later removed. In metal impellers with enclosed vanes the formation of shrouds with curved inner walls has been practiced for quite some time. Metal impellers are generally formed by sand casting, whereby the formation of curved interior walls of the forwards and rear shrouds has been easily achieved since solid core members are not used in the casting process. Thus, the achievement of a channel connecting the inlet throat with the vane passage in a manner such that the channel encounters no sharp angle restrictions has been long practiced with metal impellers. The presence of a right-angle corner such as that present in the construction illustrated in Fig. 3, may cause velocity loss as well as turbulence near the square corner and cause erosion of the elastomeric covering on the back shroud in the area directly opposite to the square corner on the front shroud. US-A-3 189 681 describes a closed shrouded rubber lined impeller having a rubber lining which is molded on a metal skeleton by using a center core plug which is located in the inlet throat of the impeller skeleton and a couple of core elements which are axially superimposed to each other within the passage between each pair of adjacent skeleton vanes. In the final molded impeller of US-A-3 189 671, each vane passage has an inlet central opening and an outlet peripheral opening having axial widths (i.e., width along the axial direction of the impeller) which are substantially identical to each other. Therefore, during the impeller operation, there is a substantial risk of undesirable flow patterns (e.g. vortices) due to the absence of guiding surfaces which can smoothly and continuously guide the flow during the 90° turn from the axial direction at the inlet to the radial direction of the outlet. Moreover, each of the two superimposed core elements used at each vane passage in US-A-3 189 671 extends through the whole radial length (i.e. the length along the radial direction of the impeller) of the vane passage. Thus, even if one would modify the structure disclosed in US-A-3 189 671 in order to overcome the risk of eddying effects, he could not improve the situation beyond a given limit, since both core elements which extend through the whole radial length of the vane passage are to be extracted radially through the vane passage outlet after molding of the rubber lining. For this reason, the axial width of the passage inlet could be rendered at maximum twice the axial width of the outlet, which prevents the designer from shaping the surface of the vane passage so as to guide smoothly the flow during the 90° turn from the axial direction to the radial direction. In order to remedy these drawbacks, the present invention provides a vane core assembly as described in the characterizing part of claim 1. In the instant invention, the vane passage is a very irregular void having an axial width near the center of the impeller which is much greater than the axial width near the periphery of the shrouds, while the circumferential spacing between adjacent vanes is exactly the opposite with the greater distance between adjacent vanes occurring at the periphery of the shrouds and a narrow spacing between adjacent vanes occurring near the center of the impeller. To accommodate the irregular three-dimensional vane passage, the vane core elements are structured to be removed along an extraction surface between adjacent cores perpendicular to and away from the central axis of the impeller. The core elements are flat members which are curved along opposed edges to conform (and to form) the curved vanes. The curvatures along opposed surfaces of adjacent vanes which form a vane passage are mating surfaces so that the vane core members, as illustrated hereinafter, may be removed along an extraction path, as viewed from the face of the impeller, which is an arc. Generally, two of the vane core elements extend substantially the whole distance from the periphery of the vane passage to the inlet portion of the vane passage. Generally the third vane core element does not extend the full radial distance of the vane passage. FIG. 1 is an elevational view of the peripheral exterior of a conventional elastomeric-covered impeller; FIG. 2 is a cross-sectional view of the impeller of FIG. 1 along section lines 2-2 illustrating the curved vanes and vane passages; FIG. 3 is a cross-sectional view of the impeller of FIG. 1 along section lines 3-3 illustrating the flow channel from the inlet throat to the vane passage; FIG. 4 is a cross-sectional view similar to FIG. 3 of an impeller having slightly curved shroud walls to provide an improved flow channel from the inlet throat to the vane passage; FIG. 5 is a cross-sectional view similar to FIG. 3 of an impeller having radially curved shroud walls forming the vane passage and a cross-sectional view of the core assembly according to the instant invention and used to form the vane passage; FIG. 6 is an elevational view of the vane core assembly member of the instant invention; FIGS. 7, 8, 9 and 10 are perspective views of an impeller with vane core members according to the instant invention illustrated in sequential steps of removal; FIG. 11 is an elevational view of the peripheral surface of an elastomeric-covered impeller; FIG. 12 is an elvational, facial view of a vane core element of the instant invention; FIG. 13 is a plan view of the peripheral edge of the vane core elements of FIG. 12. The instant invention involves a unique construction of the core used in the casting process of shrouded, elastomeric-covered impellers. Such impellers come in particularly small sizes, for example, less than about 30.5 centimeters in diameter, and are provided with a channel comprising the inlet throat and the vane passage which is essentially curvalinear. Fluid entering the pump enters axially through an opening in the front shroud and, through the centrifugal action of the pump, parts in a direction perpendicular to the original axial flow. The change of direction for fluids in the instant impellers is very gradual and distinctly different from that in existing elastomeric-covered, enclosed impellers. Theoretically, the curvature of the flow channel formed by the throat (inlet opening) through the vane passage to the peripheral exit for the impeller would ideally be a 90° arc of a circle. Such a construction would, of course, involve an impeller axial depth which was greater than about one-half the impeller diameter. While casting of metal impellers approximating such a construction in sacrificial molds is relatively straight-forward, molding complexities are encountered in making a comparable elastomeric-covered shrouded impeller. In the instant invention, a mold assembly involving a central core member and a vane core mold assembly of at least three elements is utilized to approximate an ideal flow channel from inlet to discharge for an elastomeric-covered shrouded impeller for a centrifugal pump. The technique for making the impellers of this invention and the multiple-element vane core mold assembly for forming the sweeping channel connecting the throat and vane passage is unique. Further description of the instant invention may be facilitated by reference to the attached drawings. The unique channel arrangement associated with the instant invention is illustrated in FIG. 5. The throat inlet 10 connects to the vane passage, which in FIG. 5 is occupied by vane core mold members used in molding the appropriately shaped vane passage. As illustrated in FIG. 5, a very gradual change of direction occurs between fluid entering the throat and then changing directions at 90° to exit the vane core passage at the peripheral edge of the impeller. The vane core passage 11 is formed between the front shroud interior wall 12 and the interior wall 13 of the rear shroud. The curved surface of the rear shroud 13 continues below the vane core passage 11 to form a cone-shaped projection 14 with its apex at the central axis of the impeller 15. A steel impeller skeleton is first placed in the mold before rubber is injected. In FIG. 5, the steel skeleton shroud members 16 and 17, respectively, form the front and rear shroud inserts about which the elastomer forms to form the elastomeric-covered impeller shrouds. The interior wall of the front shroud adjacent to the throat inlet has a curvature as established by radius R. Radius R is generally determined by the diameter of the impeller and the amount of sweep desired. Furthermore, the curvature of the interior wall of the front shroud may be described by several different radii having different focal points. The juncture of the inner wall 12 of the front shroud with the throat inlet wall 18 approximates a curvalinear relationship, that is, the curvature of the inner shroud wall 12 is such that the throat inlet wall 18 is substantially tangential to the curved wall 12 at the point of juncture. The interior wall 13 of the rear shroud has a curvature such that at the apex of the cone 14 the curvature of the cone wall is such that an extension of such curved wall joins with the central axis 15 of the impeller in a curvalinear fashion, that is, the axis is substantially tangential to the extended curve of the curved wall of cone 14. The center core plug 19 is substantially cylindrical in shape at the throat inlet area and has longitudinal grooves circumferentially spaced equidistantly about the plug near the distal end. These grooves accommodate the formation of extensions of the vanes into the inlet throat and are dished on the end to form the conical projection 14. The center core plug mates with vane core elements A, B and C at boundary or parting surface 20. Vane core member A joins at parting line 21 with vane core member B. Parting line 21 is, in fact, a planar surface as illustrated in other drawings, and is substantially perpendicular to the central axis 15. Vane core member A has a maximum width W A , which must not be greater than its width at the exit of the vane passage. Since the extraction of the core is in a direction perpendicular to the central axis 15, no thickness of any core member can exceed the width of the core member at its exit point. The width of W A must, of course, be less than the width of the exit as illustrated by letter "b". Preferably, the width W A is significantly less than the exit width "b". The second vane core member B should have a sufficient thickness near its exit to be sufficiently durable that it is not easily broken. For example, if core member A were substantially as thick as exit width "b" then core member B may be substantially a knife edge at its upper portions and, therefore, would be easily broken. In the core mold arrangement illustrated in FIG. 5, the width of W, which is the combined widths of core members A, B and C, is greater than the exit width "b". In the case of core member B, the width W B is not the maximum width of the element; however, the maximum width of core member B, which is near shoulder X, must be smaller than the exit width "b". Core members B and C are formed with a shoulder X so that again a sliver or knife edge is not required on core member C. Structuring core member C such that it is recessed slightly into core member B makes core member C an easier part to fabricate and assures a better seal at the sealing surfaces between core members so that the mold surface presented to form interior wall 12 is a continuous surface. Core member C and core member B are joined together by pins projecting from core member C which are recessed within bores in core member B. Conversely, the pins could be affixed to core member B and the bores recessed within core member C. Since the surfaces at the joint between core members B and C are generally machined surfaces, the parts may tend to stick together after being subjected to the pressures within the mold during injection of the elastomeric rubber material. A pry slot 26 is provided for insertion of a screwdriver or the like to pry the cores B and C apart. Also air injection port 27 is provided in core member B so that air pressure may be introduced into bores 24 and 25 to eject pins 22 and 23 to separate cores C and B. Pry slot 26 and air injection port 27 are at a boundary surface between cores B and C and the center core plug so that the elastomer in liquid form, as it is filling the mold, cannot reach the slot 26 or port 27, so that these remain open and unfilled with rubber. A frontal or elevational view of core member C is illustrated in FIG. 6. Core C has a substantially crescent shape; the central circle 28 illustrates the cylindrical wall of throat 10. FIG. 6 illustrates the central core plug 19 positioned in place with grooves 29 in the distal cylindrical surface of the plug spaced so as to form the interior tips of the vanes which protrude into the throat region. Thus, core member C at surface 30 forms a portion of the surface of a vane 31 while the surface 32 of the core C forms a surface of a vane 33. The juncture line X is illustrated showing a juncture surface between core B and core C. Surface 34 is the parting surface formed between core C and the central core plug 19. Pry slot 26 is shown in dotted lines as is injection port 27 which interconnects bores 24 and 25. FIGS. 7, 8, 9 and 10 illustrate sequentially the removal of vane core members A, B and C and central core plug 19. FIG. 7 is a perspective view of an elastomeric-covered impeller which has been molded and which has had the center core plug 19 removed. Also, three quadrants of the vane passage core sub-assemblies have been removed. In molding a complete impeller with four vanes and four vane passages, four vane core sub-assemblies similar to the sub-assembly shown in FIGS. 7 through 10 are utilized so that four vanes are formed and four vane core passages are formed in the impeller. In FIG. 7, cores A, B and C are still positioned within the impeller. In FIG. 8, vane core A has been removed. Core A has a rim 34 which is substantially as thick at its outer edge as the vane passage of the impeller. A quarter-circle, arc-like shoulder 35 is formed in the rim 34, said shoulder having a thickness substantially the same as the thickness of the rim 36 of vane core B. Thus, vane core A and vane core B mate in concentric fashion as well as in a planar fashion where the tongue 37 of vane core A extends into the vane core passage. In FIG. 9, vane core B is shown after it has been removed from the impeller. Vane core B has an outer rim 36 which is a quarter of a circle in its arc and mates with the quarter-circle shoulder 35 of vane core A. Vane core B has a tongue 38 which extends into the vane core passage. The edge surfaces of tongues 37 and 38 are illustrated in FIG. 5 and the tongues have a width, W A and W B , as indicated in FIG. 5. FIG. 10 shows vane core C removed from the impeller. The substantially crescent-shaped vane core C is illustrated. It is apparent from FIGS. 7 through 10 that similar vane core elements could be utilized to form any number of vane passages and vanes in an enclosed shrouded impeller. For example, an impeller could be formed having three, five, six, or any number of vanes and vane passages in which the rim portions of the cores A and B, respectively, would be one-third, one-fifth and one-sixth of a complete circle. A facial view of core elements B and C is presented in FIG. 12. Cores A and B have substantially the same overall shape; i.e., core B may set in with core A. The flat planar surfaces of core A are parallel to one another while core B has one flat planar surface which mates with a flat planar surface of core A. The other facial surface is a curved surface such that the width between opposed faces of the tongue increases with distance from the rim. The sweep of surface is such that it continues with the surface of core C. That edge of core C designated by the numeral 40 is that portion which mates with the center cylindrical plug of the mold. Edge 41 of the tongue of core B forms one surface of a vane while edge 42 forms a surface of an adjacent vane. The space occupied by the tongues of cores B and C between adjacent vanes is a vane passage. The view in FIG. 13 is an edge-on rim view of cores B and C illustrated in FIG. 12. The outer edge 43 of the rim has a certain width, which is the width also of the tongue member adjacent the rim. The tongue of core B increases in thickness with distance from the rim, as illustrated in FIG. 13. The joint 44 between core C and core B is at the shoulder joint between these two elements. The offset of the shoulder is about the same as the increase in thickness of the tongue of core B from the rim to the shoulder. Thus, surfaces 40 and 45 are part of core C. The depth of the shoulder 44 and thickness of core B may be viewed in FIG. 5. The thickness of edge 40 at its junction with shoulder 44 is about the same as the thickness of edge 41 at its juncture with shoulder 44. The mating surfaces between cores C and B are flat planar surfaces which have a substantially cresent-shaped outline. In FIG. 11, an edge-on elevational view of an impeller of the invention is illustrated. The center of gravity of the impeller is shifted towards the inlet opening. A consequence of the increased width of the impeller and the shift in the center of gravity is to position the center of gravity farther from the external bearing supporting the impeller shaft. Also, because of the greater cantilever effects of the impeller upon the bearing, a larger shaft and bearing are generally required. Elastomeric impellers of the type of the instant invention for an impeller of a given diameter will generally weigh more than conventional elastomeric prior art impellers, assuming the same thickness of elastomeric covering. Such increase in weight further would require an increase in shaft and bearing size. Thus, increased shaft and hearing sizes required for construction of this type may have lead those skilled in the art away from making impellers of the type of this invention. It has been found, however, that the elastomeric-covered impellers of this invention provide a significant increase in hydraulic efficiency, a significant decrease in power consumption, and improved impeller life when compared with prior art-type impellers of similar diameters. Thus, a smaller diametered impeller of the instant invention may be effectively substituted for larger diametered impellers of the prior art type, so that no real increase in shaft size or bearing size actually occurs when determined by pump performance. Molding of elastomeric-covered impellers is done under very high pressure; i.e., injection pressures of 10.3 MPa (1500 psi) to about 13.8 MPa (2000 psi) with internal mold pressure spikes of upwards of 34.5 MPa (5000 psi) to about 41.4 MPa (6000 psi). Such high pressures require mold elements which are particularly rugged, especially for vane core elements which, in the instant invention, are cantilevered from an external rim member. Should deflection or displacement of any mold element occur, several adverse conditions may result, such as: 1. The hydraulic passageways may be distorted and less efficient than desired; 2. The thickness of rubber deposited in a particular area may be less than desired, thereby diminishing the wear resistance life of the impeller; and 3. The balance and dynamics of the impeller may be adversely affected. Because of these adverse consequences, the various mold elements must fit securely together and all elements must be sufficiently strong to resist any unbalance in pressure during the molding process. Elastomeric-covered impellers of this invention have pumping efficiencies which are significantly improved over previous configurations of elastomeric-covered impellers. For example, small diameter impellers of from about 15 centimeters to 30.5 centimeters in diameter show improvements of about 30% in pump efficiency when compared to other similarly sized pumps with configurations such as those show in FIGS. 1 and 2. Another measurement of improvement for the pumps of the instant invention is that power requirements to pump a certain volumetric rate of liquid to a certain head is accomplished with signficantly less power consumption.

Claims: A vane core assembly for use in forming a vane passage (11) between a pair of adjacent vanes, in an injection-formed, elastomeric covered, closed shroud impeller; said impeller having an inlet (10) which communicates with said vane passage (11); said vane passage having a central opening contiguous said impeller inlet, a peripheral opening contiguous a periphery of said impeller and a first length defined as a distance between said central opening and said peripheral opening; a length of said central opening being substantially less than a length of said peripheral opening; said central opening being transposed from a center line alignment with said peripheral opening; said vane core assembly comprising: a first vane core element (A), said first core element (A) being dimensioned to extend substantially from said central opening to said peripheral opening of said vane passage; a second core element (B), detachably mounted on said first core element (A); said second core element (B) being dimensioned to extend substantially from said central opening to said peripheral opening of said vane passage; a center core plug (19) to be located in the inlet (10) of the impeller; characterized in that said vane core assembly comprises a third core element (C) located to the outside of the center core plug (19) and manually detachably mounted by attachment means on said second core element (B); said third core element (C) being mounted to be positionally adjacent said central opening of said vane passage (11); said third core element (C) having a length less than said first length; whereby said third core element (C) is extendable from said central opening to a location short of said peripheral opening; wherein said attachment means are configured to be accessible from said impeller inlet when said vane core assembly is within said vane passage (11) whereby said third core element (C) is rendered manually detachable from said second core element (B) by manipulation of said second (B) and third core (C) elements through said impeller inlet (10); whereby a width of said central opening is substantially greater than a width of said peripheral opening. The vane core assembly of claim 1 characterized in that attachment means include at least one pin (22, 23), releasably received within a pair of oppositely positioned bores (24, 25) defined respectively within said third (C) and second (B) core elements. The vane core assembly of claim 2 characterized in that said vane core assembly includes a pry slot (26), positioned proximate a boundary surface between said third (C) and second (B) core elements, said slot being dimensioned to receive a tool and facilitate a pried separation of said third (C) and second (B) core elements. The vane core assembly of claim 2, characterized in that said vane core assembly includes an air injection port (27); said port (27) being adapted to receive pressurized air from proximate said impeller inlet (10) and direct that air against said pin (22, 23) to effect a dislodging of said pin (22, 23) from its bore (24, 25); whereby said second (B) and third (C) core elements are forcibly separated. The vane core assembly according to claim 1, characterized in that said first and second vane core elements (A, B) are arcuately shaped defining an arc shaped vane passage (11). The vane core assembly of claim 5 characterized in that said first core element (A) and said second element (B) each include a planar mating surface whereby said respective mating surfaces are positionable co-extensive one another to permit a sliding action of said first core element (A) along said second core element (B). The vane core assembly according to claim 6, characterized in that said third core element (C) detachably mates with a portion of a surface (39) of said second core element (B), which surface (39) is opposed to a surface (21) of said second core element (B) which planarly mates with a corresponding surface of said first core element (A). The vane core assembly of claim 7 characterized in that said assembly is configured to be withdrawn element by element from said vane passage (11) substantially in a plane perpendicular to a central axis (15) of said impeller and along a generally arc-like path corresponding to said arc shaped vane passage (11). The vane core assembly of claim 8 characterized in that said first (A) and second (B) core element are configured to permit a sliding disengagement one from another whereby said first core element (A) may be initially withdrawn slidingly from said vane passage (11) and said second core element (B) may then be subsequently withdrawn whereby said withdrawal of said first core element (A) provides sufficient space within said vane passage (11) to permit a separation of said second core element (B) from said third core (C) element prior to beginning a withdrawal of said second core element (B) from said vane passage (11). The vane core assembly of claim 1 characterized in that said first core element (A) includes an outer rim (34) configured to co-act with an outer rim (34) of a second first core element (A) to thereby retain said first core (A) element stationary vis-a-vis said second first core (A) element. The vane core assembly of claim 1 characterized in that said second core element (B) has an outer rim (36) which co-acts with its respective first core (A) rim (34) to form a manually detachable union of said first core element (A) and said second core element (B). The vane core assembly according to any of the claims preceding characterized in that said central opening width is at least twice as large as said peripheral opening width.

Inventors: HYLL, JOHN
Applicants: BAKER-HUGHES INCORPORATED
Application year: 1986
Main classification: F04D29
IPC codes: F04D29, B29C33, B29C45