Molten metal pump with vaned impeller

Abstract: A molten metal pump including a vaned impeller without lateral openings which demonstrates prolonged high efficiency and reduces the likelihood of catastrophic failure.

Description: FIELD OF THE INVENTION This invention relates to molten metal pumps, and more particularly, to pumps utilizing a vaned impeller. BACKGROUND OF THE INVENTION In the processing of molten metals, it is often necessary to pump molten metal from one place to another. When it is desired to remove molten metal from a vessel, a so-called transfer pump is used. When it is desired to circulate molten metal within a vessel, a so-called circulation pump is used. When it is desired to purify molten metal disposed within a vessel, a so-called gas injection pump is used. In each of these pumps, a rotatable impeller is disposed within a volute case accessible to the molten metal in the vessel. Upon rotation of the impeller within the volute, the molten metal is pumped as desired in a direction permitted by the volute. In each of the pumps referred to, the impeller is disposed within the volute formed in a base member. Typically the volute in the base member is suspended within the molten metal by means of posts. The impeller is supported for rotation in the base member by means of a rotatable shaft. The base member includes an outlet passage in fluid communication with the impeller, and upon rotation of the impeller, molten metal is drawn into the volute and the open section of the impeller, where it then is discharged under pressure to the outlet passage. Although pumps previously known in the art operate satisfactorily to pump molten metal from one place to another, certain problems have not been addressed. Particularly, these problems relate to the efficiency of the impeller, duration of operability and consistency of performance. U.S. Pat. No. 4,940,384 shows a molten metal pump with a cup-like impeller body having vanes and lateral openings for moving molten metal. Although the impeller of this pump transports molten metal, it is prone to clogging by foreign materials such as semi-solids and solids, e.g. drosses, refractory debris, metallic inclusions, etc., (herein after referred to as "particles") contained in the vessel and frequently drawn into the molten metal pump. If a large particle is drawn into the pump, the impeller can be jammed against the volute case, causing catastrophic failure of the pump. Even if catastrophic failure does not occur, small particles eventually clog the lateral openings and degrade the performance of the impeller by reducing the volume of molten metal it can transfer. Accordingly, it is desirable in the art to have an impeller which minimizes clogging, thereby maintaining high efficiency over time and avoiding catastrophic failure. Impeller-type equipment without lateral openings has been utilized in molten metal stirring and/or submersion types of devices. U.S. Pat. No. 4,898,367 shows a gas dispersion rectangular block without openings. However, this stirring device does not achieve a directed, forced fluid flow. Particularly, the impeller must be rotatable within a housing to maximize forced flow from the impellers rotation. In addition to block type molten metal agitation devices, vaned circular equipment has been used, see U.S. Pat. No. 3,767,382. Again, however, there is no means for achieving forced directional molten metal flow. Such forced directional molten metal flow is highly necessary in the application of pumping technology in molten metal processing. In a circulation mode, better convectional heat transfer occurs (greater kinetic energy imparted by the pump), and faster melting exists as solid charge materials such as scrap or ingot is mixed more quickly and thoroughly into and with the liquid metal. In a transfer mode, the liquid metal is more strongly directed or redirected into a conveying conduit such as a riser or pipeline for more efficient transfer at a higher rate as a result of such improved forced directional molten metal flow. In summary, the molten metal treatment art described in the above two paragraphs fails to achieve important advantages of the current invention. Particularly, either there is no effective prevention of clogging and/or there is no means to achieve directional forced molten metal flow. The current invention achieves a number of advantages in directional forced molten metal flow. The impeller of the current pump is not prone to clogging of lateral openings as in prior pump impellers. Accordingly, catastrophic failure is much less likely to occur and the efficiency of the impellers operation does not degrade as rapidly over time. The design also achieves high strength by increasing the load area material thickness. Furthermore, the impeller design permits easy manufacturing processes. Accordingly, it reduces the cost of production and allows a wide selection of impeller material, such as graphite or ceramic. Also, the current impeller concept is adaptable to allow optimization as required without large scale manufacturing alteration. SUMMARY OF THE INVENTION Accordingly, it is a primary object of this invention to provide a new and improved molten metal pump. It is a further objective of this invention to provide a new and improved impeller for use in a molten metal pump. According to the present invention a molten metal pump impeller (40) comprises an imperforate substantially circular base (88) and at least two solid vanes (90) radially disposed thereon, projecting substantially perpendicularly from the surface (89) thereof and extending substantially from the center of said base to its circumference: each vane defining a first edge (93), a second edge (95) and a third edge (97; said first edge (93) being disposed on said base (88); said second edge (95) defining an inlet end of the impeller (40); said third edge (97) being a radially outer edge; said second edges (95) of adjacent vanes defining an inlet area (I) between the vanes (90) over their entire radial dimension and being generally planar, the inlet area having a circular outer boundary; said third edges (97) of adjacent vanes (90) defining an outlet area (O) between the vanes (90); and said outlet area (O) being greater than said inlet area (I). The invention also comprehends a a molten metal pump (20) comprising: (a) a shaft (30) having fist and second ends; (b) a means for rotating said shaft in communication with the first end thereof; (c) an impeller (40) in communication with said second end; and (d) a volute (38), housing said impeller, having a first opening (45) through which molten metal can be drawn and a second opening (48) through which molten metal can be discharged: characterised in that said impeller (40) is an impeller in accordance with the invention as hereinabove set forth. The molten metal pump of this invention comprises an elongated drive shaft having first and second ends, the first end extending out of a molten metal bath and the second end extending into the molten metal bath. An impeller is attached to the second end of the drive shaft. The impeller has a solid circular base portion with at least one face and at least two vanes extending substantially perpendicular from said face. Preferably, the vanes also extend radially from the center of the face. Solid as used herein means a lack of openings capable of accommodating molten metal flow. Face as used herein means a relatively flat surface. The impeller is disposed within a volute having an inlet into which molten metal can be drawn and an outlet through which molten metal can be forcibly discharged by the impeller's rotation. Volute as used herein means a casing which facilitates the impeller's convergence and expulsion of molten metal. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross-sectional view of a molten metal pump; Figure 2 is a cross-sectional view of an impeller attached to a drive shaft for use in a molten metal pump; Figure 3 is a cross-sectional view of the impeller of Figures 1 and 2; Figure 4 is a cross-sectional view of an impeller having curved vanes; Figure 5 is a cross-sectional view of impeller designs operable in a molten metal pump; and Figure 6 is a sketch of a relieved four vaned impeller. DETAILED DESCRIPTION OF THE INVENTION While the invention will be described in connection with a preferred embodiment, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications, and equivalents as may be included within the scope of the invention defined by the appended claims. Referring now to Figures 1 and 2, a molten metal pump according to the invention is indicated generally by the reference numeral 20. The pump 20 is adapted to be immersed in molten metal contained within a vessel (not shown). The vessel can be any container containing molten metal. It is to be understood that the pump 20 can be any type of pump suitable for pumping molten metal. Generally, however, the pump 20 will have a base member 38 within which an impeller 40 is disposed. The impeller of Figures 1, 2 and 3B is a cross-sectional view "X"-"X" as shown in Figure 3A. The impeller 40 is supported for rotation within the base member 38 by means of an elongated, rotatable shaft 30. The upper end of the shaft 30 is connected to a motor (not shown). The motor can be of any desired type, for example air or electric. The pump 20 is supported by means of posts 18, post sleeves 16 and a support plate 24 attached via post sockets 21. The drive shaft 30 lies within shaft sleeve 28, typically made of graphite, with a refractory coating of silicon carbide or similar material. The base member 38 includes an inlet passageway 45 and an outlet passageway 48. A riser may be connected to the base member 38 in fluid communication with the passageway 48. The pump 20 is best described as a so-called circulation pump, that is, it circulates molten metal within the vessel. As indicated earlier, however, the pump 20 is described for illustrative purposes and it is understood that the pump 20 can be of any type suitable for pumping the molten metal. The base member also includes a baffle plate 50 and a shaft mount bearing 51. The impeller 40 is secured via cement, such as Fraxset™, obtainable from Metaullics Systems Division. A first bearing ring 42 of silicon carbide or other material having bearing properties at high temperature is disposed about the lover most end of the impeller 40. A second bearing ring of silicon carbide or other material having bearing properties at high temperature is disposed at the lower most end of the base member in facing relationship to the first bearing ring 42. As will be apparent from the foregoing description, the impeller 40 is rotatable relative to the base member 38. The bearing rings 42 and 44 will prevent friction related wear of the base member 38 and the impeller 40 from occurring. This base member 38 includes volute case 39 within which the impeller 40 is disposed. The upper, or first end of the drive shaft 30 projects from the first end of shaft sleeve 28 and is connected to the motor 60 via coupling assembly 54, as shown in U.S. Pat. No. 5,092,821. Preferably, the drive shaft is of a quadralobal nature, as described in U.S. Pat. No. 5,092,821. In addition to cement attachment of the impeller to the drive shaft 30, the impeller is secured to the drive shaft via graphite dowel pins 80. The impeller is further secured to the shaft 30 via a back-up sleeve 82 which acts as reinforcement to the attachment joint and as a locator for the impeller. Both of these embodiments are covered in U.S. Patent No. 5,025,198. A further bearing ring 84, comprised of silicon carbide or other thermally resistant bearing material, encircles the upper most portion of the back-up sleeve 82. This bearing ring 84 will be opposed by another bearing ring 86 within the base member 38. The back-up sleeve 82 is generally affixed to the shaft 30 and prevented from upward movement via a collar ring 88 on the shaft 30. Referring now to Figures 3A and 3B, the impeller 40 is shown as a four-vaned circular base impeller. Typically, the impeller consists of a circular base 88 topped by at least two vanes 90. Vane generally means a flat or curved object rotated about an axis that causes or redirects fluid flow. In addition as used herein, vane means an independent surface imparting work on the molten metal. The upper portion of the impeller contains an opening 92 for acceptance of the lower end of the shaft 30. The impeller has a recessed based portion 96 for attachment of a silicon carbide bearing ring 42. Typically, the vanes are tapered with the thickest section beginning at the center most portion of the impeller adjacent the shaft. The tapering and the thickness of the vanes are important features with regard to wear from inclusions and/or sediment in the molten metal and molten metal fluid volume. Particularly, the thickness and the dimensions facilitate the durability of the vanes under stress. Figure 4 demonstrates the impeller of a molten metal pump including curved vanes in an offset design. These alternatives may further reduce the degradation to the impeller by particles in the molten metal. Figure 5 demonstrates various forms the impeller of the molten metal pump may take to achieve the objects of the invention. Preferably, the impeller is dynamically balanced. Figures 5A and 5B demonstrate that the impeller need not specifically contain vanes. In fact, any geometric shape (square, rectangle, triangle, star) will effectively force directed molten metal flow. Although the efficiency may be reduced by the limited fluid volume between the sides of the square and the circular radius created by the spinning corners, this design would demonstrate high strength and ease of manufacture. Sides as used herein means the surfaces generally parallel to the shaft axis. Fluid volume as defined herein means the area of the impeller which fills with molten metal during operation, demonstrated by the shading of Figure 5B. Figure 5A shows that a circular base portion is only a preferred embodiment. The base portion functions to direct the fluid flow into the impeller from the top and to discharge the fluid in a direction perpendicular to the rotating shaft. The base portion may also be the portion of the impeller located nearest the shaft and the top of the pump, in which case the pump is a bottom feed unit (Figure 4). Without a base plate, the pump draws molten metal from both top and bottom. This embodiment would decrease efficiency in exchange for ease of manufacture. A second purpose of the base plate is to hold a bearing ring, also a preferred embodiment. Figures 5C, 5E, and 5I demonstrate an impeller without a base plate. Figure 5D demonstrates an impeller having tapered vanes to achieve a strong central portion for shaft attachment and increased fluid volume. Figures 5A, 5F and 5G demonstrate the flexibility of this impeller design, wherein, the impeller can contain a minimum of two vanes (5H) to a very high number of vanes as demonstrated by Figure 5G. A low number of vanes, as in Figure 5H, creates a very high fluid volume, however, there may be some loss of efficiency due to a reduced force on the fluid at points distant from the two vanes. Figure 5J demonstrates curved vanes and contouring of the vanes to maximize strength and to reduce wear. Note, however, that curvature of the vanes limits the pump to unidirectional use. Vanes which are thicker in areas (vane bottoms in Figure 5J) may resist wear at those points where contact from molten metal and particles therein is the most severe. The use of thin sections again increases the fluid volume and improves efficiency. Figure 6 demonstrates an alternative means of increasing pumping capacity. Relief of a portion of the vanes near the shaft/hub provides increased fluid access, however, mechanical strength is somewhat reduced. It will be appreciated from the foregoing descriptions that the molten metal pump according to the invention, possesses the advantages of high efficiency and durability. Particularly, the impeller in relationship to the described shaft and motor mechanism is effective in the transfer of molten metal with reduced clogging and/or catastrophic failure. Thus it is apparent that there has been provided in accordance with the invention, a molten metal pump that fully satisfies the objects, aims, and advantages set forth above. While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description.

Claims: A molten metal pump impeller (40) comprising an imperforate substantially circular base (88) and at least two solid vanes (90) radially disposed thereon, projecting substantially perpendicularly from the surface (89) thereof and extending substantially from the center of said base to its circumference: each vane defining a first edge (93), a second edge (95) and a third edge (97; said first edge (93) being disposed on said base (88); said second edge (95) defining an inlet end of the impeller (40); said third edge (97) being a radially outer edge; said second edges (95) of adjacent vanes defining an inlet area (I) between the vanes (90) over their entire radial dimension and being generally planar, the inlet area having a circular outer boundary; said third edges (97) of adjacent vanes (90) defining an outlet area (O) between the vanes (90); and said outlet area (O) being greater than said inlet area (I). The impeller of claim 1 wherein said vanes (90) are straight. The impeller of claim 1 wherein said vanes (90) are curved. The impeller of claim 1 which comprises three vanes (90). The impeller of claim 1 which comprises four vanes (90). The impeller of claim 1 wherein said vanes (90) are thicker adjacent said surface (89). The impeller of claim 1 wherein said vanes are thicker adjacent the center of said base. The impeller of claim 1 which is composed of graphite. The impeller of claim 1 which further comprises a bearing (42) at least partially encasing said circular base (88). A molten metal pump (20) comprising: (a) a shaft (30) having fist and second ends; (b) a means for rotating said shaft in communication with the first end thereof; (c) an impeller (40) in communication with said second end; and (d) a volute (38), housing said impeller, having a first opening (45) through which molten metal can be drawn and a second opening (48) through which molten metal can be discharged: characterised in that said impeller (40) is an impeller in accordance with any preceding claim. The pump of claim 10 wherein a bearing (44) at least partially encases said base member (38).

Inventors: GILBERT RONALD E, MORDUE GEORGE S, GILBERT, RONALD E., MORDUE, GEORGE S.
Applicants: METAULLICS SYSTEMS CO L P, METAULLICS SYSTEMS CO., L.P.
Application year: 1993
Main classification: F04D7
IPC codes: B22D35, F04D29, F04D7