Patent Application
20060177305
This invention relates to hydrodynamic machinery in general, and in particular, to a centrifugal pump having a vane-island diffuser and a volute with a cutwater that is separate and discontinuous from the vane-islands of the diffuser.
Fluid pumps are generally considered to fall into one of two main categories, viz., positive displacement types, and rotary types, the latter including axial flow and centrifugal flow embodiments. The development of centrifugal pumps began in about the middle of the nineteenth century, and, because of their ability to provide relatively high flow rates and pressure heads from relatively small device sizes, have been used over the years in a wide variety of pumping tasks, ranging from the removal and transport of sewage and waste water, to the pumping of fuel, e.g., liquid hydrogen, to liquid rocket engines.
A centrifugal pump comprises an impeller, including a plurality of radial blades that is mounted on a shaft driven by a motor and rotated within a hollow, closely fitting housing or casing. The fluid to be pumped is continuously drawn into the center of the rotating impeller, where it is accelerated radially by centrifugal forces to a relatively high velocity by the blades of the impeller, then discharged into the hollow space of the casing, where the high-velocity stream of fluid is “diffused,” i.e., converted into a low-velocity, high-pressure stream.
In a “volute” type of pump, the casing is provided with a generally circumferential flow channel having a cross-sectional area that gradually expands from a relatively small inlet end to a relatively large outlet end corresponding to the outlet, or discharge, of the pump. A tapered “cutwater” having a leading edge is disposed in the casing between the inlet and outlet ends and serves to define opposite side walls of the inlet and outlets of the volute.
In a “vane-island diffuser” centrifugal pump, the diffusion function is achieved by the provision of an annular array of identical, fixed “vane-islands” surrounding the impeller. The vane-islands define a series of generally radial-tangential channels that expand radially outward and open into the hollow space of the casing surrounding the diffuser. The diffuser splits the flow of high-velocity fluid leaving the blades of the impeller into a plurality of radial-tangential streams that are decelerated within the expanding channels, thereby converting the kinetic energy of the streams into potential energy. The streams then recombine in a single, low-velocity, high-pressure stream in the circumferential space of the casing.
It is known to combine the diffuser designs of a volute pump with that of a vane-island pump to achieve a more efficient diffusion of the high-velocity fluid leaving the impeller, such a combination being referred to as a “vane-island-diffuser volute” pump. In such designs, it is conventional to combine the cutwater of the volute with one of the vane-islands of the diffuser in a single, continuous structure. However, such an arrangement has several drawbacks, including manufacturing complexities that render the pump difficult and expensive to make. Additionally, such a design imposes increased radial loads on the cutwater, and large tangential loads on its leading edge, relative to those on the cutwater of a conventional volute, and can additionally increase the weight of and the acoustic noise generated by the pump significantly.
Accordingly, there is need in the centrifugal pump art for design improvements that can successfully address and overcome the foregoing problems.
In accordance with embodiments of the present invention, a centrifugal volute pump with a vane-island diffuser is provided that overcomes the above and other problems of the prior art pumps by making the vane-island diffuser independent of and discontinuous from the cutwater of the volute.
In one exemplary embodiment thereof, the novel pump comprises a pump casing containing a volute channel having inlet and an outlet ends. A tapered cutwater is disposed between the inlet and outlet ends of the volute and defines a first side of a volute inlet. An annular vane-island diffuser, comprising a plurality of identical vane-islands, each having a circumferential surface, is fixed generally concentrically in the casing such that the circumferential surface of one of the vane islands of the diffuser is spaced apart from the leading edge of the cutwater of the volute and defines a second side of the volute inlet. An impeller rotatably mounted on a shaft is disposed concentrically within the diffuser.
In one embodiment, the vane-island diffuser may be formed integrally with the casing, e.g., by a casting process, such as a die-casting or an investment-casting process, and in an advantageous alternative embodiment, the diffuser may be formed independently of the casing. Additionally, the casing and diffuser may be split either axially or radially for ease of assembly, as with conventional volute pump designs.
An important advantage of the novel design is that, unlike prior art pumps, the angular position of the diffuser relative to the leading edge of the cutwater can be selectably adjusted, either during manufacture or assembly, such that the diffuser can be “clocked” relative to the cutwater to minimize the radial loads imposed on the cutwater and the dynamic imposed loads on its leading edge. Additional advantages of the novel design include reductions in pump weight, reduced frictional losses in the pump, decreased pump design and modeling time, and a decrease in the energy of outgoing acoustic waves at the pump discharge.
A better understanding of the above and many other features and advantages of the present invention may be obtained from a consideration of the detailed description thereof below, particularly if such consideration is made in conjunction with the appended drawings, wherein like reference numerals are used to identify like elements illustrated in one or more of the figures.
When the impeller 20 is rotated, a fluid (not illustrated) is drawn axially toward the center of the impeller and is accelerated in the radial direction by the vanes of the impeller. The diffuser 22 functions to split the flow of high-velocity fluid leaving the vanes of the spinning impeller into a plurality of radial streams that are caused to decelerate within the expanding channels 26, thereby converting the kinetic energy, or dynamic pressure, of the streams into potential energy, or static pressure. The streams then recombine in a single, low-velocity, high-pressure stream in the volute 14, which functions to further diffuse the flow of the stream, until it is expelled at a maximum static pressure at the outlet end 18 of the pump 10.
As illustrated in
While the foregoing arrangement achieves a satisfactory “pressure recovery,” i.e., conversion of dynamic pressure to static pressure of the pumped fluid, it is not achieved without some cost. In particular, because the leading edge 30 of the vane-island/cutwater 28 is necessarily disposed radially closer to the impeller, larger hydrodynamic radial loads are imposed on the cutwater, and larger hydrodynamic tangential loads are imposed on its leading edge, than those imposed on the cutwater of a conventional volute. Accommodating these increased loads in the pump's structure necessitates increasing the thicknesses of the structures, thereby resulting in an undesirable weight penalty and performance penalty.
In either a vane-island diffuser centrifugal pump or a vane-island-diffuser volute pump, acoustic pulses are generated as the high velocity streams leaving the passages of the rotating impeller impinge upon the leading edges of the vane-islands. At certain rotational speeds, these acoustic pulses add constructively in the circumferential space of the casing and result in undesireable high amplitude acoustic pulses propagating into the fluid beyond the pump outlet.
Yet another problem with the conventional combined vane-island/cutwater structure 28 relates to its manufacturability. As those of skill in the art will appreciate, it is frequently desirable to manufacture the casing 12 and the diffuser 22 as a single, integral piece, e.g., as an investment casting. However, as may be seen in
It has been discovered that the above and other problems of prior art vane-island-diffuser volute pumps can be overcome by the provision of a centrifugal pump in which the vane-island diffuser is arranged to be independent of and discontinuous from the cutwater of the volute. An exemplary embodiment of such a pump 100 is illustrated in the cross-sectional elevation view of
As those of skill in the art will appreciate, an important advantage of the novel design in which the cutwater 128 is separate and independent from the vane-island diffuser 122 is that, unlike the prior art pump of
The vane-island diffuser 122 may be formed integrally with the casing 112, e.g., by a casting process, such as a die-casting or an investment-casting process, and of a variety of materials, e.g., an aluminum alloy, without the need for the provision of drain holes, and in a particularly advantageous alternative embodiment, the diffuser may be formed independently of the casing, and assembled within it later. Additionally, the casing and diffuser may be split, either axially or radially, as with conventional volute pumps, thereby increasing its ease of manufacture.
The novel pump design of
Further, as described above, it enables the volute to have a selectably adjustable amount of “overturn,” or overlap between inlet and outlet, and to thereby obtain better flow performance. As above, it enables ready draining of slurry when the pump is made by a casting process, without the need for the provision of drain holes or their subsequent plugging. It also enables the provision of two separate parts (i.e., the diffuser 122 and the volute casing 112) for ease and flexibility of manufacturing.
The novel design also reduces flow friction loss by about 10%, because the flow through the channel 126 immediately preceding the “first” vane-island (indicated by 36 and 136 in the figures, respectively), i.e., the vane-island immediately adjacent to the volute inlet 116, flows directly into the volute outlet, and accordingly, is not required to travel the entire 360° length of the volute. Also, since all diffuser vane-islands may now be made identical, the design reduces pump design and modeling time. Finally, since acoustic transmission through the inlet 116 of the volute 114 bleeds away some energy from the outgoing acoustic waves at the pump discharge, the undesirable acoustic energy leaving the pump discharge is noticeably reduced.
By now, those of skill in this art will appreciate that many modifications, substitutions and variations can be made in and to the materials, apparatus, configurations and methods of implementation of the centrifugal pump of the present invention without departing from its spirit and scope. Accordingly, the scope of the present invention should not be limited to the particular embodiments illustrated and described herein, as they are merely exemplary in nature, but rather, should be fully commensurate with that of the claims appended hereafter and their functional equivalents.
The invention described herein was made in the performance of work under NASA Contract No. NAS 8-01107 and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958 (72 Stat. 435: 42 U.S.C. 2457).
