Casing for centrifugal pump

Abstract

The present invention relates to a casing of a centrifugal pump, and more particularly to a casing of a multi-stage centrifugal pump. The casing (7) includes a side plate (26) having a front surface facing to a back surface of a main plate (35) of an impeller (5), and return vanes (16) each of which has an arc shape and is projection-welded to the back surface of the side plate (26) at a plurality of welding points (PM1-PM4) provided on the upper end surface. The main plate (35) of the impeller (5) has trim edge portions (43) each of which is recessed radially inward, and the outermost welding point (PM4) farthest from the rotary shaft (1) among the plurality of welding points (PM1-PM4) overlaps at least partially with a projection point (PR1) which is projection of the outermost periphery (OM) of the impeller (1) onto the return vane (16), or is located radially inward of the projection point (PR1).

Description

Technical Field

[0001] The present invention relates to a casing of a centrifugal pump, and more particularly relates to a casing of a multi-stage centrifugal pump.Background Art

[0002] A centrifugal pump is a device that rotates an impeller housed in a casing to apply a centrifugal force to a liquid (e.g., water), thereby pressurizing the liquid. A plurality of diffusers is arranged in the casing, and a velocity energy of the liquid discharged from the impeller is converted into pressure energy as the liquid passes through a diffuser passage formed between adjacent diffusers. The casing also has guide vanes arranged therein, which are called return vanes, and liquid pressurized by the diffusers is guided by the return vanes to flow into the next impeller, or to be discharged from the discharge port. The centrifugal pump having one impeller is called a single-stage centrifugal pump, and a centrifugal pump having a plurality of impellers is called a multi-stage centrifugal pump.

[0003] In some types of centrifugal pumps, the return vane may also serve as the diffuser. In this case, the flow passage formed between adjacent return vanes serves as the diffuser passage, and thus the diffuser is omitted from the casing. In this specification, even if the return vane also serves as the diffuser, this vane is simply referred to as return vane.

[0004] The impeller generally includes a main plate (discharge-side plate member) secured to a rotary shaft, a shroud (suction-side plate member) facing to the main plate, and a plurality of blades arranged between the main plate and the shroud. A part of liquid discharged from the impeller flows into a back side of the main plate and a front side of the shroud. There is a difference in areas between the back side and the front side at which the impeller receives liquid pressure, thereby generating an axial thrust force that pushes the impeller toward the suction side.

[0005] High axial thrust force accelerates wear on bearings supporting the rotary shaft, thereby shortening their lifespan. Therefore, several technologies for reducing the axial thrust force have been proposed. For example, a main plate of an impeller assembly described in Patent Document 1 has a plurality of inclined contours projecting radially from a peripheral region of the main plate, and arcuate contours connecting adjacent inclined contours. The inclined contours and the arcuate contours form an outer periphery of the main plate. The arcuate contours are peripheral portions each of which is recessed radially inward of the main plate, and includes, for example, at least one portion whose distance from a center of the rotary shaft is increased radially. Using the main plate having such inclined contours and the arcuate contours can reduce the axial thrust force while ensuring maximum efficiency of the centrifugal pump. In this specification, such arcuate contour is referred to as "trim edge".Citation List Patent Literature

[0006] Patent document 1: Japanese Patent No. 6713460Summary of Invention Technical Problem

[0007] The plurality of return vanes mentioned above have the same arc-shaped configuration, respectively. An arc-shaped lower end surface of each return vane is secured to a surface of a bottom plate, which is formed in the casing, by projection-welding at a plurality of welding points. On the other hand, an arc-shaped upper end surface of each return vane is secured to a side plate, which is provided in the casing adjacent to the back surface of the main plate of the impeller, by projection-welding at a plurality of welding points. In this configuration, the side plate forms a communication flow passage for connecting the discharge from the impeller to the next-stage impeller or a discharge port, and the plurality of return vanes are arranged in this communication flow passage.

[0008] The inventors, through intensive research on the centrifugal pump employing the impeller with the trim edges, have found that a large pressure drop occurs in a center portion of a gap formed between the front surface of the side plate and the back surface of the main plate in the impeller (namely, near the rotary shaft). In the communication flow passage in which the return vanes are arranged, the diffusers or the return vanes cause an increase in the liquid pressure, so that a large tensile force in a direction of the rotary shaft due to the differential pressure acts on the side plate of the casing, possibly causing the side plate to peel off from the return vanes. In other words, the welding points for securing the upper surface of the return vane to the side plate may peel away, possibly resulting in damage to the side plate.

[0009] Therefore, it is an object of the present invention to provide a casing capable of effectively preventing damage to the side plate even if an impeller with trim edges is used.Solution to Problem

[0010] In one aspect, there is provided a casing of a centrifugal pump that houses an impeller secured to a rotary shaft, comprising: a bottom plate; a side plate having a front surface facing to a back surface of a main plate of the impeller and a back surface facing to the bottom plate; and return vanes each of which has an arc-shape and is projection-welded to the back surface of the side plate at a plurality of welding points provided on an upper end surface; wherein the main plate of the impeller has trim edge portions each of which is recessed radially inward; and the outermost welding point farthest from the rotary shaft among the plurality of welding points overlaps at least partially with a projection point, which is projection of the outermost periphery of the impeller onto the return vane, or is located radially inward of the projection point.

[0011] In one aspect, an arc-shaped distance from an inner end of the return vane to the outermost welding point is within 65% of an overall length of the return vane.

[0012] In one aspect, an arc-shaped distance from an inner end of the return vane to an innermost welding point, which is closest to the rotary shaft, among the plurality of welding points is in the range of 5-20% of an overall length of the return vane.

[0013] In one aspect, positions of the plurality of welding points with respect to the return vane are determined such that an arcuate spacing between adjacent welding points among the plurality of welding points is in the range of 20-40% of an overall length of the return vane, and all of remaining welding points other than the outermost welding point are located radially inward of the projection point on the return vane.

[0014] In one aspect, the projection point is a first projection point, and only the outermost welding point or both the outermost welding point and a welding point adjacent to the outermost welding point are located between the first projection point and a second projection point which is projection of a radially innermost recessed point of the trim edge portion onto the return vane.Advantageous Effects of Invention

[0015] Since the welding points for securing the return vane to the side plate are concentrated near the center of the return vane, the bonding strength between the return vane and the side plate near the center of the side plate is increased. As a result, even if using the impeller with the trim edge causes a large tensile force in the direction of the rotary shaft to be generated on the side plate, damage to the side plate can be effectively prevented.Brief Description of Drawings

[0016] [FIG. 1] FIG. 1 is a cross-sectional view showing a centrifugal pump according to one embodiment; [FIG. 2] FIG. 2 is a view of an impeller shown in FIG. 1 as seen from a suction side thereof; [FIG. 3] FIG. 3 is a view of the impeller shown in FIG. 2 as seen from a discharge side thereof; [FIG. 4] FIG. 4 is a schematic view showing a main plate of an impeller according to another embodiment; [FIG. 5] FIG. 5 is a cross-sectional view schematically showing a main section around the impeller; and [FIG. 6] FIG. 6 is a top view schematically showing a side plate shown in FIG. 5. Description of Embodiments

[0017] Embodiments according to the present invention will be described below with reference to the drawings.

[0018] FIG. 1 is a cross-sectional view showing a centrifugal pump according to one embodiment. The centrifugal pump shown in FIG. 1 includes a rotary shaft 1, a plurality of impellers 5 secured to the rotary shaft 1, and a casing 7 that houses the impellers 5. The rotary shaft 1 is coupled to a driving machine (e.g., electric motor) 8. When the driving machine 8 rotates the rotary shaft 1, the impellers 5 are rotated together with the rotary shaft 1. The centrifugal pump shown in FIG. 1 is a multi-stage centrifugal pump having the plurality of impellers 5.

[0019] The casing 7 shown in FIG. 1 has an inner casing 7A and an outer casing 7B. The inner casing 7A is disposed within the outer casing 7B, and an outer surface of the inner casing 7A is covered by the outer casing 7B. An opening in a suction-side of the inner casing 7A is coupled to a suction port 10, and an opening in a discharge-side of the outer casing 7B is coupled to a discharge port 11. A plurality of return vanes 16 are disposed on a back side of the impeller 5. The return vanes 16 shown in FIG. 1 also serves as diffusers that convert velocity energy, applied to the liquid by the impeller 5, into pressure energy.

[0020] As the impellers 5 rotate, liquid is sucked into the impellers 5 through the suction port 10. The rotation of the impellers 5 causes a velocity and a pressure of the liquid to be increased, and when the liquid passes through the flow passages (which also serve as diffuser flow passages in this embodiment) formed between adjacent return vanes 16, the velocity energy of the liquid is converted into the pressure energy (i.e., the liquid is pressurized).

[0021] The pressurized liquid is guided to the next-stage impeller 5 by the return vanes 16, and is further pressurized by the next-stage impeller 5 and the return vanes 16. The liquid leaving the final-stage impeller 5 flows into the outer casing 7B through a plurality of communication holes 20 formed in an end of the inner casing 7A. The liquid passes through a flow passage 21 formed between an inner surface of the outer casing 7B and an outer surface of the inner casing 7A toward the discharge port 11, and is discharged through the discharge port 11 to an outside of the centrifugal pump.

[0022] FIG. 2 is a view of the impeller shown in FIG. 1 as seen from a suction side thereof, and FIG. 3 is a view of the impeller shown in FIG. 2 as seen from a discharge side thereof. The impeller 5 shown in FIGS. 2 and 3 includes a main plate 35 secured to the rotary shaft 1, a shroud 37 facing to the main plate 35, and a plurality of blades 38 arranged between the main plate 35 and the shroud 37. The main plate 35 has a disk shape, and the shroud 37 is configured to cover the main plate 35. The shroud 37 has a liquid inlet 30 formed in a center thereof. A diameter of the main plate 35 is the same as or smaller than a diameter of the shroud 37.

[0023] The main plate 35 has a plurality of inclined contours 41 formed in an outer periphery thereof, and a plurality of arcuate contours 42 connecting adjacent inclined contours 41. Each inclined contour 41 has approximately a trapezoidal shape, and the inclined contours are arranged at equal intervals around the center of the impeller 5. Each arcuate contour 42 forms a radially inwardly recessed portion 43 in the outer periphery of the main plate 35. The arcuate contour 42 has, for example, at least a portion where a distance from an axis center of the rotary shaft 1 increases in a radial direction, and connects adjacent inclined contours 41 with a smooth curve. The inclined contours 41 have the same shape as each other, and the arcuate contours 42 also have the same shape as each other. In this specification, the radially inwardly recessed portions 43 formed in the outer periphery of the main plate 35 due to the arcuate contours 42 are referred to as "trim edge portions 43." These trim edge portions 43 are also arranged at equal intervals around the center of the impeller 5.

[0024] The blades 38 are arranged at equal intervals around the center of the impeller 5, and a liquid flow passage is formed between adjacent blades 38. An outer portion of each blade 38 is located between adjacent arcuate contours 42 (i.e., trim edge portions 43).

[0025] A through hole 48 for passing the rotary shaft 1 is formed in an engaging portion (i.e., boss) 45 of the main plate 35. The engaging portion 45 has a plurality of key grooves 46 that form a part of the through hole 48, respectively. These key grooves 46 are arranged at equal intervals around the center of the impeller 5. Each key groove 46 has a shape that engages with a key (not shown) formed on the surface of the rotary shaft 1. The engagement of the keys with the key grooves 46 ensures reliable transmission of torque from the rotary shaft 1 to the impeller 5.

[0026] FIG. 4 is a schematic view showing a main plate of an impeller according to another embodiment. The main plate 35 shown in FIG. 4 also has the plurality of inclined contours 41 formed in the outer periphery thereof, and the plurality of arcuate contours 42 connecting adjacent inclined contours 41. The inclined contours 41 have approximately a trapezoidal shape, respectively, and the arcuate contours 42, respectively, have at least a portion where the distance from the axis center of the rotary shaft 1 increases in the radial direction. The arcuate contours 42 connect adjacent inclined contours 41 with smooth curves, and the trim edge portions 43 each of which is recessed radially inward are formed in the outer periphery of the main plate 35 of the impeller 5.

[0027] It has been demonstrated that the impeller 5 having the main plate 35 provided with the trim edge portions 43 as shown in FIGS. 3 and 4 can reduce the axial thrust force without reducing pump efficiency.

[0028] FIG. 5 is a cross-sectional view schematically showing a main section around the impeller. FIG. 5 omits illustration of the outer casing 7B, and shows only the inner casing 7A. As shown in FIG. 5, the return vanes 16 are arranged at the back side of the impeller 5 through a side plate 26. More specifically, a back surface of the main plate 35 of the impeller 5 faces to a front surface of the side plate 26 with a small gap, and an upper end surface of the return vane 16 is secured to a back surface of the side plate 26 by projection-welding. A lower end surface of the return vane 16 is secured to a bottom plate 25, which is formed on the inner casing 7A, by projection-welding. The back surface of the side plate 26 faces to a front surface of the bottom plate 25.

[0029] The bottom plate 25 has a disk shape extending from an inner-wall surface of the inner casing 7A toward the rotary shaft 1, and has an opening in a center thereof. The rotary shaft 1 extends through the opening of the bottom plate 25, and a small gap is formed between the rotary shaft 1 and the opening in the bottom plate 25. A communication flow passage, which guides the liquid discharged from the impeller 5 to the next-stage impeller 5, is formed by the side plate 26 secured to the upper end surfaces of the return vanes 16 and the bottom plate 25 secured to the lower end surfaces of the return vanes 16. The liquid discharged from the outermost periphery of the impeller 5 flows into the next-stage impeller 5 through the communication flow passage.

[0030] In this embodiment, the return vanes 16 serve as guide vanes that guide the liquid to the next-stage impeller 5, and also serve as diffusers that convert the velocity energy of the liquid into pressure energy. In one embodiment, the return vanes 16 may serve only as guide vanes. In this case, as shown by dashed lines in FIG. 5, diffusers 9 are arranged upstream of the return vanes 16 in the communication flow passage.

[0031] It has been found that, when impellers are the impellers 5 having the trim edge portions 43 described with reference to FIGS. 2 to 4, a large pressure drop may occur in the center of gap formed between the back surface of the impeller 5 and the front surface of the side plate 26 (i.e., near the rotary shaft 1). A pressure of the liquid flowing through the communication flow passage in which the return vanes 16 are arranged is higher than a pressure of the liquid immediately after being discharged from the impeller 5. Therefore, if the pressure drop occurring in the center of gap formed between the back surface of the impeller 5 and the upper surface of the side plate 26 is too large, a large tensile force is generated in the side plate 26 in the longitudinal direction of the rotary shaft 1 due to a pressure difference between the front side and the back side of the side plate 26. Depending on magnitude of this tensile force, the side plate 26 may be peeled off from the upper end surfaces of the return vanes 16, resulting in damage to the side plate 26.

[0032] Generally, the projection-welding has many advantages, such as the ability to efficiently perform welding in a short time without relying on welding skill level of operator, low processing costs, and the ability to produce large quantities. In contrast, the projection-welding has a disadvantage that the welding equipment becomes large as the number of welding points increases. Furthermore, the projection-welding also has a disadvantage that it is difficult to align welding parts to each other when performing projection-welding at multiple welding points. Therefore, it is difficult to increase the number of welding points for welding the side plate 26 to the return vanes 16 in order to prevent damage to the side plate 26. Accordingly, in this embodiment, the welding points between the side plate 26 and the upper end surface of the return vane 16 are concentrated near the rotary shaft 1 (i.e., near the center of the side plate 26).

[0033] FIG. 6 is a top view schematically showing the side plate 26. As shown in FIG. 6, the side plate 26 also has a disk shape, and an opening for passing the rotary shaft 1 is formed in a center thereof. As shown in FIG. 6, the upper end surface of each return vane 16, which has the arcuate shape, is projection-welded to the back surface of the side plate 26 at a plurality of (four in FIG. 6) welding points PM1, PM2, PM3, and PM4. The welding point PM1 is the innermost welding point which is closest to the rotary shaft 1, and the welding point PM4 is the outermost welding point which is farthest from the rotary shaft 1. The welding points PM2 and PM3 are located between the innermost welding point PM1 and the outermost welding point PM4.

[0034] A welded portion at the outermost welding point PM4 overlaps at least partially with a projection point PR1, which is projection of an outermost periphery OM of the impeller 5 onto the return vane 16 (see FIG. 5), or is located radially inward of the projection point PR1. As a result, all of the welding points PM1-PM4 are concentrated in the center of the return vane 16, so that bonding strength between the side plate 26 and the return vane 16 near the center of the side plate 26 can be increased without increasing the number of projection-welding points. Therefore, even if a large tensile force in the longitudinal direction of the rotary shaft 1 is generated on the side plate 26, damage to the side plate 26 can be effectively prevented. Note that in this specification, the welded portion of the welding point is defined as "the portion where the side plate 26 and the return vane 16 are welded together".

[0035] In order to increase the bonding strength between the side plate 26 and the return vane 16, and / or to facilitate manufacturing of the casing 7, an arcuate spacing (or an arc-shaped distance) between adjacent welding points among the welding points PM1-PM4 is preferably in the range of 20-40% of overall length L (see FIG. 6) of the return vane 16. In this case, positions of all welding points PM1-PM4 on the return vane 16 are determined such that, while satisfying the condition that the welded portion of the outermost welding point PM4 overlaps at least partially with the projection point PR1 or is located radially inward of the projection point PR1, the remaining welding points PM1-PM3 are all located radially inward of the projection point PR1 on the return vane 16. Further, for the same purpose, all of the welding points PM1-PM4 are preferably arranged at equal intervals on the arcuate upper end surface of the return vane 16. Furthermore, for the same purpose, an arc-shaped distance from an inner end 16a of the arcuate return vane 16 to the innermost welding point PM1 is preferably in the range of 5 to 20% of the overall length L of the return vane 16. In this specification, the terms of arc-shaped distance and arcuate spacing refer to the length along the arcuate shape of the return vane 16.

[0036] In one embodiment, the arc-shaped distance from the inner end 16a of the return vane 16 to the outermost weld point PM4 is preferably within 65% of the overall length L of the return vane 16. In this case, the welded portion at the outermost welding point PM4 could be located radially inward from the projection point PR described above, but it is necessary that the welded portion at the outermost welding point PM4 not be located radially outward from the projection point PR. Here, the inner end 16a of the return vane 16 refers to an end of the return vane 16 closest to the impeller 5. In other words, the inner end 16a of the return vane 16 is a portion of the return vane 16 that is located most radially inward.

[0037] In one embodiment, it is preferable to arrange all of the welding points PM1-PM4 such that only the outermost welding point PM4, or the outermost welding point PM4 and the welding point PM3 adjacent thereto, are located between the above-mentioned projection point PR1 and a projection point PR2, which is projection of a radially innermost recessed point PD of the trim edge portion 43 (or the arcuate contour 42) onto the return vane 16. Such a configuration helps to ensure the whole bonding strength between the side plate 26 and the return vane 16.

[0038] In the embodiments described above, the multi-stage centrifugal pump having the plurality of impellers 5 have been described. However, these embodiments are also applicable to a single-stage centrifugal pump having a single impeller 5.

[0039] The previous description of embodiments is provided to enable a person skilled in the art to make and use the present invention. Moreover, various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles and specific examples defined herein may be applied to other embodiments. Therefore, the present invention is not intended to be limited to the embodiments described herein but is to be accorded the widest scope as defined by limitation of the claims.Industrial Applicability

[0040] The present invention is applicable to a casing of a centrifugal pump.Reference Signs List

[0041] 1rotary shaft 5impeller 7casing 7Ainner casing 7Bouter casing 8electric motor 9diffuser 10suction port 11discharge port 16return vane 25bottom plate 26side plate 35main plate 37shroud 38blade 41inclined contour 42arcuate contour 43trim edge portion PM1, PM2, PM3, PM4welding point PR1, PR2projection point

Claims

1. A casing of a centrifugal pump that houses an impeller secured to a rotary shaft, comprising: a bottom plate; a side plate having a front surface facing to a back surface of a main plate of the impeller and a back surface facing to the bottom plate; and return vanes each of which has an arc-shape and is projection-welded to the back surface of the side plate at a plurality of welding points provided on an upper end surface; wherein the main plate of the impeller has trim edge portions each of which is recessed radially inward; and the outermost welding point farthest from the rotary shaft among the plurality of welding points overlaps at least partially with a projection point, which is projection of the outermost periphery of the impeller onto the return vane, or is located radially inward of the projection point.

2. The casing according to claim 1, wherein an arc-shaped distance from an inner end of the return vane to the outermost welding point is within 65% of an overall length of the return vane.

3. The casing according to claim 1, wherein an arc-shaped distance from an inner end of the return vane to an innermost welding point, which is closest to the rotary shaft, among the plurality of welding points is in the range of 5-20% of an overall length of the return vane.

4. The casing according to claim 1, wherein positions of the plurality of welding points with respect to the return vane are determined such that an arcuate spacing between adjacent welding points among the plurality of welding points is in the range of 20-40% of an overall length of the return vane, and all of remaining welding points other than the outermost welding point are located radially inward of the projection point on the return vane.

5. The casing according to any one of claims 1 to 4, wherein the projection point is a first projection point, and only the outermost welding point or both the outermost welding point and a welding point adjacent to the outermost welding point are located between the first projection point and a second projection point which is projection of a radially innermost recessed point of the trim edge portion onto the return vane.

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