CN219672911U - Two-stage middle-open centrifugal pump - Google Patents

Abstract

The utility model provides a two-stage middle-open centrifugal pump which is convenient to assemble and good in sealing performance. An intermediate bushing is arranged between the first-stage impeller and the second-stage impeller, and a labyrinth structure seal is arranged on the inner circular surface of the intermediate bushing. The structure has good sealing performance, reduces axial leakage between the pump cavities of the primary impeller and the secondary impeller, and in addition, the middle bushing provided by the utility model is divided into the combination of the middle bushing of the upper half ring and the middle bushing of the lower half ring, and the middle bushing of the upper half ring and the middle bushing of the lower half ring can be buckled together and sleeved between the primary impeller and the secondary impeller directly between the two-stage impellers without disassembling the impeller to assemble the middle bushing after dynamic balance monitoring is completed. The assembly is more convenient, time-saving and labor-saving.

Description

Two-stage middle-open centrifugal pump

Technical Field

The utility model relates to the technical field of centrifugal pumps, in particular to a two-stage middle-open type centrifugal pump.

Background

The two-stage middle-open centrifugal pump is mainly used for conveying crude oil, finished oil and the like.

The two-stage split centrifugal pump comprises an upper pump body, a lower pump body, a pump shaft, a primary impeller, a secondary impeller, an intermediate bushing, a shell mechanical seal, a driving end bearing box and a non-driving end bearing box. An intermediate bushing is arranged on a pump cavity partition plate between the first-stage impeller pump cavity and the second-stage impeller pump cavity, and the outer circle of the intermediate bushing is fixedly matched and connected with a central hole of the shell pump cavity partition plate. The primary impeller and the secondary impeller are sleeved on the pump shaft and are installed at the left end and the right end of the middle bushing back to back. The liquid at the liquid outlet end of the one-stage impeller pump of the two-stage middle-open centrifugal pump is conveyed to the liquid inlet end of the two-stage impeller through an interstage flow channel in the pump body, and is discharged from the liquid outlet of the pump body after being acted by the two-stage impeller.

The pump shaft of the two-stage middle-open centrifugal pump is longer, the first-stage impeller and the second-stage impeller are the central parts of the pump shafts, and certain coaxial errors and machining precision errors always exist in the central parts of the pump shafts during machining. In order to ensure the running fit of the pump shaft and the shaft hole of the middle bushing, the fit clearance between the shaft hole of the middle bushing and the pump shaft is larger, so that the axial leakage between the primary impeller and the secondary impeller pump cavity is larger, the pump lift and the pump efficiency are greatly influenced, and the pump lift and the pump efficiency are reduced.

In view of this, there is a need for a two-stage, center-open centrifugal pump having reduced axial leakage between the primary impeller and the secondary impeller pump chambers, resulting in improved pump head and efficiency.

Disclosure of Invention

The technical problems to be solved are as follows:

in view of the above-mentioned shortcomings and disadvantages of the prior art, the present utility model provides a two-stage center-open centrifugal pump, which aims to solve the problems of the prior art that axial leakage between a primary impeller and a secondary impeller pump cavity is large, and pump lift and efficiency are greatly affected, resulting in pump lift and efficiency reduction.

The technical scheme is as follows:

in order to achieve the above purpose, the main technical scheme adopted by the utility model comprises the following steps:

the embodiment of the utility model provides a two-stage middle-open centrifugal pump, which comprises a primary impeller, a secondary impeller and a pump shaft, wherein the primary impeller and the secondary impeller are sleeved on the pump shaft, an intermediate bushing is arranged between the primary impeller and the secondary impeller, a throttling boss is arranged on the inner circular surface of the intermediate bushing, a throttling groove is arranged on the outer wall of the pump shaft, and the throttling boss is inserted into the throttling groove and is in sliding clearance fit with the throttling groove to form labyrinth seal.

Optionally, the throttling groove is directly formed on the outer circular surface of the pump shaft.

Optionally, an intermediate spacer bush is arranged between the intermediate bush and the pump shaft, and the throttling groove is formed in the outer circular surface of the intermediate spacer bush.

Optionally, a sealing ring is arranged between the middle spacer bush and the outer circular surface of the pump shaft.

Optionally, the throttle boss and the throttle groove are plural.

Optionally, the middle bushing comprises an upper half ring middle bushing and a lower half ring middle bushing, and the upper half ring middle bushing and the lower half ring middle bushing are buckled together and sleeved between the primary impeller and the secondary impeller.

Optionally, a positioning boss is arranged on the outer wall of the middle bushing and is matched with a positioning groove formed in a central hole of the pump cavity partition plate of the shell.

Optionally, a liner sealing ring is arranged between the outer circular surface of the middle liner and the inner circular surface of the central hole of the casing pump cavity partition plate.

Optionally, the left and right ends of the middle bushing are respectively provided with an inner spigot groove, the hubs of the primary impeller and the secondary impeller are provided with spigot rings, and the spigot rings are arranged in the inner spigot grooves and are in sliding fit with the inner spigot grooves.

The beneficial effects are that:

the beneficial effects of the utility model are as follows: the utility model relates to a two-stage middle-open centrifugal pump, which is characterized in that a throttling boss is arranged on the inner circular surface of a middle bushing, and a throttling groove is arranged on a pump shaft; the throttling boss is inserted into the throttling groove and is in clearance fit with the throttling groove to form a labyrinth seal. Compared with the prior art, the labyrinth type impeller pump has the advantages that the labyrinth type impeller pump has good sealing performance, reduces axial leakage between the primary impeller and the secondary impeller pump cavity, and achieves the effect of improving the pump lift and efficiency.

In addition, when the dynamic balance of the impeller shaft system is detected before the pump is assembled, the pump shaft between the primary impeller and the secondary impeller cannot be provided with the intermediate bushing, and the intermediate bushing needs to be installed on the pump shaft after the dynamic balance detection of the impeller shaft system is completed. The middle bushing in the prior art is of an integral ring structure, so that after the dynamic balance detection of the impeller shaft system is finished, the impeller on one side is required to be detached, and after the middle bushing is installed, the detached impeller is reinstalled, so that the assembly is time-consuming and labor-consuming. The bushing can be arranged into a combined structure with two semicircular bodies buckled, namely, the bushing is divided into an upper half-ring middle bushing and a lower half-ring middle bushing, and the upper half-ring middle bushing and the lower half-ring middle bushing can be arranged between the primary impeller and the secondary impeller in a buckling mode. After the dynamic balance detection is completed, the primary impeller or the secondary impeller is not required to be disassembled, and the upper half-ring middle bushing and the lower half-ring middle bushing can be directly buckled between the primary impeller and the secondary impeller. The structure makes the assembly of the middle bushing more convenient, time-saving and labor-saving, and solves the problems that the impeller needs to be disassembled and assembled during the assembly of the middle bushing and the assembly is complicated.

In conclusion, the two-stage middle-open centrifugal pump adopts the labyrinth sealing structure, has good sealing property, reduces axial leakage between the pump cavities of the first-stage impeller and the second-stage impeller, improves the pump lift and efficiency, and well solves the problems existing in the prior art.

Drawings

FIG. 1 is a cross-sectional view of the overall structure of a two-stage center-opening centrifugal pump;

FIG. 2 is an enlarged partial schematic view of the middle portion of FIG. 1;

FIG. 3 is an enlarged view showing the construction of the intermediate bushing;

FIG. 4 is a schematic view of an intermediate spacer structure;

FIG. 5 is a schematic illustration of a throttle groove disposed directly on the outer circumferential surface of a pump shaft;

fig. 6 is an enlarged schematic view of a portion of the circle in fig. 3.

[ reference numerals description ]

1a, an upper pump body; 1b, a lower pump body; 11. a housing pump chamber partition; 110. a central bore; 111. a positioning groove; 2a, a primary impeller; 2b, a secondary impeller; 21. a slip ring; 3. an intermediate bushing; 3a, an upper semi-ring middle bushing; 3b, a lower plate ring middle bushing; 31. positioning boss 32, throttle boss; 33. a bushing seal ring; 4. a pump shaft; 5a, mechanically sealing the left side of the shell; 5b, a mechanical seal 5b on the right side of the shell; 6a, driving a bearing body box; 6b, driving a bearing box; 7. an intermediate spacer; 71. a seal ring; 72. throttling groove

Detailed Description

The utility model will be better explained by the following detailed description of the embodiments with reference to the drawings. Wherein references herein to "upper", "lower", "left", "right", etc. are made to the orientation of fig. 1.

The embodiment of the utility model provides a two-stage middle-open centrifugal pump, which comprises a first-stage impeller 2a, a second-stage impeller 2b and a pump shaft 4. The primary impeller 2a and the secondary impeller 2b are sleeved on the pump shaft 4, and an intermediate bushing 3 is arranged between the primary impeller 2a and the secondary impeller 2 b. The inner circular surface of the middle bushing 3 is provided with a throttling boss 32, and the pump shaft 4 is provided with a throttling groove 72. The throttle boss 32 is inserted into the throttle recess 72 and is clearance fit with the throttle recess 72 to form a labyrinth seal. The labyrinth sealing structure can effectively reduce axial leakage between the primary impeller and the secondary impeller pump cavity, improves the pump lift and efficiency, and well solves the problems existing in the prior art.

In order to better understand the above technical solution, exemplary embodiments of the present utility model will be described in more detail below with reference to the accompanying drawings. While the exemplary embodiments of the utility model are shown in the drawings, it should be understood that: the present utility model may be embodied in various forms and should not be construed as limited to the embodiments set forth herein. These embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the utility model to those skilled in the art.

Examples:

referring to fig. 1, the utility model provides a two-stage center-open centrifugal pump, which comprises a pump body with a pump cavity baffle 11, a first-stage impeller 2a, a second-stage impeller 2b, a middle lining 3, a pump shaft 4, a shell left mechanical seal 5a, a shell right mechanical seal 5b, a driving end bearing box 6a, a non-driving end bearing box 6b and other structures.

The drive end bearing housing 6a and the non-drive end bearing housing 6b are mounted at both ends of the pump shaft 4, respectively. The pump body may be divided into an upper pump body 1a and a lower pump body 1b. The upper pump body 1a and the lower pump body 1b are vertically buckled and installed on the pump shaft 4 between the driving end bearing box 6a and the non-driving end bearing box 6 b. The housing left mechanical seal 5a is located on the left side of the upper pump body 1a and the lower pump body 1b, and the housing right mechanical seal 5b is located on the right side of the upper pump body 1a and the lower pump body 1b. The housing left mechanical seal 5a and the housing right mechanical seal 5b seal the gap between the pump body and the pump shaft 4. The primary impeller 2a, the secondary impeller 2b and the intermediate bush 3 are installed inside the upper pump body 1a and the lower pump body 1b, and the intermediate bush 3 is installed between the primary impeller 2a and the secondary impeller 2 b.

In the two-stage middle-open centrifugal pump, a throttle boss 32 is arranged on the inner circular surface of a middle bushing 3, and a throttle groove 72 is arranged on a pump shaft 4. The throttle boss 32 is inserted into the throttle recess 72 and is clearance fit with the throttle recess 72 to form a labyrinth seal.

Further, the throttle boss 32 is a boss of an annular structure, and the throttle groove 72 is a groove of an annular structure.

Here, the throttle groove 72 provided on the pump shaft 4 may be provided as follows:

setting form one: referring to fig. 5, the throttle recess 72 is directly opened at the outer circumferential surface of the pump shaft 4. The intermediate bushing 3 can be directly fitted over the pump shaft 4, in which case one or more throttle projections 32 are provided on the inner circumferential surface of the intermediate bushing 3. When the plurality of throttle bosses 32 are provided on the inner circular surface of the intermediate bushing 3, the plurality of throttle bosses 32 are arranged in an axial array along the intermediate bushing 3. The throttle groove 72 directly arranged on the pump shaft 4 corresponds to the throttle boss 32, and the throttle boss 32 is inserted into the corresponding throttle groove 72 to form clearance fit, so that a sealing labyrinth structure is formed, and a sealing effect is achieved.

The number of throttle bosses 32 and throttle recesses 72 are matched. The throttle grooves 72 are one or more corresponding to the throttle bosses 32, and when the throttle grooves 72 are plural, the throttle grooves 72 are arranged in an axial array along the pump shaft 4.

Setting form II: referring to fig. 2-4, an intermediate spacer 7 is disposed between the intermediate bushing 3 and the pump shaft 4, and a throttle groove 72 is formed in an outer circumferential surface of the intermediate spacer 7. The throttle recess 72 is not formed directly on the outer circumferential surface of the pump shaft 4, but on the outer circumferential surface of the intermediate spacer 7. The middle spacer bush 7 is sleeved on the pump shaft 4, and the middle lining 3 is sleeved on the outer side of the middle spacer bush 7. One or more throttle bosses 32 are provided on the inner circular surface of the intermediate bushing 3. When the plurality of throttle bosses 32 are provided on the inner circular surface of the intermediate bushing 3, the plurality of throttle bosses 32 are arranged in an axial array along the intermediate bushing 3. One or more throttle grooves 72 are provided on the outer wall of the intermediate spacer 7, and when the number of throttle grooves 72 is plural, the plurality of throttle grooves 72 are arranged in an axial array along the intermediate spacer 7. The number of throttle bosses 32 and throttle recesses 72 are matched.

Further, referring to fig. 2 and 4, a seal ring 71 may be provided between the inner circumferential surface of the intermediate spacer 7 and the outer circumferential surface of the pump shaft 4. Namely, a sealing groove is formed in the inner circular surface of the middle spacer 7, when the pump is installed, the sealing ring 71 is firstly placed in the sealing groove of the inner circular surface of the middle spacer 7, and then the middle spacer 7 provided with the sealing ring 71 is sleeved on the pump shaft 4 and moves to the corresponding position. The seal ring 71 may be provided in one or more. The arrangement of the sealing ring 71 reduces the axial leakage of the pump cavity parts of the primary impeller 2a and the secondary impeller 2b, and enhances the sealing effect between the middle spacer 7 and the pump shaft 4. At this time, the middle spacer 7 is used as a wearing part, so that the replacement is convenient.

For the foregoing two setting forms of "setting form one" and "setting form two": the throttling boss 32 is inserted into the throttling groove 72 with a certain gap left between them, the gap can be set according to specific conditions, for example, a single-sided gap of 0.25mm or the like can be set, the gap is called a throttling gap here, and throttling effect is generated when liquid flows through the throttling gap, so that a good sealing effect is achieved.

The seal arrangement of the throttle boss 32 and the throttle recess 72 in the form of a clearance fit is referred to as a labyrinth seal. When liquid flows through the primary impeller 2a and the secondary impeller 2b, axial leakage is easy to occur between pump cavities between the primary impeller 2a and the secondary impeller 2b, and the axial leakage can have larger influence on the pump lift and efficiency, so that the pump lift and efficiency are reduced, and the labyrinth sealing structure is arranged to strengthen the sealing between the primary impeller 2a and the secondary impeller 2b, so that the effect of axial leakage can be effectively prevented, the pump lift is guaranteed, and the working efficiency of the centrifugal pump is improved.

Further, the seal ring 71 may be made of rubber, or other known sealing materials.

In the conventional installation, since the existing intermediate bushing 3 is of a unitary ring structure, i.e., a sleeve-like structure, it is necessary to pass one end of the pump shaft 4 through the inner circular hole of the intermediate bushing 3 and then move the intermediate bushing 3 to the intermediate position of the pump shaft 4 at the time of installation. However, the primary impeller 2a and the secondary impeller 2b need to perform dynamic balance detection of the impeller shaft before assembly. The detection requires that the pump shaft between the primary impeller 2a and the secondary impeller 2b cannot be provided with the middle bushing 3, so that the middle bushing 3 can be installed after the dynamic balance detection of the impeller shaft system is completed, then the primary impeller 2a or the secondary impeller 2b is required to be detached firstly, then the middle bushing 3 is sleeved on the pump shaft 4, then the detached primary impeller 2a or the detached secondary impeller 2b is installed again, and then the rest assembly work of the centrifugal pump is performed. The process of installation, disassembly and reinstallation is tedious in assembly and time and labor consuming.

In the present utility model, referring to fig. 2, the middle bush 3 may have a structure of a snap-fit combination of two semi-circular bodies, the upper semi-circular ring middle bush 3a has one semi-circular body, and the lower semi-circular ring middle bush 3b has the other semi-circular body. After the dynamic balance detection is carried out on the impeller shaft system, when the middle bushing 3 is required to be installed, any one of the primary impeller 2a and the secondary impeller 2b is not required to be disassembled, and only the upper half-ring middle bushing 3a and the lower half-ring middle bushing 3b are required to be aligned and buckled on the pump shaft between the primary impeller 2a and the secondary impeller 2b, and the throttling boss 32 is enabled to be inserted into the throttling groove 72. Then, the upper half ring middle bush 3a and the lower half ring middle bush 3b are clamped by the buckling of the upper pump body 1a and the lower pump body 1b. The middle bushing 3 is arranged to be a structure form of buckling the upper semi-ring middle bushing 3a and the lower semi-ring middle bushing 3b, so that repeated disassembly and assembly of the rear-stage impeller 2a or the secondary impeller 2b in the shaft system dynamic balance detection are avoided, and the assembly work after the shaft system dynamic balance detection is more convenient, time-saving and labor-saving.

Further, referring to fig. 2 and 3, the outer wall of the middle bush 3 is provided with a positioning boss 31, and if the middle bush 3 is in the form of being divided into an upper half ring middle bush 3a and a lower half ring middle bush 3b, the positioning bosses 31 are provided on the outer circular surfaces of the upper half ring middle bush 3a and the lower half ring middle bush 3b. A positioning groove 111 is formed in a central hole 110 of a pump cavity baffle 11 of the pump body, and a positioning boss 31 is inserted into the positioning groove 111 to play a limiting role on the middle lining 3. During the high-speed rotation of the primary impeller 2a and the secondary impeller 2b along with the pump shaft 4, the positioning boss 31 and the positioning groove 111 control the axial movement of the intermediate bushing 3.

Further, referring to fig. 2, a liner seal ring 33 is provided between the inner circumferential surface of the center hole 110 of the pump chamber partition plate 11 of the pump body and the outer circumferential surface of the intermediate liner 3, and one or more liner seal rings 33 may be provided, and the liner seal ring 33 performs a sealing function between the primary impeller 2a and the secondary impeller 2 b. At the same time, the intermediate bushing 3 is fixed by interference extrusion of the upper and lower pump bodies 1a, 1b against the bushing seal ring 33.

Preferably, referring to fig. 2, the number of sealing rings 33 in the present embodiment may be 2, and the sealing rings are symmetrically distributed on two sides of the positioning boss 31, so as to enhance the sealing effect.

Further, referring to fig. 2, the hub of the primary impeller 2a and the hub of the secondary impeller 2b are provided with the split rings 21, and the left and right ends of the intermediate bush 3 are respectively provided with the inner split grooves 34, and the split rings 21 are slidably engaged with the inner split grooves 34 of the intermediate bush 3. When the primary impeller 2a and the secondary impeller 2b rotate along with the pump shaft 4, the orifice ring 21 prevents leakage flow, and improves the efficiency of the two-stage intermediate-opening centrifugal pump.

The assembly process of the two-stage split centrifugal pump of the present utility model will now be further illustrated by way of example only and the various foregoing modes have been selected or combined to facilitate a complete description, but such description is not intended to limit embodiments of the present utility model.

In the present description, the intermediate bushing 3 takes the form of a split, i.e. the intermediate bushing 3 comprises an upper half-ring intermediate bushing 3a and a lower half-ring intermediate bushing 3b. While the throttle recess 72 is selected to be formed in the outer circumferential surface of the intermediate spacer 7. The description is as follows:

referring to fig. 1-4, the seal ring 71 is placed in the seal groove of the inner circular surface of the middle spacer 7, and then the middle spacer 7 is sleeved on the pump shaft 4 so as to rotate together with the pump shaft 4. The first-stage impeller 2a and the second-stage impeller 2b are arranged at two ends of the middle spacer 7, and then shafting dynamic balance detection is carried out. After the detection is qualified, the upper half ring middle bushing 3a and the lower half ring middle bushing 3b are buckled between the primary impeller 2a and the secondary impeller 2b, and the throttling boss 32 is inserted into the corresponding throttling groove 72 on the middle spacer 7, so that the throttling boss 32 and the throttling groove 72 form a labyrinth sealing structure. At this time, the positioning boss 31 is inserted into the positioning groove 111, and the bushing seal ring 33 is provided between the inner circular surface of the center hole 110 of the pump chamber partition plate 11 of the pump body and the outer circular surface of the middle bushing 3, and the upper pump body 1a and the lower pump body 1b are interference-pressed against the middle bushing 3 and the bushing seal ring 33, thereby effectively preventing the rotation of the middle bushing 3.

The mouth ring 21 on the hub of the primary impeller 2a and the secondary impeller 2b is in sliding fit with the inner mouth groove 34 on the middle lining 3.

And then, installing other accessories successively until the installation is completed.

In the two-stage middle-open centrifugal pump, a labyrinth sealing structure is arranged between the first-stage impeller 2a and the second-stage impeller 2b during operation, so that the axial leakage between the first-stage impeller and the second-stage impeller pump cavity is effectively reduced, and the pump lift and efficiency are improved.

In conclusion, the two-stage middle-open centrifugal pump well solves the technical problems existing in the prior art and is beneficial to popularization and application.

In the present utility model, the terms "connected," "fixed," and the like should be construed broadly unless otherwise specifically indicated and defined. The specific meaning of the above terms in the present utility model can be understood by those of ordinary skill in the art according to the specific circumstances.

In the description of the present specification, the term "embodiment" or the like means that a particular feature, structure, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the utility model. In this specification, schematic representations of the above terms are not necessarily directed to the same embodiment or example. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples. Furthermore, the different embodiments or examples described in this specification and the features of the different embodiments or examples may be combined and combined by those skilled in the art without contradiction.

While embodiments of the present utility model have been shown and described above, it will be understood that the above embodiments are illustrative and not to be construed as limiting the utility model, and that alterations, modifications, substitutions and variations may be made in the above embodiments by those skilled in the art within the scope of the utility model.

Claims (10)

1. A two-stage middle-open centrifugal pump comprises a first-stage impeller (2 a), a second-stage impeller (2 b) and a pump shaft (4); the primary impeller (2 a) and the secondary impeller (2 b) are sleeved on the pump shaft (4); an intermediate bushing (3) is arranged between the primary impeller (2 a) and the secondary impeller (2 b), and is characterized in that: a throttling boss (32) is arranged on the inner circular surface of the middle bushing (3), and a throttling groove (72) is arranged on the pump shaft (4); the throttling boss (32) is inserted into the throttling groove (72) and is in clearance fit with the throttling groove (72) to form a labyrinth seal.

2. A two-stage split centrifugal pump according to claim 1, wherein: the throttling groove (72) is directly formed on the outer circular surface of the pump shaft (4).

3. A two-stage split centrifugal pump according to claim 1, wherein: an intermediate spacer bush (7) is arranged between the intermediate bushing (3) and the pump shaft (4); the throttling groove (72) is formed in the outer circular surface of the middle spacer bush (7).

4. A two-stage split centrifugal pump according to claim 3, wherein: a sealing ring (71) is arranged between the middle spacer bush (7) and the outer circular surface of the pump shaft (4).

5. A two-stage split centrifugal pump according to any one of claims 1-4, wherein: the throttling boss (32) and the throttling groove (72) are a plurality of annular structures arranged in an array.

6. A two-stage split centrifugal pump according to any one of claims 1-4, wherein: the middle bushing (3) comprises an upper half-ring middle bushing (3 a) and a lower half-ring middle bushing (3 b); the upper half ring middle bushing (3 a) and the lower half ring middle bushing (3 b) are buckled together and sleeved between the primary impeller (2 a) and the secondary impeller (2 b).

7. A two-stage split centrifugal pump according to claim 1, wherein: the primary impeller (2 a), the secondary impeller (2 b) and the middle bushing (3) are arranged in a pump body with a pump cavity baffle plate (11); the outer wall of the middle bushing (3) is provided with a positioning boss (31) which is matched with a positioning groove (111) formed in a central hole (110) of the pump cavity partition plate (11).

8. A two-stage split centrifugal pump according to claim 7, wherein: a liner sealing ring (33) is arranged between the outer circular surface of the middle liner (3) and the inner circular surface of the central hole (110) of the pump cavity partition plate (11).

9. A two-stage split centrifugal pump according to claim 8, wherein: the number of the bushing seal rings (33) is plural.

10. A two-stage split centrifugal pump according to claim 1, wherein: the left end and the right end of the middle bushing (3) are respectively provided with an inner spigot groove (34); the hubs of the primary impeller (2 a) and the secondary impeller (2 b) are provided with a mouth ring (21); the mouth ring (21) is arranged in the inner spigot groove (34) and is in sliding fit with the inner spigot groove (34).