CENTRIFUGAL PUMP STAGE

Technical Field

The present invention concerns a centrifugal pump stage with floating impeller, to be used in particular in multi-stage centrifugal pumps.

Background Art

It has been known that for transferring a liquid from a lower space to a higher space, centrifugal pumps with multiple stages are used. Such centrifugal pumps generally comprise an external tubular sleeve provided with a plurality of radial openings for the entrance of the liquid, usually called aspiration openings, and with an exit opening for the liquid, called outlet opening. The operative members of the pump are housed inside the tubular sleeve. The operative members are substantially made up of a pump unit and a motor unit, generally of electric type; the pump unit is suitable to carry out the transferring of the liquid from the aspiration openings to the outlet opening.

The pump unit is usually made up of a plurality of stages, arranged in series along the longitudinal axis of the motor shaft of the motor unit, to increase the hydraulic head of the liquid with respect to the pump entrance. Each of such stages of the centrifugal pump comprises a rotating member or impeller, suitable to be driven in rotation by the motor shaft, which extends coaxially to the tubular sleeve of the pump, and by a stationary member or diffuser suitable to convey the liquid from a stage to the other, until the outlet opening. More precisely, the diffuser unit usually extends at a first face of the impeller and is closed by an aspiration lid substantially having the shape of a disc extending at a second face of the impeller.

Patent US 7,290,984 discloses for example a centrifugal pump having multiple stages of this type. In such centrifugal pump, the hub of the impeller is provided with seal means, at which the impeller is supported rotatable, in use, by the diffuser unit.

Patent US 2,526,751 discloses a device for assembling the impeller on the motor shaft of the pump. The hub of the impeller is stiffly locked on the motor shaft, in use position, through a couple of expansion inserts, substantially semi-circular, mounted on a portion of reduced diameter of the shaft. In the use configuration, the expansion inserts warp to act on the opposite surfaces of the shaft and of the hub of the impeller, producing a rigid coupling.

A problem complained by users of centrifugal pumps of the cited type is the progressive wear and tear of the mobile parts being in reciprocal contact, at which the seal is carried out. In particular, the impeller is usually sustained in dragging support, for an annular portion, on the disc facing it. The total axial thrust which acts on the impeller is borne by the disc and, by the shim effect produced by functioning, it causes a progressive wear and tear of the parts. This fact obviously limits the life of the centrifugal pump, especially in presence of grains of sand in the pumped liquid.

Patent US 3,265,001 describes a multi-stage centrifugal pump in which the impeller of each stage is supported floating at the relative diffuser unit. The hub of the impeller is in fact mounted axially slidable on the motor shaft by which it is driven in rotation; the stroke of the floating impeller is limited by suitable stop means, for example of annular shape, mounted on the motor shaft. The impeller is supported in dragging contact, along an annular portion, at the cited lid of the diffuser unit, to make up a seal zone between the rotating part and the static part of the stage. More in particular, the impeller is supported by means of a thrust bearing placed beneath the hub of the same impeller, in support on a seat of the underlying diffuser.

Such solution allows to reduce the wear and tear and the scraping of the working members, in particular in case the sand or other abrasive grains are present in the liquid to be pumped. The fact that the impeller is mounted floating thus allows to prevent abrasive grains to stop at the dragging contact seal zones, thus preventing that such abrasive particles increase the wear and tear phenomena.

Nonetheless, the presence of seal zones between the rotating part and the static part of each centrifugal pump stage must be guaranteed, in order to limit as much as possible the leaking of the liquid which causes a decrease of the pump performances.

Ultimately, known solutions in the specific field do not fully satisfy user's needs in terms of performance and endurance.

Disclosure

The task of the present invention is that of solving the aforementioned problems, by devising a centrifugal pump stage with floating impeller which ensures high-level performances as well as a long endurance.

Within such task, it is a further scope of the present invention that of providing a centrifugal pump stage with reduced wear and tear between the mobile parts and the static parts, being in reciprocal contact at the hydraulic seal zones.

Another scope of the invention is that of providing a centrifugal pump stage which allows to minimize the leaking phenomena of the liquid in the above mentioned seal zones.

A further scope of the invention is that of providing a centrifugal pump stage having a simple constructive and functional conception, provided with a surely reliable functioning, versatile use as well as relative economic cost.

The cites scopes are attained, according to the present invention, by the centrifugal pump stage with floating impeller according to claim 1 .

According to the present invention, the centrifugal pump stage comprises an impeller unit which is provided with an axial hub suitable to be axially associated, freely sliding, on a shaft of the centrifugal pump and is floating, in use, inside a diffuser unit, between a first position in which said axial hub is in abutment against a first locking ring associated with said shaft of the centrifugal pump, so as to be rotatable with said shaft, and a second position in which said axial hub is in abutment against a second locking ring, similarly associated with said shaft, so as to be rotatable with said shaft.

Advantageously, the portion for the coupling of the motor shaft with the hub of the impeller has a ribbed profile, suitable to be coupled with a corresponding ribbed seat with inner toothing of the hub.

Advantageously, such coupling is of the type of the prismatic joint.

Preferably, said ribbed profile and said ribbed seat with inner toothing are made up of teeth having substantially trapezoidal shape.

Preferably, the impeller unit is made of a plastic material having high rigidity and abrasion resistance.

The locking rings are arranged respectively, in use, at a relative impeller unit of a pump stage, to be peripherally face to face with an inner surface of the axial hub, at least for a part.

The locking rings have a first and a second face transversal to the axis of the said shaft of the centrifugal pump, which are face to face, in use, with respective transversal surfaces of the axial hub of the said impeller unit and of the impeller unit of an adjacent pump stage, to act as an abutment for said transversal surfaces, respectively in said first position and second position of the said floating impeller unit.

The impeller unit has, at said second face, at least one annular rim, coaxial to the axis of the said impeller unit and suitable to shape the aspiration inlet of the liquid inside the same impeller unit, said annular rim of the impeller unit having its external edge in front of a static zone of the pump stage, to make up a seal zone at said aspiration inlet of the same impeller unit.

Suitably, said external edge of the annular rim of the impeller unit has a smoothed profile, to reduce the size of the zone of the front surface facing said static zone of the pump stage.

Preferably, said static zone of the pump stage facing said annular rim of the impeller unit comprises an edge of a circular opening of said aspiration lid.

Alternatively, said aspiration inlet for the liquid inside the impeller unit is shaped by a first inner annular rim and a second outer annular rim, coaxial to the axis of the same impeller unit, having external edges in front of corresponding static zones of the pump stage, to create respective seal zones.

Preferably, said annular rims, inner and outer, of the impeller unit have said external edges laying on a same plane transversal to the axis of the impeller unit. [28] According to a first embodiment, the locking rings are mounted at a respective annular seat obtained on the shaft of the centrifugal pump, at the impeller unit of the pump stage, in order to be protruding for an external part with respect to the same shaft.

[29] Preferably, said locking rings are respectively made up of a couple of semi-annular elements, joined at one edge by a join portion suitable to work as elastic hinge for the insertion into said annular seat and shaping at the opposite edge respective end portions suitable to join with one other.

[30] Preferably, said locking rings are arranged, in use, respectively at a vane defined by a shoulder internal to the axial hub of the impeller unit of each pump stage and open on the side of said second face of the same impeller unit.

[31 ] Preferably, said locking rings have a face, transversal to the axis of said axial hub, which is face to face with said shoulder, the shoulder being internal to the same hub.

[32] According to a different embodiment, the locking rings are made up respectively of a journal box suitable to be axially inserted on said shaft of the centrifugal pump, said axial hub of the impeller unit being coaxially mounted on the journal box.

[33] Preferably, said journal box shapes at one edge a collar, outwardly extended, which shapes a first and a second face transversal to the axis of said shaft.

[34] Preferably, the axial extension of the journal box is equal to the one of the pump stage with which it is associated, in order to be coplanar at the opposite ends with the external surfaces of said diffuser unit and of said aspiration lid.

[35] Preferably, the axial hub of the impeller unit passes at least partially with one portion through a central opening of the diffuser unit, for reaching and abutting a said locking ring at said second position of said impeller unit.

[36] Suitably, a journal box of metallic material is mounted on the inner surface of the said central opening of the diffuser unit.

[37] Advantageously, in a starting or breaking in step, a clearance is formed between a transversal surface of said axial hub of the impeller unit and a transversal face, facing it, of a said locking ring.

[38] According to another aspect of the invention, the centrifugal pump stage provides in use a starting or breaking in step in which said impeller unit is sustained in support on said aspiration lid, wherein at least one annular rim shaping the aspiration inlet of the liquid inside the same impeller unit has a seal surface in dragging contact with a static zone of the pump stage.

[39] Suitably, in the starting or breaking in step, a clearance is formed between a transversal surface of said axial hub and a transversal face in front of it of a said locking ring, the size of said clearance undergoing a progressive reduction by effect of the wear and tear of said seal surface of said annular rim until annulling in a normal working condition or speed condition, in which said impeller unit is in support on said locking ring.

In such speed condition, the impeller unit in support on the locking ring cannot further approach the static zone of the pump stage, and the rotating seal zones are spaced from such static zone by very reduced plays, such as to prevent dragging phenomena.

It is to be observed that the locking ring is driven in rotation by the motor shaft together with the impeller unit, therefore dragging phenomena do not occur between the surfaces being in reciprocal contact of the axial hub and of the same locking ring.

Therefore, further to a brief starting or breaking in step of the centrifugal pump, all the dragging frictions between the rotating impeller unit and the static parts of the pump stages are eliminated, in particular at the seal zone.

A feature on the pump stage according to the present invention is the fact that the impeller unit is floating, inside the diffuser unit, between a first position and a second position in which the axial hub is in abutment against respective locking rings rigidly associated with the motor shaft of the centrifugal pump, in order to be rotatable with the same shaft. In this way, the reciprocal contact between the rotating surfaces of the impeller unit and the fixed surfaces of the diffuser unit is prevented.

The pump stage according to the present invention also allows to reduce at the minimum the plays between the impeller unit and the static parts, at the seal zones.

Description of Drawings

Details of the invention shall be more apparent from the detailed description of a preferred embodiment of the centrifugal pump stage according to the invention, illustrated for indicative purposes in the attached drawings, wherein:

figure 1 shows an axial cross-section of a plurality of stages mounted in series on the motor shaft of the centrifugal pump;

figure 2 shows a perspective view of a detail of the centrifugal pump stage according to the invention;

figures 3 and 4 show a magnified detail of a zone of the centrifugal pump according to the invention, respectively in a starting and in a speed step.

figure 5 shows a partial axial cross-section view of a different embodiment of the centrifugal pump stage according to the invention.

Best Mode

With particular reference to such figures, the centrifugal pump stage with floating impeller according to the invention has been indicated in its entirety with 1 . The pump stage 1 comprises an impeller unit 10 provided with an axial hub 1 1 which is predisposed to be associated with a motor shaft 2 of the centrifugal pump, to be axially freely sliding, but coupled in rotation, with the same motor shaft 2. In particular, the portion for coupling with the motor shaft 2 has a ribbed profile, made for example through teeth having a substantially trapezoidal shape, suitable to be coupled, through a coupling of the type of the prismatic joint, with a corresponding ribbed seat with inner toothing of the hub 1 1 . The motor shaft 2 is suitable to be driven in rotation by a motor member of the pump, known per se and therefore not represented in the drawings.

The impeller unit 10 is mounted floating inside a correspondent diffuser unit 20. An aspiration lid 30 is fixed to the diffuser unit. The diffuser unit 20 extends at a first face of the impeller unit 10; the lid 30 extends at an opposite second face of the impeller unit 10. In vertical working position, the first face of the impeller unit 10 is arranged on the top, substantially horizontal, while the second face is placed inferiorly. Anyway, it is possible to provide the use of the pump stage according to the invention also in centrifugal pumps having horizontal axis.

More in particular, the impeller unit 10 is made up of an upper part 12 which extends transversally from the axial hub 1 1 and a lower part 13 having substantially the shape of a discoid ring. The upper part 12 shapes the above mentioned first face of the impeller unit 10; the lower part 13 shapes the above mentioned second face of the impeller unit 10. The upper part 12 of the impeller unit 10 shapes, in angularly distributed positions, a series of walls 14 which extend in spiral shape from the central zone towards the peripheral zone of the impeller unit 10. The walls 14 shape, in cooperation with the lower part 13 of the impeller unit 10, respective flow channels 15 for the liquid to be pumped. The parts 12, 13 of the impeller unit 10 are preferably made of a plastic material having high rigidity and abrasive resistance. Alternatively, it is possible to provide that such parts 12, 13 of the impeller unit 10 are made of formed steel or micro-melt steel; in this case, the surfaces of the impeller unit 10 in contact with the static surfaces are suitably made through overlapped rings made of materials having high friction coefficient, as it is specified hereinafter.

The diffuser unit 20 has an external cylindrical body 21 , preferably made of rigid plastic material. A profiled plate 22 extends inwards from the top of the cylindrical body 21 , the profiled plate 22 centrally having a sleeve 23, coaxial in assembly position to the motor shaft 2; the upper portion of the hub 1 1 passes through the sleeve 23. On the internal surface of the sleeve 23 a journal box 3 is mounted, which is preferably made of metallic material, for example stainless steel. Above the plate 22 are shaped, in angularly distributed positions, a series of keys 24 which extend in spiral form from the central zone towards the peripheral zone of the diffuser unit 20. The keys 24 shape respective flow channels 25 for a liquid to be pumped.

The aspiration lid 30 is shaped by a disc 31 centrally having a circular opening 32; the disc 31 is preferably made of metallic material, for example stainless steel. At the opening 32, the disc 31 has a flaring suitable to shape a edge 33 raised with respect to the plane of the same disc 31 . The disc 31 externally has a raised edge 34 which peripherally engages a corresponding shoulder obtained along the lower edge of the cylindrical body 21 of the diffuser unit 20.

[56] At the lower portion, the hub 1 1 of the impeller unit 10 extends in a first annular rim 16 coaxial to the axis of the impeller unit 10 and having inner diameter suitably bigger that the inner diameter of the same hub 1 1 , so as to shape a shoulder 17 on a plane transversal to the axis. The lower part 13 of the impeller shapes, at the internal edge of its annular profile, a second annular rim 18 coaxial to the axis of the impeller unit 10 and extended from the same side of the first annular rim 16 of the hub 1 1 of the impeller unit 10; in practice, the second annular rim 18 is concentric externally to the first annular rim 16 of the hub 1 1 . The annular rims 16, 18 shape the aspiration inlet for the liquid inside the impeller unit 10, being in communication with the above mentioned flow channels 15 of the liquid to be pumped.

[57] It is to be observed that the annular rims 16, 18, inner and outer, have the lower edges laying on a same plane transversal to the axis of the impeller unit and respectively facing in use an edge 3a, outwardly bent, of the journal box 3 and the raised edge 33 of the lid 30. Suitably, such lower edges of the annular rims 16, 18 have smoothed profile, to reduce the width of the front surface facing respectively the bent edge 3a of the journal box 3 and the raised edge 33 of the lid 30. As it is better specified in the following, such front surface is in dragging contact, in a starting step of the centrifugal pump, with the cited edges 3a, 33 constituting a static zone of the pump stage.

[58] The impeller unit 10 is floating in use inside the diffuser unit 20 between a first position in which the axial hub 1 1 is in abutment against a first locking ring 4 associated with the shaft 2 of the centrifugal pump, so as to be rotatable with the same shaft 2, and a second position in which said axial hub 1 1 is in abutment against a second locking ring 4 similarly associated with the shaft 2 in a suitably spaced position.

[59] The locking rings 4 are mounted at a respective seat 5 obtained on the shaft 2 of the centrifugal pump; suitably such seat 5 is made up of an annular groove of the shaft 2. More in particular, as it is possible to see in figure 2, the locking rings 4 are preferably made up of a couple of semi-annular elements 40, having a substantially rectangular cross-section, joined at one end by a joint portion 41 of small width, suitable to act as an elastic hinge. At the opposite end, the semi-annular elements 40 shape respective end portions 42, 43, complementary with one another, suitable to couple to one another. Obviously, the locking rings 4 are mounted in the respective seats 5 by keeping the semi- annular elements 40 open; the subsequent resilient hooking of the end portions 42, 43 causes the locking of the locking rings 4 inside the seats 5. In the assembly position in the respective seats 5, the locking rings 4 are protruding for an internal part with respect to the shaft 2. Such protruding portion of the locking rings 4 is housed substantially, as it is better specified in the following, at the vane shaped by the annular rim 16 of the hub 1 1 , with a face transversal to the axis of the pump facing the shoulder 17 inside the same hub 1 1 .

The functioning of the centrifugal pump stage is described in the following.

In a starting or breaking in step, the annular rims 16, 18 which shape the aspiration inlet of the liquid inside the impeller unit 10 are respectively in support on the edges 3a, 33 of the journal box 3 and of the lid 30 which constitute a static zone of the pump stage. In such condition, the locking ring 4 arranged at the same impeller unit 10, protruding inside the vane shaped by the annular rim 16 of the hub 1 1 results to be suitably spaced by the shoulder 17 inside the hub 1 1 (see figure 3). In particular, in such step, a clearance, having limited size, is formed between the shoulder 17 and the transversal face of the locking ring 4 in front of it. For more clarity, the clearance is indicated with 6.

It is to be observed that the opposite transversal face of the same locking ring 4 is, suitably spaced, in front of the top of the hub 1 1 of the impeller unit 10 of the adjacent stage, in the illustrated case arranged inferiorly.

Therefore, when the centrifugal pump is activated, in such starting step the annular rims 16, 18, inner and outer, of the impeller unit 10 have the lower edges in contact respectively with the bent edge 3a of the journal box 3 and with the raised edge 33 of the lid 30, to create the seal zones at the aspiration inlet of the same impeller unit 10. In the cited seal zones the dragging contact between the annular rims 16, 18 of the impeller unit 10 and the opposite static zones of the pump stage causes, by friction, a progressive wear and tear of the same annular rims 16, 18, made of plastic material and being in contact with steel surfaces. The lower edges with smoothed profile of the annular rims 16, 18 quicken the wear and tear process.

It is to be observed that the progressive wear and tear of the seal surfaces of the annular rims 16, 18 of the impeller unit 10 produces a corresponding progressive reduction of the size of the clearance 6 between the hub 1 1 and the locking ring 4; when the shoulder 17 of the hub 1 1 arrives in contact with the locking ring 4, in a condition of normal functioning, that is at speed, the wear and tear between the seal zones in contact ends (see figure 4). The impeller unit 10 is, as a matter of facts, in support on the locking ring 4 and consequently it cannot further approach the static zone of the pump stage; the rotating seal zones are therefore spaced from such static zone by very reduced plays, such to prevent brushing phenomena. It is further to be observed that the locking ring 4 is driven in rotation by the motor shaft 2 together with the impeller unit 10, therefore brushing phenomena do not occur between the surfaces in contact of the hub 1 1 and of the same locking ring 4.

The impeller unit 10 is floating inside the diffuser unit 20 between a fist position of normal functioning, in which the axial hub 1 1 is in abutment against the locking ring 4 associated with the shaft 2 of the centrifugal pump, at the same impeller unit 10, and a second position in which the top of the above mentioned axial hub 1 1 is in abutment against a second locking ring 4 similarly associated with the shaft 2 at the overlaying pump stage. This allows to prevent the reciprocal contact between the upper surface of the impeller unit 10 and the opposite static surface of the diffuser unit 20, in case the functioning conditions cause an upward displacement of the same impeller unit 10.

In figure 5 a different embodiment of the centrifugal pump stage according to the invention is disclosed, in which the presence of housing seats for the locking rings on the motor shaft is not required. This allows not to weaken the shaft and to facilitate the assembly in series of the pump stages, in particular in case such pump stages are relatively numerous, for example more than six or seven.

In this case, the locking rings, for simplicity indicated again with 4, are made up of a journal box 44 which axially inserts in the shaft 2; the shaft 2 has for example hexagonal profile, to create, together with the internal surface of the journal box 44, a prismatic joint. On the journal box 44 the axial hub 1 1 of the impeller unit 10 is mounted coaxial and rotatable with it. At one edge, the journal box 44 shapes a collar 45, extended outwardly, which is arranged, in use, at the sleeve 23 of the diffuser unit 20. The collar 45 shapes outwardly of the pump stage a face 46 transversal to the axis of the same pump and inwardly a shoulder 47 equally extended on a plane transversal to the axis.

The journal box 44 has axial extension equal to the one of the pump stage to which it is associated, in order to be coplanar at the opposite ends with the external surfaces of the sleeve 23 of the diffuser unit 20 and of the disc 31 of the aspiration lid 30. In practice, the axial hub 1 1 of the impeller unit 10 axially extends, at the side turned towards the aspiration lid 30, through the opening 32 of the disc 31 , in order to have the end surface facing the collar 45 of the locking ring 4 of the underlying pump stage.

The locking rings 4 are mounted is series on the shaft 2 of the pump, in contact with one another, and are locked axially at the top of a washer 7 clamped through a nut 8 screwed at the top of the same shaft 2. It is to be observed that for the sake of simplicity the drawings show a pump having two stages but, as it was said before, such solution is advantageous also in case of centrifugal pumps which provide a higher number of stages. The functioning of the pump stage is substantially the same of the functioning of the first embodiment. In a starting or breaking in step of the centrifugal pump, the annular rim 18 which shapes the aspiration inlet of the liquid inside the impeller unit 10 is in support on the internal edge 33 of the lid 30 which constitutes a static zone of the pump stage. In such condition, a clearance of limited size, for example equal to about 0,3 mm, is formed between the end surface of the axial hub 1 1 , protruding through the opening 32 of the aspiration lid 30, and the transversal face 46 facing it of the collar 45 of the locking ring 4, associated with the adjacent pump stage in the illustrated case arranged inferiorly.

It is to be observed that the opposite end of the axial hub 1 1 turned towards the side of the impeller unit 10 is suitably spaced from the shoulder 47 shaped by the collar 45 of the locking ring 4 associated with the pump stage. Such distance defines the size of the stroke permitted to the impeller unit 10 floating inside the pump stage.

Therefore, when the centrifugal pump is activated, in such starting step the annular rim 18 of the impeller unit 10 has its lower edge in contact with the raised edge 33 of the lid 30, to create the seal zone at the aspiration inlet of the same impeller unit 10. In such seal zone, the dragging contact between the annular rim 18 of the impeller unit 10 and the opposite static zones of the pump stage causes, by friction, a progressive wear and tear of the same annular rim 18, made of plastic material and in contact with a steel surface.

The progressive wear and tear of the seal surface of the annular rim 18 of the impeller unit 10 produces a corresponding progressive reduction of the size of the clearance 6 between the axial hub 1 1 and the locking ring 4 of the adjacent pump stage; when the end of the hub 1 1 arrives in contact with the upper face 46 of the collar 45 of the above mentioned locking ring 4, in a condition of normal functioning, that is at speed, the wear and tear between the seal zones in contact stops. The impeller unit 10 is, as a matter of facts, in support through the axial hub 1 1 on the locking ring 4 of the adjacent pump stage and consequently it cannot further approach the static zone; the rotating seal zone is therefore spaced from such static zone by very reduced plays, such as to prevent brushing phenomena.

Also in this case, the locking ring 4 is driven in rotation by the motor shaft 2 together with the impeller unit 10, therefore brushing phenomena do not occur between the surfaces in reciprocal contact of the hub 1 1 and of the same locking ring 4. The impeller unit 10 is floating inside the diffuser unit 20 between a first position, of normal functioning, in which the axial hub 1 1 is in abutment against the locking ring 4 associated with the adjacent pump stage, and a second position in which the top of the above mentioned axial hub 1 1 is in abutment against the collar 45 of the locking ring 4 similarly associated with the pump stage of the same impeller unit 10. This allows to prevent the contact between the upper surface of the impeller unit 10 and the opposite static surface of the diffuser unit 20, in case the functioning conditions cause an upward displacement of the same impeller unit 10.

Hence, the centrifugal pump stage according to the present invention attains the scope of ensuring high-level performance and long endurance.

As a matter of facts, after a short starting breaking in step of the centrifugal pump, all the brushing friction between the rotating impeller unit and the static parts of the pump stage are completely eliminated, in particular at the seal zones. Therefore, all the wear and tear phenomena of the impeller, usually complained, are eliminated, thus prolonging the pump life and preserving its correct functioning.

At the same time, the proposed solution allows to reduce at the minimum the plays between the impeller unit and the static parts of the pump stage, at the seal zones. In this way, the hydraulic losses by leaking are completely eliminated in the seal zones, thus evidently improving the productivity and the flow rate and hydraulic head performance of the centrifugal pump. Moreover, the absence of friction phenomena causes a considerable reduction of the energy absorption.

A feature on the disclosed pump stage is due to the fact that the impeller unit is floating, inside the diffuser unit, between a first position and a second position in which the axial hub is in abutment against respective locking rings stiffly associated with the motor shaft of the centrifugal pump, in order to be rotatable with the same shaft. This allows to avoid the contact between the rotating surfaces of the impeller unit with the fixed surfaces of the diffuser unit, preventing possible damages both on the impeller and on the diffuser.

The pump stage described for indicative purpose is susceptible of numerous modifications and variants according to the different exigencies. For instance, it is possible to foresee that the described solution can also be used in case of a mono-stage centrifugal pump. In practice, the embodiment of the invention, the materials used, as well as the shape and dimensions, may vary depending on the requirements.

Should the technical characteristics mentioned in each claim be followed by reference signs, such reference signs were included strictly with the aim of enhancing the understanding the claims and hence they shall not be deemed restrictive in any manner whatsoever on the scope of each element identified for exemplifying purposes by such reference signs.