A METHOD OF PUMPING A LIQUID MEDIUM, A CENTRIFUGAL PUMP AND AN IMPELLER THEREFOR

Abstract

An impeller for a centrifugal pump includes a hub, a shroud extending outwardly from the hub and dividing the impeller into front and rear sides, a working vane on a first face of the hub and the shroud at the front side, a rear vane on a second face of the shroud at the rear side, and a balancing conduit extending through the hub from a first opening at the first face to a second opening at the rear side of the impeller. The first opening is disposed closer to the axis than the second opening, and the first opening is located within a circle formed by a radially innermost part of the working vane, while the impeller is rotated about the axis, and the second opening or the balancing conduit at the rear side of the impeller is located at a smallest diameter of the shroud.

Claims

1-4. (canceled)

5. An impeller for a centrifugal pump, comprising: at least a hub having an axis; a shroud extending outwardly from the hub and dividing the impeller into a front side and a rear side; at least one working vane arranged on a first face of the hub and the shroud at the front side of the impeller; at least one rear vane arranged on a second face of the shroud at the rear side of the impeller; and at least one balancing conduit extending through the hub from a first opening at the first face to a second opening at the rear side of the impeller; the first opening of the at least one balancing conduit in the first face of the hub being disposed closer to the axis of the impeller than the second opening at the rear side of the impeller, and the first opening in the first face of the hub being located within a circle formed by a radially innermost part of the at least one working vane, while the impeller is rotated about the axis, and the second opening of the at least one balancing conduit at the rear side of the impeller being located at a smallest diameter of the shroud.

6. (canceled)

7. The impeller as recited in claim 5, wherein, the hub has an axial extension from the shroud in a direction opposite the at least one working vane, the extension having an outer surface, and the second opening of the at least one balancing conduit at the rear side of the impeller is located in the second face of the shroud, in a substantially cylindrical outer surface of the hub in at least one of the shaft and a shaft sleeve.

8. The impeller as recited in claim 5, wherein the first opening of the at least one balancing conduit in the first face of the hub is disposed in the rotational direction of the impeller ahead of the second opening of the at least one balancing conduit at the rear side of the impeller.

9. The impeller as recited in claim 5, wherein at least two balancing conduits having a common front opening are disposed at the axial centre of the first face of the hub.

10. (canceled)

11. The impeller as recited in claim 5, wherein the impeller is semi-open or closed.

12. The impeller as recited in claim 5, wherein the circle has a diameter, the first openings have a radially outermost point forming an other circle with a diameter, and the diameter of the other circle is 0.9 times the diameter of the circle.

13. The impeller as recited in claim 5, comprising, in parallel with the working vanes, two centrifugally acting elements configured to pump fluid in a direction parallel with the working vanes.

14. A centrifugal pump, comprising: a housing with an inlet opening, a volute and a rear wall; a shaft; a shaft sealing; and an impeller as recited in claim 5, the impeller being attached on the shaft so as to rotate with the shaft.

15. The centrifugal pump as recited in claim 14, wherein the balancing conduit has a cross sectional flow area ranging between 5 and 20% of an area of the inlet opening of the pump.

16. The centrifugal pump as recited in claim 15, wherein the balancing conduits have a combined cross sectional flow area ranging between 5 and 20% of the area of the inlet opening of the pump.

17. The centrifugal pump as recited in claim 14, wherein the shaft sealing has the same or longer diameter than that of at least one of a surface of the hub and the shaft and the shaft sleeve.

18. (canceled)

19. The centrifugal pump as recited in claim 14, wherein the hub has an axial extension from the shroud in a direction opposite the at least one working vane, the extension having an outer surface, the impeller having a shoulder area at a junction of the shroud and the hub, the at least one balancing conduit opening to the shoulder area such that the second opening is located in one of the rear face of the shroud, both the rear face of the shroud and the outer surface of the hub, and the outer surface of the hub.

20. The centrifugal pump as recited in claim 14, wherein the impeller having a shoulder area at a junction of the shroud and the shaft, the shaft having an outer surface and a diameter, the diameter extending to the second opening of the at least one balancing conduit such that the at least one balancing conduit opens to the shoulder area so that the second opening is located in one of the rear face of the shroud, the rear face of the shroud and the outer surface of the shaft and shaft sleeve, or in one of the outer surface of the shaft and the shaft sleeve.

21. The centrifugal pump as recited in claim 14 comprising, in parallel with the working vanes, two centrifugally acting elements configured to pump fluid in a direction parallel with the working vanes.

22. The impeller as recited in claim 5, wherein the circle has a diameter, the first openings have a radially outermost point forming an other circle with a diameter, and the diameter of the other circle is 0.8.

23. The impeller as recited in claim 5, wherein the circle has a diameter, the first openings have a radially outermost point forming an other circle with a diameter, and the diameter of the other circle is 0.7.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Referring now to the attached drawings which form a part of this original disclosure.

[0036] FIG. 1 is a schematical axial cross-sectional illustration of a traditional prior art centrifugal pump;

[0037] FIG. 2 is a schematical axial cross-sectional illustration of a prior art centrifugal pump in accordance with U.S. Pat. No. 7,326,029;

[0038] FIG. 3 is a schematical axial cross-sectional illustration of a centrifugal pump in accordance with a first preferred embodiment of the present invention,

[0039] FIG. 4 is a schematical illustration of the operating principle of the centrifugal pump in accordance with the present invention,

[0040] FIG. 5 is a schematical axial cross-sectional illustration of a centrifugal pump in accordance with a second preferred embodiment of the present invention,

[0041] FIG. 6 is a schematical radial cross-sectional illustration of the centrifugal pump of FIG. 3, and

[0042] FIG. 7 is a schematical radial cross-sectional illustration of the centrifugal pump of FIG. 5.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0043] FIG. 1 illustrates schematically a conventional structure of an impeller 10 of a centrifugal pump in accordance with the prior art. The figure also illustrates pump components, such as a pump volute 2, a rear wall 4 of said pump and a pump shaft 6 with an axis 8. The impeller 10 comprises a hub 12 and a shroud 14, which is the disc-shaped more or less radial extension of the hub 12. The hub and the shroud are provided with working vanes 16 arranged on the front or first face of the hub, and shroud, and balancing holes 18 extending through the hub 12 from its front or first face to its rear or second face. The rear face of the hub, or shroud, is further provided with rear vanes 20. It is a characteristic feature of the balancing holes in accordance with prior art that the centreline thereof is parallel to the axis 8 of the pump, and that the diameter of the holes is substantially constant for the entire length thereof. Moreover, the balancing holes 18 are brought relatively close to the axis 8 of the pump and located at the impeller vane passage. The purpose of the above described positioning of the balancing holes is to ensure that part of the liquid flow goes through the hole from the front side of the impeller to the rear side of the impeller 10 to raise the pressure of the sealing space 22.

[0044] FIG. 2 illustrates another prior art construction, namely that discussed in U.S. Pat. No. 7,326,029. The pump of the US patent comprises a volute 2, rear wall 4, and an impeller 30 with an impeller hub 32 and shroud 34, working vanes 36, rear vanes 40 and an axis 8 of both the pump, the hub and the impeller. The impeller structure of FIG. 2 differs from that of FIG. 1 in that the balancing holes or conduits 38 have a centreline C.sub.L in a direction that deviates from that of the axis 8 of the pump or the impeller 30. In the embodiment shown in FIG. 2 the sectional view is taken along the centreline C.sub.L of the holes or conduits 38. Thus it is clear that although FIG. 2 might give the impression that the holes or conduits are situated in an axial plane, the holes or conduits 38 may be (but are not necessarily) in reality inclined, in other words, they may be deviated from the axial plane radially as well as circumferentially. It is a characteristic feature of the balancing holes or conduits 38 of the US-patent that an opening 44 on the side of the hub 32 facing the suction conduit of the pump (left in the drawing), i.e. a so called inlet opening on the first face of the hub is closer to the axis 8 of the pump (i.e. on a smaller diameter) than the opening 46 behind the impeller hub 32 or shroud 34, i.e. a so called outlet opening at the opposite end of the balancing hole or conduit 38 at the second face of the hub or shroud. By the above discussed positioning of the balance conduits each balance conduit acts as a small centrifugal pump. It is another characteristic feature of the holes or conduits 38 of the US-patent that they are partially directed circumferentially so that the direction thereof is along the impeller vane passage i.e. along the cavity between the working vanes 36, in the flow direction of the liquid. In other words, the openings 46 of the balancing holes or conduits in the rear or second face of the impeller hub 32 or shroud 34 are located in the rotational direction of the impeller behind the opening 44 at the opposite end of the balancing hole or conduit 36, i.e. in the front or first face of the impeller hub 32 or shroud 34 and also radially outside thereof

[0045] FIG. 2 also shows the shaft sealing 48, which is arranged in the chamber of its own between the shaft and the rear wall/housing of the pump. As the shaft sealing 48 is relatively far from the second or outlet opening 46 of the balancing conduit 44 (both in axial direction and especially in radial direction much closer to the axis of the impeller than the outlet opening 46), the heat transfer to the moving liquid is limited, and a risk of running the sealing dry exists, especially when pumping liquids having a high temperature such that they easily boil.

[0046] Also, in performed experiments it has been learned that, though the arranging of the front/inlet and rear/outlet openings at different diameters slightly helps in the balancing, the pressure fluctuations in the sealing cavity 40 are too high, and there is a risk of running the seals dry. Performed experiments have shown that the solution to the problem relating to the pressure fluctuations is the correct location of the front/inlet opening to the balancing conduit. The location has to be such that the first or front or inlet opening is entirely inside the circle C (shown in FIGS. 5 and 6) formed of the points of origin of the working vanes at the front face of the impeller, i.e. the points where the leading edges of the working vanes meet the front or first face of the hub.

[0047] FIG. 3 illustrates a cross section of a centrifugal pump in accordance with a first preferred embodiment of the present invention. The centrifugal pump comprises a housing with a volute 2 and a rear wall 4. The centrifugal pump further comprises, within the housing thereof an impeller 50, a shaft 6 for rotating the impeller 50, a shaft sealing 48 and an axis 8. The impeller has a hub 52 and a shroud 54 extending radially outwardly from the hub 52 and dividing the impeller to a front side and a rear side, at least one working vane 56 arranged on the first or front face of the impeller at the front side of the impeller, i.e. on that of the hub 52 and the shroud 54, at least one balancing conduit 58 running through the hub 52 and the shroud 54 from the front side of the impeller to the rear side thereof, and at least one rear vane 60 on the rear or second face of the hub 52/shroud 54 at the rear side of the impeller 50. The rear vanes 60 may extend for the full radial extent of the shroud 54, but they may as well be arranged for only a part of the radial extent of the shroud 54. The hub 52, preferably, has a substantially cylindrical axial extension from the shroud 54 in the direction opposite the at least one working vane 56, as extending the at least one working vane 56 and the at least one balancing conduit 58 closer to the axis of the impeller 50 inevitably reduces the size of the hub 52 in front of the impeller 50, whereby, for the purpose of proper attachment on the shaft 6, the hub 52 is sometimes extended at the rear side of the impeller 50. However, in some cases, it is possible that the hub need not extend from the level of the shroud. The at least one balancing conduit 58 has a front or first opening 64 at the front or first face of the hub 52 at the front side of the impeller and a rear or second opening 66 at the rear face of the hub 52 and/or that 54′ of the shroud 54 at the rear side of the impeller. Basically, the impeller is similar to that of the pump of FIG. 2 except for the facts that now the at least one working vane 56 has been extended closer to the axis 8 of the impeller 50, and that the front opening 64 of the at least one balancing conduit 58 has been moved even closer to the axis 8 radially inside the radially innermost part 56e of the at least one working vane 56 i.e. radially inside the point where the leading edge of the at least one working vane 56 meets the front or first face of the hub 52 of the impeller 50. Furthermore, the rear or outlet opening 66 of each balancing conduit 58 opens preferably to the shoulder area at the junction of the shroud 54 and the hub 52 such that the opening is located either fully in the rear face 54′ of the shroud 54, in both the rear face 54′ of the shroud 54 and the outer more or less cylindrical surface 52′ of the hub 52, or fully in the outer more or less cylindrical surface 52′ of the hub 52. In case the hub has no axial extension as was discussed earlier above, it is advantageous that the shaft diameter extends to the rear or outlet opening 66 of each balancing conduit 58 such that it opens preferably to the shoulder area at the junction of the shroud 54 and the shaft so that the opening is located either fully in the rear face 54′ of the shroud 54, in both the rear face 54′ of the shroud 54 and the outer more or less cylindrical surface of the shaft, or fully in the outer more or less cylindrical surface of the shaft. In some cases, the shaft may be provided at its end with a shaft sleeve having an outer surface forming the surface the outlet opening 66 is in communication with. The same surface, irrespective of it being the surface of the hub extension, shaft, or shaft sleeve preferably communicates with the sealing 48 as shown in FIG. 3, too. In the latter two cases it is possible that the balancing conduit enters the shaft or shaft sleeve from the hub of the impeller such that the outlet opening is opens in the shaft or sleeve surface.

[0048] However, these modifications have required reconfiguration of the at least one balancing conduit 58 to have, in this embodiment of the present invention, a curved, i.e. non-linear configuration. The curved configuration is needed, as, if the at least one conduit 58 were taken via a linear path from the front or first or inlet opening 64 to the rear or second opening 66, the conduit could not run within the hub/shroud material, but would open a lengthy groove into the front or first face of the hub/shroud. However, the benefit of having the balancing conduit open inside the innermost parts of the at least one working vane would, then, be lost. Thus, in accordance with the preferred embodiment of FIG. 3 the at least one balancing conduit 58 is formed of three parts; a first linear part 58.sub.1 extending from the front or first opening 64 into the hub 52 in a direction substantially parallel with the axis 8 of the impeller 50, a second part, i.e. a bend 58.sub.2, turning the conduit 58 to a radially outward direction, i.e. in a direction inclined in relation to the direction of the axis 8, and a third linear part 58.sub.3 between the bend 58.sub.2 and the rear or second or outlet opening 66. Here, the cross section shown in FIG. 3 is made to run along the centreline C.sub.L of the at least one balancing conduit 58, which only means that the at least one balancing conduit 58 is not necessarily, but may, however, be, located in a plane running along the axis 8 of the impeller 50. Thus, the at least one balancing conduit 58 may run in a plane passing the axis 8 at a distance. As yet another option, the at least one balancing conduit may not be running in any single plane but be three-dimensionally curved, i.e. they may, for instance be following the direction of the at least one working vane, or rather that of the vane passage.

[0049] However, already at this stage it is worthwhile to understand that the balancing conduit may also be manufactured by drilling at least two holes such that there is no actual curved bend but a kind of a sharp bend between the linear parts of the conduit.

[0050] As to the optimal location of the second or outlet opening 66 of the balancing conduit 58 it is at the outer circumference 52′ of the hub 52 of the impeller 50 such that no shoulder is left between the outlet opening 66 and the hub surface 52′ (or the shaft surface or the shaft sleeve surface). This kind of a construction ensures that the heat generated in the shaft sealing 48 is easily conveyed away from the sealing area. The flushing of the shaft sealing 48 actually functions such that the fluid entering the rear side of the shroud 54 has a relatively high speed based on the pump inlet pressure and the balancing conduit 58 acting as a pump, whereby fluid from the space in front of the shaft sealing 48 is drawn to a cavity between the rear wall 4 of the pump and the shroud 54. The fluid that was drawn away at the outlet opening 66 is replaced with new fluid from adjacent circumferential areas, i.e. areas outside the effective range of the outlet opening 66, where the radially outward flow is weaker, whereby a small-scale liquid circulation is ensured in front of the shaft sealing 48. In other words, it is advantageous, but not totally necessary, that the shaft sealing 48 is arranged on the same hub (or shaft or shaft sleeve) surface 52′ to which the outlet opening 66 of the balancing conduit 58 opens. However, as there are various types of shaft sealings, which may be used in this position, like for instance packing box-type seals or mechanical seals, the installation of which on the shaft differs a great deal from one another, an exact dimensioning of the outlet opening 66 in relation to the sealing 48 is hard to determine. The only two ways to express that any cavity inside the diameter of the surface 52′ of the hub (or the shaft or the shaft sleeve) in front of the shaft sealing is not desired is to say that the diameter of the shaft sealing is substantially the same or longer than that of the surface 52′ of the hub or the shaft or the shaft sleeve, or that the shaft sealing 48 is arranged at the same or longer distance from the axis 8 than the outlet opening 66 of the balancing conduit 58 in the hub or the shaft or the shaft sleeve.

[0051] FIG. 4 illustrates schematically the operating principle of the impeller of the present invention using the reference numerals of FIG. 3. The fluid to be pumped enters the impeller 50 from the left along the pump inlet and is divided at the impeller eye (just in front of the working vanes of the impeller) into two flows, a first or main flow being passed or entering the effective area of the working vanes 56 that form a first centrifugal pump advancing the fluid to the pump volute 2. A second flow goes or is passed to the balancing conduits 58 that form a second centrifugally acting element, i.e. centrifugal pump, in parallel with the working vanes 56, advancing the fluid towards the pump volute 2. After having passed the balancing conduits 58, i.e. the balancing conduits pumping the second flow, the second flow communicates with the shaft sealing 48 before being passed to the effective area of the rear vanes 60, acting as the second centrifugally acting element in parallel with the working vanes 56, and is pumped by the rear vanes 60 forming a third centrifugal pump to the pump volute 2, where the first and the second flow are combined before being discharged from the volute to the pump outlet. In other words, the centrifugal pump of the present invention is, in practice, formed of the working vanes 56 and two centrifugally acting devices 58 and 60 coupled in parallel with the working vanes 56.

[0052] FIG. 5 illustrates a cross section of a centrifugal pump in accordance with a second preferred embodiment of the present invention. The centrifugal pump of FIG. 5 comprises a housing with a volute 2 and a rear wall 4. The centrifugal pump further comprises, within the housing, an impeller 150, a shaft 6 for rotating the impeller 150, a shaft sealing 48 and an axis 8. The impeller has a hub 152, a shroud 154 extending radially outwardly from the hub 152, at least one working vane 156 arranged on the front or first face of the impeller 150 i.e. on that of the hub 152 and the shroud 154, at least one balancing conduit 158 running through the hub 152 and the shroud 154, and at least one rear vane 160 on the rear or second face 154′ of the shroud 154. The hub 152, preferably, has a substantially cylindrical axial extension from the shroud 154 in the direction opposite the at least one working vane 156, as extending the at least one working vane 156 and the at least one balancing conduit 158 closer to the axis of the impeller 150 inevitably reduces the size of the hub 152 in front of the impeller 150, whereby, for the purpose of proper attachment on the shaft 6, the hub 152 is extended at the rear side of the impeller 150. However, it is also possible that that the element at the rear side of the impeller shroud is the shaft or a shaft sleeve arranged on the shaft in the manner discussed in more detail in connection with FIG. 3. The at least one balancing conduit 158 has a front or first opening 164 at the axial centre of the front or first face of the hub 152 and rear or second opening 166 at the rear face 154′ of the shroud 154 at the outer circumference of the hub 152. In case there are several balancing conduits 158 the first or front opening 164 is common to all conduits. What has been disclosed about the positioning of the second or outlet opening 66 in connection with the embodiment of FIG. 3 applies in this embodiment, too. In other words, the outlet opening 166 may be in the rear face 154′ of the shroud 154, in the substantially cylindrical outer surface 152′ of the hub 152 (or shaft or shaft sleeve) or in both. Also, the location of the shaft sealing 48 in relation to that of the outlet opening 166 may be derived from the teachings of FIG. 3.

[0053] Basically, the impeller 150 of FIG. 5 is similar to that of the pump of FIG. 3 except for the facts that now the at least one working vane 156 has been extended even closer to the axis 8 of the impeller 150, and that the front or first or inlet opening 164 of the at least one balancing conduit 158 has been located on the axis 8 of the impeller 150, and in case of several balancing conduits 158 united into one opening 164, which is naturally, radially inside the radially innermost parts 156e of the at least one working vane 156 i.e. radially inside the point where the leading edge of the working vane 156 meets the front or first face of the hub 152 of the impeller 150. Thus, the balancing conduits 158 (if more than one) have a common first part 158.sub.1, and separate second parts, i.e. a bends 158.sub.2 and third parts 158.sub.3. The rest of the options for the configuration and the direction of the at least one balancing conduit 158 is similar to those of the embodiment of FIG. 3.

[0054] FIGS. 6 and 7 illustrate front views of the impellers of FIGS. 3 and 5, respectively, such that the working vanes 56, 156 (five in this impeller) have been cut away.

[0055] The drawings illustrate with broken lines the location of the balancing conduits 58, 158 in the impeller hub 52, 152 and shroud 54, 154. The drawings of these two embodiments show that the balancing conduits 58, 158 run circumferentially inclined and curved, i.e. each conduit is turned to follow the general direction of the working vanes. Thus, each balancing conduit is inclined, and curved both in the peripheral and radially outward directions from the front or first opening 64, 164 in the front or first face of the impeller hub 52, 152. In other words, the second openings 66, 166 of the balancing conduits 58, 158 in the rear or second face of the impeller shroud 54, 154 are located in the rotational direction of the impeller behind the opening 64, 164 at the opposite end of the balancing conduit 58, 158, i.e. in the front or first face of the hub 52, 152 and also radially outside thereof The aim with the balancing conduit 58, 158 extending in the impeller hub and shroud at least substantially in the direction of the impeller vane is, on the one hand, that the speed of the liquid flowing via the conduit 58, 158 to the rear vane area is in the right direction so that less work is required from the rear vanes to pump the flowing liquid out of the space behind the impeller 50, 150. On the other hand, the aim is to increase the flow of the liquid through the balancing conduits 58, 158 to the rear vane area so that the pressure in the sealing space (discussed in connection with FIGS. 1 and 2 with reference numerals 22 and 42) would remain positive throughout the entire capacity range of the pump.

[0056] A feature worthwhile to understand is that the impeller provided with the balancing conduit/s of the present invention may not only be a semi-open impeller but also a closed one, i.e. one having another shroud, so called front shroud, arranged on the edge/s of the working vane/s facing the volute 2. Such a front shroud may, if so desired, have so called front vane/s on its face opposite to the face where the working vane/s are located. However, It is clear from the description above that irrespective of the impeller type the impeller of the present invention does not have impeller sealing/s discussed in the introductory part of the specification by referring to JP-58192995, as they result in such an operation of the centrifugal pump that is clearly different from that discussed in connection with FIG. 4, for instance.

[0057] The above description discusses very generally balancing conduits and their direction. It should be noted about the conduits that they may vary a lot, for example, in both their direction and shape. In other words, it is possible that the balancing conduits are not inclined in circumferential direction (in the front view corresponding to FIGS. 6 and 7), but are either radial or slightly curved, but mainly orienting in radial direction in the front view. Also, the balancing conduits could be linear in the front view, i.e. either in radial or in inclined direction. With regard to the direction of the balancing conduits in side view (like those in FIGS. 3 and 5), the direction of the third part of the conduit 58.sub.3 or 158.sub.3 depends on the shape of the hub of the impeller as well as to the overall size of the impeller. Thereby no exact dimension can be given. The deeper into the hub the first substantially axial part extends the steeper is the inclination in relation to axial direction, and vice versa. Also the farther out the rear opening at the rear face of the shroud is located from the axis the steeper is the inclination.

[0058] It should further be understood that the balancing conduits may be curved for all their length and that the direction and radius of the curvature may change along the length of the bend. However, the balancing conduits may as well also be formed of two linear bores meeting within the body of the hub of the impeller. As to the shape of the balancing conduits, all round, oval and angular cross-sectional shapes may come into question. The cross-sectional area of the conduits may either be constant throughout the whole length of the conduit or it may vary at least for a portion of the length of the conduit. Further, it must be noted that both in the description above and in the accompanying claims, the direction of the conduit refers more to the direction of the centreline or axis of the conduit than to the direction of any specific wall thereof. The combined cross sectional flow area of the balancing conduits should preferably range between 5 and 20% of the area of the inlet opening of the pump, the area corresponding to the area limited by the outer tips of the flow receiving, or upstream, or leading edges of the working vanes when rotating. By such dimensioning it is ensured that the balancing conduits allow free and lossless flow of medium along the conduits. By which it is ensured that the pressure fluctuations in the sealing cavity are minimized. Another dimensioning feature of the balancing conduit/s is that the radially outer point of the front or first opening thereof is within the circle C formed by the radially innermost part of the working vanes, while the impeller is rotated about the axis. Preferably, the radially outer points of the front or first openings form a circle having a diameter 0.9 times, more preferably 0.8 times, most preferably 0.7 times the diameter of the circle C formed by the radially innermost part of the working vanes.

[0059] It has to be understood that the drawings do not pay any attention to how the impeller is fastened on the shaft. One feasible option is to arrange the impeller on the shaft for rotation with the shaft by a key or alike connection and to secure the connection immobile in the axial direction by a bolt arranged to run axially through the front face of the hub to a threaded blind hole at the end of the shaft. Naturally, an option is to arrange at the end of the shaft an extension having a smaller diameter to extend through the hub such that a nut may be used to secure the impeller axially immobile on the shaft. And a third option is to use a headless bolt screwed in a blind threaded hole at the end of the shaft and use a nut to secure the impeller axially immobile on the shaft. In the case of the embodiment of FIGS. 5 and 7 the bolt, or the shaft extension, could be arranged inside the opening 164 for the balancing conduits 158, however, taking into account that the bolt, or shaft extension, may not have any negative effect on the flow via the balancing conduits.

[0060] As can be seen from the above description, a new impeller has been developed, eliminating at least some disadvantages of the prior art impellers. An impeller in accordance with the present invention enables the use of the pump also at capacities higher than that of the optimal operating point, without a risk of damaging seals. While the invention has been herein described by way of examples in connection with what are at present considered to be the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is intended to cover various combinations and/or modifications of its features and other applications within the scope of the invention as defined in the appended claims.