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

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage application of International Application No. PCT/EP2015/065355, filed Jul. 6, 2015, which claims priority to EP Application No. 14190157.9, filed Oct. 23, 2014 the contents of each of which is hereby incorporated herein by reference.

BACKGROUND

Field of Invention

The present invention relates to a method of pumping a liquid medium by a centrifugal pump, a centrifugal pump and an impeller therefor. The present invention relates especially to modifying an impeller of a centrifugal pump in such a way that the pump may be used without risk of damaging a shaft seal or like when pumping of both clean liquids and liquids containing solids, like, for instance, fibrous suspensions.

Background Information

It is already known that, when pumping liquid or a suspension by a centrifugal pump, liquid is entrained into a space behind the impeller of the centrifugal pump when working vanes of the impeller increase the pressure of the liquid while pumping such from in front of the impeller to the volute radially outside the impeller. While increasing the fluid pressure in the volute, the rotating impeller creates suction in the impeller eye tending to move the impeller towards the pump inlet. At the same time, the liquid to be pumped in addition to being discharged to the pressure outlet of the pump also tends to fill the space behind the impeller. Although the liquid between the impeller and the rear wall of the pump rotates, on the average, half the speed of the impeller (provided that there are no so called rear vanes or like ribs on the rear face of the impeller hub or shroud; the relatively thin mainly radially oriented disc-like part extending from the hub is called a shroud) and thus, while generating centrifugal force, reduces to a certain extent the pressure prevailing in the sealing space behind the impeller in the area of the shaft of the pump, a considerable pressure, however, naturally affects in the space behind the impeller. Thus, both the negative pressure, or suction created by the working vanes and the fluid pressure behind the impeller shroud subject the bearings of the pump shaft to a force directed towards the inlet of the pump. Partially, therefore, so called rear vanes have been arranged on the rear face of the impeller shroud, which rear vanes pump the liquid having entered the space outwards, whereby the pressure in the space behind the impeller is substantially decreased. In other words, the rear vanes are used for reducing the axial force the impeller subjects to the shaft of the pump by drawing the shaft towards the inlet of the pump. Thus, the rear vanes are needed in a semi-open impeller to have pressure distribution at both sides, i.e. the front side and the rear side, of the shroud as equal as possible.

The rear vanes may, however, be dimensioned so that they operate optimally only in a certain capacity range of the pump, whereby deviation in either direction from the capacity range results in that the pressure prevailing within the area of the rear vanes and also in the shaft seal space changes. If the output of the pump is increased, the rear vanes generate, in the worst case scenario, a negative pressure, which can, at its worst, also make the liquid in the shaft seal space boil, especially when pumping liquids at a higher temperature. Correspondingly, when decreasing the capacity, for example, by constricting such by a valve, the pressure behind the impeller increases and the stresses increase. At the same time, naturally also the stress on the bearings increases.

Another problem involved in centrifugal pumps is the heat generated at the shaft sealing of the pump. The shaft sealing, in practice irrespective of its type, generates heat that has to be conveyed out of the sealing. Normally, the liquid to be pumped is used to flush the sealing and convey excess heat away. If the sealing starts running hot, the sealing elements may deteriorate either directly by the heat itself or by the liquid present in the sealing cavity boiling, i.e. evaporating, whereafter the sealing starts running dry.

For affecting the above discussed problems the use of balancing holes are suggested. The balancing holes are arranged to run through the hub or shroud. The holes have traditionally been cylindrical, i.e. they have had a linear axis and a constant diameter. Furthermore, the holes have been arranged such that their linear axis is parallel to the axis of the pump close to the hub of the impeller. Thus the liquid or suspension is allowed to flow from the side of the impeller where the pressure is higher to the area of the lower pressure. In other words, the flow in the balancing holes may be in either direction depending on the pressure conditions.

JP- patent publication 58192995 discusses a specific type of a centrifugal pump, i.e. a pump where the impeller is a closed one and sealed on both axial sides, i.e. the front side of the front shroud and the rear side of the rear shroud, thereof by slide impeller sealings located radially at about half the diameter of that of the impeller outer circumference. There are holes arranged through the rear shroud of the impeller, the holes entering the rear side of the rear shroud between the impeller sealings and the shaft of the impeller. This kind of a pump functions always such that the liquid to be pumped flows at the rear side of the impeller radially inwardly towards the shaft via a narrow gap in the slide impeller sealing. The purpose of the holes in the rear shroud is to allow fluid to flow out of the rear side of the impeller back to the impeller eye in front of the working vanes. Such a flow is usually aided by arranging the inlet to the hole at the rear side of the shroud at a smaller radius than the outlet from the hole at the front side of the shroud, whereby, in practice, the hole acts as a small centrifugal pump. The construction keeps the pressure behind the impeller low reducing thus the axial load to the bearings. However, the construction has a few weaknesses. Firstly, the capacity to transfer heat from the shaft sealing, which has to be there between the shaft and the housing irrespective of the outer impeller sealing, is dependent on the amount of flow allowed to bleed through the narrow gap of the impeller sealing. Secondly, the recirculation of the liquid to be pumped via the cavity at the rear side of the impeller shroud means loss of energy, as a part, though a small one, of the liquid to be pumped has to be pumped twice through the impeller. And thirdly, the described construction works only with clean liquid, as, if liquids containing solids are pumped, the solids block the impeller sealing, stop the liquid circulation and result in the shaft sealing running dry as the hole through the shroud pumps the liquid present in the rear side of the shroud away to the front side of the shroud.

SU-A1-1751429 discusses a centrifugal pump having a curved conduit provided through its hub/shroud. In fact the pump of the Russian application resembles the above discussed JP- patent publication 58192995, where the flow direction of the fluid flowing in the conduit is from the rear sides of the shroud to the front side thereof

Furthermore, although both balancing methods, i.e. the rear vanes and the hole through the shroud, are in use, it has been noticed, as discussed in U.S. Pat. No. 7,326,029 that when moving along a so called pump curve in the H, Q (head, capacity) chart, i.e. to the right in the direction of higher capacity, the balancing in accordance with the prior art is not always capable of sufficiently preventing the pressure in the sealing space from dropping below the pressure prevailing in front of the impeller of the pump. This is problematic because the negative pressure in the sealing space leads to the fact that the lubricating effect of the liquid to be pumped or other liquid on seals decreases when the liquid escapes from the seals. Depending on the seal type, the escaping of the liquid from the seal may cause the seal to run dry, which leads with some seal types very quickly to a seal damage.

The above mentioned US-patent suggests an impeller, in which the balancing holes are located in the impeller shroud in such a manner that the opening of each hole in the front face (the face on which the working vanes of the impeller are arranged) of the shroud are both in the rotational direction of the impeller ahead of the opening located in the rear face (the face on which the rear vanes are arranged) of the shroud and closer to the axis of the pump than the opening in the rear face of the impeller shroud. The holes have still had a linear axis and a constant diameter.

SUMMARY

However, in spite of the fact that the inclined balancing hole or conduit of the US-patent is a great improvement compared to earlier, traditional balancing holes oriented parallel to the axis of the impeller, it has now been learned that the balancing hole of the US-patent may still be improved for at least the following two reasons.

Firstly, the opening in the front face of the shroud has traditionally been located between the working vanes, and in the pump of the above discussed US-patent, the opening is at least partially between the working vanes. This means, in practice, that the pressure the balancing hole or conduit “sees” is the pressure between the working vanes, and not the inlet pressure (pressure at the impeller eye, in front of the impeller). In other words, the working vane has, due to its function, subjected the medium to be pumped to a centrifugal force component that changes the pressure conditions, by reducing the pressure, between the working vanes. For instance, when the pump is operating above its optimal operating point, the pressure in a vane passage, i.e. in a cavity between two successive working vanes, is below atmospheric pressure. It means that the pump sealing behind the impeller is subjected to the reduced pressure, which may cause the sealing to run dry and result in seal failure sooner or later.

Generally speaking, the centrifugal pump is not always working at the optimal conditions it has been designed for but more or less outside its optimal operating point. Depending on which side of and how far from the optimal operating point the pump is operating the working vanes, more or less efficiently, cavitate and generate vapour in the vane passage between the working vanes. The generation of vapour means, in practice, reduced pressure and suction to the sealing cavity.

To avoid the above discussed weakness in the impeller construction the present invention suggests positioning the balancing holes such that their inlets at the front face of the impeller shroud are always inside the circumference of the leading edges of the working vanes making the balancing holes insensitive to the function of the working vanes. The pressure the inlet openings to the balancing conduits “see” is the inlet pressure of the pump, not the pressure affected by the working vanes.

Secondly, to avoid the circulation of fluid from behind the impeller shroud back into the impeller eye, and to ensure proper flushing of the shaft seal, the balancing conduits are positioned in the impeller shroud such that the inlet opening at the front face of the impeller shroud is at a smaller radius than the outlet opening at the rear face of the impeller shroud. Thus the balancing conduits function like small centrifugal pumps pumping fluid from in front of the impeller to the rear side thereof

The two above mentioned features ensure that in all operating conditions of the pump the flow in the balancing conduits is towards the rear side of the impeller shroud, form where the rear vanes further take the fluid with the heat transferred to the fluid from the shaft sealing to the pump volute.

Thirdly, when looking for efficiency improvement from the hydraulics viewpoint, the working vanes of the impeller should be brought as close to the axis of the impeller as possible. This requires moving the front openings (at the front face of the impeller) into the balancing holes or conduits closer to the axis of the impeller to be able to make the holes/conduits insensitive to the function of the working vanes. Moving the working vanes to have their origin closer to the axis of the impeller means, in practice, that material from the front surface of the hub is removed, i.e. the hub is made smaller. However, now that the shroud, i.e. the mainly radial extension of the hub is made thinner and the front openings located closer to the axis than the inner end of the leading edges of the working vanes, it is, sometimes, impossible to extend a linear balancing conduit from the front opening to the rear opening without cutting the front face of the hub or that of the shroud open. Furthermore, the above together with moving the front openings closer to the axis of the impeller may, in some cases, require constructing the shaft-impeller fastening in another manner, as there may be no room for extending the end of the shaft directly through the hub of the impeller.

Thus, an object of the present invention is to design a novel impeller for a centrifugal pump such that the balancing of the pressure conditions at the front and rear sides of the impeller could be more reliable and less sensitive to the operating point of the centrifugal pump.

Another object of the present invention is to design a novel impeller for a centrifugal pump by which the hydraulic efficiency of the impeller may be improved.

Yet another object of the present invention is to design a novel impeller for a centrifugal pump such that the working vanes may be brought closer to the axis of the impeller for improving the hydraulic efficiency of the impeller.

A further object of the present invention is to design a novel impeller such that the heat generated in the shaft sealing is reliably conveyed out of the sealing area in all operating conditions of the pump.

A still further object of the present invention is to design a novel impeller such that the flow in the balancing conduit is always from the impeller eye, i.e. area upstream of the leading edges of the working vanes, to the rear side of the impeller shroud.

A still another object of the present invention is to design a novel impeller for a centrifugal pump such that the balancing conduits, or, in fact, the front openings thereof, could be brought, at the front face of the impeller, as close to the axis of the impeller as technically possible, whereby the sealing cavity behind the impeller is arranged in communication with the pump inlet pressure and not the varying pressure prevailing in the vane passages.

A yet another object of the present invention is to design such a novel impeller for a centrifugal pump that may be used to pump both clean liquids and solids-containing liquids like fiber suspensions of pulp and paper industry reliably and with high efficiency.

At least some of the above objects of the present invention are met with a novel impeller structure for a centrifugal pump, comprising at least a hub having an axis, a shroud extending outwardly from the hub and dividing the impeller to 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 said 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 located closer to the axis of the impeller than the second opening at the rear side of the impeller, the first opening of at least one balancing conduit in the first face of the hub being located within the circle C formed by the radially innermost part of the at least one working vane, while the impeller is rotated about the axis, wherein the second opening of the at least one balancing conduit at the rear side of the impeller is located at the smallest diameter of the shroud.

Other features characteristic of the present invention become apparent from the accompanying claims.

The impeller of the present invention brings at least some of the following advantages compared to prior art impellers.

- pressure conditions in the vane passage between the working vanes do not affect the operation of the balancing conduits,
  - pressure fluctuations in the sealing chamber are minimized,
  - the risk of running the shaft sealing dry is minimized,
  - reliable heat transfer from the shaft sealing to the fluid to be pumped, o energy efficient pump, as recirculation of the fluid to be pumped is avoided, and
  - optimal balancing arrangement for pumps using hydrodynamic sealing.

BRIEF DESCRIPTION OF THE DRAWINGS

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

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

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;

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

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

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

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

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

DETAILED DESCRIPTION OF THE EMBODIMENTS

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.

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_Lin 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_Lof 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

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.

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.

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.

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 58₁extending 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 58₂, 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 58₃between the bend 58₂and the rear or second or outlet opening 66. Here, the cross section shown in FIG. 3 is made to run along the centreline C_Lof 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.

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.

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.

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.

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.

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 158₁, and separate second parts, i.e. a bends 158₂and third parts 158₃. 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.

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.

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.

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.

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 58₃or 158₃depends 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.

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.

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.

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.

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

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