Technical field

(001 ) The present invention relates to a balance ring in accordance with the preamble of claim 1 , to a balancing device in accordance with the preamble of claim 4, to a centrifugal pump and to a method of balancing an axial thrust of the centrifugal pump. More specifically the present invention relates to single- or multi-stage centrifugal pumps having a novel disc- type balancing device for balancing the axial forces of the pump.

Background art

(002) The balancing devices for balancing axial forces of centrifugal pumps are normally in use in multistage pumps, which have a high pressure head, and are provided with several subsequent centrifugal impellers on the same shaft. An axial force is generated while an impeller, or a plurality of impellers, draws liquid axially in the pump and discharges the liquid radially from the pump. The axial force tends to draw the impeller/s towards the pump inlet, whereby the bearings of the pump are subjected to a considerable axial force, when keeping the impellers and shaft in place. In order to reduce the axial force subjected to the bearings, and, thus, to make it possible to use smaller or lighter bearings or different types of bearings balancing devices for balancing the axial force have been developed.

(003) Prior art knows two basic types of balancing devices for balancing the axial force. One is a so- called drum-type balancing device, and the other a disc-type balancing device. Also hybrid balancing devices are known, i.e. one comprising both a balancing drum and a balancing disc. In most cases the balancing device is positioned on the pump shaft behind the last impeller, when viewed from the pump inlet towards the pump outlet. However, it is possible, if desired, to construct a centrifugal pump such that the balancing device is between the stages of a multi-stage centrifugal pump or in front of the impeller/s thereof. The disc-type balancing device may be considered as the preferred choice of the two basic balancing devices as it adjusts its operation automatically, i.e. slight wear does not affect the operation of the balancing device at all, whereas even the slightest wear in the drum-type balancing device results in a change in the balancing capability of the balancing device. (004) The disc-type balancing device the present invention discusses later on in more detail is formed of a rotary balancing disc fastened on the shaft of the pump and a stationary counter member. In most cases the counter member is arranged to extend from the pump volute or casing radially inwardly between the impeller or one of the impellers and the balancing disc. Often the stationary counter member is the rear wall of the last pumping stage of the centrifugal pump. The balancing disc and the counter member leave a radially extending cavity, so called balancing cavity, there between. Either the balancing disc or the counter member or both have an annular axial extension, sometimes a separate circular ring, positioned in radial direction inwards from the outer periphery of the balancing disc for reducing the axial dimension of the balancing cavity to form a so called balancing gap between the balancing disc and the counter member in order to limit the leakage flow of the pressurized liquid from the pump. However, it should be understood that the balancing device, i.e. the balancing disc, its counter member and the balancing cavity, may also be located in front of the impeller/s when viewed from the direction of the inlet opening of the pump. In such a case it is required that the pressurized liquid is taken to the balancing cavity along a separate flow passage.

(005) The disc-type balancing device functions such that a part of the liquid pressurized by the impeller or the plurality of impellers enters, as is well known in centrifugal pumps, to the cavity behind the impeller of the last pumping stage, and finds its way via the gap between the shaft of the pump or the shaft sleeve of the balancing disc and the stationary counter member to a radially extending balancing cavity between the rotary balancing disc and the stationary counter member. Now that the pressure of the liquid is, in practice, not reduced the full pressure of the pumped liquid effects on the rotary balancing disc pushing the balancing disc away from the inlet of the pump, i.e. in a direction opposite to the axial force created by the impellers. Thereby, the axial thrust loading the bearings of the pump is the difference of the two axial forces having opposite directions. By properly dimensioning the balancing device the two opposite forces may be equalized resulting in zero thrust, whereby the shaft bearings may be replaced with slide bearings that are not able to carry any axial load.

(006) However, while the pressurized liquid flows radially outwardly in the balancing cavity between the balancing disc and its counter member, the liquid reaches the annular extension or ring and enters the annular balancing gap between the annular extension or ring and its counter surface. Now that the annular balancing gap is very thin, i.e. its axial dimension is very small, and the pressure difference radially over the ring is relatively high (depending mostly on the head of the pump), the flow velocity of the liquid in the thin balancing gap is high. Due to the high velocity of the liquid the pressure in the gap between the balancing disc and the counter member is low resulting in that in the area of high flow velocity, i.e. at the ring area, the disc is not able to create any significant axial force. The result, in favorable conditions, is that a part of the liquid flow entering the gap between the ring and its counter surface evaporates temporarily to vapor. Especially in such a condition that the pressure difference over the balancing device is high compared to how far from the steam pressure the balancing device operates. The evaporation of liquid reduces heat transfer from the balancing gap, which, naturally, increases the risk of further evaporation. The temporary evaporation of the liquid in the thin balancing gap results easily in mechanical contact between the ring and its counter surface, which, in the least, increases friction losses, and raises the temperature of the surfaces. Also, sudden evaporation of the liquid may lead to impacts between the counter surfaces as they hit one another. Both the friction and the impacts may, in the long run, cause wear, which may over time lead to need for replacing the balancing device with a new one. In other words, the first problem that may be seen in the operation of the balancing device is high power consumption combined with fluctuations in the power consumption due to the balancing device operating, alternatingly, in both low-friction and high-friction situations.

(007) The above described problems, first of all the mechanical contact between the counter surfaces, have earlier been suggested to be solved by increasing the effective area of the balancing disc by increasing the diameter of the balancing disc. It results in considerable increase in the power consumption of the balancing device without, however, preventing the liquid from boiling in all operating conditions of the pump. In other words, the prior art improvement leads to increased power consumption and occasional wear-related problems.

(008) A recent way to address the above problems is discussed in WO-A1 - 2017/157702, which is incorporated herein as a reference, and which teaches that at least one of the surfaces leaving the balancing gap there between, i.e. one of the surface of the balancing disc and the surface of the counter member is provided with at least one annular groove. The actual solution to the problems the WO- publication discusses is to add resistance to flow in the annular balancing gap by dividing the total pressure difference into two or more partial pressure differences by arranging one or more annular grooves either in the balancing disc or in its counter member or in both. The one or more annular grooves in the surfaces of at least one of the balancing disc and its counter member function such that the liquid entering the balancing gap between the counter member and the balancing disc sees only the pressure difference between the entrance to the balancing gap and the first annular groove, whereby the velocity of the liquid induced by that particular pressure difference is smaller than in such a case that the total pressure difference would act in the liquid. Thereby the local pressure, due to the smaller liquid velocity, in the balancing gap is high enough for not allowing the liquid to evaporate. Furthermore, the flow velocity of the liquid in each groove is reduced close to nil, as the height or thickness of the cross sectional flow area is suddenly changed from that of the balancing gap, i.e. from a micron range, to that in the groove, i.e. to a millimeter range. For the above reason the liquid pressure everywhere in the balancing gap is able to carry some load, and as the pressure in the groove area is able to carry a substantial load, abrupt mechanical impacts of the balancing disc to its counter member are prevented. Simultaneously the risk or allowing evaporation of the liquid is reduced.

Brief summary of the Invention

(009) However, there are factors that affect the operation of the balancing device. The quality and degree of wear of the components of the pump, the process in which the pump is installed, the properties of the liquid to be pumped and the way the pump is run, just to mention a few factors, all have an effect on the operation. As to the way the pump is run, in one of the most challenging situations the discharge valve at the pump outlet is closed, whereby the pump and the balancing device should be able to run reliably for a certain period of time. In practice, when the discharge valve is closed, the maximum load carrying capacity of the balancing device is usually required. Such a condition in high power density pumps leads to a high temperature increase in the pump in just a few seconds. High power density meaning, that the fluid volume inside the pump is relatively small in relation to the motor power of the pump. As the flow heats up in the pump, also the distance to vapor pressure is changed in a short time. In other words, the cooler the liquid flow in the balancing gap is, the farther away from vapor pressure the liquid flow is and some safety margin is ensured. Now, while the liquid flow heats up the safety margin is reduced. Keeping the discharge valve closed for a certain period of time eventually leads to such a condition that vapor pressure is reached in the pump. The current invention moves this location of vaporization away from the wear surfaces of the balancing disc and the counter member. The location of the first vaporization can be, for example, the radial bearing. The vaporization leads immediately to vibration and the pump may then be stopped due to increase in vibration levels. Dry running due to vaporization in a radial bearing is a lot more convenient than in an axial bearing, i.e. the balancing device, as the forces in the radial bearing are approx. 100 times smaller.

(010) Thus an object of the present invention is to design such a novel balancing device for a centrifugal pump that is capable of solving the above discussed problem.

(01 1 ) Another object of the present invention is to design such a novel balancing device for a centrifugal pump that allows the pump to be run with a minimum capacity without a risk of having the liquid evaporate in the balancing gap.

(012) A further object of the present invention is to design such a novel balancing device for a centrifugal pump that prevents the mechanical contact between the balancing disc and its counter member.

(013) A yet further object of the present invention is to design such a novel balancing device for a centrifugal pump that makes it possible to replace the use of, for instance, silicon carbide with the use of steel as the material for the counter surfaces at the balancing gap.

(014) A still further object of the present invention is to develop such a novel balancing device for a centrifugal pump that adjusts automatically its operating clearance.

(015) A still further object of the present invention is to develop such a novel balancing device for a centrifugal pump that have different balancing modes for a high load operation and a normal load operation by weighting the role of adjustable and non- adjustable flow restrictions at different diameters between the balancing disk and the counter member. The weighting is changed by changing the axial position of the rotor (the balancing disc) in relation to the axial position of the stator (the counter member), which results in either increase or decrease of the importance of adjustable restrictions in relation to the non-adjustable restrictions. This allows both the area where the pressure is acting and the pressure to increase in case there is a need for high load capacity.

(016) At least one of the problems is solved and at least one of the objects of the present invention are met with a balance ring for a balancing device of a centrifugal pump, the balancing ring having an inner circumference, and an outer circumference, a front surface extending therebetween, and at least one annular groove dividing the front surface to an inner front surface portion and an outer front surface portion wherein at least one flow passage is provided in the inner front surface portion, the at least one flow passage extending outwardly from the inner circumference to the at least one annular groove. (017) Other characteristic features of the present invention become apparent in the appended independent and dependent claims.

(018) The present invention brings about at least some of the following advantages over the prior art balancing device

• no or very small fluctuations in the power consumption,

• lower power consumption than that in traditional disc-type balancing devices,

• smaller disc radius than that in traditional disc-type balancing device,

• continuous adjustment of the gap between the balancing disc and its counter surface,

• the balancing disc is capable of carrying load for the entire radial extension thereof,

• the balancing device may be easily adjusted for different pump properties, especially to varying head,

• the use of expensive materials for the coating and/or production of the components of the balancing device may be avoided, and

• possible wear has no effect on the function of the balancing device contrary to drum-type balancing device.

Brief Description of Drawing

(019) The present invention is discussed more in detail below with reference to the accompanying drawings, in which

Fig. 1 illustrates schematically, and in an axial cross section, a multi-stage centrifugal pump including a disc-type balancing device in accordance with a first preferred embodiment of the present invention;

Fig. 2 illustrates schematically an axial, more detailed cross section of the balancing device in accordance with a first preferred embodiment of the present invention;

Fig. 3 illustrates a detailed top view of the balance ring in accordance with a first variation of a first preferred embodiment of the present invention;

Fig. 4 illustrates an enlarged cross section of the balance ring along line A - A of

Figure 3;

Fig. 5 exemplifies the operation of a prior art balancing device, Fig. 6 exemplifies the operation of the balancing device of the present invention in normal operating conditions,

Fig. 7 exemplifies the operation of the balancing device of the present invention in abnormal operating conditions, i.e. when the axial dimension of the balancing gap is reduced.

Figs. 8a and 8b illustrate a setting of the annular groove and the outwardly extending flow passage in accordance with an optional embodiment of the present invention;

Figs. 9a and 9b illustrate a setting of the annular grooves and the outwardly extending flow passage in accordance with another optional embodiment of the present invention;

Figs. 10a and 10b illustrate a setting of the annular grooves and the outwardly extending flow passage in accordance with a still further optional embodiment of the present invention;

Fig. 1 1 illustrates a graph showing the difference in the working of the balancing device in accordance with the present invention compared to that of a prior art balancing device, and

Fig. 12 illustrates schematically the balancing device of the present invention as a collection of fixed and adjustable constrictions.

Detailed Description of Drawings

(020) Figure 1 illustrates an axial cross section of a multi-stage centrifugal pump having a casing 10 with an inlet 12 and an outlet 14, the casing 10 housing a plurality of, here four, impellers 16 attached on a shaft 18 for rotation therewith and a balancing device 20.

(021 ) Figure 2 illustrates schematically an axial, more detailed cross section of the balancing device 20 and the end part of the centrifugal pump in accordance with a first preferred embodiment of the present invention. Here in this embodiment the balancing device 20 is formed of a rotary balancing disc 22 attached on the shaft 18 for rotation therewith. In connection with the balancing disc 22 there may be a separate sleeve or the balancing disc may be provided with an integrated axial extension, i.e. a cylindrical sleeve 24, either one of the sleeves extending from the disc up to the hub of the impeller 16. The balancing device 20 further comprises a stationary counter member 28 extending from the pump casing 10 radially inwardly between the balancing disc 22 and the impeller 16. The stationary counter member 28 is, in this embodiment, either the rear wall of the centrifugal pump or a specific part attached thereto. In more general terms, the counter member is a part of the casing of the centrifugal pump or a specific part attached thereto. The stationary counter member 28 is, preferably but not necessarily, provided with a counter ring 26 attached to the counter member 28 such that it faces the area of the balancing disc 22 immediately radially inside the outer circumference of the balancing disc 22. Either one or both of the balancing disc and the counter member have, immediately inside their radially outer circumference, non-axial surfaces raised, in an axial direction, from the body of the balancing disc and the counter member, for instance in the form of an annular ring, like the counter ring 26. The surfaces being non-axial means, in practice, that the surfaces are either planar or more or less conical. The non- axial surfaces facing one another are preferably, but not necessarily, provided with an appropriate coating or manufactured of an appropriate material, like for instance silicon carbide, enduring momentary sliding contacts (for instance when starting or stopping the pump) between the surfaces.

(022) In operation, the pumped liquid is able to flow from the rear side cavity 30 of the impeller 16 to a radial clearance 32 between the sleeve 24 and the inner circumference 34 of the counter member 28 and further to an outwardly extending balancing cavity 36 between the balancing disc 22 and the counter member 28. At the radially outer part of the balancing cavity 36 the liquid flows via a thin balancing gap 36’ between the surfaces of the balancing disc 22 and the counter member 28 (here in this embodiment the counter ring 26 is the part of the counter member 28 facing the balancing disc 22) to a space radially outside the balancing disc 22. At least one of the surfaces of the balancing disc and the counter member leaving the balancing gap 36’ there between is raised, in the axial direction, from the body of the balancing disc and the counter member, respectively, to allow some wear of the surface/s. The space outside the balancing disc 22 is in flow communication with the cavity 38 axially behind the balancing disc 22 when viewed from the direction of the pump inlet 12. The liquid leaked through the balancing device 20 continues, in this exemplary embodiment, towards the slide bearing 40 of the pump shaft 18 such that the liquid is used to lubricate the slide bearing 40 while passing the bearing 40. The slide bearing 40 forms a fixed or constant throttle or constriction or resistance to the flow of the liquid having passed the balancing gap 36’ keeping the amount of leaked liquid the desired one. When having passed the bearing 40 the liquid enters the end cavity 42 of the pump from where it is either introduced via pipeline 44 to the suction of the pump or to the bearing at the opposite end of the pump shaft 18. (023) The above prior art description applies to WO-A1-2017/157702, which teaches further that an annular ring, i.e. corresponding to the above mentioned counter ring 26 in the counter member 28, may also be arranged on the balancing disc 22 to face the counter member 28 (or, possibly, the counter ring 26 thereon). Further, either the counter member 28 or the balancing disc 22, or both, may, in place of removable annular or counter rings or the like, be provided with a corresponding surface configuration, i.e. a surface portion raised from the body thereof for providing a thin balancing gap in cooperation with an opposite surface. To simplify the following description all the above mentioned variants, i.e. the one or two possible removable annular or counter rings or the one or two raised surface configurations are from here on covered by a term‘balance ring’. Thereby, both the balancing disc 22 and its counter member 28 are provided with a balance ring.

(024) Fig. 3 illustrates a partial section of the top view of the balance ring 50 in accordance with the present invention, i.e. a view taken, for instance, from the left in Figure 2. Fig. 4, on its part, illustrates an enlarged cross-section of the balance ring of Figure 3 taken along line A - A of Figure 3. In accordance with Figures 3 and 4 the balance ring 50 has a front surface 52 corresponding to at least one of the first and the second non-axial surfaces discussed in connection with Figure 2. In other words, one of the first non-axial surface and the second non-axial surface forms the front surface 56 of the balance ring 50. The front surface 52 is formed of a radially inner front surface portion 521 having a radial dimension‘hT and initiating from a radially inner circumference 54 of the front surface 52 having a radius ‘RT and an outer front surface portion 522 terminating to the outer circumference 56 and having a radial dimension‘h2’. As may be understood radius‘RT is also the inner radius of the balancing gap. The outer front surface portion 522 has an inner circumference with a radius‘R2’. Between the inner and outer front surface portions 521 and 522 there is an annular groove 58 having a radius ‘Rg’, and, as an example only, a rectangular cross section with a radial dimension or width ‘gw’ and a depth ‘gd’. Preferably, but not necessarily, the annular groove 58 is located at a radially central area of the front surface, i.e. halfway between the inner circumference 54 and the outer circumference 56 of the balance ring 50.

(025) The inner front surface 521 of the balance ring 50 is, in accordance with the present invention, provided with at least one outwardly extending flow passage 60 extending from the radius‘R1’ of the inner circumference 54 of the balance ring 50 up to the radius‘Rg’ of the annular groove 58. Here, as the actual dimension of the radius Rg is needed only for determining the length of the flow passage 60, the dimension or Rg may be considered equal with radius R2 but it may as well be considered equal with R2 - gw, i.e. Rg is between R2 - gw and R2. In other words, the radius Rg has to be long enough such that liquid flow from the flow passage 60 into the annular groove 58 is non- obstructed (applies also to the embodiments shown in Figures 8a, 8b, 10a, and 10b). The flow passage 60 has, as an example only, a rectangular cross section with a width of‘pw’ and a depth of‘pd’. The depth‘pd’ of the flow passage is, preferably but not necessarily the same or smaller than that‘gd’ of the annular groove 58. The flow passage 60 is preferably radial but it may also be inclined as long as it is capable of taking pressurized liquid to an annular groove at a diameter longer than that at the inlet to the flow passage 60. The flow passage 60 may have a constant cross-sectional area or it may have a widening or a converging cross section in the radial outward direction of flow. As already mentioned above, the balance ring may either be a removable annular ring having preferably, but not necessarily, a rectangular cross-section, or the same surface configuration with the raised front surface portions, the annular groove and the at least one outwardly extending flow passage may be accomplished by machining to the faces of the balancing disc and/or the counter member.

(026) In the following the operation of the disc type thrust balancing device is discussed in more detail. Firstly, the earlier, or traditional prior art balancing device having a rotary balancing disc and a stationary counter member have such opposing non-axial or non- cylindrical surfaces forming the balancing gap that there is no annular groove in either one of the facing surfaces. The operation of the balancing device is based on the pump pressure pushing the rotary balancing disc away from the impeller/s with a force caused by the pump pressure effecting on a surface area of the balancing disc remaining between the shaft of the pump and the inner circumference of the surfaces forming the balancing gap (corresponding to the radius‘RT of Figure 3). The problem with such a balancing device is that when the pressure in the balancing cavity (36 in Figure 2) is reduced the force moving the balancing disc away from the counter member is reduced, too, whereby the balancing gap gets narrower. It means that the flow velocity of the liquid in the balancing gap increases, whereby the pressure in the gap is reduced leading to such occasional low pressure zones that allows the liquid evaporate. When such happens there is neither any heat transfer from the balancing gap, i.e. from the surfaces on both sides of the gap, nor any lubrication between the opposing surfaces, whereby any contact between the opposing surfaces results in drastic wear of the surfaces. In the described example the resistance to flow in the balancing gap was not sufficient for preventing the flow velocity in the balancing gap from growing high and risking the operation of the balancing device. (027) In WO-A1 -2017/157702 the above discussed problem was corrected by arranging at least one annular groove, like groove 58 in Fig. 3, to at least one of the pair of counter surfaces forming the balancing gap. The annular groove 58 is, preferably but not necessarily, rectangular of its cross section and has a depth‘gd’ of 1 to tens of millimeters depending on the liquid to be pumped, i.e. the more foreign abrasive material the liquid carries the deeper the groove should be to allow the depth of the groove to wear down without losing its ability to work in the desired manner. The groove functions such that, when entering the groove the liquid loses its flow velocity almost entirely, and when entering the thin balancing gap again the liquid has to be accelerated to the flow velocity corresponding to the thin balancing gap. The thickness dimension of the flow area is increased, when entering the groove, from a micron range of, for instance, 0,02 to 0.2 mm to a millimeter range, i.e. to a value from 1 mm to tens of millimeters. In other words, the cross sectional flow area, or in fact the radial depth thereof (as the width, i.e. the circumference remains substantially the same), is increased to at least 10-fold, preferably to more than 50-fold or more than 100-fold depending again on the type of liquid to be pumped. And after the groove, the same is decreased back to micron range again. The at least one groove increases considerably the resistance to flow whereby the heat transfer from the balancing gap as well as the load the balancing disc was able to carry improved and the balancing device remained longer in operative condition.

(028) However, there are still operating conditions where the load carrying capacity of the balancing disc is not on such a level that the pump could be run in all imaginable conditions and ways without a risk of mechanical contact of the surfaces and/or evaporating of the liquid to be pumped.

(029) Now, in accordance with the present invention, the at least one outwardly extending flow passage 60 leading outwardly from the balancing cavity (36 in Figure 2) or from the inner circumference 54 of the balancing gap 36’ or that of the balance ring 50 to the annular groove 58 is the solution to the above discussed problem. The flow passage 60 is, preferably but not necessarily, rectangular of its cross section and has a depth‘pd’ of 1 to tens of millimeters depending on the size of the pump, the number of outwardly extending flow passages and the liquid to be pumped, i.e. the more foreign abrasive material the liquid carries the deeper the outwardly extending flow passage should be to allow the depth of the flow passage to wear down without losing its ability to work in the desired manner. The width‘pw’ of the outwardly extending flow passage is of the order of 1 to 10 mm. The basic property of the outwardly extending flow passage is that it forms a fixed constriction, i.e. a fixed cross sectional flow area in parallel with the balancing gap 36’ between the balancing disc 22 and the counter member 28.

(030) The balance ring 50 of prior art functions as exemplified in Figure 5 such that the pressure p, here given, in place of Pascals, Pa or MPa, i.e. SI- units, as the head of the pump, prevailing in the balancing cavity is 800 meters. The balancing gap at both the inner and outer front surface portions on both sides of the annular groove causes a pressure reduction Dr of 350 meters each, whereby the liquid pressure downstream of the balancing device is 100 meters. Now the present invention functions as follows, and as exemplified in Figures 6 and 7. In normal operating conditions, in Figure 6, when there is enough pressure, p = 800, i.e. the head of the pump is 800 meters, in the balancing cavity and the temperature of the liquid to be pumped is in normal level, the balance ring operates with a balancing gap having a normal width, i.e. axial dimension, of the order of 0,05 - 0,1 mm, whereby the inner and outer front surface portions and a groove there between form required resistances to flow, i.e. 2 ^* Dr of 350 meters such that the existence of the flow passage has no effect on the operation of the balancing device. In other words, its existence cannot, in practice, be noticed, as the cross sectional flow area of the at least one outwardly extending flow passage compared to that of the entire balancing gap is negligible. In such a case the force the balancing disc forms to counter the axial thrust of the impeller/s is the product of the pressure in the balancing cavity and the area limited by the inner radius‘R1’ of the balancing gap and the shaft of the pump. As the pressure distribution in the inner and outer parts of the balancing gap in Figures 5 and 6 is approximately the same, also the axial forces from these two areas are the same.

(031 ) However, as exemplified in Figure 7, i.e. when the width or the axial dimension of the balancing gap is reduced to a value below 0.05 mm, close to 0.02 mm or even there below, the share of the open area of the at least one flow passage 60 to the flow area into the balancing gap (without the at least one flow passage) increases considerably resulting in change in the operation of the balancing device. The open area‘pA’ of the at least one flow passage 60 may be calculated as follows: pA = n ^* pw ^* pd, where‘n’ equals to the number of the flow passages,‘pw’ the flow passage width and‘pd’ the flow passage depth. The flow area into the balancing gap‘gA’ may be calculated as follows: gA = 2 ^* p ^* R1 ^* gap, where‘RT equals to the inner radius of the balancing gap and ‘gap’ the axial dimension of the balancing gap. The performed experiments have shown that when the ratio pA/gA grows to about 0.9 ... 1.1 , the pressure of the balancing cavity starts effecting on the disc surface inside the outer radius‘R2’. In other words, when the relative share of the at least one outwardly extending flow passage 60 grows high enough the at least one outwardly extending flow passage 60 allows pressurized liquid enter the annular groove 58 such that the pressurized liquid occupies the inner part of the balancing gap 367521 , whereby the flow via the inner part of the balancing gap 36’, i.e. along the inner front surface portion 521 is significantly reduced, and, thus, the effective radius the pressure acts on is‘R2’. This means, in practice, that, now that the area the pressure is efficiently acting on is increased, the load carrying capability of the balancing disc is increased considerably. In the example shown in Figure 7, provided that the pressure in the balancing cavity is 800 meters, the pressure reduction Dr at the inner front surface portion of the balancing gap is 150 meters and in the outer front surface portion of the balancing gap is 550 meters.

(032) Example 1

(033) If we assume the radius‘RT is 100 mm and the depth or axial dimension of the balancing gap is 0.05 mm, the cross sectional flow area in the balancing gap‘gA’ is 31 .4 mm ² . Exemplary dimensions of an outwardly extending flow passage are‘pw’ = 3 mm and‘pd’ = 3 mm, when there is only one outwardly extending flow passage. Thus, the cross sectional flow area‘pA’ of the outwardly extending flow passage is 9 mm ² . Thereby, the area‘pA’ of the flow passage is less than one third of the area‘gA’ of the balancing gap, and, as a result, most of the leakage flow through the balancing device takes place via the balancing gap. However, when the axial dimension of the balancing gap is reduced to 0.02 mm, the cross sectional flow area‘gA is reduced to 12.6 mm ² , whereby the flow areas in the gap and in the flow passage are almost the same, and, in practice, the pressurized liquid flow via the flow passage fills the annular groove and causes the liquid flow via the balancing gap to stop almost entirely.

(034) Example 2

(035) If we assume that the radius‘R1’ of the inner circumference 54 of the balance ring 50 is 100 mm, the radius‘R2’ of the outer circumference of the groove 58 is 1 10 mm, i.e. ‘RT +‘hT of 7 mm +‘gw’ of 3 mm, and the radius‘r’ of the shaft of the pump is 50 mm, the areas the pressure in the balancing cavity acts on are: smaller area (with a wide balancing gap) = p ^* R1 ² - p ^* r ² , and larger area (with a narrow balancing gap) = p ^* R2 ² - p ^* r ² . The ratio of the larger area to the smaller area equals to (R2 ² - r ² )/(R1 ² - r ² ) and further to (1 10 ² — 50 ² )/(100 ² - 50 ² ) = 1.28. In other words, the load carrying capability of the balancing disc is raised by 28% with the inclusion of the at least one flow passage 60. (036) Example 3

(037) What the above examples and especially the increased load carrying capability mean, in practice, is that the opposing surfaces of the balancing gap cannot get into contact with one another in any other running situation than a short time period immediately after starting or a short time period immediately before stopping the pump, as the pumps are designed such that the surfaces rest against one another when the pump is at rest. This finding makes it possible to stop using specifically designed or manufactured, i.e. expensive, slide surfaces for the opposing or facing surfaces of the balancing disc and its counter member. In other words, the normally used silicon carbide or like surfaces or rings may be replaced with steel ones.

(038) The above principle of having at least one outwardly extending flow passage taking liquid from the radially inner part of the balancing cavity to the annular groove may also be arranged as shown in Figures 8a through 10b. In Figures 8a, 9a and 10a the embodiment shows a structure where the opposing or facing surfaces are provided in replaceable separate rings 62 and 64, and in Figures 8b, 9b and 10b the embodiment shows a structure where the opposing or facing surfaces are provided in the bodies of the balancing disc 66 and its counter member 68. Naturally it is also possible to arrange a replaceable separate ring to face a surface provided in the body of the balancing disc or its counter member. In accordance with one optional embodiment shown in Figures 8a and 8b the annular groove 70 and the at least one outwardly extending flow passage 72 are not in the same surface but in the opposing surfaces.

(039) In accordance with another optional embodiment shown in Figures 9a and 9b there may be more than one annular groove, i.e. grooves 70’ and 70” in a surface whereby the at least one outwardly extending flow passage 74 leads from the inner circumference of the balance ring or balancing gap to the radially outermost annular groove 70”. In accordance with a further variation of the present invention the outwardly extending flow passage running from the radially innermost annular groove 70’ to the radially outer annular groove/s 70” may have a cross sectional flow area smaller than that of the outwardly extending flow passage between the inner circumference of the balance ring and the radially innermost annular groove.

(040) In accordance with still another optional embodiment shown in Figures 10a and 10b the annular grooves 70’ and 70” and the at least one outwardly extending flow passage 76 and 78 are not in the same surface but in the opposing surfaces. In other words, the annular groove or grooves may be in the balancing disc or in the counter member, and the outwardly extending flow passage in the counter member or in the balancing disc, respectively. Naturally, and as shown in the Figures the outwardly extending flow passage may be divided in both opposing surfaces such that a first part 76 of the outwardly extending flow passage is in a surface facing the annular grooves and the other part 78 of the outwardly extending flow passage in the surface comprising the annular grooves 70’ and 70”, i.e. the outwardly extending flow passage 78 extending from the inner annular groove 70’ to the outer annular groove 70”.

(041 ) As to the outwardly extending flow passage it should be understood that there may be one or more such passages extending from the inner circumference of the balancing gap to the at least one annular groove. If the number of passages is more than one the passages may be preferably, but not necessarily, evenly divided on the circumference of the at least one annular groove. Also, it should be understood that there may be one outwardly extending flow passage leading to the first annular groove, and more than one passage to the second annular groove, or vice versa. Further, there may be several outwardly extending first flow passages between the inner circumference of the balancing gap and the first annular groove and several outwardly extending second flow passages between the first and the second annular grooves. The number of the first and second flow passages may be the same or different. In case the number is the same the first flow passages may extend from the first annular groove as the second flow passages to the second annular groove, or the second flow passages may not be aligned with the first ones, whereby the first and the second flow passages are arranged in a zig- zag configuration.

(042) The working of the present invention may be explained also by means of Figure 1 1 , which illustrates a graph showing several curves representing both the working of a traditional balancing device (curve A) and the working of the balancing device of the present invention (curves B and C). The vertical axis represents the head‘h’ of the pump and the horizontal axis the capacity‘Q’ of the pump. The vertical line 11 shows the best efficiency point where the pump is normally run. Line I2 shows the point below which the capacity of the pump cannot be lowered when using an ordinary balancing device. When the capacity‘Q’ is reduced below that in the best efficiency point, also the head‘h’ of the pump gets lower, which, in practice, also reduces the axial dimension or depth of the balancing gap. If the capacity is still lowered, i.e. to the left of line I2 the liquid in the balancing device is at risk of starting to evaporate, heat is generated and the balancing device is at risk of being damaged quickly.

(043) However, when using the balancing device of the present invention the head and the capacity are, when coming to the left from the best efficiency point or line 11 , reduced in the same pace with the ordinary balancing device. But, when the balancing gap reduces to a certain value, of the order of 0.01 - 0.04 mm, the curves B and C turn gradually to horizontal direction, which means that even when the capacity‘Q’ gets to zero there is still some head (pressure) left for keeping the surfaces of the balancing disc and its counter member at a distance from one another.

(044) Another way to exemplify the present invention is shown in Figure 12, where the liquid flow path via the balancing device is illustrated in view of the resistances or constrictions arranged on the flow path. In Fig. 12 the liquid enters the balancing device from the left. In the balancing cavity 36 the liquid has two flow paths to choose from, the fixed constriction in the form of the at least one outwardly extending flow passage 60 and the adjustable, or in fact, automatically adjusting constriction in the form of the first part of the balancing gap 36’ in front of the inner surface portion 521 (see Fig. 4) referred to as‘367521’. When having passed the flow passage 60 and the first part of the balancing gap 36’ the liquid enters the annular groove 58, and continues to another adjustable, or in fact, automatically adjusting constriction in the form of the second part of the balancing gap 36’ in front of the outer surface portion 522 (see Fig. 4) referred to as‘367522’. When having passed the balancing gap 36’ entirely the liquid enters the bearing 40 or some other element forming a fixed constriction on the flow path of the liquid.

(045) The opposing surfaces of the balancing disc and the counter member forming the balancing gap there between are usually in prior art balancing devices coated with or otherwise manufactured of a specific material, for instance silicon carbide, suitable for working as sliding counter surfaces. If one or both of the surfaces are in a separate annular ring or in a balance ring, such ring is fastened on the counter member or on the balancing disc or in an annular groove in the counter member or in the balancing disc by means of screws, adhesives, welding, riveting, just to name a few examples without any intention of limiting the options to the listed ones. However, the present invention makes is possible to replace the specific material with steel, as the contact between the opposing surfaces takes place only for a short period of time immediately after the starting of the pump or for a short period of time immediately before the stopping of the pump.

(046) As to the orientation of the surfaces of the balancing disc 22 and its counter member 28 facing one another it is preferably radial, as in such a case the axial dimension the balancing device requires is the smallest. However, the advantages of the present invention are available as soon as the direction of the counter surfaces of the balancing gap 36' clearly differs from axial direction. In other words, as soon as the movement of the counter surfaces relative to one another causes a change in the axial dimension of the gap 36’ the advantages of the invention are available. Thus, the basic requirement for the direction of the counter surfaces is that the direction thereof is non- axial, i.e. the surfaces are non-cylindrical. However, it could be assumed that the orientation of the counter surfaces in the balancing ring and its counter member should be between 30 and 90 degrees from the direction of the axis A (see Fig. 2) of the pump.

(047) While the invention has been described herein by way of examples in connection with what are, at present, considered to be the most preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is intended to cover various combinations or modifications of its features, and several other applications included within the scope of the invention, as defined in the appended claims. The details mentioned in connection with any embodiment above may be used in connection with another embodiment when such combination is technically feasible.