Abstract:
A double-flow centrifugal pump is disclosed, in which, using an impeller, a fluid can be suctioned from two axial sides from a negative pressure area and can be delivered in the radial direction into a positive pressure area. The negative pressure area is sealed off with respect to the positive pressure area by at least one stationary pump component, for example the pump housing and/or an insert part. According to the invention, sealing gaps are formed as axial gaps arranged axially between the impeller and the pump component and extending in the circumferential direction and in the radial direction, wherein the gap width(s) of said axial gaps is less than the radial spacing (a) of the impeller from all the components arranged with a radial spacing from the impeller.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a preferably single-stage, double-flow centrifugal pump, in particular a cooling water pump for a marine diesel engine or a ballast water delivery pump on a ship, with a pump housing and with a dual-flow impeller rotationally fixedly arranged on a rotationally driven shaft, with which impeller a fluid can be suctioned from two axial sides from a negative pressure area (suction side) and delivered in the radial direction into a positive pressure area (pressure side), whereby the negative pressure area is sealed off with respect to the positive pressure area by means of at least two sealing gaps which are spaced apart axially (via the positive pressure area) and which are formed between the impeller and at least one stationary pump component, in particular the pump housing. 
       BACKGROUND OF THE INVENTION 
       [0002]    In known dual-flow centrifugal pumps the sealing gaps, formed as annular gaps, run in the axial direction and are formed between the impeller and the pump housing. During the operation of the known centrifugal pumps a resulting radial force component acting on the shaft supported on one side occurs, in particular if the centrifugal pumps are not operated at their optimal working point, so that the shaft with the impeller rotationally fixed on it is deflected in the radial direction. In order to prevent the impeller from contacting the pump housing during this deflection movement the sealing gaps formed as axial gaps must be dimensioned to be appropriately wide. However, this results in a performance loss of the pump since constantly delivered medium from the radial positive pressure area flows in the axial direction through the sealing caps into the negative pressure area (suction area). As a consequence, the efficiency of the known centrifugal pumps is significantly impaired. Previously cited centrifugal pumps are only suited for applications, if the shaft is supported on one side, in which comparatively low volume flows are to be delivered. In the case of dual-flow centrifugal pumps for large-volume flow applications, for example, for example in the case of cooling water pumps for a marine diesel engine or of ballast water delivery pumps on a ship, the shaft carrying the impeller is supported as a rule on both axial sides of the impeller in order to minimize the radial deflection movement during operation. In the case of an only one-sided support of the shaft for these applications a shaft would have to be used with an appropriately large diameter and/or a complex support. 
       SUMMARY OF THE INVENTION 
       [0003]    Starting from the previously cited prior art, the invention solves the basic problem of indicating a dual-flow centrifugal pump, in particular for large-volume flows of at least 500 m 3 /h with which a high degree of efficiency is possible without complex constructive measures. The centrifugal pump shaft carrying the impeller should preferably be supported exclusively on one side and have the smallest possible diameter. A striking of the impeller on the pump housing should be reliably avoided. 
         [0004]    This problem is solved by a generic, dual-flow centrifugal pump in which the sealing gaps are formed as axial gaps extending in the circumferential direction as well as in the radial direction and arranged axially between the pump component and the impeller, the gap width of which gaps, preferably measured in the axial direction, is greater than the radial distances of the impeller to all components that are arranged with radial spacing radially outside of the impeller. In other words, the gap width of the sealing gaps formed as axial gaps is greater than the gap widths of all other gaps (radial gaps) that are limited on one side by the impeller. This means, expressed another way, that the distances, measured in the radial direction, between the impeller and any components of the pump are greater than the gap width of the sealing gaps formed as an axial gap. 
         [0005]    Advantageous further developments of the invention are indicated in the various claims. The scope of the invention includes all combinations of features disclosed in the specification, the claims and/or in the figures. In order to avoid repetitions, features disclosed as devices are considered as disclosed in accordance with the method and can be claimed as such. Likewise, features disclosed in accordance with the method are considered as disclosed in accordance with the devices and can be claimed as such. 
         [0006]    The invention is based on the concept that the sealing gaps between the impeller and at least one pump part with which the suction side of the centrifugal pump is sealed relative to the pressure side are to be constructed running in the radial direction relative to its longitudinal extent, i.e., as an axial gap. In other words, the impeller in accordance with the invention is distanced in the axial direction by the sealing gaps from the at least one, preferably exclusively one pump component. The width of the sealing gap, which width extends at least approximately in the axial direction, is less at least at one position, preferably over its longitudinal extent, than the distance between the impeller and all other pump components arranged with a radial distance to the impeller. In other words, the gap width of the sealing gap is less than the radial distance of the impeller to all pump components located radially outside of the impeller. The sealing gaps are distinguished in that their axial extent is (considerably) less than their radial extent. The gap width of the axial gap, (sealing gap) measured in the axial direction, is greater than the gap width, measured in the radial direction, of a radial gap arranged between the impeller and the pump component limiting the axial gap. 
         [0007]    The gap width of the sealing gap is preferably at least 20%, more preferably at least 12% and even more preferably 6% of the radial distance of the impeller  7  to the pump component limiting the axial gap, in particular to the pump housing and/or to an insertion part preferably forming a housing section. 
         [0008]    It is, of course, possible to provide several sealing gaps formed as an axial gap on both axial sides of the radial discharge area of the impeller. However, it is preferred to provide only one sealing gap formed as an axial gap, whereby the gaps with the smallest gap width are understood to be sealing gaps. 
         [0009]    A variant of an embodiment is quite especially preferred in which the preferably exclusively two sealing gaps are arranged in an area radially inside circumferentially closed radial gaps via which the impeller is distanced from the at least one, preferably exclusively one pump component. It is especially preferable if the axial gaps extend starting from the radial gaps in the radial direction inward. Therefore, a variant of an embodiment is especially preferred in which the axial gaps have, at least in a radially inner area, a lesser distance to the shaft than the radial gaps do. The sealing gaps are advantageously located inside an imaginary circular cylinder whose generated surface receives the radial gaps. As a result of such a variant of an embodiment the sealing action is improved. 
         [0010]    It is especially purposeful if the impeller has a circular, cylindrical casing contour, whereby it is even more preferred if the sealing gaps (axial gaps) are formed between a front side of the impeller comprising a cylindrical casing contour and between the at least one, preferably exclusively one pump component. 
         [0011]    Alternatively, a casing contour can also be provided in which the impeller extends with its discharge area further to the outside in the radial direction. However, as will be explained later, it is also preferred in the case of such a geometry if the axial sealing gap is arranged in an area that has a lesser radius than a possible radial gap arranged between the pump jet and the impeller. 
         [0012]    Based on the formation of the sealing gaps as axial gaps, it is possible to dimension the gap width of the sealing gaps considerably smaller than in the prior art without there being the danger that the impeller strikes the pump component limiting the sealing gaps upon a radial deflection. It is therefore possible by means of the design of the sealing gaps in accordance with the invention to achieve a high efficiency of the centrifugal pump since the amount of liquid that flows from the pressure area into the suction area (negative pressure area) is minimized by the small gap width of the sealing gaps. The distance between the impeller and the pump component and/or other components of the pump can be dimensioned in the radial direction in such a manner that even in case of the greatest possible deflection of the impeller occurring during operation there is no danger of collision. It is therefore possible, even for applications with a large-volume flow, in particular for marine applications, to realize an only one-sided support of the impeller shaft since greater radial deflections of the impeller can be accepted than previously. Furthermore, the dimensioning of the shaft as such can be minimized. 
         [0013]    An embodiment has an especially simple construction and is therefore preferred in which the sealing gaps run—in the framework of the tolerances—exactly in the radial direction relative to their longitudinal extent. However, a slightly curved or slightly oblique design of the sealing gaps by an appropriate shaping of at least one structural component (impeller and/or pump component, especially pump housing) limiting the sealing gaps is possible, in particular in such a manner that the gap geometry of the curved deflection movement of the impeller takes place especially with a one-sided shaft support so that the gap width remains at least independently constant independently of the degree of the deflection of the impeller during operation. The radius of curvature corresponds in an especially preferred manner at least approximately to the distance of the impeller to the support of the shaft carrying the impeller. 
         [0014]    It is advantageously provided in a further development of the invention that the gap width of the sealing gaps formed as axial gaps is selected from a value range between 200 μm and 2000 μm, quite especially preferably between 200 μm and 400 μm. 
         [0015]    It is especially advantageous if the minimal, i.e., the slightest radial distance of the impeller to the pump component of the centrifugal pump, which pump component limits the sealing gaps formed as axial gaps, (with the impeller standing still) is selected from a value range between 2 mm to 10 mm. In other words, the distance between the impeller and the previously cited pump component is preferably greater than the distances of the indicated value range. The previously cited minimal radial distance is especially preferably not only the minimal radial distance of the impeller to the at least one, preferably exclusively one pump component limiting the sealing gaps, but rather the minimal radial distance of the impeller to all structural components of the pump, in order to reliably prevent a collision upon a radial deflection. 
         [0016]    An embodiment of the dual-flow centrifugal pump has an especially preferred construction in which the sealing gaps are arranged between the front sides of the impeller, which front sides face in the axial direction, and between the at least one pump component. In other words, it is preferred if the sealing gaps have the greatest possible axial distance from each other. This can be realized, for example, in that the impeller has a casing contour that is at least approximately circularly cylindrical. An imaginary generated surface of a centrifugal cylinder which surface receives the radial gaps especially preferably surrounds the axial gaps radially on the outside. 
         [0017]    As initially explained, the phrase an axial gap (sealing gap) extending in the radial direction denotes not only an embodiment in which the sealing gaps run exactly in the radial direction relative to the longitudinal extent, in the framework of the tolerances, that is, they are constructed, for example, in the shape of annular disks. An embodiment is also conceivable in which the sealing gaps have a slight angle of rise or are slightly curved, i.e., have a large radius of curvature that preferably corresponds at least approximately, in particular in the case of a shaft supported on one side, to the distance of the particular sealing gap from the shaft support. The particular sealing gap is thus designed in such a manner that the gap width during the operation of the centrifugal pump does not change or changes only as slightly as possible, that is, upon a possible radial deflection of the impeller since the gap geometry follows the deflection movement. The curvature or beveling of the sealing gap can be realized by an appropriate geometric shaping of the impeller and/or of the at least one, preferably exclusively one pump component limiting the sealing gap on the axial side opposite the impeller. The angle (angle of inclination) of the particular sealing gap to an imaginary radial plane arranged orthogonally to the longitudinal extension of the shaft is quite especially preferably selected from a value range between 0.01° and 2.0°. A possible radius of curvature is preferably selected from a value range between 200 mm and 1000 mm, preferably 300 mm and 700 mm. 
         [0018]    The radius of curvature of the particular sealing gap, more precisely at least of a surface (of the impeller and/or of the pump component) limiting the sealing gap preferably corresponds at least approximately to the distance of the particular sealing gap (in particular on a radially innermost area of the sealing gap) to the shaft support, in particular in the case of one (pump shaft) supported on one side. In a corresponding manner the angle of inclination of the gap explained in the specification refers to the angle of at least one surface (of the impeller and/or of the pump component) limiting the sealing gap relative to the previously cited radial plane. 
         [0019]    As already indicated, it is an extremely cost-effective variant of an embodiment of the centrifugal pump if the shaft carrying the impeller is supported exclusively on one side, preferably on an upper side. 
         [0020]    It is especially purposeful if the centrifugal pump in accordance with the invention is designed for large-volume flow applications, in particular marine applications. The centrifugal pump is preferably designed for delivering a volume flow from a value range between approximately 500 m 3 /h and approximately 4000 m 3 /h, preferably between approximately 800 m 3 /h and approximately 1500 m 3 /h (for example, in the case of rather small cooling water pumps) or between approximately 1500 m 3 /h and approximately 2300 m 3 /h (for example, in the case of average-size cooling water pumps) or between 2300 m 3 /h and 3500 m 3 /h (for example, in the case of rather large cooling water pumps), preferably at a maximum delivery level from a value range between approximately 20 m and approximately 50 m, preferably from approximately 30 m. It is especially preferable for reasons of space, especially for marine applications, if the dual-flow centrifugal pump is realized in a vertical construction, that is, in such a manner that the shaft runs vertically to a standing surface of the centrifugal pump. 
         [0021]    It is especially preferable if the centrifugal pump is a single-stage centrifugal pump, that is, a pump comprising exclusively one impeller. 
         [0022]    It is especially purposeful if the pump housing is a so-called spiral housing that sets the flow path on the suction side to the two axial sides of the impeller and combines preferably two outlet conduits in a helical manner on the pressure side. 
         [0023]    The invention also comprises the use of a dual-flow centrifugal pump designed in accordance with the concept of the invention as a cooling water pump for a marine diesel engine or as a ballast water delivery pump on a ship. 
         [0024]    Other advantages, features and details of the invention result from the following specification of preferred exemplary embodiments as well as from the figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    The accompanying drawings illustrate preferred embodiments of the disclosed method so far devised for the practical application of the principles thereof, and in which: 
           [0026]      FIG. 1  shows a cross-section view of an exemplary embodiment of a dual-flow centrifugal pump constructed in accordance with the concept of the invention, 
           [0027]      FIG. 2  shows a schematic illustration of gap conditions for the pump of  FIG. 1 , and 
           [0028]      FIGS. 3-7  show a plurality of embodiments of the sealing gaps. 
       
    
    
       [0029]    In the figures the same elements and elements with the same function are characterized with the same reference numerals. 
       DETAILED DESCRIPTION 
       [0030]      FIG. 1  shows a sectional view of a dual-flow centrifugal pump  1  in a vertical orientation (i.e., the impeller shaft is oriented vertically). The exemplary embodiment shown is a cooling water pump for a marine diesel engine designed for delivering a volume flow of 2300 m 3 /h at a maximum delivery level of 30 m. 
         [0031]    The centrifugal pump  1  comprises a pump housing  2  designed as a spiral housing and with a suction-side inlet  3  as well as a pressure-side outlet  4 . A shaft  5  supported on one side extends into the pump housing  2  from above downward in a vertical direction and is supported by a bearing  6  constructed as a ball bearing. The shaft  5  carries on its inside a dual-flow impeller  7  with a substantially circularly cylindrical casing contour. The impeller  7  sits in a rotationally fixed manner on the shaft  5 . A shaft seal  8  is located in an area axially between the support  6  and the impeller  7 . As is apparent from  FIG. 1  the shaft  5  extends in an area above the shaft seal  8  through a cover  9  fixed by screwing on the pump housing  2 . 
         [0032]    The impeller  7  separates a negative pressure area  10  (suction side) from a positive pressure area  11  (pressure side). 
         [0033]    The shaft  5  can be rotated in a known manner by an engine (not shown), in particular by an electromotor, whereby the impeller  7  rotating with the shaft  5  sucks fluid, here cooling water, from both axial sides out of the negative pressure area  10  and delivers it in a radial direction outward into the positive pressure area  11 , whereby the positive pressure area  11  is subdivided into two helically arranged flow conduits  12 ,  13  separated from one another by a dividing wall  14 . The two flow conduits  12 ,  13  and the fluid currents are brought together again in the area of the outlet  4 . 
         [0034]    During the operation of the centrifugal pump  1 , especially when the centrifugal pump  1  is not working at an optimal working point, a loading of the shaft  5  with a radial force occurs in the area of the impeller  7  which load has the tendency to deflect the shaft  5  with impeller  7  in the radial direction. In order to prevent the impeller  7  from colliding with the pump housing  2  (pump component) in the radial direction two axially spaced radial gaps  15 ,  16  extending in the axial direction are dimensioned to be so wide that even a maximally conceivable deflection of the shaft  5  during operation cannot result in a collision of the impeller  7  with the pump housing  2 . The radial gaps  15 ,  16  are not designed as sealing gaps and fulfill no sufficient sealing function on account of their comparatively large gap width (measured at the narrowest position) from the one in the exemplary embodiment shown of approximately 5 mm. The radial gaps have the form of circular, cylindrical surfaces. If the radial gaps  15 ,  16  were the only sealing gaps, the centrifugal pump  1  would have an extremely poor efficiency on account of the comparatively large gap width since liquid, here cooling water, would constantly flow in a large amount through the radial gaps  15 ,  16  from the positive pressure area  11  into the negative pressure area  10  and would thus be directly delivered in a circuit. 
         [0035]    In order to achieve the desired sealing action while avoiding the danger of a collision between impeller  7  and pump housing  2  (pump component) the pump housing  2  (pump component) extends over the impeller  7  on both axial sides, i.e., above and below in an inward radial direction in such a manner that a sealing gap,  19 ,  20  that is constructed as an axial gap and extends as regards its longitudinal extension in the radial direction is formed between each front side  17 ,  18  of the impeller  7  and the pump housing  2  (pump component). It is essential that these sealing gaps,  19 ,  20 , measured at their narrowest position, have a smaller gap width than the radial gaps  15 ,  16 . 
         [0036]    The sealing gaps,  19 ,  20  are located radially inside the radial gaps  15 ,  16 , whereby the radial gaps  15 ,  16  merge into the sealing gaps,  19 ,  20  and the sealing gaps,  19 ,  20  border directly on the radial gaps  15 ,  16 . In the exemplary embodiment shown the width of the sealing gaps  19 ,  20  is about 400 μm. 
         [0037]    The sealing gaps,  19 ,  20  are, as explained, limited on the one hand in the axial direction by the impeller  7 , in the exemplary embodiment shown by a front side  17 ,  18  of the impeller  7  and on the opposing side by a wall surface  21 ,  22  of the pump housing  2 , which wall surface is aligned here parallel to the particular front side  17 ,  18 . 
         [0038]    If a deflection of the impeller  7  occurs in a radial direction during operation the front sides  17 ,  18  are shifted substantially parallel to the wall surfaces  21 ,  22  of the pump housing  2 , so that no collision can occur there. The radial gaps  15 ,  16  are, as explained, dimensioned to be so wide that even here a collision with the impeller  7 , even at a maximally admissible deflection, is excluded. 
         [0039]      FIG. 2  schematically shows the gap conditions. 
         [0040]    The schematically shown impeller  2  can be recognized, arranged in a rotationally fixed manner on a rotatably supported shaft  5 . 
         [0041]    The impeller  7  is surrounded by a pump component  23 , here the pump housing  2 , more precisely an inserted part  24  that forms a component of the pump housing  2 . Alternatively, the inserted part  24  may not be constructed and arranged to form a component of the housing, therefore, inside the pump housing and at a distance to an outer housing side. Upon a rotation of the impeller  7  the liquid flows in the arrow directions from the suction side (negative pressure area)  10  to the pressure side (positive pressure area)  11 . 
         [0042]    Two sealing gaps  19 ,  20  are formed between the pump component  23 , that can be designed to be monopartite or bipartite, and the impeller  7 , more precisely between the front sides  17 ,  18  of the impeller  7  comprising a circular, cylindrical casing contour. These sealing gaps  19 ,  20  are axial gaps that are formed axially between the pump component  23  and the impeller  7 . The gap widths “s” of the sealing gaps  19 ,  20  are 400 μm in the exemplary embodiment shown. The two sealing gaps  19 ,  20 , in the form of flat annular disks are distanced from one another in the axial direction and, among other things, separated from one another by the radial outlet area or areas of the impeller  7 . 
         [0043]    In the exemplary embodiment shown, in addition to the sealing gaps  19 , between the impeller  7  and the pump component  23  two radial gaps  15 ,  16  are provided whose gap width a is greater than the gap width s of the sealing gaps. In the exemplary embodiment shown the gap widths “a” are approximately 5 mm when impeller  7  is at a standstill. The sealing gaps  19 ,  20  are located radially inside the radial gaps  15 ,  16  and therefore are closer to the shaft  5  (i.e., smaller) than the radial gaps  15 ,  16 . The radial gaps have the form of circular, cylindrical jacket. The sealing gaps  19 ,  20  have approximately the form of a circular ring disk. Providing the (narrow) radial gaps  15 ,  16  can also be eliminated in a modified constructive designed of the pump component  23 . It is also conceivable to provide several sealing gaps  19 ,  20  present in parallel planes that are axial gaps on at least one of the two axial sides, preferably on both axial sides of the impeller  7 . In such cases, two axially adjacent sealing gaps are preferably connected to one another via a radial gap with a larger gap width than the gap width of the sealing gaps on at least one axial side of the impeller  7 . Thus, a stepped gap formation would result, whereby the axial gap section would represent the sealing gaps. Therefore, a stepped gap design results. 
         [0044]    Conceivable alternative sealing gap geometries are shown in the  FIGS. 3 to 7 , whereby the angles or radii of curvature are shown in an exaggerated manner for reasons of clarity. In reality, minimal rises and large radii of curvature are involved. 
         [0045]    All exemplary embodiments have the fact in common that the sealing gaps are axial gaps that run (as regards their longitudinal extension) substantially in the radial direction and their axial extension is (substantially) less than their radial extension. 
         [0046]    In the exemplary embodiment according to  FIG. 3  a sealing gap  19  is formed between the impeller  7  and a pump component  23 . The section of the impeller  7  limiting the sealing gap  19  runs relative to the longitudinal extension of the shaft exactly in the radial direction, whereas on the contrary the surface section of the pump component  23 , that limits the sealing gap  19 , is slightly inclined relative to a radial plane, here under an angle α of &lt;1°. This results in a sealing gap inclination about this angle α relative to an imaginary radial plane in which in the exemplary embodiment shown the represented surface section of the impeller  7  lies. 
         [0047]    In the exemplary embodiment according to  FIG. 4  the surface section of the impeller  7  limiting the sealing gap  19  as well as the surface section of the pump component  23  that is opposite and limits the sealing gap  19  are inclined relative to a radial plane, in the exemplary embodiment shown both under the same angle α of &lt;10° here. The use of different but similar angles of inclination is also possible. 
         [0048]    In the exemplary embodiment according to  FIG. 5 , the surface area of the impeller  7  limiting the sealing gap  19  is located in a radial plane relative to the longitudinal extension of the shaft, in contrast to which the surface area of the pump component  23  limiting the sealing gap  19  is curved, which curvature preferably has a radius that has the sealing gap  19  from the support of the shaft  5  (not shown). 
         [0049]    In the exemplary embodiment according to  FIG. 6  both surfaces limiting the sealing gap  19  as well as the surface of the impeller  7  and the surface of the pump component  23  are designed to be slightly curved. 
         [0050]    In the exemplary embodiment according to  FIG. 7  the surface of the impeller  7  limiting the sealing gap  19  is designed level but inclined at an angle α of &lt;10° to the radial plane, in contrast to which the surface of the pump component  23  limiting the sealing gap  19  is slightly curved and preferably has a radius of curvature of 500 mm.