Patent Publication Number: US-11649825-B2

Title: Multistage centrifugal pump for conveying a fluid

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to European Patent Application No. 20175833.1, filed May 20, 2020, the contents of which are hereby incorporated herein by reference in their entirety. 
     BACKGROUND 
     Field of the Invention 
     The invention relates to a multistage centrifugal pump for conveying a fluid. 
     Background Information 
     Conventional centrifugal pumps for conveying a fluid, for example a liquid such as water, are used in many different industries. Examples are the oil and gas industry, the power generation industry, the chemical industry, the water industry or the pulp and paper industry. Multistage centrifugal pumps have at least two impellers and a shaft for rotating the impellers. The impellers can be configured for example as radial impellers or as axial or semi-axial impellers or as helicoaxial impellers. Furthermore, the impellers can be configured as open impellers or as a closed impellers, where a shroud is disposed on the impeller, the shroud at least partially covering the vanes of the impeller, or as a semi-open impeller. Furthermore, it is known to configure an impeller as a single suction impeller or as a double suction impeller. In the case of a single suction impeller the fluid approaches the impeller only from one side, i.e. the impeller has only one suction side. In the case of a double suction impeller the fluid approaches the impeller from both sides, i.e. the impeller has two suction sides. It is also known to design centrifugal pumps with a combination of single suction impeller(s) and double suction impellers. 
     SUMMARY 
     In a multistage centrifugal pump a plurality of impellers is mounted to the shaft in a torque proof manner, with the impellers being arranged in series on the shaft. 
     Many multistage centrifugal pumps includes at least one balancing arrangement for at least partially balancing the axial thrust that is generated by the impellers during operation of the pump. The balancing arrangement reduces the total axial thrust generated by the rotating impellers (hydraulic forces) and acting on the axial bearing or the thrust bearing. The residual thrust needs to be absorbed by the bearing, whose size, weight and cost is proportional to the magnitude of said residual thrust, hence there is a need of reducing the residual thrust as much as possible. Besides this, high forces are associated with higher mechanical losses that ultimately reduce the efficiency of the pump. 
     It has been found that the axial thrust can be balanced to a good extent by a proper disposition of the impellers, for example by an back-to-back arrangement, whereby the thrust generated by one impeller is at least partially compensated by an equivalent but opposite force acting on another impeller fitted symmetrically on the shaft. 
     More generally, a multistage pump can comprise a first set of impellers and a second set of impellers, wherein the first set of impellers and the second set of impellers are arranged in a back-to-back arrangement, so that the axial thrust generated by the first set of impellers is directed opposite to the axial thrust generated by the second set of impellers. Of course, it is also possible that the first set of impellers and/or the second set of impellers comprises only one impeller. 
     However, in some embodiments the back-to-back arrangement is not completely effective because, as an example, the total number of stages is odd and all impellers are configured as single suction impellers or because the total number of stages is even but one of the impellers is configured as a double suction impeller and all other impellers are configured as single suction impellers double suction first stage impeller is used. In addition, there are other reasons, why the back-to-back arrangement does not fully balance the axial thrust or why the back-to-back arrangement might not be appropriate. These reasons include for example manufacturing complications associated with an back-to-back arrangement. 
     It has been found that impellers can be arranged such that the axial thrust is balanced (at least approximately) independent from the number of stages in a fully symmetric layout. The basic idea behind this design is to ‘split’ the total inflow into two distinct and equal streams, each of which feeds a different set of impellers, e.g. the first stream is guided to the first set of impellers and the second stream is guided to the second set of impellers. After each stream has passed the respective set of impellers, the two streams merge in the last stage of the pump, wherein the last stage comprises a double suction impeller. 
       FIG.  1    is a schematic representation illustrating a conventional multistage centrifugal pump  1 ′ having a symmetric layout. The multistage centrifugal pump  1 ′ has a pump housing  2 ′ with a first inlet  31 ′ and a second inlet  32 ′ as well as an outlet  4 ′ for the fluid to be conveyed. The pump  1 ′ has a shaft  7 ′ defining an axial direction A′. On the shaft  7 ′ there are mounted two first stage impellers  5 ′ and a last stage impeller  6 ′, which is arranged between the two first stage impellers  5 ′. Each impeller  5 ′, 6 ′ comprises a suction side S′ for receiving the fluid and a discharge side D′ for discharging the fluid. The two first stage impellers  5 ′ are configured as single suction impellers, i.e. each first stage impeller  5 ′ has only one suction side. The last stage impeller  6 ′ is configured as a double suction impeller  6 ′ having a first suction side S 1 ′ and a second suction side S 2 ′. Upstream of the pump housing  2 ′ the inflow I′ is split into two distinct and equal streams, namely a first stream I 1 ′ and a second stream I 2 ′. 
     The first stream I 1 ′ is guided to the first inlet  31 ′ and the second stream  12 ′ is guided to the second inlet  32 ′. The first inlet  31 ′ is in fluid communication with the suction side S′ of one of the first stage impellers  5 ′ by a first suction line  81 ′ and the second inlet  32 ′ is in fluid communication with the suction side S′ of the other first stage impellers  5 ′ by a second suction line  82 ′. After the first stream I 1 ′ has passed the first stage impeller  5 ′ it is guided to the first suction side S 1 ′ of the last stage impeller  6 ′. After the second stream  12 ′ has passed the first stage impeller  5 ′ it is guided to the second suction side S 2 ′ of the last stage impeller  6 ′. At the last stage impeller  6 ′ the first stream I 1 ′ and the second stream  12 ′ are reunited with each other and after having passed the last stage impeller  6 ′ discharged through the outlet  4 ′ as indicated by the arrow O′. 
     This fully symmetric design results—at least approximately—to a complete balancing of the axial thrust so that the residual thrust equals zero. 
     Although this symmetric design has proved its worth in practice, it has been found that the design has some drawbacks. The design requires two inlets  31 ′, 32 ′ at the pump housing  2 ′, which is more complex and requires more efforts during manufacturing. Furthermore, the design requires a Y-junction, i.e. an additional part for connecting the pump  1 ′ to a pipework. This makes the pump more expensive, heavier and might increase the susceptibility to trouble. In addition, the higher the number of stages of the pump  1 ′ is, the longer the distance between the first inlet  31 ′ and the second inlet  32 ′ becomes. Thus, also the required Y-junction increases, which can cause trouble, for example mechanical problems such as vibrations, increased weight and the like. 
     Starting from this state of the art, it is therefore an object of the invention to propose a multistage centrifugal pump for conveying a fluid, providing at least a partial balancing of the axial thrust and avoiding the drawbacks of prior art embodiments. 
     The subject matter of the invention satisfying this object is characterized by the features of the embodiments described herein. 
     Thus, according to an embodiment of the invention, a multistage centrifugal pump for conveying a fluid is proposed, comprising a pump housing with an inlet and an outlet for the fluid, two first stage impellers and a last stage impeller for conveying the fluid from the inlet to the outlet, and a shaft for rotating each impeller about an axial direction, wherein each impeller comprises a suction side for receiving the fluid and a discharge side for discharging the fluid, wherein the last stage impeller is configured as a double suction impeller, having a first suction side and a second suction side, wherein the last stage impeller is arranged between the two first stage impellers with respect to the axial direction, and wherein each first stage impeller is arranged with the suction side of the first stage impeller facing one of the first and the second suction side of the last stage impeller. 
     The configuration according to an embodiment of the invention enables a fully symmetric design of the hydraulics of the multistage pump, thus providing a balancing of the axial thrust generated by the rotating impellers. However only one inlet is required and there is no need for a Y-junction outside of the pump housing. The fluid entering the pump through the inlet is divided only inside the pump housing, e.g. by the configuration of the inlet housing, into a first stream and a second stream, wherein the first stream is guided to the suction side of one of the first stage impellers and the second stream is guided to the suction side of the other first stage impeller. The two first stage impellers are arranged in a face-to-face arrangement, i.e. the impeller eye at the suction side of the one first stage impeller faces the impeller eye at the suction side of the other first stage impeller. 
     Since there is only one inlet at the pump housing, the pump can be integrated much more easily into pipework without requiring any specific piping such as a Y-junction. This also reduces the required material and the weight of the pump, so that the multistage pump is less expensive. In addition, the effort for installing the pump is considerably reduced. 
     Furthermore, the unit comprising the inlet, the two first stage impellers and the last stage impeller may be easily extended to a multiphase pump having more than two stages without larger modifications to said unit. The inlet of the pump together with the two first stage impellers and the last stage impeller may basically remain the same independent from the total number of stages of the multistage pump. 
     Preferably the suction side of each first stage impeller is in fluid communication with the inlet by means of a suction line, with each suction line arranged within the pump housing. Each suction line can be configured as a channel, for example as a channel delimited by the pump housing, wherein the channel guides the fluid entering through the inlet to the suction side of one of the first stage impellers. 
     According to a first preferred embodiment the multistage centrifugal pump has exactly three impellers, namely the two first stage impellers and the last stage impeller, wherein the two first stage impellers are configured as single suction impellers. 
     Furthermore, it is preferred that the multistage centrifugal pump comprises two crossover lines, each of which connects the discharge side of one of the first stage impellers with one of the first and the second suction side of the last stage impeller. 
     According to a second preferred embodiment the centrifugal pump in accordance with the invention comprises a first set of impellers and a second set of impellers, wherein each set of impellers comprises one of the first stage impellers, and wherein the first set of impellers and/or the second set of impellers comprises at least one intermediate stage impeller. With such a configuration the multistage pump can be designed with three or more stages. 
     Preferably each of the first set and the second set of impellers comprises an intermediate stage impeller. 
     Furthermore, it is preferred that the first set of impellers and the second set of impellers are configured in a back-to-back arrangement, to at least partially balance the axial thrust generated by the first set of impellers and the second set of impellers, respectively. 
     In particular for embodiments, in which the residual axial thrust shall be minimized, the first set of impellers and the second set of impellers comprise the same number of impellers. 
     Also for those embodiment having more than two stages it is preferred that the pump comprises two crossover lines, each of which connects the discharge side of one of the intermediate stage impellers with one of the first and the second suction side of the last stage impeller. 
     In some embodiments at least one crossover line is configured as an external line arranged outside the pump housing. 
     It is also possible that each crossover line is configured as an external line arranged outside the pump housing. 
     Preferably the multistage centrifugal pump is configured as a between-bearing pump. 
     Further advantageous measures and embodiments of the invention will become apparent from the dependent claims. 
    
    
     
       BREIF DESCRIPTION OF THE DRAWINGS 
       The invention will be explained in more detail hereinafter with reference to the drawings. 
         FIG.  1    is a multistage centrifugal pump according to the prior art. 
         FIG.  2    is a first embodiment of a multistage centrifugal pump according to the invention, and 
         FIG.  3    is a second embodiment of a multistage centrifugal pump according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    shows a schematic representation of a multistage centrifugal pump  1 ′ according to the prior art and configured as a two stage centrifugal pump  1 ′. Since  FIG.  1    has already been explained hereinbefore in the description of the prior art no further explanations are necessary. In order to differentiate the prior art arrangement over the embodiments according to the invention, the components of the pump  1 ′ representing prior art are designated in  FIG.  1    with reference numerals having a prime (inverted comma) behind the respective reference numeral. 
       FIG.  2    shows a schematic representation of a first embodiment of a multistage centrifugal pump according to the invention, which is designated in its entity with reference numeral  1 . The pump  1  is designed as a centrifugal pump for conveying a fluid, for example a liquid such as water. 
     The multistage centrifugal pump  1  comprises a pump housing  2  having an inlet  3  and an outlet  4  for the fluid to be conveyed. The centrifugal pump  1  further comprises two first stage impellers  5  and a last stage impeller  6  interposed between the two first stage impellers  5 , as well as a shaft  7  for rotating each impeller  5 ,  6  about an axial direction A. The axial direction A is defined by the axis of the shaft  7 . Each impeller  5 ,  6  is mounted to the shaft  7  in a torque proof manner. The shaft  7  has a drive end  71 , which can be connected to a drive unit (not shown) for driving the rotation of the shaft  7  about the axial direction A. The drive unit can comprise, for example, an electric motor. The other end of the shaft  7  is referred to as non-drive end  72 . 
     Each of the first stage impellers  5  is in fluid communication with the inlet  3 . A first suction line  81  constitutes the fluid communication between the inlet  3  and the first stage impeller  5  on the left side in  FIG.  2   , and a second suction line  82  constitutes the fluid communication between the inlet  3  and the first stage impeller on the right sight of  FIG.  2   . Each suction line  81 ,  82  can be configured as a channel which is delimited by the pump housing  2  or an inlet housing (not shown) that is part of the pump housing  2 . The inflow I of the fluid to be conveyed enters the pump housing  2  through the inlet  3  and is then divided in a first stream passing through the first suction line  81  and a second stream passing through the second suction line  82 . 
     The first stage impellers  5  are each configured as single suction impellers  5 , i.e. as impellers  5  having only one suction side S. The suction side S of the impeller  5  is the side where the eye of the impeller  5  is arranged, i.e. the side, from which the fluid approaches the impeller  5 . Each impeller has a discharge side D, i.e. the side where the fluid is discharged from the impeller  5 . Regarding the first stage impellers  5  a volute or a diffusor may be arranged at the discharge side D. 
     The last stage impeller  6  is configured as a double suction impeller  6 , i.e. the fluid approaches the impeller  6  from both sides regarding the axial direction A. Thus the last stage impeller  6  has a first suction side S 1  on the left side in  FIG.  2    and a second suction side S 2  at the right side in  FIG.  2   . The last stage impeller  6  has a discharge side D which is in fluid communication with the outlet  4  of the pump. The outflow of the fluid discharged through the outlet  4  is indicated by the arrow O in  FIG.  2   . 
     The first embodiment of the multistage centrifugal pump  1  is configured as a two stage centrifugal pump  1  with the two first stage impellers  5  and the last stage impeller  6  constituting the second stage impeller  6 . 
     The two first stage impellers  5  are arranged with the suction side S of the respective first stage impeller  5  facing one of the first S 1  and second suction side S 2  of the last stage impeller  6 . The first stage impeller  5  on the left side of  FIG.  2    is arranged such that its suction side S faces the first suction side S 1  of the last stage impeller  6 . The first stage impeller  5  on the right side of  FIG.  2    is arranged such that its suction side S faces the second suction side S 2  of the last stage impeller  6 . Thus, the two suction sides S of the first stage impellers  5  are facing each other. Therefore, the axial thrust generated by one of the first stage impellers  5  compensates the axial thrust generated by the other one of the first stage impellers  5 . The residual thrust generated by the two first stage impellers  5  is at least approximately zero. 
     Since the last stage impeller  6  is configured as a double suction impeller  6  with the first suction side S 1  and the second suction side S 2 , the residual axial thrust generated by the last stage impeller  6  is at least approximately zero. 
     For guiding the first stream from the discharge side D of the first stage impeller  5  to the first suction side S 1  a first crossover line  91  is disposed between the discharge side D of the first stage impeller  5  and the first suction side S 1  of the last stage impeller  6 . For guiding the second stream from the discharge side D of the first stage impeller  5  to the second suction side S 2  a second crossover line  92  is disposed between the discharge side D of the first stage impeller  5  and the second suction side S 1  of the last stage impeller  6 . Each crossover line  91 ,  92  may include a diffuser. 
     It has to be noted that each of the crossover lines  91 ,  92  can be configured as an internal line completely arranged inside the pump housing  2  or as an external line arranged at least partially outside the pump housing  2  as it is indicated by the dash dotted lines  90  in  FIG.  2   . 
     The centrifugal pump  1  comprises bearings (not shown) on both ends of the shaft  7 , namely at or near the non-drive end  72  of the shaft, and near the drive end  71  of the shaft  7 , i.e. the centrifugal pump  1  is designed as a between-bearing pump. 
     During operation of the multistage pump  1  the inflow I of the fluid enters the pump housing  2  through the inlet  3  and is then divided into the first stream passing through the first suction line  81  to the suction side S of the left first stage impeller  5  (according to the representation in  FIG.  2   ) and the second stream passing through the second suction line  82  to the suction side S of the right first stage impeller  5  (according to the representation in  FIG.  2   ). 
     The first stream is discharged at the discharge side of the first stage impeller  5  in the first crossover line  91  and guided to the first suction side S 1  of the last stage impeller  6 . The second stream is discharged at the discharge side D of the first stage impeller  5  in the second crossover line  92  and guided to the second suction side S 2  of the last stage impeller  6 . At the last stage impeller  6  the first stream and the second stream are reunited with each other and leave the pump  1  through the outlet  4  as outflow O. 
       FIG.  3    shows a schematic representation of a second embodiment of a multistage centrifugal pump  1  according to an embodiment of the invention. In the following description of the second embodiment of the multistage centrifugal pump  1  only the differences from the first embodiment are explained in more detail. The explanations with respect to the first embodiment are also valid in the same way or in analogously the same way for the second embodiment. Same reference numerals designate the same features that have been explained with reference to the first embodiment or functionally equivalent features. 
     The second embodiment is configured as a multistage pump  1  having more than two stages. Here the multistage pump  1  is configured as a three stage centrifugal pump. It has to be noted that the number of three stages is only exemplary. In other embodiments, the multistage pump  1  may comprise more than three stages. 
     Furthermore, it has to be noted that the module comprising the inlet  3 , the two first stage impellers  5  and the last stage impeller  6  is essentially the same as in the first embodiment. It is an important advantage that said module may be configured essentially in the same manner independent from the total number of stages of the centrifugal pump  1 . 
     According to the second embodiment the multistage centrifugal pump  1  comprises a first set of impellers  51  and a second set of impellers  52 , wherein each set of impellers  51 ,  52  comprises one of the first stage impellers  5 . In addition, each set of impellers  51 ,  52  comprises at least one intermediate stage impeller  55 . 
     In other embodiments only the first set of impellers  51  or only the second set of impellers  52  comprises at least one intermediate stage impeller  55 . 
     In the second embodiment each of the first set and the second set of impellers  51 ,  52  comprises exactly one intermediate stage impeller  55 . 
     In other embodiments each of the first set and the second set of impellers  51 ,  52  comprises more than one intermediate impeller  55 . 
     Preferably and as it is shown in  FIG.  3    the first set of impellers  51  and the second set of impellers  52  are configured in a back-to-back arrangement. 
     In the second embodiment the first set of impellers  51  and the second set of impellers  52  comprise the same number of impellers  5 ,  55 . 
     In other embodiments, the first set of impellers  51  and the second set of impellers  52  comprise different numbers of impellers  5 ,  55 . 
     Each intermediate stage impeller  55  is arranged downstream of one of the first stage impellers  5  and upstream of the last stage impeller  6 . 
     In the second embodiment the first crossover line  91  connects the discharge side D of the left first stage impeller  5  (according to the representation in  FIG.  3   ) with the intermediate stage impeller  55  of the first set  51  of impellers. The second crossover line  92  connects the discharge side D of the right first stage impeller  5  (according to the representation in  FIG.  3   ) with the intermediate stage impeller  55  of the second set of impellers  52 . 
     A third crossover line  93  is configured to connect the discharge side D of the intermediate stage impeller  55  of the first set of impellers  51  with the first suction side S 1  of the last stage impeller  6 . In addition, a fourth crossover line  94  is configured to connect the discharge side D of the intermediate stage impeller  55  of the second set of impellers  52  with the second suction side S 2  of the last stage impeller  6 . 
     During operation of the multistage pump  1  the inflow I of the fluid enters the pump housing  2  through the inlet  3  and is then divided into the first stream passing through the first suction line  81  to the suction side S of the left first stage impeller  5  (according to the representation in  FIG.  3   ) and the second stream passing through the second suction line  82  to the suction side S of the right first stage impeller  5  (according to the representation in  FIG.  3   ). The first stream is discharged at the discharge side D of the first stage impeller  5  in the first crossover line  91  and guided to the suction side S of the intermediate stage impeller  55  of the first set of impellers  51 . After having passed this intermediate stage impeller  55 , the first stream is guided by the third crossover line  93  to the first suction side S 1  of the last stage impeller  6 . The second stream is discharged at the discharge side D of the first stage impeller  5  in the second crossover line  92  and guided to the suction side S of the intermediate stage impeller  55  of the second set of impellers  52 . After having passed this intermediate stage impeller  55 , the second stream is guided by the fourth crossover line  94  to the second suction side S 2  of the last stage impeller  6 . At the last stage impeller  6  the first stream and the second stream are reunited with each other and leave the pump  1  through the outlet  4  as outflow O. 
     The skilled person will easily understand how this configuration can be expanded to more than three stages.