Abstract:
A multi-stage, self-priming centrifugal pump assembly includes at least two pump stages ( 4 ) which are consecutive in a main flow direction ( 32 ), and a backflow channel ( 13 ) which lies parallel to at least one a pump stage ( 4 ). The backflow channel ( 13 ) runs out downstream of the first or a further pump stage ( 4 ), in the main flow direction ( 32 ).

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a United States National Phase Application of International Application PCT/EP2014/058643 filed Apr. 29, 2014 and claims the benefit of priority under 35 U.S.C. §119 of European Patent Application 13 168 801.2 filed May 22, 2013 the entire contents of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a multi-stage, self-priming centrifugal pump assembly with at least two pump stages which are consecutive in a main flow direction and with a backflow channel which lies parallel to at least one a pump stage and which runs out downstream of the first or a further pump stage in the main flow direction. 
       BACKGROUND OF THE INVENTION 
       [0003]    A multi-stage centrifugal pump assembly according to the preamble is known from EP 2 505 842 A1, which is designed such that a self-priming behavior is achieved with only a small quantity of delivery fluid. The centrifugal pump assembly described there has proven its worth, but requires a certain start-up time for the self-priming process. 
         [0004]    From DE 44 15 157 A1, it is counted as belonging to the state of the art, with a multi-stage centrifugal pump, to lead back the delivery fluid during the suction phase by way of opening a non-return valve. Moreover, an air separator is provided, so that the air located in the assembly, during the suction phase, can be replaced by fluid which is stored in a chamber of the pump, by which means a self-priming of the pump can be ensured. 
       SUMMARY OF THE INVENTION 
       [0005]    It is an object of the present invention, to further improve a centrifugal pump assembly of the type according to the preamble, with respect to its self-priming behavior. 
         [0006]    The multi-stage, self-priming centrifugal pump assembly according to the invention comprises at least two pump stages which are consecutive in the flow direction, as well as a backflow channel which lies parallel to at least one pump stage. According to the invention, the backflow channel is designed and arranged such that the backflow channel runs out (has a flow outlet) downstream of the first or a further pump stage, seen in the main flow direction of the pump, and specifically downstream of the diffuser (guide vane mechanism) of the pump stage, seen in the main flow direction. 
         [0007]    The basic concept of the solution according to the invention is not to arrange the backflow channel parallel to the first pump stage, as is the case with the state of the art, but to arrange this in parallel to the second or one or more further pump stages. Thereby, with regard to the centrifugal pump assembly according to the invention the pump stages are arranged vertically above one another. Surprisingly, it has seen found that if, according to the invention, the backflow channel is arranged downstream of the first or a further pump stage seen in the main flow direction of the pump, and specifically downstream of the diffuser of the pump stage, the procedure of the self-priming is effected in a significantly more intensive manner and in particular in a shorter temporal course, which is advantageous since the time of the self-priming of the pump is shortened and thus the pump is available for its envisaged application, for example for delivering extinguishing fluid, at a much earlier stage. Although a certain basic quantity of fluid is required with the centrifugal pump assembly according to the invention, in order to initiate the self-priming procedure, the subsequent suctioning i.e. the generation of a vacuum is effected significantly more quickly than with pumps according to the state of the art. Unnecessary eddying in the led-back fluid is prevented in the region of the diffuser due to the fact that the leading-back of the delivered fluid is not effected between the impeller and the diffuser, as is known from the state of the art, but downstream of the diffuser in the flow direction. The part of the fluid which is delivered through the pump stage/pump stages before the run-out of the return channel, is led through the diffuser without disturbing influences of the led-back fluid, which is to say that the kinetic energy at the exit of the impeller can be converted into pressure energy by the subsequent diffuser and only then is a mixing with the led-back fluid effected. A significant improvement of the self-priming procedure can be achieved on starting the pump by way of this. 
         [0008]    It is advantageous if the backflow channel runs out downstream of the first pump stage seen in the main flow direction, thus downstream of the diffuser of the first pump stage, at the exit of this pump stage which is at the flow side. The backflow channel according to the invention can thereby bridge one or more pump stages and preferably this should bridge at least two pump stages. A particularly quick and good suction behavior results if the suction channel bridges four pump stages, thus for example is led parallel to the second to fifth pump stage. It is advantageous if the backflow channel seen in the main flow direction runs out downstream of the first pump stage, thus downstream of the diffuser of the first pump stage, at the exit of this pump stage which is at the flow side. 
         [0009]    It is advantageous to provide a gas separator within the centrifugal pump assembly, said gas separator according to a further development of the invention preferably being arranged at the exit side of the at least second pump stage, in order to design the suction procedure as effectively as possible. With regard to the main flow direction, it is useful to arrange the gas separator downstream of the pump stages provided for the suction procedure, thus subsequently to the pump stages to which the backflow channel lies in parallel. 
         [0010]    According to one advantageous design of the invention, the gas separator is formed by a housing-fixed, tubular body which connects onto a diffuser of a pump stage and which in its wall comprises at least one recess connected to the backflow channel in a fluid-leading manner. Such an arrangement is inexpensively manufacturable and highly effective, since the conveying fluid gas mixture exiting the diffuser with swirling rises helically on the tubular body and due to the centrifugal force gets through the at least one recess in the wall and thus into the backflow channel, whereas the gas is led upwards and is thus removed from the suction circulation. 
         [0011]    According to an advantageous further development of the invention, a buffer chamber is arranged between two pump stages which are subsequent to the first pump stage in the main flow direction. Such a buffer chamber is preferably arranged downstream of the gas separator in the main flow direction. The buffer chamber serves for storing a certain quantity of water within the pump and in particular when suctioning larger air bubbles, as can occur for example on suctioning an emptying tank towards the end, ensures that these air bubbles do not lead to water necessary for the suctioning procedure being delivered out of the pump. The buffer chamber is therefore to be designed such that on the one hand it is automatically filled given flow through the pump, but one the other hand that it releases the delivery fluid stored there, at least in a delayed manner, i.e. leads it via the backflow channel back again into the pump stages provided for the suctioning procedure. 
         [0012]    According to the invention, such a buffer chamber can advantageously be formed by a housing-fixed, tubular body which surrounds the common drive of the centrifugal pump assembly at a distance and which is arranged at a distance to the outer housing wall. This tubular body is connected via an annular base which on the one hand is connected to the tubular body and on the other hand to the wall of the pump and comprises at least one recess connected to the backflow channel in a fluid-leading manner. It is therefore the case of an annular storage reservoir between the tubular body and the pump wall, in which recesses are provided on the base side and these recesses are designed such that the backflow through these recesses with regard to time runs such that an entrained large gas bubble does not lead to the self-priming behavior being compromised. 
         [0013]    According to a further development of the invention, one envisages the backflow channel being able to be shut off via a valve controlled in a pressure-dependent manner, on the one hand to ensure a good suctioning behavior and on the other hand however to ensure that no efficiency reduction of the pump is effected by the backflow channel in normal operation. 
         [0014]    Preferably, such a valve is provided at the entry side of the backflow channel, since a comparatively high pressure of the fluid delivery already prevails there at the exit of a second pump stage or one lying thereabove, and this high pressure can be used for the control of the valve, in particular for its shut-off. The valve is advantageously controlled by differential pressure, and specifically in dependence on the differential pressure at the backflow channel, so that the backflow channel is shut off on exceeding a predefined differential pressure. In this manner, it is ensured that the backflow channel is only effective for the actual suctioning procedure and has no efficiency-worsening influence in normal operation of the pump. 
         [0015]    Preferably, the backflow channel is designed as an annular channel which surrounds at least, one preferably however two to four pump stages. 
         [0016]    According to a further development of the invention, means for preventing the pump from running empty are provided. These are to be selected depending on the application of the pump. Thus, according to the invention, if the assembly is envisaged and designed exclusively for operation with pump stages arranged vertically above one another and comprises a suction connection at the foot of the pump, a pipe section can be arranged upstream of this suction connection, and this pipe section extends laterally of the assembly, preferably up to the height of the last pump stage. By way of this pipe section, it is ensured that the centrifugal pump assembly cannot run empty due to the backflow of delivery fluid. Thus, the self-priming behavior also largely ensured in this manner. Thereby, basically the pipe section arranged upstream is to be led up so high, that at least one of the pump stages lying in the region of the backflow channel and are thus are required for the self-priming behavior, is reached. 
         [0017]    According to an advantageous further development of invention, the pipe section arranged upstream is designed in a U-shaped manner and at its region connecting the limbs of the U, thus at its upper end, is provided with a ventilation opening which can be selectively opened and closed by way of a ventilation valve. The ventilation opening in particular with a suction conduit leading further downwards, ensures that the pipe section arranged upstream and thus also the pump connecting thereto are prevented from being suctioned dry due to the vacuum in the suction conduit. Then the part of the suctioning conduit leading the vacuum can be filled with air by way of opening the valve, thus by way of releasing the ventilation opening, so that the other limb of the pipe section and thus also the pump itself remain filled with fluid, with a later starting operation of the pump, and the pump starts up again in a self-priming manner. Advantageously thereby, the ventilation opening is conductively connected to the pressure space of the last pump stage amid the intermediate connection of the ventilation valve, so that given an opened ventilation valve, it is always ensured that the pipe section close to the pump, as well as the pump itself remain filled with fluid, irrespective of the pressure conditions in the other pipe section, thus at the suctioning conduit. 
         [0018]    Advantageously, an electrically controllable solenoid valve is applied as a ventilation valve. Such valves are inexpensive, reliable and simple to activate. 
         [0019]    Alternatively, a non-return valve can also be arranged on the suction side, i.e. upstream of the first pump stage, for preventing the pump from running empty. Such a non-return valve can be part of the pump assembly or also be arranged in a pipe section arranged upstream on the suction side. 
         [0020]    Advantageously, a delivery connection is arranged in the foot of the pump and is conductively connected via an annular space to the last pump stage. A pump of the inline construction manner is formed by way of this. 
         [0021]    According to the invention, advantageously an electric motor which drives a central shaft carrying the impellers is provided for the drive of the centrifugal pump assembly. The motor is advantageously arranged on the upper side of the assembly. 
         [0022]    The invention is hereinafter explained by way of embodiment examples represented in the drawing. 
         [0023]    The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    In the drawings: 
           [0025]      FIG. 1  is a greatly simplified schematic longitudinal sectional view through a centrifugal pump assembly according to the invention; 
           [0026]      FIG. 2  is an enlarged view of a region of the first four pump stages of  FIG. 1 ; 
           [0027]      FIG. 3  is an enlarged view of a region between the fourth and the last pump stage in  FIG. 1 ; 
           [0028]      FIG. 4  is an enlarged longitudinal sectional view of a pressure-side housing region behind the fourth pump stage; 
           [0029]      FIG. 5  is a cross sectional view of the housing region according to  FIG. 4 ; 
           [0030]      FIG. 6  is a greatly simplified schematic longitudinal sectional view of the centrifugal pump assembly according to  FIG. 1 , with an incorporated valve; and 
           [0031]      FIG. 7  is a greatly simplified schematic longitudinal sectional view showing one embodiment variant with a non-return valve connected upstream, according to  FIG. 1 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0032]    With regard to the centrifugal pump assembly represented by way of the  FIGS. 1 to 5 , it is the case of a multistage, self-priming centrifugal pump assembly of the inline construction type which is envisaged for vertical operation, thus standing upright. Thereby, only the pump-side part of the centrifugal pump assembly which is provided on a foot part  1  for standing placement on a horizontally aligned surface and which comprises a suction connection  2  as well as a delivery connection  3  aligned thereto, as is common with inline pumps, is represented in  FIG. 1 . A middle pump part  5  connects to this foot part  1  formed as a cast metal component and this middle pump part comprises the pump stages  4  and at its upper end is closed off by a head part  6  likewise formed from cast metal and simultaneously forming a motor base  7  for the electric motor to be connected there. This (not shown) electric motor is connected via a (likewise not represented) coupling to a central shaft  8  which passes through the pump from the head part  6  to the foot part  1 , is rotatably mounted and carries impellers  9  of the pump stages  4 . 
         [0033]    The pump represented by way of  FIGS. 1 to 5  as a whole comprises five pump stages  4  which are connected hydraulically in series so that the delivery fluid is led from the suction connection  2  firstly to the lowermost, first impeller  9   a,  from there into the diffuser  10   a  assigned to this impeller  9   a  and leading the delivery fluid to the pump stage arranged downstream, specifically to the suction port of the impeller  9   b  of the second pump stage to which second pump stage a diffuser  10   b  leading the fluid to the suction port of an impeller  9   c  of the third pump stage is assigned. The fourth pump stage consisting of the impeller  9   d  and the diffuser  10   d  connects to the third pump stage which is closed off by the diffuser  10   c.  Finally, the pump close to its upper end comprises a fifth pump stage consisting of an impeller  9   e  and a diffuser  10   e.    
         [0034]    The pump stages  4  are arranged in a cylindrical inner casing  11  which is surrounded at a radial distance by a likewise cylindrical outer casing  12 . The delivery fluid is led via the annular space formed between the inner casing  11  and the outer casing  12 , from the exit of the diffuser  10   e  of the uppermost, fifth pump stage back downwards to the lower foot part  1  and there to the delivery connection  3 . 
         [0035]    The basic construction of the pump as well as the pump stages in each case consisting of an impeller  9  and a diffuser  10  corresponds to that which is common, is counted as belonging to the state of the art and is therefore not described in detail here. 
         [0036]    In order to design the pump in a self-priming manner, i.e. to ensure with regard to design that a self-priming effect happens at least when a small quantity of fluid is located within the pump, several design measures are envisaged with the represented centrifugal pump. 
         [0037]    Thus a backflow channel  13  is provided, which is formed by a cylindrical intermediate wall arranged at a small distance to the inner casing  11  between the exit of the first pump stage and the exit of the fourth pump stage and is otherwise connected at the ends to the inner casing  11  in a fixed and sealed manner. The backflow channel  13  arises due to radial recesses  14  above the fourth pump stage, thus above the diffuser  10   d  of the fourth pump stage in the inner casing  11 . The backflow channel  13  runs as an annular channel downwards from the recesses  14 , where it runs out through recesses  15  between the diffuser  10   a  at the exit side of the first stage and the impeller  9   b  at the entry side of the second pump stage. This backflow channel  13  thus short circuits the fourth pump stage with the exit of the first pump stage, so that the delivery fluid during a suctioning phase of the pump after switching on firstly circulates between the second and the fourth pump stage, as is indicated by the interrupted lines  16  in  FIG. 2 , said lines representing the suctioning fluid circulation. The self-priming is effected in a comparatively rapid manner due to the fact that the backflow channel  13  is not led back to the entry of the first pump stage as is the case with the state of the art, but to the entry of the second pump stage. 
         [0038]    A gas separator  17  in the form of a cylindrical pipe section is formed at the exit side of the fourth pump stage within the inner casing  11 , in a manner connecting to the diffuser  10   d  of this stage, and this pipe section is arranged in a manner fixed to the housing and coaxially to the shaft  8  and in the region of the upper third of its length is provided with circular recesses  18 . The pipe forming the gas separator  17  with regard to height corresponds roughly to two pump stages. The gas separator  17  has the effect that on interruption of the flow of the fluid due to a relatively large gas bubble, this can rise centrally, whereas the fluid which exits from the diffuser  10   d,  due to the swirling which is still present and the centrifugal force resulting from this, exits through the openings  18  to the outside and then flows back at the outer periphery within the inner casing  11  or rises further upwards, without the delivery flow breaking away due to this. 
         [0039]    A buffer chamber  19  which is delimited inwards coaxially to the shaft  8  by a cylindrical pipe section  20 , is delimited to the outside by the inner casing  11  and is delimited to the bottom by an annular base  21 , connects onto the gas separator  17  to the top at a distance. The base  21  is provided with recesses  22  which are dimensioned such that the buffer chamber  19  due to the recesses  22  in the base  21  empties only very slowly but not spontaneously, thus that delivery fluid firstly remains in this region of the pump even in the case of a passage of larger gas quantities. The suction port of the impeller  9   e  of the fifth pump stage connects to the top onto the cylindrical pipe section  20  at a distance. The delivery fluid which gets through the pipe section  20  thus at least partly flows into the buffer chamber  19  arranged laterally next to it and from there, as long as these spaces are not filled with delivery fluid as in normal pump operation, back to the fourth pump stage and from there via the backflow channel  13  to the entry of the second pump stage. In this manner, even with the occurrence of larger gas bubbles, it is always ensured that sufficient delivery fluid remains within the pump, in order to ensure the continuous delivery operation. 
         [0040]    A pressure-controlled valve  23  is provided which, when the pressure at the exit of the fourth pump stage rises above a certain value, specifically when the actual suctioning procedure is completed, closes the recesses  14  in the inner casing  11 , in order to prevent losses arising after the suctioning phase in the actual delivery operation due to delivery fluid flowing back though the backflow channel  13 . For this, the valve  23  comprises a sheet-metal strip  24  which is arranged within the cylindrical outer inner contour in a limitedly movable manner, at its two ends is designed in a fork-like manner and is connected to the inner casing  11  in a limitedly movable manner within this by way of screws  25 . The sheet-metal strip  24  in the region of the screws  26  is held in a manner distanced to the inner casing  11  via a screw  26  in the inner casing  11 , said screw being arranged centrally between the screws  25  and between the two recesses  14 . The sheet-metal strip  24  which is formed from spring steel is elastically deformed with an increasing inner pressure and is pressed radially outwards in a manner closing the recesses  14 . As soon as the inner pressure drops below a certain value, the sheet-metal strip  24  again assumes its original shape represented in  FIG. 5  and thus opens the recesses  14 . 
         [0041]    A U-shaped pipe section  27  is arranged upstream of the suction connection  2  in order to prevent the centrifugal pump from running empty after switching off for example, and this pipe section with regard to height extends up to the fifth pump stage, so that the pump itself and the limb of the U-shaped pipe section  27  which is on the left in  FIG. 1  always remains filled with delivery fluid. 
         [0042]    Thereby, in a further development, the U-shaped pipe section  27  at its uppermost location, thus in the web region of the U can comprise a bleed connection  28  which is closed by way of a solenoid valve  29 . This bleed connection  28  is connected to the pressure space of the last pump stage via a flexible tube  30 . The solenoid valve  29  is closed in the non-actuated condition and is opened by way of a suitable (not shown) control, given a pressure drop in the pressure space of the last pump stage, in order to ensure that sufficient delivery fluid always remains within the pump and the self-priming capability is retained. 
         [0043]    With the embodiment according to  FIG. 7 , a non-return valve  31  is provided on the suction side instead of the U-shaped pipe section  27  and the bleed opening  28 , and this valve endures that delivery fluid can only flow into the pump but not out of this at the suction side, and the self-priming capability is also ensured by way of this. 
         [0044]    While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.