Patent Publication Number: US-2023138342-A1

Title: A rotary positive displacement pump

Description:
TECHNICAL FIELD 
     The present disclosure relates to rotary positive displacement pump. The disclosure further relates to a method for assembling a rotary positive displacement pump and to a method for providing maintenance to a positive displacement pump. 
     BACKGROUND 
     In the field of rotary positive displacement pumps, there is a continuous demand for further improved reliability and reduced maintenance effort. 
     For example, rotary positive displacement pumps with front loading seals are known and provides simplified maintenance. However, front loading seals may under certain conditions result in reduced long term reliability and pumping efficiency and/or increased manufacturing cost. 
     There is thus a need for a further improved rotary positive displacement pump in terms of improved reliability, serviceability and pumping efficiency and/or reduced manufacturing cost. 
     SUMMARY 
     An object of the present disclosure is to provide a rotary positive displacement pump, a method for assembling a rotary positive displacement pump and a method for providing maintenance to a positive displacement pump, where the previously mentioned problems are avoided. This object is at least partly achieved by the features of the independent claims. 
     In particular, according to a first aspect of the present disclosure, there is provided a rotary positive displacement pump for pumping a fluid product. The pump has a front side and a rear side and comprises a main body providing rotational support to a pair of parallel, axially extending, shafts with gears in constant mesh condition, such that the pair of shafts are arranged to rotate in opposite directions. The pump further comprises a rotor case body connected to a front side of the main body, wherein the rotor case body has a stationary interior pumping cavity defined by an axial rear wall,  a circumferential side wall, and a removable front cover, a fluid product inlet opening, a fluid product outlet opening, and a pair of cylindrical rotor case hubs extending from the rear wall, wherein each cylindrical rotor case hub receives internally one of the pair of shafts. The further comprises a pair of rotors, each having at least one rotor wing and a rotor drive element that is mounted torque proof on a rotor seat at an end region of one of the pair of shafts, wherein each of the pair of rotor seats has an axial abutment surface facing in an axial direction towards a front side of the pump and a mounting surface facing radially outwards. The pump further comprises a pair of fasteners, preferably threaded fasteners, each being engaged with a mating section, preferably a threaded section, at the end region of one of the pair of shafts, and each exerting an axial clamping force on one of the rotor drive elements against the axial abutment surface of one of the rotor seats, wherein the axial abutment surface of each rotor seat is located axially outside, towards a front side, of the associated hub. 
     Moreover, according to a second aspect of the present disclosure, there is provided a method for assembling a rotary positive displacement pump for pumping a fluid product, the pump having a front side and a rear side. The method comprises providing a main body giving rotational support to a pair of parallel, axially extending, shafts with gears in constant mesh condition, such that the pair of shafts are arranged to rotate in opposite directions. The method further comprises providing a rotor case body having: a stationary interior pumping cavity defined by an axial rear wall, a circumferential side wall, and a removable front cover; a fluid product inlet opening; a fluid product outlet opening; and a pair of cylindrical rotor case hubs extending from the rear wall, wherein the rotor case body is located on a front side of the main body, and wherein each cylindrical rotor case hub receives internally one of the pair of shafts. The method additionally comprises providing a pair of rotors, each having at least one rotor wing and a rotor drive element. Moreover, the method comprises mounting each rotor drive element torque proof on a rotor seat at an end region of one of the pair of shafts, wherein each rotor seat has an axial abutment surface facing in an axial direction towards a front side of the pump and mounting surface facing radially outwards, and mounting a fastener, preferably a threaded fastener, on an end region of each of the pair of shafts. Finally, the method comprises tightening the fasteners for exerting an axial clamping force on each rotor drive element against the axial abutment surface of one of the rotor seats, wherein the axial abutment surface  of each rotor seat is located axially outside, towards a front side, of the associated hub, and mounting the removable front cover on the rotor case body. 
     In addition, according to a third aspect of the present disclosure, there is provided a method for assembling a rotary positive displacement pump having a front side and a rear side. The method comprises providing a pump having two parallel axially extending shafts, an interior pumping cavity and a pair of cylindrical rotor case hubs extending towards the front side from a rear wall of the interior pumping cavity, and providing a pair of rotors, each having at least one rotor wing connected to a central rotor drive element. The method further comprises mounting a first part of a first pair of seal assemblies, such as mechanical face-seal assemblies, in a front seal seat of each cylindrical rotor case hub, and mounting a second part of the first pair of seal assemblies, such as mechanical face-seal assemblies, in a rotor seal seat of each rotor drive element. The method additionally comprises mounting one of the pair of rotors on each shaft, wherein each shaft has a rotor seat with an axial abutment surface facing in an axial direction towards a front side of the pump. Finally, the method comprises abutting each rotor drive element against the axial abutment surface of an associated rotor seat, wherein the axial abutment surface of each rotor seat is located axially outside, towards a front side, of the associated hub, and thereafter mounting a removable front cover on the pump. 
     In addition, according to a fourth aspect of the present disclosure, there is provided a method for providing maintenance to a sealing arrangement of a rotary positive displacement pump. The pump has a front side and a rear side and two parallel axially extending shafts, wherein each shaft is carrying a rotor having at least one rotor wing and a rotor drive element. The pump further has an interior pumping cavity including a pair of cylindrical rotor case hubs extending towards the front side from a rear wall of the interior pumping cavity, wherein each shaft has a rotor seat with an axial abutment surface facing in an axial direction towards a front side of the pump. The method comprises: removing a removable front cover of the pump, removing at least one of the pair of rotors from the associated shaft for enabling access to a sealing arrangement configured for preventing leakage along a gap between the associated shaft and the associated cylindrical rotor case hub, servicing the sealing arrangement, mounting the at least one removed rotor on the associated shaft and abutting the rotor drive element against the axial abutment surface of an associated rotor seat, wherein  the axial abutment surface of each rotor seat is located axially outside, towards a front side, of the associated hub, and mounting the removable front cover on the pump. 
     The rotary positive displacement pump and associated method of assembly described above not only enables reduced maintenance effort by means of the front loading seals, due to the design wherein the axial abutment surface of each rotor seat is located axially outside, towards a front side, of the associated hub, the rotary positive displacement pump and associated method of assembly described above additionally enable increased dimension of the first and second shafts without negative effect on pumping volume or exterior pump dimensions. 
     In particular, increased dimension of the first and second shaft, i.e. increased diameter, has positive effects in many ways. For example, the increased dimension results in increased shaft stiffness. As a result, the shafts, rotors and/or rotor case body may be manufactured in less exotic materials without sacrificing operating reliability or risk for material fatigue. For example, conventional stainless steel, such as duplex stainless steel, may be used to a larger degree. In addition, thanks to the stiffer first and second shafts, the clearance between the rotor wings and the radial and axial walls of the stationary pumping cavity may be reduced, thereby resulting in reduced pump slippage and increased pumping efficiency. 
     Further advantages are achieved by implementing one or several of the features of the dependent claims. 
     In some example embodiments, a mounting portion of each rotor drive element is radially non-overlapping the associated cylindrical rotor case hub. Thereby, space for increased shaft diameter may be accomplished. 
     In some example embodiments, a mounting portion of each rotor drive element includes an axial abutment surface facing in an axial direction towards a rear side of the pump and a mounting surface facing radially inwards, and the axial abutment surface of each mounting portion is located axially outside, towards a front side, of the associated hub. Thereby, space for increased shaft diameter may be accomplished.  
     In some example embodiments, the mounting portion of each rotor drive element does not extend radially outside of an inner diameter of the associated cylindrical rotor case hub. 
     In some example embodiments, the torque proof connection between each of the rotor drive elements and the associated shaft is a splined or keyed connection. 
     Thereby, a robust and reliable torque connection is accomplished. 
     In some example embodiments, each rotor drive element comprises an annular projection extending towards the rear side of the pump, wherein the annular projection comprises the axial abutment surface, and wherein each annular projection is arranged on a portion of the associated shaft. 
     In some example embodiments, the pump further comprises a first pair of seal assemblies, such as mechanical face-seal assemblies, each having a first part and a second part with sealing surfaces pressed against each other, and each arranged to prevent fluid product from escaping the stationary pumping cavity and flowing along one of the shafts towards the rear side of the rotor case body. Thereby, a leakage-proof pump is accomplished. 
     In some example embodiments, each cylindrical rotor case hub has a front seal seat facing towards the front side of the pump, wherein the front seal seat is located at a front region of each rotor case hub, and wherein each front seal seat has the first part of one of the first pair of seal assemblies mounted therein. Thereby, front-loading of the seal is enabled. 
     In some example embodiments, the first part of each first pair of seal assemblies faces, as seen in the radial direction, a circumferential outer surface of a portion of the associated shaft. Thereby, a compact pump design with large diameter shafts is accomplished. 
     In some example embodiments, each rotor drive element has a rotor seal seat facing towards the rear side of the pump, wherein each rotor seal seat has the second part of one of the first pair of seal assemblies mounted therein. Thereby, the seals are easily accessible from the front side of the pump.  
     In some example embodiments, the rotary positive displacement pump is configured for front-loading of the first pair of seal assemblies. Thereby, improved serviceability is accomplished. 
     In some example embodiments, an exterior diameter of each shaft in an axial region of the front seal seat of each cylindrical rotor case hub is larger than an exterior diameter of each shaft in an axial region of, and in contact with, the mounting portion of each rotor drive element. Thereby, large diameter shafts are accomplished over a wider range. 
     In some example embodiments, the pump further comprises a second pair of seal assemblies, such as mechanical face-seal assemblies, each having a first part and a second part with sealing surfaces pressed against each other, and each arranged to prevent fluid product from flowing along the shaft towards the rear side of the rotor case body. Thereby, the sealing performance is further improved. 
     In some example embodiments, the method further comprising an intermediate step, performed before mounting the rotor drive elements to the shafts, of mounting a first part of a first pair of seal assemblies, such as mechanical face-seal assemblies, in a front seal seat of each cylindrical rotor case hub, and mounting a second part of the first pair of seal assemblies, such as mechanical face-seal assemblies, in a rotor seal seat of each rotor drive element. 
     The pump according to the disclosure can be arranged for pumping a variety of different product fluids, in particular product fluids commonly known in dairy, food, beverage, pharma and personal care markets. 
     In some example embodiments, the rotary positive displacement pump is a circumferential piston pump or a lobe pump. Preferably, the rotary positive displacement pump is a circumferential piston pump. 
     Further features and advantages of the invention will become apparent when studying the appended claims and the following description. The skilled person in the art realizes that different features of the present disclosure may be combined to create embodiments other than those explicitly described hereinabove and below, without departing from the scope of the present disclosure.  
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The disclosure will be described in detail in the following, with reference to the attached drawings, in which 
         FIG.  1    shows schematically a side of a pump according to the disclosure, 
         FIG.  2    shows schematically a front view of the pump according to the disclosure, 
         FIG.  3    shows schematically a 3D view of an example embodiment of the rotor case hub, 
         FIG.  4    shows schematically a 3D view of an example embodiment of a rotor, 
         FIG.  5    shows schematically the functionality of the pump, 
         FIG.  6    shows schematically a cross-section of a front portion of an example embodiment of the pump, 
         FIG.  7    shows schematically a close-in view of a portion of  FIG.  6   , 
         FIG.  8    shows schematically an alternative embodiment of the sealing arrangement, 
         FIG.  9    shows schematically still an alternative embodiment of the sealing arrangement, 
         FIG.  10 ,  11    show the basic steps of two example embodiments of the methods for assembling a pump according to the disclosure, and 
         FIG.  12    show the basic steps of an example embodiments of a method for providing maintenance of a sealing arrangement of a pump according to the disclosure. 
     
    
    
     DESCRIPTION OF EXAMPLE EMBODIMENTS 
     Various aspects of the disclosure will hereinafter be described in conjunction with the appended drawings to illustrate and not to limit the disclosure, wherein like  designations denote like elements, and variations of the described aspects are not restricted to the specifically shown embodiments, but are applicable on other variations of the disclosure. 
       FIG.  1    schematically shows a side view of a first example embodiment of the rotary positive displacement pump  1  for pumping a fluid product according to the disclosure. The pump  1  has a main body  2  including rotational support  3  to first and second parallel shafts  4 ,  5 , which extend in an axial direction  10 . The rotational support  3  may for example be provided in form of a set of annular rolling bearings, each of which surrounds a shaft and is fastened to the main body  2 . The first axially extending shaft  4  carries a first gear  6  and the second axially extending shaft  5  carries a second gear  7 . The first and second gears  6 ,  7 , i.e. gear wheels, are arranged in constant mesh condition, meaning that they are in constant gear engagement with each other. Moreover, since the first and second gears  6 ,  7  are in directing engagement with each other they rotate in opposite directions. 
     The main body  2  has an axial direction  10 , a first lateral direction  11  that is perpendicular to the axial direction  10 , and a second lateral direction  12  that is perpendicular to both the axial direction  10  and the first lateral direction  11 . The main body further has a front side  13  and a rear side  14 , as seen in the axial direction  10 . 
     An end portion  9  of one of the first and second shafts  4 ,  5 , such as for example the first shaft  4 , may extend out through a wall of the main body  2  in the rear side of the main body  2  for rotational connection with a rotational torque source, such as for example a motor, for powering the pump  1 . 
     The main body  2  may be made of metal, such as for example cast iron, steel or aluminium alloy, and the first and second shafts  4 ,  5  may be made of steel. 
     The main body  2  may additionally include a support structure  8  for enabling attachment of the main body to an exterior support surface, for example by means of threaded bolts or other type of fasteners. The main body may be made in one piece or composed of multiple sub-parts. 
     In the example embodiment of the pump illustrated in  FIG.  1   , the pump  1  further comprises a rotor case body  15  connected to the main body  2  at the front side  13  of the main body  2 . The rotor case body  15 , which for example is made of stainless  steel, may be removably fastened to the front side  13  of the main body  2  via a suitably fastening arrangement. For example, the rotor case body  15  may be clamped against the front side  13  of the main body  2  by means of a plurality of threaded bolts or nuts  16  or similar threaded members. Alternatively, the rotor case body  15  may be permanently attached to the front side  13  of the main body  2 , of integrally formed within the main body  2 . 
     The assembled pump  1  including the main body  2  and the rotor case body  15  has a front side  17  and a rear side  18 , and the pump  1  of  FIG.  1    is shown from a front side in  FIG.  2   . As can be seen in  FIG.  2   , the plurality of threaded bolts or nuts  16  used for clamping the rotor case body  15  may extending through the entire rotor case body  15  and by visible from the front side  17  of the pump  1 . 
     In the example embodiment of  FIGS.  1  and  2   , the rotor case body  15  comprises an axial rear wall  20 , a circumferential side wall  21  and an axial front wall  22 , which jointly defines a closed stationary interior pumping cavity. 
     Since the rotor case body  15  includes first and second rotors  23 ,  24  located within the interior pumping cavity, the rotor case body  15  is openable for enabling access to the interior pumping cavity. In the example embodiment of  FIGS.  1  and  2   , this access is made possible by making the rotor case body  15  in two parts: a rotor case rear housing  25  including the axial rear wall  20  and circumferential side wall  21  of the rotor case body  15 , and a front cover  26  including the axial front wall  22  of the rotor case body  15 , wherein the removable front cover  26  is removably fastened to the rotor case rear housing  25  by suitable attachment arrangement. 
     A schematic 3D view of an example embodiment of a rotor case rear housing  25  according to the disclosure is provided in  FIG.  3   , as seen partly from a front side of the rotor case rear housing  25 . 
     The removable front cover  26  may be clamped against the rotor case rear housing  25  by means of the same plurality of threaded bolts or nuts  16  that are used for clamping the rotor case body  15  against the front side  13  of the main body  2 . Alternatively, separate attachment arrangements may be provided for attaching the front cover  26  to the rotor case rear housing  25 .  
     In the example embodiment of  FIGS.  13   , the rotor case body  15  further includes a fluid product inlet opening  30  for enabling a fluid product to enter, e.g. being sucked into, the interior pumping cavity, and a fluid product outlet opening  31  for enabling the fluid product to exit, e.g. being pumped out of, the interior pumping cavity. 
     As mention above, the rotor case body  15  furthermore includes the first and second rotors that are configured for generating the pumping functionality of the pump. The first rotor  23  is rotationally fastened to a front end of the first shaft  4  and the second rotor  24  is rotationally fastened to a front end of the second shaft  5 . Consequently, the first and second rotors  23 ,  24  are configured to rotate in mutually opposite directions, as illustrated by solid arrows in  FIG.  5   . 
     The first and second rotors  23 ,  24 , which may have substantially identical design, are schematically illustrated in  FIGS.  1  and  2   , and a 3D view of a rotor, as seen partly from a rear side, is provided in  FIG.  4   . Each of the first and second rotors  23 ,  24  has at least one, and preferably a plurality of, rotor wings  32  and a rotor drive element  33  that is configured to be mounted torque proof on a rotor seat of an associated shaft  4 ,  5 . In particular, the rotor seat is located at a front end region of each shaft  4 ,  5 . 
     The rotor drive element  33  of each rotor  23 ,  24  may be substantially disc-shaped or sleeve-shaped and including a central hole or recess  44  for mounting on the associated shaft  4 ,  5 . The hole or recess  44  may be defined by a cylindrical mounting surface  48  having splines  45 , or by a non-circular mounting surface for enabling torque proof mounting of the rotor on the rotor seat of the associated shaft  4 ,  5 . The rotor drive element  33  of each rotor  23 ,  24  may additionally include an annular rotor seal seat  46  facing towards the rear side  18  of the pump  1  and configured for housing a seal. The annular rotor seal seat  46  may for example be implemented in form of a groove machined or otherwise manufactured in a rearwards facing surface of the rotor drive element  33  of each rotor  23 ,  24 . 
     With reference to  FIG.  5   , in this example embodiment of the pump  1 , during operation of the pump  2 , the rotors are configured to rotate in opposite directions with the same rotational speed. The rotors are configured to define a pumping volume within a space  35  restricted by the neighbouring rotor wings of the same rotor and the walls  20 ,  21 ,  22  of the interior pumping cavity. Moreover, during rotation of the rotors  23 ,  24 , the fluid product is configured to be conveyed from the fluid product inlet  opening  30 , along an outer side of each rotor  23 ,  24  and to the fluid product outlet opening  32 , illustrated by the dashed arrows in  FIG.  5   . 
     In particular, when the rotor wings (pistons) rotate around the circumference of the pumping cavity, this continuously generates a partial vacuum at the product inlet opening as the rotors unmesh, causing product fluid to enter the pump. The fluid is transported around the pumping cavity by the rotor wings, and is displaced as the rotor wings re-mesh, generating pressure at the discharge port. Direction of flow is reversible. 
     The specific form and number of rotor wings  32  may vary considerably and the specific rotor twin-wing design illustrated in  FIGS.  2 ,  4  and  5    is merely one example embodiment of rotor wings, and the pump may thus have rotors  23 ,  24  with other types of rotor wing designs according to the disclosure. 
     With reference to  FIG.  3   , the rotor case body  15  comprises a first cylindrical rotor case hub  36  extending from the rear wall  20 , and second cylindrical rotor case hub  37  extending from the rear wall  20 . The first and second hubs  36 ,  37  are essentially hollow cylindrical sleeves that are open towards both axial sides thereof. Moreover, an axial direction of each cylindrical hubs is aligned with the axial direction of the pump  1 . 
     The first rotor case hub  36  is configured to receive the first shaft  4 , and the second rotor case hub  37  is configured to receive the second shaft  5 . In other words, in an assembled state, the first rotor case hub  36  is aligned with the first shaft  4 , and the second rotor case hub  37  is aligned with the second shaft  5 . The first and second hubs  36 ,  37  are thus displaced from each other in the first lateral direction  11 . 
     Prior to assembly of the main body  2  with the rotor case body  15 , the front ends of the first and second shafts  4 ,  5  protrude forwards beyond the front surface  13  of the main body. Subsequently, upon assembly of the main body  2  with the rotor case body  15 , said front ends of the first and second shafts  4 ,  5  are inserted from a rear side into the first and seconds hubs, respectively, and a rear side of the rotor case body  15  comes into contact with the front surface  13  of the main body  2 . In this state, the front ends of the first and second shafts  4 ,  5  extend through the complete axial length of the first and seconds hubs  36 ,  37 , as schematically shown in  FIG.  6   .  
     More in detail,  FIG.  6    shows a cross-sectional side view of a front portion of an example embodiment of the pump  1  in an assembled state including a front portion of the main body  2 , the rotor case body  15  composed of the rotor case rear housing  25  and the front cover  26 , threaded fasteners  16  for clamping the rotor case body  15  against the front surface  13  of the main body  2 , and first and second rotors  23 ,  24  being mounted torque proof on the rotor seats  34  of the first and second shafts  4 ,  5 , respectively. 
       FIG.  6    also shows a space  35  that is restricted by the neighbouring rotor wings of the same rotor, the axial rear wall  20 , the circumferential side wall  21 , the axial front wall  22 , and the first rotor case hub  36 . Clearly, although not showed in  FIG.  6   , also the second rotor  24  defines spaces  35  between neighbouring rotor wings  32  of the same rotor  24 . 
     In addition,  FIG.  6    also shows that each of the first and second rotors  23 ,  24  are secured to the rotor seats  34  of the associated shaft  4 ,  5  by means of a fastener  38 , preferably a threaded fastener, that is engaged with a mating section  39 , preferably a mating threaded section, at an end region of the associated shaft  4 ,  5 . Specifically, each of said fastener  38  is configured to exert an axial clamping force on a centre portion of the associated rotor  23 ,  24  for clamping the rotor  23 ,  24  against an axial abutment surface of the rotor seat of the shaft  4 ,  5 . 
       FIG.  6    further shows that each of the first and second rotor case hubs  36 ,  37  is provided with a annular sealing arrangement  40  for preventing fluid product located within the space  35  from leaking out along the first and second shafts  4 ,  5  towards the rear side of the rotor case body. 
     Each annular sealing arrangement  40  may for example be implemented in form of a seal assembly having two main sealing parts. A first annular sealing part is associated with the rotor case hub and a second annular sealing part is associated with the rotor. Preferably, the seal assembly is a mechanical face-seal assembly. Then, the first and second sealing parts are held in sealing contact against each other in the axial direction while allowing relative rotation. One or both of the first and/or second annular sealing parts may have square-shaped, L-shaped, I-shaped or P-shaped cross-sectional shape, or any other shape, as seen in a plane extending through a centre of the annular sealing arrangement  40  and aligned with the axial direction  10 .  
     In general, mechanical face seal technology involves having one seal ring remaining stationary as a shaft with a corresponding mating seal ring rotates. Thus, a dynamic seal is established between the contact faces of the seal ring and mating seal ring. However, the sealing arrangement  40  may be implemented using other types of seals. For example, an elastic seal, such as an o-ring or lip seal, may be associated with the rotor case or rotor case hub thereof and a sleeve may be associated with the rotor. Optionally, the elastic seal may be mounted on a housing associated with the rotor case hub. 
       FIG.  7    schematically shows an enlargement of the area  41  marked with dashed rectangle in  FIG.  6    for better illustrating the details of the seat  34  of the first shaft  4 , the first rotor  23 , the sealing arrangement  40  and the first rotor case hub  36 , according to an example embodiment of the pump. The dashed-dotted line  60  lines refers to a rotational centre axis of the first shaft  4 . The same design applies also to the second rotor  24 , the second shaft  5  and the second rotor case hub  37 . However, the specific design of the sealing arrangement illustrated and described with reference to  FIGS.  6  and  7    merely represent example embodiments of the sealing arrangement and other configurations and implementations of the sealing arrangement are possible within the scope of the disclosure and present claims. 
     The rotor seat  34  of the first shaft  4  has an axial abutment surface  42  facing in an axial direction  10  towards a front side  17  of the pump  1  and a mounting surface  43  facing radially outwards, i.e. in a direction perpendicular to the axial direction  10 . In the assembled state of the pump  1 , a mounting portion  47  of each rotor drive element  33  is located in the rotor seat  34  of one of the first and second shafts  4 ,  5 . The mounting portion  47  of each rotor drive element  33  is indicated by a dashed circle in  FIG.  6   . 
     The mounting surface  43  of the rotor seat  34  may be provided with splines, a key-connection, a non-cylindrical surface, or the like for rotational engagement with corresponding splines  45  or the like provided on an interior mounting surface  48  of the rotor drive element  33 . 
     A threaded fastener  38 , such as a nut, may be engaged with a mating threaded section  39 , such as a threaded pin-shaped section, at the end region  49  of the shaft  4  and configured for axially pressing the rotor drive element  33  against the axial  abutment surface  42  of the rotor seat  34 . This may also be achieved by means of a screw or bolt, possibly accompanied by a disc (similar to a washer), screwed into a threaded axial hole at the end region  49  of the shaft  4 . 
     The first annular sealing part  51  is located in a front seal seat  53  of the first hub  36 , and the second annular sealing part  52  is located in the annular rotor seal seat  46  of the rotor drive element  33 , which annular rotor seal set  46  is facing towards the rear side  18  of the pump  1 . Moreover, a rearward facing sealing surface  54  of the second annular sealing part  52  is axially pressed against a corresponding forward facing sealing surface  55  of the first annular sealing part  51  via a suitable axial pressing arrangement, such as some type of spring or resilient element, in a conventional manner. 
     As a result, product fluid that has flowed from the interior pumping cavity and having entered a gap  57  between the first rotor case hub  36  and the rotor drive element  33  is prevented from flowing further, and in particular prevented from entering a gap  56  between the interior surface of first rotor case hub  36  and an exterior surface of the first shaft  4 , because this could otherwise result in leakage of the product fluid out from the interior pumping cavity. 
     The location of the sealing arrangement  40  between the rotor  23 ,  24  and a front region of the associated rotor case hub  36 ,  37  also enables simplified maintenance because the sealing arrangement  40  is more accessible for maintenance thereof. In particular, access to the sealing arrangement  40  is accomplished by merely removing the removable front cover  26  and thereafter removing the first and/or second rotor  23 ,  24 . Thereafter, the sealing arrangement  40  is fully accessible for cleaning, replacement or maintenance, or the like, all without the need for removing the entire rotor case body  15  from the main body  2 . This is also referred to as front loading seals, or front loading sealing arrangement. 
     Furthermore, the rotary positively displacement pump  1  according to the disclosure, besides enabling reduced maintenance effort by means of the front loading seals, additionally enables improved reliability, improved pumping efficiency, improved cleanability and hygiene without disassembly, also called Clean In Place (CIP), and/or reduced manufacturing cost by means of increased dimension of the first and second  shafts  4 ,  5 , all without negative effect on pumping volume or exterior pump dimensions. 
     This is accomplished by a rotary positive displacement pump  1  for pumping a fluid product according to  FIGS.  1 - 7    of the present disclosure, wherein the pump  1  comprises a main body  2  that provides rotational support to a pair of oppositely rotating, parallel, axially extending, shafts  4 ,  5  with gears  6 ,  7  that are in constant mesh condition. The pump  1  further includes a rotor case body  15  connected to a front side  13  of the main body  2 . The rotor case body  15  comprises a stationary interior pumping cavity defined by an axial rear wall  20 , a circumferential side wall  21  and a removable front cover  26 . The rotor case body  15  further comprises a fluid product inlet opening  30 , a fluid product outlet opening  31  and a pair of cylindrical rotor case hubs  36 ,  37  extending from the rear wall  20 , wherein each cylindrical rotor case hub  36 ,  37  receives internally one of the pair of shafts  4 ,  5 . 
     The rotary positive displacement pump  1  further includes a pair of rotors  23 ,  24 , each having at least one rotor wing  32 , preferably a plurality of rotor wings  32 , and a rotor drive element  33  that is mounted torque proof on a rotor seat  34  at an end region  49  of one of the pair of shafts  4 ,  5 . The torque proof connection between each of the rotor drive elements  33  and the associated shaft  4 ,  5  may be a splined or keyed connection. Alternatively, the first and second shaft  4 ,  5  may have a non-cylindrical shape at said end region  49 , such as triangular-shaped, square-shaped, polygon-shaped, oval-shaped, or the like, for enabling the desired torque proof connection between the rotor drive element  33  and the shaft  4 ,  5 . 
     Moreover, each of the pair of rotor seats  34  has an axial abutment surface  42  facing in an axial direction  10  towards a front side  17  of the pump  1  and a mounting surface  43  facing radially outwards. 
     Furthermore, the pump  1  comprises a pair of fasteners  38 , such as threaded fasteners  38 , each being engaged with a mating section  39 , such as a mating threaded section  39 , at the end region  49  of one of the pair of shafts  4 ,  5 , and each exerting an axial clamping force on one of the rotor drive elements  33  against the axial abutment surface  42  of one of the rotor seats  34 , and the axial abutment surface  42  of each rotor seat  34  is located axially outside, towards a front side  17 , of the associated hub  36 , 37 .  
     A length of the gap  57  between the axial abutment surface  42  of each rotor seat  34  and an axial end surface  66  of the associated hub  36 ,  37 , in the axial direction  10 , may for example be about 0.05-5 mm or more, or within a range of about 0.05-50 mm, specifically 0.1-25 mm, more specifically 0.1-10 mm, or even more specifically 0.1-5 mm, or yet more specifically 0.1-1 mm. 
     Consequently, since the axial abutment surface  42  of each rotor seat  34  is located axially outside, towards a front side  17 , of the associated hub  36 ,  37 , the first and second shafts  4 ,  5  may have a relatively large diameter  63  over a wider range  73 , and in particular further towards the front side  17  of the pump  1 , thereby enabling increased shaft stiffness without negative effect on pumping volume or exterior pump dimensions. 
     As mentioned above, increased shaft diameter  63  enables manufacturing of the shafts  4 ,  5  in less exotic materials without sacrificing operating reliability or risk for material fatigue. Moreover, stiffer shafts  4 ,  5  generally enables pump design with reduced clearance between the rotor wings  32  and the radial and axial walls  20 ,  21 ,  22  of the stationary pumping cavity because stiffer or larger diameter shafts typically result in reduced shaft deflection. Reduced rotor wing clearance may be directly linked with reduced pump slippage and thus increased pumping efficiency. Stiffer shafts  4 ,  5  also reduces the risk for undesired interference between the first and second rotors  23 ,  24  during pumping operation. 
     The rotary positively displacement pump  1  according to the disclosure thus not only enables reduced maintenance effort by means of the front loading seals, the rotary positive displacement pump  1  additionally enables increased dimension of the first and second shafts  4 ,  5 , all without negative effect on pumping volume or exterior pump dimensions. 
     As a result of having the axial abutment surface  42  of each rotor seat  34  being located axially outside, towards a front side  17 , of the associated hub  36 ,  37 , a mounting portion  47  of each rotor drive element  33  is radially non-overlapping the associated cylindrical rotor case hub  36 ,  37 . 
     The term “mounting portion” herein refers to the portion of the rotor drive element  33  that is radially limited on the inside by the interior mounting surface  48  of the hole or recess  44  of the rotor drive element  33  and on the outside by the annular rotor seal  seat  46 . Hence, the mounting portion  47  of each rotor drive element  33  does certainly not extend radially outside of an inner diameter  62  of the associated cylindrical rotor case hub  36 ,  37 . By having a mounting portion  47  of each rotor drive element  33  radially non-overlapping the associated cylindrical rotor case hub  36 ,  37 , larger diameter shafts  4 ,  5  may be used over a wider range within the rotor case body  15 , as seen in the axial direction  10 . 
     The mounting portion  47  of each rotor drive element  33  includes an axial abutment surface  61  facing in an axial direction  10  towards a rear side  18  of the pump  1  and a mounting surface  48  facing radially inwards. The axial abutment surface  61  of each mounting portion is located axially outside, towards a front side  17 , of the associated hub  36 ,  37 , in particular axially outside of the axial end surface  66  of the associated hub  36 ,  37 . Thereby, larger diameter shafts  4 ,  5  may be used over a wider range within the rotor case body  15 , as seen in the axial direction  10 . 
     In  FIG.  7   , a large diameter portion  73  of the first shaft  4  is indicated and extends forwards until the axial abutment surface  42  of the rotor seat  34 , and a smaller diameter portion  74  of the first shaft is indicated and extends from the axial abutment surface  42  of the rotor seat  34  to a front end of the first shaft  4 . 
     Consequently, an exterior diameter  63  of each shaft  4 ,  5  in an axial region of the front seal seat  53  of each cylindrical rotor case hub  36 ,  37 , is larger than an exterior diameter  64  of each shaft  4 ,  5  in an axial region of, and in contact with, the mounting portion  47  of each rotor drive element  33 . 
     The mounting portion  47  of each rotor drive element  33  comprises an annular projection  65  extending towards the rear side  18  of the pump  1 , wherein the annular projection  65  comprises the axial abutment surface  61  of the rotor drive element  33 , and wherein the annular projection  65  of each rotor drive element  33  is arranged on a portion of the associated shaft  4 ,  5 , namely on the mounting surface  43  of the rotor seat  34 . 
     With reference to  FIGS.  6  and  7   , the pump  1  may comprise a sealing arrangement  40  in form of a first pair of seal assemblies, such as mechanical face-seal assemblies, i.e. one seal assembly associated with the first rotor case hub  36  and one seal assembly associated with the second rotor case hub  37 .  
     As mentioned above, each seal assembly may include a first part  51  and a second part  52  with sealing surfaces  54 ,  55  pressed against each other, and each seal assembly may be arranged to prevent fluid product from escaping the stationary pumping cavity and flowing along one of the shafts  4 ,  5  towards the rear side of the rotor case body  15 . 
     Each cylindrical rotor case hub  36 ,  37  has a front seal seat  53  facing towards the front side  17  of the pump  1 . The front seal seat  53  is located at a front region of each rotor case hub  36 ,  37 , and each front seal seat  53  has the first part  51  of one of the first pair of seal assemblies mounted therein. 
     More in detail, the front seal seat  53  may correspond to a recess having at least an axial support surface  67  facing towards a front side  17  of the pump  1  for providing an axial support to the first sealing part  51 . In addition, the recess of the front seal seat  53  may include a radial support surface  68  facing towards the associated shaft  4 ,  5 , for providing radial support to the first sealing part  51 . 
     As a result of the location of the front seal seat  53  adjacent the axial end surface  66  of the associated hub  36 ,  37 , the first sealing part  51  of each first pair of assemblies faces, as seen in the radial direction, a circumferential outer surface  71  of a portion of the associated shaft  4 ,  5 . 
     In particular, the first sealing part  51  of each first pair of seal assemblies may even face, as seen in the radial direction, a circumferential outer surface  71  of the large diameter portion  73  of the associated shaft  4 ,  5 . 
     Each rotor drive element  33  has a rotor seal seat  46  facing towards the rear side  18  of the pump  1 , and each rotor seal seat  46  has the second part  52  of one of the first pair of seal assemblies mounted therein. 
     The rotor seal seat  46 , which may be implemented in form of a groove or recess in a rearwards facing surface of the rotor drive element  33  of each rotor  23 ,  24 , may include an axial support surface  69  facing towards a rear side  18  of the pump  1  for providing axial support to the second sealing part  52 . In addition, the groove or recess of the rotor seal seat  46  may include at least one radial support surface  70  facing radially inwards and/or outwards for providing radial support to the second sealing part  52 .  
     A further embodiment of the sealing arrangement  40  is schematically illustrated in  FIG.  8   , wherein some more details of an example implementation are included. For example, the sealing arrangement  40  may include a first elastic sealing ring  75  sandwiched between a rear side of the first sealing part  51  and the axial support surface  67  and/or radial support surface  68  of the front seal seat  53  for improved sealing performance and providing more flexibility in terms of positioning and tolerances of the first sealing part  51 . Moreover, the first sealing part  51  may be rotationally fixed relative the first rotor case hub  36  for preventing any relative rotation between the first sealing part  51  and first rotor case hub  36 . For example, the rotational connection may be accomplished with a pin  76  or the like connected to the first rotor case hub  36  and configured to interact with the first sealing part  51  for preventing any relative rotation of the first sealing part  51  and first rotor case hub  36 . 
     The sealing arrangement  40  may also include a second elastic sealing ring  77  sandwiched between the second sealing part  52  and the rotor seal seat  46  for improved sealing performance and providing more flexibility in terms of positioning and tolerances of the first sealing part  51 . One of the first and second sealing parts  51 ,  52 , for example the second sealing part  52  as illustrated in  FIG.  8   , may additionally be axially preloaded with an axial spring  78 . 
     Similar to above, also the second sealing part  52  may be rotationally fixed relative the rotor  23 ,  24  for preventing any relative rotation between the second sealing part  52  and the rotor  23 ,  24 , for example by means of a pin  79  or the like rotationally connected to the rotor  23 ,  24  and configured to interact with the second sealing part  51  for preventing any relative rotation. 
     Still a further embodiment of the sealing arrangement  40  is schematically illustrated in  FIG.  9   , wherein the sealing arrangement  40  further comprises a second pair of seal assemblies, such as mechanical face-seal assemblies. Hence, each rotor case hub  36 ,  37  is provided with two internal seal assemblies, a first seal assembly  80  located adjacent the front end of the rotor case hub  36 ,  37 , and a second seal assembly  81  arranged further towards the rear side  18  of the pump  1 . The first seal assembly in  FIG.  9    may have the same configuration as described with reference to  FIG.  8   .  
     Each second seal assembly  81  of the second pair of seal assemblies, such as mechanical face-seal assemblies, includes a first sealing part  82  having a first sealing surface  84 , and a second sealing part  83  having a second sealing surface  85  pressed against each other, and each second seal assembly  81  is arranged to prevent fluid product from flowing along the shaft towards the rear side of the rotor case body  15 . 
     The second seal assembly  81  may include a first elastic sealing ring  86  sandwiched between a rear side of the first sealing part  82  and an axial support surface  87  of the shaft  4  for improved sealing performance and providing more flexibility in terms of positioning and tolerances of the first sealing part  82 . Moreover, the first sealing part  82  may be rotationally fixed relative the shaft  4  for preventing any relative rotation between the first sealing part  82  and first shaft  4 . For example, the rotational connection may be accomplished with a pin  88  or the like connected to the first shaft  4  and configured to interact with the first sealing part  82  for preventing any relative rotation there between. 
     The second seal assembly  81  may also include a second elastic sealing ring  89  sandwiched between the second sealing part  83  and the first rotor case hub  36  for improved sealing performance and providing more flexibility in terms of positioning and tolerances of the second sealing part  83 . One of the first and second sealing parts  82 ,  83 , for example the second sealing part  83  as illustrated in  FIG.  9   , may additionally be axially preloaded with an axial spring  90 . 
     Similar to above, also the second sealing part  83  may be rotationally fixed relative the first rotor case hub  36  for preventing any relative rotation there between, for example by means of a pin  91  or the like connected to the first rotor case hub  36 . 
     However, the second pair of seal assemblies may be implemented using other types of seals. For example, an elastic seal, such as an o-ring or lip seal, may be associated with the rotor case or rotor case hub thereof and a sleeve may be associated with the shaft. Optionally, the elastic seal may be mounted on a housing associated with the rotor case or rotor case hub thereof. 
     The pump shown in the drawings is a circumferential piston pump. 
     The disclosure also relates to a method of assembling a rotary positive displacement pump for pumping a fluid product as described above. With reference to  FIG.  10   , the  method comprises a first step S 1  of providing a main body  2  giving rotational support  3  to a pair of parallel, axially extending, shafts  4 ,  5  with gears  6 ,  7  in constant mesh condition, such that the pair of shafts  4 ,  5  are arranged to rotate in opposite directions. The method further comprises a second step S 2  of providing a rotor case body having a stationary interior pumping cavity defined by an axial rear wall, a circumferential side wall, and a removable front cover, a fluid product inlet opening, a fluid product outlet opening, and a pair of cylindrical rotor case hubs extending from the rear wall, wherein the rotor case body  15  is located on a front side  13  of the main body  2 , and wherein each cylindrical rotor case hub  36 ,  37  receives internally one of the pair of shafts  4 ,  5 . In addition, the method comprises a third step S 3  of providing a pair of rotors, each having at least one rotor wing, preferably a plurality of rotor wings, and a rotor drive element. Moreover, the method comprises a fourth step S 4  of mounting each rotor drive element torque proof on a rotor seat at an end region of one of the pair of shafts, wherein each rotor seat has an axial abutment surface facing in an axial direction towards a front side of the pump and mounting surface facing radially outwards. Finally, the method comprises a fifth step S 5  of mounting a fastener  38 , such as a threaded fastener  38 , on an end region of each of the pair of shafts  4 ,  5 , a sixth step S 6  of tightening the pair of fasteners for exerting an axial clamping force on each rotor drive element against the axial abutment surface of one of the rotor seats, wherein the axial abutment surface of each rotor seat is located axially outside, towards a front side, of the associated hub, and a seventh step S 7  of mounting the removable front cover on the rotor case body. 
     Clearly, the consecutive order of at least some of the steps may change without in significant change of effect, such as for example in particular the first, second and third steps. 
     In addition to above, the method may further comprise an intermediate step, performed before mounting the rotor drive elements to the shafts, of mounting a first part of a first pair of seal assemblies, such as mechanical face-seal assemblies, in a front seal seat of each cylindrical rotor case hub, and mounting a second part of the first pair of seal assemblies, such as mechanical face-seal assemblies, in a rotor seal seat of each rotor drive element. 
     Here again the consecutive order of at least some of the steps may change without a significant change of effect. For example, the step of mounting the second part  52  of  the first pair of seal assemblies in the rotor seal seat of each rotor drive element  33  may be performed any time after having provided the rotor. 
     The disclosure also relates to another method of assembling a rotary positive displacement pump, such as a circumferential piston pump or rotary lobe pump, for pumping a fluid product as described above. With reference to  FIG.  11   , the method comprises a first step R 1  of providing a pump having two parallel axially extending shafts, an interior pumping cavity and a pair of cylindrical rotor case hubs extending towards the front side from a rear wall of the interior pumping cavity. The method further comprises a second step R 2  providing a pair of rotors, each having at least one wing, preferably a plurality of wings, connected to a central rotor drive element, and a third step R 3  of mounting a first part of a first pair of seal assemblies, such as mechanical face-seal assemblies, in a front seal seat of each cylindrical rotor case hub, and mounting a second part of the first pair of seal assemblies, such as mechanical face-seal assemblies, in a rotor seal seat of each rotor drive element. The wing(s) may in case of a lobe pump be denoted lobe(s). The method additionally comprises a fourth step R 4  of mounting one of the pair of rotors on each shaft, wherein each shaft has a rotor seat with an axial abutment surface facing in an axial direction towards a front side of the pump. Finally, the method comprises a fifth step R 5  of abutting each rotor drive element against the axial abutment surface of an associated rotor seat, wherein the axial abutment surface of each rotor seat is located axially outside, towards a front side, of the associated hub, and a sixth step R 6  mounting a removable front cover on the pump. 
     In addition to above, the disclosure also relates to a method of providing maintenance to a sealing arrangement  40  of a rotary positive displacement pump  1  as described above. With reference to  FIGS.  1 - 7   , the rotary positive displacement pump  1  has a front side  17  and a rear side  18 , two parallel axially extending shafts  4 ,  5  each carrying a rotor  23 ,  24  having at least one rotor wing  32 , preferably a plurality of rotor wings  32 , and a rotor drive element  33 . The rotary displacement pump  2  further has an interior pumping cavity including a pair of cylindrical rotor case hubs  36 ,  37  extending towards the front side  17  from a rear wall of the interior pumping cavity, wherein each shaft  4 ,  5  has a rotor seat  34  with an axial abutment surface  42  facing in an axial direction towards a front side  17  of the pump  1 . With reference to  FIG.  12   , the method comprises a first step T 1  of removing a removable front cover  26  of the pump  1 , and  a second step T 2  of removing at least one of the pair of rotors  23 ,  24  from the associated shaft  4 ,  5  for enabling access to a sealing arrangement  40  configured for preventing leakage along a gap  56  between the associated shaft  4 ,  5  and the associated cylindrical rotor case hub  36 ,  37 . The method comprises a third step T 3  of servicing the sealing arrangement  40 , and a subsequent fourth step T 4  mounting the at least one removed rotor  23 ,  24  on the associated shaft  4 ,  5  and abutting the rotor drive element  33  against the axial abutment surface of an associated rotor seat  34 , wherein the axial abutment surface of each rotor seat  34  is located axially outside, towards a front side  17 , of the associated hub  36 ,  37 . Finally, the method comprises a fifth step T 5  of mounting the removable front cover  26  on the pump  1 . 
     Clearly, the method of providing maintenance to a sealing arrangement  40  of the rotary positive displacement pump  1  may include steps of removing both the first and second rotors  23 ,  24  from the associated shafts  4 ,  5 , servicing of the sealing arrangements  40  associated with both the first and second rotors  23 ,  24 , and subsequent remounting of both the first and second previously removed rotors  23 ,  24  on the associated first and second shafts  4 ,  5  while abutting each rotor drive element  33  against the axial abutment surface of the associated rotor seat  34 , wherein the axial abutment surface of each rotor seat  34  is located axially outside, towards a front side  17 , of the associated hub  36 ,  37 . Many additional alternative sequences for performing the maintenance steps of the pump are possible, such as removing the first rotor, servicing its sealing arrangement and mounting of the first rotor, and subsequently performing the corresponding steps of the second rotor and its sealing arrangement  40 , or still other sequences resulting from other mixing of the steps/actions of the method. 
     The term “enabling access to a sealing arrangement  40 ” herein refers to the fact that a sealing arrangement  40  is arranged in a front region of each of the cylindrical rotor case hub  36 ,  37  and thereby is easily accessible by service personnel from a front side of the pump  1  upon removal of the first and second rotors  23 ,  24 , thereby eliminating the need to dismount the rotor case body  15  or rotor case rear housing  25 , such that simplified servicing and maintenance of the pump is accomplished. 
     Furthermore, the term “servicing” of the sealing arrangement  40  herein refers to actions such as inspection, measurement, cleaning and/or replacement of the sealing arrangement  40  and/or associated seal seats, such as the front seal seat  56  and/or  rotor seal seat  46 . For example, the step T 3  of servicing the sealing arrangement  40  may include removing a second part  52  of a seal assembly, such as a mechanical face-seal assembly, of the sealing arrangement  40  from a rotor seal seat  46  of the at least one removed rotor  23 ,  24 , removing a first part  51  of the seal assembly, such as the mechanical face-seal assembly, from a front seal seat  53  of the associated cylindrical rotor case hub  36 ,  37 , mounting a new second part  52  of a new seal assembly, such as a new mechanical face-seal assembly, in the rotor seal seat  46  of the at least one removed rotor  23 ,  24 , mounting a new first part  51  of the new seal assembly, such as the new mechanical face-seal assembly, in the front seal seat  53  of the associated cylindrical rotor case hub  36 ,  37 . 
     It will be appreciated that the above description is merely exemplary in nature and is not intended to limit the present disclosure, its application or uses. While specific examples have been described in the specification and illustrated in the drawings, it will be understood by those of ordinary skill in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the present disclosure as defined in the claims. Furthermore, modifications may be made to adapt a particular situation or material to the teachings of the present disclosure without departing from the essential scope thereof. 
     Therefore, it is intended that the present disclosure not be limited to the particular examples illustrated by the drawings and described in the specification as the best mode presently contemplated for carrying out the teachings of the present disclosure, but that the scope of the present disclosure will include any embodiments falling within the foregoing description and the appended claims. Reference signs mentioned in the claims should not be seen as limiting the extent of the matter protected by the claims, and their sole function is to make claims easier to understand.