Patent Publication Number: US-8967960-B2

Title: Stack rotor with tie rod and bolted flange and method

Description:
BACKGROUND 
     1. Technical Field 
     Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for preventing a leakage to atmosphere of a compressed medium by a stack rotor. 
     2. Discussion of the Background 
     Turbo-machines are used extensively in the oil and gas industry for performing fluid compression, transformation of electrical energy into mechanical energy, fluid liquefaction, etc. One such machine is a compressor. Modern compressors include plural stages (e.g., plural impellers connected in series) that are configured to compress a medium, each stage compressing the medium in a certain pressure range. A single rotor (made for example, as a single solid piece of metal) may be used to hold the plural impellers. However, advanced compressors use a more complex rotor that has a couple of components in order to achieve higher pressure ratio and delivered head. 
     With regard to  FIG. 1 , such a complex rotor  10  (which is disclosed in U.S. Pat. No. 3,749,516, the entire disclosure of which is incorporated herein by reference) may include stubs  12  and  14  that sandwich between them plural impellers  16 ,  18 ,  20 , and  22 . In order to hold tight the impellers  16 ,  18 ,  20 , and  22  one to the other, a thru-bolt  30  is threaded and attached (screwed) at both ends into the stubs  12  and  14 . 
     Stub  12  is attached to first impeller  16  via a longitudinal pin  24  while stub  14  is attached to the impeller  22  via a key  26  along a radial direction. The pin  24  and key  33  provide a driving connection between the impeller assembly and the stubs  12  and  14 . It is noted that the thru-bolt is first screwed into the stub  12 , then impellers  16 ,  18 ,  20 , and  22  are added to the thru-bolt  30 , and finally the stub  14  is screwed into the thru-bolt  30 . For this reason, the pins  24  extend along an axial direction of the rotor and the keys  26  extend along a radial direction of the rotor. However, such a rotor may be difficult to compress, i.e., to connect impeller  22  to stub  14  and apply an appropriate load as an exact alignment between impeller  22  and stub  14  is needed for inserting key  26 . 
     Other existing rotors have a hollow rotor through which the thru-bolt extends fully under the bearing and seal zone and have therefore an extremity accessible from outside of rotor. In order to apply the necessary load to the thru-bolt, one end of the thru-bolt is threaded into the rotor while the other end communicates with an opening in the rotor. This arrangement creates an additional potential leaking path for the compressed medium, between the thru-bolt and the hollow rotor, which is a potential hazard especially if the compressed medium is different from air (e.g., asphyxiating, toxic, explosive or a combination of all). The potential leaking path appears as the medium compressed by the compressor is at high pressure and thus, part of the compressed medium may escape by the rotor towards an area of low pressure. Systems to seal such type of configuration can be provided but they will nevertheless have the potential to fail. 
     Accordingly, it would be desirable to provide systems and methods that provide the operator of the machine with easy access to the thru-bolt and also does not leak between the thru-bolt and the rotor or other parts of the machine. 
     SUMMARY 
     According to an exemplary embodiment, there is a rotor for a compressor. The rotor includes a solid first stub having a first end configured to engage with a corresponding bearing and a second end having a flange configured to be attached by bolts to a corresponding flange of a first impeller of the compressor; a tie rod configured to pass through the first impeller of the compressor, the tie rod having a first end having a threaded region and a second end having a threaded portion, the first end facing the second end of the solid first stub; a nut being configured to engage the threaded region of the first end of the tie rod and to apply a pre-load to the tie rod and the first impeller of the compressor; and a solid second stub having a first end configured to receive the threaded portion of the second end of the tie rod and a second end configured to engage with a corresponding bearing. The tie rod does not contact the solid first stub. 
     According to another exemplary embodiment, there is a compressor that includes a casing; first and second bearings provided at opposite ends of the casing; a solid first stub having a first end configured to engage with the first bearing and a second end having a flange; a first impeller having a flange configured to be attached by bolts to the flange of the solid first stub; a second impeller configured to be attached to the first impeller; a tie rod configured to pass through the first and second impellers, the tie rod having a first end having a threaded region and a second end having a threaded portion, the first end facing the second end of the solid first stub; a nut being configured to engage the threaded region of the first end of the tie rod and to apply a pre-load to the tie rod and the first and second impellers of the compressor; and a solid second stub having a first end configured to receive the threaded portion of the second end of the tie rod and a second end configured to engage with the second bearing, the solid second stub being attached to the second impeller. The tie rod does not contact the solid first stub. According to still another exemplary embodiment, there is a method of assembling a rotor of a compressor that includes solid first and second stubs and plural impellers. The method includes attaching a tie rod to the solid second stub; sliding the plural impellers over the tie rod such that the last impeller contacts the solid second stub, a following impeller contact the last impeller and so on until the first impeller touches a second impeller and is free on one side; tightening a nut on the one side of the first impeller on the tie rod to hold all the impellers in contact with each other and with the solid second stub; contacting the solid first stub to the first impeller such that the tie rod does not touch the solid first stub; and attaching the solid first stub to the first impeller by inserting bolts into flanges of the solid first stub and the first impeller. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate one or more embodiments and, together with the description, explain these embodiments. In the drawings: 
         FIG. 1  is a schematic diagram of a conventional rotor of a compressor; 
         FIG. 2  is an overall view of a novel rotor for a compressor according to an exemplary embodiment; 
         FIG. 3  is a schematic diagram of a solid first stub that connects to an impeller according to an exemplary embodiment; 
         FIG. 4  is a schematic diagram of a solid first stub that connects to an impeller according to an exemplary embodiment; 
         FIG. 5  is a schematic diagram of a tie rod disposes inside plural impellers according to an exemplary embodiment; 
         FIG. 6  is a schematic diagram of an impeller connected to a solid second stub according to an exemplary embodiment; 
         FIG. 7  is a schematic diagram of a compressor according to an exemplary embodiment; and 
         FIG. 8  is a flow chart illustrating a method for assembling a compressor according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The following description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to the terminology and structure of a multistage centrifugal compressor. However, the embodiments to be discussed next are not limited to this compressor, but may be applied to other type of compressors, turbines, pumps, etc. 
     Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments. 
     According to an exemplary embodiment, a rotor of a machine includes three segments connected to each other. The first segment is a solid first stub, the second segment includes one or more impellers and the third segment includes a solid second stub. The one or more impellers are sandwiched between the solid first and second stubs. To maintain the one or more impellers in tight contact with each other, a tie rod is screwed into the solid second stub through the one or more impellers. A nut is attached at the other end of the tie rod and the tie rod is preloaded with a desired tension by tightening the nut. The solid first stub covers the nut and a corresponding end of the tie rod. The solid first and second stubs are configured to come into contact with bearings for supporting a rotation of the rotor. The machine may be a compressor, an expander, a pump, etc. 
     According to an exemplary embodiment illustrated in  FIG. 2 , a compressor  40  includes a casing  42  that accommodates one or more impellers.  FIG. 2  shows a set of five impellers,  44 ,  46 ,  48 ,  50 , and  52 . However, it is noted that the exemplary embodiments discussed herein apply to a compressor having one or more impellers and is not limited to five impellers as used here as an example. Also, a compressor is illustrated in  FIG. 2  for simplicity but the exemplary embodiments apply to other machines or types of compressors. 
     A solid first stub  60  is configured to be attached to the first impeller  44 . An interface  62  between the solid first stub  60  and the first impeller  44  may include various elements for achieving the connection between the solid first stub  60  and the impeller  44 . For example, as shown in  FIG. 3 , interface  62  may include a flange  64  that is attached to the solid first stub  60  and a flange  66  that is attached to the first impeller  44 . Flanges  64  and  66  are configured to be attached to each other. According to an exemplary embodiment, flanges  64  and  66  have one or more holes  68  and  70  in which one or more bolts  72  are provided. Bolt  72  may have a threaded region that threads into a corresponding threaded region inside hole  70  of flange  66 . Alternatively, flange  66  may have a groove set up in such a way that an end of bolt  72  is accessible from outside (hole  68  goes all the way through flange  66 ). In this case, the connection of the flange is achieved by using a nut applied to the end of screw  72 . A benefit of this exemplary arrangement is to avoid filleting the flange  66  when a material not suitable for this type of machining is used. Another end  74  of bolt  72  may completely be accommodated by hole  68 , by having, for example, a first part of hole  68  drilled with a larger diameter. Alternately, the end  74  of bolt  72  may stay outside flange  64 . 
     According to another exemplary embodiment shown in  FIG. 4 , a front surface  76  of flange  64  and a corresponding front surface  78  of flange  66  may be connected to each other by providing them with teeth that mesh together, e.g., a Hirth or curvic connection (a curvic connection of coupling has precision face splines with curved radial teeth of contact depth. They are used for joining two or more members to form a single operating unit). According to another exemplary embodiment, flanges  64  and  66  are connected to each other only by bolts  74 . According to still another embodiment, both Hirth mechanism and bolts may be used to connect the two flanges. 
     Returning to  FIG. 2 , impellers  44 ,  46 ,  48 ,  50 , and  52  may be connected to each other by bolts, by Hirth or curvic connections, by both of them, or by other known mechanisms in the art. The same is true for the connections between impellers and the first and second stubs. Each impeller has an inner hole that communicates with the inner holes of the neighboring impellers. Thus, as shown more clearly in  FIG. 5 , a passage  80  is formed inside impellers  44  to  52 , in a central region of the impellers.  FIG. 5  also shows that a tie rod  82  is inserted inside the passage  80 . A first end  84  of the tie rod  82  is housed by a cavity  86  formed in the first impeller  44 . A nut  88  is provided on a threaded region  90  of the first end  84 . The nut  88  is screwed until it contacts an inside part  92  of the first impeller  44 . Thus, in one application, the tie rod  82  is configured to not touch the passage  80  formed by the impellers. A predetermined tension is applied to the tie rod  82  by appropriately tightening nut  88 . As would be recognized by those skilled in the art, other mechanisms may be used to press the impellers one against the other. 
     The other end  94  of the tie rod  82  is shown in  FIG. 6  as being screwed into a solid second stub  96 . As discussed above with regard to  FIG. 5 , the tie rod  82  may be configured to not contact passage  80 . In other words, in one application, the tie rod  82  does not contact any of the impellers of the machine. 
     Next, the assembly of the impellers  44  to  52  is discussed with regard to  FIGS. 5 and 6 . Initially, the tie rod  82  is screwed into the solid second stub  96  until the tie rod  82  is fixed, i.e., cannot be further rotated. Then, the last stage  52  is added to contact the solid second stub  96 . One by one, all stages are added on the tie rod  82  until the first stage  44  is positioned as shown in  FIG. 5 . In one application, a Hirth coupling is provided between each two adjacent stages. In another application, a Hirth coupling is provided between the solid first stub  60  and the first impeller  44  and/or between the last stage  52  and the solid second stub  96 . 
     After adding the first impeller  44  as shown in  FIG. 5 , nut  88  is screwed onto the tie rod  82  so that a predetermined tension is applied to the tie rod  82 . The applied tension ensures that the various impellers of the compressor do not slide one relative to the other when the compressor is functional. Also, the applied pre-load ensures that all the impellers trough which the tie rod passes rotate together with the solid second stub  96 . Finally, the solid first stub  60  is attached to the first impeller  44 , thus sealing cavity  86  in which the first end  84  of the tie rod  82  is present. In this way, no gas that is compressed by the impellers leaks past the tie rod as in traditional devices. Thus, in one exemplary embodiment, the tie rod  82  is fully contained inside the rotor, between the solid first and second stubs  60  and  96 . 
     According to an exemplary embodiment shown in  FIG. 7 , a compressor  100  may include four impellers  44 ,  46 ,  48 , and  52 , the solid first stub  60 , the solid second stub  96 , and the tie rod  82 . In addition, the compressor  100  may include a dry seal unit  102  that seals a flow of fluid along the solid first stub  60  and a dry seal unit  104  that seals a flow of fluid along the solid second stub  96 . The fluid that is compressed by the impellers of the compressor may escape along the rotor and the dry seal units are configured to minimize such a flow. The dry seal units are configured to receive another fluid under pressure and to interpose this other fluid under pressure between the escaping compressed fluid and the environment. According to the exemplary embodiment shown in  FIG. 7 , the tie rod  82  does not extend past the dry seal units  102  and  104 . 
     According to the exemplary embodiment shown in  FIG. 7 , bearings  106 ,  108  and  110  may be provided at ends of the solid first and second stubs  60  and  96 . For example, bearing  106  and  110  may be support bearings, i.e., bearings that support a rotation of the rotor ( 60 ,  44 ,  46 ,  48 ,  52 , and  96 ) while bearings  108  may prevent an axial displacement of the rotor. In one application, both the bearings and the dry seals are configured to face the solid first and second stubs while the impellers are configured to accommodate but not touch the tie rod. 
     With this configuration, a tie rod that is not as long as the rotor is strong enough to transfer torque to the impellers and to overcome rotor axial forces generated by axial thrust of impellers. In fact, the radial room available for tie-rod under impellers is much larger than the one available under seals or bearings. Not extending the tie-rod under seals allows the manufacturer to use a bigger diameter tie-rod with the possibility to apply higher axial pre-load and to have a stiffer tie rod that better resists at potential harmful vibrations. Further, the zone of dry gas seal can be the hottest zone in the compressor due to both the friction of seals with a very small leakage and the fact that those seals are normally supplied with filtered but hot gas from compressor to avoid potential condensate formation. Passage of the tie-rod under the dry gas seal would therefore create a thermal differential growth between the rotor under the seal and the tie-rod, with potential for thermal fatigue of tie rod. 
     According to an exemplary embodiment illustrated in  FIG. 8 , there is a method of assembling a rotor of a compressor that includes solid first and second stubs and plural impellers. The method includes a step  800  of attaching a tie rod to the solid second stub, a step  802  of sliding the plural impellers over the tie rod such that the last impeller contacts the solid second stub, a following impeller contacts the last impeller and so on until the first impeller touches a second impeller and is free on one side, a step  804  of tightening a nut on the one side of the first impeller on the tie rod to hold all the impellers in contact with each other and with the solid second stub, a step  806  of contacting the solid first stub to the first impeller such that the tie rod does not touch the solid first stub, and a step  808  of attaching the solid first stub to the first impeller by inserting bolts into flanges of the solid first stub and the first impeller. 
     The disclosed exemplary embodiments provide a system and a method for preventing leakage of a compressed medium from a compressor. It should be understood that this description is not intended to limit the invention. On the contrary, the exemplary embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the exemplary embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details. 
     Although the features and elements of the present exemplary embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein. This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims.