Patent Publication Number: US-9903374-B2

Title: Multistage compressor and method for operating a multistage compressor

Description:
BACKGROUND 
     Embodiments of the subject matter disclosed herein generally relate to multi-stage compressors and methods for operating the same. More specifically, the disclosure relates to multistage compressors having a stack rotor configuration. 
     Multi-stage compressors are widely used for industrial refrigeration, oil and gas processing and in low temperature processes and other uses. 
     Among the multitude of multi stage compressors of the know type, multi-stage compressors comprising stacked impellers held together by a tie rod are well known. A multistage compressor comprising a stack rotor is disclosed e.g. in US2011/0262284. 
       FIG. 1  illustrates an axial sectional view of a multi-stage compressor of the current art, and  FIG. 2  illustrates an enlargement of a detail of  FIG. 1 . Said compressor is labeled  100  and comprises an inlet  110 A, an outlet  110 B, a rotor  111  comprised of a plurality of stacked impellers  112 , and a stationary housing  113  housing the rotor  111 . The stationary housing comprises a diaphragm  113 A wherein each impeller discharges its gas flow to convert the kinetic energy of the gas flow into pressure recovery before returning the gas flow to the next impeller. Each impeller/diaphragm combination is usually referred to as a “stage”. The diaphragm  113 A and the rotor  111  are housed in a casing  113 B. In the compressor, a gas compression path P (indicated by a dashed line) extending from the compressor inlet  110 A to the compressor outlet  110 B and through said plurality of impellers  112  and the diaphragm  113 A is defined. The compression path P is sealed against the casing, diaphragm and rotor, using suitable seals, e.g. dry gas seals S. 
     The impellers  112  are held together by a tie rod  114 , extending axially through the impellers  112 . The first compressor stage comprises a first impeller  112 A, while the last compressor stage comprises the last impeller  112 B. The rotor  111  comprises also two terminal elements  115 A and  115 B provided at the two opposite ends of the plurality of impellers  112 . The two ends of the tie rod  114  are constrained to the terminal elements  115 A- 115 B. 
     More in particular, the hubs of the impellers  112  have through holes  116  wherein the tie rod  114  is made to pass. The holes  116  are dimensioned so as to leave a clearance  117  between the tie-rod  114  and the impellers  112 . 
     With particular reference to  FIG. 2 , each impeller  112  comprises two opposite toothed flanges  118  meshing with respective toothed flanges of two respective adjacent impellers  112  or, in the case the impeller is the first or the last impeller of the impellers stack, respectively with a toothed flange of an adjacent impeller  112  and the toothed flange  119  of one of the terminal elements  115 A,  115 B. 
     To avoid gas leakage from the compression path P to the clearance  117 , seals  120  on the meshing areas  121  of the teeth are provided. 
     The gas compressor comprises a balancing line  122  (indicated by a dash-dot line) for balancing the axial thrust of the impellers on the rotor bearings. More in particular, the compressor comprises a balancing drum  123  formed on the terminal element  115 B. The balancing drum  123  separates a balancing zone  124  from a zone in fluid communication with the outlet of the last compressor stage. The balancing zone  124  is fluidly connected with the inlet of the first impeller  112 A, such that the pressure in the balancing zone  124  is substantially equal to the pressure at the inlet of the first impeller  112 A. The balancing drum  123  is arranged in a cylindrical housing formed in the compressor casing. Between the housing and the drum a labyrinth seal  123 A is provided, so that a calibrate gas flow leakage F from the last stage towards the balancing zone  124  is allowed. The pressure difference between said balancing zone  124  and the opposite face of the balancing drum facing the last stage impeller  112 B generates an axial thrust against the balancing drum. The axial thrust on the balancing drum  123  counterbalances the axial thrust generated on the impellers by the process fluid flowing through the compressor. The balancing line  122  is formed by a pipeline, which is usually external to the casing of the compressor. 
     The compression process provokes a temperature increase of the processed gas flowing through the compressor. During startup, machine components are usually at ambient temperature and are heated up by the processed gas until a steady temperature condition is achieved. In the compressors having a stack rotor as described with reference to  FIGS. 1 and 2 , the impellers heat faster than the tie rod. This leads to high temperature gradients between the tie rod  114  and the impellers  112  during the startup transient phase. Due to this high temperature gradient, high thermal stresses are generated, which can shorten the life of the compressor or provoke malfunctioning. 
     SUMMARY OF THE INVENTION 
     To at least partly alleviate one or more of the problems of the prior art, a multi-stage compressor is provided, wherein heat developed by compressing the fluid processed by the compressor is used to heat the tie rod, which holds the stacked impellers of the compressor rotor. The multi-stage compressor comprises a return flow path, along which a fraction of the compressed process gas flows back from a downstream location to an upstream location of the gas compression path. The return flow path flows along the tie rod, so that heat generated by compression in the compressed or partly compressed processed gas is transferred to the tie-rod by forced convection. The tie rod is thus heated faster than in current art compressors. 
     According to some embodiments, a multi-stage compressor is provided, comprising a compressor rotor comprised of a plurality of axially stacked impellers, a tie rod extending through the stacked impellers and holding the impellers together and a gas compression path extending from a compressor inlet to a compressor outlet and through the plurality of impellers. The compressor further comprises a flow channel between the tie rod and the stacked impellers. The flow channel extends along at least a portion of the tie rod. The flow channel is in fluid communication with a first location and a second location along the gas compression path. During normal operating conditions, the pressure of the gas processed by the compressor at said first location is different than the pressure of the gas at the second location. The gas pressure difference between the first location and the second location in the compression path generates a gas flow along the flow channel. 
     At compressor startup, the temperature of the gas flowing from the first location to the second location is generally higher than the temperature of the tie rod, due to the temperature increase of the gas caused by compression. The gas flowing along the flow channel heats the tie rod, thus reducing the temperature gradient between the impellers and the tie rod. 
     According some embodiments, the flow channel can be used as a “balancing line” for balancing the thrust of the impellers on the bearings, as better described below. 
     In some exemplary embodiments, the first location is provided at the first compressor stage, and the second location is provided at the last compressor stage. In this way, the thermal benefits on the tie rod are maximized, since the hot gas flow contacts the tie rod along almost the entire axial extension thereof. Moreover, the compressed gas contacting the tie rod is taken from the last stage, i.e. where the gas temperature is the highest. 
     According to exemplary embodiments, each impeller comprises two opposite contacting surfaces for contacting the surfaces of two other adjacent impellers, or the surface of an adjacent impeller and the surface of a terminal element at one end of the plurality of stacked impellers. If the gas compressor comprises a first passage and a second passage, at least one of said passages is defined between the contacting surfaces of two adjacent impellers or between the contacting surfaces of one of said terminal elements and of an adjacent impeller. This configuration simplifies the construction of the compressor. In some exemplary embodiments, the first passage can be formed between mutually contacting and meshing surfaces of the hub of the first impeller and a corresponding meshing surface of the first terminal element. The second passage can be formed between mutually contacting and meshing surfaces of the hub of the last impeller and a corresponding meshing surface of the second terminal element. 
     To provide torsional constraint between the mutually stacked impellers and first and second terminal elements, torsional constraining members can be provided. In some embodiments, the contacting surfaces are provided with front toothed flanges forming the respectively meshing surfaces. The teeth of the mutually co-acting flanges form a Hirth coupling. Other connecting members can be used instead, such as curvic connections, bolts or other known mechanisms. 
     To prevent gas from flowing across meshing surfaces where no gas flow is required, e.g. at the intermediate contacting and meshing surfaces between adjacent impellers, sealing members can be provided around the meshing areas. For instance, the sealing members can be annular seals arranged on the inner surface of the through holes on the impeller hubs, wherein the tie rod is arranged, just at the meshing area. 
     According to other embodiments, at least one of the two passages can be a duct, e.g. provided, through the hub of an impeller or of a terminal element. 
     In some embodiments, the gas compressor comprises a balancing line for balancing the axial thrust of the impellers on the rotor bearing. More in particular, the compressor comprises a balance drum axially constrained to the impellers and contrasting the axial thrust of the impellers. The drum has a first face facing the last compressor stage and a second opposite face facing a balancing zone fluidly connected with the inlet of the first compressor stage, so that the pressure in the balancing zone is substantially equal to the pressure at the inlet of the first compressor stage. The pressure difference on the two faces of the balancing drum generates an axial thrust opposing the axial thrust generated on the impellers by the gas being processed through the compressor. The compressor comprises a pathway fluidly connecting the outlet of the last stage with the balancing zone associated to the balance drum. In some embodiments at least a passage fluidly connecting the flow channel and the balancing zone is provided. In this configuration, the flow channel formed between the impellers and the tie rod can function as a “balancing line”. An external balancing line is thus not required. 
     According to some embodiments, the passage fluidly connecting the flow channel and the balancing zone is provided through the balance drum. 
     According to a further aspect, the disclosure relates to a method for operating a multi-stage compressor, comprising a compressor rotor with a plurality of axially stacked impellers held together by a tie rod, and a flow channel extending along at least a portion of the tie rod. The method comprises the step of heating the tie rod by flowing compressed hot gas, e.g. drawn from the gas compression path, along the flow channel through the impellers and along the tie rod. The compressed hot gas flows from a downstream stage to an upstream stage of the compressor. 
     In some exemplary embodiments, the method provides for heating the tie rod by means of a flow of compressed gas flowing from the outlet of the last impeller to the inlet of the first impeller. 
     Features and embodiments are disclosed here below and are further set forth in the appended claims, which form an integral part of the present description. The above brief description sets forth features of the various embodiments of the present disclosure in order that the detailed description that follows may be better understood and in order that the present contributions to the art may be better appreciated. There are, of course, other features of the invention that will be described hereinafter and which will be set forth in the appended claims. In this respect, before explaining several embodiments of the invention in details, it is understood that the various embodiments of the invention are not limited in their application to the details of the construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting. 
     As such, those skilled in the art will appreciate that the conception, upon which the disclosure is based, may readily be utilized as a basis for designing other structures, methods, and/or systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the disclosed embodiments of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  illustrates an axial-sectional view of the main part of a multi-stage compressor of the prior art; 
         FIG. 2  illustrates an enlarged portion of  FIG. 1 ; 
         FIG. 3  illustrates an axial-sectional view of the main part of a multi-stage compressor according to one embodiment of the present disclosure; 
         FIG. 4  illustrates an enlarged portion of  FIG. 3 ; 
         FIG. 5  illustrates a portion of a first variant of the embodiment shown in  FIG. 3 ; 
         FIG. 6  illustrates a portion of a second variant of the embodiment shown in  FIG. 3 ; 
         FIG. 7  illustrates a portion of a third variant of the embodiment shown in  FIG. 3 ; and 
         FIG. 8  illustrates a portion of a fourth variant of the embodiment shown in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     The following detailed description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Additionally, the drawings are not necessarily drawn to scale. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. 
     Reference throughout the specification to “one embodiment” or “an embodiment” or “some embodiments” means that the 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 phrase “in one embodiment” or “in an embodiment” or “in some embodiments” in various places throughout the specification is not necessarily referring to the same embodiment(s). Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments. 
     Referring to above-mentioned  FIGS. 3 to 8 , reference number  10  indicates a multi-stage compressor as a whole. The multi-stage compressor comprises an inlet  10 A, an outlet  10 B, a rotor  11  with a plurality of stacked impellers  12 , and a stationary housing  13  housing the rotor  11 . 
     The stationary housing comprises a plurality of diaphragms  13 A wherein each impeller  12  discharges the gas flow to convert the kinetic energy of the gas flow into pressure recovery before returning the gas flow to the next impeller. Each impeller/diaphragm combination is called “stage”. The first stage of the compressor comprises the first impeller  12 A, and the last stage of the compressor comprises the last impeller  12 B. The terms “first” and “last” as used herein are referred to the direction of flow of the gas processed by the compressor. Therefore, the first stage and the first impeller are those nearest to the compressor inlet, i.e. the most upstream ones, while the last stage and last impeller are those nearest to the compressor outlet, i.e. the most downstream ones. The diaphragms  13 A and the rotor  11  are housed in a casing  13 B. The terms upstream and downstream are referred to the direction of flow of the gas processed through the compressor. 
     In the compressor  10 , a gas compression path P (indicated by a dashed line) extends from the compressor inlet  10 A to the compressor outlet  10 B and through said plurality of impellers  12  and the diaphragms  13 A. The compression path P is sealed with respect the casing, diaphragms and rotor, using suitable seals, e.g. dry gas seals S. Other kind of seals, commonly used in the art, can be used as well. 
     The impellers  12  are stacked and held together by a tie rod  14 . The tie rod  14  extends axially through the impellers. The rotor  11  comprises also two terminal elements: a most upstream, first terminal elements  15 A provided at the end of the plurality of impellers close to the first impeller  12 A; and a most downstream, second terminal elements  15 B provided at the opposite end of the plurality of impellers, close to the last impeller  12 B. The two ends of the tie rod  14  are constrained to the terminal elements  15 A,  15 B. 
     The hubs of the impellers  12  have through holes  16  wherein the tie rod is made to pass. The holes  16  are dimensioned so as to leave an interspace or clearance  17  between the tie rod and the inner surface of the holes  16 . 
     Each impeller  12  comprises two opposite contacting surfaces co-acting with the surfaces respectively of two other adjacent impellers  12 , or respectively with the surface of an adjacent impeller and the surface of a terminal element  15 A or  15 B at one end of the plurality of stacked impellers. The contact is such that the impellers are torsionally constrained one to the other and torque is transferred between the impellers. In some embodiments, each impeller  12  comprises two opposite toothed flanges  18  meshing with respective toothed flanges of two other adjacent impellers  12  or, in the case the impeller is the first  12 A or the last  12 B impeller of the stack, respectively with toothed flange  18  of an adjacent impeller  12  and the toothed flange  19 A or  19 B of a terminal element  15 A or  15 B. The toothed flanges form Hirth couplings or connections. Other kinds of connections known to those skilled in the art can be used instead of a Hirth-type coupling. 
     To avoid gas leakage from the compression path P to the interspace or clearance  17 , seals  20  are provided on the meshing areas  21 , where of the teeth of respective adjacent intermediate impellers  12  co-act. 
     The compressor comprises a balancing line  22  (indicated by a dash-dot line) for balancing the axial thrust of the impellers on the rotor bearings. More in particular, the compressor comprises a balancing drum  23  (formed on the terminal element  15 B) delimiting a balancing zone  24  from a zone in fluid communication with the outlet of the last impeller  12 B. The balancing zone  24  is fluidly connected via the balancing line  22  with the inlet of the first impeller  12 A, so that the pressure in the balancing zone  24  is substantially equal to the pressure of the inlet of the first impeller  12 A. 
     The balancing drum  23  is arranged in a cylindrical housing in the casing  13 B. Between the housing and the balancing drum  23  a labyrinth seal  23 A is provided, so that a calibrate gas flow leakage from the outlet of the last impeller  12 B towards the balancing zone  24  is allowed. The pressure difference between a first face  23 ′ of the balancing drum  23  facing the last impeller, and a second opposite face  23 ″ facing the balancing zone  24 , generates an axial thrust on the balancing drum  23 . The axial thrust on the balancing drum  23  counterbalances the axial thrust exerted by the impellers. In this embodiment the balancing line  22  is formed by a pipeline external to the compressor casing. 
     The interspace or clearance  17  forms a flow channel between the tie rod  14  and the stacked impellers  12 . The flow channel (also labeled  17 ) is in fluid communication with a first location PA and a second location PB along the gas compression path P. The first location PA is at a lower pressure than the second location PB. The pressure difference between the first location PA and the second location PB generates a gas flow along the flow channel  17 , as better explain below. 
     According to some embodiments, the first location PA is provided at the inlet of the first compressor stage where the first impeller  12 A is located, and the second location PB is provided at the outlet of the last compressor stage, where the last impeller  12 B is located. This provides for the maximum pressure difference between the first location PA and the second location PB. 
     The fluid connection between the first location PA and the flow channel  17  as well as between the flow channel  17  and the second location PB is established by respective passages. 
     In the embodiment of  FIGS. 3 and 4 , the meshing area  21 A, where the toothed flange  18 A of the first impeller  12 A meshes with the toothed flange  19 A of the first terminal element  15 A, is at least partly lacking of the seal  20 , such that at least a first gas passage  25  is established, between the first location PA and the flow channel  17 , through the co-acting teeth of the toothed flanges  18 A,  19 A. 
       FIG. 5  illustrates a modified embodiment. The same reference numbers indicate the same or corresponding components or elements, which will not be described again in detail. The first passage, again labeled  25 , which fluidly connects the first location PA of the compression path P is provided through the body or hub of the first impeller  12 A. A seal  20 A sealing the meshing area  21 A, is provided. 
     In  FIG. 6  a further modified embodiment provides for a first passage  25  arranged through the body of first terminal element  15 A. A seal  20 A sealing the meshing area  21 A, is provided. In other embodiments, the first passage can be provided in other positions and through other bodies or components of the rotor. 
     In the embodiment of  FIGS. 3 and 4 , the meshing area  21 B, wherein the toothed flange  18 B of the last impeller  12 B meshes with the toothed flange  19 B of the second terminal element  15 B, is at least partly lacking of the seal  20 , so that at least a second gas passage  26  is established between the second location PB and the flow channel  17 , through the teeth of the toothed flanges  18 B and  19 B. 
     In  FIG. 7 , a modified embodiment provides for a second passage  26  arranged through the body or hub of the last impeller  12 B. A seal  20 B sealing the meshing area  21 B, is provided. 
     In further embodiments, not shown, the second passage  26  can be provided through the body of the second terminal element  15 B, similarly to the case of the first passage  25  of  FIG. 6 . 
     In yet further embodiments, the second passage  26  can be provide in other positions and through other bodies or components of the rotor. 
     At compressor startup the rotor  11  with tie rod  14  and impellers  12  start rotating. Gas enters through the compressor inlet  10 A and flows along the compression path P through the sequentially arranged impellers  12 A,  12 ,  12  . . .  12 B and finally exits the compressor outlet  10 B. At the outlet of the last impeller  12 B, in the second location PB, the gas has reached the maximum pressure and temperature values, while at the inlet of the first impeller  12 A, i.e. in the first location PA, the gas has the lowest temperature and pressure values. The pressure difference between the first and the last stage generates a hot gas flow F (indicated by a dashed-double dotted line) from the second location PB, through the second passage  26  in the flow channel  17  and, from the flow channel  17  to the first location PA, via the first passage  25 . 
     The hot gas flowing along the flow channel  17  heats the tie rod  14  (before the startup, the tie rod is usually at room-temperature). Therefore, in this transient phase, the temperature gradients between the tie rod  14  and the impellers  12 A,  12 ,  12  . . .  12 B decrease. 
     To maximize the heating effect, as described here above, the hot gas is drawn from the last stage and is reintroduced in the gas compression path at the first stage. In other embodiments the locations PA and PB can be arranged in different positions along the compression path. 
     In  FIG. 8 , another embodiment is illustrated. In this case, the balancing line used to balance the axial thrust of the impellers is provided by the flow channel  17  and the external duct is removed. A pathway  26 ′ fluidly connects the balancing zone  24  of the balancing drum  23  to the second location PB of the compression path, arranged at the outlet of the last impeller  12 B. The pathway  26 ′ is formed, e.g. by the labyrinth seal  23 A, so that a calibrate gas flow leakage from the outlet of the last impeller  12 B towards the balancing zone  24  is generated. 
     Through a second passage  26 ″ provided in the second terminal element  15 B, the balancing zone  24  is fluidly connected with the flow channel  17 . Therefore, a gas flow F flows from the second location PB to the balancing zone  24 , with a pressure drop, and from the balancing zone  24 , via the second passage  26 ″ to the flow channel  17 . In practice, the fluid communication passage between the second location PB and the flow channel  17  is formed by the pathway  26 ′, the balancing zone  24  and the second passage  26 ″. From the flow channel  17 , the gas flows towards the first location PA at the first compressor stage, through the first passage  25 , e.g. formed in the meshing area  21 A, between the teeth of the flange  18 A of the impeller  12 A and the teeth of the flange  19 A of the first terminal element  15 A (no seal is provided in the meshing area  21 A). 
     The gas flow along the tie rod  14  heats the tie rod  14 , reducing the thermal gradients between the impellers and the tie rod during startup. At the same time, the gas flow acts as a balancing flow, balancing the thrust of the impellers on the rotor bearings. This result is achieved using the interspace or clearance  17  between the impellers  12 A,  12 ,  12 , . . .  12 B and the tie rod  14  as a flow channel connecting the first and last stage of the compressor. 
     The present disclosure concerns also a method for operating a multi-stage compressor, comprising a compressor rotor  11  with a plurality of axially stacked impellers  12  held together by a tie rod  14 , and a flow channel  17  extending along the tie rod  14 . The method comprises the step of heating the tie rod  14  by flowing a hot gas F along the flow channel  17  through the impellers  12  and along said tie rod  14 , across at least two different stages. More specifically, in some embodiments the method comprises diverting a fraction of at least partly compressed gas processed by the compressor from a high pressure location of the gas compression path, through the flow channel  17  towards a low-pressure location of the compression path. 
     In some embodiments, the compressed gas used for heating the tie rod  14  flows from the outlet of the last impeller  12 B, to the inlet of the first impeller  12 A. 
     From the last stage the heating gas flows in the flow channel  17  passing between the last impeller  12 B and the second terminal element  15 B ( FIGS. 3 and 4 ), or passing through the hub or body of the last impeller  12 B or of the second terminal element  15 B ( FIG. 7 or 8 ). 
     From the flow channel  17 , the heating gas flows in the first stage passing between the first impeller  12 A and the first terminal element  15 A ( FIGS. 3 and 4 ), or passing through the hub or body of the first impeller  12 A or of the first terminal element  15 A ( FIG. 5 or 6 ). 
     In case the stages in fluid communication with the flow channel are different from the first and last stages, the heating gas can flow passing through two adjacent impellers  12  or through the hub/body of impellers. 
     The method provides also for a balance of the thrust of the impellers against the bearings of the rotor. The gas is made to pass from the outlet of the last impeller  12 B to the balancing zone  24  defined on the balancing drum in a position opposite to said last stage impeller with respect of the drum  23 , and from said balancing zone  24  to the inlet of the first impeller  12 A, passing on and along the tie rod  14 , through said impellers, in such a way that the pressure in said inlet is substantially equal to the pressure of said balancing zone of the balancing drum. 
     While the disclosed embodiments of the subject matter described herein have been shown in the drawings and fully described above with particularity and detail in connection with several exemplary embodiments, it will be apparent to those of ordinary skill in the art that many modifications, changes, and omissions are possible without materially departing from the novel teachings, the principles and concepts set forth herein, and advantages of the subject matter recited in the appended claims. Hence, the proper scope of the disclosed innovations should be determined only by the broadest interpretation of the appended claims so as to encompass all such modifications, changes, and omissions. In addition, the order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments.