Compressor rotor structure and method for arranging the same

Abstract

Compressor rotor structures and methods for harmonizing compressor aerodynamics and rotor dynamics are provided. The disclosed embodiments benefit from compressor designs that effectively improve rotor dynamics (e.g., more rigid rotor structures) without reducing the available aerodynamic range of the compressor. Such designs can include varying the rotor structure along the rotor axis to position respective surfaces defined by respective inlet limits of one or more impellers at varying distances relative to the rotor axis based on respective ratios selected for the configuration of the impeller body. Such arrangements can effectively improve rotor dynamics while simultaneously satisfactorily meeting respective varying aerodynamic requirements at the various compression stages through the impeller body.

Description

BACKGROUND

[0001] The disclosed embodiments relate generally to the field of turbomachinery, and more specifically to a rotor structure of a turbomachine such as a compressor and a method for arranging the rotor structure.

[0002] Turbomachines are widely used in the oil and gas industry, such as for compressing process fluids, converting thermal energy into mechanical energy, liquefying fluids, etc. One example of such a turbomachine is a compressor, such as a centrifugal compressor. SUMMARY

[0003] Aspects of the disclosed embodiments relate to a rotor structure in a compressor. The rotor structure includes a connecting bolt and two rotor shafts attached to respective ends of the connecting bolt. A plurality of impeller bodies are supported by the connecting bolt. A plurality of hirth couplings are used to mechanically couple the plurality of impeller bodies to each other along a rotor axis. A first impeller body of the plurality of impeller bodies is arranged to provide a first compression stage, and each subsequent impeller body provides a subsequent compression stage. Each respective impeller body defines a respective Di / D2 ratio. The Di / D2 ratio of at least one of the impeller bodies is different from the Di / D2 ratio of the remaining impeller bodies. Based on the different Di / D2 ratio, a respective surface defined by an inlet of this impeller body is positioned at a different distance relative to the rotor axis compared to a respective surface defined by a respective inlet of the remaining impeller bodies. Di represents a respective inner diameter of a flow path entering a respective impeller body inlet, and D2 represents a respective outer diameter of the respective impeller body.

[0004] In certain embodiments, the variation of the rotor structure along the rotor axis is based on a variation of the respective Di / D2 ratio of one or more of the plurality of impeller bodies. The variation of the rotor structure along the rotor axis can include positioning a respective surface defined by a respective inlet of the one or more impeller bodies at a varying distance relative to the rotor axis. The positioning of the respective surface defined by the respective inlet of the one or more impeller bodies at the varying distance relative to the rotor axis is configured to reduce or otherwise lower an inlet Mach number in a compression stage by the one or more impeller bodies, and to adjust rotor stiffness along the rotor axis.

[0005] In certain embodiments, at least one spring biasing mechanism is arranged for adjusting a radial stiffness at a respective location of the connecting bolt. The respective location at which the at least one spring biasing mechanism is arranged can be at or proximate to a midspan section of the connecting bolt.

[0006] In certain embodiments, a multi-nut retention arrangement can be included. The multi-nut retention arrangement can be comprised of at least two retention nuts that differ from one another in diameter. The different diameters of the at least two retention nuts are effective to configure a radially outward periphery of a respective rotor shaft with a multi-stepped configuration.

[0007] The multi-stepped configuration at the radially outward periphery of the respective rotor shaft defines a plurality of axially extending segments in the respective rotor shaft, each of the axially extending segments differing in diameter relative to one another.

[0008] Other aspects of the disclosed embodiments relate to a method for arranging a rotor structure of a compressor. The rotor structure includes a connecting bolt and two rotor shafts that are respectively attached to respective ends of the connecting bolt. A plurality of impeller bodies are supported by the connecting bolt. A plurality of face gear couplings are used to mechanically couple the plurality of impeller bodies to one another along a rotor axis. A first impeller body of the plurality of impeller bodies is arranged to provide a first compression stage, and each subsequent impeller body provides a subsequent compression stage. The method allows the first impeller body of the plurality of impeller bodies to be arranged to provide the first compression stage, and also allows each subsequent impeller body to be arranged to provide the subsequent compression stage. Each respective impeller body defines a respective Di / D2 ratio. The Di / D2 ratio of at least one of the impeller bodies differs from the Di / D2 ratio of the remaining impeller bodies. Based on the different Di / D2 ratios, a respective surface defined by an inlet of the impeller body is positioned at a different distance relative to the rotor axis than a respective surface defined by a respective inlet of the remaining impeller bodies. Di represents a respective inner diameter of a flow path into the respective impeller body inlet, and D2 represents a respective outer diameter of the respective impeller body.

[0009] In certain embodiments, the method allows for setting a variation of the rotor structure along the rotor axis based on a variation of the respective Di / D2 ratio of one or more of the plurality of impeller bodies. The variation of the rotor structure along the rotor axis can include positioning a respective surface defined by a respective inlet of the one or more impeller bodies at a varying distance relative to the rotor axis. The positioning of the respective surface defined by the respective inlet of the one or more impeller bodies at the varying distance relative to the rotor axis is set to reduce an inlet Mach number in a compression stage by the one or more impeller bodies, and to adjust rotor stiffness along the rotor axis. BRIEF DESCRIPTION OF DRAWINGS

[0010] Figure 1A partial cross-sectional view of one non-limiting embodiment of a disclosed rotor structure that can be used in industrial applications involving turbomachinery, such as but not limited to centrifugal compressors, is illustrated.

[0011] Figure 2 A flow chart of a disclosed method that includes certain non-limiting steps for arranging a rotor structure of a compressor is illustrated.

[0012] Figure 3 A flow chart of one non-limiting example of a series of steps is illustrated.

[0013] Figure 4 An enlarged cross-sectional view of a portion of an impeller body that can be used to illustrate and describe certain non-limiting structural and / or operational relationships implemented in a disclosed rotor structure is illustrated.

[0014] Figure 5 A partial cross-sectional view of another non-limiting example of a disclosed rotor structure is illustrated.

[0015] Figure 6 An enlarged cross-sectional view of a midspan section of a disclosed rotor structure is illustrated.

[0016] Figure 7 Another enlarged exploded view is illustrated that shows a cross-sectional view of a non-limiting embodiment of a spring biasing mechanism, such as a tolerance ring, that can be arranged to adjust the radial stiffness at the midspan section of a connecting bolt.

[0017] Figure 8 A view of a tolerance ring around a rotor axis of a rotor structure is illustrated.

[0018] Figure 9 An enlarged cross-sectional view of one end of a connecting bolt supported by a rotor shaft is illustrated, and wherein two or more retaining nuts of different diameters can be arranged for implementing a radially outward periphery having a multi-step configuration in the rotor shaft.

[0019] Figure 10 A plot of non-limiting example values of a Di / D2 ratio as a function of compressor stage in one example application of a compressor process as a compressor stage is illustrated.

[0020] Figure 11 A plot of non-limiting example values of a Di / D2 ratio as a function of compressor stage in another example application of another compressor process is illustrated. DETAILED DESCRIPTION

[0021] As understood by those skilled in the art, turbomachinery, such as centrifugal compressors, can include rotors having a connecting bolt configuration (also referred to in the art as a through bolt or tie rod configuration), where the connecting bolts support a plurality of impeller bodies, and where adjacent impeller bodies can be connected to one another by elastic mean coupling techniques, such as including end face tooth couplings or flex couplings. These coupling types use different forms of face gear teeth (straight and curved, respectively) to create a solid coupling between two components. These couplings and associated structural members can be subjected to widely varying forces (e.g., centrifugal forces), such as from an initial rotor speed of zero revolutions per minute (RPM) to a maximum rotor speed (e.g., potentially involving tens of thousands of RPM).

[0022] The present inventors have recognized that achieving high performance and reliable operation in a centrifugal compressor can involve properly coordinating or otherwise balancing the interaction of potentially conflicting design criteria, such as can involve rotor dynamics and aerodynamics. Accordingly, the disclosed embodiments benefit from a comprehensive approach that advantageously coordinates potentially conflicting design considerations, such as involving the location of flow passages (i.e., aerodynamics) and rotor stiffness (i.e., rotor dynamics) in a centrifugal compressor.

[0023] The present inventors have further recognized that a compressor design that properly reduces the relative Mach number at the inlet of a given impeller can effectively achieve desired efficiency within the useful flow range of the compressor (e.g., satisfactory aerodynamic performance from minimum fluid flow to maximum fluid flow). Such a low Mach number design can include a reduced Di / D2 ratio, where Di represents the respective inner diameter of the flow path entering the respective impeller inlet, and D2 represents the respective outer diameter of the respective impeller. A reduced Di / D2 ratio allows the inlet region of the impeller to be positioned at a shorter distance relative to the rotor axis, and this is beneficial from an aerodynamic perspective. However, such a low Mach number design can result in reduced rotor stiffness, such as at least partially reduced rotor stiffness, due to the progressively thinner structure associated with the reduced size of Di.

[0024] The disclosed embodiments reliably and economically coordinate aerodynamics and rotor dynamics by allowing sufficiently low inlet relative Mach numbers while simultaneously maintaining sufficiently high rotor stiffness. In view of the challenging aerodynamic requirements typically encountered at the first compression stage, a reduced Di / D2 ratio substantially allows the air flow path to "sink" down onto the rotor, which can be particularly beneficial at the first compression stage.

[0025] The disclosed embodiments can additionally provide for respective varying Di / D2ratios along the rotor axis for each compression stage disposed downstream of the first compression stage. These respective varying Di / D2ratios can be adjusted to harmonize aerodynamics and rotor dynamics in an integrated and cohesive manner at each of such stages. That is, the designer can flexibly make appropriate tradeoffs in the disclosed embodiments to satisfy aerodynamic and rotor dynamic requirements using a balanced approach.

[0026] In the following detailed description, various specific details are set forth in order to provide a thorough understanding of the disclosed embodiments. However, persons of ordinary skill in the art will understand that the disclosed embodiments can be practiced without these specific details, that the aspects of the present disclosure are not limited to the disclosed embodiments, and that the aspects of the present disclosure can be practiced in a variety of alternative embodiments. In other instances, methods, procedures and components that are well known to persons of ordinary skill in the art have not been described in detail in order to avoid unnecessarily obscuring aspects of the present disclosure.

[0027] Furthermore, various operations can be described as multiple discrete steps performed in turn in a manner that facilitates comprehension. Unless otherwise indicated, however, the order of description should not be construed as to imply that these operations are necessarily order dependent. Further, the phrase "in one embodiment" does not necessarily refer to the same embodiment, although it can. Finally, the various embodiments of the disclosed application need not necessarily be mutually exclusive, as some embodiments can be combined with one another's various features.

[0028] Figure 1 A partial cross-sectional view of one non-limiting embodiment of a disclosed rotor structure 100 that can be used in industrial applications involving turbomachinery, such as but not limited to compressors (e.g., centrifugal compressors, etc.).

[0029] In one disclosed embodiment, the connecting bolt 102 extends along the rotor axis 103 between a first end and a second end of the connecting bolt 102. A first rotor shaft 1041 can be fixed to the first end of the connecting bolt 102. A second rotor shaft 1042 can be fixed to the second end of the connecting bolt 102. The rotor shafts 1041, 1042 can be referred to in the art as stub shafts. A plurality of impeller bodies 106, such as impeller bodies 1061-106n, can be disposed between the rotor shafts 1041, 1042. The impeller bodies 106 can be fixed to the connecting bolt 102. The connecting bolt 102 can be fixed to the rotor shafts 1041, 1042. The connecting bolt 102 can be fixed to the impeller bodies 106. nIn the illustrated embodiment, the number of impeller bodies is six, and therefore n = 6; it will be understood that this is merely an example and should not be construed as a limitation on the number of impeller bodies that may be used in the disclosed embodiments. Figure 1 The embodiments illustrated herein relate to a centrally suspended configuration with back-to-back impeller stages; it will be understood that this is merely an example configuration and should not be construed as limiting the applicability of the disclosed embodiments.

[0030] Multiple impeller bodies 106 are supported by connecting bolts 102 and by multiple end-face toothed connectors, such as end-face toothed connectors 1081 to 108. n-1 They are mechanically connected to each other along the rotor axis. In the illustrated embodiment, since the number of impeller bodies is six as described above, the number of end-face toothed couplings is five. It will be understood that two additional end-face toothed couplings 1091 and 1092 can be used to connect the impeller bodies 106, which are respectively close to the first and second ends of the connecting bolt 102. n 1061 is mechanically connected to rotor shafts 1041 and 1042.

[0031] Figure 2 The illustration shows a flowchart of a method for arranging rotor structural components of a compressor. Step 121 allows the arrangement of a first impeller body (e.g., impeller body 1061) among a plurality of impeller bodies. Figure 1 The impeller body is arranged to provide a first compression stage. Step 122 allows each subsequent impeller body to be arranged to provide a subsequent compression stage.

[0032] As shown in block 123, each corresponding impeller body defines a corresponding Di / D2 ratio. As shown in block 124, the Di / D2 ratio of at least one impeller body differs from the Di / D2 ratios of the remaining impeller bodies. As shown in block 125, based on the different Di / D2 ratios, the corresponding surface defined by the inlet of at least one impeller body can be positioned at a different distance relative to the rotor axis compared to the position of the corresponding surface defined by the corresponding inlets of the remaining impeller bodies. Figure 4 It is understandable in (and in Figure 2 (further shown in box 126), Di represents the corresponding inner diameter of the flow path into the inlet 110 of the corresponding impeller, and D2 represents the corresponding outer diameter of the corresponding impeller.

[0033] A reduced Di / D2 ratio allows the impeller inlet region to be positioned at a shorter distance relative to the rotor axis. Do represents the outer diameter of the flow path into the inlet 110 of the corresponding impeller body 106. It will be understood that adjustment of Di—positioning the inlet region at the desired location—can lead to adjustment of Do.

[0034] Figure 3 The disclosed method allows improving rotor dynamics in a rotor structure without reducing the available aerodynamic range of the compressor. Step 130 allows arranging a first impeller body (e.g., impeller body 1061) of a plurality of impeller bodies to provide a first compression stage. Step 132 allows selecting a Di / D2 ratio of the first impeller body, wherein the selected Di / D2 ratio is set to reduce a relative Mach number at an inlet of the first impeller body 1061. It will be appreciated that this is effective for performing a challenging first compression stage within the available aerodynamic range of the compressor. Figure 1

[0035] Returning to Figure 3 , step 134 allows selectively changing a respective Di / D2 ratio of one or more of the impeller bodies, such as changing the respective Di / D2 ratio of impeller bodies 1062-106 n ( Figure 1 ) positioned downstream of the first impeller body 1061 along the rotor axis. That is, the Di / D2 ratio of only one impeller body can be flexibly changed, for example, or the respective Di / D2 ratio of multiple impeller bodies can be changed, such as can include the Di / D2 ratio of each of the impeller bodies disposed between the rotor shafts 1041, 1042, for example.

[0036] Based on the selected respective Di / D2 ratio for one or more of the impeller bodies, step 136 allows varying the rotor structure along the rotor axis to improve rotor dynamics while simultaneously satisfying respective varying aerodynamic requirements at the respective compression stages by one or more of the impeller bodies. As shown in block 138, varying the rotor structure along the rotor axis while simultaneously satisfying respective varying aerodynamic requirements at the respective compression stages by one or more of the impeller bodies effectively coordinates the compressor aerodynamics and rotor dynamics of the rotor structure.

[0037] In one non-limiting embodiment, the respective Di / D2 ratio can range from a value of 0.2 (or approximately 0.2) to a value of 0.65 (or approximately 0.65). In another non-limiting embodiment, the respective Di / D2 ratio can range from a value of 0.25 (or approximately 0.25) to a value of 0.5 (or approximately 0.50). That is, the respective Di / D2 ratio defined by the respective impeller body can take any value within the foregoing ranges.

[0038] Figure 5 ​A partial cross-sectional view illustrating another non-limiting example of the disclosed rotor structure 100’ can be used to visually conceptualize variations in the respective ratios Di / D2 of the impeller bodies associated with the rotor structure 100’. For example, this allows for variations in the rotor structure along the rotor axis (e.g., stiffening the rotor structure, as schematically represented by the arrows labeled R1 through Rn), and in turn, allows for improved rotor dynamics while at the same time satisfactorily meeting respective varying aerodynamic requirements at the respective compression stages through the impeller bodies.

[0039] In one non-limiting implementation, varying the rotor structure along the rotor axis can include positioning respective surfaces defined by respective inlets of one or more of the impeller bodies at selectively varying distances relative to the rotor axis based on the respective Di / D2 ratios selected for the one or more of the impeller bodies. The foregoing allows for improved rotor dynamics while at the same time satisfactorily meeting respective varying aerodynamic requirements at the respective compression stages through the impeller bodies.

[0040] Figure 6 An enlarged cross-sectional view is illustrated including the midspan section 120 of the connecting bolt 102 in the disclosed rotor structure. That is, the midspan section of the connecting bolt is positioned substantially equidistant from respective opposite axial ends of the connecting bolt 102. As better understood in the further enlarged exploded view illustrated in Figure 7 Non-limitingly, a tolerance ring 154 can be provided at the midspan section of the connecting bolt 102. Such a structural feature allows one to adjust the radial stiffness at the midspan section of the connecting bolt 102, which in turn effectively shifts the natural frequency of the connecting bolt out of the rotational speed range of the rotor.

[0041] It can be shown that the natural vibration frequency of a rotating body is determined by the square root of the stiffness-to-mass ratio of that body. Thus, the increased radial stiffness provided by the tolerance ring 154 effectively reduces the likelihood that the natural vibration frequency in the disclosed rotor structure will fall within the rotational speed range of the rotor, which as will be appreciated by those skilled in the art, is beneficial for the rotor dynamics of the rotor structure.

[0042] In one non-limiting implementation, a groove 152 can be defined at a radially inner surface of the impeller body 1063 (i.e., the impeller body at which the midspan section of the connecting bolt is provided) to accommodate the wave or corrugated features in the tolerance ring 154. As better understood in the further enlarged cross-sectional view illustrated in Figure 8Each corrugation 155 on the tolerance ring 154 ("wave" or "bump") effectively acts as a rigid radial spring, and these circumferentially disposed corrugations collectively provide the desired radial stiffness at the midspan section of the connecting bolt 102, as can be better understood. It will be appreciated that the tolerance ring 154 as shown in the drawings should be understood as a non-limiting example of any one of a variety of forms of spring biasing mechanisms that can alternatively be used to adjust the radial stiffness at the midspan section of the connecting bolt 102.

[0043] It will also be appreciated that regardless of form, the spring biasing mechanism need not be limited to a single spring biasing mechanism disposed at the midspan section of the connecting bolt 102, as multiple spring biasing mechanisms can effectively be used to provide radial stiffness at multiple locations of the connecting bolt 102. For example, in certain alternative embodiments, non-limitingly, two spring biasing mechanisms (e.g., two tolerance rings 154) can each be disposed at approximately 1 / 3 of the connecting bolt length. It will thus be appreciated that the above illustrated arrangement should be understood as one non-limiting example for adjusting the radial stiffness at one or more locations of the connecting bolt 102.

[0044] Other non-limiting examples of forms of spring biasing mechanisms that can be used can include wave springs, C-shaped springs, segmented O-rings, spring- energized segmented O-rings, leaf springs, etc. It will be appreciated that any of such spring biasing mechanisms can be constructed of an open or gapped structure, e.g., that can allow fluid communication between adjacent chambers (e.g., interior chambers sharing a boundary with the tolerance ring 154), and this reduces the likelihood of pressure differentials being created between such chambers, which can occur if a grommet-type element, such as a solid O-ring, is used in place of the open structure. Non-limitingly, depending on the mechanical design of the rotor structure and the spring biasing mechanism, in certain embodiments, a pressure balance venting path can be provided around the spring biasing mechanism.

[0045] Figure 9 An enlarged cross-sectional view of the second end of the connecting bolt 102 supported by the rotor shaft 1042 is illustrated. In one non-limiting embodiment, a multi-nut retention arrangement can be used that is effective for achieving a radially outward periphery having a multi-stepped configuration in the rotor shaft 1042. Non-limitingly, such a multi-nut retention arrangement can include a primary nut 160 that provides a threaded connection relative to the connecting bolt 102 and that includes an axial face that abuts a corresponding axial face of the first impeller body 1061 and that effectively retains the stack of impeller bodies at this end of the connecting bolt 102.

[0046] Without limitation, the multiple-nut retaining arrangement can further comprise a second nut 162 having a diameter smaller than the diameter of the main nut 160. Without limitation, the second nut 162 can provide a further threaded connection with respect to the connecting bolt 102 and comprise an axial face against a corresponding axial face of the rotor shaft 1042 (e.g. at the proximal end 164) and actually retain the distal end 166 of the rotor shaft 1042 (opposite to the proximal end 164) against the first impeller body 1061.

[0047] As schematically represented by the double-headed arrows marked with S1 to S5, the multiple-nut retaining arrangement comprising different diameter sizes (e.g. comprising at least two nuts) can effectively configure the rotor shaft 1042 so that the radially outward periphery of the rotor shaft 1042 has a multi-step configuration along the rotor axis 103. This allows reducing the respective diameter of the plurality of axially extending sections in the rotor shaft 1042 (to avoid visual confusion, in the following only two of such sections are schematically represented by the double-headed arrows marked with the alphanumeric codes AS3 and AS4). Figure 9

[0048] The above arrangement in turn allows reducing the diameter of the journal bearing, the thrust bearing and the gas seal (e.g. part of a dry gas seal system) corresponding to the axially extending sections in the rotor shaft 1042. Such diameter reduction is effective to obtain a respective reduction of the sliding speed between the moving parts in the journal bearing, the thrust bearing and the gas seal, which is a feature contributing to the excellent durability and reliability of the aforementioned components.

[0049] Figure 10 is a plot of non-limiting example values of the Di / D2 ratio as a function of the compressor stage in one example application of the compressor process. In this example application, the compressor process involves a given mass flow rate, wherein the volume flow rate decreases as the process fluid is compressed as the compression stages progress downstream with respect to the first compression stage. In this application, the Di / D2 value will typically increase as the compression stages progress downstream with respect to the first compression stage.

[0050] Figure 11 is a plot of non-limiting example values of the Di / D2 ratio as a function of the compressor stage in another example application of the compressor process, wherein there is a "side stream input", i.e. an additional volume flow rate is injected into the compressor, such as at or near the middle of the rotor; let us assume before stage No. 3. In this application, the Di / D2 value will increase before the injection of the additional volume flow rate, as above in Figure 10 ​In the event that an additional volume flow is injected, the Di / D2 ratio will be adjusted (i.e., decreased) at stage 3 to account for the additional volume flow injected prior to stage 3, and then the Di / D2 value will generally increase as described above for stages downstream of stage 3.

[0051] In operation, the disclosed embodiments can exploit structural and / or operational relationships designed to reconcile potentially conflicting design considerations, such as those involving flow passages (i.e., aerodynamics) and rotor stiffness (i.e., rotor dynamics) in a centrifugal compressor, for example, by adjusting respective Di / D2 ratios for the impellers. Moreover, in operation, the disclosed embodiments can provide respective varying Di / D2 ratios in a given rotor structure that are tailored to reconcile aerodynamics and rotor dynamics at each of the compression stages in a comprehensive and cohesive manner.

[0052] In operation, the disclosed embodiments can exploit one or more spring bias mechanisms arranged to adjust radial stiffness at respective locations of the connecting bolts, which is a feature that effectively reduces the likelihood that inherent vibration frequencies in the disclosed rotor structure will fall within a rotor rotational speed range.

[0053] In operation, the disclosed embodiments can exploit a multi-nut retention arrangement for achieving a radially outward periphery having a multi-step configuration in the rotor shaft. This feature allows for a respective reduction in diameter of the plurality of axially extending segments in the rotor shaft, and in turn allows for a respective reduction in diameter of the journal bearing, thrust bearing, and gas seal corresponding to the axially extending segments in the rotor shaft. Without limitation, such a reduction in diameter is effective to obtain a respective reduction in sliding velocity between the moving parts in the journal bearing, thrust bearing, and gas seal.

[0054] While embodiments of the present disclosure have been disclosed in exemplary form, it will be apparent to those skilled in the art that many modifications, additions, and deletions, can be made to the disclosed embodiments, without departing from the scope of the present disclosure and its equivalents, as set forth in the following claims.

Claims

1. A rotor structure in a compressor, the rotor structure comprising: a connecting bolt and two rotor shafts attached to respective ends of the connecting bolt; a plurality of impeller bodies supported by the connecting bolt; and a plurality of face gear couplings for mechanically coupling the plurality of impeller bodies to each other along a rotor axis, wherein a first impeller body of the plurality of impeller bodies is arranged to provide a first compression stage, and each subsequent impeller body provides a subsequent compression stage, wherein each respective impeller body defines a respective Di / D2 ratio, wherein the Di / D2 ratio of at least one of the impeller bodies is different from the Di / D2 ratio of the remaining impeller bodies, wherein based on the different Di / D2 ratio, in which a relatively smaller Di / D2 ratio allows the inlet region of the respective impeller body to be positioned at a shorter distance relative to the rotor axis, a respective surface defined by the inlet of the at least one of the impeller bodies is positioned at a different distance relative to the rotor axis than a respective surface defined by a respective inlet of the remaining impeller bodies, wherein Di represents a respective inner diameter of a flow path into an inlet of the respective impeller body, and wherein D2 represents a respective outer diameter of the respective impeller body. A respective range of Di / D2 is from a value of 0.2 to a value of 0.

65.

2. The rotor structural member of claim 1, wherein, A respective range of Di / D2 is from a value of 0.25 to a value of 0.

50.

3. The rotor structural member of claim 2, wherein, The variation of the rotor structure along the rotor axis is based on a variation of the respective Di / D2 ratio, wherein the variation of the rotor structure along the rotor axis includes positioning the respective surface defined by the respective inlet of the at least one of the impeller bodies at a varying distance relative to the rotor axis.

4. The rotor structural member of claim 1, wherein, The positioning of the respective surface defined by the respective inlet of the at least one of the impeller bodies at a varying distance relative to the rotor axis is configured to reduce an inlet Mach number in a compression stage by the at least one of the impeller bodies, and to adjust rotor stiffness along the rotor axis.

5. The rotor structural member of claim 4, wherein, 6. The rotor structure of claim 1, further comprising at least one spring biasing mechanism arranged for adjusting a radial stiffness at a respective location of the connecting bolt. The respective location at which the at least one spring biasing mechanism is arranged is at or proximate to a midspan section of the connecting bolt.

7. The rotor structural member of claim 6, wherein, The at least one spring biasing mechanism is selected from a group comprising: a tolerance ring, a wave spring, an O-ring, a segmented O-ring, a spring energized O-ring, a C-shaped spring, and a leaf spring.

8. The rotor structural member of claim 6, wherein, The multiple nut retention arrangement includes at least two retention nuts having different diameters from each other, the different diameters of the at least two retention nuts being effective for configuring a radially outward periphery having a multi-step configuration in a respective one of the two rotor shafts.

9. The rotor structural member of claim 1, further comprising a multi-nut retention arrangement, wherein, ​ 10. The rotor structural member of claim 9, wherein, The multi-step configuration at the radially outward periphery of the respective rotor shaft defines a plurality of axially extending sections in the respective rotor shaft, each of the axially extending sections differing in diameter relative to one another.

11. A centrifugal compressor comprising a rotor structure according to any one of claims 1 to 10.

12. A method for arranging a rotor structure of a compressor, wherein, The rotor structure comprises a connecting bolt and two rotor shafts attached to respective ends of the connecting bolt, and a plurality of impeller bodies supported by the connecting bolt, the plurality of impeller bodies being mechanically coupled to one another along a rotor axis by a plurality of face gear couplings, wherein the method comprises: arranging a first impeller body of the plurality of impeller bodies to provide a first compression stage, arranging each subsequent impeller body to provide a subsequent compression stage, wherein each respective impeller body defines a respective Di / D2 ratio, wherein the Di / D2 ratio of at least one of the impeller bodies differs from the Di / D2 ratio of the remaining impeller bodies, wherein based on the different Di / D2 ratios, a respective surface defined by an inlet of the at least one of the impeller bodies is positioned at a different distance relative to the rotor axis than a surface defined by a respective inlet of the remaining impeller bodies, in the different Di / D2 ratios, a relatively smaller Di / D2 ratio allows the inlet region of the respective impeller body to be positioned at a shorter distance relative to the rotor axis, wherein Di represents a respective inner diameter of a flow path entering a respective impeller body inlet, and wherein D2 represents a respective outer diameter of the respective impeller body.

13. The method of claim 12, further comprising a change in the rotor structural member along the rotor axis based on a change in the respective Di / D2 ratio, wherein, The variation of the rotor structure along the rotor axis comprises positioning a respective surface defined by a respective inlet of the at least one of the impeller bodies at a varying distance relative to the rotor axis.

14. The method of claim 13, wherein, The positioning of the respective surface defined by a respective inlet of the at least one of the impeller bodies is arranged to reduce an inlet Mach number in a compression stage by the at least one of the impeller bodies, and to adjust rotor stiffness along the rotor axis.

15. The method of claim 12, wherein, The respective range of Di / D2 is from a value of 0.2 to a value of 0.

65.

16. The method of claim 15, wherein, The respective range of Di / D2 is from a value of 0.25 to a value of 0.50.

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