Patent Publication Number: US-11022125-B2

Title: Centrifugal compressor

Description:
TECHNICAL FIELD 
     The present invention relates to a centrifugal compressor that uses impellers to compress a fluid. 
     BACKGROUND ART 
     A centrifugal compressor used in an industrial process and a process plant causes fluid such as air and gas to flow through rotating impellers in a radial direction, and uses centrifugal force generated at that time to compress the fluid. The centrifugal compressor includes a casing and a rotor accommodated inside the casing as a basic configuration. The rotor includes a shaft rotatably provided in the casing and a plurality of impellers fixed to an outer peripheral surface of the shaft. 
     The centrifugal compressor is classified into a single-stage centrifugal compressor with a single impeller and a multistage centrifugal compressor in which a plurality of impellers are arranged in series in a rotation axis direction. The latter multistage centrifugal compressor is heavily used. 
     As an object to be compressed by the centrifugal compressor, boil off gas (BOG) is well-known as described in, for example, Patent Literature 1. For example, LNG (Liquefied Natural Gas) boil off gas is extremely low-temperature fluid. In the centrifugal compressor, in particular at the beginning of operation, a periphery of a gas suction passage is exposed to extremely low temperature, whereas an outer peripheral surface of the centrifugal compressor is exposed to atmospheric temperature, which causes large temperature difference. As a result, thermal stress associated with contraction of components occurs on the periphery of the suction passage. To reduce the temperature difference between the inside and the outside of the centrifugal compressor, Patent Literature 1 proposes to heat the periphery of the suction passage by oil as a heating medium. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: JP 2013-513064 W 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     To reduce the temperature difference between the inside and the outside of the centrifugal compressor by heating with oil, a large amount of oil is necessary, and cost increase by incidental facilities and equipment for heating with oil is not ignorable. 
     On the other hand, a casing forming an outer shell of the centrifugal compressor and internal components provided inside the casing are different in thermal responsiveness from each other based on difference of thermal capacity. Accordingly, it is necessary to consider difference in thermal deformation (or thermal expansion) between a period from activation to normal operation and a period from the normal operation to stoppage of the centrifugal compressor. 
     Accordingly, an object of the present invention is to provide a centrifugal compressor that makes it possible to reduce thermal contraction on the periphery of the gas suction passage at the beginning of the operation by a heating medium at low flow rate and to cope with thermal deformation occurred during the operation as well. 
     Solution to Problem 
     A centrifugal compressor according to the present invention includes a rotor that includes a shaft rotatably disposed inside a casing and impellers fixed to an outer periphery of the shaft, a diaphragm that surrounds the impellers from an outer peripheral side, a suction-side casing head disposed separately from the diaphragm on a side on which fluid is sucked, a temperature controlling mechanism that is provided inside the suction-side casing head and is configured to control ambient temperature through flow of a heating medium, a heat insulating body disposed between the suction-side casing head and the diaphragm, and a locking structure that locks the heat insulating body and the suction-side casing head with each other to be relatively displaceable in a radial direction. 
     The locking structure preferably includes a locking protrusion provided on one of the suction-side casing head and the heat insulating body, and a locking groove that is provided on another of the suction-side casing head and the heat insulating body and into which the locking protrusion is inserted. 
     The locking protrusion is preferably provided integrally with or separated from one of the suction-side casing head and the heat insulating body. 
     The locking protrusion preferably slides inside the locking groove when the heat insulating body and the suction-side casing head are relatively displaced in the radial direction. 
     A plurality of the locking structures are preferably radially provided on a surface of the suction-side casing head and a surface of the heat insulating body that face each other. 
     In particular, the plurality of locking structures are preferably provided at equal intervals in a circumferential direction. 
     Advantageous Effects of Invention 
     The centrifugal compressor according to the present invention includes the heat insulating body sectioning the suction passage. Therefore, it is possible to reduce thermal contraction on the periphery of the gas suction passage at the beginning of the operation. In addition, in the centrifugal compressor according to the present invention, the heat insulating body and the suction-side casing head are relatively displaceable in the radial direction. This makes it possible to cope with difference of thermal deformation between the suction-side casing head and the heat insulating body through the operation process from activation to normal operation, and further to stoppage. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross-sectional view illustrating a schematic configuration of a centrifugal compressor according to a first embodiment of the present invention. 
         FIG. 2  is a cross-sectional view illustrating a periphery of a suction passage of the centrifugal compressor of  FIG. 1 . 
         FIG. 3  is a plan view illustrating a heat insulating body of the centrifugal compressor according to the first embodiment. 
         FIG. 4A  and  FIG. 4B  each illustrate a state of interference between the heat insulating body and a suction-side casing head of the centrifugal compressor of  FIG. 1 ,  FIG. 4A  illustrating a state at activation, and  FIG. 4B  illustrating a state during operation. 
         FIG. 5  illustrates a rectification blade provided on an end surface of a diaphragm of the centrifugal compressor of  FIG. 1  as viewed from upstream side. 
         FIG. 6A  and  FIG. 6B  each illustrate a state of interference between the heat insulating body and the rectification blade of the centrifugal compressor of  FIG. 1 ,  FIG. 6A  illustrating deformation at activation and also illustrating deep interference between the heat insulating body and the rectification blade, and  FIG. 6B  illustrating deformation at stoppage and also illustrating shallow interference between the heat insulating body and the rectification blade. 
         FIG. 7A ,  FIG. 7B  and  FIG. 7C  each illustrate a modification of the first embodiment,  FIG. 7A  illustrating a configuration of the modification,  FIG. 7B  illustrating a state at activation, and  FIG. 7C  illustrating a state during operation. 
         FIG. 8  is a cross-sectional view illustrating a periphery of a suction passage of a centrifugal compressor according to a second embodiment. 
         FIG. 9  illustrates still another embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
     Some embodiments of the present invention are described below with reference to accompanying drawings. 
     In the present embodiment, a multistage centrifugal compressor including a plurality of impellers is described as an example of a centrifugal compressor. 
     A centrifugal compressor  1  according to the present embodiment is used to compress extremely low-temperature LNG boil off gas (fluid F). 
     As illustrated in  FIG. 1 , the centrifugal compressor  1  includes a casing  2  forming an outer shell, and a rotor  7  that is rotatably supported inside the casing  2 . The rotor  7  includes a shaft  71  extending along an axis C, and a plurality of impellers  72  fixed to an outer peripheral surface of the shaft  71 . The centrifugal compressor  1  includes an oil heater  8  and a heat insulating body  6  in the casing  2 . The oil heater  8  reduces temperature difference between an inside and an outside of a suction-side casing head  4 , and the heat insulating body  6  suppresses heat transfer between the suction-side casing head  4  and a suction passage  18 , in particular at the beginning of the operation. 
     The centrifugal compressor  1  according to the present embodiment can reduce a flow rate of a heating medium HM to be supplied to the oil heater  8 , and can cope with thermal contraction on the periphery of the suction passage for the fluid F at the beginning of the operation and thermal deformation occurred during the operation, by the heat insulating body  6 . 
     Components of the centrifugal compressor  1  are described below. 
     Note that, in the centrifugal compressor  1 , a direction in which the axis C of the shaft  71  extends is referred to as an axis direction, and a direction orthogonal to the axis C is referred to as a radial direction. In addition, in the centrifugal compressor  1 , upstream side U and downstream side L are defined based on a direction in which the fluid F to be compressed flows, as illustrated in  FIG. 1 . Note that the upstream side U and the downstream side L are defined relative to each other. 
     [Casing  2 ] 
     As illustrated in  FIG. 1 , a diaphragm  3  that surrounds the impellers  72  from an outer peripheral side, the suction-side casing head  4  that is disposed on the most upstream side U in the axis direction so as to be separated from the diaphragm  3 , the heat insulating body  6  that is held by the suction-side casing head  4 , and a discharge-side casing head  5  that is disposed on the most downstream side L in the axis direction so as to be separated from the diaphragm  3  are provided inside the casing  2 . 
     The diaphragm  3  according to the present embodiment includes, for example, a configuration in which a plurality of diaphragm pieces  31  are arranged in the axis direction. The diaphragm  3  includes a plurality of rectification blades  33  so as to rectify the flow of the fluid F sucked from the suction passage  18  to cause the fluid F to flow toward the downstream side L, as illustrated in  FIG. 5 . 
     Further, as illustrated in  FIG. 1 , a suction scroll  25  that sucks the fluid F and a discharge scroll  29  that discharges the fluid F are provided inside the casing  2 . 
     [Suction-Side Casing Head  4 ] 
     As illustrated in  FIG. 2 , a head end surface  41  of the suction-side casing head  4  directed to the downstream side L is an annular surface extending along a circumferential direction. The head end surface  41  includes a first plane part  42 , a first slope part  43 , a second plane part  44 , and a second slope part  45 . The first plane part  42  is a surface that is located outside in the radial direction and is orthogonal to the axis C. The first slope part  43  is located inside the first plane part  42  in the radial direction and has a conical shape in which a diameter is reduced toward the downstream side L. The second plane part  44  is a surface that is located inside the first slope part  43  in the radial direction and is orthogonal to the axis C. The second slope part  45  is located inside the second plane part  44  in the radial direction and has a conical shape in which a diameter is reduced toward the downstream side L. 
     As illustrated in  FIG. 1  and  FIG. 2 , the suction-side casing head  4  includes key grooves  46  into which respective keys  67  of the first plane part  42  are inserted, at positions facing the respective keys  67 . Combination of one key  67  and one key groove  46  corresponds to a locking structure of the present invention. 
     A dimension in the radial direction of each of the key grooves  46  is larger than a difference of thermal expansion between the suction-side casing head  4  and the heat insulating body  6 . 
     In addition, a dimension in the circumferential direction of each of the key grooves  46  is a size that enables the corresponding key  67  to be inserted without any gap. 
     A dimension (depth) in the axis direction of each of the key grooves  46  is equal to or larger than a dimension (height) in the same direction of the corresponding key  67 . Accordingly, even when the keys  67  are inserted into innermost parts of the respective key grooves  46 , a holding part  61  can come into contact with the first plane part  42 . 
     As illustrated in  FIG. 3 , the suction-side casing head  4  according to the present embodiment includes four key grooves  46 A,  46 B,  46 C, and  46 D that are arranged with equal intervals while being shifted in phase by 90 degrees in the circumferential direction. The four key grooves  46 A to  46 D are concentrically provided. 
     Note that, in a case where it is unnecessary to distinguish the key grooves  46 A to  46 D from one another, the key grooves  46 A to  46 D are simply referred to as the key grooves  46 . This is true of keys  67 A to  67 D described later. 
     As illustrated in  FIG. 1 , a dry gas seal  16  is provided inside the suction-side casing head  4  in the radial direction. The dry gas seal  16  is provided farther on the downstream side L than a first journal bearing  13 . The dry gas seal  16  is a sealing device that ejects gas such as dry gas to airtightly seal surroundings of the shaft  71 . In addition, a seal fin  17  that includes a plurality of fins is provided farther on the downstream side L than the dry gas seal  16 . 
     Note that the sealing device is not limited to the dry gas seal  16 , and a sealing device that can seal a gap between the suction-side casing head  4  and the shaft  71  can be appropriately adopted. For example, a labyrinth seal may be disposed as the sealing device between the suction-side casing head  4  and the shaft  71 . 
     If large temperature difference is sharply generated between the inside and the outside of the suction-side casing head  4  and the suction-side casing head  4  is thermally contracted at the beginning of the operation, the sealed state by the sealing device may be deteriorated. Accordingly, in the present embodiment, the oil heater  8  described later is provided and the heat insulating body  6  is also provided to prevent the large temperature difference from being generated at the beginning of the operation. 
     As illustrated in  FIG. 1 , the suction-side casing head  4  includes the oil heater  8  that is a temperature controlling mechanism heating the suction-side casing head  4 . The oil heater  8  is provided to control temperature inside/outside the centrifugal compressor  1 , in particular, to reduce the temperature difference at the beginning of the operation of the centrifugal compressor  1 . As illustrated in  FIG. 2 , the oil heater  8  includes a conduit  81  provided inside the suction-side casing head  4 , and an oil heater body  82  connected to the conduit  81 , and the heating medium HM flows to the oil heater body  82  through the conduit  81 . 
     The conduit  81  is connected to a supply source of the heating medium HM. The oil heater body  82  has an annular shape, and is provided so as to surround the shaft  71  as illustrated in  FIG. 2 . The oil heater body  82  includes a heating medium passage  83  through which the heating medium HM supplied through the conduit  81  circulates. As the heating medium HM, for example, the lubricant same as the lubricant supplied to the first journal bearing  13  and a second journal bearing  14  ( FIG. 1 ) can be supplied to the oil heater  8 . Changing the temperature of the heating medium HM makes it possible to change the heating temperature of the suction-side casing head  4 , or to cool the suction-side casing head  4  in some cases. 
     [Heat Insulating Body  6 ] 
     As illustrated in  FIG. 3 , the heat insulating body  6  is a plate member having an annular plane shape, and includes outer-diameter side and inner-diameter side. As illustrated in  FIG. 2  and  FIG. 3 , the heat insulating body  6  includes the holding part  61 , a first disc part  62 , a first conical part  63 , a second disc part  64 , and a second conical part  65 . The holding part  61  is located on the outer-diameter side. The first disc part  62  is provided on one side of the holding part  61  in the axis direction. The first conical part  63  is connected to the inner-diameter side relative to the first disc part  62 . The second disc part  64  is connected to the inside of the first conical part  63  in the radial direction. The second conical part  65  is connected to the inside of the second disc part  64  in the radial direction. 
     Respective main surfaces of the first disc part  62  and the second disc part  64  are orthogonal to the axis C. The first conical part  63  and the second conical part  65  each include a conical shape in which a diameter is reduced toward the downstream side L. 
     When the keys  67  are inserted into the respective key grooves  46 , the holding part  61  comes into contact with the first plane part  42  as illustrated in  FIG. 2 . The holding part  61  is an annular part extending in the circumferential direction. 
     The holding part  61  includes the keys  67  on the surface facing the suction-side casing head  4 . The keys  67  are provided so as to protrude from the holding part  61  toward the upstream side U. 
     As illustrated in  FIG. 3 , the heat insulating body  6  according to the present embodiment includes the four keys  67 A,  67 B,  67 C, and  67 D that are arranged with equal intervals while being shifted in phase by 90 degrees in the circumferential direction. The four keys  67 A to  67 D are concentrically provided. 
     When the keys  67 A to  67 D are respectively inserted into the key grooves  46 A to  46 D, the heat insulating body  6  is held by the suction-side casing head  4 . 
     As illustrated in  FIG. 2 , when the keys  67  are inserted into the respective key grooves  46 , the heat insulating body  6  is held by the first plane part  42  of the suction-side casing head  4  through the holding part  61  while being positioned in the circumferential direction. In this state, the heat insulating body  6  includes a cantilever structure held by the first plane part  42  only through the holding part  61 . In other words, an inner-diameter end of the heat insulating body  6  forms a free end FE, and a gap G is provided between the free end FE of the heat insulating body  6  and the outer peripheral surface of the shaft  71 . Since the inner-diameter side of the heat insulating body  6  forms the free end FE, the heat insulating body  6  is thermally expanded and thermally contracted in the radial direction without being especially restricted. 
     The keys  67  are displaced inside the respective key grooves  46  in the radial direction along with thermal expansion and thermal contraction in the radial direction of the heat insulating body  6 , or along with thermal expansion and thermal contraction in the radial direction of the suction-side casing head  4 . In other words, in the state where the keys  67  are inserted into the respective key grooves  46  as illustrated in  FIG. 4A , when the thermal expansion in the radial direction of the suction-side casing head  4  heated by the oil heater  8  is larger than that of the heat insulating body  6 , the keys  67  are relatively displaced toward the inner diameter inside the respective key grooves  46  as illustrated in  FIG. 4B  as a result of displacement of the key grooves  46  outward in the radial direction. 
     The thermal expansion in the radial direction of the suction-side casing head  4  heated by the oil heater  8  is larger than that of the heat insulating body  6 . Therefore, the keys  67  are inserted into the respective key grooves  46  so as to be relatively displaceable toward the inner diameter inside the respective key groove  46  during the operation of the centrifugal compressor  1 . 
     As illustrated in  FIG. 2 , an annular space functioning as a heat insulating space  11  is provided between the heat insulating body  6  and the head end surface  41  of the suction-side casing head  4 . 
     The heat insulating space  11  is filled with a heat insulating material  69  without any gap. The heat insulating material  69  makes the heat of the heat insulating body  6  difficult to be transferred to the suction-side casing head  4 . The heat insulating space  11 , however, is not necessarily filled with the heat insulating material  69 . 
     As illustrated in  FIG. 3 , the heat insulating body  6  includes interference maintaining grooves  66  at positions corresponding to a plurality of rectification blades  33  described later provided on the diaphragm  3 . The plurality of interference maintaining grooves  66  are provided at predetermined intervals in the circumferential direction on the second disc part  64  so as to penetrate through front and rear surfaces of the second disc part  64 . Opening areas of the respective interference maintaining grooves  66  are determined such that the rectification blades  33  are inserted into the respective interference maintaining grooves  66  with substantially no gap and are preferably slidable with receiving almost no load. 
     Note that the example in which the interference maintaining grooves  66  penetrate through the front and rear surfaces of the second disc part  64  is illustrated here; however, the interference maintaining grooves  66  do not necessarily penetrate through the front and rear surfaces of the heat insulating body  6  as long as interference between the heat insulating body  6  and the rectification blades  33  can be maintained. 
     [Rectification Blade  33 ] 
     The rectification blades  33  rectify the flow of the fluid F sucked from the suction passage  18  to cause the fluid F to flow toward the downstream side L. 
     As illustrated in  FIG. 2 , the rectification blades  33  are provided so as to protrude toward the upstream side U from an end surface  32  of the diaphragm  3  provided on the most upstream side U. 
     In the present embodiment, the plurality of rectification blades  33  are provided at predetermined intervals in the circumferential direction on the end surface  32  as illustrated in  FIG. 5 . Note that the rectification blades  33  may be formed integrally with the diaphragm  3  by, for example, machining, or may be fabricated separately from the diaphragm  3  and be joined to and fixed to the end surface  32  by an appropriate method. 
     As illustrated in  FIG. 2 , front ends of the rectification blades  33  are inserted into the respective interference maintaining grooves  66 . The relationship in which the front ends of the rectification blades  33  are inserted into the respective interference maintaining grooves  66  is constantly maintained irrespective of the operation state of the centrifugal compressor  1 . More specifically, lengths of the rectification blades  33  and depths of the interference maintaining grooves  66  are set such that the front ends of the rectification blades  33  remain in the respective interference maintaining grooves  66  of the heat insulating body  6  as illustrated in  FIG. 6B  even if the rectification blades  33  are displaced in the direction X separating from the heat insulating body  6  at a maximum. Note that, as described later, the rectification blades  33  advance and retreat in the axis C direction inside the respective interference maintaining grooves  66  and depths where the rectification blades  33  are inserted into the respective interference maintaining grooves  66  are varied. 
     [Rotor  7 ] 
     As illustrated in  FIG. 1 , the rotor  7  includes the shaft  71  extending along the axis C and the plurality of impellers  72  that are fixed to the outer peripheral surface of the shaft  71 . 
     [Shaft  71 ] 
     As illustrated in  FIG. 1 , the shaft  71  is disposed coaxially with the casing  2  inside the cylindrical casing  2 . 
     More specifically, the first journal bearing  13  is provided on the inside of the suction-side casing head  4  in the radial direction. The first journal bearing  13  is a bearing device that rotatably supports an end part of the shaft  71  on the upstream side U. Further, a thrust bearing  15  that supports the end part of the shaft  71  on the upstream side U is provided farther on the upstream side U than the first journal bearing  13 . The first journal bearing  13  is fixed to the inside of the suction-side casing head  4 , and the thrust bearing  15  is fixed to the outside of the suction-side casing head  4 . 
     A second journal bearing  14  that rotatably supports an end part of the shaft  71  on the downstream side L is provided on the inside of the discharge-side casing head  5  in the radial direction. The second journal bearing  14  is fixed to the inside of the discharge-side casing head  5 . 
     [Impeller  72 ] 
     The impellers  72  use centrifugal force that is generated when the impellers  72  rotate together with the shaft  71 , to forcibly feed the fluid F that flows from the upstream side U toward the downstream side L, toward the outside in the radial direction. Therefore, as illustrated in  FIG. 1  and  FIG. 2 , a fluid passage  12  that causes the fluid F to flow from the upstream side U toward the downstream side L is provided inside the casing  2 . 
     As illustrated in  FIG. 1 , the impellers  72  are arranged in six stages with intervals in the axis direction. As illustrated in  FIG. 2 , each of the impellers  72  includes a hub  73 , a plurality of vanes  74 , and a shroud  75 . The hub  73  has a substantially disc shape in which the diameter is gradually increased toward the downstream side L. The plurality of vanes  74  are radially attached to the hub  73  and are arranged in the circumferential direction. The shroud  75  is attached so as to cover front end side of the plurality of vanes  74  in the circumferential direction. 
     Note that the example in which the impellers  72  are provided in the six stages is illustrated; however, the present invention is applicable to the centrifugal compressor including the impellers  72  in at least one stage. 
     [Fluid Passage  12 ] 
     Next, the fluid passage  12  provided inside the casing  2  is described. As illustrated in  FIG. 1  and  FIG. 2 , the fluid passage  12  mainly includes the suction passage  18 , a diffuser passage  27 , a return passage  28 , and a discharge passage  19 . 
     As illustrated in  FIG. 1 , the suction passage  18  is provided on the end part of the casing  2  on the upstream side U in order to guide the fluid F from the outside to the inside of the casing  2 . 
     As illustrated in  FIG. 2 , the suction passage  18  is provided between the heat insulating body  6  and the diaphragm  3 . In other words, the upstream side U of the suction passage  18  is sectioned by the heat insulating body  6  held by the suction-side casing head  4 , and the downstream side L of the suction passage  18  is sectioned by the end surface  32  of the diaphragm  3 . The heat insulating space  11  is provided between the heat insulating body  6  and the suction-side casing head  4 . 
     The diffuser passage  27  and the return passage  28  are provided to cause the fluid F to flow from the upstream side U toward the downstream side L. 
     As illustrated in  FIG. 2 , an internal space  21  that communicates with each of the suction passage  18  and the discharge passage  19  and is repeatedly decreased and increased in diameter is provided inside the casing  2 . The internal space  21  functions as a space accommodating the impellers  72 , and the internal space  21  excluding the impellers  72  functions as the diffuser passage  27  and the return passage  28 . Accordingly, the suction passage  18  and the discharge passage  19  communicate with each other through the impellers  72  and the fluid passage  12 . 
     As illustrated in  FIG. 1 , the discharge passage  19  is provided on the end part of the casing  2  on the downstream side L to cause the fluid F to flow to the outside. The discharge passage  19  is provided between a shielding member  84  on the discharge side and the diaphragm  3 . 
     The fluid passage  12  is provided so as to extend toward the downstream side L while meandering in the radial direction and to connect the adjacent impellers  72  and  72  inside the casing  2  because the diffuser passage  27  and the return passage  28  are alternately provided, as illustrated in  FIG. 1 . The fluid F is stepwisely compressed every time the fluid F passes through the impellers  72  in the plurality of stages while flowing through the fluid passage  12 . 
     [Effects of Centrifugal Compressor  1 ] 
     The centrifugal compressor  1  according to the first embodiment achieves the following effects. 
     Since the centrifugal compressor  1  includes the oil heater  8 , the centrifugal compressor  1  can heat or cool the suction-side casing head  4  through selection of the temperature of the heating medium HM to be supplied. Accordingly, in a case where the centrifugal compressor  1  compresses the extremely low-temperature fluid F, supplying the heating medium HM at high temperature makes it possible to reduce the temperature difference between the inside and the outside of the centrifugal compressor  1 , more specifically, the temperature difference between the inside and the outside of the suction-side casing head  4 . 
     Further, the centrifugal compressor  1  includes the heat insulating body  6  between the suction-side casing head  4  and the suction passage  18 , which makes it possible to suppress heat transfer between the suction-side casing head  4  and the suction passage  18 . Accordingly, in the case where the extremely low-temperature fluid F is compressed, the temperature decrease of the suction-side casing head  4  caused by the fluid F is suppressed. This makes it possible to reduce the flow rate of the heating medium HM to be supplied to the oil heater  8 . In addition, since the centrifugal compressor  1  includes the heat insulating space  11  between the suction-side casing head  4  and the heat insulating body  6 , it is possible to further suppress the heat transfer between the fluid F and the suction-side casing head  4 . 
     As described above, providing the oil heater  8  as well as the heat insulating space  11  and the heat insulating body  6  enables the centrifugal compressor  1  to suppress the temperature difference between the inside and the outside of the centrifugal compressor  1  in a case where the centrifugal compressor  1  compresses the fluid F, the temperature of which is largely different from the ambient temperature. As a result, defect of the sealing device on the periphery of the suction passage  18  of the centrifugal compressor  1  caused by thermal deformation that may occur at the beginning of the operation is particularly prevented by the heating medium HM at a lower flow rate. 
     In contrast, when the operation of the centrifugal compressor  1  is continued, temperature of the suction-side casing head  4 , the heat insulating body  6 , and the diaphragm  3  are inevitably increased in turn. The thermal expansion and the thermal contraction of the suction-side casing head  4 , the heat insulating body  6 , and the diaphragm  3  are different from one another depending on the temperature during the operation of the centrifugal compressor  1  and the linear expansion coefficient. The centrifugal compressor  1  includes a structure to cope with the difference of the thermal expansion and the thermal contraction. 
     Unlike the structure of the present embodiment, a structure in which the heat insulating body  6  and the suction-side casing head  4  are fixed through, for example, fastening with bolts and relative displacement between the heat insulating body  6  and the suction-side casing head  4  is not allowed is assumed. In the structure, if the thermal expansion is different between the heat insulating body  6  and the suction-side casing head  4 , one of the heat insulating body  6  and the suction-side casing head  4  restricts the thermal expansion in the radial direction of the other, which causes thermal stress. When the centrifugal compressor  1  compresses the extremely low-temperature fluid F, large thermal stress occurs at the fastening part because the thermal expansion in the radial direction of the suction-side casing head  4  heated by the oil heater  8  is larger than that of the heat insulating body  6 . 
     In the present embodiment, however, the keys  67  as locking protrusions provided on the heat insulating body  6  are inserted into the respective key grooves  46  as locking grooves provided on the suction-side casing head  4 , and the suction-side casing head  4  and the heat insulating body  6  are mutually locked so as to be relatively displaceable in the radial direction, as illustrated in  FIG. 4A . In other words, even if the thermal expansion in the radial direction of the suction-side casing head  4  is larger than that of the heat insulating body  6 , the keys  67  displace inside the respective key grooves  46  in the radial direction as illustrated in  FIG. 4B , which makes it possible to suppress generation of thermal stress. As described above, in the centrifugal compressor  1 , the state where the heat insulating body  6  and the suction-side casing head  4  are mutually locked is maintained by the locking mechanisms including the keys  67  and the key grooves  46  as long as the operation of the centrifugal compressor  1  is continued. This makes it possible to stably suppress heat transfer between the suction-side casing head  4  and the suction passage  18 . 
     Next, measures against thermal deformation difference between the heat insulating body  6  and the diaphragm  3  are described. 
     When the rectification blades  33  are displaced in a direction separating from the heat insulating body  6  due to thermal deformation of the diaphragm  3 , a gap may be generated between the heat insulating body  6  and the front ends of the rectification blades  33  if the front ends of the rectification blades  33  are merely brought into contact with the heat insulating body  6 . If the gap is generated, the rectification effect for the fluid F due to the rectification blades  33  is not sufficiently obtainable. 
     In the present embodiment, however, the front ends of the rectification blades  33  are inserted into the respective interference maintaining grooves  66  of the heat insulating body  6  as illustrated in  FIG. 6A . Even if the thermal deformation occurs and the rectification blades  33  are displaced in the direction X separating from the heat insulating body  6  at the maximum, the front ends of the rectification blades  33  remain in the respective interference maintaining grooves  66  of the heat insulating body  6  as illustrated in  FIG. 6B . As described above, in the centrifugal compressor  1 , the interference state where the rectification blades  33  are inserted into the heat insulating body  6  is maintained as long as the operation of the centrifugal compressor  1  is continued. Therefore, the rectification effect for the fluid F by the rectification blades  33  is sufficiently obtainable, which achieves stable operation. 
     Modification of First Embodiment 
     Next, a modification of the first embodiment of the present invention is described with reference to  FIGS. 7A to 7C . In the present embodiment, components similar to those of the first embodiment are denoted by the same reference numerals as those of the first embodiment, and description of such components is omitted. 
     The key grooves  46  are provided on the suction-side casing head  4  and the keys  67  are provided on the heat insulating body  6  in the first embodiment, whereas the keys and key grooves are reversely provided in the modification. 
     In other words, as illustrated in  FIG. 7A , key grooves  68  that penetrate through the front and rear surfaces of the heat insulating body  6  are provided on the heat insulating body  6 . Keys  47  are provided on the suction-side casing head  4  so as to protrude from the first plane part  42  toward the downstream side L. As illustrated in  FIG. 7B , the keys  47  are inserted into the respective key grooves  68 . One key  47  and one key groove  68  correspond to the locking structure of the present invention. 
     In the present embodiment, when the suction-side casing head  4  thermally expands in the radial direction more than the heat insulating body  6 , the keys  47  displace inside the respective key grooves  68  outward in the radial direction as illustrated in  FIG. 7C . 
     Note that the keys  67  and the key grooves  68  may be provided on the heat insulating body  6  and the key grooves  46  and the keys  47  may be provided on the suction-side casing head  4  so as to correspond to the keys  67  and the key grooves  68  of the heat insulating body  6 . 
     Second Embodiment 
     Next, a second embodiment of the present invention is described with reference to  FIG. 8 . Note that, in the present embodiment, components similar to those of the first embodiment are denoted by the same reference numerals as those of the first embodiment, and description of such components is omitted. 
     The key grooves  46  are provided on the suction-side casing head  4  and the keys  67  are provided on the heat insulating body  6  in the first embodiment, whereas pins P as locking protrusions are detachably provided on the heat insulating body  6  in the second embodiment. 
     When the pins P are used as the parts coming into contact with the suction-side casing head  4  inside the key grooves  46 , even if the pins P are worn, it is sufficient to replace the pins P. 
     Other than the above description, the configurations described in the above-described embodiments may be selected or appropriately modified without departing from the scope of the present invention. 
     For example, in the first embodiment, four pairs of the keys  67  and the key grooves  46  are provided at equal intervals while being shifted in phase by 90 degrees in the circumferential direction; however, the present invention is not limited thereto. Three pairs of the keys  67  and the key grooves  46  may be provided at equal intervals while being shifted in phase by 120 degrees in the circumferential direction as illustrated in  FIG. 9A , or two pairs of the keys  67  and the key grooves  46  may be provided on one straight line as illustrated in  FIG. 9B . 
     In addition, the pairs of the keys  67  and the key grooves  46  may not be provided at equal intervals in the circumferential direction as long as the suction-side casing head  4  and the heat insulating body  6  are mutually locked so as to be relatively displaceable in the radial direction. 
     Further, the configuration of the oil heater  8  and the configuration of the heat insulating body  6  merely illustrate an example of the present invention, and the configurations are optional as long as the effect of reducing the temperature difference between the inside and the outside is achieved. 
     This is true of the method of maintaining the interference state between the rectification blades and the heat insulating body. The configuration thereof is optional as long as the rectification effect by the rectification blades is secured. For example, the rectification blades  33  may be provided on the heat insulating body  6  side and the interference maintaining grooves  66  may be provided on the end surface  32  side of the diaphragm  3 . 
     REFERENCE SIGNS LIST 
     
         
           1  Centrifugal compressor 
           11  Heat insulating space 
           12  Fluid passage 
           13  First journal bearing 
           14  Second journal bearing 
           15  Thrust bearing 
           16  Dry gas seal 
           17  Seal fin 
           18  Suction passage 
           19  Discharge passage 
           2  Casing 
           21  Internal space 
           25  Suction scroll 
           26  Compression passage 
           27  Diffuser passage 
           28  Return passage 
           29  Discharge scroll 
           3  Diaphragm 
           31  Diaphragm piece 
           32  End surface 
           33  Rectification blade 
           4  Suction-side casing head 
           41  Head end surface 
           42  First plane part 
           43  First slope part 
           44  Second plane part 
           45  Second slope part 
           46  Key groove 
           47  Key 
           5  Discharge-side casing head 
           6  Heat insulating body 
           61  Holding part 
           62  First disc part 
           63  First conical part 
           64  Second disc part 
           65  Second conical part 
           67  Key 
           68  Key groove 
           69  Heat insulating material 
           7  Rotor 
           71  Shaft 
           72  Impeller 
           73  Hub 
           74  Vane 
           75  Shroud 
           8  Oil heater 
           81  Conduit 
           82  Oil heater body 
           83  Heating medium passage 
           84  Shielding member 
         C Axis 
         F Fluid 
         FE Free end 
         G Gap 
         HM Heating medium 
         U Upstream side 
         L Downstream side 
         P Pin