Multi-stage centrifugal compressor

Abstract

A multistage centrifugal compressor includes: a compression passage through which fluid to be compressed flows; and a first internal passage and a second internal passage which are located between an impeller stack and a shaft body in a radial direction and extend in an axial direction. The first internal passage communicates with the compression passage through a first return passage and a first extraction passage. The first return passage is in connection with a first return position of the compression passage, and the first extraction passage is in connection with a first extraction position located downstream of the first return position of the compression passage. The second internal passage communicates with the compression passage through a second return passage and a second extraction passage. The second return passage is in connection with a second return position located downstream of the first extraction position of the compression passage, and the second extraction passage is in connection with a second extraction position located downstream of the second return position.

Description

Technical Field

[0001] The present disclosure relates to a multistage centrifugal compressor including a multistage rotor.Background Art

[0002] A multistage centrifugal compressor includes a rotor including impellers stacked on each other in an axial direction and compresses fluid by centrifugal force generated when the fluid to be compressed flows in a radial direction through the impellers that are rotating. PTL 1 discloses this type of multistage centrifugal compressor.

[0003] The multistage centrifugal compressor described in PTL 1 includes: a rotor including impellers stacked on each other; and a housing accommodating the rotor. A compression passage is formed by the housing and the impellers so as to extend from an inlet port of the housing to an outlet port of the housing through the impellers from a first-stage impeller to a last-stage impeller. The impellers are collectively held by a tie rod that extends so as to penetrate the impellers in the axial direction. There is a radial gap between the impellers and the tie rod, and this gap forms a return passage through which part of gas flowing through the compression passage returns from a downstream position to an upstream position. Since the compressed gas flows through the return passage, heat generated by the compression of the gas is transferred to the tie rod, and this heats the tie rod. Thus, a temperature gradient between the impeller and the tie rod decreases. Moreover, since the compressed gas flows through the return passage, a thrust load applied to the rotor by the compressed gas decreases.Citation List Patent Literature

[0004] PTL 1: Published Japanese Translation of PCT Application No. 2016-500420Summary of Invention Technical Problem

[0005] In the multistage centrifugal compressor, a pressure difference between the fluid which has not yet flowed into the first-stage impeller and the fluid which has flowed through the last-stage impeller is large. In PTL 1, part of the fluid which has flowed through the last-stage impeller of the compression passage returns through the return passage to an inlet of the first-stage impeller of the compression passage. Therefore, the pressure of the return passage corresponds to the pressure of the high-temperature high-pressure fluid which has flowed through the last-stage impeller. A gap between the impellers adjacent to each other in the axial direction is sealed by a seal to prevent air flow. A pressure difference between the compression passage and the return passage which are separated from each other by the seal increases toward an upstream side of the compression passage. The seal that can withstand such pressure difference is expensive and is a design restriction.

[0006] The present disclosure was made under these circumstances, and an object of the present disclosure is to reduce a load acting on a seal in a multistage centrifugal compressor which includes a multistage rotor and in which: an internal passage through which compressed fluid flows is located between impellers of the multistage rotor and a rotor shaft of the multistage rotor; and the seal seals a gap between the adjacent impellers.Solution to Problem

[0007] In order to solve the above problems, a multistage centrifugal compressor according to one aspect of the present disclosure includes: a multistage rotor including an impeller stack including impellers lined up in an axial direction, a balance piston located between the impellers, and a seal sealing a gap between the impellers adjacent to each other in the axial direction and a shaft body penetrating the impeller stack; a compression passage through which fluid to be compressed flows from an inlet port to an outlet port through the impellers including a first-stage impeller and a last-stage impeller in order; and a first internal passage and a second internal passage which are located between the impeller stack and the shaft body in a radial direction, extend in the axial direction, and are independent from each other, wherein: the first internal passage communicates with the compression passage through a first return passage and a first extraction passage, the first return passage being in connection with a first return position of the compression passage, the first extraction passage being in connection with a first extraction position located downstream of the first return position of the compression passage; part of the fluid flowing through the first extraction position flows to the first return position through the first extraction passage, the first internal passage, and the first return passage; the second internal passage communicates with the compression passage through a second return passage and a second extraction passage, the second return passage being in connection with a second return position located downstream of the first extraction position of the compression passage, the second extraction passage being in connection with a second extraction position located downstream of the second return position; and part of the fluid flowing through the second extraction position flows to the second return position through the second extraction passage, the second internal passage, and the second return passage. Advantageous Effects of Invention

[0008] According to the present disclosure, the load acting on the seal can be reduced in the multistage centrifugal compressor which includes the multistage rotor and in which: the internal passage through which the compressed fluid flows is located between the impellers of the multistage rotor and the rotor shaft of the multistage rotor; and the seal seals the gap between the adjacent impellers.Brief Description of Drawings

[0009] FIG. 1 is a schematic sectional view of a multistage centrifugal compressor according to one aspect of the present disclosure. FIG. 2 is a schematic sectional view of a multistage rotor. FIG. 3 is a partial sectional view of the multistage centrifugal compressor which explains an internal passage. FIG. 4 is a partial sectional view of the multistage centrifugal compressor which shows a modified example of a first return passage. FIG. 5 is a partial sectional view of the multistage centrifugal compressor which shows a modified example of the first return passage. FIG. 6 is a partial sectional view of the multistage centrifugal compressor which shows modified examples of a first extraction passage and a second return passage. FIG. 7 is a schematic sectional view of the multistage centrifugal compressor according to Modified Example 1. Description of Embodiments

[0010] FIG. 1 is a schematic sectional view of a multistage centrifugal compressor 10 according to one aspect of the present disclosure. FIG. 1 shows an upper half of the multistage centrifugal compressor 10 which is located at an upper side of a rotor axis A. The multistage centrifugal compressor 10 shown in FIG. 1 includes: a casing 12; a multistage rotor 2 accommodated in the casing 12; and a driver that rotates the multistage rotor 2.

[0011] FIG. 2 is a schematic sectional view of the multistage rotor 2. As shown in FIGS. 1 and 2, the multistage rotor 2 is a rotating body whose center corresponds to the rotor axis A. The multistage rotor 2 includes: a shaft body 3 whose center corresponds to the rotor axis A and which extends in parallel with an axial direction X; and impellers 4 located at a substantially middle portion of the shaft body 3 in the axial direction X and lined up in the axial direction X. A combination of the impellers 4 stacked on each other in the axial direction X is called an impeller stack 40, and the multistage rotor 2 is also called a stacked rotor.

[0012] The shaft body 3 is a stepped shaft including a stepped surface 36. The shaft body 3 includes: a first section 31 located at a middle portion thereof in the axial direction X; a second section 32 located adjacent to the first section 31; a pair of third sections 33 that sandwich the first section 31 and the second section 32 from both sides in the axial direction X; and a pair of end sections 34 located at both end portions thereof in the axial direction X. The shaft body 3 is a rigid body that is a single object. The shaft body 3 is seamless.

[0013] The first section 31 of the shaft body 3 has a first shaft diameter smaller than an inner diameter of each impeller 4. The second section 32 of the shaft body 3 has a second shaft diameter larger than the first shaft diameter and the inner diameter of the impeller 4. By the difference between the shaft diameters, the stepped surface 36 facing in the axial direction X exists at a boundary between the second section 32 and the first section 31. The shaft diameter of each end section 34 of the shaft body 3 is smaller than the first shaft diameter. The shaft diameter of each third section 33 of the shaft body 3 is larger than the shaft diameter of the end section 34 and smaller than the inner diameter of the impeller 4. The shaft diameter of the third section 33 may be larger than the first shaft diameter as long as the shaft diameter of the third section 33 is smaller than the inner diameter of the impeller 4. As above, according to the shaft body 3, the impeller 4 can be moved from the end section 34 through the third section 33 to the first section 31.

[0014] The first section 31 of the shaft body 3 is in the impeller stack 40. Each impeller 4 has a known structure, and for example, includes: a boss in which the shaft body 3 is located; a disc whose center corresponds to the boss; blades located on the disc; and a shroud attached to tips of the blades. The impeller 4 includes a passage which is defined by the disc, the blades, and the shroud and through which fluid to be compressed flows. The impeller 4 is of a closed type but may be of an open type which does not include the shroud. Both end surfaces of the boss of the impeller 4 in the axial direction X are toothed flanges including circumferential teeth. The impellers 4 located adjacent to each other in the axial direction X are coupled to each other by mechanical coupling in which the toothed flanges mesh with each other, and therefore, the impellers 4 can transmit power to each other. The mechanical coupling may be curvic coupling or hearth coupling. To prevent the flow of the fluid through a meshing portion between the adjacent impellers 4, the meshing portion between the adjacent impellers 4 is sealed by a seal 47.

[0015] The impeller stack 40 includes at least one balance piston 57 sandwiched between the impellers 4. Axial thrust of the multistage rotor 2 that is rotating is autonomously adjusted by the balance piston 57. The impeller stack 40 according to the present embodiment includes the balance piston 57 located at a portion thereof in the axial direction X and has a so-called Back-to-Back arrangement in which the direction of the impeller 4 located at an anti-load side of the balance piston 57 and the direction of the impeller 4 located at a load side of the balance piston 57 are different from each other. As with the impellers 4, the balance piston 57 is externally fitted to the shaft body 3. Both end surfaces of the balance piston 57 in the axial direction X are toothed flanges including circumferential teeth. The impeller 4 and the balance piston 57 which are located adjacent to each other in the axial direction X are coupled to each other by mechanical coupling in which the toothed flanges mesh with each other, and therefore, the impeller 4 and the balance piston 57 can transmit power to each other.

[0016] The impeller 4 located closest to the second section 32 in the impeller stack 40 is called a "leading impeller 4L" for convenience sake. In the present embodiment, the leading impeller 4L is a first-stage impeller 4A that compresses the fluid first among the impellers 4. However, the leading impeller 4L is not limited to the first-stage impeller 4A. The leading impeller 4L is in contact with the stepped surface 36 of the shaft body 3. A surface of the leading impeller 4L which faces the stepped surface 36 is a flat surface and has no teeth. The leading impeller 4L and the stepped surface 36 are in surface contact with each other. The leading impeller 4L and the stepped surface 36 are coupled to each other by friction, and power can be transmitted from the shaft body 3 to the leading impeller 4L. To give redundancy to the power transmission from the shaft body 3 to the leading impeller 4L and perform the alignment between the shaft body 3 and the leading impeller 4L, the rotation of the leading impeller 4L relative to the shaft body 3 and the movement of the leading impeller 4L in the radial direction relative to the shaft body 3 may be restricted by interference fit, spline fitting, or meshing.

[0017] The impeller 4 located farthest from the second section 32 in the impeller stack 40 is called a "trailing impeller 4T" for convenience sake. In the present embodiment, the trailing impeller 4T is a fourth-stage impeller and is not a last-stage impeller 4E that compresses the fluid last among the impellers 4. The trailing impeller 4T is in contact with a balance piston 56 fitted to the shaft body 3. The balance piston 56 may be fitted to the third section 33 of the shaft body 3 or may be fitted to the first section 31 of the shaft body 3. The trailing impeller 4T and the balance piston 56 are coupled to each other by the mechanical coupling that is the meshing.

[0018] A lock nut 6 is screwed to the third section 33 of the shaft body 3. The lock nut 6 is in contact with the balance piston 56. The impeller stack 40 and the balance piston 56 are located between the stepped surface 36 of the shaft body 3 and the lock nut 6 in the axial direction X. The impeller stack 40 and the balance piston 56 are sandwiched between the stepped surface 36 and the lock nut 6 while being pressurized in the axial direction by the axial force of the shaft body 3. Thus, the stepped surface 36 and the leading impeller 4L, the impellers 4 adjacent to each other, and the trailing impeller 4T and the balance piston 56 are kept in a tight-contact state in the axial direction X. Moreover, the alignment between the shaft body 3 and the impeller stack 40 is performed by the lock nut 6 such that the rotor axis A of the shaft body 3 and the center axis of the impeller stack 40 coincide with each other.

[0019] Dry gas seals 54 and 55 that seal between the casing 12 and the shaft body 3 are located around the respective third sections 33 of the shaft body 3. The dry gas seals 54 and 55 prevent an operating fluid, which has been compressed by the multistage rotor 2 to have high pressure, from leaking to the outside. A seal gas is supplied from the outside to the dry gas seals 54 and 55. The end sections 34 of the shaft body 3 are rotatably supported by the casing 12 through journal bearings 51 and 52. Moreover, the end section 34 of the shaft body 3 is supported by the casing 12 through a thrust bearing 53.

[0020] FIG. 1 shows part of the casing 12. The casing 12 includes connection passages 21 each of which connects the impeller 4 at a certain stage and the impeller 4 at its next stage. The connection passage 21 is connected to passages of the impellers 4. The fluid compressed by the impeller 4 at the certain stage is discharged to the connection passage 21 and then flows through the connection passage 21 into the passage of the impeller 4 at the next stage. The casing 12 includes: an inlet port 22 through which the fluid to be compressed is introduced to the first-stage impeller 4A; and an outlet port 23 through which the fluid compressed by the last-stage impeller 4E is sent to the outside. Moreover, the casing 12 includes: an intermediate outlet port 27 through which the fluid compressed at an intermediate-stage impeller is once taken out to the outside; and an intermediate inlet port 28 through which the fluid returns to the inside. As above, the multistage centrifugal compressor 10 includes a compression passage W which is defined by the casing 12 and the impellers 4 and in which: the fluid sucked through the inlet port 22 is stepwisely compressed by the impellers 4; the fluid is once taken out to the outside through the intermediate outlet port 27; the fluid returns to the inside through the intermediate inlet port 28; the fluid is further stepwisely compressed through the impellers 4; and the fluid is discharged to the outside through the outlet port 23. The configuration of the compression passage W of the multistage centrifugal compressor 10 and the number of impellers 4 are merely examples. Moreover, in the shaft body 3 according to the present embodiment, the end sections 34 supported by the bearings 51, 52, and 53 and the first section 31 supporting the impeller stack 40 are configured by a single continuous object. However, the shaft body 3 may be a so-called tie-rod shaft in which a pair of stub shafts are coupled to each other by a tie rod or a tie bolt.

[0021] There is a minute gap between an inner peripheral surface of the impeller 4 and an outer peripheral surface of the shaft body 3, and this gap is utilized by internal passages 7a and 7b through which the fluid flows. A thermal barrier coating may be on the inner peripheral surface of the impeller 4 to suppress heat transfer to the internal passages 7a and 7b from the fluid flowing through the internal passages 7a and 7b. FIG. 3 is a sectional view of the multistage rotor 2 which explains the internal passages 7a and 7b. As shown in FIG. 3, the internal passages 7a and 7b through which the fluid flows in the axial direction X are located between the impeller stack 40 and the shaft body 3.

[0022] A seal 70 is located between the balance piston 57 and the shaft body 3. The seal 70 seals a radial gap between the balance piston 57 and the shaft body 3. The seal 70 may be an annular elastic seal located between the balance piston 57 and the shaft body 3 in the radial direction or may be a labyrinth seal located on opposing surfaces of the balance piston 57 and the shaft body 3. Moreover, the radial gap between an inner peripheral surface of the balance piston 57 and the outer peripheral surface of the shaft body 3 may be sealed in such a manner that the balance piston 57 is fitted to the shaft body 3 by interference fit. Since the radial gap between the balance piston 57 and the shaft body 3 is sealed as above, the gap is divided in the axial direction X, and thus, the internal passages 7a and 7b which are lined up in the axial direction X and are independent from each other are defined. The internal passages 7a and 7b according to the present embodiment are: a first internal passage 7a located at the anti-load side of the balance piston 57; and a second internal passage 7b located at the load side of the balance piston 57.

[0023] A first return position R1, a first extraction position E1, a second return position R2, and a second extraction position E2 are defined on the compression passage W in this order from an upstream side to a downstream side along the flow of the fluid to be compressed. The first internal passage 7a communicates with the compression passage W through a first return passage 73 and a first extraction passage 71. The first return passage 73 is in connection with the first return position R1 of the compression passage W, and the first extraction passage 71 is in connection with the first extraction position E1 of the compression passage W. Since the first extraction position E1 is located downstream of the first return position R1, the pressure of the fluid at the first extraction position E1 is higher than the pressure of the fluid at the first return position R1. By this pressure difference, part of the fluid flowing through the first extraction position E1 flows through the first extraction passage 71 into the first internal passage 7a, flows through the first internal passage 7a in the axial direction X, and returns through the first return passage 73 to the first return position R1. The pressure of the first internal passage 7a is substantially equal to the pressure of the fluid at the first extraction position E1.

[0024] The second internal passage 7b communicates with the compression passage W through a second return passage 74 and a second extraction passage 72. The second return passage 74 is in connection with the second return position R2, and the second extraction passage 72 is in connection with the second extraction position E2. Since the second extraction position E2 is located downstream of the second return position R2, the pressure of the fluid at the second extraction position E2 is higher than the pressure of the fluid at the second return position R2. By this pressure difference, part of the fluid flowing through the second extraction position E2 flows through the second extraction passage 72 into the second internal passage 7b, flows through the second internal passage 7b in the axial direction X, and returns through the second return passage 74 to the second return position R2. The pressure of the second internal passage 7b is substantially equal to the pressure of the fluid at the second extraction position E2.

[0025] Since the fluid extracted from the compression passage W flows through the first internal passage 7a and the second internal passage 7b as above, the pressure in a radial gap between the shaft body 3 and the impeller stack 40 is stabilized. Thus, impurities are prevented from accumulating in the gap, and the temperature difference between the impeller stack 40 and the shaft body 3 is reduced.

[0026] In the present embodiment, the first return position R1 is on the compression passage W and between the connection passage 21 and an inlet of the first-stage impeller 4A. It is desirable that to suppress influence on the main flow, the first return position R1 be located away from and upstream of the inlet of the first-stage impeller 4A. From this point of view, the first return passage 73 extends through the inside of the shaft body 3. Specifically, a passage that connects the first internal passage 7a and the first return position R1 of the compression passage W is located inside the second section 32 of the shaft body 3 which is in contact with the leading impeller 4L, and this passage is utilized as the first return passage 73. Since the first return passage 73 extends through the inside of the shaft body 3, the degree of freedom of the position of the first return position R1 that is an outlet of the first return passage 73 improves, and the first return position R1 can be located at a position away from and upstream of the inlet of the first-stage impeller 4A that is the leading impeller 4L.

[0027] The configuration of the first return passage 73 is not limited to the above. For example, as shown in FIG. 4, the first return passage 73 may extend through contact surfaces of the impeller stack 40 and the shaft body 3. In this case, for example, regarding the contact surfaces of the impeller stack 40 and the shaft body 3, a groove is located on at least one of the impeller stack 40 or the shaft body 3, and this groove is utilized as the first return passage 73. More specifically, the shaft body 3 includes a planar seat surface orthogonal to the axial direction X, and the impeller stack 40 includes a planar contact surface. The seat surface (i.e., the stepped surface 36) of the shaft body 3 and the contact surface of the impeller stack 40 are in contact with each other in the axial direction X, and the first return passage 73 extends through the groove located on at least one of the seat surface or the contact surface. In the example shown in FIG. 4, the leading impeller 4L of the impeller stack 40 includes a surface that is in contact with the shaft body 3 in the axial direction X and a surface that is in contact with the shaft body 3 in the radial direction. Then, regarding these contact surfaces of the leading impeller 4L and the shaft body 3, the groove that serves as the first return passage 73 is located on at least one of the leading impeller 4L or the shaft body 3. The impeller stack 40 may further include a cylindrical body located between the stepped surface 36 of the shaft body 3 and the leading impeller 4L in the axial direction X. In this case, the cylindrical body of the impeller stack 40 includes a surface that is in contact with the shaft body 3 in the axial direction X and a surface that is in contact with the shaft body 3 in the radial direction. Then, regarding the contact surfaces of the cylindrical body and the shaft body 3, the groove that serves as the first return passage 73 is located on at least one of the cylindrical body or the the shaft body 3.

[0028] Or, as shown in FIG. 5, the first return passage 73 may extend through the first-stage impeller 4A. In this case, there is a passage penetrating the first-stage impeller 4A from the inside to the outside, and this passage is utilized as the first return passage 73. Moreover, the first return passage 73 may be located downstream of the inlet of the first-stage impeller 4A. In this case, a gap between meshing surfaces of the first-stage impeller 4A and the impeller 4 adjacent to the first-stage impeller 4A may be utilized as the first return passage 73.

[0029] In the present embodiment, the first extraction position E1 is located on the compression passage W and at or in the vicinity of an outlet of a first intermediate-stage impeller 4C located adjacent to the balance piston 57. The first extraction passage 71 extends through a back surface side of the first intermediate-stage impeller 4C, penetrates the balance piston 57, and reaches the first internal passage 7a. However, as shown in FIG. 6, the first extraction passage 71 may extend through a gap between meshing surfaces of the balance piston 57 and the first intermediate-stage impeller 4C instead of penetrating the balance piston 57.

[0030] In the present embodiment, the second extraction position E2 is located on the compression passage W and between the last-stage impeller and the outlet port 23. The second extraction passage 72 extends through a back surface side of the last-stage impeller, penetrates the balance piston 57, and reaches the second internal passage 7b. The second extraction passage 72 may extend through a gap between meshing surfaces of the balance piston 57 and the last-stage impeller 4E instead of penetrating the balance piston 57.

[0031] In the present embodiment, the second return position R2 is located on the compression passage W and between the outlet of the first intermediate-stage impeller 4C and an inlet of a second intermediate-stage impeller 4D. The second intermediate-stage impeller 4D is the impeller 4 at the stage next to the stage of the first intermediate-stage impeller 4C and is the trailing impeller 4T. The second return passage 74 extends through a gap between meshing surfaces of the second intermediate-stage impeller 4D and the balance piston 56. However, the second return passage 74 may extend through the shaft body 3, and then, penetrate the balance piston 56 in the radial direction.

[0032] In the above configuration, the pressure of the first internal passage 7a is substantially equal to the pressure of the fluid at the first extraction position E1, i.e., the pressure of the fluid at the outlet of the first intermediate-stage impeller 4C. Therefore, the pressure difference applied to the seal 47 that prevents the leak of the fluid to the first internal passage 7a from a portion of the compression passage W which extends from the inlet port 22 to the first extraction position E1 is adequately smaller than the pressure difference between the inlet port 22 and outlet port 23 of the compression passage W.

[0033] Moreover, the pressure of the second internal passage 7b is substantially equal to the pressure of the fluid at the second extraction position E2, i.e., the pressure of the fluid at an outlet of the last-stage impeller 4E. Therefore, the pressure difference applied to the seal 47 that prevents the leak of the fluid to the second internal passage 7b from a portion of the compression passage W which extends from the first extraction position E1 to the outlet port 23 is adequately smaller than the pressure difference between the inlet port 22 and outlet port 23 of the compression passage W. When the number of impellers 4 between the second return position R2 and the second extraction position E2 is large as in the present embodiment, a pressure reducer 48 may be located at a portion of the second internal passage 7b in the axial direction X to further reduce the pressure difference applied to the seal 47 located at a downstream portion of the second internal passage 7b. The pressure reducer 48 is, for example, a ring-shaped air-permeable seal located between the shaft body 3 and the impeller stack 40 in the radial direction.

[0034] As described above, in the multistage centrifugal compressor 10 according to the present disclosure, the pressure difference between the compression passage W and the internal passage 7a or 7b is applied to the seal 47 that seals the gap between the adjacent impellers 4. However, this pressure difference is adequately smaller than the pressure difference between the inlet port 22 and the outlet port 23, i.e., the pressure difference between the inlet of the first-stage impeller 4A and the outlet of the last-stage impeller 4E. As above, the pressure difference between the compression passage W and the internal passage 7a or 7b which is applied to the seal 47 is reduced, and the load applied to the seal 47 can be reduced. Therefore, the cost for the seal 47 can be reduced. In addition, a design restriction of the seal 47 is reduced, and the degree of freedom of the selection of the seal 47 improves.Modified Examples

[0035] Next, Modified Examples of the above embodiment will be described. FIG. 7 is a schematic sectional view of the multistage centrifugal compressor 10 according to Modified Example 1. In the explanation of the present modified example, the same reference signs are used for the same or similar components as or to the above embodiment, and the repetition of the same explanation is avoided.

[0036] The multistage rotor 2 of the multistage centrifugal compressor 10 according to the above embodiment includes the impeller stack 40 having the Back-to-Back arrangement. Instead of this, the multistage rotor 2 of the multistage centrifugal compressor 10 according to the above embodiment may include the impeller stack 40 having a straight arrangement. In the impeller stack 40 having the straight arrangement, the directions of all the impellers 4 lined up are the same as each other.

[0037] The multistage rotor 2 of the multistage centrifugal compressor 10 according to Modified Example 1 shown in FIG. 7 includes the impeller stack 40 having the straight arrangement in which the directions of the impellers 4 lined up are the same as each other. The impeller stack 40 includes the impellers 4 and the balance piston 57 sandwiched by the impellers 4. In FIG. 7, the compression passage W is shown by a dotted line, and the internal passages 7a and 7b are shown by two-dot chain lines. In the compression passage W, the fluid flows unidirectionally in the axial direction X from the anti-load side to the load side. The radial gap between the shaft body 3 and the impeller stack 40 is divided into the first internal passage 7a and the second internal passage 7b by the seal 70 that seals the gap between the shaft body 3 and the balance piston 57. In the first internal passage 7a and the second internal passage 7b, the fluid flows unidirectionally in the axial direction X from the load side to the anti-load side.

[0038] The first return position R1, the first extraction position E1, the second return position R2, and the second extraction position E2 are defined on the compression passage W in this order from the upstream side to the downstream side along the flow of the fluid to be compressed. The first internal passage 7a communicates with the compression passage W through the first return passage 73 and the first extraction passage 71. The first return passage 73 is in connection with the first return position R1 of the compression passage W, and the first extraction passage 71 is in connection with the first extraction position E1 of the compression passage W. Since the first extraction position E1 is located downstream of the first return position R1, the pressure of the fluid at the first extraction position E1 is higher than the pressure of the fluid at the first return position R1. By this pressure difference, part of the fluid flowing through the first extraction position E1 flows through the first extraction passage 71 into the first internal passage 7a, flows through the first internal passage 7a in the axial direction X, and returns through the first return passage 73 to the first return position R1. The pressure of the first internal passage 7a is substantially equal to the pressure of the fluid at the first extraction position E1.

[0039] The second internal passage 7b communicates with the compression passage W through the second return passage 74 and the second extraction passage 72. The second return passage 74 is in connection with the second return position R2, and the second extraction passage 72 is in connection with the second extraction position E2. Since the second extraction position E2 is located downstream of the second return position R2, the pressure of the fluid at the second extraction position E2 is higher than the pressure of the fluid at the second return position R2. By this pressure difference, part of the fluid flowing through the second extraction position E2 flows through the second extraction passage 72 into the second internal passage 7b, flows through the second internal passage 7b in the axial direction X, and returns through the second return passage 74 to the second return position R2. The pressure of the second internal passage 7b is substantially equal to the pressure of the fluid at the second extraction position E2.

[0040] In the present modified example, the first return position R1 is located on the compression passage W and between the connection passage 21 and the inlet of the first-stage impeller 4A. The first extraction position E1 is located on the compression passage W and at or in the vicinity of the outlet of the first intermediate-stage impeller 4C located adjacent to the balance piston 57.

[0041] The second extraction position E2 is located on the compression passage W and between the last-stage impeller and the outlet port 23. The second return position R2 is located on the compression passage W and between the outlet of the first intermediate-stage impeller 4C and the inlet of the second intermediate-stage impeller 4D. The second intermediate-stage impeller 4D is the impeller 4 at the stage next to the stage of the first intermediate-stage impeller 4C and is the impeller 4 located adjacent to the balance piston 57. The second return passage 74 extends through the second intermediate-stage impeller 4D, penetrates the balance piston 57, and reaches the second internal passage 7b. The second return passage 74 may extend through a gap between meshing surfaces of the balance piston 57 and the last-stage impeller 4E instead of penetrating the balance piston 57. The last-stage impeller is the trailing impeller 4T, and the second extraction passage 72 extends through a gap between meshing surfaces of the trailing impeller 4T and the balance piston 56. However, the second extraction passage 72 may penetrate the balance piston 56 in the radial direction.

[0042] In the above configuration, the pressure of the first internal passage 7a is substantially equal to the pressure of the fluid at the first extraction position E1, i.e., the pressure of the fluid at the outlet of the first intermediate-stage impeller 4C. Therefore, the pressure difference applied to the seal 47 that prevents the leak of the fluid to the first internal passage 7a from a portion of the compression passage W which extends from the inlet port 22 to the first extraction position E1 is adequately smaller than the pressure difference between the inlet port 22 and outlet port 23 of the compression passage W. Moreover, the pressure of the second internal passage 7b is substantially equal to the pressure of the fluid at the second extraction position E2, i.e., the pressure of the fluid at the outlet of the last-stage impeller 4E. Therefore, the pressure difference applied to the seal 47 that prevents the leak of the fluid to the second internal passage 7b from a portion of the compression passage W which extends from the first extraction position E1 to the outlet port 23 is adequately smaller than the pressure difference between the inlet port 22 and outlet port 23 of the compression passage W. As above, since the multistage centrifugal compressor 10 including the multistage rotor 2 including the impeller stack 40 having the straight arrangement includes the internal passages 7a and 7b that are independent from each other, the pressure difference between the compression passage W and the internal passage 7a or 7b which is applied to the seal 47 that seals the gap between the adjacent impellers 4 can be suppressed.Conclusion

[0043] The multistage centrifugal compressor 10 according to a first aspect of the present disclosure includes: the multistage rotor 2 including the impeller stack 40 including the impellers 4 lined up in the axial direction X, the balance piston 57 located between the impellers 4, and the seal 47 sealing the gap between the impellers 4 adjacent to each other in the axial direction X and the shaft body 3 penetrating the impeller stack 40; the compression passage W through which the fluid to be compressed flows from the inlet port 22 to the outlet port 23 through the impellers 4 including the first-stage impeller 4A and the last-stage impeller 4E in order; and the first internal passage 7a and the second internal passage 7b which are located between the impeller stack 40 and the shaft body 3 in the radial direction, extend in the axial direction X, and are independent from each other, wherein: the first internal passage 7a communicates with the compression passage W through the first return passage 73 and the first extraction passage 71, the first return passage 73 being in connection with the first return position R1 of the compression passage W, the first extraction passage 71 being in connection with the first extraction position E1 located downstream of the first return position R1 of the compression passage W; part of the fluid flowing through the first extraction position E1 flows to the first return position R1 through the first extraction passage 71, the first internal passage 7a, and the first return passage 73; the second internal passage 7b communicates with the compression passage W through the second return passage 74 and the second extraction passage 72, the second return passage 74 being in connection with the second return position R2 located downstream of the first extraction position E1 of the compression passage W, the second extraction passage 72 being in connection with the second extraction position E2 located downstream of the second return position R2; and part of the fluid flowing through the second extraction position E2 flows to the second return position R2 through the second extraction passage 72, the second internal passage 7b, and the second return passage 74.

[0044] In the multistage centrifugal compressor 10 configured as above, the pressure difference between the compression passage W and the internal passage 7a or 7b is applied to the seal 47 that seals the gap between the adjacent impellers 4. However, this pressure difference is adequately smaller than the pressure difference between the inlet port 22 and the outlet port 23, i.e., the pressure difference between the inlet of the first-stage impeller 4A and the outlet of the last-stage impeller 4E. As above, the pressure difference between the compression passage W and the internal passage 7a or 7b which is applied to the seal 47 is reduced, and the load applied to the seal 47 can be reduced. Therefore, the cost for the seal 47 can be reduced. In addition, a design restriction of the seal 47 is reduced, and the degree of freedom of the selection of the seal 47 improves.

[0045] The multistage centrifugal compressor 10 according to a second aspect is configured such that in the multistage centrifugal compressor 10 according to the first aspect, the gap between the impeller stack 40 and the shaft body 3 in the radial direction is divided into the first internal passage 7a and the second internal passage 7b by sealing the gap between the balance piston 57 and the shaft body 3 in the radial direction.

[0046] Thus, the upstream end or downstream end of the first internal passage 7a and the upstream end or downstream end of the second internal passage 7b are located in the vicinity of the balance piston 57, and the extraction passage or the return passage can be located at the balance piston 57 which is machined more easily than the impeller 4.

[0047] The multistage centrifugal compressor 10 according to a third aspect is configured such that: in the multistage centrifugal compressor 10 according to the second aspect, the first extraction position E1 is located downstream of the first intermediate-stage impeller 4C located adjacent to the balance piston 57; and the first extraction passage 71 penetrates the balance piston 57 or extends between the balance piston 57 and the first intermediate-stage impeller 4C.

[0048] According to this configuration, the fluid is extracted from the compression passage W through the balance piston 57. Therefore, as compared to when the fluid is extracted from the compression passage W through the boss of the impeller 4, extraction pressure of the fluid is further stable, and pressure equalization among the impellers 4 is relatively easy.

[0049] The multistage centrifugal compressor 10 according to a fourth aspect is configured such that: in the multistage centrifugal compressor 10 according to the second or third aspect, the second extraction position E2 is located between the outlet port 23 and the outlet of the last-stage impeller 4E located adjacent to the balance piston 57; and the second extraction passage 72 penetrates the balance piston 57 or extends between the balance piston 57 and the last-stage impeller 4E.

[0050] The multistage centrifugal compressor 10 according to a fifth aspect is configured such that: in the multistage centrifugal compressor 10 according to the second or third aspect, the second return position R2 is located downstream of the second intermediate-stage impeller 4D located adjacent to the balance piston 57; and the second return passage 74 penetrates the balance piston 57 or extends between the balance piston 57 and the second intermediate-stage impeller 4D.

[0051] In the multistage centrifugal compressor 10 according to the fourth or fifth aspect, there is a possibility that since the second extraction passage 72 is located at the balance piston 57, the pressure in the balance piston 57 is suppressed, and destabilizing force that influences the characteristics of the entire rotor 2 can be reduced.

[0052] The multistage centrifugal compressor 10 according to a sixth aspect is configured such that: in the multistage centrifugal compressor 10 according to any one of the first to fifth aspects, the first return position R1 is located between the inlet port 22 of the compression passage W and the inlet of the first-stage impeller 4A; and the first return passage 73 penetrates the shaft body 3.

[0053] Since the first return passage 73 is formed by utilizing the shaft body 3 as above, machine work that forms the first return passage 73 is easy, and the rigidity of a portion of the shaft body 3 which supports the impeller stack 40 is not influenced by the first extraction passage 71. Moreover, since the first return passage 73 extends through the shaft body 3, the first return position R1 can be located further upstream of the inlet of the first-stage impeller 4A, and the influence of the fluid, which has returned to the first return position R1, on the main flow of the compression passage W can be suppressed.

[0054] The multistage centrifugal compressor 10 according to a seventh aspect is configured such that: in the multistage centrifugal compressor 10 according to any one of the first to fifth aspects, the first return position R1 is located between the inlet port 22 of the compression passage W and the inlet of the first-stage impeller 4A; the shaft body 3 includes a planar seat surface orthogonal to the axial direction X; the impeller stack 40 includes a planar contact surface; the seat surface of the shaft body 3 and the contact surface of the impeller stack 40 are in contact with each other in the axial direction X; and the first return passage 73 extends through the groove located on at least one of the seat surface or the contact surface.

[0055] Since the first return passage 73 is formed by utilizing the shaft body 3 as above, machine work that forms the first return passage 73 is easy, and the rigidity of the portion of the shaft body 3 which supports the impeller stack 40 is not influenced by the first extraction passage 71.

[0056] The multistage centrifugal compressor 10 according to an eighth aspect is configured such that in the multistage centrifugal compressor 10 according to any one of the first to seventh aspects, the pressure reducer 48 that reduces the pressure of the fluid flowing through the pressure reducer 48 is located at at least one of the first internal passage 7a or the second internal passage 7b.

[0057] According to the internal passage 7a or 7b at which the pressure reducer 48 is located, the pressure at a downstream side of the pressure reducer 48 decreases, and the pressure difference acting on the seal 47 can be further reduced.

[0058] The foregoing discussion of the present disclosure has been presented for purposes of illustration and description. The foregoing is not intended to limit the present disclosure to the form or forms disclosed herein. In the foregoing Detailed Description for example, various features of the present disclosure are grouped together in one embodiment for the purpose of streamlining the disclosure. However, some of the features may be combined with each other. The features of the present disclosure may be combined in alternate embodiments, configurations, or aspects other than those discussed above.

Claims

1. A multistage centrifugal compressor comprising: a multistage rotor including an impeller stack including impellers lined up in an axial direction, a balance piston located between the impellers, and a seal sealing a gap between the impellers adjacent to each other in the axial direction and a shaft body penetrating the impeller stack; a compression passage through which fluid to be compressed flows from an inlet port to an outlet port through the impellers including a first-stage impeller and a last-stage impeller in order; and a first internal passage and a second internal passage which are located between the impeller stack and the shaft body in a radial direction, extend in the axial direction, and are independent from each other, wherein: the first internal passage communicates with the compression passage through a first return passage and a first extraction passage, the first return passage being in connection with a first return position of the compression passage, the first extraction passage being in connection with a first extraction position located downstream of the first return position of the compression passage; part of the fluid flowing through the first extraction position flows to the first return position through the first extraction passage, the first internal passage, and the first return passage; the second internal passage communicates with the compression passage through a second return passage and a second extraction passage, the second return passage being in connection with a second return position located downstream of the first extraction position of the compression passage, the second extraction passage being in connection with a second extraction position located downstream of the second return position; and part of the fluid flowing through the second extraction position flows to the second return position through the second extraction passage, the second internal passage, and the second return passage.

2. The multistage centrifugal compressor according to claim 1, wherein a gap between the impeller stack and the shaft body in the radial direction is divided into the first internal passage and the second internal passage by sealing a gap between the balance piston and the shaft body in the radial direction.

3. The multistage centrifugal compressor according to claim 2, wherein: the first extraction position is located downstream of a first intermediate-stage impeller located adjacent to the balance piston; and the first extraction passage penetrates the balance piston or extends between the balance piston and the first intermediate-stage impeller.

4. The multistage centrifugal compressor according to claim 2, wherein: the second extraction position is located between the outlet port and an outlet of the last-stage impeller located adjacent to the balance piston; and the second extraction passage penetrates the balance piston or extends between the balance piston and the last-stage impeller.

5. The multistage centrifugal compressor according to claim 2, wherein: the second return position is located downstream of a second intermediate-stage impeller located adjacent to the balance piston; and the second return passage penetrates the balance piston or extends between the balance piston and the second intermediate-stage impeller.

6. The multistage centrifugal compressor according to any one of claims 1 to 5, wherein: the first return position is located between the inlet port of the compression passage and an inlet of the first-stage impeller; and the first return passage penetrates the shaft body.

7. The multistage centrifugal compressor according to any one of claims 1 to 5, wherein: the first return position is located between the inlet port of the compression passage and an inlet of the first-stage impeller; the shaft body includes a planar seat surface orthogonal to the axial direction; the impeller stack includes a planar contact surface; the seat surface of the shaft body and the contact surface of the impeller stack are in contact with each other in the axial direction; and the first return passage extends through a groove located on at least one of the seat surface or the contact surface.

8. The multistage centrifugal compressor according to any one of claims 1 to 5, wherein a pressure reducer that reduces pressure of the fluid flowing through the pressure reducer is located at at least one of the first internal passage or the second internal passage.

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