JP2020033962A - Multistage centrifugal pump - Google Patents

Abstract

To improve three general evaluations of the machining cost of a seal, the vibration performance and the pump performance based on the seal performance improvement in comparison with before.SOLUTION: A multistage centrifugal pump has a first opposed part where a casing and a rotating body are opposed to each other, and a second opposed part where a distance between the second opposed part and a rotating shaft is smaller than the distance between the first opposed part and the rotating shaft. A non-contact annular seal is provided in the first opposed part and the second opposed part. The non-contact annular seal in the first opposed part includes a screw groove or a parallel groove provided at the leakage flow surface of a casing side, and a smooth cylindrical surface provided at the leakage flow surface of the rotational side of the first opposed part. The non-contact annular seal in the second opposed part includes a cylindrical surface provided at each of a leakage flow surface of the casing side and the leakage flow surface of the rotational side, and being parallel to each other.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a multi-stage centrifugal pump for handling incompressible fluid.

  2. Description of the Related Art Rotary machines (hereinafter, referred to as "pumps") for transferring liquids are widely used in various plants and facilities such as power generation, chemical processes, sewage, and water supply. FIG. 1 shows a cross-sectional view of a typical multi-stage centrifugal pump 1 (quoted from JIS Handbook Pump, 1st edition, page 74). The pump has a rotating shaft on which an impeller is mounted inside the casing, and the rotating shaft is rotatably supported by bearings. Then, the pump raises the pressure of the liquid sucked from the suction port by rotation of the impeller, and discharges the liquid from the discharge port. In the flow path inside the pump, a slight gap between the fixed part (casing) and the rotating part (rotary shaft) is sealed in a non-contact manner, so that leakage of the pressurized liquid to the low pressure side is reduced. This is because leakage of the fluid from the high pressure side to the low pressure side is a major factor in reducing the pump efficiency.

  With reference to FIG. 1, a description will be given of a mechanism of the pump and a flow leakage point of the fluid. The high-pressure pump includes a rotating shaft 11 that is connected to and rotates with an electric motor (not shown), impellers 21a, 21b, and 21c that are fitted to the rotating shaft 11 and rotate with the rotating shaft 11, and impellers 21a, 21b, and 21c. And a shaft seal 51a, 51b that is attached to a location where the main shaft 11 penetrates the casing 31 and seals the liquid in the casing 31 so as not to leak from between the casing 31 and the rotating shaft 11. ing. An electric motor for rotating this high-pressure pump is connected via a coupling 13 fitted to the left end of the rotating shaft 11.

  The first stage impeller 21 a fastened to the rotating shaft 11 rotates with the rotation of the rotating shaft 11, and guides fluid from the suction port 33 into the pump. The fluid pressurized by the first-stage impeller 21a passes through the first flow path 35a and is guided to the second-stage impeller 21b. The fluid pressurized by the second stage impeller 21b passes through the second flow path 35b and is guided to the third stage impeller 21c. The fluid is further pressurized by the third-stage impeller 21c, discharged from the discharge port 37, and transferred by a pipe (not shown). That is, the fluid is pressurized by the first-stage impeller 21a, the second-stage impeller 21b, and the third-stage impeller 21c.

  Due to the pressure difference between the high pressure side and the low pressure side generated on the suction side and the discharge side of each of the impellers 21a, 21b, 21c, if there is a slight gap between the high pressure side and the low pressure side, the fluid will move from the high pressure side to the low pressure side. Flows. This flow is a leakage because it acts to cancel the high-low pressure difference caused by the action of the impeller. Such a leak is caused by a portion surrounded by a circle in the figure, specifically, an impeller inlet portion, between the impeller and the next impeller, a final impeller outlet and a low-pressure chamber for thrust balance (this chamber). Is connected to the pump inlet pressure). In general, the greater the leakage, the greater the effect on lowering pump performance. This leakage increases as the pressure difference increases, as the distance from the center of the rotating shaft increases, and in a multi-stage pump, as the number of stages of the impeller increases.

  FIG. 2 shows a first-stage impeller 21a of the impellers 21a, 21b, and 21c shown in FIG. 1 as a typical enlarged view around the impeller. The leakage locations of the fluid around the impeller of the multi-stage pump are mainly a portion X facing the outer peripheral surface of the impeller suction port 23 and the casing 31a, and a portion Y facing the outer peripheral surface of the back surface of the impeller 21a and the casing 31b. It is.

The impeller 21 a fitted to the main shaft 11 rotates at the same time as the main shaft 11 rotates, and sucks fluid from the impeller suction port 23. The sucked fluid is energized by the rotating blades, becomes high pressure in the flow path, and is discharged. The discharged high-pressure fluid flows through the flow path of the casing 31 and is transferred to the next second stage impeller 21b.

  However, only a small part of the fluid flowing out of the discharge port 25 on the high pressure side flows out to the low pressure side from the gap formed in the opposed portions X and Y. The outflowing fluid is a leaking fluid and reduces the efficiency of the pump.

  In order not to reduce the efficiency of the pump, non-contact annular seals 41 and 43 are provided at these opposed portions X and Y. As the most basic non-contact annular seal, a smooth seal composed of a double cylinder in which a flow path by a fixed portion and a rotating portion in the seal is a parallel and smooth surface is known. Further, a parallel groove seal, a damper seal, a thread groove seal, and the like are known as seal structures in which the amount of leakage is smaller than that of a smooth seal.

  FIG. 3 is a partial sectional view of the parallel groove seal. FIG. 4 is a partial sectional view of a damper seal having a honeycomb pattern. FIG. 5 is a partial sectional view of a damper seal having a hole pattern. FIG. 6 is a partial cross-sectional view of the thread groove seal. As shown in FIG. 3, in the parallel groove seal, the outer peripheral surface of the rotating portion (rotary shaft 111) of the flow passage in the seal is smooth, but a plurality of concentric grooves are provided on the surface of the fixed portion. In the parallel groove seal 136, the fluid is affected by energy loss due to a vortex generated when the fluid flowing in the seal passes through the groove, pressure loss due to rapid expansion and contraction of the flow path, and the like. The amount of liquid leakage can be reduced.

  On the other hand, in the damper seal, as shown in FIGS. 4 and 5, the outer peripheral surface of the rotating shaft 111 of the in-seal flow path is smooth, but a plurality of uneven portions are provided on the surface of the fixed portion. As the shape of the unevenness, a honeycomb pattern 137 shown in FIG. 4, a hole pattern 138 shown in FIG. 5, and the like are employed. In the damper seal of the honeycomb pattern 137, the surface of the fixed portion of the seal has a honeycomb structure, and has many hexagonal concave portions. Due to the pressure loss of the fluid flowing on the uneven surface, leakage of the fluid is reduced. In the damper seal of the hole pattern 138, a large number of circular concave portions are provided on the surface of the fixed portion of the seal, and the pressure loss of the fluid flowing on the irregular surface reduces the leakage of the fluid.

  The thread groove seal shown in FIG. 6 has a structure in which a screw 145 is formed on one or both of the rotating portion and the fixed portion (the rotating shaft 111 in the figure). For this reason, when the rotating shaft 111 rotates, the fluid can be pushed back to the high pressure side by the pumping effect depending on the rotating direction, and the leakage amount can be greatly reduced. This is a great advantage of the thread seal. The effect is particularly exhibited when used for an incompressible fluid which is a liquid such as water.

  As mentioned above, one of the important things in the technology of the non-contact annular seal is the shaft sealing property. However, besides the shaft sealing characteristics, the vibration characteristics are also important, and it is also important to balance the shaft sealing characteristics and the vibration characteristics. However, each of the seal structures introduced above has advantages and disadvantages. If the shaft sealing characteristics are good, the vibration characteristics are bad, and if the vibration characteristics are good, the shaft sealing characteristics are bad.

  The smooth seal is inferior in shaft sealing performance and leaks more than other parallel groove seals, damper seals, and thread groove seals. However, since the liquid film in the seal produces a rigidity effect and a damping effect, the vibration characteristics are better than those of other seals. Although the parallel groove seal has better shaft sealing characteristics than the smooth seal, the rigidity and damping effects of the liquid film in the seal cannot be expected, and the vibration characteristics are worse than the smooth seal.

The thread groove seal has a more excellent shaft sealing characteristic than the parallel groove seal because the fluid is pushed back to the high pressure side depending on the direction of shaft rotation. However, when the screw 145 is formed on the rotating part as shown in FIG. 6, a circumferential force is applied to the fluid to push the fluid back to the high pressure side, which is a major cause of the generation of unstable vibration. Has a greater disadvantage in terms of vibration characteristics than the parallel groove seal. On the other hand, when a screw is formed in the fixed portion, unstable vibration does not occur. For this reason, care must be taken in how to use the thread groove seal.

  The damper seal has a large energy loss due to the unevenness of the flow path surface, has a higher vibration damping efficiency than the above-described parallel groove seal and screw groove seal, and stabilizes the vibration of the rotating shaft 11. However, the shaft sealing characteristics are not remarkably good and cannot be expected as much as parallel groove seals and screw groove seals.

  As described above, non-contact annular seals with different shaft sealing characteristics and vibration characteristics are known.However, since the structure of a smooth seal is simple, the manufacturing cost is the lowest compared to other seals. In general, a smooth seal is used. However, as a result, the amount of leakage increases in the multi-stage pump, which hinders improvement in pump performance. On the other hand, when a parallel groove seal or a thread groove seal having good shaft sealing characteristics is used for the multi-stage pump, the shaft sealing performance is improved, but the manufacturing cost is relatively high, and the vibration characteristics are generally reduced. Further, the shaft sealing characteristics and the vibration characteristics of the damper seal are located between the smooth seal and the thread groove seal, and the manufacturing cost of the damper seal is high although it is not particularly excellent. At the present time, a non-contact seal having better shaft sealing performance, vibration performance, and cost has not been commercialized.

  By the way, in the conventional multi-stage pump, the gap between the rotating body and the casing, that is, the impeller inlet, between the impeller and the next impeller, between the final impeller outlet and the low-pressure chamber for thrust balance, and the like. In addition, the seal having the same structure has been applied uniformly without considering the difference in the influence of the clearance on the vibration and the influence of the leakage amount.

  It is an object of the present invention to provide a centrifugal multi-stage pump in which a sealing structure having appropriate characteristics is provided for each gap between a rotating body and a casing, thereby making it possible to reduce the processing cost, vibration performance, and sealing performance of the seal as compared with the related art. Achieving three overall assessment improvements in pump performance based on the enhancement.

  According to the first aspect, a rotating body including a rotating shaft and a plurality of impellers provided on the rotating shaft, a casing surrounding the rotating body, and a gap between opposing portions of the rotating body and the casing are formed. And a non-contact annular seal to seal. In this multistage centrifugal pump, a first facing portion where the casing and the rotating body face each other, and a second facing portion, wherein a distance between the second facing portion and the rotating shaft is the first facing portion. A second opposing part, which is smaller than the distance between one opposing part and the rotating shaft, wherein the non-contact annular seal is provided between the first opposing part and the second opposing part. The non-contact annular seal provided at the first opposed portion is provided at a thread groove or a parallel groove provided at the leakage flow surface at the casing side, and at a rotating flow passage surface at the rotation side of the first opposed portion. A non-contact annular seal at the second opposing portion is provided on each of the casing-side leakage channel surface and the rotating-side leakage channel surface, and the cylinders are parallel to each other. Provide a surface.

  According to the second aspect, in the first aspect, the difference between the inlet pressure and the outlet pressure of the leakage flow of the non-contact annular seal at the first opposing portion is different from that of the non-contact annular seal at the second opposing portion. It is larger than the difference between the inlet pressure and the outlet pressure of the leak flow.

  According to a third mode, in the first mode or the second mode, the first facing portion is constituted by an outer peripheral portion of the suction port of the impeller and a casing facing the outer peripheral portion.

  According to the fourth aspect, in any one of the first to third aspects, the second opposing portion is provided until the fluid discharged from the impeller enters the suction port of the next stage impeller. Between the hub fixed to the casing and the rotating shaft facing the hub.

  According to the first to fourth embodiments, a screw groove or a parallel groove having a small leakage flow rate is installed at the impeller inlet where the leakage flow rate has a large effect on the pump efficiency, and the leakage flow rate has an effect on the pump efficiency. The shaft vibration of the pump main shaft is reduced by installing a smooth seal having a high liquid film rigidity in the seal between the front and rear impellers having a small size. As described above, the degree of influence of the seal and vibration in the pump is evaluated for each gap between the rotating body and the casing, and a seal structure having characteristics capable of coping with the evaluation is selected. The present invention can provide a multistage centrifugal pump having both of the above-mentioned performances.

1 shows a sectional view of a high-pressure pump (multistage centrifugal pump) according to the present embodiment. The first stage impeller 21 is shown. It is a fragmentary sectional view of a parallel groove seal. It is a fragmentary sectional view of a damper seal which has a honeycomb pattern. It is a fragmentary sectional view of a damper seal which has a hole pattern. It is a fragmentary sectional view of a screw groove seal.

  Next, an embodiment of the present invention will be described with reference to the drawings. Note that the following description is merely an example of the embodiment, and the technical scope of the present invention is not limited to the following embodiment. In addition, the components described below can constitute the present invention alone or in any combination.

[Overview of pump]
FIG. 1 is a sectional view of a high-pressure pump (multistage centrifugal pump) according to the present embodiment. With reference to this sectional view, the structure and mechanism of the high-pressure pump will be described. The high-pressure pump 1 includes a main shaft 11 that is connected to and rotates with an electric motor (not shown), impellers 21 a, 21 b, and 21 c that are fixed to the rotation shaft 11 and rotate together with the rotation shaft 11, and impellers 21 a, 21 b, and 21 c. A casing 31 to be housed; shaft seals 51 a and 51 b attached to a position where the rotating shaft 11 penetrates the casing 31 to seal liquid in the casing 31 so as not to leak from between the casing 31 and the rotating shaft 11; 31 are provided with bearings Z and Z 'for supporting the rotating shaft 11 extended to the outside. An electric motor for rotationally driving the high-pressure pump is connected via a coupling 13 connected to the left end of the rotating shaft 11.

  The casing 31 has a suction port 33 for supplying a liquid such as water from outside the casing 31 into the high-pressure pump 1, and a liquid such as water pressurized by the impellers 21 a, 21 b, and 21 c inside the high-pressure pump 1. Is provided with a discharge port 37 for discharging the liquid. The first stage impeller 21a fastened to the rotating shaft 11 rotates with the rotation of the rotating shaft 11, and a liquid such as water is pressurized by centrifugal force and discharged from the outer peripheral portion of the impeller 21a. It is guided to the second stage impeller 21b through the flow path 35a. The first-stage impeller 21a continuously discharges liquid such as water, so that liquid such as water is guided from the suction port 33 to the center of the impeller 21a.

Similarly, the fluid pressurized by the second-stage impeller 21b passes through the second flow path 35b and is guided to the third-stage impeller 21c. The fluid is further pressurized by the third stage impeller 21c, and
7 and transferred by a pipe (not shown). That is, the fluid is pressurized by the first-stage impeller 21a, the second-stage impeller 21b, and the third-stage impeller 21c. The rotating shaft 11 is supported by the bearings Z and Z ', but is not supported between the bearings Z and Z'. Therefore, the rotating shaft 11 is moved between the bearings Z and Z 'with the rotation. A whirling of which the amplitude becomes larger as the middle point of.

  Due to the pressure difference between the high pressure side and the low pressure side generated on the suction side and the discharge side of each of the impellers 21a, 21b, 21c, the fluid flows from the high pressure side to the low pressure side if there is a slight gap. This flow is a leak because it acts to cancel the high-low pressure difference generated by the action of the impeller. Such a leak is caused by a portion surrounded by a circle in the figure, specifically, an impeller inlet portion, between the impeller and the next impeller, a final impeller outlet and a low-pressure chamber for thrust balance (this chamber). Is connected to the pump inlet pressure). Generally, the greater the leakage, the greater the effect on pump performance. This leakage increases as the pressure difference between high and low pressures increases, as the distance from the center of the rotating shaft increases, and in a multi-stage pump, as the number of stages of the impeller increases.

  FIG. 2 shows, for convenience, the first stage impeller 21a of the impellers 21a, 21b, 21c shown in FIG. 1 as an enlarged view around the impeller. That is, the same description applies to the other impellers 21b and 21c. Fluid leakage points around the first stage impeller 21a of the multi-stage pump 1 mainly include a portion X facing the outer peripheral surface of the impeller suction port 23 and the casing 31a, and an outer peripheral surface of the back surface of the first stage impeller 21a. And the casing 31b. The opposing surface of the opposing portion X includes the impeller ring 41a on the rotating side and the casing ring 41b on the fixed side, and the opposing surface of the opposing portion Y includes the rotating side surface 43a and the fixed side surface 43b. In addition, a cylindrical hub may be provided on the inner surface of the casing 31b as the fixed side surface 43b in the facing portion Y. Since the first stage impeller 21a corresponds to the impellers 21b and 21c, there are a plurality of opposing portions X and Y in the multi-stage pump 1.

  Comparing the opposing part X and the opposing part Y, the opposing part X is located at a position radially farther from the rotation shaft 11 than the opposing part Y. Therefore, the flow path cross-sectional area of the gap at the opposing portion X is larger than the flow path cross-sectional area of the gap at the opposing portion Y. In addition, the pressure difference before and after the passage of the fluid passing through the gap between the opposed portions X is larger than the pressure difference before and after the passage of the fluid passing through the gap between the opposed portions Y. That is, it can be said that the leakage of the liquid such as water pressurized by the impeller 21a through the gap between the opposed portions X greatly affects the pump performance.

Table 1 shows a comprehensive evaluation using the multi-stage pump 1 shown in FIGS. 1 and 2 for the seal configuration of the opposed portion X and the opposed portion Y using vibration performance, pump performance, and cost as indices.

  In Comparative Example 1, the opposing surfaces of the opposing portions X and Y, that is, the impeller ring 41a, the casing ring 41b, the rotating side surface 43a, and the fixed side surface 43b all have a smooth cylindrical surface. That is, a conventional general smooth seal is provided at the opposing portion X and the opposing portion Y. When the vibration performance, pump performance, and cost in this case were each set to 100, the total evaluation score was 300 points.

  In Comparative Example 2, the rotation side of the opposing surface of the opposing portion X and the opposing portion Y, that is, the impeller ring 41a and the rotating side surface 43a are smooth cylindrical surfaces, and the fixed side, that is, the casing ring 41b and the fixing side surface 43b are threaded. Have been. In other words, the opposing portion X and the opposing portion Y are provided with a thread groove seal in which only the fixed side is threaded. In this case, the pump performance is improved as compared with Comparative Example 1, but the vibration performance is reduced by the amount that the liquid film rigidity is not generated. In addition, the cost is increased by forming grooves in all the seal portions. Therefore, the overall evaluation score was 300 as in Comparative Example 1.

  In Comparative Example 3, the rotation side of the opposing surface of the opposing portion X and the opposing portion Y, that is, the impeller ring 41a and the rotating side surface 43a are smooth cylindrical surfaces, and the fixed side, that is, the casing ring 41b and the fixing side surface 43b are formed by parallel grooves. Have been. That is, the opposing portion X and the opposing portion Y are provided with parallel groove seals in which only the fixed side is processed into parallel grooves. In Comparative Example 3, as in Comparative Example 2, the pump performance is improved as compared with Comparative Example 1, but the vibration performance is low and the cost is high. Therefore, the overall evaluation score was 300 points, as in Comparative Example 1.

  In the first embodiment, the rotation side of the opposing surface of the opposing portion X, that is, the impeller ring 41a is a smooth cylindrical surface, and the fixed side, that is, the casing ring 41b is threaded. That is, the opposite portion X is provided with a thread groove seal in which only the fixed side is threaded. The rotating side surface 43a and the fixed side surface 43b of the opposing portion Y are smooth cylindrical surfaces. That is, a smooth seal is provided at the opposing portion Y. The opposing portion X is located at a position farther from the rotation axis than the opposing portion Y, has a larger flow path cross-sectional area of the gap, and has a larger pressure difference before and after passage of the fluid passing through the gap. , Large leakage. According to the first embodiment, the amount of leakage at the opposing portion X having a relatively large amount of leakage is reduced, and the opposing portion Y having a small amount of leakage is originally formed by a liquid film rigidity effect of a liquid film generated in a smooth cylindrical surface. It works to suppress vibration. Although the vibration performance in this case is slightly lower than that of Comparative Example 1, the pump performance is higher than that of Comparative Example 1, and the cost is equivalent to that of Comparative Example 3 because the thread groove is formed only in the casing ring 41b at the opposing portion X. Was. As a result, the comprehensive evaluation point of Example 1 was 315 points.

  In the second embodiment, the rotating side of the opposing surface of the opposing portion X, that is, the impeller ring 41a is a smooth cylindrical surface, and the fixed side, that is, the casing ring 41b is formed with parallel grooves. That is, the opposing portion X is provided with a parallel groove seal in which only the fixed side is processed into parallel grooves. The rotating side surface 43a and the fixed side surface 43b of the opposing portion Y are smooth cylindrical surfaces. That is, a smooth seal is provided at the opposing portion Y. According to the second embodiment, the amount of leakage at the opposing portion X having a relatively large amount of leakage is reduced, and the opposing portion Y having a small amount of leakage is originally formed by a liquid film rigidity effect of a liquid film generated in a smooth cylindrical surface. It works to suppress vibration. Although the vibration performance in this case is slightly lower than that of the comparative example 1, the pump performance is higher than that of the comparative example 1, and the cost is lower than that of the comparative example 1 because the parallel groove machining is performed only on the casing ring 41b of the opposing portion X. As a result, the comprehensive evaluation point of Example 2 was 310 points.

  As described above, the multi-stage pump 1 using the seal configuration according to the first and second embodiments has a more comprehensive performance in terms of vibration performance, pump performance, and cost than the comparative examples 1 to 3. It can be seen that it has good results.

In the first embodiment or the second embodiment, at the opposing portion X, as a non-contact annular seal constituted by an impeller ring 41a attached to the outer periphery of the impeller 21a and a casing ring 41b attached to the casing so as to face the impeller ring 41a, A seal of good construction is used, ie a thread groove seal or a parallel groove seal. Specifically, a smooth cylindrical surface is provided on the impeller ring 41a, and a thread groove or a parallel groove is provided on the casing ring 41b. The gap between the opposed portion Y, that is, the gap between the outer peripheral surface of the back surface of the impeller 21a and the casing 31b, has a relatively small pressure difference between the inlet and the outlet of the gap as compared with the opposed portion X. The area is also relatively small. For this reason, even if the non-contact annular seal is a smooth seal inferior in shaft sealing performance and formed of a double cylinder having a parallel leak flow path surface, the influence of leakage is small. Rather, by providing a smooth seal at the opposed portion Y, the liquid film in the seal exhibits a vibration characteristic in which a stiffness effect and a damping effect are generated, and suppresses vibration such as whirling of the rotating shaft of the pump. By properly arranging the appropriate seals in this way, the advantages of the respective seals can be utilized and the shortcomings can be compensated for, thereby improving the pump performance and the vibration performance as a whole.

  INDUSTRIAL APPLICATION This invention can be utilized for the non-contact annular seal of rotary machines, such as a pump.

11 Pump main shaft 21a First stage impeller 21b Second stage impeller 21c Third stage impeller 33 Pump suction ports 35a, 35b, 35c Flow path 37 Pump discharge port 41 Non-contact seal 43 Non-contact seal 36 Low-pressure side flow path 45 Screw groove seal 47 Damper seal (hole pattern)
47 Damper seal (honeycomb pattern)

Claims (4)

A rotating body including a rotating shaft and a plurality of impellers provided on the rotating shaft,
A casing surrounding the rotating body,
A non-contact annular seal that seals a gap between opposed parts of the rotating body and the casing, a multi-stage centrifugal pump,
A first facing portion where the casing and the rotating body face each other,
A second facing portion, wherein a distance between the second facing portion and the rotation axis is smaller than a distance between the first facing portion and the rotating shaft, a second facing portion; , And
The non-contact annular seal is provided at the first facing portion and the second facing portion,
The non-contact annular seal at the first opposed portion has a thread groove or a parallel groove provided on the casing-side leakage flow surface and a smooth groove provided on the rotating side leakage flow surface of the first opposed portion. And a cylindrical surface,
The non-contact annular seal in the second opposed portion is provided on each of the casing-side leakage flow path surface and the rotation-side leakage flow path surface, and includes cylindrical surfaces that are parallel to each other, Multistage centrifugal pump. The multi-stage centrifugal pump according to claim 1,
The difference between the inlet pressure and the outlet pressure of the leak flow of the non-contact annular seal at the first opposed portion is larger than the difference between the inlet pressure and the outlet pressure of the leak flow of the non-contact annular seal at the second opposed portion. A multi-stage centrifugal pump. The multi-stage centrifugal pump according to claim 1 or 2,
The multistage centrifugal pump, wherein the first opposing portion is constituted by an outer peripheral portion of a suction port of the impeller and a casing opposed thereto. The multi-stage centrifugal pump according to any one of claims 1 to 3,
The second opposed portion is constituted by a hub fixed to the casing until the fluid discharged from the impeller enters the suction port of the next stage impeller, and the rotating shaft facing the hub. A multi-stage centrifugal pump, characterized in that:

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