Multistage centrifugal pump

Abstract

A multistage pump is proposed, having an external pump casing and multiple stages, including at least a first stage and a last stage. Each stage includes a stage casing, an impeller, and a diffuser. The multistage pump further includes a pump inlet for supplying fluid to the impeller of the first stage, a pump outlet for discharging fluid, and a pump shaft configured to rotate about an axial direction. Each impeller is mounted to the pump shaft in an anti-torsional manner. All impellers are arranged one after another on the pump shaft. The last stage includes a collector with a collector chamber configured to receive fluid from the diffuser of the last stage. All other stages except the last stage include guide channels configured to receive fluid from a specific diffuser and guide the fluid to the impeller of the next stage. The collector chamber is axially displaced relative to the diffuser of the last stage.

Description

Technical Field

[0001] The present invention relates to a multistage pump for conveying fluids according to the preamble of the independent claim. Background Technology

[0002] Multistage pumps are used in many different industries, especially for applications requiring high pressure. Important industries using multistage pumps include oil and gas processing, power generation, chemicals, cleaning and wastewater treatment, and pulp and paper manufacturing.

[0003] In the oil and gas processing industry, multistage pumps are designed, for example, to transport hydrocarbon fluids, such as for extracting crude oil from oil fields or for transporting oil / gas through pipelines or within refineries. Another application is the injection of process fluids (in most cases, water, and especially seawater) into oil storage units. For such applications, the pump is designed as a (water) injection pump that supplies seawater under high pressure to wells leading to underground areas of the oil storage unit.

[0004] For other applications, multistage pumps can be designed as boiler feed pumps in power plants, or as booster pumps, for example, in reverse osmosis processes used for water desalination, to name just a few.

[0005] A multistage pump comprises multiple stages, each with its own impeller, all arranged one after another on a common pump shaft. The pump shaft is driven to rotate about an axial direction, causing all the impellers to rotate together about that axial direction.

[0006] Figure 1 This is a schematic representation of a multistage pump 1' known from the prior art. Figure 1 The multistage pump 1' is shown in axial cross-section along the axial direction A. For better understanding, Figure 2 According to Figure 1 The cross section perpendicular to the axial direction A, indicated by the cutting line II-II, shows the multistage pump 1'.

[0007] The multistage pump 1' includes an outer pump casing 2' extending in an axial direction A, defined by the axis of a pump shaft 9' centrally passing through the outer pump casing 2'. The outer pump casing 2' includes a cylindrical housing 21', which is closed at its first axial end by a suction cover 22' and at its second axial end by a discharge cover 23'. The suction cover 22' and the discharge cover 23' are fixedly mounted to the cylindrical housing 21'.

[0008] The multistage pump 1' comprises multiple stages, namely a first stage 31', a last stage 33', and multiple (here, three) intermediate stages 32', wherein all intermediate stages 32' are arranged between the first stage 31' and the last stage 33'. All stages 31', 32', and 33' are arranged one after another inside the cylindrical housing 21', such that the cylindrical housing 21' encloses all stages 31', 32', and 33'.

[0009] The multistage pump 1' further includes a pump inlet 4' for supplying fluid to the first stage 31' and a pump outlet 5' for discharging the fluid. Thus, the first stage 31' is the stage closest to the pump inlet 4', and the last stage 32' is the stage closest to the pump outlet 5'.

[0010] Each stage 31', 32', and 33' includes a stage housing 6', an impeller 7' for acting on the fluid, and a diffuser 8' configured to surround and receive fluid from the impeller 7'. Each impeller 7' is mounted to the pump shaft 9' in an anti-torsional manner. All impellers 7' are arranged one after another in the axial direction A. All diffusers 8' in the first stage 31' and all intermediate stages 32' include guide channels 81', which are arranged downstream of a particular diffuser 8'. The guide channels 81' are configured to receive fluid from the particular diffuser 8' and guide the fluid to the impeller 7' of the next stage. The final stage 33' includes a collector 10' with a collector chamber 11' configured to receive fluid from the diffuser 8' of the final stage 33' and guide the fluid to the pump outlet 5'.

[0011] Collector 10' is configured to concentrically enclose the final stage 33' of diffuser 8', such that collector chamber 11' is designed as an annular collector chamber 11', which radially surrounds the entire diffuser 8' along its circumference. This in Figure 2 The best view is in the middle.

[0012] The collection chamber 11' and the diffuser 8' are aligned with respect to the axial direction A, wherein the diffuser 8' is arranged radially inward relative to the collection chamber 11' of the collector 10'.

[0013] The fluid flows through the multistage pump 1' Figure 1 and Figure 2Both are indicated by arrows without reference numerals. Fluid enters the multistage pump 1' through pump inlet 4', turns axially in direction A, and is directed to the suction side of impeller 7' of the first stage 31'. The impeller 7' acts on the fluid and discharges it radially into diffuser 8' of the first stage 31'. Downstream of diffuser 8', the fluid is directed by guide passage 81' of the first stage 31' to the suction side of impeller 7' of the first intermediate stage 32'. After passing through all intermediate stages 32' in a similar manner, the fluid is directed to the suction side of impeller 7' of the final stage 33'. The impeller 7' discharges the fluid radially into diffuser 8' of the final stage 33', from where the fluid radially outward enters collection chamber 11' surrounding diffuser 8'. The fluid is discharged from collection chamber 11' through outlet 5' of multistage pump 1. Summary of the Invention

[0014] Based on the prior art, the purpose of this invention is to propose a different multistage pump, and in particular a multistage pump that can be constructed more economically.

[0015] The subject matter of the invention that achieves this purpose is characterized by the features of the independent claims.

[0016] Therefore, according to the present invention, a multistage pump for conveying fluid is proposed, having an outer pump casing and a plurality of stages arranged in the outer pump casing, the plurality of stages including at least a first stage and a last stage, each stage including a stage housing, an impeller for acting on the fluid, and a diffuser configured to surround and receive fluid from the impeller, the multistage pump further including a pump inlet for supplying the fluid to the impeller of the first stage, a pump outlet for discharging the fluid, and a pump shaft configured for rotation about an axial direction, wherein each impeller is mounted to the pump shaft in an anti-torsional manner, wherein all impellers are arranged one after another on the pump shaft, wherein the last stage includes a collector with a collector chamber configured to receive fluid from the diffuser of the last stage, wherein all other stages except the last stage include a guide channel configured to receive fluid from a particular diffuser and guide the fluid to the impeller of the next stage, and wherein the collector chamber is axially displaced relative to the diffuser of the last stage.

[0017] The configuration with a collector chamber offset axially relative to the diffuser offers several advantages. Since the collector chamber of the last stage is no longer required to be arranged radially outward around the diffuser of the last stage, the outer diameter of the hydraulic section of the multistage pump can be significantly reduced. The outer diameter of the hydraulic section is primarily determined by the outer diameter of the fluid guiding components within the multistage pump. Therefore, the inner diameter of the outer pump casing can be significantly reduced.

[0018] Therefore, even without reducing the wall thickness of the outer pump casing, the overall external dimensions, particularly the outer diameter of the multistage pump, are significantly reduced. This reduced overall external extension of the multistage pump results in a reduced weight, a reduced mass of material required for the outer pump casing, and a reduced space requirement. These factors make the multistage pump according to the invention more cost-effective without negatively impacting its efficiency or reliability.

[0019] Furthermore, due to the reduced diameter, the fixing elements (e.g., tie rods) used to fasten the stage and the fixing elements (e.g., nuts and bolts) used to seal the outer pump housing can be arranged more significantly inward relative to the radial direction (i.e., closer to the pump shaft). Moving these fixing elements radially inward reduces the forces acting on them. Therefore, the size of the fixing elements can be reduced.

[0020] Furthermore, in terms of pressure boundaries, a reduced size of the outer pump casing, particularly a reduced diameter of the outer pump casing, is advantageous.

[0021] Given the reduction in diameter, particularly the diameter of the hydraulic components, it is advantageous that the collector chamber is axially shifted relative to the last-stage diffuser to the extent that the diffuser and the collector chamber do not overlap axially. Therefore, the collector chamber is positioned completely behind the diffuser axially and when viewed from the pump inlet toward the pump outlet.

[0022] Preferably, each impeller is configured as a radial impeller for discharging fluid in a radial direction, wherein the radial direction is perpendicular to the axial direction. Therefore, the diffusers of the plurality of stages are also designed as radial diffusers for receiving fluid from a particular impeller in a generally radial direction.

[0023] According to a preferred embodiment, the collector forms the final stage housing.

[0024] Furthermore, preferably, the final stage diffuser and collector are configured such that the fluid is diverted radially within the collector chamber, wherein the radial direction is perpendicular to the axial direction. Thus, the final stage diffuser directs the fluid toward a generally axial flow direction, and only within the collector chamber, i.e., downstream of the diffuser, is the fluid redirected radially toward the pump outlet.

[0025] According to a preferred embodiment, the collector chamber is configured as an annular collector chamber, wherein the outer diameter of the collector chamber is at most the same as the outer diameter of the diffuser of the last stage. This configuration, in particular, reduces the outer diameter of the hydraulic portion of the multi-stage pump.

[0026] Advantageously, the outer diameter of the collector chamber is equal to (or at least approximately) the outer diameter of the diffuser of the last stage.

[0027] Especially for applications requiring high head or high pressure at the pump outlet, the multiple stages include at least one intermediate stage, wherein each intermediate stage is arranged between the first stage and the last stage.

[0028] Therefore, particularly for high-pressure applications, the multistage pump is preferably constructed with at least three stages, namely a first stage, an intermediate stage, and a last stage arranged in series relative to the axial direction. It goes without saying that the multistage pump according to the invention can also be constructed with more than three stages.

[0029] Several possibilities exist for fixing the stages relative to each other. According to a preferred solution, the multistage pump includes a plurality of tie rods configured to fix the multiple stages relative to each other, wherein each tie rod extends axially through all stage housings parallel to the pump shaft. In other embodiments, there are no tie rods, but the stage housings are pushed together by hydraulic pressure generated by the multistage pump during operation. In still other embodiments, two adjacent stage housings are fixed to each other by fastening elements (e.g., screws or nuts and bolts), connecting only these specific stage housings such that the stage housings are fixed to each other in pairs.

[0030] According to a preferred embodiment, the outer pump housing includes a cylindrical housing configured to receive all stages, such that the cylindrical housing encloses the plurality of stages.

[0031] Regarding this design, the cylindrical housing is preferably constructed in a tubular shape and extends coaxially from the first axial end to the second axial end with respect to the pump shaft.

[0032] Furthermore, in this embodiment, it is advantageous that the multistage pump includes an inlet cap configured to close a first axial end of the cylindrical housing, and an outlet cap configured to close a second axial end of the cylindrical housing.

[0033] Preferably, each of the inhalation cap and the discharge cap is fastened to the cylindrical housing by means of fastening elements (e.g., nuts and bolts).

[0034] Other advantageous features and embodiments of the invention will become apparent from the dependent claims. Attached Figure Description

[0035] The invention will be explained in more detail below with reference to embodiments thereof and the accompanying drawings. As illustrated schematically:

[0036] Figure 1 A schematic cross-sectional view of a multistage pump known from the prior art, taken along the axial direction.

[0037] Figure 2 : Figure 1 Multistage pumps according to Figure 1The cross-section perpendicular to the axial direction of the cutting line II-II in the diagram.

[0038] Figure 3 A schematic cross-sectional view of an embodiment of the multi-stage pump according to the present invention, taken along the axial direction.

[0039] Figure 4 : Figure 3 The embodiments are arranged along Figure 3 The cross section perpendicular to the axial direction of the cutting line IV-IV in the diagram.

[0040] Figure 5 : Figure 3 The embodiments are arranged along Figure 3 The cross section perpendicular to the axial direction of the cutting line VV in the diagram, and

[0041] Figure 6 : Figure 1 The last stage (right side) of the multistage pump and Figure 3 A comparison of the last level (left side) of the embodiments. Detailed Implementation

[0042] Figure 1 This is a schematic representation of a multistage pump 1' known from the prior art. Figure 1 The multistage pump 1' is shown in axial cross-section along the axial direction A. For better understanding, Figure 2 According to Figure 1 The cross-section perpendicular to the axial direction A, indicated by cutting line II-II, shows the multi-stage pump 1'. This has already been explained in the description of the prior art above. Figure 1 and Figure 2 Therefore, no further explanation is needed. To distinguish prior art multistage pumps from those according to embodiments of the present invention, the components of the prior art multistage pump 1' are... Figure 1 , Figure 2 and Figure 6 In Chinese, the figure references are indicated by an apostrophe (quotation mark) following the corresponding figure reference.

[0043] Figure 3 A schematic cross-sectional view of an embodiment of a multistage pump according to the present invention is shown, generally indicated by reference numeral 1. The multistage pump 1 is designed as a centrifugal pump for conveying fluid from pump inlet 4 to pump outlet 5.

[0044] The multistage pump 1 includes an outer pump casing 2 and multiple stages 3, each of which includes an impeller 7 for acting on a fluid. All impellers 7 are arranged one after another on a pump shaft 9, which is configured to rotate about an axial direction A. The pump shaft 9 passes centrally through the outer pump casing 2 and is supported by radial bearings (also called journal bearings (not shown)) and at least one axial bearing (also called thrust bearings (not shown)). Furthermore, a shaft seal (not shown), such as a mechanical seal, is provided in a manner known in the art. The shaft seal prevents fluid leakage along the pump shaft 9 from the inside of the outer pump casing 2 to the outside of the outer pump casing 2.

[0045] The axial direction A is defined by the longitudinal axis of the pump shaft 9 (i.e., the axis of rotation about which the pump shaft 9 rotates during operation). The direction perpendicular to the axial direction A is called the 'radial direction'. The terms 'axial' or 'axially' are used in the general sense of 'along the axial direction' or 'relative to the axial direction'. Similarly, the terms 'radial' or 'radially' are used in the general sense of 'along the radial direction' or 'relative to the radial direction'.

[0046] All impellers 7 are mounted to the pump shaft 9 in an anti-torsional manner. The pump shaft 9 is driven by a drive unit (not shown) (e.g., an electric motor). Figure 1 In the illustrated embodiment, the drive unit is disposed outside the outer pump housing 2 and coupled to the pump shaft 9 in any manner known in the art. In other embodiments, the drive unit may be disposed inside the outer pump housing 2.

[0047] Figure 3 The multistage pump 1 is shown in a schematic cross-sectional view along section A in the axial direction. For better understanding, Figure 4 Press along Figure 3 The cross-section perpendicular to the axial direction A at the cutting line IV-IV in the diagram shows the multi-stage pump 1, and Figure 5 Press along Figure 3 The cross section of the cutting line VV perpendicular to the axial direction A shows the multistage pump 1.

[0048] The outer pump housing 2 includes a cylindrical housing 21, which is closed at its first axial end by a suction cover 22 and at its second axial end by a discharge cover 23. The suction cover 22 and discharge cover 23 are fixedly mounted to the cylindrical housing 21, for example, by means of nuts and bolts 24. The pump shaft 9 passes centrally through both the suction cover 22 and the pressure cover 23. The cylindrical housing 21 is pressed between the suction cover 22 and the discharge cover 23. The cylindrical housing 21 is constructed in a tubular shape and extends coaxially from the first axial end to the second axial end with respect to the pump shaft 9. Furthermore, the cylindrical housing 21 is designed to receive multiple stages 3, such that the multiple stages 3 are enclosed by the cylindrical housing 21.

[0049] The multistage pump 1 has multiple stages 3, each stage 3 including at least a first stage 31 and a last stage 33. The multiple stages 3 may further include one or more intermediate stages 32. All intermediate stages 32 are arranged relative to the axial direction A between the first stage 31 and the last stage 33. All stages 31, 32, and 33 are arranged one after another inside a cylindrical housing 21, such that the cylindrical housing 21 encloses all stages 31, 32, and 33. The first stage 31 is located near the suction cap 22 adjacent to the pump inlet 4 and receives fluid with low pressure from the pump inlet 4. The last stage 33 is located adjacent to the discharge cap 23 and discharges fluid with high pressure through the pump outlet 5. The flow of fluid through the multistage pump 1 is indicated in the figure by arrows without reference numerals.

[0050] exist Figure 3 In the embodiment shown, the multistage pump 1 includes three intermediate stages 32, thus the multistage pump 1 has five stages 31, 32, and 33. It must be understood that the number of five stages 31, 32, and 33 is merely an example. In other embodiments, the multistage pump may include fewer than five stages, such as only two stages, i.e., no intermediate stages. In still other embodiments, the multistage pump may include more than five stages, such as eight stages.

[0051] Figure 3 The multistage pump 1 further includes a balancing drum 12, which is arranged between the last stage 33 and the discharge cover 23. The balancing drum 12 is known in the art. The balancing drum 12 has a first axial surface exposed to high pressure behind the last stage 33 and a second axial surface exposed to pressure in a chamber 13, wherein the pressure in the chamber 13 is significantly lower than the high pressure. Typically, the chamber 13 is connected to the pump inlet 4 via a balancing line (not shown), such that the pressure in the chamber 13 is substantially the same as the low pressure at the pump inlet 4 on the suction side of the multistage pump 1. A portion of the pressurized fluid flows as a leakage flow from the first axial surface along the balancing drum 12 to the second axial surface through an annular gap and flows into the chamber 13. The pressure difference between the pressures at the first and second axial surfaces of the balancing drum 12 generates a force along the axial direction A on the pump shaft 9, wherein the force counteracts the hydraulic pressure generated by the rotating impeller 7.

[0052] Each of the multiple stages 3, 31, 32, 33 includes a stage housing 6, an impeller 7 for acting on the fluid, and a diffuser 8 configured to surround the impeller 7 and receive the fluid from the impeller 7.

[0053] The stage housings 6 are arranged in series with respect to the axial direction A. The stage housing 6 of the first stage 31 abuts against the stationary portion 61 of the multistage pump 1, wherein the stationary portion 61 is stationary relative to the outer pump housing 2. Each of the subsequent stage housings 6 abuts against the corresponding preceding stage housing 6. Thus, the entire stage housing 6 forms the inner pump housing.

[0054] The stage housing 6 is fixed relative to each other by a plurality of tie rods 14. Each tie rod 14 extends parallel to the pump shaft 9 in the axial direction A and passes through all the stage housings 6. The tie rods 14 are tensioned by means of tensioners 15 in a manner known in the art.

[0055] All impellers 7 are configured as radial impellers 7 having multiple impeller blades that redirect the flow of fluid from a generally axial direction to a radial direction. Each impeller 7 may also include a rear blade 71 ( Figure 4 ).

[0056] All diffusers 8 are configured as radial diffusers and arranged to radially enclose the corresponding impellers 7. Downstream of each diffuser 8 in the first stage 31 and all intermediate stages 32, in each case, a plurality of guide channels 81 are provided to redirect the generally radial flow of fluid into the axial direction A and to guide the fluid from the corresponding diffuser 8 to the suction side of the impeller 7 of the next stage. Preferably, the guide channels 81 are defined by guide vanes 82, which can be bent to smoothly redirect the fluid; that is, each guide channel 81 is arranged between two adjacent guide vanes.

[0057] The final stage 33 includes a collector 10 with a collector chamber 11 configured to receive fluid from the diffuser 8 of the final stage 33 and direct the fluid to the pump outlet 5.

[0058] According to the invention, the collector chamber 11 is displaced relative to the diffuser 8 of the last stage 33 in the axial direction A. When viewed along the axial direction A and the flow direction of the fluid, the collector chamber 11 is arranged behind the last stage diffuser 8. Preferably, the collector chamber 11 is displaced relative to the diffuser 8 of the last stage 33 to such a degree that the diffuser 8 of the last stage 33 and the collector chamber 11 do not overlap with respect to the axial direction A.

[0059] Arranging the collector chamber 11 behind the diffuser 8 of the last stage 33 has the following significant advantages: the outer diameter of the hydraulic section of the multistage pump 1 is significantly smaller compared to an arrangement in which the collector chamber is arranged radially outward around the diffuser of the last stage.

[0060] In an embodiment of the multistage pump 1 according to the invention, the outer diameter of the hydraulic section is at least substantially the same as the outer diameter D1 of the diffuser 8 of the last stage 33.

[0061] Because the outer diameter D1 decreases, the inner diameter of the cylindrical shell 21 can be reduced. Therefore, the outer diameter DA of the cylindrical shell 21... Figure 6 The outer diameter DA of the cylindrical shell 21 can also be reduced. This reduction in the outer diameter DA is due to... Figure 6 China passed Figure 6 The embodiment of the multi-stage pump 1 according to the present invention on the left side and Figure 6On the right side Figure 1 A direct comparison with the prior art multistage pump 1' is shown. The reduction of D can be, for example, about 20% of the outer diameter of the cylindrical housing.

[0062] Therefore, the overall external dimensions of the multistage pump, particularly the outer diameter DA of the cylindrical housing 21, can be significantly reduced by the present invention. This results in a reduced weight of the multistage pump 1, and consequently a reduction in the mass of materials required for the cylindrical housing 21, the suction cover 22, and the discharge cover 23. Thus, the total cost of the multistage pump 1 is reduced without compromising any efficiency or operational safety.

[0063] An additional advantage lies in the fact that the tie rod 14, along with the nuts and bolts 24, is closer to the pump shaft 9 relative to the radial direction. Moving these fixing elements radially inward (i.e., the tie rod 14 for the stage housing 6 and the nuts and bolts 24 for the outer pump housing 2) reduces the forces acting on these fixing elements. Therefore, the dimensions of the tie rod 14 and the nuts and bolts 24 can be reduced, and / or the number of tie rods 14 and / or nuts and bolts 24 can be decreased.

[0064] In addition, in terms of pressure boundary, the reduced size of the outer pump casing, especially the reduced outer diameter DA of the cylindrical casing 21, is advantageous.

[0065] The fluid flows through the multistage pump 1. Figure 3 – Figure 6 The direction is indicated by arrows without reference numerals. Fluid enters the multistage pump 1 through pump inlet 4, turns axially in direction A, and is directed to the suction side of impeller 7 in the first stage 31. The impeller 7 acts on the fluid and discharges it radially into diffuser 8 in the first stage 31. Downstream of diffuser 8, the fluid is guided by guide channel 81 of the first stage 31 to the suction side of impeller 7 in the first intermediate stage 32. After passing through all intermediate stages 32 in a similar manner, the fluid is directed to the suction side of impeller 7 in the final stage 33. The impeller 7 discharges the fluid radially into diffuser 8 in the final stage 33, from where the fluid is directed to collection chamber 11, which is arranged relative to axial direction A behind diffuser 8 in the final stage 33. The fluid is discharged from collection chamber 11 through outlet 5 of multistage pump 1.

[0066] For example in Figure 3 As can be seen, preferably, the collector 10 forms the stage housing 6 of the last stage 33.

[0067] As in Figure 6Best visible on the left side, the diffuser 8 and collector 10 of the last stage 33 are configured such that the fluid discharged from the impeller 7 in the radial direction is diverted by the diffuser 8 from the radial direction in the generally axial direction A, then diverted in the collector chamber 11 from the generally axial direction A in the generally radial direction, and then guided to the pump outlet 5.

[0068] Furthermore, preferably, such as in Figure 5 As shown, the collector chamber 11 is configured as a substantially annular collector chamber 11, wherein the outer diameter D2 of the collector chamber is at most the same as the outer diameter D1 of the diffuser 8 of the last stage 33. In particular, the collector chamber 11 may be configured such that the outer diameter D2 of the collector chamber 11 is equal to the outer diameter D1 of the diffuser 8 of the last stage 33.

[0069] Furthermore, preferably, the collector chamber 11 is shaped like a spiral, such that the collector 10 with the collector chamber 11 is constructed as a volute. This construction of the collector 10 and the collector chamber 11... Figure 5 The collector chamber 11 is best visible in the center. When viewed along the direction of fluid flow, the width of the collector chamber 11 increases in the radial direction (i.e., perpendicular to the axial direction A). Therefore, when viewed along the direction of fluid flow, the cross-sectional area perpendicular to the direction of fluid flow increases. The spiral collector chamber 11 thus forms a volute with the shell of the collector 10.

[0070] The radial impeller 7 of the final stage 33 delivers fluid radially to the diffuser 8 of the final stage 33. The diffuser 8 of the final stage 33 is configured to redirect the fluid flow from the radial direction to the axial direction A. The fluid exits the diffuser 8 of the final stage 33 along the axial direction A. The fluid enters the collector 10 along the axial direction A. In the collector 10, the fluid is redirected from the axial direction A to the radial direction. The fluid is guided by the collector chamber 11 of the collector 10 to the pump outlet 5.

Claims

1. A multistage pump for conveying fluid, having an outer pump housing (2) and a plurality of stages (3) arranged in the outer pump housing (2), the plurality of stages (3) including at least a first stage (31) and a last stage (33), each stage (31, 32, 33) comprising: The multistage pump comprises a stage housing (6), an impeller (7) for acting on the fluid, and a diffuser (8) configured to surround the impeller (7) and receive the fluid from the impeller (7). The multistage pump further comprises: a pump inlet (4) for supplying the fluid to the impeller (7) of the first stage (31), a pump outlet (5) for discharging the fluid, and a pump shaft (9) configured to rotate about an axial direction (A), wherein each impeller (7) is mounted to the pump shaft in an anti-torsional manner, wherein all impellers (7) are arranged one after another on the pump shaft (9), wherein the last stage (33) includes a collector (10) with a collector chamber (11) configured to receive the fluid from the diffuser (8) of the last stage (33), wherein all other stages (31, , 32) All include a guide channel (81) configured to receive the fluid from a particular diffuser (8) and guide the fluid to the impeller (7) of the next stage (32, 33), and wherein the collector chamber (11) is displaced in the axial direction (A) relative to the diffuser (8) of the last stage (33), wherein the diffuser (8) of the last stage (33) is configured to redirect the fluid from the radial direction to the axial direction (A) and the collector (10) is configured to redirect the fluid from the axial direction (A) to the radial direction within the collector chamber (11), characterized in that: the collector chamber (11) is configured as an annular collector chamber (11), and wherein the outer diameter (D2) of the collector chamber (11) is at most the same as the outer diameter (D1) of the diffuser (8) of the last stage (33).

2. The multistage pump according to claim 1, wherein, The collector chamber (11) is shifted in the axial direction (A) relative to the diffuser (8) of the last stage (33) to such a degree that the diffuser (8) and the collector chamber (11) do not overlap with respect to the axial direction (A).

3. The multistage pump according to claim 1 or 2, wherein, Each impeller (7) is configured as a radial impeller (7) for discharging the fluid in a radial direction, wherein the radial direction is perpendicular to the axial direction (A).

4. The multistage pump according to claim 1 or 2, wherein, The collector (10) forms the stage housing (6) of the last stage (33).

5. The multistage pump according to claim 1, wherein, The outer diameter (D2) of the collector chamber (11) is equal to the outer diameter (D1) of the diffuser (8) of the last stage.

6. The multistage pump according to claim 1 or 2, wherein, The plurality of levels (3) includes at least one intermediate level (32), wherein each intermediate level (32) is arranged between the first level (31) and the last level (33).

7. The multistage pump according to claim 1 or 2, comprising a plurality of tie rods (14) configured to fix the plurality of stages (3) relative to each other, wherein, Each tie rod (14) extends parallel to the pump shaft (9) along the axial direction (A) through all stage housings (6).

8. The multistage pump according to claim 1 or 2, wherein, The outer pump housing (2) includes a cylindrical housing (21) configured to receive all stages (31, 32, 33) such that the cylindrical housing (21) encloses the plurality of stages (3).

9. The multistage pump according to claim 8, wherein, The cylindrical housing (21) is constructed in a tubular shape and extends coaxially from the first axial end to the second axial end with respect to the pump shaft (9).

10. The multistage pump according to claim 9, comprising a suction cap (22) configured to close the first axial end of the cylindrical housing (21) and a discharge cap (23) for closing the second axial end of the cylindrical housing (21).

11. The multistage pump according to claim 10, wherein, Each of the inhalation cap (22) and the discharge cap (23) is fastened to the cylindrical housing (21) by means of a fixing element (24).

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