GB2410297A

GB2410297A - Split casing for a multi-stage pump

Info

Publication number: GB2410297A
Application number: GB0500316A
Authority: GB
Inventors: John Fleming
Original assignee: Weir Pumps Ltd
Current assignee: Weir Pumps Ltd
Priority date: 2004-01-21
Filing date: 2005-01-10
Publication date: 2005-07-27
Anticipated expiration: 2025-01-10
Also published as: GB0401188D0; GB0500316D0; GB2410297B

Abstract

A split pump casing for a multi-stage pump 170 comprises at least two parts 172, 174 which are adapted to be coupled together to define a flow passage 176 between two successive impeller stages 182, 184. The casing may be split such that an upper portion 172 and a lower portion 174 are coupled together along a horizontal plane, and the flow passage 176 may be arranged to substantially run along the same horizontal plane. A suction branch and a discharge branch may be formed in one, or both, of the upper and lower casing portions 172, 174. The pump 170 may include impellers which are arranged face to face, or back to back. The pump 170 may also include cartridge type bearings 202 and seals 204, which may be arranged such that they can be removed without disassembling the casing 172, 174. Impellers 186, 188 may be attached to a shaft 190 of the pump 170 by shrink fitting.

Description

PUMP CASING

FIELD OF INVENTION

The present invention relates to a pump casing, and in particular, to a pump casing for a multistage centrifugal pump.

BACKGROUND OF INVENTION

A typical centrifugal pump includes a casing having a suction branch and a delivery branch, a shaft rotatably mounted within the casing, and one or more pump impellers secured to the shaft and located in a volute which may be defined by the casing or by a casing insert. In use, the shaft and impellers are rotated to draw fluid into the pump through the suction branch. The fluid is pressurized by being driven radially through the impellers from a central impeller fluid inlet to a circumferential impeller fluid outlet, with high pressure fluid being discharged from the pump through the discharge branch. It is common to provide the casing in two halves which are formed to be bolted together to form the complete casing. This "split casing" arrangement allows for ease of manufacture and maintenance/repair of the centrifugal pump as access to the internal components is readily achieved.

In order to achieve relatively large fluid pressures, a multistage pump having a number of impellers may be used, where the pressure of the fluid is progressively increased as it passes from one impeller stage to the next. The impellers of a multistage pump may be mounted on the shaft such that the impellers face in the same direction, that is, the central fluid inlets of the impellers face in the same direction, and the fluid flows in a single axial direction through the pump.

Alternatively, the impellers may be mounted on the shaft to face in opposite directions, which assists in balancing thrust forces on the shaft, and in some instances may allow for easier assembly and disassembly of the impellers and shaft.

However, where the impellers are mounted on the shaft in opposing directions, the fluid will be required at some point to be delivered from one impeller stage facing in one direction to a successive impeller stage facing in an opposite direction. It is known in the art to achieve this reversed fluid delivery by providing a specially adapted casing which defines a fluid passage, separate from the pump volute, between the required impeller stages. In conventional casing arrangements, this fluid passage is known as a cross-over passage, and typically extends over the top of the pump, known as a top cross-over passage, or extends beneath the pump, known as a bottom cross-over passage. In split casings, the cross-over passage is integrally formed or cast within a respective half of the casing. This arrangement, however, has a number of disadvantages. For example, providing the cross-over passage in one portion of the split casing requires a more involved and complex casting process due to the considerable differences in shape of the separate portions of the casing. That is, a larger inventory of casting cores and boxes and the like are required to form the different portions.

Additionally, providing the cross-over passage in one portion of the casing severely restricts access to the interior surface of the passage to ensure that an optimum surface finish is achieved or to apply internal coatings or the like. Furthermore, where a top cross-over passage is provided between low and high pressure impeller stages, the cut-water of the discharge volute from the lower pressure impeller stage will typically be inclined at around 80 to 110 degrees to the cut-water of the discharge volute of the higher pressure impeller stage.

Such an alignment results in unbalanced radial loads which contributes significantly to increasing shaft deflection and bearing loading.

It is an object of the present invention to obviate or at least mitigate the aforementioned and other

problems with the prior art.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided a pump casing for a pump having a plurality of impeller stages, said casing comprising at least two portions adapted to be coupled together to define a flow passage between two successive impeller stages.

Preferably, the casing is substantially horizontally split such that the two portions of the casing are adapted to be coupled together along a substantially horizontal plane. Preferably also, the flow passage defined by the two casing portions is adapted to provide fluid communication between the two impeller stages along a substantially horizontal split line of the pump casing.

This arrangement, therefore, provides a flow passage which may be capable of being split by separating the portions of the casing along a plane substantially aligned with the intended direction of flow. This particular feature of the present invention is advantageous in that the pattern construction of the different portions of the casing is similar in many respects, thus requiring a smaller inventory of casting cores and boxes and the like in order to manufacture the casing. This in turn results in improved and less expensive manufacturing capability as simpler and lower cost equipment may be utilised. Additionally, due to the fact that the flow passage is not formed in a single, fully enclosed cast portion as with prior art designs, the resulting castings are less complex and so reduce S casting costs and improve casting integrity.

Furthermore, full access is available to the flow passage thereby ensuring optimum surface finish can be achieved, thus optimising hydraulic efficiency. Additionally, where internal coatings are to be applied to the casing, the internal access to the flow passage results in simplification of the coating process resulting in increased integrity of the coating compared with conventional or known designs.

The pump casing includes a suction branch, or inlet, and a discharge branch, or outlet. In one disclosed embodiment, both suction and discharge branches are formed in one of the at least two casing portions.

Advantageously, where a horizontally split casing is provided, the suction and discharge branches are formed in a lower casing portion. On the other hand, both branches may be formed in an upper casing portion.

Alternatively, the suction and discharge branches may be formed in different casing portions.

Advantageously, the two successive impeller stages between which the flow passage is defined include respective pump impellers which are arranged to face in opposite directions. The impellers may face towards each other, that is, a respective eye or fluid inlet of the impellers may face towards each other. Alternatively, and in a preferred embodiment, the impellers of the two successive stages may be positioned back-to-back wherein the respective impeller eyes face away from each other.

Facing the pump impellers in opposing directions reduces hydraulic loads experienced by the pump when in use, which extends the service life of pump bearings and seals.

Each impeller stage of the pump includes a suction volute and a discharge volute. The arrangement is such that fluid enters the suction volute of one impeller stage, is pressurized by an impeller, and enters the discharge volute. The fluid is then either discharged through a pump discharge branch or is alternatively directed into the suction volute of a consecutive impeller stage to be further pressurized.

Advantageously, the flow passage defined by the at least two portions of the casing between the two successive impeller stages provides fluid communication between a discharge volute of one impeller stage and a suction volute of the other, successive impeller stage.

Preferably, the pump includes two impeller stages, a first stage and a higher pressure second stage. In use, fluid enters a suction volute of the first stage, is pressurized by a first stage impeller, and enters a discharge volute of the first impeller stage. The fluid then flows through the flow passage defined by the at least two portions of the casing, and into a suction volute of the second impeller stage. The fluid is further pressurised by a second stage impeller, enters into a discharge volute of the second impeller stage, and exits the pump through a pump discharge branch.

Advantageously, the cut-waters of the discharge volutes of the two impeller stages, that is, the wedge shaped portions separating the discharge volutes from the flow passage or discharge branch, are oriented at around degrees to each other. This particular alignment results in balancing radial loads on the pump when in use, which contributes significantly to reducing shaft deflections and bearing loading. This allows lower cost components to be utilised and potentially extends the service life of shaft bearings and mechanical seals.

In an alternative embodiment, the pump may include more than two impeller stages, with the number of required stages being determined from, for example, the duty requirements of the pump.

Preferably, the pump is a multistage centrifugal pump.

Preferably, the pump casing of the present invention is adapted for use with pumps including a shaft having a plurality of impellers secured thereon by shrink fitting.

Shrink fitting of the impellers eliminates the requirement for one or more impeller keys which in turn eliminates or substantially reduces the potential of fretting and fatigue failures. In a preferred embodiment, a large diameter shaft is provided supported on a short bearing span, which assists to minimize shaft deflections, thus prolonging the service life of the seals and bearings.

Preferably also, the pump casing of the present invention is adapted for use with a pump comprising one or more cartridge bearings and seals, which cartridge bearings and seals may be assembled and removed from the pump while the at least two portions of the pump casing are coupled together. In one embodiment, individual cartridges may be provided for the bearings and seals.

Advantageously, the seal faces of the cartridge seals are positioned within a respective suction passage within the pump which offers optimum seal cooling and eliminates or substantially reduces the requirement to routinely flush the seals.

According to a second aspect of the present invention there is provided a pump comprising a pump casing and a plurality of impeller stages, said casing comprising at least two portions adapted to be coupled together to define a flow passage between two successive impeller stages.

According to a third aspect of the present invention, there is provided a pump casing comprising at least two portions adapted to be coupled together to form at least one fluid passage.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure l is a cross sectional view of a known pump incorporating a top cross-over passage; Figure 2 is a diagrammatic representation of a known pump incorporating a top cross-over passage showing the discharge cut-water positions) Figure 3 is a diagrammatic representation of a known pump incorporating a bottom cross-over passage; Figure 4 is a diagrammatic representation of a pump and pump casing in accordance with aspects of the present invention; Figure 5 is a perspective view form above of the pump shown in Figure 4, with an upper portion of the pump casing removed; Figure 6 is a perspective view from below of an upper portion of the pump casing of Figure 4; Figure 7 is a stepped cross- sectional view of the pump of Figure 4; and Figure 8 is a part cut-away perspective view of a pump and pump casing similar to that shown in Figure 4, in accordance with aspects of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Reference is first made to Figure l of the drawings in which there is shown a cross-sectional view of a known two-stage centrifugal pump lo incorporating a top cross over passage 12. The pump lo includes a pump casing which is horizontally split into two portions, a bottom portion 14, and an upper portion 16. As shown, the cross-over passage 12 is integrally formed with the upper casing portion 16. The pump lo includes two impeller stages 18, 20 which include respective impeller blades 22, 24 which are secured to a shaft 26 in a back-to-back arrangement. The first stage 18 defines a suction volute 28 and a discharge volute 30, and similarly, the second stage 20 also defines a suction volute 32 and a discharge volute 34.

The pump lo operates generally in the following manner. The shaft 26 is rotated by suitable drive means (not shown) to rotate the impellers 22, 24 and to draw a working fluid into the first stage suction volute 28 via a pump suction branch (not shown). The fluid is then driven radially through the first stage impeller 22 to impart energy to the fluid, with higher pressure fluid exiting the impeller 22 into the first stage discharge volute 30. From here the fluid passes through the cross over passage 12 and into the second stage suction volute 32, through the second stage impeller 24 where it is further pressurized, and into the second stage discharge volute 34. The fluid then exits the pump 10 through a pump discharge branch (not shown).

Integrally forming the cross-over passage 12 in the upper casing portion 16 has a number of disadvantages.

For example, providing the cross-over passage 12 as shown in Figure 1 requires a more involved and complex casting process due to the considerable differences between the shape of the upper and lower portions 16, 14 of the casing. That is, a larger inventory of casting cores and boxes and the like are required to form the different portions. Additionally, providing the cross-over passage 12 within the upper casing portion 16 severely restricts access to the interior surface 36 of the passage 12 to ensure that an optimum surface finish is achieved or to apply internal coatings or the like.

Reference is now made to Figure 2 of the drawings in which there is shown a diagrammatic representation of a known centrifugal pump 40, similar to that pump 10 shown in Figure 1, but viewed from one end. As shown, the pump of Figure 2 includes a top cross-over passage 42 forming part of an upper casing portion 41 providing a flow passage between a discharge volute 43 of one impeller stage (shown in chain-dotted outline) and a suction volute (not shown) of a successive impeller stage. The pump 40 also includes a pump suction branch 44 and a delivery branch 46 forming part of a lower casing portion 45, wherein the delivery branch 46 is in direct fluid communication with the discharge volute 48 of the pump final stage. As shown in Figure 2, each discharge volute 43, 48 includes respective cut-waters 50, 52, that is, wedge-shaped portions separating the discharge volutes 43, 48 from the flow passage 42 and discharge branch 46 respectively. In the arrangement shown, that is, where the cross-over passage 42 is provided in an upper casing portion 41, the cut-waters 50, 52 of the discharge volutes 43, 48 may be inclined at around 80 to 110 degrees to each other which conventionally results in unbalanced radial loads which contributes significantly to increasing shaft deflection and bearing loading.

A further prior art pump 60 is diagrammatically

shown in Figure 3. This arrangement is similar to that pump 10 shown in Figure 1, except the cross-over passage 62 is formed within a lower pump casing portion 64.

Thus, all those disadvantages of providing a cross-over passage in a single portion of a split pump casing described above with reference to Figure 1, also apply to the bottom cross-over passage 62 shown in Figure 3.

Reference is now made to Figure 4 of the drawings in which there is shown a diagrammatic end view of a pump and pump casing in accordance with aspects of the present invention. The pump, generally indicated by reference numeral 70, includes a horizontally split casing having an upper casing portion 72 and a lower casing portion 74.

Although not shown in Figure 4, pump 70 includes a shaft and impeller arrangement, as will be described hereinafter with reference to Figure 5. A cross-over passage 76 is provided to communicate fluid between two successive impeller stages, wherein the cross-over passage 76 is defined by both upper and lower casing portions 72, 74 when coupled together. The cross-over passage 76 is therefore substantially aligned with the horizontal plane 78 which separates the upper and lower casing portions 72, 74. Thus, the passage 76 defined by the two casing portions is adapted to provide fluid communication between two successive impeller stages along a substantially horizontal centre line of the assembled pump casing, as will be described in more detail with reference to Figures 5 and 6.

Reference is first made to Figure 5 in which the pump 70 of Figure 4 is shown with the upper casing portion 72 removed for clarity. As shown, the lower casing portion 74 defines a portion of the cross-over passage 76 which communicates fluid between two impeller stages 82, 84 which include respective impeller blades 86, 88 secured to a shaft 90 in a back-to-back arrangement. Each impeller stage 82, 84 defines a suction volute 94, 96, and a discharge volute 98, 100.

The cross-over passage 76 provides fluid communication between the discharge volute 98 of the first impeller stage 82 and the suction volute 96 of the second impeller stage 84. An internal view of the upper casing portion 72 of pump 70 is shown in Figure 6, with the shaft 90 and impellers 86, 88 shown in position. As shown, the upper casing portion 72 defines the remaining portion of the cross-over passage 76.

The arrangement shown in Figures 4, 5 and 6 is advantageous in that the pattern construction of the different portions 72, 74 of the casing is similar in many respects, thus requiring a smaller inventory of casting cores and boxes and the like in order to manufacture the casing. This in turn results in improved and less expensive manufacturing capability as simpler and lower cost equipment may be utilised. Additionally, due to the fact that the flow passage 76 is not formed in a single fully enclosed cast portion as with prior art designs (Figures 1, 2 and 3), the resulting castings are less complex and so minimize casting costs and improve casting integrity. Furthermore, full access is available to the interior surface of the flow passage 76 thereby ensuring optimum surface finish can be achieved, thus optimising hydraulic efficiency. Additionally, where internal coatings are to be applied to the casing, the internal access to the flow passage 76 results in simplification of the coating process resulting in increased integrity of the coating compared with conventional or known designs.

Furthermore, and with reference to Figure 7 in which there is shown a stepped cross-sectional view of pump 70, the cut-waters 110, 112 of the discharge volutes 98, 100 of the pump 70 of Figure 4 are aligned at around 180 degrees to each other. This particular alignment results in balancing radial loads on the pump 70 when in use which contributes significantly to reducing shaft deflections and bearing loading. This allows lower cost components to be utilized and potentially extends the service life of shaft bearings and mechanical seals.

Reference is now made to Figure 8 in which there is shown a part cut-away perspective view of a pump and pump casing according to aspects of the present invention.

The pump and pump casing of Figure 8 are substantially identical to those shown in Figures 4, 5 and 6, and so for convenience, like components share like reference numerals, incremented by 100. The pump 170 includes an upper casing portion 172 and a lower casing portion 174 which define a cross-over passage 176 between two successive impeller stages when the casing portions 172, 174 are coupled together. The pump 170 includes two impeller stages 182, 184 which include respective impeller blades 186, 188 which are secured to a shaft 190 in a back-to-back arrangement. The impellers 186, 188 are shrink fitted to the shaft, eliminating the requirement for one or more shaft keys which in turn eliminates or substantially reduces the potential of fretting and fatigue failures occurring. In the embodiment shown, the impellers 186, 188 incorporate a swept eye 192 which ensures high suction specific speeds, thus providing overall improved suction performance. The first and second stages 182, 184 each define a suction volute 194, 196, and a discharge volute 198, 200.

Pump 170 operates in a similar manner to pump 10 shown in, and described with reference to Figure 1.

Thus, a full description of the operation of pump 170 will not be given.

The shaft 190 is rotatably supported within the pump by cartridge type bearings 202, and shaft sealing is provided by cartridge type seals 204, located inboard of the bearings 202. The seal cartridges 204 are positioned such that seal faces 206 are located within a respective suction volute 194, 196 which offers optimal cooling and eliminates or substantially reduces the requirement to flush the seals.

It should be understood that the embodiment hereinbefore described in merely exemplary of the present invention and various modifications may be made thereto without departing from the scope of the invention.

Claims

CLAIMS: 1. A pump casing for a pump having a plurality of impeller stages,

said casing comprising at least two portions adapted to be coupled together to define a flow passage between two successive impeller stages.
2. A pump casing as claimed in claim 1, wherein the casing is substantially horizontally split such that the two portions of the casing are adapted to be coupled together along a substantially horizontal plane.
3. A pump casing as claimed in claim 1 or 2, wherein the flow passage defined by the two casing portions is adapted to provide fluid communication between the two impeller stages along a substantially horizontal split line of the pump casing.
4. A pump casing as claimed in claiml, 2 or 3, wherein the flow passage is adapted to be split by separating the portions of the casing along a plane substantially aligned with the intended direction of flow.
5. A pump casing as claimed in any preceding claim, wherein the pump casing includes a suction branch and a discharge branch.
6. A pump casing as claimed in claim 5, wherein both suction and discharge branches are formed in one of the at least two casing portions.
7. A pump casing as claimed in claim 5 or 6, wherein, where the casing is horizontally split, the suction and discharge branches are formed in a lower casing portion.
8. A pump casing as claimed in claim 5 or 6, wherein, where the casing is horizontally split, the suction and discharge branches are formed in an upper casing portion.
9. A pump casing as claimed in claim 5, wherein the suction and discharge branches are formed in different casing portions.
10. A pump casing as claimed in any preceding claim, wherein the two successive impeller stages between which the flow passage is defined include respective pump impellers which are arranged to face in opposite directions.
11. A pump casing as claimed in claim 10, wherein the impellers face towards each other.
12. A pump casing as claimed in claim 10, wherein the impellers of the two successive stages are positioned back-to-back.
13. A pump casing as claimed in any preceding claim, wherein each impeller stage of the pump includes a suction volute and a discharge volute.
14. A pump casing as claimed in claim 13, wherein the flow passage defined by the at least two portions of the casing between the two successive impeller stages provides fluid communication between a discharge volute of one impeller stage and a suction volute of the other, successive impeller stage.
15. A pump casing as claimed in claim 13 or 14, wherein the cut-waters of the discharge volutes of the two impeller stages are oriented at around 180 degrees to each other.
16. A pump casing as claimed in any preceding claim, adapted for use with a multistage centrifugal pump.
17. A pump casing as claimed in any preceding claim, adapted for use with a pump including a shaft having a plurality of impellers secured thereon by shrink fitting.
18. A pump casing as claimed in any preceding claim, adapted for use with a pump comprising one or more cartridge bearings and seals, which cartridge bearings and seals may be assembled and removed from the pump while the at least two portions of the pump casing are coupled together.
19. A pump casing as claimed in claim 18, wherein the bearings and seals are provided in individual cartridges.
20. A pump casing as claimed in claim 18 or 19, wherein the seal faces of the cartridge seals are positioned within a respective suction passage within.
21. A pump comprising a pump casing and a plurality of impeller stages, said casing comprising at least two portions adapted to be coupled together to define a flow passage between two successive impeller stages.
22. A pump casing comprising at least two portions adapted to be coupled together to form at least one fluid passage.
23. A pump casing as described herein and as shown in the accompanying representations.
24. A pump as described herein and as shown in the accompanying representations.