WO2014074204A1

WO2014074204A1 - Centrifugal pump with slanted cutwater for cavitation prevention

Info

Publication number: WO2014074204A1
Application number: PCT/US2013/056329
Authority: WO
Inventors: Vishnu M. Sishtla
Original assignee: Carrier Corporation
Priority date: 2012-11-10
Filing date: 2013-08-23
Publication date: 2014-05-15

Abstract

A pump (20) has a housing (22) having an inlet (26) and an outlet (28). An impeller (30) is held by the housing for rotation about an impeller axis (500). The housing defines a cutwater off parallel to the impeller axis by at least 3° over at least 50 percent of an axial span of the cutwater.

Description

CENTRIFUGAL PUMP WITH SLANTED CUTWATER FOR CAVITATION PREVENTION

CROSS-REFERENCE TO RELATED APPLICATION

[0001] Benefit is claimed of US Patent Application Ser. No. 61/724,952, filed November 10, 2012, and entitled "Pump Cavitation Prevention", the disclosure of which is incorporated by reference herein in its entirety as if set forth at length.

BACKGROUND

[0002] The present disclosure relates to water pumps. More particularly, it relates to

10 centrifugal pumps.

[0003] A centrifugal pump uses an impeller to impart kinetic energy to a flow of water and then convert the kinetic energy to pressure. Rotation of the impeller draws in a central axial flow and discharges the flow with a circumferential velocity. The flow is then diffused to form a lower velocity/higher pressure discharge stream.

15 [0004] Light weight, compact centrigugal water pumps are used in fire fighting systems.

Pumps are prone to cavitation at suction and discharge. Suction cavitation occurs at inlet and is due to water evaporating in low pressure regions and condensing in high pressure regions downstream. Discharge cavitation is due to the venturi effect, where high pressure water near tongue or cutwater of the volute recirculates into the inlet of the volute inlet through a small

20 clearance, similar to the one in venturi or orifice meters. Cavitation is accompanied by a large change in specific volume from vapor to liquid resulting in erosion due to high velocity liquid impinging on the pump surfaces. Especially in cases where the housing is made of aluminum, erosion can lead to broken components

SUMMARY

[0005] One aspect of the disclosure involves a pump having a housing having an inlet and an outlet. An impeller is held by the housing for rotation about an impeller axis. The housing defines a cutwater off parallel to the impeller axis by at least 3° over at least 50 percent of an axial span of the cutwater.

[0006] In one or more further embodiments, the cutwater is a first cutwater and the housing defines a second cutwater off parallel to the impeller axis by at least 3° over at least 50 percent of an axial span of the second cutwater.

[0007] In one or more further embodiments, there are no further cutwaters in addition to the first cutwater and second cutwater. [0008] In one or more further embodiments, the cutwater is off parallel to the impeller axis by 3-10° over said at least 50 percent of said axial span of the cutwater.

[0009] In one or more further embodiments, the cutwater is formed as an edge of a metal baffle.

[0010] In one or more further embodiments, over said at least 50% axial span, a minimum radial clearance between the impeller and a baffle on which the cutwater is formed progressively varies.

[0011] In one or more further embodiments, the impeller has an axial inlet and a radial outlet.

[0012] In one or more further embodiments, the impeller comprises a circumferential array of vanes each having an inboard leading edge and an outboard trailing edge.

[0013] In one or more further embodiments, the vane trailing edges are parallel to the impeller axis.

[0014] In one or more further embodiments, a system comprises the pump; and a water source coupled to the inlet.

[0015] In one or more further embodiments, the pump is immersed in the water source.

[0016] In one or more further embodiments, the system is a firefighting system comprising: a nozzle; and at least one secondary pump between the pump and the nozzle.

[0017] In one or more further embodiments, a method for operating the pump comprises: driving rotation of the impeller in a first direction about the impeller axis, the rotation bringing trailing edges of vanes of the impeller into progressive proximity with the cutwater so that, along said axial span of the cutwater, a gap between the cutwater and the vane trailing edge does not simultaneously minimize.

[0018] Another aspect of the disclosure involves a method for reducing pump cavitation in a centrifugal pump. The method includes driving rotation of an impeller in a first direction about an impeller axis, the rotation bringing trailing edges of vanes of the impeller into progressive proximity with a cutwater so that, along said axial span of the cutwater, a gap between the cutwater and the vane trailing edge does not simultaneously minimize

[0019] Another aspect of the disclosure involves a pump comprising: a housing having an inlet and an outlet; and an impeller held by the housing for rotation about an impeller axis. The housing defines a cutwater providing means for reducing pump cavitation.

[0020] The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIG. 1 is a view of a pump.

[0022] FIG. 1 A is an enlarged view of a portion of the pump of FIG. 1.

[0023] FIG. 2 is a sectional view of the pump of FIG. 1 , taken along line 2-2.

[0024] FIG. 3 is a partial radially-inward view of a cutwater and impeller taken along

3-3 of FIG. 1.

[0025] FIG. 4 is a view of the impeller.

[0026] FIG. 5 is a view of an outer baffle member of the pump.

[0027] FIG. 6 is a side view of the outer baffle member.

[0028] FIG. 6A is an enlarged view of the leading edge of the outer baffle member.

[0029] FIG. 7 is a plan view of a blank for forming the outer baffle member.

[0030] FIG. 8 is a view of an inner baffle of the pump.

[0031] FIG. 9 is a side view of the inner baffle.

[0032] FIG. 10 is a plan view of a blank for forming the inner baffle.

[0033] FIG. 11 is a view of a fire-fighting system including the pump.

[0034] Like reference numbers and designations in the various drawings indicate like elements.

DETAILED DESCRIPTION

[0035] FIGS. 1 (bottom perspective view) and 2 show an exemplary centrifugal pump 20.

[0036] The pump 20 has a housing (case) assembly 22. The exemplary pump includes a motor 24 (schematically shown in FIG. 2; e.g., a hydraulic or electric motor). The exemplary housing 22 has a suction port (inlet) 26 and a discharge port (outlet) 28 (FIG. 1).

[0037] The pump comprises an impeller 30 (FIG. 2) carried by the housing and mounted for rotation about an impeller axis 500 (e.g., via one or more bearings (not shown)).

[0038] The exemplary impeller comprises a circumferential array of vanes 32. The exemplary vanes extend from a radially inboard and upstream leading end/edge 34 (FIG. 4) to a radially outboard and downstream trailing edge 36. The exemplary vanes are of generally even thickness and have a convex face 38 and an opposite concave face 40 extending between the leading and trailing edges. The faces meet at respective upper and lower edges 42 and 44. Proximate these respective edges, the vanes are mounted to respective upper plate 46 and lower plate 48 (e.g., fitted into complementary apertures in the plates and welded). The exemplary upper plate bears a boss 50 (e.g., a welded assembly) for mounting to the motor shaft.

[0039] The exemplary lower plate has a large central aperture 52 at the pump inlet. In the exemplary pump, the central aperture 52 is partially covered by a cover plate or cap 54 (FIG. 2), itself having a central aperture 56 defining an inlet port. The cap 54 partially encloses the rotor compartment.

[0040] The exemplary impeller leading edges 34 (FIG. 4) are oriented at an angle θι off-axial. The exemplary trailing edges 36 are axial so that the trailing edges are all along a cylindrical boundary defining an impeller outlet. The inclined leading edge allows the end 44 to be shorter than the end 42 to allow the leading edge at the end 44 to be adjacent the opening/aperture 52. The exemplary plates and vanes may be cut from aluminum sheet or plate stock, with the plates bent to define the desired arc. The boss may be machined.

[0041] The exemplary housing comprises an outlet housing 60 (FIG. 1) one or more baffles (scrolls) 62, 64 and upper and lower face plates 66 and 68 (FIG. 2). The exemplary outlet housing comprises an aluminum casting or machining whereas the exemplary face plates may comprise cut aluminum sheet or plate stock. The housing may further include a mounting boss 70 on the upper plate for mounting to the motor. Additional mounting brackets, etc., may be present but are not shown.

[0042] The exemplary baffles 62, 64 are formed separately from the other housing members and mounted thereto during assembly. As is discussed below, the exemplary baffle 62 is the volute or outer baffle and the exemplary baffle 64 is an inner baffle. The exemplary baffle 64 is formed as a single piece. The exemplary baffle 62 is formed as two pieces, respectively an upstream piece or baffle 72 and a downstream piece or baffle 74 joined end-to-end (FIG. 1). The exemplary baffles are formed from sheet metal (e.g. aluminum alloy) cut and bent into an arcuate form. In the assembled frame of reference, each baffle 64, 72 extends from a leading edge (or cutwater) 80 to a trailing edge 82 and has an inner/inboard surface/face 84 and an outer/outboard face 86 and lateral edges 88 and 90 (upper and lower or vice versa). The lateral edges may bear tabs 92 for mounting to the outer housing. The housing face plates may be cut with through-slots complementary to the tabs and allowing external welding of the tabs at the slots when the housing is assembled. The trailing baffle piece 74 may be similarly formed extending from a leading edge 100 to a trailing edge 102.

[0043] The pump may be manufactured via otherwise conventional manufacturing techniques.

[0044] As the pump impeller is driven about its axis 500 in a first direction 502 (FIG. 1 A), the pump will draw in liquid from the inlet 26 and discharge it from the outlet 28. The baffles function to collect the discharge water from each of the impeller channels. The increasing gap between the impeller and the baffles 62 and 64 keeps the flow velocity constant. The inner baffle 64 helps to balance the pressure around the impeller circumference thereby reducing the radial load on the motor bearing.

[0045] Once the pump is primed (i.e., the impeller interior contains water at least partially immersing the impeller and the inlet 26 is at a source of the water), rotation of the impeller in the direction 502 causes the vanes to centrifugally and tangentially discharge the water from within the impeller interior, drawing in further water from the source through the inlet 26. The exemplary cutwaters 80 are approximately diametrically opposite to divide the impeller outlet into two portions 120 and 122 (FIG. 1). A first (outer) flow portion 402 exiting the impeller outlet first portion 120 passes through a channel 200 between the baffles (between the outboard/convex surface of the inner baffle 64 and the inboard/concave surface of the outer baffle 62). A second (inner) flow portion 404 exiting the impeller outlet second portion 122 passes between the inner/concave surface of the inner baffle 64 and the outer/convex surface of the outer baffle 62 (at least in a small region at the outer baffle cutwater). At the downstream end/edge 82 of the inner baffle, the two flow portions 402 and 404 merge to become a single flow 406 discharged from the outlet 28.

[0046] As so far described in this section, the pump may be illustrative of any of several prior art or yet-developed configurations. However, the baffle leading/upstream edges 80 (FIGS. 5&8) have directions 510 and 512 that are off parallel to the axis 500. The leading edge orientations may be seen relative to a baseline system.

[0047] In the baseline system, the leading edges 80' are at a right angle to the lateral edges so that the directions 510' and 512' are parallel to the axis 500. At all operating conditions (more at off-design operating conditions), water enters from the high pressure area outboard of the cutwater 80' to a lower pressure area immediately inboard. The high velocity at that location results in a low pressure and creation of vapor (cavitation along the inner surface of the baffle downstream of the cutwater). Further downstream, however, the velocity decreases and creates a high pressure resulting in collapse of bubbles to liquid. Due to the change in specific volume, the surrounding water fills the space at high velocity resulting in erosion damage. This cavitation is also enhanced by the turbulence caused by flow impinging on the cutwater 80'. One way to solve the problem is to reduce the diameter of the impeller 30 or increasing the radial gap between impeller and baffle by increasing the outer radius of the volute 62 and inner baffle 64. Reducing the impeller diameter results in inadequate head and increasing the volute size results in higher cost.

[0048] The edges 80 and their directions are off-normal to the lateral edges by an angle θ₂. Exemplary θ₂ is 3-20°, more narrowly 3-14° or 3-10° or 3-7° or 4-14° or 4-10° or 4-7° or about 5°. This may simply be achieved by cutting the baseline baffles at the angle θ₂. In the exemplary embodiments of straight cut leading edges (cutwaters), the angle θ₂ is 5° along the entire axial span of the cutwater, in other examples, it may be at such an off-axial angle over a portion such as at least 50% of the axial span of the cutwater (more particularly, at least 75% or 80-100%). This span may be continuous. With the axially aligned baffles (e.g., as distinguished from slightly frustoconical baffles) there will also be along that span, a progressive variation in minimum radial clearance between the impeller and the baffles on which the cutwaters are formed.

[0049] FIGS. 7 and 11 show sheet-metal blanks for forming the baffles, after cutting but before bending. Blank lengths are shown as LBI and LB₂, respectively. Blank overall widths are shown as H₂ whereas widths away from the tabs are shown as Hi. Hi defines the axial span of the cutwater.

[0050] After bending, the internal radii of the baffles are RBI and RB₂ over

circumferential spans of approximately 180°. The outer baffle is shown with a straight trailing portion 98 (FIG. 9).

[0051] In a baseline system where θ₂ is zero, as the vane trailing edge 36 approaches the cutwater 80', the gap between the trailing edge 36 and the cutwater 80' will decrease until, at some time, it becomes a minimum along the entire length of the trailing edge and cutwater. The angling of the cutwaters 80 at nonzero θ₂ has several effects. It may slightly offset the times at which any portion of the vane trailing edge reaches its associated minimum distance from the cutwater. FIG. 3 is a partial radially-inward view of the cutwater as a vane trailing edge passes thereby. With this particular angling, the gap is first minimum when the vane trailing edge 36 reaches the cutwater at its upper edge 88. As the vane rotates further, the location of minimum distance will move up along the cutwater to the edge 90 as the gap below this location expands and the gap above this location contracts.

[0052] The angling of the cutwater may also mean that the minimum difference is different along the length of the vane and cutwater (if the angular relief is not along a portion of the cutwater which is exactly tangential). In this example, the minimum gap/distance at the lower edge 90 will be slightly greater than the minimum gap/distance at the upper edge 88 (with progressive linear change therebetween).

[0053] The angling of the leading edges has been observed to reduce cavitation. By making an angular cut from hub to shroud, the clearance gradually increases from the upper edge 88 to the lower edge 90, instead of a constant small clearance. Because the baffles 62 and 64 spiral away from the axis, the gap between impeller vane and baffle increases as the impeller vane moves circumferentially away from the leading edge. Hence, angling the leading edge in the direction increases the minimum gap between impeller and baffle at the edge 90 relative to at the edge 88. Because the cut is carried out in both baffles 62 and 64, the area variation is symmetric and does not impose any radial load on bearing. This will not compromise the size and performance.

[0054] Alternatively characterized, the relief may be viewed from the perspective of the tangential direction rather than the axial/rotation direction. For example, FIG. 1A shows a tangential direction 506 at the cutwater (e.g., labeled at the top plate). An angle Θ3 may be defined between the directions 510, 512 for each of the two cutwaters and the tangent 506. In the main example, Θ3 will be 90° minus θ₂. However, there may be departures. For example, the baffles may be angled so that their faces are not parallel to the axis 500. For example, this might be the case if the vane OD edges were, themselves, not parallel to the axis 500.

Exemplary values for Θ3 in various such implementations would be 90° minus the exemplary values given for θ₂ in the main example.

[0055] Additionally, rounding the leading edge 80 reduces the turbulence and reduces the occurrence of cavitation. High velocity water discharged from the impeller impinges on the leading edge. In case of a square leading edge with sharp corners, the high velocity water impinges on a larger surface area and results in larger stagnation zone. The blockage due to stagnation further reduces the geometric area and increases velocity. Higher velocity results more turbulence. A rounded edge streamlines the flow and reduces the stagnation zone. The effect is a lower reduction in geometric area and less turbulence.

[0056] FIG. 11 shows the pump used as a part of a firefighting system 200. The exemplary firefighting system 200 is a multi-stage system with one or more upstream pumps 220 feeding one or more downstream pumps 222, in turn feeding one or more delivery devices 202 discharging a stream 204 from an outlet 206. The discharge stream 204 may be drawn from a source or body of water 210 such as a lake, reservoir, pond, tank, or the like. The exemplary body is an open body of water such as a lake, etc. having a surface 212. In various systems, the water flow may be mixed with a fire retardant or other agent such as a foaming agent prior to discharge. One or both stages of pumps 220 and 222 may be provided by pumps such as pump 20. The exemplary pump 220 is positioned as a submersible pump having an inlet 224 and an outlet 226. The outlet 226 is coupled (e.g., via one or more hoses 240) to an inlet 228 of the pump 222 and an outlet 230 of the pump 222 is coupled to an inlet 232 of the delivery device.

[0057] One or more of the pumps and/or delivery devices may be mounted on one or more pallets, vehicles, or the like. Although FIG. 11 shows the downstream pump 222 and delivery device 202 mounted on separate vehicles (e.g., trailers), they may be mounted on a single vehicle depending upon the implementation. Additionally, although the pump 220 is shown as a satellite pump, it may alternatively be part of the palletized or otherwise modularized system.

[0058] FIG. 11 also shows a floatation equipped housing 250 for the pump 220 containing the motor, etc. and maintaining the pump inlet below the surface 212 of the body 210. This exemplary configuration using a submersible satellite pump 220 allows the pumps and delivery device to be connected by hoses 240 instead of rigid conduit. The pump 220 provides sufficient system pressure so that the pump 222 will not collapse the intervening hose. Delivery of water from the pump 220 can, also, serve to prime the pump 222.

[0059] Although an embodiment is described above in detail, such description is not intended for limiting the scope of the present disclosure. It will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. For example, when implemented in the reengineering of an existing pump configuration, details of the existing configuration may influence or dictate details of any particular implementation. Accordingly, other embodiments are within the scope of the following claims.

Claims

CLAIMS What is claimed is:

1. A pump (20) comprising:

a housing (22) having an inlet (26) and an outlet (28); and

an impeller (30) held by the housing for rotation about an impeller axis (500), wherein the housing defines a cutwater (80) off parallel to the impeller axis by at least 3° over at least 50 percent of an axial span of the cutwater.

2. The pump of claim 1 wherein:

the cutwater is a first cutwater and the housing defines a second cutwater off parallel to the impeller axis by at least 3° over at least 50 percent of an axial span of the second cutwater.

3. The pump of claim 1 wherein:

there are no further cutwaters in addition to the first cutwater and second cutwater.

4. The pump of claim 1 wherein:

the cutwater is off parallel to the impeller axis by 3-10° over said at least 50 percent of said axial span of the cutwater.

5. The pump of claim 4 wherein:

the cutwater is off parallel to the impeller axis by 4-7° over said at least 50 percent of said axial span of the cutwater.

6. The pump of claim 4 wherein:

the cutwater is formed as an edge of a metal baffle (62; 64).

7. The pump of claim 1 wherein:

over said at least 50% axial span, a minimum radial clearance between the impeller and a baffle on which the cutwater is formed progressively varies.

8. The pump of claim 1 wherein:

the cutwater is formed as an edge of a metal baffle.

9. The pump of claim 1 wherein:

the impeller has an axial inlet and a radial outlet.

10. The pump of claim 1 wherein:

the impeller comprises a circumferential array of vanes each having an inboard leading edge and an outboard trailing edge.

11. The pump of claim 10 wherein:

the vane trailing edges are parallel to the impeller axis.

12. A system (120; 250) comprising:

the pump (20) of claim 1 ; and

a water source coupled to the inlet.

13. The system of claim 12 wherein:

the pump is immersed in the water source.

14. The system of claim 12 being a firefighting system further comprising:

a nozzle; and

at least one secondary pump between the pump and the nozzle.

15. A method for operating the pump of claim 1 comprising:

driving rotation of the impeller in a first direction (502) about the impeller axis (500), the rotation bringing trailing edges of vanes of the impeller into progressive proximity with the cutwater so that, along said axial span of the cutwater, a gap between the cutwater and the vane trailing edge does not simultaneously minimize.

16. A method for reducing pump cavitation in a centrifugal pump (20):

driving rotation of an impeller (30) in a first direction (502) about animpeller axis (500), the rotation bringing trailing edges (36) of vanes (32) of the impeller into progressive proximity with a cutwater (80) so that, along said axial span of the cutwater, a gap between the cutwater and the vane trailing edge does not simultaneously minimize

17. A pump (20) comprising:

a housing (22) having an inlet (26) and an outlet (28); and

an impeller (30) held by the housing for rotation about an impeller axis (500), wherein the housing defines a cutwater (80) providing means for reducing pump cavitation.