GB2443918A

GB2443918A - Mechanical Assembly

Info

Publication number: GB2443918A
Application number: GB0721198A
Authority: GB
Inventors: David Bilyk
Original assignee: Clyde Pumps Ltd
Current assignee: Clyde Union Ltd
Priority date: 2006-11-18
Filing date: 2007-10-30
Publication date: 2008-05-21
Also published as: GB0721198D0; GB0623042D0

Abstract

A mechanical assembly e.g. a pump or compressor comprises a stator defining a fluid cavity and a rotor 210 mounted within the cavity. The rotor comprises blades 220a-c which are uneven in number, wherein an outer portion of each blade defines a gap with an element of the stator. At least two of the blades define a gap having a first width R1 and at least one other blade defines a gap having a second width, wherein the first and second widths are different. Alternatively at least three of the blades define gaps of differing widths. The blades may have different extents and may be trimmed or machined from the rotor. In use, the present invention reduces rotational frequency vibrations within the mechanical assembly caused by blades passing a feature such as a volute cutwater.

Description

1 2443918

MECHANICAL ASSEMBLY

FIELD OF THE INVENTION

The present invention relates to a rotodynamic mechanical assembly, and in particular, but not exclusively, to a pump adapted to minimise the effects of blade passing frequency related vibrations.

BACKGROUND TO THE INVENTION

Vibration and noise from turbomachinery is common and generally occurs as a result of fluid-dynamic disturbances produced during use. For example, turbomachineiy, such as pumps, compressors, turbines, fans, blowers or the like may experience some vibration as a result of mechanical imbalances, imperfections or misalignments. The frequency of such vibrations is typically associated with the machine rotational speed. Vibration may also be caused by, for example, the occurrence of turbulence, cavitation or the like.

However, in many cases the prevailing cause of vibration within turbomachinery results from fluid-dynamic interactions between rotor and stator components. For example, the passage of a rotor component, such as a blade, past a fixed point or component of a stator, such as a diffuser blade or volute cutwater, results in flow perturbations and pulsations, the magnitude of which may be sufficient to cause noticeable noise and vibration. Vibration of this type is therefore associated with the blade passing frequency of the machine, which is the frequency at which the blades of a rotor passes the fixed point or component of the stator. Accordingly, the blade passing frequency of a machine is the product of the machine speed and the number of blades.

There are complex hydraulic and structural factors associated with turbomachinery that make it difficult to identify and control blade passing frequency related vibrations. Accordingly, it may be the case that such vibrations or their harmonics at a specific pump operating point match a natural frequency of the machinery and associated equipment and pipe work, resulting in resonance. Such resonant conditions usually produce significant noise levels and cause vibrations which may result in damage to the machinery and related equipment.

It is known in the art to seek to alter the blade passing frequency in turbomachinery which experience adverse vibrations. This may be achieved by, for example, altering the speed of the machinery, and/or by changing the number of blades present. However, changing such characteristics may significantly alter the operational conditions of the machinery which may not be viable where specific duty requirements must be met.

In circumstances where machine speed and blade numbers are preferably not altered, blade passing frequency related vibrations may be addressed by seeking to reduce the magnitude of the fluid perturbations. This may be achieved by altering the gap or clearance between the rotary and stationary components of the machinery, for example between the blade tips and one or more volute cutwaters in a centrifugal volute pump. Increasing such gaps, however, adversely affects the efficiency of the machinery and as such it has been established in the art to trim alternate blades of a rotary component such that alternate clearances are achieved. However, it is critical that the rotary components of the machinery remain mechanically balanced to avoid causing or creating an alternative source of vibration. Thus, creating alternate clearance gaps has heretofore only been attempted in rotary components having an even number of blades as the mechanical balance of such components can readily be maintained by ensuring that diametrically opposed blades are trimmed by the same amount.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided a mechanical assembly comprising: a stator defining a fluid cavity; and a rotor mounted within the fluid cavity and comprising a plurality of blades which are uneven in number, wherein an outer portion of each blade defines a gap with an element of the stator, wherein at least two of the blades define a gap having a first width and at least one other blade defines a gap having a second width, wherein the first and second widths are different.

The gap may be a radial gap. Alternatively, the gap may be an axial gap.

Alternatively further, the gap may extend in a non-radial and non-axial plane.

The present invention may therefore be utilised to alleviate the effects of blade passing frequency related vibrations in a mechanical assembly having a rotor with an uneven number of blades. In this respect, varying the gap between the blades of the rotor and an element of the stator in accordance with an embodiment of the present invention assists to vary the magnitude of the hydraulic shock created by each blade pass, thus redistributing the hydraulic energy in a generally random manner. This therefore prevents a coherent cycle of hydraulic forces being established which may otherwise generate significant vibrations.

Preferably, the rotor comprises a hub portion defining a rotational axis and adapted to be secured to a shaft, wherein the blades are circumferentially distributed around the hub portion and may extend generally radially outwardly relative to the hub portion or alternatively, or additionally, may extend generally axially relative to the hub portion. The hub portion may be separately formed and adapted to be subsequently secured to a shaft. Alternatively, the hub portion may be integrally formed with a shaft. Advantageously, each blade defines a blade root positioned adjacent the hub portion and a blade tip. Advantageously, the extent to which each blade tip extends from the hub portion may be selected in accordance with the required widths of the gaps between the blades, specifically the blade tips, and the stator element. Accordingly, the required widths of the gaps may be achieved without adjustments or variations being made to the stator element.

The at least two blades defining a gap of the first width may be adjacent each other, or alternatively may be separated by one or more other blades.

The radial and/or axial extent of the blade tips, and thus of the respective first and second widths of the gaps are preferably selected such that that the mass distribution of the rotor is balanced. In a preferred embodiment, the first clearance gap width is smaller than the second clearance gap width. Accordingly, the smaller radial and/or axial extent of the at least two blades may be balanced by the greater length of the at least one other blade.

At least one other blade may define a gap having a third width, wherein the third width differs from the first and second gap widths. Providing gaps with different widths in this manner therefore permits the energy produced by the hydraulic shocks resulting from each blade pass to be randomised in a more effective manner, while enabling mechanical balancing of the rotor to be achieved.

Further blades of the rotor may define gaps with the stator element which define widths differing from the first, second and third widths.

The tip of each blade may comprise an outwardly facing tip surface. The tip surface of at least one blade may be of a uniform profile, and may be substantially planer. The substantially planar surface may be substantially perpendicularly aligned relative to a longitudinal centre-line of the blade, or alternatively may be tapered relative to said centre-line. Alternatively, or additionally, at least one blade may comprise a tip surface having a non-uniform profile. For example, the tip surface may comprise more than one planar surface. This arrangement may therefore assist in controlling or minimising the hydraulic shock caused by the respective blade tip upon passing the stator element.

The resulting gap defined by each blade may vary across the thickness or depth of each blade in those embodiments in which one or more of the blade tips are profiled. Accordingly, it should be understood that the widths of the gaps defined above refer to the minimum gap width.

The stator element may be profiled in a suitable manner which may assist in minimising the blade passing related hydraulic shocks.

In one embodiment the rotor may be cast. Alternatively, or additionally, the rotor may be machined or fabricated. In a preferred embodiment, the rotor is cast with blades of substantially uniform length, wherein at least one of the blades is subsequently trimmed by a suitable machining operation to the required length to define the appropriate gap distribution with the stator element.

The mechanical assembly may comprise a rotodynamic assembly, such as a pump, compressor, fan, blower or the like. In a preferred embodiment the mechanical assembly comprises a centrifugal pump. The centrifugal pump may be a volute-type pump, wherein the stator element may comprise one or more volute cutwaters.

Alternatively, the centrifugal pump may be a diffuser-type pump, wherein the stator element may comprise one or more diffuser vanes.

The mechanical assembly may comprise a plurality of rotors, which rotors may be mounted in a single or a respective one of a plurality of fluid cavities defined by one or more stators. The mechanical assembly may comprise a multistage pump, such as a multistage centrifugal pump. In this arrangement, the rotors may be circumferentially off-set from each other such that misalignment of the blades in an axial direction is achieved. This arrangement may assist to prevent synchronous hydraulic shock from occurring which may otherwise result in noticeable vibration.

According to a second aspect of the present invention, there is provided a rotor for use in a mechanical assembly, said rotor comprising: a hub portion adapted to be secured to a shaft; a plurality of blades which are uneven in number and which extend generally outwardly relative to the hub portion, wherein the extension of at least two of the blades from the hub is equal and differs from the extension of at least one other blade.

The gap may be a radial gap. Alternatively, the gap may be an axial gap.

Alternatively further, the gap may extend in a non-radial and non-axial plane.

The rotor may comprise a pump impeller, such as a centrifugal pump impeller.

The hub portion may be separately formed and subsequently secured to a shaft. Alternatively, the hub portion may be integrally formed with a shaft.

According to a third aspect of the present invention, there is provided a mechanical assembly comprising: a stator defining a fluid cavity; and a rotor within the fluid cavity and comprising a plurality of blades, wherein an outer portion of each blade defines a gap with an element of the stator, wherein at least three of the blades define gaps of differing widths.

In one embodiment of the present invention all of the blades define gaps of differing widths.

The blades may be even or uneven in number.

The gap may be a radial gap. Alternatively, the gap may be an axial gap.

Alternatively further, the gap may extend in a non-radial and non-axial plane.

Various features defined above in relation to the first and second aspects may be applied to the invention defined in the third aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspect of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic representation of a conventional volute-style centrifugal pump; Figure 2 is a diagrammatic representation of a conventional diffuser-style centrifugal pump; and Figure 3 is a diagrammatic representation of a pump impeller in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

A diagrammatic representation of a conventional volute-style centrifugal pump is shown in Figure 1, reference to which is now made. The pump, generally identified by reference numeral 10, comprises a casing 12 which defines a volute 14 and a discharge passage 16. A pump impeller 18 comprising a number of blades 20 is rotatably mounted within the casing 12 and in use is rotated in the direction of arrow A. In use, fluid is drawn into the eye 22 of the impeller 18 and forced radially outwardly into the volute 14 and subsequently discharged via passage 16. A cutwater or volute tongue 24 is defined between the discharge passage 16 and the volute 14. It is preferred that the impeller 18 be mounted within the volute 14 such that the radial gap or clearance 26 defined between the tips 28 of the blades 20 and the cutwater 24 is minimised in order to maximise efficiency. However, rotor-stator fluid-dynamic interactions due to the proximity of the blade tips 28 to the stationary cutwater 24 may result in fluid perturbations which result in vibration. This vibration is associated with the blade passing frequency of the machine, which is the frequency at which rotor blades 20 pass the fixed cutwater 24. Accordingly, the blade passing frequency of the pump 10 is the product of the pump speed and the number of blades 20.

Reference is now made to Figure 2 in which there is shown a diagrammatic representation of an example of a diffuser-style centrifugal pump, generally identified by reference numeral 110. The components of the pump 110 in Figure 2 are similar to those shown in the pump 10 of Figure 1 and as such like components share like reference numerals, incremented by 100. Accordingly, pump 110 comprises a casing 112 which defines a volute 114, or any other appropriately shaped collecting chamber or annulus, and within which is rotatably mounted an impeller 118. The pump 110, however, further comprises a diffuser 130 which is fixed relative to the casing 112 and comprises a number of diffuser or stator vanes 132. The impeller 118 is mounted within the diffuser 130 and as such fluid-dynamic interactions exist between the tips 128 of the blades 120 and the roots 134 of the diffuser vanes 132. Again, these fluid-dynamic interactions may cause significant vibration levels at the blade passing frequency. It should be understood that in other diffuser pump designs the diffuser may be located within a concentric volute or an annular collecting chamber.

As discussed above, it is known in the art to trim alternate blades to alter the gap or clearance between the rotary and stationary components of the machinery in an attempt to reduce the effects of blade passing frequency vibrations. It is critical, however, that the rotary components of the machinery remain mechanically balanced to avoid causing or creating an alternative source of vibration. Thus, creating alternate clearance gaps has heretofore only been attempted in rotary components having an even number of blades as the mechanical balance of such components can readily be maintained by ensuring that diametrically opposed blades, such as blades 20a and 20b in Figure 1, are trimmed by the same amount.

Reference is now made to Figure 3 in which there is shown a diagrammatic representation of a pump impeller, generally identified by reference numeral 220, in accordance with an embodiment of aspects of the present invention. The pump impeller 220 may be utilised in the pump assemblies 10, 110 shown in Figures 1 and 2 respectively. The impeller 210 comprises seven blades 220 which extend generally radially outwardly from a central hub portion 236. In the embodiment shown, a number of the blades 220 have been radially trimmed in order to reduce the radial gap between the blades 220 and a stator of the pump, such as cutwater 24 (Figure 1) or the roots 134 of the diffuser vanes 132 (Figure 2), in order to minimise blade passing frequency related vibrations. However, as will be discussed in further detail below, the blades 220 have been trimmed in a manner which maintains the mechanical balance of the impeller 210.

Three blades 220a remain untrimmed and as such extend to the full diameter of the impeller 210. Two further blades 220b, each positioned between a pair of untrimmed blades 220a, are radially trimmed to create an effective radial reduction RI. Two further blades 220c, both positioned adjacent to each other and between a pair of untrimmed blades 220a, are radially trimmed to create an effective radial reduction R2, wherein in the embodiment shown in Figure 3 radial reduction Ri is greater than radial reduction R2. Accordingly, trimming the blades 220 of the impeller 210 in the manner described effectively varies the radial gap between the blades 220 and an element of the stator while maintaining the blade in mechanical and hydraulic balance. This arrangement acts to vary the magnitude of the hydraulic shock created by each blade pass and effectively redistributes the hydraulic energy in a generally random manner. This therefore prevents a coherent cycle of hydraulic forces being established and softens the blade wake effects to achieve a reduction in blade passing frequency related vibrations.

The embodiment shown in Figure 3 represents one specific case of an impeller incorporating an uneven number of blades wherein at least two blades 220c are trimmed by the same amount, and at least one other blade 220a, 220b is either untrimmed, or is trimmed to a different extent. Accordingly, the person of skill in the art would appreciate that a number of alternative blade arrangements may fall within the scope of the present invention.

Various alternative modifications and uses of the invention described above may be made without departing form the scope of the invention. For example, the principle of the invention may be used in other forms of turbomachinery, such as in compressors, turbines or the like. Additionally, the impeller may be utilised in alternative styles of pump, such as in multi-stage pumps or the like. Furthermore, the impeller may be provided with any number of blades which may be established and selected in accordance with the duty requirements of the pump. Additionally, the blades may be trimmed or formed by different extents such that no two blades are of the same length. Furthermore, the principle of the invention is not limited for use in varying the radial gaps between rotor and stator elements and it should be understood that any other geometric arrangement, such as axial gaps, may be modified by the present invention.

Claims

CLAIMS: 1. A mechanical assembly comprising: a stator defining a fluid

cavity; and a rotor mounted within the fluid cavity and comprising a plurality of blades which are uneven in number, wherein an outer portion of each blade defines a gap with an element of the stator, wherein at least two of the blades define a gap having a first width and at least one other blade defines a gap having a second width, wherein the first and second widths are different.
2. The mechanical assembly according to claim 1, wherein the gap comprises a radial gap.
3. The mechanical assembly according to claim 1, wherein the gap comprises an axial gap.
4. The mechanical assembly according to claim 1, wherein the gap extends in a non-radial and non-axial plane.
5. The mechanical assembly according to any preceding claim, wherein the rotor comprises a hub portion defining a rotational axis and adapted to be secured to a shaft, wherein the blades are circumferentially distributed around the hub portion.
6. The mechanical assembly according to claim 5, wherein the blades extend generally radially outwardly relative to the hub portion.
7. The mechanical assembly according to claim 5 or 6, wherein the blades extend generally axially relative to the hub portion.
8. The mechanical assembly according to claim 5, 6 or 7, wherein the hub portion is separately formed and adapted to be subsequently secured to a shaft
9. The mechanical assembly according to claim 5, 6 or 7, wherein the hub portion is integrally formed with a shaft
10. The mechanical assembly according to any one of claims 5 to 9, wherein each blade defines a blade root positioned adjacent the hub portion and a blade tip.
11. The mechanical assembly according to claim 10, wherein the extent to which each blade tip extends from the hub portion is selected in accordance with the required widths of the gaps between the blades and the stator element.
12. The mechanical assembly according to any preceding claim, wherein the at least two blades defining a gap of the first width may be adjacent each other.
13. The mechanical assembly according to any one of claims I to 11, wherein the at least two blades defining a gap of the first width may be separated by one or more other blades.
14. The mechanical assembly according to any one of claims 10 to 13, wherein the extent of the blade tips from the blade roots are selected such that that the mass distribution of the rotor is balanced.
15. The mechanical assembly according to any preceding claim, wherein the first clearance gap width is smaller than the second clearance gap width.
16. The mechanical assembly according to claim 14 or 15, wherein a smaller extent of the at least two blades from the roots is balanced by a greater extent of the at least one other blade.
17. The mechanical assembly according to any preceding claim, wherein at least one other blade defines a gap having a third width, wherein the third width differs from the first and second gap widths.
18. The mechanical assembly according to any preceding claim, wherein further blades of the rotor define gaps with the stator element with widths differing from the first, second and third widths.
19. The mechanical assembly according to any one of claims 10 to 18, wherein the tip of each blade comprises an outwardly facing tip surface.
20. The mechanical assembly according to claim 19, wherein the tip surface of at least one blade may be of a uniform profile.
21. The mechanical assembly according to claim 19 or 20, wherein the tip surface of at least one blade is substantially planer.
22. The mechanical assembly according to claim 21, wherein the substantially planar surface is substantially perpendicularly aligned relative to a longitudinal centre-line of the blade.
23. The mechanical assembly according to claim 21, wherein the substantially planar surface is tapered relative to a longitudinal centre-line of the blade.
24. The mechanical assembly according to any one of claims 19 to 23, wherein at least one blade comprises a tip surface having a non-uniform profile.
25. The mechanical assembly according to any preceding claim, wherein the resulting gap defined by each blade varies across the thickness or depth of each blade.
26. The mechanical assembly according to any preceding claim, wherein the stator element is profiled.
27. The mechanical assembly according to any preceding claim, wherein the rotor is cast.
28. The mechanical assembly according to any preceding claim, wherein the rotor is machined.
29. The mechanical assembly of any preceding claim, wherein the rotor is cast with blades of substantially uniform length, wherein at least one of the blades is subsequently trimmed by a suitable machining operation to the required length to define the appropriate gap distribution with the stator element.
30. The mechanical assembly according to any preceding claim, comprising a rotodynamic assembly.
31. The mechanical assembly according to any preceding claim, comprising a centrifugal pump.
32. The mechanical assembly according to claim 31, wherein the centrifugal pump is a volute-type pump, wherein the stator element comprises one or more volute cutwaters.
33. The mechanical assembly according to claim 31, wherein the centrifugal pump is a diffuser-type pump, wherein the stator element may comprise one or more diffuser vanes.
34. The mechanical assembly according to any preceding claim, comprising a plurality of rotors.
35. The mechanical assembly according to any preceding claim, comprising a multistage pump.
36. The mechanical assembly according to claim 35, wherein the multistage pump comprises a plurality of rotors circumferentially off-set from each other such that misalignment of the blades in an axial direction is achieved.
37. A rotor for use in a mechanical assembly, said rotor comprising: a hub portion adapted to be secured to a shaft; a plurality of blades which are uneven in number and which extend generally outwardly relative to the hub portion, wherein the extension of at least two of the blades from the hub is equal and differs from the extension of at least one other blade.
38. A mechanical assembly comprising: a stator defining a fluid cavity; and a rotor within the fluid cavity and comprising a plurality of blades, wherein an outer portion of each blade defines a gap with an element of the stator, wherein at least three of the blades define gaps of differing widths.
39. A mechanical assembly substantially as described herein and as shown in Figure 3.