AU2008253594A1

AU2008253594A1 - Seal assembly

Info

Publication number: AU2008253594A1
Application number: AU2008253594A
Authority: AU
Inventors: Andrew Philip John Collins
Original assignee: APC Co Pty Ltd
Current assignee: APC Co Pty Ltd
Priority date: 2007-05-18
Filing date: 2008-05-19
Publication date: 2008-11-27
Also published as: WO2008141377A1

Description

WO 2008/141377 PCT/AU2008/000703 Seal Assembly Field of the Invention The present invention relates to a seal assembly, and more particularly to a seal assembly for rotodynamic fluid machinery, such as pumps and compressors. 5 Further, the invention relates to a rotodynamic fluid machine incorporation such a seal assembly. The invention has been devised particularly, although not necessarily solely, as a seal assembly for rotodynamic fluid machinery used for pumping operations in the slurry, waste, pulp, paper and chemical industries. A centrifugal slurry pump 10 is a typical example of a rotating fluid machine to which the invention has particular application. Background Art A rotodynamic fluid machine, such as for example centrifugal slurry pump, requires fluid sealing around the rotating shaft against fluid pressure in the 15 machine. Various sealing arrangements are provided to establish fluid seals, including gland packing, expellers, lip seals and mechanical seals. Gland packing is oldest of the sealing technologies. It has two main deficiencies, one being that leakage is required from the gland to provide longevity to the 20 packing, and the other being that compression of the gland on to the shaft or an associated shaft sleeve can cause wear or damage. Further, gland packing is deemed to be an inappropriate sealing arrangement for a number of installations because of environmental and safety concerns. Expellers are common in many pumps used in corrosive and abrasive services 25 as they reduce the load on packing or lip seals through centrifugal force. The problem with expellers is that they do not necessarily work sufficiently with WO 2008/141377 PCT/AU2008/000703 -2 increased chamber pressure and usually require a secondary seal as gland packing which may result in leakage. Further, expellers do not seal when the machinery is not operating and can create major leakage in such circumstances. Lip seals are usually used where there is a low pressure differential across them. 5 Lip seals cannot be adjusted to compensate for wear and are more expensive to maintain than gland packing. Mechanical seals are favoured in rotodynamic fluid machinery for environmental and safety reasons. They are typically able to cope with a greater range of pressures than the other sealing arrangements referred to above. Further, 10 mechanical seals do not require an external fluid flush in circumstances where a clean fluid is being pumped. Mechanical seals are well proven and reliable for sealing rotating fluid machinery pumping water and hydrocarbons. However, mechanical seals used in rotodynamic machinery for pumping aggressive fluids have often required exotic 15 materials to establish an effective sealing arrangement and can be unreliable in circumstances where solids are present in the fluid, particularly for pumping slurry or pulp and paper. Further, all of the sealing arrangements referred to above are susceptible to catastrophic failures in circumstances where the rotating fluid machine "runs dry", 20 as the seals would not be sufficiently lubricated. It is against this background, and problems and difficulties associated therewith, that the present invention has been developed. The preceding discussion of the background to the invention is intended only to facilitate an understanding of the present invention. However, it should be noted 25 appreciated that the discussion is not an acknowledgement or an admission that any of the material referred to was or is part of the common general knowledge in Australia as at the priority date of the application.

WO 2008/141377 PCT/AU2008/000703 -3 Disclosure of the Invention According to a first aspect of the invention there is provided a seal assembly for a rotodynamic fluid machine having a rotatable shaft and a fluid chamber, the seal assembly comprising a seal chamber, a first seal and a second seal, the first 5 and second seals being spaced axially along the rotatable shaft and the seal chamber being disposed between the first and second seals, the second seal being exposed to the fluid chamber, wherein the first seal comprises a mechanical seal and the second seal comprises a hydraulic seal. Preferably, the hydraulic seal comprises a lip seal. 10 Preferably, the seal assembly further comprises means for delivering a flush fluid to the seal chamber. Flush fluid in the seal chamber may establish a fluid pressure therein. Preferably, the hydraulic seal is adapted to permit passage of fluid (such a flush fluid) from the seal chamber to the fluid chamber in circumstances where there 15 is a sufficient pressure differential therebetween arising through the fluid pressure in the seal chamber exceeding the fluid pressure in the fluid chamber. Typically, the passage of fluid comprises fluid leakage. Preferably, the lip seal is adapted to close in the absence of the pressure 20 differential, thereby isolating the seal chamber from the fluid chamber to thereby inhibit ingress of material into the seal chamber from the fluid chamber. Where the rotodynamic fluid machine comprises a pump having a pumping chamber defining the fluid chamber and a pump impeller rotatable within the pumping chamber operable to deliver fluid at a discharged pressure, the fluid 25 pressure in the seal chamber would slightly exceed the fluid pressure in the pumping chamber. The flush fluid may comprise water or other fluid compatible with the fluid with which the rotodynamic fluid machine is working. Where the rotating fluid WO 2008/141377 PCT/AU2008/000703 -4 machine comprises a pump, the flush fluid would thus be compatible with the pumpage material. Preferably, the leakage of flush fluid is at a flow rate sufficient to isolate the hydraulic seal from the fluid with which the rotodynamic fluid machine is 5 operating. In circumstances where the rotodynamic fluid machine operates to pump an aggressive pumpage material such as abrasive or corrosive slurry, the second seal is isolated from that aggressive pumpage material by the effect of the leakage of flush fluid. Preferably, the rotodynamic fluid machine comprises a portion confronting the 10 fluid chamber, the second seal being located between said portion and the rotatable shaft. With such an arrangement, only said portion confronting the fluid chamber need be of a material that is compatible or otherwise substantially unaffected by the fluid with which the machine is operating. The portion of the rotodynamic fluid machine confronting the fluid chamber may 15 have a seat for accommodating the hydraulic seal. The seat may have an axial face and a radial face, and a restrictor may be provided for confining the hydraulic seal between the two seat faces and the restrictor. Where the hydraulic seal comprises a lip seal, the latter may comprise a base portion and a lip portion, the base portion being adapted to be received on the 20 seat and confined thereon by the restrictor, and the lip portion having a radially inner edge for contact with the shaft or an associated shaft sleeve on the shaft. Means may be provided for monitoring the presence of flush fluid in the seal chamber. Such means may comprise an alarm adapted to actuate in response to an absence of fluid flow into the seal chamber. A detection means may be 25 provided for detecting fluid flow, the arrangement being that the alarm is actuated in response to the absence of fluid flow as detected by the detection means.

WO 2008/141377 PCT/AU2008/000703 -5 Preferably, the seal chamber is so configured that any particulate matter gaining entry into the seal chamber is conveyed away from the seal faces of the mechanical seal under the influence of centrifugal force. Preferably, the seal assembly further comprises a shaft sleeve for mounting on 5 the rotatable shaft for rotation therewith, the first and second seals being in sealing engagement with the shaft sleeve. Typically, the shaft sleeve presents a cylindrical outer surface for sealing engagement with the first and second seals, and an interior in which the rotatable shaft is received. 10 With this arrangement, the rotating seal of the mechanical seal is mounted on the shaft sleeve and the lip portion of the lip seal is in contact with the outer surface. Preferably, the mechanical seal comprises a stationary seal element, a rotatable seal element and a seal spring for biasing the two seal elements into intimate sealing contact, the shaft sleeve incorporating an abutment against which a 15 locating element is positioned, the seal spring being accommodated between the locating element and the rotatable seal element. Typically, the locating element is configured as a ring. According to a second aspect of the invention there is provided a seal assembly for a rotodynamic fluid machine having a rotatable shaft and a fluid chamber, the 20 seal assembly comprising a seal chamber, a first seal and a second seal, the first and second seals being spaced axially along the rotatable shaft and the seal chamber being disposed between the first and second seals, the second seal being exposed to the fluid chamber, the second seal being adapted to permit passage of fluid from the seal chamber to the fluid chamber in circumstances 25 where there is a sufficient pressure differential therebetween arising through fluid pressure in the seal chamber exceeding fluid pressure in the fluid chamber.

WO 2008/141377 PCT/AU2008/000703 -6 According to a third aspect of the invention there is provided a seal assembly for a rotodynamic fluid machine having a rotatable shaft and a fluid chamber, the seal assembly comprising a seal chamber, a mechanical seal and a hydraulic seal, the two seals being spaced axially along the rotatable shaft and the seal 5 chamber being disposed between the two seals, the hydraulic seal being exposed to the fluid chamber, means for delivering a flush fluid to the seal chamber to establish fluid pressure therein which in use is at a level exceeding the fluid pressure in the fluid chamber to establish a pressure differential between the seal chamber and the fluid chamber, the hydraulic seal being 10 adapted to permit flow of flush fluid from the seal chamber to the fluid chamber in response to a sufficient pressure differential therebetween to establish a fluid flow in the fluid chamber away from the hydraulic seal. According to a fourth aspect of the invention there is provided a rotodynamic machine having a seal assembly according to any one of the first, second and 15 third aspects of the invention. According to a fifth embodiment there is provided a pump comprising a pump casing defining a pump chamber in which a pump impeller is rotatably supported on a pump shaft, the pump shaft being rotatably supported by a bearing assembly, and a seal assembly provided between the pump casing and the 20 bearing assembly, the seal assembly being in accordance with any one of the first, second and third aspects of the invention. According to a sixth aspect of the invention there is provided a rotodynamic fluid machine comprising a rotatable shaft, a fluid chamber and a seal assembly, the seal assembly comprising a seal chamber, a first seal and a second seal, the first 25 and second seals being spaced axially along the rotatable shaft and the seal chamber being disposed between the first and second seals, the second seal being exposed to the fluid chamber, the second seal being adapted to permit passage of fluid from the seal chamber to the fluid chamber in circumstances where there is a sufficient pressure differential therebetween arising through fluid 30 pressure in the seal chamber exceeding fluid pressure in the fluid chamber.

WO 2008/141377 PCT/AU2008/000703 -7 Brief Description of the Drawings The invention would be better understood by reference to the following description by one specific embodiment thereof as shown in the accompanying drawings in which: 5 Figure 1 is a cross-sectional view of a centrifugal pump fitted with a seal assembly according to the embodiment; Figure 2 is a fragmentary view, on an enlarged scale, of that part of the centrifugal pump incorporating the seal assembly; Figure 3 is a detailed view illustrating in particular a first seal of the seal 10 assembly; Figure 4 is a detailed view illustrating in particular a second seal of the seal assembly; Figure 5 is a cross-sectional view of a seal casing forming part of the seal assembly; and Figure 6 is a cross-sectional view of a mechanical seal adaptor forming 15 part of the seal assembly. Best Mode(s) for Carrying Out the Invention The embodiment shown in the drawings is directed to a rotodynamic fluid machine in the form of a centrifugal pump 10 for pumping aggressive materials such as, for example, slurry, waste, pulp, paper and chemicals. 20 The pump 10 comprises a pump casing 11 defining a pump chamber 13 in which a pump impeller 15 is rotatably supported. The pump casing 11 incorporates a pump suction inlet 17 and a discharge outlet 18, both communicating with the pump chamber 13. The impeller 15 is rotatably supported within the pump chamber 13 on a pump shaft 19. The impeller 15 is mounted on one end 19a of 25 the pump shaft 19, the other end 19b of which is adapted to be coupled to a WO 2008/141377 PCT/AU2008/000703 motor (not shown) for operating the pump. The pump shaft 19 is rotatably supported on a bearing assembly 21 comprising a bearing housing 23 supporting bearings 25. The bearing housing 23 is closed at each end by an end cover 27. A seal assembly 31 is provided between the pump casing 11 and the bearing 5 assembly 21. The seal assembly 31 comprises a seal casing 33 having a mounting flange 35 at one end for attachment to the bearing housing 21. In the arrangement shown, the mounting flange 35 is attached to the bearing housing 21 by bolted connections 37. The other end of the seal casing 33 presents an annular rim 39 10 having a circumferential outer face 41 incorporating a circumferential recess 43 accommodating an O-ring 45. The annular rim 39 receives and supports a mechanical seal adapter 47 disposed about the shaft 19. The mechanical seal adapter 47 is located between a frame plate 48 forming part of the casing 11 and a frame plate liner insert 49 which defines a boundary surface of the pump 15 chamber 13. The frame plate liner insert 49 has a central boss 50 defining an aperture 51 through which a hub portion 52 of the impeller 15 extends for mounting on the end 19a of the shaft 19. A threaded connection 53 is provided between the hub portion 52 and the end 19a of the shaft 19. The mechanical seal adapter 47 has a radially inner portion 54 surrounding the 20 pump shaft 19. The radially inner portion 54 has two wall sections 55, 56 defining a central opening 58, and a shoulder 59 between the two wall sections 55, 56. A shaft sleeve 61 is mounted on the end section of the shaft 19 adjacent the impeller 15. The shaft sleeve 61 is secured to the shaft 19 for rotation therewith. The shaft sleeve 61 has an end section 63 at one end thereof presenting an end 25 face 65 in abutting engagement with the adjacent face of the impeller hub 52. The end face 65 incorporates a circumferential recess which accommodates an O-ring 69 for providing a fluid seal between the abutting faces. The other end section 70 of the shaft sleeve 61 has an end face 71 in engagement with a spacer 73 disposed between the shaft sleeve 71 and the adjacent bearing 25. 30 The end section 70 incorporates an inner recess accommodating an O-ring 77 WO 2008/141377 PCT/AU2008/000703 -9 for establishing a fluid seal between the shaft sleeve 61 and the shaft 19. The shaft 19 has a section 81 thereof adjacent the end 19a, the section being of reduced diameter to accommodate the shaft sleeve 61. With this arrangement, a shoulder 83 is defined in the pump shaft 19 adjacent the shaft section 81. 5 The seal assembly 31 comprises a first seal 91, a second seal 92 and a seal chamber 93 therebetween. The first seal 91 is at the atmospheric end of the seal chamber 93 and the second seal 92 is at the fluid end of the seal chamber. In other words, the second seal 92 is exposed to the pump chamber 13. The seal chamber 93 is defined by the seal casing 33 and the mechanical seal 10 adapter 47. The first seal 91 is in the form of a mechanical seal comprising a stationary seal element configured as a stationary seal ring 95 and a rotatable seal element configured as a rotatable seal ring 97, as is well known with mechanical seals. The stationary seal ring 95 is fixed with respect to the seal casing 33 and the 15 rotatable seal ring 97 which is mounted on the shaft sleeve 61 for rotation therewith. The stationary seal ring 95 is accommodated within a seat 99 defined by a rebate formed in the seal casing 33. The stationary seal ring 95 presents a stationary seal face and the rotatable seal ring 97 presents a rotatable seal face. The two sealing rings 95, 97 are in 20 intimate contact, with the seal face of the rotatable ring 97 in sliding and sealing contact with the seal face of the stationary seal ring 95. The rotatable seal ring 97 is fixed for rotation in unison with the shaft sleeve 61 (and thus the pump shaft 19 on which the shaft sleeve is mounted) but is also moveable axially along the sleeve shaft for intimate contact with the stationary seal ring 95. A biasing 25 means 105 is provided for biasing the rotatable seal ring 97 into sealing engagement with the stationary seal ring 95. In the arrangement shown, the biasing means 105 comprises a compression spring 109 acting between an abutment 111 associated with the rotatable seal ring 97 and a spring locating ring 113 mounted on the shaft sleeve 61. The spring locating ring 113 locates 30 against an abutment 115 provided on the shaft sleeve 61.

WO 2008/141377 PCT/AU2008/000703 - 10 The second seal 92 comprises a lip seal 121. The lip seal 121 comprises a radially outer base portion 123 which locates against the inner portion 54 of the mechanical seal adapter 47 and a radially inner lip portion 124 extending inwardly to terminate at a tip 125 for contact with the outer surface of the shaft 5 sleeve 61. The base portion 123 of the lip seal 121 is accommodated in a seat 126 provided on the mechanical seal adapter 47. The seat 126 has an axial face 127a defined by wall section 55 and a radial face 127b defined by shoulder 59. A restrictor 128 is provided for retaining the lip seal 121 in position. Specifically, 10 the restrictor 128 is comprises a ring 129 adapted for location as an interference fit within the central opening 58 of the mechanical seal adapter 47 to confine the base portion 123 of the lip seal 121 in position in the seat 126. The restrictor 128 surrounds the shaft sleeve 61 but is sized so as to not impede rotation of the pump shaft 19. 15 The lip portion 124 is inclined so as to extend inwardly and also towards the end 19a of the pump shaft 19, as shown in the drawings. The lip portion 124 has two inclined faces, being a first face 131 confronting the pump chamber 13 and a second face 132 confronting the seal chamber 93. With this arrangement, fluid pressure within the pump chamber 13 acting upon the first face 131 biases the 20 lip portion 124 into sealing contact with the shaft sleeve 61. In contrast, fluid pressure in the seal chamber 93 biases the lip portion 124 away from sealing contact with the shaft sleeve 61 to provide a small clearance space therebetween through which leakage can occur. The purpose of such an arrangement will become apparent later. 25 The restrictor 128 acts to retain the lip seal 121 in position while also limited flow of fluid within the seal chamber 93 past the restrictor to the lip seal 121. An inlet 141 is provided in the seal casing 33 to deliver flush fluid into the seal chamber 93. The inlet 131 is adapted for connection to a supply line (not shown). The flush fluid establishes a fluid pressure within the seal chamber 93 at a level WO 2008/141377 PCT/AU2008/000703 - 11 slightly higher than the fluid pressure within the pump chamber 13, creating a pressure differential across the lip seal 121. As a consequence of the fluid pressure differential, there is restricted fluid flow (leakage) past the lip seal 121 from the seal chamber 93 into the pump chamber 13. 5 Typically, the flush fluid comprises water or another fluid compatible with the material being pumped, such that the pumpage material is not contaminated or otherwise adversely affected by the flush fluid. The flush fluid serves the purpose of isolating the lip seal 121 from the pumpage material being pumped within the pump chamber 13. The flow of flush fluid does 10 this by diverting the pumpage material away from the lip portion 124 of the lip seal 121. In this way, the likelihood of the lip seal 121 being damaged by the aggressive pumpage material is reduced. The flow of flush fluid also inhibits ingress of the pumpage material into the seal chamber 93, so avoiding or at least reducing the possibility of the pumpage material contacting the mechanical seal 15 91. The fluid flow also serves to carry away heat generated by friction between the stationary and rotatable seal rings 95, 97 of the mechanical seal 91. The generated heat is extracted by the flush fluid and is conveyed from the seal chamber 93 by the flush fluid flow. 20 The presence of flush fluid within the seal chamber 93 can be monitored and an alarm actuated in the event of any loss of fluid. In this embodiment, such an arrangement comprises a reed switch alarm (not shown) incorporated in a flow delivery line (also not shown) connected to the seal chamber inlet. It is a particular feature of the seal assembly that the lip seal 121 can close to 25 sealingly engage the shaft sleeve 61 in circumstances where there is a loss of flush fluid pressure within the seal chamber 93, thereby sealing the seal chamber 93 against entry of pumpage material from the pump chamber 13. In this way, the mechanical seal 91 is protected from the pumpage material.

WO 2008/141377 PCT/AU2008/000703 - 12 The seal chamber 93 is configured to direct any contaminant particulate matter therein away from the stationary and rotatable seal rings 95, 97 during operation of the pump 10. In the arrangement shown, this is achieved by configuring that portion 142 of the wall of the seal chamber 93 around the mechanical seal 91 to 5 incorporate an outwardly directed taper. This configuration serves to cause any particular matter conveyed outwardly under the influence of centrifugal forces to move along the tapering portion 142 to an area within the seal chamber 93 away from the interacting seal rings 95, 97. Contaminant material can possibly enter the seal chamber either within the flush fluid or through pumpage material 10 leaking past the lip seal 121. It is a feature of the embodiment that the "working length" of the mechanical seal 91 can be adjusted according to the requirements of the specific application. This is done by positioning the shaft sleeve 61 of the end 19a of the shaft 19 at a location to establish the necessary spring force in the spring 109. The spring 15 force is established by the initial loading of the spring 109 as determined by the position of the spring loading plate 113 retained in position in relation to the sleeve shaft 61 by the abutment 115. The axial position of the spring loading plate 113 can be selectively varied by adjusting the position of the shaft sleeve 61 on the pump shaft 19. 20 From the foregoing, it is evident that the present embodiment provides a relatively simple yet highly effective seal assembly for rotodynamic fluid machines that can be used in a variety of applications, including in particular in relation to pumps capable of handling aggressive materials such as corrosive slurries without the need to use for exotic materials for most of the seal 25 components. In relation to the embodiment, the frame plate liner insert 49 is the only component of the seal assembly likely to be in prolonged contact with the pumpage material and accordingly is the only component that need be compatible with the pumpage material. It should be appreciated that the scope of the invention is not limited to the scope 30 of the embodiment described.

WO 2008/141377 PCT/AU2008/000703 -13 Modifications and improvements can be made without departing from the scope of the invention. Throughout the specification and claims, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be 5 understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Claims

1. A seal assembly for a rotodynamic fluid machine having a rotatable shaft and a fluid chamber, the seal assembly comprising a seal chamber, a first seal and a second seal, the first and second seals being spaced axially 5 along the rotatable shaft and the seal chamber being disposed between the first and second seals, the second seal being exposed to the fluid chamber, wherein the first seal comprises a mechanical seal and the second seal comprises a hydraulic seal.

2. A seal assembly according to claim 1 wherein the hydraulic seal comprises 10 a lip seal.

3. A seal assembly according to claim 1 or 2 further comprising means for delivering a flush fluid to the seal chamber.

4. A seal assembly according to claim 2 or 3 wherein the hydraulic seal is adapted to permit passage of fluid from the seal chamber to the fluid 15 chamber in circumstances where there is a sufficient pressure differential therebetween arising through the fluid pressure in the seal chamber exceeding the fluid pressure in the fluid chamber.

5. A seal assembly according to claim 4 wherein the passage of fluid comprises fluid leakage. 20

6. A seal assembly according to claim 4 wherein the hydraulic seal is adapted to close in the absence of the pressure differential, thereby isolating the seal chamber from the fluid chamber to thereby inhibit ingress of material into the seal chamber from the fluid chamber. 25

7. A seal assembly for a rotodynamic fluid machine having a rotatable shaft and a fluid chamber, the seal assembly comprising a seal chamber, a first seal and a second seal, the first and second seals being spaced axially along the rotatable shaft and the seal chamber being disposed between the first and second seals, the second seal being exposed to the fluid chamber, WO 2008/141377 PCT/AU2008/000703 - 15 the second seal being adapted to permit passage of fluid from the seal chamber to the fluid chamber in circumstances where there is a sufficient pressure differential therebetween arising through fluid pressure in the seal chamber exceeding fluid pressure in the fluid chamber. 5

8. A seal assembly for a rotodynamic fluid machine having a rotatable shaft and a fluid chamber, the seal assembly comprising a seal chamber, a mechanical seal and a hydraulic seal, the two seals being spaced axially along the rotatable shaft and the seal chamber being disposed between the two seals, the hydraulic seal being exposed to the fluid chamber, means for 10 delivering a flush fluid to the seal chamber to establish fluid pressure therein which in use is at a level exceeding the fluid pressure in the fluid chamber to establish a pressure differential between the seal chamber and the fluid chamber, the hydraulic seal being adapted to permit flow of flush fluid from the seal chamber to the fluid chamber in response to a sufficient 15 pressure differential therebetween to establish a fluid flow in the fluid chamber away from the hydraulic seal.

9. A rotodynamic fluid machine having a seal assembly according to any one of the preceding claims.

10.A pump comprising a pump casing defining a pump chamber in which a 20 pump impeller is rotatably supported on a pump shaft, the pump shaft being rotatably supported by a bearing assembly, and a seal assembly provided between the pump casing and the bearing assembly, the seal assembly being in accordance with any one of claims I to 8.

11.A pump according to claim 10 wherein the pump impeller is operable to 25 deliver fluid at a discharged pressure and wherein the fluid pressure in the seal chamber slightly exceeds the fluid pressure in the pumping chamber.

12.A pump according to claim 10 or 11 wherein the flush fluid comprises fluid compatible with fluid delivered by the pump. WO 2008/141377 PCT/AU2008/000703 -16

13.A pump according to claim 10, 11 or 12 wherein leakage of flush fluid is at a flow rate sufficient to isolate the hydraulic seal from the fluid within the pumping chamber.

14.A pump according to any one of claims 10 to 13 further comprising a 5 portion confronting the pumping chamber, the second seal being located between said portion and the rotatable shaft.

15.A pump according to claim 14 wherein the portion confronting the pumping chamber has a seat for accommodating the hydraulic seal.

16.A pump according to claim 15 wherein the seat comprises an axial face 10 and a radial face, the hydraulic seal being confined between the two seat faces and a restrictor.

17.A pump according to claim 16 wherein the hydraulic seal comprises a lip seal comprising a base portion and a lip portion, the base portion being adapted to be received on the seat and confined thereon by the restrictor, 15 and the lip portion having a radially inner edge for contact with the shaft or an associated shaft sleeve on the shaft.

18.A pump according to any one of claims 10 to 17 further comprising monitoring means for monitoring the presence of flush fluid in the seal chamber. 20

19.A pump according to claim 18 wherein the monitoring means comprises an alarm adapted to actuate in response to an absence of fluid flow into the seal chamber.

20.A pump according to claim 19 further comprising a detection means d for detecting fluid flow, the arrangement being that the alarm is actuated in 25 response to the absence of fluid flow as detected by the detection means. WO 2008/141377 PCT/AU2008/000703 -17

21.A pump according to any one of claims 10 to 20 wherein the seal chamber is so configured that any particulate matter gaining entry into the seal chamber is conveyed away from the seal faces of the mechanical seal under the influence of centrifugal force. 5

22.A pump according to any one of claims 10 to 21 further comprising a shaft sleeve mounted on the pump shaft for rotation therewith, the first and second seals being in sealing engagement with the shaft sleeve.

23.A pump according to claim 22 wherein the shaft sleeve presents a cylindrical outer surface for sealing engagement with the first and second 10 seals, and an interior in which the rotatable pump shaft is received.

24.A pump according to claim 22 or 23 wherein the mechanical seal comprises a stationary seal element, a rotatable seal element and a seal spring for biasing the two seal elements into intimate sealing contact, the shaft sleeve incorporating an abutment against which a locating element is 15 positioned, the seal spring being accommodated between the locating element and the rotatable seal element.

25.A pump according to claim 24 wherein the locating element is configured as a ring.

26.A rotodynamic fluid machine comprising a rotatable shaft, a fluid chamber 20 and a seal assembly, the seal assembly comprising a seal chamber, a first seal and a second seal, the first and second seals being spaced axially along the rotatable shaft and the seal chamber being disposed between the first and second seals, the second seal being exposed to the fluid chamber, the second seal being adapted to permit passage of fluid from the seal 25 chamber to the fluid chamber in circumstances where there is a sufficient pressure differential therebetween arising through fluid pressure in the seal chamber exceeding fluid pressure in the fluid chamber. WO 2008/141377 PCT/AU2008/000703 -18

27.A seal assembly substantially as herein described with reference to the accompanying drawings.

28.A rotodynamic fluid machine substantially as herein described with reference to the accompanying drawings. 5

29.A pump substantially as herein described with reference to the accompanying drawings.