Rotary fluid pump

Abstract: A fluid pump includes main support bearings (15) for supporting the impeller shaft and back-up bearings (40) for supporting the shaft upon failure of the main support bearings (15). The back-up bearings (40) are positioned between the main support bearings (15) and shaft driver/motor, and have a radial clearance (45) with the shaft which is greater than the radial clearance (46) between the main support bearings (15) and shaft and yet is small enough to prevent any shards that may result from failure of the main support bearings from passing through the radial clearance space between the back-up bearings and shaft and damaging other rotating parts of the pump. A diagnostic monitor detects wear or failure of the main support bearings by sensing relative shaft displacement. Also, a controller causes the driver to shut down when a predetermined amount of allowable wear of the main support bearings is reached.

Description: FIELD OF THE INVENTION This invention relates generally, as indicated, to a rotary fluid pump, and more particularly to a bearing system for a sealless pump. BACKGROUND OF THE INVENTION Bearing systems for sealless pumps are used to radially support the rotor assembly used to drive the pump. Axial loads occurring during start-up, operation, or upset conditions are also carried by the bearing system. Under normal conditions, the rotor assembly is submerged in the fluid being pumped, and by design, such fluid is circulated around the rotor assembly and bearings to cool the motor or driver and lubricate the bearing system. Fluids that are pumped by sealless pumps are generally corrosive, highly volatile, low viscosity, having very high or low temperature (ranging for example from cryogenic to 538°C (1,000°F)) and oftentimes contain hard erosive contaminants. These conditions must be mitigated by pump design, or handled by the bearing system. Generally speaking, special grades of carbon-graphite bushings or bearings operating against stainless steel provide the best performance over the widest range of conditions. However, for difficult applications or application with compatibility issues, special bearing materials may be necessary. For example, fluid compatibility may dictate the use of bearings made of Teflon or Rulon materials, while high temperature and particulate contamination may require the use of bearings made of hard bearing materials such as aluminum oxide (ALOX), tungsten carbide, or silicon carbide (SiC). These materials are characterized by high values of hardness (Vickers - 2000+), and in the case of ALOX and SiC, are extremely brittle and easy to fracture due to rough handling or vibration. Also, because hard bearings typically have relatively low thermal conductivity, they are very susceptible to heat build-up due to dry running or cavitation. These conditions, which can be common occurrences with sealless pumps, can result in bearing failures. Failure modes of hard bearings in sealless pumps can be particularly catastrophic, since when hard bearings fail, they usually shatter like glass, making hundreds of razor sharp shards which can pierce stainless steel liners or shrouds exposing motor windings or external magnets to the pumping fluid. Besides requiring very expensive repairs, the resulting leakage of volatile or corrosive pumping fluids (chemicals) can be extremely hazardous. There is thus a need for a bearing system for rotary fluid pumps including particularly sealless pumps which can advantageously use hard bearings while minimizing the dangers of bearing failure associated with their use. US 4,063,786 discloses a back-up bearing for supporting a shaft upon failure of the main bearing and addresses the problem of debris emanating from the main bearing, whereby a shield is provided to protect the back-up bearing for the debris. According to the present invention, there is provided a fluid pump comprising a pump housing containing a pump impeller supported by a rotatable shaft, a driver for rotating said shaft, main support bearings comprising a hard bushing for said shaft which are lubricated by the fluid being pumped, and back-up bearing bushings for supporting said shaft upon failure of said main support bearing bushings, said back-up bearing bushings being positioned between said main support bearing bushings and said driver wherein said back-up bearing bushings having a radial clearance space with first journal surfaces fixed to said shaft that is greater than the radial clearance space between said main support bearing bushings and second journal surfaces on said shaft or on a shaft sleeve (16) and yet small enough to prevent any shards that may result from failure of said main support bearing bushings and/or shaft sleeve (16) from passing through the radial clearance space between said back-up bearing bushings and said second journal surfaces and causing damage. The invention will be further apparent from the following description with reference to the several figures of the accompanying drawings, which show, by way of example only, one form of fluid pump embodying same. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a fragmentary longitudinal section through one form of pump in accordance with the invention. Fig. 2 is an enlarged fragmentary longitudinal section through the pump of Fig. 1 in which the clearances between the various moving parts of the pump, including particularly the bearing and rotor-stator clearances are exaggerated for illustrative purposes. Fig. 3 is a schematic diagram showing how the bearing wear detector monitors bearing wear and provides a signal when the bearing wear exceeds a predetermined amount to permit an orderly shut-down of the pump so that the bearings can be replaced before other damage to the pump can occur. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Fig. 1 shows one form of rotary fluid pump 1 in accordance with this invention including a pump housing 2 containing a pump chamber 3 having a central fluid inlet 4 into which the fluid to be pumped is drawn during rotation of an impeller 5 within the pump chamber for pressurizing the fluid prior to being discharged through the main pump outlet 6. Pump impeller 5 is suitably mounted on the output shaft 7 of a drive motor 8. At opposite ends of the motor shaft 7 are bearing systems 10, 10' for supporting the rotor assembly 11 1 of the motor as described hereafter. Only one such bearing system will be described in detail (the bearing system 10 that supports the end of the shaft 7 on which the pump impeller 5 is mounted), it being understood that the other bearing system 10' may be substantially identical. Also, for purposes of clarity, the clearances between the various components of the bearing system 10 and rotor and stator portions 12, 13 of a bearing wear detector 14 shown in Fig. 2 are exaggerated. The bearing system 10 includes a main support bearing 15 for the motor shaft 7 comprising a shaft sleeve 16 and surrounding bushing 17 and thrust ring 18. The bushing 17 is coaxially supported within a counterbore 19 in the pump housing 2 by means of a bushing retainer 20 pinned or otherwise secured to the pump housing. Suitable seals 21 may be provided between the motor shaft 7 and shaft sleeve 16 as well as between the bushing 17 and bushing retainer 20. Also, a washer 22 is disposed between a shoulder 23 of the bushing retainer 20 and one end of the bushing 17. Thrust ring 18 is supported adjacent the other end of the bushing 17 by a thrust ring retainer 26 keyed or otherwise affixed to the motor shaft 7 for rotation therewith. The thrust ring retainer 26 includes a recess 27 in which the thrust ring 18 is received, with a suitable seal 28 interposed between the outer diameter of the thrust ring and inner wall of the recess. Also, a washer 30 is disposed between shoulder 31 of thrust ring retainer 26 and adjacent ends of the shaft sleeve 16 and thrust ring 18. During normal operation, the bushing 17 and shaft sleeve 16 carry the radial load, and the bushing 17 and thrust ring 18 carry axial loads during start-up, operation or upset conditions. The bushing and thrust ring retainers 20 and 26 are designed to support and align the main support bearing components and fix them from rotation relative to their respective retainers. A plurality of radial grooves 35 in the rotating thrust ring 18 provide a pumping action to promote fluid flow through the bearing system 10 for cooling and lubrication. Also using retainers 20 and 26 for the bushing 17 and thrust ring 18 permits the bushing and thrust ring to be made with relatively thin cross sections to permit better heat removal from the main support bearing 15. Under normal conditions, the bearing system 10 is submerged in the same fluid as the pump 1, and by design, a portion of the fluid being pumped is directed through a fluid line 36 extending from the pump outlet 6 to the bearing system 10' that supports the other end of the motor shaft 7 for circulation around the rotor assembly 11 and both bearing systems to cool the motor or driver 8 and lubricate the bearing systems. Fluids that are pumped by sealless pumps are generally corrosive, highly volatile, low viscosity, and have very high or low temperature, ranging for example from cryogenic to 5'38°C (1,000°F). Also, such fluids often contain hard erosive contaminants. For difficult applications or applications with compatibility issues, special bearing materials may be necessary. For example, fluid compatibility may dictate the use of Teflon or Rulon materials for the main support bearing components, while high temperatures and particulate contamination may require the use of hard bearing materials such as aluminum oxide (ALOX), tungsten carbide, or silicon carbide (SiC). Also, the shaft sleeve 16 may be omitted, and stellite or tungsten carbide or other suitable material may be flame sprayed or welded directly to the rotor shaft 7 to provide a hard mating surface for the bushing 17. Because bearings made of such hard materials typically have low thermal conductivity, they are very susceptible to heat build-up due to dry running or cavitation. Also, bearings made of ALOX and SiC are extremely brittle and easy to fracture due to rough handling or vibration. These conditions can be common occurrences with sealless pumps, and can result in bearing failures which can be particularly disastrous, since hard bearings usually shatter like glass, making razor sharp shards which can pierce stainless steel liners or shrouds exposing motor windings or external magnets to the fluid being pumped. Also, the resulting leakage of volatile or corrosive fluids can be extremely hazardous. In the event of a failure of one or both of the radial main support bearing components of the present invention ( e.g. , the bushing 17 and/or shaft sleeve 16), a back-up bushing 40 is provided for supporting the rotor shaft 7 through a journal surface fixed to the shaft. In the embodiment shown, the journal surface is on the thrust ring retainer 26. Also, the back-up bushing 40 is located between the main support bearing 15 and motor or drive cavity 41 in a counterbore 42 in the bushing retainer 20 surrounding the thrust ring retainer 26. The back-up bearing 40 may be made of the same materials as the main support bearing components and is limited only by temperature and fluid compatibility issues. The clearance 45 between the thrust ring retainer 26 and surrounding back-up bearing 40 is small enough to prevent any of the bearing shards that may result upon failure of any of the main support bearing components 16, 17 and 18 from entering the motor cavity 41 and damaging other rotating parts. However, since long term running on the back-up bearing is not recommended, such clearance 45 is desirably set large enough so that the bearing wear detector 14, schematically shown in Figs. 1 and 2, may be used to sense excessive bearing wear and permit safe shut-down of the pump 1 before any other damage can occur. The bearing wear detector 14 may, for example, be of the type disclosed in US 5 696 444 A (= EP 0 672 886). Such detector is used in combination with the back-up bearing 40 to sense a failure of the main support bearing 15 by sensing relative shaft displacement at the bearing wear detector location and permit the pump 1 to be safely shut down before any other damage can occur as described hereafter. By way of example, the radial clearance 46 between the main support bushing 17 and shaft sleeve 16, or if the shaft sleeve 17 is omitted, between the main support bushing 17 and radial mating surface on the motor shaft 7 is approximately 0.005 cm (.002 inch), whereas the radial clearance 45 between the thrust ring retainer 26 and back-up bearing 42 is approximately 0.025 cm (.010 inch). The normal total indicated runout (TIR) of the motor shaft 7 supported by the main support bearing 15 is about twice the radial clearance 46 of the main support bearing 15 or approximately 0.01 cm (.004 inch). In this example, during normal operation, an average radial clearance of approximately 0.02 cm (.008 inch) exists between the motor shaft 7 and back-up bearing 40 ( e.g ., the difference between the back-up bearing clearance of 0.025 cm (.010 inch) and the main support bearing clearance of 0.005 cm (.002 inch). Also, the radial clearance 47 between the rotor element 12 of the bearing wear detector 14 which is mounted on the motor shaft 7, and the surrounding stationary stator liner element 13 of the detector may, for example, be approximately 0.05 cm (.020 inch). Under normal operating conditions, very little wear of the main support bearing 15 is expected, whereby the bearing wear detector 14 should be set to indicate zero output when the rotor 12 is operating within the normal TIR for the motor shaft 7. As long as a primary failure of one or both of the main shaft support bearings 10, 10' does not occur, the back-up bearing 40 will remain out of contact with the thrust ring retainer 26 and thus won't provide support for the main shaft 7. However, if a failure of one or both bearings 10, 10' should occur, the main shaft 7 will be supported by the back-up bearing 40 (through the thrust ring retainer 26). In that event, the TIR will increase dramatically from approximately 0.01 cm (.004 inch) ( e.g ., twice the radial clearance 46 of the main support bearing) to approximately 0.05 cm (.020 inch) ( e.g ., twice the radial clearance 45 of the back-up bearing). Nevertheless, the average radial clearance between the rotor 12 and stator 13 will still be about 0.025 cm (.010 inch) ( e.g ., the difference between the radial clearance 47 of 0.05 cm (.020 inch) between the rotor and stator elements 12, 13 and radial clearance 45 of 0.025 cm (.010 inch) of the back-up bearing 40). The bearing wear detector 14 may be pre-programmed to determine radial movement, for example, of 0.033 cm (.013 inch) as 100% allowable bearing wear. By setting alarm limits, for example, at 50% ( e.g ., 0.0016 cm (.0065 inch)) and 75% ( e.g ., 0.025 cm (.010 inch)) of 100% allowable bearing wear, the value of 0.02 cm (.008 inch) radial displacement that occurs upon failure of the main support bearing 15 will be between 50% and 75% of allowable bearing wear. As schematically shown in Fig. 3, the bearing wear detector (diagnostic monitor) 14 may be coupled through a controller 50 to one or more alarm outputs such as light outputs 51, 52 to provide a display indicating realtime percentage of radial movement of the motor shaft. If desired, one or more alarms/indicators such as light outputs may also be provided for showing realtime percentage of axial movement of the motor shaft. Under normal operating conditions, when the rotor element 12 of the bearing wear indicator 14 is operating within the normal TIR for the motor shaft, the bearing wear indicator will, for example, indicate zero output. This can be visually displayed, for example, by causing one of the alarms such as light outputs 51, 52 representative of radial status to emit a signal such as a steady green light. Should the main support bearing 15 fail, the increased radial displacement of the rotor element 12 of 0.02 cm (.008 inch) will cause the 50% setting of allowable bearing wear to indicate alarm status by extinguishing the signal ( e.g ., steady green light) and activating an alarm ( e.g ., a steady red light) from one of the alarm outputs 51, 52. If bearing wear should continue to the point where 75% or greater bearing wear is reached, the 75% setting may, for example, cause the steady alarm output ( e.g ., red light) to begin signalling ( e.g ., flashing), indicating that it is time to replace the bearings. Under most conditions, this will provide sufficient time to permit an orderly shut-down of the pump 1 before the rotor portion 11 of the motor 8 comes into contact with the stator liner 53 and causes additional damage to the rotating parts. Also, if desired, the controller 50 may cause both an audible alarm to be sounded through a speaker 54 and an automatic shut-down of the pump motor 8 as schematically illustrated in Fig. 3 when the 75% setting of allowable bearing wear is reached. Although the noted values given in the above example are representative, it should be understood that other clearances and alarm set points may be used in different applications as desired. From the foregoing, it will be apparent that by incorporating a back-up bearing in a rotary fluid pump in accordance with the present invention, all of the advantages of using hard bearing materials for the main support bearing including the bushing, shaft sleeve, and thrust ring such as improved load capacity, reduced wear rates, temperature capability and insensitivity to particulates can be obtained without the dangers of catastrophic bearing failures. Although the invention has been shown and described with respect to a certain preferred embodiment, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of the specification. The present invention includes all such equivalent alterations and modifications, and is limited only by the scope of the claims.

Claims: A fluid pump (1) comprising a pump housing (2) containing a pump impeller (5) supported by a rotatable shaft (7), a driver (8) for rotating said shaft, main support bearings (15) comprising a hard bushing (17) for said shaft which are lubricated by the fluid being pumped, and back-up bearing bushings (40) for supporting said shaft upon failure of said main support bearing bushings, said back-up bearing bushings being positioned between said main support bearing bushings and said driver wherein said back-up bearing bushings having a radial clearance space (45) with first journal surfaces fixed to said shaft that is greater than the radial clearance space (46) between said main support bearing bushings and second journal surfaces on said shaft or on a shaft sleeve (16) and yet small enough to prevent any shards that may result from failure of said main support bearing bushings and/or shaft sleeve (16) from passing through the radial clearance space between said back-up bearing bushings and said second journal surfaces and causing damage. The pump of claim 1 wherein the radial clearance space between said back-up bearing bushings and said first journal surfaces is several times greater than the radial clearance space between said main support bearing bushings and said second journal surfaces. The pump of claim 1 further comprising a diagnostic monitor (14) for detecting wear of said main support bearing bushings by sensing relative shaft displacement. The pump of claim 3 further comprising a controller (50) responsive to said diagnostic monitor for indicating radial displacement of said shaft as a percentage of allowable wear of said main support bearing bushings, and means (51, 52) for signalling different levels of wear of said main support bearing bushings as a percentage of allowable wear. The pump of claim 1 wherein said main support bearing bushings are supported by bushing retainers secured to said pump housing, and thrust rings are supported adjacent said main support bearing bushings by thrust ring retainers fixed to said shaft for rotation with said shaft, said bushing retainers extending around said thrust ring retainers and containing counterbores in which said back-up bearing bushings are received, said radial clearance space between said back-up bearing bushings and said first journal surfaces being located between said back-up bearing bushings and said thrust ring retainers. The pump of claim 1 further comprising a diagnostic monitor for detecting a failure of said main support bearing bushings and providing for a controlled shut down of said pump while said shaft is supported by said back-up bearing bushings. The pump of claim 6 wherein said diagnostic monitor detects a failure of said main support bearing bushings by sensing relative shaft displacement, further comprising a controller for indicating radial displacement of said shaft as a percentage of allowable wear of said main support bearing bushings. The pump of claim 7 further comprising means for signalling different levels of wear of said main support bearing bushings as a percentage of allowable wear.

Inventors: LOBACH JAMES, MATTES EDWARD W, VAUGHN PAUL E, LOBACH, JAMES, MATTES, EDWARD W., VAUGHN, PAUL E.
Applicants: CRANE CO, CRANE CO.
Application year: 1997
Main classification: F16C17
IPC codes: F04D29, F04D13, F16C17, F04D15