Hydrodynamic pivoted pad bearing assembly for a reactor coolant pump

Abstract: A nuclear reactor coolant pump utilizes a pivoted pad bearing assembly (48) to rotatably mount its shaft (22) through the pump housing (32). The bearing assembly (48) includes a plurality of arcuate bearing segments (52) disposed end-to-end in circumferentially spaced relation for encompassing the shaft (22) and providing bearing contact therewith, and a plurality of pivot pins (54) each extending radially between an outer annular split ring (50) attached to the pump housing (32) and one of bearing segments (52). The pivot pins (54) are mounted for universal pivotal movement at their inner ends (54A) with the respective bearing segments (52). The pivot pins (54) at their outer ends (54B) are mounted to the annular ring (50) for adjustable radial movement for changing the radial position of the pivot pins (54) relative to annular ring (50) and thereby of the bearing segments (52) relative to the shaft (22). A plurality of locking cups (72) are provided for coupling the pivot pins (54) in locked relation with the annular ring (50) to fix the pivot pins (54) in desired adjusted positions relative thereto.

Description: The present invention relates generally to bearings and, more particularly, is concerned with a hydrodynamic pivoted pad bearing assembly for a reactor coolant pump. In pressurized water nuclear power plants, a reactor coolant system is used to transport heat from the reactor core to steam generators for the production of steam. The steam is then used to drive a turbine generator. The reactor coolant system includes a plurality of separate cooling loops, each connected to the reactor core and containing a steam generator and a reactor coolant pump. The reactor coolant pump typically is a vertical, single stage, centrifugal pump designed to move large volumes of reactor coolant at high temperatures and pressures, for example 287°C (550°F) and 15.525 MPa (2250 psi). The pump basically includes three general sections from top to bottom -- motor, shaft seal and hydraulic sections. The upper motor section includes a motor which is coupled to drive a vertical pump shaft. The middle shaft seal section includes three tandem sealing assemblies located concentric to, and near the top end of, the pump shaft. Their combined purpose is to mechanically contain the high positive pressure coolant of the reactor coolant system from leakage along the pump shaft to the containment atmosphere during normal operating condition. The lower hydraulic section includes an impeller mounted on the lower end of a pump shaft which is operable within the pump casing to pump reactor coolant about the respective loop. The hydraulic section also includes a heat exchanger and a lower radial journal bearing assembly disposed in tandem relationship about the pump shaft above the impeller. The heat exchanger functions to reduce the rate of heat flow from the reactor coolant into the pump bearing and seal areas. The journal bearing assembly includes a two-piece horizontally split housing, a bearing cartridge, and a journal. The inside diameter of the housing is machined to a spherical shape which mates with a spherical surface on the outside diameter of the bearing cartridge to allow the bearing assembly to self-align. The bearing cartridge operates against the journal which is shrunk fit on the pump shaft. This bearing assembly is lubricated and cooled by seal injection water flow downward from the seal section to the thermal barrier area of the heat exchanger. The lower radial bearing assembly of the above-described construction has undergone many hours of operation and overall has performed satisfactorily. However, one significant drawback of the radial bearing assembly is that it tends to whirl and generate undesirable vibration due to such whirl. This is particularly evident in some pump designs as bearing radial load decreases (as the loop temperature increases). Also, over zealous pump balancing which removes all or most of the running speed component of vibration can result in a non-linearly related increase in vibration at bearing whirl frequency. One attempt to correct the problems encountered by the radial bearing assembly during operation is described in U.S. Patent No. 3,497,276 to McGrew, Jr. et al. However, McGrew et al. only discloses the use of a one-piece bearing assembly. Reference is also made to prior art document DE-A-3 622 495 which discloses a bearing assembly according to the preamble of claim 1, whereby each adjustment element is fixed to the annular member by screws and the pivot pin is fixed in the desired adjusted position by a stud screw. The bearing assembly of the present invention employs a plurality of pivoted arcuate bearing segments which are radially adjustable and universally pivoted allowing the segments to be offset from their machining center to provide for preloading of the pump shaft. Such preloading will increase the bearing radial stiffness to restrain shaft orbiting when low load exists and will also prevent pad flutter. The pivoted segments have the ability to accommodate shaft deflection or misalignment and can, therefore, carry relatively higher loads than journal bearings. Another advantage is their freedom from lubricant whirl/whip instability. This makes them suitable for such applications as vertical guide bearings or lightly loaded bearings as in the case of the reactor coolant pump. A further advantage is that the pivoted pad bearing assembly of the present invention can be employed in place of the journal bearing assembly without modification to the reactor coolant pump. Accordingly, the present invention consists in a pivoted pad bearing assembly for rotatably supporting a shaft extending through a housing, said bearing assembly comprising: (a) an annular member attachable to the housing; (b) a plurality of arcuate bearing segments disposable end-to-end in circumferentially spaced relation for encompassing the shaft and providing bearing contact herewith; (c) a plurality of pivot pins each extending radially between said annular member and one of said bearing segments; (d) means for releasably and universally pivotally mounting each of said pivot pins at an inner end thereof to one of said bearing segments; (e) means for mounting each of said pivot pins at an outer end thereof to said annular member and being adjustable for changing the radial position of said pivot pin relative to said annular member and thereby of said bearing segment relative to the shaft, said pivot pin outer end mounting means including (i) means defining a plurality of circumferentially spaced and radially extending openings through said annular member for receiving said respective pivot pins, (ii) a set of external threads formed on said outer end of each of said pivot pins, and (iii) a plurality of adjustment elements each having a set of internal threads formed thereon for threadably mating with said set of external threads on one of said pivot pin outer ends such that relative rotation between each pivot pin and its respective adjustment element causes a change of the radial position of said pivot pin relative to said annular member; and (f) means are provided for lockably coupling said pivot pins to said annular member to fix said pivot pins in desired adjusted positions relative to said annular member, characterized in that said lockable coupling means includes: means defining at least one recess in said annular member at the periphery of each of said openings which extends radially from said opening; means defining at least one notch in a peripheral edge of each of said adjustment element which extends radially to one of said openings; means defining at least one longitudinally extending slot in said outer end of each of said pivot pins which also extends radially to one of said openings; and a plurality of locking cups each being disposable in one of said openings, each of said cups having at least one projection capable of extending into said slot for maintaining said cup in non-rotatable relation to one of said pivot pins, each of said cups having a cylindrical wall with portions deformable into one of said recesses of said annular member and one of said notches of said adjustment element for maintaining said cup in non-rotatable relation thereto. inadvertent release of the pivot pins at their inner ends from the bearing segments before the bearing assembly is mounted about the shaft. More particularly, the pivot pin inner end mounting means includes a cup element having an internal bore surface and being mounted in a recess formed at an exterior surface of each of the bearing segments. The mounting means also includes an external contact surface on the inner end of each of the pivot pins. One of the cup element internal bore surface or the pivot pin inner end contact surface has an annular cylindrical contour, whereas the other has an annular spherical contour allowing universal pivoting therebetween. Further, the pivot pin outer end mounting means includes means for defining a plurality of circumferentially spaced and radially extending openings through the annular member for receiving the respective pivot pins, a set of external threads formed on the outer end of each of the pivot pins, and a plurality of adjustment elements each having a set of internal threads formed thereon for threadably mating with the set of external threads on one of the pivot pin outer ends such that relative rotation between each pivot pin and its respective adjustment element causes a change of the radial position of the pivot pin relative to the annular member. Also, the bearing assembly includes means for lockably coupling each pivot pin to the annular member to fix the pivot pin in a desired adjusted position relative to the annular member. In particular, the lockable coupling means includes means defining at least one recess in the annular member at the periphery of each of the openings therethrough which extends radially from the opening, means defining at least one notch in a peripheral edge of each of the adjustment elements which extends radially to one of the openings, means defining at least one longitudinally extending slot in the outer end of each of the pivot pins which also extends radially to one of the openings, and a plurality of locking cups each being disposable in one of the openings. Each of the cups has at least one projection capable of extending into the slot for maintaining the cup in non-rotatable relation to one of the pivot pins. Also, each of the cups has a cylindrical wall with portions deformable into one of the recesses of the annular member and one of the notches of the adjustment element for maintaining the cup in non-rotatable relation thereto. The means for universally pivotally mounting each pivot pin inner end to one of the arcuate bearing segments releasably mounts the pivot pin at its inner end. The bearing assembly also includes means for retaining the pivoted bearing segments on the annular member to preclude inadvertent release of the pivot pins at their inner ends from the arcuate bearing segments before the bearing assembly is mounted about the shaft. The invention will become more readily apparent from the following description of the preferred embodiment thereof, shown by way of example only, in the accompanying drawings wherein: Fig. 1 is an axial sectional view, partly in elevation, of a prior art nuclear reactor coolant pump. Fig. 2 is an enlarged fragmentary axial sectional view of a prior art radial bearing assembly employed in the reactor coolant pump of Fig. 1. Fig. 3 is a fragmentary axial sectional view of a pivoted pad bearing assembly in accordance with the present invention. Fig. 4 is a top plan view, partly broken away and sectioned, of the pivoted pad bearing assembly. Fig. 5 is an axial sectional view of the bearing assembly taken along line 5--5 of Fig. 4. Fig. 6 is a fragmentary axial sectional view of the bearing assembly taken along line 6--6 of Fig. 4. Fig. 7 is an enlarged view of the fragmentary sectioned portion of the bearing assembly illustrated in Fig. 4. Fig. 8 is an axial sectional of the bearing assembly taken along line 8--8 of Fig. 7. Fig. 9 is an elevational view of a clamp plate of the bearing assembly as seen along line 9--9 of Fig. 7. Fig. 10 is an enlarged fragmentary view of a pivot pin locking arrangement of the bearing assembly as seen along line 10--10 of Fig. 7. Referring now to the drawings, and particularly to Figs. 1 and 2, there is illustrated a reactor coolant pump, generally designated 10, employed in a reactor coolant system of a pressurized water nuclear power plant. The reactor coolant pump 10 is a vertical, single stage, centrifugal pump designed to move large volumes of reactor coolant at high temperatures and pressures, for example 287.5 °C (550°C) and 15.525 MPa (2250 psi). The pump 10 basically includes three general sections from top to bottom-motor, shaft seal and hydraulic sections 12-16. The upper motor section 12 includes a motor (not shown) which is supported by a motor support stand 18 and coupled by spool 20 to drive a vertical pump shaft 22. The middle shaft seal section 14 includes three axially-spaced tandemlyarranged primary, secondary and tertiary sealing assemblies 24-28 located concentric to, and near the top end of, the pump shaft 22. Their combined purpose is to mechanically contain the high positive pressure coolant of the reactor coolant system from leakage along the pump shaft 22 to the containment atmosphere during normal operating condition. The lower hydraulic section 16 includes an impeller 30 mounted on the lower end of the pump shaft 22 which is operable within the pump casing 32 to pump reactor coolant about the respective loop. The hydraulic section 16 also includes a heat exchanger 34 and a lower radial journal bearing assembly 36 disposed in tandem relationship about the pump shaft 22 above the impeller 30. The heat exchanger 34 functions to reduce the rate of heat flow from the reactor coolant into the pump bearing and seal areas. The journal bearing assembly 36 (best seen in Fig. 2) of the hydraulic section 16 includes a two-piece annular horizontally split housing 38, an annular bearing cartridge 40, and an annular journal 42. The inside diameter surface 38A of the housing 38 is machined to a spherical shape which mates with a spherical surface 40A on the outside diameter of the bearing cartridge 40 to allow the bearing assembly 36 to self-align. The bearing cartridge 40 includes an outer stainless steel sleeve 44 and an inner graphite bushing 46 which operates against the journal 42 in the form of a stellite bushing shrunk fit on the pump shaft 22. The prior art journal bearing assembly 36 is lubricated and cooled by seal injection water flow downward from the shaft seal section 14 to the thermal barrier area of the heat exchanger 34 of the hydraulic section 16. Turning now to Figs. 3-10, there is illustrated a hydrodynamic pivoted pad bearing assembly of the present invention, generally designated 48, which, as an improvement over the prior art journal bearing assembly 36, has the capability of accommodating pump shaft deflection or misalignment and of counteracting shaft orbiting or wobble, particularly bearing "water whirl". In its basic components, the pivoted pad bearing assembly 48 includes an annular member 50, a plurality of arcuate bearing segments 52, and a plurality of pivot pins 54 each extending radially between the annular member 50 and one of the bearing segments 52. In the exemplary embodiment, the respective pluralities of bearing segments 52 and pivot pins 54 are each four in number, although some other number could be utilized. More particularly, as best seen in Figs. 5, 6 and 8, the annular member 50 is in the form of a split ring having lower and upper components 50A,50B which attach to one another by fasteners 56,58 and to the pump casing 32 only by fasteners 58. As best seen in Figs. 4, 7 and 9, the arcuate bearing segments 52 are disposed end-to-end in circumferentially spaced relation for encompassing the shaft 22 and providing spaced arcuate regions of bearing contact therewith. Each bearing segment 52 includes an arcuate pad holder 60 having a pair of circumferentially spaced opposite ends 60A and radially spaced interior and exterior arcuate surfaces 60B,60C extending therebetween (best seen in Fig. 7). An arcuate bearing element 62 has a pair of circumferentially spaced opposite ends 62A and radially spaced interior and exterior arcuate surfaces 62B,62C extending therebetween. The interior surface 62B of each bearing element 62 provides the bearing contact with the shaft 22. Further, each bearing segment 52 includes a pair of clamp plates 64 attached by screws 66 to the opposite ends 60A of one of the pad holders 60 and engaged with opposite ends 62A of one of the bearing elements 62 for capturing the bearing element 62 therebetween and retaining the same at its exterior surface 62C adjacent to the interior surface 60B of the pad holder 60. Also, as best seen in Fig. 8, the bearing assembly 48 includes means 68 for releasably and universally pivotally mounting each of the pivot pins 54 at an inner end 54A thereof to one of the bearing segments 52, and means 70 for retaining the respective bearing segments 52 on the annular member 50 to preclude inadvertent release of the pivot pins 54 at their inner ends 54A from the bearing segments 52 before the bearing assembly 48 is mounted about the shaft 22. More particularly, as best seen in Figs. 3-5, 7 and 8, the pivot pin inner end mounting means 68 includes a plurality of cup elements 72 one associated with each of the respective pad holders 60 of the bearing segments 52. Each cup element 72 has an internal bore surface 72A and is mounted in a cylindrical recess 74 formed in the exterior surface 60C of the one pad holder 60. Also, the mounting means 68 includes an external contact surface 76 defined on the inner end 54A of the respective one of the pivot pins 54 associated with the pad holder 60. In the exemplary embodiment, the cup element internal bore surface 72A has an annular cylindrical contour, whereas the pivot pin inner end contact surface 76 has an annular spherical contour allowing the universal pivoting therebetween. Alternatively, the respective surface contours could be reversed. Also, as best seen in Figs. 3, 5 and 8, the retaining means 70 of the bearing assemtly 48 for preventing inadvertent release of the pivot pins 54 from the pad holders 60 before installation of the bearing assembly 48 on the shaft 22 includes first and second sets of flanges 78,80. The first set of flanges 78 project axially toward one another respectively from opposite axially spaced ends 50C of an interior side of the annular member 50. The second set of flanges 80 project axially away from one another respectively from opposite axially spaced ends 60A of the exterior side or surface 60C of each of the pad holders 60. The axial projections and respective spacing between the first and second sets of flanges 78,80 permit interfitting of the respective sets of flanges with one another for loosely retaining each of the bearing segments 52 on the annular member 50. Also, an arcuate reference rim 82 fixed on and projecting outwardly from one (but not the other) of the opposite axial ends 60A at the exterior side 60C of each bearing segment pad holder identifies to a person assembling the bearing assembly 10 the correct orientation in which to mount each of the bearing segments 52 on the annular member 50. Further, the bearing assembly 48 includes means 84 for mounting each of the pivot pins 54 at an outer end 54B thereof to the annular member 50 and being adjustable for changing the radial position of the pivot pin 54 relative to the annular member 50 and thereby of the bearing segment 52 relative to the shaft 22. A socket 85 is provided in the outer end 54B of pin 54 for receiving a complementary shaped tool (not shown) for turning the pin 54. Also, the bearing assembly 48 includes means 86 for lockably coupling each of the pivot pins 54 to the annular member 50 to fix the pivot pins 54 in the desired adjusted positions relative to the annular member 50. More particularly, as seen in Figs. 3, 5, 7, 8 and 10, and as best seen in Fig. 8 the pivot pin outer end mounting means 84 includes a plurality of circumferentially spaced and radially extending openings 88 defined through the annular member 50 for receiving the respective pivot pins 54. Also, a set of external threads 90 is formed on the outer end 54B of each of the pivot pins 54. A plurality of adjustment elements 92 in the form of nuts are also provided. Each adjustment nut 92 has a set of internal threads 94 formed thereon for threadably mating with the set of external threads 90 on one of the pivot pin outer ends 54B such that relative rotation between each pivot pin 54 and its respective adjustment nut 92 causes a change of the radial position of the pivot pin 54 relative to the annular member 50. As seen in Fig. 10, each adjustment nut 92 has a cylindrical exterior surface 92A and a plurality of radially extending notches 98 formed on the upper end of the nut 92 for receiving an end of a complementarily-shaped tool (not shown) for turning the nut 92. Also, as seen in Figs. 3, 5, 7, 8 and 10, the lockable coupling means 86 (best seen in Fig. 10) includes a pair of recesses 96 defined in the annular member 50 at the opposite sides of the periphery of each of the openings 88 and extending radially therefrom. Also, the lockable coupling means 86 uses a pair of the radially extending notches 98 formed in each adjustment nut 92 at opposite sides of the peripheral edge or exterior surface 92A thereof and extending radially toward the periphery of the respective one of the openings 88, and a pair of longitudinally extending slots 100 defined in the outer end 54B of each pivot pin 54 and extending radially toward the periphery of the respective one of the openings. The lockable coupling means 86 further includes a plurality of open-bottomed locking cups 102. Each cup 102, along with one of the adjustment nuts 92, is disposed in an outer enlarged diameter portion 88A of one of the openings 88 between its periphery and the nut. Also, each cup 102 has a pair of tabs 104 projecting inwardly toward one another from opposite sides of a bottom rim 102A of the cup 102. The projecting tabs 104 extend into the respective slots 100 in the pivot pin outer end 54B for maintaining the cup 102 in non-rotatable relation to the one pivot pin 54. Each cup 102 also has a deformable cylindrical sidewall 102B with portions deformed into the recesses 96 of the annular member 50 and the notches 98 of the adjustment nut 92 for maintaining the cup 102 in non-rotatable relation thereto. The cup 102 thus provides the structure for locking the pivot pin 54 and adjustment nut 92 to the annular member 50 so as to prevent rotation relative to the member 50.

Claims: A pivoted pad bearing assembly for rotatably supporting a shaft extending through a housing, said bearing assembly comprising: (a) an annular member (50) attachable to the housing; (b) a plurality of arcuate bearing segments (52) disposable end-to-end in circumferentially spaced relation for encompassing the shaft (22) and providing bearing contact herewith; (c) a plurality of pivot pins (54) each extending radially between said annular member (50) and one of said bearing segments (52); (d) means for releasably and universally pivotally mounting each of said pivot pins (54) at an inner end (54A) thereof to one of said bearing segments (52); (e) means (84) for mounting each of said pivot pins (54) at an outer end (54B) thereof to said annular member (50) and being adjustable for changing the radial position of said pivot pin relative to said annular member and thereby of said bearing segment (52) relative to the shaft (22), said pivot pin outer end mounting means (84) including (i) means defining a plurality of circumferentially spaced and radially extending openings (88) through said annular member (50) for receiving said respective pivot pins (54), (ii) a set of external threads (90) formed on said outer end (54B) of each of said pivot pins (54), and (iii) a plurality of adjustment elements (92) each having a set of internal threads (94) formed thereon for threadably mating with said set of external threads (90) on one of said pivot pin (54) outer ends (54B) such that relative rotation between each pivot pin (54) and its respective adjustment element (92) causes a change of the radial position of said pivot pin (54) relative to said annular member; and (f) means (86) for lockably coupling said pivot pins (54) to said annular member to fix said pivot pins (54) in desired adjusted positions relative to said annular member (50), characterized in that said lockable coupling means (86) includes: means defining at least one recess (96) in said annular member (50) at the periphery of each of said openings (88) which extends radially from said opening; means defining at least one notch (98) in a peripheral edge of each of said adjustment element (92) which extends radially to one of said openings (88); means defining at least one longitudinally extending slot (100) in said outer end of each of said pivot pins (54) which also extends radially to one of said openings (88); and a plurality of locking cups (102) each being disposable in one of said openings (88), each of said cups (102) having at least one projection (104) capable of extending into said slot (100) for maintaining said cup (102) in non-rotatable relation to one of said pivot pins (54), each of said cups (102) having a cylindrical wall (102B) with portions deformable into one of said recesses (96) of said annular member (50) and one of said notches (98) of said adjustment element (92) for maintaining said cup in non-rotatable relation thereto. The bearing assembly as recited in claim 1, further comprising: a reference rim (82) projecting outwardly from one of a pair of opposite axially spaced ends (60A) of an exterior side (60C) of each bearing segment (52) for identifying the correct orientation in which to mount each of said bearing segments (52) on said annular member (50). The bearing assembly as recited in claim 1, wherein said pivot pin inner end mounting means includes: a plurality of cup elements (72) each having an internal bore surface (72A) and being mounted in a recess (74) formed at an exterior surface (60C) of one of said bearing segments (52); and an external contact surface (76) on said inner end (54A) of each of said pivot pins (54), one of said cup element internal bore surface (72A) and said pivot pin inner end contact surface (76) having an annular cylindrical contour and the other having an annular spherical contour for allowing said universal pivoting therebetween. The bearing assembly as recited in claim 1, wherein each of said bearing segments (52) includes: an arcuate pad holder (60) having a pair of circumferentially spaced opposite ends (60A) and an interior arcuate surface (60B) extending therebetween; an arcuate bearing element (62) having a pair of circumferentially spaced opposite ends (62A) and an interior arcuate surface (62B) for providing said bearing contact with the shaft (22) ; and a pair of clamp plates (64) attached to said opposite ends (60A) of said pad holder (60) and engaged with opposite ends of said bearing element (62) for capturing said arcuate bearing element (62) therebetween and retaining the same adjacent to said interior arcuate surface (60B) of said pad holder (60).

Inventors: JENKINS LEONARD STANLEY, VERONESI LUCIANO, JENKINS, LEONARD STANLEY, VERONESI, LUCIANO
Applicants: WESTINGHOUSE ELECTRIC CORP, WESTINGHOUSE ELECTRIC CORPORATION
Application year: 1989
Main classification: F16C17
IPC codes: F16C17, F04D29, F04D7