REACTOR COOLANT PUMP AUXILIARY FLEXIBLE VACUUM SEAL FOR REACTOR COOLANT SYSTEM VACUUM DEGASIFICATION

Abstract: A coolant circulating pump for a nuclear reactor cooling system having a stationary housing (30) with an annular portion defining an opening and a shaft (34) extending through the housing opening includes an auxiliary flexible vacuum seal member (88) adapted to surround the housing (30) and the shaft (34) so as to seal the housing opening around the shaft (34) during vacuum degasification procedures on the reactor cooling system in order to prevent contaminants from entering the housing.

Description: The present invention relates generally to shaft seals and, more particularly, is concerned with a reactor coolant pump auxiliary flexible vacuum seal for reactor coolant system vacuum degasification. 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 (as disclosed in EP-A-0 286 024 for example) typically is a vertical, single stage, centrifugal pump designed to move large volumes of reactor coolant at high temperatures and pressures, for example 290°C and 17·5 MPa. The pump basically includes three general sections from bottom to top -- hydraulic, shaft seal and motor sections. 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 upper motor section includes a motor which is coupled to drive the pump shaft. The middle shaft seal section includes three tandem sealing assemblies -- lower primary, middle secondary and upper tertiary sealing assemblies. The sealing assemblies are 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. Representative examples of pump shaft sealing assemblies known in the prior art are the ones disclosed in U. S. Patents to MacCrum (3,522,948), Singleton (3,529,838), Villasor (3,632,117), Andrews et al (3,720,222) and Boes (4,275,891) all of which are assigned to the same assignee as the present invention. Thus, the sealing assemblies in the reactor coolant pumps are designed to hold high positive coolant pressures. This fact has raised some concerns about possibility of damage being done to the reactor coolant pumps during reactor coolant system vacuum degasification. Procedures for vacuum degasification of the reactor coolant system are described in U. S. Patent 4,647,425 to Battaglia et al, which is assigned to the same assignee as the present invention. Basically, in vacuum degasification of the reactor coolant system a vacuum or negative pressure is imposed on the system and thus on the reactor coolant pumps. This, in effect, pressurizes the pumps in reverse. One major concern is that reverse pressurization might draw the water used to cool the pump sealing assemblies back into the pump sealing assemblies by a reverse flow of the water through filters which might bring contamination in the form of dirt and foreign matter along with the water from the filters into the sealing assemblies. Then, when the pumps are restarted after conclusion of vacuum degasification, the sealing assemblies may become damaged by the presence of the contamination therein. It is therefore the principal object of the present invention to prevent reverse pressurization of the reactor coolant pumps so as to avoid possible damage to the pump sealing assemblies. With this object in view the present invention resides in a coolant circulating pump for a nuclear reactor cooling system having a stationary housing with an annular portion defining an opening and a shaft extending through said housing opening in spaced relation to said housing annular portion, characterized by an auxiliary flexible vacuum seal member adapted to surround said housing and said shaft so as to seal said housing opening between said housing annular portion and said shaft during vacuum degasification procedures on the reactor cooling system. The auxiliary flexible vacuum seal of the present invention is an alternative to the invention illustrated and described in the sixth patent application cross-referenced above. The flexible vacuum seal offers several advantages over the rigid segmented seal of the cross-referenced application. First, the flexible seal is embodied primarily in the form of a single flexible split boot member with a pair of axially-spaced integral sealing portions, preferably in the form of ring elements, resulting in fewer parts to handle and less sealing length to be concerned with. Second, all parts of the flexible seal are disposable, thus minimizing decontamination and storage requirements. Third, the flexible seal is easier to manipulate within the limited space of the motor stand. Fourth, deviations in concentricity between the shaft and seal housing would be of no concern with the flexible seal. Fifth, the flexible seal fits with the shaft in either its axially-displaced coupled or uncoupled positions. Sixth, the flexible seal can be installed without the need to remove any of the parts of the pump other than some piping and associated coonections. Preferably, the flexible seal includes a boot support member disposable within the boot member between the boot member and the pump for supporting the boot member when in its closed configuration. More particularly, the boot support member is annular in shape and composed of a pair of semi-annular parts. The support member also has an upper surface conformed in shape to that of an intermediate portion of the boot member located between its opposite end portions for engagably supporting the boot member at its intermediate portion. Further, the boot support member has a lower surface conformed in shape to that of the pump for mounting the support member thereon. The invention will become more readily apparent from the following description of a preferred embodiment thereof shown, by way of example only, in the accompanying drawings wherein: Fig. 1 is a schematic representation of one cooling loop of a conventional nuclear reactor coolant system which includes a steam generator and a reactor coolant pump connected in series in a closed coolant flow circuit with the reactor core. Fig. 2 is a cutaway perspective view of the shaft seal section of a reactor coolant pump, illustrating in an axial sectional form the seal housing and the lower primary, middle secondary and upper tertiary sealing assemblies which are disposed within the seal housing and surround the pump shaft in this section of the pump. Fig. 3 is an enlarged view of the axially sectioned seal housing and sealing assemblies of the reactor coolant pump of Fig. 2. Fig. 4 is a fragmentary view of an upper portion of the axially sectioned sealing housing of the reactor coolant pump of Fig. 3, illustrating in elevational form an auxiliary flexible vacuum seal of the present invention installed about the pump shaft. Fig. 5 is an enlarged axial sectional view of a flexible boot member and an annular boot support member of the auxiliary flexible vacuum seal of Fig. 4, also showing in elevational form an upper coupling hub of the pump on the pump shaft above the seal. Fig. 6 is an elevational view, partly in axial section, of the annular boot support member of the flexible vacuum seal by itself. Fig. 7 is a bottom plan view of the boot support member as seen along line 7--7 of Fig. 6. Fig. 8 is an enlarged fragmentary elevational view of the flexible boot member and a pair of clamp members of the flexible vacuum seal of Fig. 5. Fig. 9 is a top plan view of flexible boot member and clamp members as seen along line 9--9 of Fig. 8. Fig. 10 is an axial sectional view of the flexible boot member of the flexible vacuum seal by itself. Fig. 11 is a top plan view of the flexible boot member as seen along line 11--11 of Fig. 10. Fig. 12 is an enlarged view of the circled portion of the flexible boot member in Fig. 10, showing an interior upper circumferential sealing ring on the boot member. Fig. 13 is a side elevational view of one of the clamp members of the flexible vacuum seal by itself. Fig. 14 is an end elevational view of the clamp member as seen along line 14--14 of Fig. 13. Fig. 15 is a bottom plan view of the clamp member as seen along line 15--15 of Fig. 13. Referring now to the drawings, and particularly to Fig. 1, there is shown a schematic representation of one of a plurality of cooling loops 10 of a nuclear reactor coolant system. The cooling loop 10 includes a steam generator 12 and a reactor coolant pump 14 serially connected in a closed coolant flow circuit with a nuclear reactor core 16. The steam generator 12 includes primary tubes 18 communicating with inlet and outlet plenums 20,22 of the generator. The inlet plenum 20 of the steam generator 12 is connected in flow communication with the outlet of the reactor core 16 for receiving hot coolant therefrom along flow path 24 of the closed flow circuit. The outlet plenum 22 of the steam generator 12 is connected in flow communication with an inlet suction side of the reactor coolant pump 14 along flow path 26 of the closed flow circuit. The outlet pressure side of the reactor coolant pump 14 is connected in flow communication with the inlet of the reactor core 16 for feeding cold coolant thereto along flow path 28 of the closed flow circuit. In brief, the coolant pump 14 pumps the coolant under high pressure about the closed flow circuit. Particularly, hot coolant emanating from the reactor core 16 is conducted to the inlet plenum 20 of the steam generator 12 and to the primary tubes 18 in communication therewith. While in the primary tubes 18, the hot coolant flows in heat exchange relationship with cool feedwater supplied to the steam generator 12 via conventional means (not shown). The feedwater is heated and portions thereof changed to steam for use in driving a turbine generator (not shown). The coolant, whose temperature has been reduced by the heat exchange, is then recirculated to the reactor core 16 via the coolant pump 14. The reactor coolant pump 14 must be capable of moving large volumes of reactor coolant at high temperatures and pressures about the closed flow circuit. Although, the temperature of the coolant flowing from the steam generator 12 to the pump 14 after heat exchange has been cooled substantially below the temperature of the coolant flowing to the steam generator 12 from the reactor core 16 before heat exchange, its temperature is still relatively high, being typically about 290°C. The coolant pressure produced by the pump is typically 17·5 MPa. As seen in Figs. 2 and 3, the prior art reactor coolant pump 14 generally includes a pump housing 30 which terminates at one end in a seal housing 32. The pump 14 also includes a pump shaft 34 extending centrally of the housing 30 and being sealingly and rotatably mounted within the seal housing 32. Although not shown, the bottom portion of the pump shaft 34 is connected to an impeller, while a top portion thereof is connected to a high-horsepower, induction-type electric motor. When the motor rotates the shaft 34, the impeller within the interior 36 of the housing 30 circulates the coolant flowing through the pump housing 30 at pressures from ambient to approximately 17.5 MPa cover gas with minimum operating pressure of approximately 1·4 MPa. This pressurized coolant applies an upwardly directed, hydrostatic load upon the shaft 34 since the outer portion of the seal housing 32 is surrounded by the ambient atmosphere. In order that the pump shaft 34 might rotate freely within the seal housing 32 while maintaining the 175 kg/cm² pressure boundary between the housing interior 36 and the outside of the seal housing 32, tandemly-arranged lower primary, middle secondary and upper tertiary sealing assemblies 38,40,42 are provided in the positions illustrated in Figs. 2 and 3 about the pump shaft 34 and within the pump housing 30. The lower primary sealing assembly 38 which performs most of the pressure sealing (approximately 16.8 MPa) is of the non-contacting hydrostatic type, whereas the middle secondary and upper tertiary sealing assemblies 40,42 are of the contacting or rubbing mechanical type. Each of the sealing assemblies 38,40,42 of the pump 14 generally includes a respective annular runner 44,46,48 which is mounted to the pump shaft 34 for rotation therewith and a respective annular seal ring 50,52,54 which is stationarily mounted within the seal housing 32. The respective runners 44,46,48 and seal rings 50,52,54 have top and bottom end surfaces 56,58,60 and 62,64,66 which face one another. The facing surfaces 56,62 of the runner 44 and seal ring 50 of the lower primary sealing assembly 38 normally do not contact one another but instead a film of fluid normally flows between them. On the other hand, the facing surfaces 58,64 and 60,66 of the runners and seal rings 46,52 and 48,54 of the middle secondary and upper tertiary sealing assemblies 40 and 42 normally contact or rub against one another. Because the primary sealing assembly 38 normally operates in a film-riding mode, some provision must be made for handling coolant fluid which "leaks off" in the annular space between the seal housing 32 and the shaft 34 rotatably mounted thereto. Accordingly, the seal housing 32 includes a primary leakoff port 68, whereas leakoff ports 70 accommodate coolant fluid leakoff from secondary and tertiary sealing assemblies 40,42. Also, the reactor coolant pump 14 has an annular fluid blocking splash guard 72 disposed adjacent to an annular collar 74 attached to the pump shaft 34. The splash guard 72 is seated within an annular recess 76 formed about an upper portion 78 of the seal housing 32. Screws 80 (only one of which is shown) threaded into holes 82 tapped in the seal housing portion 78 serve to attach the splash guard 72 to the seal housing 32 and retain it in the recess 76 such that an inner periphery 84 of the splash guard 72 is disposed close to the exterior cylindrical surface 86 of the shaft collar 74. RCP Auxiliary Flexible Vacuum Boot Seal of the Present Invention In accordance with the present invention, an auxiliary flexible vacuum seal 88 can be temporarily installed, without the necessity of removing any parts of the pump 14 other than for some piping (not shown), to prepare the reactor coolant pump 14 for reactor coolant system vacuum degasification. Referring now to Figs. 4 to 15, the auxiliary flexible vacuum seal 88 basically includes a flexible boot member 90, clamping means 92, and upper and lower sealing portions, preferably in the form of ring elements 94,96 (Fig. 8) on the boot member 90. In the alternative, the sealing portions may be the interior surface of the boot member 90 itself at the opposite open end portions thereof since the boot member 90 is stretched when so installed. Also, preferably, the seal 88 includes a boot support member 98. for allowing flexing of the boot member between open and closed side configurations to permit its installation and removal on and from the pump. The flexible boot member 90 of the seal 88 includes a bowl-shaped body 100 (Figs. 5, 7 and 8). The body 100 has a pair of longitudinally-displaced opposite end portions 100A,100B and defines a hollow cavity 102 (Fig. 10) open at the opposite end portions. The body 100 also has a pair of side-by-side longitudinally-extending side portions in the form of flanges 104 defining a split 106 (Fig. 11) in the boot member body 100 along a side thereof. The split 106 extends between the open end portions 100A,100B rendering the body 100 openable at the split 106 for allowing flexing of the boot member 90 between open and closed side configurations to permit its installation and removal on and from the pump housing 30 and shaft 34. The flanges 104 of the boot member 90 project radially outward from and extend longitudinally along opposite sides of the longitudinal split 106 in the body 100. The flanges 104 are disposed in side-by-side contacting relation when the boot member 90 is in its closed configuration as seen in Figs. 9 and 11 and displaced from one another when the boot member 90 is flexed to its open configuration. The clamping means 92 of the flexible vacuum seal 88 are operable for releasably and sealably clamping together the flanges 104 of the boot member body 100 at the split 106 to retain the boot member 90 in its closed configuration. As seen in Figs. 8, 9 and 13-15, the clamping means 92 of the flexible seal 88 includes a pair of brackets 108,110 mountable along outer sides of said flanges 104 on the boot member body 100, and a plurality of fasteners 112 extendible through the brackets. The brackets 108,110 have a series of holes 114 therethrough which are alignable with holes 116 through the flanges 104. Also, nuts 118 are attached to the brackets 108,110 at alternating ones of the holes 114 therein. The fasteners 112 are insertable through the aligned holes 114,116 of the brackets 108,110 and flanges 104. Threading the fasteners 112 into the nuts 118 operates to draw the brackets 108,110 toward one another clamping the flanges 104 therebetween. Unthreading the fasteners 112 from the nuts 118 operates to withdraw the brackets 108,110 away from one another for releasing the flanges 104. Further, the upper and lower sealably engaging portions 94,96 (best seen in Figs. 10 and 12) on the boot member body 100 at the opposite open end portions 100A,100B thereof are rings being semi-circular in cross-section and projecting radially inwardly, howver, as mentioned above, these engaging portions 94,96 may be the interior surface of the boot member body 100. Preferably, the rings 94,96 are formed integrally on the interior surface of the body 100 of the boot member 90, extending circumferentially about the interior and projecting radially inwardly therefrom for sealably engaging the pump housing 30 and shaft 34 when the boot member 90 is installed and flexed to its closed configuration, as seen in Figs. 4 and 5. The boot member 90 and the sealably engaging rings 94,96 on the opposite end portions 100A,100B thereof are composed of resilient stretchable material such as PVC. The body 90 is in a stretched condition when it is installed and clamped to its closed configuration. In such closed configuration, it permits generation of a vacuum seal condition between the boot member 90 and the pump housing 30 and shaft 34. The body 100 has a port 120 conformable to receive the leakoff 70. Finally, the boot support member 98 (best seen in Figs. 5, 6 and 7) of the flexible seal 88 is disposable within the boot member 90 between it and the pump housing 30 for supporting the boot member 90 when in its closed configuration. More particularly, the boot support member 98 is annular in shape and split at 122 such that the member 98 is composed of a pair of semi-circular parts 124. The support member 98 also has an upper surface 98A conformed in shape to the relatively smooth surface of an intermediate portion 100C (Fig. 11) of the boot member body 100 being located between its opposite end portions 100A,100B for engagably supporting the boot member 90 at its intermediate portion. Further, the boot support member 98 has a lower surface 98B conformed in shape to the relatively interrupted shape of the pump housing 30 for mounting the support member 98 thereon.

Claims: A coolant circulating pump for a nuclear reactor cooling system having a stationary housing (30) with an annular portion defining an opening and a shaft (34) extending through said housing opening in spaced relation to said housing annular portion, characterized by an auxiliary flexible vacuum seal member (88) adapted to surround said housing (30) and said shaft (34) so as to seal said housing opening between said housing annular portion and said shaft (34) during vacuum degasification procedures on the reactor cooling system. A pump according to claim 1, characterized in that said auxiliary seal (88) comprises: (a) a flexible, boot member (90) having a bowl-shaped body (100) defining a cavity (102) which is open at its opposite longitudinally-displaced end portions (100A,100B), means defining a split (106) in a side of said body (100) extending longitudinally between said end portions (100A,100B) of said body (100), and a pair of flanges (104) connected to said body (100) along opposite sides of said split (106) and projecting radially therefrom and extending longitudinally between said end portions (100A,100B) of said body (100), said split (106) for allowing flexing of said body (100) to an open configuration to permit installation and removal of said boot member (90) at said side thereof about and from the rotatable shaft (34) of the pump (14) at one end portion (100A) of said body (100) and the housing portion (30) of the pump (14) at the other end portion (100B) of said body (100), said split (106) also for allowing flexing of said body (100) to a closed configuration about the shaft (34) and housing portion (30); (b) means (92) for releasably clamping together said flanges (104) on said body (100) of said boot member (90) to retain said body (100) in said closed configuration; and (c) axially-spaced and circumferentially-extending sealing portions (94,96) integrally formed on the interior of said body (100) of said boot member (90) at said opposite open end portions (100A,100B) thereof for sealably engaging the shaft (34) and housing portion (30) in said closed configuration of said body (100) of said boot member (90) to permit generation of a vacuum seal condition in the space within said cavity (102) bounded by said body (100) and the housing portion (30) and shaft (34) and between said sealably engaging rings (94,96) on said body (100). A pump according to claim 2, characterized in that said clamping means (92) includes: a pair of brackets (108,110) mountable along outer sides of said flanges (104) on said boot member body (100); and a plurality of fasteners (112) extendible through said brackets (108,110) and said flanges (104) therebetween and being operable for drawing said brackets (108,110) toward one another and withdrawing said brackets (108,110) away from one another for clamping and releasing said flanges (104). A pump according to claim 2 or 3, characterized in that a boot support member (98) is disposed within said cavity (102) of said boot member body (100) between said boot member (90) and the housing portion (30) for supporting said boot member (90) when in its closed configuration. A pump according to claim 4, characterized in that said boot support member (98) is annular in shape and composed of a pair of semi-annular parts (124), said boot support member (98) having an outer surface (98A) conformed in shape to that of an intermediate portion (100C) of said boot member body (100) located between its opposite end portions (100A,100B) for engagably supporting said boot member (90) at said intermediate portion (100C) and an inner surface (98B) conformed in shape to that of the pump housing portion (30) for mounting said support member (98) thereon.

Inventors: BICE, CHARLES DAVID, CAMPEN, CLIFFORD HOWARD
Applicants: WESTINGHOUSE ELECTRIC CORPORATION
Application year: 1990
Main classification: F04D7
IPC codes: F04D29, G21D1, F16J15, F04D7