Patent Number: 050770004
Section: summary

CROSS REFERENCE TO RELATED APPLICATIONS Reference is hereby made to the following copending applications dealing with related subject matter and assigned to the assignee of the present invention: 1. "Sealing Devices For The Drive Shaft Of A High Pressure Fluid Pump" by N. Bonhomme, assigned U.S. Ser. No. 379,196 and filed May 17, 1982, now U.S. Pat. No. 4,587,076, issued May 6, 1986. 2. "Nuclear Reactor Coolant Pump Impeller/Shaft Assembly" by L. S. Jenkins, assigned U.S. Ser. No. 761,447 and filed Aug. 1, 1985, now U.S. Pat. No. 4,690,612, issued Sept. 1, 1987. 3. "Improved Shaft Seal" by K. P. Quinn, assigned U.S. Ser. No. 739,745 and filed May 31, 1985, now U.S. Pat. No. 4,693,481, issued Sept. 15, 1987. 4. "Reactor Coolant Pump Hydrostatic Sealing Assembly With Improved Hydraulic Balance" by R. F. Guardiani et al, assigned U.S. Ser. No. 063,331 and filed June 17, 1987, now U.S. Pat. No. 4,838,559, issued June 13, 1989. 5. "Reactor Coolant Pump Sealing Surface With Titanium Nitride Coating" by G. Zottola, assigned U.S. Ser. No. 035,832 and filed Apr. 8, 1987, now U.S. Pat. No. 4,871,297, issued Oct. 3, 1989. 6. "Reactor Coolant Pump Hydrostatic Sealing Assembly With Externally Pressurized Hydraulic Balance Chamber" by R. F. Guardiani, assigned U. S. Ser. No. 091,224 and filed Aug. 31, 1987, now U.S. Pat. No. 4,848,774, issued July 18, 1989. 7. "Reactor Coolant Pump Shaft Seal Utilizing Shape Memory Metal" by D. J. Janacko assigned U.S. Ser. No. 197,174 and filed May 23, 1988. 8. "Reactor Coolant Pump Auxiliary Seal For Reactor Coolant System Vacuum Degasification" by J. D. Fornof, assigned U.S. Ser. No. 222,649 and filed July 21, 1988. BACKGROUND OF THE INVENTION 1. Field of the Invention 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. 2. Description of the Prior Art 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 550 degrees F. and 2500 psi. 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. Pat. Nos. to MacCrum (3,522,948), Singleton (3,529,838), Villasor (3,632,117), Andrews et al (3,720,222) and Boes (4,275,891) and in the first three patent applications cross-referenced above, 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. Pat. No. 4,647,425 to Battaglia et al, which is assigned to the same assignee as the present invention and is hereby incorporated by reference. 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. Consequently, a need exists for an effective way to prevent reverse pressurization of the reactor coolant pumps so as to eliminate these concerns about possible damage to the pump sealing assemblies. SUMMARY OF THE INVENTION The present invention provides a reactor coolant pump auxiliary flexible vacuum seal designed to satisfy the aforementioned needs. The auxiliary flexible vacuum seal of the present invention provides a simple and effective way to prepare the reactor coolant pumps so that the reactor coolant system can be vacuum degasified without applying a reverse pressure to the pump sealing assemblies. The auxiliary flexible vacuum seal is an external, temporary seal that would be installed prior to the start of vacuum degasification between the pump sealing housing and shaft, and then removed after degasification is completed. The auxiliary seal accepts the entire reverse pressure, thus preventing any possible damage to the primary, secondary and tertiary pump sealing assemblies of the pump. The auxiliary flexible vacuum seal of the present invention is an alternative to the invention illustrated and described in the eighth 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. Accordingly, the present invention is directed to an auxiliary flexible vacuum seal useful in a reactor coolant pump ,for preparing the pump for vacuum degasification of the reactor coolant system. The flexible vacuum seal comprises: (a) a flexible boot member having a pair of longitudinally-displaced opposite open end portions and a pair of side-by-side longitudinally-extending side portions defining a split in the boot member along a side thereof and extending between the open end portions for allowing flexing of the boot member between open and closed side configurations to permit its installation and removal on and from the pump; (b) means for releasably and sealably clamping together the side portions of the at the split to retain the boot member in its closed configuration; and (c) a pair of circumferentially-extending sealably portions on the interior of the boot member at the opposite open end portions thereof for sealably engaging the pump when the boot member is installed and flexed to its closed configuration to thereby permit generation of a vacuum seal condition between the boot member and the pump. Further, 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 boot member of the flexible seal includes a bowl-shaped body having the opposite end portions and defining a hollow cavity. The cavity is open at the opposite end portions and openable at the split defined in the body by the side portions of the boot member. The sealably engaging portions on the boot member at the opposite open end portions thereof are preferably rings projecting radially inwardly and formed integrally on the body of the boot member. In the alternative, these sealably engaging portions may be the interior surface of the boot member itself at its opposite open end portions. The side portions of the boot member are in the form of a pair of radially outward-projecting and longitudinally-extending flanges on the body along opposite sides of the split and disposed in side-by-side contacting relation when the boot member is in its closed configuration. The clamping means of the flexible seal includes a pair of brackets mountable along outer sides of said flanges on the boot member body, and a plurality of fasteners extendible through the brackets and flanges therebetween and being operable for drawing the brackets toward one another and withdrawing the brackets away from one another for clamping and releasing the flanges. The present invention is also directed to a method of preparing a reactor coolant pump for vacuum degasification of a reactor coolant system. The preparing method comprises the steps of: (a) sealing a seal housing of the reactor coolant pump by installing a longitudinally split boot member about a portion of the seal housing and about a shaft extending through the housing; (b) reversing the pressure of the reactor coolant system at start of vacuum degasification of the reactor coolant system, the sealing of the pump seal housing preventing damage to sealing assemblies therein by the reversing of reactor coolant system pressure; (c) terminating reversing of the reactor coolant system pressure at completion of vacuum degasification of the reactor coolant system; and (d) unsealing the pump seal housing of the reactor coolant pump by removing the split boot member. Further, the sealing includes stretching the split boot member. These and other features and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention.