Abstract:
The safety device insures the longitudinal tightness of a pump shaft in the case where the pump dynamic gasket separating the high pressure chamber from the low pressure chamber of the pump brakes. According to the invention, the device comprises a cylindrical hollow piston coaxially surrounding the pump shaft and the structure allowing the axial displacement of the piston in order to compress a toroidal safety gasket between the shaft and the piston in the event of a failure of the dynamic gasket.

Description:
The present invention concerns a safety device insuring the longitudinal tightness of the shaft of a pump in the case of the rupture of the dynamic gasket separating the high pressure fluid chamber from the low pressure fluid chamber of the pump. 
     Such a device is particularly provided for application to the primary pumps of nuclear reactors. 
     In produce, primary pumps effect the circulation of cooling fluid in the reactor. This fluid is generally contaminated with radioactivity. 
     A set of three dynamic gaskets of the leakage type, placed successively, normally assure the longitudinal tightness of the shaft. However, in case of the failure of one of these gaskets, the downstream elements are exposed to a pressure for which they are not designed and, after the possible rupture of the gaskets located downstream, the contaminated fluid under pressure is released and fills the pressure vessel itself of the the reactor. 
     SUMMARY OF THE INVENTION 
     It is the object of the present invention to eliminate this disadvantage by means of a safety device that is readily applied to primary pumps in service. 
     The device according to the invention is intended to operate only in the case of the failure of one of the gaskets so that it cannot be used when it should perform its function. 
     According to the present invention, the device comprises a cylindrical hollow piston coaxially surrounding the shaft of the pump and is normally housed in a chamber provided for this purpose in the stationary housing of the dynamic gasket separating the high pressure chamber from the low pressure chamber and means to displace said piston coaxially so as to compress a toroidal safety seal between the shaft and the piston in the event of a failure of the dynamic gasket. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood and other objects, advantages and characteristics will become more apparent from the description hereinafter of two preferred embodiments presented in a nonlimiting manner, to which two sheets of drawings are attached. 
     FIG. 1 is a schematic axial cross section of a device according to a first embodiment of the invention; and 
     FIG. 2 is a schematic axial cross section of a device according to a second embodiment of the invention insuring. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referrring now to FIG. 1, the pump, the symmetry axis of which is shown by a dot-and-dash line, comprises a rotating shaft 1 and a casing 2 of the dynamic gasket fixed in rotation. The fluid circulating in the pump is at a high pressure in the chamber 4 and leaks past dynamic gasket 3 into the chamber 5, wherein a pressure lower than that of chamber 5 prevails. The leakage of the fluid is conducted along the shaft 1 and is evacuated through the conduit 6 in communication with the low prssure chamber 5. 
     In a known manner, the dynamic gasket 3 is integral with a floating bushing 7 which moves in translation and a compressed spring 8 maintains the gasket 3 in tight contact with the part 9 of the shaft so that only a fluid film escapes from the from the high pressure chamber 4 to the low pressure chamber 5 and insures the necessary relative sliding of the parts without heating. 
     A static toroidal gasket 10 is placed between the bushing 7 and the casing 2 to prevent any nonfunctional leakage of the fluid between the two chambers 4 and 5. 
     Without the device according to the invention, in case of the rupture of the gasket 3, the chamber 5 must support the high pressure of the chamber 4. 
     The evacuating conduit 6 becomes inoperative and the fluid under pressure, escaping through the narrowing 11, floods the enclosure of the reactor. To eliminate this difficulty, the safety device provided essentially comprises a piston 12 and means to displace this piston axially so that it is applied against the rim 33 provided for this purpose on the shaft 1, thereby compressing a toroidal gasket 14 to reduce and eliminate the escape of the fluid into the reactor enclosure. The preferred means to displace the piston 12 consist of an auxiliary source of fluid under pressure, for example, air compressed to a pressure between 5 and 10 bars, connected with a conduit 15 in communication with a chamber 16. A static torodial gasket 17 prevents any mixture of the different fluids under pressure. 
     Except in a complete failure of the gasket or gaskets of the pump, the fluid does not escape through the narrowing 11 and the piston 12 may remain in the chamber 13 provided for this purpose in the casing 2. In this manner, the toroidal gasket 14 is not exposed to previous wear and retains its effectiveness when needed. 
     During a failure of a dynamic gasket and more accurately, of the gasket 3, the fluid overflows through the narrowing 11. It is then possible to limit the leakage by first slowing the pump and then stopping it after connecting the auxiliary source of fluid under pressure with the conduit 15. The piston 12 then moves axially and compresses the gasket 14 against the rim 13, which has been, optionally covered by material reducing friction, such as graphite. When the piston 12 is displaced, the chamber 13 again serves as the cylinder for the piston 12, since it is filled with fluid under pressure from the chamber 4. Means are provided to subsequently disconnect the auxiliary source from the conduit, because the necessary effect is insured by the fluid normally under pressure. 
     A second embodiment of a device according to the invention is shown schematically in FIG. 2 for a more powerful pump. In this figure, the elements similar to those in FIG. 1 carry the same references. 
     According to this embodiment, the bushing 7 constitutes the piston for the case of a failure of the gasket 3. In effect, by means of the axial displacement of the bushing 7, the toroidal gasket 14 forming the rim around the shaft of the pump is compressed by the end 18 of the bushing 7 so as to insure tightness upstream from the narrowing 11 and consequently, to prevent the leakage of fluid under pressure through the narrowing 11. 
     To effect the displacement of the bush 7, auxiliary pistons 19 are, for example, placed in the casing 2. 
     Preferably, these auxiliary pistons are three in number and are distributed around the shaft 1 of the pump, in the cylinders 20 provided for the purpose. 
     The rod 21 of each piston 19 is provided at its end with a protruding stop 22. 
     During the displacement of the piston 19, the stop 22 contacts a collar 23 integral with the bushing 7 which thereby is moved by translation of the piston 19. 
     To displace the piston 19, an auxiliary source of fluid under pressure is connected with the conduit 15 in communication with one of the parts of the cylinder 20. The segments 24 on the one hand, and the toroidal gaskets 25 on the other, insure the tightness of the device. 
     In the same manner as before, when the joint 14 is compressed, the auxiliary source may be disconnected, because the fluid under pressure in the chamber 4, communicating with the chamber 5, then tends to apply pressure to the bushing 7, thereby providing the effect required by itself. 
     Obviously, to make the operation of the device possible at low rotating velocities of the shaft, the end 18 of the bushing 7 may also be coated with a layer of a friction reducing material. 
     Such a coating, however, is not necessary if the safety device is intended to be operated only after the shaft of the pump stops completely. 
     Even though only two embodiments of the invention have been described, it is evident that any modification applied by persons skilled in the art in the same spirit will not exceed the scope of the present invention. 
     For example, the means to insure the displacement of the piston may equally be of the electrical type, an electric coil then being arranged in an appropriate manner in the casing 2 and connected with an external source of electric power.