Abstract:
A sealed stopper for an opening in a tubing for joining a chamber and a piping including a fastening ring is provided. The stopper includes a rigid bearing plate and a sealing member carried by the bearing plate, and includes a seal having a planar and flexible central portion with a reduced thickness extending below the bearing plate and a peripheral portion radially deformable by a central expander of the peripheral portion against the inner surface of the fastening ring.

Description:
The present invention concerns a sealed stopper to plug an opening of tubing connecting a chamber with piping, and more particularly an opening of tubing connecting a plenum of a steam generator of a pressurised water nuclear reactor with piping of the primary circuit. 
     BACKGROUND 
     Pressurised water nuclear reactors comprise steam generators which ensure the heating and vaporisation of feed water via the heat conveyed by the pressurised water used to cool the core of the reactor. 
     Pressurised water reactors, on each of their primary branches, comprise a steam generator having a primary part in which the pressurised coolant water of the reactor circulates, and a secondary part receiving the feed water which is heated and gradually vaporised and leaves the secondary part of the steam generator in the form of steam which is sent to a turbine associated with the nuclear reactor to drive an alternator producing electric current. 
     Said steam generators comprise an outer shell called a pressure shell of general cylindrical shape arranged with its axis positioned vertically and secured to a substantially horizontal tube sheet whose lower face or inlet face forms a wall of the plenum of the steam generator fed with pressurised water forming the primary fluid. 
     This plenum generally of semi-spherical shape has openings at which tubing is welded connecting with the hot branch and cold branch of a loop of the primary circuit of the nuclear reactor. 
     Nuclear reactors must be periodically shut down to carry out maintenance, replacement and repair operations, e.g., for refuelling operations. 
     This shutdown period is also used for maintenance of the steam generators. 
     To carry out some maintenance or repair operations on the steam generators, during shutdown periods of the nuclear reactor, it may be necessary for servicing technicians to enter inside the plenum. 
     The primary circuit and the vessel of the nuclear reactor are filled with water during maintenance operations on the nuclear reactor, which means that it is necessary to drain the plenum and then to shut off the primary circuit before it is possible for servicing technicians to work inside this plenum. 
     To enable these operators to carry out servicing inside the plenum, devices are known for the sealed plugging of the tubing of this plenum connecting with the primary circuit, these devices comprising stoppers also known as nozzle dams placed inside the tubing of the plenum prior to servicing. 
     To place this stopper in position, a fastening ring of the stopper is welded inside the plenum around the opening of the tubing, in the part thereof that opens onto the inside of the chamber in a coaxial arrangement relative to the tubing. 
     The stopper also comprises a sealing member intended to be applied onto the inner surface of the fastening ring and/or of the opening of the tubing. 
     It is generally necessary to form the stopper in several parts which are assembled inside the plenum, or in foldable form, since the stopper has to be inserted inside the plenum via a manhole whose diameter is generally smaller than the diameter of the opening of the tubing in which the stopper is placed. 
     For this purpose a stopper also known as a nozzle dam is known that is dish-shaped comprising a first cylindrical part closed by a base whose diameter is smaller than the inner diameter of the ring, and a second peripheral part of flat annular shape whose outer diameter is larger than the inner diameter of the ring. 
     In this case the seal is ensured by two inflatable, peripheral ring seals which are inserted between the inner surface of the ring and the outer surface of the cylindrical part of the stopper, when this stopper is in position, and by a static seal which comes to bear either against the inner surface of the opening of the tubing or against the upper surface of the ring and a peripheral retaining part in the form of a rim which comes to be applied against the outer surface of the fastening ring. 
     The holding in position of the stopper is generally ensured by securing members such as screws for example, which pass through the peripheral part of the stopper and are screwed into tapped orifices arranged in the ring. 
     The main disadvantage of this type of stopper lies in the fact that it requires the connecting of the seals to an inflating unit, and that it requires continuous monitoring of the inflation of the seals which guarantee the sealed plugging. 
     In addition, this type of stopper has proved to have mechanical resistance, and in particular resistance to temperature or to fluid pressure exerted on one side of the stopper, which may be insufficient for repeated use and use of long duration, or in the occurrence of accidental events. 
     SUMMARY OF THE INVENTION 
     The present invention may avoid these disadvantages by providing a sealed stopper for plugging the tubing which connects a chamber with piping, the stopper being reliable and easy to implement by operators inside a plenum of a steam generator, whilst reducing the servicing time for these operators in a hostile environment. 
     A sealed stopper is provided for plugging an opening of tubing connecting a chamber with piping, which comprises a fastening ring arranged around this connection opening of the tubing leading into the chamber and secured to the inner surface of the chamber, said stopper comprising: 
     a rigid bearing plate formed by a cylindrical part whose diameter is smaller than the inner diameter of the fastening ring, and a flat annular part whose diameter is larger than the inner diameter of said fastening ring, and 
     a sealing member carried by the cylindrical part of the bearing plate, 
     characterized in that the sealing member includes a seal comprising firstly a planar, flexible central part of narrow thickness extending underneath the cylindrical part of the bearing plate, and secondly a peripheral part deformable radially by central expansion means for expanding said peripheral part against the inner surface of the fastening ring. 
     According to other characteristics of one embodiment of the invention: 
     the peripheral part is thicker and more resistant than the central part of the seal and, together with the central part of the seal, delimits an inner housing for the expansion means, said housing being open opposite said central part, 
     the inner housing, on the peripheral part of the seal, comprises an internal frusto-conical wall whose slope is directed towards the center of the bearing plate, 
     the central expansion means is of conical press-fit type and comprises an expansion plate whose shape mates with the inner housing of the seal and can be moved in translation along the axis of the stopper by a control element between a passive position away from the housing and an active position inside said housing to cause radial movement of the peripheral part of the seal, 
     the expansion plate comprises a frusto-conical, side rim whose slope is directed towards the center of the bearing plate, 
     the upper edge of the side rim of the expansion plate has a diameter that is substantially equal to the diameter of the lower edge of the frusto-conical wall of the housing, 
     the control element is formed of a screw system or an eccentric cam system, or a system with helical ramp, 
     the bearing plate and the expansion plate are in composite material, each of their constituent elements comprising two substantially parallel cover skins delimiting volumes between them filled with foam or honeycomb, said volumes being delimited by transverse walls forming stiffeners, 
     the bearing plate and the expansion plate are in a metal alloy e.g. aluminium, 
     the bearing plate and the expansion plate are each formed of at least two hinged panels which can be folded over each other, and 
     the bearing plate is formed of five hinged panels and the expansion plate is formed of three hinged panels. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention and its advantages will be better understood on reading the following description given by way of example and with reference to the appended drawings in which: 
         FIG. 1  is a perspective exploded view of the lower part of a steam generator with its plenum, 
         FIG. 2  is a cross-sectional view of tubing of the plenum of a steam generator comprising a fastening ring for securing a stopper conforming to the invention, 
         FIG. 3  is a schematic perspective view of the stopper conforming to the invention, 
         FIGS. 4 and 5  are schematic cross-sectional views in the vertical plane of the stopper, respectively before and after expansion of the sealing member, 
         FIG. 6  is a schematic cross-sectional view in the vertical plane of the stopper bearing plate, 
         FIGS. 7 and 8  are schematic perspective and cross-sectional views respectively of the stopper sealing member, 
         FIGS. 9 and 10  are schematic perspective and cross-sectional views respectively, in the vertical plane, of the expansion means of the stopper sealing member, and 
         FIGS. 11 to 15  are schematic views showing the different steps for positioning and securing the stopper on tubing of the plenum of the steam generator. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows the lower part of a steam generator carrying the general reference  1  and comprising the semi-spherical wall of a plenum  2 . This semi-spherical wall is secured onto a tube sheet  3  in which the ends of the tubes of the bundle  4  are secured. The plenum  2  is innerly divided into two parts by a transverse partition  5 . 
     Either side of the partition  5 , openings are made in the plenum  2  in which tubing  6  is arranged connecting with two pipes of the primary circuit. 
     The tubing  6  ensures the feeding of pressurised water into one part of the plenum, and recovery by the other part of the plenum of the pressurised water which has circulated inside tubes of the bundle  4  of the steam generator  1 . 
     The wall of the plenum  2  also has manways  7  leading into each of the compartments separated by the partition  5 . As can be seen  FIG. 2 , the tubing  6  arranged at the openings passing through the plenum  2  comprises a bore of cylindrical-frustoconical shape comprising a first cylindrical part  6   a  extended by a second frusto-conical part  6   b  which flares outwardly in the direction of the inside of the plenum  2 . 
     For servicing inside the plenum  2 , it is possible for servicing operators to enter via the manholes  7  into the inside of either one of the compartments of the plenum  2  separated by the partition  5 . During such servicing, the primary circuit is generally filled with water which means that it is necessary to ensure the sealed plugging of the flared nozzle of the tubing  6  before servicing operations can begin. 
     For this purpose, a fastening ring  8  is fixed, e.g., by welding to the inner surface of the plenum  2 , in coaxial arrangement relative to the opening of the tubing  6  in the portion where the flared part of the tubing  6  opens into the inside of the plenum  2 . 
     The tubing  6  is also pierced with a channel  9  used to drain the plenum completely after it has been shut off from the primary circuit by a stopper secured to the fastening ring  8 . 
     The stopper illustrated as a whole in  FIGS. 3 to 5  must be inserted inside the plenum  2  via the manway  7  leading into the plenum. 
     As illustrated  FIGS. 4 and 5 , the stopper carries the general reference  10  and comprises four main parts, namely: 
     a bearing plate  20 , 
     a sealing member  30 , 
     central means  40  for radial expansion of the sealing member  30 , and 
     a control element  50  to control the radial expanding of said sealing member  30 . 
     Therefore, the three elements formed of the bearing plate  20 , the sealing member  30  and the central expansion means  40  are superimposed, the bearing plate  20  being arranged above the sealing member  30  and this sealing member  30  being arranged above the expansion means  40 . 
     The bearing plate  20 , respectively illustrated in perspective in  FIG. 3  and in vertical cross-section in  FIG. 6 , is formed by a cylindrical part  21  whose diameter is smaller than the inner diameter of the fastening ring  8 , and by a flat annular part  22  that is disk-shaped whose diameter is larger than the inner diameter of said fastening ring  8 . The annular part  22  comprises uniformly distributed openings  23  ( FIG. 3 ) through which the securing members are passed such as screws cooperating with threaded openings arranged in said fastening ring  8 . 
     The inner side of the cylindrical part  21  comprises a ring  21   a  ( FIG. 6 ) which forms a bearing surface for the sealing member  30  when in expansion position, as will be seen below. 
     To meet the constraints of weight and volume in order to pass the bearing plate  20  through the manway  7 , this bearing plate  20  is made in lightweight material and in at least two hinged panels which can folded over each other. 
     Preferably the bearing plate  20  is in composite material comprising two substantially parallel, cover skins  20   a  and  20   b . For example the skins  20   a  and  20   b  are formed of fabric produced by woven carbon, aramid or carbon-aramid fibres. These skins  20   a  and  20   b  determine volumes  20   c  between them which are delimited by transverse walls  24  forming stiffeners. The volumes  20   c  are filled with foam, preferably closed cell thermoplastic foam, or honeycomb. 
     To increase the impact resistance of the upper skin  20   a  of the bearing plate  20 , this upper skin  20   a  can be coated with a protective material such as a polyurethane elastomer for example. 
     The sandwich structure of the composite material forming the bearing plate  20  allows a reduction in weight to be obtained of the order of 8 to 10 kg. 
     In the example of embodiment shown in the figures and more particularly in  FIG. 3 , the bearing plate  20  is formed of five panels  25  placed side by side and separated from each other by fold lines  26  arranged at the stiffeners  24 . The panels  25  are hinged together by hinges  27  of known type or by any other suitable system. 
     The sealing member, shown in perspective  FIG. 7  and in cross-section in the vertical plane in  FIG. 8 , is formed of a seal  30  comprising a planar, flexible central part  31  of narrow thickness, and of a peripheral part  32  that is thicker and more resistant. This peripheral part  32  extends underneath the central part  31  and together with said central part  31  it delimits an inner housing  33  for the expansion means  40 . This housing  33  is open opposite the central part  31  to allow the expansion means  40  to enter therein as will be seen below. 
     The inner housing  33 , on the peripheral part  32  of the seal  30 , comprises an inner frusto-conical wall  34  whose slope is directed towards the center of the bearing plate  20  located thereabove i.e. in the direction of the axis of the stopper  10 . 
     The expansion means  40 , illustrated in perspective in  FIG. 9  and in cross-section in the vertical plane in  FIG. 10 , is of conical press-fit type and comprises an expansion plate  41  whose general shape mates with the shape of the inner housing  33  of the seal  30 . For this purpose, the expansion plate  41  comprises a frusto-conical side rim  42  whose slope is directed towards the center of the bearing plate  20  located above, i.e. in the direction of the axis of the stopper  10 . 
     The upper edge of the side rim  42  of the expansion plate  41  has a diameter that is substantially equal to the diameter of the lower edge of the frusto-conical wall  34  of the housing  33  of the seal  30 . 
     This expansion plate  41  is also made in composite material comprising two substantially parallel cover skins  41   a  and  41   b  determining volumes  40   c  between them delimited by transverse walls  44  forming stiffeners. 
     The skins  41   a  and  41   b  are preferably composed of fabric formed by woven carbon, aramid or carbon-aramid fibres. 
     The volumes  40   c  are filled with foam, preferably closed cell thermoplastic foam, or honeycomb. 
     The expansion plate  41  is formed of at least two panels  45  hinged together and able to be folded over one another. Preferably, and as illustrated  FIG. 9 , the expansion plate  41  is formed of three side-by-side panels  45  separated by fold lines  46  arranged at the stiffeners  44 . These panels  45  are hinged together by means of hinges  47  of known type, or by means of any other suitable system. 
     According to one variant, the bearing plate  20  and the expansion plate  41  can be formed of a metal alloy such as aluminium for example. 
     The expansion plate  41  can be moved in translation along the axis of the stopper  10  by the control element  50 , between a passive position away from the housing  33  and in which the peripheral part  32  of the seal  30  lies away from the ring  21   a  ( FIG. 4 ) and an active position inside said housing  33  to cause radial movement of the peripheral part  32  of the seal  30  so as to apply and hold this peripheral part  32  against the inner surface of the fastening ring  8 . In this active position, the peripheral part  32  of the seal  30  abuts the ring  21   a  ( FIG. 5 ). 
     The control element  50 , in order to move the expansion plate  41  in the direction of the bearing plate  20  and apply the peripheral part  32  of the seal  30  against the inner surface of the fastening ring  8  thereby providing ensured sealing, or to move this expansion plate  41  away from the bearing plate  20  in order to release the peripheral part  31  from the seal  30 , is formed of a known, conventional screw system in the embodiment shown in the figures. 
     This screw system comprises a threaded rod  51  of which one end is set in the expansion plate  41 , and a nut  52  is screwed onto said threaded rod  51 . 
     The control means  50  can also be formed by an eccentric cam system or helical ramp system, or by any other suitable system which can ensure this function, such as a ball lock pins. 
     The positioning of the stopper  10  in the nozzle of the tubing  6  of the plenum  2  is conducted as follows. 
     First, a servicing operator enters into the corresponding compartment of the plenum  2  as shown  FIG. 11  and inserts the expansion plate  41  in folded configuration into this compartment. As shown in  FIG. 12 , the seal  30  is previously secured onto the expansion plate  41  e.g. by screw members passing through the central part  31  of this seal  30  via holes  35  and which are screwed into said expansion plate  41 . In this configuration, the expansion plate  41  is able to pass through the manway  7  whose inner diameter is smaller than the diameter of the expansion plate  41  when deployed ( FIG. 12 ). 
     The operator then unfolds this expansion plate  41  carrying the seal  30  by causing the panels  45  to pivot around the hinges  47 , and places this expansion plate  41  and the seal  30  in its deployed configuration inside the frusto-conical part  6   b  of the tubing  6  as shown  FIG. 13 . 
     The expansion plate  41  also carries the threaded rod  51 . 
     The bearing plate  20  is then folded by causing the sections  25  to pivot on each other around the hinges  27 , and this bearing plate  20  in said configuration is inserted through the manhole  7  and brought by the operator inside the compartment of the plenum  2 . 
     As shown  FIG. 14 , the bearing plate  20  carries the screw members  55  intended to secure the stopper  10  onto the fastening ring  8 . 
     The operator then unfolds the bearing plate  20  and places this plate in position so that it bears upon the ring  8  located at the entrance to the opening of the tubing  6 , over the seal  30  and expansion plate  41  that were previously placed in position. The operator next positions the nut  52  on the threaded rod  51 . 
     The operator center positions the screw tooling  60  on the stopper  10  as shown  FIG. 15 . 
     This screw tooling is composed of a centering foot  61  on the stopper  10  and has an arm  62  whose free end is provided with a screwing machine  63 . 
     A camera  64  is also mounted on the arm  62  to enable an operator outside the plenum  2  to control the operations by means of display panel. 
     The screw tooling  60  also comprises a pole  65  so that it is possible from outside the plenum  2  to insert and position or to remove the screwing equipment  60  via the manway  7  as illustrated  FIG. 15 . 
     The angle positioning of the arm  62  carrying the screwing machine  63  and the setting in rotation of this screwing machine  63  to screw a screwing member  55  are controlled by a handle for example, located at the free end of the pole  65  and maneuvered by the operator. 
     When all the screw members  55  have been screwed to secure the stopper  10  on the fastening ring  8 , the operator proceeds with expanding the peripheral part  32  of the seal  30 . To do so, the screw system  51  is actuated by appropriate tooling which could be adapted onto the screw tooling  60  so as to cause gradual entry of the expansion plate  41  into the housing  33 . This tooling can be adapted onto the screw tooling  60 . Owing to the frusto-conical shapes of the inner wall  34  of the housing  33 , and of the side edge  42  of the expansion plate  41 , the peripheral part  32  is able to be moved radially towards the inner surface of the fastening ring  8  and comes to lie flat against the inner surface thereof so as to ensure a seal at the opening of the tubing  6  ( FIG. 5 ). The peripheral part  32  of the seal  31  is therefore compressed via the expansion plate  41  with frusto-conical side edge. 
     In addition to the above-described initial expansion of the sealing member, the pressure of the fluid exerted on the expansion plate  41  which may reach 5 bars produces an additional compression force on the peripheral part  32  of the seal  30 . 
     The stopper  10  conforming to the invention therefore allows very good sealing contact to be made on the fastening ring, and this stopper has increased resistance to pressure irrespective of the direction in which such pressure is exerted. 
     The stopper according to the invention has the advantage of being resistant and lightweight facilitating handling operations, and offers ease of use which reduces the servicing time of these operators in a hostile environment.