CLAMP ASSEMBLY AND METHOD FOR A CLAMP ASSEMBLY

Technical field

The present invention relates generally to a method and a clamp for securing two pipes at a joint and to a method for arranging a clamp over the pipes at such a joint to prevent the pipes from separating in case the joint would crack. More specifically, the present invention relates to a method and a clamp for securing two pipes in a nuclear reactor, such as in a jet pump riser in a nuclear reactor pressure vessel and to a method for arranging a clamp over a jet pump riser in a nuclear reactor pressure vessel to hold the pipes together in case it would crack.

Description of the prior art A clamp assembly to be applied at a joint between a first pipe and a second pipe, for securing the two pipes may be used on any types of pipes. Furthermore, a method for applying such a clamp assembly at a joint between a first pipe and a second pipe for securing the two pipes may also be used on any type of pipes. In the following description of the prior art the use of the clamp assembly is, however, limited to the use in nuclear reactors.

A nuclear reactor has a reactor pressure vessel, which typically has a generally cylindrical shape and is closed in both ends, e.g., by a bottom head and a removable top head, to form a closed container. A core plate and a top guide are arranged inside the reactor pressure vessel with the top guide arranged on top of the core plate. The top guide comprises a number of holes. A reactor core in the form of fuel bundles is arranged inside the reactor pressure vessel with each fuel bundle being arranged through a hole in the top guide and resting on the core plate. Some reactors are provided with a core shroud which surrounds the core, the core plate and the top guide.

The reactor pressure vessel is filled with water covering the core and the shroud. The water in the reactor pressure vessel is

arranged to flow through the core in order to cool the core during operation of the nuclear reactor. The flow is usually induced by circulation pumps arranged between the shroud and the nuclear reactor pressure vessel which makes the water flow downwards in the space between the shroud and the reactor pressure vessel (RPV) wall. The circulation pumps may be of any one of a number of different types. One type of circulation pumps used in nuclear reactors of the above described type is jet pumps, in which water is injected through a narrow nozzle in a jet pump arranged between the shroud and the reactor pressure vessel. Water enters the RPV through an inlet nozzle in the RPV wall. A jet pump riser assembly is connected to the inlet nozzle, wherein the jet pump riser assembly comprises a thermal sleeve, an elbow, and a riser pipe. One end of the thermal sleeve is arranged on the outside of the RPV while the other end of the thermal sleeve is welded to a first end of the elbow. The second end of the elbow is welded to a first end of the riser pipe. The second end of the riser pipe is connected to a transition piece with an inlet mixer. During operation of the nuclear reactor substantial amounts of water is forced through the jet pump riser assembly. The large amounts of water flowing through the jet pump riser assembly results in significant hydraulic forces and vibrations exerted on the jet pump assembly. Further, a large amount of heat is generated by the core during operation of the nuclear reactor, resulting in thermal stresses being exerted on the jet pump assembly. The hydraulic forces, vibration stresses and thermal stresses might cause stress corrosion cracking (SCC) in the riser pipe elbow, which might result in the riser pipe elbow being separated from the thermal sleeve or from the riser pipe. This is of course highly undesirable.

The European patent application EP 0 944 095 describes a clamp apparatus for performing jet pump riser repairs. The clamp apparatus includes a lower clamp element and an upper clamp element, which are configured to be positioned at the interface between the thermal sleeve and the jet pump riser elbow. The upper and lower elements include extended ridges that are configured to fit in circumferential grooves precisely machined into

the sleeve-elbow assembly on opposing sides of the interface weld. The clamp apparatus provides support in the axial direction of the pipes being supported by the clamp apparatus as well as in the direction perpendicular to the pipes. The circumferential grooves are machined by, e.g., EDM.

It is difficult to arrange a clamp apparatus according to the mentioned patent application on the riser pipe assembly in a nuclear reactor between a reactor pressure vessel and a core shroud due to the limited space between the reactor pressure vessel and the core shroud. Furthermore, as the arrangement of the clamp apparatus according to the prior art requires the machining of grooves a tool for the machining has to be directed to the pipes in order to machine the grooves.

The apparatuses according to the prior art may be used to support the interface between the riser pipe and the elbow, or the interface between the thermal sleeve and the elbow in a riser pipe assembly in a reactor pressure vessel. The clamp apparatus may also be used to support the interface between any other pipes needing a support. The problems of the prior art technique exists for all such pipes which are difficult to access.

Summary of the invention An object of the present invention is to provide a method and a clamp assembly for supporting an interface between two pipes in the axial direction of the pipes as well as in the direction perpendicular to the pipes.

Another object of the present invention is provide a method and a clamp assembly for supporting at least one of the interfaces between the riser pipe, the elbow and the thermal sleeve in a riser pipe assembly in a reactor pressure vessel in the axial direction of the pipes as well as in the direction perpendicular to the pipes.

These objects are achieved with a method and a clamp assembly according to the independent claims.

Further advantages with the present invention are achieved with the features defined in the dependent claims.

According to a first aspect of the present invention a method is provided for applying a clamp assembly at a joint between a first pipe with a length axis and a second pipe with a length axis, for securing the two pipes to each other. The clamp assembly comprises a first clamp element and a second clamp element and clamping means for clamping the first clamp element to the second clamp element. The clamp assembly comprises a length axis arranged to be parallel to the length axis of the pipes. At least the first clamp element comprises a first stud means and a second stud means, which stud means are arranged at axially spaced positions on the first clamp element. The method is characterized in that it comprises the step of arranging the first clamp element and the second clamp element around the pipes at the joint between the pipes, with the first stud means on the first clamp element in the vicinity of the first pipe and the second stud means on the first clamp in the vicinity of the second pipe. The method also comprises the steps of applying an electrical voltage between the stud means of the first clamp element and the pipes, moving the first clamp element back and forth in relation to the pipes thereby controlling electrical discharges between the stud means on the first clamp element and the pipes, thereby forming grooves in the pipes corresponding to the stud means on the first clamp element, and securing the clamp assembly to the pipes by clamping the first clamp element to the second clamp element with the clamping means, wherein the stud means on the first clamp element is positioned in the grooves formed by the electrical discharges.

With a method according to the present invention the grooves in the pipes is formed using the clamp itself as a tool in electric discharge machining. The shape of the grooves will thereby be precisely adapted to the shape of the stud means on the clamp assembly.

Furthermore, as the clamp assembly is in the correct position after the grooves have been formed the application of the clamp assembly is less complicated compared to the prior art, wherein the grooves are formed with a tool separate from the clamp assembly. This is especially important when arranging the clamp assembly on pipes in narrow positions, such as the riser pipe in a jet pump assembly in a nuclear reactor.

The first clamp element may be arranged hinged to the second clamp element. By having the first clamp element hinged to the second clamp element it is possible to arrange the clamp assembly over pipes without having to move the clamp assembly over an end of the pipes and without having to separate the first clamp element from the second clamp element.

The arrangement of the clamp elements described above is one way of arranging the clamp elements in a clamp assembly. There are also other ways of connecting the clamp elements in a clamp assembly according to the present invention. The first clamp element may be arranged slidable perpendicular to the length axis of the clamp assembly in relation to the second clamp element. By having the clamp elements arranged in this way the clamp assembly has to be assembled in position on the pipes. However, such an arrangement of the clamp elements provides a rigid connection of the clamp elements.

The second clamp element may comprise a first stud means and a second stud means, which stud means are arranged at axially spaced positions on the second clamp element. By providing stud means also on the second clamp element the connection of the pipes may be more rigid compared to when only the first clamp element comprises stud means.

The second clamp element may be arranged at the joint between the pipes, with the first stud means on the second clamp element being arranged on the first pipe and the second stud means on the second clamp element being arranged on the second pipe. The

method may also comprise the steps of applying an electrical voltage between the stud means of the second clamp element and the pipes, and moving the second clamp element back and forth in relation to the pipes thereby controlling electrical discharges between the stud means on the second clamp element and the pipes, thereby forming grooves in the pipes corresponding to the stud means on the second clamp element, wherein the stud means on the second clamp element is positioned in the grooves formed by the electrical discharges, when the clamp assembly is secured to the pipes.

By providing grooves in the pipes also for the second clamp element provides for a more rigid connection of the pipes compared to the case when grooves are formed for only the first clamp element.

Each one of the stud means may comprise a number of studs. By having studs on the stud means the grooves will locally become deep, providing a secure connection, while still not jeopardizing the mechanical strength of the pipes as the grooves will become deep only locally.

The material of the clamp elements may be chosen so that part of the stud means is eroded by the electrical discharges. As not only material from the pipes is eroded during the electric discharge machining the fit between the stud means and the grooves will become even better.

The clamp elements may be made of stainless steel. This is a suitable choice for use in a nuclear reactor. It is possible to use also other metals for the clamp, such as Inconel.

According to a second embodiment of the present invention a clamp assembly is provided which is to be applied at a joint between a first pipe with a length axis and a second pipe with a length axis, for securing the two pipes to each other. The clamp assembly comprises a first clamp element and a second clamp element and

clamping means for clamping the first clamp element to the second clamp element. The clamp assembly comprises a length axis arranged to be parallel to the length axis of the pipes. At least the first clamp element comprises a first stud means and a second stud means, which stud means are arranged at axially spaced positions on the first clamp element, which stud means are configured to create grooves in the pipes. The clamp assembly is characterized in that the first clamp element is movable in relation to the second clamp element, and in that the clamp assembly also comprises a first electrical connection in the first clamp element for connection of an electrical voltage to the stud means of the first clamp element.

A clamp assembly according to the second aspect of the invention provides for a less complicated arrangement on pipes compared to the prior art as the clamp assembly may be used as the tool for machining the grooves in the pipes. Thus no special tool has to be provided to machine the grooves in the pipes. This is especially important when arranging the clamp assembly on pipes in narrow positions, such as the riser pipe in a jet pump assembly in a nuclear reactor. Furthermore, the shape of the grooves may be more exactly adapted to the shape of the stud means if the clamp assembly is used as the tool when machining the grooves.

The clamp assembly may also comprise a second electrical connection in the second clamp element for connection of an electrical voltage to the second clamp element. This makes it possible to use also the second clamp element as a tool for providing grooves, which in turn provides for a more rigid connection of the clamp assembly to the pipes.

The first clamp element may be arranged hinged to the second clamp element. By having the first clamp element hinged to the second clamp element it is possible to arrange the clamp assembly over pipes without having to move the clamp assembly over an end of the pipes.

The arrangement of the clamp elements described above is one way of arranging the clamp elements in a clamp assembly. There are also other ways of connecting the clamp elements in a clamp assembly according to the present invention. The first clamp element may be arranged slidable perpendicular to the length axis of the clamp assembly in relation to the second clamp element. By having the clamp elements arranged in this way the clamp assembly has to be assembled in position on the pipes. However, such an arrangement of the clamp elements provides a rigid connection of the clamp elements.

The second clamp element may comprise a first stud means and a second stud means, which stud means are arranged at axially spaced positions on the second clamp element. By providing stud means also on the second clamp element the connection of the pipes may be more rigid compared to when only the first clamp element comprises stud means.

Each one of the stud means may comprise a number of studs. By having studs on the stud means the grooves will locally become deep, providing a secure connection, while still not jeopardizing the mechanical strength of the pipes as the grooves will become deep only locally.

The material of the clamp elements may be chosen so that part of the stud means is eroded by the electrical discharges. As not only material from the pipes is eroded during the electric discharge machining the fit between the stud means and the grooves will become even better.

The clamp elements may be made of stainless steel. This is a suitable choice for use in a nuclear reactor. It is possible to use also other metals for the clamp, such as Inconel.

In the following preferred embodiments of the invention will be described with reference to the appended drawings.

Short description of the drawings

Fig 1 shows schematically a boiling water nuclear reactor in which a clamp assembly according to the present invention may be arranged.

Fig 2 shows in larger detail an exploded view of a clamp assembly arranged on a pipe assembly in the boiling water reactor of Fig 1 .

Fig 3 shows a mounting tool for arranging the clamp assembly of Fig 2 on a pipe.

Fig 4 shows a clamp assembly according to a second embodiment of the present invention.

Description of preferred embodiments

In the following description of preferred embodiments of the invention the same reference numeral will be used for similar features in the different drawings, which are not drawn to scale.

The clamp assembly according to the present invention may be arranged at the joint between any pipes. However, the clamp assembly is primarily intended to be used at the joint between the elbow and the riser pipe in the jet pump assembly of a nuclear reactor. The invention will therefore be described in relation to such a joint in a nuclear reactor.

Fig 1 shows schematically a boiling water nuclear reactor 1 in which a clamp assembly according to the present invention may be arranged. The nuclear reactor 1 comprises a reactor pressure vessel 2 with a wall 3, a core 4 arranged inside the reactor pressure vessel 2, a core plate 5 on which the core 4 is arranged, a core plate support ring 6 supporting the core plate 5, a core shroud 7 surrounding the core 4 and being arranged within the reactor pressure vessel 2. The nuclear reactor also comprises a number of pipe assemblies, in the form of jet pump assemblies 8, for transferring cooling water into the reactor pressure vessel 2 for

cooling of the core 4. The jet pump assemblies 8 comprises a thermal sleeve 9 with an inlet 10 being arranged through the reactor pressure vessel wall 3, an elbow 1 1 and a riser pipe 12 extending upwardly from the elbow 1 1 to a transition piece 13 and two inlet mixers 14 extending downwardly from the transition piece. Water is pumped into the inlet 10 by external pumps (not shown). The water is forced through the elbow 1 1 and the riser pump to the transition piece 13 where the water is forced through a nozzle (not shown) in a jet pump (not shown) to force water through the inlet mixers, thereby creating a flow of water downwards from the transition piece 13 towards the bottom of the reactor pressure vessel 2 and up through the core 4. A clamp assembly 15 is arranged at the joint between the elbow 1 1 and the riser pipe 12.

Fig 2 shows in larger detail an exploded view of a clamp assembly 15 to be arranged at a joint between a first pipe 17 with a length axis 18 and a second pipe 19 with a length axis 20, in order to secure the two pipes to each other. The clamp assembly 15 comprises a first clamp element 21 and a second clamp element 22 and clamping means 23 in the form of screws for clamping the first clamp element 21 to the second clamp element 22. The clamp assembly 15 comprises a length axis 24, which is arranged in parallel to the length axis 18, 20 of the pipes 17, 19. The first clamp element 21 comprises first stud means 25, in the form of a number of studs 26, and second stud means 27 in the form of a number of studs 28. Each one of the clamp elements 21 , 22, with the stud means 25, 27, is in one single piece made of metal. The second clamp element 22 comprises first stud means 30, in the form of a number of studs 31 , and second stud means 32 in the form of a number of studs 33. The first clamp element 21 is arranged hinged to the second clamp element 22 around an axle 34. The first clamp element 21 comprises a first electrical connection 36 for connection of an electrical voltage to the first clamp element 21 . The second clamp element 21 comprises a second electrical connection 37 for connection of an electrical voltage to the second clamp element 22.

The arrangement of the clamp assembly 15 on the pipes 17, 19, will now be described. In a first step the first clamp element 21 and the second clamp element 22 are arranged around the pipes at the joint between the pipes, with the first stud means 25 on the first clamp element 21 in the vicinity of the first pipe 17 and the second stud means 27 in the vicinity of the second pipe 18. In a second step an electrical voltage is applied between the stud means 25, 27, and the pipes 17, 19. In the embodiment shown in Fig 2 the voltage may be applied to the stud means 25, 27, through the clamp elements 21 , 22 as they both consist of one piece in metal. With a voltage applied between the stud means and the pipes the first clamp element 21 is moved back and forth in relation to the pipes 17, 19, by means of first moving means (Fig 4), thereby controlling electrical discharges between the stud means 25, 27, and the pipes 17, 19. During the discharges metal is removed from the pipes 17, 19 as well as from the studs 26, 28, of the stud means 25, 27 of the first clamp element 21 . Thus, grooves (not shown) will be formed in the pipes 17, 19 corresponding to the stud means 27, 29, on the first clamp element 21 . The grooves will have a form that corresponds to the form of the stud means 25,27. When the grooves have been formed on the first clamp element the second clamp element 22 is moved back and forth in relation to the pipes 17, 19, by means of second moving means (Fig 4), thereby controlling electrical discharges between the stud means 25, 27, and the pipes 17, 19. During the discharges metal is removed from the pipes 17, 19 as well as from the studs 31 , 33, of the stud means 30, 32 of the second clamp element 22. Thus, grooves (not shown) will be formed in the pipes 17, 19, corresponding to the stud means 30, 32, on the second clamp element 22. The grooves formed by the studs 31 , 33 of the stud means 30, 32 of the second clamp element 22 will have a form that correspond to said studs 31 , 33.

It is not necessary to have studs on any one of the stud means 25, 27, 30, 32. The stud means 25, 27, 30, 32, may have a smooth form.

Fig 3 shows the clamp assembly 15 of Fig 2 arranged in a mounting tool 35. The mounting tool 35 comprises a frame 39 and a first pair of attachment arms 38 which are arranged rotatably in the frame 39 around pins 40. The mounting tool 35 also comprises a second pair of attachment arms 41 of which only one is shown. The first pair of attachment arms 38 may be arranged in contact with the pipes by arranging a first pair of attachment screws 42, which are screwed in the attachment arms 38 on the opposite side of the pins 40, so that the attachment screws 42 push on the frame 39. The second pair of attachment arms 41 may be arranged in contact with the pipes in the same way by using a second pair of attachment screws 43 of which only one is shown. By arranging the attachment arms 38, 41 , in contact with the pipes the mounting tool is fastened to the pipes. The clamp assembly 15 is arranged in the mounting tool 35 between the first pair of attachment arms 38 and the second pair of attachment arms 41 . The mounting tool 35 also comprises a first moving means 44, which is arranged in the frame 39 rotatable around an axle 45 and in contact with the first clamp element 21 . The first moving means 44 and thus also the first clamp element 21 may be moved back and forth with a first actuating means 45, which is arranged between the frame 39 and the first moving means 44. The mounting tool 35 also comprises a second moving means 46, which is arranged in the frame 39 rotatable around the axle 45 and in contact with the second clamp element 22. The second moving means 46 and thus also the second clamp element 22 may be moved back and forth with a second actuating means 47, which is arranged between the frame 39 and the second moving means 46.

An electrical voltage may be applied to the pipes through one or more of the attachment arms 38, 41 . The actuating means 45, 47 are operated one at a time while a voltage is applied over the pipes and the corresponding clamp element, in order to provide the grooves in the pipes.

Fig 4 shows a clamp assembly 48 according to a second embodiment of the present invention. Only the differences between the clamp assembly 48 according to the second embodiment of the

present invention and the clamp assembly 15 according to the first embodiment of the present invention will be described. The clamp assembly 48 comprises a first clamp element 49 comprising a first stud means 50 comprising a number of studs 51 , and a second stud means 52 comprising a number of studs 53. The clamp assembly

48 also comprises a second clamp element 54 comprising a first stud means 55 and a second stud means 56 which both have smooth surfaces without studs. The second clamp element 54 is slidably arranged in relation to the first clamp element 49. The clamp assembly 48 also comprises a frame 57, which is arranged fixed in relation to the first clamp element 49. The frame 57 comprises a through hole 58 for attachment of actuating means (not shown) which are fixed in relation to the second clamp element 54 and in contact with the first clamp element 49 arranged to move the first clamp element 49 in relation to the second clamp element 54. After having made grooves in the pipes (not shown) on which the clamp assembly 48 is to be arranged the clamp assembly may be secured to the pipes by applying a securing means, which pushes the second clamp element 54 away from the frame 57. Thus, the second clamp element 54 is pushed towards the first clamp element

49 securing the clamp assembly 48 on the pipes.

The clamp elements are made of a material suitable for electrical discharge manufacturing. As the clamp assemblies primarily are to be used in nuclear reactors it may be advantageous to use stainless steel for the clamp assemblies.

The described embodiments may be amended in many ways without departing from the spirit and scope of the present invention, which is only limited by the claims.

It is possible to form a clamp assembly having more than two clamp elements.