FR2692972A1 - Localised repair of heat exchanger tubes - uses neutral atmos. during welding of replacement section to prevent oxidn. in weld - Google Patents

Abstract

A process is claimed for repairing local deterioration in a heat exchanger tube (20) incorporating at least one damaged part close to one end of the tube fixed in a perforation (32) of a plate of tubes, consisting of:- cutting the tube beyond the damaged part; extracting via the perforation a section of the tube containing the damaged part and its end; introducing via the perforation a replacement section of tube (21); butt welding the replacement section of tube (21) to the remaining tube (20), inside the tube and under a neutral gas atmosphere. Before welding the tube, a confining component (30) for the neutral gas is introduced via the perforation and placed around the region to be welded. USE/ADVANTAGE - The process is used for repairing local damage in heat exchanger tubes, notably those associated with a nuclear reactor which are in particularly corrosive environments and thus subject to local deterioration such as cracking, notably in the zone immediately adjacent to the part of the tubes fixed to the tube plate. The use of a neutral atmosphere during the welding operation eliminates the risk of oxidn. in the weld without the need for intervention in the secondary part of the steam generator and ensures a good quality weld.

Description

METHOD FOR REPAIRING A TUBE FOR A HEAT EXCHANGER
LOCALLY DETERIORATED HEAT.

DESCRIPTION
The invention relates to a method for repairing a heat exchanger tube having local damage such as cracks.

 The invention applies in particular to the repair of tubes of steam generators fitted to nuclear power plants. In fact, these tubes are found in a particularly corrosive environment which leads to the frequent appearance of cracks or local deterioration, in particular in the area immediately adjacent to the part of the tubes fixed to the tube plate.

 When a steam generator tube is cracked under the effect of corrosion under tension or pierced by migrating bodies, intervention is necessary to avoid any contamination of the secondary circuit of the reactor by the fluid circulating in the primary circuit.

 A first type of intervention consists in plugging the damaged tube at each of its ends.

However, when such an operation is repeated on a large number of tubes of the steam generator, it leads to a significant reduction in the efficiency of this device, which may require its replacement in the medium term.

 For this reason, it appears desirable not to seal the deteriorated tubes, but to repair them in order to preserve an optimal output of the steam generator. However, this repair poses many practical difficulties, because it must be done inside the steam generator, by accesses which are essentially limited to the manholes formed in the water box through which the primary fluid enters and exits the steam generator.

 A known repair technique consists in placing inside of each deteriorated tube, in the area to be repaired, a cuff for reinforcing the tube. Usually, this cuff comprises a section of tube made of a metal alloy identical to that of the tube to be repaired. A coating made of a brazing material such as nickel or chromium can cover the tube section over at least part of its length. The fixing of cuffs inside the tubes to be repaired can also be done by welding or by mechanical expansion.

 If it does not make the repaired tubes completely inoperative, this known solution however results in a significant reduction in the efficiency of the steam generator, if it is applied to a large number of tubes.

 Another known solution for repairing damaged tubes of steam generators consists in cutting the tube beyond the damaged part, in extracting by the water box a section of tube comprising the damaged part, to be put in place by the water a section of replacement tube, and weld end to end, through the interior of the tube and under an atmosphere of neutral gas (generally argon), the section of replacement tube to the remaining part of the tube.

 In practice, however, this solution has proved somewhat satisfactory. Indeed, in the absence of confinement of the neutral gas around the tube, the oxidation of the weld in this region cannot be prevented.

 One solution to this problem could be to carry out the butt welding of the two sections of tubes separately from the inside and from the outside of the tube, as taught in document FR-A-2 655 898.

However, this technique is not applicable to the repair of a steam generator because it is practically not possible to introduce a welding device around the tube through the secondary of the steam generator until it is in close proximity to the tube plate.

 The same remark applies to the technique described in document FR-A-2 652 772, which proposes placing around the adjacent ends of two tubes to be butt welded an enclosure delimiting around this zone an annular chamber in which the atmosphere can be controlled.

 The invention specifically relates to a method for repairing a heat exchanger tube by replacing the damaged tube section with a replacement tube section which is welded end to end to the remaining part of the tube without risk of d oxidation of the weld from the outside and without intervention by the secondary part of the steam generator being necessary.

According to the invention, this result is obtained by means of a method of repairing a heat exchanger tube comprising at least one damaged part close to one end of the tube fixed in a perforation of a tube plate , consisting in - cutting the tube beyond the damaged part; - extract by said perforation a section of tube
damaged comprising the damaged part and said
end '; - Introduce by said perforation a section of tube
of substitution ; - weld end to end, inside the tube and under
neutral gas atmosphere, the replacement tube section
cementing to a remaining section of the tube; characterized in that before welding the replacement tube section to the remaining section of the tube, there is introduced by said perforation a neutral gas confinement member, and said member is placed around a region to be welded.

 In order for the neutral gas confinement member to be introduced by the perforation of the tube plate corresponding to the tube to be repaired, various embodiments are proposed.

 According to a first embodiment of the invention, a cuff made of a shape memory material is used as the confinement member, which is introduced by said perforation in a retracted state, in which the cuff and the tube have substantially the same diameter, which is then passed into an expanded state to allow the introduction into this cuff of adjacent ends of the replacement tube section and of the remaining section of the tube, then returned to its retracted state .

 According to a second embodiment of the invention, a cuff is used as the confinement member which is placed on one end of the section of replacement tube, so that a part of the cuff protruding beyond this end has a reduced diameter, the replacement tube section carrying the cuff is introduced by said perforation, then a radial expansion of said part of the cuff is carried out, before introducing it onto the remaining section of the tube.

 Finally, according to a third embodiment of the invention, one end of the replacement tube section is used as the confinement member, and this end is radially expanded after having introduced the replacement tube section by said perforation, and before introducing said end on the remaining section of the tube.

 In the second and in the third embodiments of the invention, the radial expansion is carried out after passing through a last spacer plate preceding the region to be welded, when the damaged part of the tube is located beyond a plate exchanger spacer.

We will now describe, by way of nonlimiting examples, various embodiments of the invention with reference to the accompanying drawings, in which
- Figure 1 is a longitudinal sectional view schematically illustrating a steam generator whose locally damaged tubes can be repaired using the method according to the invention;
- Figure 2 is a view illustrating on a larger scale and in partial section the principle of the repair method according to the invention;
- Figure 3 illustrates three successive stages of implementation of the method according to the invention, in a first embodiment according to which the confinement member is constituted by a cuff made of a shape memory material;
- Figure 4 is a view comparable to Figure 3 illustrating two successive stages of implementation of the method according to the invention in a second embodiment according to which the confinement member is constituted by a cuff which is placed at the end of the replacement tube section; and
- Figure 5 is a view comparable to Figures 3 and 4 illustrating a third embodiment of the invention, in which the confinement member is constituted by one end of the replacement tube section.

 There is shown schematically in Figure 1 a steam generator used in a nuclear power station, to ensure the heat transfer between the primary water contained in the primary circuit of the reactor and the secondary water-vapor contained in the secondary circuit of the reactor.

 This steam generator comprises in known manner an outer casing 10, of vertical axis, the interior volume of which is divided into two parts by a perforated plate 12 called a tube plate.

The tube plate 12 is located immediately above the convex bottom of the casing 10 and delimits with the latter a water box separated into an inlet manifold 14 for primary water and an outlet manifold 16 for primary water by a vertical partition 18.

 The primary water inlet 14 and outlet 16 collectors are connected to each other by a bundle of inverted U-shaped tubes 20 which are fixed by swaging and by welding in the perforations 32 (FIG. 2) of the tube plate 12, so that the two ends of each of the tubes 20 open respectively into the inlet manifold 14 and into the outlet manifold 16 of the primary water.

 The secondary water is admitted inside the envelope 10 of the steam generator by an inlet manifold 22 of the secondary water, of toric shape, situated above the bundle of tubes 20. The secondary water descends to the tube plate 12 in an annular space 23 formed between the outer envelope 10 and an inner envelope 24 surrounding the bundle of tubes 20.

 The secondary water then rises inside the envelope 24 by recovering, in contact with the tubes 20, the heat conveyed by the primary water circulating in the latter. The secondary water in the liquid state thus transforms into water vapor which leaves the steam generator through a tube 26 provided at the upper end of the casing 10, after having passed through drying devices (not shown). .

 Horizontal spacer plates 27 are mounted at regular intervals in the inner casing 24, to maintain the tubes 20 of the bundle and prevent their entry into vibration. These plates 27 are perforated to allow the passage of the tubes 20 and the upward circulation of the secondary water-vapor in the envelope 24.

 Between the periods of shutdown and of operation of the reactor, the tubes 20 are subjected to significant thermal stresses which tend to damage them, in particular in the zone situated in the immediate vicinity of the tube plate 12. In fact, this zone is subjected particularly intense corrosion because sludge carried by secondary water tends to settle there.

 Periodic shutdowns of the reactor make it possible to control the tubes 20 and to detect those which have defects such as cracks or perforations.

 In accordance with the invention, the tubes 20 which have local deterioration such as cracks are also repaired during these stops.

 For this and as illustrated very schematically in Figure 2, we just cut the tube 20 to be repaired at a location 28 located beyond the damaged part, relative to the tube plate 12. The cutting of the tube 20 to be repaired is carried out in a known manner using a cutting tool which is introduced, from the inlet manifold 14 or the outlet manifold 16 of the primary water, into the tube concerned, up to the location longed for.

 The damaged tube section located between the location 28 of the cutout and the underside of the tube plate 12 is then extracted by the collector 14 or 16. To do this, the corresponding end of this tube section is dewelded. tube plate 12.

 A confinement member 30 is then introduced into the secondary part of the steam generator, located above the tube plate 12, through the perforation 32 in which was fixed the damaged section of tube previously extracted. Different techniques for introducing the containment member 30 will be described later.

 A new replacement tube section is also introduced through the perforation 32, designated by the reference 21 in FIG. 2, the dimensions and construction of which are identical to those of the damaged tube section extracted previously. According to the technique used to introduce the confinement member illustrated by the cuff 30 in FIG. 2, the introduction of the replacement tube section 21 can be done after that of the confinement member or simultaneously with it.

 When the introduction of these two parts is complete, we find ourselves in the situation illustrated in FIG. 2, in which a slight clearance exists between the upper end of the replacement tube section 21 and the location 28 of the cutout on the remaining section of the tube 20. The adjacent ends of these two sections of tubes are surrounded by the confinement member 30, so that the space delimited between the opposite ends of the two sections of tubes is confined by this organ 30.

 Then is introduced by the replacement tube section 21, from the inlet manifold 14 or the outlet manifold 16, a rotary welding tool equipped with a neutral gas injection device such as argon allowing weld under a non-oxidizing atmosphere. Thanks to the confinement provided by the member 30, the atmosphere of neutral gas is also present outside the ends of the tubes to be welded, which prevents oxidation of the weld from occurring in this region. . An efficient repair of the damaged tube is thus carried out without the generator efficiency being reduced.

 In Figure 3, there is shown schematically a first embodiment of the invention for introducing the containment member 30 by the perforation 32 formed in the tube plate 12 opposite the part of the tube 20 to be repaired. In this case, the member 30 is constituted by a cuff 30a made of a shape memory material, so that its diameter increases or decreases when a given temperature threshold is crossed.

 Thus, the confinement cuff 30a has for example, beyond the temperature threshold considered, a diameter substantially equal to that of the tube 20 to be repaired. Under these conditions, and as illustrated on the left in FIG. 3, the cuff 30a can be inserted without difficulty by the perforation 32 of the tube plate 12. The replacement tube section 21 is then introduced, then increases the diameter of the confinement sleeve 30a by cooling it below the temperature threshold corresponding to its change of state. This situation is illustrated in the center in Figure 3.

 The internal diameter of the confinement sleeve 30a is then such that this sleeve can be introduced partly on the lower end of the remaining section of the tube 20 and partly on the upper end of the replacement tube section 21, as is illustrated it on the right in figure 3.

 In practice, the contact between the confinement cuff 30a and the ends of the two sections of tubes is a friction contact which makes it possible to maintain the cuff in the zone concerned before welding is carried out. Furthermore, the cuff can firstly be introduced on the lower end of the remaining section of the tube 20 using a suitable tool, after which the replacement tube section 21 is brought into its final position, this which has the effect of making it penetrate into the lower part of the confinement sleeve 30a. The confinement sleeve is then brought to a temperature above the change of state threshold, so that it retracts and is naturally maintained in the desired position to ensure the confinement of the neutral gas when the welding is carried out. This position is illustrated on the right in Figure 3.

 FIG. 4 shows a second embodiment of the invention, in which the confinement member also consists of a cuff 30b. However, in this case, the cuff 30b is made of a metal or a traditional alloy, preferably similar to that which constitutes the tubes 20.

 In this second embodiment of the invention, the sleeve 30b is previously mounted at the upper end of the replacement tube section 21. This assembly is carried out by force fitting.

The upper end of the cuff 30b located above the end of the replacement tube section 21 has a reduced diameter and, for example, a frustoconical shape facilitating the introduction of the assembly formed by the tube section replacement 21 and by the cuff 30b through the corresponding perforation 32 of the tube plate 12.

 It should be noted that the cuff 30b is sufficiently fine to be able to be introduced by the corresponding perforation 32, taking into account the fact that the diameter of the perforations of the tube plate is slightly greater than the outside diameter of the tubes.

 When the upper end of the section of replacement tube 21 carrying the confinement sleeve 30b has been introduced above the tube plate 12 or the spacer plate 27 closest to the location 28 of the cutout of the remaining section of the tube 20 (on the left in FIG. 4), the portion of reduced diameter of the cuff 30b is radially expanded. This radial expansion makes it possible to bring the upper part of the cuff 30b to an inside diameter substantially equal to the outside diameter of the tube 20.

It can be carried out by any means such as a mechanical or hydraulic device.

 When the radial expansion of the upper part of the cuff 30b has been carried out (on the right in FIG. 4), the replacement tube section 21 is put in place, which has the effect of introducing the upper part of the cuff 30b on the lower end of the remaining part of the tube 20.

 According to a third embodiment of the invention illustrated in FIG. 5, the confinement member is no longer constituted by a separate cuff but by a flange 30c, of larger diameter, formed at the upper part of the section of tube replacement 21.

 In this case, the replacement tube section 21 is introduced through the corresponding perforation 32 of the tube plate 12 while it has a uniform diameter over its entire length. When the upper end of the section 21 arrives above the tube plate 12 or the spacer plate closest to the location 28 of the cut formed on the remaining part of the tube 20, the upper part of the tube section replacement 21 is expanded radially, to form the flange 30c whose inner diameter is substantially equal to the outer diameter of the tube 20. This radial expansion can be achieved by any means and in particular by a mechanical or hydraulic device.

 The replacement tube section 21 is then placed in its final position, which has the effect of fitting the collar 30c onto the lower end of the remaining part of the tube 20, as illustrated in FIG. 5.

 As the sleeves 30a and 30b do in the two previous embodiments, the flange 30c confines the neutral gas which is introduced into the tube during welding, which prevents oxidation of the external region of the soldering and guarantees the good behavior thereof.

 Of course, the invention is not limited to the embodiments which have just been described by way of examples, but covers all the variants thereof.

Thus, the embodiment described with reference to FIG. 5 can be reversed, that is to say that it is the lower cut part of the remaining section of the tube 20 which can be expanded before the upper part of the section replacement tube 21 is introduced into the flange thus formed.

Claims (5)

 1. A method of repairing a tube (20) of heat exchanger comprising at least one damaged part close to one end of the tube fixed in a perforation (32) of a tube plate, consisting in: - cutting the tube beyond the damaged part; - extract by said perforation a section of tube  damaged comprising the damaged part and said  end; - introduce by said perforation a section of tube  replacement (21); - weld end to end, inside the tube and under  neutral gas atmosphere, the replacement tube section  cement (21) to a remaining section of the tube (20) characterized in that before welding the replacement tube section (21) to the remaining section of the tube (20), a confinement member ( 30) neutral gas, and said member is placed around a region to be welded.  2. Method according to claim 1, characterized in that a cuff (30a) made of a shape memory material is used as the confinement member, which is introduced by said perforation (32) in a retracted state, wherein the cuff and the tube (20) have substantially the same diameter, which is then passed into an expanded state to allow the introduction into this cuff of adjacent ends of the replacement tube section (21) and the remaining section of the tube, then returned to its retracted state.  3. Method according to claim 1, character  laughed at by the fact that a cuff (30b) is used as the confinement member which is placed on one end of the replacement tube section (21), so that part of the cuff protruding beyond of this end has a reduced diameter, the replacement tube section (21) carrying the cuff (30b) is introduced by said perforation (32), then a radial expansion of said part of the cuff is carried out before introducing it on the remaining section of the tube (20).  4. Method according to claim 1, characterized in that one uses as a confinement member a flange (30c) formed by expansion of one end of the replacement tube section (21) and one proceeds to the expansion radial from this end after having introduced the section of replacement tube by said perforation (32), and before introducing said collar (30c) on the remaining section of the tube (20).  5. Method according to any one of claims 3 and 4, characterized in that one proceeds to said radial expansion after passing through a last extretoise plate (27) before said region to be welded.