EP0547967A1 - Preheated steam generator - Google Patents

Abstract

In a preheated steam generator, in which the secondary water is introduced into a preheating space (34) via extension pieces (52), starting from inverted J-shaped tubes (50) fixed to a semi-toric feed collector (48), it is proposed to make the extension pieces (52) in the form of a deformable structure. This structure can be obtained in particular in the form of three tubular segments which are interconnected by swivel connections. Moreover, the mounting of the extension pieces (52) between the tubes (50) and a plate (54) which partially closes the preheating space (34) towards the top is carried out without welding by demountable fixing members. <IMAGE>

Description

The invention relates to a preheating steam generator designed for use in a pressurized water nuclear reactor.

As illustrated in document FR-A-2-477,265, a preheating steam generator fitted to a nuclear power plant usually comprises an outer envelope, of vertical axis, the interior space of which is divided into two parts, in the direction height, by a horizontal plate called "tubular plate". The ends of the tubes of a bundle of inverted U-shaped tubes are fixed on the tube plate and open below it in an inlet manifold and in a manifold for evacuating the water circulating in the primary circuit of the reactor called "primary water". The water circulating in the secondary circuit of the reactor, called "secondary water" or "edible water", is injected into the part of the steam generator located above the tube plate by a supply manifold of semi-toric shape. placed slightly above an inner envelope covering the bundle of tubes and the lower edge of which is spaced from the tube plate. More specifically, the secondary water leaves the supply manifold by inverted J supply tubes, the lower end of which opens into a preheating space formed below the supply collector, between the inner casing and a skirt. partially surrounding this envelope. This preheating space is located above the primary fluid discharge manifold.

Thanks to this arrangement, the secondary water introduced in the liquid state into the preheating space is gradually preheated as it descends in this space, before it goes back up into the inner envelope, along bundle tubes, to vaporize on contact. The steam thus formed passes through water / steam separation devices located in the upper part of the outer envelope, before leaving the steam generator. The preheating effect of the secondary water in the preheating space is mainly obtained by the water leaving the water / steam separators, called "recirculating water", which descends between the outer envelope and the inner envelope, partly through the preheating space.

In document FR-A-2 644 926, an improvement has been proposed to such a preheated steam generator, aiming to prevent the secondary water leaving the inverted J supply tubes from rising above the collector feed, due to vortices which tended to occur in the upper part of the preheating space. One of the embodiments proposed in order to prevent this rise in secondary water consists in partially obstructing the top of the preheating space by a horizontal plate fixed to the inner casing and in extending the supply tubes in J inverted so that they pass through this plate and open directly below it in the preheating space.

If this solution makes it possible to solve the problem posed by the ascent of the secondary water above the supply manifold, its implementation poses a certain number of problems.

Thus, the upper part of the steam generators, containing the water / steam separators as well as the secondary water supply manifold, is manufactured separately from the lower part containing the bundle of tubes, the inner casing which covers it and the skirt. partially surrounding this inner envelope. The two parts of the outer casing are then welded. The extension of the inverted J supply tube extensions only takes place later, after the hydraulic test on the secondary side of the steam generator. Manholes are provided for this purpose in the outer envelope, to allow the entry of workers inside this envelope.

However, the attachment of tube extenders by welding at this stage of manufacture is hardly acceptable, since it could be the source of polluting deposits on the tube plate.

Furthermore, the two-part manufacturing of the steam generators, combined with the manufacturing tolerances, results in axial misalignments of different values for each assembly formed by an inverted J supply tube and the corresponding perforation formed in the anti-plate -return. The mounting of each of the extenders therefore requires the use of mannequins making it possible to take measurements on each of these sets, in order to adapt each extension to the set which corresponds to it. This results in a significant loss of time, which is hardly acceptable at this stage of manufacture, for which the deadlines are generally very tight.

Finally, it should be noted that these problems which arise during the manufacture of the steam generator are compounded by problems of mechanical strength when the appliance is in operation. Indeed, the differential expansions which occur in particular between the periods of operation of the reactor and the periods of shutdown risk causing ruptures of the extenders.

The subject of the invention is precisely a preheating steam generator, in which the supply of the preheating space from the inverted J supply tubes is carried out by extenders whose original design makes it possible to adapt without loss of time each extension to the configuration encountered during assembly, and to avoid any risk of rupture when using the steam generator.

• - une enveloppe extérieure, d'axe vertical ;
• - une plaque tubulaire horizontale fixée de façon étanche à l'intérieur de l'enveloppe extérieure ;
• - un faisceau de tubes en U inversé, ayant chacun deux extrémités fixées sur la plaque tubulaire et débouchant en dessous de cette dernière, respectivement dans un collecteur d'admission et dans un collecteur d'évacuation de fluide primaire ;
• - une enveloppe intérieure coiffant le faisceau de tubes et dont un bord inférieur est espacé de la plaque tubulaire ;
• - une jupe entourant en partie l'enveloppe intérieure et délimitant avec cette dernière un espace de préchauffage partiellement fermé vers le haut par une plaque anti-retour ;
• - des moyens d'alimentation de l'espace de préchauffage en eau secondaire, comprenant un collecteur d'alimentation placé au-dessus de l'espace de préchauffage, des tubes d'alimentation en J inversé fixé sur le collecteur d'alimentation, et des prolongateurs raccordés sur les tubes d'alimentation et sur la plaque anti-retour ; et
• - des moyens d'extraction de vapeur secondaire au travers de parties hautes des enveloppes intérieure et extérieure ;

caractérisé par le fait que chacun des prolongateurs présente une structure déformable.According to the invention, this result is obtained by means of a preheating steam generator comprising:

• - an outer casing, with a vertical axis;
• - a horizontal tubular plate tightly fixed inside the outer envelope;
• - A bundle of inverted U-shaped tubes, each having two ends fixed on the tube plate and opening below the latter, respectively in an intake manifold and in a primary fluid discharge manifold;
• - an inner envelope covering the bundle of tubes and of which a lower edge is spaced from the tube plate;
• - a skirt partially surrounding the inner envelope and delimiting with the latter a preheating space partially closed upwards by a non-return plate;
• means for supplying the preheating space with secondary water, comprising a supply manifold placed above the preheating space, inverted J supply tubes fixed to the supply manifold, and extenders connected to the supply tubes and to the non-return plate; and
• - means for extracting secondary steam through the upper parts of the inner and outer envelopes;

characterized in that each of the extensions has a deformable structure.

By giving each of the extenders a deformable structure, it becomes possible to take into account without difficulty when mounting the extenders the particular configuration encountered, without the need to use mannequins or to perform prior measurements. In addition, the deformable nature of the extenders allows them to withstand the differential expansions that can occur between the secondary water supply manifold and the non-return plate when the steam generator is in service.

Furthermore, in order to avoid any welding during assembly, each of the extenders is preferably mounted between one of the supply tubes and the non-return plate so that a first end of the extension is able to slide according to its axis and that a second end of the extension is fixed by removable mechanical fixing means.

In this case, the first end of the extender is advantageously a high end on which is fixed a sleeve slidably received on the supply tube. The second end of the extension is then a lower end, connected by removable fixing means on a nozzle, preferably divergent, passing through the non-return plate and fixed to the latter.

In a preferred embodiment of the invention, the removable fixing means comprise
Advantageously, a screw connection, of which a rotary element is trapped on a beaten collar formed at the second end of the extension.

The deformable nature of the extenders can in particular be obtained by producing each of them using at least three tubular segments, connected to each other by swivel joints.

In order to avoid a possible space problem for the ball joint fittings closest to the supply tubes, these fittings are offset in height between the adjacent extensions.

• - la figure 1 est une vue de côté illustrant schématiquement, en coupe verticale partielle et à l'exception de la partie haute, un générateur de vapeur réalisé conformément à l'invention ;
• - la figure 2 est une vue en perspective, partiellement en coupe, représentant à plus grande échelle la partie centrale du générateur de vapeur par laquelle l'eau secondaire pénètre dans ce dernier, puis est introduite dans l'espace de préchauffage ;
• - la figure 3 est une vue à plus grande échelle illustrant en coupe partielle l'un des prolongateurs raccordant l'extrémité d'un tube d'alimentation en J inversé à la plaque anti-retour ; et
• - la figure 4 est une vue en coupe représentant encore à plus grande échelle la partie inférieure d'un prolongateur, raccordée sur la plaque anti-retour.

A preferred embodiment of the invention will now be described, by way of nonlimiting example, with reference to the appended drawings, in which:

• - Figure 1 is a side view schematically illustrating, in partial vertical section and with the exception of the upper part, a steam generator produced in accordance with the invention;
• - Figure 2 is a perspective view, partially in section, showing on a larger scale the central part of the steam generator through which the secondary water enters the latter, then is introduced into the preheating space;
• - Figure 3 is an enlarged view illustrating in partial section one of the extenders connecting the end of an inverted J supply tube to the non-return plate; and
• - Figure 4 is a sectional view showing still on a larger scale the lower part of an extension, connected to the non-return plate.

In FIG. 1, the reference 10 designates the outer envelope of revolution, of vertical axis, of a steam generator intended to ensure the transfer of heat between the primary water circuit and the secondary water / steam circuit of a pressurized water nuclear reactor. This envelope 10 delimits an enclosed interior space which is separated into a primary lower zone and a secondary upper zone by a horizontal tubular plate 12 tightly connected to the envelope 10.

A vertical partition 14 divides the primary lower zone, usually called a water box, into an intake manifold 16 and an evacuation manifold 18 of the water circulating in the primary circuit of the reactor. Tubing 20 and 22, welded to the outer casing 10 of the steam generator, connect the collectors 16 and 18 respectively to this primary circuit.

A bundle of inverted U-shaped tubes 24 is tightly connected to the tube plate 12, in the secondary upper zone delimited by the latter, so that the two ends of each of the tubes open respectively into the intake manifold 16 and in the exhaust manifold 18.

The bundle of tubes 24 is surrounded and capped by an inner envelope 26, arranged coaxially in the outer envelope 10. The horizontal upper wall of this inner envelope 26 is crossed by water / steam separators 28 which communicate the space 27 formed inside the envelope 26 with a steam outlet pipe (not shown) located at the top of the outer envelope 10. The lower edge of the inner envelope 26 is placed at a determined distance above the tube plate 12, so as to form a passage between the annular space delimited between the envelopes 10 and 26 and the space 27 inside the envelope 26.

In addition, the steam generator according to the invention comprises a skirt 30, of semi-circular section, which surrounds the part of the inner casing 26 situated vertically from the exhaust manifold 18 and in which the descending branches are placed. , called cold branches, tubes 24. More precisely, the skirt 30 is terminated at each of its circumferential ends by two radial partitions 30a (FIG. 2) which are tightly connected to the inner casing 26. The skirt 30 extends over most of the height of the inner envelope 26 and its lower edge is connected to the tubular plate 12 by a semi-sealed connection 32.

There is thus defined between the skirt 30 and the inner casing 26 a preheating space 34, open upwards and communicating with the inner space 27 by the passage formed between the lower edge of the inner casing 26 and the tube plate 12 .

The upper part of the skirt 30 has a frustoconical shape flared upward, in order to remain parallel to the outer casing 10, which has at this level a frustoconical part used to connect the cylindrical lower part to the upper part which is also cylindrical but of larger diameter, of this envelope.

A perforated horizontal plate 36 is connected both to the inner casing 26 and to the skirt 30 in the lower part of the space 34, in order to control the rate of admission of the secondary water into the space 27.

In the lower part of the space 27 formed in the inner envelope 26, a vertical plate 38 is placed in the extension of the partition 14, to separate the two branches of the tubes 24 from the bundle. The plate 38 is mounted on the tubular plate 12 by a semi-sealed connection 40 and it rises approximately halfway up in the space 27. This plate 38 makes it possible to channel the secondary water leaving the preheating space 34 along the cold branches of the bundle of tubes 24, when this water enters the space 27. It thus guarantees sufficient preheating of the secondary water before the latter flows into the entire interior space 27 , around the tubes 24.

In the lower part of the space 27 and on either side of the vertical plate 38 is placed a horizontal distribution plate 42, which is traversed by the tubes 24 and perforated so as to ensure a substantially uniform radial distribution of the speeds secondary water flow from bottom to top in space 27.

The secondary water is introduced into the secondary upper zone of the steam generator by a tube 44 tightly connected to the outer casing 10, in an upper part with a larger diameter of the latter situated at a level between the upper edge of the skirt 30 and the upper partition of the inner casing 26. This tubing 44 is extended inside the casing 10 by a pipe 46 which opens into a semi-toric supply manifold 48, located above of the supply space 34, as illustrated in particular in FIG. 2.

Supply tubes 50 in the form of an inverted J are connected at the top of the semi-toric collector 48 and descend in a substantially vertical direction outside this collector. The lower end of each of the supply tubes 50 is located at a level below the bottom of the manifold 48 and connected by an extension 52 to a non-return plate 54. This plate 54 is a horizontal plate, fixed in a sealed manner to the upper edge of the skirt 30 and extending above the preheating space 34 over the entire circumferential length of this space, that is to say approximately on a semicircle. This plate 54 has a uniform width, at most equal to about half the width of the preheating space 34 at the upper edge of the skirt 30.

The general operation of this steam generator is similar to that of existing steam generators.

Thus, the water of the primary circuit flows from the intake manifold 16 to the discharge manifold 18 passing through the tubes 24. Simultaneously, the secondary water admitted by the tubing 44 is injected at the top of the preheating space 34 by the supply tubes 50 through the extenders 52 and descends to the bottom of the space 34, in which it is preheated by the recirculating water leaving the water / steam separators 28. The preheated food water then circulates from bottom to top in space 27, gradually spreading over the entire section of the latter, until it reaches its vaporization temperature. The vapor recovered in the upper part of space 27 then passes through the water / vapor separators before being evacuated by the tube formed in the dome of the outer envelope 10.

The structure of the extenders 52 by which the supply tubes 50 are connected to the non-return plate 54 will now be described in detail with reference to FIGS. 2 to 4.

In order to solve the assembly problems as well as the problems linked to the differential expansion which occurs between the different elements of the steam generator after the start-up of the nuclear power station, instead of having a rigid structure, each of the extenders 52 has a deformable structure. In addition, the mounting of the extenders 52 between the supply tubes 50 and the non-return plate 54 is carried out without welding, to avoid any pollution of the secondary part of the steam generator.

More precisely and as illustrated in particular in FIG. 3, each of the extenders 52 is made up of three tubular segments 52a, 52b and 52c, connected together by two swivel connections 56a and 56b.

The upper segment 52a of each extension 52, of short length, is substantially rectilinear and nested, before its installation inside the steam generator, in a sleeve 58 which is welded on this section 52a and extends beyond from its end, so that it can slide into the bottom end of any of the supply tubes 50 in the form of an inverted J. The internal diameter of the sleeve 58 is dimensioned so as to allow easy mounting on the end of the supply tube 50, while reducing as much as possible the risk of leakage at this level.

When the extension 52 is put in place, this arrangement makes it possible to place the upper section 52a in alignment with the descending branch of the corresponding supply tube 50, that is to say in a substantially vertical direction, and allows a relative axial displacement between the section 52a and the supply tube 50, when differential expansions occur inside the steam generator.

The lower section 52c of each of the extenders 52 is also a straight section of short length. Like the upper section 52a, it is designed to be oriented substantially vertically inside the steam generator, after it has been fixed to the non-return plate 54.

As illustrated more precisely in FIG. 4, this lower section 52c is terminated at its lower end by a beaten collar 60 serving to retain around the section 52c a rotating ring 62 which constitutes the nut element of a screwed connection 64 by which the lower end of the extension 52 is fixed to the non-return plate 54. To this end, the rotary ring 62 comprises, in a part capable of projecting below the flanged collar 60, an internal thread 66 capable of being screwed onto a thread 68 formed at the upper end of a tubular end piece 70 fixed to the non-return plate 54, by a weld 72, before the assembly of the lower and upper parts of the steam generator. This tubular end piece 70 has an upper part 70a, of substantially vertical axis, which passes through the non-return plate 54 by extending downward the lower tubular segment 52c of the extension, and a divergent bottom part 70b (FIG. 3), of which l he axis is oriented radially inward of the steam generator.

The deformable nature of the structure formed by each of the extensions 52, linked to the presence of the ball joints 56a and 56b, as well as the mounting of the extension between the supply tube 50 and the non-return plate 54 respectively using the sleeve sliding 58 and screwed connection 64, allow the extenders to be mounted without welding and whatever the misalignment which exists between the lower end of the supply tube and the upper end of the corresponding end piece 70. To this end, the operator first puts on the sleeve 58 on the lower end of the supply tube 50, then he fixes the lower end of the extender on the end piece 70 using the screw connection 64.

The intermediate section 52b of each of the extensions 52 constitutes the longest part of the latter and has, after assembly, an inclination and an orientation which vary from one extension to the other, according to the misalignment which exists between each supply tube 50-nozzle 70 assembly.

The swivel connections 56a and 56b are identical and, as illustrated in FIG. 4, for the connection 56b, each of them is advantageously provided with an internal sealing bellows 74 preventing any risk of leakage and a deflector tubular 76 placed inside this bellows and fixed to the section of the extension situated above the ball joint considered, that is to say respectively to sections 52a and 52b for the ball joints 56a and 56b.

Each of the swivel connections 56a and 56b allows a relative angular movement between the sections of extenders which they connect, in all directions, of a maximum value which can in particular be 4 °.

As illustrated in particular in Figure 2, the extenders 52 are advantageously of two different types, an extension of a given type being placed between two extenders of the other type.

The only difference between the two types of extension relates to the location of the upper ball joint 56a closest to the supply tubes 50.

In a first type of extension, the upper sections 52a are perfectly straight and the intermediate sections 52b have at each of their ends a curved part.

On the contrary, the second type of extenders 52 comprises an upper section 52a formed by a rectilinear part extended downwards by a curved part, while the intermediate section 52b comprises a curved part only at its lower end.

This difference between the two types of extension 52 has the effect of shifting the swivel connections 56a in height between the adjacent extensions. This arrangement makes it possible to avoid a space problem arising in the case where the lower ends of the adjacent supply tubes 50 are too close to allow the side-by-side installation of two swivel fittings.

The above description clearly shows the advantages provided by the particular structure of the extenders 52 according to the invention. In particular, it is understood that the installation of these extenders after the welding connection of the upper and lower parts of the outer casing 10 of the steam generator, carrying respectively the supply manifold 48 provided with supply tubes 50 and the non-return plate 54 fitted with end caps 70, is carried out, without any welding, in a particularly rapid manner and which makes it possible to take into account the variable misalignments which necessarily exist between the lower end of each of the tubes supply 50 and the upper end of the end piece 70 which corresponds to it.

In addition, the original structure of the extenders 52 according to the invention optionally allows disassembly and reassembly of some of these extenders during the life of the steam generator for reasons of accessibility to the final seal of the steam generator in particular, this which was hardly possible in the past (welding operations).

Finally, and as already indicated, this structure allows the extenders 52 to withstand all the differential expansions which occur in the steam generator during its life.

Of course, the invention is not limited to the embodiment which has just been described by way of example, but covers all its variants. Thus, the deformability of the extenders can be obtained in a different way from that which has been described, for example by increasing the number of sections constituting the extenders as well as the number of ball joints connecting these sections, or by producing the extenders at less in part in the form of a set of rings placed end to end and articulated on each other to form a continuously deformable tube. Furthermore, the means of mounting the extenders between the supply tubes and the end pieces fixed on the non-return plate can be different and, for example, reversed with respect to those which have been described, the screw connection being able to be placed between the supply tube and the extension while the sliding sleeve would be formed by the upper part of the nozzle fixed on the non-return plate. The screw connection can also be replaced by any equivalent removable fixing means such as a bayonet fixing system, allowing rapid mounting of the extensions.

Claims (8)

1. Preheating steam generator comprising: - an outer casing (10), of vertical axis; - a horizontal tubular plate (12) sealed in the interior of the outer envelope; - a bundle of inverted U-shaped tubes (24), each having two ends fixed on the tube plate and opening out below the latter, respectively in an intake manifold (16) and in an evacuation manifold (18) of primary fluid; - an inner casing (26) covering the bundle of tubes and of which a lower edge is spaced from the tube plate; - a skirt (30) partially surrounding the inner envelope and delimiting with the latter a preheating space (34) partially closed upwards by a non-return plate (54); - means for supplying the preheating space with secondary water, comprising a supply manifold (48) placed above the preheating space, inverted J supply tubes (50) fixed on the supply manifold, and extenders (52) connected to the supply tubes and to the non-return plate; and - means for extracting secondary steam (28) through upper parts of the inner and outer envelopes; characterized in that each of the extensions (52) has a deformable structure. 2. Steam generator according to claim 1, characterized in that each of the extenders (52) is mounted between one of the supply tubes (50) and the non-return plate (54) so that a first end of the extension is able to slide along its axis and a second end of the extension is fixed by removable fixing means (64). 3. Steam generator according to claim 2, characterized in that the first end of the extender is a high end on which is fixed a sleeve (58) slidably received on the supply tube. 4. Steam generator according to any one claims 2 and 3, characterized in that the second end of the extension is a low end, connected by said removable fixing means (64) on a nozzle (70) passing through the non-return plate (54) and fixed to this last. 5. Steam generator according to claim 4, characterized in that the end piece (70) is divergent. 6. Steam generator according to any one of claims 2 to 5, characterized in that the removable fixing means comprise a screw connection (64), of which a rotary element (62) is trapped on a beaten collar (60) formed at the second end of the extender. 7. Steam generator according to any one of the preceding claims, characterized in that each of the extenders (52) comprises at least three tubular segments (52a, 52b, 52c), interconnected by ball joints (56a, 56b ). 8. Steam generator according to claim 7, characterized in that the ball joints (56a) closest to the supply tubes (50) are offset in height between the adjacent extenders.

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