EP0566477A1 - Steam generator with a system for water deflexion and for removing sludge - Google Patents

Abstract

In a steam generator which is fitted to a nuclear power station and includes inverted U-tubes connected onto a tube plate (12) and surrounded by an inner wrapper (26) which accommodates a lower flow distribution plate (41) in its bottom, the space (46) termed the "tube lane" between the tube plate (12), the flow distribution plate (41) and the hot and cold branches of the tubes, contains a deflection and blowdown (vent) box (44). This box (44) extends over the entire length of the tube lane and, in height, as far as the vicinity of the upper edge of the inner wrapper (26). It is provided on its lower face with vent orifices (50) and is both used for deflection and draining secondary fluid in the vicinity of the tube plate (12). <IMAGE>

Description

The invention relates to a steam generator designed to be used in a pressurized water nuclear reactor and the secondary part of which is fitted, in the immediate vicinity of the tube plate, with a deflection and purge device of original design.

As illustrated in particular in document FR-A-2 333 200, a steam generator equipping 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 of the height, by a horizontal plate called "tube plate" (in English, "tube sheet"). The ends of the tubes of an inverted U-shaped tube bundle are fixed on the tube plate and open below it respectively in an intake manifold and in a collector for evacuating the water circulating in the circuit. reactor primary, called "primary water". The water circulating in the secondary circuit of the steam generator, called "secondary water" is a mixture of water called "food water" coming from outside the steam generator and water called "recirculated water coming from the part upper part of the steam generator. "Food water" is injected into the part of the steam generator located above the tube plate by a supply distributor, in toric or semi-toric form. supply is usually placed above an annular recirculation space formed between the outer casing and an inner casing covering the tube bundle and the lower edge of which is spaced from the tube plate.

In such a steam generator, it is essential to prevent secondary water from stagnating in certain areas adjacent to the tube plate as much as possible. In fact, this water can contain sludge, the deposition of which on the tube plate, around the tubes of the bundle, can lead to rapid corrosion of these tubes necessitating their sealing. When too many steam generator tubes become inoperative, it becomes necessary to replace the entire steam generator. This is obviously not desirable both from the point of view of costs and with regard to the downtime of the nuclear power plant which results therefrom.

In particular in order to limit these zones of stagnation of secondary water near the tube plate, there is usually available in the lower part of the inner envelope surrounding, the tube bundle, a lower horizontal flow distribution plate comprising a opening in its central part.

Another problem which arises in the area of the steam generator situated immediately above the tube plate relates to the homogenization of the temperature of the mixture between the drinking water introduced into the generator by its supply manifold and the recirculation water from the water / steam separators placed in the upper part of the generator and which descends through the annular recirculation space. It is also desirable to ensure good regulation of the flow rates of this mixture before it rises in the inner envelope around the bundle tubes.

This problem is generally solved by placing on the tube plate, in the space, usually called "water street", formed between this plate, the flow distribution plate and the hot and cold branches of the tubes of the deflector devices. shape of profiles (in English, "tube lane blocking devi- ce").

In addition, it is also desirable to provide a drain immediately above the tube plate, in order to allow a withdrawal of the secondary fluid in this zone and to purge mainly in the zones where the flow of the secondary fluid takes place at lower speed, especially in the center of the tube plate.

To fulfill this function, there are usually in the water street two pipes pierced with holes in the lower part and connected to a purge circuit located outside the steam generator. This purge circuit makes it possible to check the chemical concentrations of the secondary fluid, in order to maintain them at acceptable values. It also allows the suspended solids contained in the secondary fluid to be removed near the tube plate.

By way of example, document US-A-4 664 178 describes a steam generator equipped with purge piping near the tube plate and comprising L-shaped deflection means mounted above the purge piping, at the base of the inner casing covering the tube bundle.

If the deflection and purge devices used until now function satisfactorily, they have the disadvantage of requiring the addition inside the steam generator of a relatively large number of separate parts, which complicates the mounting the generator, increases the cost price and complicates maintenance.

The invention specifically relates to a steam generator with inverted U tubes, comprising deflection and purging means of a new type, consisting of a single structure simultaneously fulfilling these two functions.

• - une enveloppe extérieure, d'axe vertical ;
• - une plaque à tubes horizontale fixée de façon étanche à l'intérieur de l'enveloppe extérieure ;
• - un faisceau de tubes en U inversé comprenant des branches chaudes et des branches froides ayant des extrémités fixées sur la plaque à tubes 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, dont un bord inférieur est espacé de la plaque à tubes, et formant avec l'enveloppe extérieure un espace annulaire de recirculation ;
• - des moyens d'alimentation en eau secondaire débouchant dans l'enveloppe extérieure, au-dessus de la plaque à tubes ;
• - des moyens d'extraction de vapeur secondaire au travers de parties hautes des enveloppes intérieure et extérieure ; et
• - des moyens de déflection et de purge placés dans un espace formé entre la plaque à tubes, la première plaque horizontale et les branches chaudes et les branches froides des tubes ;

caractérisé par le fait que les moyens de déflection et de purge comprennent une (ou plusieurs) boîte de déflection et de purge qui s'étend sur toute (ou une partie de) la longueur dudit espace, jusqu'à proximité de l'enveloppe intérieure, et qui s'étend en hauteur jusqu'à proximité du bord inférieur de l'enveloppe intérieure, ladite boîte étant espacée de la plaque à tubes et présentant des orifices de purge en regard de cette dernière.More specifically, according to the invention, a steam generator is proposed comprising:

• - an outer casing, with a vertical axis;
• - a horizontal tube plate tightly fixed inside the outer envelope;
• - a bundle of inverted U-shaped tubes comprising hot branches and cold branches having ends fixed to the cover plate bes and opening below the latter, respectively into an intake manifold and into a primary fluid discharge manifold;
• an inner envelope covering the bundle of tubes, a lower edge of which is spaced from the tube plate, and forming with the outer envelope an annular recirculation space;
• - secondary water supply means opening into the outer envelope, above the tube plate;
• - means for extracting secondary steam through the upper parts of the inner and outer envelopes; and
• deflection and purging means placed in a space formed between the tube plate, the first horizontal plate and the hot branches and the cold branches of the tubes;

characterized in that the deflection and purge means comprise one (or more) deflection and purge box which extends over all (or part of) the length of said space, up to the proximity of the inner envelope , and which extends in height up to near the lower edge of the inner casing, said box being spaced from the tube plate and having bleed orifices facing the latter.

Preferably, the deflection and purge box extends in height up to an intermediate level between the flow distribution plate and the lower edge of the inner casing. In addition, manholes formed in the latter as well as in the outer casing of the steam generator open immediately above the longitudinal ends of the deflection and purge box.

The deflection and purge box according to the invention is connected to drainage means by at least one drainage tube which opens at at least one of the ends of the box, at the bottom of the latter.

In a preferred embodiment of the invention, at least two drainage tubes open laterally into a central part of the deflection and purge box, on either side of this box, which has orifices facing the tube plate. This feature improves the efficiency of drainage in the central part of the steam generator located immediately above the tube plate.

According to a preferred embodiment of the invention, the deflection and purge box has a bottom forming a longitudinal recovery groove into which the purge orifices open.

Depending on the case, the deflection and purge box can have a uniform cross-section over its entire length or, on the contrary, can change.

The invention applies equally to all types of tube plate steam generators and in particular to steam generators of the "boiler" type, such as to steam generators of the "axial preheater" type.

More precisely, in the case of steam generators of the "boiler" type, devoid of any separation between the hot and cold branches of the tubes, the deflection and purge box rests on the tube plate by means of support studs. between which the drain holes are formed.

Furthermore, in steam generators of the "axial preheater" type, a vertical separation plate is arranged between the hot and cold branches of the bundle tubes and connected to the tube plate. In this case, the deflection and purge box comprises two parts located on either side of the separation plate and supported by this plate.

• - la figure 1 est une vue de côté illustrant schématiquement, en coupe verticale partielle, un générateur de vapeur de type bouilleur réalisé conformément à l'invention ;
• - la figure 2 est une vue en coupe horizontale à plus grande échelle selon la ligne II-II de la figure 1 ;
• - la figure 3 est une vue en coupe à plus grande échelle selon la ligne III-III de la figure 2 ;
• - la figure 4 est une vue en coupe selon la ligne IV-IV de la figure 3 ;
• - la figure 5 est une vue en coupe comparable à la figure 4 illustrant un deuxième mode de réalisation de l'invention, dans le cas d'un générateur de vapeur de type préchauffeur axial ; et
• - la figure 6 est une vue en perspective écorchée représentant la boîte de déflection et de purge illustrée sur la figure 5.

Two preferred embodiments of the invention will now be described, by way of non-limiting examples, with reference to the appended drawings, in which:

• - Figure 1 is a side view schematically illustrating, in partial vertical section, a boiler type steam generator produced in accordance with the invention;
• - Figure 2 is a horizontal sectional view on a larger scale along the line II-II of Figure 1;
• - Figure 3 is a sectional view on a larger scale along the line III-III of Figure 2;
• - Figure 4 is a sectional view along line IV-IV of Figure 3;
• - Figure 5 is a sectional view comparable to Figure 4 illustrating a second embodiment of the invention, in the case of a steam generator of the axial preheater type; and
• FIG. 6 is a cutaway perspective view showing the deflection and purge box illustrated in FIG. 5.

In Figure 1, there is shown schematically a steam generator of the boiler type illustrating a first possible embodiment of the invention. This steam generator is designed to transfer heat between the primary water circuit and the secondary water / steam circuit of a pressurized water nuclear reactor.

In FIG. 1, the reference 10 designates the outer envelope of revolution, of vertical axis, of the steam generator. This envelope 10 delimits an enclosed interior space which is separated into a primary lower zone and a secondary upper zone by a horizontal tube 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 discharge manifold 18. The vertical branches 24a of the tubes 24 which open into the intake manifold 16 are called hot branches and the vertical branches 24b of the tubes 24 which open into the discharge manifold 18 are called cold branches .

The bundle of tubes 24 is surrounded and capped by an inner casing 26, arranged coaxially in the outer casing 10. The upper horizontal wall of this inner casing 26 opens into water / steam separators 28 surmounted by dryers 29 which communicate space 27 formed inside the envelope 26 with a steam outlet pipe 30 located at the top of the outer envelope 10. The lower edge of the inner envelope 26 is placed at a determined distance above of the tube plate 12, so as to form a passage between an annular recirculation space 32 delimited between the envelopes 10 and 26 and the space 27 delimited in the interior envelope 26.

The supply of the steam generator with secondary water is carried out by a toroidal feed distributor 38 located immediately above the annular recirculation space 32. A feed pipe 34 sealingly passes through the outer casing 10 of the steam generator and opens into the feed distributor 38. The latter can in particular be connected to the annular recirculation space 32 by tubes 36 in the form of an inverted J.

Horizontal flow distribution plates 40, regularly spaced, are mounted inside the inner casing 26. These plates have perforations making it possible to support the tubes 24 of the bundle over their entire height and to achieve distribution radial as homogeneous as possible of the secondary water flow which rises in the envelope 26.

The lower flow distribution plate, designated by the reference 41 in FIG. 1, is situated slightly above the lower edge of the inner casing 26. It is distinguished from the other plates 40 by the fact that it has an opening central 42 of relatively large section. This lower plate 41 thus makes it possible to prevent the secondary water descending through the annular recirculation space 32 from rising immediately as soon as it has crossed the lower edge of the inner envelope 26, which would in particular have the consequence of creating above the central part of the tube plate 12 a static area.

According to the invention, a deflection and purge box 44 is mounted in the space formed between the tube plate 12, the lower flow distribution plate 41 and the hot 24a and cold 24b branches of the tubes 24. This space , usually called water street, is designated by the reference 46 in Figure 4.

As illustrated in more detail in FIGS. 2 to 4, the deflection and purge box 44 extends horizontally and radially over the entire length of the space 46, up to the proximity of the inner envelope 26. In the embodiment illustrated in FIGS. 1 to 4, which corresponds to a steam generator of the boiler type, in which no separation is provided between the hot and cold branches of the tubes 24, the deflection and purge box 44 rests on the tube plate 12 by means of support studs 48 which guarantee a spacing between the bottom of the box and the upper face of the tube plate.

This spacing allows purge orifices 50 formed in the bottom of the box 44 to drain inside this box the secondary fluid which is in the immediate vicinity of the tube plate 12 in the space 46. For this purpose , the purge orifices 50 open out between the support pads 48. In addition, they are advantageously more numerous and / or of larger section in the central part of the steam generator than in the vicinity of the outer casing 10, in order to favor drainage in this central part, in which the displacements of the secondary fluid are the smallest.

In addition, at least one vent 72 (Figures 3 and 4) is formed on the roof of the deflection box 44, in the central part of the steam generator. This vent ensures the penetration of the secondary fluid into the box 44 when the steam generator is put into water.

As illustrated in particular in FIGS. 3 and 4, in the embodiment shown, the deflection and purge box 44 has a substantially rectangular shape in cross section and extends in height from the support pads 48 to a level lower than that of the lower flow distribution plate 41 and higher than that of the lower edge of the inner casing 26. The box 44 thus provides a deflector function, that is to say that it constitutes a obstacle which opposes a horizontal flow of the secondary fluid in a transverse direction relative to this box. The deflection and purge box 44 thus fulfills the functions of homogenizing the temperature of the secondary fluid before it rises in space 27 and contributes to a good distribution of the flow rates of this fluid near the tube plate 12.

In addition to this function, which is the same as that of the deflection or blocking devices of the prior art, there is a purge function carried out thanks to the purge orifices 50 and to drainage tubes 52 (FIG. 3) opening into the bottom of the box 44, at each end of the latter. Each of the drainage tubes 52 connects the box 44 to external drainage means 54 to the steam generator, illustrated diagrammatically in FIG. 2.

The drainage means 54 make it possible to pump a small quantity of secondary fluid out of the steam generator, in order to check the chemical concentrations of this fluid to maintain them at an acceptable level and to remove the suspended matter which tends to settle in space 46 on the tube plate 12.

To facilitate the flow of the secondary fluid inside the deflection and purge box 44 up to the drainage tubes 52, the bottom of the box 44 forms a horizontal recovery groove 55, in the bottom of which open the drain holes 50. The groove 55 has for example, as illustrated in FIG. 4, a V-shaped section. This shape can in particular be obtained by machining the upper face of a rail 56 forming the bottom of the box 44 and by which the latter rests on the support pads 48. The rest of the box 44 can be constituted by a plate 58 folded to form a U whose ends are welded to the lateral faces of the rail 56. The ends of the box 44 are also formed by plates welded to the plate 58 and to the rail 56. The drainage tubes 52 are welded to the latter plates, so as to open into the bottom of the V machined on the upper face of the rail 56.

It should be noted that this particular structure of the deflection and purge box 44 is given only by way of example, the shape and the method of manufacture of this box being able to undergo numerous variants without departing from the scope of the invention. 'invention.

Thus, the substantially rectangular shape presented in section by the box 44 in the embodiment shown can optionally be replaced by a square, triangular or other shape. Furthermore, it should be noted that instead of having a constant cross section as illustrated in solid lines in FIG. 3, the box 44 can also have an evolving cross section as illustrated for example by mixed lines. This evolving section can in particular lead to reducing the height of the box in the central part of the steam generator, compared to the height presented by this box near the inner casing 26. In this way, the efficiency of the samples of the secondary fluid which are carried out in the central part of the space 46.

In addition and as illustrated in particular in FIG. 2, the withdrawals in the central part of the steam generator can be further improved by adding to the deflection and purge box 44 two purge tubes 60 which are connected to each of the lateral faces. of the box 44, in the central part of the latter and also comprise, at the bottom, purge orifices 62 turned towards the tube plate 12.

The geometry of the deflection and purge box 44 is also designed to take account of the requirements linked to the maintenance operations which must be able to be carried out on the steam generator, at the base of the bundle of tubes 24. Thus, like the illustrates Figure 3, the upper wall of the box 44 is located substantially at the same level as the lower edge of inspection holes 64 formed in the inner casing 26 of the steam generator. These inspection holes 64 are themselves formed opposite inspection holes 66 provided in the outer casing 10 of the generator and normally closed by plugs (not shown). As illustrated in FIG. 3, the inspection holes 64 and 66 are located in the extension of the space 46, in order to allow operators to access between the hot and cold branches of the tubes 24 of the bundle.

As illustrated in FIGS. 5 and 6, the invention is not limited to steam generators of the boiler type, but also applies to steam generators of the axial preheater type, in which a vertical separation plate 68 is disposed between the hot 24a and cold 24b branches of the tubes 24 and connected to the tube plate 12. This connection can in particular be ensured by fitting a profile 69 in inverted U forming the lower edge of the separation plate 68 on a rib 13 welded on or mechanically fixed to the upper face of the tube plate 12, in the axis of the water street.

In this case, the deflection and purge box 44 comprises two compartments 45, located on either side of the plate 68 and arranged symmetrically with respect to the latter. As illustrated in FIGS. 5 and 6, these two compartments 45 can be delimited by two plates 70 folded and welded on either side of the separation plate 68, to define two separate compartments.

Given the separation of the two compartments 45 by the plate 68, each of them comprises, in the bottom of the box 44, purge orifices 50 whose section and / or number decreases from the center of the generator vapor towards its periphery. As illustrated in FIGS. 5 and 6, the bottom of each of the compartments 45, in which the purge orifices 50 are made, is inclined downward as it approaches the separation plate 68, in order to promote the drainage of the fluid secondary. Furthermore, the roof of each of the compartments 45 is inclined upwards as it approaches the separation plate 68, in order to curve the flow upwards. secondary fluid.

As seen in FIG. 6, each of the compartments 45 of the deflection and purge box communicates with drainage means (not shown) situated outside the steam generator by a separate drainage tube 52, at each ends of the box 44.

As in the first embodiment of the invention, the roof of each of the compartments of the deflection and purge box 44 comprises, in the central part of the steam generator, at least one vent 72 (FIG. 5) ensuring penetration secondary fluid in the box 44 through the purge orifices 50 when the steam generator is put into water.

As shown in FIG. 6, the steam generator in this case comprises a semi-cylindrical intermediate envelope 74 which externally doubles the internal envelope 26, in the part of the latter surrounding the cold branches of the tubes 24. The feed toroid 38 of FIG. 1 then has a semi-circular shape, in order to supply with potable water only the space formed between the inner envelope 26 and the intermediate envelope 74. As illustrated schematically in the figure 6, the ends of the box 44 can then be closed off by the intermediate envelope 74.

In the embodiment illustrated in FIGS. 5 and 6, the deflection and purge box 44 fulfills the same functions as in the first embodiment described and moreover has the same characteristics.

Claims (10)

1. Steam generator comprising: - an outer casing (10), of vertical axis; - a horizontal tube plate (12) sealed in the interior of the outer casing; - a bundle of inverted U-shaped tubes (24) comprising hot branches (24a) and cold branches (24b) having ends fixed on the tube plate and opening below the latter, respectively in an intake manifold ( 16) and in a primary fluid discharge manifold (18); - an inner casing (26) covering the bundle of tubes, a lower edge of which is spaced from the tube plate, and forming with the outer casing an annular recirculation space (32); - secondary water supply means opening into the outer casing (10), above the tube plate (12); - Means (28,30) for extracting secondary steam through the upper parts of the inner and outer casings; and - deflection and purge means placed in a space (46) formed between the tube plate (12), the first horizontal plate (40,41) and the hot branches and the cold branches of the tubes;
characterized in that the deflection and purge means comprise one (or more) deflection and purge box (44) which extends over all (or part of) the length of said space, up to near the 'inner casing, and which extends in height up to near the lower edge of the inner casing (26), said box being spaced from the tube plate and having bleed orifices (50) facing the latter . 2. Steam generator according to claim 1, characterized in that the deflection and purge box (44) extends in height up to an intermediate level between the flow distribution plate (41) and the lower edge of the inner casing (26). 3. Steam generator according to claim 2, characterized in that inspection holes (64,66) formed in the inner casing (26) and in the outer casing (10) open immediately above the longitudinal ends of the deflection and purge box (44). 4. Steam generator according to any one of the preceding claims, characterized in that the deflection and purge means comprise drainage means (54) communicating with the deflection and purge box (44) by at least one drainage tube (52) opening at at least one end of the box, at the bottom of the latter. 5. Steam generator according to any one of the preceding claims, characterized in that the deflection and purge means further comprise at least two purge tubes (60) opening laterally into a central part of the deflection box and purge (44), on either side of this box, and having other purge orifices (62) opposite the tube plate (12). 6. Steam generator according to any one of the preceding claims, characterized in that the deflection and purge box (44) has a bottom forming a longitudinal recovery groove (55) into which the purge orifices open (50 ). 7. Steam generator according to any one of the preceding claims, characterized in that the deflection and purge box (44) has a uniform cross section over its entire length. 8. Steam generator according to any one of claims 1 to 6, characterized in that the deflection and purge box (44) has an evolving cross section. 9. Steam generator according to any one of the preceding claims, characterized in that the deflection and purge box (44) rests on the tube plate (12) by means of studs between which the orifices are formed purge (50). 10. Steam generator according to any one of claims 1 to 8, characterized in that, the hot and cold branches of the bundle tubes being separated by a vertical separation plate (68) connected to the tube plate (12 ), the deflection and purge box (44) comprises two parts located on either side of this separation plate and supported by this plate.

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