EP0626536A1 - Steam generator equipped with a trapping device for foreign objects - Google Patents

Abstract

In a nuclear power station steam generator a device (48) is provided for preventing objects originating from the driers (36), following fitting, subsequent intervention or else accidental detachment in operation, from being jammed between the tubes (24) of the generator. This device (48) which, for example, takes the form of a grid, is placed above a horizontal plate (34) to which are secured the top ends of the conduits (32) containing the cyclone separators. It defines passages whose largest dimensions are smaller than the minimum distance separating the tubes (24). <IMAGE>

Description

The invention relates to a steam generator adapted for use in a nuclear power plant.

As illustrated in particular in document FR-A-2 477 265, a steam generator equipping a nuclear power plant usually comprises an envelope with a 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 primary reactor circuit, 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. This edible water vaporizes on contact with the tubes, due to the heat carried by the primary water. The steam thus formed is extracted from the steam generator after having successively passed through cyclone separators placed in substantially vertical pipes, then dryers situated in the upper part of the envelope, above the pipes containing the cyclone separators.

During assembly or maintenance work on such a steam generator, it may happen that certain objects such as welding rods, screws, bolts, etc. are inadvertently introduced in the secondary circuit. It can also happen that certain objects such as screws come loose during operation. When the food water circulates in the secondary circuit of the steam generator, all these objects constitute migrant bodies which risk being trapped between the tubes of the bundle if they reach this part of the steam generator. This then results in a disturbance in the flow of drinking water and damage to the tubes, which can go as far as breaking them.

Part of the migrant bodies which circulate in the secondary circuit of a steam generator comes from the drinking water system and arrives in the steam generator by the inlet pipe of the drinking water. French patent application n ° 91 14900, in the name of the applicant, proposes a device making it possible to trap the migrant bodies thus introduced into the steam generator.

Migrant bodies can also arrive in the tube bundle through the conduits of the cyclone separators connecting the part where the tube bundle is located to the dryers. These migrant bodies coming from the dryer area are either objects such as a screw detaching from a dryer during operation, or objects introduced into the space located between the upper plate of the cyclone separators and the dryers during assembly or maintenance work in the generator and forgotten in the steam generator. However, there is currently no device capable of trapping these migrant bodies, so that these migrant bodies risk falling to the bundle of tubes by crossing the substantially vertical conduits in which the cyclone separators are located.

The subject of the invention is precisely a steam generator comprising a device making it possible to trap the migrant bodies coming from the dryers, in order to prevent them from being able to come to be trapped between the tubes of the bundle.

• une enveloppe, d'axe vertical ;
• une plaque à tubes horizontale fixée de façon étanche à l'intérieur de l'enveloppe ;
• un faisceau de tubes en U inversé, ayant chacun deux 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 ;
• des moyens d'alimentation en eau secondaire débouchant dans l'enveloppe, au-dessus de la plaque à tubes horizontale ; et
• des moyens d'extraction de vapeur secondaire placés dans l'enveloppe, au-dessus du faisceau de tubes, et comprenant des conduites d'évacuation sensiblement verticales placées en dessous de moyens de séchage ;

caractérisé par le fait que des moyens de piégeage de corps migrants sont placés entre les conduites et les moyens de séchage, au moins au-dessus de chacune des conduites.In accordance with the invention, this result is obtained by means of a steam generator comprising:

• an envelope, of vertical axis;
• a horizontal tube plate tightly fixed inside the envelope;
• a bundle of inverted U-shaped tubes, each having two ends fixed on the tube plate and opening out below the latter, respectively in an intake manifold and in a primary fluid discharge manifold;
• secondary water supply means opening into the envelope, above the horizontal tube plate; and
• secondary steam extraction means placed in the envelope, above the bundle of tubes, and comprising substantially vertical exhaust pipes placed below drying means;

characterized in that means for trapping migrant bodies are placed between the pipes and the drying means, at least above each of the pipes.

In a preferred embodiment of the invention, the means for trapping the migrant bodies delimit passages which have a maximum dimension less than the minimum distance separating the tubes from the beam.

Depending on the case, the means for trapping migrant bodies can occupy either substantially the entire section of the envelope of the steam generator, or only be located above each of the pipes.

More specifically, the means for trapping migrant bodies preferably comprise several trapping elements forming slabs, for example square, placed above a horizontal plate to which the upper ends of the conduits are fixed.

Each trapping element may in particular comprise a supporting structure forming a floor, on which a trapping grid is placed. This arrangement makes it possible to use the trapping means as a working floor during assembly or maintenance work.

Each trapping element is advantageously fixed, by removable and preferably captive fixing means such as bolts, on horizontal beams fixed above the above-mentioned horizontal plate and the outlets of the separators.

• la figure 1 est une vue en coupe verticale illustrant schématiquement un générateur de vapeur conforme à l'invention ; et
• la figure 2 est une vue de dessus en perspective, représentant, l'un des éléments de piégeage utilisés conformément à l'invention dans le générateur de vapeur illustré sur la figure 1.

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 vertical sectional view schematically illustrating a steam generator according to the invention; and
• FIG. 2 is a perspective view from above, representing one of the trapping elements used in accordance with the invention in the steam generator illustrated in FIG. 1.

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 tube plate 12 tightly connected to the envelope 10.

A vertical partition 14 divides the primary lower zone, usually called "water box", into an intake manifold 16 and an evacuation manifold 18 of the water flowing in the primary circuit (not shown) 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. More precisely, the two ends of each of the tubes 24 open respectively into the intake manifold 16 and into the exhaust manifold 18. Thus, the primary water admitted into the steam generator through the intake manifold 16 circulates in the tubes 24 before leaving the device through the evacuation manifold 18.

The bundle of tubes 24 is surrounded and capped by an interior envelope 26 arranged coaxially in the exterior envelope 10. The lower edge of the interior envelope 26 is placed at a predetermined distance above the tube plate 12, so to form a passage by which an annular space 28, delimited between the envelopes 10 and 26, communicates with a space 30 inside the envelope 26.

In the embodiment illustrated in Figure 1, drinking water is introduced into the steam generator via a supply line 42 which passes radially through the outer envelope 10 at a level close to the upper part of the inner envelope 26, covering the bundle of tubes 24. This supply line 42 opens into a manifold O-ring 44 centered on the vertical axis of the steam generator and provided over its entire periphery with inverted J-shaped tubes 46 by which the drinking water is injected into the top of the annular space 28.

It should be noted that the invention applies to steam generators of different structures and in particular to steam generators in which the introduction of drinking water takes place directly in the annular space 28, near the tube plate 12.

Drainage pipes 32 substantially vertical are connected by their lower end to the upper part of the inner casing 26, which covers the bundle of tubes 24. The upper end of these drain pipes 32 is welded to a horizontal plate 34 whose peripheral edge is supported by the outer casing 10 of the steam generator. Centrifugal or cyclone separators (not shown) are housed in each of the discharge pipes 32, in order to trap part of the water entrained by the steam, in order to recycle it towards the bottom part of the steam generator. These centrifugal or cyclone separators are usually constituted by fixed fins, in the form of propellers, housed in the evacuation pipes 32.

The steam which reaches the upper part of the outer casing 10 through the discharge pipes 32 then passes through driers 36 placed at a certain distance above the horizontal plate 34, before leaving the steam generator by a secondary steam extraction pipe 38, disposed at the top of the outer casing 10, along the vertical axis of the steam generator. The dryers 36, which can have various shapes, constitute a set of baffles which the steam must pass through before leaving the steam generator. These baffles help dry out the vapor by recycling part of the water entrained by the latter. The recycling water thus recovered, generally called "recirculation water" is brought back to the lower part of the steam generator by at least one substantially vertical pipe 40, which crosses the horizontal plate 34 and the lower end of which is situated at the above the upper part of the inner envelope 26 covering the bundle of tubes 24.

According to the invention, means 48 are provided above the horizontal plate 34 for trapping migrant bodies coming from the dryers 36.

The means 48 for trapping migrant bodies have the main function of retaining objects accidentally falling from the dryers 36, in order to prevent these objects from being able to become trapped between the tubes 24 after passing through the evacuation pipes 32. The objects which may fall out of the dryers 36 may be either objects inadvertently remaining in the dryers following the installation of the steam generator or subsequent intervention work, or objects accidentally detaching from the dryers in operation, such as screws . The trapping means also retain objects that risk accidentally falling during intervention in the upper part of the steam generator.

The means 48 for trapping migrant bodies can also have the function of forming, in the upper part of the steam generator, a work floor facilitating interventions both during assembly work of the steam generator and during maintenance operations.

Finally, the means 48 for trapping migrant bodies can also contribute to improving the efficiency of the dryers 36, by interrupting the rotating movements of the steam at the outlet of the cyclone separators housed in the evacuation pipes 32.

These last two functions are optional, however.

In the embodiment illustrated in FIG. 1, the means 48 for trapping migrant bodies occupy substantially the entire horizontal section of the steam generator, above the horizontal plate 34. Since the migrant bodies cannot descend towards the tubes 24 of the bundle that, through the evacuation conduits 32, the means for trapping migrant bodies may also have a smaller cross-section, and, for example, be formed by separate assemblies placed at the top of each of the conduits d 'Evacuation 32. It is understood, however, that the means 48 for trapping migrant bodies can only constitute an effective working floor if they occupy the major part of the section of the steam generator as illustrated in FIG. 1.

So that the means 48 can effectively fulfill their function of trapping migrant bodies, they are advantageously in the form of a grid, the structure of which opposes as weakly as possible the escape of the steam towards the pipe 38 while guaranteeing the retention of objects whose size implies a risk of jamming between the tubes 24 of the bundle, if they reached the zone 30 inside the envelope 26.

Concretely, this result is obtained by giving the meshes defined by the grid forming the means 48 for trapping migrant bodies a maximum dimension less than the minimum distance separating the tubes 24 from the beam.

In practice, the means 48 for trapping migrant bodies are advantageously formed by the juxtaposition of several trapping elements such as the element 50 in FIG. 2. Each element 50 is then in the form of a slab, for example square, which rests on the horizontal plate 34 by means of beams 52. These beams 52 bear at their ends on the outer casing 10 of the steam generator, in order to give the structure a sufficiently rigid character so that it constitutes a floor capable of supporting one or more people during assembly work or maintenance interventions.

Each of the trapping elements 50 is advantageously fixed to the beams 52 in a removable manner, for example using screws or studs 54 preferably made captive.

In the embodiment illustrated in FIG. 2, each trapping element 50 is formed of a support structure 56 forming a floor and a trapping grid 58 resting on the support structure 56.

More specifically, the support structure 56 consists of a frame in which is fixed a large mesh grid giving the structure 56 sufficient mechanical rigidity so that it can support the intervention personnel.

The trapping grid 58 ensures the trapping function of the migrant bodies. It has for this purpose a mesh defining passages whose maximum dimension is less than the minimum distance separating the tubes 24 from the beam. In the embodiment illustrated in FIG. 2, the grid 58 has in itself insufficient mechanical characteristics to support the personnel called upon to intervene in the steam generator.

When the means 48 for trapping migrant bodies occupy substantially the entire section of the steam generator as illustrated in FIG. 1, the trapping elements 50 are fixed edge to edge to form a paving as shown schematically in FIG. 2.

When the means 48 for trapping migrant bodies occupy only a part of the section of the steam generator, one or more trapping elements 50 can be placed above each of the evacuation pipes 32. One can in particular provide placing a trapping element of circular shape at the top of each of the discharge pipes 32.

It is important to note that the embodiment which has just been described with reference to FIG. 2 only constitutes an example of implementation of the invention. Thus, the means 48 for trapping migrant bodies can also be made in one piece over all or part of the section of the steam generator. In addition, in the case where separate trapping elements are used, they can have any shape. The structure ensuring the trapping of migrant bodies can also have sufficient mechanical rigidity to avoid the addition of a supporting structure. Finally, the shape of the passages and the structure which delimits them is not limited to a grid but covers all the forms and structures making it possible to define passages of limited dimensions (circular, triangular, polygonal passages, etc., structures obtained by assembling plates, rods, etc. or by machining a solid part, etc. .).

Claims (7)

Steam generator including: - an envelope (10), of vertical axis; - a horizontal tube plate (12) tightly fixed inside the envelope; - a bundle of inverted U-shaped tubes (24), each having two ends fixed to the tube plate and opening out below the latter, respectively in an intake manifold (16) and in an exhaust manifold (18) primary fluid; - means for supplying (42,44) secondary water opening into the envelope (10), above the horizontal tube plate (12); and - extraction means (32, 36, 38) of secondary steam placed in the envelope (10), above the bundle of tubes (24), and comprising substantially vertical discharge pipes (32) placed in under drying means (36); characterized in that means (48) for trapping migrant bodies are placed between the pipes (32) and dryers (36), at least above each of the pipes (32). Steam generator according to claim 1, characterized in that the means (48) for trapping the migrant bodies delimit passages having a maximum dimension less than the minimum distance separating the tubes (24) from the bundle. Steam generator according to either of Claims 1 and 2, characterized in that the means (48) for trapping migrant bodies occupy substantially the entire section of the envelope (10). Steam generator according to either of Claims 1 and 2, characterized by the fact that the means (48) for trapping migrant bodies are located above each of the conduits (32). Steam generator according to any one of the preceding claims, characterized in that the means (48) for trapping migrant bodies comprises several trapping elements (50) placed above a horizontal plate (34) to which are attached to the upper ends of the pipes (32). Steam generator according to claim 5, characterized in that each trapping element (50) comprises a support structure (56) forming a floor, on which is placed a trapping grid (58). Steam generator according to any one of claims 5 and 6, characterized in that each trapping element (50) is fixed, by removable fixing means (54), on horizontal beams (52) located above of said horizontal plate (34).

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