EP0057643A2 - Tube sheet protection device at the hot extremity of a vertical heat exchanger - Google Patents

Abstract

Device for protecting the tube plate (2) at the hot end of a vertical heat exchanger with a tube bundle between water to vaporize and overheat, circulating in the tubes (4) of the bundle, and a liquid alkali metal flowing against the current around the tubes. It comprises two parallel plates (7, 8) perpendicular to the axis of the exchanger, joined by a thin circular ferrule (9), one of which (8) is close to the tube plate, and means allowing an upward circulation of the hot liquid alkali metal through these parallel plates towards the tube plate. Application to heat exchangers for steam generators with primary fluid consisting of liquid sodium.

Description

The present invention relates to a heat exchanger, the hot end of which is provided with a device for protecting the tube plate. More specifically, the invention relates to a heat exchanger comprising an external shell closed at its ends. hot and cold by two tube plates, a bundle of tubes connected to the tube plates to open into inlet and outlet manifolds of .a fluid such as 1- eaù.circulant ^the inside of the tubes and inlet and outlet pipes for a liquid, such as an alkaline liquid metal, circulating in said ferrule and around the tubes.

It is known that such exchangers, such as those forming part of an installation for producing electrical energy from a fast neutron nuclear reactor, in which the alkali metal is generally sodium, have a hot end at a temperature significantly higher than 500 ° C, the sodium being at approximately 525 ° C, the water leaving the tube plate at around 495 ° C under a pressure of the order of 200 bars. It must therefore be very thick, and has significant thermal inertia, which results in high thermal stresses when the system is put into operation, stopped or changed in speed. It would be advantageous to constitute such a plate of a ferritic steel with 2.25% chromium and 1% molybdenum, less expensive than austenitic steels, but such a ferritic steel undergoes decarburization in contact with hot sodium as soon as the temperature of this last significantly exceeds 500 ° C. On the other hand, the temperature of the hot end of the exchanger cannot be reduced without significantly reducing the efficiency of the installation.

The object of the present invention is to remedy these drawbacks, and to provide a device for protection of the tube plate at the hot end of the exchanger, which reduces the temperature of the liquid alkali metal coming into direct contact with it, without however reducing the temperature of the superheated steam reaching the tube plate, which reduces the thermal stresses therein during changes in the regime of the installation, and which nevertheless ensures the circulation of alkali metal around the nipples joining the tubes with the tube plate. It also aims to allow the tube plate to be made of a ferritic steel with a relatively low chromium content, and to ensure a scanning of the welds of the tubes on the junction nipples by the alkali metal in circulation, which facilitates the detection of possible leaks.

To this end, the heat exchanger according to the invention is characterized in that it comprises, at its hot end, a device for protecting the tube plate, this device comprising two plates, joined by a ferrule to define a first zone filled with said liquid in the static state forming a heat shield, said plates being substantially parallel to the tube plate and a first of these plates being disposed near the tube plate, the protection device further comprising through passages said zone from one plate to another and means for creating a vacuum between said first plate and the tube plate, in order to ensure a circulation of said liquid towards the tube plate inside said passages.

• - les passages sont des passages annulaires définis entre les tubes dudit faisceau et des fourreaux à l'intérieur desquels les tubes traversent lesdites plaques ;
• - l'espace défini entre la première plaque et la plaque à tubes communique avec une zone annulaire entourant la première zone et lesdits moyens pour créer une dépression comprennent des tubes par lesquels ladite zone annulaire communique avec une seconde zone annulaire reliée par des orifices à une zone entourant les tubes du faisceau.en aval de la plaque à tubes ;
• - il comporte en outre au moins une plaque intermédiaire disposée entre lesdites plaques parallèles parallèlement à celles-ci.;
• - la deuxième plaque est raccordée à la virole externe par une virole cylindrique munie de trous de remplissage et de vidange permettant audit liquide d'entrer lors du remplissage et de sortir lors de la vidange de l'échangeur.

It also preferably meets at least one of the following characteristics:

• - The passages are annular passages defined between the tubes of said bundle and sleeves inside which the tubes pass through said plates;
• - The space defined between the first plate and the tube plate communicates with an annular zone surrounding the first zone and said means for creating a depression comprise tubes by which said annular zone communicates with a second annular zone connected by orifices to a area surrounding the bundle tubes. downstream of the tube plate;
• - It further comprises at least one intermediate plate disposed between said parallel plates parallel to the latter .;
• - The second plate is connected to the external ferrule by a cylindrical ferrule provided with filling and draining holes allowing said liquid to enter during filling and to exit when the exchanger is drained.

• - la figure 1 est une vue en coupe longitudinale schématique d'un échangeur de chaleur vertical entre de l'eau à vaporiser et surchauffer et du sodium liquide chaud, cet échangeur comportant conformément à l'invention un dispositif de protection de la plaque à tubes supérieure chaude ;
• - la figure 2 est une vue en coupe longitudinale à plus grande échelle, selon la ligne II-II de la figure 3, montrant la partie supérieure de l'échangeur de la figure 1 ;
• - la figure 3 est une vue en coupe transversale selon la ligne III-III de la figure 2 ;
• - la figure 4 représente à plus grande échelle le détail IV de la figure 2, concernant la jonction entre un tube de l'échangeur et sa tétine de liaison avec la plaque à tubes ; et,
• - la figure 5 est une vue en coupe longitudinale à plus grande échelle de la partie gauche du dispositif de protection de la plaque à tubes.

We will now describe, by way of example _; non-limiting, a particular embodiment of the invention, with reference to the accompanying drawings in which:

• - Figure 1 is a schematic longitudinal sectional view of a vertical heat exchanger between water to vaporize and overheat and hot liquid sodium, this exchanger comprising according to the invention a device for protecting the tube plate hot top;
• - Figure 2 is a longitudinal sectional view on a larger scale, along the line II-II of Figure 3, showing the upper part of the exchanger of Figure 1;
• - Figure 3 is a cross-sectional view along line III-III of Figure 2;
• - Figure 4 shows on a larger scale the detail IV of Figure 2, concerning the junction between a tube of the exchanger and its nipple connecting with the tube plate; and,
• - Figure 5 is a longitudinal sectional view on a larger scale of the left part of the tube plate protection device.

It can be seen in FIG. 1 that the exchanger comprises an outer shroud 1, with a vertical axis, closed at its upper and lower ends by tube plates 2, 2 'to which the tubes such as 4 of a bundle of tubes are connected straight arranged inside the ferrule 1. These tubes open respectively, at the lower part of the exchanger, in an inlet manifold 3 'of the water of the secondary circuit and at its upper part, in a manifold of outlet 3 of the vaporized water. The water to be vaporized therefore circulates from bottom to top inside the tubes 4 between the collectors 3 'and 3.

The liquid metal (generally sodium) circulating in the primary circuit of the exchanger moves against the current with respect to the water of the primary circuit, that is to say from top to bottom, inside the ferrule 1 and around the tubes 4, between an inlet pipe 6 and an outlet pipe 6 '.

The hottest area of the exchanger is therefore its upper part into which the liquid metal penetrates, and this part will now be described in more detail with reference to FIG. 2.

First of all, it can be seen in FIG. 2 that the upper part of the bundle of tubes 4 is surrounded by a thin internal ferrule 5 at the level of the inlet manifold 6 of the hot liquid sodium. The ferrule 5 is connected to the external ferrule 1 below the tubing 1 and its upper end ends near the protection device according to the invention, so that the hot liquid sodium rises through an annular zone 6A defined between ferrules 1 and 5, before to descend into the zone 6B internal to the shell 5, around the tubes 4 of the bundle, against the current of the water circulating in these tubes.

According to the invention, the device for protecting the tube plate 2, part of which is visible on a larger scale in FIG. 5, comprises a first flat plate 7 parallel to the tube plate 2 or, in other terms, perpendicular to the axis of the exchanger, and located above the upper end of the internal ferrule 5. The plate 7 is fixed by its periphery to the external ferrule 1 by means of a cylindrical ferrule 7A provided with a lower rim welded to the internal wall of the external shell.

The protective device according to the invention further comprises a second planar plate 8 parallel to the first plate 7 and disposed above the latter, near the underside of the tube plate 2 through which the tubes 4 penetrate. .

Although smaller in size than that of the plate 7, the plate 8 extends over the entire surface of the tube plate 2. The plates 7 and 8 are connected at the periphery of the plate 8, by a cylindrical ferrule thin 9 welded to these plates. The zone 13A thus defined between the plates 7 and 8 and the shell 9 is filled with practically static sodium playing the role of thermal shield between the hot sodium admitted by the tube 6 and the tube plate 2.

In order to further limit the convection currents of the liquid sodium present in the zone 13A, there is preferably, at mid-distance between the plates 7 and 8, a flat intermediate plate 10 of the same dimension as the plate 8 and welded to the shell 9. The role of heat shield of the sodium contained in the zone 13A is thus improved.

Given the relatively small thickness of the shell 9, provision is made to complete the joining of the plates by means of spacers 11 welded to these plates.

The tubes 4 of the tube bundle pass through the plates 7, 10 and 8 inside the sheaths 12 welded to the plate 7 and pass through the plates 10 and 8. The sheaths 12 define with the tubes 4 annular passages through which the sodium hot liquid can circulate upwards from the zone 6B of the exchanger situated below the plate 7, up to the tube plate 2 and the annular zone 13 delimited around the zone 13A and separated from the rest of the exchanger by ferrule 7A. More precisely, and as illustrated in FIG. 4, the annular passages defined between the sleeves 12 and the tubes 4 open out above the zone 13A forming a heat shield near the nipples 20 of the tube plate on which are welded tubes. It should be noted that this structure is found over the entire extent of the tube plate 2, so that the upward circulation of hot liquid sodium has the effect of ensuring an efficient scanning of the welds of the tubes on the nipples 20. From more, the liquid sodium cools during its passage in the sleeves 12, by heat exchange with the water circulating in the tubes so that it reaches the level of these welds at a temperature substantially lower than that which it had at the time. of its entry into the exchanger.

The upward circulation of liquid sodium in the passages defined between the tubes 4 and the sleeves 12 is obtained by creating a depression in the area 13, with respect to the pressure prevailing in the area of sodium circulation below the plate 7. A For this purpose, the zone 13 is connected to an annular zone 18 by a series of vertical tubes 16, crossing the zone 6A between the ferrules 1 and 5. As shown in FIG. 3, the tubes 16 are welded on lugs 17 fixed on the internal ferrule 5. The zone 18 is located below and separated from the zone 6A by a partition 19, and it communicates with a part 22 of the zone 6B internal to the shell 5 by orifices 21 formed in the latter, close to the junction of its lower end with the external shell 1. Part 22 of the zone 6B is located downstream of the part situated immediately below the plate 7, so that the pressure of the liquid sodium is lower there, due to the loss of charge.

The filling and emptying of zones 13 and 13A are carried out first of all by means of holes 7B formed in the rim of the shell 7A welded to the external shell 1 and allowing the liquid sodium to enter during filling, to leave during emptying, while practically preventing any significant circulation during the operation of the exchanger. In addition, the ferrule 9 has at its lower part orifices 9A, and there is a clearance 8A between the sleeves 12 and the plate 8. These latter arrangements allow the filling and emptying of the sodium from the zone 13A forming a heat shield.

In the usual way, the annular zone 13 surrounding the end of the tube plate is connected by a tube 14 to a reserve of argon. A perforated torus 15 for sampling sodium for analysis is connected by a tube 15A to an analysis apparatus (not shown), intended to detect possible leaks from the welds of the tubes 4 on the teats 20.

For example, while the hot liquid sodium enters the exchanger at a temperature of around 525 ° C, the protection device which has just been described brings the temperature of the liquid sodium back in direct contact with the tube plate at around 500 ° C, This allows the use of ferritic steel tube plates, 2.25% chromium and 1% molybdenum, instead of austenitic steel, while avoiding decarburization ferritic steel by hot sodium, which would be significant at 525 ° C. Furthermore, the device for protection by the parallel thin plates protects the tube plate against thermal shocks resulting from transient operating conditions, and allows it to be assigned a higher permissible working rate than if it were directly in contact with hot sodium at 525 ° C, in particular with regard to teats 20.

On the other hand, the sodium circulation ensured by the depression created in the annular zone surrounding the tube plate makes it possible to ensure an efficient scanning of the welds of the tubes of the bundle to the nipples of the tube plate, and consequently a very rapid detection of any leakage from these welds.

Although the protection device which has just been described with reference to the figures of the drawing appears to be the preferable embodiment of the invention, it will be understood that various modifications can be made to it without departing from the scope of the invention, some of its organs which can be replaced by others which would play a similar technical role.

In addition, the invention has been described on a heat exchanger in which the heating fluid is liquid sodium and the heated fluid in water, but it is understood that it relates in particular to steam generators heated by liquid metals and , more generally, to heat exchangers with tube outlets by tube plates, ferrules or manifolds.

Claims (5)

1. Heat exchanger comprising an outer shell (1) closed at its hot and cold ends by two tube plates (2, 2 '), a bundle of tubes (4) connected to the tube plates (2, 2') for lead into the inlet (3 ') and outlet (3) manifolds of a fluid circulating inside the tubes and the inlet (6) and outlet (6') tubes of a fluid circulating in said ferrule and around the tubes, the exchanger being characterized in that it comprises, at its hot end, a device for protecting the tube plate (2), this device comprising two plates (7, 8) joined by a ferrule (9) to define a first zone (13A) filled with said liquid in the static state forming a heat shield, said plates (7, 8) being substantially parallel to the tube plate (2) and a first (8) of these plates being disposed near the tube plate (2), the protection device further comprising passages passing through said zone (13A) from one plate to the other e t means (16, 18, 21) for creating a vacuum between said first plate (8) and the tube plate (2), in order to ensure a circulation of said liquid towards the tube plate inside said passages. 2. Exchanger according to claim 1, characterized in that said passages are annular passages defined between the tubes (4) of said bundle and sleeves (12) inside which the tubes pass through said plates (7, 8). 3. Exchanger according to any one of claims 1 and 2, characterized in that the space defined between the first plate (8) and the tube plate (2) communicates with an annular zone (13) surrounding the first zone ( 13A) and in that said means for creating a depression comprise tubes (16) by which said annular zone (13) communicates with a second annular zone (18) connected by orifices (21) to a zone (22) surrounding the tubes of the bundle downstream of the tube plate (2). 4. Exchanger according to any one of claims 1 to 3, characterized in that it further comprises at least one intermediate plate (10) disposed between said parallel plates (7, 8) parallel to these. 5. Device according to any one of claims 1 to 4, characterized in that the second plate (7) is connected to the external ferrule (1) by a cylindrical ferrule (7A) provided with filling and draining holes (7B ) allowing said liquid to enter during filling and to exit when emptying the exchanger.

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