EP0086695B1 - Steam generator with u-tube bundle and superheater - Google Patents

Description

The invention relates to a superheated U-tube bundle steam generator.

Such steam generators are for example used in the case of nuclear pressurized water reactors and are used for the production of steam by using, for heating and vaporizing edible water, pressurized water from the reactor as primary fluid .

These steam generators comprise a water box in two parts separated by a partition, for supplying the tubes of the bundle with hot primary fluid and for discharging the cooled primary fluid after passage through the bundle and brought into thermal contact with the food water to spray. The U-shaped tubes of the bundle are fixed to a tubular plate, one face or inlet face of which is situated on the side of the water box. The ends of the tubes are flush with this inlet face and are thus in communication, for each of the tubes, on the one hand with the supply part and on the other hand with the discharge part of the water box.

The other face of the tubular plate or outlet face is crossed by the bundle of tubes arranged vertically inside the upper part of the steam generator, the external envelope of which is called "secondary envelope".

The water box and the secondary casing are welded to the tube plate, on either side of the latter, that is to say at its entry face and at its exit face. respectively.

The bundle of U-shaped tubes is itself disposed inside an envelope coaxial with the secondary envelope and arranged inside the latter, so that an annular space remains between the external wall of the 'envelope of the bundle and the internal wall of the secondary envelope.

In superheated steam generators, the drinking water is supplied via a tube at the base of the bundle on the side of the cold branch thereof, that is to say on the side from which the primary fluid exits. For example, this edible water can be brought by the tubing into the annular space between the secondary envelope and the bundle envelope and then introduced inside the bundle envelope, on the side of the cold branch, by an opening provided in the bundle envelope above the tube plate.

In such superheat steam generators, the cold branch and the hot branch of the bundle are separated by a partition linked to the tube plate which makes it possible to channel the circulation of drinking water and then steam along the bundle.

The food water coming into contact with the cold branch of the bundle begins to heat up and rise along this cold branch until the moment when the vaporization begins, the circulation of the two-phase water-vapor mixture and then the circulation of steam continuing to descend along the hot branch of the bundle after bypassing the upper part of the partition. The circulation of the vapor along the hot branch makes it possible to obtain a dry steam, then a superheated steam which is recovered at the lower part of the hot branch of the bundle by a vapor recuperator emerging in a tube passing through the secondary envelope .

The main advantage of these overheating generators is that it is not necessary to have on the path of the steam, before it leaves the generator, a water-steam separator for the drying thereof.

However, in conventional steam generators used in pressurized water nuclear power plants where the steam is not overheated, the upper part of the secondary envelope containing the steam-water separators makes it possible to recover part of the food water entrained with the vapor which constitutes a reserve of recirculating water above the bundle envelope. This water reserve makes it possible to supply the steam generator for a sufficient time to allow the operators of the nuclear power station to intervene in the event of an accidental total interruption of the supply of drinking water to the steam generator.

Such a safety reserve does not exist in the case of overheated steam generators.

On the other hand, in some of these superheated steam generators, the emergency food water, in the event of a malfunction of the normal supply circuit, is brought to the steam generator in the vicinity of the tube plate, which creates thermal shock when using this emergency circuit whose water is at a temperature much lower than the temperature of the primary fluid.

The object of the invention is therefore to propose a steam generator with a bundle of U-shaped and overheated tubes comprising a water box in two parts for supplying the tubes with hot primary fluid and for discharging the cooled primary fluid. after passing through the bundle and placing it in thermal contact with the drinking water to be vaporized, a secondary envelope containing drinking water, a tubular plate in which the bundle tubes are fixed, secured to the water box at its entry face on which the ends of the tubes are flush with the secondary envelope, at its exit face crossed by the bundle and an envelope enclosing the bundle of tubes, disposed inside the secondary envelope to provide an annular space with the secondary casing, a space into which there is an intake pipe for food water, on the side of the bundle outlet branch, or cold branch, the bundle bundle com carrying at least one opening above the tubular plate for the passage of drinking water inside the envelope of the bundle to come into contact with the cold branch, a partition wall arranged between the cold branch and the hot branch of the bundle through which the fluid exits primary allowing the piping of the food water then of the steam along the bundle and of the superheated steam recuperator being arranged in the vicinity of the end of the hot branch passing through the tube plate, for the evacuation of the superheated steam by a tubing opening into space. annular, this steam generator must continue to operate for a sufficient time after an interruption of the water supply to ensure the extraction of heat and to allow a deterioration of the reactor core to be avoided before a supply of rescue while avoiding a thermal shock on the tube plate.

For this purpose, above the bundle envelope, a free space is provided inside the secondary envelope, to constitute a reserve of drinking water in communication with the annular space and with at least one means of water supply at a temperature lower than the temperature of the primary fluid.

• La figure 1 représente dans une vue en coupe par un plan vertical de symétrie un générateur de vapeur suivant l'invention utilisable dans une centrale nucléaire à eau sous pression.
• La figure 2 représente une vue en coupe suivant A de la figure 1.
• La figure 3 représente une vue en coupe suivant B de la figure 1.
• La figure 4 représente, dans une vue en perspective, un premier mode de réalisation d'un récupérateur de vapeur équipant un générateur de vapeur suivant l'invention.
• La figure 5 représente, dans une vue en perspective, un second mode de réalisation d'un récupérateur de vapeur associé à un générateur à surchauffe suivant l'invention.

In order to clearly understand the invention, we will now describe, by way of nonlimiting example, with reference to the attached figures, an embodiment of a steam generator with a bundle of U-shaped tubes and overheating. according to the invention.

• Figure 1 shows in a sectional view through a vertical plane of symmetry a steam generator according to the invention usable in a pressurized water nuclear power plant.
• FIG. 2 represents a sectional view along A of FIG. 1.
• FIG. 3 represents a sectional view along B of FIG. 1.
• FIG. 4 represents, in a perspective view, a first embodiment of a steam recovery unit fitted to a steam generator according to the invention.
• FIG. 5 represents, in a perspective view, a second embodiment of a vapor recuperator associated with an overheating generator according to the invention.

In Figure 1, we see a steam generator comprising a water box 1 supplied with pressurized water, on one side of the partition 2 by a pipe 3, the pressurized water being evacuated by a pipe 4 of the second part of the water box on the other side of the partition 2.

The water box 1 is fixed to a tubular plate 5 traversed by the ends of the tubes 6 of the bundle which are fixed inside the holes of this tubular plate.

The ends of the tubes are flush with the underside or entry face of the plate 5 so that one end of each tube communicates with one of the parts of the water box and the other end with the other part of this box water. In this way the circulation of water in the bundle is ensured in the direction of arrow 8, that is to say first by passing from bottom to top of the water inside the hot branch. 9 then by passing this primary water from top to bottom inside the cold branch 10.

Around the tube bundle 6 is disposed a cylindrical bundle envelope 12 and closed by a spherical bottom at its upper part. The bundle envelope 12 is itself disposed inside the secondary envelope 14 fixed to the tube plate and providing an annular space 15 around the bundle envelope 12.

As can be seen in FIGS. 1 and 2, horizontal and vertical spacers 17 make it possible to center and maintain the bundle envelope inside the secondary envelope.

At its lower part, the bundle envelope does not rest directly on the tube plate, so that on the side of the cold branch 10 there remains a passage 19 between the lower part of the bundle envelope and the tube plate.

On the side of the hot branch 9, the envelope of the bundle 12 is connected to the vapor recuperator 20 which will be described in more detail with reference to FIG. 4 or to FIG. 5.

The secondary envelope comprises a tube 21 for the supply of edible water into the annular space 15, this edible water then descending to the base of the annular space to be introduced inside the envelope of beam through the passage 19. The secondary envelope also comprises a pipe 22 for the steam outlet, this pipe 22 being in communication with the outlet of the value collector 20.

The interior volume of the bundle envelope is separated into two parts up to the upper end of the straight part of the tubes by a central partition 24 making it possible to channel the circulation of drinking water and steam inside. of the envelope 12.

The edible water, as can be seen in FIGS. 1 and 3, introduced into the annular space 15 through the tubing 21, separates into two descending streams in this annular space and enters the bundle envelope through the passage lower 19 above the tube plate. Food water is thus perfectly distributed to the lower part of the cold branch of the tubular bundle in contact with which this food water heats up.

The heated food water inside the cold branch part of the tube bundle is channeled by the bundle envelope and the central partition 24.

During its circulation in contact with the branch 10 of the bundle, the food water heats up then begins to vaporize and the two-phase water-steam mixture continues its circulation along the bundle.

The vaporization is complete after a certain course of the two-phase mixture along the upper part of the hot branch, from top to bottom, so that the superheating of the vapor has place during the last part of the course of this one along the hot branch of the bundle, before the vapor recuperator 20.

As can be seen in FIG. 3, the vapor recovery takes place at the lower part of the bundle envelope 12 by passage of the vapor inside the envelope of the recuperator.

The envelope 14 has been extended over a relatively large height above the bundle envelope 12, so as to provide a free space 25 in communication with the annular space 15 and with a tube 26 connected to a pipe 27 in bypass on the main food water pipe and a pipe 28 receiving emergency food water from the safety system of the steam generator.

In the case of a steam generator of a pressurized water nuclear reactor as currently constructed with a thermal power of the order of 1000 MW, it is desirable to have a supply of drinking water in the space 25 of the order of 50 t to allow temporary automatic emergency power supply to the steam generator in the event of a shortage of potable water, for a time sufficient to allow operator intervention.

The cross section of these steam generators is such that it is possible to store 10 water per linear meter of the secondary envelope, above the bundle envelope.

It is therefore necessary, in order to have a water reserve allowing an intervention after an accident on the food water network under very good conditions, to raise the secondary envelope above the bundle envelope, to a height of the order of 5 meters.

However, it has also been determined that for a steam generator with a power of 1000 MW, it is possible, without unacceptably reducing the operational safety of the steam generator, to reduce the water reserve up to 20 t. In this case, and always for the same type of steam generator, it is possible to raise the secondary envelope only by a height of two meters.

The thermal power of the steam generator is determined from the flow rate of the primary fluid and the inlet and outlet temperature of this fluid. In the case of nuclear reactors, these parameters are perfectly defined, so that it is possible to determine the emergency water capacity necessary to be able to intervene in the event of an interruption of water supply to the steam generator and therefore to determine the necessary dimension of the capacity 25 included inside the secondary envelope and situated above the bundle envelope.

During normal operation of the steam generator, part of the drinking water filling the secondary envelope 14 comes into contact with the bundle envelope 12, in the annular space 15, in the vicinity of the hot branch 9. It therefore produces a vaporization of part of this food water and the vapor comes to accumulate at the highest part of the casing 14. An introduction of water at the inlet temperature through the tube 26 makes it possible to condense this and to maintain a full filling of the emergency capacity 25 with water during normal operation of the steam generator.

On the other hand, if the emergency power is used, the drinking water from the pipe 28 enters the casing 14 at its upper part through the tubing 26, mixes with the water of the capacity 25 and heats up before descending into the annular space 15.

This avoids the drawback of having a cold shock at the level of the tube plate, which was the case when a backup power supply was used in the vicinity of this tube plate.

The embodiment shown in Figures 1, 2 and 3 shows the use of a central partition 24 separating the bundle into two perfectly symmetrical parts which was not the case with the overheating generators of the prior art.

In fact, in these steam generators of the prior art, the vaporization was carried out in the rising part of the circulation of the drinking water along the bundle and the steam overheated in the falling part.

This required reducing the amount of heat input and therefore the volume of the beam used for overheating, compared to those used for vaporization.

It has been found that it is possible, by varying the pressure, the temperature and the flow rate of the food water introduced into the secondary envelope, to increase or decrease the overheating while using a perfectly symmetrical beam of construction. much simpler than a beam shared by a partition in an asymmetrical way.

For example, if the pressure and the flow rate of the drinking water are increased, the overheating is reduced, delaying the start of vaporization and prolonging the presence of a two-phase state of the fluid circulating along the branch. beam hot.

It is therefore possible by varying these parameters to use a greater or lesser length of the hot branch 9 of the bundle to operate the overheating.

It is thus necessary to continue the vaporization on a descending part of the circulation of the fluid in the bundle, contrary to the technique of the prior art.

In FIG. 4, an embodiment of the vapor recuperator 20 can be seen where it constitutes the lower part of the bundle envelope 12.

The lower part of the ferrule 12 of the bundle envelope is limited so as to provide a space 30 above the tubular plate 5. The vapor recuperator 20 comprises an external ferrule 31 closed at its upper part and welded to the ferrule 12 by means of a crown 32. At its lower part, the ferrule 31 is welded continuously over its entire periphery to the tubular plate 5. The partition 24 closes the recuperator steam box in the diametrical plane of the steam generator.

This steam box is therefore completely sealed and constitutes the lower part of the bundle envelope 12. When the superheated steam reaches the base of the hot branch of the bundle, this vapor can pass through the opening 30 inside the steam box from where it is evacuated to the pipe 22 by means of a flexible sleeve 34 for connection between the steam box and the secondary envelope 14.

A flow restrictor 33 is on the other hand interposed on the passage of steam inside the pipe 22.

The flexible sleeve 34 makes it possible to make up for the differential expansions between the bundle envelope and the secondary envelope.

In Figure 5, we see a second embodiment of the vapor recuperator 20 where the steam box is integral with the lower part of the bundle casing 12 on which it is welded. This steam box 36 is in contact with the tube plate 5 by means of a rail sealing system 37, in the diametrical plane of the generator, and by means of a lamella sealing system 38 at the base of its cylindrical surface. The steam box 36 is placed in communication with the pipe 22 by means of a rigid sleeve 39 constituting a flow limiter 40 inside the pipe 22.

The sealing systems 37 and 38 make it possible to absorb the mounting and expansion play of the device.

This device is obviously not absolutely waterproof like the device shown in FIG. 4.

It can be seen that the main advantages of the device according to the invention are to allow a simple reserve of emergency water to be produced above the tube bundle and therefore to allow operators to intervene in a slightly delayed manner after a power failure. in steam generator water, introduce emergency food water in this capacity where it mixes and heats up before descending into the annular space up to the level of the tube plate, finally adopting an entirely construction symmetrical of the steam generator while allowing a regulation of the superheating by acting on the parameters of the food water introduced in the secondary enclosure.

However, the invention is not limited to the embodiment which has just been described; on the contrary, it includes all variants.

Thus it is possible to provide a capacity of any volume depending on the operating conditions of the steam generator. It suffices to raise the secondary enclosure by a sufficient height above the beam or to provide an increased capacity of frustoconical or cylindro-conical shape at the upper part of the steam generator.

It is also possible to use a steam generator according to the invention comprising a reserve capacity, with a bundle whose partition is not arranged symmetrically, although this arrangement facilitates and simplifies its construction.

It is also possible to use a vapor recuperator of any type at the bottom of the hot branch of the bundle.

Finally, the steam generator according to the invention applies not only in the case of pressurized water nuclear reactors but also in the case of other high-power installations which may require overheating of the steam produced.

Claims (5)

1. Steam generator with a bundle of U-shaped tubes and with superheating, incorporating a water tank (1) in two parts for feeding the tubes with hot primary fluid and for discharging the cooled primary fluid after passing through the bundle and coming into thermal contact with the feed water to be vaporised, a secondary enclosure (14) containing feed water, a tube plate (5) in which the bundle tubes (6) are fixed and which is integrally fixed to the water tank (1) in the region of its entry face with which the ends of the tubes are level and to the secondary enclosure (14) in the region of its outlet face through which the bundle passes, and an enclosure (12) containing the tube bundle and arranged inside the secondary enclosure (14) so as to provide, with the secondary enclosure (14), an annular space (15) into which opens a delivery pipe (21) for feed water, on the outlet branch (10), or cold branch, side of the bundle, the bundle enclosure (12) incorporating at least one opening (19) above the tube plate (5) for the passage of the feed water into the bundle enclosure (12) to come into contact with the cold branch (10), a separating partition (24) arranged between the cold branch (10) and the hot branch (9) of the bundle, through which the primary fluid leaves, permitting the channelling of the feed water and later steam along the bundle and a collector for superheated steam (20) arranged close to the end of the hot branch (9) passing through the tube plate (5) for discharging the superheated steam through a pipe (22) opening into the annular space, characterised in that a free space (25) is arranged inside the secondary enclosure (14), above the bundle enclosure (12), to form a feed water reserve communicating with the annular space (15) and with at least one means for feeding (26) water at a temperature below the temperature of the primary fluid.

2. Steam generator according to Claim 1, characterised in that the water reserve can contain at least 20 tonnes of feed water for a steam generator thermal power of 1,000 MW, the power being defined on the basis of the entry temperature, exit temperature, and primary fluid flow.

3. Steam generator according to Claim 2, characterised in that the water reserve has a capacity close to 50 tonnes, for a steam generator thermal power of 1,000 MW.

4. Steam generator according to Claim 1, characterised in that the cold branch (10) and the hot branch (9) of the bundle have planar symmetry and that the partition (24) separating this cold branch (10) from this hot branch (9) is arranged along the plane of symmetry of the bundle.

5. Process for employing a steam generator according to Claim 4, permitting a modulation of the superheating effect of the generator, characterised in that at least one of the parameters of temperature, pressure and flow rate of the feed water entering the steam generator, is varied in order to modify the length of the zone of the hot branch of the bundle along which zone superheating of the steam is produced.

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