EP0067739B1 - Method and apparatus for removing sludge from the top surface of the tube sheet of a steam generator - Google Patents

Description

The present invention relates to a method and a device for unclogging the upper face of the tube plate of a steam generator.

The steam generators of pressurized water nuclear reactors generally comprise a cylindrical envelope of great height inside which is disposed vertically a bundle composed of a large number of bent tubes in the shape of an inverted U. Each tube is connected by its two ends to a tubular plate arranged horizontally in the vicinity of the lower part of the steam generators, above the lower curved bottom of this generator.

The lower part of the steam generator located under the tube plate forms two substantially identical compartments delimited by the lower curved bottom and by a vertical partition, one of the compartments communicates by the tube plate with one of the ends of all the tubes, the other compartment communicates with the other end of all the tubes. The pressurized water bringing the heat from the core of the nuclear reactor to the steam generator is introduced into one of the compartments, circulates in the tubes, returns to the other compartment and returns to the core of the nuclear reactor. Passing through the tubes, this water gives up part of its heat by vaporizing food water which is contained in the envelope of the generator and which surrounds the tubes. The steam produced is taken from the upper part of the steam generator and sent to the turbine associated with the electric generator. This vapor is then condensed in the form of drinking water, which is reintroduced into the steam generator.

Despite the treatments carried out with potable water, its presence in the steam generator causes it to operate, over long periods of time, to form a deposit of sludge which accumulates on the tube plate around the base of the U-shaped tubes, in particular at the place of the sealed connection of the tubes and the tube plate which is produced by swaging and welding.

This sludge is mainly composed of iron oxide, in particular magnetite, which is the cause of corrosion phenomena in the tubes at the level of the tube plate. As a leak at the level of the tubes which causes a passage of the primary water in the vapor circuit being considered as a serious accident in the case of a nuclear reactor, very particular attention must be paid to avoid these phenomena of corrosion. Consequently, there is provision for periodic unclogging of the upper face of the tube plate in order to remove the sludge which is deposited there.

To carry out this operation, it has already been proposed to place inside the generator, above the tube plate, a mobile lance sending a jet of water under high pressure in the direction of the base of the tubes. It is known to arrange this lance so that it moves in a diametrical direction of the steam generator, passing through the central zone which is devoid of tubes.

On the other hand, various installations have been proposed for removing the sludge formed during the unclogging operation. It is thus known from FR-An ° 2 352 269 to cause the circulation of a water jet on the periphery of the tube plate, this water jet being intercepted and discharged outside the steam generator after having traversed a half-turn, driving along its passage and thus evacuating the sludge as they reach the periphery of the tube plate.

It has also been proposed to carry out the evacuation of the sludge, by introducing through the same orifice as that which serves for the passage of the launching head, a flexible suction pipe which is guided so as to be disposed approximately on the peripheral area of the tube plate.

Finally, it has been proposed to give the tubular plate a funnel shape so as to evacuate the sludge by gravity, the lowest point of the tubular plate being in the center and comprising at this location an evacuation.

These different arrangements have certain drawbacks, however. The evacuation process by peripheral water circulation does not make it possible to obtain a homogeneous drainage because the evacuation takes place at only one point; the evacuation device using a flexible tube temporarily inserted into the steam generator is not very reliable because this tube can twist or be arranged irregularly on the tube plate; and the device using a tubular plate in the form of a funnel is of little interest since the tubular plate being a centerpiece it is preferable to give it the simplest possible geometric shape, for example a planar shape. The latter process is further limited to a steam generator, the water box of which has a central column.

In addition, a common point in these unclogging methods is to launch the cleaning water using a single launching head which moves either along a diameter of the steam generator, or along 'a radius of this generator. In the latter case, the cleaning operation is carried out in two stages: introduction of the lance through an orifice formed in the envelope of the steam generator and cleaning of the corresponding tubular half-plate, then introduction through a diametrically opposite orifice and cleaning the other tubular half-plate. By operating in one or other of these ways, there may be a displacement of the sludge from an area being cleaned towards an already cleaned area.

The present invention relates to a method and a unclogging device intended to overcome these various drawbacks.

The present invention therefore relates to a method of unclogging the upper face of the tube plate of a steam generator comprising, in a cylindrical enclosure with vertical axis, a vertical bundle of bent tubes in the shape of an inverted U, connected to each of their ends lower than the flat horizontal tubular plate integral with the enclosure of the steam generator, a high temperature fluid being brought to the tubes of the bundle above the tubular plate to supply heat allowing the vaporization of the drinking water introduced into the steam generator.

According to an essential characteristic of the invention, provision is made in the enclosure containing the bundle of tubes, two jets of water under high pressure which can be directed towards different points of the tube plate, one of the jets of water sweeping a first zone in the form of a half-disc of the tube plate, the other water jet sweeping the complementary zone in the form of a half-disc of the tube plate, these two jets of water moving simultaneously and symmetrically by relative to the diametrical line of delimitation of the two zones of the tubular plate, provision is also made for discharging the sludge detached by the water jets by performing a suction of this sludge all along the peripheral zone of the tubular plate.

According to a particular embodiment of the invention relating to an unclogging device in accordance with said method, provision is made for two launching heads projecting water under high pressure in the direction of the tube plate, these two heads moving independently l 'one of the other along the same horizontal axis located above the tube plate, these two heads being driven in an oscillating movement around said same axis, the displacement of the heads along this axis s 'performing step by step, of a value equal to the distance separating two rows of tubes, so that the water jets leaving the lances pass successively between each row of tubes.

According to an advantageous arrangement, it is provided that the launching heads are movable in a diametrical direction of the steam generator located in the free space left between the bundle of tubes; and means are provided for simultaneous displacement of the two launching heads so that their position always remains symmetrical with respect to the central axis of the steam generator in order to symmetrically sweep the entire tubular plate.

According to another advantageous arrangement, there is provided a sludge evacuation device consisting of at least one tube situated above the tubular plate, at a relatively small distance, and disposed at the periphery of the tubular plate, in the vicinity of the vertical cylindrical envelope of the steam generator, this tube comprising orifices directed towards the bottom of the steam generator, and at least one connection of this evacuation tube to a conduit opening out of the steam generator and connected to a suction device.

• La fig. 1 représente une vue en coupe par un plan vertical de symétrie de la partie inférieure du générateur de vapeur dans laquelle se trouve le dispositif de décolmatage et de récupération de boues.
• La fig. 2 est une demi-vue simplifiée en coupe suivant A-A de la fig. 1.
• La fig. 3 est une vue en coupe suivant B-B de la fig. 1.
• La fig. 4 est une vue suivant la même coupe que la fig. 3 d'une variante du dispositif de décolmatage.
• La fig. 5 est une vue en coupe de la tête de lançage.
• La fig. 6 est une vue en coupe partielle suivant C-C de la fig. 3.
• La fig. 7 est une vue en coupe partielle suivant D-D de la fig. 3.

Other advantages specific to this invention will become apparent from the detailed description of the embodiment which follows, illustrated by the appended figures.

• Fig. 1 shows a sectional view through a vertical plane of symmetry of the lower part of the steam generator in which the unclogging and sludge recovery device is located.
• Fig. 2 is a simplified half-view in section along AA in FIG. 1.
• Fig. 3 is a sectional view along BB of FIG. 1.
• Fig. 4 is a view in the same section as FIG. 3 of a variant of the unclogging device.
• Fig. 5 is a sectional view of the launching head.
• Fig. 6 is a partial sectional view along CC of FIG. 3.
• Fig. 7 is a partial sectional view along DD of FIG. 3.

In fig. 1, the entire lower part of a steam generator is seen, consisting essentially of an external cylindrical envelope 1 arranged vertically, closed downwards by a plate 2 arranged horizontally, under this plate is an envelope in the form of a hemisphere 3 comprising a vertical watertight partition 4 which thus delimits two chambers 5, 6; the horizontal plate 2 comprises a series of pipes 7 which pass through it and which are sealed to it. Each of these pipes 7 is in the form of an inverted U, the two lower ends of which each open into one and the other of the chambers 5, 6.

The steam generator works as follows: hot water from the reactor core is introduced into chamber 6, passes through all of the tubes 7, leaves in chamber 5, then returns to the reactor core. When passing through the tubes 7, this hot water gives up its heat to a food water contained in the chamber 8, by transforming this food water into steam which rises towards the upper part of the steam generator and which is sent to the turbines. In normal operation of the generator, there is a long-term deposit of sludge on the upper face 9 of the tubular plate 2. This steam generator further comprises a set of devices for cleaning the sludge on the upper face of the tube plate 2, which we will describe in more detail below.

The unclogging principle adopted in the present invention is illustrated by schematic figure 2. In this figure, there is the tubular plate 2, on which tubes are fixed vertically 7, all these tubes being arranged equidistant so as to form a tube bundle. This tubular bundle extends in the major part of the tubular plate 2 but does not however extend to the very center of this tubular plate. The tubes 7 'closest to the central axis 10 are indeed at a certain distance, in order to respect the minimum radius of curvature of their U shape. In the vicinity of the central axis 10, there is therefore a region devoid of tubes, in which are arranged the launching heads 11 and 12. In FIG. 2 we distinguish the launching head 11 supplied with water under high pressure, and comprising a series of nozzles. In operation, the launching head is driven in an oscillating movement around the longitudinal axis of the lances, so that the water jets represented by the arrow 13 cover the entire surface of the tubular plate. The water jets which follow direction 13 lie in a vertical plane which is perpendicular to the longitudinal axis of the lances 11, 12. During the projection of the water jets, a displacement along the longitudinal axis 14 of the lances 11 allows these water jets to reach all of the connection points between the tubes 7 and the tube plate 2.

Referring to fig. 1 we will describe the arrangement and the mode of operation of the launching heads 11, 12. In the envelope 1 of the steam generator, above the tube plate, are two openings 15 and 16 allowing the introduction into the steam generator of the launching heads 11, 12, in its openings are arranged bearings 17, 18 which allow the longitudinal displacement along the axis 14 of the straight rods 19, 20 which support the launching heads 11, 12, as well as their swinging motion. The rods constitute at the same time a pipe for supplying pressurized water into these launching heads.

Referring to figs. 1 and 3, we will describe the unclogging process specific to our invention. At the start of the unclogging operation, the rods 19, 20 are fully inserted into the steam generator so that the launching heads 11, 12 are almost contiguous at the center of the steam generator. At this time it is sent into the rods 19, 20 the water under high pressure which emerges at the launching heads in the form of jets perpendicular to the axis 14. As the tubes are arranged at the tubular plate in regular grids, the water jets can pass between the rows of tubes and thus reach, due to the oscillating movement of the heads, the different zones of the tube plate being on a line perpendicular to the axis 14. After a few moments, the rods 19, 20 are moved simultaneously towards the outside of the steam generator by a distance equal to the distance which separates two rows of consecutive tubes. Successive movements are thus carried out so that the unclogging operation ends when the two launching heads 11, 12 arrive simultaneously at the peripheral zone of the tubular plate. By proceeding in this way, the sludge detached during the passage of a jet of water does not circulate on the tubular plate so as to return to an already cleaned zone to redeposit there.

Of course, the complete unclogging operation cannot stop when the sludge is removed, but must simultaneously include an operation for removing the sludge mixed with the water coming from the launching heads.

To proceed with the evacuation of sludge, there is provided according to our invention, a set of means which we will now describe.

Since the water jets intended for unclogging follow directions 13 going from the center of the steam generator towards the outside, the detached sludge is found, under the action of the water jet, projected towards the periphery of the tube plate 2 It is on this peripheral zone of the tubular plate 2 that a tube 21 has been placed which extends approximately over this entire peripheral zone.

In fig. 7 we see the detail of realization of this tube 21. We see in particular that this tube 21 is arranged above the tube plate 2, at a small and regular distance from this plate. In this tube 21, a row 22 of holes is arranged, arranged longitudinally at the bottom of the tube, which also corresponds to the zone of the tube closest to the tube plate 2.

As this tube 21 is connected to the outside by pumps intended to create a vacuum therein, the sludge 24 arriving in the vicinity of the tube 21 passes into the tube through the orifices 22.

In fig. 6 we distinguish the connection between the tube 21 and the exterior of the steam generator. The tube 21 is tightly connected to the tube plate in which are drilled various bores 25, 26 which create a communication between the inside of the tube 21 and the outside of the steam generator, these orifices being connected by a pipe 27 to a pump intended to extract the sludge during the unclogging operation.

The tube 21 is mounted and fixed permanently inside the steam generator. In the embodiment which is described here, as it can be seen in FIG. 3, the tube 21 is formed of several sections, 6 in number, closed at the ends, each section 21, 21 'comprising communication with the outside of the independent generator. A set of pipes (not shown) brings together, by pipes 27, 27 _' each tube 21, 21' to a suction device with interposition of a valve on each of the pipes, which allows, by the different setting of each valve, to locally increase the suction capacities in order to obtain good evacuation of the sludge over the entire periphery of the tube plate.

It is also possible, without departing from the scope of the invention, to have at the peripheral part of the tube plate a tube 21 making a complete turn and comprising one or more connections with the exterior of the steam generator.

It is also possible, without departing from the scope of the invention, to constitute a discharge device in a simplified manner, as described in FIG. 4. In this fig. 4 we see in fact that there is no provision at all for a peripheral tube fixed inside the generator, but that only holes provided at the periphery of the tube plate 2 are provided and which are connected to the outside the steam generator, in the same way as above, to pipes going to a suction device. Although this simplified sludge removal device shown in FIG. 4 is slightly less efficient than that shown in the other figures and comprising a peripheral tube 21, this simplified device has the advantage of not permanently comprising, inside the steam generator, an entire installation of tubes 21.

Fig. 5 shows a launching head which includes two pairs of spears 29, 29 'and 30.30'. The first pair of lances 29, 29 'being arranged in a first plane perpendicular to the axis 14 and the second pair of lances 30, 30' being arranged in a second plane perpendicular to the axis 14, the distance separating the lances 29 and 30 being equal to the distance between two consecutive rows of tubes constituting the bundle of tubes. In this way, as shown in Figs. 3 and 4, each lance unclogs by simultaneously sending two jets on either side of the same tube, which causes a highly effective cleaning. On the other hand, the lances 29 and 29 'are arranged diametrically opposite with respect to the axis 14. The same is true of the lances 30 and 30'. This arrangement has the advantage of causing the cancellation of the reaction forces at the level of the launching heads, which allows a lighter and more reliable construction of the rods which support these launching heads.

The unclogging method or device specific to our invention improves the performance of the sludge recovery operation since, due to the symmetrical action of the two heads, the jets move the sludge not only in the direction of the jets but also in a perpendicular direction. Thus, the sludge is pushed back step by step towards areas of higher transparency of the beam, that is to say areas where the number of rows of tubes is lower, areas for which the launching efficiency is increased. .

Thanks to the provisions specific to our invention it is possible to leave permanently inside the steam generator, all of the launching heads connected to their rods 19, 20, provided that they have been provided on the bearings 17, 18 sealing devices to withstand the pressure and temperature conditions prevailing inside the steam generator.

It will also be noted that the device specific to our invention makes it possible to minimize the number of removable parts located inside the steam generator, which simplifies the operating mode, saves time and reduces the risks of having to recover. elements inside the steam generator in the event of an incident. In addition, the fact of having only the lances to be introduced inside the steam generator makes it possible to use access holes of small diameter.

The device specific to our invention also makes it possible, during the period of operation of the steam generator, to operate the sludge removal device permanently mounted inside the steam generator, in order to carry out the periodic purging operations of potable water, which limits the deposition of sludge and consequently lengthens the period between two unclogging operations, which always require stopping the operation of the steam generator.

Claims (6)

1. Process for unclogging the upper face (9) of the tube plate (2) of a steam generator which incorporates, in a cylindrical enclosure with a vertical axis, a vertical bundle of tubes (7) which are bent in the form of an upturned U and which are connected at each of their lower ends to the plane horizontal tube plate fixed to the enclosure of the steam generator, a high-temperature fluid being conveyed to the tubes of the bundle located underneath the tube plate in order to supply the heat making it possible to vaporise the feed water introduced into the steam generator in contact with the tubes, characterised in that it involves an operation for loosening the sludge by providing in the enclosure containing the tube bundle two jets of water (13) under high pressure which can be directed towards different points of the tube plate, one of the jets of water sweeping over a first region in the form of a half-disc of the tube plate, the other jet of water sweeping over the complementary region in the form of a half-disc of the tube plate, these two jets of water being displaced simultaneously and symmetrically in relation to the diametral line delimiting these two regions of the tube plate, this displacement taking place from the said diametral line towards the periphery of the steam generator, and in that it involves an operation for the simultaneous discharge of the sludge loosened by the jets of water, by carrying out suction of this sludge over the entire peripheral region of the tube plate.
2. 2. Device for unclogging the upper face (9) of the tube plate (2) of a steam generator which incorporates, in a cylindrical enclosure with a vertical axis, a vertical bundle of tubes (7) which are bent in the form of an upturned U and are connected at each of their lower ends to the plane horizontal tube plate fixed to the enclosure of the steam generator, a high-temperature fluid being conveyed to the tubes of the bundle located underneath the tube plate in order to supply the heat making it possible to vaporise the feed water introduced into the steam generator in contact with the tubes, characterised in that it comprises two emission heads (11, 12) which project water under high pressure in the direction of the tube plate, these two heads being displaced independently of one another along one and the same horizontal axis (14) located above the tube plate, these heads being driven in an oscillating movement about the said same axis, the displacement of the heads taking place step by step at a value equal to the distance separating two rows of tubes (7), in such a way that the jets of water (13) issuing from the nozzles pass successively between each row of tubes (7).
3. 3. Unclogging device according to Claim 2, characterised in that the emission heads are displaceable in a diametral direction (14) of the steam generator located in the free space left between the bundle of tubes (7), and in that means for the simultaneous displacement of the two emission heads are provided, so that their position always remains symmetrical in relation to the central axis of the steam generator in order to sweep symmetrically over the entire tube plate.
4. 4. Unclogging device according to one of Claims 2 or 3, characterised in that the device for discharging the sludge (24) consists of at least one tube (21) located above the tube plate at a relatively short distance and arranged on the periphery of the tube plate (2) in the vicinity of the vertical cylindrical casing (1) of the steam generator, this tube having orifices (22), directed towards the bottom of the steam generator, and at least one branch (25, 26) for connecting this discharge tube (21) to a pipe (27) opening on the outside of the steam generator and connected to a suction device.
5. 5. Unclogging device according to Claim 4, characterised in that the tube for discharging the sludge consists of several independent, suces- sive portions (21') closed at each end, each portion being connected to the suction device by means of a separate pipe (27) incorporating a valve, and these valves can be adjusted differently from one another in order to increase the suction capacities locally.
6. 6. Unclogging device according to one of Claims 2 or 3, characterised in that the device for discharging the sludge is a simplified device comprising merely a series of holes (28) made on the periphery of the tube plate and connected, on the outside of the steam generator, to a suction device.

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