EP0077255B1 - Method and apparatus for removing sludge from the tube sheets of steam generators - Google Patents

Description

The subject of the invention is a method and a device for eliminating sludge on the tube plate of steam generators of the type comprising, in a cylindrical enclosure with vertical axis closed by the plate, a bundle of smooth parallel tubes connected to said plate. , distributed in layers and traversed by a primary fluid at high temperature. The invention finds a particularly important application in the field of recirculating steam generators, in which the water separated from the steam at the outlet of the secondary circuits is brought back to the inlet at the same time as the drinking water, which causes the accumulation on the plate of sludge containing mainly magnetite, which in the long run forms an adherent and compact layer. This sludge causes corrosion of the exchange tubes at the level of the tube plate and it is necessary to eliminate them to avoid degradation of the steam generator. In fact, the attempts made to evacuate the sludge before it forms a compact layer, for example using purge ramps (FR-A-2 285 573) have proved insufficient to prevent accumulation.

Various methods and devices have already been proposed or implemented for eliminating sludge. They are specifically intended for use in steam generators whose exchange tubes are in the form of an inverted U, the two ends of which are connected to the tube plate. The face of the latter opposite to the tubes opens into a chamber delimited by a lower dome and partitioned into two compartments, one of which communicates with one of the branches, called "hot", of each of the tubes, and the other of which communicates with the other branch, called "cold". In steam generators of this type used in pressurized water nuclear boilers, the latter, which constitutes the primary coolant of the reactor, arrives in one of the compartments, circulates successively in the hot and cold branches, then returns to the second front compartment to be returned to the reactor. In these steam generators, there remains, between the hot and cold branches, an interval greater than that which separates the current layers of tubes, from the fact that one cannot exceed a determined curvature of the U-shaped tubes.

All the sludge removal processes proposed so far take advantage of the existence of this larger interval (usually called “water street”): a hole is provided in the enclosure of the steam generator, above the tubular plate and facing the street of water, a lance which makes it possible to send jets of water under high pressure tangentially to the tubular plate. The lance is then moved radially to sweep the plate. One can in particular use a lance providing two jets directed face to face and perpendicular to the direction of introduction of the lance, which is animated by a movement of advance towards the center of the tube bundle and a movement of oscillations (FR-A-2 352 269). The lance can successively be introduced in one direction, to clean a first half of the tube plate, then in the other from a hole opposite the first to clean the second half. To drive the sludge detached by the jets, a water circulation is organized at the periphery of the plate between two holes of the enclosure placed face to face. The flow introduced on one side is divided into two fractions which circulate on the periphery on either side of the water street and meet to exit the enclosure.

Another solution already used consists in sucking the water and the mud in a flexible annular ramp pierced with holes, introduced into the space which remains between the enclosure and the peripheral tubes of the bundle.

In practice, this approach, which seems natural to those skilled in the art since it uses the relatively large interval constituted by the water street, has been shown to be imperfect.

During the oscillation of the lance, the jet which is directed upwards not only has no effect of pulling out sludge, but also causes runoff which can bring back towards the middle of the plate the sludge not immediately sucked in. the suburbs. The streams of attack with the lance and sweeping can be antagonistic in certain zones and cause a local redeposition of the sludge removed. However, due in particular to their composition, this sludge takes, as soon as it is deposited, a consistency such that it becomes almost impossible to vacuum it without resuspending it. Finally, the water pressure must be moderate (practically of the order of 150 bars) to avoid damaging the tubes by too violent jets. This pressure is often insufficient for uprooting, that is to say for attacking a consolidated and very adherent sludge deposit.

There is also known (FR-A-2445 487) a method of the type according to which at least one lance is moved radially from the center of the tube plate, parallel to the tube plate and close to the latter, two jets are directed of cleaning liquid to the plate, from the end part of the lance and in fixed and symmetrical directions with respect to the direction of the sheets, and the cleaning liquid is simultaneously discharged from the periphery of the bundle of tubes.

The method as described in this document requires inserting the lance into the “water street located between the two arms of the same U-shaped tube, much wider than the interval between two common plies of tubes. The jets are still under moderate pressure. And there is no interruption of the jets when passing in front of the tubes. This process therefore still has the drawbacks mentioned above.

The invention aims to provide a method of eliminating sludge of the type defined above, but meeting the requirements of the practice better than those previously known, in particular in that it ensures a more complete cleaning thanks to an effective attack and an circulation organized so as to avoid the return of sludges to areas already treated, and this by not using only simple means.

To this end, the invention proposes in particular a method of the type defined above, characterized in that the lance (and advantageously two lances) is moved symmetrically between two common plies of tubes, the jets are sent under high pressure, typically at least 200 bars, and the jets are interrupted when passing in front of the tubes.

Because the jets are interrupted when passing in front of the tubes, very high driving pressures can be used, allowing rapid stripping, without risking damaging the tubes: the latter can be very close to the lance, since there is no There is more to foresee the space necessary for the oscillation of this one and that they are no longer subjected to the action of the jets. They can even constitute guides for holding the lance. In a steam generator with U-tubes, the current spacing between layers of tubes will be sufficient for the passage of the lance which will therefore no longer be necessary to introduce via the water street.

This freedom of choice is of particular interest in the case of existing steam generators which include a distribution plate parallel to the tubular plate, located above and near the latter, pierced with a central opening for guiding the water preventing the formation of dead zones favorable to the deposition of sludge. This plate is located to the right of the large diameter holes, called "fist holes", made through the enclosure facing the water street. In these steam generators, the oscillating lances can only be introduced through the fist hole above the distribution plate, which limits the field of action of the jets.

On the contrary, the lances providing jets of fixed orientation used in the process according to the invention, of small thickness to pass between adjacent layers, can be introduced by the holes of small diameter, called "eye holes", which are generally provided in the enclosure of the steam generator orthogonally to the fist holes or below, between the distribution plate and the tube plate.

The cleaning will generally be carried out in several successive passes, using different lances directing jets which strike the plate in areas increasingly distant from the lance or lances. Thus, the sludge is gradually pushed back towards the periphery of the bundle where it is taken up and discharged.

The recovery and the evacuation are made so as to avoid the redeposition of sludge. A first solution consists in using four sections of piping distributed in two pairs. Each pair is introduced through a hole located 90 ° from the direction of movement of the lance or lances and each section is arranged at the periphery of the beam on a 90 ° arc. The four sections thus cover the entire peripheral space of the tube bundle. They are each connected to a suction member, which can in particular be a hydro-ejector.

A second solution consists in creating a one-way circulation of water throughout the peripheral space, by injection and aspiration using tubes passing through the enclosure at two opposite points. Finally, a last solution, which can moreover be used in combination with the first, consists in collecting the sludge using a vacuum device placed against the plate at the periphery thereof.

The invention also aims to provide a device for eliminating sludge which is both simple and effective, making it possible to implement the process defined above.

To this end, the invention provides a device comprising: at least one lance for spraying cleaning liquid which has an end portion provided with nozzles of fixed orientation, such that the nozzles direct oblique jets substantially symmetrical with respect to the direction d 'a sheet towards the tube plate; means for supplying water to the lance; a mechanism located outside the enclosure for radially moving the lance through a hole in the enclosure; and means for collecting and discharging the liquid loaded with sludge at the periphery of the tube bundle, characterized in that the lance has a thickness sufficiently small to pass between two layers of tubes and in that the supply means are designed to interrupt the supply of liquid under high pressure when the nozzles pass in front of the tubes (7).

The terminal part of the lance will advantageously consist of a head having a straight section of flat shape with two superposed nozzles, of a thickness such that it is inserted between two layers of tubes and the height of which is chosen to have a rigidity avoiding any significant bending in the vertical direction while allowing passage through a hole made through the enclosure. The head can be pierced with a high hole directing a balancing jet upwards.

The steam generators of nuclear boilers are frequently placed inside a casemate intended for protection against projectiles in the event of an incident inside the general enclosure of the boiler and / or for biological protection. The free space between the enclosure of the steam generator and the casemate is generally less than the length of lance to be introduced radially into the steam generator. To adapt to this situation, the lance will be formed so as to be deformable in plan while being rigid in the vertical direction in order to tolerate the necessary overhang. The lance may include notches regularly distributed forming a toothing for receiving teeth of a drive mechanism located outside the enclosure and which can be associated with displacement measurement means and with a tube proximity detector carried by the head allowing precise positioning of the lance before actuation .

• La figure 1 est une vue en perspective simplifiée de la partie basse d'un générateur de vapeur dans lequel est inséré un dispositif suivant l'invention,
• La figure 2 est une vue en perspective à grande échelle montrant une fraction d'une lance du dispositif de la figure 1,
• La figure 2A est une vue en coupe d'une variante de la tête de lance montrée en figure 2,
• La figure 2B similaire à la figure 2, montre une variante de lance, à trois flexibles d'alimentation,
• La figure 2C est une vue en coupe longitudinale de la tête de la figure 2B,
• La figure 2D similaire à la figure 2A, montre une disposition possible des buses constituant une variante de celle de la figure 2B,
• La figure 3 est une coupe suivant la ligne III-III de la Fig. 2,
• La figure 4 est une vue en perspective à grande échelle montrant le mécanisme d'entraînement de la lance de la figure 2,
• La figure 5 est une coupe suivant la ligne V-V de la figure 4,
• La figure 6 est une vue à grande échelle montrant une chaîne appartenant à un mécanisme d'entraînement constituant une variante de celui de la figure 4,
• La figure 7 similaire à la figure 4, montre une variante de réalisation,
• La figure 8 est une coupe suivant la ligne VIII-VIII de la fig. 7,
• La figure 9 est une vue en plan schématique depuis le dessus de la plaque tubulaire, montrant la disposition des composants du dispositif, y compris ceux destinés au balayage des boues,
• La figure 10 est un schéma en coupe verticale montrant les zones successivement nettoyées,
• La figure 11 similaire à la figure 9, montre une variante de réalisation du dispositif,
• La figure 12 est une vue en coupe schématique montrant une variante de constitution des moyens de collecte des boues.

The invention will be better understood on reading the following description of a particular embodiment given by way of nonlimiting example, intended for the elimination of sludge deposited on the tube plate of a tube steam generator U-shaped type which is currently widely used in pressurized water nuclear boilers. The description refers to the accompanying drawings, in which:

• FIG. 1 is a simplified perspective view of the lower part of a steam generator in which a device according to the invention is inserted,
• FIG. 2 is a large-scale perspective view showing a fraction of a lance of the device of FIG. 1,
• FIG. 2A is a sectional view of a variant of the lance head shown in FIG. 2,
• FIG. 2B, similar to FIG. 2, shows a variant of the lance, with three supply hoses,
• FIG. 2C is a view in longitudinal section of the head of FIG. 2B,
• FIG. 2D similar to FIG. 2A, shows a possible arrangement of the nozzles constituting a variant of that of FIG. 2B,
• FIG. 3 is a section along line III-III of FIG. 2,
• FIG. 4 is a large-scale perspective view showing the drive mechanism of the lance of FIG. 2,
• FIG. 5 is a section along the line VV of FIG. 4,
• FIG. 6 is a large-scale view showing a chain belonging to a drive mechanism constituting a variant of that of FIG. 4,
• FIG. 7, similar to FIG. 4, shows an alternative embodiment,
• Figure 8 is a section along line VIII-VIII of FIG. 7,
• FIG. 9 is a schematic plan view from above the tube plate, showing the arrangement of the components of the device, including those intended for sweeping the sludge,
• FIG. 10 is a diagram in vertical section showing the successively cleaned zones,
• FIG. 11, similar to FIG. 9, shows an alternative embodiment of the device,
• Figure 12 is a schematic sectional view showing an alternative construction of the sludge collection means.

We will first of all briefly recall the arrangement of the parts concerned by the invention of the steam generator itself, a full description of which can be found, for example, in documents FR-A-2 285 573 and 2 352 269 already cited.

The steam generator comprises a pressure enclosure of cylindrical shape 1 closed at its lower part by a tubular plate 2 in which the end portions of U-shaped heat exchange tubes 7 are expanded and welded. The enclosure extends under the plate by a dome delimiting a water box 3 separated by a vertical partition 4 in a compartment where the hot primary water coming from the reactor arrives by a tube and an outlet compartment from where the primary water escapes towards the reactor by a tubing 6. Between the two compartments, the primary water circulates in the hot branch, then the cold branch of the U-shaped tubes 7.

Between the bundle of U-shaped tubes 7 and the enclosure 1 is placed a secondary envelope 8. The secondary envelope 8 ends above the tubular plate, leaving a space allowing the secondary water and the water. recirculation which descend through the annular space between the enclosure 1 and the casing 8 to supply the area occupied by the beam, immediately above the tube plate. Spacer plates (not shown) pierced with holes for the passage of the tubes and the opening for the circulation of water and steam are arranged at regular intervals along the bundle and hold the tubes. A distribution plate 11 having a central opening 12 is disposed above the tubular plate. The hot and cold branches of the tubes are separated by a free diametrical zone 13, called “water street”, the width of which is determined by the maximum bending that can be given to the tubes 7. In this water street are placed spacers 14 and a continuous bleed ramp 15.

The cleaning device uses accesses provided in the envelope and which, in the steam generator illustrated, include two relatively large diameter holes 17, called "fist holes", diametrically opposite, located opposite the water street and at the distribution plate 11. Two smaller holes 16, usually called "eye holes and whose diameter is typically 50 mm, are aligned in a direction perpendicular to that of the fist holes, at a short distance from the tube plate 2 (typically 250 mm). The holes 17 and 16, closed in normal operation by taps, correspond to openings made in the casing 8. As a general rule, the enclosure 1 is surrounded by a casemate (not shown) whose role is notably to stop parts and debris thrown in the event of an incident.

The sludge removal device shown as an exemplary embodiment in Figure 1 can be viewed as comprising a sludge attack and suspension system and a collection system.

The attack system comprises, for the same operation, two lances 19 intended to be each introduced through an eye hole 16 and to be moved between two plies of tubes, that is to say in an interval whose width is typically of the order of 10 mm.

The lance 19 shown by way of example in FIGS. 2 to 5 comprises a guide and supply part 23 and a head 24 fixed to the part 23 by means eliminating the risk of loss of the head inside the generator . The guide part, designed so as to be flexible, comprises two superimposed tubes 27 and 28 (FIG. 3) connected by a central core pierced with holes and by two metal side walls 25 and 26. Each side wall 25 or 26 consists of a thin sheet pierced with rectangular openings formed by cutting and folding inwards fins 32 and 33. The core and the fins are coated with a coating of thermoplastic material 31 which also drowns the tubes. The fins are of such length that they trap the central core 34. The guide part 23 thus has regularly distributed openings constituting meshes of the movement mechanism which will be described later.

The head 24 is constituted by a metallic piece forming a water box, into which the tubes 27 and 28 open. In the rear part of this head are recesses for receiving tabs 42 and 43 secured to the side walls 25 and 26. Des pins allow to permanently secure the legs of the head.

The water box is provided with two cleaning nozzles constituted by orifices 38 drilled obliquely in thickened portions of the head and designed so as to provide jets inclined downwards in a substantially symmetrical manner. A lance can be formed in this way having a thickness not exceeding 10 mm, that is to say the width of the ligament of the tubular plate between two layers of tubes 7, and a height of the order of 45 mm, smaller than the diameter of an eye hole. Due to its constitution, this lance, when it is supported by a fitting link outside the enclosure, has only an acceptable arrow when it is in its minimum projecting position inside . To reduce the bending forces exerted on it, one can provide, in the head, an orifice 40 directed upwards to give rise to a balancing jet.

The lance can be used with a mechanism of the kind shown in FIGS. 4 and 5, which gives it, at the exit of the enclosure, a curvature limiting the radial size to a value compatible with the space which remains between the casemate and the enclosure. The lance can therefore remain very simple and be used without telescopic displacement or splicing of sections, which would moreover be difficult to implement given the small interval between two adjacent plies of tubes and the need to carry out these operations under control. of an operator in an area subject to significant radiation.

We will now describe, with reference to FIGS. 2A to 2D, various alternative embodiments of the invention.

The lance head shown in section in FIG. 2A differs from that shown in FIG. 2 in that it does not comprise a single set of cleaning nozzles each placed on one side and having the same orientation, but several sets of nozzles. The upper tube 27 feeds a first set of nozzles 38 whose orientation corresponds to that of the nozzles shown in FIG. 2. The lower tube 28 feeds two sets of nozzles, having an inclination of 40 ° and 70 ° respectively on the horizontal, therefore corresponding to shots at a shorter distance. The head shown in FIG. 2A can in particular be used to carry out sweeping of the loose sludge, under a lower pressure than during a preliminary stripping operation, carried out in several passes with heads with a single set of nozzles. But, as will be seen below, it is also possible to use a head of the kind shown in FIG. 2A to remove and expel the sludge by single passage of the lance, with an advance with pilgrim steps, as will be seen below.

The variant embodiment of the lance shown in FIGS. 2B and 2C (where the elements corresponding to those already shown in FIG. 2 bear the same reference number) differs in particular from FIG. 2 in that it comprises three supply hoses which are extended by corresponding tubes up to a chamber 80 provided with a balancing orifice 40 directed upwards. Partitions 81 provided in the chamber separate the flows coming from the tubes 27 and 28 until near the end of the head. The one shown in FIG. 2B, intended for shooting at a short distance from the lance introduction plane, comprises two sets of nozzles 82, of the same orientation, the longitudinal spacing of which corresponds to the distance between two successive tubes of a same tablecloth. On the head and the body of the lance may be provided engraved marks 83 for locating, the spacing of two successive marks corresponding to the interval between two tubes. Compared to the embodiment of FIG. 2, that of FIG. 2B makes it possible to provide a higher flow rate for removing the sludge.

Once the head shown in FIG. 2B has been used during a first pass, it is possible to use, in particular for removing the mud from the spaces between the tubes of the central part of the exchanger, heads projecting jets further away. In particular, it is possible to use, during a second pass, a head the nozzles of which have the arrangement shown in FIG. 2D. A last head, used for a third pass, may include nozzles having the orientation indicated in phantom in Figure _2D .

The mechanism shown in FIGS. 4 and 5, intended to push and pull the lance 19, is carried by a base ₆₀ . This base carries a guide assembly comprising two curved and parallel slides 61, between which the guide part 23 circulates by meshing on endless belts 62 notched on one side, the path of which is fixed by rollers 63. The drive mechanism further comprises two drive assemblies placed on either side of the slides on the path of the lance 19. Each of these assemblies are constituted by a set of two belts 64 notched on both sides, meshing on one side with the guide part 23, on the other side, with drive pinions 65 coupled to a gearmotor 66 by the through a torque limiter 67 and a displacement measurement device 68 of any type.

In the alternative embodiment shown in Figures 6 and 7, the lance is rigid. It consists of several superimposed tubes, two in number in the illustrated case, connected from place to place by spacers 70. The lance passes, at the outlet of the enclosure, a bender which, in the case illustrated in FIG. 7 consists of a series of bending rollers 71 carried by the base 60. The drive means consist of a chain 72 fixed to the rear spacer 70 and driven on an endless path by a pinion 73.

As the position sensor 68 may be insufficiently precise, it will frequently be supplemented by optical means, using for example reference points 74 distributed along the lance (FIG. 6) or by electrical means. These can be formed by an eddy current sensor 75 located at the base of the head (Figure 4), detecting the proximity of a tube.

The tubes are supplied with water by a flexible pipe 20 (FIG. 1) for connection with a remote pump unit (not shown). This group may have a conventional constitution, except that it is provided with means for stopping the arrival of water in response in particular to a signal received from a device for detecting passage in front of the tubes, comprising the position sensor 68, and optionally, an additional optical or electrical sensor. This group can moreover be provided to supply on demand one or the other of two pressures, one of which is very high (at least 200 bars and for example 300 bars) for the withdrawal and the other of which is lower for cleaning and suspending sludge.

The sludge collection and evacuation system comprises, in the embodiment shown in FIGS. 1 and 9, two low-pressure spraying ramps 21 introduced by the fist holes 17 above the distribution plate 11. These ramps supply water to the central part of the tube bundle and cause a flow directed towards the periphery. The sludge is taken up by four sections of flexible tubing 22 pierced with suction holes 51 which each cover a quarter of the periphery. The four sections are each connected to a hydro-ejector 52, only one of which is shown in FIG. 9. This hydro-ejector, which can use the motor water also used for cleaning as a working fluid, makes it possible to achieve a depression of up to 8 meters d water, exerting a suction effect sufficient to entrain suspended sludge.

A stream of low pressure sweeping water can also be introduced into the water street using nozzles 53 placed in the fist hole 17.

This arrangement makes it possible to organize the circulation of water in a coherent manner and to avoid dead zones and the return of sludge to the regions already cleaned.

The implementation of the method using the device which has just been described is carried out in the following manner, in the case where four successive passes are provided using four sets of lances each providing fixed jets, the incidence jets being different when going from one game to another.

During a first pass, from the center of the tube plate towards the periphery of the bundle, a set of lances is used, the nozzles of which are such that the water jets clean the zones A and repel the sludge towards the zone B. During the second pass, a lance is used which cleans zone B and pumps the sludge towards zone C and so on with a slight overlapping of the zones. Finally, a pass is made with a last lance having nozzles providing a jet which covers almost the entire tubular bundle (zone D in FIG. 10) and ensures by training the cleaning of the sludge which could have been deposited again during the first three passes.

It can be seen that the four passes with fixed jet lances allow the sludge to be evacuated progressively and always in the same direction towards the periphery of the steam generator, where they are taken up by the sections 22.

Each pass is carried out in the same way: the spears are first completely introduced. They are supplied with water under a pressure which can vary depending on whether one wishes to perform a stripping (which, in certain cases, may require a pressure of the order of 400 bars) or a simple cleaning. The lances are then moved so as to bring the nozzles 38 in front of the next ligament. During this translation, while the nozzles 38 pass in front of tubes, the jets are interrupted, which implies a stop of approximately 10 seconds in practice, the emission of jet in each intertubular space lasting approximately 30 seconds in general.

Because the jets are only sent into the ligaments of the tubular plate, very high water pressures can be used, which tear off deposits, even hardened, without risking damaging the tubes.

Furthermore, the advance of the lances and the emission of jets being done by programmed sequences with automatic remote control, the presence of an operator near the steam generator is no longer necessary except during the setting operations, loading and dismantling of equipment, resulting in a reduction in the radiation doses to which the operating personnel are subjected.

In the alternative embodiment shown in FIG. 11, an injection and suction system is used which creates a circulation of water in the same direction throughout the peripheral space. For that, we still uses four tube sections, grouped in pairs. Each pair comprises a section 75 for injecting water under high or medium pressure and a section 76 connected to a pump or to a hydro-ejector creating a suction vacuum for the sludge in suspension. The sections are oriented so as to generate a circular flow in one direction and the pressures and flow rates used are chosen so that the current has a speed greater than 1.6 m per second. This avoids redeposition of suspended sludge. The collection system is completed by ramps 21 for supplying water at low pressure, as in the previous case.

Finally, it is possible to use, in place of the previous solutions or to supplement them, a return pipe laid flat on the periphery of the plate, comprising a central suction chamber 54 of liquid charged with sludge, connected to means of suction (pump or hydro-ejector) and two chambers 55 and 56 for supplying compressed air, opening on either side of the inlet slot of the central chamber 54 (FIG. 12). The chambers 55 and 56 communicate through calibrated orifices with the flat underside of the piping, intended to be applied to the tubular plate. Thus, the compressed air leaving the chambers 55 and 56 resuspends the sludge which is entrained with the cleaning water by the central chamber 54. The compressed air jets constitute a cushion of air for supporting the piping and thereby facilitate the recovery of sludge.

It can be seen that this latter solution makes it possible to resuspend any sludge redeposited on the periphery following a temporary interruption of the attack or of the collection. It thus makes it possible to complete, if necessary, the action of the normal collection system.

The operating sequence described below assumes that several successive passes are made using lances whose nozzles have an increasingly inclined orientation in the vertical. It is also possible to carry out cleaning in a single pass, by using a lance comprising a head of the type shown in FIG. 2A. For this, the lance is advanced according to a progression in pilgrim's step such that each ligament receives the jets coming from the head according to four successive sequences of feeding of the heads. In other words, the progression can be represented by the following table where m represents the serial number of the ligament located closest to the water street, the columns correspond to the feeding sequences of the lance n ° 1 , 2 ... and the crosses indicate the ligaments which receive the jets.

Claims (10)

1. A process for removal of sludge deposits on the tube plate (2) of steam generators which comprise, in a vertical cylindrical enclosure (1) closed by the plate, a bundle of parallel tube connected to said plate, distributed into sheets and along which a hot primary fluid flows, including radially moving at least one lance from the center of the tube plate in a direction parallel to the tube plate and in close proximity to the latter, directing toward the plate two streams of cleaning liquid from the end portion of the lance (19) and in directions which are fixed and symmetrical with respect to the direction of the sheets, and simultaneously evacuating the cleaning liquid from the periphery of the bundle, characterized in that the lance is moved between two regular sheets of tubes, the streams are delivered under a high pressure, typically of at least 200 bars, and the streams are cut off as they confront the tubes (7).

2. Process according to claim 1, characterized in that several sequences are carried with different lances, which direct streams which strike the tube plate in zones which are more and more remote from the lance.

3. Process according to claim 1 or 2, characterized in that, the steam generator being of the U-tube type (7), the hot and cold legs of the tubes being separated by a tube lane, the lances are moved radially between two sheets of tubes perpendicularly to the direction of the tube lane (13) and in close proximity to the tubular plate.

4. Process according to any one of the'preceding claims, characterized in that two lances are moved simultaneously and symmetrically from the center of the steam generator for sweeping the whole of the tubular plate.

5. Process according to any one of the preceding claims, characterized in that the sludge loaded liquid is collected at the periphery of the bundle and is evacuated toward headers (22) projecting out of the enclosure (1) through holes (17) aligned perpendicularly to the hole (16) through which the lance or each lance (19) is inserted.

6. Apparatus for removing the sludge on the tube plate (2) of steam generators comprising, in a vertical cylindrical enclosure (1) closed by the plate, a bundle of parallel tubes connected to the plate, distributed into sheets and flowed by a primary high temperature fluid, the apparatus comprising : at least one lance for projecting cleaning fluid which has an end portion provided with nozzle jets of fixed angular position, selected for the nozzle jets to direct slanted streams approximately symmetrical with respect to the direction of a sheet and toward the tube plate ; means for feeding water to the lance ; a mechanism located outside of the enclosure for radially moving the lance through a hole of the enclosure ; and means for collecting and evacuating the sludge loaded liquid at the periphery of the bundle of tubes, characterized in that the lance has a transversal size which is small enough for being insertable between two sheets of tubes and in that the feeding means are arranged for cutting off feed of high pressure liquid when the nozzle jets confront the tubes (7).

7. Apparatus according to claim 6, characterized in that the end portion of the lance consists of a head having a flat cross-section with at least two lateral nozzle jets, the thickness of the head being such that it is insertable between two sheets of tubes while being unable to rotate, the height of the lance being selected for offering a rigidity which prevents substantial flexure in the vertical direction while authorizing insertion through the hole formed in the enclosure.

8. Apparatus according to claim 6 or 7, characterized in that the end portion of the lance is provided with a port directed upwardly for delivering a stream which balances the streams directed toward the plate.

9. Apparatus according to claim 6, 7 or 8, for use in a steam generator in which the space available outside of the enclosure in front of the holes through which the lance is inserted is lower than the maximum length of the lance outside of the enclosure, characterized in that the lance is constructed for being deformable in a horizontal direction while it is rigid in the vertical direction, and in that the driving mechanism bends the lance in the horizontal plane at the outlet of the enclosure.

10. Apparatus according to any one of claims 6-9, characterized in that the lance or each lance has a tube-proximity detector, for instance of the eddy current type..

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