A HEAT EXCHANGER AND A METHOD OF MANUFACTURING IT

Abstract

The wall of the water chamber is welded to the tube plate via a connection ring projecting from the tube plate. The wall of the water chamber is welded to the tube plate after the bundle of tubes has been secured to the tube plate. The connection ring presents a length in the axial direction that is equal to or greater than 250 mm.

Description

FIELD OF THE INVENTION

The invention relates to a heat exchanger, and in particular to a steam generator, and to a method of manufacturing it.

BACKGROUND OF THE INVENTION

Heat exchangers are known that are used as steam generators for heating and vaporizing feed water by heat exchange with a fluid at high temperature.

Steam generators are used in particular in nuclear power stations, and more particularly in nuclear power stations having nuclear reactors cooled by pressurized water.

The pressurized cooling water for a nuclear reactor is set into circulation in a primary portion of the steam generator, flowing inside heat-exchange tubes of a bundle comprising a very large number of tubes (e.g. 5000 or more) of great length (e.g. about 20 meters (m)) and of small diameter (e.g. about 20 millimeters (mm)). The feed water flows in contact with the outside surfaces of the tubes in the bundle, inside an outer casing of the steam generator surrounding the bundle. The tubes in the heat exchanger bundle may be U-shaped tubes, for example, or they may be straight tubes constituting a bundle in which the straight branches of the tubes or the entire tubes are parallel to one another.

For a steam generator in which the tubes are folded into U-shapes, the two straight branches of each tube are engaged in and secured to openings passing through a single tube plate, which plate separates the inside volume of the outer casing that contains the bundle from a water chamber comprising both a distribution compartment and a recovery compartment for distributing and recovering cooling water, with each tube opening out into each compartment via respective first and second ends.

For a straight-tube steam generator, the ends of the tubes in the bundle are engaged in and secured to openings passing through first and second tube plates, which plates separate the inside volume of the outer casing of the steam generator containing the bundle from first and second water chambers serving respectively for distributing and recovering the cooling water of the nuclear reaction.

The water chamber or each water chamber of a steam generator generally comprises a wall that is circularly symmetrical about an axis, which wall may be hemispherical or cylindrical-and-hemispherical in shape. The outer casing of the steam generator containing the bundle is generally cylindrical in shape of circular section and the tube plate(s) is/are disk-shaped being of considerable thickness and circular in shape.

During manufacture of the steam generator, the outer casing of the steam generator, the tube plate(s), and the water chamber(s) are assembled end to end by welding.

The tubes of the bundle are also engaged in and secured to the openings passing through the tube plate(s) going from a first face to a second face of each disk-shaped tube plate. The end portions of each tube in the bundle are engaged in the through openings in the tube plate(s) and are then secured in the through openings by swaging, by expanding the diameter of the tube wall, e.g. by hydraulic means or by pressing the wall of the tube against the wall of the through opening by means of a tube expander having rollers. Fastening is finished off by welding the end of the tube that opens out into the water chamber to one of the faces of the tube plates so as to provide sealing at a junction between the tube and the tube plate.

The operation of fitting the steam generator with its tubes is complex and requires easy access to the through openings in the tube plate(s) in order to engage, secure, and weld the ends of the tubes, and then to inspect the end portions of the tubes once secured in a tube plate. As a result, it is preferable to fit a steam generator with its tubes before assembling the wall of the water chamber to the prepared portion of the tube plate. Nevertheless, proceeding in that way presents a drawback insofar as it is then necessary to connect the wall of the water chamber to the prepared portion of the tube plate by welding with the bundle already mounted on the tube plate. The wall of the water chamber which defines a primary portion of the steam generator that is to receive the pressurized water for cooling the nuclear reactor is of considerable thickness, so welding this wall to the portion of the tube plate prepared for receiving it and presenting the same thickness as the wall of the tube plate constitutes an operation that is lengthy and complex and that leads to considerable heating of the weld zone and of the tube plate. In addition, stress in the weld of the water chamber wall needs to be relieved by heat treatment, which leads to considerable heating of the tube plate. Differential expansion thus occurs between the tube plate and the portions of the tubes that are swaged in the tube plate, such that certain tube connections can become defective after the water chamber has been welded on and the weld has been subjected to stress-relieving treatment.

Proposals have therefore been made to assemble a steam generator by welding the wall of the water chamber to the tube plate before engaging and securing the tubes of the bundle in the openings through the tube plate. That method is difficult to implement insofar as the actions of fastening by hydraulic expansion or tube rolling, of welding, and of inspecting the tube ends all need to be performed from inside the water chamber.

OBJECT AND BRIEF SUMMARY OF THE INVENTION

The object of the invention is thus to propose a heat exchanger, in particular a steam generator, comprising a bundle of tubes, at least one tube plate in the form of a generally circular disk pierced between first and second faces by openings for receiving and securing end portions of tubes of the bundle, an outer casing of generally cylindrical shape enclosing the bundle of tubes fixed to the periphery of the first face of at least one tube plate, and at least one water chamber comprising a circularly symmetrical wall secured by welding to the periphery of the second face of at least one tube plate via a cylindrical connection ring not present in the prior art, and projecting from at least one tube plate, having a first axial end lying substantially in the plane of the second face of the tube plate and having a second end that is free for connection purposes, the outer casing, the at least one tube plate, and at least one water chamber being assembled together about a common axis by welding, during welding operations that can be performed entirely after the steam generator has been fitted with its tubes and without running any risk of damaging the connections between the tubes of the bundle and the tube plate.

To this end, the connection ring for connecting to the wall of the water chamber is of a length in the axial direction between its first and second ends that is equal to or greater than 250 mm to prevent heat damage to the connections between the tubes and the tube plate during welding of the water chamber wall to the second end of the connection ring or subsequent stress relief therebetween.

According to more particular embodiments of the invention taken in isolation or in combination:

• • the connection ring is of a length equal to or greater than 270 mm;
  • the connection ring is of a length equal to or greater than 400 mm;
  • the tube plate has a diameter between approximately 3-4 meters;
  • the tubes have a U-shaped form, and the steam generator has a single tube plate and a single water chamber with a circularly symmetrical wall, and further wherein the wall of the single water chamber is welded around its periphery to the second end of the connection ring;
  • the tube plate has a diameter between approximately 3-4 meters;
  • the steam generator having a top portion of diameter greater than a bottom portion, the bottom portion comprising said single tube plate and said single water chamber;
  • the tube plate further comprising a annular flange which protrudes radially;
  • the tubes are straight tubes connected at their respective ends to first and second tube plates, the steam generator having a first water chamber with a wall welded to the first tube plate, and a second water chamber having a wall welded to the second tube plate via respective connection rings axially projecting from the first and second tube plates;
  • the connection ring of the tube plate is integral with the tube plate.

The invention also provides a method of manufacturing a steam generator comprising a bundle of tubes,

• • at least one tube plate and at least one water chamber, the tube plate being in the form of a substantially circular disk having an axial direction and pierced between first and second faces by openings for engaging and securing end portions of tubes of the bundle,
  • a water chamber with a substantially circularly symmetrical wall, and
  • an outer casing of substantially cylindrical shape around the axial direction and enclosing the bundle of tubes,
  • the method comprising the steps of:
  • providing the bundle of tubes, the tube plate and the water chamber, the tube plate having a connection ring axially projecting from the periphery of the second face, the connection ring having a first end engaging the tube plate substantially in the plane of the second face and a second end for welding to the wall of the water chamber;
  • securing the tubes of the bundle in the openings of the tube plate;
  • butt welding the wall of the water chamber to the second end of the connection ring of the tube plate having the tubes of the bundle secured thereto; the connection ring having a length in the axial direction between its first and second ends that is equal to or greater than 250 mm to prevent heat damage to the connections between the tubes and the tube plate during welding of the wall of the water chamber to the second end of the connection ring or during a subsequent relieving stress treatment of the weld between the wall of the water chamber and the second end of the connection ring; and
  • relieving stress in the weld connecting the wall of the water chamber to the connection ring of the corresponding tube plate by heating the welded zone.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to make the invention well understood, there follows a description given by way of example and made with reference to the accompanying figures, of a steam generator of the invention for a pressurized water nuclear reactor, which figures show two embodiments and a tube plate for a steam generator.

FIG. 1 is an axial section view of a steam generator for a pressurized water nuclear reactor having U-shaped tubes.

FIG. 2 is an enlarged view in axial section of the tube plate of the steam generator prior to the tubes being fitted thereto, and prior to the steam generator being assembled.

FIG. 3 is an axial section view of a steam generator having straight tubes.

MORE DETAILED DESCRIPTION

In FIG. 1, there can be seen a steam generator given overall reference 1, comprising in particular an outer casing 2 of generally cylindrical shape containing in its bottom portion the bundle of tubes 3 of the steam generator, a tube plate 4, and a water chamber 5 of cylindrical-and-hemispherical shape and comprising in particular a hemispherical wall 6.

The outer casing 2 of the steam generator has a bottom portion 2a containing the bundle of tubes 3 inside a bundle casing or “wrapper” 7, and a top portion 2b of diameter greater than that of the bottom portion 2a containing water separators and dryers for the steam produced in the steam generator.

The bottom portion 2a of the outer casing of the steam generator is welded at its bottom end to an annular portion prepared to receive it on the tube plate 4, that is itself of generally circular shape.

The tube plate 4 is of considerable thickness (about 500 mm to 600 mm) and of large diameter (about 3 m to 4 m), and it is made by forging, machining, and drilling. An ingot is forged in order to obtain a disk presenting the general shape and dimensions of the tube plate, and then the disk is subjected to finishing machining, and holes are drilled through the tubular plate in the axial direction of the steam generator between the face 4a situated beside the outer casing defining the secondary portion of the steam generator that receives the feed water, and the face 4b situated beside the water chamber.

FIG. 2 is for illustrative purposes only, and amongst the thousands of holes that pass through the tube plate 4 it shows only two openings 8 for engaging and securing tubes 9 of the bundle.

While forging an ingot to obtain a blank for the tube plate in the form of a circular disk, a blank is made for an annular portion 11 for connecting the outer casing 2 to the tube plate 4, and a blank is made for a cylindrical ring 10 for connecting the tube plate 4 to the wall 6 of the hemisperically-shaped water chamber. At the periphery of the tube plate 4, there is also formed an annular flange 12. By machining the forged part, it is possible to obtain the final shape for the tube plate as shown in FIG. 2, with the annular portion 11 for connection to the outer casing 2 situated at the periphery of the face 4a of the tube plate being separated by a groove 13 from the central portion of the tube plate through which the openings 8 pass for securing the tubes of the bundle.

In accordance with the invention, the connection ring 10 for connection to the wall 6 of the water chamber presents a height h in the axial direction between a free end portion 10a for connection purposes and a portion engaging the disk-shaped body of the tube plate in the plane of the face 4b, which height is greater than or equal to 250 mm. As a result, it is necessary to make a forging having a tubular portion that projects axially from the face 4a of the tube plate by, for example, about 250 mm in order to take account of material being removed while the tube plate is being machined.

There follows a description of the operation of assembling a steam generator having U-shaped tubes in accordance with the invention, the steam generator having a tube plate whose ring 10 constituting a stub or tab prepared for welding that is, for example approximately 250 mm in axial length.

In the above description of the steam generator, it is assumed that the steam generator is in its vertical, in-service position, i.e. that its longitudinal axis 14 is vertical.

As a matter of fact, at least some of the operations of assembling the steam generator, including the operation of fitting the tubes, are performed with the axis 14 in a horizontal position.

The various portions of the steam generator are made separately, i.e. the bottom portion 2a and the top portion 2b of the outer casing, the tube plate 4, and the wall 6 of the spherically-shaped water chamber pierced by openings for receiving tubes for coupling to the primary circuit, e.g. openings such as 15, and by openings 16 constituting manholes for inspecting the water chamber.

When implementing the manufacturing method of the invention, the tubing of the steam generator is fitted before the wall 6 of the water chamber is assembled to the connection ring 10 that is left ready to receive it. The tubing is thus connected directly to the tube plate 4 in order to constitute a subassembly comprising the bundle 3 and the tube plate 4. Each of the U-shaped tubes 9 in the bundle 3 has the free end portions of its straight branches engaged in two respective openings passing through the tube plate 4, with the tubes generally being put into place in successive plane sheets presenting radii of curvature that decrease from the periphery towards the central portion of the bundle. The end portions of the tubes 9 are engaged in the openings 8 of the tube plate 4 and are then swaged and welded.

After all of the tubes 9 of the bundle 3 have been secured to the tube plate, the wall 6 of the water chamber is assembled to the tube plate by fitting the circular peripheral surface of the wall 6 against the free connection end 10a left ready to receive it on the connection ring 10, and then the wall 6 and the connection ring 10 are connected together by circular welding. The welding is performed in successive passes, and once the welding bevel has been filled and the weld fillet has cooled, the stress in the weld and the zone affected by the welding is relieved. Stress is relieved by heating the weld junction zone to a stress-relief temperature.

It has been found that by using a connection ring of sufficient length, for example approximately 250 mm, all damage to the connections between the tubes of the bundle and the tube plate is avoided while welding to connect the wall 6 of the water chamber and during subsequent stress-relief treatment. The heat flux transmitted to the tube plate 4 and the rise in temperature of the massive disk-shaped portion of the tube plate 4 are negligible, leading to no deterioration of the connections of the tubes in the bundle, and in particular no deterioration by differential expansion.

Final inspection of the junction portions between the tubes and the tube plate, after the wall 6 of the water chamber has been secured, can be performed through the connection tube openings 15 and/or the manholes 16, the water chamber 5 itself being subdivided into two compartments by a partition 17 containing the axis of the steam generator, with the wall 6 of the water compartment having a respective connection tube 15 and manhole 16 in each of its compartments. The bundle 3 of tubes 9 is fitted in such a manner that each of the tubes has a first end opening out into a first compartment of the water chamber and a second end opening out into the second compartment of the water chamber.

After the wall of the water chamber has been assembled to the tube plate, the outer casing is engaged around the bundle 3 and the bottom portion 2a of the outer casing is welded to the tube plate around the annular connection portion 11. This welding is performed on walls defining a secondary portion of the steam generator and of thickness that is considerably smaller than the thickness of the wall of the water chamber 6 and the connection ring 10 of the tube plate. This smaller thickness, and the fact that the outer envelope 2 surrounds the secondary portion of the steam generator as generally defined by a bundle wrapper 7 makes it unnecessary to perform stress-relief treatment of the weld to the outer casing. The extent to which the tube plate 4 is heated during assembly of the bottom portion 2a of the outer casing of the steam generator is therefore very limited.

Finally, the top portion 2b of the outer casing is assembled, this portion containing the moisture separators and steam dryers and the torus 18 for feeding steam generator water to the bottom portion 2a of the outer casing and to the bundle wrapper 7, which operations can be performed on the site of the nuclear reactor.

The steam generator of the invention has a ring for connecting the tube plate to the water chamber has a length that is, for example approximately 250 mm, thereby enabling the steam generator to be assembled without running the risk of degrading the connections between the tubes of the bundle and the tube plate.

FIG. 3 shows a straight-tube steam generator for a pressurized water nuclear reactor and in accordance with the invention. Corresponding elements in U-tube steam generators (FIG. 1) and straight-tube generators (FIG. 3) are given the same references, in particular for the steam generator as a whole, the outer casing, the bundle and the tubes of the bundle which are designated respectively by references 1, 2, 3, and 9 in both cases. The straight-tube steam generator has first and second tube plates 4′ and 4″ each having a first end or to a second end of each of the tubes 9 in the bundle 3 secured thereto, and each secured respectively to a first or a second water chamber. Each tube plate has a first face (4′a or 4″a) on the secondary side of the steam generator, and a second face (4′b or 4″b) on the primary side of the steam generator inside a water chamber having a wall (6′ or 6″) that is cylindrical-and-hemispherical in shape. The hemispherical walls of the water chambers are each connected to the corresponding tube plate via a cylindrical connection ring (10′ or 10″) that is sufficiently long to prevent heat damage as previously discussed.

The various portions of the steam generator, and in particular the tube plates 4′ and 4″ are fabricated and assembled in the same manner as for a steam generator having U-shaped tubes.

A subassembly is made comprising the tubes 9 of the bundle 3 fixed at their ends by swaging and welding to the tube plates 4′ and 4″ onto which the outer casing 2 is engaged and secured by welding. The hemispherical walls of the water chambers are then secured by butt welding against the connection rings 10′ and 10″ prepared to receive them. The outer casing 2 surrounding the bundle can be welded to the tube plates before or after the water chambers are welded.

The invention is not limited to the embodiment described above.

A connection ring for connecting the tube plate to the water chamber may be provided having a shape that is other than purely cylindrical. The connection ring is preferably made integrally with the tube plate, but it is also possible to envisage using a connection ring that is a separate part that has been fitted thereto (nevertheless, this method requires two welds to be made instead of only one).

For a straight-tube steam generator where the tubes are connected to two different tube plates at opposite ends, each of the two tube plates has a respective ring for connection to the wall of a corresponding one of two water chambers, and the length of the ring in the axial direction being, for example approximately 250 mm.

Depending on the forging that can be performed on a tube plate, it is possible to envisage using connection rings having a length greater than 250 mm, preferably greater than 270 mm and of up to more than 400 mm. For connection rings of great length, the increase in the inside volume of the water chamber 5 due to the fact that the volume of the cylindrical top portion of the water chamber is greater than the volume of a spherical portion having the same diameter as the tube plate can be compensated by using a wall 6 of greater thickness or of a shape that is slightly modified.

The invention applies not only to steam generators for pressurized water nuclear reactors and having U-shaped tubes or straight tubes, but also to any heat exchanger including a bundle of tubes fixed in at least one tube plate connected to the wall of at least one water chamber.

Claims

1. A steam generator, comprising: a bundle of tubes, at least one tube plate in the form of a substantially circular disk having an axial direction and pierced between first and second faces by openings, said openings receiving end portions of tubes of the bundle to constitute connections between the end portions and the tube plate, an outer casing of substantially cylindrical shape around the axial direction and enclosing the bundle of tubes and welded to the periphery of the first face of the tube plate, at least one water chamber comprising a substantially circularly symmetrical wall, a cylindrical connection ring axially projecting from the tube plate, the connection ring having a first axial end lying substantially in the plane of the second face of the tube plate and having a second end that is welded to the wall of the water chamber, the connection ring having a length in the axial direction between its first and second ends that is equal to or greater than 250 mm to prevent heat damage to the connections between the tubes and the tube plate during welding of the wall of the water chamber to the second end of the connection ring or during a subsequent relieving stress treatment of the weld between the wall of the water chamber and the second end of the connection ring.

2. A steam generator according to claim 1, wherein the connection ring is of a length equal to or greater than 270 mm.

3. A steam generator according to claim 1, wherein the connection ring is of a length equal to or greater than 400 mm.

4. A steam generator according to claim 1, wherein the tube plate has a diameter between approximately 3-4 meters.

5. A steam generator according to claim 1, wherein the tubes have a U-shaped form, and wherein the steam generator has a single tube plate and a single water chamber with a circularly symmetrical wall, and further wherein the wall of the single water chamber is welded around its periphery to the second end of the connection ring.

6. A steam generator according to claim 5, wherein the tube plate has a diameter between approximately 3-4 meters.

7. A steam generator according to claim 5, the steam generator having a top portion of diameter greater than a bottom portion, the bottom portion comprising said single tube plate and said single water chamber.

8. A steam generator according to claim 6, the steam generator having a top portion of diameter greater than a bottom portion, the bottom portion comprising said single tube plate and said single water chamber.

9. A steam generator according to claim 5, the tube plate further comprising a annular flange which protrudes radially.

10. A steam generator according to claim 1 wherein the tubes are straight tubes connected at their respective ends to first and second tube plates, the steam generator having a first water chamber with a wall welded to the first tube plate, and a second water chamber having a wall welded to the second tube plate via respective connection rings axially projecting from the first and second tube plates.

11. A steam generator according to claim 1, wherein the connection ring of the tube plate is integral with the tube plate.

12. A method of manufacturing a steam generator comprising a bundle of tubes, at least one tube plate and at least one water chamber, the tube plate being in the form of a substantially circular disk having an axial direction and pierced between first and second faces by openings for engaging and securing end portions of tubes of the bundle, a water chamber with a substantially circularly symmetrical wall, and an outer casing of substantially cylindrical shape around the axial direction and enclosing the bundle of tubes, the method comprising the steps of: providing the bundle of tubes, the tube plate and the water chamber, the tube plate having a connection ring axially projecting from the periphery of the second face, the connection ring having a first end engaging the tube plate substantially in the plane of the second face and a second end for welding to the wall of the water chamber; securing the tubes of the bundle in the openings of the tube plate; butt welding the wall of the water chamber to the second end of the connection ring of the tube plate having the tubes of the bundle secured thereto; the connection ring having a length in the axial direction between its first and second ends that is equal to or greater than 250 mm to prevent heat damage to the connections between the tubes and the tube plate during welding of the wall of the water chamber to the second end of the connection ring or during a subsequent relieving stress treatment of the weld between the wall of the water chamber and the second end of the connection ring; and relieving stress in the weld connecting the wall of the water chamber to the connection ring of the corresponding tube plate by heating the welded zone.

13. A method according to claim 11, wherein said length is equal to or greater than 270 mm.