GB2064734A

GB2064734A - Steam-generator

Info

Publication number: GB2064734A
Application number: GB7941748A
Authority: GB
Original assignee: Breda Termomeccanica SpA; Comitato Nazionale per lEnergia Nucleare CNEN
Current assignee: Breda Termomeccanica SpA; Agenzia Nazionale per le Nuove Tecnologie lEnergia e lo Sviluppo Economico Sostenibile ENEA
Priority date: 1979-12-04
Filing date: 1979-12-04
Publication date: 1981-06-17
Also published as: GB2064734B

Abstract

A steam-generator constructed to facilitate maintenance and servicing operations comprises a vessel 1 accommodating a heat exchanger unit 3 including a bundle of U- shaped tubes carried on a tubeplate 3b and enclosed in a shell 3a, a steam separator unit 5 mounted on the heat exchanger by a flange connection 3d, 5d, and a steam dryer unit 6, a releasable cover 2 closing the top of the vessel 1, and a manifold 4 for conducting liquid to and from the heat exchanger unit, the heat exchanger unit, steam separator unit and drying device being readily separable from each other and removable from the vessel by relative vertical displacement. The manifold may be welded to the tubeplate, as shown, in which case it is removed with the heat exchanger unit, or it can be welded to the lower end of the vessel. The tubeplate rests on an internal shoulder 8 provided in the vessel, and may be held in engagement therewith by securing elements inserted radially or vertically between the vessel and tubeplate. <IMAGE>

Description

SPECIFICATION Steam-gemnerator The invention relates to steam-generators, and in particular to steam-generators which have in the interior a steam drying device, a steamseparator, and heat exchanging tubes all mounted within a common vessel.

A known steam-generator for pressurizedwater nuclear power stations includes a bundle of about 4000 U-tubes arranged on a tubeplate in such a way that the outer circumference of the bundle is nearly circular so that it can be fitted into a cylindrical vessel of the steam-generator. The tubes are arranged in such a way that one of their end sections lie in one half of the tubeplate, and their other end sections lie in the other half of the tubeplate.

The tubes are welded to the tubeplate; the latter is welded at its outer peripheral area to the cylindrical vessel of the steam-generator and along its inner peripheral area to a head piece of the steam-generator. The bundle of tubes is enclosed in a shell which at its upper section carries a steam-separator unit which in general is welded to the shell.

The shell forms inside the steam-generator vessel two cavities which are connected together at the end sections. One of these cavities is cylindrical and contains the bundle of tubes, whilst the other surrounds the first and is annular and serves to convey liquid to the base part of the bundle of tubes. These cavities together with the corresponding communicating openings are designed in such a way that convection ensures the return of liquid medium evaporating in the steam-generator. Above the steam-separator unit is the drier unit which, in general, is welded to the vessel of the steam-generator.

The steam-generator, when used in the thermo-nuclear field, is enclosed in a space which is shut off by massive concrete walls through which pipes pass for conveying liquid to and from the generator.

Although the steam-generator is designed in such a way that during its working life damage and defects should not occur, in practice it is necessary to provide for such an eventuality. For this purpose various openings are normally incorporated into the vessel of the steam-generator, through which inspections and certain repairs may be carried out, for example, the closing-off of damaged tubes in the tube bundle. But serious defects such, for example, as the fraction of a large number of tubes in the tube bundle, frequently requires complete replacement of parts of the steamgenerator, if economically feasible. During such repairs the plant itself cannot be operated which means a heavy cost burden.

In view of the importance of the problem of prevention of serious failures or defects in the region of a steam-generator and in particular of nuclear power stations, one might think of solutions which would be directed towards replacing the most sensitive components of the steam-generator, i.e., the tube bundle, and hence not the steam-generator as a whole. A first solution within the scope of the proposed solutions brought forward in this respect consists, for example, in the vessel of the steam-generator being divided up into three different sections which are connected together mechanically by conventional methods. One could then exchange only that part in which there is the tube bundle and in addition plan in advance all of the necessary steps such as separation of the tubes, removal of cladding, dismantling of supports and mountings, etc.

Another solution might consist in the steamgenerator being divided up into two sections, that is, an upper section with the dryer and steam-separator units, and a lower section with the tube bundle; this again might be divided up, for example, into modules which could be exchanged in emergency.

But both the aforementioned solutions contain considerable disadvantages which as far as the first mentioned solution is concerned, follow from the necessity of having to move, inside the space in which the reactor and the primary system of the power station are accommodated, parts of considerable mass and large bulk and having to separate part of the connections to the remainder plant such as mountings, piping, etc. Further disadvantages may be derived from the second of the solutions mentioned above because, besides the aforesaid steps of movement and conveyance, the complicated instrument equipment and the greater probability of defects in the region of the tube bundle which derive from this also have to be taken into consideration.

In accordance with the present invention there is provided a steam-generator comprising a cylindrical vessel arranged vertically and accommodating a heat exchanger including a bundle of U-shaped tubes fastened to a tubeplate, a steam-separator above the heat exchanger and above the steam-separator a device for drying the steam generated, a releasable cover closing the upper end of the vessel, a manifold located beneath the tubeplate of the heat exchanger for supplying heat transfer medium to the tubes and discharging said medium therefrom, an inlet for supplying liquid to the vessel interior above the tubeplate of the heat exchanger and outlet means for discharging steam from the vessel, wherein the heat exchanger, the steam-separator and the drying device are so supported within the vessel to be readily separable from one another and the vessel by relative vertical displacement therebetween.

Witfi such a steam-generator, the tube bun dleof the heat exchanger does not require any movement or displacement of the vessel with the exception of the top cover. In addition it is possible to minimise or eliminate any need to interrupt connections between the steam-generator and the rest of the plant, whereby the down-time of the plant may be reduced.

These advantages are obtained without any major alterations in manufacturing and opernt- ing requirements.

The steam-generator is assembled from five separate parts, that is: the heat exchanger with tube bundle and tubeplate; the steamseparator; the drying device; the dryer vessel; and the cover for the latter. The manifold may be connected to either the tubeplate or the vessel and be separate from the other one of said components. The first three parts are accommodated in the vessel and inserted in turn. All connections between the parts are not permanently anchored and hence may easily be separated. The vessel is closed by the cover which can be bolted to the vessel according to conventional methods.

It may be observed that the above-mentioned steps of assembly, and the corresponding steps in dismantling, do not in any way extend also to the connections and joints between the vessel itself and the external equipment to which it is connected. Consequently any work upon the brickwork structures in which the steam-generator is located are unnecessary.

A more complete understanding of the invention will be had from the following detailed description of some exemplary embodiments, given with reference to the accompanying drawings, in which: Figure 1 shows in longitudinal section a first steam-generator in accordance with the invention; Figure 2 is a similar view showing a second steam-generator in accordance with the invention; Figures 3 and 4 show in detail two different forms of connection between the tubeplate and the vessel for the steam-generator of Fig.

1; Figures 5, 6 and 7 show in detail three other possible forms of connection between the tubeplate and the vessel for the steamgenerator of Fig. 2; and Figure 8 shows an upper part of another steam-generator according to the invention.

With reference to the drawings, Fig. 1 illustrates in diagrammatic form the modules from which the steam-generator is composed and the connections between them when the steam-generator is completely assembled and mounted. All of the components of the steamgenerator are accommodated inside an outer shell or vessel 1. On the top is the cover 2 and on the bottom a manifold or distributor chest having the stub connectors 4a and 4b through which the heat-carrier liquid is introduced and discharged. Inside the space which is bounded by these parts, there may be seen a heat exchanger including tube bundle 3 with shell 3a and tubeplate 3b, a steamseparator unit or complex 5 and a steam drying device or complex 6.

In Fig. 1 the heat exchanger and manifold 4 are connected together. The distributor chest 4, for example, is welded onto the tubeplate 3b. The space between the two aforesaid parts is divided into two chambers by a partition 21. This partition is described again in greater detail with reference to Figs.

3 to 7.

The vessel 1 consists of a cylindrical housing having a frustoconical intermediate zone the diameter of which widens upwards and which at its upper end section runs out, for example, into a flange to which the cover 2 may be fastened by setscrews 9; in contrast this cylindrical body at its lower end section continues into an annular part 7 which exhibits a thicker cross-section and inside which there is a projection or shoulder 8 upon which the tube-bundle complex, i.e. the heat exchanger, 3 as well as the distributor chest 4 connected to it, rests via a corresponding projection. Furthermore, in this projection 8 the necessary grooves are incorporated for the sealing rings or packings.When the steamgenerator is in service the pressure from the generated stream acts upon the tubeplate and forces it against the projection 8; hence it guarantees an air-tight mechanical connection between the vessel 1 and the complex consisting of the parts 3 and 4. In the case of this latter complex there is provided on the top part of the shell 3a which surrounds the tube bundle 3 a flange 3donto which the steam-separator complex 5 is bolted by means of a corresponding flange 5d.

This latter complex consists of a series of centrifugal separators 5a which are dependent upon the capacity of the steam-generator and are welded onto the supporting plate 5b, onto the periphery of which is welded a cylindrical part So which terminates in the above mentioned flange Sod for the attachment to the tube bundle shell 3a. Under normal service conditions both the tube bundle and also part of the steam-separator complex are immersed in the water which fills the vessel to the level indicated by the reference number 10.

The dryer complex 6 is accommodated inside the region lying above the steam-separator complex, and consists, for example, of the plates of laminated dryers 6a which are of known type and which are arranged upon the vertical sidefaces of one or more cubes or similar bodies; the whole is enveloped moreover in a cylindrical shell 6b, the length of which is dimensioned so that its bottom part always lies below the level of fill 10, i.e., inside the body of water surrounding the steam-separator without prejudice to the oper ating conditions at the time. The manner and the form in which the cylindrical shell 6b is fastened to the vessel 1 are described in detail below with reference to Fig. 8.

The connections through which the steamgenerator is connected to the remainder of the plant are accommodated all together inside the housing of the vessel 1 with the exception of the inlet and outlet leads for the heatcarrier liquid, which are welded onto the distributor chest 4 which in the embodiment illustrated in Fig. 1 forms an integral component of the complex of the tube bundle 3 and the tubeplate 3b. Furthermore the most important connections or joints respectively are shown diagrammatically, which exists in relation to the installation external to the steamgenerator installation, that is, the mounting or support 1 5 of the steam-generator and the stub connections for the feedwater 1 6 and the generated steam 1 7.

The sequence of the most important working steps which are necessary in order to dismantle the steam-generator in accordance with Fig. 1 is as follows: -Removal of the cover 2 of the vessel 1 by releasing the setscrews 9.

-Dismantling of the dryer complex 6 by simple withdrawal via the top opening.

releasing the setscrews in the flanges 3d and 5don the steam-separator complex 5 and subsequent withdrawal via the opening at the top.

-Detaching of the pipes 4a and 4b which are connected to the distributor chest 4 of the tube bundle.

lifting out of the heat exchanger complex, consisting of the tube bundle, tubeplate and distributor chest via the top opening.

These steps of dismantling are simple mechanical lifting jobs, which can be carried out employing the pole crane which will probably always be available since it is needed also for other purposes inside the building in which the reactor and the steam-generator are accommodated.

Once a defective tube bundle has been removed and hence also the whole complex with the tubeplate and the distributor chest, its exchange is effected in the way that by the same crane the new unit is lowered into the vessel and let down onto the projection or shoulder 8 provided at the bottom end of the vessel. Then the stub connectors of the distributor chest are welded onto the stubs of the inlet and outlet piping (not shown) for the heat-carrier liquid.After this step has been concluded it is sufficient in order to complete the re-assembly of the steam-generator to introduce the steam-separator unit 5 into the vessel 1, setting it down on the flange 3dof the shell 3a, and bolt it to, this flange, then introduce the dryer unit 6 so that its projection 11 rests on shoulder 1 2 in the vessel 1 (see Fig. 8), and finally close the vessel by means of the cover 2.

As will be evident from the present description in connection with the exchange of the damaged tube bundles of a steam-generator the necessary interruptions and operations in the region of the structure and frame of the plant are restricted to a mere separating and re-welding process in the case of the piping for the heat-carrier liquid, which is a process which with special apparatus and devices known per se, can be performed within a very short time. All the other steps, which are mainly bolting up or freeing the flange connections and lifting and guiding the units of the steam-generator, can be performed with the aid of devices and apparatus which are normally present inside the building in which the so-called primary circuit of a pressurizedwater nuclear power station is accommodated.

With reference to Fig. 2, in which parts similar to those which have already been shown in Fig. 1 are designated by the same reference numerals, there is shown a generator in which the manifold or distributor chest 4 for the heat-carrier liquid is not welded onto the tubeplate 3dbut is welded instead to the bottom edge 7 of the vessel 1. Hence it is integral with the vessel. The tubeplate 3b rests upon the shoulder or projection 8' inside the vessel against which it is held via a number of vertical studs which, as may be seen from the detail in Fig. 7, may be screwed in from below, or by pins or pegs 18, 18' and 18" which, as may be seen more clearly from Figs. 5 and 6 extend radially.

In the embodiment described here and illustrated diagrammatically in Figs. 2, 5, 6 and 7, the sequence of the steps in connection with the exchange of a defective tube bundle is effected in essentially the same manner as has been described already in connection with the embodiment illustrated in Fig. 1with the exception that the step of separating the inlet and outlet pipes for the heat-carrier liquid is not necessary and there is required the step of releasing the studs 19, in the case of Fig.

7, or withdrawing the pins 18, 18' and 18" in the case of Figs. 5 and 6. In view of the special importances of the connection between the vessel 1 and the complex of the tube bundle, tubeplate and distributor chest, this connection is to be described in detail below.

Figs. 3 and 4 show a connection suitable for the embodiment illustrated in Fig. 1. In detail there may be seen the annular bottom edge 7 of the vessel 1, the seal 20 and the distributor chest 4 with the partition 21. Furthermore for the sake of better clarity the innermost tube 22 and the outermost tube 23 of the tube bundle are shown by way of indication, as well as one bottom edge of the shell 3a of the aforesaid tube bundle. The steam-generator rests with the lower end of the vessel on a support 1 5 to which it may be fastened by bolting. The complex 3 of the tube bundle, with the tubeplate and rigidly connected distributor chest 4, rests via the seal 20 on the shoulder 8 inside the bottom edge 7 of the vessel, and is urged against the shoulder 8 by the pressure of the steam generated by the steam-generator during service.In order to ensure this engagement with the shoulder, even under severe or catastrophic conditions, such as during an earthquake which occurs at a time when the steamgenerator is not in service, pins are inserted between the vessel and tubeplate and may be inserted radially (24) as shown in Fig. 3, or vertically (24') a shown in Fig. 4.

Figs. 5, 6 and 7 show connections between the vessel 1 and complex consisting of the tube bundle and tubeplate 3, suitable for the embodiment illustrated in Fig. 2. As already described the distributor chest 4 is connected to the vessel 1 and not to the removable tubeplate. In this case it is possible to withdraw the complex consisting of the tube bundle and tubeplate through the top opening of the vessel without having to perform any operations upon the vessel from outside and, in particular without having to carry out any separation and rewelding of the piping. The tubeplate 30 rests on the shoulder or projection 8' which is provided on the inside of the vessel and equipped with seals 20. The tubeplate is retained against the shoulder of the vessel by tierods or studs or pins which are inserted between the vessel and tubeplate.

Fig. 5 and 6 show two different ways of securing the tubeplate to the vessel using radial pins 18 or pairs of pins 18', 1 8". Fig.

7 shows an alternative method of securing the tubeplate 3b to the vessel, in which studs 1 9 are inserted vertically. In order to guarantee the sealing of the liquid medium contained in the steam-generator towards the outside, the outer ends of these studs or tension members are covered by metal caps 25 which are welded onto the outer surface of the vessel.

Similar caps can be provided for the pins 18 and 18', 18".

The sealing between the two chambers into which the distributor chest 4 is divided (and which in the solution illustrated in Fig. 2 has a cover construction which is not anchored stationary and which is not made integral with the tubeplate) is ensured via the doubly curved partition 21 which, at the point at which it comes into contact with the inner wall of the distributor chest, is welded to it along the continuous profile, whilst at the top part it is screwed to the tubeplate according to conventional methods.

The mechanical connections and joints and the sealing between the complex of the tube bundle and tubeplate 3 and the steam-separator complex 5, and between the dryer unit 6 and the vessel 1, are shown in Fig. 8. Besides the flanges 3dand 5d for the bolted connection between the complex of the tube bundle and tubeplate 3 and the steam-separator unit 5, there is a circumferential mounting collar or projection 11 on the cylindrical shell 6b, which is provided for engagement with a projection or shoulder 1 2 on the inner wall of the container 1. The parts 11 and 12 are held in contact with one another by springs 1 3.

The springs are compressed between the outer structure of the dryer unit 6 and respective supports 14 which are connected rigidly to the inner face of the cover 2. When the steam-generator is completely assembled with the setscrews 9 tightened, the springs 13 are in a compressed state and force the shell 6b of the dryer unit against the projection or shoulder 1 2 on the vessel.

The sealing between the space inside the shell 6b of the steam-separator and the annular space outside it, which is connected to steam outlet pipe 17, is guaranteed by the tight connection between collar 11 and the projection or shoulder 12 as well as by the fact that the lower end of the shell 6b is below the level 10 of the water inside the vessel .

By appropriate experts and specialists in this field further improvements and modifications may be provided for the embodiments of the steam-generator which are described above and illustrated diagrammatically in the drawings. In particular the various tierods, pins, mountings, setscrews or pegs may be substituted by mechanically equivalent parts.

Claims

1. A steam-generator comprising a cylindrical vessel arranged vertically and accommodating a heat exchanger including a bundle of U-shaped tubes fastened to a tubeplate, a steam-separator above the heat exchanger and above the steam-separator a device for drying the steam generated, a releasable cover closing the upper end of the vessel, a manifold located beneath the tubeplate of the heat exchanger for supplying heat transfer medium to the tubes and discharging said medium therefrom, an inlet for supplying liquid to the vessel interior above the tubeplate of the heat exchanger and outlet means for discharging steam from the vessel, wherein the heat exchanger, the steam-separator and the drying device are so supported within the vessel to be readily separable from one another and the vessel by relative vertical displacement therebetween.

2. A steam-generator as claimed in claim 1, in which each connection between the vessel and the heat exchanger, the steamseparator and the drying device comprises an upwardly directed shoulder on the interior of the vessel, in abutment with a cooperating shoulder on the part connected thereto.

3. A steam-generator according to claim 2, wherein said cooperating shoulders are held in abutment by studs, screw elements, or pegs.

4. A steam-generator as claimed in claim 1 wherein the manifold is attached to the tubeplate of the heat exchanger and is separate from the vessel.

5. A steam-generator as claimed in claim 4, in which the tubeplate is supported on a shoulder on the inner wall of the vessel, a seal being interposed between the tubeplate and the shoulder.

6. A steam-generator according to claim 5, wherein the tubeplate is retained against the shoulder by pins or pegs extending horizontally or vertically between the vessel and tubeplate.

7. A steam-generator as claimed in claim 1, in which the manifold is connected rigidly to the vessel and is separate from the tubeplate.

8. A steam-generator as claimed in claim 7, in which the tubeplate is supported on the inner wall of the vessel shoulder, a seal being interposed between the tubeplate and the shoulder.

9. A steam-generator as claimed in claim 8, wherein the tubeplate is retained against the shoulder by pins or screw members which extend between the vessel and tubeplate in parallel with the axis of the vessel.

10. A steam-generator as claimed in claim 9, in which the outer ends of the pins or screw members are covered by caps welded to the vessel.

11. A steam-generator as claimed in claim 8, in which the tubeplate is retained against the shoulder by a row of pegs which extend between the vessel and tubeplate and are orientated radially to the axis of the vessel.

1 2. A steam-generator as claimed in claim 11, in which the outer ends of the pegs are covered by caps welded to the vessel.

1 3. A steam-generator as claimed in any one of the preceding claims, in which the heat exchanger includes a shell enclosing the tubes, and the steam-separator is connected to the shell by a flanged joint comprising cooperating flanges on the steam-separator and shell secured together by releasable fastening elements.

1 4. A steam-generator as claimed in any one of the preceding claims, in which the drying unit is supported by springs which are held compressed by the cover.

1 5. A steam-generator as claimed in claim 14, in which the drying unit is enclosed in a cylindrical shell having means defining a projection which engages and is supported by an annular shoulder on the inner wall of the vessel, the projection being pressed against the shoulder by the springs.

16. A steam-generator as claimed in any one of the preceding claims wherein the interior of the manifold is divided into two chambers by a partition which is rigidly connected to the inner wall of the manifold and is secured by pins or screw elements to the tubeplate, the partition having a double line of curvature.

1 7. A steam-generator substantially as herein described with reference to the accompanying drawings.