GB2206959A

GB2206959A - Tube bundle restraint in heat exchangers

Info

Publication number: GB2206959A
Application number: GB08716456A
Authority: GB
Inventors: Harry Bainbridge; Clive Michael Johnson
Original assignee: National Nuclear Corp Ltd
Current assignee: National Nuclear Corp Ltd
Priority date: 1987-07-13
Filing date: 1987-07-13
Publication date: 1989-01-18
Also published as: GB8716456D0

Abstract

A tube supporting grid mounted within a tube bundle shroud of a heat exchanger e.g. a steam generator is fixed against lateral and axial movement by pins (96) spaced at 90 DEG intervals around the grid flange (92) and by rings (102) located on opposite axial sides of the grid flange (92). The pins (96) allow radial expansion and contraction of the grid, and the arrangement of pins (96) and rings (102) also facilitates alignment of the grids prior to tubing and resists movement during seismic incidents. <IMAGE>

Description

Tube bundle restraint in heat exchangers This invention relates to heat exchangers and, in particular to tube-in-shell heat exchangers of the type in which a liquid alkali metal, usually sodium, is circulated through the shellvwhile fluid such as water, in its liquid and/or vapour state, is passed through the tubes. Such heat exchangers are used for example as steam generators in liquid metal cooled fast fission nuclear reactor plants.

In such heat exchangers, the tubes are supported laterally in spaced relation by a series of supporting grids which are spaced lengthwise of the tubes. The present invention is concerned with the mounting of the tube-supporting grids within a heat exchanger.

According to the present invention there is provided a tube-in-shell heat exchanger comprising a main shell structure provided with tube plate means, a tube bundle whose opposite ends are connected to the tube plate means, a tubular shroud located within the main shell and enclosing the tube bundle and a series of tube-supporting grids mounted within the shroud in spaced relation lengthwise of the tube bundle, at least one of the grids being mounted in the shroud by means of a number of male and female connections in which the male and female formations are permitted to slide generally radially relative to one another, such connections being so spaced around the periphery of the grid that the grid is restrained against radial translational movement relative to the shroud by registry of the male formations with the female formations but is free to undergo thermal expansion and contraction in the radial direction.

In one embodiment of the invention, the male formations are provided on the shroud and the female formations are provided in a peripheral part of the grid.

The male and female formations may also serve to restrain the grid from movement in the axial direction, ie lengthwise of the tube bundle. Additionally or alternatively, axial restraint may be provided by abutment members mounted on the shroud on each side of the grid so that the grid is trapped axially but is free to expand and contract radially. The abutment members conveniently comprise a pair of axially spaced rings fastened to the shroud so as to project radially inwardly.

The male and female formations preferably take the form of pins and apertures respectively and, in a presently preferred embodiment, there are four male/female connections located at substantially 900 intervals around the grid. In the preferred embodiment, axial restraint of the grid is provided primarily by the abutment members but, if desired, the male/female connections may be employed as the sole means of providing such axial restraint.

To provide further understanding of the invention, one embodiment will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a diagrammatic in longitudinal section of a steam generating unit (SGU) for use in a liquid metal cooled fast fission reactor plant; Figure 2 is a fragmentary plan view of a tube-supporting grid mounted within the shroud of the SGU; and Figure 3 is a fragmentary view in the direction 3-3 in Figure 2.

Referring to Figure 1 of the drawings, the sodiumwater SGU illustrated is of the straight tube, bellowsin-shell type and comprises a vertically-elongated main shell 10 extending between a feed water inlet header 12 with inlet nozzle 13 and a steam outlet header 14 with outlet nozzle 15. Each header 12, 14 incorporates a tube-plate 16, 18 and a tube bundle (shown in phantom outline - see reference 19 in Figures 1 and 2) extends between the two tubeplates 16, 18 to conduct water/steam through the interior of the shell from the inlet header 12 to the outlet header 14.

The tube bundle 19 is enclosed within a circularsection flow shroud 24 which serves to limit tube bundle by-pass flow of liquid sodium, protect the main shell 10 from thermal transients and any sodium-water reaction wastage, and provide a means for locating support grids 26 for the tube bundle without impairing the integrity of the main shell 10 or resorting to tie rods. The shroud 24 is supported from the main shell by a forged-flange bolted joint 28.

Sodium enters the SGU main shell 10 via inlet nozzle 30 and is admitted to the interior of the flow shroud 24 after passage through an annular chamber 31 and an annular distribution grid 32 which is perforate and serves to produce a substantially circumferentiallyuniform velocity distribution in the sodium prior to entry into the tube bundle. The sodium flow then proceeds into the shroud and downwardly via grids 26 in heat exchange with the water/steam carrying tubes before the main bulk of the sodium flow emerges laterally at openings in section 34 of the shroud and leaves the main shell 10 through the outlet nozzle 36.

The shroud 24 continues downwardly beyond the outlet section 34 to introduce a small proportion (eg about 28) of the total sodium flow into the lower region of the SGU to form a buffer or substantially quiescent zone of sodium which acts as a thermal barrier to protect the lower tubeplate 16 from sodium temperature transients at the SGU sodium outlet, ie by forming a stratified region of relatively low temperature sodium in the region extending downwardly from the outlet shroud section 34 to the lower tube plate 16. The reduced flow of sodium into the buffer zone is achieved by grids 38, 40 and 42, the grids 38 and 40 being flow-redistributing grids for creating a substantial amount of cross flow to mix the sodium flows and avoid hot spots developing in the event of some of the steam tubes having to be plugged at some stage in the life of the SGU.The grid 42 is a high resistance grid which is penetrated by the tube bundle but permits passage of only about 2% of the total sodium flow.

The shroud 24 terminates a short distance above the lower tube plate to leave a clearance through which sodium may discharge into an annular region between the shroud 24 and the outer wall of the lower SGU shell 50 which is coupled to the main shell 10 via bellows 52. This sodium eventually re-enters to the bulk flow of sodium (see arrows) for discharge via outlet 36.

Referring now to Figures 2 and 3, this illustrates in greater detail the mounting arrangement for the tube-supporting grids, such as the grids 26 within the shroud 24. Each grid 26 is formed with an array of tube receiving openings, the centres of which are depicted, by reference numeral 90 in Figure 3, for a small area of the grid - it will be understood that such openings are provided over the full radial extent of the grid. At its periphery, each grid 26 has a circumferential flange 92 (see Figure 2) which is formed at 900 intervals with cylindrical throughbores 94 for reception of headed dowel pins 96 secured to the shroud by screwing them into threaded apertures in the shroud wall and locking the pins by means of weld fillets 98.

The pins 96 are each received as a close sliding fit in the bores 94 but, because of their spacing around the periphery of the grid, it will be seen that they restrain the grid 26 against translational movement in a radial direction relative to the shroud without impeding thermal expansion and contraction of the grid. In other words, whilst the grid is free to expand and contract radially, it is restrained against shifting bodily, as a whole, relative to the shroud. The grids 26 are restrained against axial movement within the shroud 24 by a pair of shear rings 102 each secured, as by nut and bolt connections 104, to the shroud in axially spaced relation so that the grid flange 92 is trapped between them.

During assembly of the SGU, the grids are mounted within the shroud prior to insertion of the tube bundle.

Grid installation involves locating and aligning each grid in the desired axial and radial orientation by means of a spider support, fastening the associated shear rings 102 to the shroud to restrain the grid axially, drilling aligned holes 94, 100 at 900 intervals through the shroud 24 and the grid flange 92, threading the holes 100 and threading the pins 96 into the holes 100 so that they project with clearance into the holes 94 in the grid.

It will be seen that the pins 96 restrain the grid laterally (and also axially in conjunction with the shear rings 102) so as to maintain manufacturing alignment between the grids in the cold condition thereby facilitating threading of the tubes 19 through the grids.

The lateral restraint also resists seismic movements without interfering with thermal expansion and contraction of the grids.

Claims

1. A tube-in-shell heat exchanger comprising a main shell structure provided with tube plate means, a tube bundle whose opposite ends are connected to the tube plate means, a tubular shroud located within the main shell and enclosing the tube bundle and a series of tube-supporting grids mounted within the shroud in spaced relation lengthwise of the tube bundle, at least one of the grids being mounted in the shroud by means of a number of male and female connections in which the male and female formations are permitted to slide generally radially relative to one another, such connections being so spaced around the periphery of the grid that the grid is restrained against radial translational movement relative to the shroud by registry of the male formations with the female formations but is free to undergo thermal expansion and contraction in the radial direction.

2. A heat exchanger as claimed in Claim 1 in which the male formations are provided on the shroud and the female formations are provided in a peripheral part of the grid.

3. A heat exchanger as claimed in Claim 1 or 2 in which the male and female formations also serve to restrain the grid from movement in the axial direction, ie lengthwise of the tube bundle.

4. A heat exchanger as claimed in Claim 1, 2 or 3 in which axial restraint is provided by abutment members mounted on the shroud on each side of the grid so that the grid is trapped axially but is free to expand and contract radially.

5. A heat exchanger as claimed in Claim 5 in which the abutment members comprise a pair of axially spaced rings fastened to the shroud so as to project radially inwardly.

6. A heat exchanger as claimed in any one of Claims 1-5 in which the male and female formations take the form of pins and apertures respectively.

7. A heat exchanger substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

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