GB2206958A - Steam generators - Google Patents

Abstract

An internal tube bundle shroud (24) of a sodium heated steam generator unit (SGU) is connected to the SGU main shell by an H-section annular forging (114) fitted with an external bayonet-type flange (118) which is designed to co-operate with a similar flange (112) on the inside of a lower conical section (110) of the shell in such a way that the flange (118) provided on the annular forging seats on the flange (112) of the conical section thereby avoiding placing the connecting bolts (120) in tension. <IMAGE>

Description

Heat Exchanger Support Structures 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 shell while 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 one design of steam generator, the tube bundle extends through a generally vertical tubular shroud which is mounted within the main shell of the generator.

Desirably the annular gap between the shroud and the main shell should be relatively small and bolts employed in the mounting arrangement should not be loaded in tension by the weight of the shroud. The present invention addresses the problem of providing a mounting arrangement which fulfills these requirements.

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, a first mounting member provided on the main shell, a second mounting member provided on the tubular shroud and fastening means securingsthe first and second mounting members together, the mounting members having castellated profiles such that the castellations on the second member can pass through the gaps between the castellations of the first member, the two mounting members being orientated with the castellations of the second member seating on those of the first member.

Preferably the main shell comprises a radially enlarged chamber in the vicinity of a port thereof (eg an outlet port) and the first mounting member is provided on a wall of the enlarged chamber.

The second mounting member conveniently serves to interconnect successive axial tubular sections of the shroud.

The fastening means may be engaged with the castellations of the mounting members and the fastening means may additionally serve to locate an annular cover plate for covering the gaps between the castellations.

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 an enlarged fragmentary view showing the shroud mounting arrangement of the SGU; and Figure 3 is a fragmentary underside plan view, partly broken away, in the direction 3 in Figure 2.

Referring to Figure 1 of the drawings, the sodiumwater SGU illustrated is of the straight tube, bellows-in-shell type and comprises a verticallyelongated 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 2%) 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 crossflow 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-enter to the bulk flow of sodium (see arrows) for discharge via outlet 36.

In the vicinity of the outlet port 36, the main shell 10 comprises a radially enlarged chamber which, at its upper end, comprises a generally conical forging 110 forming part of the bolted flange joint 28. The forging 110 includes a number of radially inwardly projecting lugs or castellations 112 spaced uniformly around its inner periphery. In the vicinity of the forging 110, the shroud 24 is provided with an annular H-section forging 114 which joins axially successive cylindrical sections 116 of the shroud 24 and is formed with a number of radially outwardly projecting lugs or castellations 118 which are so spaced and dimensioned in relation to those of the forging 110, that the lugs 118 can pass between the lugs 112. In this way, the forging 114 can be assembled with the forging 110 in the manner of a bayonet fitting, the forgings 110, 114 being shown in Figures 2 and 3 with the lugs 118 seating on lugs 112.

Each pair of lugs 112, 118 are secured together by a pair of bolts 120 which pass freely through bores 122 in the lugs 112 and screw-threadedly engage in aligned tapped bores 124 in the lugs 118. It will be noted that the bolts 120 will not be loaded in tension by the weight of the shroud. The gaps 126 between the lugs 112, 118 are covered by an annular plate 128 to seal against sodium flow past the joint. It will be seen that the castellated configurations of the forgings 110, 114 together with location of the joint in the vicinity of the conical section of the outlet chamber of the main shell allows the gap 130 between the shroud 24 and the main shell to be relatively small which in turn makes it feasible to reduce the main shell diameter between the sodium inlet and outlet regions of the SGU.

Claims (5)

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, a first mounting member provided on the main shell, a second mounting member provided on the tubular shroud and fastening means securing the first and second mounting members together, the mounting members having castellated profiles such that the castellations on the second member can pass through the gaps between the castellations of the first member, the two mounting members being orientated with the castellations of the second member seating on those of the first member.

2. A heat exchanger as claimed in Claim 1 in which the main shell comprises a radially enlarged chamber in the vicinity of a port thereof and the first mounting member is provided on a wall of the enlarged chamber.

3. A heat exchanger as claimed in Claim 1 or 2 in which the second mounting member serves to interconnect successive axial tubular sections of the shroud.

4. A heat exchanger a claimed in Claim 1, 2 or 3 in which the fastening means is engaged with the castellations of the mounting members and the fastening means additionally serves to locate an annular cover plate for covering the gaps between the castellations.

5. A tube-in-shell heat exchanger substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.