6.2.3 Methods of manufacture and construction - part 5
The method selected for securing tubes to the tube-plates provides leak-tightness and strength as a supporting structure for the plate. Leak-tight sliding joints are used to compensate for expansion. Various methods may be used in combination to provide characteristics consistent with the design requirements and specific service conditions.
Details of configurations commonly used on large condensers are shown on Fig 4.27. Figure 4.27 (a) shows a typical double tubeplate arrangement, using a combination of the fixed and sliding joint, the inner and outer tubeplates being of the same material. This design was extensively used on the 500 MW condensing plant.
Figure 4.27 (b) shows another typical double tube-plate arrangement, using rolled fixings at each joint; the tubeplates are of dissimilar metals, hence the need for the wide interspace gap to accommodate bending movements and to reduce shear stress in the tubes. This is the method proposed for the PWR design.
Figure 4.27 (c) shows a double tubeplate arrangement of similar design to Fig 4.27 (b), but here the tubeplates are of similar materials and the need for the wide interspace gap is eased.
Figure 4.27 (d) shows a typical double tubeplate arrangement, using a combination of fixed and sliding joints, with dissimilar metals for the inner and outer plates, as used on the more recent Heysham 2 and Torness power station designs.
Each of the above arrangements has been employed successfully. Typical tube sizes are 25.4 mm OD and 1.2 mm wall thickness; tubeplate materials are mild steel and rolled naval brass, Muntz metal or aluminum bronze for the outer plates (see Table 4.2). Titanium tubes of 25.4 mm OD and 0.7 mm wall thickness have been fixed successfully into a similar configuration to that shown in Fig 4.27 (d).
Properly made joints provide the necessary integrity to meet the design criteria for remaining leak-free in service; however, the uniformity of holding strength will vary between the methods used and it is difficult to see how this can be readily compensated in the design of the tubeplates unless some other form of fixing which possesses inherent uniformity, such as welding, can be adopted.
Tube expansion is carried out at the inlet end inner tubeplate first, using electrically-controlled roller expanders. Particular care is taken in setting up the expanders to ensure that over-expansion beyond the edge of the tubeplate does not occur. On completion of the inner tubeplate, the inlet end outer tubeplate is then packed, or expanded and belled as appropriate. The procedure for the outlet end is similar, except that the tubes are not belled. Manufacturers adopt different expanding patterns for the outlet end based on individual experience; however, a predetermined sequence is always adopted in order to minimise tubeplate distortion at the outlet end.