6.2.1 Flange design
The forces taken into account in flange design are illustrated in Fig 1.98. The flange is assumed to seal at the inside edge, with the faces subject to a contact pressure greater than the internal cylinder pressure.
To minimise bending stress on the flanges, the bolt centreline is arranged tangential to the shell as can be seen in Figs 1.93 and 1.95; this produces the typical thick narrow flange seen on HP and IP cylinders. The seal is obtained by simple metal-to-metal contact of the flange faces, after careful bedding on initial assembly.
When flanges on HP and IP cylinders are unbolted after a period in service, they are often found to have distorted so that they no longer fit, typically leaving a gap between the flanges on the inside face. This is due to temperature differences through the casing on-load causing thermal stresses which relax by creep. When off-load, opposite stresses are then induced which distort the casing. This is generally confined to inner casings as there is very little temperature differential through an outer casing on-load due to the external lagging. In addition, distortion can occur due to relaxation of residual casting stresses. If distortion causes a small flange leak on an inner casing, it may not be noticed unless large enough to have an effect on the measured efficiency of the turbine, unless it is a wet steam turbine, when rapid erosion of the joint face at the leak may occur.
LP inner casings may also distort substantially due to their complicated design of webs, bled-steam belts, etc., and to the substantial temperature differences between different parts of the casing. This can lead to crushing of the webs and distortion of horizontal joints, though leakage can be minimised by incorporating spring sealing-strips in the half-joint faces.