The design of bolting on HP and IP cylinders is complicated by the need to place the bolts close together to get the necessary distribution of stress in the flange. Special nuts are used to allow close spacing, either cap nuts with faces cut on the reduced head of the nut or entirely circular nuts with small holes drilled in them, allowing them to be turned by tommy bars.
Because of friction, the necessary bolt tension could not be achieved in the large diameter casing bolts simply by turning the nuts, even if large flogging spanners could be used. Instead, the nuts are first tightened by hand; the bolts are then heated to expand them, and the nuts again tightened by hand through an angle calculated to give the correct bolt-tension after the bolt cools down. By measuring the bolt length before and after tightening, the extension (and hence the bolt tension) can be checked. The bolts are heated by inserting an electric heating rod into a thin hole drilled down the centre of each bolt, thus heating the bolt without significantly warming the flanges (Fig 1.99). The same method can be used for undoing or retightening the bolts.
An alternative method of tightening the bolts is by hydraulic tightening. This is similar to the heating method except that the elongation is achieved by stretching it with hydraulic jacks reacting against the flange or adjacent bolts, instead of by thermal expansion. The hydraulic equipment is more cumbersome but has the advantage of allowing immediate checking of the bolt tension without waiting for temperatures to stabilise, again by measuring the extension.
Inner cylinders are assembled with the lower half of the casing resting in its final position inside the outer casing. It is thus awkward to use normal bolts on the inner casing and studs are often used instead, being screwed into the lower casing and then tightened in the normal manner. Alternatively, a square collar on the bolt at the horizontal joint allows the lower nut to be assembled and tightened before the casing is positioned.
HP and IP casings are only bolted together around the external flange, but LP casings may also be bolted along some of the internal webs if access through manholes is possible. The bolts used on LP casings, being of small diameter, are tightened by spanner in the conventional manner to a controlled torque.
High temperature bolts operating above 370°C suffer from stress relaxation in use due to creep and such bolts need periodic retightening to prevent leakage of steam at the joint, typically after 30 000 h service. Some nickel-based alloy bolts operating below 538°C may, however, increase their tension with time and require slackening and retightening to prevent early bolt failure. A record of the history of all high temperature bolts is kept, so that they can be re-tightened or replaced at appropriate times. Bolts must be replaced before the accumulated creep strain exceeds the creep ductility of the material; this often means changing steam chest cover bolts, but this is rarely necessary for turbine casing bolts.