6 Turbine casings
6.1 Forms of casing construction
A turbine cylinder is essentially a pressure vessel with its weight supported at each end on the horizontal centreline. It is designed to withstand hoop stresses in the transverse plane, and to be very stiff in the longitudinal direction in order to maintain accurate clearances between the stationary and rotating parts of the turbine.
The design is complicated by the need for internal access, all casings being split along their horizontal centreline, allowing the rotor to be inserted as a complete assembly. Substantial flanges and bolting are required to withstand the pressure forces at the horizontal joints. The relatively massive flanges respond more slowly to temperature changes than the rest of the casing, resulting in different rates of expansion and the setting-up of temperature stresses and distortion, although these are minimised by the application of flange warming steam. Further stress complexities are set up by the gland housing and steam entry and exit passages.
HP and IP casings are of cast construction and are circular in cross-section to minimise non-membrane stresses. Flanges, bolting, steam penetrations and other features are as far as possible symmetrically arranged to reduce thermal asymmetry and hence distortion. LP casings may be fabricated or a combination of castings and fabrications.
As with all pressure vessels, the integrity of the design is checked after manufacture with a hydraulic pressure test, to 150% of the highest working pressure.