The casings or shells at the high-pressure inlet end of the steam turbine must be high-strength pressure vessels to contain the internal steam pressure. The design is made more difficult by the need for a casing split at the horizontal center line for assembly. The horizontal flange and bolt design must be leakproof. Shell design problems lead to the use of small-diameter, high-speed turbines at high pressure, and the use of double-shell construction.
Turbine buckets must be strong enough to withstand high centrifugal, steam bending and vibration forces. Buckets must be designed so that their resonant natural frequencies avoid the vibration stimulus frequencies of the steam forces, or are strong enough to withstand the vibrations.
It is necessary to minimize to the greatest possible extent the wasteful leakage of steam along the shaft both at the ends and between stages. The high-peripheral velocities between the shaft and stationary members preclude the use of direct-contact seals. Seals in the form of labyrinths with thin, sharp teeth on at least one of the members are utilized.