Why does a steam turbine need a casing?

In a steam turbine, the casing serves as a critical pressure vessel that encloses the rotor and contains the high-pressure steam.12

Beyond its role as a containment structure, the casing is essential for the following functions:

• Directing Steam Flow: It accommodates fixed nozzle passages or stator vanes, which are necessary to accelerate the steam before it is directed against the rotor blading.1
• Supporting Components: It provides the structural environment within which the rotor turns.1
• Managing Forces: In certain designs, such as reaction stages, the casing must withstand the effects of pressure drops that exert axial forces on the rotor, necessitating the use of thrust bearings to absorb the resulting load.3

By housing these components, the casing ensures that the thermal energy of the steam is efficiently converted into the rotary motion of the shaft.1