6.5 Support and alignment
To achieve maximum efficiency in a steam turbine, only small clearances are permitted between fixed and moving parts at casing and diaphragm glands, and at blade tip seals. These clearances must be maintained under all operating conditions, so the inner and outer casings must be supported in such a way as to maintain concentricity with the rotor as they expand and contract.
The total axial expansion of a 500 MW turbine rotor between cold and hot conditions is about 50 mm, occurring mainly in the HP and IP cylinders, where the temperature change is greatest. The casings are supported to allow axial expansion and yet maintain the axial clearances between fixed and moving blades which may be only a few millimetres. Maintaining both concentricity and correct axial expansion leads to a complicated system of sliding supports and keys (Fig 1.101).