Abstract: A main steam inlet structure of a steam turbine has two inlet tubes which are separable from an inner casing and an outer casing of the turbine and are arranged concentrically with a spacing therebetween to constitute an inflow passage for main steam are connected at one end to the outer casing and at the other end to the inner casing, by seal rings. Cooling steam extracted from turbine stage midway is supplied to a cooling steam passage of the spacing formed between the two inlet tubes, thereby cooling the inner inlet tube.

Abstract: A stress controlling system for a rotor of a steam turbine determines thermal and rotational conditions of the rotor. Thermal and centrifugal stresses produced in the rotor during operation of the steam turbine are calculated from the thermal and rotational conditions, respectively, of the rotor. Also, brittle fracture toughness of the material of the rotor under the thermal condition determined is calculated. Operation of the steam turbine is controlled to prevent the calculated brittle fracture toughness from being exceeded by the combined stress of thermal and centrifugal stresses during operation of the steam turbine.