Abstract: In a steam turbine having a main inlet pipe which introduces steam to a nozzle chamber through the turbine casings and a bell seal assembly for sealing the inlet pipe relative to the casings and the nozzle chamber, the bell seal assembly including a bell seal disposed in the nozzle chamber, and a retainer nut sleeved by the bell seal and threadedly engaged in the inlet pipe, the improvement which comprises: the bell seal being replaced by a ring carrier, a stack of inner and outer sealing rings loosely sleeved on the ring carrier, the material of the inner rings having a lower coefficient of thermal expansion than the material of the ring carrier and the outer rings being made from a material having a higher coefficient of thermal expansion than the material of the nozzle chamber.