Of an output exceeding 40 MW

84.06 - Steam turbines and other vapour turbines. 8406.10 - Turbines for marine propulsion - Other turbines : 8406.81 - - Of an output exceeding 40 MW 8406.82 - - Of an output not exceeding 40 MW 8406.90 - Parts This heading covers steam turbines which are driven by the kinetic energy of expanding steam applied to the vanes or blades of a wheel. They consist essentially of: a (1) A rotor comprising a shaft on which is mounted a wheel (or wheels the rim of which carries a row of closely-spaced vanes or blades generally of curve cross-section and sometimes referred to as " buckets ". (2) A stator consisting of a casing, in which the rotor is supported and revolves, containing a system of stationary blades or nozzles to direct the steam on to the blading of the rotor. In " impulse " turbines the stator is provided with nozzles in which the steam expands and issues at hi h velocity tangentially to the bucket blades of the rotor. In a reaction " turbine the blades on t%e rotor revolve between similarly sha ed stationary blades mounted in the reverse form round the face of the stator, and so arranged t at the steam flows axially through the blades of the stator and on to the adjacent blades of the rotor. " R For greater efficiency two systems are oRen combined in " compound turbines ", but more often a series of rotors are mounted on a common shaft (multi-stage turbines) to allow progressive expansion of the steam. The high rotational speed of turbines makes them particularly suitable for the direct driving of machines such as electric generators (turbo-generators), compressors, ventilators or centrifugal pumps. For some purposes (e steamships and certain locomotives) turbines are fitted with reversing or reduction gear. presented separately, these reversing or reduction gears are excluded (heading 84.83). din This heading also covers mercury vapour turbines. These are of a structure and use similar to the steam turbines described above, but use mercury vapour in place of steam. PARTS An essential component of a turbine is a governing mechanism so that the supply of steam or other vapour to the turbine can be adjusted to suit the load and to maintam constant speed. This heading covers such governors and, subject to the general provisions r e g d mf .the classification of parts (see the General Explanatory Note to Section XVI), other parts of tur lnes (e.g., rotors and stators and their segments, rotor or stator blades).