NOVELTY - The turbine (1) has a case including an inlet and an outlet for working fluid. A short-pipe (4) is constrained to the case. A stator (3) is installed inside the case. A rotor (2) is moved in an axial direction (X-X) between a configuration in which a mechanical unit (5) is installed inside the short-pipe and the rotor is spaced from the short-pipe so that working fluid revolve the rotor, and another configuration in which the mechanical unit is extracted from the short-pipe and the rotor leans against the short-pipe at static seals (18, 19).    USE - Expansion turbine e.g. turbo expander and gas and/or steam expansion turbine, for generating mechanical and/or electric energy in geothermal field and in plants.    ADVANTAGE - The turbine has improved structure of mechanical unit to facilitate maintenance of the turbine and related assembling and disassembling operations, to enable maintenance of the mechanical unit without requiring the working fluid to be removed from the turbine, to simplify manufacture and reduce cost of the mechanical unit, to increase reliability of the mechanical unit and the turbine, to increase accuracy in coupling between the stator and rotor parts of the turbine and to increase efficiency of the turbine. The mechanical unit, rotor and a tie-rod are moved until bringing conical surface of the rotor into abutment against the conical surface of the short-pipe, and generates the static seal to prevent leakage of the working fluid contained in the case. The turbine performs disassembling operations carried out without emptying the turbine case by moving the mechanical unit and the rotor backward relative to the short-pipe until bringing the rotor against the short-pipe at the static seal, locking the rotor against the short-pipe, and removing the mechanical unit from the short-pipe while the rotor is locked against the short-pipe, thus enabling quick maintenance of the turbine.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a method for disassembling expansion turbine.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross-sectional view of an expansion turbine.    Axial direction (X-X)    Expansion turbine (1)    Rotor (2)    Stator (3)    Short-pipe (4)    Mechanical unit (5)    Static seals (18, 19)