Plant for the liquefaction of gas, particularly network gas, comprising: - at least one compressor adapted to increase the pressure of at least one gas in order to obtain compressed gas; - at least one cooling device connected in a fluid-operated manner to said compressor, adapted to cool said compressed gas; - at least one expansion assembly (6, 10, 20) which is connected in a fluid-operated manner to said cooling device, adapted to reduce the pressure of said compressed gas in order to obtain liquefied gas; and - at least one filling station connected in a fluid-operated manner to said expansion assembly (6, 10, 20) and adapted to fill at least one cryogenic reservoir with said liquefied gas; wherein said expansion assembly (6, 10, 20) comprises at least one piston expander comprising at least one expansion chamber which extends along a first axial direction and provided with at least one intake and exhaust mouth and at least one piston housed in said expansion chamber and moveable sliding inside said expansion chamber along said first axial direction, wherein said piston expander comprises at least one fluid-operated distributor associated with said mouth and adapted to control the flow direction of said gas; wherein said fluid-operated distributor comprises: - at least one valve body comprising at least one sliding chamber (12a, 12b) having a substantially elongated shape which extends along at least a second axial direction and provided with at least one inlet opening for the inlet of said compressed gas, at least one exhaust opening for the exhaust of said liquefied gas and at least one mouth opening associated with said mouth for the connection of said fluid-operated distributor to said expansion chamber of said piston expander; and - at least one slider (16a, 16b) having a substantially elongated shape, housed in said sliding chamber (12a, 12b), configured for moveable sliding along said second axial direction, characterised in that the at least one slider comprises at least one internal duct (17a, 17b) for the passage of said gas, configured for fluidly connecting said inlet opening with said mouth opening and said mouth opening with said exhaust opening. - at least one compressor adapted to increase the pressure of at least one gas in order to obtain compressed gas; - at least one cooling device connected in a fluid-operated manner to said compressor, adapted to cool said compressed gas; - at least one expansion assembly (6, 10, 20) which is connected in a fluid-operated manner to said cooling device, adapted to reduce the pressure of said compressed gas in order to obtain liquefied gas; and - at least one filling station connected in a fluid-operated manner to said expansion assembly (6, 10, 20) and adapted to fill at least one cryogenic reservoir with said liquefied gas; - at least one valve body comprising at least one sliding chamber (12a, 12b) having a substantially elongated shape which extends along at least a second axial direction and provided with at least one inlet opening for the inlet of said compressed gas, at least one exhaust opening for the exhaust of said liquefied gas and at least one mouth opening associated with said mouth for the connection of said fluid-operated distributor to said expansion chamber of said piston expander; and - at least one slider (16a, 16b) having a substantially elongated shape, housed in said sliding chamber (12a, 12b), configured for moveable sliding along said second axial direction, characterised in that the at least one slider comprises at least one internal duct (17a, 17b) for the passage of said gas, configured for fluidly connecting said inlet opening with said mouth opening and said mouth opening with said exhaust opening.