System for liquefying a gas

System (100) for liquefying a gas comprising:

• a gas intake (1) for connection to a gas source (101);
• at least one gas compressor;
• a gas expansion device (3) connected for being fed with compressed gas produced by the at least one gas compressor, and adapted to produce both liquefied gas and expanded gas from the compressed gas;
• and
• a return duct (97) connected for driving the expanded gas from a gas outlet (33) of the gas expansion device (3) to a duct node (10) situated between the gas intake (1) and the at least one compressor,
• characterized in that the at least one gas compressor comprises a liquid piston gas multistage compressor (2) having at least two compressor stages (21-23;
• 21-25) connected serially in an ordered chain between the gas intake (1) and an end gas outlet (29), each compressor stage comprising at least one cylinder supplied with driving liquid, and also comprising a liquid high-pressure supply device arranged for alternately increasing and decreasing a driving liquid quantity contained within the cylinder, so as to load, compress and discharge gas at the compressor stage, each compressor stage (22-23;
• 22-25) other than the first one (21) in the chain, and called higher compressor stage, being connected to process gas which is outputted by a preceding compressor stage situated in the chain just before said higher compressor stage, through an intermediate gas duct (28) connecting said preceding compressor stage to said higher compressor stage, so that gas flowing from the gas intake (1) is pressure-increased each time it is processed by one of the compressor stages, and gas outputted at the end gas outlet (29) has been processed successively by all the compressor stages of the chain,
• the gas expansion device (3) being connected for receiving compressed gas from the end gas outlet (29) of the liquid piston gas multistage compressor (2), or from an intermediate gas outlet situated at one intermediate gas duct (28) between two compressor stages (21-23;
• 21-25) successive in the chain.