A gas replacement method for an expander of a cryocooler, the expander including an expander cylinder, a pressure switching valve that switches a pressure inside the expander cylinder, a connection flow path from the pressure switching valve to the expander cylinder, and an expander motor that drives the pressure switching valve, includes connecting a nonflammable gas source to the connection flow path or the expander cylinder, and purging a residual gas in the expander cylinder with a nonflammable gas from the nonflammable gas source while the expander motor is stopped.

A blow-down method is applied to a reliquefaction system in which boil-off is compressed by a compressor, cooled and reliquefied through a heat exchanger, and returned to the tank after passing through a gas-liquid separator. The reliquefaction system includes a pressure compensation line extending from a downstream side of the compressor to an upside of the gas-liquid separator without passing through the heat exchanger; and a nitrogen blanket line along which nitrogen is supplied to the pressure compensation line. In the event of a trip of the reliquefaction system, nitrogen is supplied to the pressure compensation line along the nitrogen blanket line and is delivered to the downstream side of the compressor to pass through the heat exchanger along the reliquefaction line such that the heat exchanger and the reliquefaction line are blown down to remove compressed gas and reliquefied gas therefrom while being subjected to nitrogen (N.sub.2) purging.

A leakage detection system includes: a gas supply line along which boil-off gas is supplied to a compressor; a reliquefaction line along which the compressed boil-off gas is cooled and reliquefied through a heat exchanger and is returned to a storage tank; a refrigerant circulation line along which a refrigerant is circulated; a refrigerant compressor to compress the refrigerant discharged from the heat exchanger; a refrigerant expander expanding and cooling the compressed refrigerant compressed by the refrigerant compressor and having been cooled through the heat exchanger and supplying the cooled refrigerant back to the heat exchanger; and a refrigerant charge line connecting an inventory tank filled with nitrogen to an upstream side of the refrigerant compressor. Upon initial start-up or restart of a reliquefaction process, the nitrogen is supplied to the upstream side of the refrigerant compressor via the refrigerant charge line to detect leaks in the refrigerant circulation line.

Systems and methods for recovering natural gas liquids include a distillation column; a thermosyphon reboiler fluidly connected to the distillation column; a Joule-Thomson valve fluidly connected to the distillation column and operable to control a flow of fluid into the distillation column; and a lift gas control valve operable to control a flow of lift gas into the distillation column through a return line of the thermosyphon reboiler. An interlock system operatively couples the Joule-Thomson valve and the lift gas control valve to operate the lift gas control valve in response to operation of the Joule-Thomson valve.

A process for separating a gas mixture, a first item of equipment of the process is heated by a first flow of a first heat producer and supplies heat to a first fluid, the first flow of heat producer being heated upstream of the first item of equipment by a heat source, a second item of equipment of the process heats a second fluid while consuming heat, the heat originating from a second flow of the first heat producer heated by the heat source, which heats the second flow of the first heat producer up to an intermediate temperature lower than the second temperature, and by a heater downstream of the source, which heats the second flow of the first heat producer, which has been heated by the heat source, from the intermediate temperature up to a second temperature higher than the first temperature.