Process for the supply of pressurised gas in which a) a pressurised gas is produced at a high production pressure as a final product by separation of a gaseous mixture in a separation device (3,5), b) the liquid that is to undergo pressurisation is stored in a storage tank, c) liquid is extracted from the storage tank and pressurised by means of a pump, and at least part of the pressurised liquid is vaporised in a vaporiser to produce a pressurised backup gas featuring substantially the same purity or a higher purity than the pressurised gas that is to be produced, d) liquid is circulated through a substantially vertical duct, possibly inside the cold box of the separation device, and at least one section of the length of the duct is located above the vaporiser, and the duct is supplied, at least partially, from a column of the separation device, which is a cryogenic air separator located inside the cold box, where, in normal operating mode, the separation device supplies produced gas that has the same characteristics as the backup gas through a primary duct, by vaporisation of a liquid extracted from the air separator, a liquid flow is sent to one end of the duct, and another liquid flow is extracted from the other end of the duct and is returned to the air separator and, before or during the starting of the pump, liquid from the duct is sent to the vaporiser where it vaporised to produce the pressurised backup gas that features substantially the same purity or a higher purity than the pressurised gas that is to be produced and i. in normal operating mode the separation device provides produced gas, ii. in the event of a failure or downgraded operations of the separation device, the backup gas is provided by vaporisation of the liquid pumped in the vaporiser, iii. in normal operating mode, the liquid flow circulates continuously in the duct, iv. before and/or during the starting of the pump, a liquid flows by gravitational force from the lower end of the duct towards the vaporiser and a gas flow pressurised to a pressure equal to or greater than the pressure of the pressurised gas that is to be produced is sent to the upper end of the duct. a) a pressurised gas is produced at a high production pressure as a final product by separation of a gaseous mixture in a separation device (3,5), b) the liquid that is to undergo pressurisation is stored in a storage tank, c) liquid is extracted from the storage tank and pressurised by means of a pump, and at least part of the pressurised liquid is vaporised in a vaporiser to produce a pressurised backup gas featuring substantially the same purity or a higher purity than the pressurised gas that is to be produced, d) liquid is circulated through a substantially vertical duct, possibly inside the cold box of the separation device, and at least one section of the length of the duct is located above the vaporiser, and the duct is supplied, at least partially, from a column of the separation device, which is a cryogenic air separator located inside the cold box, where, in normal operating mode, the separation device supplies produced gas that has the same characteristics as the backup gas through a primary duct, by vaporisation of a liquid extracted from the air separator, a liquid flow is sent to one end of the duct, and another liquid flow is extracted from the other end of the duct and is returned to the air separator and, before or during the starting of the pump, liquid from the duct is sent to the vaporiser where it vaporised to produce the pressurised backup gas that features substantially the same purity or a higher purity than the pressurised gas that is to be produced and i. in normal operating mode the separation device provides produced gas, ii. in the event of a failure or downgraded operations of the separation device, the backup gas is provided by vaporisation of the liquid pumped in the vaporiser, iii. in normal operating mode, the liquid flow circulates continuously in the duct, iv. before and/or during the starting of the pump, a liquid flows by gravitational force from the lower end of the duct towards the vaporiser and a gas flow pressurised to a pressure equal to or greater than the pressure of the pressurised gas that is to be produced is sent to the upper end of the duct. i. in normal operating mode the separation device provides produced gas, ii. in the event of a failure or downgraded operations of the separation device, the backup gas is provided by vaporisation of the liquid pumped in the vaporiser, iii. in normal operating mode, the liquid flow circulates continuously in the duct, iv. before and/or during the starting of the pump, a liquid flows by gravitational force from the lower end of the duct towards the vaporiser and a gas flow pressurised to a pressure equal to or greater than the pressure of the pressurised gas that is to be produced is sent to the upper end of the duct.