Method for separating air by cryogenic distillation, wherein: i) compressed and purified air is cooled in a heat exchanger, a first portion of the air is compressed at an intermediate temperature of the heat exchanger in a compressor and sent back to the heat exchanger, where it is cooled, the first portion of the air is found liquefied and is sent to at least one first column of a double column, the double column comprising the first column and a second column, the second column functioning at a lower pressure than the first column, ii) oxygen- and nitrogen-enriched liquids are sent from the first column to the second column, an oxygen-enriched fluid is extracted in the sump of the second column and a nitrogen-enriched fluid is extracted from the head of the second column and is heated in the heat exchanger, iii) a second portion of the air (3, 5) exits from the heat exchanger at an intermediate temperature of it and possibly is then split into a first and a second fraction at a dividing point, the second portion of the air or at least one portion of the first fraction for the case where the second portion of the air is split into two fractions is expanded in a first turbine and sent to the first column, and possibly at least one portion of the second fraction is expanded in a second turbine and sent to the first column for the case where the second portion of the air is split into two fractions, and iv) the backflow of the compressor is connected to the inlet of the first turbine or for the case with two turbines at the inlet of at least one of the first and second turbines through a short-circuiting pipe and an arrival point which makes it possible to send air from the compressor to the first turbine or for the case with two turbines to one of the turbines without passing through the heat exchanger, characterised in that the second portion of the air is sent to a check valve downstream from the heat exchanger and possibly upstream from the dividing point for the case with two turbines, the valve being used to prevent air from passing in the direction opposite that of the normal operation and arrive from the arrival point in the exchanger, being arranged on a pipe between the arrival point and the exchanger. i) compressed and purified air is cooled in a heat exchanger, a first portion of the air is compressed at an intermediate temperature of the heat exchanger in a compressor and sent back to the heat exchanger, where it is cooled, the first portion of the air is found liquefied and is sent to at least one first column of a double column, the double column comprising the first column and a second column, the second column functioning at a lower pressure than the first column, ii) oxygen- and nitrogen-enriched liquids are sent from the first column to the second column, an oxygen-enriched fluid is extracted in the sump of the second column and a nitrogen-enriched fluid is extracted from the head of the second column and is heated in the heat exchanger, iii) a second portion of the air (3, 5) exits from the heat exchanger at an intermediate temperature of it and possibly is then split into a first and a second fraction at a dividing point, the second portion of the air or at least one portion of the first fraction for the case where the second portion of the air is split into two fractions is expanded in a first turbine and sent to the first column, and possibly at least one portion of the second fraction is expanded in a second turbine and sent to the first column for the case where the second portion of the air is split into two fractions, and iv) the backflow of the compressor is connected to the inlet of the first turbine or for the case with two turbines at the inlet of at least one of the first and second turbines through a short-circuiting pipe and an arrival point which makes it possible to send air from the compressor to the first turbine or for the case with two turbines to one of the turbines without passing through the heat exchanger, characterised in that the second portion of the air is sent to a check valve downstream from the heat exchanger and possibly upstream from the dividing point for the case with two turbines, the valve being used to prevent air from passing in the direction opposite that of the normal operation and arrive from the arrival point in the exchanger, being arranged on a pipe between the arrival point and the exchanger.