A U-shaped loop in a pipe or tube in which a liquid is trapped and used to provide a seal for gas flow. For example, the pipe from a gravity-fed reflux from the condenser of a distillation column contains a lute. The distillation column has a higher pressure than the condenser and the lute prevents vapour entering from the wrong direction. The depth of the lute is determined from hydrostatics and is greater than the maximum vapour pressure difference between the column and the condenser outlet.