Pipe freezing is increasingly used in a range of industries to solve otherwise intractable pipe line maintenance and servicing problems. There is, however, a limit to the size of circular pipe that may be plugged using the technique. lee, and most other fluids encountered in their frozen state, are poor thermal conductors that tend to insulate the fluid remaining in the core. For large diameters a closed plug cannot be formed no matter how much cooling is applied at the outer surface. Experiments have been conducted to investigate the advantages of freezing non-circular sections. It is shown that by reducing the distance between the fluid and cooled wall whilst at the same time maintaining the flow area of the pipe there is no limit to the nominal size of line that may be plugged by freezing. Significant advantages are gained when freezing all sizes of pipe. Strategically placed non-circular sections can eliminate the need for conventional valves and might find wide application in the hydrocarbon, utility and building services industries.