Freeze isolation of polymer pipeline using cryogenic liquids

Abstract

Pipe freezing has become well established in industry as a method of short-term isolation in order to carry out maintenance or repairs. A temporary plug is formed by freezing the contents of the pipe (usually water although other fluids, including glycols and hydrocarbons, are possible) over a short length. A review of related research and theory showed that there was little published work on pipe freezing and so a 3 phase experimental investigation was developed to with the aim of increasing knowledge in this field.
Phase 1 was to investigate the possibility of using the pipe freezing technique to form an ice plug within a polymer (PVDF) pipe. One side of the plug would then be pressurised to test if the ice plug could be used as temporary pipe isolation. Upon completion, it was found that a pressure retaining ice plug could be formed within the section of pipe and that the pipe showed no obvious signs of damage.
Phase 2 was to investigate if a solid ice plug could be formed in a selection of polymer pipes using liquid nitrogen as the coolant, without causing permanent deformation or other damage to the pipes. Secondary objectives of the experiments were to determine, the time taken for ice plugs to form and if the ice plugs could support a 10 bar differential pressure. During these experiments, the method of plug formation within the pipe was observed and the inner and outer pipe wall temperatures recorded with the aim of explaining fully the processes involved in the formation of ice plugs within polymer pipes. On completion of the experiments, it was found that, in most cases it was possible to form ice plugs without apparent damage to the pipes.
Phase 3 was to investigate the mechanisms by which the ice plug is retained within the pipe and quantify the force needed to remove a fully formed ice core from a frozen pipe section. Upon completion, it was found that the interaction of the differing rates of contraction between the ice, metallic pipe and the polymer pipes and the adhesive properties between the ice core and pipe wall were extremely difficult to quantify.

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