An aluminium puncture pipe is used as a metallic sliding liner, which tightly envelops the steam service pipe and forms a dilatation joint between the service pipe and the layers of rigidly bonded insulation and casing. The service pipe is therefore able to move in this casing due to temperature changes of the medium (is able to dilate), while the insulation layers do not move, and thus the sliding movement of the service pipe does not wear them down. This metal liner ensures the longevity of the insulation layers - unlike other designs where the insulation has direct contact with the service pipes and is subject to abrasion.