ISO 18225:2012 specifies general and performance requirements for multilayer pipe systems based on pipes made from thermoplastics and intended to be used for gas supply outdoors. It gives requirements for the design of pipe systems consisting of multilayer pipes based on thermoplastics and for which at least 60 % of the wall thickness is of a polymeric material. The polymeric material used for stress design layers and all inner layers must be polyethylene (PE) and/or crosslinked polyethylene (PE-X), in accordance with Annex A. The outer layers of metal multilayer pipes must be of either PE or PE-X. NOTE For the purposes of ISO 18225:2012, PE-RT is considered as PE, while PE-X and adhesive layers are considered as thermoplastics materials. ISO 18225:2012 is applicable to systems intended to be operated at temperatures ranging from −20 °C to 40 °C. It is applicable to pipes in these systems having a nominal diameter up to and including 630 mm, and to the supply of gaseous fuels of categories D (natural gas) and E (LPG vapour and natural gas or LPG vapour) (see ISO 13623).