This part of ISO 14531 specifies the physical properties and mechanical-performance requirements for crosslinked polyethylene (PE-X) pipes intended for use in the supply of gaseous fuels. In addition, it lays down dimensional requirements and specifies some general material properties (including chemical resistance) together with a classification scheme for PE-X produced in the form of pipe. This part of ISO 14531, when used in conjunction with the other parts of ISO 14531, is applicable as the basis for the design, manufacture and installation of PE-X piping systems (PE-X pipes, PE-X fusion fittings and mechanical fittings) for the supply of category D and category E hydrocarbon-based fuels (see ISO 13623) at a) maximum operating pressures (MOPs) up to and including 16 bar; b) a maximum operating temperature of +60 oC; c) a minimum operating temperature of i) -50 oC ii)-35 oC iii) -2+ oC This part of ISO 14531 may also be used in conjunction with the restricted specification for PE heat-fusion fittings detailed in ISO 14531-2 to support the introduction of a hybrid PE-X pipe/PE fitting system for operation within a narrower temperature range (-20 oC to +40 oC) with a maximum pressure as determined by ISO 8085-3. For installation, this standard provides for the jointing of PE-X fusion fittings and mechanical fittings to PE-X pipes within the temperature range -5 oC to +40 oC.