Which standard specifies that hydrogen pipelines must be installed above the ground?

There is no such requirement. Hydrogen pipelines can be buried directly. See GB50177 \"Code for Design of Hydrogen Stations\" 12.0.12

The hydrogen pipelines that can be seen are about half a meter above the ground; those running under roads are buried underground.

I just walked by, took a look, but I didn’t do any actual measurements.

If it is on an overhead conduit, it is usually on the top layer.

According to Chinese standards and specifications, the requirement that hydrogen pipelines must be installed above ground is mentioned in GB 50028-2012, the \"Design Code for Building Water Supply, Drainage and Heating Systems\". This specification outlines the layout and installation requirements for different types of pipelines, including the requirement that hydrogen pipelines must be installed above ground. .

The poster should consult the general layout or thermal engineering specialists. It should be remembered that pipelines for flammable and explosive gases should not be buried unless it is absolutely necessary. (It may be available in the \"Power Piping Design Manual\") ； The buried sections of the gas pipelines in our old factory were once forced to be converted to above-ground pipes ； The hydrogen pipelines seen in oxygen plants and power plants are all overhead. ）

The 2020 edition of the Power Piping Design Manual allows hydrogen pipelines to be laid underground. :L.2.9 Layout and installation of hydrogen pipelines 1) When hydrogen pipelines are installed overhead within the plant area, the following requirements shall be met: they shall be laid on supports made of non-combustible materials. ＠In cold regions, wet hydrogen pipelines should be equipped with anti-freezing measures. ＠The minimum clear distance from other overhead pipelines should be in accordance with the provisions in Appendix B of GB50177—2005, \"Code for Design of Hydrogen Stations\" ； The minimum clear distance from buildings, structures, etc., should be determined in accordance with the provisions of Appendix C in the \"Design Code for Hydrogen Stations\". 2) When hydrogen pipelines within the plant area are laid directly underground, the following requirements shall be met: The burial depth of the pipelines should be determined based on factors such as ground loads and the depth of soil freezing; the distance from the top of the pipeline to the ground surface should not be less than 0.7 m. Hydrogen pipelines carrying wet hydrogen should be laid below the frozen soil layer ； When laid in permafrost, anti-freezing measures should be taken.