Hydrogen has been transported safely through pipelines for over 50 years. There are dozens of pipeline networks in safe operation globally, with several individual networks that approach up to 1000 miles. 

Significant testing and some demonstration projects are underway to ensure safety. Some of the aspects under investigation include compatibility of the pipe and other materials, effect on odorants, and the need to modify existing residential and industrial equipment. Blending initially is at relatively low levels where its impact on the operations of the existing natural gas distribution system is minimized.

Acceptability of materials is highly dependent on the specific application. Applied stress levels, exposure to contaminants, the operating temperature, the partial pressure, and number and magnitude of material stress cycles are some of the factors that affect material selection. Guidance is provided within documents such as ISO 11114, Gas cylinders - Compatibility of cylinder and valve materials with gas contents, ASME B31.12, Hydrogen Pipe and Pipelines, and NFPA 2, Hydrogen Technologies Code. The Sandia Technical Reference site (https://h2tools.org/technical-reference-for-hydrogen-compatibility-ofmaterials)
also provides information for users to make informed selections. For ambient temperature systems designed to rules described in accepted codes and standards, austenitic stainless steel is frequently a good choice.

Hydrogen affects the mechanical properties of most materials. For example, hydrogen reduces the
fracture toughness and increases the fatigue crack growth rate in steels. There is a significant amount of
research, analytical work, and codes and standards development being undertaken to improve our
understanding of how these materials can be utilized in pipelines. The results of the efforts will be
revealed as requirements in codes like ASME B31.12, Hydrogen Piping and Pipelines, and ASME B31.8,
Gas Transmission and Distribution Piping Systems, and then addressed by national regulations, such as
those provided by the US DOT Pipeline and Hazardous Material Administration (PHMSA).