Hydrogen test lines, blending stations, metering skids and electrolyser balance-of-plant are moving from pilots to projects. Most of them are designed to ASME B31.12, Hydrogen Piping and Pipelines. From a stress-analysis point of view the method looks familiar, but several details change. These notes come from calculation work on SNAM and Saipem hydrogen-related projects and an offshore wind-farm hydrogen scope.
Structure of the code
B31.12 is organised in a general part (GR) and two application parts: Part IP for industrial piping and Part PL for pipelines. Station piping is normally designed to Part IP, the transmission line to Part PL — the boundary between them should be fixed early, because it changes the checks applied on either side.
Material behaviour
Hydrogen can reduce the ductility and fracture toughness of steels (hydrogen embrittlement), particularly at higher strength and higher pressure. The code reflects this with restrictions on materials, hardness and welding, and with a material performance factor in pressure design that can penalise wall thickness depending on the material and the design pressure. Wall thicknesses inherited from natural-gas practice must therefore be rechecked, and the stress model must use the B31.12 values.
Fatigue and pressure cycles
Hydrogen systems often cycle — storage filling, compressor start-stops, intermittent production from renewables. For pipelines, Part PL provides a prescriptive design option and a performance-based option based on fracture mechanics; in both cases the number and amplitude of pressure cycles become a design input. Ask the process team for a cycle count early.
Practical points for the stress model
- Define the IP / PL boundary and the applicable code at each node.
- Use the B31.12 allowables and factors, not those of B31.3 or B31.8.
- Pay attention to small-bore connections, instrument taps and branch connections: they concentrate stress and are frequent leak points.
- Check flange leakage carefully: hydrogen leaks more easily than natural gas.
- For repurposed natural-gas lines, verify the as-built material and welds before reusing the old calculation.
Always refer to the edition of B31.12 called up by the project specification; requirements have changed between editions.