Hydrogen Pipe Stress Analysis Considerations
Hydrogen behaves like any other gas in some ways, but when you’re doing pipe stress analysis, it’s a different beast. It’s not just about pressure and temperature — the material response, leakage risk, and code compliance all shift in subtle but important ways.
Why hydrogen is a special case
The tricky thing about hydrogen is that while it has been used in different industries for decades, the practical experience information is somewhat limited. Pipe stress as a field is based on both theory and practical experience, with many considerations derived from the latter.
If I compare it to pressure vessel design: It’s possible to add a margin of safety or take a more conservative approach with vessels. This is only partially possible for pipe stress. What is more conservative? Stiffness or flexibility? It really depends on the situation. Should I add weight as a safety margin? But then my spring hangers would be oversized. Pipe stress analysis requires creating an accurate system model and being selective about where you simplify.
In this post, I’ll walk through the main technical pitfalls I’ve run into when analyzing hydrogen piping, and how I’ve handled them.
Key stress considerations for hydrogen piping
Leakage
Hydrogen is the lightest atom, has the lowest viscosity, and a high molecular velocity (courtesy of its low weight). Hydrogen will leak much easier than nitrogen or air.
This is not really an issue for welded piping, but it is a significant concern for flanged connections. Key considerations include:
- Designing flanged connections becomes more important, requiring specific gaskets and installation procedures
- Loads induced by the piping will cause leakage much faster because the margins are tighter
- Take larger margins for error in flanged connection design
- Avoid concentrating loads on flanges in the piping design
Recommendation: In pipe stress calculations, use equivalent pressure methods and directly calculate loads against each flanged connection. Consider using EN 1591 detailed flange calculations as an additional verification.
Embrittlement
Hydrogen can cause embrittlement in steels, with some compositions being more susceptible than others:
- Hydrogen-assisted cracking at room temperature or lower
- Hydrogen reaction embrittlement at higher temperatures (over 200°C)
In the first case, this is primarily a material selection problem. If you select a material that is not susceptible to low-temperature embrittlement, there is no problem. However, this isn’t always easy to do, especially when you need stainless steel that is a stable austenite.
For austenitic stainless steels, the most issues occur with higher-pressure hydrogen and anything that can cause a structure change to martensite. Austenitic stainless steels resist hydrogen embrittlement due to their stable gamma phase. But under cold work or cyclic stress, some can shift to martensite (alpha/alpha’), which is much more susceptible, especially at high pressures. Embrittlement itself won’t significantly affect yield stress but has a large effect on tensile strength.
What to Pay Attention To:
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Stress concentrations - Areas where you sometimes depend on plastic deformation to redistribute stress, but can cause structure changes and cracks that grow over time.
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Material forming - Some forming methods can already cause structure changes, like bending, swaging, or rolling.
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High secondary stress in thermal expansion - Secondary stress is usually based on a percentage of yield strength and higher than 100% (typically 133% when not using liberal loads), meaning you assume the first temperature change will cause plastic deformation.
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Cycles and fatigue - In a hydrogen embrittlement situation, cyclic loading can make the problem much worse. This causes small cracks that hydrogen can diffuse further into, creating stress concentrations that amplify this effect. This can also change embrittlement susceptibility due to higher diffusion comparable to a higher pressure of hydrogen.
Low Temperature
This is specific to liquid hydrogen, which is cooled to -253°C, significantly colder than the common liquid temperature of nitrogen at -196°C.
Most pipe stress software bases thermal expansion calculations on codes that don’t go lower than -196°C. You’ll need to input your own information for lower temperatures. Interestingly, close to absolute zero, thermal contraction also decreases.
With these low temperatures also comes more insulation, adding extra weight to consider for the relatively small hydrogen piping.
Different Design Codes
The ASME B31.12 has been created specifically for hydrogen use. For ships, you have specialized codes like:
- Lloyd’s Register:
LR-RU-012 Rules and Regulations for the classification of ships using gases or other low-flashpoint fuels - Bureau Veritas:
NR678 Hydrogen fuelled ships - Det Norske Veritas:
Part 6 Chapter 2 Section 16
Many of these codes are loosely based on “normal” ship codes and IMO IGF and IGC rules, but there are some differences.
Dealing with these different design codes has been a challenge for me. Combining design codes when they are not a full replacement is difficult. These class societies provide some design stresses, but they’re much more limited than in a pipe stress design code like B31.3.
My approach: I generally perform the pipe stress calculation purely according to the normal design code, with the base calculations like internal pressure done according to the class rules. These classes do generally describe the extra loads to be added, like ship movements, which you must incorporate into the pipe stress analysis.