Welded systems are generally preferred, where possible, to reduce the likelihood of leaks.  Generally, even welded systems will need non-welded joints (e.g. unions, flanges, etc.) to allow maintenance replacement of components.  A low-pressure system would not be an exception to this preference.  However, piping at lower pressures and smaller sizes will leak less hydrogen and have less probability of a leak. 

It is common to have mechanical joints for small piping (1/4” to 1/2”) for gauges, transmitters, and other measuring devices.  Additionally, leak rates are lower and are usually low risk, especially in large open areas for National Pipe Taper (NPT) threads below 1.5”. NPT joints are not recommended above 1.5” since they are susceptible to larger and more frequent leaks. Compression-style joints at low pressure have been shown to be largely leak free when installed correctly but also offer a risk of complete separation when assembled improperly. It is important to match the type of fitting to the application (purity, pressure, pipe size), allowable leak rate, and maintenance needs.

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.

In laboratories, 316 stainless steel tubing is frequently the first choice for small flow and pressures less than 2800 psi (19 MPa). See Best Practices: Material Compatibility for hydrogen compatibility with various materials. Always work within manufacturer’s pressure ratings adjusted for temperature. Read and follow manufacturer’s instructions on making up tubing fittings, avoid threaded fittings, and properly purge with nitrogen, helium or other inert gas to remove air before introducing hydrogen.

We would not open the vent system to inspect the internal piping without a good reason.

It is recommended to check for water in the vent stack trap

1. At startup and daily during startup.
2. On LH2 tank system, every delivery
3. After the 1st rainstorm after a system is installed
4. LH2 vent stacks after establishing the baseline above
   1. After every large venting event
   2. Quarterly unless baseline requires more frequent
5. GH2 stack after baseline
   1. Check caps are still on quarterly for stationary tubes.

Absolutely. Vent systems will experience a variety of transient conditions of pressure, temperature, and thrust load, so stress analysis to anticipate the strength and flexibility needed are important for safe design. These issues are often overlooked and only become an issue when they are called upon to operate in emergencies. 

It is a best practice to include the vent system in the process hazards analysis (PHA)

At least three of the ASME B31 piping codes are logical choices:

• ASME B31.1, Power Piping
• ASME B31.3, Process Piping
• ASME B31.12 Hydrogen Piping and Pipelines

Considerations for code selection include:

• Requirements imposed by the authority having jurisdiction, whether by direct reference or by reference from another applicable code or standard.
• Code(s) used for other piping systems at the site. The people who have to operate and maintain the piping will be better served with fewer piping codes. The piping codes are complex and have different requirements. The people who have to operate and maintain the piping will likely be more successful if they have to learn requirements from fewer codes.

In the absence of these factors, ASME B31.12 is probably the most logical choice.

All three codes are suitable for liquid and gaseous hydrogen at pressures 15,000 psi (100 MPa) and higher. For pressures higher than 15,000 psi (100 MPa), the designation of high pressure fluid service in accordance with Chapter IX of ASME B31.3 may be a more economical choice and should be considered.

The requirements of the code used for the original construction apply. The piping may meet the requirements of more than one code. In which case, the code used for changing the rating may be different than the original code of construction. In any case, the re-rated system should meet all of the requirements of the selected code. Note that if the original proof test of the system was not high enough meet the requirement for the new service, the piping will have to be tested at the higher pressure.

The code used for repair and alterations of an existing system depends on the code used for construction as well as on the requirements imposed by the jurisdiction. Note that getting a permit from the jurisdiction may be necessary for an extensive alteration.

Note that getting a permit from the jurisdiction may be necessary for an extensive alteration.