All fittings must be suitable for hydrogen service and for the specific operating conditions.
Welding is usually preferred for joints in hydrogen piping systems since welding provides a better connection and reduces the potential for leaks compared to mechanical fittings.
Mechanical fittings are allowed, but administrative controls must be in place to ensure that leak testing is carried out on a regular basis. Make sure that the initial installation was done correctly and inspected for proper assembly per the specification. Generally, fewer mechanical joints are better.
You should never assume that every fitting is tight. In fact, it's probably a good idea to assume that some fittings are loose.
Use as few fittings as possible to minimize the potential for leaks.
Pipe fittings (tees, laterals, and crosses) should be fabricated in accordance with ASME B31.1 or B31.3 requirements.
A small leak, if ignited at a joint, can lead to a larger release. Therefore:
- Soft solder joints are not permitted for hydrogen systems (due to the low melting point of soft solder and its potential for brittle failure at cryogenic temperatures).
- Brazed joints are permitted, but such joints should be protected against the possibility of external fire.
Gray, malleable and ductile cast iron fittings should not be used for hydrogen service.
Types of fittings commonly used in hydrogen systems are listed here with the better-performing types at the top:
- Butt welding fittings meeting the requirements of ASME B16.9
- Socket welding fittings meeting the requirements of ASME B16.11
- Brazing fittings meeting the requirements of ASME B16.18, B16.22 or B16.50 [These fittings should not be used where leaks resulting from exposure to fire are not acceptable.]
- Compression fittings [These fittings should not be used in systems subjected to severe cyclic stresses.]
- Threaded fittings meeting the requirements of ASME B16.11. [The incidence of leaks in threaded joints is much higher than the other fittings. Threaded joints should not be used where leaks are not tolerable.]
Two common demountable joints are flanged joints and unions. Flanged joints with flanges meeting the requirements of ASME B16.5 are preferred. Gaskets should be fire resistant where leaks resulting from exposure to fire are not acceptable. The incidence of leaks in unions is much higher than with flanged joints. Unions should not be used where leaks are not tolerable.