After first selecting materials to minimize the effects of hydrogen exposure and cold embrittlement, fittings and joints in cryogenic systems should then be designed or selected to withstand the stresses due to thermal cycling between ambient and cryogenic temperatures. These stresses include those acting radially through pipe and fitting walls and longitudinal stresses caused by expansion and contraction of system piping runs. Apparent good seals made at ambient temperatures can readily fail after a single thermal cycle.
More detailed design information can be found in CGA H-3 -2006, Cryogenic Hydrogen Storage and Safety in the Handling of Cryogenic Fluids (Edeskuty and Stewart). At the component level, where applicable:
Bayonet joint for vacuum-jacketed transfer lines (CVI Corp., Columbus, Ohio).
- Bellows expansion joints are used in the outer jacket, but should not be extended or compressed during installation to make up for deficiencies in length, nor to compensate for improper alignment.
- Bayonet-design couplings should be used for demountable joints.
- Threaded joints should not be used.
Successful strategies for evaluating seals include:
- Cold shocking fittings with liquid nitrogen, then retightening,
- Helium mass spectrometer leak detection testing,
- Warm vacuum/cold vacuum retention tests of annular spaces.
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.