Liquid cryogens, if allowed to warm in a confined space, can create over-pressurization, and if spilled can generate a considerable volume of gas. The gas created by a liquid hydrogen spill will mix with air to form an even larger volume of gas that may be highly flammable. The following best practices should be considered:
- Indoor locations should have ventilation adequate to handle the largest anticipated spill. The space in which cryogenic systems are located should be ventilated to keep hydrogen concentrations well below explosive limits with maximum potential release rates.
- If the potential exists for flammable mixtures of hydrogen to form in enclosed areas, NFPA 68 and 69 should be consulted for control of deflagration.
- All floor openings, drains, or other low-lying enclosed spaces should be sealed or curbed to prevent liquid confinement.
- Electrical receptacles, switches, and controls should be located so they are not subject to liquid spills. Electrical equipment with potential exposure to hydrogen is classified by the National Electrical Code as Class I, Division 1, Group B or Class I, Division 2, Group B.
- Systems or apparatus handling liquid hydrogen that can cause freezing or liquefaction of the surrounding atmosphere should be designed to prevent contact of the condensed air with organic or other flammable materials.
- Liquid hydrogen for disposal should be completely vaporized and the vapor vented in a safe manner.
All system components used for cryogenic liquid hydrogen should be selected and designed for such service. (see Piping Systems in Storage & Piping).
- Design pressure for vessels and piping should not be less than 150% of maximum pressure relief. ((see Pressure Relief in Storage & Piping))
- Pressure relief of vessels and piping handling liquid hydrogen should be adequate to prevent vessel rupture and comply with applicable requirements for gaseous hydrogen service.
- Pressure relief and boil-off devices should be vented directly to the exterior of the building at a location above structures.