Cryogenic Liquid

Cryogenic liquid hydrogen (LH2) properties:

• Is a liquid with a boiling point of -253 °C (-423 °F)
• Can lead to cryogenic burns if in contact with skin
• Rapidly changes to gas when spilled or released (boils)
• If allowed to warm in a confined space, can vaporize and create over-pressurization
• If spilled can generate a considerable volume of hydrogen gas; the gas will mix with air to form large flammable mixture.
• Accidental air leakage into a liquid hydrogen storage vessel (e.g., from inadequate purging) will result in the introduction of solid (frozen) nitrogen, oxygen and water. These solids may plug lines or cause instruments to malfunction.

Best Practices for facilities that utilize LH2:

• If possible, work involving LH2 should be done outdoors
• Indoors, ventilation adequate to handle the largest anticipated spill should be installed
• If the potential exists for flammable mixtures of hydrogen and air to form, 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 subjected to liquid spills
• Even when liquid hydrogen systems are well insulated, it is difficult to eliminate exterior surfaces cold enough to condense nitrogen and oxygen out of the air. To prevent fires, such systems should be designed to prevent contact of the condensed liquids with combustible materials
• Surplus liquid hydrogen is generally disposed of by allowing it to warm slowly and venting the gas in a safe manner
• All systems components used for cryogenic liquid hydrogen should be selected and designed for such service. See Compatibility of Materials. 
• Design pressure for vessels and piping should not be less than 150% of maximum relief pressure
• Pressure relief and boil-off devices should be vented separately and directly to the exterior of the building at a location above structures. See Venting.