2.2.1 Low Temperature Storage
Liquid hydrogen storage vessels must also be well-insulated to maintain the temperature of liquid hydrogen over long periods. No matter how well-insulated, however, it is inevitable that eventually some heat will be absorbed through the vessel walls, which will cause some hydrogen in the tank to vaporize. As hydrogen vaporizes, it raises the vapor pressure inside the tank. If not relieved, the increased pressure might eventually cause the tank to rupture. 26
The extremely low temperature of liquid hydrogen poses a severe frostbite hazard to exposed skin. All vessels, hoses, and lines that carry liquid hydrogen, either on a vehicle or at a fueling station, should be protected to avoid casual contact by people.
Liquid hydrogen tanks are always equipped with pressure relief valves to relieve pressure in the tank as necessary. These valves are different from the PRD/TRDs used with gaseous hydrogen storage tanks. Liquid hydrogen pressure relief valves are designed to open when the vapor pressure inside the tank rises above a set pressure, and then to close again when the pressure inside the tank falls below a different set pressure. PRD/TRDs open as required to relieve pressure, but do not close again. Once a PRD/TRD opens, it must be replaced before the tank can be filled again.
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