Liquid hydrogen release

Linney (1990) summarized the liquid hydrogen release tests performed by A. D. Little Inc. in 1958, by Lockheed in 1956-1957, and by NASA in 1980. Both high- and low-pressure releases were studied. None of the tests resulted in a blast-producing explosion. 

The ecological consequences of an accidental (liquid) hydrogen release are harmless due to its untoxic character compared with an uncontroled spill of fossil fuels [108]. 

LH2 spills. Liquefied hydrogen releases yield dramatically larger volumes of combustible clouds (1 L of liquid produces 851 L of gas on evaporation). Therefore, the consequences of a fire or explosion are more extensive than that of a pressurized hydrogen release. 

Hydrogen can be stored as a gas, a cryogenic liquid, or, in addition, solid-state storage is also possible. A particular problem with liquid hydrogen is boiling off. As the liquid warms, boil off gas is released which must be vented from the storage tank. In confined spaces there is a risk of fire or explosion if contacted by a flame. 

Liquid hydrogen must be stored in highly insulated containers similar to those used for liquefied natural gas. These containers are made from stainless steel and may be spherical or cylindrical in shape. The containers are double-walled to allow use of vacuum insulation in addition to insulation to prevent heat transfer from conduction, convection, and radiative sources. Pressure relief devices are incorporated to release hydrogen should the internal pressure of the tank increase to the maximum safe operating limit. 

Brittle Failure (8). Brittleness is a principal consideration in selecting construction materials for liquid hydrogen service. Brittle fracture can result in the essentially instantaneous release of a vessel s contents, the hazard being a combined one of PV energy release and the possibility of fire and/or explosion. Three conditions must exist for a brittle fracture to occur 1) a stress riser, a crack, notch, or other discontinuity, 2) a section where the actual stress exceeds the yield stress of the material, and 3) a temperature below which failure occurs without appreciable plastic deformation. Metals that are satisfactory for liquid hydrogen service include aluminum, stainless steels, brass, and copper. Carbon steel is not suitable. 

The large centre tank that is attached to the shuttle orbiter has two compartments, one containing liquid hydrogen and the other containing liquid oxygen. The energy released by the reaction between these two substances provides the thrust needed to propel the orbiter into space. 

Conversion to the para form takes place at a relatively slow rate and is accompanied by the release of heat. For each pound of rapidly cooled hydrogen that changes to the para form, enough heat is liberated to vaporize approximately 1.5 1b of liquid hydrogen. However, if a catalyst is used in the liquefaction cycle, para-hydrogen can be produced directly without loss from self-generated heat. 

The main tank is filled with liquid hydrogen from a trailer. Despite the sophisticated heat insulation in any container for cryogenic liquids, the small amount of remaining heat input vfill trigger off a warming process in the tank which causes the liquid in the container to evaporate and the pressure to rise. After a certain pressure build-up time the maximum operating pressure of the tank is reached. The pressure relief valve has to be opened. From this point onwards, gas must be released (boil-off). The container now acts as an open system with gas usually being lost to the environment. 

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