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Basic Safety Precautions Regarding Compressed Gases
Cryogenic Liquids - h1015v2_115

Chemistry Volume 2 of 2
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COMPRESSED GASES DOE-HDBK-1015/2-93 Hazards of Chemicals and Gases CH-05 Rev. 0 Page 20 Proper storage and handling of containers avoids many possible incidents.  Hazards resulting from  the rupture of a cylinder or other vessel containing gas at high pressure are protected against by careful and secure handling of containers at all times.  For example, cylinders should never be struck nor allowed to fall, because if the cylinder is charged to a high pressure and the cylinder valve is broken off, it could become a projectile.  Cylinders should not be dragged or rolled across the floor; they should be moved by a hand truck.  Also, when they are upright on a hand truck, floor, or vehicle, they should be chained securely to keep them from falling over. Moreover, cylinders should not be heated to the point at which any part of their outside surface exceeds a temperature of 125   F, and they should never be heated with a torch or other open flame.    Similar  precautions  are  taken  with  larger  shipping  and  storage  containers.    Initial protection against the possibility of vessel rupture is provided by the demanding requirements and recommendations that compressed gas containers fulfill in their construction, testing and retesting. Cryogenic Liquids Most cryogenic liquids are colorless, odorless, and tasteless when vaporized to a gas.  As liquids, most have no color (except liquid oxygen which is light blue).  However, whenever the cold liquid  and  vapor  are  exposed  to  the  atmosphere  a  warning  appears.    As  the  boil-off  gases condense moisture in the air, a fog forms that extends over an area larger than the vaporizing gas.  Many cryogenic liquids are inert gases, and may inert an enclosed space.  Inert gases will not support life. Both  the  liquid  and  its  boil-off  vapor  can  rapidly  freeze  human  tissue  and  can  cause  many common materials such as carbon steel, plastic, and rubber to become brittle or fracture under stress.  Liquids in containers and piping at temperatures at or below the boiling point of liquified air (-318   F) can cause the surrounding air to condense to a liquid. Extremely cold liquified gases (helium, hydrogen, and neon) can even solidify air or other gases to which they are directly exposed.  In some cases, plugs of ice or foreign material will develop in  cryogenic container vents and openings and cause the vessel to rupture.  If a plug forms, contact the supplier immediately.  Do not attempt to remove the plug; move the vessel to a remote location. All cryogenic liquids produce large volumes of gas when they vaporize.  For example, 1 volume of saturated liquid nitrogen at 1 atmosphere vaporizes to 696.5 volumes of nitrogen gas at room temperature at 1 atmosphere. When  vaporized in a sealed container, cryogenic liquids produce enormous pressures.  If  1 volume of liquid helium at 1 atmosphere is warmed to room temperature and vaporized in a totally enclosed container, it has the potential to generate a pressure of more than 14,500 psig. Because of this high pressure, cryogenic containers are usually protected with two pressure-relief devices, a pressure-relief valve and a frangible (easily broken) disk.







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