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Photo by Robert Burke
Vertical cryogenic-liquid storage tanks located next to a building. These tanks are often found next to manufacturing buildings, hospitals, bottled-gas companies and welding-supply houses. Notice the silver-colored heat exchangers in the foreground - confirmation that these are indeed cryogenic tanks.
Propane tanks range in size from the five-gallon containers used with barbecue grills and 250-gallon tanks used for home heating to bulk-storage tanks containing thousands of gallons of product at propane facilities. Propane tanks generally have rounded ends, which is a primary indicator of a pressure vessel.
Pressure tanks are equipped with relief valves to vent excess pressure caused by increases in ambient temperature. Propane tanks have extensions on the relief valves to extend vapors well above the tank in the event the vapors catch fire. The normal relief valve height is between six and 10 inches. This height allows ignited vapors to lie on the vapor space of the tank, causing it to fail quickly. Anhydrous ammonia tanks do not have the extension, so if you see a horizontal pressure tank with tall relief valves, it is likely to contain propane; if the relief valves are near the tank surface, it is likely to contain anhydrous ammonia.
Anhydrous ammonia, propane and other petroleum gases are liquefied under pressure before being placed in these tanks. The liquid level in the tanks is generally 80% of the tank's capacity to allow for a vapor space. Pressure containers can be dangerous under fire conditions. Relief valves are designed only to relieve normal pressure increases, not those caused by flame impingement from a fire or radiant heat sources.
In the past five years, firefighters in the U.S and Canada have been killed fighting fires involving high-pressure fixed propane containers. Flame impingement on the vapor space of the tank can cause the metal to fail and a boiling liquid expanding vapor explosion (BLEVE) can occur within 20 minutes of the start of the flame impingement. The liquid level of the tank will absorb the heat from flame impingement and not fail as long as the liquid is in the tank. The flame impingement on the liquid level causes the propane in the tank to boil faster and produce more vapors. If the vapor is produced faster than the relief valve can vent it, tank failure can also occur.
Cylindrical tanks are another type of high-pressure tanks, which may be found at refineries or other locations. These tanks look like giant balls suspended in a steel support structure. Tanks of this type can be used for propane and other liquefied petroleum gases, natural gas and hydrogen.
Tube banks are a type of ultra-high-pressure tank that are often found at compressed-gas companies and distribution facilities. The pressures in this type of tank can be in excess of 3,000 psi. Tube banks are actually a series of individual tanks stacked together with valves and piping into a single outlet/inlet; similar to a fire department cascade system. Unlike the liquefied gases in high-pressure tanks, the materials in tube banks are gases. There is no liquid space to absorb heat. Under fire conditions, these tanks can fail very quickly.
Vertical cryogenic storage tanks are often found next to manufacturing buildings, hospitals, bottled-gas companies and welding-supply houses. A heat exchanger next to the tanks is confirmation that these are indeed cryogenic tanks. Heat exchangers are a series of silver-colored tubes with fins next to the tanks. The heat exchanger allows for the cold liquids to be turned back into gases for use. Ambient air around the tubes and fins of the heat exchanger warms the cold cryogenic liquids into their gas state.