Storing of Cryogenic Gas

Cryogenic gases are needed to be stored in special containers, which are usually thermally insulated containers, specifically designed to bear rapid temperature changes and extreme differences in temperature. Usually the containers are designed in such a way so that they have few of the features as given in the figure below:

Storage containers
Some of the popular storage containers of cryogenic gases are as follows:

Dewar flasks
Dewar flask, named after its inventor Sir James Dewar in 1892, is a non-pressurized, vacuum-jacketed vessels, having a loose fitting plug or cap to prevent air and moisture from entering. At the same time, such flasks allow excess pressure to vent. Dewar Flasks are also known as vacuum flasks. The vacuum is used for thermal insulation.

The gas inside is not in vacuum conditions. These flasks maintain cryogenic gases at a temperature higher or lower than ambient temperature. Dewar flasks containing hydrogen, argon, oxygen, helium, and other low-boiling liquids have an outer vessel of liquid nitrogen for insulation. Dewar flasks are available in various sizes. There are laboratory liquid dewar flasks which do not have lids or covers but wide-mouthed openings, in small sizes used in laboratories for temporary storage.

Cryogenic Liquid Cylinders
Also known as liquid containers, cryogenic liquid cylinders are portable vacuum-insulated pressurized containers, specifically designed for cryogenic liquids. The liquid cylinders are an easy and economical means of storing, transporting, and dispensing liquefied gases. These cylinders have valves for filling and dispensing the cryogenic liquid. As a backup protection these cylinders also have pressure-control valves with a frangible or bursting disk.

There are three main types of cryogenic liquid cylinders which are designed for storing:

• Only liquid
• Only gas
• Liquid or gas

Liquid cylinders have two primary advantages:

• At relatively low pressure compared to compressed gas cylinders, these cryogenic cylinders can hold a large volume of gas.
• These cylinders are an easy source of cryogenic liquid that can be easily handled.

Cryogenic products that are stored in liquid cylinders are:

Nitrogen Argon Oxygen Helium Hydrogen

Nitrous oxide Carbon dioxide Krypton Methane Xenon Neon

While nitrogen, argon, oxygen, helium, hydrogen are stored in their liquid states, nitrous oxide and carbon dioxide are kept as refrigerated liquids, but at higher temperature and pressure. These cylinders are well insulated but at times the extremely low temperatures of the cryogenic liquids can lead to constant heat leak and vaporization. The cryogenic product when not used will lead to pressure in the cylinder and often vent via the container's pressure relief device. However, this is a normal and safe function of the cylinder.

Tanks
Tanks are large double-walled, insulated storage tank, spherical or cylindrical in shape used for storing cryogenic liquids or gases. Tanks are large storing containers for cryogenic gases, capacities ranging from approximately hundreds of gallons to thousands of barrels. They are designed in such a way so that they can be easily mounted in fixed locations as stationary vessels or on truck chassis or railroad car for easy transportation. The pressure inside the tank is kept normal. The space between inner and outer surface is vacuum and there are safety relief valves to protect the tanks.

Some useful tips to consider for storing and transporting cryogenic liquids:

• Ensure that all incoming containers are not damaged before storing.
  
  
• Always use the correct name for all products. For instance, you should never call "liquid oxygen" as "liquid air". Or if the oxygen is in a mixture of other gases, refer to it as "oxygen mixture" or "medium purity liquid oxygen".
  
  
• Dewar flasks should always be kept covered with a loose fitting cap. This helps in preventing air or moisture from entering the container and at the same time allows pressure to escape.
  
  
• Make use of only the plug or stopper supplied with the container.
  
  
• Glass dewar flasks should not be used to store combustible or oxidizing cryogenic liquids.
  
  
• Never let ice form in the neck of flasks.
  
  
• Do not store containers in those areas where they have a chance to come in contact with moisture. Ensure that storing vessels are insulated from any sources of heat.
  
  
• Do not store liquid oxygen containers on asphalt, oil soaked gravel or wood.
  
  
• It is better to use concrete or clean gravel under storage areas.
  
  
• Store all cryogenic liquid containers in well-ventilated areas.
  
  
• Handle liquid cylinders carefully by avoiding rolling, dropping or tipping them on their sides.
  
  
• When transferring cryogenic liquids from one flask to another, always cool the receiving dewar flask before filling it.
  
  
• Start filling liquids slowly so as to allow the vaporization to chill the receiving container. After the liquid boiling and vaporization have declined, fill the container at the normal rate.
  
  
• Use an appropriate filling device when pouring cryogenic liquids.
  
  
• Fill storage containers only with liquids they are designed to hold.

A quick recap All cryogenic products must be properly stored and transported.The containers should be designed in such a way so that they can bear rapid temperature changes and also differences in temperature. That is why most containers are thermally insulated. Some of the popular storage containers used for storing cryogenic gases are as follows: Liquid dewar flasks Laboratory liquid dewar flasks Liquid cylinders Tanks