Due to the extreme low temperature and high rate of gas escape, a number of precaution and safety measures must be applied while working with cryogenic liquids.
Extreme Cold Hazard
Brief exposures that would not affect skin on the face or hands can damage delicate tissues such as the eyes. Prolonged exposure of the skin or contact with cold surfaces can cause frostbite.
When a tissue freezes, one may not feel pain initially. An intense pain is felt upon thawing. If the skin is unprotected it can stick to metal containers of cryogenic liquids. It may then tear when pulled away. Even non-metallic materials are dangerous to touch at low temperature.
Breathing of extremely cold air for long periods of time may damage the lungs.
Due to low temperature, many common materials such as carbon steel, rubber and plastics may become brittle upon contact with cryogenic liquids and easily break under stress.
Normally, 21% of air is composed of oxygen. Symptoms of asphyxia develop when the oxygen percentage of air drops to 15-16%. At 12% oxygen, the individual will lose consciousness without warning and may be unaware of any danger. Cryogenic liquids produce a large amount of gas upon vaporization. One volume of liquid nitrogen vaporizes to yield 694 equal volumes of nitrogen gas at standard conditions. If the oxygen level is not regulated, asphyxiation may quickly lead to death. The gaseous product formed by cryogenic liquid evaporation is very cold and heavier than air. This gas does not disperse in air, but accumulate near the floor. Although this gas is non-toxic, it replaces the air. Thus, oxygen deficiency becomes a serious hazard risk closed spaces. Therefore, cryogenic liquids should only be used and stored in well-ventilated areas.
Symptoms of asphyxiation include giddiness, mental confusion, loss of judgement, loss of coordination, weakness, and nausea, fainting and lead to death. A few breaths of oxygen-deficient air are enough to reduce the dissolved oxygen level in blood. Once blood oxygen level is dropped, mental failure and coma follow very quickly. Asphyxiation symptoms may not always be observed, even though they do, loss of mental abilities and coordination may make it impossible for people to ask for help.
Gaseous products of most cryogens are odorless, colorless and tasteless. In addition, most cryogenic liquids are colorless, except oxygen (light-blue). The most significant warning property of extremely low liquids and their vapors is that they tend to form visible fog by condensing the moisture around. In other words, a fog cloud does not define the vapor cloud but it rather defines the area where vapor is still cold enough to condense the moisture in the air. Although fog cloud is an important clue to cryogenic gas leak, it is highly probable that the leak extends beyond the fog cloud. Therefore, fog cloud must never be used to define the leak area and should never be entered by anyone.
In summary, cryogenic vapors are undetectable to human sensory system. Therefore, one should not enter a possible oxygen-deflected area without wearing an external breathing air source and without carrying and atmospheric monitor. It is strongly advised to check the safe oxygen level prior to enter these areas.
Oxygen Enriched Air
Liquid oxygen in an enclosed area vaporizes to increase the amount of oxygen in air and saturate the combustibles, such as clothing. This may easily start a fire if an ignition source is present. Oxygen is not flammable, but it starts and accelerates combustion reactions.
Release of liquids below the boiling point of air may condense the surrounding air that leads to an oxygen-enriched local atmosphere. Moreover, extremely cold cryogens, such as helium, may even freeze the surrounding air.
Explosion Due to Rapid Expansion
If a cryogenic liquid vaporizes more than expected in a sealed container, it increases the internal pressure of container, which may lead to a rupture in the container. Therefore, pressurized cryogenic containers are designed in multiple protection devices to prevent over-pressure. A pressure relief device must be installed to prevent liquid trapping. Extra care must be taken while using pressurized containers. Please make sure that you have switched off the valve completely after use.
Special Helium Precautions
The most critical safety concern in dealing with liquid helium is its extremely low temperature. Helium is so cold that once it released it freezes all gases around. This includes not only water, but also nitrogen and oxygen; freezing of these gases inside a Dewar or a pipeline cause an “ice” plug which may lead to closing up the neck and create a bomb. Therefore, it is of utmost importance the set up a procedure for helium usage and to follow it strictly. This helps prevent other gases, including air, from entering the liquid helium delivery lines. If you suspect a blockage, remove the Dewar to a safe place and immediately contact the vendor. If helium is transferred by a piping system without vacuum jacket, air surrounding the pipe may liquefy. After condensation, nitrogen evaporates first, leaving an oxygen-enriched air around the pipe. This area, where the liquid is collected, should be insulated and oxygen-compatible.