Intent: To provide a minimum standard for the safe handling
of cryogenic liquids.
Cryogenic fluids - liquid gases at temperatures nominally
Dewars are especially designed double-walled insulated
containers having provisions for pressure relief.
All cryogenic liquids are extremely cold. Cryogenic
liquids and their cold "boil-off" vapor can rapidly freeze human tissue,
and can cause many common materials such as carbon steel, plastics and
rubber to become brittle, or even fracture under stress. Liquids in containers
and piping at temperatures at or below the boiling point of liquefied air
(-318°F, -194°C) can actually condense the surrounding air to a
liquid. Extremely cold liquefied gases (liquid helium and liquid nitrogen)
can even solidify air or other gases.
All cryogenic liquids also produce large volumes
of gas when they vaporize. For example, one volume of liquid hydrogen at
one atmosphere vaporizes to 700 volumes of hydrogen gas at 70°F and
one atmosphere. If these liquids are vaporized in a sealed container, they
can produce enormous pressures which could rupture the vessel. For this
reason pressurized cryogenic containers are usually protected with multiple
devices of pressure relief, usually a pressure relief valve for primary
protection and a frangible disc for secondary protection. Vaporization
of liquid oxygen in an enclosed area can cause an oxygen-rich atmosphere
and could saturate a worker's clothing. Although oxygen is not flammable
it will vigorously support and/or accelerate the combustion of other materials.
Vaporization of liquid hydrogen in an enclosed work area can cause a flammable
or explosive mixture with air.
Most cryogenic liquids are odorless, colorless, and
tasteless when vaporized to the gaseous state. Most of them have no color
as liquids, although liquid oxygen is light blue. However, the extremely
cold liquid and vapor have a warning property that appears whenever they
are exposed to the atmosphere. The cold "boil-off" gases condense the moisture
in the air, creating a highly visible fog. The fog normally extends over
a larger area than the vaporizing liquid.
Supervisors responsible for projects using cryogenic
fluids must familiarize themselves and their personnel with appropriate
Matches, smoking materials, lighters, etc., and other
sources of ignition are prohibited where liquid hydrogen and oxygen are
present. All such areas shall be designated as "No Smoking Areas".
Safety glasses are required during transfer and normal
handling of cryogens. If severe spraying or splashing may occur, a face
shield or chemical goggles should be worn for additional protection.
Dry cryogenic gloves should always be worn when handling
anything that comes in contact with cold liquids and vapor. Gloves should
be loose fitting so that they can be removed quickly if liquids are spilled
into them. Depending upon the application, special clothing may be advisable.
It is preferable to wear trousers outside of boots or work shoes.
Keep container (Dewar) vertical at all times. Do
not roll the container on its side.
Do not enter an area or laboratory where a major
spill may have caused an oxygen deficiency, unless you are equipped with
a self-contained air supply breathing apparatus.
Relief valves on Dewars shall not be tampered with
under any circumstances!
Electronic gases (including arsine, diborane, dichlorosilane,
germane, phosphine and silane) require the following special safety provisions:
The use of vented gas cabinets with alarmed monitoring
devices is mandatory
The exhaust rate of the cabinet must be sufficient
to vent a complete cylinder discharge.
The height of the exhaust stack must be sufficient
to safely disperse the toxic gas before it reaches the ground or a receptor.
The use of these gases require special safety review
by the Safety Office before use
Evacuated glassware (Dewars) must be shielded against
implosion. Exposed glass portions of the container must be taped to minimize
flying glass hazard.
All cryogenic storage vessels shall be chosen to
withstand the weights and pressures of the material used, and shall have
adequate venting to prevent pressure build-up.
All cryogenic liquids with boiling points below that
of liquid nitrogen (particularly liquid helium and hydrogen) require specially
constructed and insulated containers to prevent rapid evaporation.