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SECTION 2G: Cryogenics
Many of the safety precautions observed for compressed gases also apply
to cryogenic liquids. Two important properties distinguish cryogenic liquids
from compressed gases and present additional potential hazards:
- By definition, all cryogenic liquids exist at very low temperatures.
The common cryogenic liquids include carbon dioxide, argon (-302°F),
hydrogen (-423°F), nitrogen (-320°F), and oxygen (-297°F).
Their cold boil-off vapor rapidly freezes human tissue. Most metals
become stronger upon exposure to cold temperatures, but materials such
as carbon steel, plastics, and rubber become brittle or even fracture
under stress at these temperatures. Proper material selection is important.
Cold burns and frostbite caused by cryogenic liquids can result in extensive
tissue damage.
- All cryogenic liquids produce large volumes of gas when they vaporize.
Liquid nitrogen will expand 696 times as it vaporizes. The expansion
ratio of argon is 847:1, hydrogen is 851:1, and oxygen is 862:1. If
these liquids vaporize in a sealed container, they can produce enormous
pressures that could rupture the vessel. For this reason, pressurized
cryogenic containers are usually protected with multiple pressure relief
devices. Primary protection is usually a pressure relief valve; secondary
protection is a frangible disc.
Vaporization of cryogenic liquids (except oxygen) in an enclosed area
can cause asphyxiation. Vaporization of liquid oxygen can produce an oxygen-rich
atmosphere. Although oxygen is not flammable, it is an oxidant and will
support and accelerate the combustion of other materials. Vaporization
of liquid hydrogen can form an extremely flammable mixture with air. Do
not purchase or plan to use liquid oxygen or liquid hydrogen without consulting
EHS.
Handling Cryogenic Liquids
(top)
Most cryogenic liquids are odorless, colorless, and tasteless when
vaporized. When cryogenic liquids are exposed to the atmosphere, the
cold boil-off gases condense the moisture in the air, creating a highly
visible fog.
Always handle these liquids carefully. Because of their extremely low
temperatures, they can produce cryogenic burns and frostbite. When spilled
on a surface, they tend to cover it completely and, therefore, cool
a large area. The vapors from these liquids are also extremely cold
and can produce burns. Exposure that may be too brief to affect the
skin of the face or hands may damage delicate tissues, such as the eyes.
Boiling and splashing always occur when charging or filling a warm
container with cryogenic liquid or when inserting objects into these
liquids. Perform these tasks slowly to minimize boiling and splashing.
Use tongs to withdraw objects immersed in a cryogenic liquid.
Never touch uninsulated pipes or vessels containing cryogenic liquids.
Flesh will stick to extremely cold materials. Even nonmetallic materials
are dangerous to touch at low temperatures. In addition to the hazards
of frostbite or flesh sticking to cold materials, objects that are soft
and pliable at room temperature, such as rubber or plastic, become hard
and brittle and are broken easily at these extremely low temperatures.
Protective Clothing (top)
Face shields are recommended during transfer and handling of cryogenic
liquids. If severe spraying or splashing could occur, safety glasses
or chemical goggles will provide additional protection.
Wear loose-fitting, dry leather gloves or Cryo-gloves® when handling
objects that come into contact with cryogenic liquids and vapor. Trousers
should be worn on the outside of boots or work shoes.
 Section
2f Section
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