Cryogenic Liquids

A cryogenic liquid is defined as a liquid with a boiling point below -130oF (-90oC).  Common cryogens and their properties are summarized in Table 1. 

Table 1

Cryogen Boiling point (1 atm) oC (oF) Critical pressure psiga Liquid density, g/L Gas density (27oC), g/L Liquid-to-gas expansion ratio Type of gas
Ar -186(-303) 710 1402 1.63 860 Inert
He -269(-452) 34 125 0.16 780 Inert
H2 -253(-423) 188 71 0.082 865 Flammable
N2 -196(-321) 492 808 2.25 710 Inert
O2 -183(-297) 736 1410 1.4 875 Oxidizerb
CH4 -161(-256) 673 425 0.72 650 Flammable

apound per square inch gauge; bAlthough oxygen does not burn, it will support combustion. Oxygen-enriched atmospheres may lead to violent reactions, such as rapid combustion or explosions, with incompatible materials.  

Hazards

Hazards may include fire, explosion, embrittlement, pressure buildup, frostbite and asphyxiation.

Many of the safety precautions observed for compressed gases also apply to cryogenic liquids. Two additional hazards are created from the unique properties of cryogenic liquids:

  • Extremely Low Temperatures –The cold boil-off vapor of cryogenic liquids 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.
  • Vaporization - 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. (See Anecdotes for an account of such an incident.) For this reason, pressurized cryogenic containers are usually protected with multiple pressure relief devices.

Vaporization of cryogenic liquids (except oxygen) in an enclosed area can cause asphyxiation. Vaporization of liquid oxygen can produce an oxygen-rich atmosphere, which will support and accelerate the combustion of other materials. Vaporization of liquid hydrogen can form an extremely flammable mixture with air.

Cryogenic Liquid PPE

EHS recommends the the following PPE for handling cryogenic liquids:

  • a full faceshield over safety glasses
  • cryogenically rated, loose-fitting gloves
    • gloves should be loose fitting so that they can be quickly removed
    • crogenic gloves are not rated for immersion into cryogenic liquids or for prolonged handling of cryogenically chilled materials 
  • long-sleeved shirt or lab coat, and pants without cuffs
  • safety shoes are recommended for people involved in the handling of containers.

Cryogenic gloves

Cryogenic Liquid Storage

Containers

Cryogenic liquids can be transported, stored, and handled in a variety of containers depending on quantity and desired use.  All three types of containers are present at Princeton University.

Dewars

Dewars are non-pressurized, double walled containers used to contain cryogenic liquids.  The are between the walls is under high vacuum for maximum thermal insulation.  They have a loose fitting insulated cap that enable gases to escape while preventing moisture buildup at the neck.  In many cases they are used at "satellite" cryogenic liquid containers within a lab.

Double-walled aluminum shell cryogenic dewars

Dewar flasks are smaller, double-walled cryogenic storage containers that usually only maintain the liquid for a few hours.  They usually have a metal outer wall and a glass inner wall, with the void space under high vacuum.  Special care should be taken when handling as these flasks can implode. 

cryogenic flasks

 

Cryogenic Liquid Cylinders

Cryogenic liquid cylinders, sometimes also referred to as Dewars, are insulated, vacuum jacketed, pressurized vessels.  They come equippmed with safety relief valves and rupture disks to alleviate pressure buildups.  These cylinders come in a variety of sizes ranging from 80-450 liters.  They can be equipped to provide gas through an internal vaporizer or provide liquid under their own internal vapor pressure.  These cylinders are usually filled by the users at filling stations throughout the campus.  All users must wear proper PPE when filling and discharging.  Cryogenic cylinders routinely relieve their internal pressure and you may observe a sudden hissing sound and the appearance of a fog.  This is completely normal.  When using cryogenic cylinders, users should ensure that all valves and pressure relief devices are in place and in good condition.  If pressure relief devices fail, catastrophic failure may result. 

cryogenic liquid cylinder

 

Cryogenic Storage Tanks

A few buildings on campus have cryogic storage tanks.  These are filled by vendors on a set schedule and are used to fill the end user crogenic cylinders and dewars. 

 

Handling Cryogenic Liquids

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.

  • Cryogenic liquids MUST be used in a well ventilated area.  All crogenic liquids produce large volumes of gas when they vaporize.  For example, one liter of liquid nitrogen dispalces 694 liters of air when it vaporizes. 
  • When used in sealed containers, this vaporization can produce enourmous pressures.
  • Always wear proper PPE.
  • Always use proper containers designed for the transport and use of cryogenic liquids. 
  • Examine containers and pressure relief valves for signs of defect. Never use a container which has defects.
  • Always handle these liquids carefully to avoid skin burns and frostbite. 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.
  • Use wooden or rubber tongs to remove small items from cryogenic liquid baths. Cryogenic gloves are for indirect or splash protection only, they are not designed to protect against immersion into cryogenic liquids.
  • When transferring into a secondary container, do not fill the secondary container to more than 80% of capacity
  • Check cold baths frequently to ensure they are not plugged with frozen material.