Cryogenic Liquids: An Overview

A cryogenic liquid is defined as a liquid with a normal boiling point below -240°F (-150°C, 123°K). The most commonly used industrial gases that are transported, handled, and stored in the liquid state at cryogenic temperatures are argon, helium, hydrogen, nitrogen, and oxygen.

There are a number of general precautions and safe practices that must be observed because of the extremely low temperatures and high rates of conversion into gas of all the cryogenic liquids. There are also specific precautions that must be followed where a particular liquid may react with contaminants or may present other hazards associated with that particular product such as asphyxiation or flammability.

As always, end users should have and be thoroughly familiar with the Material Safety Data Sheet (MSDS) for their specific product. All operators must also be trained on the equipment to be used with the cryogenic liquid using instructions provided by the manufacturer.

The two schematics below show a side and top view of a typical cryogenic liquid cylinder. Cryogenic liquid cylinders are insulated, vacuum-jacketed pressure vessels. They come equipped with safety relief valves and rupture discs to protect the cylinders from pressure build-up. These containers operate at pressures up to 350 psig and have capacities between 80 and 450 liters of liquid. Product may be withdrawn as a gas by passing liquid through an internal vaporizer or as a liquid under its own vapor pressure.

 

 

BEST PRACTICES WHEN WORKING WITH CRYOGENIC LIQUIDS

 

Wear the proper Personal Protective Equipment.      

  • Face Shield, worn over Chemical Splash Goggles

  • Insulated Cryogenic Gloves

  • Closed-Toe Shoes

  • Long pants (without cuffs

  • Lab Coat (without cuffs

  • Do not use a funnel.
 
  • Do not store container(s) of Liquid Nitrogen in a cold room or any other location where a person could physically enter an anoxic atmosphere. Cold rooms have not ventilated in any way so there is no air changing per hour.  A person entering a room with elevated nitrogen gas levels in the air can quickly pass out and die within several minutes of entering.
 
  • Liquid nitrogen will condense oxygen from the air.  This is most alarmingly demonstrated if a person leaves his/her vacuum pump's coldfinger in a Dewar of liquid nitrogen overnight.  In the morning the coldfinger will contain LIQUID OXYGEN up to the level of the nitrogen in the Dewar.
  • Guard against pressure build-up by using a pressure relief vessel or a venting lid.
  • Glass Dewars must be taped solidly around the outside.  The plastic mesh that surround smaller thermoses are sold to protect the Dewar itself, but does not protect very well against injury from glass shards resulting from an unexpected implosion.
  • Asphyxiation -- nitrogen is a simple asphyxiant. Nitrogen does not have good warning properties. You cannot see it or smell it.  In fact, 78% of the air we breathe is nitrogen with oxygen making up about 21%. The rest is trace gases. However, if sufficient liquid nitrogen is vaporized so as to reduce the oxygen levels to below 19.5%. At this level, you are at risk of oxygen deprivation. Rapid venting of liquid nitrogen can cause near-total displacement of normal air, leading to a local concentration of about 100% nitrogen. If there is too much nitrogen in the air, you might not feel light-headed, you may simply pass out without any warning whatsoever. You can then die without regaining consciousness.
  • Use only vessels designed for extreme cold.  Not all Dewars are rated for liquid nitrogen.  Always follow manufacturers' guidelines for use of cryogen vessels of any size.
  • Never not carry liquid nitrogen in a passenger elevator.
  • Always have at minimum of two people moving a large Dewar through a hallway.
  • Cryotubes containing samples stored under liquid nitrogen may explode without warning.  Tube explosions are thought to be caused by liquid nitrogen entering the tube through minute cracks and then expanding rapidly as the tube thaws.  Serious accidents have occurred around the country due to tube failures.  A researcher in California lost an eye; numerous others have suffered plastic shards embedded in their hands and faces.
  • Liquid Nitrogen is to be dispensed into smaller Dewars that (a) have carrying handles or (b) are on wheels, or (c) are 500 mL or smaller, and have pressure relief valves or pressure venting lids.  A wide-base Dewar which is stable on a wheeled cart qualifies as "on wheels."
  • Persons filling smaller dewars from a larger liquid nitrogen tank must be constantly vigil during the filling process.
  • If the receiving vessel is small enough to be placed on an adjustable table, adjust the table so that the delivery is immediately at the mouth of the receiving vessel. Do not allow the liquid nitrogen to fall a distance to reach the mouth of the receiving vessel. If the receiving vessel is too large to sit on a table during the filling process, other provisions must be made so that the vessel sit can be raised up to the delivery tube.
  • Always wear proper Personal protective equipment (PPE) while filling a vessel. Persons filling dewars should wear:  full length non-cuffed trousers (covering tops of shoes) or full length apron, shoes that will not admit spilled cryogen liquid and are easy to remove quickly, cry-gloves and safety goggles or a shield. Remove all watches and jewelry.
  • Do NOT move/bend the copper fill tube.  This will cause undue wear to the tube and will eventually cause the tube to break.

 

 

Transporting liquid nitrogen by hand or cart through a building.

Airgas is the preferred gas vendor at UVM. They have dedicated staff at UVM daily.  Contact AIrgas to have them move gas cylinders for you. If you need to move a large liquid nitrogen cylinder in the hallway, take the following preccautions:

 
  • Large mobile Dewars used for transport should always be equipped with a braking mechanism.  Do not use feet to brake.       
     
 
  • Steel toed or closed toed boots are recommended.   
 
 
  • Take care to avoid crushing hands or fingers between the vessel or cart and walls or door frames. 
 
 
  • If there is any risk of the cylinder tipping, a cart should be used. Wheeled trolleys may not be used if the vessel must pass over slots/crevasses wider than 25% of the wheel width. 
 
  • Never transport a large nitrogen Dewar outside -- over pavement, sidewalks, wheelchair curb-cuts. The casters welded to the tank, and/or the casters on the trollies in common use, are not meant for transport over pavement and concrete.  
  • Inside buildings, from room to room, the best transport is to transport Dewars that have carrying handles (4 L and less) or are on wheels (larger Dewars), and which have pressure relief valves or pressure venting lids.  (A wide-base Dewar which is stable on a wheeled cart qualifies as "on wheels.")
  • For short distances in hallways or from building to building, it is acceptable to hand-carry a pint (~ 500 mL) or smaller Dewar of nitrogen which has no handles. All other dewar sizes must have handles.  NEVER carry books, coffee, and other items while carrying a dewar.
  •  
    • the vessel has a venting lid (a cork or loose stopper is fine), and
     
     
    • WATCH carefully for people who may run into you, and
     
     
    • CARRY the vessel with both hands and as far away from your face as comfortably possible.

Dewars: Examples of liquid nitrogen vessels are shown below. All are frequently referred to as "Dewars."

Dewar Flask

HDPE 1-Litre

Cost: ~ $100

Also comes in 2 Ltr and 4 Ltr size with handle.  OK for temperatures from 196C to 100C.

 

Dewar flask

A stainless steel 1 Ltr container with Borosilicate glass inner liner. Cost: ~ $140

Dewar flask Cylindrical silvered borosilicate glass

375mL
Cost: ~ $60

 

FisherBrand Metal-Cased cylindrical, in various sizes from 0.75 qt to 4.5 qt, with and without handle, $90 - $240

From catalogue text: "Provide safe and efficient storage and handling of liquid nitrogen.  Lightweight design makes them ideal for everyday laboratory use.  Narrow neck and vacuum insulation keep evaporation rates low.  Durable aluminum, double-wall construction ensures years of trouble-free service even with the rough handling associated with portable cryogenic equipment.  Pail-style handle on the Thermo 5 and 10 makes pouring easy even in hard-to-reach places. Thermo 20 and 30 offer the same lightweight design and low liquid nitrogen consumption rate in a larger Dewar.  Thermo 35 and 50 are pressurized cylinders designed for researchers who need a continuous source of liquid nitrogen but lack space for larger cylinders.  Relief valve setting on the 35- and 50-liter models is 22psi.  Approx $1000 for the 30 L model."
Dipper, 12 mL, for removing nitrogen from the Dewar.  

El-cheapo - the plastic mesh covered version with no handle.  Borosilicate glass. Metal casing surrounds only bottom portion of flask. Protective plastic mesh covers remaining outer surface of glass. Special vented polyethylene stopper minimizes evaporation and potential for splashing when moving (If you use it.).

$67 - 205 for 665 mL - 4300 mL sizes

Stainless Steel Vacuum-Insulated Dewars

25, 35 and 50 L capacities available

With static evaporation rates as low as 0.7L/24h

photo shows ES-25 & ES-50 dewars with optional pouring trolley, tipping stand & 50KF dispensing head, from CRYOTECHNICS. Albion Industrial Estate,78 Albion Road,Edinburgh. EH7 5QZ. UK.

     

This upright tank is a stainless steel 180 L self pressurizing vessel which is used to store, fill & dispense liquid nitrogen to storage refrigerators or other containers,

Wheels such as shown here are acceptable for transport across inner hallways surfaces, elevator thresholds, or onto scales if the wheels are 3" diameter or greater and the wheel width is larger than the gaps over which the vessel must travel.